Amorphous hydrogenated silicon-carbon alloys and solar cells and other semiconductor devices produced therefrom

Amorphous hydrogenated silicon-carbon alloys having particular usefulness in the preparation of photovoltaic devices, such as solar cells, with improved properties, such as high open circuit voltage with high fill factor and improved blue response, and stability, are provided by the process of depositing the alloy on a substrate maintained at a relatively low temperature below about 260.degree. C. in a vapor deposition chamber, and introducing a gaseous mixture comprising at least one compound having the formula (SiX.sub.3).sub.3 CX.sup.1 wherein each X and X.sup.1 is selected from the group consisting of hydrogen and halogen, and a high proportion of hydrogen, in a ratio by volume of from about 50 parts to about 2000 parts hydrogen to 1 part of (SiX.sub.3).sub.3 CX.sup.1 compound, under deposition conditions of low excitation power density of less than about 50 mw/cm.sup.2, and high pressure of more than about 0.1 torr.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to hydrogenated amorphous silicon alloys, 
and, more particularly, to a process for producing improved hydrogenated 
amorphous silicon-carbon alloys having carbon-silicon bonds, which are 
useful in the production of semiconductor devices and particularly 
photovoltaic devices. 
2. Description of Related Art 
U.S. Pat. No. 4,690,830 teaches hydrogenated amorphous silicon alloys which 
are improved as compared to previously known hydrogenated amorphous 
silicon-carbon alloys, particularly those prepared by incorporating such 
gases as methane, germane, germanium tetrafluoride, diborane and phosphine 
in the deposition gas mixture. While the improvements provided by U.S. 
Pat. No. 4,690,830 have been beneficial in the production of useful alloys 
for semiconductor devices, such as photovoltaic cells, it is desirable to 
provide further improvements in undoped hydrogenated amorphous 
silicon-carbon alloys in order to attain such semiconductor devices as 
solar cells with even greater improved properties, such as higher open 
circuit voltages at desirable, active layer (i.e. the i-layer) bandgaps of 
1.8 eV or more, high fill factors, and improved solar cell stability. 
SUMMARY OF THE INVENTION 
Hence, it is one object of the present invention to provide improved 
amorphous silicon-carbon alloys from which semiconductor devices, such as 
photovoltaic cells, can be produced having additionally improved 
properties. 
It is another object of this invention to provide a method by which 
amorphous silicon-carbon alloys can be produced which are useful to 
produce semiconductor devices, and particularly photovoltaic cells, which 
have improved highly desirable properties. 
Another object of the present invention is to provide a method for 
preparing amorphous silicon-carbon alloys of improved photoconductivity as 
compared to alloys prepared from carbon feedstocks such as methane, 
silylmethane and the like. 
Still another object of this invention is to provide amorphous 
silicon-carbon alloys of higher photoconductivity and improved uniformity 
from which improved photovoltaic devices can be obtained. 
Another object of the present invention is to provide a method for 
preparing amorphous silicon-alloys from which photovoltaic devices, 
particularly solar cells, can be produced, having improved stability as 
compared to cells previously prepared from silicon-carbon alloys. 
These and other objects and advantages of the present invention will be 
apparent from the following description. 
In accordance with the present invention, improved amorphous hydrogenated 
silicon-carbon alloys are prepared by a process comprising preparing a 
vapor deposition chamber; placing a substrate in the chamber and 
maintaining the substrate at a relatively low temperature below about 
260.degree. C.; and introducing into the deposition chamber a gaseous 
mixture comprising (1) at least one compound having the formula: 
EQU (SiX.sub.3).sub.3 CX.sup.1 
wherein each X and X.sup.1 is selected from the group consisting of 
hydrogen and halogen, and (2) hydrogen, in a ratio by volume of above 
about 50 parts, and preferably to about 2000 parts hydrogen to 1 part of 
(SiX.sub.3).sub.3 CX.sup.1 compound; under deposition conditions of low 
excitation power density of less than about 50 mw/cm.sup.2, and a pressure 
of more than about 0.1 torr. The high ratio of hydrogen, high pressure, 
low substrate temperature, low excitation power in the process of the 
present invention has been found to provide amorphous silicon-carbon 
alloys, and photovoltaic devices, such as solar cells, having the improved 
properties, particularly stability, as hereinafter described. 
In a preferred process of the present invention, the gaseous mixture also 
includes silane in a ratio by volume of from about 10 parts to about 200 
parts or more silane to one part of (SiX.sub.3).sub.3 CX.sup.1 compound. 
The vapor deposition process of the present invention is preferably 
performed by chemical vapor deposition, and most preferably wherein the 
chemical vapor deposition is enhanced by being performed by glow discharge 
or by laser excitation. In the former instance, the glow discharge can be 
sustained, either by dc glow discharge or ac glow discharge, preferably 
performed at, for example, a frequency of between approximately 10 and 
approximately 200 megahertz. 
