The Effect of Defects and Dopants on Thermal Conduction in GaN Films
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The Effect of Defects and Dopants on Thermal Conduction in GaN Films J. Zou, D. Kotchetkov, A.A. Balandin1, D.I. Florescu2, and F.H. Pollak3 Department of Electrical Engineering, University of California at Riverside, Riverside, CA 92521 U.S.A. 2 Emcore Corporation, 145 Belmont Drive, Somerset, NJ 08873 3 Physics Department, Brooklyn College of the City University of New York, Brooklyn, NY 11210 ABSTRACT We present a theoretical investigation of the effects of dislocations, impurities and dopants on the thermal conductivity of GaN layers. It is shown that the experimentally observed decrease of the room-temperature thermal conductivity with increasing doping density is a result of enhanced phonon relaxation on silicon dopant atoms. Scattering of acoustic phonons on free carriers plays a relatively minor role in GaN. The functional dependence of the thermal conductivity on doping density is in good agreement with experiment. A developed model can be used for thermal budget calculation in high-power density GaN devices. INTRODUCTION GaN and other group-III nitrides are viewed as highly promising for semiconductor optoelectronic and electronic applications. The wide band gap of GaN makes it suitable for high power density and high-temperature operation. Proposed applications of GaN-based devices rely heavily on the possibility of removing high density of excess heat from the device active area. Despite significant practical importance of knowledge of thermal transport in GaN materials and a number of recent experimental reports on thermal conductivity values [1-3], the theoretical investigation of the subject lagged behind. Most of the published theoretical investigations on GaN-related materials focused on intrinsic thermal conductivity limit that does not take into account the effect of defects and impurities [4]. Recently we have reported results of our theoretical investigation of thermal conductivity of GaN films that takes into account large dislocation densities in this material system [5]. Our calculations have demonstrated that the room-temperature thermal conductivity in GaN films can be limited by phonon scattering on dislocations provided that dislocation densities are high (~ 1010 cm–2). We have also investigated the effect of point defects such as vacancies and impurities typical for GaN (oxygen, silicon, carbon, hydrogen) on acoustic phonon relaxation in wurtzite GaN [6]. In this paper we outline our model and investigate the dependence of the lattice thermal conductivity on the doping density in n-type GaN layers. The numerical results are then compared with experimental data. Details of the experiment are reported in Ref. [7]. Since the dominant contribution to thermal conductivity comes from acoustic phonons, the doping increases thermal resistance via phonon scattering on dopant atoms and free electrons. The specific goal of this paper is to demonstrate theoretically that in GaN layers, the thermal conductivity decreases approximately linearly with the logarithm of the doping concentration, 1
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