Pure silver ohmic contacts to N- and P- type gallium arsenide materials

Disclosed is an improved process for manufacturing gallium arsenide semiconductor devices having as its components an n-type gallium arsenide substrate layer and a p-type gallium arsenide diffused layer. The improved process comprises forming a pure silver ohmic contact to both the diffused layer and the substrate layer, wherein the n-type layer comprises a substantially low doping carrier concentration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to gallium arsenide solar cells, 
and more particularly to forming pure silver ohmic contacts to both n-type 
gallium arsenide semiconductor materials with substantialy low doping 
carrier concentrations and p-type gallium arsenide materials. 
2. Description of the Prior Art 
It is important for gallium arsenide solar cells which generate high 
current densities to have good ohmic contacts for efficient and reliable 
operation. The ohmic contact at a metal-semiconductor junction may be 
defined as one which exhibits linear current-voltage characteristics. A 
crucial property of the ohmic contact is its specific contact resistivity, 
that is electrical resistance between the contact and the semiconductor 
multiplied by the contact area. The specific resistivity of a good ohmic 
contact should be less than about 10.sup.-3 .OMEGA.cm.sup.2. The 
importance of good ohmic contacts becomes more significant when it is 
realized that to collect current within semiconductor solar cells, 
electrical connections must be made at the metal-semiconductor junctions, 
and that to maximize junction current flow, it is essential to use the 
lowest resistance contacts possible. 
The formation of ohmic contacts for gallium arsenide cells which exhibit 
acceptable low resistance depends on many factors. Heretofore, one primary 
such factor was the use of a highly doped semiconductor at the interface 
under the contacting metal. With regard to this factor, it will be noted 
that the present invention contemplates forming pure silver ohmic contacts 
to the n-type gallium arsenide materials with fairly low doping densities, 
as will be more fully explained hereinafter. Moreover, a number of other 
factors such as surface preparation, metal deposition conditions, 
reproducibility, cost-effective contacting techniques, and satisfactory 
electrical characteristics must also be considered in the formation of 
superior quality ohmic contacts. 
The requirements for selecting specific contacting metals for making ohmic 
contacts to gallium arsenide solar cells depends on many of the factors 
just previously mentioned. Generally, the most widely selected contacting 
metals are gold-base alloys. Gold alloys are frequently selected because 
they typically advantageously provide relatively good performing contacts 
with acceptable contact resistance. 
However, in spite of this significant advantage, there are some major 
problems associated with gold-alloy contact systems, and these problems 
generally involve cost. For instance, the cost of virtually every step 
involved in the manufacture of gallium arsenide cells presently 
substantially exceeds the cost of the manufacturing steps of silicon solar 
cell counterparts by a large factor. To illustrate further, the alloying 
process step of gold contact systems normally contributes to the high 
manufacturing cost because the gold alloys often comprise complex multiple 
element systems. Consequently, they are fairly expensive to produce. 
Similarly, the doping process step of gold contact systems also frequently 
contributes to the excessive manufacturing cost of this system. For 
example, as previously mentioned, to achieve ohmicity for the majority of 
gold-alloy contacts, the metal-semiconductor interface must necessarily be 
highly doped. Unfortunately, doping must necessarily be carefully 
performed since a heavy diffusion of donor or acceptor impurities can 
result in a deterioration of the underlying junction, which can eventually 
degrade solar performance. Consequently, this step is often costly, as 
well as time consuming. 
Incidentally, it is to be noted that the high doping carrier concentrations 
at the semiconductor interface may also have a material effect in 
electrically degrading cell performance. This is so primarily because the 
lifetime and diffusion lengths of the minority carriers are appreciably 
decreased as the carrier concentrations are increased. The result, of 
course, is a reduction in current collection efficiency. In addition to 
the above production problems, it is also fairly expensive to manufacture 
gold-alloy contacts at high volumes. 
Another major problem associated with some gold-alloy contact systems is 
aging. For instance, gold-alloy contacts which are made directly to the 
n-type surfaces are often subject to aging effects as a consequence of 
damage introduced into the n-type gallium arsenide materials by the 
alloying process. The effect of aging is generally to degrade the 
performance of the gallium arsenide cell. It also normally shortens the 
mean time of failure of the operational cell. Moreover, these conditions 
usually combine to adversely affect cell stability. 
Still another problem is that the pure gold alloys generally possess poor 
wettability (nonwetting). Poor wetting causes the liquid gold alloys upon 
heating to stand up in the form of drops at the semiconductor interface 
instead of spreading, which gives rise to a high specific contact 
resistivity. 
