NASA Technical Memorandum 103160

New Directions in InP Solar Cell Research

LInC] _q

I. Weinberg, C.K. Swartz, and D.J. Brinker

Lewis Research Center

Cleveland, Ohio

Prepared for the

25th Intersociety Energy Conversion Engineering Conference

cosponsored by the AIChE, SAE, ACS, AIAA, ASME, and IEEE

Reno, Nevada, August 12-17, 1990

NEW DIRECTIONS IN InP SOLAR CELL RESEARCH

I. Weinberg, C. K. Swartz & D. J. Brinker

NASA Lewis Research Center

Cleveland, Ohio 44135

ABSTRACT

Recent research efforts, _-epresenting new

directions in InP solar cell research, are

reviewed. These include heteroepitaxial growth

on silicon and gallium arsenide substrates,

V-grooved cells, large area high efficiency

cells and surface passivation. Improvements in

heteroepitaxial cell efficiency are describe_

together with processing oi' Ig.l%, 4 cm L

cells. Recommendations are nade for improvements in processing leading to increased

efficiencies.

I. INTRODUCTIOi_

Indium phosphide solar cells are known to be

more radiation resistant thal_ either gallium

arsenide or silicon [l]. Furthermore, AMO

efficiencies approaching 22% are predicted,

19.1% being the highest efficiency reported to

date [2,3]. In addition, InP solar cells are

currently being used to power a Japanese lunar

orbiting satellite [4]. Presently, a small 4

cell module, on board the LIPS Ill satellite,

shows no degradation after almost three years

in orbit [5]. All of these efforts have been

concerned with monolithic n/p solar cells

processed by either organometallic chemical

vapor deposition (OMCVO) or diffusion into InP

substrates. One disadvantage of these

approaches is the high cost cf the InP wafers

used as starting substrates. Current research

efforts, aimed at cost reduction, include

heteroepitaxial growth on cheaper more durable

substrates and processing aimed at multiple

substrate reuse. In additi,)n to the cost

reduction efforts, a research program aimed at

increased efficiencies using a V-grooved front

surface has produced encouraging preliminary

results. A third effort, aimed at the processing of large area high efficiency cells has

achieved notable success. These efforts, which

represent new directions in InP solar cell

research, are reviewed in the present paper.

2. LOW COST APPROACHES

From the viewpoint of reduced cost and increased strength, it would be advantageous to

use silicon as a substrate in producing a

heteroepitaxial InP solar cell. However, the

7.5% and 42% mismatches in lattice constant and

expansion coefficient, respectively, introduces

difficulties in the use of silicon substrates.

In this respect, both germanium and gallium

arsenide are preferrable. However, the significant cost reduction arising from the use of

silicon and the relative ease of substrate

thinning makes use of this substrate most

desireable. In any case, intermediate l_ers

are essential in reducing the dislocation

densities resulting from the aforementioned

mismatches.

Yamaguchi, et al, in their pioneering efforts,

used OMCVD to deposit InP directly onto a

silicon substrate. This resulted in a cell

whose AMO efficiency was 4% [6]. On the other

hand, in a feasibility stu_, sponsored by NASA

Lewis, Spire, using OMCVD, produced InP cells

on GaAs substrates (InP/GaAs) and on a silicon

substrate with an intermediate GaAs layer

(InP/GaAs/Si) [7]. In each case, the InP

layer thickness was 4 micrometers. Cell

parameters are shown in Table I, together with

values obtained from the theoretical

predictions of Yamaguchi, et al [8]. Parenthetically, it is noted that all measurements

reported in this paper were performed at NASA

Lewis, at 25 oc, using an InP flight calibrated standard c_ll and an AMO solar intensity

