Chemical Vapor Deposition Growth Of Two-Dimensional MoS₂ And Its Application In Transistors

Two-dimensional(2D)layered materials represented by graphene and transition metal dichalcogenides(TMDCs)have attracted large research enthusiasm because of their uique atomically layered structures and novel properties in recent years.Compared with graphene with zero bandgap,TMDCs with bandgaps between about 1.0 eV and 2 eV depending upon its number of layers for a specific TMDC are more favorable in logic circuits and optoelectronics.Controlled synthesis of high quality TMDCs via chemical vapor deposition(CVD)is currently the most used method for scalable and controllable synthesis of TMDCs with relatively high quality as compared to other methods such as mechanical and liquid-phase exfoliation methods.In this essay,monolayer MoS2 as well as MoS2(1-x)Se2x alloys with various composition are synthesized via CVD method and monolayer MoS2 is utilized to build field effect transistors(FETs).The main conclusions are listed as below.(1)Round-shaped MoS2 domains are synthesized by a dual-quartz-tube CVD system on the SiO2/Si substrate.The observation of round-shaped MoS2 domain is strikingly different from previously reported triangle domains.Shape evolution from the round to the triangle shapes is realized by tuning the heating temperature of precursor MoO3 powder used for the MoS2 synthesis.The round-shaped MoS2 domains were proven to be the 2H-MoS2 monolayer by Raman spectroscopy,photoluminescence spectroscopy(PL),atomic force microscopy(AFM),scanning electron microscopy(SEM),transmission electron microscopy(TEM).The formation mechanism of round-shaped MoS2 domains is the follows.Small triangle MoS2 nuclei are formed firstly due to surface reaction and the reactants rapidly are accumulated as the heating temperature of MoO3 powder is increased,which leads to a higher reaction rate beyond the thermodynamic equilibrium regime and shifts the reaction to a kinetic controlled condition.In the kinetic regime,the growth rate of MoS2 at all domain directions is equal instead of following the crystal symmetry.Large-scale uniform MoS2 film is grown by tailoring the growth mechanism and optimizing the growth conditions.(2)MoS2(1-x)Se2x alloy domains and film are synthesized with tunable bandgap and PL emission on SiO2/Si substrate by CVD method.A tunable PL emission from 675 nm to 810 nm is realized by tailoring the alloy composition and heating temperature of mixture of precursor sulfur and selenium and using H2 as the assisted gas.The compositions and its distribution are characterized by the high-resolution scanning Auger electron microscopy(AES).Inhomogeneity is found to exsist even in an individual alloy domain.(3)CVD grown MoS2 is used in Ge p-and n-MOSFETs fabrication.MoS2 has suitable energy band offsets for both the conduction and valence bands of Ge according to theorical analysis and X-ray photoelectron spectroscopy(XPS)results.As a result,quantum confined channels are realized for both hole and electron together with the reduction of recombination at the interface between MoS2 and Ge due to no dangling bond at the 2D MoS2 surface.The Ge p-and n-MOSFETs performance,including carrier mobility,on-state current and reliability,is dramatically enhanced after the MoS2 passivation.(4)Large-scale fabrication of 2D MoS2 back-gated FETs is realized by using photolithograpy and lift-off technologies.It is found that the monolayer MoS2 can endure the tough device fabrication process.The average electron mobility of our FETs with the MoS2 polycrystalline film as the channel is 4.75 cm2V-1s-1 and the highest mobility reaches 17.6 cm2V-1s-1.The current on/off ratio is between 104 and 106 and the threshold voltage is between 10 V and 30 V.Memristive effect is observed on the FETs with polycrystalline MoS2 film as the channel,showing two resistance states under the forward bias sweep and backward bias sweep.The underlying reasons of the memristive effect is most likely to the immigration of sulfur vacancy under electric field and the interaction with grain boundraies.The switching ratios were up to 103?105,which suggests that polycrystalline monolayer MoS2 film is promising for novel memory devices.