IE52206B1 - Method of making photoresponsive amorphous germanium alloys and devices - Google Patents

Abstract

A semiconductor material comprises amorphous Ge containing F (a- Ge: F). The material may also contain H, and may contain a bandgap adjusting element e.g. C or N. The material may be doped and may be utilised in Schottky, MIS, PIN and PN solar cells or in electrophotographic devices.

Description

This invention relates to a method of making amorphous germanium alloys having improved photoresponsive characteristics and devices made there from. The invention has its most important appli cation in making improved photoresponsive alloys and devices for specific applications including photoreceptive devices such as solar cells of a p-i-n, p-n, Schottky or MIS (metal-insulator-semi conductor) type; photoconducting medium such as utilized in xerography; photodetecting devices; and photodiodes including large area photodiode arrays; and multiple· solar cell constructions.

... Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 per cent) crystalline solar cells for space applications. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defectfree germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein.

This was accomplished by diffusing into such crys53206 tailine material parts per million of dondr (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar ceils and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.

These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an iir.pos6 sible economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the additon of grain boundaries and other problem defects.

In summary, crystalline devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices based upon amorphous germanium or silicon can eliminate these crystal disadvantages. Amorphous germanium and silicon have optical absorption edges having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the SO micron thick crystalline silicon. Further, amorphous germanium and silicon can be made faster, easier and in larger areas than can crystalline materials.

Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposi10 tion equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substan15 tially unproductive. Amorphous germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Addi25 tionaily, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.

In an attempt to minimize the aforementioned problems involved with amorphous alloys, W.E.

Spear and P.G. LeComber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on Substitutional Doping of Amor10 phous Silicon, as reported in a paper published in Solid State Communications, Vol. 17, pp. 11931196, 1975, toward the end of reducing the localized states in the energy gap in amorphous alloys to make the same approximate more closely in15 trinsic crystalline silicon or germanium and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.

The reduction of the localized states was accomplished by glow discharge deposition of amorphous alloys, in this case, silicon films, wherein a gas of silane (S1H4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate 53306 at a substrate temperature of about 500~600°K (227-327°C). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material a gas of phosphine (PS3) for n-type conduction or a gas of diborane (BjHg) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5 x IO*6 and 10*2 parts per volume.

The material so deposited included supposedly substitutional phosphorus or boron dopant and wa3 shown to be extrinsic and of n or p conduction type.

While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous material approximate more nearly those of the corresponding crystalline material. 53206 D.I. Jones, W.E. Spear, P.G. LeComber, S. Li, and R. Martins also worked on preparing a-Ge:H from GeH4 using similar deposition techniques.

The material obtained gave evidence of a high density of localized states in the energy gap thereof. Although the material could be doped the efficiency was substantially reduced from that obtainable with a-Si:H. In this work reported in Philosophical Magazine B, Vol. 39, p. 147 (1979) the authors conclude that because of the large density of gap states, the a-Ge:H material obtained is . . . a less attractive material than a-Si for doping experiments and possible applications.

The prior deposition of amorphous germanium, which has been doped in a manner like that of doping crystalline germanium, has characteristics which in all important respects are inferior to its doped crystalline counterpart. Thus, inadequate doping efficiencies and conductivity were achieved and the photovoltaic qualities of these films left much to be desired.

Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been pre9 58206 pared by glow discharge as fully described in U.S. Patent No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Cvshinsky and Arun Hadan which issued October 7, 1980, and by vapor deposition as fully described in U.S. Patent No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on August 12, 1980, under the sane title. As disclosed in these patents, fluorine is introduced into the amorphous silicon semiconductor to substantially reduce the density of localized states therein.

Activated fluorine especially readily diffuses into and bonds to the amorphous silicon in the matrix body, substantially to decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous silicon matrix. The fluorine bonds to the dangling bonds of the silicon and forms what is be29 lieved to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine is considered to be a more efficient compensating or altering ίο element than hydrogen when employed alone or with hydrogen because of its exceedingly small size, high reactivity, specificity in chemical bonding, and highest electronegativity. Hence, fluorine is qualitatively different from other halogens and so is considered a super-halogen.

As an example, compensation may be achieved with fluorine alone or in combination with hydrogen with the addition of these element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a siliconhydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the new alloy so formed has a lower density of defect states in the energy gap than that achieved by the mere neutralization of dangl20 ing bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other al25 loying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the density of defect states which hydrogen contributes while acting as a density of states reducing element. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.

The problem associated with the prior art preparation of amorphous germanium suitable for use in photoresponsive devices is overcome in accordance with the present invention by adding one density of states reducing element to the alloy during or after the deposition thereof. The germanium alloy can incorporate the density of states reducing element itself or the element can be added separately during the deposition. The improved germanium alloy can be deposited by vapor deposition, sputtering or glow discharge processes.

The amorphous alloy incorporates one density of states reducing element,fluorine. The compensating or altering element, fluorine,- can be added during the deposition or thereafter. Band gap adjusting element (s) can be activated and may be added during the vapor deposition, sputtering or glow discharge processes. The band gap can be adjusted as required for a specific application by introducing the necessary amount of one or more of the adjusting elements into the deposited alloy in at least the photocurrent generation region thereof.

The band gap can be adjusted without substantially increasing the number of states in the band gap of the alloy and devices, because of the presence of fluorine in the alloy. The presence of fluorine in the alloy of the invention provides a germanium alloy which differs physically, chemically and electrochemically from other germanium alloys because fluorine not only covalently bonds to the germanium but also affects in a positive manner the structural short range order of the material. This allows the adjusting elements, such as carbon or nitrogen, effectively to be added to the alloy, because fluorine forms stronger and more stable bonds than does hydrogen. Fluorine compensates or alters germanium as well as the band adjusting element(s) in the alloy more efficiently than hydrogen, because of the stronger more thermally stable bonds and more flexible bonding configurations due to the ionic nature of the fluorine bonding.

