Nitridation of Sapphire as a Precursor to GaN Growth: Structure and Chemistry

Nitridation of sapphire substrates is used as a precursor to the growth of GaN films to provide a wetting layer which is closer in terms of structure and chemistry to the overlayer. Nitridation has been carried out by metal–organic chemical vapor deposition at 530, 800, and 1100 °C in an environment of NH₃ and H₂. The structure and chemistry of the nitrided layer grown at these different temperatures have been studied by X-ray photoelectron spectroscopy, electron diffraction, high resolution electron microscopy, and electron energy loss spectroscopy. The low temperature nitridation process results in a nitrided layer in which oxygen has been partially replaced by nitrogen to form a cubic spinel-Al_<i>x</i>O_<i>y</i>N_<i>z</i> structure. Nitridation at 800° and 1100 °C results in complete substitution of oxygen atoms by nitrogen to form a cubic rock salt AlN structure. These structures are stable on thermal annealing at 1000 °C prior to epitaxial GaN growth