IE52207B1 - Method of grading the band gaps of amorphous alloys and devices - Google Patents

Abstract

A graded bandgap material comprises amorphous Si containing F(a- Si : F) together with a varying amount of a bandgap modifier (e.g. Ge, Sn, C or N). H, may also be incorporated in the material which may be doped. The graded bandgap material may be used in Schottky, MIS and PIN solar cells or in photoconductive or electrophotographic devices.

Description

This invention relates to a method of making amorphous semiconductor devices having improved photoresponsive characteristics. The invention has its most important application in making im5 proved photoresponsive alloys and devices having graded band gaps for specific photoresponsive applications including photoreceptive devices such as solar cells of a p-i-n, p-n, Schottky or MIS (metal-insulator-semiconductor) type; photocon10 ducting medium such as utilized in xerography; photodetecting devices and photodiodes including large area photodiode arrays.

Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (IB 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 abil ity of the scientist to grow substantially defect free 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 crys· tai line material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into tbe 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 cells 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 impossible 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 addition of grain boundaries and other problem defects.

An additional shortcoming of the crystalline material, for solar applications, is that the crystalline silicon band gap of about 1.1 eV inherently is below the optimum band gap of about 1.5 eV. The admixture of germanium, while possible, further narrows the band gap which further decreases the solar conversion efficiency.

In summary, crystal silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are ex25 pensive and time consuming to produce. Devices 53307 based upon amorphous silicon can eliminate these crystal silicon disadvantages. Amorphous silicon has an optical absorption edge 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 50 micron thick crystalline silicon. Further, amorphous silicon can be made faster, easier and in larger areas than can crystal silicon.

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 deposi15 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 substan20 tially unproductive. Amorphous silicon or 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 silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, 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 silicon and germanium, W.E. Spear and F.G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on Substitutional Doping of Amorphous Silicon, as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic 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 silicon films wherein a gas of silane (Sif!*) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate 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 (PH3) 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 10”® and lil”2 parts per volume. The material so deposited included supposedly substitutional phosphorus or boron dopant and was 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 cor5 responding crystalline material.

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 material obtained is . . a less attractive material than a-Si for doping experiments and possible applications.

In working with a similar method of glow discharge fabricated amorphous silicon solar cells utilizing silane, D.E. Carlson attempted to utilize germanium in the cells to narrow the optical gap toward the optimum solar cell value of about 1.5 eV from his best fabricated solar cell ma25 terial which has a band gap of 1.65-1.70 eV. (D.E 53207 Carlson, Journal of Non Crystalline Solids, Vol. and 36 (1960) pp. 707-717, given at 8th International Conference on Amorphous and Liquid SemiConductors, Cambridge, Mass., Aug. 27-31, 1979). However, Carlson has further reported that the addition of germanium from germane gas was unsuccessful because it causes significant reductions in all of the photovoltaic parameters of the solar cells. Carlson indicated that the degradation of photovoltaic properties indicates that defects in the energy gap are being created in the deposited films. (D.E. Carlson, Tech. Dig. 1977 IEDM, Washington, D.C., p. 214).

In a recent report on increasing the cell efficiency of multiple-junction (stacked) solar cells of amorphous silicon (a-Si:H, deposited from silane in the above manner, the authors reported that *[g]ermanium has been found to be a deleterious impurity in a-Si:H, lowering its Jsc exponentially with increasing Ge . . . From their work as well as that of Carlson, they concluded that alloys of amorphous silicon, germanium and hydrogen have shown poor photovoltaic properties and thus new photovoltaic film cell materials must be found having spectral response at about 1 micron for efficient stacked cell combinations with a-Si:H. (J.J. Hanak, B. Faughnan, V. Korsun, and J.P. Pellicane, presented at the 14th IEEE Photovoltaic Specialists Conference, San Diego, California, January 7-10, 1980).

Grading of the band gap of crystalline p-n junction silicon photovoltaic solar cells was suggested in 1960 by Wolf, Limitations and Possibilities for Improvements of Photovoltaic Solar Energy Converters, Proc. IRE, Vol 48, pp. 12461263; 1960. At the time of his report. Wolf concluded that the preparation of crystalline semiconductor materials with graded energy gaps was technically possible, but their application to photovoltaic cells did not appear to be very useful because of the loss in Voc.

