IE53683B1 - Forming semiconductor devices employing catenated phosphorus materials and such devices - Google Patents

Abstract

High phosphorous polyphosphides, namely MPx excluding crystalline KP15, where M is an alkalimetal (Li, Na, K, Rb, and Cs) or metals mimicking the bonding behaviour of an alkali metal, and x = 7 to 15 or very much greater than 15 (new forms of phosphorus) are useful semiconductors in their crystalline, polycrystalline and amorphous forms (boules and films). MP15 appears to have the best properties. P may include other pnictides as well as other trivalent atomic species. Resistance lowering may be accomplished by doping with Ni, Fe, Cr, and other metals having occupied d or f outer electronic levels; or by incorporation of As and other pnictides.

Description

This invention relates to forming semiconductor devices employing catenated phosphorus materials and to such devices.

In general, the materials used may include high 5 phosphorus polyphosphides (i.e. phosphides where the polymeric nature is maintained), alkali metal polyphosphides, monoclinic phosphorus and new forms of phosphorus. Vapor transport may be employed in making the crystalline, polycrystalline and amorphous phosphorus and polyphosphide materials in bulk, thick and thin films. Flash evaporation and chemical vapor deposition may be used to make thin films. A condensed phase technique may be utilized in producing crystalline and polycrystalline polyphosphides. Diffusion doping may be employed to raise the conductivity of these materials. Rectifying junctions may be formed with the materials by appropriate metal contacts. The film materials may also be used as optical coatings. Powdered crystals and amorphous materials may further be used as fire retardant fillers. The crystalline materials, especially the fibrous forms, may also be employed as the high tensile components of reinforced plastics. 53C83 During the past several decades, the use of semiconductors has become ever increasingly widespread and important. Silicon based semiconductors, for example, have generally been successful in providing a variety of useful devices, such as p-n junction rectifiers (diodes), transistors, silicon control rectifiers (SCR's), photovoltaic cells, light sensitive diodes, and the like. However, due to the high cost of producing crystalline silicon and the ever-increasing demand for semiconductors over a broadening range of applications, there has been a need to widen correspondingly the scope of available useful semiconductor materials.

Useful semiconductors of the present invention, have an energy band gap in the range of about 1 to 3 eV (more specifically 1.4 to 2.2 eV); a photoconductive ratio greater than 5, (more specifically between 100 and 10,000); -5 -12 —1 a conductivity between about 10 and 10 (ohm-cm) •8 (more specifically conductivity in the range of 10 to —9 —1 (ohm-cm) ); and chemical and physical stability under ambient operating conditions. Accordingly, while many materials may be semiconducting in the sense they are not pure metals or pure insulators, only those semiconducting materials which meet these criteria may be considered to be useful semiconductors in the context of this invention.

Given the present need to develop alternative nonpetroleum based energy sources, the potential commercial utility of a semiconductor increases dramatically when the semiconductor also exhibits an effective photovotaic characteristic, that is, the ability to economically and efficiently convert solar energy into electrical potential.

From an economic standpoint, amorphous semiconductors, particularly in the form of thin films, are more desirable than single crystalline forms due to potential lower cost of production. Amorphous semiconductors also have better electrical qualities than polycrystalline forms of the same material as used in many semiconductor devices.

The semiconductor industry has continued its search for useful new semiconductor materials beyond crystalline silicon, and the like.

In the non-silicon crystalline area, single crystals 5 of semiconducting compounds, including GaAs, GaP, and InP, are in commercial use.

Many other semiconductor materials have been utilized for specialized purposes. For example, CdS and selenium are utilized as the photoconductor in many xerographic machines.

In this application semiconductor device means a device including a semiconductor material whether the device emoloys electrical contacts, that is, is an electronic device, or whether it is a non-electronic device, such as the photoconductors employed in xerography, phosphorescent materials, the phosphorus in a cathode ray tube, or the like.

Although some of the known forms of phosphorus have been stated to have semiconducting properties, many are unstable, highly oxidizable and reactive, and no known form of phosphorus has been successfully employed as a useful semiconductor.

The group 3-5 materials such as gallium phosphide and indium phosphide are tetrahedrally bonded and thus, as will be pointed out below, are clearly distinguished from the compounds disclosed herein. Furthermore, their semiconducting properties are not dominated by phosphorus-tophosphorus bonding, i.e. the primary conduction paths are not the phosphorus-to-phosphorus bonds.

Others have disclosed hydrogenated phosphorus having a structure similar to black phosphorus and having semiconducting properties.

Considerable work on high phosphorus polyphosphides has been done by a group headed by H.G. von Schnering.

The various reports from this group indicate that the 53G83 highest phosphorus containing polyphosphide compound they have produced is crystalline MP15 (M = group la metal; These polyphosphides are produced by heating a mixture of metal and phosphorus in a sealed ampule. Von Schnering reports that based on their structure polyphosphides 53883 are classified as valence compounds in a classical sense, and that this means that these compounds are, or should be, insulators or semiconductors, i.e not metals.

Monoclinic phosphorus, also called Hittorf's phos5 phorus, is prepared according to the prior art from white phosphorus and lead as follows: lg of white phosphorus and 30g of lead are heated slowly to a melt in a sealed tube to 630°C and held for a short time at that temperature. The solution is then cooled at the rate of 10° per day for 11 days to 520eC, and cooled rapidly to room temperature thereafter. It is next electrolyzed in a solution of 2kg of lead acetate in 8 liters of 6% acetic acid, and the phosphorus is collected in a watch glass placed under the anode. Nearly square tabular crystals, about 0.2 x 0.2 x 0.05mm, are obtained in this way.

The structure of this prior art monoclinic phosphorus has been determined by Thurn and Krebs. The crystals comprise two layers of pentagonal tubes of phosphorus with all of the tubes parallel, and then another pair of layers of all pentagonal tube phosphorus,· the tubes in the second pair of layers all being parallel, but the tubes in the second pair of layers being perpendicular to the tubes in the first pair of layers. The space group of the crystals has been determined, as well as the bond angles and bond distances. See the summary of the prior art in the section Phosphorus from The Structure of the Elements by Jerry Donahue, published in 1974.

The electronic properties of Hittorf's phosphorus crystals have not been reported. Because of their small size the electrical properties cannot be readily determined.

The preparation of high purity electronic grade phosphorus according to the prior art is very complex and time consuming, thus electronic grade phosphorus is very expensive.

The prior art also exhibits a need for stable phosphorus compounds for use as fire retardants. Crystalline forms have additional utility as reinforcing additives in plastics, glasses and other materials..

There has now been discovered a family of alkali metal polyphosphide materials possessing in particular useful semiconductor, as well as optical and mechanical properties.

Useful Semiconductor Properties By polyphosphide is meant a material dominated by multiple phosphorus-to-phosphorus bonds. By useful semiconductor is meant not only that the conductivity is intermediate between insulators and metals, but also the demonstration of a host of useful properties: Stability Resilient material structure Bandgap in a useful range (typically 1 to 2.5eV) High inherent resistivity, but with ability to be doped - Good photoconductivity Efficient luminescence Ability to form a rectifying junction Ability to be formed at relatively low temperatures (for semiconductors) by processes amenable to scale-up Ability to be formed as large area amorphous thin films Ability to be formed as ductile polymeric fibers The polyphosphides are a unique family of materials possessing all of these features.

Preservation of Utility in Multiple Forms It is equally significant that the useful properties remain essentially constant over a wide range of chemical compositions and physical forms (crystalline and amorphous).

To our knowledge, polyphosphides are the only useful semiconductors in which desirable single crystal-like 3 6 8 3 properties are preserved in the amorphous form. This is of major technological significance because the amorphous form is at the least more amendable, and often essential, for large scale applications, such as photovoltaic cells, large area displays, and electrostatic copiers.

But up to now, the problem with amorphous semiconductors is that they do not form readily as a stable single phase material. And even when they are forced to, the amorphous form loses some very desirable features of its crys10 talline counterpart.

The dominant known semiconductor (silicon) has a tetrahedral coordination in its crystalline form. Any attempt to make it amorphous (to make amorphous Si) is known to be accompanied by a breaking of the tetrahedral bonds, leaving dangling bonds that destroy useful semiconducting properties. Pure amorphous Si is useless: unstable and crumbly. Attempts to satisfy the dangling bonds with Hydrogen or Fluorine have been only partially successful.

Central Role of Structure We believe that the preservation of useful properties among the multiple forms of the polyphosphides are a direct result of the structure of the materials which, in turn, is made possible by the unique properties of phosphorus, particularly its ability to form polymers dominated by 3 phosphorus to phosphorus covalent bonds at the vast majority of phosphorus sites.

In the crystalline form, the polyphosphides of the type M?i5 (with M = Li, Ha, K, Rb, Cs) have a structure formed by a phosphorus skeleton consisting of parallel tubes with pentagonal cross section. These phosphorus tubes are linked by B-M-P bridges shown in Figures 4, 5 and 6. The building block for this MP1S atomic framework can be viewed as Ρθ 3 6 8 3 (formed by 2 P4 rigid units) and MP? (formed by the association of MPj and P4 rigid units).

Using the building blocks or clusters described above, Kosyakov in a review article (Russian Chemical Review, 48(2), 1979) showed theoretically that these polyphosphide compounds could be treated as polymeric materials using their basic building blocks as monomers. Hence, in principle it is possible to construct a large number of atomic frameworks having the same phosphorus skeletons.

In our work, we have synthesized by various techniques described later, MPcrystals and also compositions of the type [MP7Jawith b much 9reater than a· These novel phosphorus rich compounds originally observed as fibers, whiskers, or ribbons are referred to in this investigation as ΜΡχ with x much greater than 15. These low metal content materials are prepared by vapor transport as thick films (greater than 10 microns) of polycrystalline fibers and large boules (greater than 1 cm3) of amorphous character. The polycrystalline fibers exhibit the same morphology as KP^g whiskers.

The structural framework of the first ΜΡχ (x much greater than 15) crystalline materials we discovered is dominated by a phosphorus skeleton similar to the phosphorus framework of the MP^^ compounds.

We have found that the useful electrical and optical properties of these crystalline materials MP^g and ΜΡχ (x much greater than 15) are similar. The properties of these materials are therefore dominated- by the multiple P-P covalent bonds of the phosphorus skeletons with a coordination number somewhat less than 3. To our surprise we have also discovered that the useful electro-optical properties of these materials were essentially preserved for MP,- and MP (x much greater than 15) crystalline materials 15 X and their amorphous counterparts.

Unlike previously known materials, this is a one dimension rigid structure and is resilient in the following 3 3 3 3 sense. The polyphosphide crystal symmetry is very low (triclinic), We believe that in the transition from the crystal to the amorphous form, the low symmetry material is capable of accommodating in a gradual way the increased. structural disorder that characterizes the amorphous state.

There is -no ripping apart of strong tetrahedral bonds (coordination number of 4) as in silicon because the phosphorus, with a much lower coordination number than silicon, can accept much greater structural disorder without the creation of dangling bonds. The polyphosphides are polymeric in nature. The result is a polymeric amorphous structure with no apparent X-ray diffraction peaks, one with longer-range local order than is achievable with conventional amorphous semiconductors. We believe that this gradual onset, in the structural sense, of amorphicity is the reason for the preservation of the desirable crystal properties in the amorphous polyphosphides.

Distinguishable from Known, Useful Semiconductors The composition and structure of the family of poly20 phosphides clearly distinguishes them from all known, useful semiconductors: Group 4a 3a-5a (IIX-V) 2b-6a (II-VI) Chalcogenides lb-3a-6a (Crystal Si, amorphous Si:H, etc.) (GaAs, GaP, InP, etc.) (CdS, CdTe, HgCdTe, etc.) (As2Se3) (CuInSe2) Distinguishable from Known Forms of Phosphorus The alkali polyphosphides (ΜΡχ, M = Li, Na, K, Rb, Cs; where x = 15 and much greater than 15) are phosphorus 30 rich. In cases of high x" material they are almost all phosphorus. Nonetheless, their structure (parallel pentagonal tubes) and their properties (stability, bandgap, 53G83 conductivity, photoconductivity) clearly distinguish them from all known phosphorus materials (black, white/yellow, red, and violet/ Hittorf). The structural relationships among these various forms are discussed below.

Our work has done much to clarify this aspect of phosphorus itself. The nomenclature in this area has been somewhat confusing. The following summarises our current usage. 1. Amorphous P or Red P Amorphous red phosphorus is a generic term 1° for all non-crystalline forms of red phosphorus, usually prepared by thermal treatment of white phosphorus. 2. Violet P This microcrystalline form of red phosphorus 15 is prepared from charges of pure P, either white or amorphous red, by extended thermal treatment. 3. Hittorf'3 P Crystalline form of red phosphorus struc20 turally identical to Violet P. Hittorf*s P is prepared in the presence of a large excess of lead. Despite this, the terms Hittorf*s P and Violet P have often been used interchangeably. The crystal structure consists of double layers of parallel pentagonal tubes, with adjacent double layers perpendicular to each other in a monoclinic cell. Hittorf's P crystals are somewhat larger (approximately 100 microns) than violet P microcrystals. 4. Large Crystal Monoclinic Phosphorus Even larger crystals (several mm), essentially isostructural with the above two, are described herein. These.novel crystals are prepared by Vapor Transport (VT) treatments 3 G 8 3 of alkali-phosphorus charges. The inclusion of the alkali is apparently essential for formation of the large crystals. Analysis confirms the presence of alkali (500 to 2000 \ ppm) in these large crystals of phosphorus.

. Twisted Fiber Phosphorus A crystalline form of phosphorus described herein prepared by VT treatments of amorphous P charges. Believed to be neaxly-isostructural with polycrystalline ΜΡχ ribbons.

ROLE OF THE METAL: WHY PHOSPHORUS IS NOT GOOD ENOUGH The many allotropic forms of elemental P are evidence for the variety and complexity of the bonds and structures 15 that are accessible with phosphorus. We lack a detailed, comprehensive model of exactly how the alkali metal works, but have developed a large body of data showing that the metal stabilizes phosphorus so that a single unique struc5 3 6 3 3 ture may be selected from the ensemble of potentially available structures.

Without at least some alkali metal, the following undesired phenomena occur: A. The phosphorus is unstable (e.g., White P).

B. To the extent that a known single phase is accessible to the P, it can only do so at high temperatures and of a size limited to microcrystals (e.g., Violet P), or C. At high pressures (e.g., black P).

D. Without an alkali metal in the charge, the ΜΡχ type of structure is not formed by vapor transport. Rather, the twisted phosphorus. fiber form we have discovered is obtained. This crystalline phase is metastable and the structure is not well defined as shown by our X-ray, Raman and photoluminescence data.

The presence of alkali metal in vapor transport favors the all-parallel untwisted phase. It also, as discovered by us, favors large crystals of monoclinic P to form at a different temperature.

The dominant role of structure, not composition, as the determinant of properties is made clear by noting that ΚΡχ (x much greater than 15) has properties (bandgap, photoluminescence, Raman spectra) that are essentially those of KP15» but are somewhat different from those of monoclinic P.

It is clear that even a little alkali metal can serve to select a stable phase. But will non-alkalis work? Krebs reported non-alkali polyphosphides, with tubular structures consisting of 2b-4a-P14 (2b = 2n, Cd, Hg and 4a = Sn, Pb). Why do these form, A speculative hypothesis is that these materials form in the tubular structure because the Group 4a element is amphoteric and can occupy a P site in lieu of P.

One can compute an effective electron affinity of the P15 framework based on the ionization energies of the alkali metals, all of which are less than or equal to 5.1 3 6 8 3 eV. One can, in turn, calculate effective ionization potentials for other possible compositions such as the 2b-4a-P^ compounds. All of Krebs' materials noted above have effective ionization less than or equal to 4.8 eV.

QSEFOL PROPERTIES Our major initial discovery was that the KP^5whis^ers (single crystal) were stable semiconductors, with an energy bandgap corresponding to red light (1.8eV) and exhibiting efficient photoconductivity and photoluminescence. These are the hallmarks of a semiconductor with potential applications in electronics and optics. Whiskers of the other alkali MP^g materials also have these properties (M = Li, Na, Rb, Cs).

To realize their potential, the materials had to be prepared in a size and form suitable for fabricating devices and for testing. We recognized that the crystal habit, however, is not conducive to growth of large, single crystals that are free of crystallographic twinning. Large, twin-free single crystals are the basis of nearly all semiconductor device technology today. Polycrystalline materials are less desirable because even if the individual grains are large, the presence of grain boundaries serves to destroy some desirable properties due to the physical and chemical discontinuities that are associated with such boundaries. Hence, our attention turned to the amorphous forms we had discovered.

Use'ful amorphous semiconductors, whether used as a junction device such as a photovoltaic cell, or as a coating such as in an electrostatic copier, have been generally made as thin films for extrinsic reasons (cost, manufacturing ease, and application need) and intrinsic reasons (material problems in the bulk amorphous state).

We have discovered that KP15 can be made as a stable amorphous thin film (by Vapor Transport). (This cannot be done with silicon: amorphous Si is not stable, while single crystal Si is.) Stable, bulk, and thin film amorphous KP (x much greater than 15) can also be made by vapor transport. 53633 There is evidence that these polyphosphides are unusual in yet another way. The useful properties of these materials MP,_ and MP„ (x much greater than 15) are similar in their crystalline forms and their amorphous counterparts as shown in Tables XVI and XVII below.

Applications utilizing amorphous thin film KP^g requiring no junctions can be readily envisioned (e.g., electrostatic copying). In fact, the high inherent O □ resistivity (approximately 10 to 10 ohm-cm) is an 10 advantage for such junctionless system applications.

Electronic and opto-electronic devices all require that some junction be formed in or with the material.

This- requires lowering the resistivity of the material by doping.

We have discovered than Ni diffused into KP^g serves the purpose of reducing the resistivity of the material by several orders of magnitude. Surface analysis has demonstrated that Ni diffusion from the solid state (KP^5 deposited onto a layer of Ni) follows a normal diffusion pattern during the growth process of the film.

Device configurations with Ni as a back contact and diffuser; and other metals, such as Cu, Al, Ug, Ni, Au, Ag and Ti as top contacts, lead to junction formation. Junction Current-Voltage (I-V) characteristics have been measured with these top contacts. Junction CapacitanceVoltage (CV) characteristics have been measured with Al and Au top contacts. The data indicates double junction formation with a high resistance layer near the top contact.

The high .resistance layer is an undoped portion of the KP1g film which results from the present doping procedure.

A small photovoltaic effect (micro amp current under a short circuit condition) has been observed.

SYNTHESIS OF POLYPHOSPHIDES Below are described the methods we have discovered that will produce polyphosphides of varying composition and morphology.

A. Condensed Phase (CP) Synthesis This refers to the process of Isothermal heat-up, soak (heating at set temperature), and cool down of starting charge carried out in a container of minimum volume. There is no vapor transport. Crystalline and bulk polycrystalline MPJ5 are produced.

B. Single Source Vapor Transport Synthesis (1S-VT) A starting reactant charge is located in one area of an evacuated tube which is heated to a temperature, Tc, which is greater than Td, where Td represents the temperature(s) of other area(s) of the tube where materials deposit from the vapor. Crystalline MP1S; crystalline, polycrystalline (bulk, and thin films) and amorphous bulk high x, ΜΡχ; monoclinic phosphorus; star shaped fiber; and twisted fiber phosphorus are produced.

C. Two Source Vapor Transport Synthesis (2S-VT) Source reactant charges loaded in an evacuated chamber are separated physically by distance with a deposition zoae between them. The two sources are heated to temperatures greater than the deposition zone (in order to get amorphous material, at least; see below). The deposition zone need not be the coldest one in the system, but a colder area should not be able to trap more than one component. 2S-VT was the first method used to make thin film amorphous KPj^. Polycrystalline and amorphous thin films of MP^ and polycrystalline thin films and bulk amorphous high x, ΜΡχ are produced.

D. Melt Quench A charge is heated in a sealed evacuated tube (isothermally, if possible) to temperatures greater than the melting point as determined by endotherms observed in DTA experiments, and held there for some period of time. The tube is then removed from the furnace and rapidly cooled.

CsPy glass has been produced.

E. Flash Evaporation A charge in powder form is fed in small amounts, under a slight Argon flow, into an RF-heated susceptor, which 3 6 8 3 is maintained at temperatures greater than about 800eC. Inside the susceptor, the material is put throi.gh a tortuous path where it is, in theory, forced to contact hot surfaces. This is intended tc rapidly and completely vaporize the charge such that the composition of the resultant vapor stream is the same as that of powder being injected. The vapor stream is directed into an evacuated chamber where it strikes cooler surfaces, resulting in condensed-product materials. Amorphous films have been produced. p. Chemical Vapor Deposition (CVD) In general, this refers to production of material by mixing two (or more) vaporized components which must undergo some chemical reaction to give products. As practiced by us, K and P^ are independently metered into furnaces where they are rapidly vaporized and carried downstream by the Argon flow to a cooler reaction chamber where the combined streams yield condensed product materials.

The significance of CVD lies in that of these methods, it is the most amenable to scale-up and to doping in situ, i.e., simultaneous synthesis and doping of material. Amorphous thin films of KP^,. have been produced.

G. Molecular Flow Deposition (MPD) This is -a multi-source vapor transport technique that draws on 2S-VT and Molecular Beam Epitaxy (MBS). Independently heated sources are used a'nd the vaporized species are allowed to reach the substrate (also independently heated) at a controlled rate not achievable with 2S-VT. The deposition takes place in an evacuated chamber with in situ monitoring of the deposition (also not available with 2S-VT). The chamber may be sealed or continuously evacuated to control pressure.

SO KP^g Materials A large variety of polyphcsphide materials of different physical forms and compositions were initially prepared during our investigations.

However, for potential useful semiconductor . applications r the emphasis of our work has changed from the preparation of single crystal materials to that of amorphous materials - either in bulk or large area thin films.

Among all the MP^g materials KP^g is a unique crystalline higher polyphosphide (x the same as or greater than 7) compound which exists for the K-P system. (In contrast, the other alkali metals can form compounds with x = 7 or x = 11, such as CsP7, NaP?, BbP^, etc.). ΚΡχχ and KP^ do not form as compounds. For this reason, the K-P system is easier to control than the other alkali-metal-P systems, where multiple compounds can form.

In addition, from the results of our experimental work, it is apparent that whenever K + P are vaporized, by whatever means, and brought in the proper ratio ([P]/[K] the same as or greater than 15) to a zone whose temperature is in the proper window, amorphous ΚΡ^5 will form. By this window we mean the temperature must be low enough to prevent crystallization of KP^g and high enough that ΚΡχ, where x is much greater than 15, is not deposited.

Based on this tenet, all synthesis methods can be seen to operate on the same general principle. Each method simply used different means to achieve control of source vaporization or control of deposition. The two source systems (2S-VT, CVD and MFD) are particularly useful as the important variables can be independently controlled.

Based on the above considerations, KP^g amorphous in tain films has been selected by us as our leading composition for the development of useful semiconductor materials.

SUMMARY In a general inquiry into the nature of polyphosphides, potassium polyphosphide whiskers of about 1 cm in length were produced by single source vapor transport. In investigating the properties of this material it was determined by x-ray diffraction of a single crystal that the crystals were KP15. It was also discovered that these crystals were semiconductors. When measuring an emission at 4’K under argon laser illumination, photoluminescence was observed having an energy of 1.8 eV, thus indicating that the material possibly had a band gap within this energy range.

