Phonon Lifetimes and Phonon Decay Channels in Single Crystalline Bulk Aluminum Nitride
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Phonon Lifetimes and Phonon Decay Channels in Single Crystalline Bulk Aluminum Nitride M. Kuball1, J.M. Hayes1, Ying Shi2, and J.H. Edgar2 1 University of Bristol, H.H. Wills Physics Laboratory, Bristol BS8 1TL, United Kingdom 2 Kansas State University, Chemical Engineering Department, Manhattan, KS 66506-5102, U.S.A. ABSTRACT We report on the Raman analysis of the phonon lifetimes and phonon decay channels of the A1(LO), E1(LO) and E2(high) phonons of single crystalline bulk AlN grown by the sublimationrecondensation method. The temperature dependence of the phonon lifetimes was investigated from 10K to 1275K. Our experimental results show that amongst the various possible decay channels, the A1(LO) phonons of AlN decay primarily into two phonons of equal energy (Klemens model), most likely longitudinal-acoustic (LA) phonons. AlN is therefore in great contrast to GaN, where a symmetric decay of the A1(LO) phonons is not possible due to a large energy gap between the acoustic and optical phonon branches. For the E2(high) phonon, we find an asymmetric phonon decay into a high-energy and a low-energy phonon. Possible decay channels of the E2(high) phonon include combinations of E2(low) and acoustic phonons. For the E1(LO) phonons, the temperature dependence of the phonon lifetime is consistent with a symmetric phonon decay. Phonon lifetimes of the A1(LO), the E1(LO) and the E2(high) phonon of 0.75ps, 1.2ps and 2.9ps, respectively, were measured at 10K.
INTRODUCTION Knowledge of the dynamical properties of phonons is important for device engineers to design better and faster devices. The lifetime of longitudinal-optical (LO) phonons in AlN or GaN, for example, can determine hot-phonon effects, which in the case of GaAs have been demonstrated to play a key role in the carrier relaxation [1]. For GaN, A1(LO) phonon lifetimes and decay channels have been investigated by Tsen et al. [2,3] finding an asymmetric decay of the A1(LO) phonon into a transverse-optical (TO) and a longitudinal-acoustic (LA) phonon, while studies on AlN are rather sparse since high quality AlN has only become available very recently [4,5]. Phonon lifetimes can be determined from the phonon linewidth in Raman experiments through the energy-time uncertainty relation [6-8]. Recently, room-temperature Raman results on AlN phonon lifetimes have been published by Bergman et al. [8], however, no temperature dependence was reported although it would provide invaluable insight into the decay process of the AlN phonons [7]. Decay processes of the phonons of AlN have not been elucidated so far. Using Raman spectroscopy, we have studied the A1(LO), E1(LO) and E2(high) phonon lifetimes on high quality bulk AlN crystals grown using the sublimation-recondensation method. Phonon decay channels of the AlN phonons were identified from the temperature dependence of the phonon lifetimes from 10K to 1275K. The temperature dependence of the phonon lifetimes was fitted using analytical phonon decay models.
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EXPERIMENTAL DETAILS Micro-Raman measurements were per
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