Selection Role of Metal Oxides into Transition Metal Dichalcogenide Monolayers by a Direct Selenization Process

Direct reduction of metal oxides into a few transition metal dichalcogenide (TMDCs) monolayers has been recently explored as an alternative method for large area and uniform deposition. However, not many studies have addressed the characteristics and requirement of the metal oxides into TMDCs by the selenization/sulfurization processes, yielding a wide range of outstanding properties to poor electrical characteristics with nonuniform films. The large difference implies that the process is yet not fully understood. In particular, the selenization/sulfurization at low temperature leads to poor crystallinity films with poor electrical performance, hindering its practical development. A common approach to improve the quality of the selenized/sulfurized films is by further increasing the process temperature, thus requiring additional transfer in order to explore the electrical properties. Here, we show that by finely tuning the quality of the predeposited oxide the selenization/sulfurization temperature can be largely decreased, avoiding major substrate damage and allowing direct device fabrication. The direct relationship between the role of selecting different metal oxides prepared by e-beam evaporation and reactive sputtering and their oxygen deficiency/vacancy leading to quality influence of TMDCs was investigated in detail. Because of its outstanding physical properties, the formation of tungsten diselenide (WSe₂) from the reduction of tungsten oxide (WO_x) was chosen as a model for proof of concept. By optimizing the process parameters and the selection of metal oxides, layered WSe₂ films with controlled atomic thickness can be demonstrated. Interestingly, the domain size and electrical properties of the layered WSe₂ films are highly affected by the quality of the metal oxides, for which the layered WSe₂ film with small domains exhibits a metallic behavior and the layered WSe₂ films with larger domains provides clear semiconducting behavior. Finally, an 8′′ wafer scale-layered WSe₂ film was demonstrated, giving a step forward in the development of 2D TMDC electronics in the industry.