Photoluminescence of exciton-polaritons in GaN

Materials Science and Engineering B50 (1997) 130 – 133

Photoluminescence of exciton-polaritons in GaN I.A. Buyanova a,*, J.P. Bergman a, B. Monemar a, H. Amano b, I. Akasaki b a

Department of Physics and Measurement Technology, Linko¨ping Uni6ersity, S-581 83 Linko¨ping, Sweden b Department of Electrical Engineering, Meijo Uni6ersity, Tempaku-ku, Nagoya 468, Japan

Abstract Temperature dependent studies of the resonant (no-phonon) and phonon-assisted radiative recombination of free excitons (FEs) in GaN are performed, and are analyzed within the polariton concept. The parameters of the exciton – phonon coupling are estimated by analyzing the thermal broadening of the no-phonon (NP) FE line. The interaction with acoustic phonons is shown to be the dominant broadening mechanism for temperatures lower than 150 K, while for higher temperatures the contribution from the interaction with optical phonons is important. Strong defect/impurity scattering of exciton-polaritons is proposed to be responsible for the revealed unusual behavior of the free A exciton in GaN, i.e. an enhanced intensity of NP FE emission in comparison with its longitudinal optical (LO) phonon replicas, as well as the narrow line shape of the 1-LO assisted transitions. © 1997 Elsevier Science S.A. Keywords: Photoluminesence; Exciton-polariton; Phonon

1. Introduction The growing interest in the III – V nitrides is partially motivated by their possible application for optical emitters and detectors, covering a broad spectral region from red (1.9 eV) to ultraviolet (UV) (6.2 eV). The emission and absorption properties of high purity direct wide band-gap semiconductors near the band edge are known to be governed by excitons. Free exciton (FE) recombination has been shown [1 – 4] to be the dominating photoluminescence (PL) process in nominally undoped GaN epilayers over a wide range of temperatures (50–300 K). Excitonic properties of wide band gap semiconductors with dipole-allowed direct band-to-band transitions can be properly described only within the polariton concept [5,6], which takes into account strong exciton–phonon coupling. The behavior of exciton-polaritons has been extensively studied in a variety of polar bulk semiconductors, such as CdS, CdSe, and GaAs. The FE recombination was shown [5,6] to require inelastic scattering of the FE from the exciton-like * Corresponding author. Tel.: + 46 13 31795; fax: +46 13 142337; e-mail: [email protected] 0921-5107/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 1 0 7 ( 9 7 ) 0 0 1 5 0 - 5

to the phonon-like region of the lower polariton branch (LPB). At low excitation densities and low doping, this scattering is usually mediated by longitudinal optical (LO) phonons (Fro¨hlich scattering), leading to an enhanced intensity of the LO-replica compared with the no-phonon (NP) FE line. Such a polariton approach has also been shown to be relevant for GaN [7], consistent with a rather large value of the estimated longitudinal–transverse splitting, DLJ, of the order 1–2 meV. However, the detailed fine structure of the exciton-polaritons in GaN, as well as parameters of the exciton– phonon interaction, are still unknown. In this paper, we present a study of optical properties of the lowest A free exciton in GaN. We shall show that the exciton system studied is in thermal equilibrium with the lattice under our experimental conditions. A number of specific properties of the FE emission in GaN, such as the shape of the 1-LO assisted transitions, as well as an enhanced intensity of the NP FE emission in comparison with its LO phonon-replicas, are revealed and will be discussed in terms of impurity scattering. The parameters of the exciton–phonon coupling are also estimated by analyzing the thermal broadening of the no-phonon (NP) FE line.

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2. Samples and experimental methods This study covers the most common types of GaN material, i.e. strained heteroepitaxial layers, and thick bulk-like layers. The heteroepitaxial layers with a thickness of about 2 mm were grown by metal organic vapor phase epitaxy (MOVPE) on optical-grade polished 6H SiC or Al2O3 substrates. A thin AlN buffer layer was introduced to diminish the effects of the lattice mismatch with the substrate. The growth temperature was about 1000°C. More than 500 mm thick undoped GaN layers grown by the hydride VPE technique at 1000°C were used as approximately strain-free bulk samples. All investigated layers were nominally undoped but showed n-type conductivity with electron concentration in the 1017 cm − 3 range from residual donors; the threading dislocation density was 108 – 1010 cm − 2. The PL measurements were performed in a variable temperature cryostat. The 334.5 nm line of an Ar + -ion laser and the second harmonic of an upconverted pulsed sapphire:Ti laser were used as photo-excitation sources. Optical spectra were dispersed with a 0.5 m double-grating monochromator and detected by a GaAs photomultiplier tube.

