Characterization and Field‐Emission Properties of Needle‐like Zinc Oxide Nanowires Grown Vertically on Conductive Zinc Oxide Films

Needle-like ZnO nanowires with high density are grown uniformly and vertically over an entire Ga-doped conductive ZnO film at 550 °C. The nanowires are grown preferentially in the c-axis direction. The X-ray diffraction (XRD) θ-scan curve shows a full width at half maximum (FWHM) value of 2°. This indicates that the c-axes of the nanorods are along the normal direction of the substrate surface. The investigation using high-resolution transmission electron microscopy (HRTEM) confirmed that each nanowire is a single crystal. A room-temperature photoluminescence (PL) spectrum of the wires consists of a strong and sharp UV emission band at 380 nm and a weak and broad green–yellow band. It reveals a low concentration of oxygen vacancies in the ZnO nanowires and their high optical quality. Field electron emission from the wires was also investigated. The turn-on field for the ZnO nanowires was found to be about 18 V μm–1 at a current density of 0.01 μA cm–2. The emission current density from the ZnO nanowires reached 0.1 mA cm–2 at a bias field of 24 V μm–1.