Browsing by Author "Wang, S. Y."

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Ablation of Copper by a Single Ultrashort Laser Pulse
(2014-06-01)
Wang, S. Y.
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Ren, Y. P.
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Chang, K. P.
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Cheng, C. W.
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Chen, J. K.
Thermal ablation of copper films by a single Ti:Sapphire femtosecond laser pulse of wavelength 800 nm and duration 100 fs was investigated experimentally and theoretically. The laser experiments were performed; the ablation depth and crater profiles were measured for laser fluences up to 1037 J/cm(2). A comprehensive axisymmetric model, including a two-temperature model, phase change models for rapid melting and evaporation, and a phase explosion model for ejecting metastable liquid and vapor, was developed to simulate the laser material ablation process. The simulated ablation depths and crater profiles agree well with the experimental measurements.
Characteristics of In(Ga)As quantum ring infrared photodetectors
(2009-02-01)
Ling, H. S.
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Wang, S. Y.
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Lee, C. P.
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Lo, M. C.
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電子工程學系及電子研究所
Characteristics of In(Ga)As quantum ring infrared photodetectors (QRIPs) were investigated under normal incidence configuration. Compared with quantum dot infrared photodetectors (QDIPs), QRIPs showed wider photocurrent spectra, more stable responsivity with temperature change, and lower dark current activation energy. The wide detection band comes from the transitions from the quantum ring (QR) ground states to different excited states. The shallow confinement states generate higher dark current and enhance the carrier flow between the QRs within the same QR layer. This carrier flow averages out the repulsive potential and makes QRIPs behave similarly to the quantum well infrared photodetectors instead of QDIPs. With an Al(0.27)Ga(0.73)As current blocking layer, the performance of QRIPs was greatly enhanced.
Confinement-enhanced dots-in-a-well QDIPs with operating temperature over 200 K
(2009-11-01)
Ling, H. S.
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Wang, S. Y.
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Lee, C. P.
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Lo, M. C.
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電子工程學系及電子研究所
LWIR InAs/Al(0.3)Ga(0.7)As/In(0.15)Ga(0.85)As confinement-enhanced DWELL (CE-DWELL) QDIPs with operation temperatures higher than 200 K are reported. A thin Al(0.3)Ga(0.7)As barrier layer was inserted above the InAs QDs to improve the confinement of QD states in the In(0.15)Ga(0.85)As DWELL structure and the device performance. The better confinement of the electronic states increases the oscillator strength of the infrared absorption. The higher excited state energy also improves the escape probability of the photoelectrons. Compared with the conventional DWELL QDIPs, the quantum efficiency increases for more than 20 times and the detectivity is an order of magnitude higher at 77 K. With better device parameters of CE-DWELL, it is possible to achieve high quantum efficiency, high operating temperature and long wavelength detection at the same time. (C) 2009 Elsevier B.V. All rights reserved.
Detection wavelength and device performance tuning of InAs QDIPs with thin AlGaAs layers
(2009-11-01)
Wang, S. Y.
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Ling, H. S.
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Lo, M. C.
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Lee, C. P.
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電子工程學系及電子研究所
QDIPs with thin inserted AlGaAs layers adjacent to the QDs were investigated for the tailoring of detection wavelength and device performance. Simple InAs/GaAs QDIPs and DWELL QDIPs with different insertion layer structure were studied. The thin AlGaAs layer is shown to effectively modify the electron wavefunction and associated confined state energies which lead to the change of the detection wavelength and the polarization dependent quantum efficiency. Furthermore. the dark current and gain also change with different device structures. The insertion of AlGaAs layers provides an additional freedom of tuning the electronics states involved in the infrared detection and also enables the improvement of the absorption and device performance. (C) 2009 Elsevier B.V. All rights reserved.
Dynamic RCST grouping for achieving high link capacities in mobile DVB-RCS networks
(2007)
Chou, C. L.
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Wang, S. Y.
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資訊工程學系
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Department of Computer Science
A DVB-RCS system provides bidirectional connections between service providers and end users. Service providers deliver broadcast data through the forward link to end users while end users send back their data through the return link. The return link is composed of multiple access channels, which can be accessed by the MF-TDMA scheme. Over each channel, a time-slot includes a guard time duration for accommodating propagation delay uncertainties among RCSTs accessing the same channel. In a system with mobile RCSTs, the guard time duration may take a considerable portion of a time-slot for avoiding data collision. Guard time represents bandwidth overhead of the system and large guard time causes significant link capacity reduction. In this paper, we first investigate the problem of link capacity reduction when the DVB-RCS standard, which is originally designed for fixed RCSTs, is directly used with mobile RCSTs without modification. Then, we propose a dynamic RCST grouping mechanism for DVB-RCS to achieve high link capacity with mobile RCSTs.
