Correlation between the surface defect distribution and minority carrier transport properties in GaN

We have studied linear dislocations and surface defects in p- and n-type metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy grown GaN films on sapphire with atomic force microscopy. The surface pits due to threading dislocations were found not to be distributed randomly but on the boundaries of growth columns. The dislocations are thought to be electrically active since the average distance between them (average column size) is comparable to minority carrier diffusion lengths as measured by electron beam induced current experiments on Schottky diodes fabricated with the same material. Diffusion lengths found for holes and electrons are on the order of Lp = 0.28 µm and Le = 0.16 µm which corresponded to the sizes of regions free from surface dislocations in both cases and can be described by a simple model of recombination on grain boundaries.