Gallium arsenide solar cell preparation by surface deposition of cuprous iodide on thin n-type polycrystalline layers and heating in iodine vapo

Ellis, S. G.

English

NASA Technical Reports Server

REPLY TO 
ATTN OF: 
TO : 
FROM: 
SPACE 
C .  20546 
A D M  INISTRAT ION 
USI/Scientific & Technical Information Division 
Attention: Miss Winnie M, Morgan 
GP/Office of Assistant General 
Counsel for Patent Matters 
SUBJECT: Announcement of NASA-Owned 
U-S, Patents in STAR 
In accordance with the procedures contained in the Code GP 
to Code US1 memorandum on this subject, dated June 8, 1970, 
the attached NASA-owned U,S. patent is being forwarded for 
abstracting and announcement in NASA STAR. 
The following information is provided: 
U,S. Patent No. : 3,340,599 
Corporate Source 
Supplementary 
Corporate Source 
NASA Patent Case NO.: XN 960 
Please note that this patent covers an invention made by an 
employee of a NASA contractor, Pursuant to Section 305(a) of 
the National Aeronautics and Space Act, the name of the 
Administrator of NASA appears on the first page of the patent;,, 
however, the name of the actual inventor (author) appears at 
the heading of Column No. 1 of the Specification, following the 
words ' le  e e with respect to an invention of, . e 
./ 
Gayle Parker 
Enclosure: M 09 p1 & - 7 2 2 - -  (THRU 
copy of Patent 
U - t2 (NASA CR OR TMX OR AD NUMBER) 
Bb 
https://ntrs.nasa.gov/search.jsp?R=19710013551 2020-03-17T03:01:31+00:00Z
This hvention relates to an improved method for pre- 
paring gallirn arsenide solar cells by the deposition of a~ 
inversion layer at its sdacc. The llpethod cam be per- 
f0IlBld 
Control. 
positing 
cuprons iodide on a relatively thin n-type polycrystal- 
te, 
to 
origrn of iilve 
The invention described berein was made in the per- 
formance of work under a NASA contract and k subject 
to the p0vkion of Section 305 of the National Aero- 
nautics and Space Act of 1958, pnblic Law 85-568 (72 
Stat. 435; 42 USC 2457). 
A photovoltaic cell geherates a voltage directiy as a 
result of the absorption of a photon. Typical junctions 
for the photovoltak cells are selenium-iron, copper oxide- 
copper. The photovoltaic cell consists of a an n-p junc- 
tion between two different semiconductors, an n-type ma- 
terial in whih  conduction is due to electrons, and a 
ptype material in which conduction is due to positive 
holes. When light is absorbed near such a junction, new 
mobile electrons and holes are released. An additional 
feature of a photovoltaic cell, however, is that there is 
an electric field in the junction region between the two 
semiconductor types. The released charge moves in this 
field. This current will then fiow in an external circuit. 
If the external circuits are broken, an open circuit photo- 
voltage appears at the break. 
The solar cell is a particular class of photovoltaic cell 
and may be considered as a constant current generator, 
the constant current being equal to that obtained on short 
circuit. This generator is shunted by the p-n junction, 
acting as a diode rectifier, and the load resistance. In 
addition, there are both shunt and series resistance ele- 
ments present in the solar cell. 
The most common solar cell produced heretofore is 
the silicon photovoltaic cell. Pure silicon can be made 
n-type by appropriate doping. By exposure to a boron 
containing vapor at high qmperature the surfaces of the 
silicon is transformed to a ptype semiconductor. This 
type has a low electron density but a high hole density, 
so that the current is a migration holes, or positively 
charged sites, through the material. The region between 
the p and n-type silicon is called the barrier region, and 
the whole structure is called a p n  junction. 
Exposure of the treated surface to light produces light 
absorption within a layer about 0.0010 centimeter thick. 
Each light photon absorbed displaces an electron, pro- 
ducing both a free electron and an electron vacancy or 
hole. Since the original surface had a low electron density 
compared to the hole density, the effect of this photon 
absorption is to increase the electron density while h- 
creasing the hole density to a much lower extent. 
A portion of the excess electrons will have sufficient 
have been investigated. 
10 
to the junction. The juuction4o-smke distance is Criti- 
16 cal in this method and is of the order of one-half micron. 
Heterojunction cells, while far less chitical in the thick- 
ness of the wide band-gap material, require high-tena- 
perature p and ]present9 other serim difficulties 
The mas of single crystal gallium arsenide cells arr 
limited to about 1 centimeter by 2 centimeters at pes- 
ent, and it hecomes expensive to construct a large area 
cell by assembling many small single crystal cells. E on 
the other hand one makes a large area polycrystalline 
25 A-type gallium arsenide iilm and attempts to form a snr- 
face junction by zinc diffusion then the rapid zinc diffu- 
