The Experts below are selected from a list of 3720 Experts worldwide ranked by ideXlab platform
Rampi Ramprasad - One of the best experts on this subject based on the ideXlab platform.
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dopants promoting ferroelectricity in Hafnia insights from a comprehensive chemical space exploration
2017Co-Authors: Rohit Batra, Tran Doan Huan, George A Rossetti, Rampi RamprasadAbstract:Although dopants have been extensively employed to promote ferroelectricity in Hafnia films, their role in stabilizing the responsible ferroelectric nonequilibrium Pca21 phase is not well understood. In this work, using first-principles computations, we investigate the influence of nearly 40 dopants on the phase stability in bulk Hafnia to identify dopants that can favor formation of the polar Pca21 phase. Although no dopant was found to stabilize this polar phase as the ground state, suggesting that dopants alone cannot induce ferroelectricity in Hafnia, Ca, Sr, Ba, La, Y, and Gd were found to significantly lower the energy of the polar phase with respect to the equilibrium monoclinic phase. These results are consistent with the empirical measurements of large remnant polarization in Hafnia films doped with these elements. Additionally, clear chemical trends of dopants with larger ionic radii and lower electronegativity favoring the polar Pca21 phase in Hafnia were identified. For this polar phase, an ad...
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dopants promoting ferroelectricity in Hafnia insights from a comprehensive chemical space exploration
2017Co-Authors: Rohit Batra, Tran Doan Huan, George A Rossetti, Rampi RamprasadAbstract:Although dopants have been extensively employed to promote ferroelectricity in Hafnia films, their role in stabilizing the responsible ferroelectric non-equilibrium Pca21 phase is not well understood. In this work, using first principles computations, we investigate the influence of nearly 40 dopants on the phase stability in bulk Hafnia to identify dopants that can favor formation of the polar Pca21 phase. Although no dopant was found to stabilize this polar phase as the ground state, suggesting that dopants alone cannot induce ferroelectricity in Hafnia, Ca, Sr, Ba, La, Y and Gd were found to significantly lower the energy of the polar phase with respect to the equilibrium monoclinic phase. These results are consistent with the empirical measurements of large remnant polarization in Hafnia films doped with these elements. Additionally, clear chemical trends of dopants with larger ionic radii and lower electronegativity favoring the polar Pca21 phase in Hafnia were identified. For this polar phase, an additional bond between the dopant cation and the 2nd nearest oxygen neighbor was identified as the root-cause of these trends. Further, trivalent dopants (Y, La, and Gd) were revealed to stabilize the polar Pca21 phase at lower strains when compared to divalent dopants (Sr and Ba). Based on these insights, we predict that the lanthanide series metals, the lower half of alkaline earth metals (Ca, Sr and Ba) and Y as the most suitable dopants to promote ferroelectricity in Hafnia.
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factors favoring ferroelectricity in Hafnia a first principles computational study
2017Co-Authors: Rohit Batra, Tran Doan Huan, George A Rossetti, Jacob L Jones, Rampi RamprasadAbstract:The surprising ferroelectricity displayed by Hafnia thin films has been attributed to a metastable polar orthorhombic (Pca21) phase. Nevertheless, the conditions under which this (or another competing) ferroelectric phase may be stabilized remain unresolved. It has been hypothesized that a variety of factors, including strain, grain size, electric field, impurities and dopants, may contribute to the observed ferroelectricity. Here, we use first-principles computations to examine the influence of mechanical and electrical boundary conditions (i.e., strain and electric field) on the relative stability of a variety of relevant nonpolar and polar phases of Hafnia. We find that although strain or electric field, independently, do not lead to a ferroelectric phase, the combined influence of in-plane equibiaxial deformation and electric field results in the emergence of the polar Pca21 structure as the equilibrium phase. The results provide insights for better controlling the ferroelectric characteristics of haf...
