| Originating in the 1960s,ion beam technology is a mature micro/nano machining technology,which realizes the fine machining of devices through collision and sputtering between the high-energy ion beam and target materials.Ion beam technology based on the use of ion beam type and energy can be divided into low energy ion implantation,ion implantation,ion irradiation,high charge in fast heavy ion injection,focused ion beam irradiation and clusters ion implantation,and so on,which can fabricate the target materials via the ion beam etching,ion beam deposition,ion plating,and ion implantation,etc.Up to now,ion beam technology is widely applied to the traditional semiconductor processing field.In recent years,with the rise of twodimensional nanomaterials,ion beam technology has attracted more and more attention in the semiconductor processing field of two-dimensional nanomaterials due to its advantages of controllable dose,high repeatability,and low cost.Two-dimensional nanomaterials refer to two-dimensional thin layered materials represented by graphene.Two-dimensional nanomaterials consist of a single or few layers of atoms whose electrons can only move in two dimensions.The atoms within the layers of two-dimensional nanomaterials are bonded by strong chemical bonds,while the layers are only bonded by weak van der Waals forces,so they are vulnerable to peeling off layer by layer under external influence.Since 2004,when Andre Geim's team at the University of Manchester in the United Kingdom first made monolayer graphene,the first truly two-dimensional nanomaterial,by hand,a variety of methods have been developed to prepare two-dimensional nanomaterials,such as liquid phase stripping,chemical vapor deposition,and wet chemical synthesis.At present,twodimensional nanomaterials have developed into a large family including graphene family,two-dimensional chalcogenide family,two-dimensional oxide family,and some other two-dimensional nanomaterials.Meanwhile,2D nanomaterials can be stacked to form van der Waals heterostructures with new properties,further expanding the range of 2D nanomaterials.The properties of two-dimensional nanomaterials cover conductors,semiconductors,and insulators.Due to the unique two-dimensional thinfilm structure and huge specific surface area,the band structure of two-dimensional nanomaterials is very sensitive to external conditions and can be easily regulated by the applied electric field,magnetic field,and doping introduction.The miniaturization of the integrated circuit is the development trend of semiconductor integrated circuits in the past several decades.Intel's founder,Gordon Moore,famously predicted that the number of components that could fit in an integrated circuit would double every 18 months at constant prices.This prediction has not been broken for the past 60 years,and by 2016,more than 1 billion transistors can fit on a single chip.However,when the size of semiconductor devices is below 10nm,the channel width of traditional 3D silicon semiconductor devices cannot be reduced to a nanometer scale.Attention has turned to two-dimensional nanomaterials,which can reduce the channel thickness of transistors to the order of atoms due to their structural characteristics of only one or a few atoms thick.Therefore,two-dimensional nanomaterials have high hopes in the scientific community as one of the most promising options to replace silicon as the base material for the next generation of semiconductor integrated circuits.Defect engineering is a traditional research direction in the semiconductor field,which aims to control the defects in semiconductors,eliminate the defects or use the defects to improve the performance of devices.During the growth and storage of twodimensional nanomaterials,various defects are inevitable,such as point defects caused by atomic loss and impurity adsorption,linear defects of grain boundary and material edge,and surface defects caused by interlayer torsion,dislocation,and contact with air substrate.These defects will reduce the properties of 2D nanomaterials,resulting in the decrease of mechanical strength,carrier mobility,and other adverse effects.Therefore,it is very necessary to conduct defect engineering research on 2D nanomaterials.The work of this thesis focuses on the application of ion irradiation technology in defect engineering of two-dimensional nanomaterials semiconductors.In the experiments involved in this paper,we use ion irradiation technology by introducing the point defects such as modification of two-dimensional nanomaterials mainly studied the defect engineering of transition metal chalcogenide and their van der Waals heterostructure with graphene in charge transfer between the layers,surface enhanced Raman scattering,photoelectric response and the influence of preparation of semiconductor devices,etc.Firstly,we investigate the effect of defects via ion irradiation on charge transfer between 2D van der Waals heterostructure between layer and layer.We irradiated the upper surface of the WSe2/graphene van der Waals heterostructure with focused ion beams,and the high-energy ions beam collided with the selenium atoms at the top of the WSe2/graphene heterostructure,sputtering some of the Selenium atoms to fly off and causing point defects.The defect density was quantitatively analyzed by measuring the ratio of Se atoms to W atoms by X-ray photoelectron spectra.The ratio of Se:W atoms has decreased to 1.96 from 2.0 after ions irradiation.Femtosecond optical probepump absorption spectrum measurement was used to measure the dynamic evolution of the energy domain and time domain excited states of the WSe2/graphene heterostructure before and after irradiation,and the test results were fitted with three kinds of dynamic photoexcitation processes that occurred when the WSe2/graphene heterostructure was probed by laser.The results of measurement and fitting show that the carrier mobility and number of the WSe2/graphene heterostructure between layers are improved after ions irradiation.First-principle calculations show that the presence of selenium defects changes the WSe2 band structure,increasing the density of electronic states of the WSe2 monolayer,which leads to an increase in the number of interlayer transport charge C0.Besides,the change of the Fermi surface of the WSe2 layer after irradiation results in a decrease in the height of the Schottky barrier between the WSe2 layer and the graphene layer,increasing the