Browsing by Author "Ko, T. S."

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AlGaN/GaN multiple quantum wells grown by atomic layer deposition - art. no. 68941V
(2008)
Lo, M. H.
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Li, Z. Y.
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Chen, J. R.
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Ko, T. S.
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Lu, T. C.
A three-pair AlGaN/GaN multiple quantum well (MQW) structure with superlattices (SLs) was grown on c-plane sapphire using metal organic chemical vapor deposition (MOCVD) system. The AlGaN barrier and GaN well of the MQW structure were grown by atomic layer deposition (ALD) and conventional growth, respectively. The HRTEM and HRXRD results show the grown structure has shape interface between SLs layers and QWs with good periodicity. The AFM and SEM data show smooth surface morphology with low RMS value and low defect density. The CL measurements also indicate uniform luminescence pattern at room temperature. The AlGaN/GaN MQW with AlN/GaN SLs structure grown by ALD could be used to improve the surface morphology by effectively suppress the threading dislocation.
Characteristics of a-plane GaN with the SiN(x) insertion layer grown by metal-organic chemical vapor deposition
(2008-11-15)
Ko, T. S.
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Wang, T. C.
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Huang, H. M.
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Chen, J. R.
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Chen, H. G.
We utilized in-situ-grown SiN(x) insertion layers to mitigate part of dislocations stretching in nonpolar a-plane GaN films using metal-organic chemical vapor deposition. Both X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements revealed that better crystal quality and smoother surface could be obtained when the in-situ SiN(x) layer was inserted closer to the r-plane sapphire substrate and indicated that the in-situ SiN(x) insertion layer could suppress dislocations caused by lattice mismatch between the sapphire and epitaxial layers. In addition, photoluminescence and cathodoluminescence measurements confirmed the effect of the in-situ SiN(x) insertion layer on the optical properties of the improved a-plane GaN thin film, which is consistent with the XRD and AFM analyses and suggested reduction in the density of nonradiative centers. (C) 2008 Elsevier B.V. All rights reserved.
Characteristics of a-plane GaN with the SiNx insertion layer grown by metal-organic chemical vapor deposition
(2008-11-15)
Ko, T. S.
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Wang, T. C.
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Huang, H. M.
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Chen, J. R.
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Chen, H. G.
We utilized in-situ-grown SiNx insertion layers to mitigate part of dislocations stretching in nonpolar a-plane GaN films using metal-organic chemical vapor deposition. Both X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements revealed that better crystal quality and smoother surface could be obtained when the in-situ SiNx layer was inserted closer to the r-plane sapphire substrate and indicated that the in-situ SiNx insertion layer could suppress dislocations caused by lattice mismatch between the sapphire and epitaxial layers. In addition, photoluminescence and cathodoluminescence measurements confirmed the effect of the in-situ SiNx insertion layer on the optical properties of the improved a-plane GaN thin film, which is consistent with the XRD and AFM analyses and suggested reduction in the density of nonradiative centers. (C) 2008 Elsevier B.V. All rights reserved.
Enhancement of InGaN-GaN indium-tin-oxide flip-chip light-emitting diodes with TiO2-SiO2 multilayer stack omnidirectional reflector
(2006-09-01)
Lin, C. H.
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Lai, C. F.
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Ko, T. S.
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Huang, H. W.
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Kuo, H. C.
Enhancement of light extraction of GaN-based flip-chip indium-tin-oxide light-emitting diodes (FC ITO LEDs) with an onmidirectional reflector (ODR) is presented. The ODR consisting of alternating layers of TiO2 and SiO2 is designed to possess a complete photonic bandgap within the blue region of interest, and it is fabricated by E-beam deposition. At a driving current of 300 mA and a chip size of 1 mm x 1 mm, the light output power of the FC ITO LEDs with the ODR reaches 156 mW. This is an enhancement of 31% when compared with the same device with an Al mirror instead. Furthermore, by examining the radiation patterns, the FC ITO LED with the ODR shows stronger enhancement around the vertical direction. Our work offers promising potential for enhancing output powers of commercial light-emitting devices.
Growth and characterization of a-plane Al(x)Ga(1-x)N alloys by metalorganic chemical vapor deposition
(2010-03-01)
Huang, H. M.
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Ling, S. C.
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Chen, J. R.
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Ko, T. S.
