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Featured researches published by Yoonho Choi.


Journal of Applied Physics | 1998

Photoluminescence studies of excitonic transitions in GaN epitaxial layers

Annamraju Kasi Viswanath; Joo In Lee; Sungkyu Yu; Dongho Kim; Yoonho Choi; Chang-Hee Hong

GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni’s equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505...


Journal of Applied Physics | 1998

Magnesium acceptor levels in GaN studied by photoluminescence

A. Kasi Viswanath; Eun-joo Shin; Joo In Lee; Sungkyu Yu; Dongho Kim; Baeyong Kim; Yoonho Choi; Chang-Hee Hong

Magnesium doped GaN epitaxial layers were grown by metal-organic chemical vapor deposition on sapphire substrate. Energy levels of these acceptors were investigated by systematic photoluminescence measurements in the temperature range of 12–300 K. Magnesium concentration was varied from <1×1019 to higher than 5×1019 cm−3. Photoluminescence measurements were made on the as-grown and annealed samples. We have observed various transitions related to donor to acceptor and their phonon replicas, conduction band to acceptors, and free excitons. Their dependence on temperature, concentration of the magnesium impurity and annealing conditions was discussed. In our study, two important observations were made. First, very deep level luminescence was not observed even in the highly magnesium doped as-grown samples. Second, free exciton transitions including valence band splittings were observed for the first time in the Mg-doped materials, demonstrating the high quality of the samples.


Applied Physics Letters | 2001

Reduction in crystallographic tilting of lateral epitaxial overgrown GaN by removal of oxide mask

Min Hong Kim; Yoonho Choi; Jaehyung Yi; Min Yang; Jina Jeon; Sungwon Khym; Shi-Jong Leem

The lateral overgrowth of GaN was carried out by low-pressure metalorganic chemical vapor deposition. SiO2 mask was removed just before coalescence and a subsequent lateral overgrowth was carried out to complete the fabrication of a SiO2-removed lateral epitaxial overgrown (LEO) GaN layer. The crystallographic tilting of (0002) plane, that was apparent in our standard LEO GaN layers, was absent in SiO2-removed LEO layer and x-ray diffraction measurement indicated a superior crystallinity for the SiO2-removed LEO layer. These results are attributed to the elimination of the interface between oxide mask and laterally grown GaN layer. The reduced crystallographic tilting in SiO2-removed LEO GaN layer also enhanced the quality of the coalesced fronts, as determined from cathodoluminescence images.


Semiconductor Science and Technology | 2012

High brightness nonpolar a-plane (11–20) GaN light-emitting diodes

Sukkoo Jung; Younghak Chang; Kyuhyun Bang; Hyunggu Kim; Yoonho Choi; Sung-Min Hwang; Kwang Hyeon Baik

We report on high brightness nonpolar a-plane InGaN/GaN LEDs using patterned lateral overgrowth (PLOG) epitaxy. High crystal-quality and smooth surfaces for a-plane GaN (a-GaN) films were achieved using PLOG with an array of hexagonal SiO2 patterns. The XRC FWHMs of as-grown PLOG a-GaN films were found to be 414 and 317 arcsec (450 and 455 arcsec for planar a-GaN films) along the c-axis and m-axis directions, respectively. Plan-view CL clearly reveals the periodic hexagonal patterns with higher band edge emission intensity, implying that the luminescence properties of a-GaN films lying above the SiO2 mask are improved. The light output powers of a-InGaN/GaN PLOG LEDs were measured to be 7.5 mW and 20 mW at drive currents of 20 mA and 100 mA, respectively. A negligible blue-shift was observed in the peak emission wavelength with increasing drive current up to 100 mA, indicating that there are no strong internal fields in nonpolar a-InGaN/GaN LEDs. We believe that nonpolar a-plane InGaN/GaN LEDs hold promise for efficient nitride emitters if the growth conditions are further optimized.


Japanese Journal of Applied Physics | 2001

The Effects of In Flow during Growth Interruption on the Optical Properties of InGaN Multiple Quantum Wells Grown by Low Pressure Metalorganic Chemical Vapor Deposition

Shi Jong Leem; Min Hong Kim; Johngoen Shin; Yoonho Choi; Jichai Jeong

In0.2Ga0.8N/In0.03Ga0.97N 3-period multiple quantum wells (MQWs) were grown by low pressure metalorganic chemical vapor deposition and the effects of pre-In flow prior to QW growth on its overall optical property were investigated. Pre-In flow did not change the structural property of the MQW structure such as In composition and period but strongly influenced its optical properties. MQW samples grown with pre-In flow exhibited longer peak wavelength and stronger peak intensity. CL measurement revealed more uniform and smaller emission centers in pre-In flow samples, implying a reduction of non-radiative defective regions and more In-rich regions. These results indicate that the In source introduced prior to QW growth preferentially gets incorporated around defect sites such as threading dislocations to inhibit their propagation while forming seeds for In-rich regions.


