Browsing by Author "Tu, Po-Min"

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Characteristics of Current-Injected GaN-Based Vertical-Cavity Surface-Emitting Lasers
(2011-11-01)
Lu, Tien-Chang
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Wu, Tzeng-Tsong
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Chen, Shih-Wei
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Tu, Po-Min
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Li, Zhen-Yu
This paper reviews the fabrication technology and performance characteristics of current-injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with hybrid distributed Bragg reflectors (DBRs). The GaN-based VCSEL consists of a ten-pair Ta(2)O(5)/SiO(2) top DBR, a 7 lambda-thick optical cavity embedded with 10 InGaN/GaN multiquantum wells, and a 29-pair AlN/GaN bottom DBR. Lasing action is observed under continuous-wave operation at room temperature. The laser characteristics, such as temperature-dependent laser threshold current, emission wavelength, and spontaneous emission coupling factors, have been measured and discussed.
Continuous wave operation of current injected GaN vertical cavity surface emitting lasers at room temperature
(2010-08-16)
Lu, Tien-Chang
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Chen, Shih-Wei
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Wu, Tzeng-Tsong
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Tu, Po-Min
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Chen, Chien-Kang
We report the demonstration of the continuous wave laser action on GaN-based vertical cavity surface emitting lasers at room temperature. The laser structure consists of a ten-pair Ta(2)O(5)/SiO(2) distributed Bragg reflector (DBR), a 7 lambda-thick optical cavity, ten-pairs InGaN/GaN multiquantum wells with an AlGaN electron blocking layer, and a 29-pair AlN/GaN DBR. The laser has a threshold current of about 9.7 mA corresponding to the current density of about 12.4 kA/cm(2) and a turn-on voltage about 4.3 V at 300 K. The lasing wavelength was 412 nm with a linewidth of about 0.5 nm. A spontaneous emission coupling efficiency factor of about 5x10(-3) and the degree of polarization of about 55% were measured, respectively. The laser beam has a narrow divergence angle of about 8 degrees. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483133]
High Q microcavity light emitting diodes with buried AlN current apertures
(2011-07-25)
Cheng, Bo-Siao
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Wu, Yun-Lin
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Lu, Tien-Chang
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Chiu, Ching-Hsueh
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Chen, Cheng-Hung
We demonstrate a GaN-based high-Q microcavity light emitting diode (MCLED) with a buried AlN current aperture. A thin AlN layer is inserted on the InGaN/GaN multiple quantum wells as a current blocking layer and an optical confinement layer. The GaN-based MCLED is composed of a 29-pair GaN/AlN distributed Bragg reflector (DBR), an eight-pair of SiO(2)/Ta(2)O(5) dielectric DBR, and a three-lambda optical thickness InGaN/GaN active region. The current can be injected more effectively in the MCLED with a buried AlN current aperture. The output emission has a dominant emission peak wavelength at 440 nm with a very narrow linewidth of 0.52 nm, corresponding to a cavity Q-value of 846 at a driving current of 5 mA. (C) 2011 American Institute of Physics. [doi:10.1063/1.3617418]
High quality vertical light emitting diodes fabrication by mechanical lift-off technique
(2011)
Tu, Po-Min
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Hsu, Shih-Chieh
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Chang, Chun-Yen
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光電工程學系
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Department of Photonics
We report the fabrication of mechanical lift-off high quality thin GaN with Hexagonal Inversed Pyramid (HIP) structures for vertical light emitting diodes (V-LEDs). The HIP structures were formed at the GaN/sapphire substrate interface under high temperature during KOH wet etching process. The average threading dislocation density (TDD) was estimated by transmission electron microscopy (TEM) and found the reduction from 2x10(9) to 1x10(8) cm(-2). Raman spectroscopy analysis revealed that the compressive stress of GaN epilayer was effectively relieved in the thin-GaN LED with HIP structures. Finally, the mechanical lift-off process is claimed to be successful by using the HIP structures as a sacrificial layer during wafer bonding process.
High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers
(2014-05-01)
Li, Zhen-Yu
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Lee, Chia-Yu
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Lin, Da-Wei
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Lin, Bing-Cheng
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Shen, Kun-Ching
In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-meansquare value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (1012) reflections in the double crystal X-ray rocking curve were similar to 330 and 450 arcs, respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <6 x 10(8) cm(-2). From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaNbased LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of similar to 110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.
Highly Efficient and Bright LEDs Overgrown on GaN Nanopillar Substrates
(2011-07-01)
Chiu, Ching-Hsueh
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Tu, Po-Min
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Lin, Chien-Chung
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Lin, Da-Wei
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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.
