Observation of localization complexes and phonons replicasin heavily doped GaAs1xNx
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
We studied the photoluminescence (PL) from GaAsN with the nitrogen content of 2 1018 cm3 grown by molecular beamepitaxy (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
Heavily nitrogen- (N) doped GaAs, often consid-
ered to as a dilute GaAs1xNx 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  and the appearance of
the resonant band  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 . 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
. In the impurity limit, an isolated N impurity
introduces a resonant state (Nx) 150180 meV above
the GaAs conduction-band edge, which was first
observed by Wolford et al.  and later confirmed
by Leroux et al.  and Liu et al. , and qualita-
tively agrees with the theoretical prediction that the
Applied Surface Science 226 (2004) 4144
* Corresponding author. Tel.: 216-98-901-722;fax: 216-71-560-723.E-mail address: email@example.com (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
In this paper, we study the photoluminescence
(PL) of GaAs doped with nitrogen concentration
of 2 1018 cm3. Low-temperature (LT) PL revealsseveral 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
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
250450 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 11.5 mmthick GaAs1xNx layer, grown at two different tem-peratures 420 and 470 8C. The post-growth anneal wascarried out on the samples for 10 min under a nitrogen
gas ambient at 750 8C. During thermal annealing, thesamples 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-
3. Results and discussion
Fig. 1 shows the low-temperature photolumines-
cence spectra for dilute GaAs1xNx alloys, withnitrogen concentration of 2 1018 cm3, 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, whichwere obtained at 420 8C growth temperature. Asseen 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
GaAs1xNx band gap energy at 1.505 eV. References have observed these features in a dilute
GaAs1xNx alloy and have attributed to N-relatedtransitions 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. , 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
, 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 ). 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 . 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 GaAs1xNx layers in alloyregion 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 GaAs1xNx layers withx 2 1018 cm3 considered at a temperature of 10 K.
42 F. Bousbih et al. / Applied Surface Science 226 (2004) 4144
form. In our case and for GaAs1xNx in dopedregion, 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 GaAs1xNx samples grown at 420 and470 8C. These samples have the same nitrogen con-centration of 2 1018 cm3 and the same substratemisorientation 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 richnature of the spectra, peak lines Y1, Y5, Y6 ,
NNA, NNC, NND  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 GaAs1xNx in dopedregion 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 toan improvement of the GaAs1xNx 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 GaAs1xNx samples, with the same nitrogenconcentration of 2 1018 cm3, 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
GaAs1xNx series grown respectively at 420 and470 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 GaAs1xNx (x 21018 cm3) for two samples grown at 420 and 470 8C temperatures,with substrate misoriented 28 towards (1 1 1)Ga plane, terminatedwith single Ga bond.
Fig. 3. Low-temperature PL spectra of GaAs1xNx layers withx 2 1018 cm3 grown at 420 8C (a) and 470 8C (b) as afunction of substrates misorientation 2, 4 and 68 towards (1 1 1)Asplanes, each terminated with single As bond.
F. Bousbih et al. / Applied Surface Science 226 (2004) 4144 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, asseen 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 NNAlines 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.
In summary, we present low-temperature photolu-
minescence measurements carried out on a set of
GaAs1xNx samples grown by molecular beam epi-taxy, with N concentration of x 2 1018 cm3. Thelow-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 growthtemperature. 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)Asplanes, each terminated with single As bond, have
shown two types of features, some are sensitive to
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Observation of localization complexes and phonons replicas in heavily doped GaAs1-xNxIntroductionExperimental procedureResults and discussionConclusionReferences