Physica Status Solidi (a) | 2002

Improvement of Structural Properties of GaN Pendeo‐Epitaxial Layers

Hung-Seob Cheong; Young-Kue Hong; Chang-Hee Hong; Yoonho Choi; Shi-Jong Leem; Hyung Jae Lee

The effects of growth parameters on growth behavior of Pendeo-epitaxial GaN layers in (1100) patterned GaN seed stripes on sapphire were systematically investigated to improve the structural properties of the overgrown layers. It was found that the ratio of lateral to vertical growth rate (y) is strongly affected by changes in growth temperature, reactor pressure, and V/III ratio, which have an effect on the distribution of crystallographic tilt among the wing regions. Also, the crystallographic anisotropy, which was determined from the results of (0002) X-ray diffraction rocking curves taken perpendicular and parallel to the seed stripe direction, can be improved with relatively high V/III ratio.


Applied Physics Letters | 2013

Defect recombination induced by density-activated carrier diffusion in nonpolar InGaN quantum wells

Fan Yang; Chunfeng Zhang; Chentian Shi; Min Joo Park; Joon Seop Kwak; Sukkoo Jung; Yoonho Choi; X.L. Wu; Xiaoyong Wang; Min Xiao

We report on the observation of carrier-diffusion-induced defect emission at high excitation density in a-plane InGaN single quantum wells. When increasing excitation density in a relatively high regime, we observed the emergence of defect-related emission together with a significant efficiency reduction of bandedge emission. The experimental results can be well explained with the density-activated carrier diffusion from localized states to defect states. Such a scenario of density-activated defect recombination, as confirmed by the dependences of photoluminescence on the excitation photon energy and temperature, is a plausible origin of efficiency droop in a-plane InGaN quantum-well light-emitting diodes.


Optics Express | 2014

Reducing the efficiency droop by lateral carrier confinement in InGaN/GaN quantum-well nanorods

Chentian Shi; Chunfeng Zhang; Fan Yang; Min Joo Park; Joon Seop Kwak; Sukkoo Jung; Yoonho Choi; Xiaoyong Wang; Min Xiao

Efficiency droop is a major obstacle facing high-power application of InGaN/GaN quantum-well (QW) light-emitting diodes (LEDs). In this paper, we report the suppression of efficiency droop induced by the process of density-activated defect recombination in nanorod structures of a-plane InGaN/GaN QWs. In the high carrier density regime, the retained emission efficiency in a dry-etched nanorod sample is observed to be over two times higher than that in its parent QW sample. We further argue that such improvement is a net effect that the lateral carrier confinement overcomes the increased surface trapping introduced during fabrication.


Journal of Applied Physics | 2002

Correlation between the type of threading dislocations and photoluminescence characteristics at different doping concentrations of Si in GaN films

Chinkyo Kim; Sungwoo Kim; Yoonho Choi; Shi-Jong Leem

At various doping concentrations of Si in GaN films, the correlation between the type of dislocations and photoluminescence (PL) characteristics was investigated. A different broadening behavior of symmetric and asymmetric Bragg peaks as a function of carrier concentration provided qualitative evidence that the type of threading dislocations generated in GaN layers was strongly dependent on the doping levels of Si. PL spectra in conjunction with x-ray rocking curve measurements suggested that the yellow luminescence associated with deep levels was more strongly related with edge dislocations than screw or mixed ones.


Applied Physics Letters | 2000

Inversion domains triggering recovery of luminescence uniformity in epitaxially lateral overgrown thick GaN film

Chinkyo Kim; Jaehyung Yi; Min Yang; Minhong Kim; Jina Jeon; Sungwon Khym; Meoungwhan Cho; Yoonho Choi; Shi-Jong Leem; Seon Tai Kim

A 150 μm-thick GaN layer was grown by halide vapor phase epitaxy utilizing selective lateral overgrowth on a SiO2-prepatterned sapphire substrate. A series of optically active regions above the SiO2 mask was observed in cross sectional monochromatic cathodoluminescence images taken at 367 nm. These bright regions were, however, consistently terminated by triangular shaped domains at 60 to 80 μm thickness, leaving no sign of luminescence nonuniformity beyond the thickness. In conjunction with the recent results on the characteristics of inversion domains in GaN, we proposed that these triangular regions might be inversion domains.

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Chang-Hee Hong

Chonbuk National University

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