The process of the present invention deposits on the substrate one or more 
regions of amorphous hydrogenated silicon-carbon alloy having 
silicon-carbon bonds. Photovoltaic devices of improved properties as 
heretofore described can be readily prepared from the substrate having one 
or more regions of alloy as described by applying front and back contacts 
to the substrate in a manner known to those skilled in the art. 
The improvement in the alloys of the present invention is unexpected as no 
improvement was found in the properties of solar cells prepared from 
alloys using a process in which the process parameters described above 
were individually varied into the range of parameters in combination 
required in the present invention. The alloys were prepared by depositing 
amorphous hydrogenated silicon-carbon alloys using an initial gaseous 
mixture of trisilylmethane and silane, in a ratio by volume of about 40 
parts of silane to 1 part of trisilylmethane, in a vapor deposition 
chamber on a substrate initially maintained at a temperature at about 
260.degree. C. and an initial pressure below about 0.4 torr. Improvement 
in properties of solar cells from the alloys prepared were sought by 
adding hydrogen gas to the gaseous mixture and varying separately the 
substrate temperature to a lower temperature of about 80.degree. C. and 
raising the pressure to 0.5 torr. Hydrogen gas was added to the gaseous 
mixture starting at zero parts and increasing the ratio by volume of 
hydrogen to about 1000 parts hydrogen to 1 part of trisilylmethane. Also 
the power applied to the vapor deposition to sustain glow discharge was 
reduced to about 10 mW/cm.sup.2 from about 50 mW/cm.sup.2. In each 
instance, the stability of solar cells prepared from the deposited 
amorphous silicon-carbon alloy was poorer than the alloys deposited under 
the indicated in initial conditions. However, upon depositing alloys in 
accordance with the process of the present invention improvement in the 
properties of the solar cells produced from the alloys were unexpectedly 
obtained, as will be hereinafter shown. 
The alloys prepared according to the process of the present invention can, 
for example, provide an improved i-layer in a p-i-n photovoltaic device 
having more uniform microstructure and stability than in previous 
amorphous hydrogenated silicon-carbon alloys. The improvement in the 
i-layer is achieved without the effect of dopants, such as diborane, as 
such dopants, if present, may have a beneficial effect on the p-layer, but 
are undesirable for the i-layer. Therefore, dopants are not used with the 
present invention and are to be avoided. Photovoltaic devices prepared 
from the alloys of the present invention are believed to have improved 
desirable properties, in large part, due to the improved i-layer of the 
alloys resulting from the process described in this application.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment of the method of the present invention, vapor 
deposition of amorphous hydrogenated silicon-carbon alloy is achieved in a 
conventional rf glow discharge deposition chamber. A suitable substrate, 
for example, conductive transparent oxides of metal, such as tin oxide, on 
a transparent base, e.g. glass, or non-transparent metals, such as 
stainless steel, is placed in the chamber. In the preferred embodiment, 
the substrate is maintained at a temperature below about 250.degree. C., 
and preferably above about 85.degree. C., and the deposition chamber is 
activated at an excitation power density of less than about 50 
mW/cm.sup.2. A gaseous mixture comprising at least one compound having the 
formula (SiX.sub.3).sub.3 CX.sup.1, wherein each X and X.sup.1 is selected 
from the group consisting of hydrogen and halogen, and hydrogen, in a 
ratio by volume of from about 50 parts to about 2000 parts hydrogen to 1 
part of (SiX.sub.3).sub.3 CX.sup.1 compound, is introduced into the 
deposition chamber at a pressure of more than about 0.1 torr, for example 
between about 1 and about 10 torr. Preferably, silane is included in the 
gaseous mixture introduced into the chamber, in a ratio by volume of from 
about 10 parts to about 200 parts silane to one part of (SiX.sub.3).sub.3 
CX.sup.1 compound. 
In the process of the present invention, it is preferred to utilize as the 
carbon-silicon bond feedstock (SiX.sub.3).sub.3 CX.sup.1 compounds wherein 
X.sup.1 is hydrogen, for example, trisilylmethane and halogenated 
trisilylmethanes, such as chlorinated trisilylmethanes and fluorinated 
trisilylmethanes. A useful example of the latter is 
tris(trifluorosilyl)methane having the formula (SiF.sub.3).sub.3 CH. Most 
preferably, the carbon-silicon bond feedstock utilizes the 
(SiX.sub.3).sub.3 CX.sup.1 compound wherein all of X and X.sup.1 are 
hydrogen i.e. trisilylmethane. Also preferably, the amount of hydrogen gas 
to (SiX.sub.3).sub.3 CX.sup.1 compound is present in the gaseous mixture 
in a ratio by volume of from about 50 parts to about 2000 parts hydrogen 
to one part (SiX.sub.3).sub.3 CX.sup.1 compound, and the gaseous mixture 
includes silane, preferably in a ratio by volume of from about 10 parts to 
about 200 parts silane to one part (SiX.sub.3).sub.3 CX.sup.1 compound. 
