The effects of temperature and radiation on n(+)p InP solar cells and mesa diodes grown by metallorganic chemical vapor deposition (MOCVD) were studied. It was shown that MOCVD is capable of consistently producing good quality InP solar cells with Eff greater than 19 percent which display excellent radiation resistance due to minority carrier injection and thermal annealing. It was also shown that universal predictions of InP device performance based on measurements of a small group of test samples can be expected to be quite accurate, and that the degradation of an InP device due to any incident particle spectrum should be predictable from a measurement following a single low energy proton irradiation

Statler, Richard L.

Summers, Geoffrey P.

Walters, Robert J.

English

NASA Technical Reports Server

N91-30245
Temperature Coefficients and Radiation Induced DLTS
Spectra of MOCVD Grown n+p InP Solar Cells
Robert J. Waiters and Richard L. Statler
SFA Incorporated
Landover, MD 20785
and
Geoffrey P. Summers
Naval Research Laboratory
Washington, DC 20375
and
Department of Physics University of Maryland Baltimore County
Baltimore, MD 21228
Abstract
The effects of temperature and radiation on n+p InP solar cells
and mesa diodes grown by metallorganic chemical vapor deposition
(MOCVD) have been studied. Prior to irradiation, the performance of
several solar cells as a function of temperature from 90 to 400 K was
measured and temperature coefficients of the photovoltaic parameters
will be presented. The solar cells and diodes were then electron and
proton irradiated, and the radiation induced defects (RID) were
characterized by Deep Level Transient Spectroscopy (DLTS) and by I-V
measurements on the cells thus providing the most direct evidence
available of a relationship between the changes in the solar cell
performance with changes in the H4 defect concentration. In contrast
to previous work (Yamaguchi, 1990), the DLTS spectra induced by the
proton and the electron irradiations were essentially the same. Also,
the relative number of defects found as a result of the electron and
proton irradiation is shown to be proportional to the calculated non-
ionizing energy loss (NIEL) in InP, indicating that the same defect
structure was produced in both cases, in agreement with the DLTS
results. Minority carrier injection annealing experiments were then
performed on the irradiated diodes at 200 K. The results showed that
in both the electron and proton irradiated samples, the injection
caused a substantial reduction in the major RID labeled H4. However,
the H4 defect could not be completely eliminated but instead displayed
a non-zero, asymptotic limiting concentration. Furthermore, this
residual defect concentration was about 50% greater in the proton than
in the electron irradiated diodes. In contrast, thermal annealing of
the diodes at 375 K showed no residual defect concentration.
42-1
https://ntrs.nasa.gov/search.jsp?R=19910020931 2020-03-19T17:01:56+00:00Z
Introduction
Due to the superior radiation resistance of InP over Si and GaAs
(Yamamoto, 1984 and Weinberg, 1985) and its ability to anneal radiation
damage at relatively low temperatures and by minority carrier injection
(McKeever, 1991 and Walters, 1991), InP has been considered as a space
solar cell material since 1984. Progress in developing these cells has
of 4 c cells of overbeen extremely rapid resulting in the production m 2
19% efficiency (air mass zero (AM0), 25 °C) in the U.S. in 1989
(manufactured by the Spire Corp. under an NRLcontract) and the launch
of the MUSES-A Lunar Orbiter spacecraft powered by 1300 InP solar cells
in 1990 (Yamaguchi, 1990). Despite this rapid device development, the
effects of temperature and irradiation on InP devices has not been
fully studied. The only temperature effects studies are those of
Weinberg et al. which produced results which either varied
substantially among different InP cells (Weinberg, 1987) or covered
only a limited temperature range (Weinberg, 1990). Three proton
irradiation studies have been published (Weinberg, 1986; Takamoto,
1990; and Yamaguchi, 1990). However, since they used room temperature
I-V measurements to characterize the irradiated solar cells and InP
readily anneals under minority carrier injection, the interpretation
of their results is not clear. Furthermore, the recent DLTS study of
proton irradiated InP diodes (Yamaguchi, 1990) showed that protons in
the range 2 to 7 MeV produced a DLTS spectrum significantly different
than that produced by 1 MeV electrons which, from a kinematic
viewpoint, is quite unexpected.
