Browsing by Author "Chang, L."

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Amorphous carbon coated silicon nanotips fabricated by MPCVD using anodic aluminum oxide as the template
(2007)
Chen, Te-Ming
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Pan, Fu-Ming
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Hung, Jui-Yi
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Chang, L.
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Wu, Shich-Chuan
We have used nanoporous anodic aluminum oxide (AAO) as a template to fabricate amorphous carbon (alpha-C) coated silicon nanotips by microwave plasma chemical vapor deposition (MPCVD). During the preparation of the well-ordered AAO pore channel array, an underlying TiN layer was anodically oxidized as well in the late stage of the AAO anodization, forming titanium oxide nanomasks for Si nanotip fabrication. The titanium oxide nanomasks were then used to transfer the arrangement pattern of the AAO pore channel array to the Si substrate by plasma etch in the MPCVD system and, therefore, a well-ordered Si nanotip array was produced. An alpha-C layer similar to 5 nm thick was in situ deposited on the Si nanotips during the MPCVD process. The alpha-C layer was rich in nanocrystalline graphitic carbons according to Raman and Auger electron spectroscopies. The nanocrystalline graphitic carbons in the coating and the sharp tip shape made the Si nanotip a good field emitter and a field enhancement factor of similar to 659 was obtained. (c) 2007 The Electrochemical Society.
Atomic scale characterization of buried In(x)Ga(1-x)As quantum dots using pulsed laser atom probe tomography
(2008-06-09)
Mueller, M.
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Cerezo, A.
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Smith, G. D. W.
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Chang, L.
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Gerstl, S. S. A.
Atom probe tomography (APT) has been used to study In(x)Ga(1-x)As quantum dots buried in GaAs. The dots have an average base width of 16.1 +/- 1.1 nm and height of 3.5 +/- 0.3 nm, but a wide range of sizes. APT composition profiles across the dots are similar to a previous study by cross-sectional scanning transmission electron microscopy, but show significant gallium incorporation (average x=0.22 +/- 0.01). The direct three-dimensional nature of the APT data also reveals the complex spatial distribution of indium within the dots. Data such as these are vital for optimizing the performance of quantum dot materials and devices. (c) 2008 American Institute of Physics.
Bimodel onset strain relaxation in InAs quantum dots with an InGaAs capping layer
(2010-08-30)
Chen, J. F.
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Chen, Ross C. C.
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Chiang, C. H.
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Chen, Y. F.
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Wu, Y. H.
Capping InAs quantum dots (QDs) with an InGaAs layer allows strain relaxation to induce a low-energy electron state below a set of fine dot family states, which is consistent with photoluminescence (PL) spectra. The evolution of InAs thickness suggests a bimodal onset relaxation, i.e., a fine dot family that is strain-relieved by indium outdiffusion from the QDs, as suggested by transmission electron microscopy, and a low-energy dot family that is strain relaxed by the generation of lattice misfits. The indium outdiffusion can explain an abnormal PL blueshift in 70 meV in the fine dot family at onset of strain relaxation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483757]
Effect of antimony incorporation on the density, shape, and luminescence of InAs quantum dots
(2008-07-15)
Chen, J. F.
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Chiang, C. H.
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Wu, Y. H.
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Chang, L.
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Chi, J. Y.
This work investigates the surfactant effect on exposed and buried InAs quantum dots (QDs) by incorporating Sb into the QD layers with various Sb beam equivalent pressures (BEPs). Secondary ion mass spectroscopy shows the presence of Sb in the exposed and buried QD layers with the Sb intensity in the exposed layer substantially exceeding that in the buried layer. Incorporating Sb can reduce the density of the exposed QDs by more than two orders of magnitude. However, a high Sb BEP yields a surface morphology with a regular periodic structure of ellipsoid terraces. A good room-temperature photoluminescence (PL) at similar to 1600 nm from the exposed QDs is observed, suggesting that the Sb incorporation probably improves the emission efficiency by reducing the surface recombination velocity at the surface of the exposed QDs. Increasing Sb BEP causes a blueshift of the emission from the exposed QDs due to a reduction in the dot height as suggested by atomic force microscopy. Increasing Sb BEP can also blueshift the similar to 1300 nm emission from the buried QDs by decreasing the dot height. However, a high Sb BEP yields a quantum well-like PL feature formed by the clustering of the buried QDs into an undulated planar layer. These results indicate a marked Sb surfactant effect that can be used to control the density, shape, and luminescence of the exposed and buried QDs. (C) 2008 American Institute of Physics.
