Study On Photodetectors And Neuromorphic Devices Based On Narrow Bandgap Two-dimensional Materials

High-performance broad-spectrum photodetectors are central to diverse optoelectronic applications such as imaging,remote sensing,and optical communications and have critical applications.However,conventional detection materials are limited by their band gaps and processing processes to achieve broad-spectrum detection using a single device.For example,gallium nitride,silicon,and indium gallium arsenide are used for UV,visible,and near-infrared light detection.These three detectors must be integrated to achieve broad-spectrum detection,leading to complex device structures and processes.Benefiting from excellent optoelectronic properties,good mechanical flexibility,and compatibility with traditional semiconductor processing,emerging two-dimensional（2D）materials compensate for the shortcomings of traditional photodetectors and become one of the crucial platforms for the realization of multifunctional broad-spectrum photodetectors.The narrow bandgap 2D material can respond to light in the mid-infrared band because of its smaller forbidden bandwidth,making it even more advantageous for broad-spectrum light detection applications.However,influenced by the intrinsic properties of the material,such as low optical absorbance and quantum efficiency,the narrow bandgap 2D material-based photodetectors have some drawbacks,such as high dark current,low optical response,and low functionality,which restrict their application in light detection and related functional devices.Chemical doping or heterojunctions can reduce dark currents and increase light absorption,optimizing the performance of photodetectors based on narrow bandgap 2D materials and expanding the applications of light detection and related functional devices,such as information processing and logic operations.In this context,this thesis systematically studies graphene and Ta₂Ni S₅ for light detection and neuromorphic device applications.The main research work is as follows:1.Preparation and performance study of graphene in-plane p-n-p junction-based photodetectorsDirectional doping of graphene is achieved using patterned Si/Si O₂ substrates to form graphene in-plane p-n-p junctions to reduce the dark current in graphene photodetectors.The p-n-p junction can not only effectively reduce the dark current of the device through the built-in barrier but also improve the optical response of the device through the combined effect of the photovoltaic effect and the grating pressure effect.Graphene in-plane p-n-p junction-based photodetectors exhibit excellent photoresponse to 1.0～4.0μm IR light illumination with ultra-low dark current at the order of 10⁹,which is three orders of magnitude lower than pristine graphene photodetectors.This approach provides ideas for the design of graphene and silicon-integrated broadband photodetectors.2.Preparation and performance study of Ta₂Ni S₅/Mo S₂ based heterojunction photodetectorsThe Ta₂Ni S₅-based photodetector was prepared by mechanical exfoliation have high responsivity(13.07 A W^-1)and detectivity（1.19×10⁹ Jones）,but the response time of the device is long（>50 s）.The built-in electric field of the Ta₂Ni S₅/Mo S₂heterojunction reduces the device’s dark current and accelerates photogenerated carrier separation,improving the light detection capability.The photodetectors based on Ta₂Ni S₅/Mo S₂ heterojunction achieves high responsivity(up to 3167.49 A W^-1)and detectivity(up to 1.03×10¹³ Jones)over a wide spectral range（532 nm～808 nm）,and a shortened optical response/recovery time of 4.35/3.54 s.This work demonstrates the great potential of narrow bandgap 2D materials for light detection and the critical role of heterojunction energy band design in modulating device performance,providing a positive impetus to applying new narrow bandgap 2D materials for light detection.3.Performance study of Ta₂Ni S₅/Mo S₂ heterojunction-based synaptic devicesThe Ta₂Ni S₅/Mo S₂ heterojunction-based devices successfully enable the simulation of synapses by exploiting the charge trapping/release of great trap states at the material interface.The Ta₂Ni S₅/Mo S₂ heterojunction based synaptic device can simultaneously implement two-terminal synapses,three-terminal synapses（optical and gated synapses）,and multi-terminal synapses,successfully simulating biological synaptic behavior,including excitatory/inhibitory postsynaptic currents,paired-pulse facilitation/depression,and the transition from short-term to long-term potentiation.The synaptic device can operate at low power consumption（17.193 f J/spike）and also features high-pass filtering.The coupling of different pulse signals enables the simulation of more complex behaviors,including biological emotions,“withdrawal reaction”and logic operations.The artificial neural network based on the device can achieve an accuracy of 91.52%in recognition of handwritten digits.This work demonstrates the great potential of narrow bandgap 2D materials for multifunctional synaptic devices and provides a new idea for the preparation of heterojunction-based synaptic devices,playing a positive role in the development of next-generation neuromorphic computing systems.