Research Of Uncooled Infrared Detection Based On Their Confinement Or Interfacial Effect Of Two-dimensional Materials

Photodetector is a device that converts electromagnetic signals into electrical signals.According to the working-bands of photodetectors,it can be classified into three categories:ultraviolet detectors,visible light detectors and infrared detectors.At present,the research and industrialization hotspots of photodetectors are mainly focused on infrared detectors.The traditional infrared detectors mainly include Hg Cd Te,quantum wells,and superlattice photodetectors.However,these traditional infrared photodetectors have the disadvantages of high cost and poor performance,which cannot meet the requirements for the next generation photodetector with miniaturized,broad-spectrum,excellent sensitivity,transparent and printable flexible properties.With the discovery of graphene,more and more two-dimensional（2D）materials have been discovered.It’s excellent electronic and optical properties and unique atomic structure make 2D materials have tremendous potential for next-generation photodetector,semiconductor industry and microelectronic.These are the unavoidable challenges in applying 2D material photodetectors to practical production or life,such as simultaneously improving the photoresponsivity and specific detectivity,and broadening their detection spectral range.To explore the photoresponse mechanism and improve the comprehensive performance of 2D material photodetectors,the novel optical and electrical properties of 2D materials and their photodetectors were furtherly investigated in this dissertation,including:h-BN/MoTe₂/graphene/SnS₂/h-BN heterojunction broad-spectrum detectors,h-BN protected MoS₂/WSe₂ type-II heterojunction photodetector and FePSe₃ bolometer,have been designed and explored.In additon,the physical mechanism of the photodetor has been systematically studied.These photodetectors not only meet the current requirements for high sensitivity,fast response speed,wideband of photodetectors,but the systematic study of their working principle can also provide a viable approach for the next generation of ultrahigh-sensitivity and broadband optoelectronics.MoTe₂ and SnS₂ bulk single crystals were synthesized by chemical vapor transport method,and h-BN/p-MoTe₂/graphene/n-SnS₂/h-BN p-g-n photodeterctors were prepared by standard mechanical exfoliation,dry transfer,and electron beam lithography processes.The fundamental physical properties and photoresponse performance of h-BN/p-MoTe₂/graphene/n-SnS₂/h-BN p-g-n heterostructure were systematically investigated.There are several unique designs in the p-g-n junction:1)there is an vertical built-in electrical field between p-MoTe₂ and n-SnS₂,which can reduce the charge trapping effect;2)The zero-bandgap graphene interlayer is switched between the MoTe₂ and SnS₂ not only expending the detection spectrum range,but also enhancing light absorption and charge transport in van der Waals（vd W）p-g-n junctions by reducing charge traps and improving the interface quality;3)The h-BN sandwich structure is used to reduce the response time,noise of the device and enhance the performance of the detector by shielding the detrimental impact of the charged impurities from the substrate surface or the environment contamination.Finally,the roles of the sandwiching graphene and the origin of the high performances were systematically studied,an optimized structure of the vd W h-BN/p-MoTe₂/graphene/n-SnS₂/h-BN p-g-n junction containing 5-7-layer graphene interlayer have been achieved.The combined effect of these device designs is profound as illustrated in the extraordinary broadband responsivity exceeding2.6×10³ AW^-1 and specific detectivity D*up to～10¹³ Jones have been achieved in the ultra-wide spectrum from ultrvoilet to near infrared（NIR）,which is at least two orders of magnitude higher than that of the MoTe₂-based photodetectors reported so far.Under 1550 nm short wave infrared（SWIR）light illumination,the D*of the detector is decrease slightly,but it also can reach up to 1.06×10¹¹ Jones,which is comparable to the commercial SWIR uncooled Ge-on-Si photodetector.This work can provide a feasible design idea for new type photodetectors with broadband and ultra-high sensitivity.MoS₂ and WSe₂ bulk single crystals were grown by chemical vapor transport method.The h-BN protected WSe₂,MoS₂ and MoS₂/WSe₂ II type heterojunction photodetectors were prepared by mechanical exfoliation,dry transfer,and standard electron beam lithography processes,respectively.The photoresponse performance of three devices were measured separately.Notably,the interlayer exciton in the MoS₂/WSe₂ type II heterojunction structure successfully broadens spectral range to the near-infrared band exceeding the cutoff wavelengths of the individual MoS₂ and WSe₂.Comparing the photoresponse performance of the WSe₂,MoS₂ and MoS₂/WSe₂ II type heterojunction photodetector,the responsivity of h-BN-protected MoS₂/WSe₂ heterojunction photodetector can reach up to 10⁵ AW^-1,which is also nearly one orders of magnitude larger than that of MoS₂ photodetector and WSe₂photodetector under 532 nm light illumination.Under 1064 nm light illumination,the responsivity of the detector is up to 4.8×10³ AW^-1,which represents the best performance so far achieved on the 2D/2D van der Waals p-n junction NIR photodetector.These results not only obtains a photodetector with high responsivity beyond the limit of intrinsic bandgap of single material,but also provides a scheme to design new type and high sensitivity photodetectors.FePSe₃ single crystals were prepared by chemical vapor transport method,and FePSe₃ bolometer with different thicknesses were prepared by standard mechanical exfoliation method,and electron-beam lithography process.The fundamental and photoresponse performance of FePSe₃ bolometer with different thicknesses were investigated.The FePSe₃ bolometer contains several features:1)the temperature coefficient of resistance（TCR）of material is nearly independent of the sample thickness at room temperature and can reach up to-2.96%K^-1,which is slightly higher than the TCR value of the commercial VO_x bolometer;2)the ⁵T_2g-⁵E_gtransition of Fe²⁺in an octahedral ligand field,whose energy is 0.75-0.89 e V,broadens the absorption band of the material to the short-wave infrared band;3)The2D van der Waals stacking of FePSe₃ material inherit a natural suspension structure that can reduce the heat exchange between FePSe₃ and the external environment,which further enhances the photoresponse performance of FePSe₃ bolometer.The responsivity and D*of the FePSe₃ bolometer are 10¹⁰-10¹¹ VW^-1 and 10¹¹-10¹² Jones under the illumination wavelengths ranging from ultraviolet to visible spectra,respectively,and exhibit an inverted-bell shaped thickness dependence.The responsivity and D*of the FePSe₃ bolometer exceeding～1×10⁸ VW^-1 and～1×10⁹Jones under 1550 nm light illumination,which is 2-3 orders of magnitude higher than that of current commercial VO_x bolometer.This result not noly reveals the photoresponse mechanism of FePSe₃ bolometer,but also paves the way toward new type of bolometers with high sensitivity.These works provide a viable route toward new type of 2D photodetectors with boradband and high sensitivity.Base on the researches on the photoresponse mechanisms,the structure of photodetectors were optimized,which underpins strategies and references for the next generation of optical communication technology.