Configuration And Photodetection Of Two-dimensional MoS₂ PN Junction Modulated By Ferroelectric Polarization

Since the discovery of graphene,the family of two-dimensional(2D)materials grows rapidly,the quantum confinement effect has endowed 2D materials with novel physical and chemical properties,which brings in great potential in electric,photoelectronic,catalysis,energy storage and harvesting,thereby making 2D materials become one of the most important directions of electronic functional devices in the future.However,the modification of 2D materials by doping is a prerequisite for the configuration of electronic functional devices.There are mainly several doping methods reported at present:ion intercalation,surface charge transfer,element substitution,and electrostatic field doping.Among them,the electrostatic doping from the gate modulation shows good compatibility with 2D materials,this charge doping method will not introduce additional defects in 2D material to avoid the degradation of materials.In this method,the doing concentration regulated by the gate voltage is volatile,which means that the gate voltage needs to be applied continuously to maintain the doping effect,but this also brings additional energy consumption to the device.Therefore,the realization of large-scale controllable and reversible non-volatile electrostatic doping is very important for the development and application of 2D materials.In this paper,we introduce the non-volatile doping of electrons or holes in 2D materials by the remanent polarization of organic ferroelectric(FE)materials.On this basis,2D PN junction,bipolar junction transistor(BJT)and junction field effect transistor(JFET)has been defined with a piezoelectric force microscope(PFM).and their electronic and optoelectronic properties have been further characterized and analyzed,and the main achievements are listed as follow:(1)FE modulation of the concentration and types of carrier in 2D TMDCs.By optimizing the annealing conditions,an organic P(VDF-TRFE)FE thin film with high ferroelectric crystallinity was obtained.The carrier concentration of 2D TMDCs are tuned widely(MoS₂:10⁹-10¹² cm^-2,WSe₂:10⁷-10¹¹ cm^-2)by the remanent polarization of the coupled FE film,and even realize the reversal of the carrier polarity.Different from traditional doping methods,this FE doping is non-volatile and rewritable,which provides a universal doping method for electrons and holes in 2D TMDCs materials.(2)2D MoS₂PN junction based on FE modulation.Owning to the rewritable feature of FE polarization,the influence of P/N doping ratio of channel on the carrier collection efficiency of the PN junction is studied by designing the scanning pattern of PFM.Besides,the photoelectric performance of the PN junction photodiode was characterized under the illumination of 532 nm light source,the separation efficiency of photo-generated electrons and holes reaches?12%with a fast response speed of 10?20?s within a self-driven photoresponse.In the meantime,the maximum of V_oc and photoelectric conversion efficiency are 650 m V and 0.61%,respectively,which is better than other homogenous PN junctions or heterojunctions based on MoS₂ reported currently.(3)2D MoS₂ BJT based on FE modulation.For the rewritable feature of FE polarization,the BJT is configured based on 2D MoS₂ PN junction by designing the scanning pattern of PFM.Benefitting from the transistor gain,the limitation of the lack of gain of the PN diode is compensated with a photogain of?1000.Owning to the reconfiguration feature,the influence of base width and base current on the electrical and optoelectronic properties of BJT was studied.On this basis,the optimization of transistor performance is realized(electrical gain:5076,photogain:1703).By comprehensively considering the influence of base width on gain and dark current,BJT in this paper shows the best photodetection performance with W_B=1040 nm,where the detectivity reaches1.3×10¹² cm?Hz(2^-1while maintains the fast response speed of?10?s in the meantime.(4)2D MoS₂ JFET based on FE modulation.Through the design of the PFM scan pattern and the integration of the top gate,2D MoS₂ JFET is configurated.The influence of gate voltage on the width of space charge region(W_D)of PN junction is explored.The photoelectric performance of JFET is also discussed(subthreshold swing?110 m V dec^-1,threshold voltage shift?0.7 V)under 680 nm illumination.Besides,the shielding effect of the top gate on the depolarization field of the FE material is used to improve their retention.On this basis,silver nanowire was taken as the top gate to pole the FE materials,thereby realizing the vertical PN homojunction and JFET of 2D materials,with their optoelectronic properties characterized.