Phonon Decay in GaN and AlN and Self-Heating in III-N Devices
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Phonon Decay in GaN and AlN and Self-Heating in III-N Devices M. Holtz1, D. Y. Song2, S. A. Nikishin2, V. Soukhoveev3, A. Usikov3, V. Dmitriev3, E. Mokhov4, U. Makarov4, and H. Helava4 1 Texas Tech Univeristy, Lubbock, TX, 79409 2 Texas Tech University, Lubbock, TX, 79409 3 TDI, Inc., Silver Spring, MD, 20904 4 Fox Group, Piedmont, CA, 94610
ABSTRACT We report studies of the temperature dependence of Raman lines in high quality GaN and AlN. The temperature dependence of the phonon energies and linewidths are used to produce consistent phonon decay properties of zone center optic phonons. In GaN we observe the E22 phonon to decay into three phonons, while the A1(LO) phonon is well described according to the so-called Ridley process–one TO and one LA phonon. For AlN the E22 phonon decays by two phonon emission and the A1(LO) line also exhibits a dependence consistent with the Ridley process. Along with the phonon decay processes, it is important in each case to take into account the contribution of the thermal expansion, including the temperature dependence, to describe observed temperature shifts in the phonon properties. INTRODUCTION Self-heating in optoelectronic and electronic devices is a substantial problem, particularly in situations with high current density. This typically occurs where current crowding takes place via miniaturization or in devices based on two-dimensional electron gases [1-5]. The self-heating degrades performance and is an important failure mechanism. In polar AlGaN hot electron energy is dissipated predominantly through Fröhlich electron-phonon scattering. Due to the form of the Fröhlich interaction, phonons produced by this carrier relaxation process are primarily zone-center longitudinal-optic (LO) vibrations. The LO phonons behave like standing waves, making them ineffective for dissipating heat created in high current density device channels. The LO phonons must decay into traveling acoustic waves to dissipate energy. Understanding of the intrinsic phonon decay properties of high-quality crystalline materials is thus critical to selfheating and any phonon engineering efforts to mitigate the associated device problems. The q=0 selection rule, where q is wavevector, ideally suits first-order Raman scattering for studying phonons which are important for carrier relaxtion in polar semiconductors. Phonon lifetime studies may be accomplished through Raman linewidths (Γ) measurements, since Γ is inversely proportional to the overall phonon lifetime (τTOTAL). The phonon lifetime is influenced by anharmonic decay and inhomogeneous impurity phonon scattering according to 1 1 1 = + (1) 2π cΓ =
τ TOTAL τ DECAY τ i where c is the speed of light in cm/s and Γ is in cm-1. The average phonon decay time is τDECAY, while τi represents the net effect of impurity and defect scattering processes limiting the phonon lifetime. In high-quality crystals, the latter is generally considered small and temperature
independent. When impurity scattering is negligible in comparison to the anharmonic p
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