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Aug 15, 2003 - Paul-Drude-Institut für Festkoerperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany. M.-A. Pinault and E. Tourniéb). CRHEA-Centre ...
Annealing effects on the crystal structure of GaInNAs quantum wells with large In and N content grown by molecular beam epitaxy A. Hierro, J.-M. Ulloa, J.-M. Chauveau, A. Trampert, M.-A. Pinault, E. Tournié, A. Guzmán, J. L. Sánchez-Rojas, and E. Calleja Citation: Journal of Applied Physics 94, 2319 (2003); doi: 10.1063/1.1591416 View online: http://dx.doi.org/10.1063/1.1591416 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/94/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in InN/InGaN multiple quantum wells emitting at 1.5   μ m grown by molecular beam epitaxy Appl. Phys. Lett. 98, 061901 (2011); 10.1063/1.3552195 Characteristic of rapid thermal annealing on Ga In ( N ) ( Sb ) As ∕ Ga As quantum well grown by molecular-beam epitaxy J. Appl. Phys. 99, 034903 (2006); 10.1063/1.2164539 Thermal excitation effects of photoluminescence of annealed Ga In N As ∕ Ga As quantum-well laser structures grown by plasma-assisted molecular-beam epitaxy J. Vac. Sci. Technol. B 23, 1434 (2005); 10.1116/1.1935533 Effect of temperature on the optical properties of GaAsSbN/GaAs single quantum wells grown by molecularbeam epitaxy J. Appl. Phys. 93, 4475 (2003); 10.1063/1.1560574 Effect of rapid thermal annealing on GaInNAs/GaAs quantum wells grown by plasma-assisted molecular-beam epitaxy Appl. Phys. Lett. 77, 1280 (2000); 10.1063/1.1289916

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JOURNAL OF APPLIED PHYSICS

VOLUME 94, NUMBER 4

15 AUGUST 2003

Annealing effects on the crystal structure of GaInNAs quantum wells with large In and N content grown by molecular beam epitaxy A. Hierroa) and J.-M. Ulloa ISOM–Universidad Polite´cnica de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain

J.-M. Chauveau and A. Trampert Paul-Drude-Institut fu¨r Festkoerperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany

M.-A. Pinault and E. Tournie´b) CRHEA-Centre National de la Recherche Scientifique, Parc Sophia Antipolis, F-06560 Valbonne, France

A. Guzma´n, J. L. Sa´nchez-Rojas,c) and E. Calleja ISOM–Universidad Polite´cnica de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain

共Received 31 March 2003; accepted 21 May 2003兲 The impact of rapid thermal annealing on the optical emission of GaInNAs/GaAs quantum wells 共QWs兲 grown by molecular beam epitaxy with high In and N content is shown to be highly dependent on the crystal structure of the QWs, as determined by transmission electron microscopy. Due to the presence of higher concentrations of nonradiative recombination centers, the annealing temperature required to obtain maximum photoluminescence emission is higher for the QW with strong structural modulation of the upper interface 关at the onset of three-dimensional 共3D兲 growth兴, intermediate for the two-dimensional 共2D兲 grown QW with compositional fluctuations, and lower for the homogeneous 2D grown QW. Moreover, the transition from homogeneous 2D growth, to 2D growth with compositional fluctuations, and finally 3D growth, leads to progressively deeper carrier localization states below the conduction-band edge. Increasing annealing temperatures gradually shifts the localization states closer to the conduction-band edge, predominantly when compositional fluctuations are present. These results suggest a link between the formation of carrier localization centers and the presence of alloy fluctuations along the QW. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1591416兴

I. INTRODUCTION

GaInNAs-based materials have received increasing attention in the past few years due to their large potential for optoelectronic applications. In particular, the possibility of achieving long-wavelength luminescence emission, up to 1.55 ␮m, is of great interest for laser diode 共LD兲 applications in optical communications.1 Indeed, there are already reports that show the feasibility of room-temperature continuouswave operation of GaInNAs-based LDs in the 1.3 ␮m region.2– 4 Even though lasing emission at 1.55 ␮m has also been reported,5 there seems to be an important fundamental barrier to exceed 1.3 ␮m emission and reach longer wavelengths. In order to reach such wavelengths, large N concentrations are needed, leading to large non-radiative recombination.6,7 Thus, achieving a larger incorporation of N or In into the GaInNAs quantum well 共QW兲 requires a moredetailed knowledge of the role of the growth mode on defect formation, and how it impacts the optical properties. Because of the large concentration of in-grown defects in GaInNAs films,8 –10 thermal annealing is also commonly used in order to improve the crystal quality.11–13 However, it is still unclear a兲