Later, in order to determine the conductivity of these whiskers, leads were attached with silver paint. In order to see if the leads were actually attached to a very small crystal, it was placed under a microscope while the conductivity was measured. Surprisingly the conductivity changed dramatically when the crystal was moved in the microscope, changing the illumination. A photoconductivity ratio of 100 was measured with the unilluminated conductivity of the -8 -1 whisker being about 10 (ohm-cm) . To establish whether the whiskers had a band gap, measurements were then made of the wavelength dependence of the photoconductivity, the wavelength dependence of the optical absorption and the temperature dependence of the conductivity of the whisker. These measurements, together with the photoluminescence measurement at 4°K, established that the whiskers had a band gap of approximately 1.8 eV. Thus it was established that KP|g crystalline whiskers were potentially useful semiconductors.

An amorphous film was formed on the inside of the quartz tub-· during the vapor transport production of the KP15 whiskers. This amorphous film was also found to have a band gap on the order of 1.8 eV and a photoconductivity ratio on the order of about 100. Like the whiskers, the 3 8 8 3 53G83 The problem then presented to the inventors was whether could be produced as large crystals, such as silicon, used in semiconductor production; whether polycrystalline or amorphous films of KP15 could be reproducibly made and utilized for semiconductor production; and the full characterization of the materials produced by the vapor transport experiment and any analogous materials which might have the same useful properties.

After many vapor transport experiments the inventors were astonished to find that the polycrystalline and amorphous materials that were produced by vapor transport where a single source' of a mixture of potassium and phosphorus is heated and material condensed at the other end of a closed tube, were not KPjj but when measured by wet analysis were ΚΡχ where x seemed to range from about 200 to about 10,000.

The inventors have since made the amazing discovery that the affinity of phosphorus for potassium, or any alkali metal for that matter, in single source vapor transport causes initial deposition of MP^g as the most stable polyphosphide. If there is an excess of phosphorus, then a new form of phosphorus will be deposited. (ΜΡχ where x is much greater than 15) This new form of phosphorus has the same electronic qualities as KP^S and i3 a useful semiconductor.

During the course of their investigations the inventors, in an effort to form thin films of polycrystalline and amorphous and other alkali metal analogs which could not be formed by single source vapor transport, conceived of a two source (separated source) vapor transport method in which the alkali metal and the phosphorus are spaced apart and separately heated. By controlling the temperature of a separate intermediate deposition zone, thin films of MP15 where M is an alkali metal, have been made in polycrystalline and amorphous forms. This technique has also led to the production of thin, films of polycrystalline and thick films of amorphous phosphorus material of the new form, and other materials presumably polymer-like having the formula ΜΡχ, where M is an alkali metal and x is much greater than IS. ο 3 3 4 we have also used Flash Evaporation, Chemical Vapor Deposition, and propose to use Molecular Flow Deposition methods for synthesizing these materials.

We use MF as the formula for all phosphosphides. As 5 will be pointed out below, for useful semiconductors, x may range from 7 to infinity.' Known alkali polyphosphides have the formula MP MP^^, and MP^^. We have discovered that presumably polymer forms exist having the formula ΜΡχ where x is much greater than 15.

Also during these investigations single source vapor transport has been improved over the prior art by controlling the deposition temperature to be constant over a large area, so that large area thick films and boules of polycrystalline and amorphous ΜΡχ where x is much greater than 15 15 have been formed.

Large quantities of crystalline and polycrystalline MPig, where M is an alkali metal, have been made by isothermolly heoting together stoichiometric proporfions of an alkali metal and phosphorus. This condensed phase method produces excellent ΜΡχ where x ranges from 7 to 15 for use in single source vapor transport. The condensed phase method itself is facilitated by the prior mixing and grinding together an alkali metal and phosphorus in a ball mill which is preferably heated to a temperature in the neighbor25 hood of 100°C. This ball milling surprisingly produces relatively stable powders.

All of the parallel tuba polyphosphides have a band gap of approximately 1.8 eV, photoconductivity ratios much greater than 5, (measured ratios having a range from 10C to —8 ,000) , and low conductivity in the order of 10 to -9(ohm-cm)-1.

Since we have discovered that the amorphous forms of these materials, i.e. alkali polyphosphides MP where x is greater than 6 formed in the presence of an alkali metal have substantially the same semiconductive properties, we conclude that the local order of the amorphous materials is the same, i.e.al 1 parallel pentagonal tubes substantially throughout their extent. 3 8 8 3 In all the polyphosphides, the 3 phosphorus-to-phosphorus (homatomic) covalent bonds at the majority of phosphorus sites dominate any other bonds present to provide the conduction paths and they all have semi5 conductor properties.

The covalent bonds of the phosphorus atoms,all of which are used in the catenation providing the dominant conduction paths and the parallel local order in these materials, provide the good semiconducting properties.

The phosphorus atoms are trivalent and the catenations form spirals or tubes having channel-like cross sections. The alkali metal atoms, when present, join the catenations together. Atomic species other than phosphorus, particularly trivalent species capable of forming 3 covalent hom15 atomic bonds, should also form semiconductors.

Thus we have invented new forms of phosphorus and methods of· making the same, solid films of amorphous and polycrystalline ΜΡχ and methods and apparatus for making the same, methods and apparatus for making metal poly20 phosphides by multiple temperature single source techniques, methods and apparatus for making high phosphorus polyphosphides by multiple separated source techniques, methods and apparatus for making MPlg by condensed phase techniques in polycrystalline forms, semiconductor devices comprising polyphospbide groups of seven or more phosphorus atoms covalently bonded together inpentagonal tubes having a band gap greater than 1 eV and photoconductivity ratios of 100 to 10,000, semiconductor devices comprising ΜΡχ where M is an alkali metal and x is greater than 6, and materials having a band gap greater than 1 eV and photoconductivity ratios of 100 to 10,000, semiconductor devices formed of a high pioportion of catenated covalently bonded trivalent atoms, preferably phosphorus, where the catenated atoms are joined together in multiple covalent bonds, the local order of which comprises layers of catenated atoms which are 8 ο 8 3 parallel in each layer and the layers are parallel to each other, the catenations preferably being pentagonal tubes, semiconductor devices comprising an alkali metal and said 53G83 catenated structures wherein the number of consecutive covalent catenated bonds is sufficiently greater than the number of non-catenated bonds to render such material semiconducting, semiconductor devices formed of compounds comprising at least two catenated units, each unit having a skeleton of at least 7 covalently bonded catenated atoms, preferably phosphorus, and having alkali metal atoms, conductively bridging the skeleton of one unit to another, junction devices, methods of forming such semiconductor devices, methods of doping such semiconductor devices, methods of conducting electrical current and generating electrical potential utilizing such devices.

We have therefore discovered a whole class af materials to be useful semiconductors, some members of the class having been first produced or properly characterized by us, and others of which have been produced in the prior art with their useful semiconductor properties being unknown until our discoveries and inventions.

All of these materials have a band gap within the range of 1 to 3 eV, preferably within the range of 1.4 to 2.2 eV and most preferably about 1.8 eV. Their photoconductivity ratios are greater than 5 and actually range between 100 and 10,000. Their conductivities are within -5 -12 -1 the range of 10 -10 (ohm-cm) , being in the order of 108(ohm-cm)-1.

Those skilled in the art will readily understand that the alkali metal component M of polyphosphide or any appropriate trivalent ide capable of forming homatomic covalent bonds, and having the formula ΜΥχ may comprise 30 any number of alkali metals, (or combination of metals mimicking the bonding behavior of an alkali metal) in any proportion, without changing the basic pentagonal tubular structure and thus without significantly affecting the electronic semiconductor properties of the material.

We have further discovered and invented methods of doping the materials of the invention utilizing doping with iron, chromium and nickel, to increase the conductivity. Junctions have been prepared using Al, Au, Cu, Mg, Ni, Ag, Ti, wet silver paint, and point pressure contacts. 53883 The incorporation of arsenic into the polyphosphides (all parallel tubes) has also been demonstrated to increase conductivity.

These doping methods are also part of our invention .5 and discovery.

The semiconductor materials and devices of the present invention have a wide variety of uses. These include photoconductors such as in photocopying equipment; light emitting diodes; transistors, diodes, and integrated circuits; photovoltaic applications; metal oxide semiconductors; light detection applications; phosphors subjected to photon or electron excitation; and any other appropriate semiconductor application.

In the course of our work we have also produced for the first time large crystals of monoclinic phosphorus.

These crystals are obtained from vapor transport technique using an MP^g charge or mixture of M and P (M/P) in varying ratios.

Surprisingly, these large crystals of monoclinic phosphorus contains a significant amount of alkali metals (500 to 2000 ppm have been observed), tinder the same conditions, these crystals cannot be grown without the presence of alkali metals in the charge.

Two different crystal habits have been observed for these large crystals of phosphorus.

One crystal habit was identified as truncated pyramidal shape crystals as shown in Figure 39. These crystals are hard to cleave. The other form is a plateletlike crystal and is cleavable as shown in Figure 40.

The largest crystals we have produced in the habit shown in Figure 39 are 4 x 3 mm x 2 mm high. The largest crystals we have produced in the habit shown in Figure 40 are 4 mm square and 2 mm thick.

The crystals are metallic looking on reflection and deep red in transmission. Chemical analysis indicates that they contain anywhere from 500 to 2000 parts per million of alkali metal. Their powder X-ray diffraction patterns, Raman spectra and differential thermal· analysis are all consistent with the prior art Hittorf's phosphorus.

Photoluminescence of crystals grown in the presence of Cesium in Figure 41 and crystals grown in the presence of Rubidium in Figure 42 show peaks at 4019 and 3981 cm 1, which indicate a band gap of about 2.1 eV at room temper5 ature for this monoclinic phosphorus.

The crystals may be utilized as a source of phosphorus; as optical rotators in the red and infra-red portion of the spectrum (they are birefringent); as substrates for the growth of 3-5 materials such as Indium Phosphide and Gallium Phosphide. They may be utilized in luminescent displays or as lasers.

We have grown from the same charge and deposited at a slightly lower temperature the star shaped fibrous crystals shown in Figures 44 and 45.

We have also grown by vapor transport a crystal allotrope of phosphorus, the twisted fiber of phosphorus shown in Figure 46.

The polyphosphides may also be used as fire retardants and strengthening fillers in plastics, glasses, and other materials. The twisted tube and star shaped fibers should be of particular value in strengthening composite materials because of their ability to mechanically interlock with the surrounding material. The platelets should be of particular value in thin sheet material where glass flakes are now employed.

The film materials of the invention may also be utilized as coatings for their chemical stability, fire retardant, and optical properties.

Having illustrated the field of the present invention, it will now be discussed more specifically.

The present invention provides a method of forming a semiconductor device which comprises: (a) providing a material comprising, at least as one component thereof, a polyphosphide having the formula: ΜΡχ wherein M represents an alkali metal; and x represents the atom ratio of P to M, x being at least 7; the number of consecutive covalent phosphorus-tophosphorus bonds being sufficiently greater than the number of non-phosphorus-to-phosphorus bonds to render the said material semiconducting; and (b) attaching to the said material means for electrically communicating with the said material to utilize the said material as a semiconductor; the said semiconductor having chemical and physical stability under ambient operating conditions, an energy band gap of from 1 to 3 eV, a photoconductive ratio greater than 5 and a conductivity between 10 0 and 10-12 (ohm-cm)-1.

The present invention also provides a semiconductor device which comprises: (a) a material as defined above; and (b) means attached to the said material for electrically communicating with the said material.

The present invention may be regarded as being more convenient than the prior art and less expensive. reference may also be made to a divisional Patent Specification No. ' which, inter alia, relates to similar materials.

In accordance with the present method, there is preferably provided a material comprising, at least as one component thereof, at least two polyphosphide units, each unit having a skeleton of at least 7 covalently bonded phosphorus atoms, the said units having associated 53633 therewith at least one alkali metal atom, the said alkali metal atoms conductively bridging the phosphorus skeleton of one unit with the phosphorus skeleton of another unit, and the said material having a band gap primarily determined by the said phosphorus-to-phosphorus bonds. More preferably, there is provided a material comprising, at least as one 4 component thereof, a polyphosphide having the formula ΜΡχ wherein M represents an alkali metal; and x represents the atom ratio of P to M, x being at least 7; and wherein the said material has an energy band gap of from 1 to 3 eV.

The material provided may have a bandgap of from 1.4 to 2.2 electron volts, e.g. substantially 1.8 electron volts, and/or a photoconductivity ratio of from 100 to 10,000. It may comprise a single alkali metal or at least two different alkali metals. The material may be polycry10 stalline or amorphous.

In certain preferred embodiments of the present method, in the said material at least 7, e.g. 15, phosphorus atoms may be bonded to other phosphorus atoms per each of a metal atom In the said material. There may be at least 500 phosphorus atoms per each of a metal atom in the said material.

It is particularly preferred that the polyphosphide correspond to the formula; ΜΡχ wherein M represents an alkali metal and x is from 7 to 15. The subscript x may be substantially equal to 15 or greater than 15. The material may be defined by the formula; [MP7]a [Ρθ]&, wherein b:a is the atom ratio of [Pg] to [MP?.].

In the present devices, it is preferred that at least 6/7ths of the atoms have 3 homatomic bonds exclusively, more preferably at least 14/15ths have 3 homatomic bonds.

In certain preferred instances, the material in the present devices may be characterized in its local order by tubular columnar structure, more preferably the tubular structures in a local order are all generally parallel.

The columnar structure may be channel-like and/or pentagonal when viewed on end.

Preferably, the major component atoms of the material are trivalent, more particularly they are one or more pnictides, in particular they are predominantly phosphorus.

The average bond angle is generally greater than 98° and the bonds may be spaced at an angle of from 87 to 109°. Additional atoms of one or more different elements than atoms of the catenations may be bonded between two or more of the said catenations and may form conduction paths therebetween. Preferably, the catenations in each local order are all generally parallel.

The material may be formed as the deposition product from vapour transport in a deposition zone from separated sources of phosphorus and an alkali metal. The atom ratio of phosphorus to metal may be substantially 50 or greater, more preferably substantially 200 or greater, most preferably substantially 1000 or greater. The amount of metal is preferably less than 1,000 parts per million.

The material may be formed of a single crystal or may be in the form of a thin film.

A device in accordance with the present invention may comprise a junction, which may comprise a metal selected from Cu, Al, Mg, Ni, Au,. Ag and Ti, preferably Ni. The material may be doped with atoms of another pnictide, such as As. Also, the material may be doped by diffusing therein a metal having occupied outer f or d electronic levels, such as nickel, iron or chromium.

The present devices may comprise metal contacts selected fromCu, Al, Mg, Ni, Au, As and Ti. Preferably, the devices in accordance with the present invention comprise a material corresponding to the formula: MPx wherein M represents Li, Na„ K, Rb or Cs; and x is at least 7.

Having defined the present invention, it will now be discussed and illustrated in more general terms.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in connection with the accompanying drawings, in which: FIGURE 1 is a diagrammatic view partly in cross section of single source vapor transport apparatus; FIGURE 2 is a diagrammatic view of a portion of the vapor transport apparatus of FIGURE 1; FIGURE 3 is a diagrammatic view of another single 10 source vapor transport apparatus; FIGURE 4 is a computer diagram from X-ray diffraction data of phosphorus atoms in MP^g where M is an alkali metal; FIGURE 5 is a computer diagram from X-ray diffraction data of a cross-section of KP^g showing how the covalent bonding of the phosphorus atoms of FIGURE 4 forms a pentagonal tubular structure; ' FIGURE 6 is a computer diagram from X-ray diffraction data in longitudinal section of KP^g! FIGURES 7 and 8 are photomicrographs of KPlg crystal 20 whiskers; FIGURE 9 is a powder X-ray diffraction fingerprint of crystalline KP^5; FIGURE 10 is a powder X-ray diffraction fingerprint of crystalline ΚΡχ where x is much greater than 15; FIGURE 11 is a diagrammatic view of an experimental reaction tube for two source vapor transport; FIGURE 12 is a plot of temperature versus length for the reaction tube of FIGURE 11; FIGURE 13 is a diagram of the P to K ratio of the 30 reaction products in the reaction tube of FIGURE 11, FIGURE 14 is a schematic diagram of apparatus for a two source vapor transport; 53833 FIGURE 15 is a diagram of one of the elements of the apparatus illustrated in FIGURE 14; FIGURE 16 is a diagrammatic view of another reaction tube for two source vapor transport; FIGURE 17 is a diagrammatic view of a ball mill; FIGURE 18, 19 and 20 are scanning electron micrographs (SEM's) of a film of a new form of phosphorus ΜΡχ where x is much greater than 15; FIGURE 21 is a photomicrograph of an etched amorphous 10 surface of such high x ΜΡχ synthesized by single source vapor transport; FIGURE 22 is an photomicrograph of an etched amorphous surface of such high x ΜΡχ synthesized by two source vapor transport; FIGURE 23 is an photomicrograph of the same surface as shown in FIGURE 22; FIGURE 24 is an photomicrograph of an etched surface perpendicular to the surface shown in FIGURES 22 and 23; FIGURE 25 is an SEU photomicrograph of the upper 20 surface of an amorphous thin film of KP^g synthesized by two source vapor transport; FIGURE 26 is a cross-sectional view partly in diagrammatic form illustrating the formation of a junction; FIGURE 27 is an illustration of the oscilloscope 25 screen in the experiment illustrated in FIGURE 26; and FIGURE 28 is a cross-sectional view partly in diagrammatic form illustrating the formation of a junction; FIGURE 29 is an illustration of the oscilloscope screen in the experiment illustrated in FIGURE 28; FIGURE 30 is a diagram of a photosensitive resistor; FIGURES 31, 32 and 33 are illustrations of oscilloscope screens showing junction activity in devices; 8 S2S83 FIGURES 34, 35 and 36 are plots of capacitance versus applied electrical potential in junction devices; FIGURE 37 is a plot of capacitance and resistance as a function of frequency of applied potential of devices; FIGURE 38 is a diagram of a preferred form of sealed ampoule utilized to form monoclinic phosphorus; FIGURE 39 is a photomicrograph of a crystal of monoclinic phosphorus; FIGURE 40 is a photomicrograph of a crystal of mono10 clinic phosphorus; FIGURE 41 is a diagram of the photoluminescence response of a crystal of monoclinic phosphorus; FIGURE 42 is a diagram similar to FIGURE 6 of the photoluminescent response of a crystal of monoclinic phos15 phorus; and FIGURE 43 is a Raman spectrum of monoclinic phosphorus; FIGURES 44 and 45 are SEM photomicrographs of another new form of phosphorus; FIGURE 46 is an SEM photomicrograph of still another 20 new form of phosphorus; FIGURE 47 is a side diagrammatic view of flash evaporation apparatus; FIGURE 48 is a cross-sectional view taken along the line 48-48 of FIGURE 47; FIGURE 49 is a cross-sectional view taken along the line 49-49 of FIGURE 48; and, FIGURE 50 is a diagram of chemical vapor deposition apparatus.

The same reference numbers refer to the same elements 30 throughout the several views of the drawings. 3 6 8 3 The high phosphorus materials exemplified by the high phosphorus polyphosphides MPlg where M is an alkali metal, and the new forms of phosphorus formed, are all believed to have similar local order, whether crystalline, •5 polycrystalline or amorphous. We believe that in both crystalline and amorphous MPlg, this local order takes the form of elongated phosphorus tubes having pentagonal cross sections as shown in Figures 4, 5 and 6. All of the pentagonal tubes are generally parallel on the local scale and in MPlg double layers of the pentagonal phosphorus tubes are connected to each other by interstitial alkali metal atoms. In the new forms of phosphorus, many, if not most of the alkali metal atoms are missing However, it appears that one new form of phosphorus formed in the presence of very small amounts of alkali metal atoms grows from vapor deposition in the same form as MP^g. One experiment to be discussed below indicates that at least one form of this is by growth of the new form of phosphorus on a layer of MP15· The MPlg may act as a template causing the phosphorus to organize in the same structure. All of the materials having these all parallel pentagonal phosphorus tubes have been found by us to have a band gap between 1.4 and 2.2 eV and most on the order of 1.8 eV. Photoconductivity ratios range from 100 to 10,000. Thus it is indicated that all high phosphorus alkali metal polyphosphides from MP? through MP^g and more complex forms and mixed polymers of MP^g and the new form of phosphorus discovered by us (ΜΡχ where x is much greater than 15), which all have the all parallel pentagonal. tube structure, if stable, will be useful semiconductor materials, barring the inclusion of elements that would act as traps, cause the formation of grain boundaries, or the like.

In all of these materials having the all parallel pen35 tagonal tubular structure, our investigations indicate 3 3 3 3 that the multiple continuous covalent phosphorus-tophosphorus bonds of the tubes being substantially greater in number than the number of other bonds will provide primary electrical conduction paths for electrons and holes and thus provide good semiconductor properties. It is further our opinion that the presence of alkali metals in the charge, even when resulting in trace amounts in the new forms of phosphorus we have discovered, promote growth of the materials in forms that maintain the same structural and electronic properties as KP^g or as monoclinic phosphorus, depending on deposition conditions.

The family of semiconductor members to which the subject invention is directed comprises high phosphorus polyphosphides having the formula ΜΡχ wherein M is a Group la alkali metal, and x is the atomic ratio of phosphorusto-metal atoms, x being at least 7. Metallic elements of Group la most suitable are Li, Na, K, Rb, and Cs. Although francium presumably is suitable, it is rare, has not been involved in any known synthesis of ΜΡχ and is radioactive.

High phosphorus polyphosphides where M includes Li, Na, K, Rb or Cs have been formed and tested by the inventors.

The polyphosphide compounds as presently defined must contain an alkali metal. Some of the new forms of phosphorus must be formed in the presence of minor amounts if not unmeasurable amounts of alkali metal. However, other metals may be present in minor amounts as, for example, dopants or impurities.

KP^g and, as we later .learned, a new form of phosphorus was first synthesized as follows.

Referring to Figure 1, a two temperature zone furnace comprises an outer sleeve 12 preferably constructed of iron. Outer sleeve 12 is wrapped in a thermally insulative coating 14 which can comprise an asbestos cloth. The furnace was constructed in the laboratory shop of the inventors. 53333 We used a P/K atom ratio of about twelve as reactants 36 in furnace 10. As one illustrative example, 5.5 g of red phosphorus and 0.6 g of potassium were trans5 3 0 8 3 ferred under nitrogen to quartz tube 32. Prior to transfer, the phosphorus was washed repeatedly with acetone, and air dried. However, this washing is considered optional, as is the solvent selected.

After being charged with reactants 36, tube 32 was evacuated to, for example, 10 · Torr, sealed, and then placed in furnace 10. Tube 32 was mounted at a slight incline in the furnace. Power supplied to conductors 24 and 26 was adjusted to establish a temperature gradient of, for example, 650°C to 300°C from heat zone 28 to heat zone 30. With the above described inclination of furnace 10, reactants 36 were assured of being located in the hotter temperature heat zone 28.

