What causes rough surface in AlN crystal growth?

The surface morphologies of several AlN single crystals which build an AlN polycrystalline sample grown by physical vapor transport (PVT) method is observed by the AFM. The SIMS measurements showed that the concentrations of the nitrogen vacancies in the rough crystallites are higher than those in the smooth crystallites. The Raman measurements in the back scattering geometry showed that the single crystals oriented along the c-axis present smooth surfaces, while the single crystals which are not oriented along high symmetry directions present rough surfaces. The reflectivity spectra of these single crystals are analyzed by using the Kramers–Kronig technique and classical dispersion theory. Single crystals characterized by rough surface present signs of Al2O3 bond, while those characterized by smooth surface present signs of Al–O configurations other than the Al2O3 bond. A model is proposed in which we postulate that the rough surface is caused by the formation of the Al2O3 bond, which becomes more favorable when the nitrogen vacancies become higher than the oxygen concentration.