Hyeonjun Baek

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 ZnO nanowalls via epitaxial lateral overgrowth. Furthermore, whispering-gallery-mode lasing from the GaN microdisk with a Q-factor of 1200 was observed at room temperature.

Position- and morphology-controlled ZnO nanostructures grown on graphene layers.

Hybrid heterostructures of low-dimensional semiconductor nanostructures with two-dimensional (2-D) graphene layers are emerging as new materials for fabricating transferable and/ or fl exible optoelectronic and electronic devices. [ 1–6 ] The semiconductor nanostructures work as effi cient channels for carrier transport and electrical pumping to radiative recombination, thereby improving the device performances greatly in optoelectronics and electronics. Furthermore, the graphene layers, which have excellent electrical and thermal conductivity, high mechanical strength and elasticity, and/or optical transparency, act as novel substrates offering new functionalities such as transferability or fl exibility. [ 7–13 ] In those previous reports, however, the nanostructures were randomly formed on graphene layers since nucleation sites on the graphene layers could not be controlled, preventing practical manipulation of individual nanostructure. It is well known that control of positions and morphology of nanostructures is a prerequisite for the nanostructure to be exploited as building blocks for fabricating various types of nanodevices. [ 14–17 ] Accordingly, the position and morphology control of the low-dimensional nanostructure on the graphene layers is crucial for practical applications. The well-controlled nanostructure on the graphene layers could serve as templates for achieving heteroepitaxy of ultrathin layer on the surface, yielding various quantum heterostructures for diverse applications including optoelectronics, electronics, and photovoltaics. Furthermore, precise position control of the individual nanostructures is a great advantage for realizing the integration of semiconductor nanodevices with graphene electronics for fl exible and/or transparent display applications. Here, we report the positionand morphologycontrolled growth of ZnO nanostructures using artifi cially formed graphene step-edges. Then, the resulting structural and optical properties of the ZnO nanostructure are also discussed. Furthermore, in order to demonstrate the feasibility of those

Selective excitation of Fabry-Perot or whispering-gallery mode-type lasing in GaN microrods

Lasing from long semiconductor nanorods is dictated by Fabry-Perot (FP) resonances whereas that from large-diameter microrods is determined by whispering gallery modes (WGMs). Lengths and diameters intermediate between the two systems represent an important size regime for photonics and electronics, but have not been studied in detail. Here, we report on the detection of FP and WGM lasing emissions from a single GaN microrod, and demonstrate the ability to switch between the two lasing mechanisms by translating the excitation beam along the microrod. The competition between FP and WGM-type lasing was studied by finite-difference time-domain simulation and statistical analysis by measuring microrods of various diameters. Finally, control over the relative lasing intensities originating from either FPs or WGMs was demonstrated by tuning the polarization of the emission.

Flexible GaN Light-Emitting Diodes Using GaN Microdisks Epitaxial Laterally Overgrown on Graphene Dots

The epitaxial lateral overgrowth (ELOG) of GaN microdisks on graphene microdots and the fabrication of flexible light-emitting diodes (LEDs) using these microdisks is reported. An ELOG technique with only patterned graphene microdots is used, without any growth mask. The discrete micro-LED arrays are transferred onto Cu foil by a simple lift-off technique, which works reliably under various bending conditions.

Growth and optical characteristics of high-quality ZnO thin films on graphene layers

We report the growth of high-quality, smooth, and flat ZnO thin films on graphene layers and their photoluminescence (PL) characteristics. For the growth of high-quality ZnO thin films on graphene layers, ZnO nanowalls were grown using metal-organic vapor-phase epitaxy on oxygen-plasma treated graphene layers as an intermediate layer. PL measurements were conducted at low temperatures to examine strong near-band-edge emission peaks. The full-width-at-half-maximum value of the dominant PL emission peak was as narrow as 4 meV at T = 11 K, comparable to that of the best-quality films reported previously. Furthermore, the stimulated emission of ZnO thin films on the graphene layers was observed at the low excitation energy of 180 kW/cm2 at room temperature. Their structural and optical characteristics were investigated using X-ray diffraction, transmission electron microscopy, and PL spectroscopy.

