Two-dimensional Tungsten Diselenide Photodetector With Ultra-low Dark Current And High Switching Ratio

Transition metal dichalcogenides（TMDs）are material systems formed by the arrangement of certain transition metals and chalcogen elements,and their molecular formula is usually defined as MX₂.M mainly represents transition metals,such as Mo,W,etc,and X represents chalcogen elements,such as S,Se,Te,etc.As a member of layered TMDs,tungsten diselenide（WSe₂）can break the weak van der Waals force between layers by mechanical exfoliation to prepare few-layer or single-layer thin film materials.In addition,WSe₂also has the characteristics of high carrier mobility,strong light absorption,band gap and energy band structure adjustable with external modification,which makes WSe₂ become one of the“star”materials in the field of optoelectronics and photodetection.Most of the existing WSe₂ photodetectors are based on the photovoltaic effect or the photoconductive effect.Among them,photovoltaic detectors need to build a built-in electric field,which has the advantages of fast response speed,high detection rate,and large switching ratio.However,there are obvious shortcomings such as complicated preparation process.Photoconductive-based detectors have been widely studied by researchers because of their simple preparation method and large optical gain.However,due to the shortcomings of this type of detectors,such as slow response speed,low switching ratio,and low detection rate,their further applications are limited.Therefore,it is very important to introduce a new regulation method to take into account the performance of multiple indicators of the photoconductive detector for its practical application.As a simple and easy-to-operate regulation method,strain engineering can realize continuous and reversible adjustment of material structure.To a certain extent,it can affect the optoelectronic properties and carrier transport mode of the material,and then achieve the purpose of regulating the photodetection performance.Based on this,we utilize the difference in thermal expansion coefficient between flexible PDMS and thin-layer WSe₂ materials to promote WSe₂ to form surface nano-wrinkled structures during the transfer process,thereby generating stress and strain.Subsequently,a series of optical properties detection and regulation of the constructed device were carried out,and finally a photoconductive WSe₂ photodetector with low dark current,high performance and stable operation was obtained.The main research progress of this paper is as follows:1.Fabrication and characteristics of nano-wrinkled tungsten diselenide photodetectors.First,device preparation and characterization:The article elaborates the preparation process and related characterization of the nano-wrinkled structure of WSe₂ thin films.The results show that there are Raman characteristic peaks at 252.9 cm^-1,260.6 cm^-1 and 310.8 cm^-1 and a band gap energy of 1.50 e V,and the thickness of the step between the material and the substrate are 2.44 nm,which determines that WSe₂ is a three-layer flakes.The electrical characterization shows that the constructed transistor exhibits N-type semiconductor characteristics,has excellent gate control capability,and has an overall on-off ratio as high as 10⁸.Secondly,the performance test:In the experimental process of the photocurrent changing with time,the photocurrent is dependent on the light intensity and bias voltage.In addition,the device exhibits fast optical switching characteristics under the entire optical excitation,and remains stable after periodic cycling.When the device is subjected to the same illumination conditions,its photocurrent and source-drain bias voltage show a positive correlation.The results show that the optical switching ratio was as high as 1.38×10⁶,the specific detection rate D^*is1.744×10⁹ Jones,and the response and recovery times are as low as 4.52 ms and 6.97 ms,respectively.More importantly,the entire test process reveal a low dark current phenomenon,and the value is about 0.01 f A.Based on the above performance parameters,it is shown that the photodetector designed and prepared in this paper can achieve fast response detection,strong optical switching ability,ultra-low dark current and weak light detection ability（0.096μW）.Compared with the singleness and excellent performance of traditional photoconductive detectors,the device can take into account multiple performance indicators at the same time,and the overall detection is at a high level.2.Study on the mechanism of nano-wrinkled tungsten diselenide photodetector.On the basis of the above performance studies,new devices were constructed to further explore the influence of the contact metal and WSe₂ surface structure on the detection performance.First,contact metals with different metal work functions are investigated.The Cu electrode with a metal work function of 4.65 e V and the Ag electrode with a metal work function of 4.26 e V were replaced respectively.Optical test shows that the device performance of Cu electrode and Ag electrode is basically consistent with that of Au electrode device.So,it has been shown that the nano-wrinkled device performance does not depend on the metal work function under van der Waals contact conditions.Secondly,the optoelectronic properties of WSe₂ devices with different structures were compared and analyzed.The results show that the introduction of the nano-wrinkled structure leads to a red shift of the photoluminescence,and the luminescence intensity is significantly enhanced,and the indirect band gap luminescence peak can be enhanced by a maximum of 10.52 times.Compared with the nano-wrinkled structure,the photocurrent of the planar WSe₂ device is reduced by one order of magnitude,and the dark current is increased by two orders of magnitude,which in turn leads to a three-order reduction in the optical switching ratio.In addition,there is an obvious relaxation phenomenon in the photocurrent recovery stage.Based on the above investigations,it shows that the nano-wrinkled structure on the surface of WSe₂ plays a dominant role in high-performance detection.Finally,according to the performance characterization and photoelectric test comparison under the wrinkle morphology,it is proposed that the stress gradient generated by the WSe₂ nano-wrinkled structure can generate polarized charges,thereby inducing a local electric field to tune the detection performance of the device.In summary,this thesis successfully designed and constructed a WSe₂ photodetector with low dark current and high switching ratio.At the same time,the detector has high detection capability and can be applied to low-power working scenarios,laying a foundation for the further development of WSe₂.