Electronic mail: [email protected] b兲 Present address: Universite´ Montpellier II, CEM2, F-34095 Montpellier cedex 5, France. c兲 Present address: Dpto. Ingenieria Electronica, Electrica y Automatica, ETSI Industriales-Campus Universitario, 13071 Ciudad Real, Spain.

what the relation is between the commonly required annealing cycles and the microstructure of the QW, in particular, when large N and In concentrations are present in the alloy. Thus, we analyze in this paper the impact that rapid thermal annealing 共RTA兲 has on GaInNAs QWs, grown by molecular beam epitaxy 共MBE兲 at different temperatures, which present very different microstructures: strong structural modulation of the QW interfaces, sharp interfaces but compositional fluctuations along the QW, or a high crystal quality with no apparent alloy fluctuations. The QWs were grown with large In and N concentrations, up to 40% and 2.5%, respectively, such that the low-temperature luminescence from the as-grown material ranged between 1.46 and 1.62 ␮m. These films are investigated by transmission electron microscopy 共TEM兲 and temperature-dependent photoluminescence 共PL兲 spectroscopy. The band anticrossing 共BAC兲 model14 is also used to determine the band-gap energy of the QWs and its dependence with temperature, and it serves as a tool to estimate carrier localization effects in the QW. II. EXPERIMENTAL PROCEDURE

The GaInNAs films were grown by MBE on 共001兲 GaAs substrates in a Riber system using Ga, As, and In solid sources, and a N Addon radio-frequency plasma source. The sample structure consisted of GaInNAs single QWs 88 –92 Å thick 共from cross-sectional TEM analysis兲, surrounded by

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GaAs barriers, and with a top AlGaAs cladding layer for carrier confinement. The QWs were grown at temperatures ranging from 400 to 450 °C, with nominal In and N concentrations of 25%– 40% and 2%–2.5%, respectively. The growth runs were monitored by in situ reflection high-energy electron diffraction 共RHEED兲. The nominal In concentrations were determined by recording RHEED intensity oscillations.15 Rapid thermal annealing using an AET furnace was performed on all samples with temperatures and times varying from 700 to 950 °C and 15 to 30 s, respectively. During annealing, the films were capped with GaAs and exposed to a N2 atmosphere. TEM analysis was performed in a JEOL 3010 microscope operating at 300 kV. Cross-sectional TEM was used to determine the QW thicknesses and microstructure, both for the as-grown and annealed samples. In particular, a g⫽002 reflection was used for the TEM observations because of its sensitivity to In–N compositional fluctuations in the GaInNAs alloy.16,17 The QWs were analyzed by means of PL spectroscopy. The samples were excited with ⬃6 mW light from the 488 nm line of an Ar⫹ laser, and the luminescence was dispersed with a 1 m monochromator and detected with a cooled Ge photodetector using a lock-in amplifier. III. RESULTS AND DISCUSSION A. Growth mode and TEM analysis of as-grown and annealed GaInNAs QWs

In order to understand the impact that annealing has on GaInNAs QWs, we use three films deposited under different growth modes, which were determined by two-beam darkfield TEM analysis 共with a g⫽002 diffraction vector兲 of cross-sectional samples, and by observation of the RHEED reconstruction patterns during growth. The high TEM sensitivity to chemical composition is expressed as a variation of the image contrast that is dependent on In–N concentrations. This contrast, together which the lattice distortion in the QW obtained through high-resolution TEM 共HRTEM兲 analysis, can allow the unambiguous determination of the In and N contents.17,18 Figure 1 shows the TEM images for as-grown samples S135, S199, and S200, where a transition from threedimensional 共3D兲 island to a smooth two-dimensional 共2D兲 growth mode is observed, corresponding to a decrease in growth temperature from 450, to 410 and 400 °C, respectively, calibrated against the (2⫻4)/c(4⫻4) transition. Indeed, a similar change of growth mode has been previously observed in GaAsN by means of examination of the RHEED pattern:19 for a given growth rate, low temperatures lead to smooth 2D growth, that turned into 3D at higher growth temperatures, even if the growth had been initiated in the 2D mode. This 2D–3D transition is related to alloy phase separation,19 and it is likely preceded by strong compositional fluctuations that may be originated by surface spinodal decomposition.20 As seen in Fig. 1, S135 shows a strong modulation of the top GaInNAs/GaAs interface, whereas both S199 and S200 present sharp GaInNAs/GaAs inter-

