Growth of ZnO nanowires without catalyst on porous silicon

Porous silicon (PS) technology is, for the first time, used to grow ZnO nanowires on the surface of a PS substrate with a rough morphology without any catalyst. The characteristics of these nanowires were investigated by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), grazing-angle X-ray diffraction (XRD) and photoluminescence (PL) measurements. Zn vapor condenses easily on the PS surface and forms a wetting layer, but not on the flat Si surface. The PS surface provides a rough surface morphology to form a wetting layer by decreasing the surface energy so that ZnO nanowires can grow without any catalyst. The Zn-rich composition changed during growth; the ratio of zinc to oxygen was near to one at the top part of the nanowires. The probable growth mechanism was the vapor-solid (VS) process. In principle, the selective growth of ZnO nanowires on Si-base devices for optoelectronic application is possible.