Research Of Improving The Response Performance Of Monolayer MoS₂ Photodetectors

Photodetectors are a crucial link in optoelectronic signal transduction,and consequently they have extensive applications in areas including optical communication,video imaging,biomedical imaging,night-vision,security,gas sensing and motion detection.As a result,world-wide researchers and technicians have been working on highsensitivity,high-speed,broadband photodetectors for decades.However,the next generation photodetectors need to be micro-scale,integratable and flexible.Traditional Si-based photodetectors work at visible and near-infrared wavelengths while compound semiconductors-based?e.g.,Hg Cd Te?and Type-? superlattice?e.g.,In As/Ga Sb?photodetectors responding across almost the whole infrared range have been thoroughly studied and the industrialized.However,these existing devices have relatively big volumes and low transparency and some of them require complex cooling systems.Therefore,they cannot meet the current demands for photodetectors.Fortunately,some emerging 2-dimensional?2D?materials appear to be able to meet these new requirements.Several two-dimensional materials have: single-atomic-layer thickness,high mechanical flexibility,high transparency,dangling-bond-free surfaces,unique electronic and optical properties,wafer-scale productivity and CMOS-compatibility.These characteristics make them promising candidates for photodetectors.Monolayer MoS₂ an important member of this class of 2D materials.It is a semiconductor with a direct bandgap of about 1.8 e V,which ensures that it has a high absorption coefficient and hence offers a high electron-hole generation rate under illumination.To create high-performance monolayer MoS₂ photodetectors,the material growth method,the material optoelectronic properties,the device fabrication procedure,the device performance characterization system and the performance-enhancing approaches must all be understood and optimized.This thesis provides a study of monolayer MoS₂ photodetectors at the micro-nano scales,and includes an investigation of methods for growing and characterizing the materials,fabricating the devices and finally building a proper measurement system.A monolayer MoS₂ photodetector which responds across the visible wavelength range and based on a field effect transistor structure is designed and fabricated.By tuning the applied source-drain bias and gate voltage,a maximum responsivity of 13.84 A/W is realized with a specific detectivity of 1011 Jones.Meanwhile,the effect of the channel length and incident illumination power are also studied.The results show that as the channel length is shortened,the responsivity increases but the dark current noise also increases drastically.As the incident power is increased,the responsivity decreases.To increase the photoresponsivity of monolayer MoS₂ photodetectors,surface plasmon resonances in metal nanostructures are employed.A gold nanoparticle grating structure is designed,which further enhances the localized surface plasmon resonance effect of Au nanoparticle arrays.The intensity and wavelength of the resonant peak can be tuned by selecting the grating period and the fabrication conditions.The fabricated nanoparticle grating structure can more than double the light absorption under 532 nm illumination,compared to corresponding single-nanoparticle arrays.When integrated with a nanoparticle grating,the hybrid monolayer MoS₂ photodetector displays a 111-fold enhancement in the photocurrent compared to a bare MoS₂ photodetector.This enormous enhancement in sensitivity can be ascribed to the increase in the local electric field,but it is also contributed by the heating effect of the surface plasmons on the MoS₂ conductivity.Though possessing excellent optoelectronic properties,MoS₂ only responds to wavelengths shorter than 690 nm.To extend the response to infrared wavelengths,the plasmonic hot electron effect at the metal/semiconductor interface is used.The hot electrons generated inside a metal nanostructure can be injected into MoS₂ as free carriers.Photons with energies higher than the Schottky barrier between the metal and MoS₂,which is significantly lower than the bandgap of MoS₂,are sufficient for exciting electrons with enough momenta and thus the photoresponse can be extended into the NIR.Finite-different time-domain simulations show the optimal structure is an Au film coated Si O2 grating with a period of 700 nm,width-to-depth ratio of 0.14 and Au film thickness of 10 nm.Theoretical analysis shows the devices should respond to light with wavelengths up to 1550 nm.In addition,modelling predicts there is a significant contribution from hot electrons to the photocurrent.A corresponding device is fabricated and characterized and its performance is consistent with the modelling predictions.This work makes near-infrared detection by monolayer MoS₂ possible.This thesis contains a theoretical and experimental study which addresses the design,fabrication,and characterization of high-performance monolayer MoS₂ photodetectors.In addition,valid methods to improve the responsivity and response wavelength of the detectors are proposed.The research here demonstrates that 2D materials are indeed an excellent material for next generation photodetectors and will further allow them to be miniaturized and directly integrated into conventional CMOS devices,while maintaining a high response speed and offering a broad wavelength response window.