Simultaneous heteroepitaxial growth of SrO (001) and SrO (111) during strontium-assisted deoxidation of the Si (001) surface

Epitaxial integration of transition-metal oxides with silicon brings a variety of functional properties to the well-established platform of electronic components. In this process, deoxidation and passivation of the silicon surface are one of the most important steps, which in our study were controlled by an ultra-thin layer of SrO and monitored by using transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), synchrotron X-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED) methods. Results revealed that an insufficient amount of SrO leads to uneven deoxidation of the silicon surface i.e. formation of pits and islands, whereas the composition of the as-formed heterostructure gradually changes from strontium silicide at the interface with silicon, to strontium silicate and SrO in the topmost layer. Epitaxial ordering of SrO, occurring simultaneously with silicon deoxidation, was observed. RHEED analysis has identified that SrO is epitaxially aligned with the (001) Si substrate both with SrO (001) and SrO (111) out-of-plane directions. This observation was discussed from the point of view of SrO desorption, SrO-induced deoxidation of the Si (001) surface and other interfacial reactions as well as structural ordering of deposited SrO. Results of the study present an important milestone in understanding subsequent epitaxial integration of functional oxides with silicon using SrO.

Arrows point to areas of interest.

Calculation of reciprocal space distances
The determination of the distances in the reciprocal space is based on the feature of the RHEED method to probe the bulk of the crystal in the case of higher tilt angles. In this way, not only the distances in reciprocal space can be calculated but also the high-symmetry azimuthal direction can be identified. Because of the periodicity, the marked distances in Figure S4a and Figure  The length of segment is 9.3550 and 6.6825 for Figure S4a and Figure S4d, respectively, with ratio close to . Thus, it can be concluded that azimuthal directions in case of Figure S4a and √2 Figure S4d are <100> and <110>, respectively. Since some RHEED patterns in our study contained both streaks (contribution from the smooth, 2×1 Sr-reconstructed silicon surface) and spots (contribution from 3D structure), to elaborate the reciprocal space distances the two approaches based on: i) surface lattice and ii) formalism of transmission electron microscopy can be used. For this purpose, we will use measured distances ( Figure S4).

Surface lattice formalism
To calculate reciprocal space distances we first need to calculate screen constants from the measured distances of Si surface lattice ( Figure S4a,d). For Si(001) surface lattice, whose lattice spacing is a=5.43 Å/√2, the reciprocal lattice spacing is (2π/a)=1.63642 Å -1 . Thus, the screen constants, k1 and k2, from Figure S4a and Figure S4d are, respectively: (1) 18.71 26.73 These constants can be used to calculate reciprocal spacing of 3D structure and 2×1 Srreconstruction.

Formalism of transmission electron microscopy
If we consider that spotty RHEED pattern is a result of transmitted e-beam, then diffraction pattern should follow selection rules based on structure factors. Let us apply the same approach for silicon substrate before (with native oxide) and after deoxidation (2×1 Sr-reconstructed surface).
Let us first calculate screen constants using the stated approach. Thus, the screen constants, k1 and k2, from Figure S4a and Figure  In Figure S4e,f a spotty pattern of 3D structure can also be observed: 18.34 13.10 2 • 2 = 0.5107 Å -1 As can be observed, in the case of 2×1 Sr-reconstructed silicon surface, the experimental values (eqs. 13 and 14) are nicely matching the theoretical ones (0.738 Å -1 and 0.261 Å -1 ). The same can be concluded if calculated values (eqs. 11, 12 and 15,16) are compared to theoretical one ( Figure   5b). This method is more convenient since both surface and island contributions are considered based on structure factors characteristic for Si and SrO, because of which this method was used in the manuscript.