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Excitonic recombination processes in GaAs grown by close-space vapour transport L. Bouzrara a, * , R. Ajjel a , H. Mejri b , M.A. Zaidi a , S. Alaya a , J. Mimila-Arroyo c , H. Maaref a a Laboratoire de Physique des Semiconducteurs et des Composants Electroniques, Faculte ´ des Sciences, Monastir 5019, Tunisia b Ecole Pre ´paratoire aux Acade ´mies Militaires, Avenue Mare ´chal Tito 4029 Sousse, Tunisia c Centro de Investigacio ´n y de Estudios Avamados del Instituto Polite ´cnico Nacional, AP 14-740, Me ´xico D.F. CP07000, Mexico Received 19 January 2004; received in revised form 27 February 2004; accepted 1 March 2004 Abstract Epitaxial GaAs layers were grown using the close-space vapour transport. From deep level transient spectroscopy measurements, the native EL2 donor has been observed in all of the layers with deposition temperature-dependent concentration. On the GaAs samples, also performed are photoluminescence experiments in the temperature range 10 – 300 K. Two peculiar features were revealed: (i) the radiative recombination in GaAs layers is increasingly dominated by bound – exciton transitions, (ii) the excitonic luminescence is found to be very sensitive to the growth conditions. A study of the near-band-edge photoluminescence as a function of power excitation and temperature has been done in an attempt to elucidate the origin of the enhanced bound – exciton luminescence. q 2004 Elsevier Ltd. All rights reserved. PACS: 61–68; 78 Keywords: Close-space vapour transport technique; GaAs; Deep level transient spectroscopy; Photoluminescence; EL2 centre; Bound– exciton transitions; Near-bound-edge photoluminescence 1. Introduction The growth of epitaxial layers in III-V semiconductors by close-space vapour transport (CSVT) has been paid recently a great deal of interest for both fundamental and applied physics. The CSVT technique is characterised by a close spacing between source and substrate. This arrange- ment in space provides a large mass transfer during the decomposition and the recomposition reactions. Addition- ally, the rate of transport can be easily controlled by monitoring both the source and substrate temperatures and the water vapour pressure. Epitaxial GaAs layers of high crystalline quality and having a reasonable electron mobility have been realised using CSVT [1]. Electrical and optical properties of CSVT deposited GaAs have been investigated [2–4]. To judge the interest of this material, it is required to know in more detail the effects of the growth conditions on the characteristics of deposited layers. On the other hand, it was evidenced from thermodynamical considerations that the CSVT GaAs is As-rich [5]. This property can favour the formation of the EL2 centre in these layers [6,7]. Indeed, standard deep level transient spectroscopy (DLTS) measurements have shown unambiguously the existence of the EL2 level in all of the CSVT samples studied [4]. The assignment of this deep donor is also supported by optical quenching [8]. It is worth to notice that the occurrence of deep acceptor states related to possible gallium vacancies ðV Ga Þ has been invoked as well in materials grown by CSVT, based on experimental and theoretical investigations [9–13]. This paper reports on a DLTS and photoluminescence (PL) study of CSVT deposited GaAs layers. The deep EL2 donor has been observed with substrate temperature- dependent concentration. From PL measurements, the bound–exciton (B–E) recombination was revealed to be dominant at low temperature. It was also found that the B – E PL intensity increases with the substrate temperature. An attempt to assign the latter behaviour to increased concentration of impurities and/or stoichiometric defects that bind the excitons will be presented. 0026-2692/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2004.03.002 Microelectronics Journal 35 (2004) 577–580 www.elsevier.com/locate/mejo * Corresponding author.

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Page 1: Excitonic recombination processes in GaAs grown by close-space vapour transport

Excitonic recombination processes in GaAs grown

by close-space vapour transport

L. Bouzraraa,*, R. Ajjela, H. Mejrib, M.A. Zaidia, S. Alayaa, J. Mimila-Arroyoc, H. Maaref a

aLaboratoire de Physique des Semiconducteurs et des Composants Electroniques, Faculte des Sciences, Monastir 5019, TunisiabEcole Preparatoire aux Academies Militaires, Avenue Marechal Tito 4029 Sousse, Tunisia

cCentro de Investigacion y de Estudios Avamados del Instituto Politecnico Nacional, AP 14-740, Mexico D.F. CP07000, Mexico

Received 19 January 2004; received in revised form 27 February 2004; accepted 1 March 2004

Abstract

Epitaxial GaAs layers were grown using the close-space vapour transport. From deep level transient spectroscopy measurements, the

native EL2 donor has been observed in all of the layers with deposition temperature-dependent concentration. On the GaAs samples, also

performed are photoluminescence experiments in the temperature range 10–300 K. Two peculiar features were revealed: (i) the radiative

recombination in GaAs layers is increasingly dominated by bound–exciton transitions, (ii) the excitonic luminescence is found to be very

sensitive to the growth conditions. A study of the near-band-edge photoluminescence as a function of power excitation and temperature has

been done in an attempt to elucidate the origin of the enhanced bound–exciton luminescence.

q 2004 Elsevier Ltd. All rights reserved.

