Scanning Kelvin force microscopy was applied to study the charge nature of threading dislocations on GaN surfaces. On the oxidized surface, the surface potential maps showed little change near dislocations, indicating that if the dislocations were charged in the bulk, the charges were either screened or depleted due to band bending. After cleaning in hot

H3PO4, the potential near dislocations located at domain boundaries and inside domains was 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 H3PO4 treatment. Thus, either these screw dislocations had no gap states, or if they do have gap states, the positions were higher in energy (closer to conduction band edge) than the gap states of other dislocations.

Scanning Kelvin Force Microscopy Imaging of Surface Potential Variations near to Threading Dislocations in GaN. J.W.P.Hsu, H.M.Ng, A.M.Sergent, S.N.G.Chu: Applied Physics Letters, 2002, 81[19], 3579-81