Structural and electrical characterization of epitaxial CdF2 layers grown on Si(111) and CaF2(111) substrates

The wide band-gap, group-II fluoride CdF2 exhibits semiconducting properties when doped with some rare earth impurities and annealed under appropriate conditions. In addition, CdF2 is closely lattice matched to Si and epitaxial layers of CdF2 on Si are of potential interest for integration of optoelectronic devices (such as light emitting diodes) with Si ULSI technology. In this work, approximately 300 nm-thick CdF2 films were grown on both Si and CaF2 substrates by molecular beam epitaxy (MBE). Characterizations of the films by reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), Rutherford back-scattering (RBS) and atomic force microscopy (AFM) allowed the growth conditions to be optimized. After growth, the structures were annealed in Cd vapor and their resistivity was measured by a simple two-probe technique both before and after annealing. After annealing in Cd vapor at 500°C for 1 min, the resistivity of CdF2 films grown at 200°C on CaF2 substrates dropped below 0.1 Ω · cm from an as-grown value of 106 Ω · cm. On the other hand, films grown at 400°C had an as-grown resistivity of about 10 Ω · cm but did not decrease with annealing.