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Li, J. C.
The non-polar a-plane Al(x)Ga(1-x)N alloys on GaN epitaxial layer with different Al compositions (0 <= x <= 0.2) were grown on r-plane (1 (1) over bar 0 2) sapphire substrates by using low-pressure metalorganic chemical vapor deposition (MOCVD) and the Al composition x were estimated from the X-ray diffraction measurements. According to the result of asymmetric X-ray reciprocal space mapping, AlGaN layer was coherently strained to the underlying GaN template. The a-plane AlGaN alloy with relatively lower Al composition showed a flat surface with reduction of pits. The best mean roughness of the surface morphology was 1.18 nm. The photoluminescence (PL) result revealed that the PL peak position shifted from 3.42 to 3.87 eV with 0 <= x <= 0.2. Apart from the shifted peak position with increasing Al content, the PL emission intensity and surface morphology of the a-plane AlGaN alloy with relatively low Al content show slightly better characteristics than that of the a-plane GaN and AlGaN with higher Al composition. (C) 2010 Elsevier B.V. All rights reserved.
Growth and characterization of a-plane AlxGa1-xN alloys by metalorganic chemical vapor deposition
(2010-03-01)
Huang, H. M.
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Ling, S. C.
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Chen, J. R.
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Ko, T. S.
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Li, J. C.
The non-polar a-plane AlxGa1-xN alloys on GaN epitaxial layer with different Al compositions (0 <= x <= 0.2) were grown on r-plane (1 (1) over bar 0 2) sapphire substrates by using low-pressure metalorganic chemical vapor deposition (MOCVD) and the Al composition x were estimated from the X-ray diffraction measurements. According to the result of asymmetric X-ray reciprocal space mapping, AlGaN layer was coherently strained to the underlying GaN template. The a-plane AlGaN alloy with relatively lower Al composition showed a flat surface with reduction of pits. The best mean roughness of the surface morphology was 1.18 nm. The photoluminescence (PL) result revealed that the PL peak position shifted from 3.42 to 3.87 eV with 0 <= x <= 0.2. Apart from the shifted peak position with increasing Al content, the PL emission intensity and surface morphology of the a-plane AlGaN alloy with relatively low Al content show slightly better characteristics than that of the a-plane GaN and AlGaN with higher Al composition. (C) 2010 Elsevier B.V. All rights reserved.
InGaN/GaN nanostripe grown on pattern sapphire by metal organic chemical vapor deposition
(2007-01-01)
Ko, T. S.
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Wang, T. C.
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Gao, R. C.
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Lee, Y. J.
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Lu, T. C.
The authors have used metal organic chemical vapor deposition to grow InGaN/GaN multiple quantum well (MQW) nanostripes on trapezoidally patterned c-plane sapphire substrates. Transmission electron microscopy (TEM) images clearly revealed that the MQWs grew not only on the top faces of the trapezoids but also on both lateral side facets along the [0001] direction defined by the selected area electron diffraction pattern. Meanwhile, dislocations that stretched from the interfaces between the GaN and the substrates did not pass through the MQWs in the TEM observation. Microphotoluminescence measurements verified that the luminescence efficiency from a single nanostripe was enhanced by up to fivefold relative to those of regular thin film MQW structures. Observation of the cathodoluminescence identified the areas of light emission and confirmed that enhanced emission occurred from the nanostripes. (c) 2007 American Institute of Physics.
Observation of strong red photoluminescence with broadband in indium oxynitride nanoparticles
(2006-07-01)
Ko, T. S.
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Chu, C. P.
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Chen, H. G.
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Lu, T. C.
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Kuo, H. C.
Indium oxynitride nanoparticles were synthesized on a silicon substrate in nitrogen atmosphere using the method involving thermal evaporation of pure indium in a two-zone reactor. Nanoscale compositional analysis by energy dispersion, spectrum showed the existence of indium oxynitride compound. X-ray diffraction analysis further confirmed high crystallization and nitrogen atom existence within the nanoparticles. Scanning electron microscopy investigations showed shape transformation from amorphous sphere to well-shaped octahedron with an average nanoparticle size ranging from 50 nm to 1 mu m when the growth temperature of the substrate was increased from 600 to 900 degrees C. Photoluminescence study was performed on the indium oxynitride nanoparticle samples grown at different temperatures. It was found that with increasing growth temperatures there was not only the formation of high quality indium oxynitride nanoparticles but also an increase in the intensities of emissions. These nanoparticles grown at 900 degrees C could emit a strong photoluminescence spectrum centered around 700 nm with a broad full width at half maximum of about 250 nm, spanning the whole red segment. (c) 2006 American Vacuum Society.
