(Invited) Molybdenum Sources Containing Alkali-Metals for the Low Temperature Growth of Optical Grade Monolayer MoS2

Abstract

Two dimensional (2D) transition metal dichalcogenides (TMDCs) based on molybdenum (Mo) and tungsten (W) are optimum building blocks for low power optoelectronic devices due to their particular electrical and optical properties when approaching to the monolayer. Unfortunately, those outstanding properties are usually associated with the synthesis of highly crystalline films at high temperature on specific substrates. In particular, for the synthesis of molybdenum disulphide (MoS2), temperatures over 650 °C are usually required to reach the sublimation point of the commonly used Molybdenum (VI) Oxide (MoO3). Different approaches such as plasma have demonstrated the possibility of the formation of MoS2 at low temperature at expenses of poor crystallinity displaying poor electrical and optical properties reflected in low mobility devices and decreased photoluminescence (PL). Here, we show that using alkali-metal containing sources mixed with MoO3, the sublimation point can be further decreased below 500 °C. The reactive Mo source sublimated at this temperature allows the formation of highly crystalline monolayer films with a PL comparable to those films synthesized at high temperature. Thermogravimetric measurements support the decrease of the sublimation point of Mo using alkali containing sources compared to the sublimation point of pure MoO3. In addition, Raman and PL mappings supported by X-ray Photoelectron Spectroscopy and TEM observations, confirm the quality and crystallinity of the synthesized films. More important, by using a low temperature process, the range of substrates used for synthesis can be further increased. As a proof of concept MoS2 was grown on low cost commercial borosilicate glass suitable for TFT applications.