Synthesis of large monolayer MoS2 film and its application to field-effect transistors

Abstract

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have gained considerable attention as an emerging semiconductor due to their promising atomically thin film characteristics with good field-effect mobility and a tunable bandgap energy. Among TMDC materials, molybdenum disulfide (MoS2) has gained significant attention due to its direct bandgap of 1.8 eV as a single layer. Herein, numerous studies have explored the application of MoS2 in nanoelectronic devices. To make full use of its unique optical and electrical merits in practical applications, however, synthesis of large and uniform monolayer MoS2 is highly necessary. In this regard, a chemical vapor deposition (CVD) technique has been intensively used to produce large and uniform monolayer MoS2. Meanwhile, their electronic applications have been generally realized with conventional inorganic electrodes and dielectrics implemented using conventional photolithography or transferring processes that are not compatible with large-area and flexible device applications. To facilitate the advantages of 2D TMDCs in practical applications, novel strategies for realizing flexible and transparent 2D electronics using low-temperature, large-area, and low-cost processes should be developed. Here, in this dissertation, the study on the atomically thin MoS2 synthesis and its application to FETs will be discussed. First, the effect of irradiation on MoS2 FETs with 10 MeV high energy proton beams will be discussed. The electrical characteristics of the devices were measured before and after proton irradiation with different fluence conditions. The electrical changes were explained by the proton-irradiation-induced traps, including positive oxide-charge traps in the SiO2 layer and trap states at the interface between the MoS2 channel and the SiO2 layer. Second, the CVD synthesis of large and monolayer MoS2 film will be discussed. The predominantly monolayer character of the CVD-grown MoS2 film was verified by atomic force microscopy (AFM), Raman, and photoluminescence (PL) spectroscopy measurements. Third, the electrical properties of synthesized large-area monolayer MoS2 field-effect transistors with low-cost inkjet-printed Ag electrodes will be discussed. The monolayer MoS2 film was grown by CVD method, and the top-contact Ag source/drain electrodes (S/D) were deposited onto the films using a low-cost drop-on-demand inkjet-printing process without any masks and surface treatments. The electrical characteristics of FETs were comparable to those fabricated by conventional deposition methods such as photo or electron beam lithography. Last, the fully printed transparent CVD-synthesized monolayer MoS2 phototransistor arrays on flexible polymer substrates will be discussed. All the electronic components, including dielectric and electrodes, were directly deposited with mechanically tolerable organic materials by inkjet-printing technology onto transferred monolayer MoS2. By integrating the soft organic components with ultra-thin MoS2, the fully printed MoS2 phototransistors exhibits excellent transparency and mechanically stable operation.