Energy dependent carrier relaxation in self-assembled InAs/GaAs quantum dots
(2008)
Ling, H. S.
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Lee, C. P.
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Wang, S. Y.
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Lo, M. C.
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電子工程學系及電子研究所
We performed the selective excitation photoluminescence (SEPL) spectroscopy studies on InAs/GaAs self-assembled quantum dots (QDs). Under different excitation energies, different groups of QDs are selected and emit light. The excited carriers relax to the ground state through different mechanisms when excited at different energies. Three distinct regions with different mechanisms in carrier excitations and relaxation are identified in the emission spectra. These three regions can be categorized, from high energy to low energy, as continuum absorption, electronic state excitation, and multi-phonon resonance. The QDs special joint density-of-state tail extending from the wetting layer peak facilitates the carrier relaxation and was suggested to explain these spectral results. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
EVALUATING INFORMATION QUALITY AND VALIDITY OF VALUE LINE STOCK RATINGS USING DOMINATION CONES
(2013-09-01)
Wang, S. Y.
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Yu, P. L.
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資訊管理與財務金融系註：原資管所+財金所
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Department of Information Management and Finance
Investment in stock market involves many decision criteria and variables; hence investors are increasingly relying on the ratings provided by rating agencies to guide their stock selections. However, do these stock ratings have information value? Are these agencies' ratings valid? We establish the dominance cone principle and use the Value Line stock ratings to demonstrate the application of the dominance principle. Our results based upon limited data show that the Value Line rating does not support the notion that better rating results in better rate of return during 2006-2007.
Fabricating a n(+)-Ge contact with ultralow specific contact resistivity by introducing a PtGe alloy as a contact metal
(2015-09-14)
Hsu, C. C.
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Chou, C. H.
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Wang, S. Y.
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Chi, W. C.
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Chien, C. H.
In this study, we developed an Ohmic contact structure to an in situ n(+)-Ge film that has an ultralow specific contact resistivity of [(6.8 +/- 2.1) x 10(-8) Omega.cm(2)]. This structure was developed by introducing a PtGe alloy as the contact metal. We observed that Ohmic contact behavior can be achieved with several other metals, and the contact resistance is related to the work function of the metal. A physical model of the band diagram was created for the Schottky tunneling width, which can provide insight into the validation and explanation of work function-dependent specific contact resistivity. Dopant segregation at the interface and increased interface roughness induced by the formation of the alloy are crucial in further reducing the specific contact resistivity. As a result, a stable PtGe alloy and high doping concentration in Ge are critical in pursuing a lower contact resistance for a Ge n-channel device. (C) 2015 AIP Publishing LLC.
Fabricating a n(+)-Ge contact with ultralow specific contact resistivity by introducing a PtGe alloy as a contact metal (vol 107, 113503, 2015)
(2015-12-07)
Hsu, C. C.
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Chou, C. H.
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Wang, S. Y.
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Chi, W. C.
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Chien, C. H.
Femtosecond laser ablation of copper at high laser fluence: Modeling and experimental comparison
(2016-01-15)
Cheng, C. W.
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Wang, S. Y.
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Chang, K. P.
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Chen, J. K.
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機械工程學系
Thermal ablation of a copper foil surface by a single femtosecond laser pulse of duration 120 fs and wavelength 800 nm was investigated herein both theoretically and experimentally. A 1D two-temperature model with temperature-dependent material properties was considered, including the extended Drude model for dynamic optical properties. The rapid phase change and phase explosion models were incorporated to simulate the material ablation process. The simulated ablation depths agree well with the experimental measurements for the high laser fluence regime ranging from 6.1 to 63.4 j/cm(2). (C) 2015 Elsevier B.V. All rights reserved.
High quantum efficiency dots-in-a-well quantum dot infrared photodetectors with AlGaAs confinement enhancing layer
(2008-05-12)
Ling, H. S.
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Wang, S. Y.
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Lee, C. P.
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Lo, M. C.
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電子工程學系及電子研究所
We demonstrate the high quantum efficiency InAs/In(0.15)Ga(0.85)As dots-in-a-well (DWELL) quantum dot infrared photodetectors (QDIPs). A thin Al(0.3)Ga(0.7)As layer was inserted on top of the InAs quantum dots (QDs) to enhance the confinement of QD states in the DWELL structure. The better confinement of the electronic states increases the oscillation strength of the infrared absorption. The higher excited state energy also improves the escape probability of the photoelectrons. Compared with the conventional DWELL QDIPs, the quantum efficiency increases more than 20 times and the detectivity is about an order of magnitude higher at 77 K. (c) 2008 American Institute of Physics.
