Growth regimes of (0001) AlN by ammonia molecular beam epitaxy

AlN is an emerging ultra-wide bandgap semiconductor, which is attractive due to its large critical electric field and thermal conductivity. Due to the high cost of single crystal AlN substrates, AlN-on-sapphire templates are a viable option for the development of epitaxial layers. In this work, AlN films were grown by ammonia molecular beam epitaxy (NH3-MBE) with varying V/III ratios and substrate temperatures. The surface morphology was examined by atomic force microscopy, and the growth rate was calculated from the Pendellösung fringe spacing obtained from high resolution x-ray diffraction ω-2θ scans. Rocking curve widths were also measured for both on-axis and off-axis peaks. Three growth regimes based on different surface morphologies were identified, most importantly the N-rich step flow growth regime at high temperatures. The differences in surface morphology illustrate the effects of the growth parameters on adatom surface mobility. Growth rates of N-rich AlN homoepitaxial films decreased with increasing NH3 flux, which is consistent with prior observations of this effect being caused by gas-phase collisions prior to the Al flux reaching the substrate. On- and off-axis rocking curve widths of films grown at different V/III ratios were compared to those of the bare substrates, and no additional broadening was observed which indicates that no significant defect formation takes place in the MBE-grown films, which is confirmed by planview transmission electron microscopy. This work shows that NH3-MBE AlN can serve as an insulating buffer layer for high quality electronic devices enabled by precise control over growth regimes.