Browsing by Author "Li, Heng"

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3D numerical modeling of the carrier transport and radiative efficiency for InGaN/GaN light emitting diodes with V-shaped pits
(2016-05-01)
Li, Chi-Kang
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Wu, Chen-Kuo
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Hsu, Chung-Cheng
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Lu, Li-Shuo
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Li, Heng
In this paper, influence of a V-pit embedded inside the multiple quantum wells (MQWs) LED was studied. A fully three-dimensional stress-strain solver and Poisson-drift-diffusion solver are employed to study the current path, where the quantum efficiency and turn-on voltage will be discussed. Our results show that the hole current is not only from top into lateral quantum wells (QWs) but flowing through shallow sidewall QWs and then injecting into the deeper lateral QWs in V-pit structures, where the V-pit geometry provides more percolation length for holes to make the distribution uniform along lateral MQWs. The IQE behavior with different V-pit sizes, threading dislocation densities, and current densities were analyzed. Substantially, the variation of the quantum efficiency for different V-pit sizes is due to the trap-assisted nonradiative recombination, effective QW ratio, and ability of hole injections. (C) 2016 Author(s).
3D structural construction of GaN-based light emitting diode by confocal micro-Raman spectroscopy
(2016-01-01)
Li, Heng
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Chang, Chiao-Yun
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Cheng, Hui-Yu
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Chen, Wei-Liang
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Huang, Yi-Hsin
The key issue for light emission strength of GaN-based LEDs is the high defect density and strain in MQWs causing the electric polarization fields. In this work, we construct 3D confocal microspectroscopy to clarify strain distribution and the relationship between photoluminescence (PL) intensity and pattern sapphire substrate (PSS). From 3D construction of E-2(high) Raman and PL mapping, the dislocation in MQW can be traced to the cone tip of PSS and the difference in E-2(high) Raman mapping between substrate and surface is also measured. The ability to measure strain change in 3D structure nondestructively can be applied to explore many structural problems of GaN-based optoelectronic devices.
Design of photonic crystal surface emitting lasers with indium-tin-oxide top claddings
(2018-02-05)
Huang, Shen-Che
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Hong, Kuo-Bin
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Chiu, Han-Lun
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Lan, Shao-Wun
;
Chang, Tsu-Chi
Electrically pumped GaAs-based photonic crystal surface emitting lasers were fabricated using a simple fabrication process by directly capping the indium-tin-oxide transparent conducting thin film as the top cladding layer upon a photonic crystal layer. Optimization of the separate-confinement heterostructures of a laser structure is crucial to improving characteristics by providing advantageous optical confinements. The turn-on voltage, series resistance, threshold current, and slope efficiency of the laser with a 100 x 100 mu m(2) photonic crystal area operated at room temperature were 1.3V, 1.5 Omega, 121mA, and 0.2W/A, respectively. Furthermore, we demonstrated a single-lobed lasing wavelength of 928.6 nm at 200mA and a wavelength redshift rate of 0.05 nm/K in temperature-dependent measurements. The device exhibited the maximum output power of approximately 400 mW at an injection current of 2 A; moreover, divergence angles of less than 1 degrees for the unpolarized circular-shaped laser beam were measured at various injection currents. Overall, the low threshold current, excellent beam quality, small divergence, high output power, and high-operating-temperature (up to 343K) of our devices indicate that they can potentially fill the requirements for next-generation light sources and optoelectronic devices. Published by AIP Publishing.
Double superstructures in InGaN/GaN nano-pyramid arrays
(2015-10-01)
Chang, Chiao-Yun
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Li, Heng
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Hong, Kuo-Bin
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Yang, Ya-Yu
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Lai, Wei-Chih
We have fabricated and investigated InGaN/GaN nanopyramid arrays grown on c-plane sapphire by selective area growth technique. The double InGaN/GaN superstructures were formed in one GaN pyramid observed in transmittance emission microscopy. The two growth faces on GaN nanopyramind showed different growth rates, resulted in double superheterostructures of semi-polar facets for InGaN/GaN multiple quantum wells (MQWs) and c-plane InGaN/GaN nano-hetero-disk (NHD) under the nanoscale growth condition. The c-plane InGaN/GaN NHD had high Indium composition due to the pulling effect, exhibiting double wavelength emissions from GaN nanopyramid. And InGaN/GaN MQWs and c-plane InGaN/GaN NHD showed extremely low quantum confinement Stark effect, yielding great light emission efficiency. This result is beneficial for the development of blue, green and white light-emitting diodes. (C) 2015 Elsevier Ltd. All rights reserved.
