The apparently unrelated phenomena of temperature dependency, carrier removal and photoluminescence are shown to be affected by the high dislocation densities present in heteroepitaxial InP solar cells. Using homoepitaxial InP cells as a baseline, it is found that the relatively high dislocation densities present in heteroepitaxial InP/GaAs cells lead to increased volumes of dVoc/dt and carrier removal rate and substantial decreases in photoluminescence spectral intensities. With respect to dVoc/dt, the observed effect is attributed to the tendency of dislocations to reduce Voc. Although the basic cause for the observed increase in carrier removal rate is unclear, it is speculated that the decreased photoluminescence intensity is attributable to defect levels introduced by dislocations in the heteroepitaxial cells

Brinker, D. J.

Curtis, H. B.

Faur, M.

Jenkins, P.

Swartz, C. K.

Weinberg, I.

English

NASA Technical Reports Server

N91-30209
EFFECT OF DISLOCATIONS ON PROPERTIES OF
HETEROEPITAXIAL InP SOLAR CELLS
I. Weinberg, C. K. Swartz, H. B. Curtis and D. J. Brinker
NASA Lewis Research Center
Cleveland, Ohio
P. Jenkins and M. Faur
Cleveland State University
Cleveland, Ohio
ABSTRACT
The apparently unrelated phenomena of temperature dependency, carrier
removal and photoluminescence are shown to be affected by the high
dislocation densities present in heteroepitaxial InP solar cells. Using
homoepitaxial InP cells as a baseline, it is found that the relatively
high dislocation densities present in heteroepitaxial InP/GaAs cells
leads to increased values of dVoc/dt and carrier removal rate and
substantial decreases in photoluminescence spectral intensities. With
respect to dVoc/dt, the observed effect is attributed to the tendency
of dislocations to reduce Voc. Although the basic cause for the
observed increased in carrier removal rate is unclear, it is speculated
that the decreased photoluminescence intensity is attributable to defect
levels introduced by dislocations in the heteroepitaxial cells.
INTRODUCTION
Several research programs, now underway, are aimed at producing InP
solar cells from thin layers of InP epitaxially deposited on cheaper,
more durable substrates (refs. 1,2,3). The motivation for this research
lies in the high cost and relative fragility of InP. Efforts to date
have focused on the use of Si and GaAs substrates. Although intervening
lattice matching layers have been used , the lattice constant mismatch
between InP and these foreign substrates introduces dislocations which
tend to adversely affect cell performance. It is anticipated that the
adverse effect of dislocations will eventually be minimized. However,
in the present state of the art, dislocations are a dominant factor in
adversely affecting cell performance and in contributing to increased
radiation resistance (refs. 1,3,4). Although information exists
concerning the effects of dislocations on cell performance and radiation
resistance, little or nothing is known concerning their effects on such
cell properties as temperature dependence, carrier removal and photolu-
minescence spectral intensities. The present paper is concerned with
our initial results concerning the effects of dislocations on these
properties.
EXPERIMENTAL DETAILS
The cells were produced by organo-metallic vapor phase epitaxy (OMVPE)
at the Spire Corporation under contract to NASA Lewis. Both homoepitax-
ial and heteroepitaxial n+p+ cells were processed, the latter consisting
of InP cells on GaAs substrates. Etch pit densities, determined by
electrochemical etching, were 4XI0 7 cm -2 for the heteroepitaxial cells
6-I
https://ntrs.nasa.gov/search.jsp?R=19910020895 2020-03-19T17:30:50+00:00Z
and 4XI03 cm -2 for the homoepitaxial cells. Performance parameters of
both cell types are listed in table I. Temperature dependencies were
determined over a range from 25 to 75°C. Over this temperature range,
a pulsed Xenon arc solar simulator was used to determine cell perfor-
mance. Carrier concentrations were determined by capacitance-voltage
(C-V) measurements after irradiation by i0 MeV protons in the Lewis
cyclotron. Photoluminescence (PL) spectra were obtained at ii and 298K.
The PL spectrometer covered the wavelength range from 850 to 3000 nm
while the excitation wavelength was 514 nm.