After maintaining furnace 10 at these conditions for a sufficient period of time, for example approximately 42 hours, power to conductors 24 and 26 was terminated and tube 32 was allowed to cool. Upon reaching ambient temperature, tube 32 was cut open under a nitrogen atmosphere and the contents of tube 32 were removed. The contents were washed with CS2 to remove pyrophoric materials, leaving approximately 2.0g of stable product. This resulted in a yield of approximately 33 percent.

Using this form of synthesis, various phases of resultant product occur at well defined positions within tube 32 as illustrated in Figure 2. A dark gray-black residue 40 coupled with a yellow-brown film 42 is typically produced at the extreme end of hot zone 30, where reactants 36 are initially located. - Moving in a direction of decreasing temperature along tube 32, there is next found black to purple film deposits 42 which are a polycrystalline material. Next to film deposits 42 is an abrupt dark ring of massed crystallites 44 and immediately adjacent crystallites 44 is a clear zone wherein whiskers 46 are grown. A highly reflective coating or film deposit 48 is found on the lower portion of tube 32 in the beginning of cold zone 28. Above film deposit 48 a deep red film deposit 50 'occasionally occurs depending on the temperature maintained in the zone. The deposits 48 and 50 can' be polycrystalline. 53633 amorphous or a mixture of polycrystalline and amorphous material depending on the reactants and temperature. At the extreme end of cold zone 28 is a mass or film deposit 52 which is amorphous material.

Since there is a continuous temperature gradient from the hot zone to the cold zone of the reaction tube shown in Figures 1 and 2, the nature of the materials deposited actually varies continuously from high quality crystalline whiskers to polycrystalline to amorphous. In order to manipulate the reaction and attempt to deposit large areas of uniform layers of material, a three zone furnace was constructed and is illustrated in Figure 3. As herein embodied, the three zone furnace 54 is essentially identical to furnace 10 illustrated in Figure 1, in that furnace 54 comprises an outer iron sleeve 56, a tube 60, and a reaction tube 58. For purposes of simplicity, asbestos wrappings of outer sleeve 56 and tube 58 have been omitted from Figure 3. Furnace 54 is primarily distinguishable from furnace 10 in that tube 58 is much longer in comparison to tube 32, and is preferably on the order of 48 cm in length. In addition, furnace 54 has associated with it three distinct heat zones, 62, 64 and 66 which are individually controllable to create a more definitive heat gradient along tube 60. Tube 60 may be supported by asbestos blocks 68 and 70 in a manner so as to provide for an inclination of tube 60 and reaction tube 58 toward heat zone 62, in order to keep reactants 36 in proper position.

Very good quality preparation of KP15 whiskers were obtained using temperature set points of 550, 475, and 400 degrees centigrade in heat zones 62, 64 and 66 respectively. It was also found that bulky deposits generated in furnace 10, when loaded into inner sleeve 60 of furnace 54 and reheated in the above-identified temperature gradient, would sublime to form film deposits like those of films 48-52 illustrated in Figure 2, but only when a high zone temperature of at least 400-475°C was used.

Unit cell structural information on KP-j_5 crystals produced in accordance with the method described above was 53883 4S obtained by single crystal X-ray diffraction data, and collected with an automated diffractometer. A fibrous single crystal of 100 microns diameter was selected and mounted on a glass fiber. The structure was determined by direct methods using a total of 2,544 independent reflections. All the atoms were located by an electron map and differential Fourier synthesis.

Typical needle-like crystals were examined by high magnification and scanning electronic microscopy (SBM). The resultant SEM photographs of the cross section of the needles show that the needles are apparently composed of dense fibrils rather than hollow tubes. Marked twinning of the whisker crystals is also discernible on the microphotographs of KP^g whiskers in Figures 7 and 8. The diameter of the primary fibrils of the whisker-type crystals is estimated to be approximately 0.1-0.2 microns. Larger fibrils seem to have a fine structure consisting of parallel lamellae of approximately 500 angstroms thickness.

From the initial crystal data refinement study, the stoichiometry of the studied potassium phosphide compound appears to be KP^g.

The phosphorus atomic framework of the compound is formed of identical unit tubes with a pentagonal cross section. The tubes are unidimensional along the needle axis direction. The phosphorus tubes are parallel to one another. In the simplest description, double layers of separated phosphorus tubes are connected by a layer of potassium atoms. As judged by the inter atomic distances, the K atoms are at least partially ionically bonded to P atoms. A cross sectional view of a whisker is presented in Figure 5.

More specifically, each potassium site is associated with a rigid unit of 15 consecutive phosphorus atoms having a structure as illustrated in Figure 4. In this rigid unit all the phosphorus atoms but one are bound to three other phosphorus atoms. The other phosphorus atoms are chained, with the missing bonds linked to a potassium atom as shown in Figure 5. Thus, the potassium atom appears to link CS3 tubular phosphorus units through a missing P-P bridge. In the investigated structure, potassium has phosphorus atoms as nearest neighbors at distances of 3.6A, 2.99A and. 2.76A, respectively. The P-P distances vary from 2.13A to 2.58A.

The bond angles at the phosphorus chains vary between 87° to 113° and average 102°.

Arsenic forms a layered structure having an average bonding angle of 98° and this is not known to be a useful semiconductor. Black phosphorus has a similar structure and an average bonding angle of 96®. Trivalent atoms which can form their three bonds within the range of 87® to 113® with the average above 98® may form- the same catenated structure as ΜΡχ. If the bonds are covalent the material can be expected to have the same electronic properties as ΜΡχ.

Table I gives the crystal lattice parameters and atomic positions we found, for crystalline Κ?15· TABLE I Crystal Lattice Parameters For KP^5 Triclinic system Onit cell parameters 9.087 A® (+ 0.15) A® 11.912 A® (+ 0.10) A® 7.172 A® (+ 0.15) A® 101.4 (+ 0.1)® 107.9 (+ 0.2)® 89.3 (+ 0.1)® The unit cell is primitive with one molecule per unit cell and a volume of 723.3 Cubic Angstroms Space group P1 The highest attainable symmetry in the above structural configuration is a centrosymmetric Pj^ space group with the stoichiometry given by KP15· The corresponding X-ray powder diffraction data for KP15 polycrystalline material with copper illumination is shown in Figure 9. This shows the d spacing with the corresponding X-ray intensities.

Similar X-ray powder diffraction data have been observed for whiskers and polycrystalline MP15 materials with M= Li, Na, K, Ub and Cs.

In all these isostructural compounds, the structural framework can be viewed as formed of parallel pentagonal phosphorus tubes. These tubes are linked by a P-M-P bridge.

The rigid units for this type of structure are P4 and MPg. The building block for the atomic framework can be viewed as [P4 - MPg] or [MP?].

Therefore: [MP?] + 2[P4] —-> [P4 - MP? - P4] which represents the basic structure MPjg.

Of course, one of the building blocks in such compounds may be present in much larger quantities than the other. In the case of ΜΡχ, for example, there may exist building blocks of [MP?] and [Pg], which are present in a ratio of a to b, respectively. In such a case ΜΡχ could be expressed in the form [MP?]a[Pg]b, wherein mathematically x = (7a+8b)/(a).

It is also possible for a compound to have b much greater than a and have the same basic structural framework.

This type of polymer like tubular structure will result in fibers or whiskers of the type ΜΡχ with x much greater than 15.

Whiskers and polycrystalline fibers of the type ΜΡχ with x greater than 1000 (M3Li, Na, K, Rb, Cs) have been observed to crystallize at low temperature (about 400°C) using the vapor transport technique. The X-ray powder diffraction data of these materials are substantially the same. Data for ΚΡχ where x is much greater than 15 under copper illumination is shown in Figure 10.

We can compare the structure described above to other structures based on pentagonal cross section phosphorus tubes. The KP15 compound is isostructural to LiPls, NaP15, PbP,cf CsP1c. The other alkali metals appear to'play the lb 15 same role as K.

From structural data we concluded that numerous compounds can be formed which will be based on pentagonal cross section tubular building blocks. We also found that in phosphorus materials, at least partially, the phosphorus 10 atoms can be replaced by other pnictides, such as As, Bi or Sb. Substitution under. 50 atom percent is possible, without adversely affecting the basic structure of the high phosphorus polyphosphides.

In Table II are shown the various ΜΡχ carpounds synthesized that we have found the same structure as crystalline KP15 as shown by XRD powder diffraction fingerprint analysis.

Rigid units Building'blocks Basic structure Ms TABLE II MP3 and P4 [P4-MP3] or [MP71 and [Ρθ] IP4-MP7-P41 or [MP151 Li, Na, K, Rb, Cs Compounds Isostructural With Crystalline KP^ with: 0 fz x 1 Υ < 7-5 M and M' from Group la P* from Group 5a (As, Bi, Sb) Initially the inventors found, as previously stated, that the crystalline whiskers produced in the apparatus of Figures 1, 2 and 3 were However, analysis of the polycrystalline and amorphous materials, although indicating that these materials had the same semiconducting properties •as the MPwhiskers, had widely variable stoichiometric proportions from Μ?20θ to ΜΡ^θ θθθ, and surprisingly no manipulation of the temperatures in the three zone furnace illustrated in Figure 3 would produce amorphous forms of was therefore necessary to greatly refine the methods of producing these materials and to invent a new two source vapor transport apparatus in order to successfully produce polycrystalline and amorphous MP15 materials. The very high x materials which are now thought to be a new form of phosphorus, have also been prepared by this method by initially depositing MP^S and thereafter cutting off the source of alkali metal so that only phosphorus vapor is present for deposition of phosphorus. Additionally, a condensed phase process has been extensively investigated using molar charges of ΜΡχ materials where x varies from 7 to 15. In this method the stoichiometric mixtures are heated isothermally to reaction and then cooled. We have produced a wide variety of ΜΡχ materials in this manner which are crystalline or polycrystalline powders.

There follows a detailed description of the methods we have employed -to synthesize high phosphorus materials and how we have measured the electro-optical characteristics and demonstrated that they are useful semiconductors. 53G33 Preparation of Stable High Phosphorus Materials by the Vapor Transport ( Technique from a Single Source Introduction The technique of applying sufficient energy to a system to create vapor species which yield products on condensation or deposition, at appropriate temperatures, is called vapor transport. For the following discussion, where the source materials are held in close contact and heated together at about the same temperature, the further description as a single source technique is applicable.

The methodology described by Von Schnering was essentially a single-source vapor transport technique, although the charge sometimes consisted of separate ampoules of metal and phosphorus heated to nearly the same temperatures. However, the flow of vapor species to the deposition zones was effectively the same as when the metal and the phosphorus are first mixed together. More specifically, in single source vapor transport the vapor species are first brought together at a high temperature and then are deposited at a lower temperature.

The following indicates our development of the technique as it has been applied to the preparation of alkali metal polyphosphides and the departure from von Schnering's method, which results in improved, more selective preparation of: crystalline metal polyphosphides of the type low alkali-metal content polyphosphides, polycrystalline material, of the type ΚΡχ, where x is much greater than 15; and a new form of amorphous phosphorus, in which the alkali metal content can be less than 50 ppm (parts per million).

The studies we have made fall into several categories: type of charge, charge ratio, tube length and geometry, and temperature gradient profile. The following examples illustrate the temperature dependent product deposition relationships we have discovered and our improved temperature controlling methods that result in the selective preparation of desired products. 53CK3 General Methods: An alkali metal and red phosphorus are sealed in -4 quartz tubes, at reduced pressures (about 10 Torr).

Atom ratios of the two elements range from P/M = 5/1 to 30/1, with 15 to 1 as the most common charge. The elements are generally ball-milled together, prior to loading in the quartz tubes. The millings are carried out with stainless steel halls and mills and last for at least 40 hours. The mills are usually heated to 100°C for the duration of the milling, to assist in the dispersion of the metal in the red phosphorus powd The milling achieves an intimate contact of the two elements in as homogeneous a manner as possible.

The products of the milling are generally fine powders which are easily manipulated in a dry box and may be stored with little noticeable deterioration. The powders show remarkable stability when exposed to air and moisture, compared to the stability of their constituents, especially the alkali metals. For instance, direct addition of water to the powders only results in combustion of materials in random cases and on a small scale.

Preparation fo MP^g single crystals, polycrystalline and 25 amorphous materials__ A mixture of the elements (alkali metal and red phosphorus) is sealed at reduced pressure (less than -4 Torr) in a quartz tube 58 (Figure 3), about 50 cm long by 2..5 cm in diameter. Tube 58 is supported 30 inside the heating chamber of a Lindberg (Registered Trade Mark) Model 24357 3-zone furnace in one of two ways. One method employs a second quartz tube 60 as 53883 a support piece, which is, in turn, held in the chamber, away from the heating elements, by asbestos blocks 68 and 70, such that the coupled tubes rest at an incline, ensuring the reactants remain in the hottest zone. The other method (Figure 14) is to use supports built of woven tape 137,139 wrapped about the reaction tube in an expanding spiral, an inch (2.54 cm) wide, and filling the circular crosssection of the 3 C 3 3 •heating charter. This woven tape nay be made of a variety of materials: Asbestos, Fiberfrax, (Registered Trade Mark, fraa Carborundum Company), or woven-glass. The latter is preferred primarily on safety and performance criteria. The implications of 5 using the two different methods are described below.

The reactants are driven to products by applying energy to the system via the resistance elements of the furnace.

If a sufficiently high temperature is applied to the reactants, while other portions of the tube are held at appro10 priate lower temperatures, products will deposit, or condense, from vapor species. The temperature differential which drives this so-called vapor-transport synthesis, is achieved in a 3-zone furnace by selecting different temperatures for the individually controlled heating elements.

METHOD 1. See Figure 3. The 50 cm tube, containing the reactants, is held by the second quartz tube in the 61 cm long heating chamber. Application of a thermal gradient by manipulation of the 3 set-points results in a generally linearly-falling gradient. That is, the slope of the gradient, ^ΪΤ/d, where T is temperature and d is distance along the chamber, is approximately constant between the centers of the two outside heating elements. This linear gradient, applied over the long dimensions of the tube, functions to cleanly separate the variety of product materials formed in the reaction. The products occur in a characteristic pattern of decreasing temperature of deposition: dark purple to black polycrystalline films; a ring of massed crystallites; single crystals or whiskers; red films of small-grain, polycrystalline morphology; and, at coldest temperatures, dark grey amorphous material.

A series of experiments have shown that the amorphous material will not form in these sealed tubes if the coldest temperature is greater than about 375"C. Similarly, the occurrence of the red polycrystalline material could be greatly reduced by keeping the lowest temperatures at or above 450°C. 3 6 8 3 We have also found that polycrystalline MP15 will not form in single source apparatus. The polycrystalline and amorphous materials formed are all high x materials where x is much greater than 15.

METHOD 2. The woven tape holders serve not only to orient the reaction tube but also as effective barriers tc heat-transfer between the three heating zones. These barriers give rise to steeper drops between the zones, but a flatter gradient within the center zone. The result is a step-like temperature profile, which can be manipulated to selectively produce products by providing appropriate ranges of deposition temperatures.

A. Determination of Product Deposition Temperatures In von Schnering's announcement of the preparation of single crystals (whiskers) of KPi5' he described the preparation from the elements as entailing the heating of the elements—potassium and red phosphorus—in a temperature gradient of 600/200eC, in a 20 cm or so quartz tube. He further states the crystals form at 300 to 320°C. The furnaces used were apparently single element furnaces in which the gradient arises via heat loss from one end of the tubes sticking out of the furnace.

In the first improvement on this procedure, a threezone furnace as shown in Figure 3, with independently controlled heating elements, and a 61 cm long heating chamber (Lindberg Model 54357 3-zone furnace) was .employed, to achieve and control the applied gradient. By supporting the reaction tube, which was now extended to approximately 52 cm long, in a second, open quartz tube, which was, in turn, supported by asbestos blocks, a generally linear temperature gradient, A T/d, was approximately constant between the centers of the two outside heating elements. The power to these elements were controlled .by a Lindberg Model 59744-A Control Console, which uses three, independent SCR-proportional band controllers to maintain the temperatures selected on manually set thumb-wheels. 3 ίί ο 3 The linearly-falling gradient, applied over the long dimensions of the reaction tube, served to cleanly separate the variety of product materials formed in the reaction.

The products occur in a characteristic pattern of decreasing temperature of deposition: dark purple to black polycrystalline films; a ring of massed crystallites; single crystals or whiskers; red films of small-grain, 'polycrystalline morphology, and, at the coldest temperatures, dark grey, amorphous materials.

Example I A Lindberg Model 54357 3-zone furnace as shown in Figure 3 comprising heating elements embedded in a refractory material in separate cylindrical sections of 15.3 cm, 30.6 cm and 15,3 cm lengths, for a total heating chamber length of 61 cm,was used for this example. The diameter of the chamber is 3 cm. Controlling thermocouples (not shown) are located at about 7.0, 30,5, and 53.5 cm along the 61 cm length.

The ends of the heating chamber were plugged with glass wool to minimize heat loss from the furnace. A 60 cm long by 4.5 cm diameter quartz tube was held at a slight angle, by asbestos blocks, in the heating chamber.

The quartz reaction tube was round bottomed, 49 cm long by 2.5 cm in diameter, and reduced to a narrow addition tube 10 cm long by 1.0 cm wide. Under a dry nitrogen atmosphere, 6.51 g of red phosphorus and 0.62 g of potassium were transferred into the tube. The atom to atom ratio of phosphorus to metal was 13.3 to 1. The phosphorus was reagent grade -4 (J.T. Baker) . The tube was evacuated to 10 Torr and sealed by fusing the addition tube several cm's from the wider part of the tube such that the total length was 51.5 cm. The sealed tube was placed in the 3-zone furnace as described above and the set point temperatures of the three zones brought to 650’C, 450°C, and 300eC over a period of 5 hours, and held there for.another 164 hours. The power was S3S83 shut off and the oven allowed to cool to ambient temperatures at the inherent cooling rate of the furnace. The tube was cut open under a dry nitrogen atmosphere in a glove bag. The products consisted of crystalline, polycrystalline, and amorphous forms.

In Table III, the different processing parameters used for several other runs are listed, along with the types of products observed in each run. Prior to being cut open, the tubes from the first three runs were inspected as to the po10 sitions along the tubes of several products: the dark ring of massed crystallites, and the start of red polycrystalline films. The whiskers were always observed between these two points. These positions were later correlated to the temperatures along the gradients created by the noted set 15 points. These data are recorded in Table IV. 0) X Μ (0 ε φ οζ tn u ο — 3Ό Ό Ο k 0« JS Φ 4J Δ Ο»! 3 3 < 0) « 0) k -Ρ — S3 0 0 •Η Ο Ε-»~ £ΗΚ\ ΓΠ e< CM Ο β ω · w Μλ g μλ k Ο *-* ft &< k cn k O' ο ο Ο'Ή^ \ k 4J nJ ft <0 φ ** JC « υ ·* <Η · Φ Ο οζζ ra Φ X Η q α CM Ο »0 φ φ ΛΗ 3 ·Η & # Ή υ CU Ό CA CM Γ* *β· ΙΟ Ο Ο C*1 ο ΙΛ *9· Ο ΙΛ ΙΟ ο ι4 X ιΛ ΙΟ 3 (0 <0 • O • υ • u u • CU • ο< s· cu ft • * • a • • α • u ϋ • CA 03 CA co X Ο o o • • k CM CM ft tn tn ft Φ t·* tit •Μ· tn t-i CM \ X CM © GO VO r* © cn cn *4 r4 CM O o tn © in © r* in cn Μ» cn cn in in o o ΚΏ r- m 10 -a· *r M· o © o O o in o © 10 tn 10 10 cn M· I tn tn o 1 © 1 © 1 © «4 r4 *4 r4 X X X X »-1 tn Γ- r4 10 CM ΙΟ © in o CD © * • • • 10 GO tn 10 US « (0 "S’; %-*· z Z i Γ* Γ* ** · 10 co σι tn i • • CM •4 i o o • * o O < tn rH © © • • • « CM tn tn o »—i r4 i—l cn CM cn *8· tn Φ CD 6 ·Η Π (1 • φ φ α ω c Ξ C&h ο Φ d n Eh es o O u O o o o o o o © to to tO to CO *d* to τρ of High x Products as a Function of Set Temperatures in 3-Zone furnace Ό Φ μ ε g ε ε « μ H 43 a o o ο (0 >1 •Η W -P g μ μ 10 o tuo ε O tO © to to 0 ΓΗ 0 a g o • • G d a φ φ Tp co co co co Ό G φ Ω J H μ μ 0-Η Ό (4 Μ a*rt Φ 43 Q Q o o ο Φ 0 μ a o o o o ο ϋ ε to © o (0 ο G CO •ΰ S CO IO co to 10 Ό G g-l TP TP Tp tp Tp 0 ♦ d Μ 0 az 60 β g g g ε ε 0 · μ 0 ϋ 0 υ ο μ «η h w ε μ μ hO C Φ g μ E01 ι-l Eh > G a w 0 τΗ W μ ♦Η • CQ (0 (Η » <3 0 Φ 0 &< a OS ζ Ο O tO tO O O CO CD TP Cd CO CO CO CO CO t> 00 O to 0) μ tO tO Tp O to to O to CO (0 CO CD GO Cd 01 03 03 01 Φ co es Φ μ G a g S h Λ co r o φ μ «Η μ <3 Φ h a d Φ a h φ co Φ G +» 0 G >k 0 μ μ Ό G Φ Φ CO Φ μ C Φ Φ μ Φ Φ ¢3 a CO Φ Ο ε 0 ο to ιη + t 6 0 μ +1 Φ 0 CO (0 co CQ to *0 s co μ μ >» 5» >1 >> G Φ d d d os ta ca Φ (0 μ fa Φ Ό Ό Ό Ό Ό a φ a G μ φμ ε ϋ d e- to o co co *tj μ d 0 μ G μ co d s μ 0 0 Η μ Ό Φ ό a μ Φ d Ο ο o O o φ d μ ο ΙΟ o O o CO ho d *0 <0 co Tp τρ Tp G C c g Φ μ μ μ μ μ μ μ d μ d ho CO ε ο to to to IO d Φ G Φ μ to co c* 00 00 Λ μ μ τρ τρ TP Tp Tp Φ Φ Φ CO h h 0 h Φ Φ (4 d ? φ 60 Φ ο ο o o to (0 h (0 ο to H μ CO 4 •a tt d co co to co co G G XJ ΙΟ G μ co G 0 •a tt ca ό co μ d G G Φ μ cd d Φ - Φ G Ό μ h d « «τΗ Φ Φ φ μ τρ μ h μ ζ-\ z-o /-*·» z—' o G μ cd co TP to CQ CO fc co μ G φ φ a u d 0 μ μ Φ · μ μ μ ho 44 CO Φ μ μ μ G CO E a μ ο ο μ μ μ ε co h R CO Λ·* φ 0 μ cd co co co aa g * μ Eh a The information from these two tables was used to establish the relationship of temperature and product-type.

The single crystals of KPj5 appear to form over a temperature range of about 40 + 10°C, the center of which varies from run to run, but which lies around 465-475’C. Similarly, the onset of deposition of red, polycrystalline materials appears to be about 450 + 10°C. Finally, amorphous material deposited even when the lowest temperature was around 350°C. When this was raised to 40Q°C, no amorphous material was observed. (Although the run in which this temperature was used eventually ended in a failure of the reaction tube, before products could actually be harvested, this temperature-product relationship for the amorphous material was confirmed in later runs using more advanced techniques). Assuming a mid-range value, an upper limit for deposition of amorphous material was taken as about 375°C. The pressures in the heated tubes were not measured.

