Browsing by Author "Yang, C. -H."

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A Strain-Driven Morphotropic Phase Boundary in BiFeO(3)
(2009-11-13)
Zeches, R. J.
;
Rossell, M. D.
;
Zhang, J. X.
;
Hatt, A. J.
;
He, Q.
Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO(3)) films. Electric field-dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.
A Strain-Driven Morphotropic Phase Boundary in BiFeO3
(2009-11-13)
Zeches, R. J.
;
Rossell, M. D.
;
Zhang, J. X.
;
Hatt, A. J.
;
He, Q.
Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field-dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.
Above-bandgap voltages from ferroelectric photovoltaic devices
(2010-02-01)
Yang, S. Y.
;
Seidel, J.
;
Byrnes, S. J.
;
Shafer, P.
;
Yang, C. -H.
In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.
Domain Wall Conductivity in La-Doped BiFeO(3)
(2010-11-05)
Seidel, J.
;
Maksymovych, P.
;
Batra, Y.
;
Katan, A.
;
Yang, S. -Y.
The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO(3)) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.
Domain Wall Conductivity in La-Doped BiFeO3
(2010-11-05)
Seidel, J.
;
Maksymovych, P.
;
Batra, Y.
;
Katan, A.
;
Yang, S. -Y.
The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO3) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.
Electric modulation of conduction in multiferroic Ca-doped BiFeO(3) films
(2009-06-01)
Yang, C. -H.
;
Seidel, J.
;
Kim, S. Y.
;
Rossen, P. B.
;
Yu, P.
Many interesting materials phenomena such as the emergence of high-T(c) superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO(3) through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of similar to 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO(3).
Electric modulation of conduction in multiferroic Ca-doped BiFeO3 films
(2009-06-01)
Yang, C. -H.
;
Seidel, J.
;
Kim, S. Y.
;
Rossen, P. B.
;
Yu, P.
Many interesting materials phenomena such as the emergence of high-T-c superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of similar to 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO3.
Full Low Temperature Microwave Processed Ge CMOS Achieving Diffusion-Less Junction and Ultrathin 7.5nm Ni Mono-Germanide
(2012-01-01)
Lee, Y. -J.
;
Chuang, S. -S.
;
Liu, C. -I.
;
Hsueh, F. -K.
;
Sung, P. -J.
For the first time, Ge CMOS with all thermal processes performed by microwave annealing (MWA) has been realized. The full MWA process is under 390 degrees C. It significantly outperforms conventional rapid thermal annealing (RTA) process in 3 aspects: (1) Diffusion-less junction: for easily diffused n-type dopant, phosphorous (P), the ion implantation dopant profile after the MWA activation process remains unchanged. (2) Increased C-ox and lower gate leakage: the low temperature activation process leads to less Ge out-diffusion during MWA than RTA, suppressing the degradation of gate dielectric/ Ge channel interface. (3) Ultrathin 7.5nm Ni mono-germanide with low sheet resistance (Rs) and contact resistivity: after two-step MWA, a thin mono-NiGe layer was obtained which has larger crystallite size to lower Rs. Ge n- and p-MOSFET were also demonstrated. Compared to conventional RTA, the MWA gives 50% and 24% drive current enhancement for p- and n-MOSFET, respectively. These data show that the low temperature MWA is a very promising thermal process technology for Ge CMOS manufacturing.
Interface control of bulk ferroelectric polarization
(2012-06-19)
Yu, P.
;
Luo, W.
;
Yi, D.
;
Zhang, J. X.
;
Rossell, M. D.
The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.
Interface Ferromagnetism and Orbital Reconstruction in BiFeO(3)-La(0.7)Sr(0.3)MnO(3) Heterostructures
(2010-07-06)
Yu, P.
;
Lee, J. -S.
;
Okamoto, S.
;
Rossell, M. D.
;
Huijben, M.
We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO(3) at the interface with ferromagnet La(0.7)Sr(0.3)MnO(3). Using x-ray magnetic circular dichroism at Mn and Fe L(2,3) edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related to an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at the oxygen K edge.
Interface Ferromagnetism and Orbital Reconstruction in BiFeO3-La0.7Sr0.3MnO3 Heterostructures
(2010-07-06)
Yu, P.
