It is demonstrated that a combination of techniques can be conveniently employed to evaluate the crystalline quality, and to detect, identify, and verify the role of defects and impurities in the structural, optical, and electronic properties of thick freestanding GaN, bulk AlN, and homoepitaxial layers. The sharpness and line-shapes of X-ray diffraction and Raman scattering phonons of AlN and GaN were adequately used as figures of merit to evaluate the crystalline quality and homogeneity of the bulk substrates. Variable temperature photoluminescence and cathodoluminescence were successfully applied to obtain information about the nature of impurity and extended related defects and their pervasive character in bulk and thin homoepitaxial films of AlN and GaN fabricated by different techniques. (c) 2005 Elsevier B.V. All rights reserved.