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

In this Letter, we investigate the atomic and electronic structure of a Hf-doped beta-gallium oxide (beta-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 beta-Ga2O3. Finally, the bandgap values of the Hf doped beta-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 beta-Ga2O3 crystals. All these results make Hf an excellent dopant candidate for beta-Ga2O3.