interlayer carrier mobility.Furthermore,we have measured the vertical interlayer photocurrent of the WSe2/graphene heterostructure,and the test results show that the vertical interlayer photocurrent of the WSe2/graphene heterostructure after irradiation was much higher than that of the pristine sample.Our experimental results show that introducing a certain number of Se point defects through ion irradiation can enhance the interlayer charge transfer of the WSe2/graphene heterostructure,which provides a new research idea for defect engineering in the field of two-dimensional nanomaterials.Besides,we have studied the effect of defects via ion irradiation on the enhancement effect of 2D material surface enhanced Raman scattering.We irradiated the upper surface of the WSe2 monolayer with focused ion beams,and the high-energy ions beam collided with the selenium atoms at the top of the WSe2 monolayer,sputtering some of the Selenium atoms to fly off and causing point defects.The defect density was quantitatively analyzed by measuring the ratio of Se atoms to W atoms by X-ray photoelectron spectra.We used the CuPc as the probe molecule and quantitatively measured the enhancement factors of the monolayer WSe2 as a surface-enhanced Raman scattering substrate before and after irradiation,respectively.The experimental results show that when the atomic ratio of Se:W atoms is 1.96,the SERS enhancement factor of the WSe2 monolayer reaches the maximum 120,which is 40 times higher than that of the pristine sample.The point defects induced by ion irradiation increased the number of excitons of the WSe2 monolayer and enhanced the exciton resonance with the exciting light.Meanwhile,the presence of the point defects enhanced the charge transfer between the WSe2 substrate and CuPc probe molecule and enhanced the charge transfer resonance with the exciting light.Therefore,we have found that introducing a certain number of point defects through ion irradiation can enhance the enhancement factor of the monolayer WSe2 as the SERS substrate,which promotes the application of two-dimensional nanomaterials in the field of surface enhanced Raman scattering.Next,we have done the research on the effect of defects caused by ion irradiation on the photoelectric response of two-dimensional materials.We irradiated the upper surface of the WSe2/graphene and MoSe2/graphene van der Waals heterostructure with focused ion beams,and the high-energy ions beam collided with the selenium atoms at the top of the heterostructures,sputtering some of the Se atoms to fly off and causing point defects.The defect density was quantitatively analyzed by measuring the ratio of Se atoms to W(Mo)atoms by X-ray photoelectron spectra.We constructed photodetectors based on pristine and irradiated heterostructures and measured their photoelectronic performances,respectively.The experimental results show that for the WSe2/Graphene heterostructure when the Se to W atom ratio deceases to 1.96,the photoresponsivity of the irradiated WSe2/graphene heterostructure increases by two orders of magnitude compared with the pristine sample,and the response range expands from visible band to near-infrared band.However,for the MoSe2/graphene heterostructure,the situation is different.The presence of Se point defects affects the surface potential of the MoSe2 monolayer,changes the relative position of the surface potential between the MoSe2 layer and the graphene layer,and then changes the direction of the built electric field between the MoSe2/graphene heterostructure layers,resulting in a change in the direction of the photocurrent.When the atomic ratio of Se:Mo decreases to 1.8,the photoelectric response direction of MoSe2/graphene heterostructure changes from positive to negative compared with the pristine sample,and their responsivities are in the same order of magnitude.At the same time,the specific detectivity(D*)of the photodetector based on irradiated MoSe2/graphene heterostructure is one order of magnitude or higher than that of the pristine sample due to the decreased dark current and the change of band structure brought by the Se point defects.The negative/positive photoresponse of MoSe2/graphene heterostructure can be exploited for optical logic operations.As a proof-of-concept,we achieve optical logic OR and AND gates based on the irradiated and pristine MoSe2/graphene heterostructures.At last,we propose a method of direct-writing lateral diodes based on MoSe2/graphene van der Waals heterostructure via focused ion beam technology.We irradiated the upper surface of the MoSe2/graphene heterostructure with focused ion beams,sputtering some of the Se atoms to fly off and causing point defects.The defect density was quantitatively analyzed by measuring the ratio of Se atoms to Mo atoms by X-ray photoelectron spectra.First-principles calculation and transport characteristics measurement results show that the ion irradiation region of MoSe2/graphene heterostructure is affected by selenium defects,causing the changes of the surface potential and band structure.When the atomic ratio value of Se:Mo decreases to 1.4,the band structure of MoSe2/graphene heterostructure in the irradiated region becomes a quasi-metallic state,while the band structure in the non-irradiation region is similar to the simple sum of the isolated graphene and MoSe2 monolayer.Therefore,a Schottky barrier similar to graphene-metal contact is formed between the irradiated region and the non-irradiated region,which causes the rectification of the source-drain current.The 2D material lateral diode direct wrote by focused ion beam has a rectification ratio of 104,a maximum operating voltage of more than 20V,and a reverse recovery time of only 0.29?s.Its performance is comparable to that of some traditional commercially available diodes.We have realized the application of the 2D material lateral diode in semiconductor integrated circuits(such as half-wave rectifier circuit,high-frequency filter circuit,and simple logic gate circuit).Compared with other methods of preparation of 2D nanomaterials diode(such as electrostatic doping,chemical doping),this method has advantages of simple operation,low cost,does not need additional energy input to maintain in effect,can realize the fully-automated fabrication,and so on,which provides a solution to the realization of the real application of two-dimensional nanomaterials integrated circuit. |
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