"Deposition of device quality, low hydrogen content, amorphous silicon films by hot filament technique using ""safe"" silicon source gas"

A method of producing hydrogenated amorphous silicon on a substrate by flowing a stream of safe (diluted to less than 1%) silane gas past a heated filament.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the deposition of thin films of material 
on a substrate, and more specifically to the deposition of device quality 
hydrogenated amorphous silicon (a-Si:H) containing low hydrogen content 
for use as photovoltaic and other semiconducting devices. 
2. Description of the Prior Art 
In the manufacture and construction of microelectronic semiconductor 
devices and photovoltaic solar cells, amorphous silicon is often a 
feasible alternative to the use of silicon crystals for layers of a 
device, due to economics, flexibility in manufacture, and higher 
through-put. However, amorphous silicon tends to react chemically with its 
environment, causing it to become contaminated, and thereby deteriorating 
the chemical, electrical, and mechanical properties of the intrinsic or 
undoped silicon. This reactivity of the amorphous silicon can be 
passivated by the incorporation of hydrogen into the amorphous silicon 
layer, which is usually accomplished during the deposition process, and it 
considerably improves the electrical properties of the individual layers 
and the device. 
Two measures of these electrical properties of hydrogenated amorphous 
silicon layers are the Urbach tail width and the density of midgap states, 
both of which should be minimized to achieve device quality semiconductor 
films. Although exact mechanisms are not known, there has appeared to be a 
relationship in glow discharge deposited films between the amount of 
hydrogen incorporated and both the Urbach tail width and density of midgap 
states. At hydrogen concentrations too low, the amorphous silicon film 
exhibits very poor electrical properties due to the high density of midgap 
states and is thus not suitable for use in practical devices. At hydrogen 
concentrations too high, these films show an increased density of 
microvoids and once again inferior electrical properties. 
Incorporation of this hydrogen into the amorphous silicon film is not 
without its costs, however. Specifically, when this hydrogenated amorphous 
silicon is used in photovoltaic solar cells, these solar cells over time 
degrade electrically upon exposure to sunlight. This degradation, which is 
referred to as the Staebler-Wronksi effect, has been strongly linked to 
the concentration of hydrogen within the amorphous silicon film. The 
prevalent model for the Staebler-Wronksi effect suggests that the 
degradation is due to movement of hydrogen within the film. 
In the last decade or so, since the development of the glow discharge (GD) 
technique as the standard means for producing device quality hydrogenated 
amorphous silicon films for solar cells and other applications, there has 
been considerable progress made in increasing the efficiencies of these 
solar cells. However, most of this progress has been in improved 
techniques in manufacturing and utilization of these solar cells, such as 
better uniformity of deposition, better light utilization, and better 
doping of layers. The U.S. Pat. No. 4,237,150, issued to H. Weismann, and 
the U.S. Pat. No. 4,237,151, issued to Strongin et al. illustrate attempts 
to improve amorphous silicon as a photovoltaic material by using silane as 
a silicon source gas in a hot wire deposition technique to eliminate 
impurities, non-uniformities, and clusters of silicon that they thought 
limited the utility of the material. J. Doyle et al, in their article, 
Production of High Quality Amorphous Silicon Films by Evaporative Silane 
Surface Decomposition, published in the Journal of Applied Physics, Vol. 
64, p. 3215-3223, 1988, gave credit to H. Weismann and carried the 
improvements to better temperature and vacuum ranges, but they apparently 
were not able to control the degradation from the Staebler-Wronski effect. 
No improvements either in the material quality or in the Staebler-Wronski 
effect were achieved along with the other improvements that were reported 
in those publications. Also, silane gas is toxic and very explosive and 
flammable, so it is not considered to be a safe gas for use in even 
laboratory work without stringent safety controls, let alone industrial or 
commercial applications. Therefore, there is still room, in fact a need, 
for additional attention and improvement to this technology to increase 
efficiencies and usefulness, particularly for solar applications. 
SUMMARY OF THE INVENTION 
Accordingly, a general object of the present invention is to provide a 
hydrogenated amorphous silicon film which demonstrates state-of-the-art 
material quality of the intrinsic or undoped material. 
Another general object of the present invention is to provide a viable, 
economical, and high through-put method of depositing hydrogenated 
amorphous silicon films for solar cells and other applications, which 
films demonstrate significantly improved electrical, chemical, and 
structural qualities. 
It is also an object of this invention to provide a hot wire or other 
deposition process in which a safe silicon and hydrogen atom source gas 
can be used to produce device quality hydrogenated amorphous silicon film 
on a substrate. 
