Fig. 2: XPS characterization and band diagrams of the nBn unipolar barrier heterostructure.

a Valence band maximum (VBM) spectra and the core levels for the Nb:STO bulk, MgO bulk, Ga2O3 bulk, and Ga2O3/MgO/Nb:STO interface. Band diagram of Ga2O3/MgO/Nb:STO heterostructure in equilibrium conditions (b) and in avalanche condition (c). d Illustration of the avalanche process in the nBn unipolar barrier APDs.