Electronic states and optical transitions in small Si quantum boxes

Electronic and optical properties of small Si quantum boxes (QBs) with hydrogen saturators (referred to as a Nx×Ny×Nz structure, where Nx, Ny and Nz are the number of Si monolayers along the [100], [010] and [001] directions, respectively) are studied using the extended Hückel-type nonorthogonal tight-binding method. It is found that a clear transition between the bulk-like and surface-like conduction band (CB) edge states takes place alternately by changing Nz, among others, in the 5×5×Nz (Nz=5,7,9,…) QBs, resulting in an oscillation behavior in band gap and oscillator strength. The occurrence of the surface-like CB edge state, which is responsible for the enhanced oscillator strength, is found to be ascribed to an inter-hydride interaction between the trihydride units at the side of the (001̄) surface.