Microstructure and optoelectronic properties of Cu-Li codoped ZnO film: Role of CuZn and Lii defects

Wurtzite Cu-Li codoped ZnO (Cu-Li:ZnO) films with Cu concentrations of 0-3 at.% were grown by sol-gel method. The conductivity, band gap and transparency of the Cu-Li:ZnO films decrease with increasing the Cu concentration due to the substitution of Cu for the Zn sites (CuZn) and the interstitial Li atoms (Lii). The CuZn defects generate a fully occupied impurity band above the valance band maximum (VBM), resulting in an upward shift of the VBM and a decrease of the band gap. The CuZn acceptors can compensate for the Lii donors and further form complex self-compensation defects [CuZn + Zni]. The carrier mobility of the Cu-Li:ZnO film is about 2-5 orders of magnitude lower than that of the intrinsic one due to the grain boundary and surface scattering.