Epitaxial growth of AlN on c-plane sapphire by High Temperature Hydride Vapor Phase Epitaxy: Influence of the gas phase N/Al ratio and low temperature protective layer

AlN is epitaxially grown on c-plane sapphire by High Temperature Hydride Vapor Phase Epitaxy (HT-HVPE) at constant growth rate and thickness, while varying the N/Al ratio in the gas phase at 1500 °C. The influence of an additional low temperature (1200 °C) protective layer on AlN crystal quality is also assessed. The experiments and thermodynamic calculations show that the sapphire substrate is unstable at high temperature under hydrogen and ammonia while it is stable at low temperature or under a few hundred nanometers of AlN protective layer even at high temperature. In terms of AlN crystal quality, the optimal process developed here consists in depositing a 170 nm low temperature protective AlN layer with N/Al = 3 followed by a high temperature thick AlN layer grown with N/Al = 1.5. In this case, the interface between AlN and sapphire remains continuous (no etching) and the stress in the grown layer at room temperature is minimized by a balance of the growing tensile stress with the cooling compressive stress.