Selected Energy Epitaxial Deposition and Low Energy Electron Microscopy of AIN, GaN and SiC Thin Films.

Epitaxial AIN films were grown on as-received and hydrogen etched 6H-SiC00Ol substrates using an ammonia supersonic seeded beam. The films on the latter substrates exhibited a higher degree of order. Cross-sectional electron microscopy revealed sharper SiC-AiN interfaces with extended flat terraces. The few stacking mismatch boundaries originated from the 1.5 nm steps corresponding to the 6H stacking sequence. in situ LEEMLEED studies were conducted on GaN homoepitaxial growth on MOCVD GaN substrates by plasma-assisted gas-source MBE. After 6 hours of growth at 6600C, LEEM images showed a GaN layer with a hexagonal parquet structure and a large number of dark spots on the surface. Ex situ AFM studies revealed that the former was a result of spiral growth and the latter were due to surface pits of hexagonal shape. The surface density of the pits is l-2x1O9 CZn.2. In situ cleaning of MOCVD-grown GaNAlN6H-SiC substrates using NH3-seeded supersonic molecular beams was investigated. Complete oxygen removal was achieved by heating in vacuum at 73O0C. Surface carbon concentrations of -3, as evidenced by xPS, were achieved by heating at 73O0C under a hyperthermal NH3 flux. Modifications have been made to the SEE system to provide a cleaner vacuum ambient and are detailed in this report A new substrate holder was fabricated that uses Mo clips and a polished Mo block to ensure good thermal contact without Ag paste. Outgassing of the colloidal Ag paste is believed to be one source of carbon contamination at high temperatures. The design of the N plasma source and testing chamber is also reported.