In the above referenced article, D.E. Carlson Journal of Noncrystalline Solids,.Vol. 35 and 36 (1980) pp. 707-717, Carlson also suggested that grading the band gap in the silane deposited devices may improve the device performance. No suggestion was made on how to grade the band gap over any significant range, other than the suggestion of adding Ge which Carlson also has reported as a failure. No suggestion of increasing 52307 the band gap was made whatsoever.

Later, Carlson suggested that the band gap of amorphous silicon solar cells in a silane environment may be graded to provide a potential gradient that would assist in the collection of photogenerated carriers. However, it was not known how such grading could be accomplished without adding germanium to the a-Si:H which, as previously noted, was recognized by Carlson to cause significant reduction in all of the photovoltaic parameters of the solar, cells. The incorporation of hydrogen in the above silane method not only has limitations based upon the fixed ratio oi hydrogen to silicon in silane, but, most importantly, various Si:H bonding configurations introduce new antibonding states which can have deleterious consequences in these materials. Therefore, there are basic limitations in reducing the density of localized states in these materials which are particularly harmful in terms Of effective p as well as n doping. The resulting density of states of the silane deposited materials leads to a narrow depletion width, which in turn limits the efficiencies of solar cells and other devices whose operation depends on the drift of free carriers. The method of making these materials by the use of only silicon and hydrogen also results in a high density of surface states which affects all the above parameters. Further, the previous attempts to decrease the band gap of the material, while successful in reducing the gap, have at the same time added states in the gap. The increase in the states in the band gap results in a decrease or total loss in photoconductivity and is thus counterproductive in producing photoresponsive devices.

After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in the atmosphere of a mixture oi argon {required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as a compensating agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states in the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process 53207 described above. The above described p and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process. Neither process produced efficient p-doped materials with sufficiently high acceptor concentrations for producing commercial p-n or p-i-n junction devices. The n-doping efficiency was below desirable acceptable commercial levels and the p-dop.ing was particularly undesirable since it reduced tbe width of the band gap and increased the number of localized states in the band gap.

The prior deposition of amorphous silicon, which has been altered by hydrogen from the silane gas in an attempt to make it more closely resemble crystalline silicon and which has been doped in a manner like that of doping crystalline silicon, has characteristics which in all important respects are inferior to those of doped crystalline silicon. Thus, inadequate doping efficiencies and conductivity were achieved especially in the ptype material, and the photovoltaic qualities of these silicon 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 prepared by glow discharge as fully described in U.S. Patent No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan 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 same 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 amorphous body to substantially 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 body. The fluorine bonds to the dangling bonds of the silicon and forms what is believed 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 53207 by hydrogen and other compensating or altering agents. Fluorine is considered to be a more efficient compensating or altering 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 fluorirte 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 dangling 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 alloying 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 relation15 ships.

An enhanced spectral response of amorphous silicon photoresponsive devices and improved collection of photogenerated carriers therein is provided in accordance with the present invention by adding one or more band gap adjusting elements in continuously varying amounts to an amorphous photoresponsive alloy at least in one or more portions of the photocurrent generation region thereof to grade the band gap width for particular applications without substantially increasing the 3 2 0 7 deleterious states in the gap. Thus, the high quality electronic properties of the material are not affected in forming the graded band gap alloy.

The amorphous alloy incorporates at least one density of states reducing element, fluorine. The compensating or altering element, fluorine, and/or other elements can be added during deposition or thereafter. Tlie adjusting element(s) can be activated and may be added in vapor deposition, sputtering or glow discharge processes. The band gap can be graded as reguired for a specific application by introducing varying amounts of one or more of the adjusting elements into the deposited alloy in at least one or more portions of the photocurrent generation region thereof.

The band gap is graded 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 prior silane deposited films typically are deposited on substrates heated to 250°C to 350°C to maximize the incorporation and compensation of silicon with hydrogen in the films. The prior attempts to decrease the band gap by adding germanium to this film failed because the hydrogen germanium bond is too weak to be stable at the required deposition substrate temperature.