Effects of Nanoscale V-Shaped Pits on GaN-Based Light Emitting Diodes
(2017-02-01)
Chen, Shuo-Wei
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Li, Heng
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Chang, Chia-Jui
;
Lu, Tien-Chang
;
光電工程學系
This paper reviews the formation of nanoscale V-shaped pits on GaN-based light emitting diodes (LEDs) grown by the metal organic chemical vapor deposition (MOCVD) system and studies the effect of V-shaped pits on quantum efficiency. Since V-pits could provide potential barriers around threading dislocations to lessen non-radiative recombinations in such a high defect environment. In our study, multiple InGaN/GaN quantum well samples with different emission wavelengths of 380, 420, 460, and 500 nm were grown, each with different nanoscale V-shaped pits of three diameters for 150, 200, and 250 nm, respectively. It was found that the multiple quantum well (MQW) sample with larger V-pits had a lower pit density, but a relatively larger total V-pits defected area. The optimum diameter of V-pits showing the highest quantum efficiency from the MQW sample depended on the emission wavelength. MQW samples with wavelengths of 380 and 500 nm exhibited the best internal quantum efficiency (IQE) performance at the smallest V-pits area; however, the best performance for MQW samples with wavelength around 420 and 460 nm occurred when large V-pit areas were presented. Photoluminescence (PL) peak shifts and Raman shifts can provide a relationship between quantum-confined Stark effect (QCSE) and IQE, as well as a comparison between strain and IQE. The results obtained in this phenomenological study shall provide a useful guide line in making high-performance GaN-based LEDs with wide emission spectra.
Effects of thickness on optical characteristics and strain distribution of thin-film GaN light-emitting diodes transferred to Si substrates
(2016-04)
Li, Heng
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Shi, Yang-Da
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Feng, Meixin
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Sun, Qian
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Lu, Tien-Chang
We investigated the effect of device thickness on the internal quantum efficiency (IQE) of thin-film GaN light-emitting diodes (LEDs), which were grown on Si substrates and transferred to other Si substrates with reduced film thickness. It was confirmed by Raman spectroscopy and photoluminescence measurement that the compressive strain is released and the quantum-confined Stark effect (QCSE) is suppressed after reducing the thickness. The best IQE of 62.9% was reached with a large suppression of the band tilting by QCSE, from 7.9meV in the original structure to 2.4meV in the thinnest sample, and this value can compete with that of GaN-based LEDs grown on a sapphire substrate. (C) 2016 The Japan Society of Applied Physics
Effects of three-dimensional strain distribution on the performance of GaN-based light-emitting diodes on patterned sapphire substrates
(2019-06-01)
Chen, Shuo-Wei
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Li, Heng
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Chang, Chia-Jui
;
Lu, Tien-Chang
;
光電工程學系
Depth-resolved confocal Raman and micro photoluminescence (PL) are adopted to analyze GaN-based light-emitting diodes (LEDs) with an ex situ sputtered AlN nucleation layer on patterned sapphire substrates (PSSs). The strain on PSSs is measured by Raman spectral mapping, and the results demonstrate that the strain distribution within the LED structures strongly correlates to the pattern array of the PSS. Cone regions with less dislocations have lower strain than plane regions. The array pattern of the PSS's surface can directly affect strain existing in the later crystal growth even at the position of the layer before the p-GaN growth. Moreover, the two-dimensional micro-PL mapping of the GaN-based LED samples on a PSS reveals a close relationship with the strain distribution on the sample surface. It is found that the strain as well as the PL intensity difference can be mitigated by proper pre-strained layers in order to realize high efficiency LEDs. (C) 2019 The Japan Society of Applied Physics
Flexible Organometal-Halide Perovskite Lasers for Speckle Reduction in Imaging Projection
(2019-05-01)
Wang, Yu-Chi
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Li, Heng
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Hong, Yu-Heng
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Hong, Kuo-Bin
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Chen, Fang-Chung