Additional objects, advantages, and novel features of the invention shall 
be set forth in part in the description that follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by the practice of the invention. The objects 
and the advantages of the invention may be realized and attained by means 
of the instrumentalities and in combinations particularly pointed out in 
the appended claims. 
To achieve the foregoing and other objects and in accordance with the 
purpose of the present invention, as embodied and broadly described 
herein, the method of this invention may comprise producing a thin film of 
hydrogenated amorphous silicon with a low hydrogen content by flowing a 
silicohydride gas past a filament which has been heated to a sufficient 
temperature to thermally decompose the silicohydride on the filament into 
mostly, atomic silicon and atomic hydrogen. The filament should be heated 
to at least 1500.degree. C., but is preferably heated to about 
2,000.degree. C. This gaseous mixture of mostly atomic silicon and atomic 
hydrogen is then evaporated from the filament and is deposited onto a 
substrate heated to between 200.degree. and 600.degree. C., but which is 
preferably heated to about 400.degree. C. (about 300.degree. to 
450.degree. C. surface temperature) for concentrated silane, but 
preferably about 290.degree. C. when safe or diluted silane is used as a 
source of silicon and hydrogen. Safe gas or diluted silane comprises less 
than one percent (1%) silane in ninety-nine percent (99%) inert gas. This 
procedure of thermally decomposing the silicohydride on the heated 
filament and then depositing the resultant gaseous mixture onto a heated 
substrate occurs within a deposition chamber maintained at an optimal 
pressure in the range of about 5-50 millitorr, preferably about 8 
millitorr for concentrated silane, or about 50-500 millitorr, preferably 
about 80 millitorr, for safe or diluted silane. 
The silicohydride gas may be silane, disilane, noble gases, or other gases 
or combinations of gases containing silicon and hydrogen. The heated 
filament is preferably made from tungsten, but can be made from other high 
temperature materials such as graphite or silicon carbide. When the 
silicohydride gas is decomposed at those temperature and pressure ranges 
described above, the hydrogen content of the film appears to be controlled 
by the temperature of the substrate, such that the higher the temperature 
the lower the hydrogen content. Despite hydrogen contents as low as one 
atomic percent hydrogen, the hydrogenated amorphous silicon films produced 
with concentrated silane in the process of this invention still exhibit 
photovoltaic device quality electrical, chemical, and mechanical 
properties which will not as readily degrade upon exposure to sunlight. 
These same results of better resistance to degradation in sunlight have 
not yet been achieved with the diluted silane in the process of this 
invention. However, the safe (diluted) silane gas used according to this 
invention has been used to produce results as good as those that were 
achievable prior to this invention only by using unsafe, concentrated 
silane gas, and even better results are believed to be achievable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A device quality low hydrogen content, hydrogenated amorphous silicon film 
is produced by the high temperature decomposition of a gas containing 
silicon and hydrogen, under a vacuum, by depositing the decomposed gas 
onto a heated substrate. Heretofore it has been believed that to achieve a 
device quality hydrogenated amorphous silicon film, as determined by 
measuring such parameters as Urbach Tail widths, Tauc's Bandgap, 
Photo-conductivity, and Dark Conductivity, that the film had to contain at 
least between 10 and 15 atomic percent (at. %) hydrogen. However, the low 
hydrogen content, hydrogenated amorphous silicon films produced with 
concentrated silane according to the principles of the present invention, 
at about 1 at. % hydrogen, exhibits superior device quality films despite 
their low hydrogen content. 
An alternate embodiment of this invention also achieves device quality 
hydrogenated amorphous silicon, but with safer materials, including the 
use of highly diluted source gas comprising less than one percent (1%) 
silane or other silicohydride gas in helium or some other inert gas. This 
diluted gas mixture is sometimes referred to herein as helium-diluted safe 
gas or simply as safe silane or safe gas. At present, films produced with 
the helium-diluted safe silane gas according to this invention have not 
exhibited such superior results at low hydrogen content, but they are at 
least equal to state-of-the-art films with 10 at. % produced by other 
methods that require concentrated silane and that do not achieve even 
those results from diluted silane. Consequently, the process of this 
invention achieves better than state-of-the-art results for low (1 at. %) 
hydrogen content from concentrated silane. It also achieves at least 
state-of-the-art results for "standard" (10 at. %) hydrogen from very 
diluted silane, which has not been achieved prior to this invention. 