of 137.2 mW cm -(. The low efficiencles are

due to the high density of dislocations which

in turn result in extremely small values of

diffusion lengths [8]. The lower efficiency of

the InP/GaAs/SI cell is attributed to doping of

the GaAs by SI [7]. Since deposition of the

GaAs layer occurs at lO00 oc, silicon, which

is an n-dopant in InP, diffuses into the GaAs

layer, creating a reverse diode. To circumvent

this effect, the GaAs layer was appropriately
shorted out [7]. This, however, creates a high

resistance path for holes arriving at the back

surface, leading to increased series resistance. This is illustrated in figure 1 where

the effects of series resistance are clearly

evident in the I-V curve of the InP/GaAs/Si

cell. The radiation resistance of the heteroepltextal cells is shown to be significantly

higher than that of the usual homoepitaxtal InP

cells after 10 HeV electron irradiation [9].

However, this is attributable to the high

dislocation densities which dominate the

present heteroepltaxtal cell performance. In

fact, the present c_ ls h;_ve dislocation densities in the 10 ul cm-c range. Hence a

reduction by several orders of magnitude is

required to reach efftcfenctes over 18%.

Obviously additional transition layers are

needed to reduce the effects of mismatch

between cell and substrate. This approach has

been pursued by Wanlass at SERI (10). The

initial result, using a GaAs substrate, was a

heteroepitaxial InP cell whose AMO efficiency

was 13.7% [I0]. A major difference between

this latter cell and both of the Spire heteroepitaxlal cells is the use of a Gaxlnl.xAS

transition layer. In fact this latter semiconductor with x--0.47 is lattice matched to

InP, resulting in an order of magnitude

decrease in dislocation density. This preliminary result is highly encouraging and cell

performance should improve with additional

effort. Although a GaAs substrate was used,

future efforts will be directed towards use of

the more desireable silicon substrate. There

still remains the problem of autodoping by the

silicon. However, it is felt that appropriately heavy doping of the p+ buffer layer,

next to the silicon substrate, should overcome

this difficulty.

The preceding approaches use OMCVD to deposit

the cell structure and transition layers on the

relatively thick substrate. In another

approach, liquid phase epltaxy (LPE) is used to

deposit the thin, single crystal, films on a

silicon substrate [11]. The potential of this

technique has been demonstrated by fabricating

LED's from InP films grown on silicon with

transition layers grading from GaP to

,InxGal_x P. The future near term goals of

this research include the processing of solar

cells using other types of transition layers.

However, at the time of this writing, no cell

fabrication results have been reported.

Another approach towards cost reduction makes

use of processes whlch a11ow for multlple

substrate reuse. In this regard, the CLEFT

[12] and chemical lift-off techniques (13) are

designed to produce thin cells which are lifted

off reusable substrates. The substrates can

then be reused to produce additional cells.

The CLEFT process has produced GaAs cells whose

efficiency, measured at NASA Lewis, was 20.5%.

The chem lift off process has also been used to

produce device quality GaAs films. In the

expectation that the success with GaAs could be

transferred to InP, the Lewis Research Center

has initiated a program, at the Kopin Corporation, in which InP cells are to be produced by

both processes. To date, InP films have been

successfully lifted off from InP substrates

using the chemical lift-off process. In this

latter case, a thin pseudomorphic AIAs release

layer was used with HF as the releasing agent.

The CLEFT process has been demonstrated for an

InP layer grown on an intermediate GaAs layer

which was laterally overgrown on a GaAs substrate. The next step includes demonstration

of lateral overgrowth of InP on an InP substrate with subsequent cleavage of the InP

film. After successful demonstration Of film

separation, InP solar cells will be produced by

both lift-off processes. Although in their

embryo stages, it is expected that both

processes will eventually result in lower cost,

lightweight and highly efficient InP solar

cell s.

3. V-GROOVED InP SOLAR CELLS

Preliminary research on V-grooved InP solar

cells has been conducted at NASA Lewis [14].

The cell geometry is shown schematically in

figure 2. The grooves are formed by anisotropic etching, the etch rate being dependent

on the orientation of the etched face [14, 15,

16]. Before etching, the cell surface is in

the (lO0) direction. After etching, the

slanted surface is in the (Ill) direction. The

emitter of the n+p InP cell was formed by

closed ampoule diffusion of sulphur using

In2S3 as a diffusion source [14].

