Field emission properties of zinc oxide nanowires fabricated by thermal evaporation

Arrays of randomly oriented zinc oxide (ZnO) nanowires were fabricated on silicon wafers via a simple thermal evaporation method. During the fabrication, the temperature around the substrate was below 500 °C. The products were analyzed by conventional means and determined to be single crystals of wurtzite-type ZnO that grew along the c-axis. These nanowires were 10–100 nm in diameter and 10–100 μm in length, suggesting a possible high field enhancement factor. The dependence of the field emission current on the anode–cathode voltage (I–V behavior) of the ZnO nanowire arrays was measured in a lab-built ultrahigh vacuum system with a base pressure of 10−7 Pa. After surface cleaning by heat treatment, two characteristic electric fields, under which 10 μA/cm2 and 1 mA/cm2 current densities were extracted, were measured to be 4.0 and 4.7 V/μm, respectively. As observed with a transparent anode, emission occurred uniformly over the whole sample surface. A 72 h-long test on emission stability was performed under a constant voltage of 2.75 kV. The current dropped occasionally to approximately 80% of the initial value during the test owing to the poor adherence of the nanowires to the substrate. These preliminary results have shown the perspective of, as well as a major drawback to, a ZnO nanowire array being developed into a cold electron source to be used in future electronic devices.