*Corresponding author. Tel./fax: #81-52-809-1877.E-mail addresses: a.khan@toyota-ti.ac.jp (A. Khan), bour-

goin@ccr.jussieu.fr (J.C. Bourgoin)

Journal of Crystal Growth 210 (2000) 264}267

Recombination centers in electron irradiated GaInP:application to the degradation of space solar cells

Aurangzeb Khan!,*, Masafumi Yamaguchi!, Tatsuya Takamoto!, N. de Angelis",J.C. Bourgoin"

!Semiconductor Laboratory, Toyota Technical Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan"Laboratoire des Milieux De& sordonne& s et He& te& roge%nes, Universite& Pierre et Marie Curie, CNRS, UMR 7603, Tour 22, Casier 86,

4 place Jussieu, F-75252 Paris Cedex 05, France

Abstract

Native recombination centers, as well as those introduced by electron irradiation, in p-type GaInP layers have beencharacterized using combined lifetime measurements and deep level transient spectroscopy. The data, acquired usingjunctions of solar cells, provide information on the degradation of such cells in space conditions. ( 2000 ElsevierScience B.V. All rights reserved.

PACS: 71.55.Eq; 61.80.Fe

Keywords: GaInP; Solar cell; Electron irradiation; Lifetime; Recombination centers

1. Introduction

GaInP is a material which is now widely used inmicroelectronics. It is important to know the mi-nority carrier lifetime q of this material since, often,the properties of an active device depend on q. It istherefore important to characterize the recombina-tion centers responsible for this lifetime in GaInPlayers.

In this communication we describe a "rst at-tempt to measure the minority carrier lifetime q, todetect and to characterize the associated recombi-nation centers in GaInP layers. The detection of thedefects being performed using deep level transient

spectroscopy (DLTS), i.e. with the help of a junc-tion, we take advantage of the existence of thisjunction to extract q from its current}voltage (I}<)characteristics and hence to correlate the electronand hole capture rates with q. Electron irradiationis used to introduce recombination centers in a con-trolled manner to evaluate this technique of charac-terization. Because the junctions used are solarcells, this study provides at the same time informa-tion on the degradation of such cells in space.

2. Experimental procedure

The solar cells used are 1 cm2 n`}p junctions,where the n` emitter is Si doped at a level of3]1018 cm~3 and the base (0.7 lm thick) is Zndoped at a level of 1.5]1017 cm~3 (for a fulldescription see Ref. [1]).

0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 9 3 - 4

Fig. 1. DLTS spectra obtained for an emission rate of 693 s~1

prior irradiation (curve 1), and after irradiation with a #uence of1016 cm~2, 1 MeV, electrons. Curves 2 and 3 correspond respec-tively to conditions where the "lling pulse amplitude is smalleror larger than the built-in voltage. The variation of the emissionrate for the peak emitting at 150 K after irradiation is given asinsert.

For irradiation the cells are placed on the copperhead of a liquid-nitrogen cryostat allowing irradia-tion and in situ measurements between 90 and400 K, in dark or under illumination. The cryostatconstitutes a Faraday cup allowing to measure the#uence of irradiation. One MeV electrons, produc-ed by a Van de Graa! accelerator, are used for theirradiation. The electron beam is scanned in orderto insure a uniform irradiation over the wholesurface of the cells.

Conventional I}< and DLTS measurementsare performed, coupled with capacitance (C)-voltage measurements in order to obtain accuratelythe doping concentration of the base of the cells.Extrapolation to <"0 of the log I versus <plot in dark allows to deduce q as we shallsee below. The concentrations and characteristics(energy level) of the defects are extracted fromthe DLTS spectra. Finally, the I}< characteristicsof the cells under 0.1 AM0 illumination is usedto study the change in q versus the #uence ofirradiation.

3. Results

3.1. Native recombination centers

In n-type GaInP previous studies [2}5] haveshown that this material contains a main defect,identi"ed as the DX center, which emits around150 K and is characterized by an ionization energystrongly varying with an electric "eld [2]. When thedoping concentration is low enough, a second de-fect is detected, at &0.9 eV below the conductionband [3}5] with a concentration evaluated to be inthe 1014}1015 cm~3 range. In p-type material,where to our knowledge DLTS has not yet beenapplied, we observe (see Fig. 1) also a peak around150 K. This peak is present even when the "llingpulse amplitude leaves the junction in reverse bias.We attribute this peak to the DX center present inthe n` emitter. Indeed, the ionization energy(0.29 eV), measured from the variation of the emis-sion rate versus temperature, is in agreement withthe value of the ionization energy associated withthe DX center in the high electric "eld of the junc-tion used here [2]. We think it is "lled by hole

injection from the p side, which is not negligibleeven in reverse bias.

