Self-assembled growth and luminescence of crystalline Si/SiO_x core-shell nanowires

Crystalline Si/SiO_x core/shell nanowires(NWs) are self-assembled by annealing Ni-coated hydrogenated Si-rich SiO_x (SRO:H) films at 1100 °C in the presence of Si powder. Plasma-enhanced chemical vapor deposition is used to grow 100nm SRO:H thin films with varying silicon concentration (n_Si). The NWs vary from SiO_x nanowires to Si/SiO_x core/shell structures depending on the composition of the SRO:H substrate, with the fraction of core/shell structures increasing with increasing Si concentration. As n_Si increases from 37 to 43at.%, the average diameter of the NWs also increases from 48 to 157nm. A growth model based on the diffusion-assisted vapor-liquid-solid mechanism is proposed to explain how the core/shell structures are self-assembled. Photoluminescence(PL) spectra of the individual NWs have two major emission bands in the near UV (381nm) and blue (423nm) ranges at n_Si = 43at.%, named as UV and BL PL bands, respectively. In contrast, only the BL PL band is observed at n _Si ≤ 39at.%. These results suggest that the BL and UV PL bands can be attributed to the defect states in the SiO_x shell and at the Si core/SiO_x shell interface, respectively, and that the BL band is closely related to the growth process of the NWs.