B. Temperature Gradients Which Favor Growth of Single Crystals (Whiskers) Using the knowledge of the deposition temperatureproduct morphology relationships of Tables III and IV, improvements in the synthetic technique were sought which would allow greater selectivity of product type. Methods were sought for manipulating the temperature profiles in the furnaces which would result in larger areas of the tube surface being within the appropriate temperature ranges for given products. Several available materials with low thermal conductivities, and in easily manipulatable forms were checked for use as barriers to heat transfer in the fur30 naces. Woven tapes of asbestos proved a suitable product for both supporting the reaction tubes and creating complex gradients, consisting of areas of fairly flat, or isothermal, temperatures, separated by areas (across the barriers) of steep drops or gradients. These so-called step-like profiles were applied in all the'subsequent examples where specific products were being sought in maximum yields. 53G33 Another improvement which helped gain more reproducible temperature profiles from run to run was to use a more solid, ceramic type of material to fill the gaps in the heating chamber walls. In early runs, these were plugged with glass wool, which helped stem loss of heat, but not very efficiently. The large cylindrical gaps are present in the chamber walls because the furnaces are actually designed to hold a process tube along its length, for flow-through applications, rather than for enclosed systems, as are being run in these methodologies.

The following examples were all aimed at trying to promote growth of single crystals, both larger in size, and in greater yields, both as a percentage of product forms, and in absolute yields. These results were indeed achieved.

Example II A Lindberg Model 54357 3-zone furnace identical in design and size as that of Example I was also used in this example. The elements were likewise driven by the same manually set model 59744-A Control Console. The ends of the heating chamber were plugged with a heat resistant ceramiclike material, to minimize heat loss from the furnace. The reaction tube was supported in the heating chamber by two rings of woven tape of asbestos. One of these was located between 16-19 cm and the other between 42 and 45 cm along the chamber. This put both rings completely inside the center heating section, just beside the junctions of the center elements and those of the two outer sections. The rings were constructed such that the tube was held at a slight angle. The ring served to insulate the heating zones from each other by acting as barriers to heat transfer.

The quartz reaction tube (Figure 3) was round bottomed, 48 cm long by 2.5 cm in diameter, and reduced to a narrow addition tube 162, 10 cm long by 1.0 cm wide. Under a dry nitrogen atmosphere, 5.47 g of red phosphorus and 0.50 g of potassium were transferred into the tube. The atom to atom ratio of phosphorus to metal was 15.1. The phosphorus was 99.9999% pure. The potassium was 99.95% pure. The tube was 3 υ ώ ·> evacuated to 10-^ Torr and sealed by fusing the addition tube several cm's from the wider part of the tube such that the total length was 52 cm. The sealed tube was placed in the 3-zone furnace as described above and the set point temperatures of the three zones brought to 600eC, 475®C and 450®C over a period of 4 hours, and held there for another 76 hours. The power was shut off to all three zones at once and the oven allowed to cool to ambient temperatures at the inherent cooling rate of the furnace. The tube was cut open under a dry nitrogen atmosphere in a glove bag. The products consisted of crystalline and polycrystalline forms.

Table V lists the processing parameters for a number of other such runs (data for the above example are from the run with reference number 10). 53CS3 x X Φ cn XI c 3 Φ I <0 X o Φ 6-« gX~ ?:g:s Γϋ 6<· ΙΌ £« ο KJ e*o ο co b φ MOO ft K*"* © ~ Cn 0 b Ή «R3 4J^ XX « rt ft o « *-* CS *β· *r tn in X X X P 3 3 If) o o o o 3 3 O 0 • • • • • 0 0 o X X Οί CM H H o X J3 tn 3 3 If) If) *in in in 3 « VO VO 3· GO o o GO GO X 60 CM rb X X ο- Ο- 3X cb X X X 3· χ Χ O 3· \ X n· VO VO *· CM CM X 3» CM CM CM σι σν ib rb Γ* 0- 3 8-4 r- O- O o o o o © © o tn © in in in in in in in © Γ- in O' *0· 3» 3* Ί· 3 If) tn m m in O O in © o GO 60 r* r* p* f* o· o· in in Ό* M* *6· ’T M* 3· 3* 3» 3* 3 O O O o o © o o © © o o O o o o o tn o © VO VO VO vo vo vo vo in vo VO 3 O 3» 3 3· 41» tn in tn tn σν I 1 1 1 1 t 1 1 1 1 σν O © © © © © © © © o σν rt rb rb rb rb rb rb rb rb • - σν κ X κ κ κ κ κ κ X X σν o* rb o- cM rb m H rb o* o* Κ ft X X © rb cn GO o- o VO O CM r* in co O © σν σν © cn 0- H co σν • • • • • » • • • • • vo VO vo m m vo VO in vo in σν σν * »«» *·*» Κ X 3 •b 2 2 X X co GO V cn o o in o· 60 © σν Η ' in in in in in CM CM rb' cn tb 3 1 « • • • • • • • • • •b o rb rb © o © o O o O Μ n b O X OV If) fi If) r-4 o cn σν CM at gradienit/total resident time in Φ H X 3 3 Φ eew o >P PP <8 O 3 U p as n e xs a a o o< u e p-k H-a <8 Λ ftp if) · «Μ IB Λ 0Π3 333 All of those runs resulted in crystalline and polycrystalline forms. The yields of the single crystals were always greater than in Example I. The polycrystalline materials were always in the form of films deposited in the colder ends of the tubes and were usually limited to the last 10 or so cm of the tube, though there was usually some overlap with the single crystals. Single crystals from these runs were characterized by X-ray powder diffraction patterns as having the same structure as KP^g as determined from XRD data. Wet chemical analysis of the crystals were difficult to obtain with great accuracy, in part because of their stability, which required extreme conditions for digesting the materials for analysis. (See the tables VIII through XI on analytical data below) The polycrystalline films were also characterized by X-ray powder diffraction methods and wet methods. The films showed varying degrees of crystallinity, and the patterns were similar in several aspects to that of KP^g, but yet were distinctly different in others. Furthermore, the wet analysis, coupled with flame emission spectroscopy consistently showed the alkali metal content to be in the part per million range (i.e. less than 1000 ppm and often less than 500 ppm), and with P/K ratios ranging from about 200 to 1 to about 5000 to 1.

C. Thermal Gradients Which Favor Growth of Polycrystalline and Amorphous Materials Following the successful improvements in production of single crystal materials, a similar series of experiments was carried out to manipulate the 3-zone furnace and asbestos rings to find the stepped thermal gradients appropriate to selectively produce the polycrystalline and amorphous materials observed in earlier runs.

These earlier runs suggested the temperatures necessary for obtaining the desired products. What remained to be shown was how to optimize these products. Table VI shows . the type of profiles used and the products observed. 3 υ 6’ 3 ο η Λ Ο Ο «σ ο Μ Λ Λ ti Μ ε 9~· •Η Ο Φ frix — <*ο CO £40 r-U E«o to to k O a « k 0« O' to e n k tf O' ti O' 0 k Ή tf ti JJ^ Χχί ti ti 04 U « ~ tf ti tf * tf in © in ti ti •H JJ to 0* ti ti •H JJ to 0) 0* tf tf C O g 6 rH ti’K w c ti Ή E35 (0 JJ Δ ti ti O •H JJ V σ 4/ g is c* © r* IO 10 r* CM Ol Ol tn p* fJ X X • X X Cl Cl CM Ol o CM c* r* r* 01 tn C* o © o © tn tn m o in m ci CM cn ti co cn Cl cn tn tn tn © tn © 10 Cl r* in Cl ti ti ti cn Cl ti © o o © © © o © o © © © Ό IO © © © © in I © ti O ti ti k g in c- rJ tn tn C «Η 33 ti m © o O in u 6 © © ti © σι * •H • • • • 33 ti JJ m tn tn © ti ««» k ti £ n 9 fJH g ca JJ JJ ti tifl Λ 1 C X © cn A CM tn rd 0>·Η<** 0 X** Λ in ti ti in CM t 0 E E 0 4* -HU • * • « • © H 01 X 0) 0) k « © H rJ H rj ** **rtflrt**rt C« > ® e> o> « c 0>*H 4* k k Γ* β SBH«f « CJIH ti®k*k rj g ti E ti *H 04 cu 04 o< Ol Ol X X X X X X tf tf tf Ϊ4 ti ti On k (β M W «ι-l O ____ o*l Λ HVM O 0,9 *> ax 3 m in tiufl « -h ο o E« I « «Λ O gk ok_.»e»ou8o. 4* 9 S k-rl I I I β cun cu·**so, «« H >0 4* Φ Hi O G6 The first run, which is the subject of the Example III, just duplicated the temperatures of the ranges used in Example I, the linear falling gradients now changed to a stepped gradient. Not surprisingly, all product types were found, with some variation in quantity, compared to those of section A. When the coldest temperature was raised to 400°C, as in the second run of Table VI, no amorphous material was found, as anticipated. With the 425°C centersection temperature, however, nearly two-thirds of the tube's interior was covered with polycrystalline films, and only a small number of whiskers were found, meaning the films could be produced almost exclusively.

In the third and fourth runs, though, where the coldest temperatures were held at 350eC (cold enough for amorphous material to be formed in the first run), and the center zone temperatures were lowered to 375 and 350°C, the amorphous materials were not formed in large amounts at all. Instead, large amounts of both single crystals and polycrystalline material were found over a fairly short space of the tube, and at best, only thin films of amorphous materials may have formed in’ the rest of the tubes. The same phenomenon was observed in the next two runs as well, although there were definitely thin amorphous films in one run. Apparently most vapor species are condensed out in the polycrystalline and single crystal forms and no significant vapor travels to the region which is cold enough to form amorphous forms.

Example III A Lindberg Model 54357 3-zone furnace identical in design and size as that of Example I, was also used in this example. The elements were likewise driven by the same manually set Lindberg Model 59744-A Control Console. The ends of the heating chamber were plugged with heat resistant material . to minimize heat loss from the furnace. The reaction tube was supported by two rings of woven asbestos tape. One of the rings was located between 16-19 cm and the other between 42 and 45 cm along the chamber. This puts 7 both rings completely inside the center heating zone, just beside the junctions of the center elements with those of the two outer sections. The rings were constructed such that the tube was held at an angle. The rings also served to insulate the heating zones from each other, by acting as barriers to heat transfer.

The quartz reaction tube was round bottomed, 48 cm long by 2.5 cm in diameter, and reduced to a narrow addition tube 10 cm long by 1.0 cm wide. Under a dry nitrogen atmosphere, 5.93 g of red phosphorus and 0.50 g of potassium were transferred into the tube. The atom ratio of phosphorus to metal was 15. The phosphorus was 99.9999% pure. The potassium was 99.95% pure. The tube was evacuated to 3 x 10-4 Torr and sealed by fusing the addition tube several cm’s from the wider part of the tube such that the total length was 51 cm. The sealed tube was placed in the 3-zone furnace as described above. The temperature gradient was driven to 600°C, 465°C and 350°C over a period of hours and held there for 72 hours. The power to the elements was then shut off simultaneously and the furnace allowed to cool to ambient temperatures at the inherent cooling rate of the furnace. The tube was cut open under a dry nitrogen atmosphere in a glove bag. The products consisted of single crystals, polycrystalline films,and· amorphous material.

D. Production of Cylindrical Boules of Amorphous Polyphosphides It was evident from the experiments described in secI tion C that to obtain large amounts of amorphous material improvements needed to be made in the processes already being used. It was recognized that in order to get bulk forms of the material, as opposed to thin films, the conditions appropriate for growth had to be confined to a smaller space than previously allowed. This translated into allowing only the extreme end of the tube to be at or below 375*C or so. This was accomplishable in principle by use of 52633 the thermal barriers. However, it was also recognized that if the conditions for formation of other materials, i.e. single crystalline MP^g or polycrystalline ΜΡχ (x is much greater than 15), were also available over a large area of the tube, these materials would act as traps for vapor species. It was therefore, also necessary to discourage the formation of the other materials. This was accomplished by raising the center zone temperatures to levels which would be too high for formation of polycrystalline or single crystals. The only area then where these materials were favored were through the area of the thermal barrier, where rapid temperature drops occurred.

As shown by the following exmaple, and other experiments summarized in Table VII below, further improvements in the procedure were worked out. The first was the use of Honeywell Model DCP7000 Digital Control Programmers to drive the heating elements. This allowed the pre-programming of the temperature changes such that reporducible treatments could be made from run to run. Both controlled heat-ups and cool-downs could be accomplished, eliminating tube failures, and production of white phosphorus. The latter often occurred when tubes were cooled rapidly and phosphorus vapor condensed as P4. This was often the reason materials appeared reactive. This reactivity could often be removed by soaking the materials in solvents which would dissolve away the white phosphorus. The second improvement was the routine of applying an inverted gradient of 300-490-500°C across the tube from the metal/phosphorus source 'to the deposition zones before vapor transport, which cleared the deposition zones of materials, which might affect nucleation processes.

By far^ the most important improvement, however, was redesigning the geometry of the tube. Instead of a long tube of nearly uniform 2.5 cm diameter, the body of the tube was shortened to about 30-32 cm and the 10 mm diameter K3e;J3 addition tube 160 (Figure 2) lengthened and sealed such that about 5-7 cm of this tube remained as available space in the interior of the tube. When this latter section was placed 2 6 3 3 in zone 3, and the vapor transport gradient applied, this section became filled with solid, bulky cylinders of increasing length, as the conditions for growth were improved.

Example IV A Lindberg Model 54357 '3-zone furnace, identical in design and size as that of Example I was also used in this example. The elements, however, were driven by a Honeywell Model DCF-7700 Digital Control Programmer which enabled processing to be pre-programmed and carried out in a reproducible fashion.

The ends of the heating chamber were plugged with heat resistant material to minimize heat loss from the furnace. The reaction tube was supported by two rings of asbestos tape. The rings were constructed such that the tube was held at a slight angle. The rings also served to insulate the heating zones from each other.

The quartz reaction tube was round bottomed, 33 cm long by 2.5 cm in diameter, and reduced to a narrow addition tube 162, 20 cm long by 1.0 cm wide. Under a dry nitrogen atmosphere, 7.92 g of a ball milled charge of atom to atom ratio of 15 to 1 was loaded into the tube which was evacuated to 1 χ 10-4 Torr and sealed by fusing the addition tube 10 cm from the wider part such that the total length was 43 cm long. The sealed tube was placed in the 3-zone furnace using the woven barriers described above.

With the tube between 6 and 49 cm, one thermal barrier at 16-19 cm and the other at about 38-40 cm, the Honeywell Programmer was used to apply an inverted gradient of'300, 490, 500*C for 10 hours. After the furnace cooled at the inherent rate of the furnace, the tube was moved to lie between 12 and 55 cm. The thermal barriers were also rearranged to lie at 18.5-21.0 cm and 44.5-47 cm. The programmer then drove the gradient to 600®, .485®, 300®C for 64 hours. The programmer then took the tube through a controlled cool-down sequence to a 180, 190, 200®C gradient, which was held for 4 hours. The furnace was then allowed to cool to ambient temperatures at the inherent cooling rate of the furnace.

The tube was cut open under a dry nitrogen atmosphere and 4.13 grams of a 2-3 cm long solid homogeneous amorphous boule recovered from the addition tube 162 (Figure 3).

The results of several other runs are shown in Table VII.

Ul Ο Λ CU Μ Ο Ό Φ C *H Φ 4J 4J Ο Φ C Ό ε φ OH © 3 • O •ΗΛ e φ ε ο 0 H OH © 3 © 3 3* «* • 0 • 0· © © CM Ή Λ Λ © tn © © © tn 0) Λ Λ 4J 0'S C ο φ >4 O Γ· in © in in « CM © Φ k iH f* ε 3 \ ♦H O CM M* e< as P* © r* β r* ** © IO rH <Ί Ο &* β ca ο &ι ο U) U) φ k CU 4J k k O e< « I k © in CM © CM © © <*) o © CM \ O o o o © © o o © © © © o © © tn © © © © © o in ** © e*· M* in « M* in m » co M1 in © © © to © © o © © o o © o © © © © © © © © © rH X X in © in I © X K in I I © © •Η H X in tn '© '© Η H X X H © n· © I I © o in o CM C* in o CM © tn © Φ O' k (8 Λ O se s sj ©on © © © *H pH © CU CU £ £ £ a © © *H CU « a © a © © Η t* a. o. « « a © © rH CU S % § % in -I w © CU CU \ \ \ « « fctf © CM © CM f* CM •ri ίΜ to Ο A Cb M Q «w «* <* o H r- r· o 4» (Ml 4* • • • r • to F"1 r-l n Γ* to n tn © r» tn tn in H re n M a s OJ Q β m η u H Or M M O Eh O) § © in © m tn tn © • « « • • ♦ ♦ m n* tn tn tn «e w n tn tn n* tn tn n tn Μ» O *J· o r-l o o tn © o tn o © o © © © o © © o o o © © tn .tn in in cn tn tn © o tn © © tn o © tn © o in o © © © © © «η in tn © o © © © O O © o © © O O o © © © © © © © © © © V cn M ««J· © © © © © <0 1 1 1 1 1 1 | A © © o o © o © © O H r4 rd H rH r-l cH o O' © © «4 © r4 r4 r-l © tJ © o © © © a © tn r* Ot © H © r* 0 • ♦ • • • • • • r» © © r* r- co t* ot & w nt o S3 ·§ g, B* <9 JG M β ϋ Or a φ *8 w h flj ‘ Φ CQ U Or © U © a © 43 43 0» © © U r4 © H © H 44 0< Ό 44 M tn CM © 0« i—1 Or «-< 0« O' Φ cd ta Or H r4 \ Or \ c Ό fH X Or 0« Λ 43 <0 A to 0 a r4 Φ o « X X & S z Z U r-4 w si •r4 ε M a V G) n r4 Ό r-t a o a 0 W Φ 3 0 o (0 H Λ u A Or M-t Φ o W CM tn' *«· © © Γ- Oi X tn tn tn tn tn tn η H u a * The results showed the yields of material to be fairly independent of the charge type - i.e. ball milled, or the pre-reacted condensed phase products. However, there was a distinct dependency of yield on the P/M ratio. The greater 5 the relative amount of metal in the charge, the lower the yield of material. As the amorphous material is essentially phosphorus, this reflects a lower vapor pressure of phosphorus over a metal-phosphorus charge the greater the metal content; hence, a slower rate of growth for identical thermal conditions.

Table VIII contains some analytical results on amorphous boules prepared. It shows potassium content, as determined by wet methods. It also shows trace constituents shown to be present by Flame Emission Spectroscopy. ρ φ «Ρ Φ S Trace Constituents of MP Amorphous to 0) η β β r-| o Ή > (0 * ή ε ε α μ a _ c Ό « Φ «β 4J +J U Φ Μ Ρ Φ Φ «Ρ Q Φ Φ η Ρ •Ρ θ' β Φ ·Ρ β Φ «ρ ε •η e a <ρ aa co cu β β ο β·Η ο ο AO CO < g · >10« Λ β «-Η © c « Λ Ή P o to o o to o O o o CM © n r-| «*» v 1 1 © 1 I 1 1 © © 4 w w CM o © M M ·· ·· >· ·· CM Φ - •P tt «Η tt •Ρ Φ •P 1 fatt cn CO . cn z cn © P P *0 CM 0 0 Φ M M •P ·· CO tt X Φ O o O' © Φ Φ o © o *P © Λ Λ © CM o •0 ©CM © o <*) 0 CM K CM β M* i CM 1 1 1 1 P | I o 1 «Ρ © CM O XT CM CM O « 0 CM H CM β • β β 0 ,. .· .. nJ ·* r0 .. ·. Φ rH -H n®H Φ-Η to ·· ·· CQ fa < cn < Q fa Q tt Φ Ή η η 0«>» 6 Η 8 φ tt β φ *β M· Φ 0 CM ·Ρ·Ρ CM η λ r- | in r-l Φ 04 CM (0 O © © © θ' Φ CO CM •Μ» CM H •Ρ Μ Ό» β Ρ Ο U Φ cn Μ Φ Λ Ο Φ a ο. ιη ιη ιη m «η >|O 01 rp pH ρ4 pH Γ* pH rd M fa X X fa X cm C 0 ft 0 tt tt ti tt tt « U-H Ό O a Φ co η Η Λ Ή SO Φ -Η «Η η ο φ ε Φ +1 Η 3 α < fa η rd cm <*> Ν Μ «Μ -φ tn to CM CM CM ΗΝΠ Tables IX, X and XI are of analytical data obtained by wet methods on product from vapor transport synthesis.

The P/M ratios in the tables are atom ratios unless otherwise noted.

TABLE IX SINGLE CRYSTALS (WHISKERS) FROM VAPOR TRANSPORT Ref. No._ Charge P/M_Total* 38 */>15 19.1 94.5 6 */>15 19.1 98.8 10 */>15 19.1 99.4 11 */>30 16.4 96.1 17 K/P5 11.3 97.7 * Analytical mass balance SM + %P detected TABLE X AMORPHOUS MATERIALS FROM VAPOR TRANSPORT Ref. No. Charge P/M Total* 39 K/PlS 2500 W 100.3 16 K/Pis 1750 W 99.7 21 */>15 2300 W 92.8 22 */>15 12200 W 97.0 25 K/P7 3500 W 97.9 26 */>15 6200 W 97.8 23ΚΛ15 greater than 4500 W 98.2 24 K/P,5 wg 7000 W 93.3 24 25000 W 99.5 27 K/P5 greater than 84000 E 99.7 28 */>15 7800 W 82500 E 98.2 29 V»u 25000 E 94.8 W) Wet analysis Ξ) Flame emission spectroscopy * Analytical mass balance %M + %P detected 536S3 Bt ξ P* s« & 01 οο οο cm m ΝΠ ΝΠ «β ** ί c Μβι· cm« MWM CNF Ο co ο βο Μ JSO ο»*ιρ» •o r» *» *< m o «μ«π W HH » VA •Λ "2^ Ol tf«b «•I Ο e. x κ « COO Ο ·ΟΛ WCmCM 0»4 CM eo ο cowo Ο «ΟΗΟ Η Cm CM CM p»*J η r»r» mis σι σι J* nr* «μ «Α XX O ft) 044 44 4 Λ» 4460446000 OU 0U Η«βιή*14βΟ CifOCilOSCB «£Ν 4 C 4Α 4« 4 « 4 · UUM«UUWN«V 44 40 σ> ο χο χό χο 0 4 60 4 60 • 4 · 4 Λ Ο U£*MCO 0*40 0«4Γ» r» re SSo σ·4?* 4 4 2 6 4 6 4 6 O' ««Ο4.4Ο44ΟΠ* mco mco Mcen 0*4 r- ο»4γ» e*4r-*H< ns e*x 41 4 • 4 Μ Η 4 Ο 4 Cfl 4 60 44004m wcw Mere 9»444 6*4*4 6 4 4 uc &*» 6 4 4 O UC r· o»4 C 0X00 ο eoonoo ucoo -nm 0*4r- H^HH r4 44 n ns -. o nSre 44 4 44 44 X Ο δ u Μ 9 4 04 OH »4 0« Β." *22 £3. i-ί m «: ξ X X ¢:3 X Cb S3 4 (Β Η Μ 4 U ΑΗ 4U Ό?** 4 U Η 4 9S Ο·* 4 S S 9 4 4 Ϊ52 33£ J58S *4 4C 4 4 Οι 6 4-Η 4ΖΧ SO Preparation of Metal Polyphosphides by Two Source Techniques iolyphosphides have been prepared in two fundamentally different types of equipment which are both identified herein as Two Source or separated source techniques because in both types of equipment, the metal and phosphorus are separated and heated independently on either side of a deposition zone. All examples have been carried out on the K-P system.

