Correlation between photoluminescence and structure in silicon nanowires fabricated by metal-assisted etching

Structural and optical properties of silicon nanowires synthesized by metal-assisted chemical etching are investigated. Macroscopic properties of nanowire arrays as well as microscopic photoluminescence (PL) and Raman scattering spectra of single nanowires are studied. The morphology of silicon nanowires changes depending on etching duration and concentration of H2O2 in the etching solution. In thick nanowires having small length and large diameter, red PL appears. Typical length and diameter are 90m and 600nm, respectively. The PL spectrum is composed of two bands with the peaks at 635 and 705nm. In thin nanowires having large length and small diameter with their ratio larger than 210, no PL appears. Clear correlation between the morphology of nanowires and the PL appearance is demonstrated. Transmission electron microscopy reveals the presence of porous structures of silicon at the top of nanowires. In the middle part of nanowires, the interface between silicon crystalline core and surrounding silicon oxide is smooth and no nanocrystal is present. Optical Raman microscopy for a single nanowire with the space resolution of approximate to 1m clarifies positions where the red PL appears. At the top of nanowires where porous structures of silicon are present, the red PL appears. At these positions, the Raman peak of the optical phonon mode of crystalline silicon becomes broad and shifted. These Raman spectra reflect silicon nanocrystals present at the top of the nanowire. The origin of the red PL in silicon nanowires is considered to be silicon nanocrystals present at the top of nanowires.