BANDGAP CONTROL IN AMORPHOUS SEMICONDUCTORS

Abstract: Method of producing amorphous semiconductor hydrides (hydrogenated amorphous semiconductors) with specified bandgaps. The desired bandgap is achieved by controlling the temperature and partial pressure of higher order semiconductanes which are introduced into, for example, in a hotwall epitaxial reactor, to create a deposit on a substrate held in the reactor, said deposit being created by pyrolytic decomposition of said semiconductanes.

Description: BANDGftP OCWTROL IN AMORPHOUS SEMICONDUCTORS Background of the Invention This invention relates to the production of amor- * phous semiconductors having a specified bandgap, and, more particularly, to amorphous silicon having such a bandgap. Semiconductors have an extended region of disconti uity in their internal energy levels between their valence a conduction bands. This region of discontinuity is known as a "bandgap". It determines how a device made from the mater ial will respond to external stimuli. in order to realize the most efficient interchange or interaction between a device and its source of activation for example, between the solar spectrum and a photo respon¬ sive device, it is advantageous to match the bandgap of the device to that of the energizing spectrum. This procedure i analogous to the matching of electrical elements in order to obtain maximum energy transfer. Unfortunately, previously produced amorphous semi¬ conductors have been characterized by undesirable bandgaps. It has not been possible in the case of silicon, for example, to provide bandgaps that could achieve the most efficient use of the solar spectrum with devices made with the semiconduc¬ tors. An object of the invention is to achieve precision control over the bandσaus of amorphous silicon. Prior attempts have been made to produce amorphous silicon with desirable characteristics by hydrogenating the silicon. Examples are Strongin et al., U.S. patent 4,237,151 issued December 2 T 1980; Wies ann U.S. patent 4,237,150 issued December 2 T 1980; and Scott et al., "Deposition of a-Si:H by Homogeneous CVD" Journal de Physique, Collogue C4, supplinent au n°/0 7 Tome 42, Octobee 1981 (427/86). These examples, however, suffer from the disadvantage of requiring the use of relatively high temperatures. Accordingly, it is another object of the invention to achieve the production of hydrogenated amorphous silicon at comparatively low temperatures. _OMPI Summary of the Invention In accomplishing the foregoing and related object the invention provides a method of preparing an amorphous semiconductor with a specified bandgap by the pyrolytic decomposition of one or more gaseous phase polysemiconduc¬ tanes at a specified temperature for each desired bandgap. In accordance with one aspect of the invention, t pyrolytic decomposition can be of a mixture of semiconducta including a monosemiconductane and one or more polysemicond tanes. In accordance with another aspect of the inventio the pyrolytic decomposition forms a deposit over a limited range of temperature within the total range of temperature ■ over which the deposition can take place. In accordance with a further aspect of the inven- tion, the deposition takes place in a reactor which is main tained at a different temperature than the average tempera¬ ture of the deposit that is formed. The deposit is desirab made on a substrate within the reactor, and the deposition temperature is substantially that of the substrate. In accordance with yet another aspect of the inve tion, the temperature of the reactor is maintained at a different level than the temperature of the substrate. The reactor may be maintained at a temperature which is greater than or less than that of the reaction, with a difference in the two temperatures being maintained over the time of the reaction. The reaction desirably takes place in the temperature range between about 300°C and 450°C. The reac¬ tion temperature can be permitted to vary within a prescrib range, which can vary between about 5 percent above and about 5 percent below the average temperatue of the reactio In accordance with still another aspect of the invention, the semiconductanes which are pyrolytically deco posed can be diluted with an inert gas, such as argon, heli neon, xenon, or hydrogen. The semiconductanes are desirabl CMΠ selected from the class of silanes., germanes and mixtures o the two. The gas from which the deposit takes place can be in the form of a stream which flows continuously into and out of the reactor. The gas may be made to directly imping on a heated substrate by, for example, the use of a direc¬ tive nozzle. Alternatively, the gas may be static within the reactor. In accordance with a still further aspect of the invention, the gaseous