Implementing and evaluating three routing protocols in dual-radio-dual-mode IEEE 802.11(b) wireless mesh networks
(2008-06-25)
Wang, S. Y.
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Lin, C. C.
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Chou, C. L.
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資訊工程學系
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Department of Computer Science
In a dual-radio-dual-mode IEEE 802.11(b) wireless mesh network (WMN), each mesh access point has two IEEE 802.11(b) interfaces. One interface operates in infrastructure mode to serve IEEE 802.11(b) client devices so that they can readily use the mesh network without any software modification or installation. The other interface operates in ad hoc mode to forward packets among mesh access points until they reach an Internet gateway or the access point with which the destination client device is associated. Compared with a single-radio WMN, such a WMN offers ease of use and higher network throughputs for client devices. In this paper, we implement and evaluate the performance of three routing protocols (OSPF, AODV, and STP) operating in dual-radio-dual-mode IEEE 802.11(b) WMNs. Our simulation results show that among the tested protocols, OSPF provides the highest system total througput and results in the largest number of stable connections in such a WMN. (c) 2008 Elsevier B.V. All rights reserved.
Improving the Channel Utilization of IEEE 802.11p/1609 Networks
(2009)
Wang, S. Y.
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Chou, C. L.
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Liu, K. C.
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Ho, T. W.
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Hung, W. J.
The IEEE 802.11p/1609 standards specify a suite of communication protocols for vehicular communication networks. A new operational mode, called the WAVE (Wireless Access in Vehicular Environments) mode, is defined to enable communication among high-speed vehicles or between a vehicle and a roadside infrastructure network. In the WAVE mode, a mobile/stationary station uses a combined FDMA/TDMA channel access scheme to switch its channel between the control channel and a service channel every 50 ms. The primary purpose of this design is to detect new services announced by other stations. However, if a station would like to use all of its service channel bandwidth to transmit a large volume of data to another station, switching back to the control channel every 50 ms without doing anything useful only unnecessarily reduces the maximum possible transmission throughput by a half. To overcome this problem, in this paper we propose an improved channel access scheme to allow a station to stay on a service channel as long as it requires before returning to the control channel. Our simulation results show that the proposed scheme significantly improves the service channel utilization under various network conditions.
Increasing wide-area download throughputs on the roads by trunking multiple cellular channels over a vehicular ad hoc network
(2009-02-12)
Wang, S. Y.
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Chou, C. L.
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Lin, C. C.
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資訊工程學系
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Department of Computer Science
To obtain infotainment services (e.g., traffic, maps, accidents, weather, etc.) on the roads, vehicle drivers and back-seat passengers may want to use wide-area networks (e.g., GPRS) to download useful information from the Internet when needed. In this paper. we design and implement a scheme to increase GPRS download throughput for a vehicle user while he (she) is moving on the roads. This scheme integrates inter-vehicle communication with trunking mechanisms to achieve this goal. This scheme can be readily deployed for any real-world GPRS network or any 3G network without any support from the network operator. Our field trial results show that when this scheme is applied in a motorcade of four vehicles and all of the GPRS channel provided by each vehicle are used to download a file simultaneously, the download speed is 3.92 times faster than that achieved when only one GPRS channel is used. (C) 2008 Elsevier B.V. All rights reserved.
NCTUns 4.0: An integrated simulation platform for vehicular traffic, communication, and network researches
(2007)
Wang, S. Y.
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Chou, C. L.
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Chin, Y. H.
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Tzeng, Y. S.
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Hsu, M. S.
In this paper, we present an integrated simulation platform, called NCTUns, for vehicular traffic, communication, and network researches. This platform combines the capabilities supported by a network simulator and those supported by a traffic simulator. With these simulation capabilities, NCTUns can be used to design protocols for Intelligent Transportation Systems (ITS) communication networks such as a wireless vehicular communication network. Besides, the novel architecture of the platform enables the real-world Linux protocol stack and any real-world application to be used in simulations of such networks. In this paper, we present the design of NCTUns for supporting ITS researches and show its scalability.
NCTUns 5.0 Network Simulator for Advanced Wireless Vehicular Network Researches
(2009)
Wang, S. Y.
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Chou, C. L.
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資訊工程學系
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Department of Computer Science
NCTUns is a high-fidelity network simulator and emulator By using a novel kernel re-entering simulation methodology, it provides many unique advantages over traditional network simulators and emulators. For this reason, since its public release on November 1, 2002, as of February 2, 2009, more than 13,838 people from. 130 countries in the world have registered at its web site to download it and these numbers are still fast increasing. NCTUns is open-source and runs on Linux. It can simulate many different types of networks. In this paper, we will present its supports for wireless vehicular network researches. In particular it supports IEEE 802.11(p)/1609 standards, which are proposed for wireless vehicular networks. More information about this tool is available at its web site at (http://NSL.csie.nctu.edu.tw/nctuns.html).
