Browsing by Author "Kuo, H. C."

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2-Step self-assembly method to fabricate broadband omnidirectional antireflection coating in large scale
(2011-02-01)
Yeung, K. M.
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Luk, W. C.
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Tam, K. C.
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Kwong, C. Y.
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Tsai, M. A.
We have developed a simple and reproducible 2-step self-assembly method for the fabrication of broadband, omnidirectional antireflection coating on glass substrate with 4 in. size. The glass surface has been modified to be positively charged, then the negatively charged nano silica is self-assembled to the glass substrate by electrostatic attraction. The nanostructure on the glass substrate reduces the reflection significantly, which results in enhanced transmittance. Transmittance as high as 97.7% at 499 nm has been obtained for a double-side coated glass substrate. Obvious reduction in weighted reflectance is still observed up to 60 degrees incident angle. (C) 2010 Elsevier B.V. All rights reserved.
40 GHz Phase Shifter based on Semiconductor Laser
(2008-04-10)
Peng, P. C.
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Wu, F. M.
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Lin, C. T.
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Chen, J. H.
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Kao, W. C.
This investigation experimentally demonstrates a 40 GHz phase shifter based on a semiconductor laser. The phase shift is achieved by adjusting the bias current and wavelength detuning. The device has an optical delay of 23 ps and a phase shift of about 331 degrees.
A Full-Color, White Light Emission of Quantum-Dot-Based Display Technology Using Pulsed Spray Method with Distributed Bragg Reflector
(2012)
Chen, K. J.
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Chen, H. C.
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Lin, C. C.
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Tsai, H. H.
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Tsai, K. A.
A colloidal quantum dot light-emitting multi-layer structure with HFO2/SiO2 distributed Bragg reflector is manufactured by pulse spray method. A pure white light with strong enhancement due to better utilization of UV pump was demonstrated. (C)2012 Optical Society of America
A HIGHLY EFFICIENT HYBRID CDS-GAAS SOLAR CELL
(2012)
Chen, H. C.
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Lin, C. C.
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Tsai, Y. L.
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Han, H. V.
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Yu, Peichen
A novel design of hybrid solar cell combining traditional GaAs-based solar cell with colloidal CdS quantum dots (ODs) is demonstrated. By applying CdS ODs on the surface of GaAs device, we can obtain an effective antireflection coating at long wavelength. Moreover, due to its strong absorption at ultraviolet range and efficient light emission at blue photon, CdS quantum dots can down-convert the high-energy UV photons into visible ones, which are more easily to be collected in GaAs solar cell. The short circuit current and power conversion efficiency of this new device increases by 21% and 18.9%, respectively, when we compare them to the uncoated GaAs solar cell. An increase of the surface photoconductivity was also detected due to UV presence, and the fill factor of the CdS ODs solar cell can be improved accordingly.
Abnormal PL spectrum in InGaN MQW surface emitting cavity
(2008-01-01)
Chu, J. T.
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Cheng, Y. -J.
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Kuo, H. C.
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Lu, T. C.
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Wang, S. C.
We report the observation of an abnormal photoluminescent (PL) spectrum from a HeCd laser pumped InGaN multiple quantum well (MQW) vertical cavity. The device is fabricated using standard MOCVD deposition on a (0001)-oriented sapphire substrate. The layer structures are: 10nm nucleation layer, a 4um bulk GaN layer, InGaN MQWs, and a final 200nm GaN cap layer. The InGaN MQWs consist of 10 pairs of 5 nm GaN barrier and 3 run In0.1Ga0.9N well. The peak emission of the as-grown MQWs sample was similar to 420nm. A dielectric distributed Bragg reflectors (DBR) were then coated on the top layer, followed by a laser lift off from sapphire substrate, and subsequently another DBR coated on the bulk GaN bottom surface. The cavity has a quality factor of similar to 520 from 400-490nm. The device was pumped by a focused CW HeCd laser from the bulk GaN side. When the laser is focused onto the InGaN MQWs, a photoluminescent spectrum centered at the designed MOW wavelength was observed as expected. However, when the focused position was moved toward the bulk GaN region, an additional abnormal PL peak around 460nm was observed. This is far outside the designed MQW wavelength.
Abnormal PL spectrum in InGaN MQW surface emitting cavity - art. no. 69080L
(2008)
Chu, J. T.
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Cheng, Y. -J.
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Kuo, H. C.
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Lu, T. C.
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Wang, S. C.
