In situ measurements of GaN nucleation layer decompostion

GaN nucleation layer (NL) decomposition was measured using optical reflectance over a wide range of pressure $P,$ temperature $T,$ and $H2/NH3$ gas mixture. The GaN NLs show measurable decomposition above $800 °C$ and do not significantly roughen until above $960 °C.$ The NL decomposition rates increase with increasing $P,$ increasing $T,$ and decreasing $NH3$ flow. An activation energy $EA$ of 2.68 eV was measured (from 820 to $960 °C)$ for NL decomposition and an $EA$ of 2.62 eV was measured (from 900 to $1075 °C)$ for decomposition of thick, high-T bulk GaN films. Depending on $P,$ the pre-exponential factor $A0$ was four to nine times larger for NL decomposition compared to bulk GaN decomposition. The $EA$ measured for both NL and bulk GaN decomposition in mixed $H2$ and $NH3$ flows is similar to the $EA$ for Ga desorption, suggesting that the rate-limiting step for both NL and bulk GaN decomposition is Ga desorption.