| The energy band structure of a semiconductor determines the movement state and migration law of its internal electrons.It exhibits various properties such as optics,electricity,and magnetism.It is one of the most important parameters of semiconductor materials and the basis for the research of electronics and optoelectronic semiconductor devices.With the development of semiconductor technology and the popularization of applications,the idea of energy band modulation has been paid more and more attention.A variety of physical or chemical modulation methods have been developed and applied,and the physical properties of many types of materials have been improved and modified,correspondingly expanding the scope of application of devices and making a variety of new devices appear,which plays an important role in many fields related to national economy and national security.Photoelectric detection is an important technology for photoelectric conversion utilizing the electronic transition and transmission process in semiconductors.It can capture,identify and process optical information,which is an important part of semiconductor optoelectronics with small size,light weight,fast response speed,high sensitivity and easy to integrate.Among them,semiconductor detectors suitable for infrared bands have important requirements in the fields of video imaging,optical communication,night vision,remote sensing,biomedical imaging.Due to the mature application of traditional semiconductor materials such as Si,the near-infrared photoelectric detection has basically met the application needs,but the mid-and far-infrared detection in room temperature is always an important challenge in the field,and the development of related technologies requires modulation from the aspect of material essence-band structure,because of the limit by the photoelectric effect on the semiconductor bandgap requirements and the influence of the electronic Fermi-Dirac distribution on dark current.Transition metal sulfide materials are an important class of two-dimensional semiconductor materials.They have adjustable energy band structures and excellent optical and electrical performance characteristics which have shown great potential in the field of electronics and optoelectronic devices,and have received widespread attention in recent years.However,due to the energy band characteristics of such materials,the band gap cannot achieve absorption and light detection in the mid-infrared band.This paper aims at the bottleneck problem of photoelectric detection technology with MoS2 film as the research object,systematically studied the modulation effect of vacancy defects(vacancy charge)on its energy band structure,and the variation rule of defect concentration and band structure is revealed.Based on this rule,the strategy of reducing the room-temperature dark current of MoS2 materials to make it suitable for the production of mid-infrared long-wave detectors at room temperature was developed.Moreover,the photoelectric detection materials with ultra wide response bandwidth that can be operated at room temperature in the range of visible light,near infrared,mid infrared and even terahertz electromagnetic wave are developed,which provides theoretical reference for the research of the application of infrared band photoelectric devices using band modulation.The main work is as follows:1、Band structure design,controllable preparation and characterization of multilayer MoS2 films with S or Mo vacancy defectsStarting from the basic idea of semiconductor physics that defects can cause changes in the bandgap and produce "shallow"/"deep" energy levels,the energy band structure of MoS2 materials with different vacancy defects is systematically analyzed using the first principle method based on the DFT function.The energy band structure modulated by defects of the MoS2 material reveals the effect of vacancy defects on the narrowed band gap and the modulated electronic state density,which provides a pre-judgment standard and theoretical reference for the rational design of the electronic structure of the MoS2 material for mid-infrared photodetection.And as a guide,the controllable preparation of MoS2 thin films containing vacancy defects by pulse laser deposition method was explored from both theoretical and experimental aspects.The surface morphology and structural properties of the films were comprehensively tested and analyzed by means of XPS,SEM,HRTEM,Raman,optical absorption spectroscopy and other characterization methods.2、Study on the performance of mid-infrared photoelectric detection at room temperature of Vs-MoS1.89 photodetectorThe intrinsic MoS2 has a relatively large bandgap which determines the optical response localized in the near infrared spectral region with the cutoff wavelength edge about 1 μm.The band structure theory calculation of defective MoS2 material shows the introduced vacancy defects reduced the band gap which makes a broadband optical response possible.Under the theoretical results,a multilayer MoS1.89 thin film was designed and controllable prepared with a modulated band gap of 0.26 eV by introducing S defect energy levels into layered MoS2,corresponding the theoretically response wavelength was broadened to 4.7 μm.Its photodetection in the wavelength range from 445 to 2717 nm at room temperature are investigated with a photoresponsivity of 28.9 mA/W in the mid-infrared region.3、Study on the performance of mid-far infrared photoelectric detection at room temperature of VMo-MoS2.15 photodetectorNowadays,the underdeveloped mid-infrared photodetection at room temperature is primarily a result of the large dark currents unavoidably generated by the Fermi-Dirac distribution in narrow-bandgap semiconductors.To sovel it,the formation mechanism of dark current has been studied and the close relationship between dark current and electronic state density has been found.Under the theoretical guidance of the defect modulated band structure calculation,an electronic structure strategy is proposed to reduce the dark current for designing ultrabroadband covering mid-and even far-infrared photodetection materials operating at room temperature.A layered MoS2.15 thin film was manifested and characterized with an engineered bandgap of 0.13 eV and modulated electronic state density by introducing Mo defect energy levels into layered MoS2.Its photodetection was demonstrated for wavelengths from 445 nm to 9.5 μm at room temperature with an electronic state density-dependent peak photoresponsivity of 21.8 mA/W in the mid-infrared region.4、Study on the defect-induced THz photodetection performance of MoS2 at room temperatureBased on a new detection mechanism of carrier redistribution induced by THz electromagnetic radiation to change the carrier concentration of the semiconductor,the key physical parameters that caused the carrier concentration change of the semiconductor were analyzed,and it was found that the change in carrier concentration was restricted by bandgap and resistivity of a material.Comprehensively considering and utilizing the characteristics of vacancy defects that can induce narrow bandgaps,low resistance,and high-concentration trapping potential wells,the pioneering RT MoS2-based photodetection in the THz region via introducing Mo and S vacancies for rational band gap engineering was fabricated.Utilizing the balance between the carrier concentration fluctuation and carrier-scattering probability,a high RT photoresponsivity of 10 mA/W at 2.52 THz was realized in a Mo-vacancy-rich MoS2.19 sample.In summary,this paper mainly theoretically studies the modulation law of vacancy defects on the energy band structure of two-dimensional MoS2 semiconductor materials.Based on the defect modulation function as a theoretical guidance and pre-judgment standard,the defective MoS2 thin film material is designed reasonably and achieved controllable preparation.The ultrabroadband optical response in the mid-infrared,far-infrared and even terahertz electromagnetic wave band at room temperature was realized step by step,which greatly expanded its application range in the field of room-temperature photodetector devices.This work provides a useful reference for the development of modern mid-infrared optoelectronic elements and systems. |
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