Observation of localization complexes and phonons replicasin heavily doped GaAs1�xNx

F. Bousbiha,*, S. Ben Bouzida, R. Chtouroua, J.C. Harmandb

aUnite de Recherche de Physique des Semiconducteurs, Institut Preparatoire aux Etudes Scientifiques et Techniques,

BP 51, 2070 La Marsa, TunisiabLaboratoire de Photonique et de Nanostructures, CNRS Route de Nozay 91460, Marcoussis, France

Abstract

We studied the photoluminescence (PL) from GaAsN with the nitrogen content of 2 � 1018 cm�3 grown by molecular beam

epitaxy (MBE). The low-temperature (LT) photoluminescence spectra are composed of several features of excitons associated to

nitrogen complexes and phonons replicas. These features were studied as a function of thermal annealing, growth temperatures

and substrate misorientation. We have shown that these nitrogen bound-excitonic transitions are very sensitive to these

parameters and could be used to study the statistical distribution of nitrogen in nominally uniform layers.

# 2003 Elsevier B.V. All rights reserved.

PACS: 71.55.-I; 78.55.Cr; 78.55.-m

Keywords: GaAsN; Molecular beam epitaxy; N complexes; Misoriented substrate, thermal annealing; Growth temperature

1. Introduction

Heavily nitrogen- (N) doped GaAs, often consid-

ered to as a dilute GaAs1�xNx alloy, has been inten-

sively studied during the past decade to understand the

giant band-structure changes in the host semiconduc-

tor GaAs by the incorporation of small amounts of

nitrogen. However, the mechanisms underlying the N-

induced band gap reduction [1] and the appearance of

the resonant band [2] remain contentious. Also, the

origins of various N-related transitions observed

below the GaAs band gap at relatively low N doping

levels are either unclear or controversial, which in turn

obscures the understanding of the mechanisms under-

lying the band-structure changes.

From the point of view of isoelectronic doping, the

behavior of N impurities in the dilute N limit of

GaAs:N is relatively less well understood as compared

to the case for GaP:N [3–8]. A better understanding of

the behavior of N impurities in the dilute doping limit

and the evolution of N-related transitions on increas-

ing the N doping level into the intermediate region

between the impurity limit and the alloy region has

been shown to be critical for understanding the N-

induced band-structure effects in the GaP:N system

[9]. In the impurity limit, an isolated N impurity

introduces a resonant state (Nx) 150–180 meV above

the GaAs conduction-band edge, which was first

observed by Wolford et al. [10] and later confirmed

by Leroux et al. [11] and Liu et al. [12], and qualita-

tively agrees with the theoretical prediction that the

Applied Surface Science 226 (2004) 41–44

* Corresponding author. Tel.: þ216-98-901-722;

fax: þ216-71-560-723.

E-mail address: bousbih_fatma@yahoo.fr (F. Bousbih).

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2003.11.029

isolated N center generates a resonant state in GaAs

[13].

In this paper, we study the photoluminescence

(PL) of GaAs doped with nitrogen concentration

of 2 � 1018 cm�3. Low-temperature (LT) PL reveals

several features attributed to excitons bound to iso-

electronic traps. The dependence of these features

on the thermal annealing, the growth temperature

and the substrate misorientation is presented and

analysed.

2. Experimental procedure

The samples used in this work were grown in a

conventional molecular beam epitaxy (MBE) system

with solid sources, except for nitrogen. The arsenic

cell was equipped with a cracking zone. A radio

frequency (rf) plasma cell was used to generate nitro-

gen species active for the growth. A 7 N purity N2 gas

flow, ranging from 0.1 to 0.6 sccm was excited by a

250–450 W rf power. The samples were realized on

(0 0 1)-oriented GaAs substrates misoriented 2, 4 and

68 towards (1 1 1)As and (1 1 1)Ga planes, each ter-

minated with single As and Ga bonds, respectively.