The band adjusting element (s) are tailored into the material without adding substantial deleterious states because of the influence of fluorine. The new alloy therefore maintains high quality electronic qualities and photoconductivity when the adjusting element(s) are added to tailor the wavelength thereshold for a specific photoresponse application·.

While the principles of this invention apply to each of the aforementioned deposition processes, for purposes of illustration herein a vapor and a plasma activated vapor deposition environment are described. The glow discharge system disclosed in U.S. Patent No. 4,226,898 has other process variables which advantageously can be utilized with the principles of this invention.

Accordingly, this invention provides a method of making an improved photoresponse amorphous alloy, said method comprising depositing on a substrate a material including at least germanium and incorporating in said material a single density of states reducing element, said element being fluorine.

The invention also provides an improved photoresponse amorphous alloy, said alloy including germanium and incorporating a single density of states reducing element therein, said element being fluorine.

The invention further provides an improved photoresponse device, said device comprising superimposed layers of various materials including an amorphous germanium semi-conductor alloy body having an active photoresponse region including a band gap therein upon which radiation can impinge to produce charge carriers, said germanium amorphous alloy including a single density of states reducing element, said element being fluorine.

In the following are described, with reference to the accompanying drawings, methods, alloys and devices wherein more than one density of states reducing element is used. However, the invention is limited to there being a single density of states reducing element, said element being fluorine. In the drawings: Fig. 1 is a diagrammatic representation of more or less conventional vacuum deposition equipment to which has been added elements for carrying out the addition of fluorine (and hydrogen) by the addition of molecular or fluorine compounds containing fluorine such as GeF4, and hydrogen inlets and activated fluorine and hydrogen generating units which decompose the molecular fluorine and hydrogen within the evacuated space of the vapor deposition equipment, to convert molecular fluorine and hydrogen to activated fluorine and hydrogen and to direct one or both against the substrate during the deposition of an amorphous alloy containing germanium.

Fig. 2 illustrates vacuum deposition equipment like that shown in Fig. 1, with activated fluorine (and hydrogen) generating means comprising an ultraviolet light source irradiating the substrate during the process of depositing the germanium amorphous alloy, such light source replacing the activated fluorine and hydrogen generator units shown in Fig. 1 and adjusting element generating means; Fig. 3 illustrates the vacuum deposition equipment for Fig. 1 to which has been added additional means for doping the depositing alloy with an n or p conductivity producing material; Fig. 4 illustrates an application wherein the deposition of the germanium amorphous alloy and the application of the activated fluorine and hydrogen may be carried out as separate steps and in separate enclosures; Fig. 5 illustrates exemplary apparatus for diffusing activated fluorine into a previously deposited germanium amorphous alloy; Fig. 6 is a fragmentary sectional view of an embodiment of a Schottky barrier solar cell to illustrate one application of the germanium amorphous semiconductor photoreceptive alloys made hy the process of the invention; Fig. 7 is a fragmentary sectional view of a p-n junction solar cell device which includes a doped germanium amorphous semiconductor alloy made by the process of the invention; Fig. 9 is a fragmentary sectional view of a photo-detection device which includes an amorphous germanium semiconductor alloy made by the process of the invention; Fig. 9 is a fragmentary sectional view of a xerographic drum including an amorphous germanium semiconductor alloy made by the process of the invention; Fig. 10 is a fragmentary sectional view of a 10 p-i-n junction solar cell device; Fig. 11 is a fragmentary sectional view of an n-i-p junction solar cell device; Fig. 12 is a diagrammatic representation of a plasma activated vapor deposition system for depo15 siting the germanium amorphous alloys with the adjusting element(s) of the invention incorporated therein; and Fig. 13 is a solar spectral irradiance chart illustrating the standard sunlight wavelengths available for various photoresponsive applications.

Referring now more particularly to Fig. 1, there is shown vapor deposition equipment generally indicated by reference numeral 10, which may be conventional vapor deposition equipment to which is added an activated compensating or altering material injecting means to be described. This equipment, as illustrated, includes a bell jar 12 or similar enclosure enclosing an evacuated space 14 in which is located one or more crucibles like crucible 16 containing the amorphous semiconductor film-producing element or elements to be deposited on a substrate 18. In the form of the invention being described, the crucible 16 initially contains germanium for forming an amorphous alloy containing germanium on the substrate 18 which, for example, may be a metal, crystalline or polycrystalline semiconductor or other material upon which it is desired to form the alloy to be deposited by the process of the present invention.

An electron beam source 20 is provided adjacent to the crucible 16, which electron beam source, diagrammatically illustrated, usually includes a heated filament and beam deflection means (not shown) which directs a beam of electrons at the germanium contained in the crucible 16 to evaporate the same.

A high voltage DC power supply 22 provides a suitable high voltage, for example 10,000 volts DC, the positive terminal of which is connected through a control unit 24 and a conductor 26 to the crucible 16. The negative terminal of which is connected through the control unit 24 and a conductor 28 to the filament of the electron beam source 20. The control unit 24 including relays or the like for interrupting the connection of the power supply 22 to the conductors 26 and 28 when the file thickness of an alloy deposition sampling unit 30 in the evacuated space 14 reaches a given value set by operating a manual control 32 on a control panel 34 of the control unit 24. The alloy sampling unit 30 includes a cable 36 which extends to the control unit 24 which includes well known means for responding to both the thickness of the alloy deposited upon the alloy sampling unit 30 and the rate of deposition thereof. A manual control 38 on the control panel 34 may be provided to fix the desired rate of deposition of the alloy controlled by the amount of current fed to the filament of the electron beam source through a conductor 40 in a well known manner.