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stabilization of metastable phases in Hafnia owing to surface energy effects
2016Co-Authors: Rohit Batra, Huan Doan Tran, Rampi RamprasadAbstract:The recent empirical observation of ferroelectricity in Hafnia is rather surprising since all of its known ground-state phases are nonpolar. In this letter, we show that finite size effects, relevant under experimental conditions, may themselves lead to this unexpected phenomenon due to stabilization of metastable polar phase(s). Using surface energies computed from first principles, we determine the thermodynamic stability of a parallelepiped shaped particle constructed from various low energy nonpolar and polar phases of Hafnia. We find that at small dimensions, surface effects may stabilize either one of the polar phases or the nonpolar tetragonal phase (the parent phase of the polar phases), suggesting a possible explanation of the ferroelectric behavior observed in pure Hafnia films. These results also explain the stabilization of the metastable tetragonal phase in nanoparticles of zirconia, the twin oxide of Hafnia. While a comprehensive understanding of the origins of ferroelectricity in Hafnia wil...
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stabilization of metastable phases in Hafnia owing to surface energy effects
2016Co-Authors: Rohit Batra, Huan Doan Tran, Rampi RamprasadAbstract:The recent empirical observation of ferroelectricity in Hafnia is rather surprising since all of its known ground-state phases are nonpolar. In this letter, we show that finite size effects, relevant under experimental conditions, may themselves lead to this unexpected phenomenon due to stabilization of metastable polar phase(s). Using surface energies computed from first principles, we determine the thermodynamic stability of a parallelepiped shaped particle constructed from various low energy nonpolar and polar phases of Hafnia. We find that at small dimensions, surface effects may stabilize either one of the polar phases or the nonpolar tetragonal phase (the parent phase of the polar phases), suggesting a possible explanation of the ferroelectric behavior observed in pure Hafnia films. These results also explain the stabilization of the metastable tetragonal phase in nanoparticles of zirconia, the twin oxide of Hafnia. While a comprehensive understanding of the origins of ferroelectricity in Hafnia will require inclusion of other relevant factors (such as strain and dopants), this work highlights the importance of finite size effects as a possible key factor.
Yi Wei - One of the best experts on this subject based on the ideXlab platform.
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comparative genomic analysis of the Hafnia genus reveals an explicit evolutionary relationship between the species alvei and paralvei and provides insights into pathogenicity
2019Co-Authors: Zhiqiu Yin, Chao Yuan, Pan Yang, Chengqian Qian, Yi Wei, Si Zhang, Di Huang, Bin LiuAbstract:The Hafnia genus is an opportunistic pathogen that has been implicated in both nosocomial and community-acquired infections. Although Hafnia is fairly often isolated from clinical material, its taxonomy has remained an unsolved riddle, and the involvement and importance of Hafnia in human disease is also uncertain. Here, we used comparative genomic analysis to define the taxonomy of Hafnia, identify species-specific genes that may be the result of ecological and pathogenic specialization, and reveal virulence-related genetic profiles that may contribute to pathogenesis. One complete genome sequence and 19 draft genome sequences for Hafnia strains were generated and combined with 27 publicly available genomes. We provided high-resolution typing methods by constructing phylogeny and population structure based on single-copy core genes in combination with whole genome average nucleotide identity to identify two distant Hafnia species (alvei and paralvei) and one mislabeled strain. The open pan-genome and the presence of numerous mobile genetic elements reveal that Hafnia has undergone massive gene rearrangements. Presence of species-specific core genomes associated with metabolism and transport suggests the putative niche differentiation between alvei and paralvei. We also identified possession of diverse virulence-related profiles in both Hafnia species., including the macromolecular secretion system, virulence, and antimicrobial resistance. In the macromolecular system, T1SS, Flagellum 1, Tad pilus and T6SS-1 were conserved in Hafnia, whereas T4SS, T5SS, and other T6SSs exhibited the evolution of diversity. The virulence factors in Hafnia are related to adherence, toxin, iron uptake, stress adaptation, and efflux pump. The identified resistance genes are associated with aminoglycoside, beta-lactam, bacitracin, cationic antimicrobial peptide, fluoroquinolone, and rifampin. These virulence-related profiles identified at the genomic level provide insights into Hafnia pathogenesis and the differentiation between alvei and paralvei. Our research using core genome phylogeny and comparative genomics analysis of a larger collection of strains provides a comprehensive view of the taxonomy and species-specific traits between Hafnia species. Deciphering the genome of Hafnia strains possessing a reservoir of macromolecular secretion systems, virulence factors, and resistance genes related to pathogenicity may provide insights into addressing its numerous infections and devising strategies to combat the pathogen.