The presence of fluorine in the alloy of the invention provides a silicon alloy which differs physically, chemically, and electrochemically from other silicon alloys, because fluorine not only covalently bonds to the silicon but also effects in a positive manner the structural short range order of the material. This allows adjusting elements, such as germanium, tin, carbon or nitrogen effectively to be added to the alloy, because fluorine forms the stronger and more stable bonds than does hydrogen. Fluorine compensates or alters silicon 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 use of fluorine produces the alloy or film, described in U.S. Patent No. 4,217,374 in which the density of states in the band gap are much lower than those produced by a combination of silicon and hydrogen, such as from silane. Since the band adjusting element(s) has been tailored into the material to grade the band gap without adding sub52207 atantial deleterious states, because of the influence of fluorine, the new alloy maintains substantially high quality electronic qualities and photoconductivity and further provides an electric field throughout the entire semiconductor to improve solar photovoltaic energy conversion by increasing the carrier collection efficiency. Hydrogen further enhances the fluorine compensated or altered alloy and can be added during dep10 osition with fluorine or after deposition, as can fluorine and other alterant elements. The post deposition incorporation of hydrogen is advantageous when it is desired to utilize the higher deposition substrate, temperatures allowed by fluo15 rine.

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. 4,226,896 has other process variables which advantageously can be utilized with the principles of this invention. 53207 According to the invention there is provided a method of making an improved photo-responsive amorphous alloy, said method comprising depositing on a substrate a material including at least silicon and incorporating in said material at least one density of states reducing element, said element being fluorine, introducing at least one band gap adjusting element into said material in varying amounts without substantially increasing the states in the band gap and varying the composition of the deposition gas· mixture introduced into the deposition chambers to produce an alloy having a graded band gap with a specified range of photoresponse wavelength function.

The invention also provides an improved photo15 responsive amorphous alloy, said alloy including silicon and incorporating at least one density of states reducing element therein, said element being fluorine, said alloy having a band gap adjusting element incorporated therein in varying amounts without substantially increasing the states in the gap, the amount of silicon and fluorine varying relative to the amount of the band gap adjusting element incorporated in said alloy, and said alloy having a graded band gap with a specified range of photoresponse wavelength function.

The invention further provides an improved photoresponsive device, said device comprising superimposed layers of various materials including an amorphous semiconductor alloy body having an active photoresponsive region including a graded band gap therein upon which radiation can impinge to produce charge carriers, said amorphous alloy including at least one density of states reducing element, said element being fluorine, said alloy further including a band gap adjusting element in varying amounts therein at least in said photoresponsive region to enhance the radiation absorption thereof without substantially increasing the states in the gap, the amount of silicon and fluorine varying relative to the amount of the band gap adjusting element incorporated in said alloy, and the band gap of said alloy being graded for a specified range of photo15 responsive wavelength functions. 52307 The preferred embodiments of this invention will now be described by way of example with reference to the drawings accompanying this specification in which Fig. 1 illustrates one type of a PIN solar 5 cell having a band gap which is graded in accordance with the present invention; Fig; 2 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 hydro52207 gen) by the addition of molecular or fluorine compounds containing fluorine such as SiF4, 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 amor30 phous alloy containing silicon; Fig. 3 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 amorphous alloy, such light source replacing the activated fluorine and hydrogen generator units shown in Fig. 1 and adjusting element generating means; Fig. 4 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. 5 illustrates an application wherein the deposition of the amorphous alloy and the applica52207 tion of the activated fluorine and hydrogen may be carried out as separate steps and in separate enclosures; Fig. 6 illustrates exemplary apparatus for diffusing activated hydrogen into a previously deposited amorphous alloy; Fig. 7 is a fragmentary sectional view of an embodiment of a Schottky barrier solar cell to illustrate one application of the amorphous semiconductor photoreceptive alloys made by the process of the invention; Fig. 8 is a fragmentary sectional view of a p-n junction solar cell device which includes a doped 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 semiconductor alloy made by the process of the invention; Fig. 10 is a fragmentary sectional view of a xerographic drum including an amorphous semiconductor alloy made by the process of the invention; Fig. 11 is a fragmentary sectional view of a p-i-n junction solar cell device; 53207 Fig. 12 is a fragmentary sectional view of an n-i-p junction solar cell device; Fig. 13 is a diagrammatic representation of a plasma activated vapor deposition system for- depositing the amorphous alloys with the adjusting element(s) of the invention incorporated therein; and Fig. 14 is a solar spectral irradiance chart illustrating the standard sunlight wavelengths available for various photoresponsive applications.

