Scanning Kelvin Force Microscopy Imaging of Surface Potential Variation Near Threading Dislocations in GaN

Scanning Kelvin force microscopy is applied to study the charge nature of threading dislocations on GaN surface. On the oxidized surface, the surface potential maps show little change near dislocations, indicating that if the dislocations are charged in the bulk, the charges are either screened or depleted due to band bending. After cleaning in hot H_3PO_4, the potential near dislocations located at domain boundaries and inside domains is found to be lower, consistent with excess local negative fixed charges. Curiously, no contrast was seen for the screw dislocations at the centers of growth spirals even after H_3PO_4 treatment. Thus, if gap states are associated with these screw dislocations, their positions are higher in energy (closer to conduction band edge) than the gap states of other dislocations.