Fabrication And Electrical Characteristics Of Van Der Waals Heterostructures Based On Two-dimensional Materials

In recent years,in order to meet the development of big data,artificial intelligence and other fields,the demand for semiconductor devices and chips with low power consumption and high performance is increasing.The node of the existing silicon-based semiconductor chips has reached 2-3 nm.However,enormous challenges have emerged in terms of integration and R&D in the nanoscale process.It is of great significance to find materials suitable for the sustainable development,explore their physical properties and construct new principal devices.Two-dimensional atomic crystal materials have excellent properties such as atomically thin thickness,self-passivation,dangling bonds free surfaces,adjustable band gap,high mobility,and low dielectric constant,thus making semiconductor devices have the properties of efficient electrostatic regulation and natural immunity to short channel effects,which are expected to solve the serious challenges brought about by the further miniaturization in silicon-based devices.In this article,heterostructures such as semi-floating-gate homojunctions,all-two-dimensional,back-gate,floating-gate memories,top-gate memories,and type II van der Waals heterojunctions are constructed by combining two-dimensional channel materials（InSe,MoS₂,Te）,semimetals（graphene）and insulators（h BN）,achieving various functions such as rectification,data storage,multi-valued logic,photoelectric detection,and photovoltaics.The main results are briefly described as follows:1.We focus on the usage of the semi-floating-gate device structure.InSe/MoS₂ is selected as the channel material,h BN is used as the tunneling layer,multi-layer graphite is used as the（semi-）floating gate,achieving a semi-floating-gate homojunction that integrates storage,rectification,logic,photovoltaic and photoelectric.The device has an ultrafast programming speed of～20 ns and a high on-off ratio of 10⁹.Three homojunctions with different doping states can be constructed by changing the polarities of the voltage pulses.The homojunction p-n junction performs a high rectification ratio up to 10⁵ and can function as a rectifier with the function of forward voltage conducted and reverse voltage cut-off.In addition,the device achieves an optical-to-electrical power conversion efficiency of 2.66%and a photovoltage responsivity of 5.72×10⁹V/W at 0.61 n W and 0.06 n W laser power,respectively.The atomically sharp interfaces between the 2D functional layers in the van der Waals heterostructure are observed by scanning transmission electron microscopy.Computational simulation is used to calculate the physical quantities as functions of the widths and amplitudes of control-gate voltage pulses.The results provide new ideas and avenues for designing novel homojunctions in the future.2.We replaced all the functional layers of traditional floating gate memory with two-dimensional materials,thus achieving an all-two-dimensional,floating-gate memory with ultrafast programming/erasing operations speed of 20 ns.The device maintains stable currents over a period of more than 8000 s and 1200 cycles with an on-off ratio up to 10⁸.Improvements in the device structure result in a smaller unit device area.Multiple-bit data storage can be achieved by changing the pulse amplitudes.Optically programming/erasing operations can also be implemented.Importantly,we have experimentally demonstrated the“OR”logic gate on a single device by synergistically utilizing electrical and optical operations.Scanning transmission electron microscopy reveals five functional layers and sharp,clean interfaces with low defect densities between each layer,which greatly extends the data retention time（up to 60 years）.Computational simulation is used to calculate the physical quantities as functions of the widths and amplitudes of control-gate voltage pulses.These results provide new ideas and approaches for designing new non-volatile memories.3.We designed a top-gate,floating-gate heterojunction structure.The device exhibits a non-volatile memory effect opposite to the conventional floating-gate memory.The device maintains stable current states after more than 6000 s and 1000cycles,with a high on-off ratio up to 4×10⁷.Multi-bit data storage can also be achieved by changing the amplitudes of voltage pulses.The fastest programming/erasing operation speed is also explored.The processes of long-term potentiation and long-term depression in synaptic neurons are also simulated by applying multiple positive/negative voltage pulses.To verify the universality of the device structure,monolayer-layer MoS₂ grown by CVD is used as the channel material,and the same electrical characteristics are achieved.This result provides possibilities for designing new types of floating-gate memory in the future.4.We constructed InSe-Te van der Waals heterojunction and explored the optoelectronic and photovoltaic properties of the device,which has a large rectification ratio of 1.56×10⁷ at the drain-source voltage of±2 V.The device has a stable light-to-dark current switching behavior and a high responsivity and detectivity of 0.07 A/W and 3.73×10¹⁵ Jones,a high electrical power conversion efficiency and photovoltage responsivity of 0.91%and 5.72×10⁹V/W under the laser power of 72.11 n W,respectively.These experimental results provide a new pathway for realizing devices with high rectification ratio and visible-light-detection capability.In this dissertation,we fabricated van der Waals heterostructures based on two-dimensional materials with excellent electronic and optoelectronic properties,we also proposed new floating-gate storage mechanism.All of these provide new designs for the development of the next-generation field-effect transistors.