Degenerate doping in beta-Ga2O3 single crystals through Hf-doping

n-type conductivity of beta-Ga2O3 grown from the melt is typically achieved using Sn and Si. In this paper, we experimentally and computationally investigate Hf doping of beta-Ga2O3 single crystals using UV-vis-NIR absorption and Hall effect measurements and hybrid functional calculations. Unintentionally-doped and Hf-doped samples with a nominal concentration of 0.5at% were grown from the melt using vertical gradient freeze and Czochralski method in mixed Ar + O-2 atmosphere. We demonstrate Hf dopants, predicted to incorporate on the octahedral Ga-II site as a shallow donor, achieve degenerate doping in beta-Ga2O3 with a measured electron concentration similar to 2 x 10(19) cm(-3), mobility 80-65 cm(2) V-1 s(-1), and resistivity down to 5 m omega cm in our samples. The concentration of Hf was measured to be 1.3 x 10(19) atoms cm(-3) using glow discharge mass spectroscopy on doped samples, confirming Hf to be the cause of n-type conductivity (electron concentration similar to 2 x 10(19) cm(-3)).