NCTUns tool for wireless vehicular communication network researches
(2009-08-01)
Wang, S. Y.
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Chou, C. L.
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資訊工程學系
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Department of Computer Science
Several goals such as improving road safety and increasing transport efficiency are being pursued in intelligent transportation systems (ITS). Wireless vehicular communication is one technology to achieve these goals. Conducting vehicular experiments on the roads is an approach to studying the effectiveness of wireless vehicular communication. However, such an approach is costly, hard-to-control (repeat), dangerous, and infeasible when many vehicles and people are involved in the field trial. In contrast, the simulation approach does not have these problems. It is a very useful approach and complements the field trial approach. This paper presents NCTUns, an open source integrated simulation platform, for wireless vehicular communication network researches. This tool tightly integrates network and traffic simulations and provides a fast feedback loop between them. Therefore, a simulated vehicle can quickly change its driving behavior such as moving speed and direction when it receives a message from the wireless vehicular communication network. This capability is required by several novel ITS applications such as active collision avoidance systems. In this paper, we present the design, implementation, validation, and performance of this tool. (C) 2009 Elsevier B.V. All rights reserved.
On the characteristics of information dissemination paths in vehicular ad hoc networks on the move
(2008-11-01)
Wang, S. Y.
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Chou, C. L.
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資訊工程學系
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Department of Computer Science
Recently, Intelligent Transportation Systems (ITS) is becoming an important research topic. One goal for ITS is to disseminate important information among vehicles in a timely and efficient manner. In the ITS research community, inter-vehicle communications (IVC) is considered a way that may be able to achieve this goal. An information network built on top of vehicles using IVC can be viewed as a type of mobile ad hoc networks. Although several information dissemination protocols for mobile ad hoc networks have been proposed, how well they can be applied to a vehicle-formed mobile ad hoc network is still poorly studied. Without doubt, no matter what methodology is taking for designing an IVC suitable information dissemination protocol, the protocol performance is fundamentally limited by the potential degree of physical-link connectivity. Thus, in this paper, some potential link attributes of IVC networks, such as the lifetime of a transmission path between two vehicles, are studied based on reasonable vehicle traces generated by a microscopic traffic simulator. Although no specific protocol is focused on in this paper, the studied results can potentially contribute to performance improvement and/or protocol efficiently for any existed or developing protocol.
On the characteristics of routing paths and the performance of routing protocols in vehicle-formed mobile ad hoc networks on highways
(2010-02-01)
Wang, S. Y.
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Chou, C. L.
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Lin, C. C.
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資訊工程學系
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Department of Computer Science
Recently, intelligent transportation systems (ITS) is becoming an important research topic. One goal of ITS is to exchange information among vehicles in a timely and efficient manner. In the ITS research community, inter-vehicle communications (IVC) is considered a way that may be able to achieve this goal. An information network built on the top of vehicles using IVC can be viewed as a type of mobile ad hoc networks (MANETs). In the past, several unicast routing protocols for MANET have been proposed. However, most of them are designed for general MANETs rather than for IVC networks. In this paper, we first used more realistic vehicle mobility traces generated by a microscopic traffic simulator (VISSIM) to understand the characteristics of routing paths in an IVC network. Based on the insights gained from the derived path characteristics, we designed and implemented an intelligent flooding-based routing protocol for small-scale IVC networks. Via several field trials conducted on highways, we compared the performance of ad hoc on-demand distance vector (AODV) and our protocol. Our experimental results show that (1) our protocol outperforms AODV greatly in IVC networks and (2) our protocol can provide text, image, audio, and video services for small-scale IVC networks (e.g., a platoon) quite well. Copyright (C) 2009 John Wiley & Sons, Ltd.
PC-CTS: A power-controlled-CTS MAC scheme to improve spatial reuse in wireless mesh networks
(2007)
Wang, S. Y.
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資訊工程學系
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Department of Computer Science
Spatial reuse is important in wireless mesh networks to allow multiple transmissions to occur simultaneously without interfering with each other. The design of IEEE 802.11 (a/b/g) MAC scheme does not optimally reuse space and may result in low network throughputs. This paper proposes PC-CTS, which is a power-controlled-CTS MAC scheme, to improve spatial reuse in wireless mesh networks. PC-CTS is simple and easy to implement. Our simulation results show that, even with some extra cost for making it more robust against noise, fading, and multiple interferences, on average it still outperforms IEEE 802.11 by a factor of 2 under more realistic random traffic patterns.