NASA Technical Reports Server (NTRS) 19920015559: Sinterless contacts to shallow junction InP solar cells

8 Pages·0.46 MB·English

by  NASA Technical Reports Server (NTRS)

/I/W ",z_"f
,/_o
NASA Technical Memorandum 105670
Sinterless Contacts to Shallow Junction InP
Solar Cells
V.G. Weizer
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio
N.S. Fatemi
Sverdrup Technology, Inc.
Lewis Research Center Group
Brook Park, Ohio
and
A.L. Korenyi-Both
Calspan Corporation
Cleveland, Ohio
Prepared for the
Fourth International Conference on Indium Phosphide
and Related Materials
sponsored by the Institute of Electrical and Electronics Engineers
Newport, Rhode Island, April 21-24, 1992
(_ ::,A-| '_-._0')_570) ...... _)_,L_SS CON/ACTS 1"0 N92-24802
_,_%_L.__,_ JU_+CIIL)N IrtP SOLAR C_:LLS [NASA)
7 _ CSCL I0 A
Unc|_s

SINTERLESS CONTACTS TO SHALLOW JUNCTION InP SOLAR CELLS
V.G. Weizer N.S. Fatemi A.L. Korenyi-Both
National Aeronautics and Space Administration Sverdrup Technology, Inc. Calspan Corporation
Lewis Research Center Lewis Research Center Group Cleveland, Ohio 44130
Cleveland, Ohio 44135 Brook Park, Ohio 44142
_i In the past, the achievement
Before discussing our
of good electrical contact to InP
results, however, let us consider
has inevitably been accompanied by
the metal-semiconductor reactions
mechanical degradation of the InP
that take place between InP and Ni
itself. Most contact systems
and between InP and Ag.
require heat treatment after metal
deposition that results in the
The InP-Ni Interaction
dissolution of substantial amounts
of InP into the metallization.
Figure 1 shows the variation
Devices such as the solar cell,
of R c (TLM measurements) for Ni
where shallow junctions are the
contacts (2000 A thick) on (i00)
rule, can be severely degraded if
n-InP (Si: 1.7 x 1018 cm 3) during
the damage to the semiconductor
heat treatment at 400 C. Within a
substrate is not precicely
matter of minutes R c is seen to
controlled.
drop several orders of magnitude
While there are several
to the mid 10 -7 ohm cm 2 range. As
remedial approaches to control the
the sintering proceeds, however,
sinter-induced metallurgical
R c eventually rises back up to the
interactions such as the use of
10 -4 ohm cm 2 range.
rapid thermal annealing (RTA)
To determine the causes of
techniques or the imposition of
the R c variations we attempted to
diffusion barriers between the InP
profile the sintered contacts
and the current carrying
using both AES and EDS. We
metallization, their use adds
performed these measurements on
complexity and an element of
trial-and-error to the contacting
process.
...,. 10 "a ...
A better solution would be
to eliminate the need to sinter
Ni
the contacts and thus avoid the
device-destroying metallurgical 10"_
interactions that accompany high
temperature processing. In this
paper we describe two contact
systems that provide low contact _ 1O'S
resistance to InP solar cells that
do not require subjecting the
current carrying metallization to _ lO'e
a post deposition sintering
process. We show that these two z 400°C
systems, one nickel-based and the _ 10-7
I , I * !
other silver-based, provide 0 10 20 3O
contact resistivity (Rc) values in
TIME (rain.)
the low 10 -6 ohm cm 2 range, as
fabricated, without the need for
Fig. 1 Thevariation of R forNi-contacted InPduring sintering
sintering. at400°C. c
1
samples that had been sintered for and the previous samples was that,
several minutes (low Rc) and for in this case, only one phosphide
extended periods (high Rc). Prior layer is observed between the InP
to EDS examination we chemically and the In-rich layer. The data
etched the samples using HN03- and indicates that this layer is
Thiourea-based etches so as to
composed of Ni2P. The resulting
expose the subsurface structure. structure is described
Both the EDS and the AES schematically in figure 2b.
