Study On The Photodetection Mechanisms In Quasi-one Dimensional Structures

The development of low-dimensional materials promotes the study on electronic and optoelectronic devices based on novel nanostructures.Because of the one-dimensional carrier transportation channel,large surface-to-volume ratio,and sub-wavelength diameter,quasi-one-dimensional semiconductors exhibit distinct electrical and optical properties,showing great potential to be used in high-performance photodetection with high gain,fast speed,polarization-sensitivity,and broadband response.However,limited by the relatively high dark current and weak optical absorption,the performances of the current photon-detectors based on quasi-one-dimensional structures are still not satisfying.Particularly,there is plenty of room to be improved when such devices are used for infrared detection.In this work,photodetectors based on several quasi-one-dimensional semiconducting structures including InAs nanowires,CdS nanowires,SnX?X=Se,S?nanowires,and single-walled carbon nanotubes were fabricated and studied,of which the detection wavebands cover the spectrum from ultraviolet,visible to mid-infrared.The photocurrent generation mechanisms were clearly illustrated and the detector performances were further optimized.The detailed results are as follows:1.The back-gated field effect transistors based on single n-type CdS nanowires and -type SnX?X=Se,S?nanowires synthesized via chemical vapor deposition?CVD? method were separately fabricated,and the corresponding photogain reach up to 10⁵ and 10⁴,respectively.Both the numerical simulation on the optical absorption of single nanowire and the newly-established ultraviolet-laser induced photocurrent microscopy were conducted.This high gain is mainly attributed to the surface charges and defects induced photocurrent multiplication.To suppress the dark current,side-gated single CdS nanowire phototransistors using ferroelectric polymer P?VDF-TrFE?as the dielectric layer was fabricated,of which the dark current is suppressed by the ferroelectric field to be lower than 1 pA and able to be kept at this value for a long time without applying any gate voltage.The noise urrent power achieves as low as 4.6×10^-28 A²,while the photogain maintains as igh as 10⁵,thus leading to an excellent signal-to-noise ratio.2.Photodetectors based on single InAs nanowires were fabricated,and their hotoresponse behaviors including the negative photoconductance were nvestigated.By separately studying the impacts of gas molecule adsorption and orking temperature,it has been confirmed that the trap levels on the nanowire surface can capture the photoexcited hot electrons leading to negative photoconductivity?NPC?and that the releasing of the trapped electrons is a thermal-assisted process.On the basis of this,the visible light assisted infrared detection method was subsequently put forward and further verified——irradiation by visible light at low temperature will result in persistent NPC,and the device current from the persistent NPC can be regarded as the new dark current for further infrared detection.Finally,the visible light assisted single InAs nanowire photodetectors have achieved a fast response time of less than 100ms,a high peak detectivity of¹0¹² cm Hz^1/2 W^-1 as well as a broad detection wave band from near-infrared to mid-infrared and even beyond the bandgap limitation.3.Field effect transistors based on single-walled carbon nanotube?SWCNT?films using Cr or Pd as the symmetric electrode contacts were separately fabricated and scanning photocurrent mapping was conducted to study the photocurrent generation.When Cr is used as the contacts,the photocurrent with opposite signs would be localized at the source and drain electrode edges,respectively.By hybridizing SWCNTs with graphdiyne,a nonlocal photoresponse has been achieved with a responsivity of 0.4 mA/W.When Pd is used as the contacts,the photocurrent generates uniformly from the SWCNTs channel.The responsivity at l=785 nm achieves 2 A/W and the response time is as fast as¹0ms,while the photoresponse atl=2mm is too weak to be observed.By further hybridizing SWCNTs with PbS quantum dots,the responsivity atl=2mm was enhanced to³9 mA/W.The study concludes that for SWCNTs phototransistors with symmetric electrode contacts,the photocurrent could be more efficiently collected if the work function of the electrode metal is far away from the bandgap center of SWCNTs.