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sion is typically rather weak due to existence of a so-called polariton bottleneck in the vicinity of the transition region between exciton- and phonon-like dispersion of the LPB [5,6]. The relatively high intensity of the resonant FE emission in GaN (Fig. 1) points to an enhanced scattering of the exciton-polariton into the radiative branch of the A exciton. This scattering might be induced by point or extended defects, assisting the momentum transfer. An indication in favor of this assumption in our GaN samples is the higher strength of the NP FE line relative to the LO-replica detected in GaN epilayers with a higher donor density, as evident from the more intense shallow donor related BE emission (see Fig. 1(a) and (b,c) for comparison). The enhanced intensity of the NP FE emission is observed not only in heteroepitaxial GaN structures but also in homoepitaxial samples [9]. Since the dislocation density in these structures differs by nearly 4 orders of magnitude, dislocations are unlikely to be strongly involved in the polariton scattering in GaN. Thus, we propose that the enhanced impurity scattering of exciton-polaritons in GaN is induced by the high concentration of residual point defects, presumably mainly the residual donors, in the epilayers.

3. Experimental results and discussion

3.1. Resonant emission of free excitons—defect induced polariton scattering Fig. 1 shows typical PL spectra recorded from the thick GaN samples (Fig. 1(a)), as well as from GaN epilayers grown on Al2O3 and SiC substrates (Fig. 1(b,c) respectively). In addition to the bound exciton (BE) emission related to residual donors, which rapidly quenches with increasing temperature, the PL spectra contain three excitonic bands with an energy separation of : 92 meV. These emissions have been previously identified as the NP recombination of the intrinsic A exciton and its 1-LO and 2-LO phonon-assisted transitions, respectively. (The weaker excitonic emissions detected at elevated temperatures at the high energy side of the A-FE related lines are due to recombination of B- and C-FEs related to the higher valence subbands). The intensity of the NP emission relative to the phonon-assisted FE transitions is higher in the structures showing higher intensity of the shallow donor related BE recombination (Fig. 1). However, for all investigated structures, the PL intensity of the NP FE recombination in a broad range of temperatures is much higher than the LO phonon-assisted transitions. This behavior is opposite to the experience from other comparable polar semiconductors, e.g. CdS [5,8], CdSe [8] and ZnO [5], with a similar strength of DLT and exciton–phonon coupling, where the resonant FE emis-

Fig. 1. PL spectra at 60 K recorded from bulk GaN sample (a) and GaN epilayers grown on Al2O3 (b) and SiC (c) substrates. Dashed and dotted lines represent curve fitting using equations specified in the figures.

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3.2. Phonon-assisted emission of free excitons The FE PL emission in polar semiconductors occurs efficiently with the emission of LO phonons due to the rather strong Fro¨hlich coupling between excitons and LO phonons, as is also observed for GaN (Fig. 1). The spectral shape of the phonon-assisted PL of the FE can be used [5] to determine the energy distribution in the exciton system, as well as to analyze the exciton– phonon coupling in the material. According to the established theory for exciton-polariton–phonon coupling [5], the line shape of the mth LO phonon replica for the exciton system, being in thermal quasi equilibrium with the crystal lattice, can be described as I m-LO :E 1/2 exp(− E/kT)Wm (E) pl

(1)

where I m-LO is the PL intensity of the mth LO phononpl replica, E is the kinetic energy of the exciton, and Wm (E) is the energy-dependent probability of exciton recombination with emission of m LO phonons. For the 1-LO phonon-assisted transitions W1 is typically proportional to E, because of the increase of the density of final states for the LO phonons [5]. For the 2-LO phonon-replica, an independence of W2 on the exciton kinetic energy is usually observed as a result of the parity-allowed character of the 2-LO scattering. This makes it possible to use the 2-LO line as a probe to study the energy distribution of the exciton system, since its shape directly reproduces the energy distribution of the excitons. Experimentally, the Wm (E) dependence can be found by fitting the measured PL spectra of the LO phononreplicas with Eq. (1). For GaN this is conveniently done in the temperature range 50 – 100 K, where the donor bound exciton replicas are weakened, and the B exciton intensity is still much lower than the A exciton intensity, and therefore does not influence the PL lineshape significantly. The results of such a fitting for the A exciton are demonstrated in Fig. 1 by the dotted and dashed lines. For T ] 40 K, the line shape of the 1-LO and 2-LO phonon-assisted FE emission can be fitted as I 1,2-LO : E 1/2 exp( − E/kT). For lower temperpl atures, the 1-LO replica is markedly broadened, due to the contribution of the BE phonon-assisted transitions. The obtained fit implies that the probability W(E) is independent of the exciton kinetic energy E for both 1-LO and 2-LO phonon-assisted transitions. As was mentioned above, the independence of the transition probability for the 2-LO phonon-assisted FE recombination on the exciton energy proves that the A exciton system is in thermal equilibrium with the crystal lattice under the experimental conditions, in agreement with [7]. This is typical for polar semiconductors, where the exciton relaxation time is typically much shorter than the PL decay time. For the 1-LO scattering, the