Disorder is emerging as a strategy for fabricating random laser sources with very promising materials, such as perovskites, for which standard laser cavities are not effective or too expensive. We need, however, different fabrication protocols and technologies for reducing the laser threshold and controlling its emission. Here, we demonstrate an effectively solvent-engineered method for high-quality perovskite thin films on a flexible polyimide substrate. The fractal perovskite thin films exhibit excellent optical properties at room temperature and easily achieve lasing action without any laser cavity above room temperature with a low pumping threshold. The lasing action is also observed in curved perovskite thin films on flexible substrates. The lasing threshold can be further reduced by increasing the local curvature, which modifies the scattering strengths of the bent thin film. We also show that the curved perovskite lasers are extremely robust with respect to repeated deformations. Because of the low spatial coherence, these curved random laser devices are efficient and durable speckle-free light sources for applications in spectroscopy, bioimaging, and illumination.
Full-Spectrum Analysis of Perovskite-Based Surface Plasmon Nanolasers
(2020-03-29)
Cheng, Pi-Ju
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Zheng, Qi-Yan
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Hsu, Chu-Yuan
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Li, Heng
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Hong, Kuo-Bin
We systematically studied the characteristics of hybrid perovskite-based surface plasmon nanolasers. If one changes the anion composition of perovskites, the emission wavelength can be easily tuned. We conducted in full-spectrum modeling that featured hybrid perovskite nanowires placed on different SiO2-coated metallic (Au, Ag, and Al) plates. The proposed nanocavities that supported plasmonic gap modes exhibited distinguished properties of nanolasers, such as low-transparency threshold-gain and low lasing threshold. The corresponding experimental results for the MAPbBr(3) nanolaser on Ag revealed the low-threshold operation. These superior features were attributed to enhanced light-matter interaction with strong coupling. Therefore, the proposed scheme, integrated with hybrid perovskite as gain material, provides an excellent platform for nanoscale plasmon lasing in the visible to near-infrared spectra.
High efficiency GaN-based near UV light emitting diodes with asymmetric triangular multiple quantum wells
(2018-01-01)
Li, Heng
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Chang, Chia-Jui
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Kuo, Shiou-Yi
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Chen, Jun-Rong
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Lu, Tien-Chang
Near-ultraviolet (NUV) light-emitting diodes (LEDs) have found many applications in the areas such as UV curing, bio-chemical sensors etc. However, the internal quantum efficiency (IQE) of NUV-LEDs show relatively lower value than blue LEDs. In previous research, asymmetric triangular MQWs with gallium face-oriented inclination in the blue wavelength band are demonstrated to have higher emission efficiency and lower efficiency droop. In this study, we surprisingly found different trend in NUV-LEDs. Compared to blue LEDs, NUV-LEDs tend to have shallower quantum wells and less ability to localize holes. In the simulation results, holes are more confined within nitrogen face-oriented inclination than that in MQWs with gallium face-oriented inclination and the IQE are improved about 10%.
High Output Power GaN-Based Green Resonant-Cavity Light-Emitting Diodes With Trapezoidal Quantum Wells
(2020-09-01)
Wu, Haocheng
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Li, Heng
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Kuo, Shiou-Yi
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Chen, Bo-Yan
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Lu, Tien-Chang
Green resonant-cavity light-emitting diode (RCLED) has great potential in optical communication but suffers low efficiency. In this study, GaN-based flip-chip green RCLED incorporated with nitrogen face-oriented inclination asymmetric trapezoidal quantum wells (NOAT-QWs) is proposed to enhance the light-output power (LOP). Samples with NOAT-QWs and normal symmetric square quantum wells (SS-QWs) were fabricated and characterized. Although their electrical characteristics and emission spectra are similar, the LOP and emission efficiency are significantly improved. At a driving current of 450 mA, the improvement of LOP for RCLED with NOAT-QWs can be as high as 1.44 times in comparison to the sample with SS-QWs. The unprecedented high output power of 115 mW and the narrow full-width-at-half-maximum (FWHM) of 6.4 nm in the emission spectrum give great potential in optical communication. The inherent mechanism was investigated by the finite element analysis, from which the simulation results match well with the experimental measurements. The simulation results reveal that the NOAT-QWs are beneficial in alleviating the quantum confined stark effect and facilitating easier hole carriers' flow across the barriers, leading to more electron-hole wave function overlaps and higher radiative recombination rate.
Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers
(2016-04-01)
Chen, Shuo-Wei
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Li, Heng
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Lu, Tien-Chang
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光電工程學系
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Department of Photonics
The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study shows the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Improved Performance of Near UV GaN-Based Light Emitting Diodes With Asymmetric Triangular Multiple Quantum Wells
(2019-02-01)
Li, Heng
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Chang, Chia-Jui
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Kuo, Shiou-Yi
;
Wu, Hao-Cheng
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Huang, Huamao
Near-ultraviolet (NUV) light-emitting diodes (LEDs) have been used in several potential applications such as UV curing and biochemical sensors. However, the internal quantum efficiency (IQE) of NUV-LEDs is still a crucial issue. To improve the IQE, in this paper, an asymmetric triangular multiple quantum well (MQW) structure was used, which exhibited a higher emission efficiency and lower efficiency droop with nitrogen face-oriented inclination. This is in contrast to the trend observed in blue LEDs. Furthermore, we demonstrated that holes are more confined in MQWs with nitrogen face-oriented inclination than in MQWs with gallium face-oriented inclination. Moreover, simulations revealed that the IQE improved by approximately 32% compared with that of symmetric square MQWNUV-LEDs; this trend was also confirmed through experimental results. The external quantum efficiency of thin-film flip-chip LEDs with nitrogen face-oriented inclination MQWs was 52%.
Manipulation of nanoscale V-pits to optimize internal quantum efficiency of InGaN multiple quantum wells
(2015-03-02)
Chang, Chiao-Yun
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Li, Heng
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Shih, Yang-Ta
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Lu, Tien-Chang
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光電工程學系
We systematically investigated the influence of nanoscale V-pits on the internal quantum efficiency (IQE) of InGaN multiple quantum wells (MQWs) by adjusting the underlying superlattices (SLS). The analysis indicated that high barrier energy of sidewall MQWs on V-pits and long diffusion distance between the threading dislocation (TD) center and V-pit boundary were crucial to effectively passivate the non-radiative centers of TDs. For a larger V-pit, the thicker sidewall MQW on V-pit would decrease the barrier energy. On the contrary, a shorter distance between the TD center and V-pit boundary would be observed in a smaller V-pit, which could increase the carrier capturing capability of TDs. An optimized V-pit size of approximately 200-250 nm in our experiment could be concluded for MQWs with 15 pairs SLS, which exhibited an IQE value of 70%. (C) 2015 AIP Publishing LLC.
Metallic subwavelength grating on GaN-based edge emitting lasers
(2015)
Huang, Shen-Che
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Hong, Kuo-Bin
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Li, Heng
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Lu, Tien-Chang
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光電工程學系
Metallic subwavelength gratings as limit-element linear array can provide the great improving output performance of optical light emitter. We investigated that the device with metallic subwavelength grating can be used to modulate the beam of edge-emitting laser with a patterned facet. Utilizing GaN-based semiconductor laser as a model system, the defined metallic subwavelength slits and gratings is a type of superior reduction method of beam divergence and coupling into other optical components effectively. We believe this study will be helpful for development of the novel light emitting devices in the blue or UV region.
Plasmonic nanolasers based on graphene-insulator-metal platform
(2019-01-01)
Li, Heng
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Hong, Kuo-Bin
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Hsu, Chu-Yuan
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Huang, Zhen-Ting
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Lu, Tien-Chang
As the development of nanoscale optics, the ability of surface plasmons to break the diffraction limit receives numerous attention from researchers. In this work, we demonstrate a graphene-insulator-metal (GIM) nanolaser structure. The graphene can modify the permittivity of metal films which would further influence the nanolasers characteristics.