The substrate 18 is carried on a substrate holder 42 upon which a heater 44 is mounted. A cable 46 feeds energizing current to the heater 44 which controls the temperature of the substrate holder 42 and substrate 18 in accordance with a temperature seating set on a manual control 48 on the control panel 34 of the control unit 24.

The bell jar 12 is shown extending upwardly from a support base 50 from which the various cables and other connections to the components within the bell jar 12 may extend. The support base 50 is mounted on an enclosure 52 to which connects a conduit 54 connecting to a vacuum pump 56. The vacuum pump 56, which may be continuously operated, evacuates the space 14 within the bell jar 12. The desired pressure of the bell jar is set by a control knob 58 on the control panel 34. In this form of the invention, this setting controls the pressure level at which the flow of activated fluorine (and hydrogen) into the bell jar 12 is regulated. Thus, if the control knob is set to a bell jar pressure of 10“4 torr, the flow of fluorine (and hydrogen) into the bell jar 12 will be such as to maintain such pressure in the bell jar as the vacuum pump 56 continues to operate.

Sources 60 and 62 of molecular fluorine and hydrogen are shown connected through respective conduits 64 and 66 to the control unit 24. A 53206 pressure sensor 58 in the bell jar 12 is connected by a cable 70 to the control unit 24. Flow valves 72 and 74 are controlled by the control unit 24 to maintain the set pressure in the bell jar. Con5 duits 76 and 78 extend from the control unit 24 and pass through the support base SO into the evacuated space 14 of the bell jar 12. Conduits 76 and 78 respectively connect with activated fluorine and hydrogen generating units SO and 82 which convert the molecular fluorine and hydrogen respectively to activated fluorine and hydrogen, which may be atomic and/or ionized forms cf these gases. The activated fluorine and hydrogen generating units 80 and S2 can be heated tungsten filaments which elevate the molecular gases to their decomposition temperatures or a plasma generating unit well known in the art for providing a plasma of decomposed gases. Also, activated fluorine and hydrogen in ionized forms formed by plasma can be accelerated and injected into the depositing alloy by applying an electric field between the substrate and the activating source.

In either event, the activated fluorine and hydrogen generator units 80 and 82 are preferably placed in the immediate vicinity of the substrate 53206 18, so that the relatively short-lived activated fluorine and hydrogen delivered thereby are immediately injected into the vicinity of the substrate 18 where the alloy is depositing. The activated fluorine (and hydrogen) as well as other compensating or altering elements also can be produced from compounds containing the elements instead of from a molecular gas source.

As previously indicated, to produce useful 10 amorphous alloys which have the desired characteristics for use in photoresponsive devices such as photoreceptors, solar cells, p-n junction current control devices, etc., the compensating or altering agents, materials or elements produce a very low density of localized states in the energy gap without changing the basic intrinsic character of the film. This result is achieved with relatively small amounts of activated fluorine and hydrogen so that the pressure in the evacuated bell jar space -4 can still be a relatively low pressure (like 10 torr). The pressure of the gas in the generator can be higher than the pres- --52206 sure in th· bell jar by adjusting the size of the outlet of the generator.

The temperature of the.substrate IS is adjusted to obtain the maxinun reduction in the density of the localized states in the energy gap of the amorphous alloy involved. The substrate surface temperature will generally be such that it ensures high mobility of the depositing materials, and preferably one below the crystallization tem10 perature of the depositing alloy.

The surface of the substrate can be irradiated by radiant energy to further increase the mobility of the depositing alloy material, as by mounting an ultraviolet light source (not shown} in the bell jar space 14. Alternatively, instead of the activated fluorine and hydrogen generator units 80 and 82 in Fig. 1, these units can be replaced by an ultraviolet light source 84 shown in Fig. 2, which directs ultraviolet energy against the substrate 18. This ultraviolet light will decompose the molecular fluorine (and hydrogen) both spaced from and at the substrate 18 to form activated fluorine (and hydrogen) which diffuses into the depositing amorphous alloy condensing on the substrate 18. The ultraviolet light also enhances the surface mobility of the depositing alloy material.

In Figs. 1 and 2, the band gap adjusting elements can be added in gaseous form in an iden5 tical fashion to the fluorine and hydrogen by replacing the hydrogen generator 82 or by adding one or more activated adjusting element generators 86 and 83 (Fig. 2). Each of the generators 86 and 88 typically will be dedicated to one of the ad10 justing elements such as carbon or nitrogen. For example, the generator 86 could supply carbon as in the form of methane gas (CH4), and generator 88 could supply nitrogen as in the form of ammonia gas (NH3).

Referring now to Fig. 3 which illustrates additions to the equipment shown in Fig. 1 for adding other agents or elements to the depositing alloy. For example, an n-conductivity dopant, like phosphorus or arsenic, may be initially added to make the intrinsically modest n-type alloy a more substantially n-type alloy, and then a pdopant like aluminum, gallium or indium may be added to form a good p-n junction within the alloy. A crucible 90 is shown for receiving a dopant like arsenic which is evaporated by bombarding the same with an electron beam source 92, like the bean source 20 previously described. The rate at which tbe dopant evaporates into the atmosphere of the bell jar 12, which is determined by the intensity of the electron beam produced by the electron beam source 92, is set by a manual control 94 on the control panel 34, which controls the current fed to the filament forming part of this beam source to produce the set evaporation rate. The evaporΒΙΟ tion rate is measured by a thickness sampling unit 96 upon which the dopant material deposits and which generates a signal on a cable 98 extending between the unit 96, and control unit 24, which indicates the rate at which the dopant material is deposited on the unit 96.

