The characterization and understanding of electrically active defects in the HfO2/β-Ga2O3(201) MOS system

We have investigated the properties of the Au/Cr/HfO2/β-Ga2O3(2¯ 01) MOS (metal-oxide-semiconductor) system after annealing (450 °C) in different ambient conditions (forming gas, N2, and O2). Defect properties have been analyzed using an approach combining experimental impedance measurements with physics-based simulations of the capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics of β-Ga2O3/HfO2 MOS capacitors. The analysis demonstrates that the electrically active defects are not confined to the β-Ga2O3/HfO2 interface but are comprised of two defect bands in HfO2 characterized by thermal ionization energies of ∼1.1 eV (acceptor-like) and ∼2 eV (donor-like) attributed to a polaronic self-trapping state and an oxygen vacancy in HfO2, respectively. The adopted methodology also enabled the extraction of the spatial distribution of defects across the HfO2 thickness and Cr/HfO2 interface. The high concentration of oxygen vacancies close to the Cr/HfO2 interface extracted from experimental and simulated electrical data is confirmed by in situ XPS analysis, which shows how Cr is scavenging oxygen from the HfO2 and creating the donor band confined near the Cr/HfO2 interface. This donor band density is observed to be reduced after annealing, and the reduction is not ambient dependent.