Browsing by Author "Lee, Hsin-Yi"

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A Cache Scheme for Femtocell Reselection
(2010-01-01)
Lee, Hsin-Yi
;
Lin, Yi-Bing
;
資訊工程學系
;
Department of Computer Science
In a cellular network, femto base stations may be deployed to improve indoor coverage that cannot be accommodated by the macro base stations. However, to switch from the macro-tier to the femto-tier, a user equipment may be required to scan the whole femto radio spectrum, which is an expensive operation. We propose a cache scheme that may significantly speed up the macro-tier to femto-tier switching.
Annealing effect on the optical response and interdiffusion of n-ZnO/p-Si, (111) heterojunction grown by atomic layer deposition
(2010-11-01)
Ku, Ching-Shun
;
Huang, Jheng-Ming
;
Cheng, Ching-Yuan
;
Lin, Chih-Ming
;
Lee, Hsin-Yi
Optical and structural properties of n-ZnO films grown on a p-Si (111) substrate by atomic layer deposition were observed using in situ synchrotron x-ray diffraction during annealing. The photoluminescence showed a complicated photon response with increasing annealing temperature. In situ x-ray diffraction indicated the growth of grains for an annealing temperature from 500 to 800 degrees C with the orientation altering from polycrystalline to preferential (200). Measurements with a time-of-flight secondary-ion mass spectrometer indicated that the outgassing of hydrogen atoms and ZnO/Si interdiffusion behavior were correlated with the intensity and position of emissions in photoluminescence spectra. (c) 2010 American Institute of Physics. [doi:10.1063/1.3511284]
Annealing treatment improves the morphology and performance of photovoltaic devices prepared from thieno[3,4-c]pyrrole-4,6-dione-based donor/acceptor conjugated polymers and CdSe nanostructures
(2011-06-01)
Kuo, Chih-Yin
;
Su, Ming-Shi
;
Chen, Guan-Yu
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Ku, Ching-Shun
;
Lee, Hsin-Yi
We have prepared photovoltaic devices based on blend films of CdSe tetrapods and the donor/acceptor conjugated polymer PDTTTPD, which comprises 2,5-di(thiophen-2-yl) thieno[3,2-b]thiophene and thieno[3,4-c]pyrrole-4,6-dione units. The AM1.5 power conversion efficiency (PCE) of a photovoltaic device containing a PDTTTPD/CdSe tetrapod blend (1:9, w/w) that had experienced thermal annealing (130 degrees C, 20 min) was three times greater than that of the corresponding device incorporating the as-prepared PDTTTPD/CdSe tetrapod blend (2.9% vs. 1.0%). Synchrotron X-ray reflectivity revealed that annealing (i.e., removal of pyridine ligands from the surfaces of the CdSe tetrapods) caused the thickness of the PDTTTPD/CdSe tetrapod blend film to decrease (and its average density to increase) relative to that of the as-prepared blend film. Transmission electron microscopy and atomic force microscopy revealed that thermal annealing enhanced the degree of aggregation of the CdSe tetrapods and induced denser morphologies, leading to substantially increased charge transport, which enhanced the PCE of the device.
Broadband Omnidirectional Light Trapping in Gold-Decorated ZnO Nanopillar Arrays
(2017-04-05)
Chang, Yuan-Ming
;
Lin, Man-Ling
;
Lai, Tung-Yen
;
Chen, Chang-Hung
;
Lee, Hsin-Yi
The photoluminescence (PL) and reflectivity characteristics of zinc oxide nanopillars- (ZnO-NPs) grown on indium tin oxide (ITO)-coated glasses were investigated. The room temperature PL showed bright -white -light emission for the undoped ZnO-NPs grown at 600 degrees C, suggesting the close relation between the optical characteristic and "the growth conditions being, carried out for,obtaining the present ZnO-NPs. The reflectivity of the as -grown ZnO-NPs array was about, similar to 29% with the wavelength of the incident light ranging from 200 to 1800 nm. Nevertheless, the reflectance reduced significantly to less than 9.9% when a layer of gold (Au) was deposited on ZnO=NPg by sputtering for 5 min, corresponding to more than 65% reduction in Au -coated ZnQ-NPs (Au/ZnO-NPs). Moreover, the angle -resolved reflectance measurement's on the present Au/ZnO-NPs array show an omnidirectional light -trapping characteristic. These remarkable characteristics, broadband and omnidirectional light -trapping -of Ati/ZnO-NPs, are attributed to the extended effective optical path of the incident light due to subwavelength scattering resulting,from the presence of Au nanoparticics.