In this study, high efficiency MOCVD InP solar cells (=18%) were
illuminated with simulated solar light (AM0) in a DLTS cryostat. The
cell temperature was varied over the range of 400 to 90 K, I-V curves
were measured, and temperature coefficients for the photovoltaic
parameters were determined. There was little variation in the measured
values over all the cells. The solar cells were then irradiated with
3 MeV protons and 1 MeV electrons along with diodes of identical
structure. The RID were characterized by DLTS measurements on both the
cells and diodes, and the solar cell degradation was characterized by
I-V measurements made at 90 K. At this low temperature, the AM0, 1 sun
I-V measurement did not induce minority carrier injection annealing.
It is shown that 3 MeV proton irradiation of MOCVD InP produces a DLTS
spectrum essentially identical to that of 1 MeV electron irradiated
InP. Also, the results show, for the first time, the actual DLTS
spectrum corresponding to the changes in the performance of MOCVD grown
InP solar cells.
Experimental Notes
Figures la and b show a schematic drawing of the solar cells and
diodes, respectively. Both device types were grown by metallorganic
chemical vapor deposition (MOCVD) with identical internal structure.
The 3 _m thick base p region was Zn doped to a level of 3 xl016 cm 3, and
the n ÷ region doping level was > i0 Is cm 3. The inset of figure 2 is a
typical set of pre-irradiation photovoltaic parameters measured at room
temperature.
The cells were illuminated through a sapphire window in the DLTS
42-2
cryostat by an Oriel I000 W, Xe arc lamp solar simulator at 1 sun
intensity, air mass zero (AM0). The simulator intensity was calibrated
by standard cells and was constant to within 0.15 % during the
measurements.
DLTS measurements were made using a Bio-Rad DL 4600 spectrometer.
For all DLTS scans, a -2 v reverse bias was used which defined a = 0.5
#m depletion region. In the solar cells, hole traps were detected with
a 200 ms, 0 v fill pulse while electron traps were detected with a 50
ms, 1.5 v fill pulse. These pulses saturated the DLTS peak heights. In
the diodes, the same voltage pulses were used with a 1 ms pulse width
(unless stated otherwise).
The 3-MeV proton irradiation was performed at room temperature,
in the dark, with a 60 nA current using the Pelletron Accelerator at
the Naval Research Laboratory. A Faraday cup was used for dosimetry.
The 1 MeV electron irradiation was performed using the Van de Graaff
accelerator at NASA Goddard with a current density of 0.2 #A/cm 2 . A
Faraday cup and calibrated radiochromatic films were used for
dosimetry.
Results
The first results of this study are the temperature coefficients
of the photovoltaic parameters of the solar cells. Prior to
irradiation, the solar cell temperature was changed from 90 to 400 K
in 25 K steps and the I-V curve was measured at each step (figure 2).
From these curves, the value of the short circuit current (Isc), open
circuit voltage (Voc), maximum power (Pmax), fill factor (FF), and
efficiency (Eff) were determined and plotted vs. temperature (figures
3 (a)and(b)). For each cell measured, the data for each parameter over
the entire temperature range could be fit to a straight line to within
<5 %. Therefore, the temperature coefficient was determined as the
slope of the best fit straight line. The coefficients were then
averaged over all of the cells to give the final values (table i). The
errors in table 1 are the standard deviations of the averages over the
different cells. The relatively large error for the FF may be due to
loose top metalization contacts on some of the cells which introduced
a temperature dependent series resistance into the cell contacts.
The solar cells and mesa diodes were then irradiated with 3 MeV
protons up to a fluence of 5 xl0 n cm 2. Several other mesa diodes were
irradiated with 1 MeV electrons up to a fluence of 3 xl015 c_ 2.
Immediately after irradiation, the DLTS hole trap spectrum was measured
in each set of devices. The minority carrier traps were not measured
until after the radiation damage was characterized to avoid minority
carrier injection annealing effects. The DLTS spectra measured in the
proton irradiated solar cells was similar to that measured in the
proton irradiated diodes (figure 4) (for an example of the spectrum
measured in the diodes see Walters, 1991). The spectrum measured in the
electron irradiated diodes is shown in figure 5. Comparison of figures
4 and 5 shows that all the defects seen following electron irradiation
are seen in the proton irradiated samples. The defect labeled HPI in
the proton irradiated spectrum is detected in electron irradiated
samples at higher fluences than used here, and EC was seen in the
electron irradiated samples after an annealing stage (private
communication from S. Messenger). The measured parameters of the
42-3
defects in figure 4 (table 2) match well with values measured in
several other studies (McKeever, 1991; Yamaguchi, 1988; and Sibille
1982).
