Study On The Low Dark Current In Two-dimensional Semiconductor Photodetectors

Due to excellent opoelectronic properties,conventional semiconductor photodetectors are widely used in civil and military fields.However,the complicated process,low temperature operation,brittle and poisonous properties resulting in a high cost limit to further development.Moreover,there is a certain lattice dislocation between conventional semiconductors and substrates,degrading the performance of the detector.In recent years,two-dimensional?2D?semiconductors have attracted enormous attention due to unique properties,such as high mobility,flexibility,easily tailored van der Waals heterostructures,broadband and uncooled detectors.Nonetheless,unintentional doping and crystal defects will lead to large free carrier,which limits to the detectivity and response of the detectors.This thesis focused on suppressing dark current,improving photoresponsivty,detetectivity and response speed.There are main four parts below:1.The research of the floating-gate phototransistor based on WS₂.The large dark current degrades the performance of WS₂ detectors such as detectivity and photoresponsivity.In order to suppress the dark current,the high-performance WS₂phototransistors based on floating-gate memeory structure are fabricated.The devices not only demonstrate stable memory performance,but also exhibit outstanding photodetection capabilities.In the program state,the dark current decreases from 10^-8A to 10^-1111 A.The high photoresponsivity and detectivity of 1090 A W^-1 and 3.5×10¹³Jones are obtained at the wavegenth of 520 nm under zero gate voltage and small voltage bias?20 mV?,which is better than the most photodetectors based on WS₂.Moreover,the same floating-gate memory structure is applied to the MoS₂,and the performance of devices are improved compared with the conventional MoS₂photodetectors,indicating the universality of the structure based on the floating-gate memory investigated here.These results demomstrate that the work provides a new way to design and fabricate the low dark current and high-sensitivity photodetectors.2.The research of the vertical Schottky junctions photodetector based on multilayer MoS₂.The large dark current degrades the detectivity of the MoS₂photodetector and the response speed of detectors is within the order of millisecond affected by localization effect of minority carriers.In order to suppress the dark current and improve the response speed,the photodetectors based on MoS₂ vertical Schottky junctions are fabricated.The vertical Schottky junctions suppress the dark current as low as 10^-12 A,which enhances the detectivity as high as 10¹² Jones.Moreover,The photoresponse of 68?s which is faster than most MoS₂ photodetectors is obtained and can be ascribed to two reasons:?1?The photoinduced carries are quickly accelerated by the built-in electric field.?2?The shorter channel length?<100nm?can reduce the transit time.These results demonstrate that the work provides an efficient way for photodetectors to achieve the low dark current,fast photoresponse and high detectivity.3.The research of high-performance infrared photodetectors based on black phosphorus?BP?.The BP photodetector can not reach the fast photoresponse and high photoresponsivity duo to the surface defects and instability in air.Moreover,the larger dark current degrades the detectivity.In order to improve photoresponse,responsivity,detectivity and reduce the dark current,multilayer BP photodetectors are demonstrated with bottom electrodes structure consisting of Au-BP-BN.Scanning photocurrent microscopy is appied to analysis the mechanism of the detector,and there are two distinct states:photoconductive mode and photovoltaic mode.In photoconductive mode,the photocarries are driven by the external electric field.The photoresponse,responsivity and detectivity of 16?s,1.55 A W^-1 and¹0⁸ Jones are achieved.The photoresponse is better than most BP photodetector in this mode.However,the large dark current?10^-55 A?degrades the detectivity of the device.In photovoltaic mode,the photocarries are driven by the built-in electric field formed between Au and BP.The photoresponse,responsivity and detectivity of 68?s,0.319A W^-11 and¹0⁹ Jones are achieved.Though the responsivity decreases somewhat,the detectivity is improved to¹0⁹ Jones duo to the relative low dark current.Moreover,the response speed is faster in photoconductive mode than in photovoltaic mode.It can be explained the photoresponse is dominated by drift velocity.The results prove that the BP interface is protected with high quality and the photogain can be neglected resulted from defects.4.The research of hybrid WSe₂-In₂O₃ phototransistors.In order to reduce the dark current and extend the detection spectra range,the hybrid WSe₂-In₂O₃phototransistors are designed and fabricated.The function of WSe₂ and In₂O₃ are the absorption layer and carrier transport layer,respectively.Due to the differences of work function,the photoinduced carriers accumulate the interface between multilayer WSe₂ and the In₂O₃ nanowire to control the conductivity of the nanowire,which is called photogating effect.Compared with photogating effect in other materials,the back gate voltage not only suppresses the dark current,but also enhances the built-in electric field between the two materials to improve the sensitivity.The high responsivity of 7.5×10⁵ A W^-1 and 3.5×10⁴ A W^-11 and detectivity of 4.17×10¹⁷ Jones and 1.95×10¹⁶ Jones are obtained at the wavegenth of 637 nm and 940 nm under ultralow dark current(10^-1310^-14 A).This work provides a new way for photodetectors to achieve low dark current,high responsivity and detectivity.