High Performance Field-effect Transistors Based On Van Der Waals Epitaxy Of High-Quality MoS₂ Crystals

In recent years,two-dimensional materials,including graphene,black phosphorus?BP?,and transition metal dichalcogenides?TMDs?,have attracted much attention due to their unique structures and remarkable properties.As a member of the TMD family,molybdenum disulfide?MoS₂?with a tunable bandgap ranging from 1.2 eV to 1.9 eV suggests a great potential for logic devices,integrated circuits,and optoelectronics.Thus,it is highly desirable to develop controllable synthesis methods for MoS₂.In this thesis,we mainly focus on scalable growth,material characterization and electrical properties of monolayer MoS₂ crystals.The thesis is mainly constituted by the following three aspects:?1?We report a scalable van der Waals epitaxy of monolayer MoS₂ crystals on soda-lime glass substrates by chemical vapor deposition system.Typical MoS₂ growth conditions on SiO₂ substrates were pressure of 760 Torr,carrier gas flow rate of 40 sccm,growth temperature of 670?and the MoS₂ crystals size is around 20?m.By changing the 90 nm SiO₂ substrates into soda-lime glass,the grain size increases to 200?m.We also investigate the effect of growth temperature and the carrier gas flow rate on the crystal size.And We theoretically explained the results.This research provides a potential towards large-area synthesis of monolayer MoS₂.?2?We transferred the MoS₂ crystals to 300 nm SiO₂ for material characterization.We use atomic force microscopy,Raman spectroscopy and photoluminescence at room temperature,temperature-dependent Raman spectroscopy and photoluminescence spectroscopy,transmission electron microscopy to characterize material properties of MoS₂ crystal.This research demonstrate that high-quality MoS₂ crystal is achieved.?3?We transferred the MoS₂ crystals on soda-lime glass and 90 nm SiO₂ substrates to18 nm high-k HfLaO substrates for fabrication of back-gated field-effect transistors and electrical characterization,respectively.Back-gated field-effect transistors based on MoS₂crystals on SiO₂ shows on/off ratio of 10⁶,field mobility of 0.34 cm²V^-1s^-1,threshold voltage of 0.98 V,drive current of 18?A/?m.Back-gated field-effect transistors based on MoS₂ crystals on soda-lime glass shows on/off ratio of 10⁸,field mobility of 11.9cm²V^-1s^-1,threshold voltage of 0.06 V,drive current of 266?A/?m.And the latter has a smaller hysteresis than the former at the same channel length.The superior performance of MoS₂ crystals grown on soda-lime glass can be attributed to the following two aspects:On one hand,the melting process of soda-lime glass erases the high-energy sites such as defects and kinks;On the other hand,the MoS₂ crystals grown on the soda lime glass can be transferred by a green non-etching deionized water transfer method,thereby avoiding the destruction of the MoS₂ crystals by an etchant such as hydrofluoric acid.This research demonstrate that soda-lime glass is better than SiO₂ for chemical vapor deposition of monolayer MoS₂ crystals.