FIG. 1. Dark-field TEM images of the GaInAsN QWs for the as-grown and annealed samples. The growth temperatures for S135, S199 and S200 were 450, 410 and 400 °C, respectively. The annealed cycles were those that led to maximum PL emission. In all cases a g⫽(002) diffraction vector, pointing from the GaInNAs to the AlGaAs was used.

faces. However, S199 shows a periodic contrast variation in the GaInNAs QW that corresponds to compositional fluctuations. These fluctuations are not present 共within the detection limit of dark-field TEM兲 in S200, likely resulting from the slightly lower QW growth temperature. After rapid thermal annealing of the samples using the cycles that produced maximum PL emission 共Sec. III B兲, two different behaviors were observed depending on the crystal quality of the QW 共Fig. 1兲. In sample S135, where the upper interface was rough, indicating the onset of 3D growth, there is some additional interface roughening due to annealing whose origin is still uncertain. In contrast, both S199 and S200, which have sharp QW as-grown interfaces, still maintain their high crystal quality and sharp interfaces upon annealing, and the QW thickness remains unchanged. The high crystal quality observed after annealing in S199 and S200 correlates well with the large improvement of the PL integrated intensity and full width half maximum 共FWHM兲 共Figs. 2 and 3兲, and the observed high-temperature luminescence in the 1.5 ␮m range. It is unclear at this point if the compositional fluctuations in S199 are fully smoothed out after annealing, but we do not observe the formation of additional stronger compositional fluctuations. Future HRTEM

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FIG. 3. Comparison of the integrated intensity and FWHM of the PL at 20 K for the GaInNAs QWs as a function of annealing cycle. The dashed lines indicate the annealing temperature range where optimum PL emission is obtained for each QW. Connecting solid lines are drawn to guide the eye.

FIG. 2. Low-temperature PL spectra of the GaInNAs QWs as a function of annealing. PL units are comparable between different graphs.

analysis of these films, following the procedure presented in Refs. 17 and 18, may clarify this question. B. Growth mode, annealing and non-radiative recombination

We focus first on the impact that annealing has on the PL emission for each growth mode 共Fig. 2兲, and specifically, we analyze the FWHM and integrated intensity for each sample as a function of anneal treatment 共Fig. 3兲. We see that: 共1兲 for the as-grown samples only the QWs grown under 2D conditions 共S199 and S200兲 present luminescence emission, whereas S135 共3D兲 does not emit light; 共2兲 the optimum anneal cycle required to obtain the narrowest PL emission with the largest integrated intensity has a strong dependence on growth mode. Indeed, while S200 共smooth 2D growth兲 requires a rather low anneal temperature (750– 800 °C) to produce optimum PL emission, this temperature rises to 850 °C for the 2D growth mode QW with compositional fluctuations 共S199兲, and reaches⭓900 °C for the QW grown under 3D conditions 共S135兲. Annealing at higher temperatures than 900 °C for S135 leads to the formation of a very broad (⬃150 meV) higher energy band that shadowed the

GaInNAs PL emission. This band is present to some degree in all samples after annealing at precisely the same energy and is therefore most likely not originated in the GaInNAs QW. From these results we see that the growth mode plays a key role in determining the PL efficiencies, and that the efficiency for radiative recombination in the as-grown GaInNAs QW is quenched by strong nonradiative recombination.10,19 The difference in nonradiative recombination between the films grown under 2D and 3D modes can be linked to the presence of higher point defect concentrations in the 3D growth mode film 共S135兲. Extended defects that could arise from a difference in strain between GaInNAs and GaAs, e.g., misfit dislocations, are not observed by TEM in any of the films analyzed here and are, therefore, ruled out as the source. By comparison of S199 and S200 we also observe that larger compositional fluctuations in the GaInNAs QW lead to higher anneal temperatures to obtain maximum luminescence emission, consistent with higher concentrations of point defects in the QW. Thus, the formation of long-range compositional fluctuations seems to correlate with the generation of nonradiative centers during growth. Indeed, in the high quality 2D QW, sample S200, the lower nonradiative recombination after annealing leads to roomtemperature PL emission at 1.522–1.545 ␮m, close to the frequently targeted 1.55 ␮m wavelength. C. Growth mode, annealing and carrier localization

Analysis of the temperature dependence of the PL emission provides further insight into the effect that annealing has on GaInNAs. Figure 4 shows this dependence for as-grown and representative annealed films from each growth mode: the as-grown, the ‘‘optimum’’ annealed, and two annealed samples at lower and higher temperatures. First, we observe an ‘‘s’’ shaped behavior for all samples consistent with that previously attributed to radiative emission of localized carriers found in states below the conduction band edge.9,21 As the temperature is increased there is a transition from the

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TABLE I. Dependence of T trans , E local , and E act , which define the carrier localization regime, on annealing and growth mode for GaInNAs QWs.