Characteristics of highly orientated BiFeO(3) thin films on a LaNiO(3)-coated Si substrate by RF sputtering
(2010-10-01)
Liu, Yen-Ting
;
Chen, San-Yuan
;
Lee, Hsin-Yi
;
材料科學與工程學系
;
加速器光源科技與應用學位學程
BiFeO(3) (BFO) films were grown on LaNiO(3)-coated Si substrate by a RF magnetron sputtering system at temperatures in the range of 300-700 degrees C. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. For a substrate temperature below 300 degrees C and at 700 degrees C only partially crystalline films and completely randomly polycrystalline films were grown, whereas highly (001)-orientated BFO film was obtained for a substrate temperature in the range of 400-600 degrees C. The crystalline quality of BFO thin films increase as the deposition temperature increase except for the film deposited at 700 degrees C. The fitted result from X-ray reflectivity curves show that the densities of the BFO films are slightly less than their bulk values. For the BFO films deposited at 300-600 degrees C, the higher the deposition temperature, the larger the remnant polarization and surface roughness of the films present. (C) 2010 Elsevier B.V. All rights reserved.
Characteristics of highly orientated BiFeO3 thin films on a LaNiO3-coated Si substrate by RF sputtering
(2010-10-01)
Liu, Yen-Ting
;
Chen, San-Yuan
;
Lee, Hsin-Yi
;
材料科學與工程學系
;
加速器光源科技與應用學位學程
BiFeO3 (BFO) films were grown on LaNiO3-coated Si substrate by a RF magnetron sputtering system at temperatures in the range of 300-700 degrees C. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. For a substrate temperature below 300 degrees C and at 700 degrees C only partially crystalline films and completely randomly polycrystalline films were grown, whereas highly (001)-orientated BFO film was obtained for a substrate temperature in the range of 400-600 degrees C. The crystalline quality of BFO thin films increase as the deposition temperature increase except for the film deposited at 700 degrees C. The fitted result from X-ray reflectivity curves show that the densities of the BFO films are slightly less than their bulk values. For the BFO films deposited at 300-600 degrees C, the higher the deposition temperature, the larger the remnant polarization and surface roughness of the films present. (C) 2010 Elsevier B.V. All rights reserved.
Credit Pre-Reservation Mechanism for UMTS Prepaid Service
(2010-06-01)
Lee, Hsin-Yi
;
Lin, Yi-Bing
;
資訊工程學系
;
Department of Computer Science
Online Charging System (OCS) supports multiple prepaid and postpaid sessions simultaneously. Through credit reservation, the OCS assigns some credit units to a session. These credit units are decremented based on the traffic volume or the duration time. If the assigned credit units are consumed before the session is completed, an reserve units (RU) operation is executed to obtain more credit units from the OCS. If the credit at the OCS is depleted, the prepaid session is forced to terminate. During the RU operation, packet delivery is suspended until extra credit units are granted from the OCS. To avoid session suspension during credit reservation, we propose the credit pre-reservation mechanism (CPM) that reserves credit earlier before the credit at the GGSN is actually depleted. Analysis and simulation experiments are conducted to investigate the performance of the mechanism. Our study indicates that the CPM can significantly improve the performance of the OCS prepaid mechanism.
Diagonals and numerical ranges of direct sums of matrices
(2013-11-01)
Lee, Hsin-Yi
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應用數學系
;
Department of Applied Mathematics
For any n-by-n matrix A, we consider the maximum number k = k(A) for which there is a k-by-k compression of A with all its diagonal entries in the boundary partial derivative W(A) of the numerical range W (A) of A. If A is a normal or a quadratic matrix, then the exact value of k(A) can be computed. For a matrix A of the form B circle plus C, we show that k(A) = 2 if and only if the numerical range of one summand, say, B is contained in the interior of the numerical range of the other summand C and k(C) = 2. For an irreducible matrix A, we can determine exactly when the value of k(A) equals the size of A. These are then applied to determine k(A) for a reducible matrix A of size 4 in terms of the shape of W (A). (C) 2013 Elsevier Inc. All rights reserved.
