H. K. Cho

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.

Exciton recombination in ZnO nanorods grown on GaN/sapphire template

The authors have employed variable temperature photoluminescence (PL) and time-resolved PL spectroscopy to probe the exciton recombination in high density and vertically aligned ZnO nanorods grown on p-type GaN/sapphire template. The low-temperature PL characterizes the dominant near-band-edge excitonic emissions from such nanorod arrays. At 4.3 K, a PL decay time of 432 ps reveals improved crystalline quality. The PL decay time shows irregular behavior due to different types of excitonic transitions dominating the PL spectra at different temperatures and a competitive effect of radiative recombination and nonradiative relaxation processes.

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.

Effect of topographical control by a micro-molding process on the activity of human Mesenchymal Stem Cells on alumina ceramics

BackgroundNumerous studies have reported that microgrooves on metal and polymer materials can affect cell adhesion, proliferation, differentiation and guidance. However, our knowledge of the cell activity associated with microgrooves on ceramics, such as alumina, zirconia, hydroxyapatite and etc, is very incomplete, owing to difficulties in the engraving of microgrooves on the hard surface of the base material. In this study, microgrooves on alumina were fabricated by a casting process using a polydimethylsiloxane micro-mold. The cell responses of Human Mesenchymal Stem Cells on the alumina microgrooves were then evaluated.ResultsMicrogrooves on an alumina surface by micro-mold casting can enhance the adhesion, differentiation of osteoblasts as well as gene expression related to osteoblast differentiation. The ALP activity and calcium concentration of the cells on alumina microgrooves were increased by more than twice compared to a non-microgrooved alumina surface. Moreover, regarding the osteoblast differentiation of hMSCs, the expression of ALP, RUNX2, OSX, OC and OPN on the microgrooved alumina were all significantly increased by 1.5u2009~u20092.5 fold compared with the non-microgrooved alumina.ConclusionAltering the topography on alumina by creating microgrooves using a micro-molding process has an important impact on the behavior of hMSCs, including the adhesion, differentiation of osteoblasts and osteoblast-specific gene expression. The significant increase in hMSC activity is explained by the increasing of material transportation in parallel direction and by the extending of spreading distance in perpendicular direction.

Influence of Buffer Layer on Structural and Optical Properties of ZnO Nanorods on Glass Substrates

In this article, we report on the structural and optical properties of ZnO nanorods grown at a relatively low temperature on glass substrates with and without a ZnO buffer by metallorganic chemical vapor deposition for transparent optoelectronic application. The thickness of the buffer layer strongly influences the aspect ratio and alignment of the ZnO nanorods. The nanorod growth rate becomes fast when the optimized stress-free buffer thickness is used. Photoluminescence measurements show strong bandedge excitonic features. The visible and UV resonant Raman scattering measurements suggest that the structural and optical properties of the nanorods on amorphous substrates are improved.