They consist of GaAs buffer layer, and a 1–1.5 mm

thick GaAs1�xNx layer, grown at two different tem-

peratures 420 and 470 8C. The post-growth anneal was

carried out on the samples for 10 min under a nitrogen

gas ambient at 750 8C. During thermal annealing, the

samples were put on a GaAs wafer face to face to

prevent loss of arsenic at high temperatures. The PL

measurements were performed at 10 K using a solid

state diode-pumped, frequency-doubled Nd:Vanadate

(Nd:YNO4) laser operating at 532 nm. The excitation

power was always kept less than 5 mW on the sample.

The PL emissions were dispersed using a double

grating Jobin-Yvon monochromator and detected by

a Ge detector associated with a sensitive lockin ampli-

fication system.

3. Results and discussion

Fig. 1 shows the low-temperature photolumines-

cence spectra for dilute GaAs1�xNx alloys, with

nitrogen concentration of 2 � 1018 cm�3, grown on

(0 0 1)GaAs substrates for the same as-grown sam-

ples, as before and after thermal annealing per-

formed for 10 min at 750 8C, respectively, which

were obtained at 420 8C growth temperature. As

seen here, the low-temperature PL spectrum of the

as-grown sample is formed by several features posi-

tioned between 1.32 and 1.50 eV and below the

GaAs1�xNx band gap energy at 1.505 eV. References

[14–17] have observed these features in a dilute

GaAs1�xNx alloy and have attributed to N-related

transitions resulting from exciton localization in

nitrogen arrangement as clusters and pairs. The

energies of the N-related transitions are independent

of N composition. Taking into account notation of

ref. [14], we have identified these states as NNA

(1.4760 eV), NNC (1.4623 eV), NNE (1.4583 eV)

and NND (1.4493 eV). An additional peak (labeled

Y) appears at 1.4390 eV as seen in the spectra of

[16], but not in the spectra of [15,18]. The origin of

this peak is not clearly explained. However, it could

be related to other impurities (e.g. an acceptor-

related transition [15]). Three additional transitions

at 1.4440, 1.4120 and 1.4080 eV shown in Fig. 1 are

also likely to be N-related and they have been

reported in the literature [16]. As seen in Fig. 1,

the thermal annealing treatments performed for

10 min at 750 8C, affect drastically the low-tempera-

ture PL spectra: The NNA line becomes dominant

and the PL spectrum is forth time more intense than

the as-grown PL spectrum. Generally, the thermal

annealing treatment on GaAs1�xNx layers in alloy

region is used to improve the photoluminescence

emission [19,20], without affecting the PL spectra

Fig. 1. Photoluminescence spectra depicting the influence of post-

growth treatments at 750 8C for 10 min on GaAs1�xNx layers with

x ¼ 2 � 1018 cm�3 considered at a temperature of 10 K.

42 F. Bousbih et al. / Applied Surface Science 226 (2004) 41–44

form. In our case and for GaAs1�xNx in doped

region, the thermal annealing changes dramatically

the PL spectra form. We believe that the thermal

annealing treatment affects considerably the statis-

tical distribution of nitrogen atoms by decreasing the

local nitrogen content and the exciton binding ener-

gies in nitrogen complexes.

Fig. 2 shows low-temperature photoluminescence

spectra of two GaAs1�xNx samples grown at 420 and

470 8C. These samples have the same nitrogen con-

centration of 2 � 1018 cm�3 and the same substrate

misorientation 28 towards (1 1 1)Ga plane, termi-

nated with single Ga bond. Both spectra show sev-

eral sharp features positioned below the expected

band gap energy of GaAsN (�1.505 eV). The rich

nature of the spectra, peak lines Y1, Y5, Y6 [21],

NNA, NNC, NND [14] and the phonon replicas, and

the sensitivity to growth temperature, lead us to

assume that these spectral features are related also

to nitrogen atoms acting as isoelectronic traps. The

peaks at 1.4966 and 1.4850 eV appearing in the

spectra of Fig. 2 are respectively attributed to a

carbon (C)-related transition and to the carbon trans-

verse acoustic (TA) phonon replicas. These experi-

mental results reveal that GaAs1�xNx in doped

region is also very sensitive to growth temperature.

