P/N InP homojunction solar cells have been prepared by using both liquid phase epitaxy (LPE) and metallorganic chemical vapor deposition (MOCVD) growth techniques. A heavily doped p-In sub 0.53Ga sub 0.47As contacting layer was incorporated into the cell structure to improve the fill factor and to eliminate surface spiking at the front surface. The best conversion efficiencies (total area) obtained under AM 1 illumination are 14.2 percent for a LPE cell and 15.4 percent for a MOCVD cell

Choi, K. Y.

Miller, B. I.

Shen, C. C.

English

NASA Technical Reports Server

N8 7- 26 445
P/N InP HOMOJUNCTION SOLAR CELLS BY LPE AND MOCVD TECHNIQUES
K.¥. Choi and C.C. Shen
Arizona State University
Tempe, Arizona
and
B.I. Miller
A1 & T Bell Laboratories
Princeton, New Jersey
P/N InP homojunction solar cells have been prepared by using both
Liquid Phase Epitaxy (LPE) and Metalorganic Chemical Vapor Deposition
(MOCVD) growth techniques. A heavily doped p-Ino.saGao.4zAs contacting
layer was incorporated in the cell structure to improve the fill factor and
to eliminate surface spiking at the front surface. The best conversion
efficiencies (total area) obtained under AM I illumination are 14.2% for a
LPE cell and 15.4% for a MOCVD cell.
INT RODUCT ION
_nu_m _,,_,,ide (InP) has recently emerged as an attractive solar cell
material for space application, owing to the high conversion efficiencies
(ref. I) and radiation resistance (ref. 2) which were demonstrated in the
past few years.
Most of the InP homojunction solar cells reported so far consist of a
shallow homojunction. In order to obtain low contact resistance as well as
low sheet resistance, the top layer was usually doped heavily; as a result,
the minority-carrier diffusion length in this region is very short. To
maximize the short-circuit current, the top layer has to be made very thin;
however, the reverse saturation current and the diode's ideality factor of
InP homojunction solar cells were observed to increase markedly when the p-n
junction depth is less than 0.15 um (ref. 3). Also, the surface spiking at
the surface layer of shallow junction solar cells can sometimes cause the
deterioration of diode characteristics (ref. 4). To circumvent these
problems associated with shallow homojunction cells, we have modified the
conventional InP homojunction solar cells by adding an additional contacting
Ino.saGao.4zAs layer between the front grid contact and the thin top InP
layer (This is an extension of our previous work (ref. 5), in which an
InGaAsP layer was used as contacting layer). The advantages of this extra
InGaAs contacting layer are twofold: firstly, the bandgap of Ino.saGao.4zAs
(0.75 eV) is much smaller than that of InP (1.35 eV) at room temperature, so
*The work at Arizona State Univ. is supported by NASA Lewis Research Center.
301
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it can be doped more heavily to facilitate the contacting processes.
Secondly, the front grid contact is now in direct contact with the InGaAs
layer instead of the InP surface layer, thus it eliminates possible surface
spiking problems. In the following sections, we present the experimental
details of the fabrication and characterization of this type of P/N InP
homojunction solar cell prepared by LPE and MOCVDtechniques.
EPITAXIALGROWTHANDCELLFABRICATION
A schematic diagram of the cross section of a mesa-type P/N InP
homojunction solar cell is shownin Figure I. The cell contains an InP p-n
junction and an InGaAs contacting layer. To eliminate optical absorption
