Research On Design And Performance Of Two-dimensional WS₂/Si Van Der Waals Heterojunction Broadband Photodetector

In the past decade,two-dimensional materials（2D）have been used as the unique,promising new electronic and optoelectronic device materials by their attractive physical and chemical properties.Recently,two-dimensional transition metal chalcogenides（TMDs）have aroused researcher’s interests in photoelectric detection due to their excellent air stability,mechanical flexibility and adjustable band gap with layers,showing excellent photoelectric propertie.Tungsten disulfide（WS₂）,as a typical two-dimensional TMDS,has direct band gaps of 2.1 e V and 1.3 e V in monolayer and bulk,respectively.This relatively small band gap makes multi-layer WS₂ one of the ideal materials for broadband photodetection.In addition,WS₂ has no suspension bonds on the surface,which makes it can construct van der Waals heterojunction devices with other semiconductor materials at room temperature,without the restriction of lattice matching.But at the same time,the poor light absorption resulting from the atomic thin layer limits total generating photogenerated carriers,resulting in a smaller photoresponse and current on/off ratio（Ion/Ioff）,thus limiting the performance of the photodetector.In addition,two-dimensional samples prepared by traditional chemical vapor deposition（CVD）and mechanical peeling methods are small in size（micron level）,out of control and reproduce,which severely restricts the wide application of two-dimensional materials.Therefore,it is of great significance to explore the synthesis of large-area two-dimensional materials and design and construct new high-performance wide-band photodetectors.In this paper,the methods of metal vulcanization and thermal decomposition for the synthesis of large-area 2D WS₂ films were explored.And designed and constructed the two-dimensional/three-dimensional（2D/3D）mixed-dimensional van der Waals heterojunction photodetector with two-dimensional WS₂ film and Si and Si pyramids,high-performance wide-band photodetection is achieved due to the interlayer transition and defect-induced narrow band gap of type II heterojunction.The main research contents are as follows:Large-area WS₂ films were respectively synthesized by thermal decomposition and metal vulcanization.And the field emission scanning electron microscope（FESEM）,X-ray diffraction（XRD）,Raman spectroscopy（Raman）,high resolution transmission electron microscopy（HRTEM）,atomic force microscope（AFM）,X-ray photoelectron spectrometer（XPS）,and energy dispersive spectrometer（EDS）of the characterization methods was used to characterize the morphology,structure and composition of the film.2.Designed and constructed a mixed-dimensional van der Waals heterojunction device with WS₂/Si and WS₂/Si pyramid structure,and studied its photoelectric detection performance.（1）The large-area WS₂ film was prepared by metal vulcanization,and the heterojunction photodetector was constructed with planar Si.The WS₂/Si heterojunction photodetector has a type II band arrangement structure,which can realize the transition of carriers between layers,thus breaking through the limitation of its band gap,and the detection of wide-band optical signals ranging from 200 nm to3043 nm.In addition,The detector is illuminated by 980 nm（204 n W cm-2）light irradiation,the Ion/Ioff ratio is up to 106 at zero bias voltage.The calculated responsitivity（R）is 20 m A W-1 and the specific detection D* is 4.3×1013 Jones,with a fast response speed of 4.5/21.7 μs.Moreover,the detector also has excellent infrared imaging capability.（2）In order to further enhance device’s performance,the 2D WS₂ film synthesized by thermal decomposition method and Si pyramid are used to construct heterojunction photodetector.In the process of synthesis of WS₂ films by thermal decomposition method,narrow-bandgap WS₂ films induced by sulfur vacancy defects were successfully synthesized by controlling the vulcanization time,thus realizing the optical response of ultra-wide band from ultraviolet to mid-infrared（200 nm to 3043nm）.Meanwhile,benefiting from the enhanced light absorption of the Si pyramid structure,the photodetector achieves a responsitivity of 290 m A/W,a high specific detection of 2.6×1014 Jones and a fast response of 5.2/22.3 μs,under the irradiation of980 nm（16.5 μW/cm2）light signal.