Observation of localization complexes and phonons replicas in heavily doped GaAs1−xNx

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  • 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

    1. Introduction

    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 [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 [38]. 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) 150180 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) 4144

    * Corresponding author. Tel.: 216-98-901-722;fax: 216-71-560-723.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


    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-

    fication system.

    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[1417] 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. [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 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 [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 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.

    4. Conclusion

    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

    substrate misorientation and others independent to

    substrate misorientation.


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    Observation of localization complexes and phonons replicas in heavily doped GaAs1-xNxIntroductionExperimental procedureResults and discussionConclusionReferences