in the InGaAs layer, major portion of the InGaAs layer, which was not
covered by the front grid contact, was etched away chemically. The two InP
layers and the InGaAs layer were grown successively on a (100) oriented,
n+-InP single crystal substrate with a typical carrier concentration of
18 w3
2 x 10 cm by either LPE or MOCVD growth techniques.
The LPE growth was performed using a conventional horizontal LPE
system. Three epitaxial layers were grown successively on a n+-InP
substrate. An undoped n-type InP layer about 3 to 5 um thick was grown
first on the substrate, which was followed by a p-InP layer and a p÷-InGaAs
layer. Zinc was used as p-type dopants for the second and the third
epitaxial layers. For the samples we have prepared so far, the two p-type
layers have typical thickness values of 0.5 _m. The LPE layers were grown
by a supercooling growth technique with a cooling rate of 1°C/min. The
growth temperature ranged from 650°C to 620°C. To suppress surface
dissociation at the InP substrate surface, the substrate was covered by an
InP cover piece during the saturation period, prior to the growth cycle.
The MOCVD wafer was grown in a horizontal atmospheric MOCVD system
featuring a fast switching run-vent manifold, allowing for very abrupt
interfaces. Trimethylindium (TMI) and trimethylgallium (TMG) were used as
group III sources while pure arsine and phosphine were used as group V
sources. Hydrogen sulfide (H2S) and triethylzinc (TEZ) were used as n- and
p-type dopant sources respectively. The growth temperature was around 625°C
with a growth rate of 5 A/sec for InP and a growth rate of 10 A/sec for
InGaAs. The background concentrations were about 5 x 10 Is -3cm for InP
layers and 2 x 10Is cm -3 for InGaAs layers.
The layer thicknesses and p-n junction locations for several grown
wafers were determined by using a scanning electron microscope. Figure 2
shows the electron-beam induced current (EBIC) signals superimposed on the
secondary emission images for the cleaved and stained cross sections of a
LPE sample (Fig. 2a) and a MOCVD sample (Fig. 2b). The gentle slopes of the
EBIC signals in the lightly doped InP layers indicate that the minority
carrier diffusion lengths in this region, which are estimated to be several
microns long for both samples, are much longer than the minority carrier
diffusion lengths in other layers as expected. The typical carrier
302
concentrations of the grown layers for the LPEsamples are 3.5 x 1018cm-3,
4.0 x 1017 cm-3, and 4.0 x 10'6 cm-3 for p+-InGaAs, p-InP, and n-InP layers,
respectively; while those for the MOCVDsamples are 10'8 cm-3 ,8, I0 cm-3,
and 10'6 cm-3 for p+-InGaAs, p-InP, and n-InP layers, rspectlvely.
The ohmic contacts to the backside were obtained by thermal evaporation
of Au/Sn alloy onto the entire substrate, while the front grid contacts were
achieved by first evaporating Au/Zn alloy onto the front surface which was
previously coated with photoresist and patterned by conventional
photolithographic techniques. A subsequent lift-off process completed the
deposition of the front contact. The cells were subsequently annealed at
250°C for I minute in a flowing nitrogen atmosphere. The contact grids are
50 pmwide and the spacings between the grid lines are 450 pm. The front
contact for these cells covers about 12.5 percent of the whole surface. The
cell area was defined photolithographically and the cells were made into
mesa-type devices by etching the edges of the cells with IN K2Cr20T : HBr :