Effect of gate sinking on the device performance, of the InGaP/AlGaAs/InGaAs enhancement-mode PHEMT
(2007-02-01)
Chu, L. H.
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Chang, E. Y.
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Chang, L.
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Wu, Y. H.
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Chen, S. H.
An enhancement-mode InGaP/AlGaAs/InGaAs pseudomorphic high-electron mobility transistor using platinum (Pt) as the Schottky contact metal was investigated for the first time. Following the Pt/Ti/Pt/Au gate metal deposition, the devices were thermally annealed at 325 degrees C for gate sinking. After the annealing, the device showed a positive threshold voltage (V-th) shift from 0.17 to 0.41 V and a very low drain leakage current from 1.56 to 0.16 mu A/mm. These improvements are attributed to the Schottky barrier height increase and the decrease of the gate-to-channel distance as Pt sinks into the InGaP Schottky layer during gate-sinking process. The shift in the V-th was very uniform across a 4-in wafer and was reproducible from wafer to wafer. The device also showed excellent RF power performance after the gate-sinking process.
Electron delocalization of tensily strained GaAs quantum dots in GaSb matrix
(2010-12-15)
Lin, T. C.
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Wu, Y. H.
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Li, L. C.
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Sung, Y. T.
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Lin, S. D.
The magneto-optical response of type-II tensily strained GaAs self-assembled quantum dots in GaSb was investigated in magnetic fields up to 14 T. By depositing different GaAs amount, the dot sizes and the corresponding emission energies were varied. We analyzed the carrier wave function extent of different dots using the diamagnetic shift results. It was found that, with the increase in the energy (the reduction in the dot size), the diamagnetic coefficient first rises quickly and then saturates at around 21 mu eV/T(2). Based on a simple calculation model, this unusual tendency is attributed to the electrons gradually spilling out of the quantum dot to the wetting layer as the dots get smaller. This delocalization effect is enhanced in this material system due to the tensile strain relaxation within the dots, which raises the conduction band edge over that in the wetting layer. (c) 2010 American Institute of Physics. [doi: 10.1063/1.3520669]
Fabrication and field-emission characteristics of TiN nanorods with a concave top surface
(2008)
Chen, Te-Ming
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Hung, Jui-Yi
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Pan, Fu-Ming
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Chang, L.
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Sheu, J. -T.
Well-ordered titanium nitride nanorods were fabricated by reactive ion etch using titanium oxide nanodots as the mask, which were prepared using the anodic aluminum oxide templation method. The TiN nanorods exhibited a concave top surface with a protruding edge. Due to the protruding top edge and a high aspect ratio, the TiN nanorods showed a low turn-on voltage of 1.6 V/mu m. The ellipsoidal cylinder model was used to evaluate the field-enhancement effect of the protruding edge, and an underestimation by similar to 26% was found as compared with the enhancement factor derived from the Fowler-Nordheim plot. (C) 2008 The Electrochemical Society.
Geometrical correlations of quantum dots in InAs/GaAs superlattice structure from electron tomography
(2008-10-13)
Wu, Y. H.
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Chang, L.
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Chen, L. C.
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Chen, H. S.
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Chen, F. R.
In this study, the three-dimensional (3D) information about the structural properties of quantum dots (QDs) in InAs/GaAs superlattice structure has been illustrated using electron tomography in the mode of high-angle angular dark-field scanning transmission electron microscopy. Comparison of this 3D reconstruction with the two-dimensional projection at the same positions is made. The structural properties of embedded quantum dots have been evaluated from electron tomography. The correlation relationship of QDs in superlattice structure has been understood by accurate measurements of 3D geometric positions, which can be free of the overlapping effect from 2D cross section along different crystallographic orientations. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2998693]
How do InAs quantum dots relax when the InAs growth thickness exceeds the dislocation-induced critical thickness?