To cope with the aforesaid problems, particularly that of nonwetting, a 
layer of nickel is deposited over some gold-alloy contacts, such as 
gold-germanium, to suppress the balling-up effect. Unfortunately, nickel, 
despite its usefulness in enhancing wetting, is a fast diffuser in gallium 
arsenide materials, and therefore excessive amounts degrade the gold alloy 
contact performance. 
Another approach to overcome the problems associated with gold-alloy 
contacts is to replace them with less expensive metal alloys. To this end, 
silver base alloys are commonly used as an alternative to gold-alloys 
essentially because they provide quality ohmicity to both n-type and 
p-type cells at fairly high doping carrier concentrations. Unfortunately, 
in virtually all silver-alloy contact systems, the complexity of the 
metallization process and the high cost associated with manufacturing them 
still remain a severe problem. Additionally, some silver-alloy contacts, 
such as tin-silver, tarnish when exposed to air. This problem is 
compounded when the contact is to be bonded by thermal compression to a 
heat sink. 
In a similar approach, a number of pure metals have also been considered as 
alternative contact systems for gallium arsenide cells. Some of the most 
widely used pure metal contacts are molybdenum, chromium, titanium, tin, 
indium, gold, and silver. These metals are attractive because they usually 
form good performing ohmic contacts to either the p-or n-type materials. 
Unfortunately, most pure metal contact systems require substantially high 
levels of doping carrier concentrations to achieve ohmicity, and thus fail 
to satisfactorily solve the aforesaid associated problems concerning the 
reduction in current collection efficiency and the adverse effects of 
excessive diffusion of donors and acceptors during the doping process. 
In this regard, it is reiterated that the present invention contemplates 
using pure silver to form ohmic contacts to the n-type gallium arsenide 
materials at substantially low density carrier concentrations, as well as 
to the p-type gallium arsenide materials. Heretofore, it has been well 
established that for superior quality ohmic contacts to be achieved for 
the n-type materials, a highly doped semiconductor interface was 
absolutely necessary. To this end, all known prior art teachings indicate 
that pure silver will form either rectifying contacts (nonohmic) or 
contacts with poor conductivity on gallium arsenide materials unless the 
carrier concentration is equal to or higher than 1.times.10.sup.18 
carriers/cm.sup.-3 for n-types and 6.times.10.sup.18 carriers/cm.sup.-3 
for p-types. Hence, applicant's ability to obtain good ohmic contacts with 
pure silver on n-type gallium arsenide materials at one order of magnitude 
lower than that taught by the prior art was totally unexpected, as will be 
more fully discussed hereinafter. 
To continue, some pure metal contact systems such as molybdenum and 
chromium are problematic because they are extremely difficult to deposit. 
Some pure metal contact systems such as titanium and platinum are 
generally just as expensive as the gold alloy contact systems. Some pure 
indium contacts frequently have very low current drops at thresholds, and 
consequently are very unstable with time. Moreover, with the latter 
contacts failure by metal migration from the anode occurs rapidly. 
Notably, some pure tin contacts on bulk n-type materials often fail under 
bias by metal migration from the anode in the same way as pure indium 
contacts fail. 
Some articles containing information relating to the forming of ohmic 
contacts for gallium arsenide semiconductor materials include: R. P. Gupta 
and J. Freyer, Metallization systems for ohmic contacts to p- and n-type 
GaAs, Int. J. Electronics, Vol. 47 No. 5, 459-467, July 1979; K. L. Kohn 
and L. Wandinger, Variation of Contact Resistance of Metal-GaAs Contacts 
with Impurity Concentration and Its Device Implications, J. Electrochem. 
Soc., Solid State Science, Vol. 116, No. 4 507-508, April 1969; H. Matino 
and M. Tokunaga, Contact Resistances of Several Metals and Alloys to GaAs, 
J. Electrochem. Soc., Electrochemical Technology, Vol. 116, No. 5, 
709-711, May 1969; J. Palau, E. Testemale, Al Ismail, and L. Lassabatere, 
Surface and contact properties of GaAs overlaid by silver, J. Vac. Sci. 
Technol., Vol. 21, (1), 6-13, May-June 1982; and B. Schwartz, editor, 
Ohmic Contacts to Semiconductors, The Electrochemical Society, Inc., 1969. 
