Surface modification for improvement of crystallinity of MoS2 using ultraviolet–ozone treatment

Abstract

The efficiency of ultraviolet–ozone (UVO) treatment as a surface modification method for improving the crystallinity of MoS₂ thin films was studied. MoS₂ thin films were prepared using a multi-step chemical vapor deposition method, and highly crystalline MoS₂ thin films were grown by varying the UVO exposure time of molybdenum trioxide (MoO₃) thin films with unstable Mo-O bonds. X-ray diffraction results revealed that the crystallinity of MoS₂ thin films improved with increasing UVO exposure time, but long-term exposure (> 5 min) decreased crystallinity due to surface etching. Cross-sectional transmission electron microscopy images of MoS₂ thin films also showed that short-term UVO exposure (5 min) led to significant surface crystallization without structural defects. In contrast, long-term exposure caused damage to crystal layers. The short-term UVO exposure to unstable MoO₃ thin films induces the formation of many highly reactive MoO₃^- groups, which then react with sulfur to form highly crystalline MoS₂ thin films. These results were confirmed by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analysis. Overall, this study improved the crystallinity of the MoS₂ thin films by increasing the crystallinity of the MoO₃ thin films using a simple and short-term UVO treatment, and these results are attracting attention for their potential application in various industrial fields.