Metallic conduction in ZnO/Cu/ZnO thin films

ZnO/Cu/ZnO multilayer films have been obtained with carrier concentration of 1.2×10²² cm^-3 and resistivity of 6.9×10^-5 Ω-cm at the optimum copper layer thickness. The conduction mechanism involves metal to oxide carrier injection at low copper layer thickness prior to the formation of a continuous metal conduction pathway at higher copper thicknesses. The peak transmittance, photopic averaged transmittance, and Haacke figure of merit are 88%, 75%, and 8.7×10 ^-3 Ω^-1, respectively. Optical transmission is elucidated in terms of copper's absorption due to d-band to Fermi surface transitions at short wavelengths and reflectance combined with scattering losses at long wavelengths. A Burstein-Moss shift in the band gap of the films is seen to take place with increase in thickness of the copper layer. Annealing at 150°C for up to 24 hours in a reducing ambient(Ar + 5%H₂) was found to have negligible effect on the properties of the films adding to the significance of the room temperature results obtained.