Exploring the surface of gallium oxide

Since the 1990s, gallium oxide has been explored as a gas sensing material. Recently, it has also garnered interest as an oxide semiconductor with an ultrawide energy band gap, about 4.7 eV at room temperature, which makes it a promising material for many applications, including high-power electronics and solar-blind ultraviolet detectors.

But some fundamental properties of gallium oxide remain unknown, especially its surface electronic behavior, which determines the material’s interfacial properties for heterojunctions and metal contacts.

Swallow et al. investigated the surface electron properties of gallium oxide, and observed downward band bending and electron accumulation at the surface of as-received gallium oxide crystals for the first time.

They found that this is not the inherent state of the surface of gallium oxide. When vacuum thermal annealing is used to clean the surface, its natural state is electron depletion. However, before cleaning, the unintentional presence of adsorbed hydrogen provides an extrinsic source of surface donors, which causes an accumulation layer of electrons on the surface.

Co-author Tim Veal said that their work suggests that previous data has also indicated electron accumulation is often present at the surface of gallium oxide, but this detail has heretofore been overlooked. Swallow et al. put the surface electronic properties of gallium oxide into the context of other post-transition metal oxides. Their results also provide evidence that changes in conductivity of gallium oxide during gas sensing are caused by changes in surface electronic properties.

Source: “Transition from electron accumulation to depletion at β-Ga₂O₃ surfaces: The role of hydrogen and the charge neutrality level,” by J. E. N. Swallow, J. B. Varley, L. A. H. Jones, J. T. Gibbon, L. F. J. Piper, V. R. Dhanak, and T. D. Veal, APL Materials (2019). The article can be accessed at https://doi.org/10.1063/1.5054091 .