Gradient doping in amorphous silicon

An amorphous silicon semiconductor having a gradient doping profile is produced by thermo-electrically diffusing an ionizable deposit material such as antimony or aluminum, for example, into the amorphous silicon layer. Embodied in a photovoltaic device, the gradient doping profile increases the width of the depletion or barrier region and concurrently ensures an ohmic contact between amorphous silicon and current carrying electrodes.

FIELD OF THE INVENTION 
The present invention relates to amorphous silicon and more particularly to 
amorphous silicon having a gradient doping profile. 
BACKGROUND OF THE INVENTION 
Hydrogenated amorphous silicon has been demonstrated as having favorable 
photoconductive properties, promising a viable alternative to crystalline 
materials such as single crystal silicon and germanium. Produced typically 
in a thin film form, amorphous silicon provides substantial material 
savings over its crystalline counterparts. The existing impediment to its 
widespread use is a low device efficiency relative to other materials. 
Although the material displays a favorable quantum efficiency of 
photogenerated charge carriers, other fundamental electrical properties of 
the semiconductor such as mobility, lifetime and diffusion length of 
carriers, for example, limit the efficiency of an amorphous silicon 
device. The resultant effect upon a device such as a solar cell is that 
the effective collection of photo-generated charge carriers is limited to 
the barrier region or the non-junction region is not electronically 
neutral, thereby impeding the transport of carriers generated in the 
barrier or depletion region. 
The present invention deals with these deficiencies by selectively altering 
the amorphous silicon with an extrinsic dopant which both extends the 
effective field region of charge collection substantially throughout the 
amorphous silicon layer and concurrently improves the electrical 
characteristics of the non-junction or bulk region of the device. The 
dopant comprises an ionizable material such as antimony, for example, 
which is thermo-electrically diffused into the amorphous silicon layer 
during the sputter deposition of the silicon film. 
The two principal methods of producing hydrogenated amorphous silicon are 
the glow discharge decomposition of silane and reactive sputtering in a 
plasma consisting of a mixture of argon and hydrogen. In either case the 
material has been doped N or P-type by adding to the discharge an amount 
of phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6); respectively. 
Solare cell structures have been fabricated from these materials utilizing 
abrupt junctions formed by gas phase doping. Such structures include for 
example Schottky barriers, p-i-n junctions and hetero-junction 
configurations. The Schottky barrier structure is a multilayer 
configuration consisting of a metallic substrate, a 500A heavily phosphine 
doped a-Si layer (n.sup.30 ), an intrinsic amorphous silicon layer, and a 
high work function semi-transparent metal contact. The thin n.sup.+ 
amorphous silicon layer, obtained by doping from a discharge containing 
PH.sub.3, is used to form the ohmic contact to the intrinsic amorphous 
silicon layer. 
PRIOR ART 
The sputter deposition of photoconductive amorphous silicon is well known 
in the art. For example, Moustakas et al, in a technical publication 
entitled "Preparation of Highly Photoconductive Amorphous Silicon by 
Reactive Sputtering" Solid State Communications, Vol. 23, June 1977, teach 
the sputter deposition of photoconductive amorphous silicon in hydrogen. 
SUMMARY OF THE INVENTION 
The present invention provides a method of doping amorphous silicon. An 
extrinsic dopant such as antimony is thermo-electrically diffused into the 
amorphous silicon, altering its intrinsic semiconductor characteristics. 
The thermo-electrically diffused material provides a gradient doping 
profile through the majority of the silicon layer. Embodied in a 
photovoltaic device such as a solar cell, the present invention extends 
the effective field region of photogenerated charge collection 
substantially throughout the silicon layer while concurrently increasing 
the conductivity of the non-barrier or bulk region of the device. The 
doping process also produces an improved ohmic contact between the 
semiconductor and a conventional metal electrode.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to an improved semiconductor device having a 
body of amorphous silicon which has been altered by thermo-electrically 
diffusing an extrinsic dopant material into the silicon layer. 