In the first method, as shown in Figure 11, the phosphorus and potassium charges are held at opposite ends of a sealed quartz tube 100. The tube is subjected to a temperature profile as shown in Figure 12, achieved by use of a three zone furnace. The profile takes the independent charges to elevated temperatures, relative to che center zone between the two constituents. In this zone, the vaporized constituents combine to form the deposited product of KP^g, in the form of films on the reactor walls. (More complete details appear in Example V below) In the second apparatus, as illustrated in Figure 14, a substantial section generally indicated at 102 15 at ambient temperature held outside the three zone furnace 104.

This section includes a stopcock 106 and ball-joint 108 arrangement used to achieve the low-pressures desired to carryout the reaction. This alternate sealing technique requires lower temperatures for this portion of the set up, but allows, for rapid and nondestructive insertion of a glass boat which holds the phosphorus and metal sources. The boat 112 (see Figure 15) also is designed to hold metal on glass substrates 114 (Figure 14) upon which the films are to be deposited. These film/substrate configurations serve as initial starting points for device designs, as indicated below.

The section outside the furnace provides a cold trap for vapor species. Specifically, phosphorus, which is loaded into the zone closest to the outside section, is deposited 8ί in the outside section in large amounts, generally as the highly pyrophoric white form. Because this trap exists, the vapor pressure conditions of the system are quite different from the totally-heated systems described above. It follows that the temperature conditions which successfully yield desired products in the first apparatus, are not appropriate for the second apparatus. The conditions appropriate for the latter were independently determined.

Example V In the 54 cm long by 2.5 cm diameter quartz tube 100, with a 10 cm long by 1.0 cm diameter neck 116, shown in Figure 11, phosphorus and potassium were loaded, under dry nitrogen conditions, into opposite ends of the tube, in an atom to atom ratio of 15 to 1. The potassium (99.95% pure) was loaded first by dropping small pieces, totaling 0.28g in weight, into a cup 118 with the tube oriented vertically. The pieces were then melted and allowed to resolidify in the cup. The phosphorus (99.9999%) was then added to the tube, the 3.33 grams of pieces easily being manipulated around the cup 118. · The tube was then sealed by fusion of the neck 116, at 5 x 10-5 Torr.

The tube was then arranged in a Lindberg Model 54357-S 3-zone furnace to lie centered amongst the three zones. Unlike the Model 54357, which has zone lengths of 6, 12 and 6 inches (15.2, 30.5, and 15.2 cm), the 5 model has zones of 8, 8 and 8 inches (20.3, 20.3, and 20.3 cm). Two woven asbestos tapes, spiraled around the tube, held it at the junctions of zones 1 and 2, and zones 2 and 3. Not only did these tapes support the tube, they insulated the center zone from the higher temperatures of the outside zones. A schematic representation of the resultant temperature profile is shown in Figure 12. A Honeywell Model DCP-7700 Digital Control Programmer was used to drive the three heating zones through an appropriate warm-up period, to the 450, 300, 450’ gradient, which' was held for 72 hours, and then through a 15 hour cool down sequence to ambient temperature.

The materials formed in the tube were analyzed by the following procedure. First, in a dry nitrogen atmosphere, the tube was cut into seven tubular sections, of approximately equal lengths, by use of a silicon carbide saw, Pieces of the films found in the sections (generally 10 microns or greater in thickness), were removed and individually examined by X-ray diffr ction techniques. The remainder of each section was subjected to analysis by wet methods.

The P/K ratios of the deposits found for the sections are indicated in Figure 13. For the center regions, where T was approximately 300eC, the bulk compositions were about 11/1» which falls within the accuracy limits of the methods employed to identify the material as KP^g. More revealing were the X-ray powder diffraction patterns for the materials found having a P/K of about 14, which clearly showed they matched those of KP^g, either from single whiskers or bulk polycrystalline material. Furthermore, the patterns clearly showed the presence of both polycrystalline and amorphous materials in about a one to one ratio, as manifested by broadening of the peaks.

Example VI The apparatus used in this example was modified relative to that of Example V. The quartz tube 119 was fabri25 cated with nozzles 120 and 122 segregating the two end chambers from the center one (see Figure 16). Under dry nitrogen conditions, melted potassium (0.47 g, 99.95% purity) was added to the outside chamber indicated at K, and allowed to resolidify. The addition tube 124 was then fused 30 shut. Phosphorus (5.58 g, 99.9999% purity) was then added to the other outside chamber indicated at P and the whole apparatus evacuated and sealed at 1 x 10-5 Torr, by fusion of the second addition tube 126. The phosphorus to potassium ratio in the system was 15 atoms to 1 atom.

The sealed tube 119 was 41 cm long, and was centered amongst the three consecutive 20.3 cm zones of a Lindberg 3 8 8 3 8 Model 54357-S 3-zone furnace. Two thermal barriers (TB) of woven asbestos tapes, spiraled around the tube held it at the junctions of zones 1 and 2, and zones 2 and 3. jn atj_ dition to holding the tubes, they insulated the center zone from the higher temperatures of the outside zones. A Honeywell Model DCP-7700 Digital Control Programmer was used to drive the three heating zones through a warm up period, to a 500, 355, 700°C gradient. (The phosphorus was at 500°C, the potassium at 700eC. The center zone temperature was selected as 300eC, but because the insulating characteristics of the woven tape are limited, heat spillover from the side chambers raised the center zone temperature to the 355‘C level.) This gradient was held for 80 hours, and then a 24 hour cool-down sequence was followed.

When tube 119 was cut open, under dry nitrogen conditions, using a silicon carbide saw it was found that nozzle 122 between the potassium zone K and the center zone had become clogged with material, which looked like polyfibrous KP^g. The center zone contained thin, light red films; thicker, darker red films; and several, relatively large, monolithic boules. The two largest pieces were each about 4 cm long, by 1 cm wide, with a maximum thickness of about 4mm. One side of each piece is relatively planar, while the other has a convex configuration, associated with growth against the inside walls of the circular reaction tube.

Wet analysis of this material showed the potassium content to be extremely low, as a bulk analysis, at less than 60 parts per- million. Electron Spectroscopy for Chemical Analysis (ESCA) indicated that the potassium content of this material decreased rapidly outwardly of the tube wall on which it was first deposited. At 100 angstroms the ratio of P-K was about 50. As measured by ESCA the P-K ratio on the final surface deposited was in the order of 1000. X-ray diffraction studies showed the material to be amorphous. 3 8 8 3 8-1 Example VII Under dry nitrogen conditions, 0.19 g of melted potassium (99.95% purity) were transferred to one of the outermost sections 128 (5 cm long) of a ’Pyrex (Registered Trade Mark) boat 112 (Figure 15). The metal was allowed to resolidify. Two plain glass substrates 114 (see Figure 14), each about 7.5 cm long by 1 cm wide, were laid end to end, filling the 15.3 cm long center section 130. Next, 1.36 grams of phosphorus (99.999% purity obtained form Johnson Matthey) were added to the opposite outside section 132 of the boat. The phosphorus is in a mixed-size granular form which readily pours out and fills in the bottom of section 132. Pyrex dividers 113 keep the P and K and 'ubstrates from sliding in the boat 112. The 35 cm long boat 112 was then carefully slid into the 60 cm long by 2.5 cm diameter Pyrex reaction chamber 134 of Figure 14, until section 128 with the potassium abutted the round bottom, closed end of the chamber 136. A Buna-N O-ring, size 124 was then clamped into the O-ring joint 102, and the Teflon (Registered Trade Mark) stopcock 106 (supplied by ChemVac, Inc) screwed down tightly. On a vacuum line, the stopcock 106 was re-4 opened and the chamber pumped down to 8 x 10 Torr. The stopcock was then re-closed, sealing the reaction chamber.

The reaction chamber is arranged in a Lindberg Model 54357-S 3-zone furnace. As shown in Figure 14, two wovenglass tapes 137 and 139, spiraled around the tube, supported the chamber at the junctions of zones 1 and 2, and zones 2 and 3. These tapes forming thermal barriers (TB) were set to just lie completely within the center zone. A third spiraled tape 138 was used to support and thermally insulate the point where the apparatus exits the heating chamber of the furnace. A cylindrical plug 140 of a ceramic like material was used to stem heat loss out of the furnace opening at the other end of the chamber.

This arrangement of the apparatus results in section 128 of the boat 112 containing the potassium to lie within the third heating zone, section 130 containing substrates to lie in the center, or second, heating zone and section 132 of the boat containing phosphorus to lie in the first heat5 53SS3 SC ing zone. It also results in a large segment of the apparatus being outside the furnace, at ambient temperature.

A Honeywell Model DCP 7700 Digital Control Programmer was used to drive the three heating sections through a warm5 up period in which the temperatures were brought to 100, 150, 100®C· in the phosphorus zone, the substrate zone, and the potassium zone, respectively. Then, as rapidly as possible (approximately 18 minutes) the gradient was driven to 500, 300, 40Q°C, where it was held for about 8 hours. The furnace was then allowed to cool at its inherent rate, to a profile of 100, 100, 100"C, which took about 10 hours. The furnace then was allowed to cool to room temperature.

The tube 134 was removed from the furnace. The section outside the furnace contained deposits of white, yellow, and 15 yellow-red materials, all of which were probably phosphorus in varying stages of polymerization. The phosphorus heating zone was clear of material, while the potassium zone contained a variety of materials, ranging in color from tan, to yellow, to orange.

The latter extended slightly into the center zone, which otherwise was covered through one-half of its length, next to the potassium zone, with a dark film, which transmitted red light when a source lamp was shone through it. The remaining half of the zone was clear of material. The apparatus was opened under dry nitrogen conditions, the pyrex boat 112 withdrawn, and the glass substrates, covered with the red film, removed from the boat, and placed in a tightly sealed bottle, for later analysis. (When the remainder of the materials were exposed to ambient conditions, the phosphorus deposits in the exposed section of tube would generally burn vigorously, though those closest to the phosphorus source did not exhibit such reactivity. The materials which were in the potassium-source section of the apparatus were very reactive when exposed to moisture. They generally burned vigorously, apparently by the production of hydrogen via reduction of water.) The technique was repeated several times. Further examples are noted' in Table XII. 53G33 FURNACE < < 01 a a ft < Sx ω e-< in © o © © © tn O in OU E • • • • • • • • ο. z o «ω Q U © © in H cn © P* © co w i w O CO S CO Η (4 EH S ο CO Μ Ο © DZ η ω υ gg° W Μ ft ϋ •S Η U &4 ft Ο ω *α μ β «β 4 I * • < < Ο ο ο ο ο <Π <Η -Η\ Η\ (0 O' O' (0 n n w a n (Q n n » 4 « c β 4 4 4 4 rH 4 o o ^4 »-4 r4 r4 O' ^4 O' O' O' O' O'-Η -H z Z 3© 3© ι<Ο <Ο V.I*· Χ,ι** •Η \ < < Ο Ο Ο ο ο ο Γ» cn CS CS © CS *4» © «-4 5»· 5J· © co © Γ- tn m m • ♦ • 9 • r4 r4 i4 r4 r4 r4 i-4 r4 σι σι © r* tn o •H r4 O r4 rH CS cs i-4 cs CS CS CS • • « • • • • © © o © O o © O o o o o © © o o © © • • • • • • • • • 00 co © © a © © © © o © o to © © o o o © © o p- © © o © © mt *r n *4· «4» <4· o o © © O © O o O o o o o © o o o o n cn m cn cn cn cn cn o © © m © © m m a o o o r* © © cs CS o tn in in *3* tn tn in m tn co σι *9· Ο γΜ cs in in in η tn 8 There exist limiting conditions for the preparation of the dark films which transmit red light. If the temperatures in the two source zones are dropped slightly, as in run number 49 of Table XII, the amount of material formed, as manifested by the length of the deposit, drops dramatically. Similarly, subtle differences between the performance characteristics of two otherwise identical Model 54357S 3-zone furnaces require that in the second furnace (BJ, the temperature of the phosphorus source be raised to higher temperature (see run numbers 50, 51 and 52). Raising the phosphorus source temperature to 550"C gives a good result, raising it to 525®C gives a better result.

Analysis of materials from runs 46, 47 and 48, by Scanning Electron Microscope with electron diffraction analysis (SEM-EDAX) methodologies revealed the material to be KP^g films, on the order of 6-7 microns in thickness, and to be of an amorphous character, with no discernible structure evident in the micrographs.

Summary of Vapor Transport Conditions Processing features for controlling product types are: 1) Use of a three zone furnace for more uniform temperature control; 2) Extended tube length; 3) Use of thermal barriers for temperature gradient control; 4) Use of thermal plugs at ends of oven; and 5) Use of extended narrow 25 addition tube to obtain cylindrical boules.

Ranges of conditions for one source vapor transport are: 1) Reaction zone temperatures range from 650-550°C; Cold zone deposition temperatures range from 450-30(^0. 2) Deposition temperature for single crystals of KP,-. were found to range plus and minus 25 °C around a center value of 465-475’C. 3) Deposition temperature for polycrystalline films were found to range from about 455°C down to 375°C. 4) Deposition temperature for amorphous forms of the new form of phosphorus range from about 375 °C down to at least 300°C. (No lower temperatures were investigated to date).

The conditions for two source vapor transport for forming bulk KPjj materials are (Figure 11 apparatus); Phosphorus, temperature at 450°C, Potassium at 450eC, and deposit zone at 300®C; deposits were thick films of mixed polycrystalline and amorphous KP^; f°r bulk amorphous ΚΡχ (x much greater than 15 the new form of phosphorus, Figure 16 apparatus): Phosphorus at 500°C, Potassium at 700eC and deposit zone at 355°C. K source became plugged, deposit was bulk amorphous ΚΡχ; for thin films of amorphous KP15 (Figure 14 apparatus) Phosphorus at 500*C, Potassium at 400°C, and substrate at 300°C.

For thin films of KP15, the Phosphorus source may be raised to 525°C and amorphous KP^,. is still produced. If the Phosphorus source temperature is dropped to 475°C, the system does not. yield KP^^. If the Potassium source temperature is dropped to 375°C, the system does not yield ΚΡ^5< The substrate temperatures may be raised to 315eC and the system will still yield KP15, hut not if they are raised to 325’C.

Preparation of Polycrystalline Metal Polyphosphides in Large Amounts Via Condensed Phase Synthesis Although not formed in a physical state appropriate to the tapping of their useful semiconducting properties, alkali-metal polyphosphides of the type MP^, MP?, and MP^, can readily be prepared in gram or more quantities by a technique we call condensed-phase synthesis. Before using this technique, the reactants are generally brought in intimate contact by a ball-milling procedure. Decagram or more quantities of the elements arc loaded in ball-mills, under dry nitrogen conditions, in the desired metal to phosphorus, atom to atom ratio, e.g. P/M 15 to 1 for mpj5· 'The sealed mills are then utilized for 40 or more hours to reduce the components to a well-mixed, homogeneous, free-flowing powder. The mills are generally heated during 20 hours or so o of the milling, to about 100 C. This is done to increase the fluidity of metal component during the milling.

A portion of the milled mixture, generally 10 grams or more, is transferred to a quartz ampoule, under dry nitrogen conditions. The ampoule ranges in size from 2.5 cm in diameter by 6.5 cm in length, to 2.5 cm in diameter by 25 cm in length, depending on the charge size to be processed.

The tube is sealed at reduced pressure (generally less than 10-4 Torr).

The reaction is carried out by subjecting the tube to an ever increasing temperature, under isothermal conditions, until the applied temperature reaches 500 or 525°C. By isothermal conditions we mean that the whole mass of material is always as nearly as practicable at the same temperature to prevent vapor transport from hot to cold portions which would result in non-uniform products. The highest soaking temperature is held for a substantial time, during which a powdery polycrystalline or crystalline.product is formed. A typical soaking time is 72 hours. The longer the reaction, or soaking time, the more crystalline the product (as manifested by grain size, sharpness of X-ray powder-diffraction lines, etc). The hot tube is also taken through a cooling period (more than 10 hours) to ambient temperature. Slow cooling is not necessary for the reaction, but prevents tube breakage due to the different thermal coefficients of the products and the quartz·ampoule.

Both the heat-up and cool-down periods have been ob30 served to best be devised as relatively long (more than 10 hours) with soaking at intermediate temperatures (e.g., 200, 300, 400, 450*C.) for 4-6 hours. Failure to follow these slow heat-ups or cool-downs often resulted in explosions of the reaction tubes. However, the products of the condensed phase reactions were the same as in slow cool down except that a small quantity of residual phosphorus would be white rather than red phosphorus.

Example VIII 19.5 grams of a ball milled mixture of reagent grade phosphorus and potassium, in an atom to atom ratio of 15 to 1, was transferred into a 6,5 cm long by 2.5 cm diameter quartz tube, which tapered to a 8 cm long by 1.0 cm diameter section. The transfer was carried out under dry nitrogen conditions. The tube was sealed at reduced pressure —4 (1 x 10 Torr) by fusing the narrow section a centimeter or so above the wider part of the tube.

The tube was supported in the center zone of a Lindberg Model 54357 three-zone furnace by a second quartz tube, or liner, which was, in turn, supported in the radial center of the heating chamber by asbestos blocks. The 3-zone furnace heating elements were driven by a Honeywell Model DCP-7700 Digital Control Programmer which enables processing to be preprogrammed and carried out in a reproducible fashion. Using the programmer, the reaction tube was subjected to the following temperatures for the indicated lengths of time: 100°C, 1 hr; 450eC, 6 hrs.; 500°C, 18 hrs.; 525°C, 72 hrs.; 300°C, 2 hrs.; and 200°C, 4 hrs. (When all three zones are controlled at the same temperature, the center zone is highly isothermal, with a temperature variance of less than 1°C across the zone).

After the furnace cooled to ambient temperature, at the inherent cooling-rate of the furnace, the reaction tube was removed from the furnace. Under dry nitrpgen conditions, the quartz ampoule was cut open using a silicon-carbide saw, and the dark purple, polycrystalline mass removed. A sample of the material was subjected to compositional analysis. Wet analysis gave a P/K ratio of about 14.2 to 1, which is accurate to about 6% of the theoretical value of 15 to 1. Products from similar runs on K/P^^ charges fell in the some range values, as shown in Table XIII, ο 3 ϋ k λ 3* U Κ ΙΑ Λ ω ο «Η t I I I I I I OO0000QQ ‘ Μ Ή »4 r4 »4 «Η »»rt^i/»^«n-v*n®r*n*«"· τ * < I I I I I I ( I I t t 1 I ( < ooooooooooeooooo SS28 bOt*S κχκχχκκκαχχχχχχχχχχχχχχχχ hviuiu»«4Mtom oeonooortooonoooooooooooo^ *W(OC\«N«C\WO\rtC\’i4nnnffl»N'»(*l«r»N »4(*ΐηΝΗι-ΙΗΝΗΠί>Νί'«ΗΗΗΝΠΗΜί1Ν«Η U7QOOOOOOQQOQOGQQ OOOOOOOOO ΟΐΛΟ»ίΝ««ιβΝΟΟ<Λθ4ΝΝΝΝΒΝΤβ\<*Ν Γ4Ον0«-4Γ*Γ·Γ*·Ηί*ί4Μ»·-4«β^Ρ*Γ·Γ*Γ*'βΡ’’Τ ΰ>^ -‘'met C4 H W · u α» κζυ Ο&3· Mb OirtmunfltftinifttAooino «loco oo iflmofliflfl ©ννννγινννοονονοοοοομνονμν tntf)«nm«n«ninininininmv)u)tntfiMtAvi(Atf%witn«AtA U Ο ΙΛ (CMS <μ2 ζηκ υ«« irtNNeNr'f»4H«o»»mnonNy)^inr,riei<»H *^δ « 8§* HBS Ο X «>% (Ufa U go ΚΜ σ*· ·,·’η’ΚίΜΐΑ2Λ4Φ'β2Γ·2«22Ζ2Ζ2 *4 ^4 i-4 ·4 *4 a4 *4 *4 *4 »4 *4 a· a« «· w v « « 9 9 b b b b a* a* fa 9 9 9 9 **** ** ο. fa fa fa Ort a a c a - . w _ 9 fa ** fa fa fa fa fa —* ——»»— m iniAMtnin(n(AfM»4»4^4a4»>t»4r*t*«i«>r*r*fa fafafafafafafafafafafa fa fa fa fa fa fa 3Λ » β * «-1ΛΛ a a a\\\\\\ ·κνοζζ*422ϋϋζχχχχχχ ax> Ob 9 -4 ab aba κ ox uz? In addition, several samples from different runs were subjected to morphological analysis. The XRD powder diffraction patterns for these materials were readily matched to those obtained from the single crystal KP^ samples produced by the vapor-transport methods cited elsewhere.

The methodology was carried over to other metalphosphorus systems, as is indicated in the table. Comparisons of the XRD data of these materials, both with each other and that obtained on single crystals established the analogous nature of the products, i.e. they all have basically the same all parallel pentagonal tubes of covalently bonded phosphorus.

Milling Metals with Red Phosphorus Introduction We have utilized ball milling to prepare homogeneous, intimately contacted mixtures of red phosphorus with Group la and group 5a metals.

The milled products are relatively air stable and they provide conveniently handled starting materials for the previously described condensed phase and single source vapor transport techniques. Their stability indicates that polyphosphides have formed at least in part during the milling process.

Summary The Group la metals (with the exception of lithium) have proved to ball mill easily with red phosphorus. The facility of milling becomes even more pronounced with the lower melting metals, typified by rubidium and cesium. A problem arises when the Group la M/P ratio is varied from 1/15 down to 1/7. The increased metal content generally results in severe agglomeration of the charge onto the walls of the ball mill. Fortunately, the agglomerated products are easily scraped from the mill and crushed through a 12 mesh sieve. Lithium and arsenic are somewhat difficult to 53S83 mill using the standard ball milling procedure due to their hardness and higher melting points.

Reagent Purity The initial experimental work used reagent grade metals and regaent grade phosphorus. We now use only high purity metals and electronic grade (99.999% and 99.9999% pure) red phosphorus obtained from Johnson Matthey.

Mode of Milling A. Standard Ball Milling (Rotation) This was originally the method of choice for the alkali M/P systems. However, we have used more intensive grinding processes (cryogenic and vibratory milling) for the other group 5a metals.

The stainless steel ball mills were fabricated in house and as shown in Figure 17 comprise a cylinder 150 with these dimensions —4.5 (11.4 cms) O.D. X 6 (15.2 cms) X 1/4 (0.63 cms) wall thickness. The top of the mill is provided with an inner flange 151 to accept a Viton 0-ring 152. A stainless steel top 154 is held in place by a bar 155 tightened down with a screw 156.