(2012-01-01)
Chen, J. F.
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Lin, Y. C.
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Chiang, C. H.
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Chen, Ross C. C.
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Chen, Y. F.
A simple critical thickness for generating lattice misfits is insufficient to describe the onset strain relaxation in InAs quantum dots (QDs). A predominant dot family is shown to relieve its strain by In/Ga interdiffusion, rather than by lattice misfits, at the onset of strain relaxation. This argument is based on photoluminescence spectra, which show the emergence of a fine blueshifted transition at the onset of strain relaxation, along with a low-energy transition from a dot family degraded by lattice misfits. From the analysis of the temperature-dependent blueshift and energy separation between the ground and excited-state transitions, the blueshift is attributed to In/Ga interdiffusion. Transmission electron microscopy suggests a relaxation-induced indium migration from the interdiffused dot family to the dislocated dot family. Post-growth thermal annealing can further relieve strain by inducing more In/Ga interdiffusion in the interdiffused dot family and more dislocations in the dislocated dot family. This study explains the co-existence of strong carrier confinement in the QDs and enormous misfit-related traps in the capacitance-voltage spectra, and an elongated QD electron-emission time. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3675519]
Improving the photoluminescence properties of self-assembled InAs surface quantum dots by incorporation of antimony
(2011-08-15)
Chiang, C. H.
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Wu, Y. H.
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Hsieh, M. C.
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Yang, C. H.
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Wang, J. F.
This study investigates the effects of surfactant and segregation from InAs surface quantum dots (SQDs) by incorporating antimony (Sb) into the QD layers. The Sb surfactant effect extends planar growth and suppresses dot formation. Incorporating Sb can reduce the density of SQDs by more than two orders of magnitude. Photoluminescence (PL) reveals enhancement in the optical properties of InAs SQDs as the Sb beam equivalent pressure (BEP) increases. This improvement is caused by the segregation of Sb on the surface of SQDs, which reduces non-radiative recombination and suppresses carrier loss. The dark line at the SQDs surface in the transmission electron microscopic image suggests that the incorporated Sb probably segregates close to the surface of the SQDs. These results indicate a marked Sb segregation effect that can be exploited to improve the surface-sensitive properties of SQDs for biological sensing. Crown Copyright (C) 2011 Published by Elsevier B. V. All rights reserved.
Magnetotransport properties, electronic structure, and microstructure of La0.7Sn0.3MnO3 thin films
(2006-10-01)
Cheng, T. Y.
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Lin, C. W.
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Chang, L.
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Hsu, C. H.
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Lee, J. M.
Single-phase La0.7Sn0.3MnO3 (LSnMO) thin films were fabricated on LaAlO3 (LAO) substrates by pulsed laser deposition (PLD). The as-deposited films, though showed insulating characteristics with no sign of insulator-metal transition (IMT) down to very low temperatures, did display a paramagnetic-ferromagnetic transition (PFT) around 180 K. The x-ray absorption spectroscopy (XAS) of the as-deposited LSnMO films shows signature of Mn3+/Mn2+ mixed valence indicating that tetravalent Sn ions may have resulted in electron doping into the e(g) band of Mn. The transmission electron microscopy (TEM) analyses on the as-deposited LSnMO films further confirmed that the films are epitaxial with uniform composition distributions. It is suggestive that the doping level of x=0.3 in La1-xSnxMnO3 can be achieved without disrupting the perovskite structure or any composition inhomogeneity. On the other hand, ex situ annealing in oxygen as well as in argon atmosphere, though both drive the films to display IMT and a marked enhancement in the transition temperature, the preservation of LSnMO phase is somewhat doubtful. In the oxygen-annealing case, the evidence from the XAS measurements on Sn ions though showed the existence of tetravalent characteristics, the Hall measurements indicated that the obtained LSnMO films are p type in nature. Furthermore, the TEM analyses also revealed the emergence of the Sn compounds, which may ultimately drive the obtained films into La-deficient La1-xMnO3 phases.
