Modeling of Thermal Conductivity of Polycrystalline GaN Films
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Modeling of Thermal Conductivity of Polycrystalline GaN Films D. Kotchetkov1 and A. A. Balandin Nano-Device Laboratory, Department of Electrical Engineering, University of California - Riverside, Riverside, CA 92521 U.S.A. ABSTRACT We present preliminary results of a theoretical investigation of the thermal conductivity of polycrystalline GaN films. It is assumed that grain boundaries play a major role in limiting the thermal conductivity, which is calculated using the phononhopping transport approach. The effect of the grain size, size dispersion, and inter-grain interface structure on the thermal conductivity values is analyzed. The obtained results are compared with available experimental data for polycrystalline films and model predictions for crystalline GaN films.
I. INTRODUCTION Gallium nitride (GaN) is a wide-bandgap semiconductor, which is well suited for applications in high-power electronic and optoelectronic devices. Due to the high-power density involved the thermal budget constrains is a very important consideration in designing GaN-based devices. The estimates of temperature rise, and corresponding degradation of the device performance, depend crucially on the values of thermal conductivity used. The modeling of temperature rise is complicated by the discrepancy of the reported experimental values of the thermal conductivity and its apparent strong dependence on the material quality and concentration of impurities [1]. We have recently calculated thermal conductivity in wurtzite GaN crystalline films as the function of temperature and density of dislocation lines, dopants and impurities [1-2]. The calculation was based on Klemens’ formulas for the phonon scattering rates [3] and explicitly included three-types of dislocation lines and four impurities typical for GaN. The functional dependence of the thermal conductivity on doping density and temperature in crystalline GaN compared favorably with experimental data quoted in the Ref. [1-2]. In this paper we investigate thermal conductivity of polycrystalline GaN films, which currently attract significant attention. It is known that the temperature dependence of the thermal conductivity of polycrystalline films is very different from that of crystalline materials. Moreover GaN samples with very large concentration of defects can be considered as disordered material rather than crystal. This requires a different theoretical approach for calculating the thermal conductivity. In this work we adopt a general formalism for the thermal conductivity in polycrystalline materials proposed by Braginsky et al. [4], apply it to GaN films, and compare the results with experimental data for polycrystalline GaN reported by Daly et al. [5]. The results are then analyzed in order to extract the thermal conductivity 1
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