medium from which the deposit takes place may include a suitable dopant. Appropriate dopants are the trihydrides or trihalides of phosphorus f arsenic an tin, as well as diborane. Organometallic compounds of ele¬ ments such as aluminum and gallium may also be employed. OM?I Description of the Drawings Other aspect of the invention will become apparen after considering several illustrative embodiments taken in conjunction with the drawings in which: FIGURE 1 is a flow chart of a method in accordanc with the invention for preparing amorphous silicon with a specified bandgap; FIGURE 2 is a schematic diagram of an illustra¬ tive reaction chamber for preparing amorphous silicon in accordance with the invention; FIGURE 3 is an infra-red spectrum indicating the presence of hydrogen in amorphous silicon produced with a specified bandgap; FIGURE 4 is a schematic diagram of an illustra- tive hot-wall epitaxial reaction chamber for preparing amorphous semiconductors in accordance with the invention; FIGURE 5 is a mass-spectrometer plot of an illus¬ trative gas mixture for preparing amorphous semiconductors in accordance with the invention; FIGURE 6 is a set of parametric curves showing absorption data for amorphous semiconductors with controlle bandgaps in accordance with the invention; FIGURE 7 is an absorption graph for films deposit in a cold-wall reactor and for a hot-wall reactor; FIGURE 8 is an infrared spectrum for an amorphous silicon hydride film deposited on a crystalline silicon substrate within a hot-wall reactor; FIGURE 9 is a plot of conductivity for doped deposits-in accordance with the invention as a function of temperature; and FIGURE 10 is a plot of conductivity for undoped deposits in accordance with the invention as a function of temperature. OMH Detailed Description With reference to the drawings, a flow chart 100 for the general practice of the invention is set forth in FIGURE 1. The process begins with the introduction of a gaseous phase mixture containing at least one higher order silane .into a reaction chamber, as illustrated in * block 101. In the reaction chamber the gaseous phase mixture has its desired temperature and/or pressure controlled in accordance with block 102. The result, as indicated in:■ block 103, is the pyrolytic decomposition of the gaseous phase mixture to deposit amorphous silicon having a speci¬ fied bandgap. It will be understood that the process is egually applicable to other semiconductanes, such as germanes, with the deposition temperature being adjusted to be suitable for the material being pyrόlytically decomposed." A similar consideration applies to the pressure that is employed. An illustrative arrangement 250 for producing semiconductors wi'th controlled bandgaps is illustrated in FIGURE 2. The semiconductanes to be converted, including higher order silanes, are prepared in a preliminary section 210 (not shown) and introduced at a mixing valve 251. The latter permits the controlled introduction either simultane¬ ously or at desired intervals of time, of dopant gases from source 252 and/or 255. The result is a gaseous stream which is applied at an inlet 263 of a reaction chamber 260. The incoming gaseous stream, indicated by the arrow G, is drawn over the substrate 262 by the action of a vacuum pump 280. In addition, the substrate 262 is subjected to controlled heating by a heater 2664 in a holder 266. A current is applied to the heater 266r from an adjustable source 290. The temperature of the substrate is monitored by a guage 291 which illustratively extends to the substrate 262. A further guage 264 for the vacuum pump 280 gives an indica- tion of the pressure within the reaction chamber 260. OMPI As the incoming gaseous phase mixture G is drawn over the heated substrate 262 it pyrolytically decomposes and forms an amorphous semiconductor on the upper surface of the substrate, as indicated by the dashed line arrows D. The portion of the gaseous phase mixture that does not participate in the pyrolytic decomposition is drawn from the chamber into the vacuum pump 280 as exhaust gase E. With the pressure indicated by the pressure guage 270 held at a fixed level by constant operation of the vacuu pump 280, and with the substrate maintained at a prescribed temperature T as indicated by the temperature guage 291, the semiconductor material that becomes pyrolytically de¬ posited on the su state has a prescribed bandgap. As the substrate temperature in changed and held constant, the bandgap of the deposited material changes accordingly. The control that is achieved in accordance with the invention over the bandgap appears to result from the changes that are controllably made in the amount of hydrogen bonded to the semiconductane atoms of the deposit. While it has been known that