The Optical And Electrical Properties Of The Interface In 2D Transition Metal Dichalcogenides Heterojunctions Based On AFM Technology

The two dimensional transition metal dichalcogenides sulfide?2D TMDCs?semiconductor materials have been extensively investigated due to the layered stacking structures,typically like MoS₂ and WS₂.Some important properties of monolayer TMDCs,such as bandgap,doping concentration and polarity,can be tunable which makes them have been successfully applied in the photoelectric devices,such as photodiodes,photoelectric detectors,photovoltaic cells and field effect transistors.So the 2D TMDCs materials are considered to be the candidates for applying in the next generation of semiconductor devices.However the contact barrier and resistor formed in the interface of the 2D TMDCs heterojunctions will obviously influence the response speed and consumption of the devices.Thus it is of importance for the investigations of the 2D TMDCs heterojunctions.In this work,the kelvin probe force microscope?KPFM?and the conductive atomic force microscope?CAFM?,as well as the Raman scattering spectra and the photoluminescence?PL?spectra have been exploited to exploring the electrical and optical properties of the 2D TMDCs heterojunctions relating MoS₂,WS₂,WSe₂ materials.The follow will introduce the main work and innovations:?1?The optical properties of the interface in the WSe₂-WS₂ lateral heterojunction are explored by the PL and Raman scattering spectra in different temperature.The dynamic transition behaviors of the electrons across the interface and the relating parameters are investigated by KPFM in different temperature.It is concluded that there might exist the alloy compound of W,Se and S elements by the different shifting of the PL peak of WSe₂ and WS₂ at the interface.And a direct to indirect transition of the photoelectrons of WSe₂ and WS₂ at the interface is found through the PL spectra in different temperature.The shifting of Raman phonons with temperature are investigated by the fitting of the Raman spectra.The strain effect may exist due to the thermal expansion difference between the WSe₂and WS₂ at the interface.The KPFM measurement results shows that the contact barrier between the WSe₂ and WS₂ at interface decreases with the temperature,and the ohmic contact arise at the critical temperature Tc.The surface potential is fitted with pn junction modeling and find that the poling of built-in electric field has reversed.It is because that the thermal expansion extent is different between the WSe₂and WS₂ at the interface which can induce the strain effect.Besides,the drop of the carrier concentration with temperature can also be supposed to be the electron-withdrawing effect of the Si/SiO₂ substrate.?2?The carrier transport behaviors have been investigated by CAFM together with variation of strain effect.The screening effect of the layered MoS₂is also explored by KPFM.The gold film with thickness of 30nm is deposed by magnetron sputtering method acting as conducting electrode.The exfoliated MoS₂ flakes are separated from the bulk MoS_2.The KPFM results show the discrepancy of the surface potential between the different layers of MoS₂.This verifying the screening effect between the layers of MoS_2.The current-voltage properties of local MoS₂-gold sample have been explored by CAFM.The schottky diodes rectification curves have been obtained and this proves the back-to-back heterojunction characteristics.Furthering,with the increasing of the strain imposed by the probe on the surface of MoS₂,the current has been weakened at the same voltage.A current image of the bulk MoS₂-gold surface is gained by the peak force tunneling CAFM technology,and found that the conductance of the MoS₂ is better than that of the unannealing gold film.Considering the current is very small on the bulk MoS₂ sample surface,a greater voltage is imposed on it and this induce the breakdown effect,which drastically increase current.?3?The system is designed for in-situ characterization of both the optical and electrical properties with the error under 1.And the anti-oxidation system is also realized for AFM measurements in high temperature.A movable station is controlled by the instructions in the AFM computer which can direct the movements of the sample between the camera of AFM and Raman laser.Using a silicon grating to calibrate the coordinate of selecting point in the silicon grating before the experiments.And then the in-situ exploration of the sample can be realized through the linear computation with the error under 1?m.Considering the atmospheric environment can result the oxidation at the surface of the 2D TMDCs sample that can seriously impact the surface potential,a closed chamber full of nitrogen gas is designed to solve this problem.With the help of gas flowmeter,the gas flowing speed is controlled within 50⁵00ml/min for best image by the probe.