Pre-avalanche Ultraviolet Photoconduction Properties of Transitional-Metal-Doped ZnO Nanowires

The ultraviolet (UV) photoelectric characteristics of transitional metal (Cu) doped ZnO nanowires produced by the self-catalytic vapor–liquid–solid (VLS) method were investigated by performing a series of photoconduction and time-resolved measurements. The photocurrent voltage characteristics obtained on the nanowires configured as two-terminal metal–semiconductor–metal photodetectors exhibited a nonmonotonic behavior attributed to the interplay of several limiting mechanisms: Schottky contacts and trapping/detrapping effects that take place at low and intermediate (pre-avalanche) bias regimes, respectively. In the intermediate biases, the photocurrent was power-law dependent, i.e., changed with voltage as and for excitation wavelengths of 365 nm, 302 nm, and 254 nm, respectively. The dependence of the exponent on the wavelength of the light is analyzed and explained based on the detailed consideration of the contribution of different deep-defect Cu levels formed within the band gap of ZnO. The study will be important to those working in the area of ZnO-based nanophotodetectors, optical switches, and sensors.