CA1279471C - Gas mixtures for the vapor deposition of semiconductor material - Google Patents

Abstract

ABSTRACT

Precursor gaseous mixtures from which wide and narrow band gap semiconductor alloy material may be deposited by a glow discharge process, said material characterized by improved photoconductivity and stability. There is also disclosed a method of fabricating a narrow band gap semiconductor that avoids differential depletion of the components of the precursor gaseous mixture.

Description

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GAS J5IXTURES AND PROCESSE:S FOR
DE~OS I TI ON OF AMC)RPHOUS SEMI CONDUC'rORS

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BAC~GROUND OF THE INVENTION

Amorphous ~hin film semiconductor alloys hav~ gained acc~ptance a~ a u~eful ~aterial for fabricating electronic devices such ~5 photovoltaic oells, photore~ponslve and photoconductive dsvices, transistors, diodes, integrated circui~s, memory arrays and th~ like. Amorphous thin film s~miconductor alloys can be manufactured by relatively low cost continuou~ processes, pos~ess ~ wide range of controllable ele~trical, opti~al ~nd structural prop~r~ie~ and can be deposited over relatively large areas. Amorphous ~ilicon, germanium ~nd ~ilicon-germanium based alloys exhibit the greatest pr~sant Go~mercial ~ignific~nce.

These alloy~ can be deposited over large are~ by ostablishinq ~ glow d~scharg~ in a ~ixture of precursor gases that include the semiconductor ele~ents. Depletion o~ certain ele~ents is a ~ignificanS problem when thick (over approxim~tely 190 nanometers) l~yer~ of thesc alloys are deposited in ~ plasma over a l~rge ~rca. Depletion occurs when ~ome of the components of the precur~or gas mixtur~ decompo~e ~t greater rate than othQrs. Depl~tion r~sul~s in ~pa~ial lrr~gularities or inhomogeneities in th~ depo~ited ~iconductor alloy, producing layer~ of semiconductor alloy material h~ving non-uni~orm ~lEctrical, ch~ical ~nd op~ical properties. The proble~ of gas d~pletion is particularly ~ani~ested when a relatively thick (over 300 nano~t~rs) layer o~ n3rrow band gap silicvn:germanium alloy ~terial iG d~po~it~d f~o~ silane.and g~r~ano pr~cursor gaseg in apparatu~ ~mploying ~ continuou web sub~trate. G~rmane ga8 i8 d~co~pc~ed ~uch ~ore ~a~lly th~n ~ilane ~o that g@rmanium i8 dopo~it~d rom a germane-~ilane Qixture ~aster than 18 ~ilicon. D2plction i~ particul~rly signi~icAnt in a m$crowave ~nerg~z0d glow di~charg~ ~lnc~ th~ d~position r~tæ~ incr~a~e ~igni~icantly when microwave ~n~rgy powers ~he glow dl~chargeO

For example, in order to depo~it a semicondue~or ~lloy that is 60 per~ent ~illcon and 40 percent germanium, a ga~
mixture of silane and germane having far l~ss than 40 percent germane must b~ employed. Depl~tion rates ~re also af~ect~d by the amount ~ power ~upplied to the glow discharge, the geometry of th~ deposition appar~tus ~nd other f~ctor~. Th~ ~ppropri~t0 gas ~ixture is usually de~rmined by trial and error.
According to the prs6ent invention, in one aspect, there is provided a ga8~0u5 precursor mixture from which a silicon-based narrowed band gap semiconductor alloy may be deposited in a glow discharge decomposition proceas, said mixture comprising disilane and germane.

According to the present invention, in another aspect, there is provided a gaseous precursor mixture from which a silicon-based semiconductor alloy may be deposited in a glow discharge decomposition process, said mixture comprising disilane and a gaseous source of fluorine atoms.

According to this invention, in still another aspect, there is provided a method of fabricating a narrowed band gap silicon-based semiconductor alloy by the glow discharge decomposition of a proces~ gas mixture, said method comprising astablishing a glow di~charge in a gae mixture containing a silicon-containing compound and a germanium-containing compound decomposing at substantially similar rates.

The present invention further provides a m~thod of fabricating a silicon-based semiconductor alloy by the glow discharge decomposition of a process gas mixture, said method comprising establishing a glow discharge in a gas mixture including disilane and a gaseous source of fluorine atoms.

