The chemistry of sapphire nitridation in relation to the GaN structural quality: Why low temperature 200 degrees C nitridation?

The effect of c-plane sapphire nitridation upon exposure to a rf N-2 plasma at temperatures in the range 100-700 degreesC on the quality of GaN epilayers grown by MBE is investigated. A homogeneous AIN laver is formed at 200 degreesC. Nitridation at higher temperatures causes rough and non-homogeneous nitridated laver including both AIN and NO. Lowering the nitridation temperature to 200 degreesC results in the improvement of the GaN structural and photoluminescence properties. The results are interpreted in the framework of a chemical model based on the competition between formation of AIN and NO whose adsorption/desorption equilibrium on the sapphire surface strongly depends on temperature.