RESULTS AND DISCUSSION
Temperature Dependencies
The temperature dependency of Voc, from 25 to 75°C is shown in fig.l.
With the exception of Isc (fig.2) all of the parameters shown in table
I were linear over this temperature range. The non-linear behavior of
Isc is consistent with our previous data obtained over a much wider
temperature range (ref. 5). A summary of temperature dependencies at
328 K is shown in table II. This temperature was chosen to avoid the
non linearity in Isc. In addition, it falls within the temperature
range of several space orbits of interest. As seen from the table, the
temperature dependencies of all parameters, except Voc, are equal within
the standard deviations. Clearly, dVoc/dT is greater for the heteroe-
pitaxial cell.
The temperature dependency of Voc can be discussed using the relation
(ref. 6) ,
dVoc/dT = ((Voc-Eg(T))/T) - 3k/q
+ (kT/qIsc) (dIsc/dT)
- _ T(T+2 @)/(T+ @)2
(1)
where Eg(T) is the bandgap at temperature T, k is the Boltzmann
constant, while _ and @ are constants in the expression
Eg(T) = Eg(O) - _ T2/(T+@) (2)
Eg(0) is the bandgap at 0 K (1.421 eV) and _ = 6.63XI0 -4 ev/K with _ =
552 K (ref. 5). Values calculated for dVoc/dT, at 328 K, are shown in
table III where it is seen that the measured and calculated values
differ by 9.7 and 13% for the homoepitaxial and heteroepitaxial cells
respectively. Despite this, equation 3 is useful in correlating values
of Voc with its temperature coefficient. Detailed calculations indicate
that the first term in equation 3 is dominant. Hence cells with higher
values of Voc should have smaller values for dVoc/dT. The data of table
III is in agreement with this prediction. Furthermore, since increased
dislocation densities result in smaller values of Voc (ref.7) the data,
and equation I, tend to support the conclusion that increased disloca-
tion densities result in higher values of dVoc/dT.
Carrier Removal
Carrier removal, after i0 MeV proton irradiations, is shown in fig.3.
The carrier removal rate is obtained using the relation,
B-2
_p = R_ (3)
where the_p are carriers removed at the fluence _ and Rc is the carrier
removal rate. From (3) a slope of one is indica%ed for the plot shown
in the figure. Since this is indeed the case, Rc can be determined from
points on the straight lines of fig.3. The results shown in table IV
indicate that the cell with the highest dislocation density has the
highest carrier removal rate. Although the increased carrier removal
is correlated with the increased dislocation density, the basic
mechanism responsible for this effect is unclear at present.
photoluminescence
The photoluminescence spectrum of an unirradiated InP/GaAs cell, at II
K, is shown in fig.4. The peaks at 1.382 eV and 1.419 eV are attributed
to the conduction band to acceptor and interband radiative transitions
respectively. The remaining peak is the so called phonon replica of the
conduction band-acceptor peak. Except for additional structure in the
interband peak, the peak positions and slope of the homoepitaxial cell
are similar to those shown in fig.4. The relative intensities, at Ii
K, for each spectral component, except the phonon replica, are shown in
fig.5. It is readily seen that the intensities for each component of
the heteroepitaxial cell are at least an order of magnitude less than
the spectral intensities for the homoepitaxial cell. The room tempera-
ture peaks of fig.6 confirm the tendency for the cell with greatly
increased dislocation density to exhibit a considerably reduced
photoluminescence intensity. The decreased intensity for the heteroepi-
taxial cell can be attributed to the presence of additional transitions
outside the range of the spectrometer and/or to additional non-radiative
transitions, both effects attributed to the effects of dislocations.
In either case, it is assumed that the undetectable transitions are to
defects caused by the increased presence of dislocations in the
heteroepitaxial cells. It is noted that we have been unable to find
evidence, in the literature, for the presence of additional defects,due
to dislocations in p-type InP. However, for n-type InP, DLTS measure-
ments indicate the presence of defects attributable to the presence of
dislocations (refs. 8,9). Lacking such evidence for the p-type base of