The low hydrogen content, hydrogenated amorphous silicon film may be 
produced according to this invention with any suitable apparatus as will 
be understood readily by persons skilled in the art upon gaining an 
understanding of the features of this invention. However, to facilitate 
the explanation of this invention, suitable apparatus is illustrated in 
FIG. 1. A typical deposition chamber 10 enclosed by a housing 14 is 
illustrated from a top plan view with a transparent window 30 mounted in 
the top flanged opening 31 and with portions of the housing 14 broken away 
to show the operative components in the chamber 10. A substrate table or 
holder 18 is positioned in the chamber 10 to support a substrate 12 on 
which the hydrogenated amorphous silicon film is to be deposited. A 
heating element (not shown) is provided under the substrate table 18, 
preferably outside the vacuum system, to heat the substrate 12 to the 
desired temperatures, which are described in more detail below. A wire 
filament 16, preferably tungsten, is supported between two electrodes 20, 
22 a spaced distance over the substrate holder 18, and electric power is 
supplied to the electrodes 20, 22 by wire leads 32, 33. A gas inlet 24 
mounted in flange 34 is connected by a conduit 35 to a feed gas source 
(not shown), and an outlet port 26 mounted in the diametrically opposite 
flange 36 is connected by a conduit 37 to a vacuum pump (not shown). 
In operation, the substrate 12 is placed on support table 18, and the 
chamber 10 is sealed. The vacuum pump (not shown) is turned on, and the 
chamber 10 is evacuated. A silicohydride gas, as will be described in more 
detail below, is made to flow into the chamber 10 through tube 35 and 
inlet 24, as indicated by arrows 28. The silicohydride gas flows across 
chamber 10 preferably transverse to the filament 16. When electric power 
is applied to the filament 16, it gets very hot, and the silicohydride gas 
is absorbed onto the hot filament 16 similar to a catalytic converter, 
where it is decomposed and re-evaporated substantially in the form of its 
atomic species, silicon and hydrogen. The silicon and hydrogen are then 
coated onto the substrate, as will be described in more detail below. 
There are four important identified deposition parameters that have to be 
optimized to produce good device quality films of hydrogenated amorphous 
silicon, according to this invention. These four parameters, all of which 
have to be coordinated and kept within the preferred ranges, include 
filament temperature, chamber pressure, distance between filament and 
substrate, and substrate temperature, as will be described in more detail 
below. 
The filament temperature at which the decomposition of the silicohydride 
gas begins will vary somewhat, depending on the specific silicohydride gas 
used, such as, for example, silane, disilane, or other gases or 
combinations of gases containing silicon and hydrogen. The preferred gas 
is silane (SiH.sub.4), and even more preferred safe (helium-diluted) 
silane or "safe gas", for which the filament 16 temperature should be at 
least 1,500.degree. C. and preferably 2,000.degree. C. That temperature 
provides the most efficient and effective decomposition of the silane gas 
to its constituent elements, silicon and hydrogen. The flow of silane, 
preferably at a rate of about 20 sccm can be regulated by a flow 
controller and is pumped through the chamber by means of a turbo-molecular 
pump. The temperatures at the ends of the filament 16 are usually not as 
high as in the center, and, where such end temperatures are less than 
1,500.degree. C., such as around 1,400.degree. C., the silicon and 
possibly even the hydrogen, tend to alloy at those outer ends, which is an 
undesirable effect that should be kept to a minimum for the purposes of 
this invention. 
As the decomposed silicon and hydrogen atomic species are evaporated off 
the hot filament 16 and migrate toward the substrate 12, some collisions 
occur among the silicon and hydrogen atoms and the undecomposed SiH.sub.4 
gas molecules due to normal Brownian movements. A few of these collisions 
seem to be desirable, even necessary, according to this invention, to 
produce a good, device quality, hydrogenated amorphous silicon film. 
However, too few collisions and too many collisions are deleterious to the 
quality of the film. The number of atomic collisions is a function of the 
pressure in chamber 10 and of the distance between the filament 16 and the 
substrate 12, so those parameters have to be controlled carefully. It has 
been found, according to this invention, that, when using concentrated 
silane, a pressure in the range of about 5 to 50 millitorr (preferably 
about 8 millitorr) and a distance between the filament 16 and substrate 12 
in the range of about 2 to 30 cm (preferably about 5 or 6 cm) produces the 
best results. When using the diluted silane or "safe gas" according to 
this invention, a pressure in the range of about 50 to 500 millitorr is 
preferred. The preferred pressure and spacing produce about 20 to 40 
atomic collisions between silicon and hydrogen species as the atoms 
migrate from the filament 16 to the substrate 12, based on the statistical 
relationship of mean free path of an atomic particle being about eight 
divided by the pressure in milliorr, which, as mentioned above, appears to 
produce the best results for device quality film. Collisions of silicon 
and hydrogen species with helium species do not appear to affect the 
results, possibly because of the relatively low atomic weight of helium in 
relation to silicon, which accounts for the higher pressure needed when 
using the helium-siluted safe silane gas according to this invention. 