Plasmonic Nanolasers Enhanced by Hybrid Graphene-Insulator-Metal Structures
(2019-08-01)
Li, Heng
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Li, Jhu-Hong
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Hong, Kuo-Bin
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Yu, Min-Wen
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Chun, Yi-Cheng
Graphene is a two-dimensional (2D) structure that creates a linear relationship between energy and momentum that not only forms massless Dirac fermions with extremely high group velocity but also exhibits a broadband transmission from 300 to 2500 nm that can be applied to many optoelectronic applications, such as solar cells, light-emitting devices, touchscreens, ultrafast photodetectors, and lasers. Although the plasmonic resonance of graphene occurs in the terahertz band, graphene can be combined with a noble metal to provide a versatile platform for supporting surface plasmon waves. In this study, we propose a hybrid graphene-insulator-metal (GIM) structure that can modulate the surface plasmon polariton (SPP) dispersion characteristics and thus influence the performance of plasmonic nanolasers. Compared with values obtained when graphene is not used on an Al template, the propagation length of SPP waves can be increased 2-fold, and the threshold of nanolasers is reduced by 50% when graphene is incorporated on the template. The GIM structure can be further applied in the future to realize electrical control or electrical injection of plasmonic devices through graphene.
Rapid separation of gold nanorods in multilayer aqueous systems via centrifugation
(2016-01-01)
Yao, I-Chun
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Chang, Chun-Wei
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Ko, Hao-Wen
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Li, Heng
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Lu, Tien-Chang
In this work, we demonstrate multilayer aqueous systems as media to separate gold nanoparticles of different shapes and sizes. The multilayer aqueous systems are composed of CTAB/EG solutions with different viscosities. By centrifugation, gold nanoparticles with different shapes and sizes can be dispersed in different layers of the aqueous systems. Gold nanorods synthesized by different conditions, such as changing the amount of AgNO3 or different aging times, are used to examine the multilayer aqueous systems. Gold nanorods with higher aspect ratios can be obtained by increasing the concentration of AgNO3, as confirmed by UV-Vis spectra and TEM studies. The separated gold nanorods are also characterized by surface-enhanced Raman scattering (SERS), showing lower activity for gold nanorods with higher aspect ratios, due to the size-dependent absorption of the longitudinal plasmon band. The multilayer aqueous systems can effectively separate gold nanorods prepared by different conditions and can be applied to separate other types of nanoparticles.
Reliability Analysis of GaN-based UVLEDs under forward bias operations in salty vapor environment
(2017-01-01)
Huang, Shen-Che
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Li, Heng
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Lee, Yu-Shan
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Hung, Chen-Hao
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Wang, Shing-Chung
We illustrated the reliability of longer wavelength ultraviolet light-emitting diodes (UVLEDs) in salty vapour environment under manipulation of forward-bias stress via consistent electrical and material characterizations. By manipulating the unprecedented operation in salty vapour ambient, we investigated a series of remarkable findings related to origin of failure behaviours.
Significant Improvement of GaN Crystal Quality with Ex-situ Sputtered AlN Nucleation Layers
(2016-01-01)
Chen, Shuo-Wei
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Yang, Young
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Wen, Wei-Chih
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Li, Heng
;
Lu, Tien-Chang
Ex-situ sputtered AlN nucleation layer has been demonstrated effective to significantly improve crystal quality and electrical properties of GaN epitaxy layers for GaN based Light-emitting diodes (LEDs). In this report, we have successfully reduced X-ray (102) FWHM from 240 to 110 arcsec, and (002) FWHM from 230 to 101 arcsec. In addition, reverse-bias voltage (Vr) increased around 20% with the sputtered AlN nucleation layer. Furthermore, output power of LEDs grown on sputtered AlN nucleation layer can be improved around 4.0% compared with LEDs which is with conventional GaN nucleation layer on pattern sapphire substrate (PSS).