Characteristics of transparent conducting nano-scaled thin films based on ZnO

Different from the common method used for improving the conductivity of ZnO through doping and/or heat treatments, we have used an alternative, i.e., through the introduction of an Al mid-layer without any heat treatment, to enhance the electrical conductivity. We have investigated a nano-scaled sandwich structure consisting of two outer layers of ZnO and a mid-layer of an ultra-thin aluminum thin film for the improvement of the electrical conductivity of ZnO. The nano-scaled ZnO/Al/ZnO thin films were obtained using an RF sputter deposition method. A number of different RF sputter deposition pressures were used to deposit the ZnO layers with thicknesses ranging from 5 to 180 nm. Al layers with various thicknesses ranging from less than 30 to 210 Å were also obtained using the same RF sputter deposition system. The optical transmittance and electrical resistivity of ZnO films, Al films, and ZnO/Al/ZnO thin films were investigated and compared. We have shown that the use of an ultra-thin Al mid-layer enhances the electrical conductivity of ZnO without scarifying its optical transmittance. Furthermore, the electrical transport in ZnO/Al/ZnO films is believed to be dominated by the electrons in the Al but not by the carrier concentration in the ZnO.