Field emission and optical properties of Ga-doped ZnO nanowires synthesized via thermal evaporation

Ga-doped ZnO (GZO) nanowires have been synthesized by thermal evaporation with gallium metal as the dopant source. The morphology, microstructure and chemical composition were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The investigation confirmed that the GZO nanowires were the wurtzite hexagonal structures. These doped nanowires have diameters in the range 30–70 nm and lengths of several hundreds of nanometers with growth direction along the (1 0 0) crystal plane. The optical properties from the cathodoluminescence (CL) and photoluminescence (PL) spectrum show that GZO nanowires exhibit a relative weak ultraviolet emission (UV) and a strong green emission. The UV emission for ZnO and GZO nanowires is attributed to near band-edge emission from recombination of free excitons. Furthermore, the green emission is attributed to oxygen vacancy and gallium impurity energy levels. Field emission measurements demonstrate that the GZO possesses good performance with a turn-on field of 3.4 V/μm at a current density of 10 μA/cm2, a threshold field of 5.4 V/μm at a current density of 1 mA/cm2, and a field-enhancement factor β of 5945. These results are very helpful for the design, fabrication and optimization of integrated optoelectronic nanodevices using GZO nanowires.