Browsing by Author "Chang, Jet-Rung"

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Coreshell InGaN/GaN MQW Nanorod Photovoltaic Device
(2015)
Li, Yun-Jing
;
Tsai, Yu Lin
;
Chang, Jet-Rung
;
Chang, Shih-Pang
;
Lin, Chien-Chung
We report the fabrication and study of coreshell InGaN/GaN multiple quantum well (MQW) nanorod photovoltaic device. The nanorods were fabricated from a GaN substrate by top-down etch, followed by InGaN/GaN MQW growth. The 3D geometry allows higher Indium incorporation and extends the solar absorption spectral range as compared with a planar reference sample. The proof-of-concept coreshell nanorod photovoltaic device has a fill factor of about 54.2%, the short current density of 1.16 mA/cm(2), the open circuit voltage of 0.68 V, and the power conversion efficiency of 0.38%, respectively.
Dependence of GaN Defect Structure on the Growth Temperature of the AlN Buffer Layer
(2008)
Wong, Yuen-Yee
;
Chang, Edward Yi
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Yang, Tsung-Hsi
;
Chang, Jet-Rung
;
Chen, Yi-Cheng
The defect structure of the GaN film grown on sapphire by plasma-assisted molecular beam epitaxy (PAMBE) technique was found to be dependent on the AlN buffer layer growth temperature. This buffer growth temperature controlled the defect density in GaN film but had shown contrary effects on the density of screw threading dislocation (TD) and edge TD. The density of screw TD was high on lower temperature buffer but low on the higher temperature buffer. Meanwhile the density of edge TD had shown the opposite. Further examinations have suggested that the defect structure was closely related to the stress in the GaN film, which can be controlled by the growth temperature of the AlN buffer. Using the 525 degrees C AlN buffer, optimum quality GaN Film with relatively low screw and edge TDs were achieved.
Dislocation reduction in GaN film using Ga-lean GaN buffer layer and migration enhanced epitaxy
(2011-07-29)
Wong, Yuen-Yee
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Chang, Edward Yi
;
Wu, Yue-Han
;
Hudait, Mantu K.
;
Yang, Tsung-Hsi
A GaN buffer layer grown under Ga-lean conditions by plasma-assisted molecular beam epitaxy (PAMBE) was used to reduce the dislocation density in a GaN film grown on a sapphire substrate. The Ga-lean buffer, with inclined trench walls on its surface, provided an effective way to bend the propagation direction of dislocations, and it reduced the dislocation density through recombination and annihilation processes. As a result, the edge dislocation density in the GaN film was reduced by approximately two orders of magnitude to 2 x 10(8) cm(-2). The rough surface of the Ga-lean buffer was recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N(2) radicals, during the PAMBE growth. By combining these two methods, a GaN film with high-crystalline-quality and atomically-flat surface can be achieved by PAMBE on a lattice mismatch substrate. (C) 2011 Elsevier B.V. All rights reserved.
Effect of Nitridation on the Regrowth Interface of AlGaN/GaN Structures Grown by Molecular Beam Epitaxy on GaN Templates
(2012-08-01)
Wong, Yuen-Yee
;
Huang, Wei-Ching
;
Trinh, Hai-Dang
;
Yang, Tsung-Hsi
;
Chang, Jet-Rung
AlGaN/GaN structures were regrown on GaN templates using plasma- assisted molecular beam epitaxy (PA-MBE). Prior to the regrowth, nitridation was performed using nitrogen plasma in the MBE chamber for different durations (0 min to 30 min). Direct-current measurements on high-electron-mobility transistor devices showed that good pinch-off characteristics and good interdevice isolation were achieved for samples prepared with a 30-min nitridation process. Current-voltage measurements on Schottky barrier diodes also revealed that, for samples prepared without nitridation, the reverse-bias gate leakage current was approximately two orders of magnitudes larger than that of samples prepared with a 30-min nitridation process. The improvement in the electrical properties is a result of contaminant removal at the regrowth interface which may be induced by the etching effect of nitridation.
