Influence of Doping of Different Metals on the Photoconducting Properties of ZnO Thin Films

Ultraviolet (UV) photodetectors based on pure Zinc oxide (ZnO) thin film and metal doped ZnO (M:ZnO) thin film (M: Co, Mn, Ni, Pb, Ag and Au) have been prepared by sol–gel technique. The photoresponse characteristics have been studied for UV radiations of λ = 365 nm and intensity = 24 μwatt/cm². The Au:ZnO thin film based photodetector exhibits a high photocoductive gain (K = 3.7×10²) in comparison to other prepared photodetectors. Au doping in ZnO through substitution of Au⁺ ions at the Zn²⁺ sites in ZnO and also through occupation of Au at interstitial sites. A very low dark current is observed (0.06 μA) for the Au:ZnO photodetector which may be related to the formation of Au–ZnO schottky contacts inside the ZnO bulk. Besides, substituted Au⁺ at Zn²⁺ ions creates traps for free electrons which also decrease the dark current of the film. Upon UV illumination, these trapped electrons are released and contribute in photocurrent. Gold is known to be possessing plasmon propagating properties in a semiconducting medium. Increased absorption cross section of Au clusters in the presence of UV rays leads to increased absorption of UV rays and hence enhanced photoresponse characteristics.