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comparative genomic analysis of the Hafnia genus reveals an explicit evolutionary relationship between the species alvei and paralvei and provides insights into pathogenicity
2019Co-Authors: Zhiqiu Yin, Chao Yuan, Pan Yang, Chengqian Qian, Yi WeiAbstract:Background The Hafnia genus is an opportunistic pathogen that has been implicated in both nosocomial and community-acquired infections. Although Hafnia is fairly often isolated from clinical material, its taxonomy has remained an unsolved riddle, and the involvement and importance of Hafnia in human disease is also uncertain. Here, we used comparative genomic analysis to define the taxonomy of Hafnia, identify species-specific genes that may be the result of ecological and pathogenic specialization, and reveal virulence-related genetic profiles that may contribute to pathogenesis.
Rohit Batra - One of the best experts on this subject based on the ideXlab platform.
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dopants promoting ferroelectricity in Hafnia insights from a comprehensive chemical space exploration
2017Co-Authors: Rohit Batra, Tran Doan Huan, George A Rossetti, Rampi RamprasadAbstract:Although dopants have been extensively employed to promote ferroelectricity in Hafnia films, their role in stabilizing the responsible ferroelectric nonequilibrium Pca21 phase is not well understood. In this work, using first-principles computations, we investigate the influence of nearly 40 dopants on the phase stability in bulk Hafnia to identify dopants that can favor formation of the polar Pca21 phase. Although no dopant was found to stabilize this polar phase as the ground state, suggesting that dopants alone cannot induce ferroelectricity in Hafnia, Ca, Sr, Ba, La, Y, and Gd were found to significantly lower the energy of the polar phase with respect to the equilibrium monoclinic phase. These results are consistent with the empirical measurements of large remnant polarization in Hafnia films doped with these elements. Additionally, clear chemical trends of dopants with larger ionic radii and lower electronegativity favoring the polar Pca21 phase in Hafnia were identified. For this polar phase, an ad...
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dopants promoting ferroelectricity in Hafnia insights from a comprehensive chemical space exploration
2017Co-Authors: Rohit Batra, Tran Doan Huan, George A Rossetti, Rampi RamprasadAbstract:Although dopants have been extensively employed to promote ferroelectricity in Hafnia films, their role in stabilizing the responsible ferroelectric non-equilibrium Pca21 phase is not well understood. In this work, using first principles computations, we investigate the influence of nearly 40 dopants on the phase stability in bulk Hafnia to identify dopants that can favor formation of the polar Pca21 phase. Although no dopant was found to stabilize this polar phase as the ground state, suggesting that dopants alone cannot induce ferroelectricity in Hafnia, Ca, Sr, Ba, La, Y and Gd were found to significantly lower the energy of the polar phase with respect to the equilibrium monoclinic phase. These results are consistent with the empirical measurements of large remnant polarization in Hafnia films doped with these elements. Additionally, clear chemical trends of dopants with larger ionic radii and lower electronegativity favoring the polar Pca21 phase in Hafnia were identified. For this polar phase, an additional bond between the dopant cation and the 2nd nearest oxygen neighbor was identified as the root-cause of these trends. Further, trivalent dopants (Y, La, and Gd) were revealed to stabilize the polar Pca21 phase at lower strains when compared to divalent dopants (Sr and Ba). Based on these insights, we predict that the lanthanide series metals, the lower half of alkaline earth metals (Ca, Sr and Ba) and Y as the most suitable dopants to promote ferroelectricity in Hafnia.
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factors favoring ferroelectricity in Hafnia a first principles computational study
2017Co-Authors: Rohit Batra, Tran Doan Huan, George A Rossetti, Jacob L Jones, Rampi RamprasadAbstract:The surprising ferroelectricity displayed by Hafnia thin films has been attributed to a metastable polar orthorhombic (Pca21) phase. Nevertheless, the conditions under which this (or another competing) ferroelectric phase may be stabilized remain unresolved. It has been hypothesized that a variety of factors, including strain, grain size, electric field, impurities and dopants, may contribute to the observed ferroelectricity. Here, we use first-principles computations to examine the influence of mechanical and electrical boundary conditions (i.e., strain and electric field) on the relative stability of a variety of relevant nonpolar and polar phases of Hafnia. We find that although strain or electric field, independently, do not lead to a ferroelectric phase, the combined influence of in-plane equibiaxial deformation and electric field results in the emergence of the polar Pca21 structure as the equilibrium phase. The results provide insights for better controlling the ferroelectric characteristics of haf...
