Fabrication of photovoltaic devices by solid phase epitaxy

Fabrication of photovoltaic devices by solid phase epitaxy; devices produced by this method consisting of a semiconductor base and a semiconductor junction-forming epitaxial layer. The epitaxial layer grown by solid phase means from a metal-semiconductor alloy or from a sandwich structure of semiconductor/metal on the semiconductor base.

BACKGROUND AND OBJECT OF THE INVENTION 
At the present time, most photovoltaic energy conversion devices, or solar 
cells, as they are also known, are constructed from the semiconducting 
material silicon. Such constructions are accomplished by the high 
temperature diffusion of suitable impurity atoms into a single crystal 
silicon base to produce a p-n junction near the surface of the base and in 
a plane parallel to said surface. Light incident on said diffused surface 
produces the photovoltaic effect--the direct conversion of light into 
electricity. Typically such diffusions are carried out at temperatures on 
the order of 1100.degree. C. The depth of the diffusion and the 
concentration of the diffused impourity atoms are determined by trade-offs 
between the sheet resistance of the diffused layer and the optical 
abosorption of the base material. Values of the junction depth and 
impurity atom profile will vary depending upon the particular material 
utilized for the base. 
The primary problem that exists in using solar cells produced by this 
method for large-scale earth-based power systems has been the high cost of 
the cells. This high cost can be attributed to three factors: (1) the 
current low production rate of solar cells, (2) the complex processes 
involved in the production of the devices, and (3) the high cost of the 
single crystal semiconductor wafers used as the starting material for cell 
fabrication. At present, these factors combine to make the cost of solar 
cells approximately thirty times the cost which is generally considered to 
be necessary to make photovoltaic power supplies competitive with nuclear 
power. 
It is therefore a primary object of the present invention to reduce the 
cost of photovoltaic devices by a simplification of the fabrication 
process, which will also permit the utilization of a lower cost starting 
material, such as polycrystalline material. 
Other objects, features and advantages of the invention will become 
apparent as the description thereof proceeds when considered in connection 
with the accompanying illustrative drawings.

