Enhancement in optoelectrical properties of polycrystalline ZnO thin films by Ar plasma

In this study, the effects of Ar plasma treatment on the electrical, optical, morphological, and crystalline structural properties of polycrystalline ZnO thin films have been investigated. The simultaneous enhancements in conductivity and near band edge emissions (NBEs), accompanied by a suppression of deep level emissions (DLEs), suggest a passivation effect of the Ar plasma. The dependences on plasma treatment time showed correlations between the DLE and Hall mobility as well as between carrier concentration and the NBE. The results suggested that not only radiative defects but also nonradiative defects could be passivated. The plasma treatment also led to a distortion and deterioration of crystalline structure, as well as damage on the surface, which may account for the decrease in Hall mobility for long treatment time. The incorporation of hydrogen into the ZnO film after Ar plasma treatment has been confirmed using a secondary ions mass spectroscopy. The hydrogen incorporation could be the most probable factor accounting for the significant improvement in optoelectrical properties of ZnO thin film. In addition to the role of passivant, hydrogen atoms can also act as shallow donors, contributing to the eventual increase in carrier concentration.