Electrically driven green, olivine, and amber color nanopyramid light emitting diodes
(2013-10-07)
Chang, Shih-Pang
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Chang, Jet-Rung
;
Sou, Kuok-Pan
;
Liu, Mei-Chun
;
Cheng, Yuh-Jen
We report the fabrication and studies of electrically driven green, olivine, and amber color nanopyramid GaN light emitting diodes (LEDs). InGaN/GaN multiple quantum wells (MQWs) were grown on the nanopyramid semipolar facets. Compared with the commonly used (0001) c-plane MQWs, the semipolar facet has lower piezoelectric field, resulting in much faster radiative recombination efficiency. This is important for high In content MQWs. The measured internal quantum efficiencies for green, olivine, and amber color LED are 30%, 25%, and 21%, respectively. The radiative and non-radiative lifetime of the semipolar MQWs are also investigated. (C) 2013 Optical Society of America
Epitaxial Overgrowth of Gallium Nitride Nano-Rods on Silicon (111) Substrates by RF-Plasma-Assisted Molecular Beam Epitaxy
(2010)
Ku, Jui-Tai
;
Yang, Tsung-Hsi
;
Chang, Jet-Rung
;
Wong, Yuen-Yee
;
Chou, Wu-Ching
Strain-free gallium nitride (GaN) overgrowth on GaN nano-rods is realized by RF-plasma assisted molecular beam epitaxy (RF-MBE) on silicon (Si) substrate. The strain-free condition was identified by the strong free A exciton (FX(A)) photoluminescence (PL) peak at 3.478 eV and the E(2) high phonon Raman shift of 567 cm(-1). It is clearly demonstrated that the critical diameter of GaN nano-rods is around 80nm for the overgrowth of strain-free GaN. The blue-shift of PL peak energy and phonon Raman energy with decreasing the diameter of nano-rod result from the strain relaxation of overgrowth GaN. (C) 2010 The Japan Society of Applied Physics
Growth of free-standing GaN layer on Si(111) substrate
(2009-03-15)
Yang, Tsung Hsi
;
Ku, Jui Tai
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Chang, Jet-Rung
;
Shen, Shih-Guo
;
Chen, Yi-Cheng
This investigation demonstrates the epitaxial growth of a free-standing GaN layer on Si(1 1 1) using a funnel-like GaN nano-rod buffer structure. The funnel-like GaN nano-rods were directly grown on Si substrates by RF-plasma molecular beam epitaxy. Free-standing GaN layers were achieved through the coalescence of funnel-like GaN nano-rods by the metalorganic chemical vapor deposition. This study examines the structure, optical characteristics and stress of GaN nano-rods and free-standing GaN layers. The c-axis lattice constant of the strain-free Ga-face GaN layer on Si is 5.1844 angstrom, as determined by high-resolution X-ray diffraction. The fully relaxed band edge at 3.468 eV without deep-level emission around 2.3 eV, was revealed in a free-standing GaN layer on Si, using photoluminescence. (C) 2008 Elsevier B.V. All rights reserved.
Highly Efficient and Bright LEDs Overgrown on GaN Nanopillar Substrates
(2011-07-01)
Chiu, Ching-Hsueh
;
Tu, Po-Min
;
Lin, Chien-Chung
;
Lin, Da-Wei
;
Li, Zhen-Yu
We presented a study of high-performance GaN-based light emitting diodes (LEDs) using a GaN nanopillars (NPs) structure grown on sapphire substrate by integrating RF-plasma molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Nanoscale air voids were clearly observed at the interface between GaN NPs and the overgrown GaN layer by cross-sectional scanning electron microscopy. It can increase the light-extraction efficiency due to additional light scattering. The transmission electron microscopy images suggest the air voids between GaN NPs introduced during nanoscale epitaxial lateral overgrowth of GaN can suppress the threading dislocation density. Moreover, Raman spectrum demonstrated that the strain of the GaN layer grown on GaN NPs was effectively eliminated, resulting in the reduction of quantum-confined Stark effect in InGaN/GaN quantum wells. Consequently, the LEDs fabricated on the GaN NPs template exhibit smaller electroluminescent peak wavelength blue shift and great enhancement of the light output (70% at 20 mA) compared with the conventional LEDs.