many amorphous semiconductors, such as those produced by gaseous discharge, have hydrogen compen sated defects because of ion bombardment, it has been believ that amorphous films produced by pyrolytic. decomposition contained little hydrogen bonding. It was further believed that at low pressure levels where there was virtually no ion bombardment in the pyrolytic process, there was no hydro gen bonding in the resulting amorphous silicon. Our tests of the infra-red spectrum of pyrolytic amorphous silicon have indicated the presence of slight amounts of hydrogen. We believe that this is only necessary to compensate for certain of the amorphous bonds that are created in the pyrolytic decomposition process. In other words, because of the amorphous nature of the resulting material an occasional hydrogen bond appears to be created because of the need to compensate for structural irregular- ities in the resultant materials. In any event, our specu lation as to the nature of the hydrogen bonding created in amorphous semiconductors for e'd by pyrolytic decompositi appears to be confirmed by the illustrative infra-red spec trum of FIGURE 3, in which the percentage transmittance is plotted against wave numbers in reciprocal centimeters. The distinct absorption at 1978 wave numbers is characteris tic of silicon hydride (Si:H) stretched bonding in which the bond between Si and H obtained is subject to linear oscillatory extensions. FIGURE 3 is a computer print in which the compute was programmed to note any significant absorptions. FIGURE 3 shows significant absorptions at 2355, 1682 and 1104 wave numbers, in addition to the significant silicon hydride absorption at 1978 wave numbers. By contrast with amorphous semiconductors produce for example, by techniques which cause a significant amount of ion bombardment and a substantial number of defects • which require compensation, the relative absence of such defects in the case of pyrolytically decomposed semiconduc¬ tanes results in bandgap control by changing the amounts of hydrogen, and not defect compensation. We have found that control over temperature during pyrolytic decompositio allows the resulting material to retain its photovoltaic properties. This is significantly different than, for exam glow discharge amorphous silicon in which an increased temp ture results in the reduction in the desired photoelectric effect. Onemeasure of the improved performance of materia with controlled bandgaps in accordance with the invention is that as temperature is increased we have been able to measure both a reduction in bandgap and an increase in short circuit current. We have speculated that this results becau an increase .in temperature leads to greater coherence and ordering in the semiconductor structure, requir.ing a reduced 1 amount of hydrogen for defect, compensation. This is opposit to what has been observed for amorphous silicon produced in other ways, such as glow discharge, in which the excessive defect condition caused by ion bombardment appears to preven 5 the material from achieving greater coherence as temperature is increased. An illustrative reaction chamber 100 for producing semiconductors with controlled bandgaps is illustrated .in FIGURE 4. The reaction chamber 100 is used in a process 10 that begins with the introduction of a gaseous medium con¬ taining at least one higher order semiconductane. In the reaction chamber 100 the gaseous medium is controlled in temperature and/or pressure to being about a pyrolytic depos of semiconductive material having a specified bandgap. 15 It will be understood that the process is equally applicable to other semiconductanes, such as germanes, with the deposition temperature being adjusted according to the material that is being pyrolytically decomposed. Similar adjustments are made in the pressure that is employed. 20. The reactor..100 is of the hot-wall epitaxial type, i.e., the decomposition of gases takes place at the wall of the reactor, and a deposit is formed on the substrate 130. The wall or closure 110 of the reactor 100, typically of quartz, is surrounded over its principal cylindrical 25 body portion by a heating coil 120. The latter is used to apply heat directly to the wall 110 and maintain it at a prescribed temperature level. This is by contrast with the similar reactor 250 shown in FIGURE 2. The heating effect from the coil 120 is radiated 30 from the wall 110 onto the substrate 130. Where desired the substrate may have an additional heating effect produced by an internal heating coil 132, or a cooling effect by an internal coolant pipe, " which is substituted for the coil 132. 