One mill has smooth inside walls. The second mill was constructed with three baffles welded onto the walls from top to bottom. These act as lifters for the balls and reagents and result in more efficient grinding.

A total of less than 50-60 g reagent charge is desirable. Initial milling experiments used 1/4 (0.63 cms) stainless steel balls; we have since achieved better results with a mixture of 1/4 (0.63 eras) and 1/8 (0.32 cms) stainless steel balls.

Cryogenic Milling (~196°C) This was accomplished using the Spex freezer mill (available from Spex Industries, Metuchen, N.J.).

Due to equipment limitations, only small quantities (2-3 g) can be milled in a single operation—however, this can be done quickly at liquid nitrogen temperatures (in a matter of a few minutes). Thus, this technique finds applicability in reducing to powder form, the harder and higher melting metals such as lithium and arsenic. These can then be co-ground with red phosphorus in the rotating ball mill or vibratory mill.

Vibratory Milling The equipment (Vibratom) is available from TEMA, Inc., Cincinnati, Ohio.

This is essentially a ball mill, but instead of using a rotating motion, circular vibrations are generated—similar to that of a paint shaker. The dimensions of the mill are 5I (13.3 cms) O.D. X 3.5 (8.9 cms) height X 1/8 (0.32 cms) wall thickness.

The mill does not contain baffles. We have used this mill for the difficult to mill elements such as As.

Time of Milling There has been considerable variation here. Generally, the duration of hot milling is not less than 40 hrs. Nor more than 100 hrs. To some extent, this has been determined by the system being milled. Less time is required for the lower melting Cs and Rb systems.

Temperature of Milling This has either been at ambient temperature or the mills have been externally heated to approximately 100°C with a heat lamp. Ambient temperatures are suitable for low melting point metals such as Cs (28.7°C) and Rb (38.9°C). External heat lamp application to 75-100°C for 3-4 hours was definitely beneficial for the Na(97.8°C) and K (33.7°C) systems. Heating to 100°C was of no value with Li (108.5°C). We conclude that stable products are the result of milling melted alkali metal and phosphorus. 9G Ball Milling of K/P^g Example IX (Reference No. 88, Table XIV) Under nitrogen in a dry box, an unbaffled stainless steel ball mill containing 884 g of 1/4 (0.635 cm) stain5 less steel balls was charged with 6.14 g (.157 atom) 99.95% pure K (from United Mineral and Chem. Co.) and 72.95 g (2.36 atom) of 99.9999% pure red P (from Johnson Matthey Chemicals). The mill was sealed and rotated on a roll station for a total of 71 hours. The mill was heated to 10 approximately 100°C for 4 hours by playing a heat lamp on its surface. The mill contents were discharged in the dry box to a 12 mesh sieve and pan. No agglomeration of the product was observed. The steel balls were separated from the product on the sieve. A total of 76.4 g of black powder product was obtained.

Ball Milling of Cs/P? Example X (Reference No. 115, Table XIV) Under nitrogen in a dry box, a baffled stainless steel ball mill containing 450 g 1/4 (0.635 cm) and 450 g 1/8 20 (0.3175 cm) stainless stell balls was charged with 12.12 g (.0912 atom) of 99.98% pure Cs (from Alfa/Ventron Corp.) and 19,77 g (.638 atom) of 99.999% pure red P (from Johnson Matthey Chemicals). The mill was sealed and rotated on a roll station for 46.5 hours at ambient temperature, (no external heat source applied). Upon opening the mill in the dry box, almost total agglomeration of the product was observed on the mill walls. This material was scraped off with a spatula and discharged to a 12 mesh sieve and pan. The chunks of product were then crushed through the sieve. A total of 27.8 g of product was collected in the pan. 7 Table XIV summarizes the results of milling various metals with red phosphorus. As previously noted, these materials are surprisingly stable. ► ts o ft ' fi « H ι O b O ι μ ο μ 'POP ,« » β ' h S Η s a § * -ss SSB OUM f* wx K *tt as m ca μ u cu < a. ca a k ca a o 2 CU s ca oo u uos Kw3 < S- H gss ta μ ο μ O w 22 o §i Od a «Ρ «5 ~ ~ W I ti * ft A ft A b O ω§ ttriI " O HO01ω. -gU07 δύ co ti tw c4 TP ti cq 3 ti ia I b ti Φ ο Ό μ &ρ Ο ti -a ·ώ fe3 c< 6fl μ ti 3 ti b b © « MM • 9 «2 ti © *- 55 55 ** *s»s u © •ox β « b b 33 P P e e © © 0» 3 a ti © v b b I I XCU © ti t (J © © Ό Ό Ό Τί «Β Ό ti ti titi ti ti b b b b b b 3 3 3 3 3 3 P P P P P P c c c c e e © © © © « © 3 3 3 3 3 3 titi titi « » ti © V © © © b b m b b b ii ii ii ti i rt Ο φ o , •HOd P tfc P 1 ti 0 d : b A b ' §ug' o o ' M°Ti· fcwxp ω4 ti Q ti ( TJ * | d £ § S3 ">·§ S o> o g _ η ft o I b e © o XM £S Ab © utO K © « ** b 5 Λ JS 3 b X 3 b X 3 « b X H b X « b X *b X 3 n 3 — 3 3 3 «r 3 X μ o ? o o O U V U « o 3 μ • 3 c* • 3 r* o o μ O o μ © © μ o © μ O 3 3 3 3 3 *r o CD CO e μ e* 3 μ ·+ 3 *▼ o 3 Γ» ft ti > X « ux 1 P 6 > © e fiS ft « X Mb e ti © © μχ I titi e ®« ΌΟ b b © · OCX ti « OS b b ti © MM « ti a a 1 P c a X o b b ti ti 4CX ti ti η n b b « o MM « ti a a T Sti© μ «μ XM ti 3P TS b © ** μχ • · c O si* ci* P s μχ e o oo « a ft p ft © μ ox *>3 p rj 33 bb »3 3- CPU ©< © a ΌΟ ti ti b b 33 « © XX 4» ti b b 33 ti © X X ti « b b 33 P © ©*0 *0 ti « b b 3 3 ti © Ό Ό ti ti b b 3 3 P 3 3 3 P P 3 titi P 3 PP c- c PP ts e 3 * 3 P P c e © P P C C 3 P P 3 3 3 0 0 3 μ 3 O · 4© 3 • tfl ·<) »O -w ομ μ cc hn m 3 3 3 titi · ti © 3 3 b b 3 3 I I II XCU KCU 3 3 3 © CD CD n 3 io · ti V 33 e a © © b b I I Kfe titi 3 3 3 3 3« Q ti © U b b t I ti 3 ti ti 3 ti 33 cu £ 3 3 | I CU X © b b I i0· _3 b b I I X CU Sg ♦X ti 3 | ( CU X 3 3μ 3 φ cu X X St X *ax« SS5 ggS sa Qu ae.

M3 RS a 3 S cu gg S3 v e r* i > Ο β A V *· O Λ--4 JJ -4 a Μ β A rt M © u-s « rt © Ο B 5βϊ, 5 2r* σ> 9 h « cn (3. © v 0» rt rt cr A V Nil > • © · ο Ό μ n rt v I * rt rt « 9 XX HK ©a *a β a rt rt 0» p A 6 e u v 0» e»« © 9 9 rt rt i A* cn re R 3 H 3 © M tU> Lt > <® Λ ® *" « p m li o ?«§ A > H a Ή » © A « ©-P 3 «4 A 3 1-10} u « r w a R ©Ofi) ^ss; s son Ο«τ«η «β e a O A rt rt rt © rt « • 9 9 OH 44 X XM XH 1« β « · β 0144 © c a A A it 9 > o • Β V© • · · e 9 9 e H H fr· 045 *M«5 • * · 44 U HO ►3»? <>? 9 9 9 tn cn e»o 9 9 9 A 9 9 9 9 rt rt rt I ( I I cu KCU X ffl I I B> X 0* O’re et* R > © R w Λ I +» © a > o Ό © Ή ©•rt -P Λ w ¢) 01 R 3 Λ © R w S © © o M C o o *OH «-· 45 U · XHH « cn© a rt 4i .21 ►Js s rt rt rt X A com 9 rt β» e*a · 0»σ» © « Ok rt i i* O’ +»3 ί 0Ή f a f> I τίτί ♦rt »*3 t>> ) h J Hr) • cu « 44 «4 · -C © 9k iNrt »ll -4» e ©«r · O> ·<ο « ΟΌ X—· B a 0 < >10 4) <-i c kA·* © 44 rt Η Q.H £ O OAW044OHC4HO © f* n 0 *· £ ~ H W ϊκ> © © Λ Λ C 44 44 0 44 44 rt « 0 9 9 © fi C n<° <88 H M fi © rt Ok rt 0» · © Ok 9 • 0k rt rt i * i σ» A 0.0* in e 0k Ok 3 · Ok © | 0» pie Ok E « 6H Η H A 9 hr· agglomeration 100 as ·©.-» fa© lit «Μ • E n c bfi C* © «4 a·*© X w O x JjA fa fan 0** X** ου X· ao ao x c Om 0 X · -4 ar* a aid £ — 0 cs F«U 2« U»3 « eu SS Zfc U uu M < u u xe a. · O CSX . o X X X «41 « *CJ< 0 0 0 «4>β • «4 · fiS 0 4» X X O fifiX • « c «· « e A X A e « β t»*o o o*x m SaS v.v KACb 0*0» Ft 40 mm a a a met a et oi tn a a A A ffe n WON § §~3 s«p «il 0O fi MW«4 45 ww I 0 <<**n a « aa 4" 0* o»o •48 0 ea *5*3 _A A CA U 0 A I x 0 C 0 4» X ~l ©«4 fi X-4 0 0 s * ttA > Ο Ό X O K« 0 0 E * fi A E 0 *« A w OA WO β AQ <»3 9< A A fi C 0 0 mm 0 0 « X X I t «fa • e tt tt AAA XA moi mom aa a x a • et · a · m · a ao» a a a 0 a i? AA mm mm ttm am •0 A § s X X Xm · A 0 A A 0 r-l ex e o-4 © St β «a mA mA Ό X " 0«.^. -^A 8 0 « · a 6 ΰ -2SS ** * PA * a · a « a am · ο A < Ol Ok X • » A I 0 flu io. •4 m a 10 mm ©ι a · «ο Wn (AB |0« «L 0· m X A a · am 0 A ίο: «·» ^ «Η *« w« «4 Π M Hit 0<Λ αζα < w s Sab BH U a. ss sss <*1*2 u»a o J Om XX O 04* 0 41 hh h « h« t5 e* Sx • 0 6 C 9 C < <8 <8 ο · β c * c wh Λ RV 04* <§ Ο 6 ι σι « ι υα* ο»σ» η» «ιη >1 > • ο βδ βδ Ο 4* 0 4* . gs C · Β <8 ζ.8 • Β 0 Β «X MX *4 0 *4 0 A C riJ O 4* Μ β 4*« Sc <8 β c 4X MX JO MO C 4* 04* ?§ ?s Nrt ne φ »ο Λ Λ I 0» Ο I Ufa σι · σι 01 I 01 Ο I Ufa eno» NI* HN « ci u o JO HH > MM H MM 0 xx8 MOW M >»X 0 WM au> Ο «Η 102 Analysis of Products Table XV summarizes the various ΜΡχ (X = 15 and x much greater than 15, then®? form of phosphorus) materials synthesized from vapor transport with one source (1S-VT), 5 from vapor transport with two ’source (2S-VT), condensed phase processes and chemical vapor deposition (CVD).

TABLE XV MP X M ® XiXf NHf Rf Rb r Cs X x = IS X much greater than iS single X X 1S-VT crystals poly.B, TF amorphous B single 2S-VT crystals poly. TF TF amorphous TF B Condensed single crystals X Phase poly. B* CVD amorphous TF X: = crystals/whiskers B: = Bulk greater than 10 micrometers thick TF: = Thin film less than 10 micrometers thick B* = Powder 103 The materials obtained from these techniques were crystals or whiskers, referred to as X, solid polycrystalline bulk, referred to as B; solid thin film, referred to as TF; solid amorphous, referred to as B and TF; and bulk powder from condensed phase synthesis referred to as B*.

The analysis of MP^g crystalline materials was given above with reference to Figures 7-10. As indicated in Table XV, the polycrystalline and amorphous MP15 materials have only been produced in the form of thin films.

Polycrystalline bulk and thin films of ΚΡχ (x much greater than 15, were obtained by vapor transport (one source and two sources). These polycrystalline thin films nucleate on glass substrates (or glass walls) and show dense packing of parallel whiskers growing perpendicular to the substrate. SEM photomicrographs, Figures 18, 19, and 20 of such materials show a large physical separation between the ΚΡχ whiskers.

These polycrystalline thin films are formed at low temperatures from around 455°C to 375°C where the amorphous phase begins to form.

Analysis on these materials wet chemical, XRD and EDAX consistently show x to be much greater than 15 (typically greater than 1000). A typical powder XRD diagram fingerprint of crystalline ΜΡχ (x much greater than 15) is shown in Figure 10.

As indicated in Table XV, amorphous ΜΡχ materials can be formed in bulk form (boules) by the vapor transport techniques. These boules are formed in the narrow end 160 of tube 32 (Figures 1 and 2),.the narrow end 162 of tube 58 of Figure 3, or as pieces of material in zone 2 of Figure 16.

These materials show no X-ray diffraction peaks.

XRD powder diagrams were used in our study to characterize the degree of amorphicity of the materials obtained 104 by these techniques. These amorphous ΜΡχ materials where x is much greater than 15 can be cut, lapped and polished using conventional semiconductor techniques for wafer processing. This is even true of material containing no more than 5Q to 500 parts per million of M, a new form of phosphorus.

The resulting high x, ΚΡχ amorphous wafers or substrates were shown to have useful semiconductor properties with electro-optical response almost identical with whiskers of KP^. We therefore conclude that the local order of all ΜΡχ materials where x = 15 or is very much greater than 15 {when solidified in the presence of alkali metal) exhibit the same local order substantially throughout their extent. This local order is the all parallel pentagonal phosphorus tubes.

Amorphous high x, ΚΡχ materials were prepared with mirror finish surfaces for electro-optical evaluation. Routine surface preparation of these amorphous materials includes several processing steps such as cutting, embedding, lapping,polishing, and chemical etching. 'Surface work damage induced during such processing steps are known to affect the electro-optical performance of semiconductor materials. Therefore, attention was focused on assessing techniques and processing steps leading to a damage free surface. The following processing steps have been found to be suitable for the preparation of high quality mirror finish surfaces.

Embedded boules of high x, ‘KP (about 1 to 2 cm in length) from Table VII were cut with a slow speed diamond saw using minimum pressure. Each wafer was sliced to a thickness of approximately 1 mm. The wafer was then immersed in a bromine/HNO^ solution. To remove- sufficient cutting damage the thickness of each wafer was reduced by this chemical etching by approximately 50 micrometers. The wafers were then washed and checked for inclusions and voids. The high x, ΚΡχ amorphous material appears to be void free. 105 A standard low temperature wax (melting point about 80*C) was used to mount the high x, ΚΡχ wafers onto a polishing block. The wafers were then lapped at 50 rpm at 2 minute intervals individually with a 400 and 600 SiC grit 2 using distilled water as a lubricant with a 50 g/cm weight until a smooth surface was achieved.

The final polishing step was carried out for one hour 2 at 50 rpm with 50 g/cm weight on a Texmet cloth with 3 micrometers diamond compound and lapping oil as extender.

This polishing step was followed by an additional fifteen 2 minute polishing step at 50 rpm with 50 g/cm weight on a microcloth with a slurry of 0.05 micrometers of gamma alumina suspension in distilled water. All procedures require scrupulous in between cleaning steps in a sonic bath with subsequent rinsing and drying.

Samples prepared by this technique have a high quality mirror finish surface. The final polishing step was performed on standard metallographic Buehler* polishing equipment.

Chemical etching plays a prominent role in wafer preparation, surface treatment, pre-device preparation, metallization and device processing.

Numerous review articles are available covering the chemistry and the practical aspects of etching processes. However, most information on specific etchants is widely scattered throughout the scientific literature. An attempt was made to bring together essential information that should be useful to the selection of an etching process relevant to these amorphous high x materials. Special attention was placed on etching procedures and processes used for surface preparation. It was found that some of the etching solutions and procedures currently used for GaP and InP are applicable but with different etching rates.

The following etching solutions were selected and tested: * Trade Mark 53S83 '100 -10% Br2 95-90% CHgOH for general etching and. polishing - 1% Br2, 99% CHgOH for polishing high quality surface (approximately 1 micron minute) - 5% by weight NaOCl solution for chemical polishing - 1 SCI : 2 HNOg (1% Br2) for removing work damage after cutting and lapping - 1 HC1 : 2 HNO3 for removing surface layer.

Several samples were prepared for optical absorption 1° measurements. The above technique was used to slice and polish on both sides amorphous wafers of high x material as thin as 0.5mm. Reference samples of GaP and GaAs crystals were also polished on both sides and used to measure the band gap by optical absorption.

Etching techniques were developed to reveal microstructures and to thin down small areas to 0.2mm thick for optical absorption.

Several etching solutions were selected and tested. The best chemical solution was found to be a mixture of 6.0 g potassium hydroxide, 4 g red potassium ferric cyanide and 50 ml distilled water at 70°C. Application to reveal an etching pattern takes less than 60 seconds. This solution is very stable and can be used with reproducible etching rates.

After embedding, cutting and polishing, several samples of amorphous ΚΡχ, (x much greater than 15) from Tables VII and VIIX have been etched. Typical microstructures were revealed from this chemical etching treatment after 30 seconds.

Figure 21 is a photomicrograph at 360. magnification of the etching pattern on a surface cut perpendicular to the axis of an amorphous boule of high x material grown by single source vapor transport (Reference No. 28, Table VII) showing honeycomb microstructures with well defined domains a few microns in size. These honeycomb microstructures are characteristic for an etching pattern on a material having a two dimensional atomic 'framework (such as parallel tubes). 3 6 8 3 107 Figure 22 is a photomicrograph at 360 magnification of the etching pattern on a surface cut perpendicular to the axis of growth of the amorphous high x material grown by two source vapor transport in Example VI. Figure 23 is a photomicrograph of the same etched surface as shown in Figure 22 at 720 magnification. Figure 24 is a photomicrograph at 360 magnification of an etched surface perpendicular to the surface shown in Figures 22 and 23 and shows an etching pattern characteristic of tubular packing.

Thus we conclude from the available evidence that our MP materials where M is an alkali metal where x is much greater than 15, i.e. where the amount of alkali metal is as little as 50 parts pet million all have as their local order the pentagonal phosphorus tubes either all parallel (the MPis form) or doub.le alternating perpendicular layer (monoclinic phosphorus).

• Electro-Optical Characterization of High Phosphorus Materials from One Source Vapor Transport The electro-optical characterization was carried out on single crystal whiskers, on polycrystalline films, and amorphous films and boules. The characterization consists of (1) optical measurements on samples with no electrical contacts (absorption edge, photoluminescence) (2) electrical measurements with simple contacts of linear behavior (conductivity, temperature dependent conductivity, photoconductivity, wavelength dependence of photoconductivity, conductivity type) (3) electrical measurements with non-linear or rectifying contacts with metals which are indicative of the semiconducting behavior.

From the above data we extracted the properties which indicate that all materials produced have electrical criteria for useful semiconductors, that is, they all have an energy band gap from 1-3 eV; conductivity between 10_5-10-'1'2 (olim-cm)rl; a photoconductivity ratio from· 100 to 10,000, and chemical and physical stability under ambient operating conditions. 3 ί> 8 3 10G Measurements were carried out on the following equipment ; (1) Absorption edge Photoluminescence (2) Conductivity Temperature Dependent conductivity Photoconductivity Zeiss 2 beam IR and visible spectrometer Low temperature (4°K) cryostat and laser excitation probe and 4 probe measurements from 300°K to 550 ®K in an evacuated chamber with light source of 2 approximately lOOmW/cin Wavelength dependent photoconductivity - Xe lamp light source and monochromator Conductivity type thermoelectric power measurement with hot and cold probes (3) wet silver paint was used to provide a temporary junction to materials, with a photovoltaic open circuit voltage of 0.2V measured under illumination.

Metallic and pressure contacts forming junctions were evaluated for their current voltage characteristics on a Tektronix (Registered Trade Mark) and tracer.

Data on samples from the broad class of materials under 25 investigation are summarized in Tables XVI, XVII, XVIII and XIX.