Orthorhombic BiFeO3
(2012-12-14)
Yang, J. C.
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He, Q.
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Suresha, S. J.
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Kuo, C. Y.
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Peng, C. Y.
A new orthorhombic phase of the multiferroic BiFeO3 has been created via strain engineering by growing it on a NdScO3(110)(o) substrate. The tensile-strained orthorhombic BiFeO3 phase is ferroelectric and antiferromagnetic at room temperature. A combination of nonlinear optical second harmonic generation and piezoresponse force microscopy revealed that the ferroelectric polarization in the orthorhombic phase is along the in-plane < 110 >(pc) directions. In addition, the corresponding rotation of the antiferromagnetic axis in this new phase was observed using x-ray linear dichroism. DOI: 10.1103/PhysRevLett.109.247606
Pulse Electrodeposition of Iridium Oxide on Silicon Nanotips for Field Emission study
(2009-05-01)
Chen, Te-Ming
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Hung, Jui-Yi
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Pan, Fu-Ming
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Chang, L.
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Wu, Shich-Chuan
To improve electron field emission properties of anodic aluminum oxide (AAO) templated Si nanotips, IrO(2) nanoparticles were deposited on the nanotips using bipolar pulse electrodeposition method. The IrO(2) nanoparticles had a uniform size distribution with an average value of similar to 4 nm, and well dispersed on the ordered Si nanotips. Due to the small radius and a lower work function, the IrO(2)/Si nanotip exhibited field emission properties better than the bare Si nanotip with a field enhancement factor of similar to 128. The SEM and TEM were utilized to investigate the structure and morphology. The physical and chemical properties were evaluated XRD and X-ray photoelectron spectroscopy (XPS).
Rapid Synthesis of Semipolar ZnO Nanorod Arrays on M-Sapphire by Microwave-Assisted Chemical Bath Deposition
(2018-01-01)
Tsai, C. J.
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Do, H.
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Chang, C.
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Chiu, K. A.
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Hsiang, C. C.
Microwave-assisted chemical bath deposition (CBD) is a newly developed technique for fast growth of single-crystalline ZnO nanorods in a solution containing zinc nitrate at temperature below 100 degrees C. ZnO nanorods have many applications for devices, however, semipolar oriented ZnO nanorods have been rarely reported. In this study, epitaxial growth of well-aligned arrays of inclined semipolar ZnO nanorods on a ZnO seed layer on m-plane sapphire substrates is successfully fabricated by using microwave-assisted chemical bath deposition for 10 min at 90 degrees C. TEM analysis reveals that The ZnO nanorods have an inclined angle about 30 degrees with m-sapphire surface and an average length about 500 nm which gives a growth rate of 3 mu m/h. Micro-photoluminescence measurements show that ZnO nanorods are of a reasonably good quality.
Relaxation-induced lattice misfits and their effects on the emission properties of InAs quantum dots
(2007-09-05)
Chen, J. F.
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Wang, Y. Z.
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Chiang, C. H.
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Hsiao, R. S.
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Wu, Y. H.
Strain relaxation in InAs/InGaAs quantum dots (QDs) is shown to introduce misfits in the QD and neighboring GaAs bottom layer. A capacitance -voltage profiling shows an electron accumulation peak at the QD with a long emission time, followed by additional carrier depletion caused by the misfits in the GaAs bottom layer. The emission-time increase is explained by the suppression of tunneling for the QD excited states due to the additional carrier depletion. As a result, electrons are thermally activated from the QD states to the GaAs conduction band, consistent with observed emission energies of 0.160 and 0.068 eV which are comparable to the confinement energies of the QD electron ground and first-excited states, respectively, relative to the GaAs conduction band. This is in contrast to non-relaxed samples in which emission energy of 60 meV is observed, corresponding to the emission from the QD ground state to the first-excited state.
Strain relaxation and induced defects in InAsSb self-assembled quantum dots
(2006-06-05)
Chen, J. F.
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Hsiao, R. S.
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Huang, W. D.
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Wu, Y. H.
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Chang, L.
