Vibrational and electron-phonon coupling properties of β-Ga2O3 from first-principles calculations: Impact on the mobility and breakdown field

Abstract

The wide band gap semiconductor β-Ga 2 O 3 shows promise for applications in high-power and high-temperature electronics. The phonons of β-Ga 2 O 3 play a crucial role in determining its important material characteristics for these applications such as its thermal transport, carrier mobility, and breakdown voltage. In this work, we apply predictive calculations based on density functional theory and density functional perturbation theory to understand the vibrational properties, phonon-phonon interactions, and electron-phonon coupling of β-Ga 2 O 3 . We calculate the directionally dependent phonon dispersion, including the effects of LO-TO splitting and isotope substitution, and quantify the frequencies of the infrared and Raman-active modes, the sound velocities, and the heat capacity of the material. Our calculated optical-mode Grüneisen parameters reflect the anharmonicity of the monoclinic crystal structure of β-Ga 2 O 3 and help explain its low thermal conductivity. We also evaluate the electron-phonon coupling matrix elements for the lowest conduction band to determine the phonon mode that limits the mobility at room temperature, which we identified as a polar-optical mode with a phonon energy of 29 meV. We further apply these matrix elements to estimate the breakdown field of β-Ga 2 O 3 . Our theoretical characterization of the vibrational properties of β-Ga 2 O 3 highlights its viability for high-power electronic applications and provides a path for experimental development of materials for improved performance in devices.

Isotopic study of Raman active phonon modes in β-Ga2O3

“…64 Raman tensor elements were determined in angular-resolved measurements 60 using a modified Raman tensor formalism proposed in a preceding publication. 68 Though the effects of isotope substitution on the frequencies of the Raman active phonon modes in β -Ga 2 O 3 were investigated theoretically, 69 no corresponding experimental study is available in the literature. Raman vibrations may be excited by oscillations of Ga or O atoms.…”

First and Second Order Raman Spectroscopy of Monoclinic $β-\mathrm{Ga}_2\mathrm{O}_{3}$

“…Polar phonons induce an oscillating macroscopic electric field in the direction of atomic displacements, leading to a splitting of the IR-active A u and B u modes into transverse-optical (TO) and longitudinal-optical (LO) phonons. 11,12,24 The non-polar A g and B g modes, in turn, exhibit no LO-TO-splitting. The inclusion of the LO-TO-splitting into our calculations leads to a step-like behavior in the phonon dispersion for a number of branches at the Γ-point, which is fully consistent with previous theoretical studies.…”

Structural, electronic, elastic, power, and transport properties ofβ−Ga2O3from first principles

“…A common approximation known as the self-energy relaxation time approximation (SERTA) consists in neglecting the second term on the right-hand side of Eq. (22). The mobility then takes the simpler form:…”

“…The Dirac delta function in Eqs. (22) and (23) were computed using the adaptive smearing method of Refs. [64,69,70].…”

“…the magnitude of the external electric field that a material can sustain before incurring permanent damage. The breakdown field can be computed from first-principles using the von Hippel low energy criterion [26][27][28], and was recently computed ab-initio by Mengle and Kioupakis [22] to be 5.4 MV/cm in β-Ga 2 O 3 considering only the dominant LO phonon mode. They further estimated that considering all modes would increasing the theoretical intrinsic breakdown field by 20% to 6.8 MV/cm.…”