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stabilization of metastable phases in Hafnia owing to surface energy effects
2016Co-Authors: Rohit Batra, Huan Doan Tran, Rampi RamprasadAbstract:The recent empirical observation of ferroelectricity in Hafnia is rather surprising since all of its known ground-state phases are nonpolar. In this letter, we show that finite size effects, relevant under experimental conditions, may themselves lead to this unexpected phenomenon due to stabilization of metastable polar phase(s). Using surface energies computed from first principles, we determine the thermodynamic stability of a parallelepiped shaped particle constructed from various low energy nonpolar and polar phases of Hafnia. We find that at small dimensions, surface effects may stabilize either one of the polar phases or the nonpolar tetragonal phase (the parent phase of the polar phases), suggesting a possible explanation of the ferroelectric behavior observed in pure Hafnia films. These results also explain the stabilization of the metastable tetragonal phase in nanoparticles of zirconia, the twin oxide of Hafnia. While a comprehensive understanding of the origins of ferroelectricity in Hafnia wil...
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stabilization of metastable phases in Hafnia owing to surface energy effects
2016Co-Authors: Rohit Batra, Huan Doan Tran, Rampi RamprasadAbstract:The recent empirical observation of ferroelectricity in Hafnia is rather surprising since all of its known ground-state phases are nonpolar. In this letter, we show that finite size effects, relevant under experimental conditions, may themselves lead to this unexpected phenomenon due to stabilization of metastable polar phase(s). Using surface energies computed from first principles, we determine the thermodynamic stability of a parallelepiped shaped particle constructed from various low energy nonpolar and polar phases of Hafnia. We find that at small dimensions, surface effects may stabilize either one of the polar phases or the nonpolar tetragonal phase (the parent phase of the polar phases), suggesting a possible explanation of the ferroelectric behavior observed in pure Hafnia films. These results also explain the stabilization of the metastable tetragonal phase in nanoparticles of zirconia, the twin oxide of Hafnia. While a comprehensive understanding of the origins of ferroelectricity in Hafnia will require inclusion of other relevant factors (such as strain and dopants), this work highlights the importance of finite size effects as a possible key factor.
Uwe Schroeder - One of the best experts on this subject based on the ideXlab platform.
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Ferroelectric hafnium oxide for ferroelectric random-access memories and ferroelectric field-effect transistors
2018Co-Authors: Thomas Mikolajick, Stefan Slesazeck, Min Hyuk Park, Uwe SchroederAbstract:Ferroelectrics are promising for nonvolatile memories. However, the difficulty of fabricating ferroelectric layers and integrating them into complementary metal oxide semiconductor (CMOS) devices has hindered rapid scaling. Hafnium oxide is a standard material available in CMOS processes. Ferroelectricity in Si-doped Hafnia was first reported in 2011, and this has revived interest in using ferroelectric memories for various applications. Ferroelectric Hafnia with matured atomic layer deposition techniques is compatible with three-dimensional capacitors and can solve the scaling limitations in 1-transistor-1-capacitor (1T-1C) ferroelectric random-access memories (FeRAMs). For ferroelectric field-effect-transistors (FeFETs), the low permittivity and high coercive field E _c of Hafnia ferroelectrics are beneficial. The much higher E _c of ferroelectric Hafnia, however, makes high endurance a challenge. This article summarizes the current status of ferroelectricity in Hafnia and explains how major issues of 1T-1C FeRAMs and FeFETs can be solved using this material system.
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analysis of performance instabilities of Hafnia based ferroelectrics using modulus spectroscopy and thermally stimulated depolarization currents
2018Co-Authors: Franz P G Fengler, Thomas Mikolajick, Robin Nigon, Paul Muralt, Everett D Grimley, Xiahan Sang, Violetta Sessi, Rico Hentschel, James M Lebeau, Uwe SchroederAbstract:The discovery of the ferroelectric orthorhombic phase in doped Hafnia films has sparked immense research efforts. Presently, a major obstacle for Hafnia's use in high-endurance memory applications like nonvolatile random-access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field-induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake-up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in Hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance.
