Low Dimensional Material Graphene Photodetector Research

Graphene is a two-dimensional nanomaterial composed of a single layer of carbon atoms,which breaks the scientific community’s perception that two-dimensional materials can not exist.It has many properties that traditional photodetection materials do not have,such as submicron ballistic transport,high mobility and specific surface area,zero band gap,adjustable Fermi level and so on.Low-dimensional materials,represented by graphene,provide more possibilities for the development of new detection technologies and theories.With the continuous development of graphene and low-dimensional nanomaterials,a new generation of photoelectric detection technology is bound to usher in.However,there are still some limitations in the application of graphene and low-dimensional materials in the field of photodetection.For example,single-layer graphene has only 2.3%light absorption rate,high conductivity and unstable surface charge distribution.As a result,graphene-based photodetectors generally have low responsivity,high dark current,high noise and unstable performance.The problem of low responsivity of graphene photodetectors can be solved by combining graphene with materials with high absorption coefficient.The dark current and noise of graphene photodetectors can be effectively reduced by using graphene/other materials to form a barrier structure.Using the gate voltage to control the surface charge of graphene can well analyze the photoresponse mechanism of the device and find out the key factors affecting the performance of the device.This dissertation prepared and studied graphene field effect transistors,and studied the basic properties of graphene film.Based on the principle of field effect,the Fermi level of graphene was adjusted electrically by gate voltage bias,and chemically by using inorganic salts.Based on the chemical doping of graphene,a graphene PN junction photodetector was prepared,and the pure graphene structure achieved a good photoelectric response in the 405～980 nm band.In addition,the patterning of graphene had been realized through micro processing techniques such as photolithography and etching.This dissertation reported CuInS₂/ZnS quantum dot-graphene photodetectors,CdS quantum dot-graphene photodetectors and FAPbBr₃ perovskite quantum dot-graphene photodetectors were prepared using graphene-quantum dot composite structure,which solved the problem of low response of graphene photodetectors.Among them,the FAPbBr₃ perovskite quantum dot-graphene composite photodetector obtained a responsivity of 1.15×10⁵ A/W and an external quantum efficiency of 3.42×10⁷%（incident wavelength of 520 nm,illumination power of 3μW）.The high photoconductive gain of the FAPbBr₃ perovskite quantum dot-graphene photodetector not only originated from the high mobility and wide spectral absorption of graphene materials,but also from the long charge carrier lifetime and low dark carrier concentration in FAPbBr₃ QDs.In addition,the photoelectric conversion mechanism of this composite photodetector was digged from the perspective of the built-in electric field of the quantum dot-graphene interface,revealing the role of photoconductive gain in the graphene-quantum dot composite structure.Then,in order to solve the problem of high photoconductive gain and fast response time in graphene-quantum dot composite devices,this dissertation proposed a graphene/C₆₀ film/graphene all-carbon photodetector(the thickness of C₆₀ film is only 2nm～10 nm),which greatly improved the response time of the device without greatly sacrificing the responsivity.The device has both high responsivity and fast response time（3.4×10⁵ A/W@23 ms@405 nm）,and the response band covers 405～1550 nm.The device not only obtained sensitive and fast response characteristics,but also realized adjustable bidirectional（positive and negative）response.In addition,large-area graphene/C₆₀/graphene photodetector arrays were prepared by patterning the graphene/C₆₀/graphene structure.In addition,a 250×250 large area graphene/C₆₀/graphene photodetector array was fabricated by patterning the graphene/C₆₀/graphene structure,and the graphene/C₆₀/graphene structure was coupled with the bottom circuit of 384×288 focal plane.In addition,the graphene/germanium/WS₂ photodetector greatly improved the response speed of graphene device.The device got a response speed of 100μs and a detection rate of up to 1×10¹⁴ Jones,and could work under high frequency illumination of 800 Hz.In the graphene/germanium/WS₂ structure,graphene/germanium/WS₂ formed a barrier structure in the vertical direction,and graphene formed a photoconductive channel in the horizontal direction.By combining the graphene conductive channel with the vertical barrier,the device could work with zero bias,effectively reducing dark current and noise,and improving detection rate.The method of adjusting the surface charge of graphene through the top gate structure effectively controled the size and polarity of the photoelectric response of the device.In this dissertation,the optical response of the detector was improved by using the structure of quantum dot-graphene photodetector,but the response time of the device has not been improved.After that,graphene/C₆₀/graphene photodetectors were prepared.The response time was greatly reduced while the high response degree was maintained,the problem of long response time was solved,and the coexistence of high response and fast response time was realized.However,there was still a problem of high dark current in graphene/C₆₀/graphene devices.Through the structure of graphene/Ge/WS₂,the response time of graphene devices was further improved,and the dark current and noise are greatly reduced,and the ability to work under high frequency light of 800 Hz was realized.Furthermore,the response current of the device was adjusted by controlling the surface charge of graphene,and the working principle of each graphene photodetector was explained in detail.In addition,the large area of arrayed devices are prepared by using graphene/C₆₀/graphene structure,which provides a new way for the array application of low dimensional materials.In general,this dissertation provides a new way of thinking and method for the application of graphene and related low dimensional materials in the field of photoelectric detection through the research of graphene and low dimensional composite photodetectors.