Vertical growth of ZnO nanowires on c-Al₂O₃ substrate by controlling ramping rate in a vapor-phase epitaxy method

Vertically well-aligned ZnO nanowires were synthesized on c-Al₂O₃ substrates at 950 °C by the vapor-phase epitaxy (VPE) method. An Au thin film with a thickness of 3 nm was used as a catalyst. The growth direction and length of the ZnO nanowires were successfully controlled by adjusting the ramping rate. Tilted nanowires with a shorter length were grown by increasing the ramping rate, while vertically well-aligned nanowires with a longer length were uniformly formed by decreasing the ramping rate. The X-ray diffraction (XRD) and high-resolution transmission electron microscope measurements showed that the vertically well-aligned ZnO nanowires on the c-Al₂O₃ substrate have a single-crystalline hexagonal structure and preferred c-axis growth orientation.