Improved Output Power of InGaN-Based Ultraviolet LEDs Using a Heavily Si-Doped GaN Insertion Layer Technique
(2012-02-01)
Chiu, Ching-Hsueh
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Lin, Chien-Chung
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Tu, Po-Min
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Huang, Shih-Cheng
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Tu, Chia-Cheng
In this paper, a high quality ultraviolet lightemitting diodes (UV-LEDs) at 375 nm was developed using a heavy Si-doping technique with metal organic chemical vapor deposition. By using high-resolution X-ray diffraction, the full width at half-maximum of the rocking curve shows that the GaN film inserting a heavily Si-doped GaN layer (Si-HDL) had high crystalline quality. From the transmission electron microscopy image, the threading dislocation density was decreased after inserting a Si-HDL between undoped and n-doped GaN layers by nanoscale epitaxial lateral overgrowth. As a result, a much smaller reverse current and a higher light output were achieved. The improvement of light output at an injection current of 20 mA was enhanced by 40%. Therefore, we can use an in-situ nano pattern without complex photolithography and etching process and improve the internal quantum efficiency of UV-LEDs.
Improved performance of 375 nm InGaN/AlGaN light-emitting diodes by incorporating a heavily Si-doped transition layer
(2012-01-01)
Huang, Shih-Cheng
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Shen, Kun-Ching
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Tu, Po-Min
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Wuu, Dong-Sing
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Kuo, Hao-Chung
High performance 375-nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) was developed using a heavy Si-doping technique with metalorganic chemical vapor deposition (MOCVD). From the transmission electron microcopy (TEM) image, the dislocation density was reduced after inserting a heavily Si-doping growth mode transition layer (GMTL) between un-doped GaN layer and Si-doped Al0.02Ga0.98N contact layer. The internal quantum efficiency (IQE) of the sample with GMTL measured by power-dependent photoluminescence shows 39.4% improved compared with the sample without GMTL. When the vertical type LED chips (size: 1mmx1mm) driving by a 350-mA current, the output powers of the LEDs with and without GMTL were measured to be 286.7 mW and 204.2 mW, respectively. As much as 40.4% increased light output power was achieved. Therefore, using the GMTL to reduce dislocation defects would be a promising prospective for InGaN/AlGaN UV LEDs to achieve high internal quantum efficiency.
Investigation of efficiency droop for InGaN-based UV light-emitting diodes with InAlGaN barrier
(2011-05-23)
Tu, Po-Min
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Chang, Chun-Yen
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Huang, Shih-Cheng
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Chiu, Ching-Hsueh
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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
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Yang, Shun-Kuei
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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
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Tu, Po-Min
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Huang, Shih-Cheng
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Lin, Ya-wen
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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
Large Area of Ultraviolet GaN-Based Photonic Quasicrystal Laser
(2012-04-01)
Chen, Cheng-Chang
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Chiu, Ching-Hsueh
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Tu, Po-Min
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Kuo, Ming-Yen
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Shih, M. H.
In this study, large-area GaN-based photonic quasicrystal (PQC) nanopillars were fabricated on an n-GaN substrate using the nanoimprint lithography (NIL) technique. Under optical pumping condition, a high lasing action from the GaN photonic quasicrystals was observed. The lasing wavelength is at 366nm with a low threshold power density of 0.009 kW/cm(2). To confirm the band-edge lasing mode, the finite-element method (FEM) was used to perform the simulation for the 12-fold symmetry photonic quasicrystal lattices. (C) 2012 The Japan Society of Applied Physics
Light Output Enhancement of GaN-Based Light-Emitting Diodes by Optimizing SiO2 Nanorod-Array Depth Patterned Sapphire Substrate
(2012-04-01)
Chiu, Ching-Hsueh
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Tu, Po-Min
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Chang, Shih-Pang
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Lin, Chien-Chung
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Jang, Chung-Ying
In this study, we investigated high-efficiency InGaN/GaN light-emitting diodes (LEDs) grown on sapphire substrates with SiO2 nanorod arrays (NRAs) of different heights. The GaN film showed an improved crystal quality through X-ray diffraction (XRD) full-width at half-maximum (FWHM), photoluminescence (PL), and cathodoluminescence (CL) measurements. The light output power and internal quantum efficiency (IQE) of the fabricated LEDs were increased when compared with those of conventional LEDs. Transmission electron microscopy (TEM) images suggested that the voids between SiO2 nanorods and the stacking faults introduced during the nanoscale epitaxial lateral overgrowth (NELOG) of GaN can effectively reduce the threading dislocation density (TDD). We believe that the improvements could be attributed to both the enhanced light extraction by utilizing SiO2 NRAs and the improved crystal quality through the NELOG method. We found that the sample with SiO2 NRA structures of 200 nm height can increase the LED output power by more than 70% in our study. (C) 2012 The Japan Society of Applied Physics
Natural substrate lift-off technique for vertical light-emitting diodes
(2014-04-01)
Lee, Chia-Yu
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Lan, Yu-Pin
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Tu, Po-Min
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Hsu, Shih-Chieh
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Lin, Chien-Chung
Hexagonal inverted pyramid (HIP) structures and the natural substrate lift-off (NSLO) technique were demonstrated on a GaN-based vertical light-emitting diode (VLED). The HIP structures were formed at the interface between GaN and the sapphire substrate by molten KOH wet etching. The threading dislocation density (TDD) estimated by transmission electron microscopy (TEM) was reduced to.1 x 10(8) cm(-2). Raman spectroscopy indicated that the compressive strain from the bottom GaN/sapphire was effectively released through the HIP structure. With the adoption of the HIP structure and NSLO, the light output power and yield performance of leakage current could be further improved. (C) 2014 The Japan Society of Applied Physics
Output Power Enhancement of Light Emitting Diodes with Defect Passivation layer
(2010)
Lo, Ming-Hua
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Tu, Po-Min
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Cheng, Yuh-Jen
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Wang, Chao-Hsun
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Hung, Cheng-Wei
We demonstrate high efficiency blue light emitting diodes with defect passivation layers. The defect passivation layers were formed by defect selective wet etching, SiO(2) deposition, and chemical mechanical polishing process. The process does not require photolithography patterning. The threading dislocation density of grown sample was reduced down to similar to 4x10(7) cm(-2). The defect passivated epi-wafer is used to grow light emitting diode (LED) and the output power of the fabricated chip is enhanced by 45% at 20 mA compared to a reference one without using defect passivation.