After the desired thickness of amorphous alloy having the desired degree of n-conductivity has been deposited, evaporation of germanium and the n-conductivity dopant is terminated and the crucible 90 (or another crucible not shown) is provided with a p-conductivity dopant described, and the amorphous alloy and dopant deposition process then proceeds as before to increase the thickness of the amorphous alloy with a p-conduc25 tivity region therein.

The band adjusting element (s) also can be added by a similar process to that described for the dopant by utilizing another crucible similar to the crucible 90.

In the case where the amorphous alloys comprise two or more elements which are solid at room temperature, then it is usually desirable to separately vaporize each element placed in a separate crucible, and control the deposition rate thereof in any suitable manner, as by setting controls on the control panel 34 which, in association with the depositon rate and thickness sampling units, controls the thickness and composition of the depositing alloy.

As previously indicated, although it is pre52206 ferred that compensating and other agents be incorporated into the amorphous alloy as it is deposited, in accordance with another aspect of the invention, the amorphous alloy deposition process and the process of injecting the compensating and other agents into the semiconductor alloy can be done in a completely separate environment from the depositing of the amorphous alloy. This can have an advantage in certain applications since the conditions for injecting such agents are then completely independent of the conditions for the alloy deposition. Also, as previously explained, if the vapor deposition process produces a porous alloy, the porosity of the alloy, in some cases, is more easily reduced by environmental conditions quite different from that present in the vapor deposition process. To this end, reference should now be made to Figs. 4 and 5 which illustrate that the amorphous deposition process and the com20 pensating or altering agent diffusion process are carried out as separate steps in completely different environments, Fig. 5 illustrating apparatus for carrying out the post compensation diffusion process.

As there shown, a low pressure container body 100 is provided which has a low pressure chamber 102 having an opening 104 at the top thereof.

This opening 104 is closed by a cap 106 having threads 108 which thread around a corresponding threaded portion on the exterior of the container body 100. A sealing O-ring 110 is sandwiched between the cap 106 and the upper face of the container body. A sample-holding electrode 112 is mounted on an insulating bottom wall 114 of the chamber 100. A substrate 116 upon which an amorphous germanium alloy 118 has already been deposited is placed on the electrode 112. The upper face of the substrate 116 contains the amorphous allay 118 to be altered or compensated in the manner now to be described.

Spaced above the substrate 116 is an electrode 120. The electrodes 112 and 120 are connected by cables 122 and 124 to a DC or RF supply source 126 which supplies a voltage between the electrodes 112 and 120 to provide an activated plasma of the compensating or altering gas or gases, such as fluorine, hydrogen, and the like, fed into the chamber 102. For purposes of simplicity, Fig. 5 illustrates only molecular fluorine being fed into the chamber 102 by an inlet conduit 129 passing through the cap 106 and extending fro· a supply tank 130 of aolecular fluorine. Other compensating or altering gases (such as hydrogen and the like) also may be similarly fed into the chamber 102. The conduit 128 is shown connected to a valve 132 near the tank 130. A flow cate indicating gauge 134 is shown connected to the inlet conduit 128 beyond the valve 132.

Suitable means are provided for heating the interior of the chamber 102 so that the substrate temperature is elevated preferably to a temperature below, but near the crystallization temperature of the film 118. For example, coils of heat15 ing wire 136 are shown in the bottom wall 114 of the chamber 102 to which coils connect a cable (not shown) passing through the walls of the container body 100 to a source of current for heating the same.

The high temperature together with a plasma of gas containing one or more compensating elements developed between the electrodes 112 and 120 achieve a reduction of the localized states in the band gap of the alloy. The compensating or alter25 ing of the germanium amorphous alloy 118 may be enhanced by irradiating the amorphous alloy 118 with radiant energy from an ultraviolet light source 138, which is shown outside of the container body 100 directing ultraviolet light be5 tween the electrodes 112 and 120 through a quartz window 140 mounted in the side wall of the container body 100.

The low pressure or vacuum in the chamber 102 can be developed by a vacuum pump (not shown) such as the pump 56 in Fig. 1. The pressure of the chamber 102 can be on the order of .3 to 2 torr with a substrate temperature on the order of 200 to 450°C. The activated fluorine (and hydrogen) as well as other compensating or altering elements also can be produced from compounds containing the elements instead of from a molecular gas source, as previously mentioned.

Various applications of the improved amorphous alloys produced by the unique processes of the invention are illustrated in Figs. 6 through 11. Fig. 6 shows a Schottky barrier solar cell 142 in fragmentary cross-section. The solar cell 142 includes a substrate or electrode 144 of a material having good electrical conductivity prop25 erties, and the ability of making an ohmic contact 53206 with an amorphous germanium alloy 146 compensated or altered to provide a low density of localized states in the energy gap and with a band gap optimized by the processes of the present invention.

The substrate 144 may comprise a low work function metal, such as aluminum, tantalum, stainless steel or other material matching with the amorphous alloy 146 deposited thereon which preferably includes germanium, compensated or altered in the manner of the alloys previously described so that it has a low density of localized states in the energy gap. It is most preferred that the alloy have a region 148 next to the electrode 144, which region forms an n-plus conductivity, heavily doped, low resistance interface between the electrode and an undoped relatively high dark resistance region 150 which is an intrinsic, but low n-conductivity region.

The upper surface of the amorphous alloy 146 as viewed in Fig. 6, joins a metallic region 152, an interface between this metallic region and the amorphous alloy 146 forming a Schottky barrier 154. The metallic region 152 is transparent or semi-transparent to solar radiation, has good electrical conductivity and is of a high work function (for example, 4.5 eV or greater, produced, for example, by gold, platinum, palladium, etc.) relative to that of the amorphous alloy 146. The metallic region 152 may be a single layer of a metal or it may be a multi-layer. The amorphous alloy 146 may have a thickness of about .5 to 1 micron and the metallic region 152 may have a o thickness of about 100A in order to be semi-transparent to solar radiation.

