Thermal transport properties of GaN with biaxial strain and electron-phonon coupling

Abstract

Strain inevitably exists in practical GaN-based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth and band engineering, and it significantly influences the thermal properties of GaN. In this work, thermal transport properties of GaN considering the effects from biaxial strain and electron-phonon coupling (EPC) are investigated using the first principles calculation and phonon Boltzmann transport equation. The thermal conductivity of free GaN is 263 and 257 W/mK for in-plane and cross-plane directions, respectively, which are consistent better with the experimental values in the literature than previous theoretical reports and show a nearly negligible anisotropy. Under the strain state, thermal conductivity changes remarkably. In detail, under +5% tensile strain state, average thermal conductivity at room temperature decreases by 63%, while it increases by 53% under the −5% compressive strain, which is mostly attributed to the changes in phonon relaxation time. Besides, the anisotropy of thermal conductivity changes under different strain values, which may result from the weakening effect from strain induced piezoelectric polarization. EPC is also calculated from the first principles method, and it is found to decrease the lattice thermal conductivity significantly. Specifically, the decrease shows significant dependence on the strain state, which is due to the relative changes between phonon-phonon and electron-phonon scattering rates. Under a compressive strain state, the decreases of lattice thermal conductivity are 19% and 23% for in-plane and cross-plane conditions, respectively, comparable with those under a free state. However, the decreases are small under the tensile strain state, because of the decreased electron-phonon scattering rates and increased phonon anharmonicity.

Phonon-boundary scattering and thermal transport in AlxGa1−xN: Effect of layer thickness

“…[20][21][22][23] The contribution of the optical phonons is expected to have a minor effect on the thermal conductivity of III-nitride semiconductors due to i) the large energy gap between the acoustic and optical phonons and ii) the flat dispersion of the optical phonons. 24,25 In modified Callaway's model, the thermal conductivity is given by 22,23…”

Vibrational density of states of free and embedded semiconducting GaN nanoparticles

“…Last but not least, it is worth to mention at this point, that the optic peaks obtained with this interatomic potential is blue shifted when compared to the one obtained from experimental results [46] and even more compared to ab-initio computation [47,44]. The empirical potential used here shows a good agreement for the acoustic modes but is less accurate for the optic modes.…”

“…FigureB1: Comparison of the VDOS of bulk GaN using KPM, direct diagonalization and VACF with results from ab-initio calculation by Tang et al[47] and experimental results by Nipko et al[46] …”

Epitaxial Lift-Off of Flexible GaN-Based HEMT Arrays with Performances Optimization by the Piezotronic Effect

“…An improved thermal property will reduce the electron scattering. In principle, strain can influence the thermal conductivity by tuning the phonon properties based on the phonon Boltzmann transport equation [ 34 ]: where is the Bose–Einstein distribution function, is the phonon angular frequency in wavevector q and polarization , and is the phonon group velocity. F is the product of the phonon group velocity and converged relaxation time , over distance, and can be simplified into under relaxation time approximation (RTA), where is the phonon relaxation time for the perturbation theory.…”

“…An improved thermal property will reduce the electron scattering. In principle, strain can influence the thermal conductivity by tuning the phonon properties based on the phonon Boltzmann transport equation [34]:…”