2 ( ~) In the invention, disilane, ~i2H6, as well 3~ other higher order ~ilane6 such as Si3H8, Si4H1o, and neubstituted"
higher order ~ilanes ~uch as fluorinated higher order 6ilanes, collec~ively referred to herein as polysilanes, ~re omployed with germane to eliminate problems oP gas depletion. Poly~ilanes decompose and deposit ~ilicon in a glow di~charg~ at a higher rate than does ~ilane or ~ilicon tetrafluoride. It is known th~t polysilan~s can be u~ed alone to increase the d~po~it~on r~te of amorphous ~ilicon. See U.S. Patent 4,363,828 tQ ~rodsky et al.
In ~ multi-component process gas mix~ure including a germa~ium-containing gas such as germane and a polysilane g~s ~uch as disilane, the germanium-containing gas and the silicon-containing gas deco~pose at ~pproximately ~imilar rates. Since the two gases decompose at approximately the same rate, the sillcon-germanium ratio in the gas mixture does not change throughout thP
deposition proce s~ ellminating differential depletion problems.

Small amountC of fluorine gas significantly affect the composition of the glow di~charge plasma ~separate ~rom any influ~nce the fluorine exert in or on the depositing film), resulting ln a ~ignificant improvement in the photoelectronic properties of the deposited semiconductor alloy m~terial.
Fluorine is believed to act as a ~plas~a catalyst~ facilitating, through its high electronegativity ~nd high reactivity, the formation and maintenance of desirable free radical and/or ionic species in the pl~ma, improving the propertie~ of the deposited material. In the Invention, the precursor ga~ ~ixture preferably includ~s a ~ource of fluorine to organize the pla6ma into desirable speci~s and produce improv~d ~emiconductor alloy ~aterial .

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A precursor gas mix~ure ac~ording ~o ~h0 invention includes a silicon-containinq gas ~uch as ~ilane or ~ polysilane and a ~luorine-containing gas ~uch as silicon tetrafluorid~, fluorosilanes (including fluoropolysilanes), fluorine, fluorocarbons, and, when doped alloys are being depo~ited, fluorinated dopant gases ~ueh as boron 1uorides, phosphorus fluorides and aluminum fluorides. The gas mixture ~y includs a germanium-eontaining gas if a n~rrow band 93p 6ilicon:germanium alloy is being prepar~d. One preferred mixture from which narrow band gap amorphous ~emiconductor material may be d~po it~d by glow discharge includes, by volume, approximat~ly 0.5 to 3.0 p~rts of the silicon-containing gas, 0.5 to 3.0 parts of the germanium-containing gas, ,~nd approxi~ately 3.0 to 15.0 parts of the diluent gas. Semiconduc~or ~lloy materials having a band gaps of approxi~tely 1.3 to 1.S5 eV may be depo~ited from thi6 mixture. A precursor gaseous mixture including a polysilane compound, such as disilane, and a ~ource of fluorine, produces a ~emicondu~tor alloy mat~rial having improved photoelectric properties. In a pre~srr~d eMbodiment of the i.nvention, the pr~cursor saseous ~ixture comprises by volume, approximately 0.5 to 3 part~ sf disilan~, ~pproxi~ately 0.05 to 0.5 parts of silicon .tetrafluoride ~nd approxi~ately 3.0 to 15.0 parts of hydrogen gas. ~luorine may be directly incorporated into the polysilane molecule it~elf. A polysilane precursor g3s formula $s SiXRyHzl where R is ~hosen from the group consis~ing essentlally of fluorine, chlorine, $odine, bromine and combinations of the~, x i~ an ~nteger greater than one, y and z are ~ach an integer, not ze;o, and y+z~72x+20 The fo egoing formula $nclud~s metastable co~pounds and species as well as stable molecules.

