Due to the lattice mismatch between sapphire and GaN, extended defects (mainly threading dislocations) were generated at the sapphire/epilayer interface, and a degenerate layer, characterized by high defect density and high conductivity, was observed. Moving toward the top surface, the density of the extended defects, which seem to greatly affect the material properties, gradually decreased. This fact mainly caused the commonly observed electrical and optical inhomogeneities. A comparative study was made here, by means of optical and electrical characterization, of two HVPE-grown layers with differing thickness (2.6 and 55μm) in order to check the effects of the extended defect distribution across the sample.

Electrical and Optical Characterization of GaN HVPE Layers Related to Extended Defects. A.Castaldini, A.Cavallini, L.Polenta, C.Díaz-Guerra, J.Piqueras: Journal of Physics - Condensed Matter, 2002, 14[48], 13095-104