H. Medina, S.W. Tong, C. Manzano, W. Liao, W. Yang, L.K. Tan, W-W Tjiu, C. Zhu, D-Z Chi
Institute of Materials Research & Engineering,
Keywords: monolayer MoS2, low temperature chemical vapor deposition, transition metal dichalcogenides
Summary:Low cost and low power electronics are major requirements for endpoint applications of the internet of things (IoT). Two dimensional (2D) molybdenum disulphide (MoS2) has been considered as a potential material for low power and transparent electronic devices due to their electrical and optical properties close to the monolayer film thickness. Unfortunately, those outstanding properties are usually associated with the synthesis of crystalline films in high temperature system on specific substrates such as sapphire and rely on further transfer process that will increase the cost associated with technology development. In particular, temperatures over 650 °C are usually required for the synthesis of highly crystalline MoS2 films by the standard chemical vapor deposition (CVD) process. Metal-organic CDV (MOCVD) approaches have shown good electrical properties; however, long deposition time (unit in days) is required in order to achieve low temperature growth of MoS2. Plasma enhanced CVD (PECVD) have demonstrated the formation of MoS2 at low temperature, at expenses of poor crystallinity reflected in low ON/OFF current ratio field effect transistors (FETs) and decreased or almost no photoluminescence (PL). Here, we show that using sodium containing sources mixed with MoO3, the sublimation temperature of the Mo source can be decreased to 500 °C. By tuning the growth parameters, MoS2 single domains up to ~100 µm in size can be achieved. The reactive Mo source is crucial for the formation of crystalline films with a strong PL emission at low temperature comparable to those films synthesized at high temperature. In addition, the high quality and crystallinity of the synthesized films was confirmed by the Raman and PL mappings, X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Electrical characterization of as-synthesized and non-transferred MoS2 film as backgated FETs displaying ON/OFF current ratio up to 106 orders of magnitude show the potential for low power applications. Significantly, a low temperature and a fast process growth method allows a wider choice of substrates for synthesis. As a proof of concept, MoS2 continuous films with excellent uniformity and transparency, was grown on low cost commercial borosilicate glass, which is suitable for Thin Film Transistor (TFT) applications.