Thermal Conductivity of Wurtzite Zinc-Oxide from First-Principles Lattice Dynamics – a Comparative Study with Gallium Nitride

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.

An alternative, faster and simpler method for the formation of hierarchically porous ZnO particles and their thermoelectric performance

“…6b), which is similar to the reported values for undoped bulk ZnO 6,7 but done at a lesser pressure with lower sintering temperature by SPS when compared to the referred ZnO datas (ref. [17][18][19]. As observed, with the increase in the sintering temperature of the samples, the PF increases correlates with the decrease in the electrical resistivity.…”

“…2,22,48 The (k) thermal conductivity value (especially T-NZnOP-750C and TNZnOP-950C) is considerably lower compared with the previous reported data of normal bulk ZnO-based materials ($100 W m À1 K À1 at RT). 6,7,[17][18][19] We also note that for our present T-ZnOP samples, the sintering temperatures of T-ZnOP were lower compared to typical bulk ZnO-based material wherein their sintering temperature required to reach as high as 1200-1400 C. 6,[12][13][14][15] Generally, the low (k) thermal conductivity achieved by the T-ZnOP samples were mainly attributed to the relative suppression of grain growth due to our low-temperature sintering and the formation of macro/nanopore structuring owing to our (synthesized) starting nanoporous ZnO powders. These results clearly validate that the macro/nanopore structuring and interface scattering of the raw powders (ex: NZnO) are very important in reducing the total thermal conductivity in fabricating a TE materials.…”

“…7 However, the simple wurtzite crystal structure and light Zn and O elements result in thermal conductivity as high as $100 W m À1 K À1 (ref. [17][18][19] at RT (for example in ref. [17][18][19].…”

“…[17][18][19] at RT (for example in ref. [17][18][19]. The reduction of k becomes a primary (associated with phonon selective scattering 4,17,[20][21][22] ) and essential task towards improving the ZT of ZnO.…”

“…[17][18][19]. The reduction of k becomes a primary (associated with phonon selective scattering 4,17,[20][21][22] ) and essential task towards improving the ZT of ZnO. A low-resistance n-type ZnO sintered body is usually obtained by doping ($0.5-5 mol%) Al 2 O 3 followed by burning at a high temperature of $1673 K ($1400 C).…”