InGaN/GaN multiple-quantum-well nanopyramid-on-pillar light-emitting diodes
(2015-04-01)
Li, Yun-Jing
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Chang, Jet-Rung
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Chang, Shih-Pang
;
Sou, Kuok-Pan
;
Cheng, Yuh-Jen
We report the study of an electrically driven nanopyramid-on-pillar green light-emitting diode (LED). The pyramid-on-pillar nanostructure was fabricated by top-down etching from a GaN template, followed by epitaxial regrowth. High-In-content InGaN/GaN multiple quantum wells (MQWs) were grown on semipolar pyramid and nonpolar pillar surfaces. The measured radiative lifetimes of MQWs on these two surfaces were 2 orders of magnitude shorter than that of a conventional c-plane (0001) MQW owing to their lower polarization field. Electrical injection performance was also demonstrated and discussed. (C) 2015 The Japan Society of Applied Physics
Investigation of efficiency droop for InGaN-based UV light-emitting diodes with InAlGaN barrier
(2011-05-23)
Tu, Po-Min
;
Chang, Chun-Yen
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Huang, Shih-Cheng
;
Chiu, Ching-Hsueh
;
Chang, Jet-Rung
The efficiency droop in InGaN-based UV light emitting device (LED) with AlGaN and InAlGaN barrier is investigated. Electroluminescence results indicate that the light performance of quaternary LEDs can be enhanced by 25% and 55% at 350 mA and 1000 mA, respectively. Furthermore, simulations show that quaternary LEDs exhibit 62% higher radiative recombination rate and low efficiency degradation of 13% at a high injection current. We attribute this improvement to increasing of carrier concentration and uniform redistribution of carriers. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3591967]
Investigation of Efficiency Droop for UV-LED with N-type AlGaN Layer
(2012-01-01)
Tu, Po-Min
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Chang, Jet-Rung
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Huang, Shih-Cheng
;
Yang, Shun-Kuei
;
Lin, Ya-wen
The efficiency droop in InGaN-based 380nm UV light emitting device (LED) with n-GaN and n-AlGaN underlayer grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD) was investigated. From simulation result of high resolution x-ray diffraction (HRXRD) omega-2 theta. curve by using dynamical diffraction theory, the Al composition in the n-AlGaN layer was determined to be about 3%. The experimental results of temperature dependent photoluminescence (PL) demonstrated that the internal quantum efficiency (IQE) of n-GaN and n-AlGaN UV-LEDs are 43% and 39%, respectively, which are corresponding to an injected carrier density of 8.5 x 10(17) #/cm(3). It could be explained that the crystal quality of n-GaN is better than of n-AlGaN. In addition, the observation of pit density from atomic force microscopy (AFM) surface morphology is consistent with the interpretation. It was well-known that the pits appearing on the surface in the virtue of the threading dislocations. Thus, it means that defects induce the non-radiative centers and deteriorate the IQE of the UV-LED with n-AlGaN underlayer. Therefore, the light output power of n-GaN UV-LED is slightly higher below the forward current of 250 mA. Nevertheless, the output power was enhanced about 22% as the injection current was increased to 600 mA. Furthermore, the external quantum efficiency (EQE) of n-AlGaN UV-LED was nearly retained at the 600 mA (only 20% droop), whereas the UV-LED with n-GaN exhibits as high as 33%. We attributed this improvement to the less self-absoption by replacing n-GaN underlayer with n-AlGaN.
