Fabrication And Research Of Photodetectors Based On Two-Dimentional Layered Transition Metal Dichalcogenides/Silicon Heterojunctions

Two-dimentional(2D)layered transition metal dichalcogenides(TMDs)exhibit many exceptional properties,such as tunable and diverse electronic structures,ultrafast carrier dynamics and large absorption coefficients,thus accommodating a great potential in various photodetection platforms.However,owing to the difficult controllable doping,low incident photon-to-electron conversion efficiency and limited absorption spectrum,the 2D TMDs-based photodetectors suffer from slow response speed,weak specific detectivity and inferior detectability to infrared light.As such,the devices and applications in optical communication,imaging and night vision have been hindered.Meanwhile,along with the development of functional flexible electronics,highperformance,transparent and flexible photodetectors remains to be further explored.In view of the mentioned challenges,this thesis focuses on the following researches:1.Ultrafast,broadband photodetectors based on graphene/MoSe2/silicon(Gr/MoSe2/Si)heterojunctions with vertically standing layered structure using graphene as transparent electrodeIn this work,high-quality MoSe2/Si heterojunctions were fabricated by depositing n-type MoSe2 film with vertically standing layered structure on p-type Si substrate through magnetron sputtering method.On this basis,high-performance Gr/MoSe2/Si heterojunction photodetectors were constructed by implementing annealing heat treatment on MoSe2 and using graphene as transparent electrode.The advantages of the photodetectors including:(ⅰ)Strong built-in electric field in the Gr/MoSe2/Si heterojunction photodetectors can greatly facilitate the separation and transport of photogenerated carriers.(ⅱ)The distinct vertically standing layered structure of MoSe2 film ensures the fast transport of photogenerated carriers along the vertical direction due to the high in-plane mobility.(ⅲ)The graphene transparent electrode can further enhance the carrier collection and thus reduce the carrier recombination.Owing to the unique device structure,the Gr/MoSe2/Si photodetectors exhibited ultrafast response speed of 270 ns and the ability to identify 1 MHz high frequency pulsed light signal,which are significantly better than mono-/multilayer MoSe2-based photodetectors.Meanwhile,the photodetectors exhibited broadband response ranging from ultraviolet(UV)to visible and to near-infrared(NIR)light(365-1310 nm)due to the relatively narrow bandgap of MoSe2,broadening the photoresponse spectrum of the traditional Si-based photodetectors.In addition,the Gr/MoSe2/Si photodetectors exhibited outstanding device characteristics in terms of high responsivity(0.27 AW-1)and specific detectivity(7.13×1010 Jones).This work paves the way for development of high-quality p-n heterojunctions and highperformance optoelectronic devices based on novel 2D layered materials and traditional semiconductors.2.Infrared light trapping in conformal MoS2/Al2O3/silicon nanowire arrays(MoS2/Al2O3/SiNWs)heterojunctions for ultraweak infrared signal detectionIn this work,vertically standing silicon nanowire arrays with～600 nm length were fabricated by metal assisted chemical etching and novel construct of conformal MoS2/Al2O3/SiNWs heterojunctions were carried out through atomic layer deposition and magnetron sputtering technique.Owing to the conformal light-trapping structure,interface passivation of Al2O3 layer and energy band engineering of MoS2 film,selfdriven MoS2/Al2O3/SiNWs heterojunction photodetectors were fabricated with a broadband photoresponse range(365-1550 nm),an ultralow noise current(1.1 fA Hz-1/2),and an extremely high specific detectivity(1013-1014 Jones)in the NIR spectrum.With the outstanding properties,the devices could effectively detect ultraweak NIR illumination with light power intensities of 100 pW at 808 nm and 1 nW at 1310/1550 nm,which represent the best result thus far for 2D materials-based NIR photodetectors.This work demonstrates an attractive strategy to fabricating 2D light-trapping structures,which will promote the development of low-cost and high-performance optoelectronic systems for weak infrared light detection based on 2D materials.3.Freestanding hollow-out single-crystalline silicon membranes for substrate-free and self-driven transparent flexible photodetectorsThis work reported the fabrication of thin Si by alkaline etching and the preparation of freestanding hollow-out single-crystalline Si membranes(sc-SiMs)with good flexibility and transparency by combining photolithography and reactive ion etching technologies.Through the rational design of hollow-out structures,the sc-SiMs exhibited high transparency in all wavelength range(ultraviolet-visible-infrared)and the optimal transparency in the visible spectrum up to 96%.Based on hollow-out sc-SiMs,substratefree and self-driven transparent flexible photodetectors have been achieved for the first time through the construction of conformal Au-Ag asymmetry electrodes.These devices not only obtain the fastest response speed(29 μs)among reported transparent flexible photodetectors,but also exhibit high responsivity(0.11 A W-1)and excellent detectivity(5.5×1012 Jones),superior to the results of most of reported devices.Furthermore,integrated multi-band photo-detecting systems have been demonstrated to reveal the potential of the substrate-free and self-driven transparent flexible photodetectors for integrated optoelectronic applications.The strategy of processing high-property Si wafers into transparent flexible hollow-out membranes is expected to promote the development of transparent flexible optoelectronics based on 2D TMDs/Si heterojunction for integrated applications such as smart windows,artificial visual cortex,virtual reality and E-skins.