the InP/GaAs cell we tentatively assume the presence of additional
defects due to the high dislocation density in this cell.
CONCLUSION
The present data indicates that large differences in dislocation density
lead to increased values of dVoc/dT and carrier removal rate together
with a drastic decrease in photoluminescence intensity. Considering the
limited data set, it is perhaps premature to overgeneralize concerning
the effects of dislocations on these quantities. On the other hand, the
present data set tends to indicate that the increased value of dVoc/dT
is due to the tendency of dislocations to reduce minority carrier
diffusion length and thus Voc. Considering photoluminescence, it is
speculated that the decreased intensity in the heteroepitaxial cells is
due primarily to defects associated with the increased dislocation
density. However, the basic cause of the increased carrier removal rate
is relatively unclear at present.
6-3
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REFERENCES
C. Keavney, S. Vernon and V. Haven, "Tunnel Junctions for InP-on
-Si Solar Cells," Proceedings llth Space Photovoltaic Research
and Technology Conf., NASA Lewis Research Center, May 7-9,1991,
to be Published•
M. W. Wanlass, T. J. Coutts, S. S. Ward, K. A. Emery and
G. S. Horner, "High Efficiency, Thin-Film InP Concentrator
Cells," Proceedings 3rd Int'l Conf. on InP and Related Materi-
als, Cardiff, Wales, April 8-11, 1991, IEEE to be Published.
T. J. Coutts, M. W. Wanlass, T. A. Gessert, X. Li and
J. S. Ward, "Progress in InP-Based Solar Cells," Ibid 1991.
I. Weinberg, C. K. Swartz, D. J. Brinker and D. M. Wilt,
"Effects of Radiation on InP Cells Epitaxially Grown on Si and
GaAs Substrates," Proceedings 21st IEEE Photovoltaic Specialists
Conf., p 1235, 1990.
I. Weinberg, C. K. Swartz, R. Hart, Jr., and R. L. Statler,"Rad-
iation and Temperature effects in Gallium Arsenide, Indium
Phosphide and Silicon Solar Cells," Proceedings 19th IEEE
Photovoltaic Spec. Conf.,p548, 1987.
J. C. C. Fan, "Theoretical Temperature Dependence of Solar Cell
Parameters," Solar Cells 17, p309, 1986.
M. Yamaguchi, A. Yamamoto, N. Uchida and C. Uemura "A New Appro-
ach for Thin Film InP Solar Cells," Solar Cells 19 p 85,
1986-1987•
A. Zozime and W. Schroter, "Deep Levels Associated with and
Dislocations in p-Type InP," Appl. Phys. Lett. 57, p1326, 1990.
M. Sugo, Y. Takanashi M. M. Al-jassim and M. Yamaguchi, "Hete-
roepitaxial Growth and Characterization of InP on Si Sub-
strates," J. Appl. Phys. 68, p 540, 1990.
6-4
T'An_Z X. ¢i_.,?]umnmTlms IT z98z
CELL TYPE
InP/InP
InP/GaAs
NUMBER
OF CELLS
4
4
Jsc
32.3±0.1
28±0.2
Voc
(mv)
0.874±.001
0.7±.006
FF
(_)
83.3±1.4
69.8±3.9
el i i
EFFICIENCY
%
17.1±0.3
10±0.6
CELL
TIBL3 II.
dPM/dT
mW/cm2K
CELL T_ER_TURX COEFFICIENT8 &T 320K
dIsc/dT
mA/cm2K
dVoc/dT
mV/K
dFF/dT
%/K
InP/InP -(5.46±.21)Xi0 -2 -2.07±.02 +(2.21±.4)XI0 -2 -5.43±1.56
X10-2
InP/GaAs -(5.63±.25)X10 -2 -2.51±.01 +(1.99±. ll)Xl0 -2 -7.5±1.97
XI0 -2
T]&BIJ III. _TXD]&IDwtlmURZD_UI8 01 dVOC/dT
CELL EPD
cm-2
Voc/dT (my/K)
CALCULATED
Voc
mV MEASURED
InP/InP 4 X 103 874±1 -2.07±.02
InP/GaAs 4 X 107 700±6 -2.51±.02
-2.27±.01
-2.84±03
I
6-5
REMOVAL RATE
cm-1
EPD
cm-2
InP/GaAs 8.8 X 10 2 4 X 10 7
InP/InP 5 X 10 2 4 X 10 3
Voc
(V)
0.85
0.80
0,75 i I I I I
25 35 _5 55 65 75
TEMPERATURE (oc)
FIGURE i - TEMPERATURE DEPENDENCE OF OPEN
CIRCUIT VOLTAGE (InP/InP CELL)
6-6
Jsc
(M/VCM2)
33.5 -
33.0
32.5
32,0
25
I I i I I
35 45 55 65 75
TEMPERATURE (°C)
FIGURE 2 - TEMPERATURE DEPENDENCE OF SHORT
CIRCUIT CURRENT (InP/InP CELL)
!
E
U
t_
c_
t_
t/I
I016
1015
1012
0 InP/GaAs //_
i ,I, I
1013 4xiO 13
I0 HeY PROTON FLUENCE (cm-2)
FIGURE 3 - CARRIER REMOVAL BY 10 MeV
PROTONS
6-7
I--
Z
laJ
I,--
z
]
.... i .... i .... i .... i--I--i .... • .... • .... • .... i .... i .... i ....
1.32 1.36 1.4 1.44
ENERGY(eV)
FIGURE 4 - PHOTOLUMINESCENCE AT 11 K
(InP/GaAs CELL)
.01
°_
.001
.O001
m_--Band to acceptor
[_-Band edge
InP/inP InP/GaAs
FIGURE 5 - RELATIVE INTENSITIES AT 11 K
6-8
I .Oe-3
*¢-.I
G)
8.0e-4
6.0e-4
4.0e-4
2.0e-4
InP/InP
' InP/GaAs
O.Oe+O
1.300 1.325 1.350 1.375 1.400 1.425 1.450
FIGURE 6 - ROOM TEMPERATURE PHOTOLUMINESCENCE
6-9



Effect of dislocations on properties of heteroepitaxial InP solar cells

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