Bo-Hyun Kong

Enhanced light output from aligned micropit InGaN-based light emitting diodes using wet-etch sapphire patterning

The authors have demonstrated an effective method to obtain high light output power of GaN-based light-emitting diodes (LEDs) by simultaneous enhancement of internal quantum efficiency and light extraction efficiency. Micropit InGaN∕GaN LEDs were fabricated on hexagonal-shaped GaN template through wet-etched substrate patterning. The result indicated that under optimized growth condition of high temperature GaN template, micropits could be formed and distributed in an aligned manner by growing on wet-etch patterned sapphire substrate. The LED structures showed superior optical output power, which directly resulted from not only effective elimination of threading dislocation of the epitaxial layers but also significant increase in light extraction efficiency via the inclined facets of aligned micropits.

A Nonpolar a-Plane GaN Grown on a Hemispherical Patterned r-Plane Sapphire Substrate

A high-quality a-plane GaN layer with a pit-free, mirror-like surface was grown on a hemispherical patterned r-plane sapphire substrate (PSS) by metal–organic chemical vapor deposition. The use of the r-plane PSS reduced the density of defects such as basal plane staking faults and threading dislocations of the a-plane GaN. The low-temperature-photoluminescence intensity of the emissions at 3.25 and 3.39 eV related with the defects increased along with the near-band-edge emission at 3.46 eV. The intensity of yellow emission at 2.2 eV on the PSS was remarkably decreased, which indicates the improvement of the crystal quality because of the defect reduction. The InGaN light emitting diode grown on the PSS showed an output power of 7.4 mW at 100 mA, which was about 7 times higher than that on the planar substrate.

Regrowth of Semipolar GaN on Nanoporous GaN Template by Metal Organic Chemical Vapor Deposition

A nanoporous semipolar GaN template has been fabricated by photoenhanced electrochemical etching to obtain porous GaN with nanoscale pores. The surface morphology of regrown semipolar GaN on a nanoporous GaN template was enhanced by surface modification. Cross-sectional transmission electron microscopy (TEM) images showed reductions in the densities of dislocations and basal stacking faults at the regrown interface. Photoluminescence measurement also revealed that the crystallinity of regrown GaN was improved by reducing the density of defects. Our results suggest that the photoenhanced electrochemical etching and regrowth technique is promising for high-quality semipolar GaN growth with a reduced defect density on a sapphire substrate.

Emission characteristics of ZnO nanorods on nanosilicon-on-insulator: competition between exciton–phonon coupling and surface resonance effect

We investigated the optical properties of ZnO nanorods on nanosilicon-on-insulator using variable temperature photoluminescence (PL) spectroscopy, and explored the contribution of exciton–phonon coupling and surface resonance effect on the emission characteristics of the nanorods. The low-temperature (<100 K) PL spectra revealed different strengths of exciton–phonon interaction for nanorods of different surface structures. The exciton–phonon coupling strength was stronger for nanorods of rougher surfaces with enhanced contribution of longitudinal optical phonon replicas of free exciton. Despite exhibiting different coupling strengths of exciton–phonon interactions, the room-temperature PL showed an unchanged energy position at 3.28 eV for nanorods of different surface structures. The unchanged energy position of band-edge emission was caused by the competitive effect of the surface defects induced exciton–phonon interaction and the surface resonance effect in faceted nanorods.

Shape Control and Characterization of One-dimensional ZnO Nanostructures through the Synthesis Procedure

The one-dimensional ZnO nanostructures prepared through thermal evaporation under various cooling down procedures by changing the flow rates of the carrier gas and the reactive gas were investigated. The nanorod structures were changed into the nanonail types with a broad head through the reduction of the flow rate of the carrier gas. The decrease of the reactive gas reduced the length of the nail heads due to the limited mass transport of reactive gas. The intensity ratio of the ultraviolet emission/green emission of photoluminescence was proportional to the length of the broad head showing a larger surface area. The vertically aligned nanostructures were grown along the [0001] direction of ZnO regardless of the aligned directions. The crystal direction of the nanostructures was determined by that of the initial ZnO crystal.

Effects of the Gas Atmosphere of ZnO Buffer Layers in the ZnO films grown on Si Substrates by RF Magnetron Sputtering

The effects of gas atmosphere and in-situ thermal annealing in buffet layers on the characteristic of the ZnO grown by RF magnetron sputtering have been investigated. It was shown that the introduction of buffer layers grown at the gas atmospheres of the mixed and the in-situ thermal treatment of the ZnO buffer layer improved the structural and optical properties. In addition, the ZnO films on the buffer layer thermal-annealed at gas ambience showed the strong emission of the near band gap exciton with narrow linewidth by combining the high-temperature growth of the ZnO film.