Abstract
Wurtzite Zinc-Oxide (w-ZnO) is a wide bandgap semiconductor that holds promise in power electronics applications, where heat dissipation is of critical importance. However, large discrepancies exist in the literature on the thermal conductivity of w-ZnO. In this paper, we determine the thermal conductivity of w-ZnO using first-principles lattice dynamics and compare it to that of wurtzite Gallium-Nitride (w-GaN) – another important wide bandgap semiconductor with the same crystal structure and similar atomic masses as w-ZnO. However, the thermal conductivity values show large differences (400 W/mK of w-GaN vs. 50 W/mK of w-ZnO at room temperature). It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity. Compared to w-GaN, w-ZnO has a smaller frequency gap in phonon dispersion, which is responsible for the stronger anharmonic phonon scattering, and the weaker interatomic bonds in w-ZnO leads to smaller phonon group velocities. The thermal conductivity of w-ZnO also shows strong size effect with nano-sized grains or structures. The results from this work help identify the cause of large discrepancies in w-ZnO thermal conductivity and will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.
Highlights
Compared to SThM, the laser flash method has measured much lower values of thermal conductivity
Phonon frequency gap (THz) dominated by interface scattering, the thermal conductivity of those sintered samples showed a distinct 1/T temperature dependence, which indicates transport behavior is dominated by anharmonic phonon scattering– a feature generally seen in perfect or large grain crystals
The harmonic force constants are calculated from density functional perturbation theory (DFPT) while the cubic force constants are derived using the finite difference method from a set of force-displacement data obtained from density functional theory (DFT) calculations[25,26]
Summary
Compared to SThM, the laser flash method has measured much lower values of thermal conductivity. In studying the phonon properties of w-ZnO, we have made a detailed comparison to w-GaN – another wide bandgap semiconductor whose intrinsic thermal conductivity was found to be very high (~400 W/mK at room temperature)[13] and share many similarities with w-ZnO in terms of crystal structure, lattice constants and atomic masses. Thermal conductivity values and trends calculated using first-principles lattice dynamics (see Methods section) are shown in Fig. 1 as a function of temperature for both ZnO and GaN.
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