Effect of oxygen modification on the switching of ZnO TFT

Abstract When zinc oxide thin films lack the O 2− ions required to maintain electrical neutrality (i.e., forming oxygen vacancy defects) during low-temperature physical vapor deposition, the Zn 2+ ions that are left in a dangling bond state donate two electrons and act as donors. As a result, undoped zinc oxide films exhibit n-type conductivity, which reflects a form of structural degradation. When such films are used to fabricate thin-film transistors, they tend to exhibit pronounced depletion-mode electrical characteristics and negatively affect the device’s switching behavior. In this work, two methods for incorporating oxygen atoms to improve the quality of zinc oxide thin films are proposed. In one, the film is modified by oxygen plasma: the plasma bias allows for the mild implantation of oxygen atoms into the film, which helps compensate for oxygen vacancy defects. O 2 plasma treatment can also lead to increased oxygen vacancy defects due to ion bombardment of the zinc oxide film, and simultaneously damage the gate insulator–channel interface. This results in an increase in subthreshold swing and a shift of the threshold voltage toward more negative values, ultimately degrading the switching performance of the thin-film transistor. In the other method, oxygen gas is introduced in-line to actively participate in the zinc oxide formation process. The in situ doping method effectively suppresses the formation of oxygen vacancies and controls carrier concentration, resulting in a positive shift in the threshold voltage. Additionally, it improves the gate insulator–channel interface, which optimizes the switching behavior of the thin-film transistor and leads to a reduction in subthreshold swing value.