The graphs in FIG. 2 show the types of Si-H bonding versus the approximate 
number of collisions that the atomic species evaporated from the 
2000.degree. C. filament undergo as they traverse the filament-substrate 
distance in the deposition chamber. The graphs are offset vertically to 
show differences in curve shapes. It is known in the hydrogenated 
amorphous silicon field that a dip in the transmission curve in the 
neighborhood of 2000 wave numbers is the signature of H bonded in the 
monohydride, or SiH mode. This characteristic is indicative of H bonded in 
a compact Si lattice, and it is observed traditionally in device quality 
a-Si:H. It is also known that a dip in the transmission curve in the 
neighborhood of 2070-2100 wave numbers can be the signature of H bonded 
polyhydride, or (SiH.sub.2).sub.n mode, which is indicative of H bonded in 
a porous lattice and is observed traditionally in non-device quality 
a-Si:H. As can be seen in FIG. 2, when the number of collisions is either 
too few or too many, the polyhydride signature is clearly evident. It is 
only when the number of collisions are limited, as discussed in this 
invention, that device quality a-Si:H is produced. 
While it is not entirely clear at this point exactly why these pressure and 
distance parameters produce the best device quality hydrogenated amorphous 
silicon films, it is believed that the pure atomic species which are 
evaporated off the filament collide either with themselves or with the 
dissociated or undissociated silicohydride gas and produce a different 
mixture of radical species than is produced in the SiH.sub.3 dominated 
glow discharge process. It is these radical species, which in turn hit the 
substrate, that become integrated into the growing film. Fewer atomic 
collisions, it is believed, would increase the likelihood of pure atomic 
silicon hitting the substrate, and more atomic collisions would produce 
higher order silicon-hydride clusters (microparticulates) to form in the 
gas phase and also hit the substrate. Both of these extremes would produce 
a-Si:H of inferior electronic and structural quality. 
Experiments have shown that about 8 millitorr pressure when using 
concentrated silane gas seems to produce the best device quality 
hydrogenated amorphous silicon films, and such films produced at 1 
millitorr and those produced at 100 millitorr and higher were not as good. 
However, as discussed above, about 80 millitorr pressure seems to produce 
the best results when the helium-diluted safe gas is used according to 
this invention. 
The fourth parameter, the substrate temperature, also seems to be a 
controlling factor in the quantity of hydrogen finally incorporated into 
the hydrogenated amorphous silicon film. The relationship between 
substrate temperature and hydrogen content appears to be that the higher 
the substrate temperature, the lower the hydrogen content of the 
hydrogenated amorphous silicon film. In fact, experiments that lead to the 
development of this invention indicate that the hydrogen content, measured 
in atomic percent hydrogen, in the hydrogenated amorphous silicon film 
decreases monotonically as the temperature of the substrate 12 increases. 
Consequently, it has been found that, as used in this invention with the 
other parameters described above, the temperature of the substrate 12 
should be maintained in the range of 200.degree. to 600.degree. C., and 
preferably at about 400.degree. C., during the deposition process. Heating 
the substrate table or holder 18 to the temperature of about 400.degree. 
to 600.degree. C. actually results in slightly lower temperatures in the 
range of about 300.degree. to 450.degree. C. at the substrate surface due 
to the high vacuum, infrared radiation from the substrate surface, and 
other thermal conduction inhibitions. At this preferred substrate 
temperature range, a significant proportion of the hydrogen atoms that hit 
the substrate 12 retain sufficient thermal mobility to diffuse back out of 
the forming hydrogenated amorphous silicon film and are released as 
molecular hydrogen from the growing film. 
One additional factor has been identified, which might contribute to 
allowing a superior a-Si:H film to be produced with such low H content 
according to this invention. As opposed to the glow discharge process, 
where particles in the discharge are energetic electrons, neutral radical 
species and ions, the maximum energies obtainable in the process of the 
present invention are the thermal energies that the atomic species receive 
as they are evaporated off the substrate. In addition, since these species 
undergo a few collisions in the gas phase, the energies of the mix of 
radical species in the deposition chamber is reduced even further. 