Additionally, some articles containing information relating to the 
annealing process used in forming ohmic contacts for gallium arsenide 
semiconductors include: C. Lindstrom and P. Tihanyai, Ohmic Contacts to 
GaAs Lasers Using Ion-Beam Technology, IEEE Transaction on Electron 
Devices, Vol. ED-30, No. 1, 39-44, January 1983; B. L. Sharma, Ohmic 
Contacts to III-V Compound Semiconductors, Semiconductors and Semimetals, 
Vol. 15 1-39, 1981; and J. G. Werthen and D. R. Scifres, Ohmic contacts to 
n-GaAs using low-temperature anneal, J. Appl. Phys. 52(2), 1127-1129, 
February 1981. 
SUMMARY OF THE INVENTION 
Against the foregoing background, it is therefore a general object of the 
present invention to provide a process for forming pure silver ohmic 
contact systems for gallium arsenide devices which overcomes many of the 
aforesaid shortcomings and disadvantages of the prior art metallization 
systems. 
It is another general object to provide an improved process for forming 
pure silver, low resistance, ohmic contacts on n- and p-type gallium 
arsenide semiconductor materials. 
It is a further general object to provide a simplified process for 
fabricating pure silver contacts on p- and n-type gallium arsenide 
materials usable in the manufacture of solar cells and having acceptable 
electrical cell performance characteristics, structural performance 
characteristics, reproducibility, and stability. 
It is a specific object of the present invention to provide an improved 
simplified process for fabricating pure silver, low resistance, ohmic 
contacts to n-type gallium arsenide materials at substantially low doping 
carrier concentrations. 
It is another specific object of the present invention to use a vacuum 
evaporation deposition process in combination with an annealing process to 
achieve ohmic contacts with pure silver to both n-type gallium arsenide 
materials at substantially low doping carrier concentrations and p-type 
gallium arsenide materials. 
It is still another specific object to provide pure silver, low resistance, 
ohmic contacts for gallium arsenide solar cells with doping carrier 
concentrations less than 1.times.10.sup.18 cm.sup.-3 for n-type gallium 
arsenide materials and less than 6.times.10.sup.18 cm.sup.-3 for p-type 
gallium arsenide materials. 
It is yet another specific object to replace gold alloy ohmic contact 
systems for gallium arsenide solar cells with substantially less expensive 
metal contact systems that can be applied to n-type and p-type gallium 
arsenide materials with simplified processing techniques. 
It is still another specific object to replace gold alloy ohmic contact 
systems for gallium arsenide solar cells with a cost-effective ohmic metal 
system which provides satisfactory electrical and structural cell 
performance characteristics that are substantially equal to those obtained 
with the gold alloy contact systems. 
The above objects, as well as still further objects and advantages, are 
obtained from the present invention which may be briefly described as a 
method for manufacturing gallium arsenide semiconductor devices having as 
its components an n-type gallium arsenide substrate layer and a p-type 
gallium arsenide diffused layer. The method includes forming a pure silver 
ohmic contact on both the diffused layer and the substrate layer, wherein 
the n-type substrate layer comprises a substantially low doping carrier 
concentration. 
Additional objects, advantages, and novel features of the present invention 
will be set forth in part in a detailed description that follows, and in 
part will become apparent to those skilled in the art upon examination of 
the following description or upon practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the elements or a combination of elements particularly pointed out in the 
appended claims. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The accompanying drawings, which are incorporated in and form part of the 
specification, illustrate preferred embodiments of the present invention 
and together with the description serve to explain the principles of the 
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring initially to FIG. 1, a gallium arsenide solar cell 2 is 
illustrated having a pure silver, low resistance, ohmic contact system 
which is generally denoted by the reference numeral 4. In the present form 
of the invention, the solar cell 2 includes a liquid phase epitaxially 
grown gallium-arsenide-aluminum window layer 14 approximately 2 .mu.m 
thin, which is heavily doped with magnesium to a carrier concentration of 
about 1.times.10.sup.19 cm.sup.-3. The cell 2 further includes a p.sup.+ 
diffused layer 8 of about 3 .mu.m to 4 .mu.m deep on a commercially 
available gallium arsenide n-type substrate 10 which possesses fairly low 
carrier concentrations of about 3.times.10.sup.17 cm.sup.-3, a broad area 
back ohmic contact of pure silver 12, and a finger-type front ohmic 
contact of pure silver 6. 
In practicing the inventive process, the substrate layer 10 is initially 
chemically polished, cleaned, and etched with a hydrofluoric acid. 
