Room temperature photoluminescence in the visible range from silicon nanowires grown by a solid-state reaction

Black silicon with high density and high aspect ratio nanowhiskers

The physical properties of black silicon (b-Si) formed on Si wafers by reactive ion etching in chlorine plasma are reported in an attempt to clarify the formation mechanism and the origin of the observed optical and electrical phenomena, which are promising for a variety of applications. The b-Si consisting of high density and high aspect ratio sub-micron length whiskers or pillars with tip diameters of well under 3 nm exhibits strong photoluminescence (PL) both in the visible and the infrared, which is interpreted in conjunction with defects, confinement effects and near band-edge emission. Structural analysis indicates that the whiskers are all crystalline and encapsulated by a thin Si oxide layer. The infrared vibrational spectrum of Si-O-Si bondings in terms of transverse-optic (TO) and longitudinal-optic (LO) phonons indicates that disorder induced LO-TO optical mode coupling can be an effective tool in assessing the structural quality of the b-Si. The same phonons are likely coupled to electrons in visible region PL transitions. Field emission properties of these nanoscopic features are demonstrated indicating the influence of the tip shape on the emission. Overall properties are discussed in terms of the surface morphology of the nanowhiskers.

The growth of silica and silica-clad nanowires using a solid-state reaction mechanism on Ti, Ni and SiO(2) layers

A large area compatible and solid-state process for growing silica nanowires is reported using nickel, titanium and silicon dioxide layers on silicon. The silica nanowires also contain silicon, as indicated by Raman spectroscopy. The phonon confinement model is employed to measure the diameter of the Si rich tail for our samples. The measured Raman peak shift and full width at half-maximum variation with the nanowire diameter qualitatively match with data available in the literature. We have investigated the effect of the seedbed structure on the nanowires, and the effect of using different gas conditions in the growth stages. From this, we have obtained the growth mechanism, and deduced the role of each individual substrate seedbed layer in the growth of the nanowires. We report a combined growth mechanism, where the growth is initiated by a solid-liquid-solid process, which is then followed by a vapour-liquid-solid process. We also report on the formation of two distinct structures of nanowires (type I and type II). The growth of these can be controlled by the use of titanium in the seedbed. We also observe that the diameter of the nanowires exhibits an inverse relation with the catalyst thickness.