Formation Characteristics of Silica Nanowires Grown by Annealing Double Layers of ZnO/SiO_x without Precursors

Double layers of ZnO/SiO_x (Si-rich oxide) were fabricated on a Si wafer by radio-frequency (RF) sputtering and ion beam sputtering, respectively, and were subsequently coated with a 10-nm Au film by RF sputtering. The Au/ZnO/SiO_x layers were annealed at 1100 °C for 20 min without a Si source to form nanowires (NWs). The oxygen content (x) of SiO_x was varied from 0.4 to 1.8, which was controlled and determined by using an X-ray photoelectron spectroscopy analysis. After annealing, NWs with 40 ~ 60 nm in diameter were vertically oriented with their lengths increasing with increasing x from 0.8 to 1.4, but saturating at values of x above 1.4 for all annealing times except 5 min. In contrast, for x < 0.8, no NW growth was observed under the same annealing conditions. The NWs had a composition of amorphous-phase silica (SiO_x), as confirmed by using several structural-analysis techniques. Almost all Zn atoms in the ZnO layer were diffused out to the SiO_x layer during annealing, as determined by using secondary ion mass spectroscopy. The experimental results are discussed based on possible growth mechanisms of silica NWs.