In ~Photoconductive a-SlGe:F:H and ~ransparent InOx for High ~f~ieiency Amorphous Solar Cells", publi~hed in the Technical Digest of the International PVS~C-l, Xobe, J~p~n (1984) pp. 429-432, Oda, et al. r~ported the fabrication o~ a narrow band gap silicon:g~r~anium:fluorin~:hydrogen alloy for photovoltaic u~ ro~ a ~ixture of ~ilicon tetr~fluoride, g~r~anium t~trafluorlde nnd hydrogen. V~rious combinations o~
ga~e6 w~re ex~mined es precur~or~ for the deposition of the a ~y, including ~ilane, di~ilane, silicon Setrafluoride, germane and germanium ~otrafluoride. ~t is reported that the be6t results were ob~ained throu~h ~he use of silicon tetrafluoride, germanium ~e~rafluoride and hydrogen, where the concentration of hydrogen is typically 30 percent. The Oda group repo~ted on deps~ltion only on ~mall ~r2~s whers depletion is n~t~a problem.
Oda~s best amorphou~ ~ilicon:~ermanium:fluorine:hydrogen alloy was reported to hav~ a photoconductivity of 9 x 10-5 ohms-centimeters -1, thres times that of similar material produced according to the invention. However, the Oda material has in~erior photovoltaic properties. Oda reported elsewhere that his narrow gap material discu~sed contained a high density of states adjac~nt the conduction band which reduces the lifetime of minority charge carriers and incr~a~es the r~combinat$on rate of charge carrier~ within the band gap. Material pr~pared according to the inv~ntion ha~ photovoltaic properties ar ~uperior to Oda's material.
In the drawings appended to the specification:
Figure l is a fragmentary, cro~s-sectional view o a tandem photovolt~ie device including a plurality of photovoltaic cells; and Figure 2 is a chart contrasting some properties of photovoltaic ~aterials deposited from variou~ precur~or gaseous mixtures.

Figure 1 show~ ~n n-i-p type photovoltaic device 10 ~ade up of individual n-$-p typ~ cells 12a, 12b and 12c. Below the low~rmo~t cell 12a is æ sub~trate 11 which ~ay be for~d of ~
tran6par2nt glass or synthetic poly~eric me~ber; rom a metallic material ~uch as stainless teel, aluminum, tantalum, molybdenum, chrome; or from met~llic particles embedded within an in ul~tor.

~ ach of the cells 12a, 12b and 12c is pr~f~rably fabrica~cd with a ~hin ~ semiconductor body containing at l~a6t ~ ~ilicon or ~ilicon:ger~aniu~ alloy. Each o the semiconductor bodies includes a p-type conductivity ~mlconductor layer 20~, 20b ~nd 20c; a sub~tantially in~rinsic ~e~iconductor layor 18~, 18b ~nd 18c; ~nd An n-~ype co~duc~ivity 6emico~ductor ~ 3~7~

layer 16a, 16b and 16c. Cell 12b is an intermediate cell and additional intermediate cells may be stacked on the illustrated cells. Similarly, the tandem cell may include only two cells. A
TCo (transparent conductive oxide) layer 22, prefe~ably formed of indium tin oxide, is disposed on p-layer 16c of cell 12c. An electrode grid 24 may be added to the device to increase current collection efficiency.

~ precursor gas mixture for depositing high quality silicon based (approximately 1.7 eV band gap) semiconductor alloys includes a silicon-containing gas, a fluorine-containing gas and a diluent gas. Such a gaseous mixture preferably includes approximately 0.5 to 3.0 parts by volume of the silicon-containing gas, which may be silane or a polyatomic silicon-containing species such as disilane or other higher order silane.
The fluorine-containing gas is present in approximately 0.01 to 3.0 parts by volume. A wide variety of fluorine-containing gases may be used. Silicon tetrafluoride has demonstra~ed particular utility; however, elemental or molecular fluorine, other fluorinated silicon compounds, fluorinated-boron compounds, fluorinated phosphorus compounds, fluoro-carbons and combinations of them may also be employed. The fluorine-containing gas may also contain silicon, i.e. fluorinated silane and polysilane compounds. A general formula for such silicon-containing gases is Si~RyHz, where x is an integer, R is chosen ~ro~ the group consisting essentially of halogens, y and z are integers, not zero, and y+z~2x+2. ~ diluent gas, such as hydrogen, controls the rate of deposition, adjusts the concentration of the depositing species, and facilitates the decomposition of the silicon-containing and/or fluorine-containing gases. Other gases such as nitrogen, helium, argon, neon and xenon may be used as diluents. These precursor gaseous mixtures pre~erably include approximately 3.0 to 30.0 parts of the diluent gas.

Dopant gases may also be included in the mixture.
Typical dopant gases such as, phosphine, diborane, phosphorus trifluoride, phosphorus pentafluoride, and boron trifluoride may be employed. It should be noted that the latter three dopant ~ases also function as a source of fluorine.

