STRUCTURAL IDENTIFICATION OF A CUBIC PHASE IN HEXAGONAL GAN FILMS GROWN ON SAPPHIRE BY GAS-SOURCE MOLECULAR-BEAM EPITAXY

The structural characteristics of gallium nitride (GaN) films grown on sapphire(0001) substrates by gas source molecular beam epitaxy (GSMBE ) have been investigated using high-resolution synchrotron irradiation X-ray diffraction and cathodoluminescence with a variable energy elec tron beam. Besides the well-known GaN hexagonal structure, a small por tion of cubic phase GaN was observed. The X-ray measurements provide a n essential means for the structural identification of the GaN layers. Arising from the variable penetration depth of the electron beam in t he cathodoluminescence measurements, it was found that the fraction of the GaN cubic-phase typically increased as the probing depth was incr eased. The results suggest that the GaN cubic phase is mostly located near the interface between the substrate and GaN layer due to the init ial nucleation.