PACS: 61–68; 78

Keywords: Close-space vapour transport technique; GaAs; Deep level transient spectroscopy; Photoluminescence; EL2 centre; Bound–exciton transitions;

Near-bound-edge photoluminescence

1. Introduction

The growth of epitaxial layers in III-V semiconductors

by close-space vapour transport (CSVT) has been paid

recently a great deal of interest for both fundamental and

applied physics. The CSVT technique is characterised by a

close spacing between source and substrate. This arrange-

ment in space provides a large mass transfer during the

decomposition and the recomposition reactions. Addition-

ally, the rate of transport can be easily controlled by

monitoring both the source and substrate temperatures and

the water vapour pressure. Epitaxial GaAs layers of high

crystalline quality and having a reasonable electron mobility

have been realised using CSVT [1]. Electrical and optical

properties of CSVT deposited GaAs have been investigated

[2–4]. To judge the interest of this material, it is required to

know in more detail the effects of the growth conditions on

the characteristics of deposited layers. On the other hand, it

was evidenced from thermodynamical considerations that

the CSVT GaAs is As-rich [5]. This property can favour the

formation of the EL2 centre in these layers [6,7]. Indeed,

standard deep level transient spectroscopy (DLTS)

measurements have shown unambiguously the existence

of the EL2 level in all of the CSVT samples studied [4]. The

assignment of this deep donor is also supported by optical

quenching [8]. It is worth to notice that the occurrence of

deep acceptor states related to possible gallium vacancies

ðVGaÞ has been invoked as well in materials grown by

CSVT, based on experimental and theoretical investigations

[9–13].

This paper reports on a DLTS and photoluminescence

(PL) study of CSVT deposited GaAs layers. The deep EL2

donor has been observed with substrate temperature-

dependent concentration. From PL measurements, the

bound–exciton (B–E) recombination was revealed to be

dominant at low temperature. It was also found that the B–E

PL intensity increases with the substrate temperature. An

attempt to assign the latter behaviour to increased

concentration of impurities and/or stoichiometric defects

that bind the excitons will be presented.

0026-2692/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mejo.2004.03.002

Microelectronics Journal 35 (2004) 577–580

www.elsevier.com/locate/mejo

* Corresponding author.

Page 2: Excitonic recombination processes in GaAs grown by close-space vapour transport

2. Results and discussion

The epitaxial set-up used for this investigation consists of

CSVT deposited GaAs layers. The samples are undoped.

The source and substrate are both a high-purity (001)

oriented GaAs. The substrate temperature ðuÞ was varied in

the range 750–800 8C. A nominal temperature difference of

50 8C has been established between source and substrate.

The water vapour pressure is fixed at 1.25 mm Hg during the

growth. The DLTS measurements were performed in the

temperature range 77–450 K using a double lock-in

amplifier and a PAR 410 capacitance meter. The lumines-

cence was excited by a 514.5 nm line from an argon ion

laser. The emission spectra were analysed using a 0.6 m

double monochromator. The samples were mounted in a

helium cryostat for the temperature dependence studies.

