Method for the manufacture of gallium arsenide thin film solar cells

A method for the manufacture of gallium arsenide thin film solar cells on inexpensive substrate material whereby an intermediate layer of highly doped, amorphous germanium is employed in order to promote the growth of the gallium arsenide layers. A high-energy radiation is directed to specific, prescribed points on the highly doped, amorphous germanium layer thereby generating centers having a defined crystal orientation, so that the epitaxial layer spreads laterally from these centers in a surface-covering fashion during the epitaxial vapor phase deposition. The solar cells produced by designational grain growth can be manufactured with high purity in a simple way and have an efficiency (greater than 20%) comparable to known mono-crystalline solar cells.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for the manufacture of gallium 
arsenide thin film solar cells and in particular to a method wherein a 
germanium layer is applied to the substrate before the gallium arsenide 
layers are applied. 
2. Description of the Prior Art 
Research to develop a solar cell having a high conversion efficieny, 
efficiencies of higher than 20%, that is inexpensive to manufacture, and 
that is stable has not led to any satisfactory result. One such result, a 
thin film solar cell of amorphous silicon, can be cost-favorably 
manufactured by a glow discharge method, but these cells are unstable and 
have a very low conversion efficiency, roughly 5%. 
Another such result, a crystalline silicon solar cell can also be 
cost-favorably manufactured in band or film form by growing the crystals 
using a high growth rate, but these cells have a low conversion 
efficiency. However, a high conversion efficiency can be obtained for 
these cells if the crystalline silicon solar cells are produced in accord 
with conventional crystal growing methods, that is, using a low growth 
rate. This in turn renders the cells relatively expensive to manufacture 
and unsuitable for mass production. 
Another important factor to consider when manufacturing crystalline solar 
cells is the selection of a suitable initial material. This material 
should be inexpensive, simple to work with and have the purity appropriate 
to achieve a high conversion efficiency. 
While thin film solar cells of gallium arsenide have a higher theoretical 
conversion efficiency (24 to 28%) than the aforementioned silicon solar 
cells, the manufacture of gallium arsenide epitaxy layers with an adequate 
grain size, preferably in columnar structure, presents difficulties. Also, 
the selection of an inexpensive substrate material having lattice matching 
properties is a further problem. 
It would therefor be an improvement in this art to manufacture a solar cell 
which is stable, has a high conversion efficiency, and is inexpensive. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for the 
manufacture of a solar cell having a high conversion efficiency, and which 
is stable and inexpensive to manufacture. 
It is a further object of the present invention to provide a method of 
making solar cells utilizing gallium arsenide as a semi-conductor body. 
In accordance with the foregoing objectives, the present invention provides 
a method where a highly doped germanium layer is deposited, in its 
amorphous state, on an inexpensive substrate, such as metallized glass or 
planar silicon. The amorphous germanium layer is then divided into zones 
having a defined crystal orientation by irradiating the layer with 
high-energy radiation at defined, prescribed points. A vapor phase epitaxy 
is then performed originating at these prescribed points, wherein the 
parameters of the epitaxy are set such that, proceeding from the 
prescribed points, the epitaxial layer laterally spreads in 
surface-covering fashion until the crystallization fronts of neighboring 
zones meet. 
The invention thereby utilizes the perception gained from the article by 
Shirley S. Chu et al., Journal of Applied Physics, Vol. 48, No. 11, 
November 1977, pages 4848-49, of promoting the growth of gallium arsenide 
layers on recrystallized germanium layers. The present method calls for 
the use of high-energy radiation for crystallization, whereby the 
crystallization zones produced, these preferably lying in the region of 
100 um, prescribe the grain size of the gallium arsenide epitaxy layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The Figure of the drawings shows a sectional view of a thin film gallium 
arsenide solar cell produced in accordance with the method of the present 
invention. 
A substrate 1 of, for example, metallized glass or planar silicon produced 
by tape drawing is used. A highly doped, amorphous germanium layer 2 
(a-Ge:P:H) having a phosphorous content of at least 1%, which is formed by 
the decomposition of GeH.sub.4 and phosphine (PH.sub.3), is deposited in a 
low-pressure glow discharge reactor in a layer thickness of 0.2-0.5 um. 
This germanium-phosphorous layer 2 is crystallized into zones, germanium 
having &lt;001&gt; and &lt;111&gt; preferred orientations, at specific, prescribed 
points 3 by using a pulsed or continuous wave laser having a suitable 
wavelength, in the region of 1 um for germanium. This crystallization can 
also be achieved at the points 3 by using an electron beam. 
Next, a gallium arsenide epitaxy suitable for the manufacture of solar 
cells is executed in a known fashion, for example from the metallo-organic 
compound Ga(CH.sub.3).sub.3 and arsine (AsH.sub.3) or from arsine and 
gallium trichloride (GaCl.sub.3), the accretion performed at the 
crystallization points 3. Deposition of a gallium arsenide layer 4 on the 
amorphous regions of the germanium-phosphorous layer 2 cannot occur due to 
the lack of lattice matching between the two layers. 
The gallium arsenide layer 4, proceeding from the prescribed 
crystallization points 3, each point being about 5 um long and spaced from 
the adjacent point a distance in the range between 20 and 1000 um, 
preferably of 100-200 um between each point, spreads laterally in the 
directions of arrows 7 in a surface-covering fashion until the 
crystallization fronts of neighboring zones in the germanium phosphorous 
layer 2 meet. The growth surfaces are of either &lt;001&gt; or &lt;111&gt; preferred 
crystallization orientation. Grain boundaries 8 arise at the meeting 
points of the crystallization fronts, but these boundaries are formed 
perpendicularly or nearly perpendicularly to the substrate surface 1. 
Thus, these boundaries do not exert any noteworthy harmful effect on the 
current collecting properties of the solar cell. Furthermore, the grain 
boundaries 8 can be passivated by the hydrogen diffusing out of the 
amorphous germanium-phosphorous layer 2 during the epitaxy. 
When the deposition of the gallium arsenide is performed at temperatures 
between 700.degree. and 800.degree. C., the gallium arsenide is deposited 
in crystalline form. Also, the amorphous germanium can crystallize at 
these temperatures, thereby promoting the gallium arsenide grain growth. 
The highly doped germanium-phosphorous layer 2 forms the ohmic contact of 
the gallium arsenide solar cell to the substrate 1. 
The spacings between adjacent crystallization points 3 generated by the 
laser are selected in the region of 100 um because these points will 
determine the grain site. The first gallium arsenide layer 4 is deposited 
in a layer thickness of 3-5 um. This first layer is doped with zinc and is 
n-type conductive. A second gallium arsenide layer 5 is deposited on this 
first layer in a known fashion having a layer thickness of 0.2-1 um. This 
second layer is doped with tin and is p-type conductive. Finally, this 
arrangement is covered with a gallium aluminum arsenide mixed crystal 
layer 6. The gallium aluminum arsenide layer 6, having a layer thickness 
of 0.5-l um, is p-type conductive and serves as a window layer. It 
broadens the band gap between the conduction band and the valency band and 
prevents surface recombination. 
Because the individual process steps are executed successively in a closed 
system by simply connecting the various reaction gases, a high purity of 
the amorphous, highly doped germanium layer 2 is assured in a 
cost-favorable manner. This means that the otherwise sensitive gallium 
arsenide vapor phase epitaxy will not be initiated by disruptive foreign 
nuclei. 
Although other modifications and changes may be suggested by those skilled 
in the art, it is the intention of the inventors to embody within the 
patent warranted hereon all changes and modifications as reasonably and 
properly come within their contribution to the art.