On the surface of the metallic region 152 is deposited a grid electrode 156 made of a metal having good electrical conductivity. The grid may comprise orthogonally related lines of conductive material occupying only a minor portion of the area of the metallic region, the rest of which is to be exposed to solar energy. For example, the grid 156 may occupy only about from 5 to 10% of the entire area of the metallic region 152. The gri,d electrode 156 uniformly collects current from the metallic region 152 to assure a good low series resistance for the device.

An anti-reflection layer 158 may be applied over the grid electrode 156 and the areas of the metallic region 152 between the grid electrode areas. The anti-reflection layer 158 has a solar radiation incident surface 160 upon which impinges the solar radiation. For example, the anti-reflection layer 158 aay have a thickness on the order of aagnitude of the wavelength of the maxi5 Bum energy point of the solar radiation spectrum, divided by four times the index of refraction of the anti-reflection layer 158. If the metallic o region 152 is platinum of 100A in thickness, a suitable anti-reflection layer 158 would be ziro conium oxide of about 500A in thickness with an index of refraction of 2.1.

The band adjusting element(s) are added to the photocurrent generating region 150. The Schottky barrier 154 formed at the interface between the regions 150 and 152 enables tbe photons from the solar radiation to produce current carriers in the alloy 146, which are collected as current by the grid electrode 156. An oxide layer (not shown) can be added between the layers 150 and 152 to produce an MIS (metal insulator semiconductor) solar cell.

In addition to the Schottky barrier or MIS solar cell shown in Fig. 6, there are solar cell constructions which utilize p-n junctions in the body of the amorphous alloy forming a part thereof formed in accordance with successive deposition, compensating or altering and doping steps like that previously described. These other forms of solar cells are generically illustrated in Fig. 7 as well as in Figs. 10 and 11.

These constructions 162 generally include a transparent electrode 164 through which the solar radiation energy penetrates into the body of the solar cell involved. Between this transparent electrode and an opposite electrode 166 is a deposited amorphous alloy 168, preferably including germanium, initially compensated in the manner previously described. In this amorphous alloy 168 are at least two adjacent regions 170 and 172 where the amorphous alloy has respectively oppositely doped regions, region 170 being shown as a n-conductivity region and region 172 being shown as a p-conductivity region. The doping of the regions 170 and 172 is only sufficient to move the Fermi levels to the valence and conduction bands involved so that the dark conductivity remains at a low value achieved by the band adjusting and compensating or altering method of the invention. The alloy 168 has high conductivity, highly doped ohmic contact interface regions 174 and 176 of the same conductivity type as the adjacent region of the alloy 168. The alloy regions 174 and 176 contact electrodes 164 and 166, respectively. The adjusting element(s) are added to regions 170 and/or 172.

Referring now to Fig. 8, there is illustrated another application of an amorphous alloy utilized in a photo-detector device 178 whose resistance varies with the amount of light impinging thereon.

An amorphous alloy 180 thereof is band gap adjusted and compensated or altered in accordance with the invention, has no p-n junctions as in the embodiment shown in Fig. 7 and is located between a transparent electrode 182 and a substrate elec15 trode 184. In a photo-detector device it is desirable to have a minimum dark conductivity and so the amorphous alloy 180 has an undoped, but compensated or altered region 186 and heavily doped regions 188 and 190 of the same conductivity type forming a low resistance ohmic contact with the electrodes 182 and 184, which may form a substrate for the alloy 180. The adjusting element(s) are added at least to the region 186.

Referring to Fig. 9 an electrostatic image producing device 192 (like a xerography drum) is illustrated. The device 192 has a low dark conductivity, selective wavelength threshold, undoped or slightly p-doped amorphous alloy 194 deposited on a suitable substrate 196 such as a drum. The adjusting element(s) are added to the alloy 194.

As used herein, the terms compensating agents or materials and altering agents, elements or materials mean materials which are incorporated in the germanium amorphous alloy for altering or changing the structure thereof, such as, activated fluorine (and hydrogen) incorporated in the amorphous alloy containing germanium to form an amorphous germanium/fluorine/hydrogen composition alloy, having a desired band gap and a low density of localized states in the energy gap. The activated fluorine (and hydrogen) is bonded to the germanium in the alloy and reduces the density of localized states therein and due to the small size of the fluorine and hydrogen atoms they are both readily introduced into the amorphous alloy without substantial dislocation of the germanium atoms and their relationships in the amorphous alloy.

This is true most particularly because of the extreme electronegativity, specificity, small size and reactivity of fluorine, all of which char52206 acteristics organize the local order of the alloys. In creating this new alloy the strong inductive powers of fluorine and its ability to act as an organizer of short range order -is of impor5 tance. The ability of fluorine to bond with both germanium and hydrogen results in the formation of alloys with a minimum of localized defect states in the energy gap. Bence, fluorine and hydrogen are introduced without substantial formation of other localized states in the energy gap to form the new alloy.

Referring now to Fig. 10, a p-i-n solar cell 198 is illustrated having a substrate 200 which may be glass or a flexible web formed from stain15 less steel or aluminum. The substrate 200 is of a width and length as desired and preferably at least 3 mils thick. The substrate has an insulating layer 202 deposited thereon by a conventional process such as chemical deposition, vapor deposition or anodizing in the case of an aluminum substrate. The layer 202 for instance, about 5 microns thick can be made of a metal oxide. For an aluminum substrate, it preferably is aluminum oxide (AI2O3} and for a stainless steel substrate it may be silicon dioxide (S1O2) or other suitable glass.

