Hydride vapour-phase epitaxy growth and cathodoluminescence characterisation of thick GaN films

Hydride vapour-phase epitaxy of thick (12-120 mu m) GaN layers grown on unbuffered sapphire was examined. We found growth parameters such as the HCl flow rate and the distance from the mixing point to be critical determinants of the film quality. Good-quality films have been further examined by scanning electron microscopy, cathodoluminescence spectroscopy and imaging. We have carried out spatially resolved studies of film cross-sections and top surfaces, as well as the interface side of free standing layers. The top surface of the films show narrow bound exciton emission lines, while the cathodoluminescence spectra near the interface are broad and extend to energies above the band gap. We were able to quantify the electron concentration in those regions and found evidence of strong n-type doping (in the 10(19) cm(-3) range). Close to the interface we were able to directly observe a region about 10 mu m thick containing columnar structures which are highly n-type doped. The highly doped columns occasionally protrude from the layer surface. The relation between the defective region close to the substrate interface and the features observed at the top surface has been analysed. (C) 1999 Elsevier Science B.V. All rights reserved.