The main advantage of the V-grooved cell is

lower total surface reflection. A disadvantage

is the increased dark current due to the Increased Junction area. This latter effect

leads to decreased cell voltage. Preliminary

calculations indicate an expected increase of

10% in Isc and a 14 mV decrease in Voc.

Although not spectacular, the initial results

for a few unoptlmized, low efficiency cells,

are encouraging. As shown in figure 3, the

quantum efficiency of the V-grooved cells are

greater than that of the planar InP cell. In

comparing three V-grooved cells to two planar

cells the increase in Isc was 8% while the

decrease in Voc was 37 millivolts. The much

greater than expected reduction in Voc may be

due to poor surface quality of the grooved

cells, inadequate diffusion into the (Ill) face

and striations and other imperfections in the

grooved surface. Since the V-grooved cell

processing is still in the experimental stage,

additional effort is needed to effect improvements. In addition, the antlreflection coating

needs to be optimized before any meaningful

comparision in cell efflciencies can be made.

Eventually, the cells will be processed by

OMCVD to attain the highest efficiencies.
LARGER AREA-HIGH EFFICII:NCY CELLS

In previous years, the highest efficiency InP

solar cell had an efficienc) of 18.8% [17].

This ce11, ho)(ever, was of relatively small

area (0.25 cm_). Since cells of this size

are unsuited for practical use, it is essential

to show that similar results can be attained

with larger area cells. Mo_t recently, the

cell _rocessing has been scaled up to produce

4 cmL cells whose efficiency was 1g.1% [3].

The AMO parameters for this cell are shown in

table II. The previous, smal'Ier area, highest

efficiency cell was processed by a coni)ination

of OMCVD and ion implantation the latter process being used to form the emitter region of

the n+pp + cell [17]. On the other hand,

the 4 cm2 n+pp + cell was processed

entirely by OMCVD [3]. The present high

efficiency cell includes a graded emitter

str_cture incorporating a doping of 3xi018

cm -_ near the maiTq junct_o_ and a doping

density of l.SxlO '9 cm -_ at the front

surface. In effect, this :reates a front

surface field which tends to reduce the flow of

emitter minority carriers to the front surface. A surface recombination velocity of

4.5xi06 cm sec-, was obtained when fitting

the measured short wavelength quantum efficiency to calculated results (3). This indicates that surface passivation is necessary to

achieve AMO efficlencies over 20%. Hence

efforts to achieve the necessary passivation

are underway in several laborat)ries.

Since efforts to achieve larger area, surface

passivated, high efficiency InF' cells have only

begun to be seriously pursued during the past

year, they represent a new ¢irectlon in InP

so_ar cell research. In fact, since areas of 8

cmc are the norm in GaAs ,:ell production,

further scale up of the present 4 cmZ, high

efficiency InP cells is highly ,_esireable.

B. CONCLUSION

When compared to previous homcepitaxial planar

InP cell research, the efforts in heteroepitaxial growth, CLEFT, chemical lift off and

V-grooved cells represent entirely new directions. Efforts aimed at larger area, surface

passivated, high efficiency cells are not

entirely new. However, because of their recent

emergence, they represent new cirectlons in InP

solar cell research. Although present efficiencles of the heteroepitaxial cells are admittedly low, the recent increase in efficiency

from 9 to 13.7%, for cells bas;ed on GaAs substrates, demonstrates the progress which can be

affected by the use of more appropriate transition layers. A similar demor_stration, using

silicon substrates, has yet Lo be achieved.

However, assuming a solution of the problem

presented by silicon autodoping, there appears

to be no insurmountable obstacle toward achieving high efficiency cells using silicon as a

substrate. InP cells produced by the lift-off

processes have yet to be demonstrated. Successful efforts will result in lower cost,

lightweight, high efficiency cells, qualities

which make them attractive to the spacecraft

array designer. The few V-grooved cells, produced to date, although demonstrating higher

currents, are of relatively low efficiency.