Therefore, at these low thermal energies, no ions or energetic electrons 
are produced in the gas phase. Since it is generally believed that film 
bombardment by energetic species damages the growing film surface, films 
grown by the present technique may avoid the effects of such bombardment. 
It has generally been believed, prior to this invention, that the "normal" 
hydrogen content in amorphous silicon films not only passivate the film by 
filling bonds, but also was necessary to reduce lattice strain in the 
amorphous silicon. The method of this invention, however, may allow the 
deposition of lower hydrogen contact hydrogenated amorphous silicon with 
reduced randomness of the deposited silicon, thus with reduced lattice 
strain between the silicon atoms within the film. This reduced silicon 
lattice strain, coupled with the higher substrate temperature, may also 
allow much of the hydrogen which strikes substrate 12 to have sufficient 
thermal mobility to escape being incorporated into the hydrogenated 
amorphous silicon film. 
A lowered hydrogen content in the hydrogenated amorphous silicon film is 
desired because of the observed link between hydrogen content and the 
subsequent degradation of photovoltaic cells upon exposure to sunlight. 
This degrading effect is called the Staebler-Wronksi effect and is 
strongly linked to the hydrogen content of the amorphous silicon film. It 
is believed that the degradation occurs as a result of movement of the 
hydrogen. Accordingly, if the hydrogen content can be lowered while still 
retaining good device quality amorphous silicon film, as can be 
accomplished with the present invention, the tendency or extent of the 
degradation of electrical properties due to the Staebler-Wronski effect 
can also be reduced. As mentioned above, hydrogenated amorphous silicon 
films produced according to this invention from concentrated silane can 
have as little as one atomic percent (1 at. %) hydrogen, yet have better 
electrical properties, including better transport of charge carriers with 
band gaps comparable to prior art films that need as much as 10 to 15 at. 
% hydrogen to have acceptable electrical properties for device quality 
film. While such improved electrical properties have not yet been achieved 
for low hydrogen content hydrogenated amorphous silicon films from the 
helium-diluted safe gas, such results are believed to be attainable. 
EXAMPLE I 
Presented below is a comparison between a deposition of a low hydrogen 
content, amorphous silicon film produced according to the principles of 
the present invention and one produced by the more traditional glow 
discharge (GD) deposition process. The use of hot wire or filament to 
assist a vapor deposition, as in the current invention, is commonly 
referred to as a hot-wire-assisted chemical vapor deposition or hot wire 
(HW) deposition. It is to be understood that the examples given below are 
for illustrative purposes only, and are not intended to limit the scope of 
the invention as herein described or as set forth in the appended claims. 
Some hot wire (HW) films were deposited using the method of the present 
invention, as described above, using silane gas, a deposition chamber 
pressure of 8 millitorr, and a filament temperature of about 2,000.degree. 
C. The substrate temperature, however, was varied to change the 
atomic-percent of hydrogen contained within the various films. Each sample 
at a particular substrate temperature, and therefore particular atomic 
percent hydrogen content, was simultaneously deposited onto 7059 glass and 
crystalline silicon substrate. The deposition rates for these HW films, 
deposited according to the principles of the present invention, were 5-10 
.ANG./sec. 
The glow discharge (GD) films were deposited on the anode of a capacitively 
coupled, parallel plate, radio frequency deposition apparatus, which was 
operating at 13.56 MHz. The other operating conditions were selected to 
produce a high quality, or device quality, hydrogenated amorphous silicon 
film. These conditions include a 70 mW/cm.sup.2 discharge power, silane at 
a flow rate of 100 sccm, and a 500 millitorr chamber pressure. Similar to 
the HW films, several GD films were deposited over a range of substrate 
temperatures, and thus, hydrogen contents, simultaneously onto 7059 glass 
and crystalline silicon. The deposition rates for the GD films deposited 
were in the range of 1.5-2.5 .ANG./sec. 
The hydrogen content of each of these samples were determined from the 
films deposited onto the crystalline silicon substrate by the magnitude of 
the absorption of the SiH 630 cm.sup.-1 infrared mode. Film thickness for 
all samples were typically 1.5-2.5 .mu.m, and Raman measurements showed 
all films to be amorphous. 
Several measurements were taken on these HW and GD films in order to 
compare their electrical, chemical, and mechanical properties. These 
measurements included Urbach tail widths (E.sub.o), Tauc's bandgaps 
(E.sub.g), photoconductivity, dark conductivity, Electron Spin Resonance 
(ESR), and ambipolar diffusion lengths. 
