IN-SITU TRANSMISSION ELECTRON-MICROSCOPY OF ALN GROWTH BY NITRIDATIONOF (0001)ALPHA-AL2O3

Using a novel ultrahigh vacuum transmission electron microscope (TEM) with in situ reactive molecular beam epitaxy system, we report the suc cessful synthesis of epitaxial AlN on the (0001) sapphire surface upon exposure to ammonia. The substrate, an electron transparent sapphire foil, was annealed in oxygen at high temperature to eliminate damage i nduced during preparation and to create large atomically flat regions. The sample was then held at 950 degrees C in flowing ammonia inside t he microscope for 2 h during which periodic observations of microstruc tural development were made. We report direct observation of the forma tion of epitaxial AlN with the orientation relationship (0001)(AlN)//( 0001)(sub), [(1) over bar 010](AlN)//[1120](sub) and present both TEM and atomic force microscope images of the sample before and after nitr idation. The results are consistent with a diffusion-limited reaction model involving transport of oxygen and nitrogen ions through the grow ing AlN epilayer between the free surface and the unreacted alpha-Al2O 3. (C) 1998 American Institute of Physics.