Referring initially to Fig. 1, a PIN solar cell having a band gap which is graded in a manner and a method embodying the present invention is there illustrated. The band gap of the intrinsic region from the n+ region to the p+ region is graded from 1.2 eV to 1.8 eV, for example by adding one or more band adjusting element(s). The intrinsic (i) region may be formed from amorphous Si:F deposited in a manner to be described subsequently. The lowest band gap of 1.2 eV may be achieved by adding germanium or tin to the intrinsic region as it is deposited. The gap may be increased from that value by reducing the amount of germanium added during the intrinsic layer deposition. The final band gap value of 1.6 eV may be reached by terminating the addition of the band adjusting element(s) germanium or tin near the end of the intrinsic layer deposition prior to depositing the p+ layer. Further, by adding adjusting element(s) which increase the band gap such as nitrogen or carbon, the band gap may be increased to 1.8 eV or above near to the p+ region. The nitrogen may be added in the form of ammonia (NH3) and the carbon may be added in the form of methane (CH4). As will be described in detail hereinafter, the band adjusting element(s) may be added to provide the graded band gap while still maintaining high quality electronic qualities and photoconductivity in the cell due to the inclusion of fluorine (and hydrogen) into the alloy.

By virtue of the graded band, gap, improved utilization of the sun energy for the generation of electron-hole pairs is acheived. Further, because the band adjusting element(s) may be added in any amounts, various forms of grading (i.e. continuous, intermittent or stepped) can be achieved Additionally, the n+ and p+ layers may also be graded in the same manner or by selective doping.

The band gap can be narrowed below 1.2 eV by introducing tin into the alloy.

Not only does the graded band gap provide improved sun energy utilization, it also creates an electric field substantially throughout the device which assists in carrier collection. As will be appreciated, volume recombination predominates in a-5i:F in the absence of electric fields. This reduces the collection efficiency since carriers photogenerated outside the depletion regiqn are lost. However, by virtue of the graded band gap, a built in electric field is created decreasing the transit time compared to the carrier lifetime so that substantially all of ehe carriers are collected, provided the field exceeds a minimum value throughout the semiconducting region.

It will also be noted in Fig.· 1 that the valence band has been tilted. As illustrated, it is inclined from the n+ layer to the p+ layer. This slight tilting allows the photon-generated holes to drift towards the p+ layer for collection. Such tilting may be accomplished by selectively doping in small amounts as the intrinsic layer is deposited. Such an effect may also be achieved not by doping but instead by the space charge build up. Once such a space charge region is present, the generated holes will be free to move toward the p+ layer for collection.

By grading the band gap in the manner represented in Fig. 1, there will be some slight reduction in Voc over a single band gap alloy of 1.6 eV, since the lowest band gap in the graded alloy essentially determines Voc. Jsc, however, is increased due to the more efficient use of the sun energy for electron-hole generation and the more efficient collection of the generated carriers. Therefore, the overall device operation can be enhanced especially for utilization where higher currents, but lower voltages are required or acceptable. Widening the band gap above 1.8 eV also increases the amount of the sunlight available for use in the device.