The Urbach tail widths (E.sub.o), for the various hydrogen concentrations 
of the GD films and the HW films deposited according to the principles of 
the present invention, were determined by photothermal deflection 
spectroscopy. These measurements of Urbach tail widths in millielectron 
volts (meV) are plotted in FIG. 3 against a logarithmic scale of hydrogen 
concentrations or "H content" in atomic percent, to show a wide range of 
data and to better emphasize the differences between the GD and HW samples 
at low H content. The discrete data for the HW samples deposited according 
to the principles of the present invention are represented by the solid 
dots. The trends of this data are approximated by continuous line 42. 
Similarly, the discrete data for the GD samples are represented by the 
hollow dots and approximated by the continuous line 44. There are two 
regions of interest in the comparison of these two sets of samples. First, 
for that region above 10 at. % hydrogen, both the HW and GD samples 
display similar characteristics, in that the Urbach tail widths increase 
rapidly in this region. For that region below 10 at. % hydrogen, the GD 
films again show increasing Urbach tail widths. However, the HW films, 
deposited according to the principles of the present invention, remain 
approximately at a 50 meV minimum until well below a hydrogen 
concentration of 1 at. %. This 50 meV value of Urbach tail width is 
respectable and is typical of device quality films. Therefore, this 
graphic representation in FIG. 3 illustrates that the HW film with 
substantially lower H content (1 at. %) according to this invention is of 
comparable device quality to the more conventional GD films having more 
moderate content (10 at. %). Yet, as described above, the HW film 
according to this invention is less susceptible to Staebler-Wronsik effect 
degradation because of its much lower H content. 
It is important to note that it was impossible, in making the two sets of 
films, to lower the H content of the GD films below the values of 2-3 at. 
%, and thus enable a comparison with the low H constant HW films, without 
the GD films becoming microcrystalline. 
FIG. 4 shows a graph of the H contents of the respective films as a 
function of the surface temperature of the growing film. For hydrogen 
contents less than about 10 at. %, the predominant mode of binding was in 
the SiH, or monohydride, mode (see FIG. 2) for both sets of films. 
However, for temperatures in excess of 400.degree. C., almost all the H is 
removed from the HW films, while a significant amount of H still remains 
incorporated in the GD films. These data suggest basic differences in how 
the H in the monohydride mode is bonded in the two sets of films as the 
substrate temperature is increased, which may explain in part the 
differences in the structural, electronic, and light soaking properties 
observed between the low H content HW and GD films. 
The measured values of the Tauc's bandgaps (E.sub.g), in electron volts, 
for the two sets of samples are plotted in FIG. 5 once again against a 
logarithmic scale of hydrogen content in atomic percent. As with the prior 
graph in FIG. 2, the solid dots represent the discrete HW data and the 
hollow dots represent the discrete GD data. This HW data is approximated 
by continuous line 52 and the PECVD data is approximated by continuous 
line 54. Both show the traditional linear dependence of bandgap (E.sub.g) 
and at. % hydrogen. However, the HW films produced in accordance with the 
principles of the present invention, show a more flattened linear 
relationship, which is indicative of a film with greater integrity and 
less dependence on the number of hydrogen bonds. 
The Photo and Dark Conductivity, expressed in S/cm, for both the HW and GD 
sets are plotted on the same graph, in FIG. 6, against a logarithm scale 
of hydrogen content in at. %. Once again the HW data for the films 
produced in accordance with the present invention are the solid discrete 
points, which, in this case are circular dots for the Dark Conductivity 
and diamond shapes for the Photoconductivity. Similarly, the hollow 
circular dots represent the Dark Conductivity for the GD films samples, 
and the hollow diamonds represent the Photoconductivity for the GD films. 
Line 62 approximates the Dark Conductivity for the HW films, and line 64 
approximates the Dark Conductivity for the GD films. Line 66 represents an 
approximation of the Photo-conductivity of the HW film samples, and line 
68 the Photo-conductivity of the GD film samples. 
In that region above a hydrogen content of 10 at. % hydrogen, both sets of 
data in FIG. 6 show similar characteristic rapidly decreasing conductivity 
for both Photo and Dark Conductivity for increasing at. % H. However, for 
that region below a hydrogen concentration of 10 at. % hydrogen, including 
1 at. % hydrogen and below, the films of the HW samples deposited 
according to the principles of the present invention show much higher 
levels of Photoconductivity and lower levels of Dark Conductivity, both of 
which are marked improvements over the GD films. Note that the only way of 
reaching H contents less than 1 at. % for the GD films is to take a film 
previously deposited at a lower substrate temperature (e.g., 290.degree. 
