Crystallite size and film-substrate interface mediated structural evolution of silicon thin films

Abstract

The effect of crystallite size and the film-substrate interface on the crystal structure of Si nanoparticles leading to the stabilization of wurtzite structure in Si films is reported. Bilayers of Ni(50 nm)/Si(400 nm) were deposited on fused silica (FS) and borosilicate glass (BSG) substrates by electron beam evaporation. The as deposited films, which were amorphous, crystallized at a post deposition annealing temperature of 400 C on BSG and 500 C on FS substrates. Electron diffraction studies showed that the nanocrystalline Si on BSG crystallized into the wurtzite structure with a crystallite size of 45 nm. In contrast, the nanocrystals of Si on FS crystallized in the diamond cubic structure with a crystallite size of 220 nm. The presence of the wurtzite phase is confirmed by the Raman line at 504 cm-1, whereas the signature of the cubic phase appears at 520 cm-1. The composites with Si in the wurtzite structure exhibit higher refractive index of 2.10 (at 1000 nm) than the films with Si in the diamond cubic structure 1.95 (at 1000 nm). The optical band gap in both cases is 2.1 eV. The stabilization of the wurtzite phase is attributed to the lower crystallite size of Si on the borosilicate glass substrate. © 2013 Elsevier Ltd.