Scanning tunneling microscopy study of initial growth of CaF₂ and BaF₂ on Si(111)

Scanning tunneling microscopy (STM) has been used to investigate nucleation and initial growth in the heteroepitaxies of calcium fluoride (CaF₂) and barium fluoride (BaF₂) on Si(111) surfaces in situ. The fluoride depositions and the STM measurements are performed at a substrate temperature of about 400°C. STM images clearly show that a BaF2-deposited surface has a different morphology from that of CaF₂-deposited surface. Preferential nucleation and island growth of BaF₂ only occur at steps and domain boundaries on a Si(111)-7 x 7 reconstructed surface. On the other hand, CaF₂ islands nucleate not only at steps and domain boundaries but also in domain-boundary-free regions of a Si(111) surface. We attribute the difference in the morphologies to the much higher mobility and diffusion length of a BaF₂ molecule in comparison to those of a CaF₂ molecule on a Si(111) surface at 400°C. We also report the first STM measurement of a (CAF₂ + BaF₂)-coexisting surface at 480°C.