Metallization contacts to nonpolar a -plane n -type GaN

We have investigated the electrical characteristics of metallization contacts to nonpolar a -plane and polar c -plane n -type GaN. Pd Schottky diodes and x-ray photoemission spectroscopy measurements show that the Schottky barrier height of the a -plane GaN is lower than that of the c -plane GaN by 0.24 and 0.30 eV, respectively. TiAl Ohmic contacts to the a -plane n-GaN produce lower contact resistivity than that of the c -plane samples when annealed at 500 °C. However, TiAl contacts to the c -plane and a -plane GaN show opposite electrical behavior when annealed at temperatures above 500 °C, which is attributed to the absence of polarization-induced surface charges for a -plane GaN.