Enhanced electrical properties and field emission characteristics of AZO/ZnO-nanowire core-shell structures
(2016-06-14)
Huang, Jheng-Ming
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Tsai, Shang-You
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Ku, Ching-Shun
;
Lin, Chih-Ming
;
Chen, San-Yuan
The electrical properties and field-emission characteristics of ZnO nanowires (ZnO-NWs) fabricated using a vapor-liquid-solid method were systematically investigated. In particular, we explored the effects of Al-doped ZnO (AZO) films (thickness 4-100 nm) deposited on ZnO-NWs using an atomic layer deposition (ALD) method on the optoelectronic properties. The results show that the sheet resistance of net-like ZnO-NW structures can be significantly improved, specifically to become similar to 1/1000 of the sheet resistance of the as-grown ZnO-NWs, attaining less than 10 Omega Sq(-1). The emission current density measured at the maximum field was roughly quadrupled relative to that of the as-grown ZnO-NWs. The data of the enhanced field-emission characteristics show that, with the ALD system, the AZO films of small resistance are readily coated on a structure with a high aspect ratio and the coating radius is controlled relative to the turn-on voltage and current density. The ultrathin AZO film from a one-monolayer coating process also significantly improved emission properties through modification of the effective work function at the AZO/ZnO-NW surface.
Enhanced field emission characteristics in metal-coated Si-nanocones
(2013)
Chang, Yuan-Ming
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Kao, Pin-Hsu
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Tai, Hung-Ming
;
Wang, Hau-Wei
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Lin, Chih-Ming
Metallic gold (Au) and platinum (Pt) thin films were deposited on silicon nanocones (Si-NCs) by sputtering to elucidate the effects of work function and conductivities on the field electron emission characteristics of surface-modified Si-NCs. The results showed that for Pt/Si-NCs and Au/Si-NCs, although the turn-on field defined at a corresponding current density of 10 mu A cm(-2) only improved from 4.20 V mu m(-1) for bare Si-NCs to 3.65 and 2.90 V mu m(-1), respectively, the emission current density measured at 5.00 V mu m(-1) was enhanced by orders of magnitude, reaching 1.82 mA cm(-2) for Au/Si-NCs. Compared to those obtained from various surface-modified Si-nanostructures, such as ZnO/Si-nanopillars and ferroelectrics/Si-nanotips, the current results represent an interesting alternative route for producing surface-modified Si-NCs that might be useful for optical and electronic applications.
Enhanced Free Exciton and Direct Band-Edge Emissions at Room Temperature in Ultrathin ZnO Films Grown on Si Nanopillars by Atomic Layer Deposition
(2011-11-01)
Chang, Yuan-Ming
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Shieh, Jiann
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Chu, Pei-Yuan
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Lee, Hsin-Yi
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Lin, Chih-Ming
Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 degrees C, an intensive UV emission corresponding to free-exciton recombination (similar to 3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 degrees C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by similar to 60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 degrees C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is similar to 2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions.
Enhanced visible photoluminescence from ultrathin ZnO films grown on Si-nanowires by atomic layer deposition
(2010-09-24)
Chang, Yuan-Ming
;
Jian, Sheng-Rui
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Lee, Hsin-Yi
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Lin, Chih-Ming
;
Juang, Jenh-Yih
Bright room temperature visible emission is obtained in heterostructures consisting of similar to 3.5 nm thick ZnO ultrathin films grown on Si-nanowires produced by means of self-masking dry etching in hydrogen-containing plasma. The ZnO films were deposited on Si-nanowires by using atomic layer deposition (ALD) under an ambient temperature of 25 degrees C. The orders of magnitude enhancement in the intensity of the room temperature photoluminescence peaked around 560 nm in the present ZnO/Si-nanowire heterostructures is presumably due to the high aspect (surface/volume) ratio inherent to the Si-nanowires, which has, in turn, allowed considerably more ZnO material to be grown on the template and led to markedly more efficient visible emission. Moreover, the ordered nanowire structure also features an extremely low reflectance (similar to 0.15%) at 325 nm, which may further enhance the efficiency of emission by effectively trapping the excitation light.
Enhancement of epitaxial LaNiO3 electrode on the ferroelectric property of La-doped BiFeO3/SrTiO3 artificial superlattice structure by rf sputtering
(2013-04-01)
Chiu, Shang-Jui
;
Liu, Yen-Ting
;
Yu, Ge-Ping
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Lee, Hsin-Yi
;
Huang, Jia-Hong
Improvement of ferroelectric property by LaNiO3 (LNO) electrode on superlattice films was shown in this study. High quality asymmetric multiferroic 5 at% La-doped BiFeO3 (BFO) and paraelectric SrTiO3 (STO) superlattice structure was deposited on LNO electrode by rf magnetron sputtering system. High crystalline quality of the La-doped BFO/STO artificial superlattice structure on LNO electrode have been confirmed by clear main feature and satellite features around (0 0 2) STO substrate Bragg peak shown in (0 0 2) radial scan spectrum. The results of in-pane crystal truncation rod (CTR) spectrum and the rocking curve indicated that superlattice films with high crystal quality behaved in-plane strain closed to fully strained state on both LNO and Nb-doped STO electrode. The epitaxial relation between the La-doped BFO and STO layers in the superlattice was examined through a (1 0 3) azimuthal scan and (1 0 3) Lscan. The results show a four-fold symmetry and slight distorted of gamma angle of the BFO unit cell, confirming the pseudocubic structure of the La-doped BiFeO3 structure. The measurement of hysteresis loops showed that La-doped BFO superlattice structure on LNO electrode behaved lower, but stable and saturated ferroelectric properties than that of films with Nb-doped STO substrate in our previous study. The measurement of resistivity and leakage current density of superlattice films demonstrated that resistivity of electrode is the main factor to determine the leakage properties of films. The superlattice films behaved larger piezoelectric coefficient (d(33)) on LNO electrode than films on Nb-doped STO substrate, which indicated the enhancement of intrinsic ferroelectric properties in La-doped BFO/STO superlattice films. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.
