Research On Ultrafast And Sensitive Photodetectors

Ultrafast and sensitive photodetectors have important applications in the field of security imaging technology,such as unmanned driving technology,long-distance detection of biological and chemical weapons and airport security scanner.However,traditional silicon photodetectors are limited by that silicon is an indirect band gap semiconductor with low quantum efficiency and narrow absorption spectrum.Also silicon is difficult to be realized in flexible devices.In addition,the fabrication technology of photodetectors with silicon and germanium is quite mature,and the main properties of those photodetectors are close to the theoretical limit.However,two-dimensional materials only have the thickness of single or several atomic layers.They show unique properties that many three-dimensional materials do not have,which provides infinite possibilities for the preparation of new optoelectronic devices.Although single-layer two-dimensional photodetectors show strong light matter interaction and high electron mobility.However,the absorption path of the two-dimensional material is very short,which leads to the lack of absorption of light.The photogenerated excitons also have large binding energy,which makes them difficult to separate into free electrons and holes.These shortcomings lead to the low photoelectric conversion efficiency of two-dimensional photodetectors.What's more,the devices could not have both high sensitivity and ultrafast response speed,which limits the application of two-dimensional materials in photoelectrical field.This thesis mainly focus on the zero dimensional quantum dot and two dimensional graphene.Through the construction of hybrid heterostructure,novel photodetector based on two-dimensional materials was fabricated,and its photoelectric response mechanism was studied and analyzed,the details are as follows:(1)Using ultraviolet(UV)contact lithography and thermal evaporation technology,the metal electrode was prepared on silicon / silica substrate,and then graphene was transferred onto the electrode by wet transfer technology.Through spin coating method,the graphene / CdSe quantum dot hybrid heterostructure photodetector was fabricated.(2)The photoelectric properties of graphene / CdSe quantum dot heterostructure photodetector was characterized by B1500 semiconductor analyzer.Due to the highabsorption coefficient of quantum dots,the quantum dots will generate excitons after absorbing photons,then the excitons efficiently transfer into graphene.Under the function of bias,the excitons separate in the graphene leading to the high responsivity of the device,which makes up the single-layer graphene photodetectors for the lack of responsivity.In addition,the results show that the responsivity of the photodetector can be improved effectively with small electrode gap and large bias voltage.The persistent photoconductivity effect(PPC)was also observed in the device.(3)Using the PPC effect,a non-volatile optical memory device with integration of photoelectric detection and storage is designed and fabricated.The device structure is graphene / quantum dot / graphene three-layer vertical van der Waals heterostructure.The study of the device shows that the device has low power consumption and stable multi-level optical storage capacity,and the storage time can last for 450 s.Due to the existence of Schottky barrier in the graphene / quantum dot heterostructure,carriers will gather in the interface state brought by Schottky barrier,which leads to the non-volatile performance of the device.This research proves that the device has great application prospects in the field of artificial vision and artificial neural network,and provides a reference for designing and preparing new two-dimensional optoelectronic devices.