Abstract
The wide band gap semiconductor β-Ga2O3 shows promise for applications in high-power and high-temperature electronics. The phonons of β-Ga2O3 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-Ga2O3. 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-Ga2O3 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-Ga2O3. Our theoretical characterization of the vibrational properties of β-Ga2O3 highlights its viability for high-power electronic applications and provides a path for experimental development of materials for improved performance in devices.
Highlights
An increasing amount of recent experimental and theoretical research has focused on the β phase of gallium oxide ( β-Ga2O3), with a primary focus on its applications in highpower electronic devices.1 Because of its wider band gap and, correspondingly, its larger estimated breakdown voltage, βGa2O3 has been identified as a promising alternative for power electronics compared to other wide-gap semiconductors such as GaN and SiC.2–7 The band gap of β-Ga2O3 is an especially important property to consider, with various reports placing it within a broad energy range of 4.4-5.0 eV.2,3,8–10 Reasons for this uncertainty could include experimental growth conditions, sample type, sample quality, and light polarization
We 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
Comparing our calculated transverse optical (TO) modes to other calculations, the majority of the frequencies agree within 5% compared to the results of Liu et al, and within 7% compared to Schubert et al The majority of our reported TO frequencies agree with experiment (Onuma et al.) better than the calculations reported by Liu et al, and five of our reported values are closer by more than 5% to experiment
Summary
An increasing amount of recent experimental and theoretical research has focused on the β phase of gallium oxide ( β-Ga2O3), with a primary focus on its applications in highpower electronic devices. Because of its wider band gap and, correspondingly, its larger estimated breakdown voltage, βGa2O3 has been identified as a promising alternative for power electronics compared to other wide-gap semiconductors such as GaN and SiC. The band gap of β-Ga2O3 is an especially important property to consider, with various reports placing it within a broad energy range of 4.4-5.0 eV. Reasons for this uncertainty could include experimental growth conditions, sample type (i.e. film, bulk, etc.), sample quality, and light polarization. Since the breakdown field of a material increases strongly with increasing band-gap value, a reinvestigation of the breakdown field estimate is needed This is achievable by studying the phonon and electron-phonon coupling properties. Limited carrier mobility and breakdown field can both be explained by the electron-phonon coupling in a material Each of these properties is important for high-power electronic applications. While the band gap and estimated breakdown field of β-Ga2O3 make it promising for power electronics, its electron mobility and thermal conductivity are lower than desired for such applications. Our evaluated electron-phonon coupling matrix elements point to a specific low-frequency polar optical phonon mode that limits the electron mobility at room temperature These matrix elements are used to estimate a value of 6.8 MV/cm for the breakdown field of β-Ga2O3.
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