| Wide bandgap Ⅲ-nitride semiconductors,including gallium nitride(GaN),aluminum nitride(AIN),indium nitride(InN),and their Al-Ga-In alloy nitrides,have merits of direct and tunable bandgaps,excellent electrical and optical properties,and good stability.These materials can be grown on low-cost substrates such as silicon and sapphire using heteroepitaxy to form thin films(planar structures)or nanostructures such as nanowires.As a result,they are widely studied and applied in the fields of semiconductor optoelectronic devices,power electronics,and radio frequency devices.Compared to their thin films or planar structures,Ⅲ-nitride nanowires benefit from the effective release of radial stress during their growth process,enabling them to be grown on almost any substrate without strict lattice matching requirements.Furthermore,nitride nanowires grown by molecular beam epitaxy have intrinsic advantages such as high crystal quality(single crystal),precise morphology and size control,high light absorption efficiency,high carrier extraction efficiency,and large surface area,making them an ideal material platform for developing novel semiconductor optoelectronic devices.On the one hand,the precise control of the morphology,size,and internal structure of nanowires provides a solid material foundation for regulating their optical,electrical,and optoelectronic properties.On the other hand,the large amount of exposed surface area of nanowires facilitates efficient regulation of their surface physical and chemical properties,providing an ideal material platform for developing various optoelectronic devices based on Ⅲ-nitride nanowires,such as photoelectrochemical catalysis,photodetection,and chemical and biological photoelectronic sensing.Specifically,by combining semiconductor physics with materials,chemistry,biology,and other disciplines,through surface modification and hetero-integration,the charge generation,separation,and transport in nitride nanowires can be controlled from multiple perspectives.With continuously improving the optoelectronic performance of the devices,it is promising to innovate the functionality of traditional nitride semiconductor optoelectronic devices,developing novel device functions and applications.On this basis,in this thesis we focuse on single crystal GaN nanowires grown on silicon substrate by molecular beam epitaxy.To fully utilize the material and structural advantages of nitride nanowires,we construct novel photoelectrochemical(PEC)photo detector with the design of GaN nanowire structures and surface modification.By precisely controlling the carrier transport process at the surface and interface of the nanowire,multifunctional PEC photodetectors are designed and fabraicated,and their application in underwater optical communication,especially in dual-channel encrypted optical communication systems,is explored.Specifically,this thesis first demonstrates the research foundation of the mechanism of aqueous redox reactions,the design of reactive sites,and the structure-performance relationship.Then we propose to leverage these research experiences into the design and fabraication process of PEC photodetectors.Using different nanowire structures and surface modification methods,the carrier transport behavior,PEC reaction properties,and device response characteristics are deeply studied to better understand the carrier-transport mechanism that affects the optoelectronic performance of the device.Furthermore,based on the carrier-transport mechanism of the device,the band structure,doping characteristics,and surface modification methods of GaN nanowires are designed and optimized to tune the dynamic processes of carrier separation and transport,thereby controlling device performance of PEC photodetectors,such as responsivity,response speed,photocurrent polarity,and light-wavelength dependence.Ultimately,we can achieve the control of device functions and the development of application scenarios.The detailed research are as follows:(1)Using platinum(Pt)-based nanomaterials as the model system,we fully understand the thermodynamic and kinetic mechanisms of redox reactions in aqueous solutions.The dynamic evolution of atomic and electronic structures of Pt nanostructures in working states are studied at the atomic scale,and their influence on the electrochemical reaction activity,namely the structure-performance relationship between material structure and device performance are revealed.These research lay a foundation for future surface modification of GaN nanowires.Afterwards,we use molecular beam epitaxy(MBE)technique to grow GaN nanowires.With optimizing the growth conditions of nanowires,using Mg and Si dopants for effective p-type and n-type doping,and designing the energy band structure of nanowires,we construct a high-quality material platform based on GaN nanowires.On this basis,surface modification strategies are used to hetero-integrate nanowires with various cocatalysts.On the one hand,the reaction activity of the nanowire surface is improved by designing the reactive sites.On the other hand,the carrier separation efficiency is improved by forming heterojunctions.(2)Based on GaN nanowires,we construct PEC photodetectors with modification of Pt nanoparticles on the nanowire surface.Using photodeposition and magnetron sputtering methods to load Pt nanoparticles on GaN nanowires with different structures,we realize the improvement device performance and the reverse of the photocurrent polarity,respectively.Firstly,by modifying the surface of p-n junction GaN nanowires with Pt nanoparticles through photodeposition,the photoresponse of the PEC photodetector is improved from 4 μA/cm2 to-80 μA/cm2 under 365 nm light illumination.Then,by depositing Pt nanoparticles on p-type GaN nanowires using magnetron sputtering,the photocurrent polarity of the PEC photodetector is reversed under 365 nm and 340 nm light illuminations.The device output was positive current under 365 nm light illumination and negative current under 340 nm light illumination.Importantly,through series of PEC tests,electrical tests,and spectroscopic characterization,the light absorption of the device,the generation,separation and transport of photogenerated carriers,and surface PEC reaction are deeply studied,and the carrier competition mechanism is proposed,revealing the underlying working mechanism of device.(3)Based on the understanding of the structure-performance relationship and carrier transport mechanism,GaN nanowires were designed from their composition and band structure,and the carrier transport behavior was targetedly regulated according to the specific application scenarios and device functionality requirements.Combined with precise design of reactive site,the responsivity and response speed were simultaneously improved in unipolar PEC photodetectors,and the positive and negative current were simultaneously enhanced in bipolar PEC photodetectors.Their applications in high-efficiency underwater optical communication and dual-channel encrypted optical communication were also demonstrated.Firstly,using n-type GaN nanowires with high carrier mobility and reasonable surface modification of iridium oxide,the responsivity of the device was improved from 51.4 mA/W to 110.1 mA/W,and the response/recovery time was optimized from 14 μs/27 μs to 2 μs/4 μs.Taking full advantage of the feature that PEC devices can directly work in aqueous solutions,the constructed fast-speed PEC photodetector was applied to an underwater optical communication system,demonstrating high communication bandwidth(>100 kHz)as an optical communication receiver.Then,based on the light-induced carrier competition and balance mechanism in p-n junction GaN nano wires,the bipolar photoresponse of the PEC photodetector under 365 nm and 265 nm light illumination was achieved by loading bifunctional ruthenium-based cocatalysts on the nanowire surface.The positive and negative photocurrent were simultaneously enhanced(with improvement of 775%and 3000%,respectively).To fully utilize the excellent bipolar photoresponse of the device and explore its potential application in receiving and processing dual-band optical signals in optical communications,the high efficiency and encryption characteristics of the device were demonstrated in a dual-channel optical communication system. |
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