Further development is needed here in the area

of cell processing. In fact, it appears necessary to use epitaxial growth rather than diffusion, into Czochralski substrates, to produce

the cells. The drive toward larger area cells

has shown that s_ale up is feasible. In fact,

the 19.1%, 4 cmL cells produced to date are

slightly better than the small area cells which

demonstrated the previous record high efficlencies. Present efforts, directed at surface

passivation, should result in efficlencies

greater than 20%.

In summation, despite the relatively few laboratories engaged in InP solar cell research, the

field is dynamic in its exploration of new concepts. Undoubtedly, high efficiencies comparable to GaAs will result from this activity.

Furthermore, glven sufficient time, one would

expect the resultant cells to be low cost,

lightweight, and with improved strength to

weight ratios. Successful completion of this

research effort will result in cells which are

prime candidates for extensive use on satellites which experience degrading space

radiation environments.

REFERENCES

[l.] !. Weinberg, C. K. Swartz & R. E. Hart,

Jr., "Radiation and Temperature Effects in

GaAs, InP and Si Solar Cells," Proceedings Igth

Photovoltaic Spec. Conf., 548 (1987).

[2.] C. Goradia, W. Thesling & I. Weinberg "Key

Factors Limiting the Open Circuit Voltage of

n+pp + InP Solar Cells," To Be Published in

Proceedings 21st Photovoltaic Specialists

Conf., Kissimee FL, May 21-25, (1990).

[3.] C. J. Keavney, V. E. Haven & S. M. Vernon,

"Surface Recon_)ination and Hlgh Effi- ciency in

InP Solar Cells," Proceedings 2nd Int'l. Conf.

on InP and Related Materials," To Be Published,

(IEEE-LEOS, Iggo).

[4.] T. Takamoto, H. Okazaki, H. Takamura, M.

Ohmori, M. Ura and M. Yamaguchi, "Radiation

Effects on n+p Homejunction InP Solar

Cells," Ibid, To Be Published.
[5.] D. J. Brinker, "RecentResults From the

InP homojunctlon Cell Module on the LIPS III

Spacecraft," To Be Published in Proceedings

20th IEEE Photovol talc Special ists Conf.,

Kisslmmee, FL, May 21-25, (1990).

[6.] M. Yamaguchi, "Present Status and Future

Prospects of InP Solar Cells," Proceedings 20th

IEEE Photov_talc Specialists Conf., 880 (1988).

[7.]. C. J. Keavney, S. M. Vernon, V. E. Haven,

S. J. Wojtczuk & M. M. AI-Jassim, "Fabrication

of n+/p InP Solar Cells on Silicon Substrates," Appl. Phys. Lett., 54, 1139 (1989).

[B.] M Yamaguchi, A. Yamamoto, N. Uchida & C.

Uemura, "A new Approach for Thin Film Inp Solar

Cel Is," Solar Cel Is l9, 85 (1986-I 887 ).

[9.] I. Welnber9, C. K. Swartz, D. J. Brinker &

D. M. Wilt, "Heteroepltaxialy Grown InP Solar

Cells," Proceedings 2nd Int'l Conf. on InP and

Related Materials," To Be Published (IEEELEOS,1990).

[10.] M. W. Wanlass, T. A. Gessert, G. S.

Horner, J. S. Ward, T. J. Co_tts, G. F. Virshup

& M. L: Rostow, "Recent Advances in Epitaxial

InP Based Solar Cells," Ibld.

[ll.] S. R. Collins & A. M. Barnett, "Selective

Liquid Phase Epitaxial Growth of InP on

Silicon," Ibid.

[12.] J. C. C. Fan, C. O. Bozler & R. W.

Mclelland, "Thin Film GaAs Solar Cells,"

Proceedings 15th IEEE Photovoltaic Specialists

Conf., 666 (1981).