As expected, the defect introduction rate for 3 MeV protons was
substantially higher than for 1 MeV electrons- the measured ratio is
_760. For predictive purposes, it is necessary to determine whether the
defect introduction rate is proportional to the calculated NIEL value.
The NIEL calculation estimates the average number of vacancy-
interstitial pairs initially produced by the irradiation (Burke, 1986).
For the energies involved here, the cross sections for both 3 MeV
protons and 1 MeV electrons are essentially Rutherford-like and the
calculation is relatively straight forward. Similar calculations of the
NIEL as function of incident electron and proton energy for Si and GaAs
have been discussed in detail previously (Summers, 1988 and references
therein). Briefly, the calculation involves a product of the cross
section for interaction and the recoil energy, corrected for Lindhard
energy partition (Lindhard, 1963). The calculation shows that the ratio
of the NIEL for 3 MeV protons to 1 MeV electrons in InP is =740. The
agreement of this calculated ratio with the measured ratio of
introduction rates indicates that the H4 defect introduction rate is
directly proportional to the number of defects initially produced by
the irradiation. This is a important result for predicting radiation
induced degradation in InP devices in general.
To further characterize the radiation damage, I-V measurements
were made at 90 K on an irradiated cell (figure 6). The measurement
shows that the irradiation reduced the Isc by 54%, the Voc by 2.5%, and
the Pmax and Eff by 71%. A DLTS spectrum taken immediately following
this measurement was identical to that of figure 4 which insures that
no minority carrier injection annealing was induced by the
illumination. This is the first data which clearly shows the defect
spectrum corresponding to the radiation induced degradation of the
solar cell performance before injection annealing.
After the radiation induced defects were measured, minority
carrier injection annealing studies were performed on the diodes.
Figure 7 shows the results of injecting an electron irradiated InP mesa
diode with 6.4 mA/cm 2 and a proton irradiated InP mesa diode with 30
mA/cm 2 at 200 K. While the sensitivity of the H4 defect to injection is
well known (Ando, 1986; McKeever, 1991; and Walters, 1991), figure 7
shows that not all of the H4 defect concentration will anneal under
injection at this temperature. In both cases, the H4 defect
concentration approaches a non-zero, limiting value, and this residual
defect concentration seems to be about 50% larger in the proton
irradiated samples than in the electron irradiated samples. However,
also depicted in figure 7 is the results of isothermal annealing at 375
K of a proton irradiated diode. Over 5 hours of annealing, this data
shows a first order annealing process. The same activation energy was
measured for the H4 defect by DLTS before and after the injection
annealing. The cause of this annealing behavior and its effect on the
long term solar cell performance in a space environment is uncertain
and still under investigation.
Discussion
When measuring the properties of a material, the main concern is
42-4
the consistency of the measurement from one sample to the next. The
uncertainties of the temperature coefficients presented in table 1
indicate that this is a reliable data set describing the temperature
variation of InP solar cell performance over a large temperature range.
The magnitude of the coefficients indicate the necessity of considering
the operating temperature of the cell when predicting its performance.
It is concluded that, given a single measurement at room temperature
on a good quality MOCVD n+p InP cell, these coefficients enable an
accurate estimate of the cell performance through the 90-400 K
temperature range and will be useful in any modeling study.
To completely characterize a solar cell for space fight, the
effects of radiation on the cell performance must be well understood.
To this end, this study has shown that the defects produced by 1 MeV
electron irradiation of MOCVD InP solar cells are the same as those
produced by low energy proton irradiation. Also, since the defects were
measured in the actual solar cells, the degradation of the solar cell
performance has been clearly associated with the introduction of the
major RID labeled H4. Until now, this conclusion was based on
circumstantial evidence measured on different samples. Furthermore,
this study has suggested a linear dependence of the defect introduction
rate on the calculated NIEL value for a given incident particle. Since
the NIEL value can be relatively easily calculated for a any given
incident particle spectrum, this result would allow the calculation of
the damage rate due to an entire spectrum of irradiating particles from
a measurement of the damage due to irradiation at a single energy.