Investigation of Efficiency Droop in GaN-based UV LEDs with N-type AlGaN Underlayer
(2012)
Yang, Shun-Kuei
;
Tu, Po-Min
;
Huang, Shih-Cheng
;
Lin, Ya-wen
;
Hsu, Chih-Peng
The efficiency droop in InGaN-based 380nm UV light emitting device (LED) with n-GaN and n-AlGaN underlayer grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD) was investigated. From simulation result of high resolution x-ray diffraction (HRXRD) omega-2 theta curve by using dynamical diffraction theory, the Al composition in the n-AlGaN layer was determined to be about 3%. The experimental results of temperature dependent photoluminescence (PL) demonstrated that the internal quantum efficiency (IQE) of n-GaN and n-AlGaN UV-LEDs are 43% and 39%, respectively, which are corresponding to an injected carrier density of 8.5 x 10(17) #/cm(3). It could be explained that the crystal quality of n-GaN is better than of n-AlGaN. In addition, the observation of pit density from atomic force microscopy (AFM) surface morphology is consistent with the interpretation. It was well-known that the pits appearing on the surface in the virtue of the threading dislocations. Thus, it means that defects induce the non-radiative centers and deteriorate the IQE of the UV-LED with n-AlGaN underlayer
Lasing action in gallium nitride photonic quasicrystal nanorod arrays
(2012)
Chang, Shih-Pang
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Sou, Kuok-Pan
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Chang, Jet-Rung
;
Cheng, Yuh-Jen
;
Li, Yuh-Jing
We report the observation of lasing action from optically pumped gallium nitride nanorod arrays in a quasicrystal pattern. The nanorods were fabricated from a GaN substrate by nanoimprint patterned etching, followed by epitaxial regrowth to form crystalline facets. The imprint was a 12-fold symmetric quasicrystal pattern. The regrowth grew a multiple quantum well core-shell structure on nanorods. The cathodoluminescent emission of quantum wells red shifts from the bottom to top region of nanorod. Under optical pumping, multiple lasing peaks were observed. The lasing modes formed by 12-fold symmetric photonic quasicrystal nanorod arrays are discussed.
Low efficiency droop of InGaN/GaN blue LEDs with super-lattice active structure
(2012)
Chang, Shih-Pang
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Sou, Kuok-Pan
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Chang, Jet-Rung
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Cheng, Yuh-Jen
;
Li, Yuh-Jing
We report the efficiency droop behaviors of InGaN/GaN blue LEDs with different thickness of GaN quantum barriers (QBs). The droop percentage from efficiency peak to 70 A/cm(2) is only about 10% as reducing the thickness of GaN QBs from 104 angstrom to 33 angstrom. A less carrier localization has been observed from wavelength dependent time resoled photoluminescence measurement as reducing the thickness of GaN QBs. The alleviation of droop percentage may due to more uniform distribution of electron and hole carrier in the active region, which resulted from super-lattice (SL) like active structure. The crystalline quality does not become worse from the results of v-pits density even thickness of GaN QBs is as low as 33 angstrom. The SL like active structure could be a potential structure to alleviate the efficiency droop for the application of solid state general lighting.
Multifacet Microrod Light-Emitting Diode With Full Visible Spectrum Emission
(2016-09)
Li, Yun-Jing
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Chang, Jet-Rung
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Chang, Shih-Pang
;
Lin, Bo-Wen
;
Yeh, Yen-Hsien
We demonstrated a full visible spectrum emission from a 3D multifacet microrod light-emitting diode (LED). The microrods were fabricated by top-down patterned etched and regrowth. Hexagonal {11-20} and {11-22} facets were first formed on the microrods, then gradually transformed to {10-10} and {10-11} facets. This facet evolution was attributed to the growth competition among different crystal planes. The multiple quantum wells grown on the microrods also followed this facet evolution and resulted in broad emission spectrum. The device can have direct white light emission covering full visible spectral range from 460 to 660 nm under electrical injection.
Photoluminescent Study of High Indium Content Nanopyramid Light Emitting Diodes
(2013)
Chang, Shih-Pang
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Chang, Jet-Rung
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Sou, Kuok-Pan
;
Li, Yun-Jing
;
Cheng, Yuh-Jen
The radiative and non-radiative lifetime of high In content nanopyramid GaN LEDs are investigated by time resolved and temperature dependent photoluminescent measurement. The radiative recombination efficiency is much improved compared with the conventional c-plane LED.
