Structural transitions and relaxation processes during the epitaxial growth of ultrathin CaF2 films on Si(111) Citation

Citation Deiter, Carsten et al. Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. The structure and morphology of ultrathin lattice matched CaF 2 films of very few monolayers thickness, which were deposited on Si͑111͒ substrates by molecular-beam epitaxy, have been studied in situ by synchrotron based grazing incidence x-ray diffraction. Even for the thinnest investigated film of three monolayers thickness, the in-plane structure of the CaF 2 film is determined by a lateral separation in two domains: a pseudomorphic phase assuming the lateral lattice constant of the Si͑111͒ substrate and a completely relaxed phase. Analysis of the crystal truncation rods verifies that both phases adopt the entire homogeneous CaF 2 film thickness. Therefore, we propose that atomic steps of the substrate bypass the nucleation barrier for the formation of ͑Shockley partial͒ dislocations so that the film starts to relax below the classical critical film thickness. While the relaxed phase assumes also the CaF 2 bulk lattice constant for the vertical direction, the vertical lattice constant of the pseudomorphic phase increases due to the compressive lateral strain at the interface. This vertical expansion of the pseudomorphic phase, however, is larger than expected from the elastic constants of the CaF 2 bulk. The fraction of the pseudomorphic CaF 2 phase decreases with increasing film thickness. The interface between the pseudomorphic CaF 2 phase and the Si͑111͒ substrate is characterized by Ca on T 4 sites, a smaller distance between the Si͑111͒ substrate and the CaF interface layer and an expanded layer distance between CaF interface layer and the completely stoichiometric CaF 2 film.