Electronic excitations in Si and Ge nanocrystals: Parameterfree calculations

Abstract

We describe the modelling of free and oxidized Si and Ge nanaocrystals with up to 363 group-IV atoms using the density functional theory and a supercell method. Electronic excitations are described within the picture of independent Kohn-Sham particles and the ∆SCF method. Matrix elements are computed by means of projector augmented waves. We discuss the influence of several effects such as quantum confinement, structural relaxation, strain, alloying, and oxidation.

A hybrid model for the origin of photoluminescence from Ge nanocrystals in a SiO2matrix

“…The ab-initio electronic structure calculation by Niquet et al [14] for NC-Ge using sp 3 tight binding description presented an analytical law : (a) the blue-green luminescence (above 2 eV) of NC-Ge crystallites originates from the defects in the oxides and (b) the observed size dependent PL energy (as mentioned in [7,8,9,10]) near the infra-red region may be due to the deep trap in the band gap of the nanocrystallites. The above confusion was supplemented with the fact that unlike nanocrystalline Si for NC-Ge the oscillator strength of electronic transitions are quite high at the absorption edge, even for dots with a diameter of about 2.2 nm [15]. Thus, it is difficult to rule out the possibility of getting strong luminescence from NC-Ge itself (as could be done in explaining the origin of visible PL from Si nanostructures [15]).…”

“…The above confusion was supplemented with the fact that unlike nanocrystalline Si for NC-Ge the oscillator strength of electronic transitions is quite high at the absorption edge, even for dots with a diameter of about 2.2 nm [15]. Thus, it is difficult to rule out the possibility of getting strong luminescence from NC-Ge itself (as could be done in explaining the origin of visible PL from Si nanostructures [15]). It is to be noted that the origin of cathodoluminescence from mechanically milled Ge has also been attributed to oxide-related defect states in the system [16].…”