Double layers of ZnO/SiOx (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/SiOx layers were annealed at 1100 ¡É for 20 min without a Si source to form nanowires (NWs). The oxygen content (x) of SiOx 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 £¼ 0.8, no NW growth was observed under the same annealing conditions. The NWs had a composition of amorphous-phase silica (SiOx), as confirmed by using several structural-analysis techniques. Almost all Zn atoms in the ZnO layer were diffused out to the SiOx layer during annealing, as determined by using secondary ion mass spectroscopy. The experimental results are discussed based on possible growth mechanisms of silica NWs.