The increase of the growth temperature from 420 to

470 8C affects drastically the PL spectra and leads to

an improvement of the GaAs1�xNx optical quality.

Then, we can deduce that the increase of the growth

temperature affects also the statistical distribution of

nitrogen atoms by decreasing of the local nitrogen

content then, inducing a blue-shift of the local

nitrogen traps energies.

In the same context, to study the nitrogen distribu-

tion as function of growth conditions, two series of

three GaAs1�xNx samples, with the same nitrogen

concentration of 2 � 1018 cm�3, are grown respec-

tively at 420 and 470 8C temperatures and for dif-

ferent substrate misorientations 2, 4 and 68 towards

(1 1 1)As planes, each terminated with single As

bonds. Fig. 3(a) and (b) show the evolution of the

low-temperature PL spectra recorded for the two

GaAs1�xNx series grown respectively at 420 and

470 8C. For the samples grown at 420 8C(Fig. 3(a)), PL spectra are essentially formed by:

(i) some features that are very sensitive to substrate

misorientation as NNA with high intensity, Y1 and the

Fig. 2. Low-temperature (10 K) PL of GaAs1�xNx (x ¼ 2�1018 cm�3) for two samples grown at 420 and 470 8C temperatures,

with substrate misoriented 28 towards (1 1 1)Ga plane, terminated

with single Ga bond.

Fig. 3. Low-temperature PL spectra of GaAs1�xNx layers with

x ¼ 2 � 1018 cm�3 grown at 420 8C (a) and 470 8C (b) as a

function of substrates misorientation 2, 4 and 68 towards (1 1 1)As

planes, each terminated with single As bond.

F. Bousbih et al. / Applied Surface Science 226 (2004) 41–44 43

fundamental GaAsN transition; (ii) some other tran-

sitions insensitive to substrate misorientation, as

carbon (C)-related transition, Y6, NNC, NND,

1.4440 and 1.4400 eV. We note also that when the

substrate misorientation angle increases from 2 to 68,C-TA and Y1 lines appear slightly. For the second

series of samples grown at 470 8C temperature, as

seen in Fig. 3(b), low-temperature PL spectra

changes the form where Y1, carbon (C)-related tran-

sition and C-TA peaks features become dominant,

and the NNA line intensity decreases and others

resolved lines next NNA feature as Y5 are observed.

We note that for these two series of samples, the

GaAsN band gap and the positions of Y1 and NNA

lines are slightly dependant on the growth tempera-

ture and substrate misorientation angle. These results

confirm that some nitrogen complexes and their

binding energies are sensitive to growth orientation.

4. Conclusion

In summary, we present low-temperature photolu-

minescence measurements carried out on a set of

GaAs1�xNx samples grown by molecular beam epi-

taxy, with N concentration of x ¼ 2 � 1018 cm�3. The

low-temperature spectra show features which we attri-

bute to nitrogen complexes involving at least two

nitrogen atoms. The composition and configuration

of these complexes are very sensitive to thermal

annealing at 750 8C for 10 min and to the growth

temperature. We have deduced that the growth tem-

perature plays a major role in the local nitrogen

distribution. Then, low growth temperature favors

the formation of localized regions with high nitrogen

content inducing a strong exciton binding energy. The

increase of the growth temperature from 420 to 470 8Cinduces a blue-shift of local nitrogen trap energies.

Photoluminescence measurements as function of sub-

strate misorientations 2, 4 and 68 towards (1 1 1)As

planes, each terminated with single As bond, have

shown two types of features, some are sensitive to

substrate misorientation and others independent to

substrate misorientation.

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