Reduction of efficiency droop in InGaN-Based UV Light-Emitting Diodes with InAlGaN Barrier
(2012)
Chiu, Ching-Hsueh
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Tu, Po-Min
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Chang, Jet-Rung
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Chang, Wei-Ting
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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 375 nm ultraviolet InGaN/AlGaN light-emitting diodes with heavily Si-doped GaN transition layer in growth mode, internal quantum efficiency, and device performance
(2011-12-15)
Huang, Shih-Cheng
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Shen, Kun-Ching
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Wuu, Dong-Sing
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Tu, Po-Min
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Kuo, Hao-Chung
High performance 375 nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) were demonstrated with inserting a heavy Si-doped GaN transition layer by metal-organic chemical vapor deposition. From transmission electron microcopy (TEM) image, the dislocation densities were significantly reduced due to the existence of the heavily Si-doping growth mode transition layer (GMTL), which results in residual stress relaxation and 3D growth. The internal quantum efficiency (IQE) of the LEDs with GMTL was measured by power-dependent photoluminescence (PL) to be 40.6% higher than ones without GMTL. The GMTL leads to the superior IQE performance of LEDs not only in decreasing carrier consumption at nonradiative recombination centers but also in partially mitigating the efficiency droop tendency. When the vertical-type LED chips (size: 1 mm x 1 mm) was driven with a 350 mA injection current, the output powers of the LEDs with and without GMTL were measured to be 286.7 and 204.2 mW, respectively. A 40.4% enhancement of light output power was achieved. Therefore, using the GMTL to reduce dislocations would be a promising prospective for InGaN/AlGaN UV-LEDs to achieve high IQE. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3669377]
Study of Efficiency Droop in InGaN-based Near-UV LEDs with Quaternary InAlGaN Barrier
(2012)
Tu, Po-Min
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Huang, Shih-Cheng
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Lin, Ya-wen
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Yang, Shun-Kuei
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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 effect of free-standing GaN substrate on carrier localization in ultraviolet InGaN light-emitting diodes
(2014-12-13)
Tsai, Ming-Ta
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Chu, Chung-Ming
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Huang, Che-Hsuan
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Wu, Yin-Hao
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Chiu, Ching-Hsueh
In this study, we have grown 380-nm ultraviolet light-emitting diodes (UV-LEDs) based on InGaN/AlInGaN multiple quantum well (MQW) structures on free-standing GaN (FS-GaN) substrate by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD), and investigated the relationship between carrier localization degree and FS-GaN. The micro-Raman shift peak mapping image shows low standard deviation (STD), indicating that the UV-LED epi-wafer of low curvature and MQWs of weak quantum-confined Stark effect (QCSE) were grown. High-resolution X-ray diffraction (HRXRD) analyses demonstrated high-order satellite peaks and clear fringes between them for the UV-LEDs grown on the FS-GaN substrate, from which the interface roughness (IRN) was estimated. The temperature-dependent photoluminescence (PL) measurement confirmed that the UV-LEDs grown on the FS-GaN substrate exhibited better carrier confinement. Besides, the high-resolution transmission electron microscopy (HRTEM) and energy-dispersive spectrometer (EDS) mapping images verified that the UV-LEDs on FS-GaN have fairly uniform distribution of indium and more ordered InGaN/AlInGaN MQW structure. Clearly, the FS-GaN can not only improve the light output power but also reduce the efficiency droop phenomenon at high injection current. Based on the results mentioned above, the FS-GaN can offer UV-LEDs based on InGaN/AlInGaN MQW structures with benefits, such as high crystal quality and small carrier localization degree, compared with the UV-LEDs on sapphire.