Thereafter, the p.sup.+ diffused layer 8 is formed by liquid phase 
epitaxially growing the window layer 14 on the substrate layer 10. To this 
end, the cell 2 is conventionally heated at a temperature range of from 
about 810.degree. C. to about 820.degree. C. for around about 10 minutes 
to effect diffusion of the magnesium from the window layer into the n-type 
substrate layer 10, and thereby to also effect forming the p-n junction 
15. Although the diffused layer 8 is preferably formed using liquid phase 
epitaxial growth techniques, it should be understood that other suitable 
techniques for preparing the diffused layer and the p-n junction, such as 
chemical vapor deposition techniques, may be employed as will occur to 
those skilled in the art. 
Following the formation of the window layer 14, photolithographic and 
photoresist techniques are utilized to define a front grid contact 
pattern. Afterwards, a plurality of channels 16 are etched in the window 
layer 14 for the contact grid pattern with a dilute hydrofluoric acid. 
Thereafter, the pure silver is deposited onto the bottom of the substrate 
layer 10 in any manner well known in the art so as to form the back 
contacts 12. The pure silver is next evaporated into the channels 16 under 
a vacuum in a range of from about 5.times.10.sup.-6 to about 
9.times.10.sup.-6 Torr for a depth of approximately 1 .mu.m thick. 
Thereafter, pursuant to the inventive process, the two deposited pure 
silver contacts 6, 12 are subjected to an anneal to achieve ohmicity. To 
this end, the deposited contacts 6, 12 are treated with heat using a 
forming gas comprising about 10% hydrogen and about 90% nitrogen. The 
annealing temperatures are in a range of from about 430.degree. C. to 
about 460.degree. C. with the preferred annealing temperature being about 
450.degree. C. The annealing cycle is from about 8 minutes to about 10 
minutes long. 
The following examples serve to illustrate certain preferred embodiments of 
the present process invention, as well as serving to compare the 
electrical performances obtained from these embodiments with the 
electrical performances obtained from prior art gold alloy ohmic contacts 
used with gallium arsenide cells and are not to be construed as limiting 
the scope of the present invention. 
EXAMPLE I 
Three ohmic contact systems for gallium arsenide cells are presented by the 
first example. One is a pure silver contact system, and the remaining two 
are gold alloy contact systems, namely, gold-beryllium and 
gold-germanium-gold. The pure silver contacts were fabricated using 
evaporation deposition techniques and annealing techniques in accordance 
with the invention. The two gold-alloy contacts were similarly fabricated 
with modifications to effect ohmicity thereof. 
To accomplish these objectives, gallium arsenide wafers were used for 
investigating the basic electrical characteristics of the three ohmic 
contact systems. Doping densities for the wafers were chosen to coincide 
with surface concentrations required in typical gallium arsenide 
photovoltaic devices. Hence, the doping carrier concentration of the 
n-type gallium arsenide wafers was 3.times.10.sup.17 cm.sup.-3 (tellurium 
doped). The doping carrier concentration of the p-type gallium arsenide 
wafers was 2.times.10.sup.18 cm.sup.-3 (zinc doped). 
All the gallium arsenide wafers had the same cleaning cycle, being 
degreased and etched with a hydrofluoric acid prior to the deposition of 
the pure silver and the gold alloys. Thereafter, the pure silver and the 
two gold alloys were sequentially evaporated onto the wafers used. The 
silver was deposited using a conventional electron beam evaporator at a 
vacuum of between 5.times.10.sup.-6 torr and 9.times.10.sup.-6 torr. A 
layer of gold-beryllium alloy approximately 5 .mu.m thick was evaporated 
onto each p-type wafer using resistance heating techniques. The 
gold-germanium-gold alloys were deposited onto the n-type wafers through 
successive evaporations of 100 .ANG., 1300 .ANG., and 2400 .ANG. layers. 
It will be noted that the gold-beryllium contacts were not fabricated on 
the n-type wafers and the gold-germanium-gold contacts were not fabricated 
on the p-type wafers. 
Thereafter, the deposited pure silver contacts and the deposited gold-alloy 
contacts were subjected to an anneal to achieve ohmicity. Ohmicity of the 
pure silver contacts was obtained at an annealing temperature of about 
450.degree. C. with an annealing cycle of about 10 minutes. Ohmicity of 
the two gold alloys was obtained at an annealing temperature of about 
350.degree. C. with an annealing cycle of about 15 minutes. 
