Raman Analysis of Single Crystalline Bulk Aluminum Nitride: Temperature Dependence of the Phonon Frequencies
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Raman Analysis of Single Crystalline Bulk Aluminum Nitride: Temperature Dependence of the Phonon Frequencies Jonathan M. Hayes1, Martin Kuball1, Ying Shi2, and James H. Edgar2 1 University of Bristol, H.H. Wills Laboratory, Bristol BS8 1TL, United Kingdom 2 Kansas State University, Chemical Engineering Department, Manhattan, KS 66506-5102, U.S.A.
ABSTRACT The frequencies of the E2(high), A1(LO), A1(TO), E1(TO) and E1(LO) phonons of singlecrystalline bulk AlN were measured using micro-Raman spectroscopy over a temperature range from 10K to 1275K. A modeling of the temperature dependence of the AlN phonon frequencies considering the thermal lattice expansion and two-phonon decay mechanisms gave results in good agreement with the experimental data. At temperatures in excess of ~300K an approximate linear shift of the phonon frequencies with temperature was found. In this high temperature regime, we determined a frequency shift of the E2(high) phonon of (-2.22 ± 0.02) x10-2cm-1/K, which is very similar to values reported for bulk GaN. This suggests that similar parameters will be suitable for AlxGa1-xN alloys, commonly used in high-power high-frequency electronic devices. The results provide the basis for non-invasive local temperature monitoring in highpower III-nitride devices using micro-Raman scattering techniques.
INTRODUCTION Great interest has been shown in GaN, AlN and the ternary alloy AlGaN for deep ultraviolet optoelectronic [1] and high-power high-frequency electronic device applications [2]. However, relatively little is known about their basic material properties, particularly for AlN. Knowledge of material properties is essential for the advancement of device technology. Raman scattering probes the phonons of AlN giving valuable information on the temperature, the strain and the free-carrier concentration. Basic properties of bulk AlN phonons such as the phonon frequencies [3] and the phonon dispersion curves [4] have been investigated. However, no information is available on the temperature dependence of bulk AlN phonons, whereas results on the temperature dependence of bulk GaN phonons have been reported in the literature [5]. AlN, like GaN, normally crystallizes in the hexagonal wurtzite structure which belongs to the space group C46v. At the zone-center group theory predicts eight sets of phonon modes: 2E2, 2A1, 2E1 and 2B1 of which both E2 sets, one A1 set and one E1 set are Raman active, whilst the B1 modes are silent [6]. We report on the temperature dependence of the phonon frequencies of high quality singlecrystalline wurtzite bulk AlN measured using Raman scattering spectroscopy. The results not only provide insight into the material properties of AlN but also provide the basis for local temperature measurements via the phonon frequency using micro-Raman scattering. This is particularly relevant to high-power devices where it is desirable to obtain fast and accurate temperature measurements of the active regions under operation. G6.38.1
EXPERIMENTAL DETAILS Micro-Raman measurements were perf
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