analyses indicated that the Since the presence of the
contacts sintered for only a Ni3P interlayer is correlated with
minute or so (low R c) consist of the achievement of low R c values,
three distinct layers. The we suggest that the formation of
outermost layer consists almost this layer in the early stages of
entirely of In metal containing a the sintering process is the cause
small amount (15%) of P. Beneath
of the dramatic drop in Rc, and
the In layer in these low R c that its disappearance (via
samples we found two Ni-P layers. conversion to Ni2P) upon extended
The analysis indicates that the heating results in the observed
layer which is in contact with InP rise in Rc. Similar Rc-lowering
is the compound Ni3P with a characteristics have been reported
phosphorus content of 25 at%. The for Au2P 3 in the Au-InP system. (I)
analysis further indicates that
the more remote layer consists of The Ag-InP Interaction
the 33 at% phosphorus compound
Ni2P. A schematic diagram of the In a manner similar to the
suggested structure is given in Ni-InP system, the specific contact
figure 2a. The relative positions resistivity of Ag-contacted InP
of the two phosphide layers is drops quickly upon sintering at 400
somewhat surprising in light of the C. Figure 3 shows the variation of
fact that the phosphorus source is R c with time at 400 C for 2000 A
the InP. One would have expected thick Ag contacts on the same InP
the more P rich compound to form substrate material described
adjacent to the InP, and not previously. R c values in the low
vice-versa. 10 -6 ohm cm 2 range are readily
A similar analysis was
performed on samples subjected to
extended sintering (high Rc). The
• | - i - i - I - z - i
only major difference between these _ 10"2
Ag 400°C
10"
In('P) In (P) Ag (In)
¢_ 10"
Ni_ _Pz _ On) m
Ni3P _ 10"
ImdP InP InP
O 10" • ! - i , i , i , i , i
10 20 30 40 50 60
TIME (rain.)
Fig. 2 Schematic description of sintered contacts, a)Ni-InP, 400°C,
Fig. 3 The variation of R_for Ag-contacted InP during
4 rain., b) Ni-InP, 400°C, 30min., and c) Ag-InP, 400°C.
sintering at400°C._
achieved after a minute or so at sintering schedule sufficient to
400 C. This system differs from produce low R c values (several
the Ni-InP system, however, in that minutes at 400 C) results in the
extended sintering does not cause a dissolution of large amounts of
rise in R c. InP into the contact
As we did in the previous metallization. The result is
case, we attempted to profile the emitter perforation and a shunting
reacted metallization by combining of the electrical characteristics
various chemical etches with EDS of the device. The diodes used in
and _<PS analyses. Employing KI- figures 1 and 3, for example, were
and Thiourea-based etches to reveal seriously shunted after a minute
subsurface details, we have found at 400 C.
that there are two major layers in Since the reason for the
the sintered metallization. Our sintering step is thought to be to
analysis indicates that at the free generate certain metal phosPhides
surface of the contact metal there at the metal-InP interface, ii
is a layer of Ag containing some follows that one should be able to
In. Between this layer and the InP eliminate the need for sintering
surface we have found a second by introducing the appropriate
layer consisting of regions of an phosphide layer by some other
Ag-P compound coplanar with regions means. While it is no doubt
of Ag(In). This structure (figure possible to introduce a phosphide
2c) is present after sintering for layer via vapor or chemical
1 minute at 400 C and remains deposition, there is a simpler,
qualitatively unchanged during albeit slightly more destructive,
extended sintering at that way to deposit the layer.
temperature. The Ag-P compound Since the electrical
which we have shown to be in characteristics of the contact
intimate contact with the InP and system are controlled by the
to account for about 50 % of the nature of the interface between
metal-semiconductor contact area, the InP and the metal phosphide,
has been observed by others and it seems reasonable to conclude
positively identified as AgP2.(2) that a very thin phosphide layer
The correlation between the would suffice. Thus, if a very
drop in Rc and appearance of AgP 2 thin (Angstroms thick) metal layer
at the metal-InP interface leads
were deposited on the InP and
one to suspect that there is a sintered, the required phosphide
cause-and-effect relationship would form, but it would be
between the two events. It is
accompanied by only a very small
suggested, therefore, that in the amount of emitter dissolution.
Ag-InP system, as in the Au-InP and The contacts could then be built
the Ni-InP systems, significant up with a conductive metal. In
reductions in the specific contact this way gross damage to the
resistivity can be achieved by semiconductor device could be
introducing an appropriate avoided while still achieving the
metal-phosphide layer between the low R c values attendant to
InP and the current carrying interfacial phosphide formation.
metallization. It should be noted that the use of
a thin interfacial phosphide layer
Non Destructive Contacts
has proven successful in the
Au-InP system.(3)
In the foregoing we have To test this idea in the
shown that low contact
present systems we fabricated a
resistivities can be achieved with
number of TLM patterns by first
either Ni or Ag contacts. In both evaporating 40 A of either Ag or
of these systems, however, a Ni. The thin layers were sintered
3
at 400 C for two minutes to previous results employing a thin
generate the respective phosphide gold phosphide interlayer(3) and
layers. The samples were then with Rc values typical of Au
carefully remasked and the contacts prior to sintering. When
patterns built up with a the current-voltage
conducting metal (Au used here). characteristics of the diodes upon
Figure 4 shows electron which these contacts were
micrographs of the thin sintered deposited (2000 A junction depth)
layers prior to remasking and were inspected, there was no sign
buildup. The Ag structure of device shunting. The
consists of unconnected islands conduction voltages (see ref i)
that cover about one third of the for all of the Ni- and
surface area. Ni, on the other Ag-contacted samples in figure 5
hand, forms an open dendritic were in the 900-to-950 mY range,
lattice with about a 50 % surface indicating negligible diode
coverage. The resulting degradation.
as-fabricated Rc values (after Au We can conclude, therefore,
buildup) are shown in figure 5 that it is indeed possible to
where they are compared with achieve specific contact
resistivities in the low 10-6 ohm
cm2 range without compromising
emitter integrity through the
introduction of any of a number of
metal-phosphide interlayers
including Ni3P, AgP2, and Au2P3.
