Vapor epitaxy of CaF2 on NaCl

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Abstract
CaF2 films, 30–1500 Å thick, were vapor deposited at 25–380 °C onto air-cleaved and vacuum-cleaved NaCl in residual gas pressures of order 10−5, 10−7, and 10−9 Torr. The structures of the films were examined by in situ ultrahigh-vacuum reflection high-energy electron diffraction, conventional transmission electron microscopy, transmission electron diffraction, and surface replication techniques. The films consisted of many crystallites, the complex {110} and {001} orientations of which depended on the deposition parameters. The transition from the complex {110} orientation to the {001} orientation is described in detail in terms of its dependence on the deposition temperature, film thickness, vacuum, and substrate conditions. The {001} orientation was identified with two sets of thin needle-shaped crystallites oriented at 90° to each other. The {110} orientation was identified with two sets of larger crystals also oriented at 90° to each other. At higher deposition temperatures the larger crystals formed predominantly at cleavage steps on the NaCl substrate, while the needlelike crystals tended to form on the surface of the films.

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