For our analysis, we have selected results of two

representative samples prepared at u ¼ 750 and 800 8C and

which are labelled A and B respectively. Capacitance–

voltage CðVÞ measurements at room temperature showed

that these epilayers are n-type with a net donor concentration

in the range 2–8 £ 1016 cm23. A typical DLTS spectrum of

the CSVT GaAs layers is shown in Fig. 1. It is obtained for an

emission rate en ¼ 426 s21, a reversed bias V0 ¼ 23 V, a

pulse amplitude DV ¼ 3V and a filling time tp ¼ 0:5 ms. As

clearly seen, this spectrum exhibits a single peak at 377 K,

which corresponds to an electron trap. The ionization energy

of this trap, evaluated from the signature, is in the order of

0.76 eV (see the inset of Fig. 1). These observations are

consistent with EL2 centre formation in the CSVT GaAs

layers investigated. From the plot of the logarithm of T21=2 �

lnð1 2 DCm=DC0Þ versus 1=T ; we have deduced a capture

barrier energy of 60 meV for this donor level. It was also

found that the concentration of this electron trap increases

from 1012 to 1014 cm23 as the substrate temperature varies

between 750 and 800 8C; result in an agreement with that

reported in Ref. [4]. As well demonstrated from the DLTS

study, the dominant defect in the samples is the native EL2

donor. Which means that the usual growth conditions in

CSVT epitaxy favours the formation of As antisites. This

does not, however, exclude the presence of deep acceptors

related to possible Ga-vacancies.

It is obvious that stoichiometry-induced defects have also

an influence on the optical behaviour of materials grown by

CSVT. In the following, we analyse PL properties of the

GaAs layers studied. Fig. 2 shows the near-bound-edge

(NBE) luminescence at low temperature of samples A

(spectrum a) and B (spectrum b). As can be noticed, the

dominant emission is at 1.512 eV. It corresponds most

probably to donor bound excitons, since the GaAs layers are

found to be n-type. The PL spectrum also shows a series of

relatively weak luminescence lines of extrinsic nature. A

brief analysis of the extrinsic luminescence reveals the

presence of free-to-bound (F-B) transition of CAs [14] with its

1 LO phonon replica. The PL peak at 1.477 eV, has been

ascribed to a donor–acceptor (D–A) transition associated

with Ge as an acceptor [15]. But Ge seems to be a rare non-

intentional impurity in GaAs compared to C, Si or Zn for

instance. As to the peak at 1.418 eV, its photon energy is

close to the energy associated to the complex SiGa–As

vacancy [16]. The luminescence band at 1.405 eV could be a

(D0 2 MnGa0 ) pair transition [17]. This PL line is not,

however, broad enough. The reason is that Mn acceptors do

not show an increased coupling to the lattice. According to

Fig. 1. Typical DLTS spectrum of CSVT deposited GaAs. In the inset, is

shown the signature of the observed electron trap.

Fig. 2. Photoluminescence intensity as a function of the photon energy in

CSVT GaAs samples grown at a substrate temperature u ¼ 7508C (a) and

u ¼ 8008C (b).

L. Bouzrara et al. / Microelectronics Journal 35 (2004) 577–580578

Page 3: Excitonic recombination processes in GaAs grown by close-space vapour transport

this analysis, stoichiometry defect-related donor and accep-

tor levels are expected to be present in epitaxial GaAs layers

prepared by CSVT. In Fig. 2, also reported is the evolution of

the NBE luminescence as a function of the growth

conditions. As shown, an increase in the substrate tempera-

ture u does not affect appreciably the photon energy as well as

the intensity of the extrinsic PL lines. In contrast, the B–E

luminescence shows a rapid grow as u increases from 750 to

800 8C. Moreover, in the high temperature u sample, a new

luminescence line labelled L–E is resolved at 1.499 eV. A

study of this band versus excitation power shows a linear

increase with a change in the slope, similar to that of the

1.512 eV B–E luminescence (see Fig. 3). This implies that

the new PL peak at 1.499 eV arises from a radiative

recombination involving localised excitons. Stoichiometric

defects could be at the origin of the localised exciton

formation. While, the increasing of the B–E PL intensity

with the substrate temperature is due to increased concen-

tration of residual impurities that bind excitons. This

proposal is supported by the CðVÞ obtained data which

show that the net donor concentration increases with u:

Another recombination mechanism able to affect the B–E

luminescence of the GaAs layers is the non radiative

transitions. For this purpose, we have investigated the

luminescence at 1.512 eV in sample B as a function of

temperature between 10 and 300 K. As a result, the B–E PL

intensity shows a thermal quenching at high temperature (see

Fig. 4a). From the plot of the PL intensity versus 1000=T ; we

have deduced an activation energy in the order of 56 meV. It

is to be noticed that this energy is close to the electron capture

barrier of the EL2 centre as measured by DLTS. This can lead

to assign the B–E PL quenching to capture on EL2. If this is

true, as the EL2 concentration increases with the substrate

temperature, an increase in this growth parameter would lead

to increased non-radiative recombination. Such a process can

limit the performance of the CSVT GaAs epilayers. What is

observed in the set-up of samples investigated is that the NBE

PL usually shows an enhancement, due probably to increased

concentration of bound–excitons. The latter mechanism

competes the non-radiative recombination. In the GaAs

layers studied, the increasing of u between 750 and 800 8C for

a partial water pressure fixed at 1.25 mm Hg operates in

favour of the B–E binding. A study of the B–E PL peak

energy versus temperature has been also done. The results are

depicted in Fig. 4b. As shown, the maximum of the B–E line

does not exhibit a significant shift in energy with respect to

the band gap edge in the temperature range 10–60 K. It,

however, shifts towards the band gap beyond this tempera-

ture. The latter behaviour of the B–E PL peak shows, at

relatively high temperature, that the radiative recombination

is increasingly dominated by electron-hole pair transitions.

