| Summary: | 博士 === 國立交通大學 === 電子研究所 === 107 === In previous decades, researchers followed the prediction of Moore’s Law, which states that the number of transistors doubles every 18 months, thus reducing the cost per chip. Currently, the state-of-the-art semiconductor manufacturing technology uses 7-nm nodes for producing iPhone Xs, which includes 7 billion transistors per chip. The miniaturization of transistors is confined because of physical limitation. More than Moore and beyond complementary metal oxide semiconductor get more attention and need more effort especially <5 nm technology node.
Two-dimensional transition metal dichalcogenide (TMD) materials comprise a transition metal and two chalcogens in the form MX2 (M: Mo, W; X: S, Se, Te). Atoms are bonded in-plane with covalent bonds; however, atoms are stacked in layers through van der Waals forces. The stacking results in an atomic thin single layer of 0.7 nm and a dangling bond-free surface. Polymorphs describe the phase diversity of TMDs, including semiconductors (H phase) and metals (T phase). MoS2 field effect transistors (FETs) have a high carrier mobility and on/off current ratio, which are competitive with those of silicon. Among various TMDs, only MoTe2 can be synthesized in a 2H phase or T phase. The energy bandgap of 2H-MoTe2 is approximately 1 eV, which is close to that of silicon. Phase reversibility can be induced by an external strain or electric voltage. MoTe2 has not been thoroughly studied; however, numerous distinct quantum properties of MoTe2, such as Weyl semimetals and spin-orbit coupling, have been discovered. Synthesizing MoTe2 through the traditional chemical vapor deposition or physical vapor deposition method is difficult because of weak bonding energies between tellurium and molybdenum atoms. Decomposition or oxidization occurs at high temperature during synthesis. Thus, no high-quality synthesis method is available.
In this study, a sputter-deposited method was used to synthesize MoS2. 2H-MoS2 exhibits a uniform thin film (20 cm2). The electronic characteristics of 2H-MoS2 indicate that it has n-type conduction with an on/off current ratio of 5. The advanced solid-phase crystallization method, which is optimized from the sputter-deposited method, for the synthesis of MoTe2 demonstrates advantages in terms of hardware design requirements and reactive gas precursors. MoTe2 FETs exhibit p-type electrical characteristics, with a mobility of 10 cm2/V·s for 2H-MoTe2 and conductivity 100 Ω−1cm−1 for 1T′-MoTe2. The proposed solid-phase crystallization method used a composite SiO2 layer as a capping layer to prevent tellurium out-diffusion and film oxidation at high temperature. Thus, the elements were recrystallized in the designed encapsulated structure. The recrystallization process was similar to solid-phase crystallization. Without a gas-phase reactive precursor or hardware design, the synthesis of TMDs is simplified through solid-phase crystallization, which provides a common method to synthesize oxygen-sensitive materials, such as Te-based TMDs. When an adequate amount of oxygen is used during synthesis, it acts as a p-type dopant and dominates phase growth. MoTe2 is favorable for studying electronics and spintronics. The development of solid-phase crystallization provides a large-area, consecutively uniform, and high-quality film for the synthesis of TMDs compatible with the modern semiconductor industry.
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