Referring now to Fig. 2, 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 en52207 closure 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 silicon for forming an amorphous alloy containing silicon on the substrate 18 which, for example, may be 9 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 silicon 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 includes relays or the like for interrupting the connection of the power supply 22 to the conductors 26 and 28 when the film 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 setting 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 56 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^ 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 pressure sensor 68 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. Conduits 76 and 78 extend from the control unit 24 and pass through the support base 50 into the evacuated space 14 of the bell jar 12. Conduits 76 and 78 respectively connect with activated fluorine and hydrogen generating units 80 and 82 which convert the molecular fluorine and hydrogen respectively to activated fluorine and hydrogen, which may be atomic and/or ionized forms of these gases. The activated fluorine and hydrogen generating units 80 and 82 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 18, so that the relatively short-lived activated fluorine and hydrogen delivered thereby are immediately injected into the vicinity of the substrate 18 5S307 where the alloy is depositing. As indicated previously, at least fluorine will be included in the alloy and hydrogen preferably also will be included. 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 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 14 can still be a relatively low pressure (like 10“4 torr). The pressure of the gas in the generator can be higher than the pressure in the bell jar by adjusting the size of the outlet of the generator.

The temperature of the substrate 18 is adjusted to obtain the maximum 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 temperature 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 S2 in Fig. 2, these units can be replaced by an ultraviolet light source 84 shown in Fig. 3, 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. 2 and 3, the band gap adjusting elements can be added in gaseous form in an iden52207 tical fashion to the fluorine and hydrogen by replacing the hydrogen generator 62 or by adding one or more activated adjusting element generators 86 and B8 (Fig. 3). As previously mentioned, the amount of the adjusting element or elements introduced into the system is varied during deposition to provide the graded band gap. Each of the generators 86 and ,88 typically will be dedicated to one of the adjusting elements such as germanium or carbon. For example, the generator Θ6 could supply germanium as in the form of germane gas (GeH^) or carbon in the form of methane gas (CH4).

Referring now to Fig. 4, it illustrates additions to the equipment shown in Fig. 2 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 p-dopant 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 beam source 20 previously described. The rate at which the dopant S2207 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 evaporation rate is measured by a thickness sampling unit 96 upon which the dopant material deposits and which gen10 erates a signal on a cable 98 extending between the unit 9.6, 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 silicon 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-conductivity region therein.

The band adjusting element(s) also can be added by a similar process to that described for S2207 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, when 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 deposition rate and thickness sampling units, controls the thickness and composition of the depositing alloy.

As previously indicated, although it is preferred that compensating and other agents be incorporated into the amorphous alloy as it is de38 posited, 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. 5 and 6 which illustrate that the amorphous deposition process and the compensating or altering agent diffusion'process are carried out as separate steps in completely different environments. Fig. 6 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. 53207 This opening 104 is closed by a cap 106 having' threads 100 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 semiconductor alloy 118 has already been deposited is placed on the electrode 112. The upper face of the substrate 116 contains the amorphous alloy 118 to be altered or compensated in the manner now to be described.

Spaced above the substrate 116 is an elec15 trode 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. 6 illustrates only molecular hydrogen being fed into the chamber 102 by an inlet conduit 128 passing through the cap 106 and ex25 tending from a supply tank 130 of molecular hydro40 gen. Other compensating or altering gases (such as fluorine 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 rate 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 heating 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 altering of the 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 between 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. 2. 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. 7 through 12. Fig. 7 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 properties, and the ability of making an ohmic contact with an amorphous 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 silicon, 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 of preferably no more than 1016 per cubic cen10 timeter per eV. 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 re15 sistance region 150 which is an intrinsic, but low n-conductivity region having a graded band gap as previously described.

The upper surface of the amotphous alloy 146 as viewed in Fig. 7, 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, pro52207 duced, 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 grid 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 may have a thickness on the order of magnitude of the wavelength of the maximum 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 in varying amounts during the depostion thereof. The Schottky barrier 154 formed at the interface between the regions 150 and 152 enables the 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. 7, 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. 8 as well as in Figs. 11 and 12.

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 silicon, 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 at least to regions 170 and/or 172 for grading the band gap.

Referring now to Fig. 9, 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 graded and compensated or altered in accordance with the invention, has no p-n junctions as in the embodiment shown in Fig. 6 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 to grade the band gap thereof for better sensitivity.

Referring to Fig. 10 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 to grade the band gap to improve the sensitivity.

