Structure and electronic properties of SiO ₂/Si multilayer superlattices: Si K edge and L _3,2 edge x-ray absorption fine structure study

We report an x-ray absorption fine structure study at the Si K and L _3,2 edges of a series of Si/SiO ₂ superlattices (SL). The SL system comprises four periods of elemental silicon with a spacing of 1, 1.4, 2.2, and 2.6 nm sandwiched by a 1.5 nm silicon oxide and capped by a 3 nm silicon oxide layer. These systems exhibit electroluminescence and photoluminescence. X-ray absorption near edge structure (XANES) at both the Si K and L _3,2 edge confirms that the Si layers are amorphous. Polarization dependent measurement at the Si K edge reveals that a distinct Si/SiO ₂ interface exists with strong Si-O bonding oriented preferentially closer to the surface normal. High resolution XANES at the Si L _3,2 edge shows a noticeable blueshift of the edge threshold as the lattice spacing decreases, in good accord with quantum confinement. The results and their implications for the origin (quantum confinement and interface/oxide defects) of luminescence in these superlattice systems are discussed.