Epitaxial Growth of m-Plane ZnO Thin Films on (10(1)over-bar0) Sapphire Substrate by Atomic Layer Deposition with Interrupted Flow
(2010-04-01)
Ku, Ching-Shun
;
Lee, Hsin-Yi
;
Huang, Jheng-Ming
;
Lin, Chih-Ming
;
材料科學與工程學系
Nonpolar ZnO films were grown epitaxially on (10 (1) over bar0) sapphire substrates at 200 degrees C by atomic layer deposition with interrupted flow. The latter method improved the crystalline quality of ZnO films, transformed the structure from polycrystalline to epitaxial, and enhanced the optical properties of near-band-edge emission. The interfacial structure shows multiple domain phases along sapphire (020) and the disappearance of a minor phase near the surface. As determined by X-ray diffraction, the epitaxial relation between ZnO and sapphire follows [002](ZnO)vertical bar vertical bar[020](sapphire) and [020](ZnO)vertical bar vertical bar[006](sapphire). The photoluminescence intensity increased with increasing crystalline quality and thickness of ZnO films.
Epitaxial growth of ZnO films at extremely low temperature by atomic layer deposition with interrupted flow
(2010-04-15)
Ku, Ching-Shun
;
Lee, Hsin-Yi
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Huang, Jheng-Ming
;
Lin, Chih-Ming
;
材料科學與工程學系
Thin crystalline films of zinc oxide (ZnO) of high quality have been grown epitaxially on a (0 0 0 1) c-plane of a sapphire substrate with atomic layer deposition (ALD) at extra-low temperature. With diethylzinc (DEZn) and deionized water as precursors in combination with interrupted flow, we obtained ZnO thin films with an optimal growth window in a range 25-160 degrees C, so effectively lowering the growth temperature by about 120 degrees C relative to the conventional method involving a continuous-flow. We characterized the microstructure of these films with X-ray reflectivity and high-resolution X-ray diffraction (XRD) measurements. The XRD results indicate that the stock time might extend the reaction of DEZn and water through an increased duration. This low temperature for growth results in increased crystalline quality and reduced the non-radiative recombination process to enhance the optical properties of ZnO films. (c) 2009 Elsevier B.V. All rights reserved.
Fabrication and ferroelectric properties of BiFeO3/LaNiO3 artificial superlattice structures grown by radio-frequency magnetron-sputtering
(2013-02-01)
Liu, Yen-Ting
;
Chiu, Shang-Jui
;
Lee, Hsin-Yi
;
Chen, San-Yuan
;
材料科學與工程學系
Symmetric superlattice structures of multiferroic BiFeO3 and conductive LaNiO3 sublayers were grown on a Nb-doped SrTiO3 substrate with radio-frequency magnetron-sputtering. The formation of a superlattice structure was confirmed from the appearance of satellite lines on both sides of the main line in the X-ray diffraction pattern. X-ray measurements show that these superlattice films become subject to greater tensile stress along the c-axis and to increased compressive stress parallel to the surface plane with a decreasing thickness of the sublayer. The smaller is the thickness of the sublayer, the greater is the crystalline quality and the strain state. The hysteresis loops show a large leakage current at frequencies of 0.5 and 1 kHz; the polarization decreases with an increasing frequency. (C) 2012 Elsevier B.V. All rights reserved.