Sample

Annealing cycle

S135 共3D growth兲

As grown

S199 共2D growth with In–N fluctuations兲

S200 共2D smooth QW兲

FIG. 4. Temperature dependence of the PL peak energy for the different GaInNAs QWs as a function of annealing. The dashed lines correspond to the band-gap energy for the same QWs calculated using the BAC model. ⌬T defines the temperature regime where there is a transition from localized to band edge emission, and the onset to this regime is defined as T trans .

localized to band-edge emission due to thermal excitation to the conduction band of localized carriers that have enough energy to overcome the potential barrier associated with the states. If we define T trans as the onset of the transition temperature regime from localization to full delocalization, we see that in the as-grown films once T trans is exceeded, nonradiative recombination quenches the band-edge luminescence and does not allow higher-temperature emission 共Fig. 4兲. This effect is clearly decreased after annealing, where PL emission can reach higher temperatures. However, the growth mode still plays a role in the annealed samples, and for 3D growth conditions the density of nonradiative recombination centers remains high even after annealing, and are responsible for quenching the luminescence 共Fig. 4兲. It is expected that the localization centers and their energy distribution in the band gap will also be highly dependent on both annealing and growth mode. We follow two different approaches to estimate this localization energy: 共i兲 modeling of the temperature dependence of the band-edge emission of the GaInNAs QW, and 共ii兲 analysis of the tem-

T trans 共K兲

850 °C-15 s 900 °C-15 s

75 73

As grown

⭓84

800 °C-30 s 850 °C-15 s 950 °C-15 s

69 51 30

As-grown

74

750 °C-30 s 800 °C-30 s 900 °C-15 s

62 57 55

E local 共meV兲

E act 共meV兲

32 26 21

28 20 12 ⭓38

26 25 29

27 24 20

perature dependence of the integrated PL intensity in the transition region from localization to band edge emission, defined by ⌬T 共Fig. 4兲. 共i兲 The first approach uses the band-anticrossing model,14 where GaInNAs is represented by the incorporation of N into an InGaAs matrix that results in a strong interaction of the conduction-band-edge states of the matrix and a narrow resonant band formed by localized N states with energy E N ⫽E N0 - ␥ x, where x is the N concentration and ␥ is a constant. The interaction between the matrix and the N level is given by V M N ⫽ ␤ x 1/2, where ␤ is a constant.8 As it is discussed in Ref. 22, even though the BAC model may not portrait the microscopic structure of GaInNAs accurately, the temperature dependence of the band-gap energy is well predicted. We use the BAC model to obtain the confined energy levels in the GaInNAs/GaAs QW with the BAC parameters from Ref. 22 ( ␤ ⫽2400 and ␥ ⫽3900). Strain effects are included in the calculations. Within the frame of the change of environment of N upon annealing,8,13 which is discussed in Sec. III D, the energy position of the N localized state, E N0 , and the ␤ parameter, are varied8 until a good fit is obtained to the high-temperature PL emission, and the variation of the band gap with T is subsequently modeled. The results are shown in Fig. 4, and excellent agreement with the temperature dependence of the PL emissions is observed. An estimation of the largest energy of the localization centers, E local , present in the QWs is then obtained by subtracting the modeled band gap and the smallest PL energy within the localization temperature regime 共Table I兲. 共ii兲 The second approach analyzes the thermal activation of the integrated PL intensity across the temperature range where the transition from localization to band-edge emission occurs (⌬T), i.e., where most of the carriers that contribute to the PL emission are thermally excited from the localization centers to the conduction-band edge. The energies calculated this way, E act , correspond to an average of the thermal emission of carriers from all recombination centers, and are therefore, smaller than the energies obtained with method

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GaInNAs well, and annealing clearly removes the deepest localization levels. D. Crystal structure reorganization upon annealing

FIG. 5. Arrhenius plot for the integrated PL intensity across ⌬T in S199 and S200 as a function of annealing.