This defect cannot be an e!ective recombinationcenter, owing to its energetical location inthe gap (0.02 eV below the conduction band ac-cording to Ref. [2]). Hence, it is reasonableto conclude that the recombination center isthe defect located at 0.9 eV below the conductionband, in concentration 1014}1015 cm~3 [3}5],which cannot be detected because its concentra-tion is too small compared to the free carrierconcentration.

The log I(<) characteristics exhibit the e/k¹ ande/2k¹ behaviors (see Fig. 2) from which it is pos-sible to extract the pre-exponential factors I

01and

I02

, respectively, whose expressions are

I01

"en2*Dth(X/¸)~1N~1

$¸~1

and

I02

"

p

2n*=

0k¹(q

/q1)~1@2<~1@2

$,

A. Khan et al. / Journal of Crystal Growth 210 (2000) 264}267 265

Fig. 2. Current}voltage characteristics of a cell, exhibitingclearly the two regimes of di!usion and recombination priorirradiation (h) and after irradiation with 3]1015 (r) and3]1016 (L) 1 MeV electrons cm~2.

Fig. 3. Variation of the short-circuit current measured under0.1 AM0, normalized to the unirradiated value (0.91 mA cm~2)versus the #uence of irradiation.

where n*

is the intrinsic carrier concentration,=

0the width of the space-charge region under 0 V,

<$

the built-in voltage of the junction, X the widthof the base, D the di!usion coe$cient of the minor-ity carriers, ¸ their di!usion length and N

$the

doping concentration of the base. From theI02

value, we deduce the product of the electronand hope capture rates q

/and q

1: q

/q1"

3]10~21 s2. Because for deep levels playing therole of recombination centers the rates of electronand hole capture are of the same order of magni-tude, we can evaluate the minority carrier lifetimeq as

q"Jq/q1,

i.e. 5]10~11 s. The corresponding cross section is2.5]10~9 cm2 for a concentration of 5]1014 cm~3

recombination centers.

3.2. Electron irradiation

After irradiation two additional defects are cre-ated, as illustrated in Fig. 1. One minority carriertrap emits above 300 K and has not been character-ized. This trap could be the defect labeled IE4emitting around 350 K in electron irradiated n-typematerials, when its ionization energy is 0.8 eV. Itsintroduction rate R (0.41 cm~1) and its electroncapture cross section p

/(2.5]10~12 cm2) are also

known [6]. Because deep, this defect could be therecombination center. The other peaks induced bythe irradiation, the main one being associated withan ionization energy of 0.48 eV (see Fig. 1), are notrelated with recombination centers because theycorrespond to lower ionization energies. For thisreason, they are not described here.

The dependence of the short-circuit current in-duced by illumination /"0.1 AM0, versus the #u-ence of irradiation (see Fig. 3) is given by

ISC"e/C1!

exp(!a=)

1#a¸ D(a is the absorption coe$cient). Because the irradia-tion does not modify signi"cantly the free carrierconcentration, as demonstrated by C}< measure-ments, the only parameter which varies with theirradiation is the di!usion length

¸"JDq.

The knowledge of ISC

for zero #uence, coupled withthat of= and ¸ allows to calculate a (3.3]103 cm~1)Then q(u) can be determined from the variation ofISC

versus the #uence u. The data obtained after thecontribution of the native recombination centers,given by q

0, has been removed and show that, as

expected, q~1 varies linearly with u (because theconcentration of recombination center is propor-tional to u)

q"ku.

In practice, as illustrated in Fig. 4, k takes twodi!erent values (8]10~7 and 4]10~11 s~1 cm2)depending on the #uence range.

266 A. Khan et al. / Journal of Crystal Growth 210 (2000) 264}267

Fig. 4. Variation of the inverse of the component of the minoritycarrier lifetime induced by the irradiation versus #uence.

References

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[2] J. Krynicki, M.A. Zaidi, M. Zazoui, J.C. Bourgoin, M. DiForte-Poisson, C. Brylinski, S.L. Delage, H. Blanck, J. Appl.Phys. 74 (1993) 260.

[3] E.C. Paloma, A. Ginoudi, G. Kiriakidis, N. Franc7 ois, F.Scholz, M. Moser, A. Christer, Appl. Phys. Lett. 60 (1992)2749.

[4] S.L. Feng, J.C. Bourgoin, F. Omnes, M. Razeghi, Appl.Phys. Lett. 59 (1991) 941.

[5] E.C. Paloma, A. Ginoudi, G. Kiriakidis, A. Christer, Appl.Phys. Lett. 59 (1991) 3127.

[6] M.A. Zaidi, M. Zazoui, J.C. Bourgoin, J. Appl. Phys. 73(1993) 7229.

A. Khan et al. / Journal of Crystal Growth 210 (2000) 264}267 267