The characterization and understanding of electrically active defects in the HfO₂/β-Ga₂O₃(2̄01) 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.