Atomic-scale characterization of structural and electronic properties of Hf doped β-Ga₂O₃

In this Letter, we investigate the atomic and electronic structure of a Hf-doped beta-gallium oxide (β-Ga₂O₃) single crystal using high resolution scanning transmission electron microscopy imaging and electron energy loss spectroscopy. Ultraviolet-visible (UV-Vis)-near-infrared absorption measurements and density functional theory calculations are performed to further connect the nanoscale observation to the macroscale properties arising from the atomic structure. The Hf-doped sample was grown from the melt with a nominal Hf concentration of 0.5 at. %. We show that the Hf dopants prefer to occupy octahedral over tetrahedral sites by 0.68 eV and have some resistance to form precipitates due to a repulsive interaction of 0.17 eV between Hf atoms on neighboring sites. Also, the presence of Hf atoms on either tetrahedral or octahedral sites do not significantly affect the crystal structure of β-Ga₂O₃. Finally, the bandgap values of the Hf doped β-Ga₂O₃ obtained by electron energy loss spectroscopy and UV-Vis-spectroscopy were E_g = 4.83 ± 0.1 and 4.75 ± 0.02 eV, respectively, similar to the values reported for unintentionally doped β-Ga₂O₃ crystals. All these results make Hf an excellent dopant candidate for β-Ga₂O₃.