_.. 10-2
E
Y
o
_._ 10-3
o
I-,-
_ 10.4
ul
0
I-- lO-S 8
0
°
F-
Z
0 o
0 10" s I | I I
Au Au2 PS NI 3P AgP2
Fig. 4 Structure of thin metallization layers after sintering Fig. 5 As-fabricated contact resistivities for contacts
at 400°C for 2min. Upper: Ni, Lower: Ag. with Au2P 3 ,Ni3P, & AgP2interlayers.
Acknowledgements
The authors wish to thank Frank S. Honecy for providing the
AES analysis and Douglas T. Jayne for providing the XPS analysis.
Work performed by N.S.F. and A.L,K. was done under contracts
NAS3-25266 and NAS3-30759, respectively, with the NASA Lewis
Research Center.
References
i) V.G.Weizer and N.S.Fatemi, "The Influence of Interstitial Ga and
Interfacial Au2P3on the Electrical and Metallurgical Behavior of Au
Contacted III-V Semiconductors", J. Appl. Phys. 69, 8253 (1991).
2) D.L.Kirk and J.S.K.MilIs, "The Appearance of AgP 2 Within the
Interface of Silver Metal and (i00) Indium Phosphide", Thin Solid
Films 67, L29 (1980)
3) N.S.Fatemi and V.G.Weizer, "The Formation of Low Resistance
Electrical Contacts to Shallow Junction InP Devices Without
Compromising Emitter Integrity", J. Electron. Mat. 20, 875 (1991).
Form Approved
REPORT DOCUMENTATION PAGE OMB NO.0704-0188
Public reporting burden for this collection of information is estimated to average 1hour per response, including the time for reviewing instructions, searching existing data sources,
gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this
collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for information Operations and Reports, 1215 Jefferson
Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.
1. AGENCY USE ONLY(Leaveblank) 12. REPORT DATE
I
1992 3. REPORT TYPEATNeDchDniAcTalESMCeOmVoErRanEdDum
4. TITLE ANDSUBTITLE 5. FUNDING NUMBERS
Sinterless Contacts to Shallow Junction InP Solar Cells
WU-506-41-11
6. AUTHOR(S)
V.G. Weizer, N.S. Fatemi, and A.L. Korenyi-Both
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
REPORTNUMBER
National Aeronautics and Space Administration
Lewis Research Center
E-7036
Cleveland, Ohio 44135-3191
9. SPONSORING/MONITORING AGENCY NAMES(S) ANDADDRESS(ES) 10. SPONSORING/MONITORING
AGENCY REPORTNUMBER
National Aeronautics and Space Administration
Washington, D.C. 20546-0001 NASA TM- 105670
11. SUPPLEMENTARY NOTES
Prepared for the Fourth International Conference on Indium Phosphide and Related Materials sponsored by the Institute of Electrical and Electronics Engineers, Newport,
Rhode Island, April 21-24, 1992. V.G. Weizer, NASA Lewis Research Center; N.S. Fatcmi, Svcrdrup Technology, Inc., Lewis Research Center Group, 2001 Aerospace
Parkway, Brook Park, Ohio 44142 (work funded under NASA contract NAS3-25266); A.L. Korcnyi-Both, Calspan Corporation, 7550 Lucerne Drive, Suite 4(11,
Cleveland, Ohio 4413(I (work funded by NASA contract NAS3-3(I759). Responsible person, V.G. Weizer, (2161433-2230.
12a. DISIHIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE
Unclassified -Unlimited
Subject Category 44
13. ABSTRACT (Maximum 200 words)
In the past, the achievement of good electrical contact to lnP has inevitably been accompanied by mechanical
degradation of the lnP itself. Most contact systems require heat treatment after metal deposition that results in the
dissolution of substantial amounts of InP into the metallization. Devices such as the solar cell, where shallow
junctions are the rule, can be severely degraded if the damage to the semiconductor substrate is not precisely con-
trolled. In this paper we describe two contact systems that provide low contact resistance to InP solar cells that do
not require subjecting the current carrying metallization to apost deposition sintering process. We show that these
two systems, one nickel-based and the other silver-based, provide contact resistivity (Re) values in the low
10-6 ohm cm 2 range, as fabricated, without the need for sintering.
14. SUBJECT TERMS 15. NUMBER OF PAGES
6
Indium phosphide; Electrical contacts; Silver; Nickel
16. PRICE CODE
A02
17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITYCLASSIFICATION 20. LIMITATION OFABSTRACT
OFREPORT OF THIS PAGE OFABSTRACT
Unclassified Unclassified Unclassified
NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)
PrescribedbyANSI Std.Z39-18
298-102