Finally, this study has shown that, at 200 K, minority carrier
injection annealing is unable to completely eliminate the H4 defect,
and the residual defect concentration was about 50% larger in the
proton than in the electron irradiated samples. It seems that while
there is only a single defect level producing the H4 DLTS peak, there
is some portion of the H4 defect concentration which is insensitive to
the injection annealing. While the reason for this behavior is still
uncertain, it does seem to pose a limiting factor on the long term
radiation resistance of InP at low temperatures. However, since the
thermal annealing behavior at 375 K, which is near the normal operating
temperature of a space solar cell, did not show this behavior, it is
expected that, under normal space solar cell operating conditions, the
combination of minority carrier injection and thermal annealing of the
H4 defect will make InP extremely radiation resistant. This conclusion
confirms the results of several previous studies (Yamamoto, 1984 and
Weinberg, 1985).
Conclusions
This study has shown that MOCVD is capable of consistently
producing good quality InP solar cells with Eff > 19% which display
excellent radiation resistance due to minority carrier injection and
thermal annealing. It has also been shown that universal predictions
of InP device performance based on measurements on a small group of
test samples can be expected to be quite accurate, and that the
degradation of an InP device due to any incident particle spectrum
should be predictable from a measurement following a single low energy
proton irradiation.
42-5
References
Ando, K., M. Yamaguchi, and C. Uemura. 1986 Phys. Rev B. 34, 15 Nov.
3041-3044
Burke, E.A. 1986 IEEE Trans. on Nuclear Science NS-33, No. 6, Dec.
1276-1281
Lindhard, J., V. Nielsen, M. Scharff and P.V. Thomsen. 1963 Mat. _.
_edd. Dan. Vid. Selsk., 33, n 10
McKeever, S.W.S, R.J. Walters, S.R. Messenger, and G.P. Summers. 1991
J. Appl. Phys. 69(3), 1435-1439, 1 Feb.
Sibille, A. and J.C. Bourgoin. 1982 Appl. Phys. Lett. 41, 15 Nov.
Summers, G.P., E.A. Burke, M.A. Xapsos, C.J. Dale, P.W. Marshall, and
E.L. Peterson. 1988 IEEE Trans. on Nuclear Science N8-35,
1221-1226
Takamoto, T., H. Okazaki, H. Takamura, M. Ohmori, M. Ura, and M.
Yamaguchi. 1990 IEEE Proc. 2nd International Conf. on
InP and Related Materials, 62-65
Walters, R.J. and G.P Summers. 1991 J. Appl. Phys. 1 May
Weinberg, I., C.K. Swartz, and R.E. Hart, Jr. 1985 IEEE 18th
Photovoltaic Specialists Conf., 1722-1724
Weinberg, I., C.K. Swartz, R.E. Hart, Jr. and M. Yamaguchi 1986 5th
European Symposium on Photovoltaic Generators in Space,
415-420
Weinberg, I., C.K. Swartz, R.E. Hart, Jr. and R.L. Statler, 1987 IEEE
19th Photovoltaic Specialist Conf., 548-557
Weinberg, I., C.K. Swartz, H.B. Curtis, P. Jenkins, and D.J. Brinker.
1990 IEEE Proc. 3rd International Conf. on InP and Related
Materials
Yamaguchi, M. and K. Ando, 1988 J. Appl. Phys. 63, 1 June 5555-5562
Yamaguchi, M., T. Hayashi, A. Ushirokawa, Y. Takahashi, M. Koubata,
M. Hasimoto, H. Okazaki, T. Takamoto, M. Ura, M. Ohmori,
S. Ikegami, H. Arai, and T. Orii. 1990 IEEE 21st
Photovoltaic Specialist Conf.