The following examples illustrate the process of the present invention, the 
alloys prepared therefrom, and solar cells and their properties, which can 
be prepared from the alloys. 
EXAMPLES 
Into an rf glow-discharge chemical vapor deposition chamber using diode 
reactors, each reactor having a substrate of conductive tin oxide on 
glass, the substrates being maintained at temperatures between 200.degree. 
C. and 260.degree. C., was introduced a gaseous mixture of 
trisilylmethane, hydrogen and silane in ratios of about 400 parts hydrogen 
to one part trisilylmethane and from about 44 parts silane to one part 
trisilylmethane, at a pressure of about 0.4 torr. The deposition of the 
amorphous hydrogenated silicon-carbon alloy was continued until alloy 
films having thicknesses of between about 700 and about 12,000 .ANG.were 
attained. The bandgap of the alloy films produced was between 1.83 eV and 
1.96 eV. 
p-i-n solar cells were prepared from alloys deposited on the substrate, 
prepared as noted above, by deposition of the alloy film, and by 
subsequent back contact metallization, i.e. by deposition, of zinc oxide 
and silver on the surface of the alloy film opposite the substrate, in the 
manner known to those in the art. Solar cells as thus prepared are 
illustrated in FIG. 1, which is not drawn to scale. Light (arrow 1) enters 
through the glass support substrate 2 as noted above, and through the 
conductive tin oxide contact layer 3. Next, the light passes into the 
amorphous silicon region 4. The back contact layer noted above is 
represented by numeral 5. The amorphous silicon region 4 is a p-i-n 
junction, i.e., the layer 4 includes a positively doped layer 6, an 
intrinsic (i-) layer 7, and a negatively doped layer 8. 
Solar cells as prepared above from the alloys of the preferred embodiment 
have superior properties as compared to solar cells prepared under 
comparable process conditions from methane (as the carbon source) and 
silane, and from methane, hydrogen and silane, as the feedstock gaseous 
mixture, in ratios of 10 and 50 parts hydrogen to one part methane and 
silane. Thus, solar cells prepared from alloys using trisilylmethane and 
halogenated trisilylmethanes as the carbon source in accordance with the 
present invention are preferred and provide improved properties as 
compared to cells prepared from methane and hydrogen alloys. For example, 
p-i-n cells having a buffer layer between the p and i layers and with 
the/-layer having a combined thickness of from about 700 .ANG.to about 
1000 .ANG.prepared from alloys deposited using trisilylmethane in 
accordance with the preferred embodiment were found to have a short 
wavelength quantum efficiency or blue response measured at about 400 nm of 
from 0.73-0.75, open circuit voltage of from about 0.93 volts to about 
0.99 volts, short circuit current of from about 7.3 mA/cm.sup.2 to about 
7.9 mA/cm.sup.2, fill factor of from about 0.62 to about 0.75, and 
efficiency of from about 4.7% to about 5.5%. At least one cell prepared in 
this manner had considerably poorer properties and is believed to be an 
anomaly. 
As noted above, through the process of the present invention, in accordance 
with this embodiment, high quality alloy films of amorphous hydrogenated 
silicon-carbon alloys were obtained and high quality, improved solar cells 
were prepared using the alloys. The solar cells described are single 
junction p-i-n solar cells having the amorphous hydrogenated 
silicon-carbon alloys as the i-layers and/or the buffer layers between the 
p and i layers, using trisilylmethane as the carbon source in the 
feedstock for the alloy. The combined thickness of the buffer and the 
i-layer is comparable to what would be needed for the top absorber in 
triple junction solar cells. 
The high open circuit voltage together with the high fill factor, improved 
blue response and stability, attained in accordance with the present 
invention, is not attainable using alloys prepared from such hydrocarbons 
as methane as the carbon source under comparable conditions. It is 
unexpected that through the process of the present invention, the 
improvement in the i-layer quality alone, provides alloys from which solar 
cells can be obtained having open circuit voltages approaching one volt 
with only mild loss in fill factor, and improved stability. 
While a particular embodiment of the amorphous hydrogenated silicon-carbon 
alloys and solar cells and other semiconductor devices produced therefrom 
of the invention has been shown and described, it will be appreciated by 
those skilled in the art that changes and modifications may be made 
thereto without departing from the invention in its broader aspects and as 
set forth in the following claims.