Browsing by Author "Wang, S. C."

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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.
;
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.
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]
Characteristics of emission polarization in a-plane nanorods embedded with InGaN/GaN multiple quantum wells
(2010-09-15)
Li, J. C.
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Lu, T. C.
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Huang, H. M.
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Chan, W. W.
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Kuo, H. C.
Nonpolar (a-plane) GaN nanorod arrays with embedded In(x)Ga(1-x)N/GaN (x=0.09, 0.14, 0.24, and 0.30) multiple quantum wells (MQWs) grown on r-plane sapphire have been fabricated successfully by self-assembled Ni nanomasks and subsequent inductively coupled plasma reactive-ion etching. After nanorod fabrications, the polarization ratio of the emission from MQWs with lower indium composition (x=0.09 and 0.14) slightly decreases but apparently increases by at most 79% for the samples with higher indium composition (x=0.24 and 0.30). Competition between the effect of multiple scattering, strain relaxation and reduction in localized centers, expected in a-plane MQW samples, are attributed to the variations in the polarization ratios after the nanorod formation. (c) 2010 American Institute of Physics. [doi:10.1063/1.3483239]
Characteristics of emission polarization in a-plane nanorods embedded with InGaN/GaN multiple quantum wells
(2010-09-15)
Li, J. C.
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Lu, T. C.
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Huang, H. M.
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Chan, W. W.
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Kuo, H. C.
Nonpolar (a-plane) GaN nanorod arrays with embedded In(x)Ga(1-x)N/GaN (x=0.09, 0.14, 0.24, and 0.30) multiple quantum wells (MQWs) grown on r-plane sapphire have been fabricated successfully by self-assembled Ni nanomasks and subsequent inductively coupled plasma reactive-ion etching. After nanorod fabrications, the polarization ratio of the emission from MQWs with lower indium composition (x=0.09 and 0.14) slightly decreases but apparently increases by at most 79% for the samples with higher indium composition (x=0.24 and 0.30). Competition between the effect of multiple scattering, strain relaxation and reduction in localized centers, expected in a-plane MQW samples, are attributed to the variations in the polarization ratios after the nanorod formation. (c) 2010 American Institute of Physics. [doi:10.1063/1.3483239]
Design and fabrication of a InGaN vertical-cavity surface-emitting laser with a composition-graded electron-blocking layer
(2014-08-01)
Lin, B. C.
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Chang, Y. A.
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Chen, K. J.
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Chiu, C. H.
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Li, Z. Y.
In this study, the design and fabrication of a InGaN vertical-cavity surface-emitting laser (VCSEL) with a composition-graded electron blocking layer (GEBL) are revealed experimentally and theoretically. It has been demonstrated that laser output performance is improved by using a GEBL when compared to the typical VCSEL structure of a rectangular EBL. The output power obtained at 20 kA cm(-2) is enhanced by a factor of 3.8 by the successful reduction of threshold current density from 12.6 to 9.2 kA cm(-2) and the enlarged slope efficiency. Numerical simulation results also suggest that the improved laser output performances are due mainly to the reduction of electron leakage current and the enhanced hole injection efficiency in the multiple-quantum-well (MQW) active region.
Dichromatic InGaN-based white light emitting diodes by using laser lift-off and wafer-bonding schemes
(2007-04-16)
Lee, Y. J.
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Lin, P. C.
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Lu, T. C.
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Kuo, H. C.
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Wang, S. C.
An InGaN-based dual-wavelength blue/green (470 nm/550 nm) light emitting diode (LED) with three terminal operations has been designed and fabricated by using sapphire laser lift-off and wafer-bonding schemes. The device is equivalent to a parallel connection of blue and green LEDs; thus the effective electrical resistance of the device could be reduced. The luminous efficiency is 40 lm/W at 20 mA, accompanied by a broad electroluminescence emission with a combination of blue and green colors. This monolithically integrated dichromatic lighting structure has great potential in the application of the solid-state lighting. (c) 2007 American Institute of Physics.
Effect of adding Sc and Zr on grain refinement and ductility of AZ31 magnesium alloy
(2008-02-01)
Wang, S. C.
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Chou, C. P.
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機械工程學系
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Department of Mechanical Engineering
Grain size in the microstructure dominates the ductility of Mg-Al-Zn magnesium alloy. The strengthening and grain refinement of aluminum alloys by Sc have been extensively studied recently. As is well known, Zr refines grains in magnesium alloys. This work presents a novel approach for enhancing the grain refinement of magnesium alloy by adding Sc and Zr. Rolling and equal channel angular extrusion (ECAE) are performed to refine the grains. Experimental results indicate that adding 0.03-0.06 wt% of Sc and 0.05-0.15 wt% of Zr reduced the mean grain size of AZ31 to 2.47 mu m. The ductility of AZ31-0.15Zr-0.06Sc modified alloy can be increased to 324% at 300 degrees C. Results of this study demonstrate that both Sc and Zr are important in controlling grain size and ductility. (C) 2007 Elsevier B.V. All rights reserved.
Effect of growth conditions on the Al composition and quality of AlGaN film
(2007-01-15)
Huang, G. S.
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Yao, H. H.
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Kuo, H. C.
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Wang, S. C.
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光電工程學系
Effects of growth conditions on Al composition and quality of AlxGa1-xN epilayer grown by low-pressure matelorganic vapor phase epitaxy (MOVPE) have been investigated. The dependences of Al composition, growth rate and quality on the NH3 flow rate, TMAl flow rate and growth temperature were studied. The Al composition and quality of AlxGa1-xN film depend not only on the TMAl flow rate, but also on the NH3 flow rate and on the growth temperature. The Al composition of AlxGa1-xN film becomes saturated when the gas-phase composition TMAl/(TMAl + TMGa) increases to 0.4, while quality of AlxGa1-xN film becomes much worse. The Al composition of AlxGa1-xN film increases with increase in the NH3 flow rate. The quality of AlxGa1-xN epilayer is improved when the growth temperature increases. Possible Al incorporation mechanism is discussed. (c) 2006 Elsevier B.V. All rights reserved.
Effect of nitrogen contents on the temperature dependence of photoluminescence in InGaAsN/GaAs single quantum wells
(2006-07-01)
Lai, Fang-I
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Kuo, S. Y.
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Wang, J. S.
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Kuo, H. C.
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Wang, S. C.
A series of InGaAsN/GaAs single-quantum wells (SQWs) with N contents varied from 0% to 5.3% were grown by molecular-beam epitaxy using a solid As and a nitrogen plasma sources. The impact of nitrogen concentration on the optical properties, as determined by the temperature dependence of photoluminescence (PL), of a 6 nm SQW was investigated. In the low-temperature region, a pronounced temperature-dependent S-shaped peak position was observed in PL spectra while increasing the nitrogen concentration. Quenching behavior reveals that the defect-related nonradiative processes might be enhanced in the highly nitrogen incorporated samples and thus influence the recombination dynamics. In addition, the evolution of the peak position of the InGaAsN/GaAs samples was in agreement with the empirical Varshni model in the high-temperature region. A significant reduction in the temperature dependence of the emission peak position is analyzed as well, and further confirms the prediction of proposed band anticrossing model of the electronic structure of III-N-V alloys. (c) 2006 American Vacuum Society.