Structural and photoluminescence properties of superlattice structures consisting of Sn-rich SiO₂ and stoichiometric SiO₂ layers

We compare the nanostructural and photoluminescence (PL) properties of Sn-rich silicon oxide (SiO₂) single layer and superlattice structures grown by a co-sputtering process. The superlattice structure consists of alternating thin Sn-rich SiO₂ and stoichiometric SiO₂ layers. Both transmission electron microscopy (TEM) and X-ray diffraction studies demonstrated the formation of β-Sn nanocrystals in the single layer structure after annealing at 600 °C, whereas no β-Sn nanocrystals were found in the superlattice structure. Sn oxide nanoparticles of ~ 3.0 nm in diameter were also observed by TEM in both structures. UV-violet PL enhancement around 3.2 eV was observed at room temperature from the superlattice structure compared with the single layer after annealing at the same temperature. The PL property is attributed to the triplet–singlet transition of the oxygen vacancy typical for Sn-related defects in the Sn-rich SiO₂ films. The formation of Sn oxide nanoparticles also contributes to PL in this region with its near band edge emission and defect centre emission. The enhancement of the UV-violet PL is due to the pronounced difference in the nanostructural properties between the two structures. The formation of β-Sn nanocrystals in the single layer can significantly reduce the number of the Sn-related defect centres, hence reducing the PL intensity. On the other hand, the precipitation of Sn nanocrystals is strongly suppressed in the superlattice structure and so also their effect on reducing PL intensity. In addition, more pronounced interface diffusion of Sn atoms to SiO₂ barrier layers in the superlattice structure causes more Sn-related defect centres near the layer interfaces so as to increase PL emission further.