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Electronic and optoelectronic nanodevice arrays using semiconductor nanostructures

Electronic and optoelectronic nanodevice arrays using semiconductor nanostructures
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Semiconductor nanostructures, such as nanorods, nanowires, nanotubes, nanowalls, and so forth, have attracted much attention as vital components for fabricating various electronic and optoelectronic devices due to their several potential advantages over semiconductor thin films. The excellent material characteristics of single-crystalline nanostructures, such as high carrier mobility, high radiative recombination rate, and long minority carrier life time, have enabled the fabrication of high-performance electronic and optoelectronic devices. In addition, the ability to fabricate composition-modulated heterostructures with a high-quality interface made it possible to fabricate sophisticated nanoscale electronic and optoelectronic devices. For practical applications, however, several critical issues still remain, including reliable and precise characterization of physical properties of individual nanostructures and integration of functional nanodevices in a controlled manner. The electrical properties of individual nanostructures, such as carrier concentration and mobility, were measured typically by field-effect transport measurements. However, they are strongly influenced by surface and interface states, also cannot be used for the heavily-doped nanostructures. Meanwhile, temperature-dependent thermoelectric power measurements allowed us to determine the carrier concentration of highly-doped ZnO nanorods and GaN nanowires, even when the conventional field-effect estimation is not possible. In addition, electronic device applications using a single ZnO nanorod and its coaxial heterostructure have been investigated. In the top-gate geometry, first, the device characteristics of ZnO nanorod field-effect transistors (FETs) were significantly enhanced due to geometrical enhancement of gate capacitance. In addition, ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructure FETs exhibited the excellent electrical characteristics with much higher mobility and smaller subthreshold swing values, compared with bare ZnO nanorod FETs. Such enhancement in device performances was mainly attributable to both in-situ surface passivation and carrier confinement effects through the heteroepitaxial growth of a lattice-matched Mg0.2Zn0.8O shell layer with a wider band gap than ZnO nanorods. Furthermore, the modulation doping with a site-specific doping profile not only enabled the control of electrical conductivity in both ZnO nanorod and ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructure, but also showed great promise in fabricating high-mobility electrical devices. For more practical applications, vertical nanostructures grown on various substrates, including ZnO nanowall networks and GaN/ZnO coaxial nanorod heterostructures, were used to fabricate electronic and light-emitting devices. Selective-area growth of ZnO nanowall networks on the insulating AlN/Si substrate enabled the fabrication of network electrical devices with the controlled electrical characteristics, which were useful for high-sensitive gas sensing. In addition, on selectively-grown ZnO nanowall networks on the patterned graphene films, high-quality GaN microscale film arrays were fabricated by lateral overgrowth, which enabled the fabrication of transferable GaN light-emitting diodes (LEDs) on arbitrary substrates. Meanwhile, high-quality GaN/ZnO coaxial nanorod heterostructures grown randomly on large graphene films were used to fabricate flexible inorganic nanostructure LEDs. The nanostructure LEDs were transferred onto flexible plastic substrates, which operated reliably in a flexible form without significant degradation of the LED performance. Considerable advances in nanodevice fabrication have been achieved using position-controlled coaxial nanomaterial heterostructure arrays. Nanoarchitecture LED microarrays were fabricated on many different substrates, including sapphire, Si, and graphene substrates using position-controlled GaN/In1-xGaxN/GaN/ZnO coaxial nanotube heterostructure arrays. The nanoarchitecture LED microarrays emitted strong visible light originated from individual nanoarchitectures. The position-controlled coaxial nanoarchitecture arrays provide the significant opportunities for the fabrication of high-efficiency LEDs and integrated optoelectronic devices. More generally, they may be employed in the fabrication of many other optoelectronic devices, including laser diodes and solar cells.
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