First- and second-order Raman spectroscopy of monoclinic β−Ga2O3

We employ a combined experimental-theoretical study of the first- and second-order Raman modes of monoclinic $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. Gallium oxide has attracted considerable interest due to its deep-UV band gap paired with a high critical field strength, offering promising applications in power electronics. A crucial prerequisite for the future development of ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$-based devices is a detailed understanding of the lattice vibrations, i.e., phonons, as they govern important material properties such as elasticity, thermal conductivity, temperature-dependence of the band gap, or free-carrier transport. Polarized micro-Raman spectroscopy measurements on the (010) and ($\overline{2}01$) planes of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ single crystals enable the determination of the phonon frequencies of all 15 first-order and more than 40 second-order Raman modes. The experimental results are correlated with calculations of the mode frequencies, the phonon dispersion relation, and the phonon density of states using density functional perturbation theory. By applying a group-theoretical analysis, we are able to distinguish between overtones and combinational modes and identify the high-symmetry points in the Brillouin zone that contribute to the observed second-order modes. Based on this information, we demonstrate the simultaneous determination of Raman, IR, and acoustic phonons in $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ by second-order Raman spectroscopy.