Low contact resistivity of metal/n-GaN by the reduction of gap states with an epitaxially grown GaOx insulating layer

We investigated the contact properties of metal/n-GaN and metal/GaOx/n-GaN in terms of Fermi level pinning (FLP) by metal-induced and disorder-induced gap states (MIGS and DIGS). The work function of ten different metals spanned a wide range from 4.2 to 5.7 eV. The measured Schottky barrier height vs metal work function in metal/n-GaN showed the linear relationship with the slope parameter of S = 0.26 ± 0.01 for the doped and the undoped GaN, indicating a strong FLP. The insertion of a GaOx layer increased S to 0.35 corresponding to the decrease of the gap state density by ∼1.1 × 1013 states/(cm2 eV). A contact resistivity of 1.3 × 10−5 Ω cm2 was obtained in Al/GaOx (2.3 nm)/n-GaN (doped with 2 × 1018 cm−3 of Si), which was smaller by nearly three orders than that without the GaOx layer. The insulating property and partially epitaxial structure of GaOx were considered to be responsible for the reduction of the MIGS and DIGS densities, respectively, thereby relaxing FLP and leading to low contact resistivity. Optimization of the GaOx growth parameters may further improve the contact property.