Nanocrystallites of Si Embedded in CaF₂ by Molecular Beam Epitaxy (MBE)

Abstract

Thin films of CaF₂ containing layers of Si nanocrystals were grown epitaxially on Si(111) substrates by MBE. When Si is deposited epitaxially on CaF₂ it forms islands and, by choosing appropriate growth conditions, a composite of Si nanocrystals embedded in a CaF₂ host crystal is obtained which we have observed with TEM. We varied the effective Si concentration in the composite region from 1% to 25%. The CaF₂ could be grown with high crystal quality as demonstrated by ion channeling results which gave Ca x_min’s as low as 4.7%. Raman spectra of the CaF₂ containing Si nanocrystallites showed significant shifts toward lower energies. Ellipsometry measurements were performed on a number of samples over a range of effective Si concentrations and Maxwell-Garnett effective medium theory was used to fit the data. A key technological advantage to this approach is that the epitaxial Si/CaF₂ composite material is readily integrable into VLSI processing.