Ultrahigh-vacuum scanning electron microscopy characterization of the growth of Fe on CaF2/Si(111): Selective nucleation on electron-beam modified surfaces

Abstract

The initial stages of Fe island growth on electron-beam modified and unmodified CaF2/Si(111) surfaces were studied with a nanometer lateral spatial resolution ultrahigh-vacuum scanning electron microscope. Fe coverages between 7 and 8 ML (deposition rates from 0.12 to 0.19 ML/min, 1 ML=7.7×1014 atom/cm2) grown on room temperature through 300 °C CaF2/Si(111) relaxed and unrelaxed substrates produced a relatively uniform distribution of islands that cover 23% of the substrate with an island density of 7.4×1012 island/cm2. Chemical or defect dominated Fe growth on the CaF2/Si(111) substrates is indicated by the temperature independence of the Fe island distributions for 20 °C≤T≤300 °C. Substrate temperatures near 400 °C yielded mottled surfaces and an altered island distribution relative to those formed during growth at temperatures between 20 and 300 °C. Parallel step edges separated by 25–75 nm were observed for unrelaxed films of CaF2 on Si(111), while relaxed CaF2 films exhibited a saw-toothed step pattern. Fe coverages of Θ=21.4 ML produced a percolation network of connected islands rather than a continuous film covering the CaF2 substrate. The production of nanometer-sized surface structures was evaluated for electron-beam modified growth of Fe on CaF2/Si(111) substrates. Pregrowth (100 keV, 8.2–140 pA) electron irradiation doses as low as 1.14 C/cm2 altered the Fe film morphology on the selectively irradiated regions. Areas dosed with electron irradiation prior to Fe growth were more stable to the damaging effects of post-growth electron irradiation as compared to regions that had not been exposed.

Preferential Sputtering Induced Stress Domains and Mesoscopic Phase Separation onCaF2(111)

“…The growth behavior of Ca on CaF 2 ͑111͒ has not yet been reported in the literature, however, similar elongated island structures have been observed for the growth of Fe on CaF 2 ͑111͒ [18,19]. The lattice constant of calcium is ϳ2% larger than that of CaF 2 .…”

Structured surfaces of wide band gap insulators as templates for overgrowth of adsorbates

“…A straightening of the substrate steps and the formation of regular stripe patterns was observed [99]. A similar expression of the film-substrate symmetry in strain relief structures was found by Heim et al, who observed saw-tooth pattern upon annealing of stepped structures of CaF 2 grown on Si (111) [91].…”

Cobalt on calcium fluoride: Initial stages of growth and magnetic properties

“…The formation of Co islands on the CaF 2 surface (instead of 2D growth) is basically determined by the much higher surface free energy of the metal with respect to CaF 2 [13,14]. Co wets the buffer layer insufficiently to provide coalescence of the islands.…”