Initial nucleation of AlN grown directly on sapphire substrates by metal-organic vapor phase epitaxy

Abstract

AlN layers were grown directly on sapphire (0001) substrates using three different growth sequences based on metal-organic vapor phase epitaxy with an emphasis on initial nucleation processes. These three methods were simultaneous, alternating supply of aluminum and nitrogen sources, and a combination of the two. In all the methods, nucleation was initiated by three-dimensional (3D) islands with a typical diameter of ~20 nm. Enhanced migration by the alternating source supply caused highly 3D AlN ridge structures at the sapphire molecular steps. These ridge structures prevented a flattened AlN surface and, in addition, moderated lattice relaxation, suggesting the importance of controlling the initial nucleation in determining the film's properties. In fact, the hybridized method, derived from the simultaneous and alternating supply methods, was able to control the initial nucleation, and provided the best film quality; the 600-nm-thick AlN grown by this method had an atomically flat surface free of pits and particles, and the x-ray diffraction line widths were ~45 and ~250 arcsec for the (0002) and (10[overline 1]2) planes, respectively.