Plasma assisted MBE growth and characterizations of GaN

A newly installed molecular beam epitaxy system has been used to grow GaN epifilms on c-sapphire. Initial efforts are on the nitridation and the low-temperature buffer layer growth. Considerable nitridation with a high V/III flux ratio results in pure near-bandgap (NBG) transition without the usual yellow emission in photoluminescence spectra. Spotty reflection high-energy electron diffraction patterns and field emission scanning electron microscopy show that the surface is rough. No or less nitridation with a lower V/III flux ratio gives a streaky reflection high-energy electron diffraction patterns and a smooth surface. Photoluminescence measurements yield a narrower NBG emission with certain yellow emission. The images of transmission electron microscopy ensure the thickness of the low temperature GaN buffer layer to be around 200 Angstrom, and the diffraction patterns indicate increasing better quality of the GaN epifilm away from the buffer layer and a single crystal near the surface. Both van der Pauw and capacitance-voltage measurements reveal that the undoped GaN epilayers are natively n-type. X-ray diffraction in theta/2theta mode is performed, which gives a sharp GaN (0002) peak with a full-width-at-half-maximum of 150 arcsec. Other details on the growth and characterizations are discussed.