Prototyping And Characterization Of Two-dimensional Devices Based On Transition Metal Dichalcogenides

Atomically thin two-dimensional（2D）materials,such as graphene,Mo S₂,WSe₂,and h-BN,have recently attracted tremendous attention due to their intriguing electrical,mechanical,and optical properties,and demonstrated great potential in realizing novel2D devices.Among 2D devices,transistors,inverters,and resonators are some of the widely studied types.The 2D devices properties are closely related to the fabrication process.Currently,one of the most typical fabrication processes involves the exfoliation of 2D materials,dry-transfer of 2D materials,and defining/metalizing electrodes.However,the commonly used fabrication process has some limitations:it requires a number of different equipment,such as 2D material transfer system,spin coater,and e-beam lithography/photolithography systems,which is rather complicated.Further,the spin-coating and lithography processes are not compatible with device structures such as 2D resonator,as the spin-coating,developing,and liftoff processes all involves solution or liquid processing,which could cause the suspended structure in 2D resonators to collapse.In order to overcome these limitations,a novel multifunctional high-efficiency fabrication system is designed and demonstrated.Based on this system,a number of 2D devices are prototyped and characterized,including Mo S₂ transistor,inverter,and WSe₂resonator.Specifically,this dissertation included the following parts:1.A multifunctional high-efficiency fabrication system is designed and demonstrated.It can achieve the transfer,stacking,and metallization functions,all using the same system.The multifunctional high-efficiency fabrication system is used to prototype 2D devices,like 2D transistor,inverter and resonator.2.Using the multifunctional high-efficiency fabrication system,transistors based on graphene/h-BN/Mo S₂ heterostructure back gate are demonstrated.From the transfer curve of Mo S₂ transistor,it is found that the devices exhibit typical N-type transistor behavior:the output current increases with the gate voltage.The device mobility is estimated to be 11.3 cm² V^-1 s^-1,demonstrating the potential of fast prototyping of 2D transistors using the multifunctional fabrication system.3.Using the multifunctional high-efficiency fabrication system,dual-gate transistors based on Mo S₂/h-BN/graphene heterostructures are fabricated.The top gate Mo S₂transistor shows more powerful channel switch control,due to the atomically thin and superior smooth surface properties of top gate dielectric h-BN.Inverter based on two top-gated Mo S₂transistors is constructed and measured:the fabricated Mo S₂ inverter can realize logic NOT correctly.Furthermore,to improve the electrical performance of the Mo S₂ transistor and inverter,another type of Mo S₂ inverter is fabricated,where the inverter consists of an Mo S₂ transistor and an Mo S₂ resistor connected in series with graphene flakes serving as electrodes.It is found that the Mo S₂ transistor exhibits an I_ON/I_OFF>10⁵;the Mo S₂ inverter shows a typical logic-conversion feature with voltage gain>6.The characteristic of Mo S₂ inverter can further verify the great potential of realizing 2D electronics.4.Using the multifunctional high-efficiency fabrication system,WSe₂ resonators with different diameters and thicknesses are fabricated.The measured response reveals three mechanical regions and the frequency scaling law.Furthermore,the Young’s modulus of WSe₂（130 GPa）and device pre-tension（0.05 N/m to 0.4 N/m）are extracted from the measurements.The frequency scaling law sheds light on the practicality of tuning device frequency using electric gate,using which a broad frequency tuning range of more than 200%and gate tuning efficiency of 23.1%/V are demonstrated,among the highest in resonators based on 2D semiconductors.This work can offer important guidelines of designing high-frequency WSe₂ resonator circuits at the wafer scale.In conclusion,a multifunctional high-efficiency fabrication system has been demonstrated,using which a number of 2D devices including Mo S₂transistor,inverter,and WSe₂NEMS resonator have been prototyped.This multifunctional fabrication system offers a versatile and promising platform for fast prototyping 2D devices and exploring device properties in these atomically thin devices.