Electrical Properties Of Two-Dimensional Transition Metal Dichalcogenides

In recent years,the unique structures and properties of low-dimensional materials have attracted intense academic interest.As typical layered 2D materials,two-dimensional（2D）transition metal dichalcogenides（TMDCs）are of great interest in fundamental research as well as in device applications because of extraordinary properties,such as flexible displays based on flexibility and semiconducting,new-type photoelectric devices based on direct band gap and valley polarization,phase transition devices based on charge density wave phase transition.In this thesis,we first have established an advanced device fabrication process applied to unstable 2D materials（sensitive to air,electron beam,etc.）and then have studied the electrical properties of several typical 2D TMDCs,which exhibits extremely different properties from the bulk counterparts.Moreover,high performance field effect transistors（FETs）and oscillators have been demonstrated on the TMDC materials.The main content of this dissertation concludes:1.Electrical measurement and applications of 2D TMDCs alloy based FETs.Single layer and few-layers MoS_2（1-x）Se_2x,Mo_（1-x）W_xSe₂,Mo_（1-x）W_xS₂ 2D alloy were synthesized on SiO₂/Si substrates by physical vapor deposition and mechanical exfoliation.In Raman spectra of Mo_（1-x）W_xS₂,we discovered that A′₁ mode showed one-mode behavior and E′mode showed tow-mode behavior,and calculated the relationship between the component x and frequencies by MRET model.Fitting the data with MREI result,we obtained the formula between the component x and Raman shift.FETs of MoS_2（1-x）Se_2x and Mo_（1-x）W_xSe₂ were fabricated by electron beam lithography（EBL）method.We measured electrical properties of devices with different x at room temperature,and the temperature-dependent electrical properties of MoS_2（1-x）Se_2x with fluorescence peak centered at 705 nm.The electrical data illustrates that the type of FETs based on MoS_2（1-x）Se_2x change from n-type to p-type with the increasing x.2.Electrical measurement of 2D ZrS₂ based FETsZrS₂ films were prepared by chemical vapor deposition on hexagonal boron nitride substrates.Damage to ZrS₂ films caused by electron beam irradiation was observed.Then EBL process was improved by vaporing a layer of Au film on materials.Electrical properties of ZrS₂ devices with different thicknesses were tested at room temperature,which showed that the properties vary with thickness.Threshold voltages decrease gradually with the increase of thickness,while the conductivity and field effect mobility displayed an opposite tendency.When the thickness is over 6 nm,these parameters will be saturated,which illustrated that the upper layers of 2D ZrS₂ can effectively screen the scattering of impurities on the upper side of samples.In addition,a metal-semiconductor phase transition was observed in thick samples（8.1 nm and 12.1nm）.The relation between the mobility and temperature can be described by the formulaμ^T-γ.Through the results,we speculated the main scattering mechanisms in ZrS₂ film including intrinsic phonon scattering,charge-impurity scattering,interface-charge-trap scattering.3.Electrical measurement of 2D TaS₂ based oscillators.We established a room-temperature FETs fabrication process based on air-sensitive2D materials,which was successfully applied to 1T-TaS₂.Electrical testing illustrated temperature-induced transition between nearly commensurate charge density wave（NC-CDW）and incommensurate charge density wave（IC-CDW）states and electric-field-activated transition among multiple metastable states in materials.Moreover,the critical electric field for the phase transition increasedas the device channel length decreased.With a homemade high frequency cuicirt,we observed electrical oscillation from the negative resistance effect.The phase change mechanism of 1T-TaS₂ was speculated and the solution of fabrication of oscillator with higher frequency was proposed.