Growth conditions, stoichiometry, and electronic structure of lattice-matched SrO/BaO mixtures on Si(100)

The low temperature growth conditions of mixed Ba0.7Sr0.3O layers on Si(100) were investigated using the combination of low energy electron diffraction, x-ray photoemission (XPS), and electron energy loss spectroscopy. With these methods crystallinity, stoichiometry, and electronic structure of both occupied and unoccupied levels were studied as a function of layer thickness. Oxide layers were generated by evaporating the metals in oxygen ambient pressure with the sample at room temperature after determination of the minimum oxygen pressure necessary for full oxidation. Good crystallinity with perfect lattice matching was only obtained starting with preadsorbed metallic layers at a concentration close to one monolayer (ML), with best results starting from the Sr(5x1). The XPS analysis shows that a sharp interface is formed during oxidation. The chemical species present at the interface are up to 1 ML of mono-oxidized Si and 1 ML of a mixed Sr-O-Si species. No silicide and silicate species or SiO2 formation at the interface after oxidation were found. Both interface and mixed oxide layers turned out to be stable to temperatures up to at least 600 deg. C. Starting already at 1 ML of Ba0.7Sr0.3O, the band gap was found to be 4.3 eV, independent of layer thickness. We determined the valence band offset with respect to n-Si to be -2.2 eV, resulting in a conduction band offset of +1.0 eV