C.) and anneal it in vacuum to drive out the bonded H. When this is done, 
enabling H contents in the range 1.0-0.5 at. % to be produced, the ratio 
of photo or light to dark conductivity for the GD films is on the order of 
2-3, while the photo or light to dark conductivity ratio for as grown HW 
films of comparable H contents is on the order of 10.sup.4. This result 
again illustrates the superior electronic nature of the low H constant HW 
films. 
Preliminary Electron Spin Resonance (ESR) measurements were taken on a 7 
.mu.m thick sample of the HW film at a hydrogen content of 10 at. %, and 
on a 1.5 .mu.m thick sample of a 0.6 at. % hydrogen content sample of a HW 
film, both deposited with concentrated silane in accordance with the 
principles of the present invention. The former film contained 
3.times.10.sup.15 /cm.sup.3 spins, which is typical of device quality 
hydrogenated amorphous silicon films deposited by the GD Technique, while 
the latter HW film had a spin density of 1.3.times.10.sup.16 /cm.sup.3 
spins, which is considerably lower than has been previously observed for a 
GD film in this range of hydrogen content. 
The final comparison of the quality for these two samples, the ambipolar 
diffusion lengths, were measured by the steady state photograting 
technique. The HW films produced with concentrated silane according to the 
principles of the present invention gave values for the ambipolar 
diffusion lengths as high as 2000 .ANG. for the films with low hydrogen 
concentrations, which are greater than those values observed for device 
quality GD hydrogenated amorphous silicon films (about 1500 .ANG.) 
containing larger (10 at. %) hydrogen contents. 
In summary, the data shows that for a hydrogenated amorphous silicon film 
deposited with concentrated silane according to the principles of the 
present invention, with a hydrogen content as low as 1 at. %, the 
photoconductivity is between 1 and 2.times.10.sup.-5 /cm, the light to 
dark conductivity ratio is greater than 10.sup.5, the Urbach tail width is 
50 meV, the bandgap is 1.67 eV, and the ambipolar diffusion length is as 
high as 2000 .ANG.. All of these data are indicative of device quality 
electronic performance. 
Finally, in FIG. 7, we show values of the midgap defect density for 
selected HW and GD films, plotted versus a linear scale of hydrogen 
concentration in at. %, in the samples (N(A)) as grown, and also for 
samples (N(B)) when they are light soaked to saturation in a way such that 
no further increase in midgap state density is observed to occur upon 
additional light soaking. A linear scale of H concentration was chosen 
here, because no light soaking comparisons were made for samples 
containing H contents &lt;1 at. %. The important point illustrated in FIG. 7 
is that the saturated defect densities for the HW films are consistently 
lower than those for the GD samples containing comparable H contents, and 
the saturated values for the HW samples containing low H contents (1-4 at. 
%) are lower than the values for state of the art GD samples deposited at 
substrate temperatures of 290.degree. C. and containing approximately 10 
at. %H. This data supports the observation that degradation of 
photoelectric cells upon exposure to sunlight can be reduced when the low 
H content a-Si:H material, which is the subject of the present invention, 
is incorporated into such a cell. 
Accordingly, a process has been provided which demonstrates fast rate, 
large area deposition of device quality, low hydrogen content, 
hydrogenated amorphous silicon films, particularly when using concentrated 
silane. The low hydrogen content, hydrogenated amorphous silicon films 
exhibit greater material integrity and stability because of the lowered 
hydrogen content, while still demonstrating improved electrical, chemical, 
and structural properties, is not as subject to Staebler-Wronski effect 
degradation as the conventional device quality films, which prior to the 
invention, had to be produced with substantially higher hydrogen content 
to achieve device quality electronic characteristics. 
While the technique described above is very efficient and effective in 
producing device quality hydrogenated amorphous silicon films, there can 
be significant safety problems associated with the use of silane gas in 
the process. Unfortunately, concentrated silane gas is not only toxic, but 
it is also very explosive. Safety controls and procedures for handling 
silane gas are available, but they are very expensive and require rigid 
monitoring and controls. Such mandatory safety controls include the use of 
a toxic gas monitor, a liquid scrubber that decomposes the exhaust gases, 
a well ventilated system enclosure, nitrogen dilution of the exhaust gases 
before and after the exhaust pumps, multiple in-series regulators with 
both manual and pneumatic shut-off valves on the gas inlet lines, and a 
chamber overpressure monitor. There must be system interlocks to 
immediately shut off the gas flow in the event of a failure of any one of 
the safety controls. 