Microtube Light-Emitting Diode Arrays with Metal Cores

We report the fabrication and characteristics of vertical microtube light-emitting diode (LED) arrays with a metal core inside the devices. To make the LEDs, gallium nitride (GaN)/indium gallium nitride (In(x)Ga(1-x)N)/zinc oxide (ZnO) coaxial microtube LED arrays were grown on an n-GaN/c-aluminum oxide (Al2O3) substrate. The microtube LED arrays were then lifted-off the substrate by wet chemical etching of the sacrificial ZnO microtubes and the silicon dioxide (SiO2) layer. The chemically lifted-off LED layer was then transferred upside-down on other supporting substrates. To create the metal cores, titanium/gold and indium tin oxide were deposited on the inner shells of the microtubes, forming n-type electrodes inside the metal-cored LEDs. The characteristics of the resulting devices were determined by measuring electroluminescence and current-voltage characteristic curves. To gain insights into the current-spreading characteristics of the devices and understand how to make them more efficient, we modeled them computationally.

GaN/ZnO Nanotube Heterostructure Light-Emitting Diodes Fabricated on Si

We report the fabrication and luminescent characteristics of GaN-based visible light-emitting diode (LED) arrays on Si substrates. For the fabrication of the LEDs, high-quality GaN/ZnO coaxial nanotube heterostructures were prepared by the heteroepitaxial growth of GaN layers on position-controlled ZnO nanotube arrays grown on 1-μm-thick crack-free GaN/Si substrates. The nanostructured LEDs were composed of GaN-based p-n homojunction with GaN/In1-xGaxN multiple quantum wells (MQWs) coaxially coated on GaN/ZnO nanotube heterostructures. The fabricated micro-LEDs emitted visible green light that originated from the MQWs of individual coaxial LEDs. In addition, the origin of the light emission was investigated by measuring cathodoluminescence and electroluminescence spectra.

Nanoscale Single-Element Color Filters

Visible-light filters constructed from nanostructured materials typically consist of a metallic grating and rely on the excitation of surface plasmon polaritons (SPPs). In order to operate at full efficiency, the number of grating elements needs to be maximized such that light can couple more efficiently to the SPPs through improved diffraction. Such conditions impose a limitation on the compactness of the filter since a larger number of grating elements represents a larger effective size. For emerging applications involving nanoscale transmitters or receivers, a device that can filter localized excitations is highly anticipated but is challenging to realize through grating-type filters. In this work, we present the design of an optical filter operating with a single element, marking a departure from diffractive plasmonic coupling. Our device consists of a ZnO nanorod enclosed by two layers of Ag film. For diffraction-limited light focused on the nanorod, narrow passbands can be realized and tuned via variation of the nanorod diameter across the visible spectrum. The spectral and spatial filtering originates from scattering cancellation localized at the nanorod due to the cavity and nanorod exhibiting opposite effective dipole moments. This ability to realize high-performance optical filtering at the ultimate size introduces intriguing possibilities for nanoscale near-field communication or ultrahigh resolution imaging pixels.

Real-time device-scale imaging of conducting filament dynamics in resistive switching materials.

ReRAM is a compelling candidate for next-generation non-volatile memory owing to its various advantages. However, fluctuation of operation parameters are critical weakness occurring failures in ‘reading’ and ‘writing’ operations. To enhance the stability, it is important to understand the mechanism of the devices. Although numerous studies have been conducted using AFM or TEM, the understanding of the device operation is still limited due to the destructive nature and/or limited imaging range of the previous methods. Here, we propose a new hybrid device composed of ReRAM and LED enabling us to monitor the conducting filament (CF) configuration on the device scale during resistive switching. We directly observe the change in CF configuration across the whole device area through light emission from our hybrid device. In contrast to former studies, we found that minor CFs were formed earlier than major CF contributing to the resistive switching. Moreover, we investigated the substitution of a stressed major CF with a fresh minor CF when large fluctuation of operation voltage appeared after more than 50 times of resistive switching in atmospheric condition. Our results present an advancement in the understanding of ReRAM operation mechanism, and a step toward stabilizing the fluctuations in ReRAM switching parameters.

Direct observation of Li diffusion in Li-doped ZnO nanowires

The direct observation of Li diffusion in Li-doped zinc oxide nanowires (NWs) was realized by using in situ heating in the scanning transmission electron microscope (STEM). A continuous increase of low atomic mass regions within a single NW was observed between 200 °C and 600 °C when heated in vacuum, which was explained by the conversion of interstitial to substitutional Li in the ZnO NW host lattice. A kick-out mechanism is introduced to explain the migration and conversion of the interstitial Li (Lii) to Zn-site substitutional Li (LiZn), and this mechanism is verified with low-temperature (11 K) photoluminescence measurements on as-grown and annealed Li-doped zinc oxide NWs, as well as the observation of an increase of NW surface roughing with applied bias.