Etching, Raman and PL study of thick HVPE-grown GaN

HVPE-grown homo-epitaxial 2 mm thick GaN single crystals were examined using different techniques, including orthodox defect-selective etching in molten bases and electroless photo-etching (PEC method) in aqueous KOH solution, micro-Raman spectroscopy and photoluminescence (PL). Examination was performed on the (1 0 −1 0) longitudinal cross-section, which was parallel to the macroscopic [0 0 0 1] growth direction of the crystal. Surface features formed during etching were examined using DIC optical microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Different types of crystallographic and electrically active defects were recognized and analyzed. These are: individual and aligned dislocations, large areas of remarkably different carrier concentration and local growth striation patterns. Both etching methods revealed the presence of individual dislocations and low-angle grain boundaries with dislocation lines parallel and perpendicular to the [0 0 0 1] growth directions. Photo-etching showed remarkable difference of etching rate and defect type/density in the central part of the GaN crystals and in so-called wings. Very low rate of PEC etching of wings resulted from remarkably increased carrier concentration in this part of the crystal, as was established by micro-Raman spectroscopy. In the wings other peculiar structural features were revealed, namely areas with clear, but randomly oriented growth striation patterns. Micro-Raman measurements showed large local differences in carrier concentration inside and outside these areas. PL examination remained in good agreement with the results of etching and Raman spectroscopy: the spectra taken from the central part of the crystal were typical for low carrier concentration GaN (clear DBA pick at 3.47 eV), while spectra from the wings were very broad, typical for high carrier concentration material.