Browsing by Author "Chang, C. F."

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A solid-state cation exchange reaction to form multiple metal oxide heterostructure nanowires
(2016)
Chen, Y. H.
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Huang, C. W.
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Yeh, P. H.
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Chen, J. Y.
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Lin, T. Y.
Metal oxide nanostructures have been investigated extensively due to their wide range of physical properties; zinc oxide is one of the most promising materials. It exhibits fascinating functional properties and various types of morphologies. In particular, ZnO heterostructures have attracted great attention because their performance can be modified and further improved by the addition of other materials. In this study, we successfully transformed ZnO nanowires (NWs) into multiple ZnO/Al2O3 heterostructure NWs via a solid-state cation exchange reaction. The experiment was carried out in situ via an ultrahigh vacuum transmission electron microscope (UHV-TEM), which was equipped with a video recorder. Moreover, we analyzed the structure and composition of the heterostructure NWs by Cs-corrected STEM equipped with EDS. Based on these experimental results, we inferred a cation exchange reaction ion path model. Additionally, we investigated the defects that appeared after the cation reaction, which resulted from the remaining zinc ions. These multiple heterostructure ZnO/Al2O3 NWs exhibited excellent UV sensing sensitivity and efficiency.
Charge-transfer energy in iridates: A hard x-ray photoelectron spectroscopy study
(2020-07-13)
Takegami, D.
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Kasinathan, D.
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Wolff, K. K.
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Altendorf, S. G.
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Chang, C. F.
We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir4+ or Ir5+ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight-binding calculations including only the Ir 5d, O 2p, and O 2s orbitals with parameters based on the downfolding of the density-functional band structure results. We found that, regardless of the A and B cations, the A(2)BIrO(6) iridates have essentially zero O 2p to Ir 5d charge-transfer energies. Hence double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.
CMOS 0.18 mu m Standard Process Capacitive MEMS High-Q Oscillator with Ultra Low-Power TIA Readout System
(2014)
Kuo, F. Y.
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Chang, C. F.
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Wen, K. A.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
This paper presents the first monolithic ultra-low power MEMS oscillator that can be manufactured in the ASIC compatible standard CMOS process and monolithically integrated with TIA circuitry. It is designed for high Q value and moderate motional impedance under strict design constraints of the standard fabrication process. A high gain ultra-low power sustaining TIA amplifier circuit is compactly integrated with the resonator structure on a single die for low-power 32 kHz clock generation. The proposed 1.69 mu W MEMS oscillator can be embedded in common SoC applications monolithically to provide clock sources.
Comparison of MiM performance with various electrodes and dieletric in Cu dual damascene of CMOS MS/RF technology
(2006)
King, M. C.
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Chang, C. F.
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Lin, H. J.
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Chin, Albert
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電子工程學系及電子研究所
In this paper, we have compared the performance of metal-insulator-metal (MiM) capacitors for different bottom electrode materials including Cu, TaN, and Al in Cu-back-end-of-the-line (BEOL) process. A high-performance and low-defect-density MiM capacitor for mixed-signal and radio frequency (MS/RF) technology based on a 130 nm complementary metal oxide semiconductor (CMOS) process was demonstrated. Q-factor can achieve > 100 for both Cu and Al at 2.4 GHz with 0.7 pF MiM capacitors. TaN showed a low Q-factor (< 60) due to high resistivity. The process incorporates aluminum electrode into Cu BEOL for MiM capacitor with a cost-effective process. The roughness of electro-dielectric interface by a thin aluminum electrode is critical for MiM performance due to field enhancement by roughness of a thin aluminum electrode. We have demonstrated a way to eliminate the roughness effect of thin Al and provide a MiM capacitor with high performance and low defect density. In particular, a method is demonstrated to achieve better matching, leakage, electrical breakdown, and temperature coefficient of capacitance performance for MiM capacitors. (c) 2006 The Electrochemical Society.
Molecular beam epitaxy preparation and in situ characterization of FeTe thin films
(2020-02-28)
Pereira, V. M.
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Wu, C. N.
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Liu, C. E.
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Liao, S-S
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Chang, C. F.
We have synthesized Fe1+y Te thin films by means of molecular beam epitaxy (MBE) under Te-limited growth conditions. We found that epitaxial layer-by-layer growth is possible for a wide range of excess Fe values, wider than expected from what is known from studies on the bulk material. Using x-ray magnetic circular dichroism spectroscopy at the Fe L-2(,3) and Te M-4(,5) edges, we observed that films with high excess Fe contain ferromagnetic clusters while films with lower excess Fe remain nonmagnetic. Moreover, x-ray absorption spectroscopy showed that it is possible to obtain films with very similar electronic structure as that of a high quality bulk single crystal Fe-1.(14) Te. Our results suggest that MBE with Te-limited growth may provide an opportunity to synthesize FeTe films with smaller amounts of excess Fe as to come closer to a possible superconducting phase.
Orbital polarization and Jahn-Teller distortion of strained La0.5Sr0.5MnO3 thin films
(2007-03-01)
Wu, W. B.
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Huang, D. J.
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Huang, C.-M.
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Hsu, C.-H.
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Chang, C. F.
We report spectroscopic evidence for orbital-mediated phases in strained La0.5Sr0.5MnO3 thin films by combining soft X-ray spectroscopy and synthesis of manganite thin films. Measurements of polarization-dependent soft X-ray absorption reveal that electronic states responsible for the lowest-energy excitations in C-type antiferromagnetic La0.5Sr0.5MnO3 films have an orbital symmetry of 3z(2) - r(2), while those in A-type AFM. films have an orbital symmetry of x(2)-y(2). Such orbital polarizations in strained films of manganite result from a combined effect of the Jahn-Teller distortion and the electron correlations of Mn 3d electrons. (c) 2006 Published by Elsevier B.V.