Annealing of nm-thin Si1−xCx/SiC multilayers

Abstract

Amorphous hydrogenated silicon carbide (a-SiC:H) multilayers consisting of nm-thin, alternating Si-rich Si1−xCx and stoichiometric SiC layers were prepared using plasma enhanced chemical vapour deposition. Such multilayers are possible precursor layers for forming Si nanocrystals (NCs) for Si-based tandem solar cells. The multilayers were annealed at temperatures up to 1000°C for 30min and the crystallization of Si NCs in a SiC matrix was investigated. The annealing temperature affected the nanostructure of the multilayers and was studied using various techniques: by scanning electron microscopy we observed that the total thickness of the multilayers was reduced during annealing by 30%. X-ray diffraction revealed the crystallization of Si and SiC NCs when annealing temperatures exceeded 900°C. The crystal structure and the nanostructure were investigated by Transmission Electron Microscopy (TEM) bright-field imaging, high resolution TEM and electron diffraction. By electron diffraction it was proved that SiC crystallized in the cubic polytype 3C-SiC during the annealing process. Energy-filtered TEM showed a roughening of the multilayer structure when crystalline phases were formed. Aberration-corrected high-resolution images yielded lattice fringe images of the SiC and Si NCs. The size of the NCs was 5nm for Si and 2nm for SiC. Raman spectroscopy and electron diffraction showed that a large fraction of Si was still amorphous after annealing at 900°C. Annealing at 1000°C reduced the fraction of the amorphous phase by ∼35% for Si and ∼25% for SiC and strongly affected the crystalline perfection of the NCs and also the integrity of the layers. In contrast to Si NCs in a SiO2 matrix, Si NCs in a SiC matrix did not yield significant photoluminescence possible reasons will be discussed.