Browsing by Author "Chang, Chia-Jui"

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Effect of Strains and V-Shaped Pit Structures on the Performance of GaN-Based Light-Emitting Diodes
(2020-04-01)
Chen, Shuo-Wei
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Chang, Chia-Jui
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Lu, Tien-Chang
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光電工程學系
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Department of Photonics
Strains and V-shaped pits are essential factors for determining the efficiency of GaN-based light-emitting diodes (LEDs). In this study, we systematically analyzed GaN LED structures on patterned sapphire substrates (PSSs) with two types of growth temperature employed for prestrained layers and three different thickness of n-type GaN layers by using cathodoluminescence (CL), microphotoluminescence (PL), and depth-resolved confocal Raman spectroscopy. The results indicated that V-pits formation situation can be analyzed using CL. From the emission peak intensity ratio of prestrained layers and multiple quantum wells (MQWs) in the CL spectrum, information regarding strain relaxation between prestrained layers and MQWs was determined. Furthermore, micro-PL and depth-resolved confocal Raman spectroscopy were employed to validate the results obtained from CL measurements. The growth conditions of prestrained layers played a dominant role in the determination of LED performance. The benefit of the thick layer of n-GaN was the strain reduction, which was counteracted by an increase in light absorption in thick n-type doped layers. Consequently, the most satisfactory LED performance was observed in a structure with relatively lower growth temperature of prestrained layers that exhibited larger V-pits, leading to higher strain relaxation and thinner n-type GaN layers, which prevent light absorption caused by n-type GaN layers.
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
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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 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
Fabrication and Thermal Dissipation Properties of Carbon Nanofibers Derived from Electrospun Poly(Amic Acid) Carboxylate Salt Nanofibers
(1970-01-01)
Li, Jia-Wei
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Chiu, Yu-Jing
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Chang, Chia-Jui
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He, Hung-Chieh
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Tu, Yi-Hsuan
Polyimides (PIs) possess excellent mechanical properties, thermal stability, and chemical resistance and can be converted to carbon materials by thermal carbonization. The preparation of carbon nanomaterials by carbonizing PI-based nanomaterials, however, has been less studied. In this work, the fabrication of PI nanofibers is investigated using electrospinning and their transformation to carbon nanofibers. Poly(amic acid) carboxylate salts (PAASs) solutions are first electrospun to form PAAS nanofibers. After the imidization and carbonization processes, PI and carbon nanofibers can then be obtained, respectively. The Raman spectra reveal that the carbon nanofibers are partially graphitized by the carbonization process. The diameters of the PI nanofibers are observed to be smaller than those of the PAAS nanofibers because of the formation of the more densely packed structures after the imidization processes; the diameters of the carbon nanofibers remain similar to those of the PI nanofibers after the carbonization process. The thermal dissipation behaviors of the PI and carbon nanofibers are also examined. The infrared images indicate that the transfer rates of thermal energy for the carbon nanofibers are higher than those for the PI nanofibers, due to the better thermal conductivity of carbon caused by the covalent sp(2) bonding between carbon atoms.
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%.
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
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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%.
Intra-Vehicle Network Routing Algorithm for Wiring Weight and Wireless Transmit Power Minimization
(2015)
Huang, Ta-Yang
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Chang, Chia-Jui
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Lin, Chung-Wei
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Roy, Sudip
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Ho, Tsung-Yi
As the complexity of vehicular distributed systems increases rapidly, several hundreds of devices (sensors, actuators, etc.) are being placed in a modern automotive system. With the increase in wiring cables connecting these devices, the weight of a car increases significantly, which degrades the fuel efficiency in driving. In order to reduce the weight of a car, wireless communication has been introduced to replace wiring cables between some devices. However, the extra energy consumption for packet transmissions by wireless devices requires frequent maintenance, e.g., recharging of batteries. In this paper, we propose an intra-vehicle network routing algorithm to simultaneously minimize the wiring weight and the transmit power for wireless communication. Experimental results show that the proposed method can effectively minimize the wiring weight and the transmit power for wireless communication.
