“…A review of the study of the morphology of the heteroepitaxially grown GaN layers using the scanning tunnel microscopy could be found in Ref. (Bakhtizin et al, 2004). Threading dislocation are electrically active with the charge density of approximately 2 elementary charges per nanometer dislocation length (Müller et al, 2006); surface depressions caused by the high strain-energy density near dislocations are observed on the surface of GaN films (Heying et al, 1999).…”