An electrode 204 is deposited in one or more layers upon the layer 202 to form a base electrode for the cell 198. The electrode 204 layer or layers is deposited by vapor deposition, which is a relatively fast deposition process. The electrode layers preferably are reflective metal electrodes of molybdenum, aluminum, chrome or stainless steel for a solar cell or a photovoltaic device. The reflective electrode is preferable since, in a solar cell, non-absorbed light which passes through the semiconductor alloy is reflected from the electrode layers 204 where it again passes through the semiconductor alloy which then absorbs more of the light energy to increase the device efficiency.

The substrate 200 is then placed in the deposition environment. The specific examples shown in Figs. 10 and 11 are illustrative of some p-i-n junction devices which can be manufactured util20 izing the improved methods and materials of the invention. Each of the devices illustrated in Figs. 10 and 11 has an alloy body having an overall thickness of between about 3,000 and 30,000 angstroms. This thickness ensures that there are no pin holes or other physical defects in the structure and that there is maximum light absorption efficiency. A thicker material may absorb more light, but at some thickness will not generate more current since the greater thickness allows more recombination of the light generated electron-hole pairs. (It should be understood that the thicknesses of the various layers shown in Figs. 6 through 11 are not drawn to scale.) Referring first to forming the n-i-p device 198, the device is formed by first depositing a heavily doped n+ alloy layer 206 on the electrode 204. Once the n+ layer 206 is deposited, an intrinsic (i) alloy layer 208 is deposited thereon. The intrinsic layer 208 is followed by a highly doped conductive p+ alloy layer 210 deposited as the final semiconductor layer. The alley layers 206, 208 and 210 form the active layers of the ni-p device 198.

While each of the devices illustrated in Figs. 10 and 11 may have other utilities, they will be now described as photovoltaic devices. Utilized as a photovoltaic device, the selected outer, p+ layer 210 is a low light absorption, high conductivity alloy layer. The intrinsic alloy layer 208 has an adjusted wavelength thresh40 old for a solar photoresponse, high light absorption, low dark conductivity and high photoconductivity including sufficient amounts of the adjusting element(s) to optimize the band gap.

The bottom alloy layer 204 is a low light absorption, high conductivity n+ layer. The overall device thickness between the inner surface of the electrode layer 206 and the top surface of the p+ layer 210 is, as stated previously, on the order of at least about 3,000 angstroms. The thickness of the n+ doped layer 206 is preferably in the range of about 50 to 500 angstroms. The thickness of the amorphous adjusting element containing intrinsic alloy 208 is preferably between about 3,000 angstroms to 30,000 angstroms. The thickness of the top p+ contact layer 210 also is preferably between about 50 to 500 angstroms. Due to the shorter diffusion length of the holes, the p+ layer generally will be as thin as possible on the order of 50 to 150 angstroms. Further, the outer layer (here p+ layer 210) whether n+ or p+ will be kept as thin as possible to avoid absorption of light in that contact layer and generally will not include the band gap adjusting element(s).

A second type of p-i-n junction device 212 is illustrated in Fig. 11. In this device a first p+ layer 214 is deposited on the electrode layer 204' followed by an intrinsic germanium amorphous alloy layer 216 containing the bend gap adjusting eleS ment(s) in the desired amount, an n amorphous alloy layer 218 and an outer n+ amorphous alloy layer 220. Further, although the intrinsic alloy layer 208 or 216 (in Figs. 10 and 11) is an amorphous alloy the other layers are not so restricted and may be polycrystalline, such as layer 214.

(The inverse of the Figs. 10 and 11 structure not illustrated, also can be utilized.) Following the deposition of the various semiconductor alloy layers in the desired order for the devices 198 and 212, a further deposition step is performed, preferably in a separate deposition environment. Desirably, a vapor deposition environment is utilized since it is a fast deposition process. In this step, a TCO layer 222 (trans20 parent conductive oxide) is added which, for example, may be indium tin oxide (ITO), cadmium stannate (CdjSnOj), or doped tin oxide (Sr.O2).

The TCO layer will be added following the post compensation of fluorine (and hydrogen) if the films were not deposited with one or more of the desired compensating or altering elements therein. Also, the other compensating or altering elements, above described, can be added by post compensation.

An electrode grid 224 can be added to either of the devices 198 or 212 if desired. For a device having a sufficiently small area, the TCO layer 222 is generally sufficiently conductive such that the grid 224 is not necessary for good device efficiency. If the device is of a sufficiently large area or if the conductivity of the TCO layer 222 is insufficient, the grid 224 can be placed on the layer 222 to shorten the carrier path and increase the conduction efficiency of the devices.

Referring now to Fig. 12, one embodiment of a plasma activated vapor deposition chamber 226 is illustrated in which the semiconductor and band adjusting element(s) of the invention can be de20 posited. A control unit 228 is utilized to control the deposition parameters, such as pressure, flow rates, etc., in a manner similar to that previously described with respect to the unit 24 (Fig. 1). The pressure would be maintained at about IO3 torr or less.

One or more reaction gas conduits, such as 230 and 232, can be utilized to supply gases such as germanium tetrafluoride (GeE^) and hydrogen (B2> into a plasma region 234. The plasma region 234 is established between a coil 236 fed by a DC power supply (not illustrated) and a plate 238. The plasma activates the supply gas or gases to supply activated fluorine (and hydrogen) to be deposited on a substrate 240. The substrate 240 may be heated to the desired deposition temper10 ature by heater means as previously described.

The band adjusting element(s) and germanium can be added from two or more evaporation boats, such as 242 and 244. The boat 242 could, for example, contain germanium and the boat 244 would contain carbon. The elements in boats 242 and 244 can be evaporated by electron beam or other heating means and are activated by the plasma.

If it is desired to layer the band adjusting element(s) in the photogenerating region of the film being deposited, a shutter 246 can be utilized. The shutter could rotate layering separate band adjusting elements from two or more of the boats or can be utilized to control the depositing of the band adjusting element from the boat 244 (or others) to provide layers in the film or to 52200 vary the amount of band adjusting element deposited in the film. Thus, the band adjusting element (s) can be added discretely in layers, in substantially constant or in varying amounts.