To illustrate this invention, FIG. 1 represents a Schottky junction 
photodiode having a body of amorphous silicon which has been altered by 
the doping technique taught herein. Accordingly, in FIG. 1, a substrate 10 
provides a supportive base for the deposition of thin film materials. 
Substrate 10 comprises a material capable of withstanding the requisite 
processing temperatures of the overlaying layers described hereinafter. 
The substrate is preferably free from surface discontinuities of the order 
of one micron or less to avoid pinhole or similar deficiencies in the 
subsequently deposited films. The substrate is coated with a layer 12 of 
chromium (approximately 1000 A in thickness) and a layer 13 of an 
ionizable dopant comprising antimony, phosphorous, aluminum, lithium, 
arsenic or a mixture thereof approximately 50 A to 100 A in thickness. In 
a preferred embodiment, layer 13 comprises approximately 100 A of antimony 
deposited by thermal evaporation. For the purpose of the present invention 
layer 13 comprises the ionizable dopant and the chromium layer 12 is 
primarily used for current conduction. This sequence of the two metallic 
layers is used since thick antimony layers do not adhere well to a glass 
substrate. 
The substrate 10, coated with the layers 12 and 13, is secured to the anode 
electrode of a conventional radio frequency vacuum sputtering apparatus 
which is suitably modified to permit controlled heating and D.C. 
electrical biasing of the substrate. The substrate is heated to a 
temperature of between 200.degree. C. and 300.degree. C. and a D.C. 
positive bias ranging from about 30 volts to about 100 volts is applied to 
the substrate. Preferred ranges are 250.degree.-300.degree. C. and 50-100 
volts. In the event that substrate 10 comprises an electrically insulating 
material, direct electrical contact is made between the biasing anode and 
layer 13, ensuring the application of the bias voltage to the dopant layer 
13. 
The biased and heated substrate is then subjected to a sputter deposition 
of intrinsic amorphous silicon layer 14. This deposition comprises, for 
example, evacuating the sputtering apparatus to a pressure of about 
1-3.times.10.sup.-7 Torr and back filling same with argon and hydrogen. 
The partial pressure of argon may range from about 10mTorr to about 
20mTorr; the partial pressure of hydrogen may range from about 0.6mTorr to 
about 1mTorr, outside which range the doping technique taught herein will 
be ineffective due principally to the dominating effect of a deficiency or 
overabundance of hydrogen in the amorphous silicon film. The target, a 
polycrystalline silicon disk, 5" in diameter, is located 4.5 cm above the 
anode and is supplied with a power ranging from about 100 to 500 watts 
from a radio frequency, (hereinafter RF) power supply. At higher power 
densities (approximately 500 watts) the cathode is water cooled. At the 
lower power densities the cathode is permitted to reach an equilibrium 
temperature of about 200.degree. C. These conditions result in a 
deposition rate between 2 to 4 Angstroms per second the film thickness 
varies between 1 micron and about 3 microns. 
It is believed that the antimony reacts and dopes n-type the subsequently 
deposited layers of amorphous silicon. The elevated temperature of the 
substrate permits the dopant atoms to become positively ionized. During 
the subsequent silicon deposition, the ionized impurities assisted by the 
voltage bias diffuse and dope the depositing silicon layers under the 
driving force of the supplied positive bias. The thermo-electric diffusion 
produces a gradient composition of the dopant material, which is most 
concentrated near the origin of the dopant supply, decreasing in 
concentration through the film thickness. The gradient profile and the 
depth of penetration is controlled by the temperature-bias combination as 
taught herein. 
A conventional junction forming layer 16, is deposited onto amorphous 
silicon layer 14. The junction may comprise a Schottky junction, p-n 
junction, heterojunction or similar such semiconductor junction, well 
known in the art. In one embodiment, a semi-transparent layer of a metal 
having a work function above about 4.5 electron volts (eV) is deposited 
onto the amorphous silicon to form a Schottky junction. 
