Figure 4a shows the Si2p spectra for silicon substrates with native oxide after annealing at 800 °C for 5 minutes at different pO2. An increase in the intensity of SiO2 with respect to Si from the bulk of the substrate was notable above a pO2 of 1*10−4 mbar. Using the model described in the methods section, the silicon oxide thicknesses were calculated from these spectra. A linear increase of oxide thickness with pO2 was observed, as shown in Fig. 4b. Growth of YSZ was normally started within 30 seconds after reaching 800 °C. This is especially important for the higher pO2, since the thickness of the silicon oxide is expected to increase approximately linearly with time27. For example, when growth is performed at a total pressures of 2*10−2 mbar O2, the increase in oxide thicknes is expected to be only 0.04 nm when the substrate is kept at 800 °C for 30 s, instead of the observed 0.4 nm when the substrate is kept at 800 °C for 5 min. Indeed, polycrystalline growth was observed in the latter case (data not shown), while epitaxial growth was observed when the growth started immediately after reaching 800 °C (see Fig. 2).Figure 4(a) XPS Si2p spectra of Si substrates after heating to 800 °C at different pO2. Increase of oxide thickness was observed above 1*10−4 mbar. (b) Calculated silicon oxide thicknesses for as received and annealed substrates, and after growth of YSZ. The silicon oxide thicknesses for the samples with YSZ film were calculated from the samples shown in Fig. 1a, i.e. 6 nm YSZ films grown at a total pressure of 2*10−2 mbar with a laser repetition rate of 14 Hz. The black line is a linear fit of the data..