3. Conclusion

Epitaxial GaAs layers were grown by CSVT under

different growth conditions. They have been investigated

using DLTS and PL. The native EL2 donor is consistently

observed in all of the layers. It was found thatFig. 3. Excitation power dependent PL intensity at T ¼ 10 K of both B–E

(K) and L–E (O) lines in sample B.

Fig. 4. (a) Temperature dependent B–E PL intensity (K) in sample B. (b)

Peak energy of the B–E line versus temperature (B) in the same sample.

The curve in solid line represents the band gap of GaAs versus temperature

as calculated from Ref. [18].

L. Bouzrara et al. / Microelectronics Journal 35 (2004) 577–580 579

Page 4: Excitonic recombination processes in GaAs grown by close-space vapour transport

the concentration of this centre increases with the substrate

temperature. The PL study led to two main observations:

(i) the NBE luminescence is increasingly dominated by B–

E transitions at low temperature, (ii) the efficiency of the

B–E luminescence increases with the substrate tempera-

ture. The latter feature has been explained as due to an

increase in the concentration of residual impurities and/or

stoichiometric defects that bind the excitons. It was also

found that non-radiative centres are present in the samples

and could cause a PL quenching. In technological device

applications, an increase in the B–E PL intensity is of

great interest in CSVT epitaxy, since it allows to realise

active layers of high performance using GaAs.

References

[1] J. Mimila-Arroyo, J.A. Reynoso, R. Legros, F. Chavez, Proceedings

of the 16th IEEE Photovoltaıc Specialists Conference, IEEE, New

York, 1983, 952 p.

[2] J. Mimila-Arroyo, L. Kratena, F. Chavez, F. de Anda, Solid State

Commun. 49 (1984) 939.

[3] J. Mimila-Arroyo, R. Legros, J.C. Bourgoin, F. Chavez, J. Appl. Phys.

58 (1985) 3652.

[4] B.A. Lambos, T. Bretagnon, A. Jean, R. Le Van Mao, S. Bourassa,

J.P. Dobelet, J. Appl. Phys. 67 (1990) 1879.

[5] D. Cote, J.P. Dodelet, B.A. Lambos, J.I. Dickson, J. Electrochem. Soc.

133 (1986) 1925.

[6] M.D. Miller, G.H. Olsen, M. Ettenberg, Appl. Phys. Lett. 31 (1977)

538.

[7] J. Lagowski, H.C. Gatos, J.M. Parsey, K. Wada, M. Kaminska, W.

Walukiewicz, Appl. Phys. Lett. 40 (1982) 342.

[8] G. Vincent, D. Bois, A. Chantre, J. Appl. Phys. 53 (1982) 3643.

[9] E.W. Williams, Phys. Rev. 168 (1968) 922.

[10] S.Y. Chiang, G.L. Pearson, J. Appl. Phys. 43 (1975) 2986.

[11] S. Chichibu, N. Ohkubo, S. Matsumoto, J. Appl. Phys. 64 (1988)

3987.

[12] K.C. Shin, M.H. Kwark, M.H. Oh, Y.B. Tak, J. Appl. Phys. 65 (1989)

736.

[13] F.D. Auret, A.W.R. Leitch, S. Vermaak, J. Appl. Phys. 59 (1986)

158.

[14] W.M. Theis, K.K. Bajaj, C.W. Letton, W.G. Spitzer, Appl. Phys. Lett.

41 (1982) 70.

[15] D. Bois, D. Beaudet, J. Appl. Phys. 46 (1975) 3882.

[16] S.Y. Yin, D.B. Wittry, J. Appl. Phys. 54 (1983) 360.

[17] W. Schairer, M. Schmidt, Phys. Rev. B 10 (1974) 2501.

[18] M. Guzzi, J.L. Staehli, Solid State Phenom 10 (1989) 25.

L. Bouzrara et al. / Microelectronics Journal 35 (2004) 577–580580