Simulation Investigation And Optimization Of Strained Black Phosphorus Photodetector

As we all know,the commonly used materials for photodetectors in near infrared band mainly refer to Ge,which has a relatively high absorption coefficient.However,it still fails to cover the entire L band,even introducing in-plane tensile stain.Ge Sn,as another group IV alloy,shows its tunable band gap by adjusting the composition of Sn or strain engineering,which can extend the absorption band to the U band experimentally or almost 4?m theoretically.However,none of them can extend the cut-off wavelength any further within the whole mid-infrared(MIR)range.In recent years,two-dimensional materials including graphene,transition metal dichalcogenides,and black phosphorus(BP)has been regarded as the future material of new electronic and optoelectronic devices.As for BP,it owns direct band gap,which can be tuned from 0.3 to 2 e V by changing the number of layers from its bulk material to monolayer.Therefore,in principle,layer engineering of BP can contribute to the realization of photo-detection over an ultra-broadband spectrum.However,it is a challenge to precisely control the number of layers of BP in experiments and practical devices fabrication.Moreover,there still remains a considerable broad MIR spectrum of the blank to fill up,such as above 4?m.Due to two-dimensional materials owns excelent capacity of maintaining integrity under unprecedented deformations before rupture than their bulk counterparts,strain engineering is an exciting avenue for most optoelectronic materials to further tailor the band gap to a smaler value.Especially,for BP,due to its super flexibility with an order of magnitude smaler Young's modulus than other two-dimensional materials,it can sustain a tensile strain up to 30%theoretically,which is larger than other two-dimensional materials,such as graphene.By applying in-plain or vertical strain,the band gap can be tuned continuously,which will be accompanied by the semiconductor-metal-transition(SMT)and a considerable change of effective mass,and thus,this unique energy band tailoring suggests a great potential of optoelectronic devices.In particular,due to this remarkable property,BP has attracted a lot of interests for the extensive applications for the photodetectors.However,none specialized studies have been given to extend the optical absorption edge to MIR range.This paper mainly studies the uniaxially and biaxially strained black phosphorus(BP)photodetectors.Different strains applied in the zigzag or armchair direction can effectively tune the direct band gap of 5-layer of BP.The optical field intensity is modeled to determine the absorption for the BP layer.The cut-off wavelength of strained 5-layer of BP pin photodetector is extended to middle infrared range with a high responsivity of 66.29 A/W,which means that the strained black phosphorus photodetector provides a new approach for the middle-infrared range optoelectronic devices.This paper also compares optical characteristics of black phosphorus photodetectors integrated with a stripe waveguide and a ridge waveguide by optical field intensity and absorption spectrum,which proves that the stripe waveguide is better for enhancing the optical absorption of black phosphorus photodetector.The strain effect on the band structure of black phosphorus is investigated using the first-principles method based on density functional theory(DFT).The band structure of 5-layer BP experiences a direct-indirect-direct transition and a semiconductor-metal transition(SMT)when applied different strains.As a result,the cut-off wavelength and the responsivity of this strained BP photodetector can reach 3.76?m and 0.48 A/W respectively.In a word,the waveguide-integrated black phosphorus photodetector under strain for mid-infrared range may promote potential novel optoelectronic device applications based on two-dimensional materialsin the future.Finally,this paper also investigates the whispering galery modes(WGMs)of a 14-layer black phosphorous(BP)phototransistor based on a silicon microdisk.The transmission characteristics of the waveguide-coupled microdisk resonator with and without BP are analyzed to determine the resonance wavelength.The effect of BP on the electric field distributions of the WGMs of the Si microdisk resonator is simulated by using the finite-element method.In addition,the enhanced optical absorption of the BP-covered Si microdisk resonator is further analyzed by the coupled mode theory.Contrastingly,the device also functions as a phototransistor with a peak responsivity of 328.1 A/W and high field-effect mobility of nearly 466.6 cm² V^-1 s^-1.Our proposed device paves the path for the exploitation of BP optoelectronics devices with the assistance of optical microresonators in the near-infrared range(NIR).