Band alignment between NiOx and nonpolar GaN plane and between NiOx 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 NiOx/GaN interface presents obvious crystalline plane dependency although they are coated with the same NiOx films.By fitting the Ga 3d spectrum obtained from the NiOx/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 NiOx.On the other hand,the affinity difference between Ga and O forms a dipole which will introduce an extra energy band bending.