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 degrees C in an environment of NH3 and H-2. 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(x)O(y)N(z) structure. Nitridation at 800 degrees and 1100 degrees C results in complete substitution of oxygen atoms by nitrogen to form a cubic rock salt AIN structure. These structures are stable on thermal annealing at 1000 degrees C prior to epitaxial GaN growth.