Two-dimensional Semiconductors And Their Optoelectronics Based On Field Effect Transistors

With the discovery of graphene,atomic-thin two dimensional?2D?materials have attracted extensive research interest because of their rich physical properties and broad application prospects.Graphene has a series of characteristics such as high mobility,high thermal conductivity,high strength,high electrical conductivity,and low optical absorptivity.It can be used as transparent conductive films,supercapacitor electrode materials and so on.But due to the absence of band gap,it is difficult to make a logic device with graphene,which limits its application in electronic circuit devices.However,most 2D transition metal dichalcogenides?TMDC?are semiconductors,they have high carrier mobility and large on/off ratios.They are good candidates for electronic devices.At the same time,the bandgap of TMDC corresponds to the spectral range from ultraviolet to infrared light,and TMDC has high optoelectronic conversion efficiency make it a good candidate for optoelectronic devices.The emerging two-dimensional semiconductor black phosphorus?bP?has the characteristics of high carrier mobility similar to graphene and scalable bandgap.Therefore,it has a wide application prospect and attracts widely attention.To study two-dimensional materials,on the one hand is to understand the basic properties of 2D materials.When thickness of the material is reduced to the atomic scale,it will emerge novel properties in strain,thermal,sound,optical and electrical due to the quantum confinement effect.On the other hand,based on their novel properties,developing of new 2D material electronic devices such as flexible electronic devices,and small-size devices.In this paper,starting from the basic properties of 2D materials,we studied the determination of the number of layers through optical method,as well as the Raman spectrum properties and estimated the thermal conductivity of the few layer MoTe₂.Then,photodetectors based on few layer TMDC and black phosphorus were developed to realize the photodetection from visible to infrared light,and the mechanism of photoconductivity was discussed.The ferroelectric material is applied as the gate dielectric material of the transistor to improve the performance of the 2D material based photodetectors.With the same structure,we studied the memory device based on the ferroelectric gated 2D material field effect transistors?FET?and the interface engineering.In chapter two,the Raman spectrum properties of few layer MoTe₂ were systematically studied,including the relationship with the number of layers,the relationship with the excitation powder density,the relationship with the temperature,and discovered the thermal effect on the Raman spectrum.By comparing the power dependent and the temperature dependent behavior,the thermal conductivity was roughly estimated,without considering the SiO₂ substrate effect on the thermal transport.The thermal conductivity of a single layer of MoTe₂ is 462 Wm^-1K^-1,and the thermal conductivity of four layers of MoTe₂ is 43 W?m^-1K^-1.As the thickness of the material increases,the thermal conductivity decreases.The study of thermal conductivity provides important thermal parameters for the development of MoTe₂-related optoelectronic devices.In chapter three,a variety of two-dimensional based photodetectors were studied.Firstly,the design of the interpolated shape electrode is used to solve the problem that channel signal current is too low to read out in GaSe FET due to the low carrier mobility.In this way,the GaSe phototransistor realized the high sensitivity photoresponse for450 nm and 520 nm visible light,and the responsivity reaches as high as 2200 mA/W,superior to many other two-dimensional materials.Next,the MoTe₂ based backgate phototransistor was studied.The device achieves photodetection from visible light 600nm to short-wavelength infrared light 1550 nm with a high photoresponsivity and detectivity.For visible light at 637 nm,the device's responsivity can reach 50 mA/W and the detectivity can reach 3.1×10⁹ cm·Hz^1/2·W^-1,and for 1060 nm near-infrared light,the responsivity can reach 24 mA/W,and the detectivity is 1.3×10⁹ cm·Hz^1/2·W^-1.The device responds quickly,the rise and fall time are 1.6 ms and 1.3 ms,respectively.By measuring the photoresponse for incident light with different intensity,it's found not only the photocurrent increased but also the threshold voltage shifted with the increasing intensity of light,which is due to the photogating effect associated with the trap states in the channel materials.Photogating effect helps increase the photoresponse of the detector and is an important mechanism in 2D materials based photodetectors.This chapter also studied photodetectors based on narrow bandgap 2D materials,black-phosphorous?bP?.Due to the narrow bandgap of bP,the bP based phototransistors achieve photodetection of infrared light of 1.5?m and 2.0?m.This provides the possibility of infrared photodetection at room temperature.In order to improve the performance of the photodetector,ferroelectric material is used as a dielectric material of the 2D materials?MoTe₂?based phototransistors.The remnant polarization of ferroelectric material could deplete the carrier in the channel and suppress the channel current to a low level,therefore enhancing the detectivity of the detector.Moreover,the device could work at zero gate voltage,which is good for reducing the power consumption and increase the stability of the device.Based on the structure of ferroelectric gated 2D materials optoelectronics,the 2D materials based ferroelectric memory FET has been studied.The perovskite-type ferroelectric material PZT gated 2D bP transistor has realized the multi-state memory due to existing of the remnant polarization and interface states.By inserting of few-layer BN between the ferroelectric material and the 2D material as a buffer layer,it improved the interface quality effectively and eliminates the non-ferroelectric hysteresis.Similarly,in the organic ferroelectric P?VDF-TrFE?gated bP FET,the insertion of BN buffer layer helps realize the non-volatile ferroelectric memory.In this work,the characteristics of 2D semiconductors and their optoelectronics devices has been studied,and additionally some work about the high temperature superconductivity in cuprate has been done,the related result are shown in the appendix part.