35 The temperature of the wall is monitored by a OMH thermocouple 111, while the temperature of the substate is monitored by a thermobouple 131. semiconductanes that are to be pyrolytically decomposed in accordance with the invention are introduced into the reactor 100 at an inlet 140 through an appropriate mixing valve 150. The gaseous medium is exhausted through a vacuum pump 160 through an outlet valve 170. Pressure within the chamber 100 is moni¬ tored by a guage 180. The pyrolytic deposit formed : on the substrate 130 may be produced by either static or dynamic action of the σaseous medium. In static deposition the semiconductan are introduced into the evacuated reactor 100 through the valve 150. The exhaust valve 170 and the inlet valve 150 are then shut, causing a specified volume of gaseous medium to be trapped in the chamber 100. Because of the heating effect of the coil 120, the trapped gas decomposed on both the wall 110 and the substrate 130. The result is a desired amorphous silicon hydride film having a specified bandgap. The static system has the advantage of economizing on the volume and pumping of the gaseous medium. In the dynamic processing arrangement, the gaseous medium flows continuously through the chamber 100 at a desir rate determined by the settings of the inlet valve 150 and the exhaust valve 170. As a result a new reaction mixture is continuously introduced into the chamber 100 and all components of the gaseous medium maintain their relative partial pressures. The dynamic arrangement, however, has the disadvantage of some waste of the gaseous medium since a considerable amount of the gas is exhausted through the vacuum pump 160 without decomposing to form the semiconducto deposit. An illustrative gaseous medium has the mass-spectr meter plot of FIGURE 5. As noted from FIGURE 5 the medium is a mixture of monosilanes, disilanes and trisilanes. The monosilanes are indicated by peaks between mass numbers OMPI 28 and 34. The disilanes are indicated by peaks between mas numbers 56 and 64, while the trisil ' anes have peaks between mass numbers 84 and 93. The mass-spectrometer plot of FIGUR 5 also indicates the absence of moisture and hydrogen chlori For example, if moisture were present, it would be indicated by peaks in the vicinity of mass number 18. As is apparent, there is no indication at that particular mass number positi Since disilanes, e.g. the gaseous components peaking between mass numbers 56 and 64 are ordinarily accompanied by signifi cant amounts of moisture and hydrogen chloride, the mixture of FIGURE 5 results from the preparation of the higher order silanes under conditions which limit the creation of impurit or result from trapping of-the impurities in standard cold traps. When the gaseous medium of FIGURE 5 is introduced into the hot-wall reactor of FIGURE 4 and controlled pyrolyt decomposition takes place at three different substrate tem¬ peratures, the results are as plotted in FIGURE 6, where the square root of the absorption coefficient (alpha) t.imes energy in electron volts (E) is plotted against energy. The illustrative curves T. through T where T represents the relatively fixed temperature at which pyrolytic decompositio takes place, are shifted in the direction of lower energy, corresponding to the bandgap shift, as the temperature of the controlled deposition takes place. The curve T. for a temperature of 37.5°C. , the curve T, for a temperature of 425°C, and the curve T., for a temperature of 450°C. , all have essentially the same slope measured in terms of the ration of the change in the square root of the absorptio coefficient times energy to the change in energy. This is significantly different from experience with glow dischar and other materials in which a significant decrease in energ gap is accompanied by a shallower slope and hence a reduced photovoltaic effect. The distinct variations in energy gap for the different temperatures of pyrolytic decompositio is attributed to the different concentrations of hydrogen that result in the deposits. ' It is to be noted that for the plots of FIGURE 6 the various readings are indicated by dots to represent absorption data for films deposted using a heated wall reactor. Substantially similar results are obtained for operations using a cold-wall reactor as indicated in FIGUR 7 where the data for cold-wall operation is represented by crosses. In all cases, the temperature at which the pyrolytic decomposition takes place is about 425°C. for the substrate where the deposit forms. Both hot-wall and cold-wall deposition produce an absorption curve with a relatively constant slope. The test results indicate that the controlling factor in determining the bandgap of the deposited material is that the substrate temperature be the same whether the reaction chamber is of the cold-wall or the hot-wall type. The substrate temperature should be maintained within ten degrees above