Ordered growth of tilted ZnO nanowires:: morphological, structural and optical characterization

Tilted ZnO nanowires have been grown by high-pressure pulsed laser deposition (PLD) on m-plane sapphire without employing any catalyst species. The nanowires are inclined 30 degrees. with respect to the substrate normal and show well-defined epitaxial relationships with the m-plane sapphire substrate, the projection of the nanowires' [0001] axis being parallel to the in-plane sapphire [(1) over bar2 (1) over bar0] direction. Two sets of nanowires, differing in diameter and length, coexist within the initial growth stages, but only the narrowest and longest nanowires are found to keep on growing as deposition proceeds. A systematic study of the effects of growth conditions, including number of pulses, temperature, total pressure and oxygen partial pressure, has been carried out to determine those conditions under which a completely ordered array of tilted nanowires, with a minimum angular distribution, is obtained. Among the growth conditions, temperature and oxygen partial pressure are seen to mainly affect the nanowires' axial and lateral growth rates, respectively, while the total chamber pressure allows monitoring of the evolution from thin film growth, at low pressure, to a mixture of nanowire/ nanobelt growth, at high pressure. Finally, cathodoluminescence ( CL) shows that low growth temperatures and high oxygen partial pressures improve the overall optical quality of the ZnO nanowire arrays.