As used herein, the terms compensating agents or materials and altering agents, elements or materials mean materials which are incorporated in the amorphous alloy for altering or changing the structure thereof, such as, activated fluorine (and hydrogen) incorporated in the amorphous alloy containing silicon to form an amorphous silicon/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 silicon 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 silicon atoms and their relationships in the amorphous alloy. This is true most particularly because of the extreme electron48 egativity, specificity, small size and reactivity of fluorine all of which characteristics help influence and 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 importance. The ability of fluorine to bond with both silicon and hydrogen results in the formation of new and superior alloys with a minimum of localized defect states in the energy gap. Hence, fluorine and hydrogen are introduced without substantial formation of other localized states in the energy gap to form the new a.lloys.

Referring now to Fig. 11, a p-i-n solar cell 198 is illustrated having a substrate 200 which may be glass or a flexible web formed from stainless steel or aluminum. The substrate 200 is of a width and length as desired and pfeferably 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 (SiOj) 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. 11 and 12 are illustrative of some p-i-n junction devices which can be manufactured utilizing the improved methods and materials of the invention. Each of the devices illustrated in Figs. 11 and 12 has an alloy body having an over52207 all 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 ab5 sorption 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. 7 through 12 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 2Γ0 deposited as the final semiconductor layer. The alloy layers 206, 208 and 210 form the active layers of the ni-p device 198.

While each of the devices illustrated in Figs. 11 and 12 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 a graded wavelength threshold for a solar photoresponse, high light absorption, low dark conductivity and high photoconductivity including sufficient and varying amounts of the adjusting element(s, to grade 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 206 is preferably between about 3,000 angstroms to 30,000 angstroms. The thickness of the top p* contact layer 210 also is pref20 erably 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. 12. In this device a first p+ layer 214 is deposited on the electrode layer 204 followed by an intrinsic amorphous alloy layer 216 containing the band gap adjusting element(s) in the varied amounts to grade the band gap, an n amorphous alloy layer 218 and an outer n+ amor10 phous alloy layer 220. Further, although the intrinsic alloy layer 208 or 216 (in Figs. 11 and 12) is an amorphous alloy the other layers are not so restricted and may be polycrystalline, such as layer 214. (The inverse of the Figs. 11 and 12 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 (transparent conductive oxide) is added which, for example, may be indium tin oxide (ITO), cadmium stannate (Cd2SnO4), or doped tin oxide (SnO2>· 53307 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 de10 vice 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. 13. 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 deposited. A control unit 228 is utilized to control the deposition parameters, such as pressure, flow rates, etc., in a manner similar to that 53207 previously described with respect to the unit 24 (Fig. 1). The pressure would be maintained at about IO-3 torr or less.

One or more reaction gas conduits, such as 5 230 and 232, can be utilized to supply gases such as silicon tetrafluoride (SiF^ and hydrogen IH2> 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 temperature by heater means as previously described.

The band adjusting element(s) and silicon 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 silicon. The elements in boats 242 and 244 can be evaporated by electron beam or other heating means and are activated by the plasma.

A shutter 246 can be utilized to vary amounts of the band adjusting element(s) deposited in the photogenerating region of the film for grading the band gap. The speed of rotation of the shutter 52307 could be selectively controlled to vary the amount of band adjusting element deposited in grading the film. Thus, the band adjusting element(s) can be added discretely in stepped or in continuously varying amounts.

Fig. 14 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. Crystalline silicon has an indirect band gap of about 1.1 to 1.2 eV, which corresponds to the wavelength of about 1.0 micrometer (microns). This equates to losing, i.e. not generating useful photons, for substantially all the light wavelengths above 1.0 microns. As utilized herein, band gap or E optical is defined as the extrapolated intercept of a plot of (a hv)l/2, where a is the-absorption coefficient and hv (ore) 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. 2 2 Ο *7 Another photoresponsive application is for laser wavelengths such as for infrared response.

A photoresponsive material used in a high speed electrophotographic computer output device util5 izing a laser, such as a helium neon laser, should have a wavelength threshold about .6 microns. For fiber optic use, such as with GaAs lasers, the photoresponsive material threshold should be about one micron or below. The addition of the band gap adjusting element(s) of the invention allows the tailoring of alloys having a graded 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 procSss. 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 tetrafluoride (S1F4), hydrogen (Hj) and germane (GeHf). The glow discharge plasma preferably is obtained from the gas mixture. The deposition system in Patent No. 4,226,698 preferably is oper5 ated 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 utilized. 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. 53207 The doping concentrations are varied to pror duce the desired p, p , n or n type conductivity 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 conventional dopants· 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.

