In order to investigate the influence of the initial chemistry on the crystalline properties of YSZ, 100 nm YSZ films were grown on top of 5 nm films which were grown under varying pO2 and a fixed laser repetition rate of 14 Hz. This laser repetition rate was chosen because of the clear oxygen pressure dependent differences in chemistry during initial growth, as shown by the XPS measurements above. The 100 nm films were all grown with the same depostion conditions (p = 2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ast $$\end{document}⁎ 10−2 mbar O2, f = 14 Hz). In this way, XRD measurements of the thick films acted as a tool to indicate the crystalline properties of the first 5 nm. The XRD measurements presented in Fig. 2 show a clear trend in crystalline properties depending on the pO2 at 2*10−2 mbar Ar during initial growth. At low pO2, (111) oriented YSZ was measured besides the epitaxial (001) orientation (besides the RHEED patterns shown in Fig. 2, see Supplementary Fig. S2 for a ϕ-scan confirming the epitaxial relation between Si and YSZ). The intensity of the (111) peak decreased with increasing pO2. At the same time, the FWHM of the rocking curve of the (002) peak decreased. At a pO2 of 5*10−3 mbar, the lowest FWHM was measured, while no (111) orientation was visible anymore. Increasing the pO2 above this value led to increased values of the FWHM and the presence of (111) oriented YSZ again.Figure 2XRD measurements of 100 nm thick YSZ films on top of 5 nm YSZ films grown in varying pressures. (a) XRD θ − 2θ scans of the films with the first 5 nm grown at a total pressure of 2*10−2 mbar Ar. At 33° a multiple reflection peak of the Si can be observed. The variation in intensity of this peak is only related to the in-plane orientation of the sample in the XRD40. (b) FWHM of the YSZ (002) rocking curves of the samples with the initial 5 nm grown in total pressures of 2*10−2 or 1*10−1 mbar. The dashed lines are inserted for visual reference. (c–f) RHEED images corresponding to the films shown in subfigure a, after growth of 5 nm in respectively 1*10−5, 1*10−4, 5*10−3 and 2*0−2 mbar. The images were taken after adding the 0.02 mbar O2 to the growth chamber for growth of the 100 nm layer. (g–i) RHEED images of the same samples after growth of the 100 nm layer in 0.02 mbar O2. The rings indicated by a green arrow are artefacts formed by reflection of the electron beam at its entrance pinhole to the growth chamber.