Reduction of efficiency droop in InGaN-Based UV Light-Emitting Diodes with InAlGaN Barrier
(2012)
Chiu, Ching-Hsueh
;
Tu, Po-Min
;
Chang, Jet-Rung
;
Chang, Wei-Ting
;
Kuo, Hao-Chung
In this study, we fabricated and compared the performance of LEDs of InGaN-based UV MQWs active region with ternary AlGaN and quaternary InAlGaN barrier layers. HRXRD and TEM measurements show the two barriers are consistent with the lattice, and smooth morphology of quaternary InAlGaN layer can be observed in AFM. The electroluminescence results indicate that the light performance of the InGaN-based UV LEDs can be enhanced effectively when the conventional LT AlGaN barrier layers are replaced by the InAlGaN barrier layers. Furthermore, simulation results show that InGaN-based UV LEDs with quaternary InAlGaN barrier exhibit higher radiative recombination rate about 62% and low efficiency droop about 13% at a high injection current. We attribute this change to a drastic improvement from increasing of carrier concentration and redistribution of carriers, because of reduction of scatterings due to better morphology in the transverse carrier transport through the InGaN/InAlGaN MQWs.
Study of Efficiency Droop in InGaN-based Near-UV LEDs with Quaternary InAlGaN Barrier
(2012)
Tu, Po-Min
;
Huang, Shih-Cheng
;
Lin, Ya-wen
;
Yang, Shun-Kuei
;
Hsu, Chih-Peng
In this study, we demonstrate high efficient near-UV LEDs by replacing low-temperature AlGaN by InAlGaN barrier in active region. The efficiency droop in InGaN-based near-UV LED with AlGaN and InAlGaN barrier is investigated. High-resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM) measurements show the two barriers are consistent with the lattice, and smooth morphology of quaternary InAlGaN layer can be observed in atomic force microscopy (AFM). Electroluminescence results indicate that the light performance of quaternary LEDs can be enhanced by 25 % and 55 % at 350 mA and 1000mA, respectively. Furthermore, simulations show that quaternary LEDs exhibit 62 % higher radiative recombination rate and low efficiency degradation of 13 % at a high injection current. We attribute this improvement to increasing of carrier concentration and more uniform redistribution of carriers.
The Crystalline and Optical Properties of (11(2)over-bar2) Semipolar GaN and InGaN/GaN MQWs on (1(1)over-bar00) M-Sapphire
(2013-01-01)
Li, Yun-Jing
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Chang, Shih-Pang
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Sou, Kuok-Pan
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Chang, Jet-Rung
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Chang, Ruey-Wen
We report the growth temperature effect on the crystalline and optical properties of (11 (2) over bar2) semipolar GaN and InGaN/GaN MQWs. It shows that a lower growth temperature at 1020 degrees C produces better crystalline and optical properties.
The effect of AlN buffer growth parameters on the defect structure of GaN grown on sapphire by plasma-assisted molecular beam epitaxy
(2009-03-01)
Wong, Yuen-Yee
;
Chang, Edward Yi
;
Yang, Tsung-Hsi
;
Chang, Jet-Rung
;
Chen, Yi-Cheng
The defect structure of GaN film grown on sapphire by plasma-assisted molecular beam epitaxy (PAMBE) depends on the growth temperature and thickness of the aluminum nitride (AlN) buffer layer. High-resolution X-ray diffraction was used to measure symmetric (0 0 0 2) and asymmetric (1 0 (1) over bar 2) rocking curve (omega-scans) broadening, which allowed the estimation of screw threading dislocation (TD) and edge TD densities, respectively. For GaN grown on lower-temperature buffer, the density of screw TD was increased while the density of edge TD was decreased. Further examinations revealed that the edge TD was closely related to stress in GaN film and the screw TD was controlled by AlN surface roughness. Since the GaN defect was dominated by edge TD, the total TD was also effectively suppressed with the use of lower-temperature buffer with appropriate thickness. (C) 2009 Elsevier B.V. All rights reserved.