[13.] E. Yablanovitch, T. Omitter, J. P.

Harblson and R. H. Batt, "Extreme Selectivity

in the Lift-off of Epitaxial GaAs," Appl. Phys.

Lett. 51, 2222 (1987).

[14.] S. Bailey, N. Fatemi, G. A. Landis, D. J.

Brinker, M. Faur and M. Faur, "Application of

V-groove Technology to InP Solar Cells,"

Proceedings 2nd Int'l Conf. on InP & Related

Materials, To Be Published, (IEEE-LEOS, 1990).

[15.] C. R. Baraona and H. W. Brandhorst,

"V-grooved Silicon Solar Cells," Proceedings

lOth IEEE Photovoltaic Specialists Conf. 44

(1975).

[16.] C. V. Keavney & H. I. Smith, "A 3.8

Micrometer Period Sawtooth Grating in InP by

Anisotropic Etchlng," J. Electrochem. Soc.,

131, 452 (1984).

[17.] C. J. Keavney & M. B. Spitzer, "Indium

Phosphide Solar Cells Made by Ion Implantation," Appl. Phys. Lett., 52, 1439 (1988).

Table I. Measured and predicted pre-irradiation cell parameters

Cell Jsc Voc FF Efficiency

ma/cm 2 mv % %

InP/GaAs a 26.6±.7 667)4 69±I 8.9_.I

InP/GaAs/Si a 25.8±.3 642)4 57.7_I 6.95±.I

predicted b 29.6 600 77 9.9

aAverage 8 cells

bND=3X108/cm2 , p=3XlO16 cm -3, 5% front contact coverage

Table II Parameters of High Efficiency

4 cm2 InP Cell 2

Jsc Voc FF Efficiency

ma/cm 2 mV % %

36.34 876 82.4 19.1

4
A

v

FZ

W

r.,

OC

(J

10

• l

$

0 .2 .4

\,i

,I

,\t

.6

VOLTAGE (V)

Fig. I.

CELL EFFICIENCY (%)

----- n/p InP 17.8

.... InP/GaAs 9,i

--.-- InP/GaAs/SI 7.1

I-V curves of n/p homojunction and present heteroepitaxial cells

INCIDEN"

_.--Z_..-----.{LIGHT.

_L

Fig. 2. Schematic of V-grocved cell

IO0

80 I

_ 6o

_ 40

X

I...-

Z

N 2o

V-GROOVED CELL

400 GO0 800

WAVELENGTH (NM)

lOOO

Fig. 3. Quantum efficiency-V-groovedand planar cell.

(no antireflection coating)
National Aeronautics and Report Documentation Page

Space Adminlstrahon

1. Report No. 2. Government Accession No.

NASA TM-103160

4. Title and Subtitle

New Directions in InP Solar Cell Research

7. Author(s)

I. Weinberg, C.K. Swartz, and D.J. Brinker

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

3. Recipient's Catalog No,

5. Report Date

6. Performing Organization Code

8. Performing Organization Report No.

E-5528

10. Work Unit No.

506-41-11

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

Prepared for the 25th Intersociety Energy Conversion Engineering Conference cosponsored by the AIChE, SAE,

ACS, AIAA, ASME, and IEEE, Reno, Nevada, August 12-17, 1990

16. Abstract

Recent research efforts, representing new directions in InP solar cell research, are reviewed. These include

heteroepitaxial growth on silicon and gallium arsenide substrates, V-grooved cells, large area high efficiency cells

and surface passivation. Improvements in heteroepitaxial cell efficiency are described together with processing of

19.1%, 4 cm 2 cells. Recommendations are made for improvements in processing leading to increased efficiencies.

17. Key Words (Suggested by Author(s))

Solar cells

Indium phosphide

Heteroepitaxial growth

Thin films

18. Distribution Statement

Unclassified- Unlimited

Subject Category 33

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages

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