The onset of strain relaxation and induced defects in InAs0.94Sb0.06 quantum dots are investigated. We show that the relaxation causes partial carrier depletion in the dots and drastic carrier depletion in the top GaAs layer due to the introduction of two defect traps At 0.35 and 0.64 eV. This result is consistent with transmission electron microscopy data which show misfit dislocations on the edges of the dot upper boundary and threading dislocations in the top GaAs layer. The bottom GaAs layer is dislocation-free, and thus the strain relaxation may initially occur on the edges of the dots. (c) 2006 American Institute of Physics.
Strain relaxation in InAs self-assembled quantum dots induced by a high N incorporation
(2008-11-15)
Chen, J. F.
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Yang, C. H.
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Wu, Y. H.
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Chang, L.
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Chi, J. Y.
The effect of a high N incorporation in self-assembled InAs quantum dots (QDs) is investigated by analyzing the electronic and structural properties around QD region. Capacitance-voltage profiling and admittance spectroscopy shows that N incorporation into the InAs QD layer leads to drastic carrier depletion in the QD layer and neighboring GaAs layers due to the formation of a deep defect state at 0.34-0.41 eV. The signature of this defect state is similar to those defects observed in strain relaxed QDs or InGaAs/GaAs quantum wells when the InAs deposition thickness exceeds a critical thickness. Accordingly, the N incorporation might result in strain relaxation either by increasing localized strain or by inducing composition inhomogeneities, which provide nucleation sources for strain relaxation. The argument of strain relaxation is supported by transmission electron microscopy that reveals lattice misfits at the QD layer and neighboring GaAs layers.
The Effect of Tensile Strain on Optical Anisotropy and Exciton of m-Plane ZnO
(2015-04-01)
Wang, H. H.
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Tian, J. S.
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Chen, C. Y.
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Huang, H. H.
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Yeh, Y. C.
The near band edge emission of the tensile-strained m-plane ZnO film grown on (112)LaAlO3 substrates shows abnormal low polarization degree (rho = 0.1). The temperature dependency of polarization degree clarifies the origins of different emission peaks. In tensile-strained m-plane ZnO, the [0001] polarized state is upper shifted and is overlapping with the [11 (2) over bar0] polarized state. This phenomenon causes the abnormal low polarization degree and reveals the effect of strain on the emission anisotropy of m-plane ZnO.
The muon system of the Daya Bay Reactor antineutrino experiment
(2015-02-11)
An, F. P.
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Balantekin, A. B.
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Band, H. R.
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Beriguete, W.
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Bishai, M.
The Daya Bay experiment consists of functionally identical antineutrino detectors immersed in pools of ultrapure water in three well-separated underground experimental halls near two nuclear reactor complexes. These pools serve both as shields against natural, low-energy radiation, and as water Cherenkov detectors that efficiently detect cosmic muons using arrays of photomultiplier tubes. Each pool is covered by a plane of resistive plate chambers as an additional means of detecting muons. Design, construction, operation, and performance of these muon detectors are described. (C) 2014 Elsevier By. All rights reserved.
Valley-dependent resonant inelastic transmission through a time-modulated region in graphene
(2013-11-18)
Liu, T. L.
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Chang, L.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We investigate resonant sideband processes in the transmission through a time-modulated-potential region in graphene. Valley-dependent features in the time-dependent transmission due to trigonal-warping effects in the electronic structures are explored within a tight-binding model. Three main results obtained are dip structures in the transmission, valley dependence of the dip structures, and nontypical-Fabry-Perot behavior in the dip-structure amplitudes. Dip structures in the transmission are obtained when a relevant band edge is available for the sideband processes. The relevant band edges are shown to become valley dependent, when the incident flow is formed from states of the same group-velocity direction. This is a consequence of the trigonal-warping effects, and it leads to the valley dependence in the dip structures. The dip-structure amplitudes, on the other hand, are found to exhibit a nontypical Fabry-Perot oscillatory behavior, in their dependence on the width of the time-modulated region. This is shown, in our multiple sideband scattering analysis, to result from resonant sideband processes to a relevant band edge. As such, the nontypical Fabry-Perot oscillatory behavior serves as another evidence for the key role the relevant band edges play in the transmission dip structures.