Band alignment between NiO_x and nonpolar/semipolar GaN planes for selective-area-doped termination structure

Band alignment between NiO_x and nonpolar GaN plane and between NiO_x and semipolar GaN plane are measured by x-ray photoelectron spectroscopy. They demonstrate that the maximum value of the valence band in the unintentional-doped a-plane, m-plane, and r-plane GaN are comparable to each other, which means that all the substrates are of n-type with similar background carrier concentrations. However, the band offset at the NiO_x/GaN interface presents obvious crystalline plane dependency although they are coated with the same NiO_x films. By fitting the Ga 3d spectrum obtained from the NiO_x/GaN interface, we find that relatively high Ga-O content at the interface corresponds to a small band offset. On the one hand, the high Ga-O content on the GaN surface will change the growth mode of NiO_x. On the other hand, the affinity difference between Ga and O forms a dipole which will introduce an extra energy band bending.