Observations on surface morphologies and dislocations of a-plane GaN grown by metal organic chemical vapor deposition
(2007)
Ko, T. S.
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Wang, T. C.
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Chen, H. G.
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Gao, R. C.
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Huang, G. S.
In this study, we grew non-polar a-plane GaN thin films on r-plane sapphire using a series of growth conditions by metal-organic chemical vapor deposition. The results showed that high temperature and low-pressure conditions benefited two-dimension growth could lead to a fully coalesced a-plane GaN layer with a very smooth surface. The best surface morphology with an excellent mean roughness of 10.5 angstrom was obtained. The different thickness AlN as a nucleation layer and the different delta/beta ratio were also considered. The results revealed that the surface morphology would get worse when the thickness of nucleation layer and delta/beta ratio were away from the values of optimal condition. The observation of transmission electronic microscopy shown the lowest density of threading dislocations was 1.85x10(10) /cm(2). (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Optical characteristics of a-plane InGaN/GaN multiple quantum wells with different well widths
(2007-04-30)
Ko, T. S.
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Lu, T. C.
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Wang, T. C.
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Lo, M. H.
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Chen, J. R.
a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm were grown on r-plane sapphire by metal organic chemical vapor deposition for investigation. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL study shows that the sample with 3-nm-thick wells has a better optical property with a fast exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify the more uniform and stronger luminescence intensity distribution observed for the samples of thinner quantum wells, indicating that the important growth parameters for a-plane InGaN/GaN multiple quantum wells could be dominated by the In fluctuation and crystal quality during the epitaxial growth. (c) 2007 American Institute of Physics.
Optical characteristics of a-plane ZnO/Zn(0.8)Mg(0.2)O multiple quantum wells grown by pulsed laser deposition
(2010-10-01)
Ko, T. S.
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Lu, T. C.
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Zhuo, L. F.
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Wang, W. L.
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Liang, M. H.
We reported optical properties of a-plane ZnO/ZnMgO multiple quantum wells (MQWs) structure grown by the pulse laser deposition system. The emission peak energy of a-plane ZnO/ZnMgO MQWs kept invariant in the power-dependent photoluminescence (PL) measurement, indicating the nonpolar characteristics due to the lack of built-in electric fields. Large exciton binding energy of 68 meV was deduced and no apparent S-curve appeared in temperature-dependent PL results, demonstrating less carrier localization effect in a-plane ZnO/ZnMgO MQWs. Large difference in electronic transition levels of 45 meV due to the valence band splitting was observed in the polarization dependent absorption spectrum. Furthermore, the high degree of polarization of 92% and 56% at 20 and 300 K in PL emission of a-plane ZnO/ZnMgO MQWs were obtained. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488898]
Optical characteristics of a-plane ZnO/Zn0.8Mg0.2O multiple quantum wells grown by pulsed laser deposition
(2010-10-01)
Ko, T. S.
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Lu, T. C.
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Zhuo, L. F.
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Wang, W. L.
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Liang, M. H.
We reported optical properties of a-plane ZnO/ZnMgO multiple quantum wells (MQWs) structure grown by the pulse laser deposition system. The emission peak energy of a-plane ZnO/ZnMgO MQWs kept invariant in the power-dependent photoluminescence (PL) measurement, indicating the nonpolar characteristics due to the lack of built-in electric fields. Large exciton binding energy of 68 meV was deduced and no apparent S-curve appeared in temperature-dependent PL results, demonstrating less carrier localization effect in a-plane ZnO/ZnMgO MQWs. Large difference in electronic transition levels of 45 meV due to the valence band splitting was observed in the polarization dependent absorption spectrum. Furthermore, the high degree of polarization of 92% and 56% at 20 and 300 K in PL emission of a-plane ZnO/ZnMgO MQWs were obtained. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488898]
Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition
(2008-11-01)
Ko, T. S.
;
Lu, T. C.
;
Wang, T. C.
;
Chen, J. R.
