Surface band bending of a-plane GaN studied by scanning Kelvin probe microscopy

We report the value of surface band bending for undoped, a-plane GaN layers grown on r-plane sapphire by metalorganic vapor phase epitaxy. The surface potential was measured directly by ambient scanning Kelvin probe microscopy. The upward surface band bending of GaN films grown in the [11 (2) over bar0] direction was found to be 1.1 +/- 0.1 V. Because polarization effects are not present on a-plane GaN, we attribute such band bending to the presence of charged surface states. We have modeled the surface band bending assuming a localized level of surface states in the band gap on the surface. It should be noted that the band bending observed for a-plane layers is comparable to that obtained on polar c-plane layers, and both a-plane and c-plane GaN films with similar surface treatments demonstrate comparable band bending behavior, indicating that charged surface states dominate band banding in both cases.