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The previously describea precursor gaseous mixture~ may also include a germanium-containing gas. The germanium-containing gas may be chosen ~rom germane, which is preferred, or germanium tetra luoride ~nd other 1uorinated germanium-containing compounds tha~ may provide ~ 60urce of flub~ine. Th~
germanium-containing gas is pre~erably pre6ent in the mixture in the same concentration range a~ the silicon-containln~ gas, from 0.5 to 3.0 parts by volume.

Another class of novel precursor gaseous mixtures include disilane or other higher order 6ilanes, as well as variously ~ubstituted analogues of th~m, in conjunction with a germanium-containing gas uch as germane. In one preferred precursor gaseous mixture, the ~ilicon-containing component is di~ilane and the germaniu~-containing component is germane. ~t ha~ been ~ound th~t these gases deco~pose at about the same r~tes in a glow disch~rge. Other ~ilicon-con~ining qa~eE such as trisilane or other higher order sil~nes ~ay also be advantageouPly employed. The precursor ga~eous mixture also typically includes ~ diluent gas ~uch a~ hydrogen or an inert gas.

The inclusion of a fluorine-containing gas in the mixture w~ll organize plasma conditions and assure the deposition o~ desir~d species ~rom the plasma. Fluorine may b~ provided from any o~ tha previously di~cussed gaseous source~. Generally sp~ak$ng, the pr~cursor gaseou~ mixture~ will includ~ 0.3 to 3.0 part~ by volu~e of the silicon-containing gas, 0.3 to 3.0 parts by volume of the germanium-cont~ining g~ and 3 to 30 parts by volume of the diluent gas. If fluorine is included, the fluor~ne-containing gas i~ present in 0.01 to 3.0 parts by volume. As previously discussed, the mixture m~y also include a dopant gas therein, which dopant may al50 serve as the source of fluorine.

The glow di~ch~rge deposition of ~emiconductor alloy terinl ~ay be carried out $n either a b~tch qr continuous mode of production. Sn a batch d~po~ltion proce~, a ~inqle d~po~ition ch~mber is ge~erally employed for the deposition of ' .

the material on a discrete substra~e. In the continuous mode of deposition, a semiconductor layer is deposited in each of a plurality of interconnected deposition chambers. The atmosphere in each chamber is isolated from the others to avoid contamination. ~n elongated web of substrate material is continuously advanced through the deposition chambers and a plurality of layers of doped or intrinsic semiconductor alloy material is successively deposited. The problem of differential depletion of various gaseous species is most signi~icant in continuous deposition apparatus.

The following experimental examples illustrate the advantages of the invention.

Example 1 A series of five layers of silicon:germanium alloy material having an optical band gap of 1.5 eV were deposited by the glow discharge deposition of the precursor gaseous mixtures in Table I. The photoconductives of each of the five layers is reported in the last column of the Table. The layers were deposited from a gas mixture created from gas sources having the nominal flow rates shown in columns 2-5 of Table 1.

Table I
Flow Rates Flow Rates (std. cubic cm.)Photoconductivity Sample Sl2H6 SiF4 GeH4 H2(ohm-cm)~
I. 5 0 5 501 X 10-5 II. 5 5 S 503 - 4 X 10-5 III. S 2.5 5 503 - 4 X 10-5 IV. 5 1.25 5 503 - 4 X 10-5 V. 5 .6 5 5~2 X 10-5 For these layers, no significant differences in activation energy between any of the samples was seen.
Therefore, for these layers, the measured photoconductivity is a good indication of the actual photovoltaic qu~lity of the particular samples ~ ~ ~ 7 ~

The photoconductivity of sample I versus sample II
increased by a factor of three to four as a result of the addition of a source of fluorine atoms to the gas mixture. The presence of the silicon tetrafluoride source gas did not significantly change the silicon content in the deposited film.
The sample II film had a fluorine cont0nt of only .07 percent (plus or minus .02 percent).

The amount of silicon tetrafluoride present in the precursor gas mixture was successively decreased by approximately one-ha'f, compared to the previous sample, in preparing samples III-V, No reduction in the enhanced photoconductivity is observed until there is almost a ten fold reduction in the amount of silicon tetrafluoride introduced into the precursor gas mixture. Even at that reduced level of fluorine (sample V), the photoconductivity is still twice that of non-fluorinated sample I. The amount of fluorine in sample V is too small to be detected. Clearly, even very small amounts of fluorine have a very significant effect on the photovoltaic quality of glow discharge-deposited silicon germanium films and alloy films.