sion down the crystal bou~daris hort circuits tlhe p-tyipe 
layer to the substrate holding the film. The present inven- 
tion disclases a low-temperature method of forming a 
It has been knowr~ for a long time that the surface 
potential of a semiconductor is Muenced by the chemical 
phase or phases with which the semiconductor is in con- 
tact. Under some conditions, the semiconductor may have 
35 an inversion layer at its surface; that is, the surface has 
the opposite conductivity type to the bulk Given the 
right combination of properties, a chemical phase (pref- 
erably solid) formed on a semiconductor can produce a 
photovoltaic junction. The first of these properties is that 
40 a strong inversion layer should be produced in the semi- 
conductor. The added phase should make an ohmic con- 
tact to the inversion layer. Also, the added layer should 
have high transmission for the radiation which will excite 
the semiconductor. Finally, the added layer should have 
45 low lateral resistance. There are, of course, other desir- 
able properties which the added layer should have. For 
example, these may include ease of application, and 
chemical and electrical stability. The fistmentioned char- 
acteristics are, however, essential. 
The present invention provides a novel method for 
the fabrication of a photovoltaic semiconductor having 
improved properties over similar devices made hereto- 
fore. In a preferred embodiment this comprises a large- 
55 area solar cell having a transparent conducting layer of 
cuprous iodide on a polycrystalline layer of n-type gal- 
lium-arsenide. The combination forms a barrier having 
improved performance as well as good rectifying proper- 
ties. Other characteristics will be discussed hereinafter. 
Accordingly, it is a principal object of this invention 
to provide direct contact to a polycrystalline layer of 
gallium arsenide and more particularly a cuprous-iodide 
contact to n-type gallium arsenide. 
A further object of this invention is to provide a novel 
66 method for forming a rectifying and photovoltaic barrier 
with an n-type gallium arsenide. 
Another object of the invention is to provide a novel 
and improved method of forming a p t y p e  inversion layer 
Yet another object of the invention is to provide a 
solar cell of novel and improved characteristics comgris- 
in preparat1on. 
20 
30 barrier which avoids this di5dty. 
60 
?0 on an n-type semiconductor. 
3,~Q,5~ 
ing a layer of p-type cuprous iodide on a crystal of a 
n-type gallium arsenide. seconds with some agitation. AIcohol wash and dry. 
Jt is still a further object of the invention to provide 
a novel and improved gallium arsenide semico&nctor wash and transfer wet to copper plating solution. 
device. 6 (4) Electroplate copper layer onto G a h  sheet. 
It is a general object of this invention to provide novel 
and improved methods of fabricating photovoltaic devices '~ 
which overcome disadvantages of previous means and $ 
methods heretofore intended to accomplish generally 
\ similar purposes. IO Many other advantages, features, and additional ob- invdves fie use of a G& sheet pre- 
jects of the present invention wiiI become manifest to pared as in Example 11. ss example, however, the 
those versed in the art upon malring reference to the r aa i&g  method of applying the CUI layer is used. The 
detailed description which follows: third method of forming cuprous iodide layer ki the direct 
The invention is based on a transpant condufting 15 evaporation of cuprous io&& to a thickness between 3 
layer of cuprou~ iodide (cd), which is a P-WP semi- and 4 microns- me device is subsequently exposed to 
conductor, on an n-type gallium arenside (G*) Poly- iodine vapM at 70" centigrade for 2 minutes after which 
Crystalline klyer. The Combination forms a barrier With a gold grid is evaporated on the surface to give ohmic 
gmd rectifying prOpedeS. The junction between the -1 to cuprous iodide, as indicated in the previous 
and the GaAs is referred to as a "barrier" rather than a 20 examples. It is Once again given a 2 minute exposure to 
"heterojunction" since it comprises a rectifying structure iodine vapor at 700 e. and is &owed to age in dry air for 
resultin tact of two dissimilar chemical 
phases. which are characteristic of prior p r e - m h e n a  &&bed in the foregoing .ex- 
art dev special form Of "barrier" Where mples are designed PO athe amonnt of oxide 
the junction exists at, or on both sides, Or the Chemical e5 left on the ms c r y s ~  
interface. The value of I, depends not only on the Pre-tmment 
Briefly the method of making a large area d l  ctlnskts of the G&, but also on the optical transmission and 
of the following steps: sheet resistance of the 011 layer. These deepend on more 
(1) Deposit a large area n-type PoIycrystaEne layer than tbic-s of the layer. 
4 mils thick on a conducting substrate and making ohmic 30 of V, with a barrier 