dependence W1(E)zE is usually observed in other materials, which reflects the forbidden character of this scattering. Interestingly, such a dependence is not observed for the GaN samples studied in this work (Fig. 1). The transition probability is found to be insensitive to the exciton energy, even for the 1-LO line, pointing to a parity-allowed character of the 1-LO scattering. The observed lineshape of the 1-LO emission can be explained assuming strong impurity scattering in the investigated GaN structures. The defects have been shown [5] to increase the 1-LO scattering intensity, introducing additional intermediate states of alternating parity into the Fro¨hlich scattering. Thus, the 1-LO emission in a crystal with internal strain or/and a high defect density can change its character to parity allowed, as has been previously observed in ZnO samples containing a high defect density [5]. The above explanation seems to be relevant for the existing GaN epilayers, which typically have rather high concentrations of both point and extended (dislocations) defects, i.e. at least 1017 cm − 3 donors and 106 –1010 cm − 2 dislocations, respectively. The enhanced scattering in this case, however, is most likely induced by the point defects (such as the residual donors), since the anomalous shape of the 1-LO replica is detected in a very similar way also in homoepitaxial GaN layers, where the dislocation density is lower by about 4 orders of magnitude (see [9]).

3.3. Temperature-dependent linewidth of the resonant FE emission The efficiency of different exciton–phonon interactions in GaN can be estimated by analyzing the thermal broadening of the NP FE emission. The measured temperature dependence of the full width at half maximum (FWHM) for the A exciton NP line is shown in Fig. 2 for the GaN epilayers grown on Al2O3 and SiC substrates. The analysis of the NP FE line shape was performed using numerical deconvolution of the PL spectra assuming (for T\30 K) the Lorentzian shape of the FE lines. The temperature dependence of the NP FE linewidth G(T) can be described as [10]: G(T)= Gi + sT + g/(exp(ELO/kT)−1)

(2)

The first term in Eq. (2) arises from scattering due to impurities and is here assumed to be temperature independent within 2–260 K temperature range due to a high compensation ratio of donor and acceptor impurities in nominally undoped GaN. The second term in Eq. (2) is due to the interaction of excitons with acoustic phonon modes, where s is a temperature independent parameter of the interaction. The last term arises from exciton interaction with LO phonons, where ELO is the LO phonon energy.

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4. Conclusions

Fig. 2. Temperature dependent full width at half maximum (FWHM) of the A exciton no-phonon line. Symbols (“) and () represent experimental data for GaN/Al2O3 and GaN/SiC, respectively, while solid and dashed lines show corresponding fits using Eq. (2).

The parameters s and g, as well as the impurity-related broadening Gi can be estimated from the analysis of experimental data. The results of the fits using Eq. (2) for the A exciton are shown in Fig. 2, by the solid and dashed lines for one GaN/Al203 sample and one GaN/SiC sample, respectively. The following parameters are obtained: Gi =4 9 1 meV, s=0.04 90.01 meV K − 1, g=4009 60 meV. The error bars are determined by the difference in the estimated values for the GaN epilayers grown on sapphire and silicon carbide, which are probably due to differences in the internal strain in the epilayers, affecting the exciton – phonon interaction. From the analysis performed here it is possible to conclude that the acoustic-phonon scattering determines the temperature dependent part of the FE linewidth for temperatures B150 K. For higher temperatures, the contribution of the interaction with LO phonons to the line broadening is essential. This result is in agreement with previously reported transport measurements [11], demonstrating the capability of PL linewidth analysis to study the scattering mechanisms in the material.

We have performed temperature dependent studies of the intrinsic A exciton PL emission in state of the art quality GaN epilayers. It is shown that the spectral properties of the FEs can be described within the polariton concept. The unusual behavior of the excitonpolariton PL in GaN, such as the energy independent probability of the 1-LO assisted transitions, as well as an enhanced intensity of the no-phonon FE emission in comparison with its LO phonon-replicas, is revealed and is discussed in terms of strong defect/impurity scattering. This scattering is suggested to be mediated by residual impurities (such as shallow donors), which are present in relatively high concentration (in the 1017 cm − 3 range) in typical GaN material.

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