Fig. 13 illustrates the available sunlight spectrum. Air Mass 0 (AMO) being the sunlight available with no atmosphere and the sun directly overhead. AMI corresponds to the same situation after filtering by the earth's atmosphere. Crys10 tailine germanium has an indirect band gap of about .7 eV, which corresponds to the wavelength of about 1.8 micrometer (microns). This equates to losing, i.e. not generating useful photons, for substantially all the light wavelengths above 1.8 microns. As utilized herein, band gap or E optical is defined as the extrapolated intercept of a plot of whereeX. is the absorption coefficient and^LuJ (or e) is the photon energy.

For light having a wavelength above the threshold defined by the band gap, the photon energies are not sufficient to generate a photocarrier pair and hence do not add any current to a specific device.

Calculations for the maximum theoretical conversion efficiency as a function of the width of the band gap were done by J.J. Loferski, as reported in Journal of Applied Physics, Vol. 27, p. 777, July, 1956. For single band gap materials, depending upon the assumptions made, the optimum band gap is on the order of 1.4 to 1.5 ev for solar applications. To produce the desired photovoltaic band gap of 1.5 eV in the amorphous devices, the band adjusting element(s) of the invention, such as carbon or nitrogen are added to the photogenerating regions, as previously des10 cribed.

Another photoresponsive application is for laser wavelengths such as for infrared response.

A photoresponsive material used in a high speed electrophotographic computer output device uti15 lizing a laser, such as a helium neon laser, should have a wavelength threshold greater than .6 microns. For use with GaAs or other infrared semiconductor lasers, the photoresponsive material threshold should be greater than one micron. The addition of the band gap adjusting element(s) of the invention allows the tailoring of germanium alloys having the optimum band gap for the desired application.

Each of the device semiconductor alloy layers can be glow discharge deposited upon the base electrode substrate by a conventional glow discharge chamber described in the aforesaid U.S. Patent No. 4,226,898. The alloy layers also can be deposited in a continuous process. In these cases, the glow discharge system initially is evacuated to approximately 1 mtorr to purge or eliminate impurities in the atmosphere from the deposition system. The alloy material preferably is then fed into the deposition chamber in a compound gaseous form, most advantageously as fluorine (Fj), hydrogen (Hj) and germanium tetrafluoride (GeF4). The glow discharge plasma preferably is obtained from the gas mixture. The deposition system in U.S. Patent 4,226,898 preferably is operated at a pressure in the range of about 0.3 to 1.5 torr, preferably between 0.6 to 1.0 torr such as about 0.6 torr.

The semiconductor material is deposited from a self-sustained plasma onto the substrate' which is heated, preferably by infrared means to the desired deposition temperature for each alloy layer. The doped layers of the devices are deposited at various temperatures in the range of 200°C to about 1000°C, depending upon the form of the material used. The upper limitation on the substrate temperature in part is due to the type of metal substrate utilised. For aluminum the upper temperature should not be above about 600°C and for stainless steel it could be above about 1000°C. For an initially hydrogen compensated amorphous alloy to be produced, such as to form the intrinsic layer in n-i-p or p-i-n devices, the substrate temperature should be less than about 400°C and preferably about 300°C.

The doping concentrations are varied to produce the desired p, p+, n or n* type conductivi ty as the alloy layers are deposited for each device. For n or p doped layers, the material is doped with 5 to 100 ppm of dopant material as it is deposited. For n+ or p* doped layers the material is doped with 100 ppm to over 1 per cent of dopant material as it is deposited. The n dopant material can be deposited at their respective optimum substrate temperatures and preferably to a thickness in the range of 100 ppm to over 5000 ppm for the p+ material.

The glow discharge deposition process can include an AC signal generated plasma into which the materials are introduced. The plasma preferably is sustained between a cathode and substrate anode with an AC signal of about 1kHz to 13.6 MHz.

Although the band adjusting method and element (s) of the invention can be utilized in devices with various amorphous alloy layers, it is preferable that they are utilized with the fluo5 rine and hydrogen compensated glow discharge deposited alloys. In this case, a mixture of germanium tetrafluoride and hydrogen is deposited as an amorphous compensated alloy material at or below about 400°C, for the n-type layer. The band ad10 justed intrinsic amorphous alloy layer and the p+ layer can be deposited upon the electrode layer at a higher substrate temperature above about 450°C which will provide a material which is fluorine compensated. For example, a mixture of the gases GeF4+H2 having ratios of 4 to 1 to 10 to 1 may be utilized. Additional fluorine may be added such as from other fluorine compounds so that the mixture includes up to 10% fluorine. The amount of each gas utilized may vary depending upon the other glow discharge parameters, such as temperature and pressure.

Although the band gap adjusting element(s) are added at least to the photoresponsive region of the devices, the element(s) also can have util25 ity in the other alloy layers of the devices. As previously mentioned, the alloy layers other than the intrinsic alloy layer can be other than amorphous layers, such as polycrystalline layers. (By the term amorphous*' is meant an alloy or material which has long range disorder, although it may have short or intermediate order or even contain at times some crystalline inclusions.}

Claims (42)

CLAIMS:

1. A method of making an improved photo-responsive amorphous alloy, said method comprising depositing on a substrate a material including at least germanium and incorporating in said material a single density of states 5 reducing element, said element being fluorine.

2. A method according to claim 1 further including the step of introducing at least one band gap adjusting element into said material without substantially increasin 10 the states in the band gap to produce an alley having a band gap adjusted for a specified photoresponse wavelength function.

3. The method according to claim 2 wherein said 15 adjusting element is either carbon or nitrogen.

4. The method according to any one of claims 2 or 3, wherein said alloy is deposited with an active photoresponsive region therein and said adjusting element is 20 introduced at least in said region.

