The roles of rare-earth dopants in solution-processed ZnO-based transparent conductive oxides

Zinc oxide (ZnO) doped with group III A elements such as boron, aluminum, gallium, and indium forms a transparent conductive oxide with low sheet resistance. The group III B rare-earth elements scandium (Sc) and yttrium (Y) also have three valence electrons and thus may similarly act as n-type dopants. Here, we use an ink process to deposit Sc:ZnO and Y:ZnO nanocrystalline films with Sc and Y concentration of a few atomic percent. After annealing in forming gas, the resistivity of all films was reduced by four orders of magnitude. However, the Sc or Y-doped ZnO films are more resistive than undoped ZnO, and their resistivity is more strongly dependent on film thickness; Sc and Y do not act as electron donors. The cause of this anomalous behavior was studied by electrical measurements, x-ray photoelectron spectroscopy, and scanning spreading resistance spectroscopy. The high resistivity of Y:ZnO and Sc:ZnO compared to ZnO is attributed to phase segregation of insulating yttria and scandia at the nanocrystalline grain boundaries. The strong thickness dependence of Sc:ZnO and Y:ZnO film resistivity is due to surface and bulk depletion, which may be enhanced by the slowing of grain growth caused by the insulating grain boundary layer.