Surface states on (001) oriented β -Ga2O3 epilayers, their origin, and their effect on the electrical properties of Schottky barrier diodes

Surface states on (001) oriented halide vapor phase epitaxy (HVPE) grown β-Ga2O3 epilayers were explored through the determination of the Schottky barrier height (SBH) as a function of the metal work function using Cr, Cu, Ni, and Au Schottky barrier diodes. SBH is found to be nearly pinned between 1.2 and 1.35 eV in the HVPE grown epilayers. The position of the Fermi level pinning is closely matched with the energy level of the oxygen vacancy [VO(III)] state (EV + 3.57 eV) in the energy bandgap of β-Ga2O3, indicating that Fermi level pinning is due to oxygen vacancy type surface states on (001) oriented β-Ga2O3 epitaxial layers. The Fermi level is found to be relatively unpinned on the bulk β-Ga2O3 (001) substrate, suggesting the presence of lower density of oxygen vacancy states on its surface. Hence, the HVPE growth process was found to be responsible for the presence of oxygen vacancy states [VO(III)] in the epilayer. Moreover, this work highlights the role of these surface states in determining the SBH on β-Ga2O3 (001) epilayers and also explains the reason behind the scattered data of SBH values reported in the literature. In addition to these results, we also showed an increment in the built-in potential and the reduction of reverse leakage current for the epilayer with lower surface state density, which gives a direct evidence of the effect of surface states on the properties of β-Ga2O3 (001) Schottky barrier diodes.