In this study, the interaction of undoped ZnO films with (NH4)2Sx treatment have been investigated by x-ray photoelectron spectroscopy, photoluminescence, optical transmittance, and four-point probe. According to the experimental results, we find that the formation of Zn–S bonds and the reduction of oxygen vacancies (i.e., the S occupation of oxygen vacancies) near the ZnO surface might lead to an increase in the upward band bending, resulting in an increase in the sheet resistance and work function of ZnO.
