Understanding the role of potassium incorporation in realizing transparent p-type ZnO thin films

Developing p-n homojunction zinc oxide (ZnO) is an effective approach for delivering high-performance ZnO-based optoelectronic devices; however, difficulties associated with preparing p-type ZnO severely hinder the development of such devices. Herein, we report the fabrication of potassium-incorporated transparent zinc oxide (K-ZnO) thin films by subjecting ZnO thin films to high-pressure molten-salt treatment (hp-MST) in potassium nitrate (KNO₃), which successfully led to p-type semiconductor films. With raising the hp-MST temperature, X-ray diffraction (XRD) results show that the ZnO (002) plane clearly becomes more crystalline, accompanying by larger grains. While the hp-MST temperature significantly influences surface morphology, as further revealed by scanning electron microscopy (SEM). More importantly, the carrier type of K-ZnO thin films was successfully transferred from n-type to p-type with raising the hp-MST temperature, while maintaining excellent optical performance. The oxygen species on the surface of the ZnO thin films change with raising the hp-MST temperature; in particular, the oxygen vacancies increase in number, as evidenced by X-ray photoelectron spectroscopy (XPS), which indicates that K is successfully incorporated into the surface of the ZnO thin films. Notably, the change in K 2p energy combined with the realization of p-type character by the raised hp-MST temperature suggests that K incorporation changes from interstitial to substitutional. This study introduces a simple and efficient alternative strategy for homojunction semiconductors that can be used to prepare next-generation highly efficient semiconductor devices.