Si and Ge Nanocrystallites Embedded in CaF 2 by Molecular Beam Epitaxy (MBE)

Thin films of CaF 2 containing layers of Si and Ge nanocrystallites were grown epitaxially on Si( 111) substrates by MBE while varying the substrate temperature. The sticking coefficient of both Si and Ge to CaF 2 are less than unity at the temperatures studied. Evidence that chemical reactions are minimal between Si and CaF 2 leading to the formation of volatile species such as SiF x is presented and it is surmised that the dominant mechanism for re-evaporation is thermal desorption. Both Si and Ge sticking coefficients vary exponentially with 1/T and activation energies are determined. A cluster growth model describing the evolution of the amount of Si in clusters over time is given. Solving in the limit of low cluster coverage, a solution that varies linearly with time and exponentially with 1/T is obtained. Weak room temperature photoluminescence from Si nanocrystallites in CaF 2 is seen, however, it is unclear whether the luminescence is coming from the Si nanocrystallites or the CaF 2 . Second harmonic generation is observed from samples containing single layers of Ge in CaF 2 .