Table XVI summarizes the basic physical, chemical and electro-optical properties of the prototype material,namely ΚΡχ, x ranging from 15 to much greater than 15, in various physical forms and chemical composition. 3 P s 3 109 PROPERTIES SINGLE CRYSTAL (KP,-) POLYCRYSTALLINE (KP„) AMORPHOUS (KP a >» eh I fi I I I I * I * cH fi O GO 1 > Φ o a r4 © « I H r-l > > > S* Γ* 1 1 Φ Φ Φ Φ 1 fM ’φ CM 00 CO r* o O • • • • • r4 r4 rH r4 •H r-i rH r-i o' -b >,+» p <8 •rl M $· •d a 1 0 SD.C o»e « >. m w a p 44 RJ > > ο *> e -H Q-d o •d •d C 4) > * * •d fi •d 44 44 4» M 6 Οι·Η Φ Φ Φ 0 4» 44 1 υ U o OOP υ O 44 h a •d 0 fl) a C Kfl 0 c (3 c id Ή —* o > 6 *0 a *0 0 0 Φ Φ 0 U 4) 1 •d 8 c\ C 4* rl 41 Ό •H y υ t> O' o 44 1 0 44 O fl) P — M C 44 V) w 0 3 3 h 0 ε O A u 3 O’ 3 0 0« Φ Φ X 0) 4J 3 P 44 a A 0 tp 0 X > W P 0 d Φ C fi 0 Q. 0 ΟιΗ υ Ό 0 4>-d 44 rt •rl 3 0 O' •d X •d O s,x: ο o φ c 0 J 0 Φ P N P W Ό 6o Ε Ο Η 0i — > rd o Λ- A P< O " 41 0 4 w fi* fi n ω o α* CU < 2 ►4 110 φ Cn tn Μ rH ra Χ3 c υ XJ 0' 4J α -* •μ •Η ΧΜ μ (0 μ cu Φ ra φ μ « δ Φ A <0 - ra A 4J Φ Ό 03 CA μ μ c O bi tn rH ft TABLE XVI CONTINUED ε o μ UH 4J M O CU C (0 μ μ μ O 0« ra > o M □ o 0) O' q . Λ Ό Φ q •H Φ +» A O μ Φ 4J ra ε <*H o 4J ra *0 rH •H cu >1 in —»rH X ft q « 0 U 0 •Η O UH 03 I I o o σι σι in tn CU X5 ία bi cn A <α μ cn bi bi ra © φ Φ Φ J3 o XS μ *3 «μ q ft 03 0 3 μ u -Η φ uh φ Φ rH φ rH II I in ο ι I Λ 1 Φ XJ •η σι J to in j 1 bi ι cn ra μ cn 4J μ Φ Φ φ Φ A A bi q 03 O μ a ♦Η φ UH 03 1 1 uu U u 9 9 o o o © O © co co in in to to ^3· 5Γ o 4J cu A cn Λ « «μ cn A <α +> cn Λ <α μ cn φ «-Η Λ «α μ cn φ rH Χ3 φ δ co Λ <0 μ X o A bi O Ό C Φ CU M XJ W < e« Q M Φ μ Φ o ° ο ω h •rf rf *) u s u rf 4J ω W Ιβ CU o rf to cu ω e an Ο Ε «1 □ a 0 a> fr>a o *-* 0) μ A co \ O μ q φ tr « M A Ο cn η « tn σι Μ XJ CU ε φ ε ο ο κ Ο ft η χ ιη r* η a ο • * <0 ft ζ Ο η» X σ» c dP Ο ο ιη ιη -Η Ο « q ο •Η bi υ φ cu 0) q-π ι •Η 03 1 >ι< •η «η ; Φ Φγ 3 α ι 0) (0 ι •Η ι >η Φ I c Ο W ! ft Φ J 3 Λ ι * I Ό ι φ «-ι: 03 Φ I Φ·Η | Λ bi , q Φ* •η μ < >45.! Ρ ι« < -Η Ή j Ω ί •Η Λ~»< <α <« P&4C cn ο ( Iii Table XVII shows the properties of Group la (alkali metal, polyphosphides of various compositions and physical form. We observed that the electro-optical properties are independent of the metal whether it be Li, Na, K, Rb, Cs, physical form - crystal, polycrystal, amorphous (boule or film); and chemical composition, x - 15 or much greater than 15. ^683 Αι ο > Q Φ ζ~ < η ft Η > Η ft I υ 0 9 0 SO Η as Ο ic ft U S in CT rb © G ft 4i 3 Hrb P US > 1 © 41 H ft ft 2 0 P U 8 H P © 9 l M ft 41 O s > 3 2 K X »b © O o 3 P o H 0 cn lb •b O •P Λ < ft 0 z ft 0 9 a 1 ε H 0 ft P CT o 4J •b oi a O > w « © «b © © P Ό © c x; β 3 tn •b HX 3 ft ft PKK Si ϋ 0 0 0 4) P p p «Ρ 3 3 CTrb ib Ή *b I 6 6 •b -b © » w Ό •b β β Λ μ μ Οι© © W XI 4J Ο 4» 41 Λ 3 3 ft ft ft b Η II 0 ft < tn < Q ft &< « Σ2 ft ft 1 O K < Xft Qft O to co u < H ft w § © o n cm I I © cs co rb ib rb © © © • · · bbb CM CM CM © © © ib ib «b GV Cl GV I t I ©o© Η H ib I I I © Γ» © I I I o o © »b rb ib n £ p o in in ib ib κ κ tn η κ κ ft cu ο« « x « © c •b rb © «b G 3 •b 41 in ib » & Η >t o flPX -P 0 ft w >,p >nb O nog © ft 3 CM © rb I CM rbCH O O © rb ib ib GV I © rb I © I I I o o © «b ib rb CT Λ ft P ->i0 « Ρ χ; •ο o a oi >ih >,H o u ο ε o am CM O co o rb I r*» I © tn rb ft · © •b rb ©rb G 3 b 41« rb CT 0 h>,o 3 Ρ X3 P 0 ft « >iM >«b O ρ ο ε o ft 3 CM O b ft a □ 3 6 8 3 113 Table XVIII summarizes the properties of mixed polyphosphides and shows those formed of mixed alkali metals have no substantial changes in properties; partial, substitution of As on P sites is possible and produces a reduction in resistivity and possibly in the band gap (i.e. substitutional doping). o J'i Β 3 & < Ο Q Ζ < > η ο !μ £« Βι U © Q Β ο ο SK Β < CU Β ε* Η ιΗ H μ > I H k H Λ H ti £ > cu U u tf o © 1 k o £ M CU tf tf P o O Q rd u Se* rf ti υ •P CU Ο I ο Μ μ Ο ti « <3 Η W 3SB I ο < X Λ 04 CM I co co rd X o k cu cu ti CM Ο CM O r-i CM CM O © rd rd CM O rd CM O σι ι ο rd I α ι ο rd σι I © CO I o rd © I © rd to μ a k o r* ι © σι I o rd to O A CU k o 0) ti Ό •Η Λ α to ο £ Οι >1 »k Ο Ό ti X •Η £ »J 0 < tf Ο Η Η Η Ε-ι Ο £ tf OS Η rf rf Κ Εμ« UWO ►3 rf Η tf W © rd ti O 0« \ CM td © tf o o η m \ tf A *e & & cn en •k rd o, a, CM CM to to 5! to rf ti C -Η X -4 04 Η > ti I μ rd 01 ti >1 2 k >ι ο tf ti c k X i-l 04 rd >1 ti I μ H to U k tf* ° ti x -5 U 1-1 >t rk I ti μ to & υ XJ « to g jC cu k ti c ♦k M tk I rd X ti Pi μ N (0 >1 k to A 0« k O § ti μ to >, k O pattern similar to crystalline KP 3 f? 8 3 115 Table XIX summarizes materials and properties obtained from different starting charge ratios. We find that the best properties are obtained with materials formed from starting charge proportions of P to K of about 15 (i.e. between 10 to 30). Below 10 the yield decreases; above 30 the physical properties of the amorphous boules begin to deteriorate.

TABLE XIX KP from different starting charges analyzed in Tables IX, X, and XI above STARTING CHEMICAL X-RAY CONDUCTIVITY PHOTOCHARGE ANALYSIS POWDER (ohm-cm) 1 CONDUCTIVITY PATTERN RATIO K/Pj.5 reagent crystalline x - 15 A io8-io9 102-103 polycrystalline X» 15 B 107-10-9 102 amorphous X» 15 amorphous 10-8-10-9 102K/P15 pure crystalline X = 15 A IO9 102 polycrystalline x» 15 B IO8 102 amorphous X» 15 amorphous IO8 103K/P30 crystalline x = 15 A io9 102 polycrystalline X» 15 B 10-9 102-103 amorphous X» 15 k/p5 crystalline x = 15 A IQ9 10 polycrystalline X» 15 B 108 10 amorphous K/,p125 crystalline polycrystalline amorphous x » 15 poor physical properties A = pattern similar to crystalline ΚΡ·.5 B = pattern similar to crystalline1KP 3 6 8 3 7 We conclude that all these materials in whatever form have a band gap between 1 and 3 eV, more particularly in a range from 1.4 to 2.2 eV, since 1.4 eV is the lowest photoconductivity peak we measured and 2.2 eV is the estimated band gap of red phosphorus. The data further indicate that · the band gap of the best form of these materials is approximately 1.8 eV. Furthermore, their surprising high photoconductivity ratios of from 100 to 10,000 indicate that they are very good semiconductors.

Doping Bulk amorphous ΜΡχ boules obtained by single source vapor transport (Tables VI, VII, X and XI above) in our three zone furnace having a composition x much greater than 15 can be processed by cutting, lapping, polishing, and etching into high quality, mirror finish wafers of about 0.5 cm diameter.

It is on these samples that we have been able to perform electrical measurements with different geometrical arrangements of electrical contacts to determine accurately the bulk conductivity of the materials. By 2 probe and 4 probe measurements, we ascertained the bulk conductivity of —8 —9 —1 these materials to be 10 to 10 (ohm-cm) . This conductivity is too low for the material to be able to form a sharp junction with rectifying properties. Therefore, it was our aim to find a foreign element (dopant) which would affect the conduction mechanism in the material and increase conductivity. . As is typical of other amorphous semiconductors, the presence of small amounts of impurities in the material do not affect the conductivity and, above room temperature, we find intrinsic behavior with an activation energy equal to approximately half the bandgap, indicative of a midgap Fermi level. The low conductivity and large photoconductivity ratio indicate a small number of dangling bonds. This indicates that a strong perturbation 31> G 3 113 of the electronic wave function of the P-P bond will be required to modify the conductivity and conductivity type.

Two approaches were taken: (1) substitute As or Bi into the P site; {2) diffuse a foreign element into the amorphous matrix.

In the first method K/As2/P^3 has As incorporated into the matrix. The conductivity is increased by 2 orders of magnitude (Table XVIII), and the material remains n type.

In the second method, after trying many conventional 10 diffusers (e.g. Cu, Zn, Al, In, Ga, KI) in vapor, liquid and solid phase diffusion with no success, we found a surprising success with the diffusion of Ni and then Fe and Cr from the solid phase. For example, a layer of Ni was deposited by vacuum evaporation onto a well prepared surface of a high x, κΡχ wafer. After annealing for several hours, the Ni was found to diffuse for about 0.5 micrometers into the substrate and the conductivity increased by more than 5 orders of magnitude. The conductivity is still n type.

More specifically, 1500 angstroms of Niwere deposited onto the wafer in a Varian* resistance heated vacuum evaporator under pressure of 10 8Torr. The sample was sealed in an evacuated Pyrex tube and heated for 4 hours at 350°C. The top Ni layer was removed. The conductivity measured by the two probe method showed an increase from 25 10 to greater than 10 . Electro spectroscopy for chemical analysis (ESCA) depth profiling of the sample showed the diffusion depth to be 0.4 micrometers and the chemical bonding of the Ni to be Ni°, i.e. free Ni in the material. The wavefunction of the Ni overlaps with electronic wavefunctions in the P-P matrix, affecting the conduction (mobility). The Ni concentration is greater than about 1 atom percent.

Evaporated gold top contacts or dry silver paint in coplanar fashion form ohmic contacts to the doped layer.

Variations in the diffusion temperature show 350°C to be optimum for Ni diffusion.

Variation in the diffusion time follow the diffusion equation (diffusion depth is proportional to square root of * Trade Mark 3 6 8 3 119 time) and 1500 angstroms of Ni heated at 350°C for 60 hours, showed diffusion depth of 1.5 micrometers as measured by ESCA. 350°C approaches the highest temperature these amorphous materials may be subjected to.

Ni diffusion can also be accomplished from the liquid phase, such as from a Ni-Ga melt, or from the vapor phase, such as from Ni carbonyl gas.

It was further found that Fe and Cr show similar behavior under the above processing procedures.

For example, we took a cut wafer from a bulk amorphous high x boule obtained by the single source vapor transport and evaporated 500 angstroms of iron onto it and then diffused it into the wafer at 350°C for sixteen hours. Applying two pressure probes to the doped material gave a full non-linear characteristic on the Tektronix curve tracer.

On another wafer of high x material we evaporated 300 0 angstroms of nickel and 200 A of iron, then heated the wafer to 350°C for sixteen hours. We then evaporated two 1 mm radius aluminum contacts 2000 angstroms thick and upon measuring the current voltage characteristic with the Tektronix curve tracer between the aluminum dots, again obtained a full non-linear characteristic.

On another wafer of high x material produced by single, source vapor transport, we evaporated 500 angstroms of nichrome and then heated the wafer for diffusion at 350°C for sixteen hours. We then evaporated two aluminum 1 mm radius dots 2000 angstroms thick onto the wafer and again measured a full non-linear characteristic between the two aluminium dots.

We thus conclude that nickel, iron and chromium are useful diffusants in these materials for lowering conductivity and that on the lower conductivity material junctions can be effected with wet silver paint, pressure contacts and aluminum contacts. 120 Other elements besides Ni, Fe and Cr with occupied d or f outer electronic levels that can overlap with the phosphorus levels are expected to be able to affect the (· conductivity in these materials such as to give p-type material and form p/n junctions for solid state devices.

Amorphous High Phosphorus Material By Two Source Vapor Transport Two types of materials were obtained by'this method and the properties of these were investigated. 1} Amorphous bulk ΚΡχ, (Example VI) where x equals approximately 50 on one side and x is much greater than 15 on the other. Surface analysis supports the hypothesis of the template effect, which is very strong in this instance.

The surface of a cut and polished sample is of very high quality, low number of defects and voids, uniform etching pattern.

The conductivity measured was by the two probe technique 10-^(ohm-cm)and the photoconductivity ratio 3 under illumination of 100 mW/cm is greater than 10 . The photoconductivity peak is approximately at 1.8eV, indicating a bandgap of that order. The data indicates that the P-P bond dominates the electrical and optical properties of this material as well as those in Tables XVI, XVII, XVIII and XIV, and its strong photoconductivity ratio is consistent with a highly reduced level of dangling bonds. 2) Amorphous thin films of ΚΡ^5, (Reference Ho. 47 Table XII) deposited onto glass slides which have a metal layer deposited on them for a back contact to the thin film.

The success in the thin film deposition of opens the opportunity to manufacture many types of thin film devices.

The amorphous KP15 thin films deposited by the 2 source technique have a thickness of approximately 0.5 micrometers over an area of 3 cm\ The film is uniform and the surface roughness does not exceed 2,000 angstroms. The film is chemically stable. Figure 25 is a photomicrograph at 2000 magnification of the surface of one of these KP15 films. The adhesion to the substrate is excellent.

Quantitative analysis of the film was performed using a Scanning Electron Microscope (SEM) and an Energy Dispersive £83 X-ray (EDAX) measurement. The composition of the film was found to be in agreement with the KP^^ nominal composition.

The uniform composition, homogeneity, and pinhole free surface leads to uniform electro-optical properties across the films.

In view of the diffusing capability of Ni into bulk amorphous ΚΡχ, an Ni film 172 was evaporated onto the glass substrate 170 to form a back contact for the amorphous KP^g layer 174 as shown in Figure 26.

The Ni serves as a back contact and a diffuser. ESCA and SEM profiling shows Ni to diffuse significantly into the KP^3 film 174 at a rate of 200 angstroms per hour during the KP15 growth process.

In more detail,we deposited by vacuum evaporation 1500 "6 angstroms of Ni 172 onto a glass slide 170 at 10 Torr. pressure. Part of the Ni surface is then masked with a Ta mask in order to have a material free zone for electrical contact.

Two micrometers of amorphous KP^^ 174 is deposited in our two source apparatus onto the Ni film 172. The composition of this film has been identified to be KP^^, it is amorphous and has more than 1% Ni diffused into the film.

Pressure contact with an electrical probe was applied to the top of the KP^^ film. The two leads, from the back contact and the top pressure contact, were connected to a Tektronix Curve Tracer 176 to observe the current voltage characteristics. The forward characteristic of the rectifying pressure contact junction is shown in Figure 27, which indicates a junction with a barrier height Of 0.5eV and 30 current in the mA range.

As shown in Figure 28, we also deposited by vacuum evaporation a 2 mm radius Cu contact 178 onto the top surface 180 of a KF^g amorphous layer 182 grown by the two source technique on a Ni layer 184 deposited on a glass substrate 186. We connected the Tektronix curve tracer 176, as shown, and measured the full forward and reverse biased 3 o o 3 junction curve shown in Figure 29, which thus indicated that Cu forms junctions with these materials.

Subsequently, smaller metal dots were deposited as top contacts in order to reduce the effect of leakage currents —3 2 at the edges of the contacts, 10 cm area top contacts -5 2 and 10 cm top contacts were deposited in the vacuum evaporator through mechanical masks. The I-V characteristics shown in Figure 31 were observed with Cu, Au, and Al top contacts. They appear as the breakdown voltages of two back to back diodes in each instance.

Similar curves were obtained with Ni, Ti, Mg, and Ag as the top contacts.

The most significant difference appears in the fact that Au contacts change the I-V characteristic after applying 10V to the device. The I-V characteristic become asymmetric, as shown in Figure 32, and a more ohmic contact is formed at the Au interface after this forming process. The forming is consistently observed with Au, and intermittently observed with Ag and Cu top contacts. The forming does not permanently affect the device, but it reappears every time a voltage is applied. Heating the device at 300°C does not affect the phenomenon. Cooling the device to -20°C results in very sharp I-V characteristics (Figure 33).

It appears as if the forming may be a breakdown of a high resistance layer remaining between the diffused part of the device and the top contact. Capacitance - voltage (C-V) characteristics shown in Figures 34,35, and 36 point in the same direction. Al and Au top contacts have C-V characteristics of double diodes, but convert into single diode behaviour in the case of Au contacts. If we assume a dielectric constant of approximately 10, we can extract a 16 carrier concentration of approximately 10 carriers per 3 cm near the junction and a carrier mobility of -2 2 approximately 10 to 1cm /volt second. Frequency dependence of the capacitance and resistance in Figure 37 can be used tomodel the multiple junctions that can form in such a structure with a graded diffusion profile in the ιυΰ 3 « 8 3 active material. In addition, poor bulk material quality (low density) and rough surface morphology could contribute to the complex observations. Nonetheless, junction formation capability on amorphous two source thin film KP^g has been demonstrated.

Some of the above phenomena, such as forming with Au top contacts was also observed with flash evaporated thin films deposited on Ni. This film is not pure Kp^g, but has excellent quality. No C-V dependence was seen in this case. The device, which was very thin, had a good —β response to light and a small (10 amps) current was drawn from it under short circuit conditions when illuminated with visible light.

We expect that KP^g thin films made by CVD technique will result in similar behavior when the films are sufficiently thick. At the moment they have been too thin and have been found to short out.

The formation of junctions with these materials indicates that they may be utilized to form pn junctions, Schottky diodes, or Metal Oxide Semiconductor (MOS) devices.

We expect that by utilizing the above noted classes of dopants, that the materials can be converted to p-type conductivity and thus will be useful in the entire range of semiconductors.

The photoconductivity ratio was obtained in all these by forming a semiconductor device comprising our material and means attached to the material for electrically communicating with it. This means comprised two single electrodes 80 and 82 attached to the material, as illustrated in Figure 30.

More specifically, for a single crystal of MPlg, two copper strips 80 and 82 were adhesively attached to a glass substrate 84. A sample 86, of KP^g, made according to the above teachings, as bridged across strips 80 and 82 at one end thereof and attached thereto by silver paint 88. 3 0 8 3 <1 Meter 90 attached to the opposite ends of strips 80, 82 introduces an electrical potential to the KP^g, and thereby permits measurement of the resistivity of the KP1K.

Xo 3 6 8 3 7 The resultant device of Figure 30 and similar devices using our other materials established that our high phosphorus materials can in fact be used to control the flow of electrical current, at least as a photosensitive resistor.

In addition, our materials show luminescence characteristics with an emission peak at 1.8 eV at temperatures of four degrees K, and luminescence at ambient temperatures.

Preparation of Large Crystal Monoclinic P Rubidium We have found that the can be utilized to produce large crystal monoclinic phosphorus.

A 0.62 g sample of RbPjS encapsulated, in vacuo, in a 10 mm O.D. x 6 nun I.D. x 5.0 cm quartz tube was vertically positioned in a crucible furnace and subjected to a temperature gradient such that the RbPjg charge was maintained at 552°C while the top of the tube was maintained at 539°C. After heating for approximately 22 hours, the tube was opened and single crystals of monoclinic phosphorus, as large as 3.0 mm on edge, in the form of truncated pyramids were found in the upper (cooler) region of the tube.

We found that large crystal monoclinic phosphorus can also be prepared from mixtures of Rb and P in the atom ratio of 1 to 15 (RbP^g).

Cesium and Sodium Large single crystals of monoclinic phosphorus were also grown via vapor transport using either CsP15 or NaP13 charges formed in our condensed phase process. In each run approximately O.S g of the appropriate alkali metal polyphosphide was sealed in vacuo 3 6 8 3 i£8 in a quartz tube (10 ie O.D. x 6 mm I.D.) of length 8.9 cm. The tubes were then subjected to a temperature gradient such that the alkali metal polyphosphide charges were maintained at 558°C while the tops of the tubes were maintained at 514°C.

After 48 hours, large deep-red crystalline stacked square platelets of monoclinic phosphorus formed from the charges. 130 The morphologies of the monoclinic phosphorus crystals: grown from CsPi5 and NaP15 condensed phase charges appear to be very similar, that is, stacked square platelets. This is in contrast to the truncated pyramidal habit of the monoclinic phosphorus crystals grown from a RbP|5 charge.

We found that large crystal monoclinic phosphorus can also be prepare from Cs/Pj^, and Cs/Pj^ and Cs/P15 mixtures maintained at. high temperatures.

Potassium Using similar processes we have also produced mono- . clinic phosphorus crystals from condensed phase KP^g, and from mixtures of Κ/Ρ^θ and κ/ρχ25 Lithium No experiments have been conducted with lithium/phosphorus charges. However, we expect that large crystal monoclinic phosphorus can be prepared from the materials under similar conditions.

Effect of Temperature While the nature of the alkali metal present seems not to be important, the temperature at which the charge is maintained is apparently very important to the crystal growth process. In the case of the Cs/P^ ball milled system, large crystals were produced in experiments where the charge was maintained at 555’C and 554*C. However, in experiments where the charge was held at 565’C and 545’C, no large monoclinic crystals were produced.

Referring to Figure 38, using our preferred apparatus, we sealed a 0.6gm sample of RbP^,. prepared by our condensed phase process in vacuo in a 12mm O.D. x 6mm I.D. x 8cm long glass tube 270. The top was sealed with a 16mm diameter flat glass surface 272. Fill tube 274 is provided with a constriction 276 at which it is sealed after charging and evacuation.

The tube was subjected to a temperature gradient such that the flat surface 272 at the top of the tube was maintained at 462°C, while the charge at the bottom of the tube was maintained at 550°C. After heating for 140 hours approximately half of the original charge had been transported to the flat surface.

The resulting button-like boule was cleaved and exam5 ined. It was made up entirely of uniform light-red fibers — not the desired large crystal monoclinic phosphorus. Figures 44 and 45 are SEM photomicrographs of this product at 200 and lOOOx magnification,respectively.

The SEM photomicrographs of Figures 44 and 45 proved to 10 be a surprise. The individual fibers consist of bundles of long platelets which are attached such that they appear to be star-shaped rods when viewed from the end. This material is thus quite different in appearance from the twisted tube fibrous phosphorus produced via vapor 15 transport from a 99.9999% red phosphorus charge (see below).

We conclude that the condensing temperature to form large crystal monoclinic phosphorus should be in the range of 500° to 56Q°C. Further experiments indicate that the preferred condensing temperature is about 539®C.

The charge must be heated to a temperature above 545®C and below 565°C as previously indicated. Our preferred range is 550® to 560®C with about 555®C giving the best results.

Effect of Composition we have produced monoclinic phosphorus from charge ratios of P to alkali metal of 11 to 125. However, a ratio of about 15 seems to work best.

Characteristics of Monoclinic Phosphorus Condensed from Vapor in the Presence of an Alkali Metal Figure 39 is a photomicrograph at 50X magnification showing a pyramidally shaped monoclinic crystal of phosphorus prepared from a RbP^ charge. These crystals are hard to cleave. Similar crystals are produced from charges utilizing sodium as the alkali metal. We have produced crystals as large as 4 x 3 x 2mm.

Figure 40 is a photomicrograph, at 80X magnification, of a crystal of monoclinic phosphorus produced from a ball milled mixture of Cs/P^. These platelets are easy to cleave into mica-like sheets. Similar crystals can be pro5 duced from a charge of K/P^g. We have produced crystals in this habit as large as 4mm on a side and 2mm thick.

We have determined that the crystals are birefringent. When placed between crossed polarizers in a polarizing microscope, they rotate the light and allow some of it to pass through. Thus they may be utilized as birefringent devices such as optical rotators in the red and infra-red portion of the spectrum.