The alternative embodiment of this invention that also achieves device 
quality hydrogenated amorphous silicon, but with safer materials, includes 
the use of highly diluted source gas comprising less than one percent (1%) 
silane or other silicohydride gas in helium or some other inert gas. This 
alternative embodiment process can still utilize the hot wire technique 
described above in which the silane or other silicohydride gas is 
decomposed into atomic silicon and atomic hydrogen with a wire filament 
heated above 1,500.degree. C. (preferably about 2,000.degree. C.) in a 
vacuum in the range of about 60 to 300 millitorr and deposited on a 
substrate at a surface temperature in the range of about 200.degree. to 
600.degree. C. 
A concentration of less than one percent (1%) silane in a inert gas, such 
as helium, is considered to be "safe" gas that is not toxic, explosive, or 
flammable. Below a concentration of 0.77%, it is classified by the United 
States Department of Transportation as merely a compressed gas and 
requires only ordinary safety precautions for handling and storing 
compressed gases. 
Because of the very diluted silane gas concentration, the flow rate is 
increased by as much as a factor of twenty over the highly concentrated 
silane gas flow rates and volumes described above. However, both 
deposition rate and device quality of the resulting hydrogenated amorphous 
silicon film remain comparable to those of the high concentration silane 
process described above. While any silane concentration in an inert gas, 
such as helium, of less than one percent (1%) is considered to be a "safe" 
gas, use of silane concentrations of less than about one-fifth of a 
percent (0.20%) in this process would not be feasible because the 
deposition rates would drop to unacceptably low levels. Therefore, the 
preferred concentration of silane in inert gas for purposes of this 
alternate embodiment of this invention is in the range between one percent 
(1%) at the high end and about one-fifth of a percent (0.20%) at the low 
end. A concentration of about one percent (1.0%) silane in helium has 
provided good results, as shown in the example described below. The 
presence of helium can also have some other subtle effects that might 
require some adjustments in process parameters. For example, it is 
believed that the radicals, which come off the hot filament and collide 
with lighter helium gas molecules, might diffuse different distances than 
when they collide with residual silane gas molecules. This difference can 
affect cooling and energy dissipation, thus requiring slight adjustments 
in vacuum and substrate temperatures. 
EXAMPLE II 
For a gas flow rate of 300 sccm (0.95% silane in helium), a chamber 
pressure of 80 millitorr, a filament temperature of about 2,000.degree. 
C., a substrate surface temperature of 290.degree. C., a filament to 
substrate spacing of 5.5 cm, and deposition rates on the order of 1 to 1.5 
.ANG./sec., a hydrogenated amorphous silicon film was obtained with a 
hydrogen content of about 10 at. %, as indicated by data point 72 in FIG. 
4. The Urbach tail width 74, Tauc's bandgap 76, photo-conductivity 78, 
dark conductivity 80, and electron spin resonance (ESR) 82 data points for 
that a--Si:H film are shown in FIGS. 3 and 5-7 to be comparable to 
state-of-the-art a--Si:H films with 10 at. % hydrogen content from 
concentrated silane by either glow discharge (GD) or hot wire (HW) method. 
The ambipolar diffusion length of that film was found to be 1,700 to 1,800 
.ANG., and the density of defect states was about 3.times.10.sup.5 
defects/cm.sup.3. 
EXAMPLE III 
An a-Si:H film was deposited from safe (diluted) silane gas substantially 
the same as in Example II, except the substrate spacing to the filament 
was set closer at only about 1 cm, and a higher pressure of about 300 
millitorr was used. The hydrogen content of the a-Si:H film again was 
about 10 at. %, and the electronic properties of Urbach tail width, Tauc's 
bandgap, photo-conductivity, and ESR were about the same as those in 
Example II. However, at this closer spacing (1 cm) and higher pressure 
(300 millitorr), deposition rates as high as 3 .ANG./sec. were obtained. 
The foregoing is considered as illustrative only of the principles of the 
invention. Further, since numerous modifications and changes will readily 
occur to those skilled in the art, it is not desired to limit the 
invention to the exact construction and operation shown and described, and 
accordingly all suitable modifications and equivalents may be resorted to 
falling within the scope of the invention as defined by the claims which 
follow.