Study On Chemical Vapor Deposition Growth Of Layered Two-dimensional MoS₂ And Its Photodetection Device At Room Temperature

Molybdenum disulfide(MoS₂)is a new two-dimensional semiconductor with layer dependent bandgap ranging from 1.2-1.8 e V.Recently,MoS₂ has attracted much attention due to its atomic thickness,high carrier mobility and mechanical flexibility,and it has broad application prospects in electronic and opto-electronic devices.Chemical vapour deposition(CVD)is one of the preferred methods for the synthesis of two-dimensional semiconductor MoS₂ due to its high quality of as-synthesized films,ability to grow large area films,controllable growth,and so on.The impact of Mo precursor's concentration on the growth of MoS₂ crystals was obtained by exploring the CVD growth process.The shape of MoS₂ crystal is closely related to the ratio of precursor S:Mo,when S concentration was stable,the shape and size of MoS₂domain altered as the concentration of Mo elevated.Regulating the ratio of S:Mo leed to the growth of MoS₂ crystal with desired shape and large size.The influence of reaction time to the morphology of MoS₂ was studied.By observing the morphology varing with reaction time,the growth process was unveiled,thus helped to reveal growth mechanism.The synthesis of continuous large-area films highly depends on the distribution of the reactive intermediate molybdenum oxide(Mo O_3-x)with lower saturation vapor density.Monolayer MoS₂ crystals with the size over 100?m were synthesized by optimizing the Mo concentration and adjusting the ratio of S:Mo.The crystals were complete and theirs structures were defectless.Continuous monolayer MoS₂ films were synthesized by optimizing the growth parameters,and back-gated field effect transistors were fabricated,showing typical n-channel device characteristics.The effective,controllable and stable doping of MoS₂ is the key to the application of its based devices and circuits,and the heterojunction formed between MoS₂ and other layered semiconductors can meet the requirements of circuits application.However,further improvement of device performance is limited by lattice mismatching and surface limitations between two heterogeneous materials in MoS₂ heterogeneous PN junctions.A selective nitrogen plasma doping new technique using a toroidal-magnetic-field was adapted to realize stable p-type doping of MoS₂,lateral homojunctions based on monolayer MoS₂ were fabricated.This homojunction presented desired electronic properties with rectify ratio of 20.Using the PN junction as photodetector to detect visible light at room temperature,it showed maximum photoresponsivity of 129.6 A W^-1,detectivity of 2.44×10¹² Jones,external quantum efficiency(EQE)of 3.02×10⁴%and response time of 60 ms.In order to improve the photodetection performances of devices based on monolayer 2H phase MoS₂,a novel vertically stacked 2H-MoS₂/1T@2H-MoS₂ structure was proposed.Using the monolayer 2H-MoS₂ to absorb light and produce photogenerated carriers.Due to the fact that 1T@2H-MoS₂ is a mixture of metallic 1T-MoS₂ and semiconducting 2H-MoS₂,using the 1T-MoS₂ in 1T@2H-MoS₂ to transport carriers faster and reduce recombination of carriers.The interface between 2H-MoS₂ and 1T@2H-MoS₂ ensures fewer dangling bonds and reduced lattice mismatching because of the existence of 2H-MoS₂ in both layers.Vertically stacked 2H-MoS₂/1T@2H-MoS₂ photodetectors,monolayer 2H-MoS₂ field effect transistor photodetectors and 1T@2H-MoS₂ field effect transistor photodetectors were fabricated,the experiments'results showed that the photoresponsivity of the 2H-MoS₂/1T@2H-MoS₂ vertically stacked photodetector increased dramatically,to a value of1917 A/W,which was 34 times of that of monolayer 2H-MoS₂ photodetector.The performance of the vertically stacked photodetector was voltage bias-modulated with a maximum EQE of 4.48×10⁵%and a detectivity of(?)10¹¹ Jones.This photodetector with novel vertically stacked structure paves the way for obtaining photodetectors exhibiting high performance.