Electronic structure of silicon nanowires: A photoemission and x-ray absorption study

Photoemission and x-ray absorption spectroscopy have been used to study silicon nanowires prepared by a laser ablation technique together with Si(100) and porous silicon. Si $2p$ and valence-band spectra show that the Si nanowires are essentially crystalline Si encapsulated by silicon oxide. HF etching removes the surface oxide but leaves the morphology intact. Si K-edge x-ray absorption near-edge structures show that the characteristic Si K-edge whiteline doublet in Si(100) smears out in the nanowires and blurs entirely in porous silicon and that the whiteline exhibits a small blueshift. This observation indicate a progressive degradation in long-range order going from bulk Si to nanowires to porous Si and a wider band gap for a fraction of the nanowires. The extended x-ray absorption fine structures show that despite an increased disorder relative to bulk Si, Si nanowire remains essentially crystalline, in good accord with recent transmission electron microscopy and x-ray powder diffraction studies.