Example 2 A single n-i-p type photovoltaic cell was formed including a 150 nanometer thick intrinsic layer deposited from a precursor gaseous mixture of 2 percent Si2H6, 96 percent H2 and 2 percent SiF4. The intrinsic layer was sandwiched between n- and p-doped silicon alloy layers 15 nanometers thick. ~11 layers were deposited in 13.56 MHz radio frequency energiæed glow discharge decomposition process. The efficiency of the photovoltaic cell was approximately 6.2 percent as l~easured under AM-1 illumination. The open circuit voltage was 9.24 volts and the fill factor was .730.

~ similarly prepared photovoltaic cell havinq an approximately 200 nanometer thick intrinsic layer and Pxhibiting an initial efficiency of approximately 8.4 percent was exposed to AM-l illumination to assess its photodeqradation. After 75 hours of illumination, a loss of only 5 percent of the initial operating efficiency occurred. This level of photodegradation compares with a typical loss in photoconversion efficiency exceeding 25 percent for similar photovoltaic cells deposited from a silane and hydrogen mixture that excludes fluorine.

Example 3 A precursor qaseous mixture was formed from nominal gaseous flows in standard cubic centimeters units of 5 disilane, 5 silicon tetrafluoride, 4 germane and 70 hydrogen, and subjected to a glow discharged at 13.56 MHz. A similar precursor mixture, devoid of fluorine, was used to deposit of a second layer of semiconductor alloy material. A third mixture of 10 silane, 5 silicon tetrafluoride, 1.5 germane and 70 hydrogen was used to deposit a third layer of semiconductor alloy material. The two mixtures containing disilane produced deposition rates of approximately 0.3-0.5 nanometers per second. The silane-containing mixture deposited an alloy at a rate of only 0.1-0.2 nanometers per second. No gas depletion problems were presented by either of the disilane mixtures, however, the gases in the silane-containing mixture produced differing deposition rates.

The photoconductivity of the sample deposited from the disilane-fluorine mixture was 3 x 10-5 ohms-centimeters~1. The photoconductivities of the disilane and the fluorinated silane prepared samples were only 1 x 10-5 ohms-centimeters~l. The sub-band gap absorption of the latter two samples was approximately 3-4 times higher than for the sample prepared from the disilane-fluorine mixture. Approximately 10-15 percent hydrogen was present in all three samples. Fluorine was present in the disilane-fluorine prepared sample and the silane-fluorine prepared sample at no more than Ool percent. The sample prepared from the disilane-fluorine mixture included approximately 40 percent germanium, the largest amount reported in a silicon alloy film without detrimentally affecting electronic properties.

Example 4 .
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The table of Figure 2 illustrates the relationship of gas precursor mixture to characteristics of silicon:germanium alloys deposited from the mixtures. The headings on the columns list the components o~ the mixtures used to deposit the films having the characteristics shown in the table. For each film deposited, photoconductivity, dark conductivity, band gap, activation energy, sub-band gap absorption (which is a measure of the density of tail states in the band gap), and deposition rate of the film is listed. ~he photodegradation of n-i-p photovoltaic cells into which the layers of semiconductor alloy material prepared from the precursor mixtures are incorporated is also listed. Trace amounts of boron were included in the two silane-produced samples (columns 1 and 4) to manufacture photovoltaic devices that exhibited an acceptable level of photoconductivity. In the absence of boron doping, the photoconductivity was too low and the defect densi~y ~oo high in these films to manufacture a photovoltaic device having measurable electronic properties.

The first group of samples, i.e., the results reported in the first column of Figure 2, was prepared from a precursor gaseous mixture including 10 sccm (standard cubic centimeters) of hydrogen, 1 sccm of germane, 8 sccm of silane and 3 sccm of a mixture of 500 ppm of diborane in hydrogen (where used). The second group, i.e. second column, of samples were prepared from a precursor gaseous mixture including 32 sccm of hydrogen, 2 sccm of germane, and 2.5 sccm of disilane. The third group, i.e., third column, of samples was prepared from a precursor gaseous mixture including 4 sccm of hydrogen, 7 sccm of silane and 3 sccm of a mixture of 500 ppm of diborane in hydrogen (where used).
The fourth group, i.e. fourth column, of samples was prepared from a precursor gaseous mixture including 52 sccm of hydrogen, 2.5 sccm of silicon tetrafluoride, 2.0 sccm of germane, and 2.5 sccm of disilane. The flow rates are nominal values. All samples were deposited by glow discharge deposition at 13.S6 MHz.