共i兲. This approach also neglects the increase with temperature in nonradiative recombination across ⌬T, which should also manifest a decrease of E act . The results are shown in Fig. 5 and summarized in Table I, where it is clear that the same dependence with annealing is observed with both methods 共i兲 and 共ii兲. The energy position of the localization band of states could not be analyzed in the as-grown QWs and annealed S135 due to quenching of the luminescence by nonradiative recombination. However, an approximate indication of the depth of this band is given by T trans : lower values indicate a shallower position in the band gap because a smaller thermal energy is needed to produce the transition. We see in Table I that for the annealed films T trans is always larger for S135 than for S199 and S200, indicating that the localization minima extend deeper into the band gap, consistent with the marked island structure present in S135. Furthermore, the calculated band gap energy at 20 K for S135 using the BAC model and nominal QW parameters is 0.993 eV, 144 meV above the measured PL energy. This result is also indicative of the large energy of the localization minima in S135. Regarding the impact of annealing, we focus first on the QW grown in the 2D mode containing compositional fluctuations 共S199兲. Figure 4 shows that as the thermal cycle that the well is exposed to is increased, both T trans and ⌬T get smaller. Consistent with this trend, Table I shows that the depth of the localization band of states is reduced for increasing anneal temperature calculated with approach 共i兲, and consequently, the thermal activation energy gets smaller. However, a slightly different scenario is observed for the perfectly 2D grown well, where annealing has a small effect on the energy of the band of states. Thus, the generation of localization centers seems strongly coupled to the growth mode of the

From the results presented above we see that there is an energy barrier that needs to be overcome in order to produce a reorganization of the crystal lattice. This barrier is related to the microstructure of the crystal: in QWs that present very strong fluctuations, like the case of S135 where strong modulations of the top interface are observed, higher thermal energies are needed to obtain a reorganization of the lattice leading to partial removal of the point defects responsible for nonradiative recombination. Even in the case of S199, with sharp interfaces and only minor compositional fluctuations in the QW, a higher anneal temperature is needed compared to S200, which shows no fluctuations 共within the detection limit兲. In parallel, higher annealing temperatures lead to smaller carrier localization effects. This effect seems enhanced when stronger compositional fluctuations are present in the QW, likely linked to both the formation of point defects and clusters. The theoretical work by Kent et al.23 indicates that the incorporation of N into the GaAs matrix leads to the formation of a large number of different types of N clusters. These clusters generate states along the conduction-band edge and lead to the formation of a fluctuation potential that would be responsible for the carrier localization states. When In is added to this semiconductor, Kim et al.24 predict the formation of a statistical distribution of Ga-rich N-centered N–In4⫺x Gax clusters. Only when the so-called ‘‘short-range order’’ is taken into account, they show that the clusters are dominated by In–N bonds. Several experimental reports indicate that upon annealing there is a change of local environment of Ga–N in favor of In–N bonds.8,13 In particular, Klar et al.8 observe different conduction-band-edge states in GaInAsN depending on the environment of the N–N pairs: for as-grown material the environment is dominated by Ga atoms, whereas annealing changes the environment to In rich, producing an energy increase in the conduction-bandedge states. Thus, it is likely that the low GaInNAs growth temperature kinetically limits the formation of clusters leading to an statistical distribution rich in Ga–N bonds. Postgrowth annealing then provides enough energy for atoms to minimize strain and rearrange, and would lead to an In-rich distribution of clusters, dominated by In–N bonds. IV. CONCLUSIONS

The impact of annealing on the optical emission of GaInNAs QWs has been shown to be highly dependent on the microstructure observed by TEM, and to correlate to the growth regime. The QW that shows a very rough upper interface, at the onset of 2D to 3D growth, requires the highest anneal temperature in order to achieve luminescence emission, but cannot reach high-temperature PL emission. In contrast, the QWs with sharp interfaces require lower anneal temperatures and are able to produce luminescence at high temperature. In particular, the presence of small compositional variations in the QW leads to higher anneal tempera-

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tures in order to obtain light emission. Thus, the generation of nonradiative recombination centers is likely enhanced not only by 3D growth but also by the appearance of compositional fluctuations along the QW. Both the presence of carrier localization centers and the role of annealing strongly correlate to the microstructure of the QW. Two important conclusions can be drawn. First, the localization states extend deeper into the band gap for QWs with strong compositional fluctuations, and is closer to the conduction-band edge for smooth 2D QWs. Second, when large compositional fluctuations are present in GaInNAs, annealing has a large impact on this band of states, which becomes narrower and closer to the conduction-band edge. However, this effect is weaker in the smooth 2D QW, consistent with the fact that as grown this same sample has fewer localization minima below the conduction band. ACKNOWLEDGMENTS

This work has been supported by the European Union, Project No. IST-2000-26478-GINA1.5, by Comunidad de Madrid and Spanish Ministerio de Ciencia y Tecnologı´a, and by the French Ministry for Education and Research 共Project No. RMNT-REGINAL兲. 1

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