;
Lee, J. -S.
;
Okamoto, S.
;
Rossell, M. D.
;
Huijben, M.
We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO3 at the interface with ferromagnet La0.7Sr0.3MnO3. Using x-ray magnetic circular dichroism at Mn and Fe L-2,L-3 edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related to an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at the oxygen K edge.
Photovoltaic effects in BiFeO(3)
(2009-08-10)
Yang, S. Y.
;
Martin, L. W.
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Byrnes, S. J.
;
Conry, T. E.
;
Basu, S. R.
We report a photovoltaic effect in ferroelectric BiFeO(3) thin films. The all-oxide heterostructures with SrRuO(3) bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages similar to 0.8-0.9 V and external quantum efficiencies up to similar to 10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO(3)/tin doped indium oxide interface.
Photovoltaic effects in BiFeO3
(2009-08-10)
Yang, S. Y.
;
Martin, L. W.
;
Byrnes, S. J.
;
Conry, T. E.
;
Basu, S. R.
We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages similar to 0.8-0.9 V and external quantum efficiencies up to similar to 10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.
Probing the evolution of antiferromagnetism in multiferroics
(2010-04-01)
Holcomb, M. B.
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Martin, L. W.
;
Scholl, A.
;
He, Q.
;
Yu, P.
This study delineates the evolution of magnetic order in epitaxial films of the room-temperature multiferroic BiFeO(3) system. Using angle-and temperature-dependent dichroic measurements and spectromicroscopy, we have observed that the antiferromagnetic order in the model multiferroic BiFeO(3) evolves systematically as a function of thickness and strain. Lattice-mismatch-induced strain is found to break the easy-plane magnetic symmetry of the bulk and leads to an easy axis of magnetization which can be controlled through strain. Understanding the evolution of magnetic structure and how to manipulate the magnetism in this model multiferroic has significant implications for utilization of such magnetoelectric materials in future applications.
Selective A- or B-site single termination on surfaces of layered oxide SrLaAlO4
(2013-02-04)
Biswas, A.
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Rossen, P. B.
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Ravichandran, J.
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Chu, Y. -H.
;
Lee, Y. -W.
We demonstrate that thermal annealing in cation controlled environments is an effective means to obtain atomically flat and chemically single terminated surfaces of a layer structured substrate. The effectiveness of the cation controlled annealing method is proved with SrLaAlO4, which is a representative layer structured substrate of A(2)BO(4) type. Potassium ion scattering, in particular, shows that the method allows not only single termination but also selective termination of either A-or B-site on the substrate. We further demonstrate that the chemical nature of underlying SrLaAlO4 substrates is of critical importance in the growth of SrRuO3 thin films resulting in different morphologies and transport properties. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790575]
Two-phased meta-heuristic methods for the post-mapping yield control problem
(2006-11-15)
Su, C. -T.
;
Yang, C. -H.
;
工業工程與管理學系
;
Department of Industrial Engineering and Management
Yield control plays an important role in the TFT-LCD manufacturing firms, and the post-mapping operation is a crucial step. The post-mapping operation combines one TFT plate and one CF plate to form a LCD. Each TFT and CF plate is divided into a number of panels. The LCD panel is acceptable only when both TFT and CF panels are good. The TFT-LCD manufacturing firms use the sorter, a kind of robot, to increase the yield for matching TFT and CF plates. Evidently, there will be a great loss if a random mapping policy is executed. In this study, we first apply two of the most popular meta-heuristic methods to solve the post-mapping problem: Genetic Algorithm (GA) and Simulated Annealing ( SA). However, when the number of matched cassettes is large, the number of ways for choosing different matched objects will become so enormous that the initial population in GA ( or initial solution in SA) should be selected with a proper procedure. That is, we propose a two-phased GA and SA to improve the performance of the initial population. The basic concept of phase one is to generate an efficient initial population ( or initial solution). In phase one, the initial population is created based on the optimal solution to the cassette-matching problem. In phase two, we perform GA ( or SA) with the initial population created in phase one. The four different heuristic algorithms are tested for the same data to compare the various ports in the post-mapping yield control problem. The result shows that proposed two-phased algorithms provide a more excellent solution than GA and SA.