GROWTH-STUDIES OF CAF2 AND BAF2 CAF2 ON (100)SILICON USING RHEED AND SEM/

Molecular beam epitaxial growth of CaF2 and BaF2/CaF2 stacked layers o n (100) silicon substrates was studied using reflection high-energy el ectron diffraction and scanning electron microscopy. Both in-situ anne aling and a two-stage growth method were applied to improve the crysta llinity and surface morphology of the epitaxial layers. Smooth, facet- free, (100)-oriented CaF2 layers on (100) silicon were obtained using both methods. BaF2 layers grown on smooth CaF2-coated (100) silicon su rfaces in the range 580-800 degrees C, however, exhibited mixed (111) and (100) orientations, whereas epitaxial growth of (100)-oriented BaF 2 was achieved on rough CaF2 surfaces. It is believed that exposed (11 1) facets on rough CaF2 layers provide favorable low-energy surfaces f or nucleation of epitaxial BaF2. Smooth BaF2 layers were obtained usin g a two-stage growth method where an initial BaF2 layer, 1 000-3 000 A ngstrom thick, was grown at 580 degrees C on a 300 Angstrom thick CaF2 layer, annealed at 820 degrees C for a few minutes, and followed by 1 000 Angstrom of BaF2 grown at 700-800 degrees C. Annealing the initia l BaF2 layer above 870 degrees C resulted in mixed (111) and (100) ori entations, thus imposing an upper limit on this processing step.