Study On Photoelectric Properties Of MoS₂-based P-n Junction

Different from the zero band gap of the graphene,two-dimensional?2D?layered transition metal chalcogenides?TMDs?have tunable band gap with thickness because of the d-electrons' interactions,thus becoming a star material system for novel electronic and optoelectronic devices.Molybdenum disulfide?MoS₂?,as a typical layered d-electronic material,can achieve an adjustable band gap of 1.3 e V to 1.9 e V.The quantum confinement effect arising from its atomic layer thickness,contributes to the strong light-matter interaction,which is beneficial for the application of photoluminescence,photodetection,and solar energy harvesting etc.MoS₂-based p-n junction is a promising photodetector structure.The built-in electric field at its interface not only promotes the separation and transmission of photo-generated carriers,but also enables the device to achieve the self-powered function.However,the S vacancy defects,as the most stable point defect in MoS₂,reduce the optical absorption of MoS₂ materials and the carrier separation/transmission efficiency,which become the root of suppressing the photoelectric signal conversion.On the other hand,although the light absorption efficiency of MoS₂ and other TMDs is high,the total amount of light absorption is limited due to its vertical two-dimensional scale.Based on the above questions,we have started to work on two aspects.?1?To suppress the generation of S vacancy?VS?defects during the growth preparation stage of MoS₂,we controlled the concentration of S steam by adjusting the low temperature zone?S source?from 150 ? to 180 ?.On the one hand,high concentrations of S steam accelerate the growth of MoS₂?S₂+Mo O₃?MoS₂+SO₂?,and we can obtain a larger size of MoS₂.On the other hand,in the environment of S steam with higher concentration,S is easier to replace O and covalently bond with Mo,and it is beneficial to reduce the density of VS defects in MoS₂.We used Raman spectroscopy and photoluminescence spectroscopy to prove that low concentration of VS defect of the CVD-grown MoS₂ at high S vapor concentration.More importantly,the low defect density MoS₂ sample were transferred to the p-Si substrate by wet transfer technique to obtain a high-quality p-n junction interface free of impurities and traps.The results showed that we use a p-Si/MoS₂-based p-n junction with low VS defect to achieve a self-driven photodetector with high photoresponsivity,ultrafast response speed and good air stability.?2?In order to enhance the local optical field by localized surface plasmon resonance?LSPR?,we constructed a photodetector based on p-Si/MoS₂@Au by sputtering Au nanoparticles?Au-NPs?on the surface of MoS₂.Transmission electron microscopy observed the uniform distribution of Au-NPs,and the diameter of the Au-NPs is about 2 nm.Raman signal enhancement showed that light can be trapped and enhanced by LSPR.For p-Si/MoS₂-based p-n junction photodetectors,the simple method of sputtering Au-NPs enhanced the optical absorption to improve the photogenerated current,and helped the built-in electrical field of p-n junction to promote the photogenerated carrier separation efficiency and improve the photovoltaic voltage.The results showed that the photogenerated current increases by nearly 3 orders of magnitude and the photovoltaic voltage increases by nearly 5 times of the p-Si/MoS₂@Au photodetector.