| Two-dimensional(2D)semiconductor materials,such as graphene,black phosphorus(BP)and transition metal disulfides,have presented new opportunities for the development of high-performance optoelectronic devices due to their atomic size,high carrier mobility,broadband optical response and strong light-matter interaction.Meanwhile,the layers of 2D material are combined with weak van der Waals(vdW)forces,and have a naturally smooth and non-hanging bond surface.Therefore,2D materials with different physical properties can be flexibly assembled together by vdW integration technology,creating various 2D vdW heterojunctions without limitation.These heterojunctions are able to integrate the benefits of each material and fully unleash device performance at the atomic scale,which are considered to be important units in next-generation integrated optoelectronic applications.Therefore,for the research of 2D optoelectronic devices,how to reasonably construct a new heterostructure to improve the performance and expand the function is an unavoidable scientific problem.In view of this,this thesis has fully investigated the physical properties of a variety of 2D semiconductors,and then used vdW integration techniques combined with the energy band design,defect engineering and local-field modulation to construct a series of new heterostructure optoelectronic devices.Based on this,this thesis further explored the connection between device structure and device performance and function.Also,this thesis designed the heterostructures from the target performance and optimized the heterointerfaces to achieve the optimal performance of optoelectronic devices.The main research contents and innovative points of this thesis are as follows:(1)Constructed a 2D multiple stacked heterostructure to unleash the performance advantages of each material.Specifically,the 2D semiconductor MoS2 has high carrier mobility and large transistor switching ratio,but poor broadband response,especially in the infrared region,while BP has good infrared response,but relatively small switching ratio and large dark current.Based on this,we used MoS2 as the main channel material of the device and BP as the absorbing layer to vertically stack a MoS2/BP/MoS2 multi-heterojunction,and made the source-drain electrodes of the device located on both sides of MoS2 to ensure that the back-gate voltage can modulate the whole heterojunction.Such a device structure can maintain the excellent transistor performance and obtain excellent light absorption capability.Importantly,to maintain the photoconductive state of the device and further improve the photogain,a large number of trap states at the upper MoS2/BP interface have been induced by BP oxidation.In the light,the type-Ⅱ vdW hetero-interface allowed photogenerated electrons to be transferred from BP to MoS2,while photogenerated holes were trapped within the BP layer,and the device obtained high photogain as the electrons were circulated in the channel.As a result,the device has exhibited excellent optoelectronic performance,including a large switching ratio(1.5×107),high responsivity(1.3×107 A W-1),and ultra-high specific detectivity(1.2×1016 Jones).More importantly,the BP oxidationinduced trap state enabled the device to obtain a photocurrent storage time of more than 6×104 s as well as multi-bit storage performance(11 storage states,546 nC state-1),while the optical anisotropy of the BP intrinsic enabled the device to have infrared polarization resolution.These data results consistently demonstrate that the designed multiple stacked heterostructures can achieve high performance optoelectronic storage and would provide a new research direction for future high-density data storage.(2)Based on 2D heterojunction defect engineering for realizing the optoelectronic function switching of devices.Building multifunctional devices can effectively simplify the integration process and improve integration density.For the growing needs of complex optoelectronics,a single device with switchable functionality is very attractive.This work has presented a new concept of multifunctional operation through 2D heterojunction defectengineered devices.The device can be quickly switched between photodetector and photomemory by simply changing the gate-voltage.The switch is achieved by using vdW transfer and assembly technology to oxidize the BP on both sides,inducing more defect states and building a planar BP/MoS2 p-n heterojunction with the source-drain electrodes located on BP and MoS2,respectively.Under illumination,the trapped photogenerated-holes can induce a gate-voltage modulated photogating effect,which provides the device with optoelectronic memory and detector operation.In memory mode,the device has an ultra-low dark current(0.13 pA),an ultra-high writing/erasing ratio(3.5×107),excellent multi-bit storage(90 memory states),and excellent broadband storage capability(UV to NIR).In detector mode,the device still exhibits a fast response(130/260 μs),stable detection cycles(750 cycles),self-driven broadband detection(375-1550 nm),and IR polarization resolution(maximum polarization ratio of 6.98).These results are much better than those previously reported for 2D single-function memories and detectors,achieving the best balance of all performance parameters for a multifunctional device.(3)Designed the heterojunction bilateral contact barriers to achieve the dark current limit of the photodetector.The dark current is an important indicator that presents the operating power consumption,sensitivity and signalto-noise ratio of the photodetector.This work has constructed a WSe2/PdSe2/MoS2 multi-stacked heterojunction as a two-terminal-operated photodetector by vdW integration strategy.The large hetero-interface barrier at the contact between WSe2,MoS2 and PdSe2 can effectively suppress the device dark current to the limit level(20.6 fA)and obtained a high switching ratio(1.8×108).The near-zero bandgap PdSe2 serves as an efficient adsorption layer for the device,enabling a broadband response from the UV to the IR(375-2240 nm),while the anisotropic PdSe2 allows for ultra-sensitive IR polarization detection with a polarization ratio of up to 2.12 at 1550 nm.More importantly,the strong built-in electric field on both sides allowed the device to have fast photogenerated carrier separation,obtaining an ultra-fast response of 65/68μs that does not change with the detection wavelength,and have excellent photovoltaic behavior with a short-circuit current of 780 nA and an ultra-high photovoltaic conversion efficiency of 13%.The device architecture provides a new idea for 2D heterojunction-based high-performance photodetectors,which is expected to drive the future development of optoelectronic integrated chips. |
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