DESCRIPTION OF THE INVENTION 
Since silicon is the most common semiconductor material used in commercial 
solar cell production, we utilize silicon as the base material in our 
description of the current invention. However, it will be understood that 
the present invention is applicable to semiconductors other than silicon, 
such as germanium and gallium arsenide, among others. The applicability of 
the present invention to other semiconductor materials will be apparent to 
those skilled in the art. As discussed previously, commercially-available 
silicon solar cells are fabricated from high cost single crystal 
materials, through processes which are complex, energy intensive, and 
expensive. The present invention reduces the complexity and cost of solar 
cell production by utilizing an epitaxial growth and simultaneous doping 
process which permits fabrication of p-n junctions at temperatures 
significantly below those used in the standard diffusion process. The use 
of lower temperatures in the p-n junction formation permits the present 
invention to be also applied to polycrystalline base materials, offering 
further cost reductions. 
There follows a description of the physical phenomenon which is exploited 
in the fabrication of solar cells according to the present invention. The 
aluminum-silicon (A1-Si) system is considered in this description. The use 
of the A1-Si system in the description is not intended to imply that this 
is the only materials system which exhibits this phenomenon; rather, it is 
merely intended to be illustrative of the type of materials systems which 
demonstrate this behavior. A wide variety of metal-semiconductor 
combinations exhibit this behavior, including, but not limited to, A1-Si, 
A1-Ge, Sb-doped Au-Si, Sb-doped Au-Ge, and Sn-GaAs. The phenomenon 
requires semiconductors which have a sufficiently high diffusivity in the 
metal and a metal which has a sufficient solid solubility in the 
semiconductor. The choice of metal-semiconductor combinations should be 
readily predictable by anyone versed in semiconductor/materials science. 
When a system consisting of aluminum in contact with silicon is heated to a 
temperature in the range from approximately 300.degree. C. to 576.degree. 
C., i.e., at an elevated temperature but one where only the solid phases 
of the materials are present, a high rate of diffusion of silicon into 
aluminum occurs. At such temperatures, silicon diffusion into aluminum 
occurs by a vacancy mechanism along the grain boundaries of the aluminum 
at a much higher rate than either the bulk diffusion of silicon into 
aluminum or the diffusion of aluminum into silicon. The diffusion of 
silicon into aluminum is a function of time and temperature up to the 
solid-solubility limit of silicon in aluminum, which is approximately 1.6 
atomic percent at the eutectic temperature of 576.degree. C. 
This solid phase transport phenomenon can be exploited to produce 
semiconductor layers at low temperatures. If the solid solubility 
requirements of the semiconductor in the metal, e.g., silicon (Si) in 
aluminum (A1), are exceeded by employing a super-saturated combination of 
semiconductor-in-metal prior to the heat treatment, the excess 
semiconductor can be caused to segregate out of metal upon heating. 
Normally, the segregation will take the form of "islands" of semiconductor 
material distributed through the metal. When the super-saturated 
metal-semiconductor combination is in contact with an underlying substrate 
material, careful control of the temperatures gradient across the 
interface between said saturated metal and said semiconductor can cause 
substantially all of the diffusing semiconductor to migrate to the 
interface between the metal-semiconductor combination and the substrate, 
where the diffusing semiconductor can be caused to grow as a continuous 
epitaxial semiconductor layer on the substrate. Returning to the Al-Si 
system, the deposition of a layer of Al supersaturated with Si on a 
Si-substrate, and the application of an appropriate heat treatment can 
cause the excess Si to migrate to the Al/Si-substrate interface and to 
form a continuous epitaxial layer of Si on the Si base; said layer 
containing Al at the solid solubility of Al in silicon (.about.1.6%). 
Referring now to the drawings, FIG. 1 shows a conventional single crystal 
solar cell comprising a single crystal base wafer 10, silicon in this 
example, the top surface of same, or the surface to which illumination is 
to be applied, having diffused therein a suitable impurity layer 12 to 
produce a p-n junction 14 near the surface to be illuminated and in a 
plane parallel to said surface. A bottom surface 16 of electrically 
conductive metallic material is deposited on the underside of wafer 10, 
i.e., the surface where no illumination is to be applied, and functions as 
one of the ohmic contacts for the cell. The other ohmic contact 18, also a 
thin metal deposition, is applied to the top surface of the wafer 10 in 
the form of a finger or grid array. 
Referring now to FIGS. 2 and 3 there is shown the construction of a solar 
cell fabricated in accordance with the present invention. A layer of 
aluminum-silicon (Al-Si) alloy 22 is deposited on the surface of a silicon 
base wafer 20 of either single crystal or polycrystalline material. The 
application of a suitable heat treatment causes a solid-phase migration of 
the silicon to the interface 34 of the alloy layer 22 and the base wafer 
20, effecting the epitaxial growth of a silicon layer 36 at the interface 
34. The migrating silicon carries with it aluminum up to the solid 
solubility limit of aluminum in silicon, causing the epitaxially grown 
silicon layer to be p-type. If the base wafer 20 in n-type, a p-n junction 
is formed at the interface 34. 
FIGS. 4 and 5 show an alternate manifestation of the present invention. The 
doped epitaxial layer produced by solid phase migration and epitaxial 
crystal growth can be fabricated by the deposition of a layered structure 
46 on the base wafer 40. This layered structure 46 consists of a metal 
layer 42 in contact with the base wafer 40 and a semiconductor layer 44, 
deposited on the metal layer 42. Following the example of the 
aluminum-silicon system, the base wafer 40 is n-type silicon, the metal 
layer 42 is aluminum, and the semi-conductor layer 44 is silicon. The 
application of a suitable heat treatment causes the silicon layer 44 to 
migrate through the aluminum 42 and to grow as a p-type epitaxial layer 54 
at the surface 52 of the base wafer 40. A p-n junction is formed at the 
interface 52 of the base wafer 40 and the solid phase epitaxially grown 
layer 54. The transport of the surface silicon layer 44 through the 
aluminum layer 42 causes the aluminum to remain as a surface layer 56 
following the transport and growth of the aluminum-doped silicon epitaxial 
layer 54. 
After the solid phase growth of the epitaxial layer, the residual surface 
aluminum can be stripped away and ohmic contact fingers 68 can be applied 
by utilizing techniques which are well known in the industry. 
Alternatively, the aluminum could be selectively removed, with the 
remaining material serving as an ohmic contact to the grown p-type silicon 
layer 54. In addition, an ohmic contact 66 must be applied to the back of 
the base wafer 40, i.e., the side that is not to be illuminated. The 
techniques for producing ohmic contacts on semiconductors are well known 
in the art and form no part of the present invention. The preceding 
discussion refers to FIG. 6. 
While there is shown and described herein certain specific structure 
embodying the invention, it will be manifest to those skilled in the art 
that various modifications and rearrangements of the parts may be made 
without departing from the spirit and scope of the underlying invention 
and that same is not limited to the particular forms herein shown and 
described except insofar as indicated by the scope of the appended claims. 
If the solid solubility requirements of the semiconductor in the metal, 
e.g., Si in Al, are exceeded by employing a super-saturated combination of 
semiconductor-in-metal prior to the heat treatment the excess 
semiconductor can be caused to diffuse out of the metal. When the 
super-saturated metal-semiconductor is in contact with an underlying 
substrate material, the diffusing semiconductor will migrate to the 
interface between the metal-semiconductor combination and the substate and 
grow as an epitaxial semiconductor layer on the substrate. Returning again 
to the Al-Si system, the deposition of a layer of Al super-saturated with 
Si on a Si base, or substrate, and the application of an appropriate heat 
treatment will cause the excess Si to migrate to the Al/Si-Si interface 
and to form an epitaxial layer of Si on the Si base.