New insights into the electronic structure and dielectric response of wide-band-gap Ga2O3 polymorphs

In this work, the electronic structure and optical response of four Ga 2 O 3 polymorphs (rhombohedral, monoclinic, orthorhombic, and cubic) are systematically investigated using density functional theory with the modified Becke–Johnson exchange potential within the independent-particle approximation, combined with time-dependent density functional theory employing the bootstrap kernel to describe excitonic and local-field effects in the optical response. The calculated band structures predict direct band gaps at the Γ point for all phases, with values of 5.4 eV for the rhombohedral phase, 5.0 eV for the monoclinic phase, 4.8 eV for the orthorhombic phase, and 5.0 eV for the cubic phase. For all phases, the top of the valence band is predominantly composed of O 2p states, whereas the bottom of the conduction band is mainly derived from Ga 4s states. The inclusion of excitonic and local-field effects leads to optical band gaps of about 5.1 eV (rhombohedral), 4.7 eV (monoclinic and orthorhombic), and 4.8 eV (cubic). The calculated static dielectric constants are 3.79, 3.63, 3.84, and 3.72 for the rhombohedral, monoclinic, orthorhombic, and cubic phases, respectively. Moreover, the full Vis–UV optical response (5-25 eV) of the Ga 2 O 3 polymorphs, revealing strong optical anisotropy and substantial phase-dependent variations in their dielectric spectra.