Preparation and thermal dissipation of hollow carbon fibers from electrospun polystyrene/poly(amic acid) carboxylate salt core-shell fibers
(2020-05-05)
Li, Jia-Wei
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Hsu, Hsun-Hao
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Chang, Chia-Jui
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Chiu, Yu-Jing
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Tseng, Hsiao-Fan
Polyimides (PIs) possess great mechanical properties, outstanding thermal stability, solvent inertness, and the ability to be converted into carbon by thermal carbonization. Although studies on carbon materials derived from PIs have been conducted, PI-derived one-dimensional (1D) carbon materials with high surface areas, especially carbon tubes or hollow carbon fibers, have been rarely investigated. In this work, we provide a simple and facile method to prepare hollow carbon fibers by carbonizing hollow PI fibers. Blend solutions of poly(amic acid) carboxylate salts (PAAS) and polystyrene (PS) are electrospun to form core-shell PS/PAAS fibers, in which the PS and PAAS domains are used as sacrificial and precursor materials, respectively. By imidizing the PAAS to PI and selectively removing PS, hollow PI fibers can be obtained. Finally, the hollow PI fibers are carbonized to form hollow carbon fibers. The fiber samples at different stages are examined by thermal gravimetric analysis (TGA), Raman spectrometry, Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The infrared images show that the thermal energy transfer rates of the hollow carbon fibers are higher than those of the hollow PI and PS/PI core-shell fibers, which can be attributed to the better thermal conductivity of carbon resulting from the covalent sp(2) bonding between carbon atoms and the high surface area of the hollow structure.
Reproducible and Bendable SERS Substrates with Tailored Wettability Using Block Copolymers and Anodic Aluminum Oxide Templates
(1970-01-01)
Lin, Yu-Liang
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Karapala, Vamsi Krishna
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Shen, Ming-Hui
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Chen, Yi-Fan
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He, Hung-Chieh
Surface properties are essential for substrates exhibiting high sensitivity in surface-enhanced Raman scattering (SERS) applications. In this work, novel SERS hybrid substrates using polystyrene-block-poly(methyl methacrylate) and anodic aluminum oxide templates is presented. The hybrid substrates not only possess hierarchical porous nanostructures but also exhibit superhydrophilic surface properties with the water contact angle approximate to 0 degrees. Such surfaces play an important role in providing uniform enhanced intensities over large areas (relative standard deviation approximate to 10%); moreover, these substrates are found to be highly sensitive (limit of detection approximate to 10(-12) m for rhodamine 6G (R6G)). The results show that the hybrid SERS substrates can achieve the simultaneous detection of multicomponent mixtures of different target molecules, such as R6G, crystal violet, and methylene blue. Furthermore, the bending experiments show that about 70% of the SERS intensities are maintained after bending from approximate to 30 degrees to 150 degrees.
Surface plasmon polariton nanolasers: Coherent light sources for new applications
(2018-11-01)
Chou, Yu-Hsun
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Chang, Chia-Jui
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Lin, Tzy-Rong
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Lu, Tien-Chang
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光電工程學系
The invention of the Internet and mobile devices has caused tremendous changes in human lives over the past two decades. Information technology has broken through limitations of geospatial space, enabling extremely high-speed data transmission and new types of data services. In recent years, demands for data processing have shown an increasing trend. Furthermore, data generated from internet-related applications such as cloud services and self-driving technology are likely to grow exponentially over the coming years. Currently, data transmission inside integrated circuits mainly relies on metal wires. However, the substantial resistive-capacitive delay and energy loss that are caused by metal wires limit data transmission speeds. Optical interconnection has been regarded as a major solution to efficiently reduce energy consumption and increase data transmission speeds. The size of conventional semiconductor laser devices, which are the key component in optical interconnection, cannot be smaller than the wavelength of light, which is a fundamental physical obstacle to lasers integrating with current electronic integrated circuits in reasonable volumes. To realize optical interconnection, the volume of the laser device must match the existing electronic components. Recently, the use of diffraction-unlimited plasmonic lasers has been successfully demonstrated, and these have great potential in different applications. In this paper, we discuss the recent progress toward surface plasmon polariton lasers and provide practical insights into the challenges in realizing these novel devices.