In situ monitoring of GaN metal-organic vapor phase epitaxy by spectroscopic ellipsometry

Epitaxy of high-quality GaN on sapphire requires a rather sophisticated substrate preparation prior to the GaN epilayer growth, namely nitridation of the substrate's surface, growth of a GaN nucleation layer at a relative low temperature, and reduction of the defect density of this layer by a subsequent annealing step. For studying both, the detailed mechanisms of this complex procedure and its growth parameter dependencies, we attached an in situ spectroscopic ellipsometer to a nitride metal-organic vapor phase epitaxy reactor. First, the high-temperature dielectric function of GaN was measured using samples from different suppliers. Based on these data, the effect of growth parameter variations on the crystal quality of GaN epilayers could be monitored in situ. In particular, we determined the threshold temperature and the duration of the substrate nitridation under ammonia as well as the thermal threshold and duration of the nucleation layer transformation. Additionally, based on the in situ measurements a qualitative estimate for the crystalline quality of the nucleation layer and the epilayer is provided. Finally, the surface roughness of differently prepared GaN layers was evaluated by using the high-energy spectroscopic range of our vacuum-ultraviolet ellipsometer (3.5-9.0 eV). (C) 2000 American Institute of Physics. [S0021-8979(00)00619-8].