Study of Electronic Band Structure and Optical Properties Al-F co-doped ZnO

Background: Al-F co-doped ZnO systems are investigated by the first principles calculations. Synchronously, we successfully prepared Al-F co-doped ZnO thin films using the aerosolassisted chemical vapor deposition technique. The computational results reveal that the Fermi energy of the Al-F co-doped ZnO system shifts to the conduction band in the electronic band structure, which illustrates that the Al-F co-doped ZnO system is an n-type semiconductor. Furthermore, Al-F co-doped ZnO system has much smaller minimum band gap than pure ZnO system and Al, F monodoped ZnO systems, which indicates its better conductive performance. Experimental results confirm the Al-F co-doping ZnO thin film has the smallest sheet resistance. More importantly, for the optical properties, the strong absorption of Al mono-doped and F mono-doped ZnO systems occurs in the UV region, while the obvious absorption of Al-F co-doped ZnO system happens in the visible-light region. Experimental results of photoluminescence spectroscopy can confirm the conclusion. These results suggest that Al-F co-doped ZnO system has excellent electrical conductivity and optical properties. Objective: Our work mainly concentrates on the Al-F co-doping ZnO system and its electronic band structure and optical properties in terms of experimental studies and theoretical calculations simultaneously. Methods: The ultrasoft pseudopotentials and CASTEP code of plane wave are used to execute all calculations, and depositing ZnO thin films on a glass substrate used by a cold wall aerosol assisted chemical vapor deposition method. Results: We use first principles and experimental results to study ZnO:Al, ZnO:F and ZnO:Al-F systems. We found that F and Al co-doped ZnO thin films resulted in the decrease of both the resistivity and the optical absorption in the visible range compared with the mono-doping systems (ZnO:Al, ZnO:F). It indicates that the Al-F co-doping ZnO system shows better conductive and optical performances in the visible light range. Conclusion: In summary, first principles calculations and experimental results have been performed to study the electronic band structure and optical properties of the ZnO:Al-F system. The electronic band structures show that ZnO:Al-F system exhibits n-type semiconductor, whereas the Fermi level shifts to the conduction band and exhibits metal-like characteristics with Al-F co-doping. The calculated optical properties indicate that the optical energy gap increases with Al-F co-doping. More importantly, a strong absorption in the visible-light region has been found with Al-F co-doping, which originates mainly from the transition between F 2p and Al 3p states. Our calculations provide electronic structure evidence that, in addition to the usage as optoelectronic devices, the ZnO:Al-F system could be a potential candidate for photo-electrochemical application due to the nature of the activity in the visible-light region.