Surface band bending and interface alignment of plasma-enhanced atomic layer deposited SiO₂ on Al_xGa_1-xN

Al_xGa_1-xN is characterized by a significant spontaneous and piezoelectric polarization, which increases with the aluminum content. As a result, a surface bound charge is present, which favors compensation by surface states and influences the reliability of AlGaN/GaN devices. This work, therefore, focused on the effects of the polarization charge for GaN and AlGaN with three different aluminum concentrations 15%, 25%, and 35%. The band bending of Al_xGa_1-xN surfaces was measured after a N₂/H₂ plasma pretreatment, which reduced the carbon and oxygen contamination below the detection limit of x-ray photoelectron spectroscopy. Surface band bending was then related to surface states, where the band bending of oxygen-free surfaces - as obtained with a high-temperature, immersed hydrogen/nitrogen plasma clean - scales with the aluminum content. In addition, the band offsets at the plasma-enhanced atomic layer deposited SiO₂/Al_xGa_1-xN interface were measured, giving 3.4 eV, 3.3 eV, 3.3 eV, and 3.0 eV for respective 0%, 15%, 25%, and 35% aluminum concentrations. These values are in accordance with the charge neutrality level model, which implies that SiO₂ will confine carriers over nearly the full range of the aluminum content.