We report the observation of an abnormal photoluminescent (PL) spectrum from a HeCd laser pumped InGaN multiple quantum well (MQW) vertical cavity. The device is fabricated using standard MOCVD deposition on a (0001)-oriented sapphire substrate. The layer structures are: 10nm nucleation layer, a 4um bulk GaN layer, InGaN MQWs, and a final 200nm GaN cap layer. The InGaN MQWs consist of 10 pairs of 5 nm GaN barrier and 3 run In0.1Ga0.9N well. The peak emission of the as-grown MQWs sample was similar to 420nm. A dielectric distributed Bragg reflectors (DBR) were then coated on the top layer, followed by a laser lift off from sapphire substrate, and subsequently another DBR coated on the bulk GaN bottom surface. The cavity has a quality factor of similar to 520 from 400-490nm. The device was pumped by a focused CW HeCd laser from the bulk GaN side. When the laser is focused onto the InGaN MQWs, a photoluminescent spectrum centered at the designed MOW wavelength was observed as expected. However, when the focused position was moved toward the bulk GaN region, an additional abnormal PL peak around 460nm was observed. This is far outside the designed MQW wavelength.
AlGaN/GaN Multiple Quantum Wells Grown By Atomic Layer Deposition
(2008)
Lo, M. H.
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Chen, S. W.
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Li, Z. Y.
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Lu, T. C.
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Kuo, H. C.
Three-pair AlGaN/GaN multiple quantum well structure with superlattice was grown using metal-organic chemical vapor deposition system. The AlGaN barrier and GaN well of the MQW were grown by atmoic layer deposition and conventional growth, respectively. (C) 2008 Optical Society of America
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.
Analysis of Strain Relaxation and Emission Spectrum of A Free-Standing GaN-based Nanopillar
(2008-01-01)
Wu, Yuh-Renn
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Yu, Peichen
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Chiu, C. H.
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Chang, Cheng-Yu
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Kuo, H. C.
We have made a, GaN-based single nanopillar with a, diameter of 300nm using the focused ion beam (FB) technique. The micro-photoluminescence (mu-PL) from the embedded GaN/InGaN multi-quantum wells reveals a blue shift of 68.3 meV in energy. In order to explain the spectrum shift, we have developed a valence force field model to study the strain relaxation mechanism in a, single GaN-based nanopillar structure. The strain distribution and strain induced polarization effect inside the multiple quantum wells is added to our self-consistent Poisson, drift-diffusion, and Schrodinger solver to study the spectrum shift of mu-PL.
Anisotropy of light extraction from two-dimensional photonic crystal light-emitting diodes
(2007-09-17)
Lai, Chun-Feng
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Kuo, H. C.
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Chao, C. H.
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Hsueh, H. T.
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Wang, J.-F. T.
Anisotropic light extraction of photonic crystal (PhC) light-emitting diodes in the azimuthal direction has been investigated with an annular structure of triangular PhC lattice. The optical images of the photoluminescence light extraction are obtained with laser excitation. For increasing lattice constant, sixfold symmetric patterns with varying numbers of petals in multiples of six are observed and analyzed. A map of the anisotropy for various lattice constants and numerical apertures is constructed. Several features of light propagations associated with the PhC are observed including the focusing and collimating behaviors.(c) 2007 American Institute of Physics.
Antireflection and Light Trapping of Periodic Subwavelength Surface Structures Formed by Colloidal Monolayer Spheres Lithography on Thin Film Solar Cells
(2010)
Tseng, P. C.
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Tsai, M. A.
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Yu, Peichen
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Kuo, H. C.
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光電工程學系
Solar cell efficiency enhancement by gradient periodic TiO(2) nanoislands formed with the facilitation of polystyrene-spheres was studied. The enhancement mechanism was categorized by antireflection and light-trapping and achieves 72% relative increase of short-circuit current density. @ 2010 Optical Society of America
Azimuthal anisotropy of light extraction from photonic crystal light-emitting diodes
(2008)
Lai, Chun-Feng
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Kuo, H. C.
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Chao, C. H.
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Hsueh, H. T.
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Wang, J. -F. T.
Photonic crystal (PhC) light-emitting diodes (LEDs) exhibiting anisotropic light extraction have been investigated experimentally and theoretically. It is found that the anisotropic light extraction strongly depends on the lattice constant and orientation. Optical images of the anisotropy in the azimuthal direction are obtained using annular structure with triangular lattice. 6-fold symmetric light extraction patterns with varying number of petals are observed. More petals in multiple of 6 appear in the observed image with lattice constant increasing. This anisotropic behavior suggests a new means to optimize the PhC design of GaN LED for light extraction.
Azimuthal anisotropy of light extraction from photonic crystal light-emitting diodes
(2007)
Hsuch, H. T.
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Wang, J. -F. T.
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Chao, C. H.
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Yeh, W. Y.
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Chi, J. Y.