To produce fluorine and hydrogen compensated glow discharge deposited alloys, a mixture of silicon tetrafluoride and hydrogen may be deposited as an amorphous compensated alloy material at or below about 400°C, for the n-type layer. The band gap graded in59 53307 trinsic 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 GeH4 + Ar + SiF4 + H2 having relative percentage of GeH4 to SiF4 in a varying range of .001 to II produces photoresponsive. alloys having the graded band gaps without losing the desired electronic properties. The mixture has a ratio of SiF4 to H2 of 4 to 1 to 10 to 1. The amount of the grading or adjusting element(s), here germanium, in the resulting alloy is much greater than the gas percentages and can be significantly above twenty per cent depending upon the application. Argon is useful as a dilutent but is not essential to the process of the invention.

Further, although the band gap adjusting element(s) are added at least to grade the photoresponsive region of the devices, the element(s) also can have utility 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 (44)

1. CLAIMS;

1. A method of making an Improved photoresponsive amorphous alloy, said method comprising depositing on a substrate a material including at least silicon and incorporating in said material at least one density of states reducing element, said element being fluorine, introducing at least one band gap adjusting element into said material in varying amounts without substantially increasing the states in the band gap and varying the composition of the deposition gas mixture introduced into the deposition chambers to produce an alloy havino a graded band gap with a specified range of photoresponse wavelength function.

2. The method according to claim 1 wherein said adjusting element is one of the group consisting of germanium, t'in, carbon or nitrogen.

3. The method according to claim 1 or 2 wherein said alloy is glow discharge deposited from at least a mixture of SiF^, Hj and GeH^.

4. The method according to claim 3 wherein said mixture includes up to one percent GeH^.

5. The method according to claim 3 or 4 wherein said mixture of SiF, and E_ has a ratio of 4 to 1 to 10 4 2 to 1.

6. The method according to any one of claims 1 to 5 wherein said alloy is deposited with an active photoresponsive region therein and said adjusting element is introduced at least in said region.

7. The method according to any one of claims 5 1 to 6 wherein said method forms one step in a multi-step process for forming successively deposited alloy layers of opposite (p and n) conductivity type,, the n-type layer being formed by introducing during the deposition of the layer an 10 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 the layer a p-dopant element which is deposited with the deposited layer to produce a 15 p-type layer .

8. The method according to claim 7 wherein there is deposited between said p and n doped layers an intrinsic amorphous alloy layer without a ρ or n dopant element present therein, at least 20 a portion of said intrinsic layer containing said adjusting element.

9. The method according to any one of claims 1 to 8 further including introducing a second density of states reducing element, said second 25 element being hydrogen. 53807

10. The method according to claim 9 wherein both said density of states reducing elements are incorporated into said, depositing alloy substantially simultaneously with said band gap adjusting 5 element.

11. The method according to any one of claims 1 to 10 wherein said reducing element is incorporated into said alloy after deposition thereof.

12. The method according to any one of claims 10 1 to 11 wherein said adjusting element is introduced into said alloy in substantially discrete layers.

13. The method according to any one of claims 1 to 12 including evaporating said adjusting element prior to introducing it into said alloy.

14. 15 14. The method according to any one Of claims 1 to 13 including plasma activating said adjusting element as it is being introduced into said alloy. 15. The method according to any one of claims 1 to 14 including activating said adjusting element 20 by plasma activated vapor deposition.

15. 16. The method according to any one of claims 1 to 6 or 8 to 15 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 alloy.

16. 17. An amorphous alloy made according to any one of the processes of claims 1 to 16. 5

17. 18. An improved photoresponsive amorphous alloy, said alloy including silicon and incorporating at least one density of states reducing element therein, said element being fluorine, said alloy having a band gap adjusting element incorporated therein in varying j_q amountswithout substantially increasing the states in the gap, the amount of silicon and fluorine varying relative to the amount of the band gap adjusting element incorporated in said alloy, and said alloy having a graded band gap with a specified range cf photo15 respone wavelength function.