Chemical analysis indicates that they contain anywhere from 500 to 2000 parts per million of an alkali metal. They are made in a process which takes as little as 22 hours versus the 11 days employed in the process of the prior art to produce Hittorf's phosphorus.

The powdered X-ray diffraction pattern of these crystals is consistent with that of the prior art Hittorf's phosphorus.

The photoluminescence spectra shown in Figures 41 and 42 were taken with an Argon laser Raman spectrometer. A broad peak at 1.91 eV is clearly observed with a half width of about 0.29 eV. This indicates a band gap of about 2.0 eV at room temperature.

The Figure 41 spectrum was taken utilizing a monoclinic crystal of phosphorus prepared in the presence of cesium, while the Figure 42 spectrum was taken using monoclinic phosphorus condensed in the presence of rubidium.

The Raman spectrum of Figure 43 was taken utilizing a monoclinic phosphorus crystal formed in the presence of Rubidium. The peaks 280, 282, 283, 284 and 285 are at wave numbers 285, 367, 465, 483, and 529.

Evaporated dots about 25 micrometers in diameter were deposited on large crystals of monoclinic phosphorus (from a 2 Rb/Pig source) for electrical measurements. The resistance of the crystals was found to be 10® ohm to 7 ohm and practically independent of the geometry of the .ί ίί y 3 crystal and the size of the contacts. This reflects surface resistance.

These crystals may be utilized as the substrate for depositing 3-5 materials such as Indium Phosphide or Gallium . 5 Phosphide. They may be utilized as phosphors in luminescent displays, semiconductors, lasers, and as starting materials for other semiconducting devices.

Twisted Fiber Phosphorus The presence of the alkali metal in the charge appears to be critical to the production of large crystal monoclinic phosphorus. We attempted to produce large single crystals of monoclinic phosphorus from 99.9999% pure red phosphorus by mimicking the conditions used successfully with the various alkali metal/phosphorus systems. This attempt failed. No monoclinic phosphorus was produced. For example, a 0.6 g sample of 99.9999% pure red phosphorus was heated at 552’C in a sealed evacuated tube in a vertically positioned 10mm outside diameter x 6mm inside diameter quartz tube. The temperature gradient between the bottom and top of the two and three-quarter inches (6.985 cms) long· tube was 43°C. After heating for 24 hours more than half of the charge had been transported to the top third of the tube where a boule had formed.

Surprisingly, the boule was found to consist entirely of a red fibrous material. Several long (approximately 1.5mm) fibers were found in the vapor space at the bottom of the boule. Microscopic examination of the deep red fibers revealed that they are twisted.

XRD data secured on the fibrous material were found to match those secured earlier on polycrystalline ΚΡχ where x is much greater than 15. Figure 46 is an SEM photomicrograph at 500 magnification of these fibers.

Differential thermal analytical data was found to be similar to that secured on polycrystalline high x material. 53G Jl 3 ‘1 For two DTA determinations, the first heat plot consists of a single endotherm at 622°C (average). The second heat plot consists of a single endotherm in both cases - at 599°C 3 6 8 3 The DTA data secured earlier on polycrystalline high x material consists of a first heat single endotherm — at 614°C and a second heat single endotherm — at 590°C. Thus, we observed substantial similarities between the fibrous phosphorus prepared from 99.9999% red phosphorus and polyctystalline high x material.

Flash Evaporation We have succeeded in forming stable thin film amorphous coatings on glass and nickel coated glass substrates using a flash evaporation process. The flash evaporation apparatus is generally indicated at 302 in Figure 47. It comprises a glass cylinder 304 connected to a vacuum system (not shown) through tubing 306. Argon is supplied at inlet 308 of supply tube 310. Reservoir 312 is filled with powdered KP15 formed by the Condensed Phase method. It is agitated by means of a vibrator generally indicated at 314 and picked up by the flow of Argon gas through the venturi generally indicated at 316. It then flows into the reactor 304, passing through tube 317 into a steel susceptor 318. The susceptor is heated by means of a RF coil 319 to a temperature of at least 900°C, which causes the KP^g to vaporize. At the end of tube 317, as shown in Figure 49, a nozzle is formed by incorporating a plurality of small tubes generally indicated at 320 in Figure 48, having a plurality of small orifices 321. Tubes 317 and 320 are alumina and tubes 320 are held within the end of tube 317 by means of magnesium oxide cement 322.

The KP-£2 upon vaporization dissociates into its constituents and the vapor is carried by the Argon gas through the orifices 321. The film is deposited on a cooler substrate 324. The substrate may be heated by means of hot wires 326 fed by electrical connections 328.

Alumina tube 317 has a one-quarter inch (0.635 cm) outside diameter and one-eighth inch (0.3175 cm) inside diameter. Tubes 320 have a one-sixteenth inch (0.159 cm) outside diameter, are one-quarter inch (0.635 cm) long and have four one-sixteenth inch (0.159 cm) diameter holes through them. 3 6 8 3 The apparatus is operated under a vacuum of 0.1 to 0.5mm Hg. Amorphous films of up to 1 micron thick may be formed in runs of up to fifteen minutes. At the end of a run, the substrate 324 reaches a temperature of 200-300"C, depending on whether it starts out at room temperature or is initially preheated to 200"C.

Chemical Vapor Deposition We have prepared thin films of KP^g by means of Chemical Vapor Deposition.

A typical chemical vapor deposition reactor is shown in Figure 50. It is constructed of Pyrex. The reactor chamber 401 is a 26 mm I.D. x 27.0 cm long tube in the center of which is positioned a 6.0 mm I.D. x 30.0 cm long tube 402 which serves as both a thermowell and substrate holder. The thermowell is held in position by an adjustable O-ring collar 403. The vent tube 404 allows for the continuous removal of the gaseous exhaust stream. It is attached to a trap (not shown) which removes the unreaeted phosphorus before venting of the stream to air. The vent tube 404 and O-ring collar 403 are attached to the reactor chamber 401 through a 2.0 cm I.D,. O ring joint 405. The reactor chamber 401 is located in a resistance furnace generally indicated at 406.

Molten phosphorus is metered by a piston pump (not 25 shown) through a 1.0 mm I.D. capillary tube 407 into a vaporization chamber 408. The molten phosphorus is evaporated in the vaporization chamber 408 by a stream of argon which is injected into the vaporization chamber 408 through the 6.0 mm I.D. inlet tube 409. The gaseous phosphorus/argon stream enters the reactor chamber through nozzle 410. The nozzle 410 has an opening of 4.0mm. The evaporation chamber 408 is located in a resistance oven generally indicated at 411.

A gaseous mixture of potassium and argon is metered into the reactor chamber 401’ through inlet tube 412 which has a 6.0 mm I.D. Neat argon, which acts as a shroud for the potassium/argon stream, enters the system through 6.0 mm I.D. tube 413. Both the potassium/argon stream and neat 3 6 8 3 argon stream enter the reaction chamber 401 at 414. The potassium/argon and neat argon lines (412, 413) are located in a resistance oven generally indicated at 415.

The substrate 416 is positioned on the thermowell 402. The temperature of the substrate 416 is determined by a thermocouple 417 positioned directly below the substrate 416 on the thermowell 402.

During operation, ovens 406, 411, and 415 are maintained at appropriate temperatures. The gaseous reactant streams enter the reactor chamber at 410 and 414. The exhaust gas mixture leaves the reaction chamber through the vent tube 404. The desired film forms on the substrate 416.

The substrates are maintained at a temperature of 310-350°C, the temperature being maintained constant to plus or minus 2eC.

In a typical run 1.24g of white phosphorus and 0.13g of potassium are delivered into the reactor over a two hour period. The total Argon flow rate is maintairied at 250ml per minute during the run.

A number of experiments were conducted in which phosphorus/argon and potassium/argon were fed simultaneously into the reactor. The phosphorus/argon stream was maintained at approximately 290°C and the potassium/argon stream at approximately 410®C. The calculated atom ratio of reactants in the reactor was P/K approximately 15. The reactor was maintained at 300-310°C. In a typical experiment the liquid phosphorus feed rate was 0.34ml per hour.

Amorphous KP^ films were prepared using nickel-onglass substrates. The films are about 0.3 millimeters thick.

With a run time of 1.0 hour, the films produced - were found to have nominal KPlg composition. The thickness of the film was dependent on the position, of the particular substrate in the reactor. Examination of the films using SEM showed them to be quite uniform. 683 Purification of Phosphorus grams of Atomergic phosphorus, 99.95% pure, was subjected to a 450"300C gradient for 75 days. After this admittedly very long time, 21% of the material remained behind and 60% of the charge ended up as amorphous, bulk deposits.

Previous analysis showed the Atomergic phosphorus to be less than 99.90% pure, probably closer to 99.80%, with aluminum, calcium, iron, magnesium, sodium, and silicon as major impurities (all greater than 0.01%, and some greater than 0.05%). This material costs about $220 per kilogram. In comparison, 99% P from Alpha Ventron is $17/lb., or $37.5/Kilo.

Table XX summarizes the results of flame emission spectroscopy on three materials generated by the aforementioned treatment.

S 3 « (‘ 3 130 t—l Cl rd rd rd O Cl rd β β C β β β β rd rt rt A rt rt rt rt x X x X A X X •rd 41 4» 41 41 41 41 41 M Φ U a 1Λ w rt rt rt n 4J ω (0 rt M m rt rt rt Φ Φ Φ Φ Φ Φ Φ £ rd rd rd rd rd rd rd IMPURITY LEVELS BY FES* <3 •H M Q 4J o © CJ I c rt X! 4i w (0 c x P 0) w Φ rt η I o o •u £ Cl Cl rt rd rt rd Γ1 © n dP rd β rt X X dP dP Ό M d rt Φ Φ rt 41 41 •rd © © © © dP O rt Id I o © rt to β Φ Φ < CJ M* o CJ rt o © © to © © 1 u 41 o VO o rt to rt rt rt rd to Φ Ch rt Φ © o • IO u « o • M £ o H* to Cl © rt to rt • rt S o S rd o Φ X 4> rt >3 0« Ό •id s O’in id o\ Φ · 6* 0 Ok 41 S < 0« w Φ 0 0 Μ X 4J Φ U © o © © © I I O m n dp o Φ ·» a M· 1 ag O 1 V © o V) »4 to o to rd Qi I • • 1 1 β to © o to rd 0 c •rl «Η Φ Id tt rt Φ rd rt «J 3 4aOUfeA£S£Z:zCfl(QE'<> Material A was a residue, dark brown in color, throughout the charge zone, which did not undergo vapor transport. Material designated Material B was a hard boule of material, light in color, which did not vaporize, primarily because its position in the charge zone results in it being at a slightly lower temperature than 450’C.

Material C was an amorphous boule in the cold zone.

Clearly, most of the impurities of the charge remain in Material A at fairly concentrated, amounts. The high impurity levels would be expected to give rise to a lower vapor pressure of phosphorus at a given temperature. The impurity level of the material pretty well reflects the values for the initial charge material. The boule, Material C, is a pretty pure material, with the sodium content being the major observed contaminant. Taking the sum of contaminants, at their maximum indicated levels, this material has a purity level of 99.997%, at worst. The comparable material, obtainable from commercial sources, as 99.999% P, costs about $1,800/kilogram.

Clearly, we have illustrated a cost effective method for purifying red phosphorus to a high degree which may be regarded as being particularly applicable to the present invention. il Thus we have disclosed an entirely new class of high phosphorus semiconductor materials. These semiconductors comprise catenated covalent atoms where the catenated covalent bonds serve as the primary conduction paths in the materials. The catenated atoms form parallel columns as the predominant local order. Preferably the atoms are trivalent and have bonding angles that permit tubular, spiral or channel-like columns. The columns may be joined by atoms of one or more different elements bonded to two or more of the catenated columns.

We have disclosed in particular high phosphorus and mixed pnictide semiconductor materials of this class. These include high phosphorus polyphosphides of the formula ΜΡχ where x ranges from 7 to 15 and entirely new materials where x is very much greater than 15 — for all practical purposes pure phosphorus.

These materials can be characterized as containing groups of seven or more atoms organized into pentagonal tubes. They may be characterized as having the formula ΜΡχ where x is greater than 6 and they may be characterized as comprised of phosphorus in a molar ratio of phosphorus to any other atomic constituent greater than 6; they may be characterized as high phosphorus materials where the phosphorus atoms thereof in substantially all local orders comprise phosphorus atoms joined together by multiple covalent p-p bonds organized into layers of all parallel pentagonal tubes. They may be characterized as polyphosphides containing alkali metal atoms wherein the number of consecutive covalent phosphorus-to-phosphorus bonds is sufficiently greater than the number of non-phosphorus-to-phosphorus bonds to render the material semiconducting. They may be characterized as having a skeleton of at least seven covalently ' bonded phosphorus atoms having associated therewith at least one alkali metal atom, conductively bridging the phosphorus skeleton of one 142 unit with the phosphorus skeleton of another unit; they may be characterized as a polyphosphide having the formula MP where M is an alkali metal and x is at least 7. x These materials may be further characterized by 5 having a band gap greater than 1 eV, that is, from 1.4 to 2.2 eV, and for the best materials we have discovered, approximately 1.8 eV. They may be characterized by having a photoconductivity ratio greater than 5; more particularly, within the range of 100 and 10,000.

These materials may be further characterized by their trivalent dominant atomic species; the homatomic bonds formed by the dominant species; the covalent nature of these bonds; the materials' coordination number of slightly less than 3; the materials’ polymer nature; their formation in the presence of an alkali metal or metals mimicking the bonding of alkali metals with the dominant species; in the crystalline form, their pentagonal parallel tubes either all parallel in KP^g-like materials, paired parallel crossed layers in monoclinic phosphorus, or all parallel twisted tubes in twisted fiber phosphorus; their ability to form amorphous films and boules retaining their electronic qualities; and by their methods of manufacture; and other qualities made apparent in the preceding description.

The amorphous materials we have discovered maintain the electronic qualities of the KP^,. all parallel pentagonal tube structure and in theory, at least, it appears that that structure is maintained in the local scale in our amorphous materials. However, we do not wish to be bound by any particular theory in this matter.

We have disclosed junction devices, photoconductive (resistive) devices, photovoltaic devices and phosphors made from these materials.

We have disclosed resistance lowering dopants, namely Nickel, Chromium and Iron, leading to the conclusion that substantially the entire group of atomic species having occupied d or f outer electronic levels may be utilized if of the appropriate atomic size.

We have disclosed resistance lowering substitutional doping with Arsenic which indicates that all Group 5a metals may be utilized.

Ke have disclosed junction devices having a back contact of Ni, Ni diffused therefrom, and top contacts of Cu, Al, Mg, Ni, Au, Ag, and Ti.

We have disclosed new forms of phosphorus wherein the local orders are all substantially parallel pentagonal tubes, twisted fiber phosphorus and monoclinic phosphorus. We have formed these new forms of phosphorus by vapor deposition. The all parallel and monoclinic form require the presence of an alkali metal during deposition.

We have disclosed both amorphous and polycrystalline films of MP^g where M is an alkali metal. We have constructed various semiconductor devices from all of the all parallel pentagonal tube materials, including wafers of MP where x is much greater than 15, including a new form of phosphorus, amorphous thin films of KP^g and amorphous thin films of ΚΡχ.

We have disclosed methods of making metal polyphosphides and two new forms of phosphorus by controlled two temperature single source techniques.

We have disclosed methods of making our high phosphorus materials by two source vapor transport.

We have disclosed a method of making high purity phosphorus.

We have disclosed methods of making crystalline anu amorphous forms of ΜΡχ where x ranges from 7 to 15 by condensed phase methods.

We have disclosed chemical vapor deposition, flash evaporation and molecular flow deposition methods.

Industrial applications of the semiconductor materials and devices we have discovered are manifest, running the whole gamut of semiconductor applications. The crystalline materials may also be used as reinforcing fibers and flakes . for plastics, glasses and other materials. The materials may also be used as coatings on metals, glass, and other materials. Such coatings may protect a substrate from fire, oxidation, or chemical attack. The coatings may be employed for their infrared transmitting, visible light absorbing qualities. They may be employed with other materials as antireflection coatings on infrared optics. The-materials may also be used as fire retardant fillers and coatings. Monoclinic phosphorus may be used as an optical rotator.

It should be understood that we have used crystalline to mean single crystals or polycrystalline material unless otherwise stated. Amorphous as distinct from single crystal or polycrystalline, means amorphous to X-ray diffraction. All periodic table references are to the table printed on the inside front cover of the 60th edition of the Handbook of Chemistry and Physics published by the CRC Press Inc., Boca Raton, Florida. Alkali metals are identified thereon and herein in Group la and pnictides in Group 5a. All ranges stated herein are inclusive of their limits.

By semiconductor device we mean any device or apparatus utilizing a semiconductor material. In particular, semiconductor device includes Xerographic surfaces and phosphors regardless of how they are excited, as well as photoconductors, photovoltaics, junctions, transistors, integrated circuits and the like.

Reference may also be made to Divisional Patent Specification No. 53624·.

Claims (46)

CLAIMS:

1. A method of forming a semiconductor device which comprises: (a) providing a material comprising, at least as one 5 component thereof, a polyphosphide having the formula: ΜΡ χ wherein M represents an alkali metal; and x represents the atom ratio of P to M, x being at least 7; the number of consecutive covalent phosphorus-tophosphorus bonds being sufficiently greater than the 10 number of non-phosphorus-to-phosphorus bonds to render the said material semiconducting; and (b) attaching to the said material means for electrically communicating with the said material to utilize the said material as a semiconductor; the said semi15 conductor having chemical and physical stability under ambient operating conditions, an energy band gap of from 1 to 3 eV, a photoconductive ratio greater than -5 -12 -1 5 and a conductivity between 10 and 10 (ohm-cm)

2. A method as claimed in claim 1 wherein there is 20 provided a material comprising, at least as one.component thereof, at least two polyphosphide units, each unit having a skeleton of at least 7 covalently bonded phosphorus atoms, the said units having associated therewith at least one alkali metal atom, the said alkali metal atoms conductively 25 bridging the phosphorus skeleton of one unit with the phosphorus skeleton of another unit, and the said material having a band gap primarily determined by the said phosphorus-to-phosphorus bonds. 146

3. A method as claimed in claim 1 or claim 2 wherein the said material has a bandgap of from 1.

4. To 2.2 electron volts and/or a photoconductivity ratio of from 100 to 10,000. 5. 4. A method as claimed in claim 3 wherein the said material has a bandgap of substantially 1.8 electron volts.

5. A method as claimed in any of claims 1 to 4 wherein the said material comprises a single alkali metal.

6. A method as claimed in any of claims 1 to 4 wherein the said material comprises at least two different alkali metals.

7. A method as claimed in any of claims 1 to 6 wherein the said material is polycrystalline.

8. A method as claimed in any of claims 1 to 6 wherein the said material is amorphous.

9. A method as claimed in any of claims 1 to 8 wherein in the said material at least 7 phosphorus atoms are bonded to other phosphorus atoms per each of a metal atom in the said material. 20

10. A method as claimed in claim 9 wherein 15 phosphorus atoms are bonded to other phosphorus atoms per each of a metal atom in the said material.

11. A method as claimed in claim 9 wherein there are at least 500 phosphorus atoms per each of a metal atom in the 25 said material.

12. A method as claimed in any of claims 1 to 11 wherein in the said polyphosphide ΜΡ χ 5 3β83 Id' and x is from 7 to 15.

13. A method as claimed in any of claims 1 to 12 wherein in ΜΡ χ x is substantially equal to 15.

14. A method as claimed in any of claims 1 to 12 wherein 5 in ΜΡ χ x is greater than 15.

15. A method as claimed in any of claims 1 to 14 wherein the said material is defined by the formula: [MP?] a [P g ] b , wherein b:a is the atom ratio of [P g ] to [MP?].

16. A method as claimed in any of claims 1 to 15 10 substantially as herein described.

17. A semiconductor device which comprises: (a) a material as defined in any of claims 1 to 16; and (b) means attached to the said material for 15 electrically communicating with the said material.

18. A device as claimed in claim 17 wherein at least 6/7ths of the atoms have 3 homatomic bonds exclusively.

19.L9. A device as claimed in claim 18 wherein at least 20 14/15ths of the atoms have 3 homatomic bonds. »> τ> ν. ν- b

20. A device as claimed in any of claims 17 to 19 wherein the said material is characterized in its local order by tubular columnar structure.

21. A device as claimed in claim 20 wherein the said 5 tubular structures in a local order are all generally parallel.

22. A device as claimed in any of claims 17 to 21 wherein the major component atoms of the said material are trivalent.

23. A device as claimed in any of claims 20 to 22 wherein 6. 10 the said columnar structure is channel-like when viewed on end.

24. A device as claimed in any of claims 20 to 23 wherein the said columnar structure is pentagonal when viewed on end.

25. 15 25. A device as claimed in any of claims 17 to 24 wherein one or more pnictides other than P are present.

26. A device as claimed in any of claims 17 to 25 wherein the bonds of the atoms are spaced at an average angle of greater than 98°. 20

27. A device as claimed in any of'claims 17 to 26 wherein the bonds of the : atoms are spaced at an angle of from 87 to 109° .

28. A device as claimed in any of claims 17 to 27 wherein additional atoms of one or more different elements than 25 atoms of the said catenations are bonded between two or more of the said catenations. 14 9

29. A device as claimed in claim 28 wherein the said additional atoms form conduction paths between the catenations to which they are bonded.

30. A device as claimed in any of claims 17 to 29 wherein 5 the said catenations in each local order are all generally parallel.

31. A device as claimed in any of claims 17 to 30 wherein the said material is formed as the deposition product from vapour transport in a deposition zone from separated sources 10 of phosphorus and an alkali metal.

32. A device as claimed in any of claims 17 to 31 wherein the atom ratio of phosphorus to metal is substantially 50 or greater.

33. A device as claimed in claim 32 wherein the said atom 15 ratio is substantially 200 of greater.

34. A device as claimed in claim 33 wherein the said atom ratio is substantially 1,000 or greater.

35. A device as claimed in claim 34 wherein the amount of the said metal is less than 1,000 parts per million.

36. 20 36. A device as claimed in any of claims 17 to 35 wherein the said material is formed of a single crystal.

37. A device as claimed in any of claims 17 to 36 wherein the said material is in the form of a thin film.

38. A device as claimed in any of claims 17 to 37 25 comprising a junction.

39. A device as claimed in claim 38 wherein the said junction comprises a metal selected from Cu, Al, Mg, Ni, Au, Ag and Ti. 5 8 6:. 8 150

40. A device as claimed in claim 39 wherein the said junction metal is Ni.

41. A device as claimed in any of claims 17 to 40 wherein the said material is doped with atoms of another pnictide. 5

42. A device as claimed in. claim 41 wherein the said pnictide is As.

43. A device as claimed in any of claims 17 to 42 wherein the said material is doped by diffusing therein a metal having occupied outer f or d electronic levels. 10

44. A device as claimed in claim 43 wherein the said dopant is selected from nickel, iron and chromium.

45. A device as claimed in any of claims 17 to 44 comprising a metal contact selected from Cu, Al, Mg, Ni, Au, As and Ti. 15

46. A device as claimed in any of claims 17 to 45 substantially as herein described.