The diffused dopant is believed to displace the Fermi level of the 
intrinsic amorphous silicon toward the conduction band. The gradient 
composition doping profile through the silicon layer produces a monotonic 
decrease in the displacement of the Fermi level through the film. This 
results in a built-in field through the amorphous silicon layer, extending 
the region of photogenerated charge barrier collection substantially 
throughout the silicon layer. The displacement of the Fermi level toward 
the conduction band in the nonbarrier, bulk region of the device also 
increases the conductivity of the semiconductor material in this region, 
reducing the internal dissipative load of the device. In addition, the 
heavy doping of the amorphous silicon layer at the interface with the 
metal layer 13 ensures the ohmicity of this contact. 
The performance of the previously described photovoltaic device was 
compared with an abrupt junction device formed by techniques known in the 
art. 
In this comparison photovoltaic device, the prior art technique of 
interposing an amorphous n.sup.+ a-Si layer between a current carrying 
metal layer and the intrinsic amorphous silicon layer is used to form an 
ohmic contact between the metal and the semiconductor. Referring to FIG. 
2, there is shown a comparison of the photovoltaic current versus voltage 
characteristics of the two devices. Trace 20 represents the current versus 
voltage characteristics of an illuminated photovoltaic device have an 
antimony doping gradient as taught herein. Shown in contrast as trace 22 
is the identically illuminated current versus voltage characteristic for a 
conventional amorphous silicon device, evidencing a substantial decrease 
in short circuit current and generally a decreased ability to deliver 
power to dissipative load. 
To assist one skilled in the art in the practice of this invention the 
following example details the construction and electrical evaluation of 
photovoltaic devices having a body of amorphous silicon produced according 
to the present invention. 
EXAMPLE 1 
A plurality of borosilicate glass substrates were scrupulously cleaned to 
remove surface debris and then coated sequentially with a layer of 
chromium, approximately 1000 A in thickness and a layer of antimony, 
approximately 100 A in thickness. Both of these layers were deposited by 
resistance heating evaporation. The coated substrates were then placed on 
the anode of a conventional RF sputtering system. Electrical contact 
between the anode and the antimony layer was assured by physically 
contacting the antimony layer with electrically conductive screws secured 
to the anode. Another set of substrates coated with 1000 A sputtered 
nichrome and 500 A of n.sup.30 a-Si deposited by glow discharge of silane 
containing 1% phosphine were affixed to the anode of the sputtering 
system, described heretofore. The anode and substrates were heated to 
275.degree. C. and supplied with a positive D.C. bias voltage of about 50 
volts. The vacuum chamber, which had been evacuated to below 
5.times.10.sup.-7 Torr, was backfilled with partial pressures of hydrogen 
and argon of about 7.0.times.10.sup.-4 Torr and 15.times. 10.sup.-3 Torr 
respectively. The target, a polycrystalline silicon disk, 5" in diameter, 
was not water cooled as in conventional sputtering units, and was supplied 
with R.F. power of 200 watts during the silicon deposition. The antimony 
layer, under the combined driving force of the applied bias voltage and 
the elevated temperature, diffused and doped the silicon layer. In 
contrast, the silicon deposited on the substrates coated with the n.sup.- 
a-Si substrates remained undoped. 
The samples were cooled to room temperature, about 23.degree. C., and then 
coated with a pellucid layer of palladium, which is known to form a 
Schottky junction to amorphous silicon. The samples were then subjected to 
conventional photoelectric measurements. A comparison of the antimony 
doped versus undoped samples revealed an increase of 65% in the short 
circuit photocurrent of the doped samples when illuminated with 100 mWatt 
cm.sup.2 of simulated solar spectrum light. Conventional spectral 
dependence of collection efficiency measurements indicated a substantial 
increase in the width of the barrier or depletion region.