Direct observation of large-scale nonuniformities in hydride vapor-phase epitaxy-grown gallium nitride by cathodoluminescence

Hydride vapor phase epitaxial GaN films grown on sapphire without a buffer are found to contain large-scale regions with high electron concentration l ocated close to the interface. These regions are composed of individual col umns forming an irregular but quasicontinuous layer, while the rest of the film has a much lower carrier concentration. The highly doped regions are e asily visualized using cathodoluminescence. The coexistence of regions with low and high electron concentration allows us to explain the concurrent ev idence of high film quality in photoluminescence, Raman spectroscopy and x- ray diffraction, and a high electron concentration measured in transport st udies. (C) 1998 American Institute of Physics. [S0003-6951(98)03850-9].