Impact of sapphire nitridation on formation of Al-polar inversion domains in N-polar AlN epitaxial layers

In this work, we study the basic processes during the initial stages of growth which control polarity in N-polar AlN films grown on c-plane sapphire substrates by metalorganic chemical vapor deposition. More specifically, we study the morphology and atomic structure of the films as dependent on nitridation conditions, i.e., duration and temperature, by atomic force microscopy, high resolution transmission electron microscopy ,and high resolution high-angle annular dark field scanning transmission electron microscopy. Our experimental results show that beyond a critical temperature of 1000 °C in addition to an omnipresent two-dimensional aluminum-oxynitride layer, three-dimensional Al-polar AlN islands form. While the aluminum-oxynitride layer is unstable under high temperature growth conditions and results in N-polar films, Al-polar islands are stable and induce Al-polar columnar inversion domains in the N-polar AlN films. Appropriate nitridation conditions (approximately 10 minutes at T = 850 °C–950 °C) and adjustment of growth parameters (reactor pressure, NH₃ flux, etc.) prevent the formation of Al-polar islands, which is essential for achieving N-polar films free of inversion domains.