Surface fermi level pinning of semipolar (1122) n-type GaN surfaces grown on m-plane sapphire substrates

The Schottky barrier height and the S-parameter of the semipolar (1122) n-type GaN grown on m-plane sapphire substrate were investigated by using Schottky diodes fabricated with the different work functions of metals including Cu, Pd, and Pt. The Barrier inhomogeneity model applied to temperature dependent current - voltage characteristics of Schottky diodes revealed the mean barrier heights of 0.86, 0.77, and 0.82 eV for the Cu, Pd, and Pt contact, respectively. The extracted S-parameter was nearly zero, indicating a pinning of the surface Fermi level at approximately 0.8 eV below the conduction band. This could be attributed to the substantial crystallographic defects of semipolar GaN as verified from the atomic force microscope and x-ray diffraction measurements.