Chemical kinetics and thermodynamics of the AlN crystalline phase formation on sapphire substrate in ammonia MBE

Chemical kinetics of a two-dimensional (2D) AlN layer formation on the (0001) sapphire (Al₂O₃) surface during nitridation as function of ammonia flux and temperature is investigated by reflection high-energy electron diffraction. The process on the surface is described in framework of a chemical reactions kinetic model including interaction between partially reduced aluminum oxide species (AlO) and chemisorbed NH₂ particles for the temperature range < 1210 K. The experimentally determined AlN formation rates as functions of both the temperature and the ammonia pressure are successfully described by a simple set of kinetic equations. Calculated maximum rate of the process well agrees with the experimental values. It was found that AlN formation rate is independent from temperature for the temperature range > 1210 K. In this range, the process is described as a phase transition in frame of lattice gas model. Precision measurement of 2D AlN lattice parameters during the nitridation process detects the value of 0.301 nm. This value strongly differs from bulk value of wurtzite AlN structure 0.311 nm, but it coincides exactly with a characteristic structure parameter of the oxygen-deficient (0001) Al₂O₃ surface with reconstruction (√31×√31)R ± 9°. We assume that this coincidence is the result of minimizing the elastic stresses at the heterojunction of the 2D AlN and the (0001) Al₂O₃ layer.