The results of investigations of undoped GaN heteroepitaxial layers with Atomic Force Microscopy and Kelvin Force Microscopy are presented. The measurements of surface potential were performed in the dark and under illumination below and above the energy gap of GaN. In the dark no fluctuations of potential were observed. Moderate illumination revealed fluctuations of (1) short range (~ 100 nm and ~ 0.1 V), related to threading dislocations with a screw component and (2) long range, independent of morphology (~ 5 µm and ~ 0.3 V). Intense illumination removed long range fluctuations, while dislocations remained surrounded by areas of lower potential.