Gallium nitride growth by RF-plasma assisted molecular bean epitaxy: Determination of surface stoichiometry by RHEED-TRAXS, annealing of gallium nitride:beryllium and the effects of active nitrogen species, surface polarity, and excess gallium-overpressure on high temperature limits.

Growth of GaN by molecular beam epitaxy is ultimately limited by thermal decomposition. Factors influencing thermal decomposition are growth species (atomic versus metastable molecular nitrogen), surface polarity (N- vs. Ga-polar), the presence of atomic hydrogen and varying Ga-overpressure. Surface polarity and growth species are the predominant influence determining the onset of thermal decomposition. Decomposition for Ga-stable Ga-polar and N-polar growth both have the same activation energy as that observed for thermal decomposition of GaN in vacuum even though N-polar growth exhibits a significant decrease in thermal decomposition rate. Decomposition may be rate-limited by desorption of surface-adsorbed Ga. Improvement in electrical quality may occur for Ga-stable growth in part due to suppression of point defect formation due to thermal decomposition. The upper temperature limit for GaN growth by rf-plasma MBE may be significantly increased through greatly increasing the Ga-overpressure. Reflection high-energy electron diffraction total-reflection-angle X-ray spectroscopy (RHEED-TRAXS) is an effective tool for measuring chemical composition of a growing surface with sub-monolayer sensitivity. The high-energy electrons from the RHEED measurement also excite x-ray fluorescence. Since the RHEED electrons just penetrate the surface, with a geometry that measures x-ray takeoff at the total reflection angle, RHEED-TRAX probes primarily the top 20 to 30 A of material. Thus, the surface stoichiometry of layers of GaN and InGaN are directly probed during growth, including direct measurement of the amount of adsorbed excess Ga or surface ratios of Ga to In. High quality N-polar and Ga-polar GaN grown by MBE is grown under gallium stable conditions, which results in the formation of an excess gallium layer on the surface. There are several models of the amount of Ga in the excess layer. Too much gallium incident on the surface during growth results in gallium droplet formation. RHEED-TRAX is a viable tool to test predictions and also serves as an in-situ diagnostic to determine the optimal growth regime. There have been many indirect measurements indicating that near-monolayer coverage of Mg occurs during heavy doping for Ga-polar growth. RHEED-TRAX is used as a diagnostic to work on techniques for altering surface segregation of dopants