18. 19. The alloy according to claim 18 wherein said adjusting element is one of the group consisting of germanium, tin, carbon or nitrogen.

19. 20. The alloy according to claim 18 or 19 20 wherein said alloy has an active photoresponsive region therein and said adjusting element is included at least in said region.

20. 21. The alloy according to any one of claims 18 to 20 wherein said alloy is a multi-layer alloy 25 of successively deposited layers of opposite (p 53207 and n) conductivity type, the n-type layer including an n-dopant 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-typelayer.

21. 22. The alloy according to cliam 21 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.

22. 23. The alloy according to any one of claims 18 to 22 further including a second density of states reducing element incorporated therein, said element being hydrogen.

23. 24. The alloy according to any one of claims 18 to 23 deposited by glow discharge deposition.

24. 25. The alloy according to any one of claims 18 to 24 including said adjusting element in substantially discrete layers.

25. 26. The alloy according to any one of claims 18 to 20 or 22 to 25 including at least one of an n or p conductivity portion therein, said portion including an n or p dopant element therein.

26. 27. An improved photoresponsive device, said 53207 device comprising superimposed layers of various materials including an amorphous semiconductor alloy body having an active photoresponsive region including a graded band gap therein upon which 5 radiation can impinge to produce charge carriers, said amorphous alloy including at least one density of states reducing element, said element being fluorine, said alloy further including a band gap adjusting element in varying amounts therein at 10 least in said photoresponsive region to enhance the radiation absorption thereof without substantially increasing the states in the gap, the amount of silicon and fluorine varying relative to the amount of the band gap adjusting element 15 incorporated in said alloy, and the band gap of said alloy being graded for a specified range of photoresponse wavelength functions.

27. 28. The device according to claim 27 wherein the band gap-of said photoresponsive region is 20 less than 1.6 eV.

28. 29. The device according to claim 27 or 28 wherein said adjusting element is one of the group consisting of germanium, tin, carbon or nitrogen.

29. 30. The device according to any one of claims 53207 27 to 29 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 5 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 the layer a p-dopant element which is deposited with the deposited layer to produce a p-type 10 layer.

30. 31. Tbe device according to claim 30 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 15 of said intrinsic layer containing said adjusting element.

31. 32. The device according to any one of claims 27 to 31 further including a second density of states reducing element incorporated therein, said 20 element being hydrogen.

32. 33. The device according to any one of claims 27 to 32 deposited by glow discharge deposition.

33. 34. The device according to any one of claims 27 to 33 wherein said alloy body includes said 25 adjusting element in substantially discrete layers. 53307

34. 35. The device according to any one of claims 27 to 29 or 31 to 34 wherein said alloy body includes at least one of an n or p conductivity region therein, said region including an n or p dopant element therein. 5

35. 36. The device according to any one of claims 27 to 29 or 31 to 34 wherein said alloy body forms part of a Schottky barrier solar cell.

36. 37. The device according to any one of claims 27 to 29 or 31 to 34 wherein said alloy body forms 10 part of an MIS solar.cell.

37. 38. The device according to any one of claims 27 to 30 or 31 to 35 wherein said alloy body forms part of a p-n junction device.

38. 39. The device according to any one of claims 15 27 to 35 wherein said alloy body forms part of a p-i-n device.

39. 40. The device according to any one of claims 27 to 35 wherein said alloy body forms part of a photo-detector. 20

40. 41. The device according to any one of claims 27 to 35 wherein said alloy body forms part of an electrostatic image producing device.

41. 42. A method as claimed in claim 1 of making an improved photoresponsive amorphous alloy, substantially as hereinbefore 25 described.

42. 43. An improved photoresponsive amorphous alloy as claimed in claim 18, substantially as hereinbefore described.

43.

44. An improved photoresponsive device as claimed in claim 27, substantially as hereinbefore described. F.R. KELLY & CO., ASMS FOR THE APPLICANTS.