Gyu-Chul Yi

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(00⋅1) 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.

Dry etching of ZnO films and plasma-induced damage to optical properties

To study the effects of plasma chemistries on etch characteristics and plasma-induced damage to the optical properties, dry etching of ZnO films has been carried out using inductively coupled plasmas of Cl2/Ar, Cl2/H2/Ar, and CH4/H2/Ar. The CH4/H2/Ar chemistry showed a faster etch rate and a better surface morphology than the Cl2-based chemistries. Etched samples in all chemistries showed a substantial decrease in the PL intensity of band-edge luminescence mainly due to the plasma-induced damage. The CH4/H2/Ar chemistry showed the least degradation of the optical properties.

ZnO nanorods: synthesis,characterization and applications

This paper presents a review of current research activities on ZnO nanorods (or nanowires). We begin this paper with a variety of physical and chemical methods that have been used to synthesize ZnO nanorods (or nanowires). There follows a discussion of techniques for fabricating aligned arrays, heterostructures and doping of ZnO nanorods. At the end of this paper, we discuss a wide range of interesting properties such as luminescence, field emission, gas sensing and electron transport, associated with ZnO nanorods, as well as various intriguing applications. We conclude with personal remarks on the outlook for research on ZnO nanorods.

Ferromagnetic properties of Zn1−xMnxO epitaxial thin films

We report on ferromagnetic characteristics of Zn1−xMnxO (x=0.1 and 0.3) thin films grown on Al2O3(00⋅1) substrates using laser molecular-beam epitaxy. By increasing the Mn content, the films exhibited increases in both the c-axis lattice constant and fundamental band gap energy. The Curie temperature obtained from temperature-dependent magnetization curves was 45 K for the film with x=0.3, depending on the Mn composition in the films. The remanent magnetization and coercive field of Zn0.9Mn0.1O at 5 K were 0.9 emu/g and 300 Oe, respectively. For Zn0.7Mn0.3O, the remanent magnetization at 5 K increased to 3.4 emu/g.

Excitonic emissions observed in ZnO single crystal nanorods

We report on the photoluminescent characteristics of ZnO single crystal nanorods grown by catalyst-free metalorganic vapor phase epitaxy. From photoluminescence (PL) spectra of the nanorods at 10 K, several PL peaks were observed at 3.376, 3.364, 3.360, and 3.359 eV. The PL peak at 3.376 eV is attributed to a free exciton peak while the other peaks are ascribed to neutral donor bound exciton peaks. The observation of the free exciton peak at 10 K indicates that ZnO nanorods prepared by the catalyst-free method are of high optical quality.

Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices

Gallium Nitride Grown on Graphene Nitride semiconductor materials used in light-emitting diodes and lasers are usually grown on single-crystal sapphire substrates with intermediate buffer layers. Instead, Chung et al. (p. 655) used graphene as a substrate for gallium nitride growth and found that nucleation of the gallium nitride layers was enhanced by first depositing zinc oxide, which grew as vertical nanowalls on the graphene. The gallium nitride layers displayed strong photo- and electroluminescence and, even better, the layers could be transferred to flexible substrates such as plastic. Graphene can replace sapphire crystals as the substrate for the growth of gallium nitride layers. We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) using graphene-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 on graphene 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 graphene substrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.

Metalorganic vapor-phase epitaxial growth and photoluminescent properties of Zn1−xMgxO(0⩽x⩽0.49) thin films

High-quality Zn1−xMgxO(0.00⩽x⩽0.49) thin films were epitaxially grown at 500–650 °C on Al2O3(00⋅1) substrates using metalorganic vapor-phase epitaxy. By increasing the Mg content in the films up to 49 at. %, the c-axis constant of the films decreased from 5.21 to 5.14 A and no significant phase separation was observed as determined by x-ray diffraction measurements. Furthermore, the near-band-edge emission peak position showed blueshifts of 100, 440, and 685 meV at Mg content levels of 9, 29, and 49 at. %, respectively. Photoluminescent properties of the alloy films are also discussed.

Fabrication and electrical characteristics of high-performance ZnO nanorod field-effect transistors

We report on fabrication and electrical characteristics of high-mobility field-effect transistors (FETs) using ZnO nanorods. For FET fabrications, single-crystal ZnO nanorods were prepared using catalyst-free metalorganic vapor phase epitaxy. Although typical ZnO nanorod FETs exhibited good electrical characteristics, with a transconductance of ∼140nS and a mobility of 75cm2∕Vs, the device characteristics were significantly improved by coating a polyimide thin layer on the nanorod surface, exhibiting a large turn-ON/OFF ratio of 104–105, a high transconductance of 1.9μS, and high electron mobility above 1000cm2∕Vs. The role of the polymer coating in the enhancement of the devices is also discussed.

Schottky nanocontacts on ZnO nanorod arrays

We report on fabrication and electrical characteristics of ZnO nanorod Schottky diode arrays. High quality ZnO nanorods were grown for the fabrication of the Schottky diodes using noncatalytic metalorganic vapor phase epitaxy and Au was evaporated on the tips of the vertically well-aligned ZnO nanorods. I–V characteristics of both bare ZnO and Au/ZnO heterostructure nanorod arrays were measured using current-sensing atomic force microscopy. Although both nanorods exhibited nonlinear and asymmetric I–V characteristic curves, Au/ZnO heterostructure nanorods demonstrated much improved electrical characteristics: the reverse-bias breakdown voltage was improved from −3 to −8 V by capping a Au layer on the nanorod tips. The origin of the enhanced electrical characteristics for the heterostructure nanorods is suggested.

Visible‐Color‐Tunable Light‐Emitting Diodes

However, conventional inorganic thin-fi lm light-emitting diodes (LEDs) emit only a single color that is determined by the quantum well layer thickness and composition. Achieving multiple color generation from inorganic LEDs on a substrate is a major obstacle to using inorganic semiconductors in fullcolor displays. To overcome this obstacle, we used multifacetted gallium nitride (GaN) nanorod arrays with In x Ga 1 − x N/GaN multiple quantum wells (MQWs) anisotropically formed on the nanorod tips and sidewalls. For various electroluminescence (EL) colors, current injection paths were controlled through a continuous p-GaN layer depending on the applied bias voltage. Here, we report on the fabrication and characteristics of monolithic, full-color, tunable LEDs, whose EL color can be tuned continuously from red to blue by adjusting the external electric bias. The basic strategy for epitaxial growth of multifacetted GaN nanostructures and fabrication of color-tunable LEDs is shown in Figure 1 a. To obtain the LED structure, the GaN nanorod arrays were grown on n + -GaN/Al 2 O 3 (0001) substrates with a submicrometer-hole-patterned SiO 2 growth-mask layer using catalyst-free, selective metal–organic vapor-phase epitaxy (MOVPE). As shown in the scanning electron microscopy (SEM) image in Figure 1 b, a vertically aligned GaN nanorod array exhibited excellent uniformity, with a mean length, dia meter, and neighbor spacing of 520, 220, and 550 nm, respectively, all of which could be controlled by changing the lithographic