GaN epilayers have been grown using low pressure MOVPE (76 Torr) onto (0001) sapphire substrates. Triethylgallium (TEGa) and Ammonia (NH3) were used as precursors. The growth rate has been determined versus growth parameters. The growth rate versus NH3 flow displays a rather unusual behavior, in particular at low growth temperature (buffer layer), where the growth rate decreases strongly when the NH3 flow increases. A growth mechanism involving a competitive adsorption step onto the surface is proposed, and results in a strong dependence of the growth rate on the V/III ratio.
The structural quality of the layers has been assessed by X-ray diffraction versus the growth parameters, and will be reported here. A substrate nitridation step, prior to buffer layer deposition has been introduced in our growth process. The effect of substrate nitridation on the structural and optical properties of the epilayers is studied and we propose an optimized pre-treatment for the MOVPE growth of GaN onto (0001) sapphire substrates.