CH3COOH(I : I : I by volume). The surface area of InGaAs layer, which was
not covered by the grid contact, was later removed by selective etching.
The frontside of the cells were then covered with 650 A vacuum-deposited
Sb203 as an antireflection (AR) coating. The photograph of the frontside
of a completed cell processed from a MOCVDsample is shown in Figure 3.
The total cell area for the MOCVDcells is 25 mm2 while that for the LPE
cells, which is 4 mm2, is somewhat smaller.
DEVICE CHARACTERIZATION
The dark current density - voltage (J-V) characteristics under forward
bias for two solar cells prepared by both LPE and MOCVD are shown in
Figure 4. The diode's ideality factors are about 1.7 for both cells. The
reverse saturation current densities are 3.97 × 10-'I A/cm 2 for the LPE cell
and 1.67 x 10-'1 A/cm 2 for the MOCVD cell.
The completed cells were tested under simulated sunlight using
ELH lamp as light sources. The incident power density was adjusted to
100 mW/cm 2 using a calibrated silicon solar cell. The photovoltaic
characteristics of six InP homojunction solar cells prepared from one LPE
w_fer are summarized in Table I. The average values for Voc , J sc , and FF
are 0.812 volt, 25.0 mA/cm 2, and 0.63, respectively. In Table II, we list
the photovoltaic characteristics of InP homojunction solar cells processed
from one MOCVD wafer. The average values for Voc , Jsc , and FF are
0.840 volt, 22.5 mA/cm 2, and 0.72, respectively. The highest conversion
efficiencies (total area) achieved are 14.2% for a LPE cell and 15.4% for a
MOCVD cell. Figure 5 shows the J-V characteristics under illumination of
the best LPE cell and the best MOCVD cell.
The spectral responses for these cells were characterized by using a
grating monochromator equipped with a calibrated tungsten light source. The
spectral responses of a representative LPE cell and a representative MOCVD
cell with and without AR coating are shown in Figure 6. For those uncoated
303
cells, the spectral response curves exhibit a peak at 0.9 um, and then drop
gradually as the wavelength decreases. The spectral response curves of
those coated cells exhibit a flat zone for photon wavelengths between 0.6
and 0.9 um, and drop off for wavelengths less than 0.6 um. The overall
efficiencies for our cells were improved more than 305 with the aid of an
AR coating.
DISCUSSION AND CONCLUSION
Although the open-circuit voltages for our cells are excellent, the
short-circuit current values are less than satisfactory. We believe that
short-circuit current can be improved by optimizing the cell parameters such
as layer thicknesses and doping levels of the epitaxial layers. A larger
fill factor can also be expected by increasing the doping level of the
InGaAs layer. We have observed very good yield for cells processed from
grown wafers prepared by both LPE and MOCVD techniques. However, solar
cells processed from MOCVD wafers exhibit better yield and uniformity
regarding overall device performance. The preliminary experimental results
we obtained from these modified P/N InP homojunction solar cells are
encouraging. Such a structure can be easily extented to N/P InP
homo junction solar cells. The technical advantages it offers may make it
more suitable for large-scale production than the conventional shallow
homojunction InP solar cells.
REFERENCES
I •
•
.
.
1
It•h, Y.; Yamaguchi, M.; and Uemura, C.: 17.2-Percent Efficient (AMO)
p+-i-n InP Homojunction Solar Cells. IEEE Electron Device Lett.,
vol. EDL-7, no. 2, Feb. 1986, pp. 127-128.