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lanthanum doped hafnium oxide a robust ferroelectric material
2018Co-Authors: Uwe Schroeder, Min Hyuk Park, Franz P G Fengler, Claudia Richter, Tony Schenk, Milan Pesic, Michael J Hoffmann, Darius Pohl, Bernd Rellinghaus, Chuanzhen ZhouAbstract:Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all Hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped Hafnia. La:HfO2 appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.
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Lanthanum-Doped Hafnium Oxide: A Robust Ferroelectric Material
2018Co-Authors: Uwe Schroeder, Min Hyuk Park, Franz P G Fengler, Claudia Richter, Tony Schenk, Darius Pohl, Bernd Rellinghaus, Milan Pešić, Michael Hoffmann, Chuanzhen ZhouAbstract:Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all Hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped Hafnia. La:HfO2 appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices
Zhiqiu Yin - One of the best experts on this subject based on the ideXlab platform.
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comparative genomic analysis of the Hafnia genus reveals an explicit evolutionary relationship between the species alvei and paralvei and provides insights into pathogenicity
2019Co-Authors: Zhiqiu Yin, Chao Yuan, Pan Yang, Chengqian Qian, Yi Wei, Si Zhang, Di Huang, Bin LiuAbstract:The Hafnia genus is an opportunistic pathogen that has been implicated in both nosocomial and community-acquired infections. Although Hafnia is fairly often isolated from clinical material, its taxonomy has remained an unsolved riddle, and the involvement and importance of Hafnia in human disease is also uncertain. Here, we used comparative genomic analysis to define the taxonomy of Hafnia, identify species-specific genes that may be the result of ecological and pathogenic specialization, and reveal virulence-related genetic profiles that may contribute to pathogenesis. One complete genome sequence and 19 draft genome sequences for Hafnia strains were generated and combined with 27 publicly available genomes. We provided high-resolution typing methods by constructing phylogeny and population structure based on single-copy core genes in combination with whole genome average nucleotide identity to identify two distant Hafnia species (alvei and paralvei) and one mislabeled strain. The open pan-genome and the presence of numerous mobile genetic elements reveal that Hafnia has undergone massive gene rearrangements. Presence of species-specific core genomes associated with metabolism and transport suggests the putative niche differentiation between alvei and paralvei. We also identified possession of diverse virulence-related profiles in both Hafnia species., including the macromolecular secretion system, virulence, and antimicrobial resistance. In the macromolecular system, T1SS, Flagellum 1, Tad pilus and T6SS-1 were conserved in Hafnia, whereas T4SS, T5SS, and other T6SSs exhibited the evolution of diversity. The virulence factors in Hafnia are related to adherence, toxin, iron uptake, stress adaptation, and efflux pump. The identified resistance genes are associated with aminoglycoside, beta-lactam, bacitracin, cationic antimicrobial peptide, fluoroquinolone, and rifampin. These virulence-related profiles identified at the genomic level provide insights into Hafnia pathogenesis and the differentiation between alvei and paralvei. Our research using core genome phylogeny and comparative genomics analysis of a larger collection of strains provides a comprehensive view of the taxonomy and species-specific traits between Hafnia species. Deciphering the genome of Hafnia strains possessing a reservoir of macromolecular secretion systems, virulence factors, and resistance genes related to pathogenicity may provide insights into addressing its numerous infections and devising strategies to combat the pathogen.
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comparative genomic analysis of the Hafnia genus reveals an explicit evolutionary relationship between the species alvei and paralvei and provides insights into pathogenicity
2019Co-Authors: Zhiqiu Yin, Chao Yuan, Pan Yang, Chengqian Qian, Yi WeiAbstract:Background The Hafnia genus is an opportunistic pathogen that has been implicated in both nosocomial and community-acquired infections. Although Hafnia is fairly often isolated from clinical material, its taxonomy has remained an unsolved riddle, and the involvement and importance of Hafnia in human disease is also uncertain. Here, we used comparative genomic analysis to define the taxonomy of Hafnia, identify species-specific genes that may be the result of ecological and pathogenic specialization, and reveal virulence-related genetic profiles that may contribute to pathogenesis.
