Study of the Temperature Dependence of E2 and A1(LO) Modes in ZnO
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0957-K07-11
Study of the Temperature Dependence of E2 and A1(LO) Modes in ZnO Esther Alarcon-Llado1, Ramon Cusco1, Jordi Ibanez1, Luis Artus1, Juan Jimenez2, Buguo Wang3, and Michael Callahan4 1 Inst. Jaume Almera, C.S.I.C., C. Sole Sabaris sn, Barcelona, 08028, Spain 2 Departamento Física Materia Condensada, Univ. Valladolid, P. del Cauce sn, Valladolid, 47011, Spain 3 Solid State Scientific Corporation, 27-2 Wrigth Road, Hollis, NH, 03049 4 Sensors Directorate, Air Force Research Laboratory, Hanscom AFB, MA, 01731-2909
ABSTRACT Raman scattering measurements were carried out on a bulk, single crystal of wurtzite ZnO over a temperature range from 80 to 760 K and the temperature-dependent shift and broadening of the E2high and A1(LO) modes was analyzed. The E2high mode exhibits a visibly asymmetric line shape that can be related to the interaction with the continuum of acoustic twophonon density of states. A Fermi resonance model was used to describe the E2high temperature dependence. On the other hand, the anharmonic shift and broadening of the A1(LO) mode are adequately accounted for by a decay model with a dominating Ridley channel involving TO and LA modes. Phonon lifetimes of ∼0.9 and 0.5 ps are found for the E2high and A1(LO) modes, respectively, which corroborates that anharmonic decay involves in both cases a three-phonon process. The A1(LO) lifetime is one order of magnitude lower than that of GaN, which suggests that hot phonon effects should be expected to play a less relevant role in carrier relaxation in ZnO as compared with GaN. INTRODUCTION ZnO is a wide band gap semiconductor with great potential as an alternative to GaN for optoelectronic devices operating in the blue and UV spectral region, since it has a much higher free exciton binding energy (60 meV), it is more resistant to radiation damage and large native substrates are available. The achievement of large-area bulk growth of high-quality ZnO single crystals has renewed the interest on this semiconductor. While the dynamics of the phonon population strongly affect the performance of high-speed optoelectronic devices, our knowledge of ZnO lattice dynamics is still rather limited. Detailed measurements of the optical phonon branch by neutron scattering are still lacking, and only recently a density functional theory (DFT) calculation of the phonon dispersion in ZnO has been published [1]. Raman scattering measurements are well suited to obtain information about the sample quality as well as to analyze more specific aspects of lattice dynamics, such as isotopic effects and phonon lifetimes [2]. In the present study, we present the temperature dependence of Raman scattering in ZnO. We analyze the anharmonic shift and broadening of the E2high and A1(LO) modes as a function of temperature, and we obtain phonon lifetime values for both modes.
EXPERIMENT The experiments were performed on a high quality ZnO single crystal obtained by the hydrothermal growth method [3]. The Raman spectra were excited with the 514.5-nm line of an Ar+ laser in the usua
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