Molecular Origin of Valence Band Anisotropy in Single β-Ga₂O₃ Nanowires Investigated by Polarized X-ray Absorption Imaging

The origin of anisotropy at the molecular and electronic structure levels in individual β-Ga₂O₃ nanowires grown along the crystallographic b direction was studied using linearly polarized X-ray absorption imaging at both gallium L₃ and oxygen K edges. The O K-edge spectrum shows significant linear dichroism for the electric field vector of the X-rays oriented parallel and perpendicular to the nanowire long axis. The contributions from the three nonequivalent oxygen sites to the observed spectral anisotropy were elucidated by using FDMNES calculations in the framework of the multiple scattering theory. The role of relevant O and Ga orbitals in the linearly polarized O K-edge absorption was determined based on the point group symmetry arguments. The results of this work suggest mixed covalent and ionic character of the Ga-O bond in individual nanowires, with the dominant contribution of O 2p_z orbitals to the absorption spectrum for the electric field vector oriented perpendicular, and O 2p_x,y orbitals for the electric field vector oriented parallel to the nanowire long axis. Gallium 4p and d orbitals were found to contribute mostly to the antibonding states. These results improve the understanding of the origin of anisotropy in complex transparent metal oxide nanostructures, and could lead to the prediction of physical properties for different nanowire growth directions.