Electronic structure of light-emitting superlattices composed of quantum films of silicon: Theoretical approach

Abstract

The electronic structure of a Si-based superlattice structure composed of ultrathin Si(001) quantum films is calculated by the extended H\"uckel-type nonorthogonal tight-binding method. The structure is made of alternating Si(001) quantum films of two different thicknesses, in which a thicker film acts as a quantum well $({\mathrm{Si}}^{w})$, while a thinner film as a barrier $({\mathrm{Si}}^{b})$, and a monolayer of oxygen which is sandwiched between the ${\mathrm{Si}}^{w}$ and ${\mathrm{Si}}^{b}$ films. The superlattice structure consisting of the ${\mathrm{Si}}^{w}$ film with $m$ atomic layers and the ${\mathrm{Si}}^{b}$ film with $n$ layers is referred to as a ${({\mathrm{Si}}^{w})}_{m}∕\mathrm{O}∕{({\mathrm{Si}}^{b})}_{n}$ superlattice, which is studied with special emphasis placed on the determination of the value and nature of the band gap, together with oscillator strength for transitions across the band gap. Calculated results are presented for the superlattices with period lengths corresponding to $m+n=8$, 12, and 16. We find that some proposed superlattices become direct band-gap semiconductors by Brillouin-zone folding and have significantly high oscillator strengths in the visible regime. The calculations show that the electronic and optical properties of the superlattices with direct band gap are not very sensitive to the angle of the Si-O-Si bridge bond at the ${\mathrm{Si}}^{w}∕\mathrm{O}∕{\mathrm{Si}}^{b}$ interface, demonstrating that those of the proposed superlattices are determined mostly by the thicknesses of the ${\mathrm{Si}}^{w}$ and ${\mathrm{Si}}^{b}$ films and that the O monolayer serves as a good partition between the ${\mathrm{Si}}^{w}$ and ${\mathrm{Si}}^{b}$ films without giving any lattice-mismatch problem. The possibility of fabricating the proposed superlattice structures is also described.