and below the nominal value of the desired bandgap. An infra-red spectrum of an amorphous silicon hydride film deposited in the reactor of FIGURE 4 on a crystalline silicon substrate is shown in FIGURE 8. The spectrum has a characteristic silicon hydride stretch bond at 1980 wave numbers(reciprocal centimeters. It is to be noted that other absorption bands centered at 1170 and 750 wave numbers are attributable to suitable oxide bonding in the crystalline silicon substrate and consequently play no role in the silicon hydride that produces bandgap con- trol according to the invention. OMFI ,/ ' 1PO FIGURE 8 also shows a minor absorption peak at 870 wave numbers. This is likely to be substrate absorption In any event, even if it could be associated with silicon- dihydrogen bonding, its effect would be too insignificant to adversely influence the silicon hydride that is formed. The invention is illustrated further with referenc to the following non-limiting examples: EXAMPLE I A mixture of semiconductanes including monosilanes disilanes and trisilanes was prepared. The mixture was subjected to pyrolytic decomposition in a heated-wall reacti chamber to form a semiconductor film on a silicon crystallin substrate at a temperature of about 425°C. and a pressure of 40 Torrs. The body of the reaction chamber was maintaine at a fixed temperature above that of the substrate. An epitaxial film of amorphous silicon was desposited on the substrate and found to have a bandgap in the range from about 1.58 to about 1.60 electron volts. EXAMPLE II Example I was repeated except that the reaction chamber was maintained at a relatively constant temperature below that of the substate. The result was substantially the same as the result for Example I. EXAMPLE III Example I was repeated except that the reaction chamber was unheated, and was of the cold-wall type. The results were the same as for Example I. EXAMPLE IV The preceeding examples were repeated except that the substrate temperature was permited to vary in the range from about 2 percent above and 2 percent below the reaction temperature. The result was substantially the same as for Example I. EXAMPLE V The foregoing examples were repeated except that CM?I inert gases were used as diluents of the gaseous medium. The results were substantially the..same as before, except that the growth rates of the resulting film were lower. EXAMPLE VI The foregoing examples were repeated except that n-type dopant gases diluted in argon or hydrogen were intr duced into the gases medium. The results were substantial the same as before, except that the reaction rate was slow EXAMPLE VII The foregoing examples were repeated using a sta gaseous medium in the reaction chamber. The results were substantially the same as for the preceeding examples. EXAMPLE VIII The foregoing examples were repeated varying the flow rate through the reaction chamber. * The results were substantially the same as before. EX-AMPLE IX The foregoing examples were repeated except that the substrate temperature was increased to about 475°C. and held at substantailly that level during pyrolytic decom position. The resulting amorphous silicon deposit was foun to have areduced bandgap in the range from about 1.48 to about 1.50 electron volts. EXAMPLE X ' The foregoing examples were repeated except that the temperature of pyrolytic decomposition was decreased to 375°C. The result was a substantial increase in bandgap to the level of about 1.63 to about 1.65 electron volts. As noted above in connection with Example V the presence of monosilanes in the gaseous medium appeared to have only the effect of slowing the rate of pyrolysis. In any case, there was no adverse effect on the amorphous sili con hydride film that resulted. It is believed that the monosilanes do not participate in the decomposition since they do not decompose substantially at the test temperature in the range from about 300°C to about 475°C. It is apparent that if the composition is successfully maintained at higher temperatures where monosilanes do decompose, their presence will add to the resulting film deposit. In addition, disilan is known to decompose into silane and silane-dihydride, so that the presence of independent silanes merely supplements the silanes that are produced by the decomposition of higher order silanes, and may slow the reaction down somewhat. In connection with Example V, it has been noted that the addition of inert gases such as argon only affect the growth rate of the pyrolytically produced films. This comes about because there is a reduction in the partial pressure of the higher order silanes for a given total pres¬ sure. Other inert gases such as helium, nenon, xenon and hydrogen have a similar effect. In connection with the introduction