5. The method according to any one of claims 2 to 4 wherein said method forms one step in a multi-step process for forming successively deposited alloy layers of 25 opposite (p and n) conductivity type, the n-type layer being formed by introducing during the deposition of the layer an n-dopant element which is deposited with the deposited layer to produce an n-type layer and the p-type layer being formed by introducing during deposition of 5 the layer a p-dopant element which is deposited with the deposited layer to produce a p-type layer.

6. The method according to claim 7 wherein there is deposited between said p and n doped layers an 10 intrinsic amorphous alloy layer without a p or n dopant · element present therein, at least a portion of said intrinsic layer containing said adjusting element.

7. The method according to any one of claims 2 15 to 6 wherein said adjusting element is introduced into said material in substantially discrete layers.

8. The method according to any one of claims 2 to 7 wherein said adjusting element is introduced into 20 said material in varying amounts.

9. The method according to any one of claims 2 to 8 including evaporating said adjusting element prior to introducing it into said material.

10. The method according to any one of claims 2 to 9 including plasma activating said adjusting element as it is being introduced into said material.

11. The method according to any one of claims 2 to 10 including activating said adjusting element by plasma activated vapour deposition as it is being introduced into said material.

12. The method according to any one of claims 1 to 11 wherein said method includes depositing at least a portion of said alloy with one of a p or n dopant element therein to form a p or n conductivity type 10 alloy.

13. The method according to any one of claims 1 to 12 wherein said reducing element is incorporated into said material after deposition thereof.

14. An amorphous alloy made according to the process of any one of claims 1 to 13.

15. An improved photoresponsive amorphous alloy, 20 said alloy including germanium and incorporating a single density of states reducing element therein, said element being fluorine.

16. The alloy according to claim 15, further including 25 a band gap adjusting element incorporated therein without substantially increasing the states in the gap, said alloy -having a band gap adjusted for a specified photoresponse wavelength function.

17. The alloy according to claim 16 wherein said adjusting element is either carbon or nitrogen.

18. The alloy according to claim 16 or 17, wherein said alloy has an active photoresponsive region therein and said adjusting element is included at least in said region.

19. The alloy according to any one of claims 16 to 18 wherein said alloy is a multi-layer alloy of successively deposited layers of opposite (p and n) conductivity type, the n-type layer including an ndopant element in the layer to produce an n-type layer and the p-type layer including a p-dopant element in the layer to produce a p-type layer.

20. The alloy according to claim 19 wherein there is deposited between said p and n doped layers an intrinsic amorphous alloy layer without a p or n dopant element present therein, at least a portion of said intrinsic layer containing said adjusting element.

21. The alloy according to any one of claims 16 to 20 including said adjusting element in substantially discrete layers.

22. The alloy according to any one of claims 16 to 21 including said adjusting element in varying amounts.

23. The alloy according to any one of claims 15 to 18 or 21 or 22 including at least one of an n or p conductivity portion therein, said portion including an n or p dopant element therein.

24. The alloy according to any one of claims 15 to 23 deposited by glow discharge deposition.

25. An improved photoresponsive device, said device comprising superimposed layers of various materials including an amoprhous germanium semi-conductor alloy body having an active photoresponsive region including a band gap therein upon which radiation can impinge to produce charge carriers, said germanium amorphous alloy including a single density of states reducing element, said element being fluorine.

26. A device according to claim 25 further including a band gap adjusting element therein at least in said photo-responsive region to enhance the radiation absorption thereof without substantially increasing the states in the gap, the band gap of said alloy being adjusted for a specified photoresponse wave-length function.

27. The device according to claim 26 wherein said adjusting element is carbon.

28. The device according to any one of claims 25 to 27 wherein said alloy body is a multi-layer body formed of successively deposited layers of opposite (p and n) conductivity type, the n-type layer being formed by introducing during the deposition of the layer an n-dopant element which is deposited with the deposited layer to produce an n-type layer and the ptype layer being formed by introducing during deposition of the layer a p-dopant element which is deposited with the deposited layer to produce a p-type layer.

29. The device according to claim 28 wherein there is deposited between said p and n doped layers an intrinsic amorphous alloy layer without a p or n dopant element present therein, at least a portion of said intrinsic layer containing said adjusting element.

30. The device according to any one of claims 25 to 29 wherein said alloy body includes said adjusting element in substantially discrete layers.

31. The device according to any one of claims 25 to 30 wherein said alloy body includes said adjusting element in varying amounts.

32. The device according to any one of claims 25 to 31 deposited by glow discharge deposition.

33. The device according to any one of claims 25 to 27 or 30 to 32 wherein said alloy body includes at least one of an n or p conductivity region therein, said region including an n orP dopant element therein.

34. The device according to any one of claims 25 to 27 or 30 to 33 wherein said alloy body forms part of a Schottky barrier solar cell. 10

35. The device according to any one of claims 25 to 27 or 30 to 32 wherein said alloy body forms part of an MIS solar cell.

36. The device according to any one of claims 15 25 to 28 or 30 to 32 wherein said alloy body forms part of a p-n junction device.

37. The device according to any one of claims 25 to 32 wherein said alloy body forms part of a p-i-n 20 device.

38. The device according to any one of claims 25 to 32 wherein said alloy body forms part of a photodetector .

39. The device according to any one of claims 25 to 32 wherein said alloy body forms part cf an electrostatic image producing device.

40. A method of making an improved photo-responsive amorphous alloy substantially as hereinbefore described with reference to the accompanying drawings. 5

41. A photoresponsive amorphous alloy substantially as hereinbefore described.

42. A photoresponsive device according to claim 25, substantially as hereinbefore described with reference 10 to Figure 6 to 11 of the accompanying drawings.