Wide-bandgap Semiconductor AlGaN-based Nanowires For Novel Optoelectronic Device Applications

Ⅲ-nitride(including GaN,AlN,InN,and corresponding alloys)nanowires grown by bottom-up molecular beam epitaxy(MBE)exhibit continuously tunable bandgap and could be doped as n-type or p-type.Due to nanowires’ unique one-dimensional morphology,they have been widely applied in nanophotonic and electronic devices owing to their excellent lattice mismatch tolerance(could be epitaxy on many substrates without lattice match restrictions),high crystal quality,large surface-to-volume ratio,and easily controlled size.In particular,owing to their unique one-dimensional geometry and large surface-to-volume ratio,Ⅲ-nitride nanowires demonstrate unique properties compared to thin-film materials,such as high light extraction/absorption efficiency,strong quantum and photon localization effect,easy to be decorated,multiple structure design possibilities(such as core-shell structure),etc.Expanding the application scenarios of Ⅲ-nitride materials in optoelectronics is our continual goal,which may be achieved by taking advantage of their intrinsic material properties and unique large surface-to-volume ratio.Among numerous semiconductor optoelectronic device applications,the multifunctional and high-performance photodetector is one of the key technologies for information capture,collection,analysis,and transmission in today’s electronic information society.It is also an important "window" for human beings to perceive the external world.The development of next-generation self-driven photodetectors with diversified functionalities,portability,and miniaturization has become one of the innovation frontiers and research hotspots in the future information systems,which plays a key role in the future information society.In recent years,photoelectrochemical photodetectors(PEC PD)have attracted considerable attention,showing great research value and application prospects,which are expected to promote the development of new generation photodetection technology.Importantly,not only the generation,separation,and transport of photogenerated carriers in the semiconductor but also the chemical reaction processes at the semiconductor/electrolyte interface play critical roles in their photodetection process.If we could combine the photoelectric conversion process inside the nanowire and the chemical reaction process in the electrolyte environment together,by precisely designing the chemical reaction process at the nanowire/electrolyte interface through surface modification methods,the carrier transport properties inside the nanowire can be regulated,which is expected to realize the multi-dimensional regulation of carrier transport behavior,finally achieving the function expansion and the performance improvement of the device.In conclusion,this dissertation focuses on group Ⅲ-nitride nanowire materials,aiming to design and prepare photodetectors with excellent performance and diverse functions,conducting research on Ⅲ-nitride nanowire-based photoelectrochemical photodetectors.Through the precise design of the energy band structure of nanowires,making full use of its inherent advantages and one-dimensional structural properties,a new type of semiconductor interface is designed to achieve precise control of the transport process of photogenerated carriers at the surface/interface,to construct photoelectrochemical photodetector with excellent performance and tunable photocurrent polarity,which aims to provide new ideas for the research and design of multifunctional and highperformance photodetectors in the future.The corresponding research contents of this thesis are as follows:1.Construction of highly efficient solar-blind photoelectrochemical-type photodetector based on defect-free,high scalable p-AlGaN nanostructures grown by molecular beam epitaxyPrevious investigations of semiconductor-based PEC PDs mainly focus on powder or nanosheet materials synthesized through chemical methods.Such PEC PDs suffer from slow response and low responsivity owing to the poor material quality,inefficient carrier separation,and collection capability.Herein,we adjusted the molecular beam epitaxy conditions to obtain defect-free,high scalable p-AlGaN nanostructures to construct novel PEC PD for solar-blind DUV detection.With the proper surface platinum(Pt)decoration through in situ photodeposition method,the carrier separation and collection efficiency and the redox reaction rate at the surface of nanowires are effectively improved,demonstrating the superior performance of solar-blind photodetection.2.Bidirectional photocurrent in p-AlGaN/n-GaN p-n heterojunction nanowiresBased on the research basis of the photoresponse behavior of p-AlGaN and nAlGaN nanowires in photoelectrochemical photodetectors,we combined them with semiconductor bandgap engineering to construct p-AlGaN/n-GaN p-n heterojunction nanowires photodetector,and we provided a deep understanding of the transfer and separation processes of photogenerated carriers in p-n junction nanowires and the nanowire/electrolyte solution interface.More importantly,based on our previous research on active-site design and interaction between active sites and the semiconductor substrate,we further improve the surface chemical properties of IIInitride nanowires through surface decoration.We finally observed bidirectional photocurrent in semiconductor p-n heterojunction under different light illumination(254 nm and 365 nm),which is,negative photocurrent under 254 nm and positive photocurrent under 365 nm,overcoming the intrinsic limitation of the conventional pn junction.3.Achieving multi-band distinctive photodetection through Ⅲ-nitride/MoSx coreshell nanowiresTaking advantage of the large specific surface area and controllable morphology of group Ⅲ nitride nanowires,we combined dislocation-free Ⅲ-nitride nanowires with amorphous molybdenum sulfides(a-MoSx)by electrodeposition method to constructⅢ-nitride/a-MoSx core-shell nanostructure.The transport process of photogenerated carriers in this structure was deeply analyzed and the polarity reversal of the photocurrent under different ultraviolet light wavelengths was realized.Upon 254 nm light illumination,the nanowires demonstrate negative photoresponsivity of-100.42 mA/W.Under 365 nm illumination,the nanowires demonstrate positive photoresponsivity of 29.5 mA/W.This unique bidirectional photocurrent response allows the devices to carry more information than conventional solid-state photodetectors and such devices could be used to determine and identify multiple optical bands(wavelengths),further promoting the multi-functionalities of the detector.