contact to it by techniques well known in the art. depends on the chemical &tory of the GaAs 
(2) Clean the upper surface of this layer and deposit up to the time at which the or is deposited on 
on it a layer of cuprous iodide by methods described it  elmdeposition method is the fstest one for 
below. producing an experimental cell. However, the preferred 
(3) Make ohmic contact to the cuprous iodide by 36 method of CUI layer has been by direct 
evaporating on a gold grid. o a tbickness between 3 and 4 
(4) Heat the cuprous iodide layer with iodine vapor microns. s of the method are that it can 
+$o lower its resistance. be done at low temperatures (i.e., not above 70" C. 
The same procedures Can be applied to *de CrYSbI and it is relatively simple to control. Also, because it 
-gallium arsenide. can be &ne by vacuum evaporation, it can be used to 
. TO better mderstand the present invention, a detailed pmdum sob cells of special shapes for such applica- 
'description of a specific example will now be given. tions as direction sensing and pattern recognition. 
Although the invention has been described in terms 
of specific preferred embodiments, nevertheless, it will 
Of pO1yc~stafline forma- 45 be appreciated that various changes and modifications 
will occur to in the art which do not in 
fact depart from the teachings in the present invention. 
Such changes are deemed to and scope 
of the inventim as defined by the appended claims. 
(2) Etch in 1% bromi in methyl alcohol for 20 
(3) Wash in hydrofluoric acid for 30 seconds water 
( 5 )  Expose copper layer to iodine vapr at 70" e. for 
(6) Apply electrode for ohmic contact to CUI layer as 
2 minutes. 
in m p l e  L 
EXAMPLE 
m i s  
to the cu deMtion. 
a 
E E 1  
tion was prepared having a thickness of about 20 mils 
and an area about 130 mils square. The material exhibited 
n-type conductivity. The wafer was placed in a suitable 
vacuum evaporating apparatus, and by conventional 
VaCUUm deposition process, a thin f i h  of copper is evapo- 60 mat claimed is: 
rated onto one surface of the gallium arsenide wafer. 
The copper layer is subsequently exposed to iodine vapor 
at 70" C.  for two minutes and then a gold grid is 
evaporated onto the exposed surface to give ohmic con- 
tact to the CUI. The device as prepared k~ the foregoing 55 
Example I was tested, and exhibited V,=0.82, Z,=20 
ma. under a focused microscope light. 
The gallium arsenide should be a n-type with carrier 
concentration in the range of 1016 to 1017 centimeters. 
The cuprous iodide layer can be formed by various 
techniques. These include: (1) evaporating copper onto 
the gallium arsenide and exposing it to iodine vapor at 
70" centigrade (2) electroplating copper onto gallium 
arsenide and exposing it to iodine vapor at 70" centigrade 
(3) evaporating CUI 
A wafer of gallium 
within 
1. The method of making a photovoltaic junction corn- 
forming an *type gallium arsenide layer on a metal 
+sing the of: 
substrate; and 
posed surface of said layer. 
dewitins a a m  of ptype cuprous iodide on & ex- 
2. The method of making a photovoltaic e l l  com- 
mounting a layer of polycxystalline gallium arsenide 
forming an inversion layer by depositing cuprous iodide 
applying an ohmic contact to said inversion layer. 
3. The method of forming a barrier solar cell corn- 
depositing a polycrystallii film of n-type gallium 
arsenide, having gain boundaries, onto a metal 
substrate; 
f cugrous iodide on said 
prising the steps of: 
on a metal substrate; 
on said gallium arsenide layer; and 
65 prising the steps of: 
The gallium arsenide is cut into sheets with parallel 
surfaces and the upper si& is used for the cuprous iodide 
deposition. The following etching p r d m  is then fol- 10 
lowed: 
(1) Etch in a solution comprising 5 parts W # Q .  I 
part W20, 1 part H2Q (all full strength) until the worked 
surface is removed. and t h ~  rrystal is SWJ& Water wash 
and dry. 15 
p 
evaporating cuprous iodide onto said 
layer to a tbkkness hetween 
layer. 
contact to said cuprous iodide 
5. The methud of f d n g  a ~~~~1~ cell oom- 
forming a sheet from a galium arsenide crystal so 
as to have parallel surfaces; 
etching said sheet until the worked surface is removed 10 
and the crystal is smooth; 
washing said etched surface to remove the etchant and 
to minimize the amount of oxide on said crystal; 
depositing a layer of cuprous iodide onto said gallium 
Blsenide layer to a thickness between 3 and 4 16 
prising the steps of: 
to said eupron% iodide layer to 
thereto; 
exposing said cnprous iodide layer to 
a i rdy ing  said ceM for approxmiately 43 hours. 
6. The method of  a photovoltaic junction a m -  
ana 
least QEE of said copper-plateca surfaces 
to iodine vapor at approximately 7 0 ° C  to form 
3,114,@88 12/1963 Abercrombs ________ 317-237 
3,264,707 8/1966 jElie __- ______-_____ 29-155.5 


Simple method of making photovoltaic junctions Patent

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server