Yamamoto, A. ; Yamaguchi, M.; and Uemura, C.: High Conversion Efficiency
and High Radiation Resistance InP Homojunction Solar Cells.
Appl. Phys. Lett., vol. 44, no. 6, Mar. 1984, pp. 611-613•
Sugo, M.; Yamamoto, A.; Yamaguchi, M.; and Uemura, C.: High-Efficiency
InP Solar Cells with n+-p-p + Structure Grown by Metalorganic Chemical
Vapor Deposition• Jpn. J. Appl. Phys., vol. 24, no. 9, Sept. 1985,
pp. 1243-1244.
Schroder, D. K.; and Meier, D. L. : Solar Cell Contact Resistance u
A Review. IEEE Trans. Electron Devices, vol. ED-31, no. 5, May 1984,
pp. 637-647.
Shen, C. C.; and Choi, K. Y.: High Conversion Efficiency p-n + InP
Homojunction Solar Cells• IEEE Electron Device Lett., vol. EDL-6,
no. 2, Feb. 1985, pp. 78-80.
304
Table I. Photovoltaic characteristics for InP homojunction solar cells
prepared by LPE (AMI, 100 mW/cm 2 at 27°C).
Cell no. Voc (V) Jsc (mA/cm2) FF (%) Efficiency (%)
I 0.81 7 25.4 68.0 14. I
2 0.818 25.1 59.2 12.1
3 0.817 24.1 60.5 11.9
4 0.815 25.5 68.2 14.2
5 0. 805 24.6 61 .9 12.3
6 0.800 24.9 62.1 12.4
Table II. Photovoltaic characteristics for InP homojunction solar cells
prepared by MOCVD (AMI, 100 mW/cm 2 at 27°C).
f
Cell no. Voc (V) Jsc (mA/cm2) FF (%) Efficiency (%)
I O. 832 21 .9 70.4 12.9
2 0. 840 21 .6 77.9 14. I
3 0. 845 22.6 69.4 13.3
4 0. 849 22.7 79.7 15.4
5 O. 840 22.8 76.5 14.7
6 0. 844
7 0.837
22.2 76.6 14.4
22.7 68.9 13.1
8 0.840 23.8 63.8 12.8
9 0. 832 23.0 72.0 13.8
10 0.839 21 .7 66.8 12.1
305
Sb203 AR COATING / 
FRONT GRID CONTACT 
p+- InGaAs: Z n  
p -InP: Zn - n - InP: UNDOPED, OR S 
- n+- InP SUBSTRATE 
BACK CONTACT 
F i g u r e  1 .  Schematic diagram of the  c r o s s  s e c t i o n  of 
a P/N InP homojunct ion s o l a r  c e l l .  
+ + 
p -1nGaAs p-InP n-InP n -1nP 1 f / , s u b i t r a t e  
I l l  
+ 
p-InP n-InP n -1nP , I su;st\ te 
F i g u r e  2 .  Secondary emis s ion  and E B I C  images of the  c r o s s  s e c t i o n s  of 
two P/N InP s o l a r  c e l l s  p r e p a r e d  by ( a )  LPE and ( b )  MOCVD. 
ORTGmAt. pA- 
OJ2 QU- VOLTAGE ( V I  
WR? m 
000 0 2 0  040 060 080 I00 
0 0 0 ,  I I I I  1 
F i g u r e  3. Pho tograph  of t h e  f r o n t  
s i d e  of a comple t ed  P I N  InP 
homoj u n c t  i o n  so la r  ce l l .  
I I I I I 
0.00 0.20 0.40 0.60 0.80 1.00 
VOLTAGE ( V I  
F i g u r e  4.  D a r k  c u r r e n t  d e n s i t y  
v e r s u s  v o l t a g e  p l o t  unde r  
forward b i a s  f o r  two P/N 
I n P  homojunc t ion  s o l a r  
c e l l s  p r e p a r e d  by ( a )  LPE 
a n d  ( b )  MOCVD. 
- -5.00 - 
E < 
a -10.00 - E 
> 
I- 
% -15.00 - 
Z 
W 
0 
(u 
- 
5 -20.00 - 
( b )  W (L 
(L 
3 
-25.00 - 
(a) 
Figure  5. C u r r e n t  d e n s i t y  v e r s u s  v o l t a g e  
p l o t  f o r  two P/N InP homojunc t ion  
s o l a r  c e l l s  p r e p a r e d  by ( a )  LPE and  
( b )  MOCVD u n d e r  AM 1 i l l u m i n a t i o n .  
The c o n v e r s i o n  e f f i c i e n c i e s  f o r  
c e l l s  ( a )  a n d  ( b )  a re  14.2% and  
15.4%, r e s p e c t i v e l y .  
0.80 - w 
ul 
Z 
0 a 
0) 0.60 - 
W 
E 
-I 9 0.40 - 
w a 
ul 0.20 
(COATED CELLS) 
/ CUUCOATED CELLS) I 
0.00 I 1 I 1 I I 
0.40 0.50 0.60 0.70 0.80 090 1.00 
WAVELENGTH CUM) 
F i g u r e  6. S p e c t r a l  r e s p o n s e s  of two 
t y p i c a l  P I N  InP  homojunc t ion  
solar c e l l s  p r e p a r e d  by ( a )  LPE 
and ( b )  MOCVD w i t h  a n d  w i t h o u t  
AR c o a t i n g .  
307 


P/N InP homojunction solar cells by LPE and MOCVD techniques

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