Photonic crystal light-emitting diodes exhibiting anisotropic light extraction efficiency have been investigated experimentally and theoretically. Images of the anisotropy in the azimuthal direction are obtained using annular structures with triangular lattice. Depending on the lattice constants, 6-fold symmetric patterns with varying number of petals are obtained.
Beam Shaping of GaN/InGaN Vertical-Injection Light Emitting Diodes via High-Aspect-Ratio Nanorod Arrays
(2009)
Tsai, M. A.
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Yu, Peichen
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Chao, C. L.
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Chiu, C. H.
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Kuo, H. C.
The enhanced light extraction and collimated output beam profile from GaN/InGaN vertical-injection light emitting diodes are demonstrated utilizing high-aspect-ratio nanorod arrays. The nanorod arrays are patterned by self-assembled silica spheres, followed by inductively-coupled-plasma reactive-ion-etching. (C) 2008 Optical Society of America
Blue GaN-based vertical cavity surface emitting lasers by CW current injection at 77K
(2009)
Kuo, H. C.
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Chen, S. W.
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Kao, T. T.
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Kao, C. C.
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Chen, J. R.
In the paper, we describe the fabrication and performance characteristics of GaN-based vertical-cavity surface-emitting lasers (VCSELs) by optical pumping and current injection. According to the employment of high-quality and high-reflectivity AlN/GaN DBRs in the whole structure, the lasing action of optically pumped GaN-based VCSELs with hybrid mirrors has been observed at room temperature. Due to the excellent results of optically pumped GaN-based VCSELs with hybrid mirrors, we further demonstrated the lasing behavior of GaN-based VCSELs by continuous-wave current injection at 77 K. The laser has one dominated blue wavelength located at 462 nm with a linewidth of about 0.15 nm and the threshold injection current at 1.4 mA. The divergence angle and polarization ratio of the GaN-based VCSELs with hybrid mirrors are about 11.7 degrees and 80 %, respectively. A larger spontaneous coupling efficiency of about 7.5x10(-2) was also measured.
Broad Angular and Spectral Anti-Reflection Employing GaN Nano-Pillar Structures
(2008)
Chiu, C. H.
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Yu, Peichen
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Chen, C. C.
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Kuo, H. C.
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Lu, T. C.
Angular and spectral reflectivities of GaN nano-pillar structures are investigated for heights of 350nm, 550nm and 720 nm. Calculations based on rigorous coupled-wave analysis show excellent agreement with the measured reflectivities for s- and p-polarizations. (c) 2008 Optical Society of America
Broadband and omnidirectional antireflection employing disordered GaN nanopillars
(2008-06-09)
Chiu, C. H.
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Yu, Peichen
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Kuo, H. C.
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Chen, C. C.
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Lu, T. C.
Disordered GaN nanopillars of three different heights: 300, 550, and 720 nm are fabricated, and demonstrate broad angular and spectral anti-reflective characteristics, up to an incident angle of 60 degrees and for the wavelength range lambda=300-1800nm. An algorithm based on a rigorous coupled-wave analysis (RCWA) method is developed to investigate the correlations between the reflective characteristics and the structural properties of the nanopillars. The broadband and omnidirectional antireflection arises mainly from the refractive-index gradient provided by nanopillars. Calculations show excellent agreement with the measured reflectivities for both s- and p-polarizations. (c) 2008 Optical Society of America.
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.
Characteristics of efficiency droop in GaN-based light emitting diodes with an insertion layer between the multiple quantum wells and n-GaN layer
(2010-12-20)
Chang, S. P.
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Wang, C. H.
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Chiu, C. H.
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Li, J. C.
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Lu, Y. S.
We have studied the characteristics of efficiency droop in GaN-based light emitting diodes (LEDs) with different kinds of insertion layers (ILs) between the multiple quantum wells (MQWs) layer and n-GaN layer. By using low-temperature (LT) (780 degrees C) n-GaN as IL, the efficiency droop behavior can be alleviated from 54% in reference LED to 36% from the maximum value at low injection current to 200 mA, which is much smaller than that of 49% in LED with InGaN/GaN short-period superlattices layer. The polarization field in MQWs is found to be smallest in LED with InGaN/GaN SPS layer. However, the V-shape defect density, about 5.3 x 10(8) cm(-2), in its MQWs region is much higher than that value of 2.9 x 108 cm(-2) in LED with LT n-GaN layer, which will lead to higher defect-related tunneling leakage of carriers. Therefore, we can mainly assign this alleviation of efficiency droop to the reduction of dislocation density in MQWs region rather than the decrease of polarization field. (C) 2010 American Institute of Physics. [doi:10.1063/1.3531957]