Thereafter, the contact resistivity of the three contact systems was 
measured so as to enable comparisons in the contact resistivities of the 
silver and the gold alloy ohmic contacts. All of the resistivity 
measurements were accomplished using conventional equations in a well 
known manner. 
The following values of the contact resistivity were obtained by 
fabricating the aforedescribed pure silver and gold-alloy ohmic contacts 
on gallium arsenide semiconductor materials. 
______________________________________ 
Type of 
Ohmic Contact 
P--type GaAs 
N--type GaAs 
System (.OMEGA. cm.sup.2) 
(.OMEGA. cm.sup.2) 
______________________________________ 
Au(Be) 5 .times. 10.sup.-4 
Au/Ge/Au 3 .times. 10.sup.-4 
Ag 1 .times. 10.sup.-4 
4 .times. 10.sup.-4 
______________________________________ 
EXAMPLE II 
Example II provides the averaged results of the current-voltage 
characteristics of 16 ohmic contact systems on gallium arsenide cells; 8 
of the 16 being pure silver contacts, and the remaining 8 being 
gold-germanium contacts on the p.sup.+ gallium arsenide materials and 
gold-germanium-gold contacts on the n-type gallium arsenide materials. The 
current-voltage characteristics show the current-voltage relationship for 
the measured gallium arsenide cells and were accomplished for Example II 
using standard equations in a well-known manner. 
The 16 ohmic contact systems were fabricated in substantially the same 
manner as the three contact systems of FIG. 1, one difference being that 
the gallium arsenide semiconductor structures used were cleaved solar 
cells. Another difference being that each cleaved n-type substrate 
included at least 4 cells. The last difference being that the p-type 
materials comprised magnesium doped diffused layers. 
The following electrical characteristics were obtained by fabricating the 
aforesaid pure silver and gold alloy ohmic contacts on the n-type gallium 
arsenide semiconductor materials. 
______________________________________ 
Current Fill Series 
Type of Density Efficiency 
Factor Resistivity 
Contact System 
mA-Cm.sup.2 
n(%) 
##STR1## 
.rho..sub.s (.OMEGA. cm.sup.2) 
______________________________________ 
Ag 14.45 10.4 0.79 0.575 
Au Alloys 14.48 10.2 0.77 0.756 
______________________________________ 
From the foregoing, it may now be appreciated that the pure silver, upon 
being evaporated on low-doped n-type gallium arsenide semiconductor 
materials becomes ohmic after an anneal. Moreover, it may be further 
appreciated that the ohmicity obtained from such a pure silver contact 
system is usually substantially equal to the ohmicity realized from 
gold-base-alloy contact systems. 
Furthermore, it will now be apparent that it was also both unexpected and 
surprising that applicant's simplified metallization process would 
attractively provide pure silver ohmic contacts on both low doped n-type 
and p-type gallium arsenide materials, which may effectively be 
substituted for gold-alloy ohmic contact systems with substantially no 
degradation of gallium arsenide device performance. Additionally, from the 
aforesaid, it will now be apparent that it was both unexpected and 
surprising that the vacuum evaporation deposition technique in combination 
with the annealing technique would provide a simplified metallization 
process, which advantageously provides a more cost-effective ohmic contact 
system for both p-type and n-type materials. 
Additionally, it is evident that the ability to provide ohmic contacts for 
n-type gallium arsenide materials at fairly low doping densities indicates 
a promising potential for simplifying the doping processing step, as well 
as for minimizing electrical degradation due to the adverse effects of 
heavy doping concentrations on the life of the minority carriers. It is 
equally as evident that the ability to form ohmic contacts on the p-type 
gallium arsenide materials alone is not particularly advantageous due to 
the fact that the magnitude of the carrier concentrations is fairly 
normal. However, the ability to form pure silver contacts on both the n- 
and p-type materials is extremely advantageous since solar cell-type 
devices normally must necessarily employ both types of materials. 
The foregoing description of the preferred embodiments of the present 
invention have been presented for purpose of illustration and description. 
It is not intended to be exhaustive or to limit the present invention to 
the precise forms disclosed, and obviously many modifications and 
variations are possible in the light of the above teaching. 
The embodiments were chosen and described in order to best explain the 
principles of the invention and its practical application to thereby 
enable others skilled in the art to best utilize the invention and various 
embodiments and with various modifications as are suited to the particular 
use contemplated. It is intended that the scope of the invention be 
defined by the claims appended hereto.