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Gao, R. C.
a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm grown on r-plane sapphire by metal-organic chemical vapor deposition were investigated. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3-nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify that the more uniform and stronger luminescence intensity distribution are observed for the samples of thinner quantum wells. In addition, more effective capturing of excitons due to larger localization energy E(loc) and shorter radiative lifetime of localized excitons are observed in thinner well width samples in the temperature dependent TRPL. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3013435]
Phase transformation and optical characteristics of porous germanium thin film
(2008-03-31)
Ko, T. S.
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Shieh, J.
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Yang, M. C.
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Lu, T. C.
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Kuo, H. C.
In this study, we proposed a method to prepare GeO2 by treating porous Ge thin film with thermal annealing in O-2 ambient. After annealing, the morphological transformation from porous thin film to an island structure was observed. The crystallization and composition of the porous Ge thin film prepared using different annealing time in O-2, ambient were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectra. Initial Ge composition was gradually oxidized to GeO2 with increasing annealing time. Comparing the photoluminescence (PL) results between Ge and GeO2, it was found that the visible photoluminescence originated from the germanium oxide. Photoluminescence measurements obtained at different temperatures exhibited a maximum integrated PL intensity at around 200 K. A possible explanation for this behavior might be the competition between radiative recombination and nonradiative hopping process. (C) 2007 Elsevier B.V. All rights reserved.
Study on optimal growth conditions of a-plane GaN grown on r-plane sapphire by metal-organic chemical vapor deposition
(2007-03-15)
Ko, T. S.
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Wang, T. C.
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Gao, R. C.
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Chen, H. G.
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Huang, G. S.
Non-polar a-plane (1 1 (2) over bar 0) GaN thin films were grown on r-plane (1 (1) over bar 0 2) sapphire substrates by metal-organic chemical vapor deposition. In order to obtain a-plane GaN films with better crystal quality and surface morphology, detailed comparisons between different growth conditions were investigated. The results showed that high-temperature and low-pressure conditions facilitating two-dimensional growth could lead to a fully coalesced a-plane GaN layer with a very smooth surface. The best mean roughness of the surface morphology was 10.5 angstrom. Various thickness values of AlN nucleation layers and the V/III ratios for growth of the a-plane GaN bulk were also studied to determine the best condition for obtaining a smooth surface morphology of the a-plane GaN layer. (c) 2007 Elsevier B.V. All rights reserved.
Synthesis of In2O3 nanocrystal chains and annealing effect on their optical properties
(2007-07-01)
Ko, T. S.
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Chu, C. P.
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Chen, J. R.
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Chang, Y. A.
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Lu, T. C.
The authors reported the synthesis of indium oxide nanocrystal chains on silicon substrates using gold nanoparticles as catalyst on native silicon dioxide. Scanning electron microscopy investigations showed that In2O3 nanocrystal chains were formed. The nanocrystal chains comprised many well shaped octahedron nanoparticles with their size ranging from 20 to 100 nm connected with each other to form a network. Both high resolution transmission, electron microscopy and diffraction pattern revealed that the In2O3 nanocrystal chains were single crystalline grown along the [111] direction. Photoluminescence study of In2O3 nanocrystal chains exhibited main photoemission at 544 nm due to the recombination between electrons in localized state induced by oxygen vacancies and holes in valence band. After the annealing treatment, the main photoluminescence peak not only slightly redshifted to 558 nm but also became narrower, which was attributed to the total localized state away from the conduction band caused by less defect distribution. (c) 2007 American Vacuum Society.
ZnO nanopowders fabricated by dc thermal plasma synthesis
(2006-09-25)
Ko, T. S.
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Yang, S.
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Hsu, H. C.
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Chu, C. P.
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Lin, H. F.
Ultrafine ZnO nanopowders were successfully synthesized by the de thermal plasma process with a high production rate of 1.2 kg/h. The combination of plasma was found to affect the nanoparticles morphology. Both X-ray diffraction and Raman spectrum analysis confirmed a high quality wurtzite structure of the ZnO nanopowders. Photoluminescence study exhibited strong ultraviolet emission corresponding to the near band edge emission in the ZnO nanopowders. Random laser action observed in the ZnO nanostructures by optical pumping demonstrated high optical and crystal quality of the ZnO nanopowders fabricated by the dc thermal plasma synthesis. (c) 2006 Elsevier B.V. All rights reserved.