ZnO transparent conducting thin films codoped with anions and cations

Abstract

In the past two decades, ZnO related materials and devices are widely used for various applications, such as thin-film transistors, photodetectors, light-emitting diodes, and transparent conductors because ZnO has several appealing features like abundant raw materials, environmental friendliness, wide and direct band gap, high electron mobility, and large exciton bonding energy. Especially, ZnO has been recognized as one of the most promising candidates for substituting commercial transparent conductors ITO which is exploited as transparent electrodes in solar cells and flat planar display and transparent heaters etc. For example, aluminum doped ZnO (AZO) transparent conducting films have been commercialized successfully. However, the figure of merits and stability of ZnO transparent conducting films still need to be further improved in order to meet the strict demands of practical large-scale applications. So far, various techniques have been developed to alleviate that contradiction between conductivity and visible light transmittance. In the past few years, a new strategy called anion and cation co-doping in ZnO has been widely implemented to improve the figure of merit, in which IIIA group elements are used as cation dopants and fluorine is used as anion dopants. It is worth noting that F has several advantages as anion dopant in ZnO. First, fluorine has a similar ionic radius with oxygen, which will avoid large lattice distortion. Second, the substitution for oxygen with fluorine only disturbs the valence band of ZnO and leaves the conduction band unaffected, which is beneficial for achieving high electron mobility. In this review, the latest researches of F and boron, or aluminum, or gallium, or indium co-doped ZnO thin films are summarized from both the theoretical and the experimental aspects. Effects of anion and cation co-doping on the optical and electrical properties and thermal stability of ZnO thin films are systematically discussed. Both theoretical and experimental results show that cation and anion co-doping can effectively improve the conductivity and visible light transmittance, no matter ZnO thin films were prepared by physical vapor deposition or synthesized by chemical solution processs. Taking Al and F co-doped ZnO (AFZO) thin films prepared with pulse laser deposition as an example, the mobility of AFZO thin film was apparently higher than that of Al-doped ZnO at the same carrier concentration. The high carrier mobility of AFZO is due to that the F can passivate the surface defects and acceptor-like complex defects in ZnO. In addition, the highly electronegative F makes the AFZO thin films exhibit excellent thermal stability, e.g. maintaining high conductive even after air-annealing at 600°C. These results indicate that anion and cation co-doping can effectively improve the performance of ZnO transparent conductive thin films and broaden their applications. Despite these exciting achievements, challenges still remain in the field of anion and cation co-doped ZnO thin films. For example, the mechanism of the co-doping strategy has not been clarified because the formation energy of intrinsic defects in ZnO is relatively low and the defects configurations for anion and cation co-doping are more complicated than single element doping case. Besides, we are still short of effective techniques and theoretical methods to reveal and simulate the behavior of defect clusters in co-doped ZnO. It is also urgent to establish the industrial standard of ceramic targets for co-doped ZnO to eliminate the discrepancies between different groups. More efforts should also be made in developing new equipment for large-scale manufacturing co-doped ZnO thin films. We hope this paper will provide reference to the readers who are interested in transparent conducting oxides.