Structural transitions and relaxation processes during the epitaxial growth of ultrathinCaF2films on Si(111)

Abstract

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.

Surfactant enhanced solid phase epitaxy of Ge/CaF2/Si(111): Synchrotron x-ray characterization of structure and morphology

“…In agreement with Ref. 35, we conclude from the complete analysis of the L dependence of the in-plane scans (L ¼ 0.10-0.35) that both phases are laterally arranged and have a thickness of 12 TL. The lateral grain size of the relaxed and pseudomorphic CaF 2 phases is 60 nm and 100 nm, respectively, as concluded from the FWHM of both peaks.…”

“…This effect has recently been demonstrated for thinner CaF 2 films (thickness < 7 TL, mainly deposited at 600 C) and attributed to the formation of partial edge dislocations at substrate steps. 35 Before, the formation of dislocation lines and the correlation were reported from plan-view TEM studies. 36 Compared to these former studies, on one hand, the amount of relaxed CaF 2 is larger here, due to the increased film thickness driving the system to a higher degree of relaxation.…”

“…Therefore, the peaks can be assigned to relaxed and pseudomorphic CaF 2 phases, respectively, as previously reported for thinner CaF 2 films. 35 The smaller intensity of the peak due to relaxed CaF 2 (K ¼ 0.994), shows that the majority of the CaF 2 film is pseudomorphic. In agreement with Ref.…”

“…Following Ref. 35, we modeled the CaF 2 films by two lateral separated CaF 2 phases (two columns). One column models the relaxed CaF 2 , while the second column models the pseudomorphic part.…”

Nanostructured CaF2:Ln3+ (Ln3+ = Yb3+/Er3+, Yb3+/Tm3+) Thin Films: MOCVD Fabrication and Their Upconversion Properties

“…Most of the above‐mentioned applications require the calcium fluoride material in the form of thin films. Several studies are available on the deposition of CaF 2 thin films using physical vapor deposition techniques, such as sputtering, electron beam evaporation, pulsed laser deposition, and molecular beam epitaxy . Calcium fluoride films have also been deposited through sol–gel chemical routes using spin coating or dip coating .…”