Fabrication and optoelectronic properties of core-shell biomimetic ZnO/Si nanoball arrays
(2012)
Chang, Yuan-Ming
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Kao, Pin-Hsu
;
Liu, Mao-Chen
;
Lin, Chih-Ming
;
Lee, Hsin-Yi
We report an effective process scheme comprising a single-step Ag sputtering with subsequent dry etching and atomic layer deposition (ALD) process for fabricating biomimetic ZnO/Si nanoball (ZnO/Si-NB) core-shell nanostructures directly on Si substrates. The obtained ZnO/Si-NB core-shell nanostructures consist of similar to 30 nm thick ZnO films grown on Si frustums produced by means of dry etching masked by the self-assembled silver nanodots created by single-step sputtering. The ZnO films were deposited using atomic layer deposition under an ambient temperature of 200 degrees C. The photoluminescence (PL) measurements on these ZnO/Si-NB core-shell nanostructures showed that the visible range emission was almost completely absent and only the ultraviolet emission (3.28 eV peak) resulting from the free excitons was observed, indicating that the films indeed have high crystalline quality. Moreover, a dramatic improvement of the field emission performance was observed for ZnO/Si-NB core-shell nanostructures as compared to the bare Si frustum arrays. The detailed analyses on the field enhancement factor (beta value) based on the Fowler-Nordheim field emission model indicate that the effective work function of the ZnO/Si-NB core-shell nanostructures might be significantly different from that of either ZnO or Si.
Fabrication of epitaxial ZnO films by atomic-layer deposition with interrupted flow
(2009-12-31)
Ku, Ching-Shun
;
Huang, Jheng-Ming
;
Lin, Chih-Ming
;
Lee, Hsin-Yi
;
材料科學與工程學系
We formed epitaxial ZnO thin films on a (0001) c-plane sapphire substrate through deposition of atomic layers (ALD) at 25-160 degrees C using diethylzinc (DEZn) and deionized water as precursors in combination with interrupted flow. High-resolution X-ray diffraction measurements were employed to characterize the microstructure of these films. With interrupted flow, we obtained ZnO thin films with an optimal growth window in a range of 40-160 degrees C, effectively decreasing the growth temperature by about 120 degrees C relative to a conventional method involving continuous flow. X-ray reflectivity measurements showed that the rate of growth increased also between 20 degrees C and 120 degrees C. The XRD results indicate that the stock time might extend the reaction of DEZn and water through an increased duration. All results show that a low temperature for growth improves the crystalline quality and is consistent with thermodynamically blocked self-compensation. (C) 2009 Elsevier B.V. All rights reserved.
Field Emission in Vertically Aligned ZnO/Si-Nanopillars with Ultra Low Turn-On Field
(2012-03-01)
Chang, Yuan-Ming
;
Liu, Mao-Chen
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Kao, Pin-Hsu
;
Lin, Chih-Ming
;
Lee, Hsin-Yi
An effective method of fabricating vertically aligned silicon nanopillars (Si-NPs) was realized by using the self-assembled silver (Ag) nanodots as natural metal-nanomask during dry etching process. The obtained Si-NPs were preferentially aligned along the c-axis direction. Ultrathin ZnO films (similar to 9 nm) were subsequently deposited on the Si-NPs by atomic layer deposition (ALD) to enhance the field emission property. The average diameter of the ZnO/Si-NPs is in the order of tens of nanometers, which enables efficient field emission and gives rise to marked improvement in the field enhancement factor, beta. The turn-on field defined by the 10 mu A/cm(2) current density criterion is similar to 0.74 V/mu m with an estimated beta approximate to 1.33x10(4). The low turn-on field and marked enhancement in beta were attributed to the small radius of curvature, high aspect ratio, and perhaps more importantly, proper density distribution of the ZnO/Si-NPs.
Field Emission Properties of Gold Nanoparticle-Decorated ZnO Nanopillars
(2012-12-01)
Chang, Yuan-Ming
;
Lin, Man-Ling
;
Lai, Tung-Yen
;
Lee, Hsin-Yi
;
Lin, Chih-Ming
The structural and optoelectronic properties of ZnO nanopillars (ZnO-NPs) grown on Si substrates by the vapor transport deposition method were investigated. In particular, by varying the deposition duration and hence the morphology of the vertically aligned ZnO-NPs, the resultant field emission characteristics were systematically compared. In addition to identifying the advantageous field emission properties exhibited in the pencil-like ZnO-NPs, we observed that by adhering Au nanoparticles on the surface of the ZnO-NPs the turn-on field and the maximum current density can be drastically improved from 3.15 V/mu m and 0.44 mA/cm(2) at 5 V/mu m for the best ZnO-NPs to 2.65 V/mu m and 2.11 mA/cm(2) at 5 V/mu m for Au/ZnO-NPs, respectively. The enhancement of field emission characteristics that resulted from Au-nanoparticle decoration is discussed on the basis of charge-transfer-induced band structure modifications.