The photoconductivity of the silicon:germanium:fluorine~hydrogen alloy deposited from the mixture including disilane and fluorine, 2-3 x 10-5 ohms-centimeters-1, is at least one order of magnitude higher than the photoconductivity of the other three samples, excluding boron.
The inclusion of small amounts of boron effects an increase in the photoconductivity of samples of columns 1 and 3, but is not reflected in improved electronic properties.

The dark conductivity of all layers, in the range of 10-8 _ lO-9 ohms-cen~imeters~1, is near the limit of measurement.
The band gaps of all samples is approximately 1.5 eV, indicating that similar amounts of germanium were incorporated into each.
The activation energy of all layers is similar, al~hough the samples deposited from the disilane-containing mixtures have a slightly lower activation energy. This difference is activation energy is comparable to that produced by ~race amounts of boron so that the samples are substantially intrinsic in conductivity type.

The relative sub-hand gap absorption indicates that density of tail states is low. Tail states are those defect states (such as dangling bonds, strained bonds, vacant bonds, etc.) that occur at the edge of the band gap. A high density of tail states impedes charge carrier mobility because o~ traps and other recombination centers. Sub-band gap absorption was measured by photodeflection spectroscopy at an energy of 1.1 eV
to indicate the density of states proximate the edge of the valence band. The relative density of tail states of the samples prepared from the fluorinated and non-~luorinated silane mixtures, and from the non-fluorinated disilane mixture, is 5 to lO times higher than that of the samples prepared from disilane and fluorine. The addition of trace amounts of boron has no effect on the densit~ of tail states. The combination of a ~luorine-containing gas and disilane in a precursor ~ixture results in a reduction in the density of tail states that is not observed when disilane or ~luorine are used separately.

Samples o~ the various layers were fabricated into single n-i-p type photovoltaic cells. ~ layer of n-type microcrystalline silicon alloy about 150 nanometers thick was deposited on an electrically conductive stainless steel substrate. An approximately 200 nanometer thick layer of substantially intrinsic semiconductor alloy material was then deposited, followed by the deposition of a layer of microcrystalline p-~ype silicon alloy material, approximately 15 nanometers thick. A transparent, electrically conductive electrode of indium tin oxide 45 nanometers thick was deposited on the p-type layer.

The silicon:germanium alloy samples deposited from silane mixtures and not containing boron, had a high density of defect states so that photodegradation of the photovoltaic devices in which they were incorporated could not be measured.
The inclusion of small amounts of boron in those samples permitted measurement of photodegradation. The layers deposited from the disilane, germane, hydrogen mixture also contained a fairly high density of defects and could not be fabricated into a useable photovoltaic cell; accordingly no data can be presented for photodegradation of these samples.

The samples incorporating a layer deposited from a silane mixture without a fluorine source exhibited the highest photodegradation, 34 percent after only 16 hours of AM-1 illumination. Incorporation of fluorine into the precursor gaseous mixture resulted in a cell which degraded only 12 percent after that 16 hours of exposure. The best results were obtained from the use of a precursor gaseous mixture of disilane and fluorine. Photovoltaic cells produced from an ailoy deposited from that mixture exhibited only a 5 percent degradation after 16 hours of exposure.

The measured results show that use of disilane in the precursor mixture increases the deposition rate of silicon-containing germanium alloys and eliminates differential depletion; the addition o fluorine to silicon:germanium alloys improves the stability and efficiency of devices containing those alloys; and the combination of disilane and fluorine in the precursor mixture achieves not only a high deposition rate, but greatly increased resistance to photodegradation, a lower density of defect state5 in the band gap and increased photovoltaic efficiency~

Claims (5)

1. A gaseous precursor mixture from which a silicon-based narrowed band gap semiconductor alloy may be deposited in a glow discharge decomposition process, said mixture comprising disilane and germane.

2. The mixture of claim 1 including a gaseous source of fluorine.

3. The mixture of any one of claims 1 or 2 including a gaseous precursor source of dopant elements.

4. The mixture of claim 1 including, by volume, 0.3 to 3 parts disilane, 0.3 to 3 parts germane and 0 to 3 parts of silicon tetrafluoride.

5. The mixture of any one of claims 1 or 2 including, by volume, approximately 2.5 parts of disilane, 2.5 parts of silicon tetrafluoride and 2 parts of germane.