Sol-Gel-Derived Cu-Doped ZnO Thin Films for Optoelectronic Applications

Pristine and Cu-doped ZnO thins films were deposited on Si(100) and quartz substrates by the sol-gel method, followed by post annealing at 450 °C. Structural analysis shows that all grown films are polycrystalline in nature and the crystallite size of the doped film increases with the doping concentration. Narrowing of the band gap is seen with Cu doping, and electrical analysis shows an increase in the conductivity and carrier concentration with Cu doping. As compared to pristine ZnO, Cu-doped thin films show p-type conductivity with a maximum carrier concentration of 1.34 × 10¹⁵ cm^-3 for 6% doping at room temperature. The p-type conductivity in these films easily degraded with time, which may be due to the interaction of films with ambient conditions or may be due to the diffusion of indium inside the material. After 15 days, ambient-exposed films were totally converted into n-type, whereas the vacuum-placed film still shows p-type behavior with good mobility. This study shows that sol-gel-derived Cu-doped thin films show low electrical resistivity, p-type conductivity, and high transmittance and can be used for optoelectronics devices if these films were well prepared, protected, and properly passivated.