Yamamoto, A., M. Yamaguchi, C. Uemura. 1984 Appl. Phys. Lett. 44 15
March 611-613
42-6
TABLE 1
Temperature coefficients for InP Solar Cells
m
disc _A -dVoc mV -dPmax _W -dFF I0 _ -dEff 10-2%
dT K dT K dT K dK K dK K
7.81 ± 0.68 1.939 ± .051 10.45 ± .85 5.3 ± 1.3 3.07±.25
TABLE 2
Parameters of Defects Measured in Irradiated
MOCVD Grown n+p InP Solar Cells and Diodes by DLTS
HP1 H2 H3 H4 H5 H7 EA EB EC
E i
(ev)
.15 .20 .30 .37 .54 .61 .26 .74 .16
xl0 is cm -2
.048 .011 .66 .14 6.0 5.3 30 500 2.0
i
z.sa_F' 2' Off <100) Cr-Au-Ag
/
/,
0.5 Mm--_ ....
SUBSTRATE 5X10" Znlcm 3
j/- _ R"
P
CrAuAg
,iz_-__dw n + Emitter
P
p_÷
p+
_- Base
_- Buffer
"- Substrate
Q'/////////////////.)/////F///////////////////////_
AuZnPdAg
(b)
Figure i. Schematic drawings of the (a) solar cells and (b) mesa diodes
42-7
,o 40 .....
_ _ _ _Z____..._0350 K
",, _,_'q"300 K
\ \ _,._.25oz
" I :I _ i _ \ \--'_ql----zOO K
l sun intensity _ [
fill
i J. I ,I I I i | I AI |_ u t [ l J% o.l 0.2 0.70.80.9l.o 1.10.30.40.50.6
._--1 O0 K
t
1.2 1.3 1.4
Cell Voltage (V)
.5
Figure 2.
from 400 to 90 K
A typical set of I-V curves measured vs. temperature
25 1.5 10
._,20 g
1.0
_o _ 4
,_ 0.5
5 - 2
-9_---- tb I_
0 a , r, a i i I 0.0 0
75 125 175 225 275 325 375 425
Temperature (K)
2
Pmax • Acea = 0.25 cm
"_ 1 sun AM0
•
Voc _ _
75
2.0
1.8
1.6
1.4
_.}
L_
0._
0.6
0.4
0.2
r , L I I i I , 0.0
125 175 225 275 325 375 425
Temperature (K)
(a) (b)
Figure 3. The plots of the photovoltaic parameters gained from
the above I-V curves vs. temperature.
42-8
0.100.09l ' % ' ' ' ' ' "' ............... "_ ..... ts 4
3MeVl_ot_Zmdimod s=x.2_o'_"[] 3.0 .tuevm,_-ontn_.p-m_
0.08 n_p lzzP Sol_ Cd m " Na. a.TxtOam
tle•50, | 2.5 - 4
12 .34 " H4 t/,.so, s ,
o ot .o o:
_, 0.06
_o.os 1.5
'_ 0.04
_0.03 1.0_
0.00 .....
-0.01 _ 0.0-0.02
-0.03 / , , -0.5
75 I00 125 150 175 200 225 250 275 300 -I.0
Temperature (K) 75 100125 150 175 200 225 250 275 300 325 350
Temperature (K)
Figure 4. Typical DLTS spectrum
of a proton irradiated InP solar
cell
I0
9
8
7
g,
5
_3
2
1
0
0.0
i i i
i i r _ i i i , , i
2
Area = 0.25 cm
Ira. AM0
T=90K
lrrm_liat_
3 MeV proton,
t2 -2
=5xlO cm
0.2 0.4 O.i; o.g 1.0 i.2 1.4
CellVoltage (V)
Figure 6. Typical I-V
measurements at 90 K before and
after proton irradiation
Figure 5. Typical DLTS spectrum of
an electron irradiated InP diode.
I
g
$
7
4
0.1
Thermal Annealing Time (home)
1 2 3 4 5
ated InP diode at 375 K •
inj_tioa areal.of 3 MeV p
Injection tnne.4dof 1 MeV e"
i irr_ated InP diode
,I I I t , I
0 5 I0 15 20 25
Injection Annealing time (minutes)
6
30
Figure 7. Comparison of the
annealing behavior of the H4 defect
42-9



Temperature coefficients and radiation induced DLTS spectra of MOCVD grown n(+)p InP solar cells

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