Role of interface states in electronic properties of (ZnSe)n/(Si2)n (001) superlattices

Abstract

In a semiempirical tight-binding scheme, the detailed electronic structure and optical properties of (ZnSe)n/(Si2)n (001) superlattices (SLs) are studied with a wide range of n=1–20 giving special attention to the role of the interface states at the Zn–Si and Se–Si polar interfaces. The states at the conduction and valence-band edges are confined two dimensionally in the Si layers. Furthermore, we have found two interface bands in the lower and upper regions of the gap. The states of the lower interfaces band are located at the Zn–Si interface, while those of the upper interface band are located at the Se–Si interface. The energies of the interface states depend on the parameters representing the Zn–Si and Se–Si bond lengths and the valence band discontinuity between ZnSe and Si, but the interface states do not disappear from the gap with reasonable choices of the parameters. It is shown that the heterointerface bond relaxation strongly affects interface band in the band gap. In this system, relaxed Si bonds at the heterointerface induce a vacant interface band and a filled interface band in the band gap. By decreasing the SL period n, the energy gap between the confined band-edge states increases (2.07 eV at the Γ point for n=2) due to the quantum confinement effect. A sudden shrinkage in the band gap (Eg=1.76 eV at the M point) is obtained for n=1. The origin of the band-gap shrinkage is related to the fact that the interface states increasingly overlap and combine as band states. Furthermore, the calculated absorption spectra of the superlattices are found to be quite different from those of bulk ZnSe and Si, but fairly close to their average.