of dopants into the gaseous medium as in Example VI, FIGURES 9 and 10 compare the conductivities of n-type doped and undoped films as a function of temperature. In the case of FIGURE 9, the doped film was prepared using phosphine and the gaseou medium had the approximate composition of FIGURE 5, including monosilanes. As seen from FIGURE 10, the conductivity of an undoped film is comparatively low and the activation energy is comparatively high, indicating that the material is intrinsic or undoped. It has a Fermi level of approximate ly 0.61 electron volts below the conduction band and is in fact in the middle of the bandgap. By contrast, for the doped film of FIGURE 9, there is a comparatively smaller activation energy of 0.18 electron, volts, indicating that the Fermi level has moved towards the conduction band as a result of the doping. Other suitable dopants, in addition to phosphine, include stibine, arsine'and phosphorus tri¬ chloride for producing n-type material, and diborane, togethe with organometallic compounds of aluminum and gallium for producing p-type material. While various aspects of the invention have been set forth by the drawings and specification, it is to be understood that the foregoing detailed description is for illustration only and that various changes in parts, as well as the substitution of equivalent constituents for those shown and described may be made without departing from the spirit and scope of the invention as set forth in the appended claims. _ CMFI

Claims: 1 WHAT IS CLAIMED IS: 1. A method of producing an amorphous semiconduc tor with a specified bandgap which comprises the steps of (a) commencing the pyrolytic deposition of said 5 semiconductor at a first temperature which would produce a first bandgap; (b) adjusting the temperature of deposition to a second value and holding said second value temperature with a limited range to achieve a pyrolytic deposit having a 10 second prescribed bandgap. 2. A method of producing a semiconductor with a speci ied bandgap which comprises producing a semiconductor hydride and controlling the amount of hydrogen in said hydride by controlling the temperature thereof below about 15500°C. ' 3. The method in accordance with claim 2 wherein said semiconductor is selected from the class consisting of silicon or germanium, or an alloy thereof. 4. The method of claim 1 wherein the bandgap de- 20 creases as the midpoint of said limited range of temperatur is increased. 5. The method of claim 2 wherein the bandgap de¬ creases and the amount of hydrogen decreases as the tempera¬ ture increases. 25 6. The method of cla.im 1 wherein the limited ran has a variation of less than two percent about its midpoint value. 7- The method of claim 2 wherein said temperature is modulated about a specified value. 30 8. The method of claim 2 wherein said temperature is varied by no more than two percent about a specified tem¬ perature value. 9. The method of producing an amorphous semicon¬ ductor with a specified bandgap which comprises forming a 35pyrolytic deposit of said semiconductor over a limited range of temperature within the total range " of temperature \Y of about 300°C. to about 500°C over which the deposition can take place using a reactor which is maintained at a different temperature than the average temperature of said deposit. 10. The method of claim 9 wherein said deposit is made on a substrate within said reactor. 11. The method of claim 10 wherein the temperatu of said reactor is different than that of said substrate. 12. The method of claim 9 wherein said different temperature is that of the body of said reactor. 13. The method of claim 9 wherein the reactor temperature is greater than that of said reactor. 14. The method of claim 9 wherein the temperatur of said reaction is less than that of said reactor. 15. The method of claim 9 wherein said reaction takes place in the temperature range between about 300°C and about 475°C. 16. The method of claim 9 wherein the reaction temperature varies within a prescribed range. 17. The method of claim 16 wherein the range varies between about 2 percent above and -2 percent below the average temperature of the reaction. 18. The method of claim 9 wherein the amorphous semiconductor is formed by the pyrolytic decomposition of a mixture of semiconductanes including a monosemiconductane and/or one or more polysemiconductanes, and the amorphous semiconductor is selected from the class consisting of sili¬ con, germanium and alloys thereof.

Inventors: AKHTAR M, DALAL VIKRAM L, GAU SHEK-CHUNG, RAMAPRASAD KACKADASAM R, AKHTAR, M., DALAL, VIKRAM L., GAU, SHEK-CHUNG, RAMAPRASAD, KACKADASAM, R.
Applicants: CHRONAR CORP, CHRONAR CORPORATION
Application year: 1982
Main classification: H01L21
IPC codes: H01L21, H01L29, H01L31