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

Epitaxy of high-quality GaN on sapphire requires a rather sophisticated sub strate 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 subseq uent annealing step. For studying both, the detailed mechanisms of this com plex procedure and its growth parameter dependencies, we attached an in sit u spectroscopic ellipsometer to a nitride metal-organic vapor phase epitaxy reactor. First, the high-temperature dielectric function of GaN was measur ed using samples from different suppliers. Based on these data, the effect of growth parameter variations on the crystal quality of GaN epilayers coul d be monitored in situ. In particular, we determined the threshold temperat ure 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 provide d. Finally, the surface roughness of differently prepared GaN layers was ev aluated by using the high-energy spectroscopic range of our vacuum-ultravio let ellipsometer (3.5-9.0 eV). (C) 2000 American Institute of Physics. [S00 21-8979(00)00619-8].