Quasiparticle bands and spectra ofGa2O3polymorphs

Within the framework of density functional theory and Hedin's $GW$ approximation for single-particle excitations, we present quasiparticle band structures and densities of states for two gallium oxide polymorphs: rhombohedral $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and monoclinic $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. The gap problem is attacked. In addition, their electron effective mass tensors are given. Solving the Bethe-Salpeter equation we also calculate excitonic optical spectra of the two polymorphs. The treatment of excitonic effects allows for a trustable prediction of optical properties from the band gap to the ultraviolet region. In addition, for few other polymorphs we also discuss the frequency-dependent dielectric tensor within the independent-particle approximation (random phase approximation) and densities of states on density functional level. We demonstrate that apart from subtle details, the overall densities of states and optical spectra, in particular the isotropically averaged spectra, are rather similar for all polymorphs, while the electronic dielectric constants vary with the structure. For all polymorphs, complete sets of elastic constants are given.