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 on (100) silicon substrates was studied using reflection high-energy electron diffraction and scanning electron microscopy. Both in-situ annealing and a two-stage growth method were applied to improve the crystallinity 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 surfaces in the range 580-800 degrees C, however, exhibited mixed (111) and (100) orientations, whereas epitaxial growth of (100)-oriented BaF2 was achieved on rough CaF2 surfaces. It is believed that exposed (111) facets on rough CaF2 layers provide favorable low-energy surfaces for nucleation of epitaxial BaF2. Smooth BaF2 layers were obtained using a two-stage growth method where an initial BaF2 layer, 1 000-3 000 Angstrom 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 initial BaF2 layer above 870 degrees C resulted in mixed (111) and (100) orientations, thus imposing an upper limit on this processing step.