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Selected Publications

Catalyst free 1D nanostructure growth

Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods

 

We report metalorganic vapor-phase epitaxial growth and structural and photoluminescent characteristics of ZnO nanorods. The nanorods were grown on Al2O3(001) substrates at 400 °C without employing any metal catalysts usually needed in other methods. Electron microscopy revealed that nanorods with uniform distributions in their diameters, lengths, and densities were grown vertically from the substrates. The mean diameter of the nanorods is as narrow as 25 nm. In addition, x-ray diffraction measurements clearly show that ZnO nanorods were grown epitaxially with homogeneous in-plane alignment as well as a c-axis orientation. More importantly, from photoluminescence spectra of the nanorods strong and narrow excitonic emission and extremely weak deep level emission were observed, indicating that the nanorods are of high optical quality.



Electroluminescence in n-ZnO Nanorod Arrays Vertically Grown on p-GaN 

 

Electroluminescent (EL) devices have been fabricated using n-ZnO nanorod arrays grown on p-GaN epilayers. Simple heteroepitaxial growth yields vertically aligned ZnO nanorods with an abrupt interface on GaN. The p?n heterojunction EL device shows a high current density and strong electroluminescence even at a reverse-bias voltage of 3 V. 



Quantum Confinement Observed in ZnO/ZnMgO Nanorod Heterostructures 

 

Multiple quantum well nanorods have been fabricated via heteroepitaxial growth of ZnO and ZnMgO (see Figure and also cover). Simple yet accurate thickness control allows the realization of nanosized well structures in individual nanorods that are tunable through the effects of quantum confinement. This approach should be readily extendible to other heteroepitaxial semiconductor nanorods.



Shape-Controlled Nanoarchitectures Using Nanowalls

 

A novel method for shaping and positioning ZnO nanoarchitectures using conventional lithography and catalyst-free metal organic vapor-phase epitaxy is demonstrated. Nanowalls and nanotubes of desired shapes and arrangements can be grown heteroepitaxially on Si substrates, and their electron-emission characteristics were optimized by changing their diameter and spacing. This method can be readily expanded to create many artificial 1D and 2D structures, as required for various device applications.



GaN/In1-xGaxN/GaN/ZnO nanoarchitecture light emitting diode microarrays

 

We studied the fabrication and electroluminescent EL characteristics of GaN/In1?xGaxN/GaN/ZnO nanoarchitecture light emitting diode (LED) microarrays consisting of position-controlled GaN/ZnO coaxial nanotube heterostructures. For the fabrication of nanoarchitecture LED arrays, n-GaN, GaN/In0.24Ga0.76N multiquantum well (MQW) structures and p-GaN layers were deposited coaxially over the entire surface of position-controlled ZnO nanotube arrays grown vertically on c-plane sapphire substrates. The nanoarchitecture LEDs exhibited strong green and blue emission from the GaN/GaN/In0.24Ga0.76N MQWs at room temperature. Furthermore, the origins of dominant EL peaks are also discussed.



Visible-Color-Tunable Light-Emitting Diodes

 

Visible-color-tunable light-emitting diodes (LEDs) with electroluminescent color that changes continuously from red to blue by adjusting the external electric bias are fabricated using multifacetted GaN nanorods with anisotropically formed 3D InGaN multiple-quantum wells. Monolithically integrated red, green, and blue LEDs on a single substrate, operating at a fixed drive current, are also demonstrated for inorganic full-color LED display applications.



2) Electronics and optoelectronics on 2D van der Waals materials
Epitaxial GaN Microdisk Lasers Grown on Graphene Microdots

 

Direct epitaxial growth of inorganic compound semiconductors on lattice-matched single-crystal substrates has provided an important way to fabricate light sources for various applications including lighting, displays and optical communications. Nevertheless, unconventional substrates such as silicon, amorphous glass, plastics, and metals must be used for emerging optoelectronic applications, such as high-speed photonic circuitry and flexible displays. However, high-quality film growth requires good matching of lattice constants and thermal expansion coefficients between the film and the supporting substrate. This restricts monolithic fabrication of optoelectronic devices on unconventional substrates. Here, we describe methods to grow high-quality gallium nitride (GaN) microdisks on amorphous silicon oxide layers formed on silicon using micropatterned graphene films as a nucleation layer. Highly crystalline GaN microdisks having hexagonal facets were grown on graphene dots with intermediate ZnOnanowalls via epitaxial lateral overgrowth. Furthermore, whispering-gallery-mode lasing from the GaNmicrodisk with a Q-factor of 1200 was observed at room temperature.



Position- and Morphology-Controlled ZnO Nanostructures Grown on Graphene Layers

 

Position- and morphology-controlled ZnO nanostructures are grown on an oxygen plasma-treated selective area of graphene layers using metal-organic vapor-phase epitaxy. The structural and optical characteristics examined by electron microscopy, cathodoluminescence and photoluminescence techniques indicate that high-quality nanostructures are prepared on graphene layers. This approach to grow the controlled ZnO nanostructures selectively on graphene layers enables us to fabricate variousnanodevices including GaN/ZnO coaxial nanotube LED microarrays.






High-quality GaN films grown on chemical vapor-deposited graphene films 

 

We report the growth of high-quality GaN films on large-size graphene films for visible light-emitting diodes (LEDs). The graphene films were synthesized by chemical vapor deposition and then transferred onto amorphous silica (SiO2) substrates that do not have an epitaxial relationship with GaN. Before growing the high-quality GaN thin films, ZnO nanowalls were grown on the graphene films as an intermediate layer. The structural and optical characteristics of the GaN films were investigated, and the films exhibited stimulated emission even at room temperature, a highly c-axis-oriented crystal structure, and a preferred in-plane orientation. Visible LEDs that emitted strong electroluminescence under room illumination were fabricated using the GaN thin films.





Flexible Inorganic Nanostructure Light-Emitting Diodes Fabricated on Graphene Films

 

Flexible inorganic nanostructure light-emitting diodes (LEDs) are fabricated using high-quality GaN/ZnOcoaxial nanorod heterostructures grown directly on large graphene films. The nanostructure LEDs fabricated on graphene films are readily transferred onto flexible plastic substrates, which operated reliably in a flexible form without significant degradation of the LED performance.




Transferable GaN Layers Grown on ZnO-Coated Graphene Layers for Optoelectronic Devices

 

We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) usinggraphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films ongraphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphenesubstrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.


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