Symposium Organizers
Elizabeth Dickey, North Carolina State University
Ulrich Aschauer, University of Bern
Nicole Benedek, Cornell University
Sakyo Hirose, Murata Manufacturing Co Ltd
EM05.01: Defect Segregation and Space Charges
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 1, Room 109
8:30 AM - *EM05.01.01
Impact of Native Point Defect Movement and Segregation on Oxide Interface Electronics
Leonard Brillson 1 , Geoffrey Foster 1 , Jonathan Cox 1 , William Ruane 1 , Alexander Jarjour 3 , Hantian Gao 1 , Yuanyao Zhang 2 , Holger von Wenckstern 4 , Marius Grundmann 4 , Buguo Wang 6 , David Look 6 , Alana Hyland 5 , Martin Allen 5
1 , The Ohio State University, Columbus, Ohio, United States, 3 Physics, Cornell University, Ithaca, New York, United States, 2 Nanoengineering, University of California, San Diego, San Diego, California, United States, 4 Institute for Experimental Physics, University of Leipzig, Leipzig Germany, 6 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States, 5 Electrical and Computer Engineering, University of Canterbury, Christchurch New Zealand
Show AbstractNanoscale optical and electrostatic techniques are now able to measure the movement of native point defects inside oxide semiconductors directly. These defects are electrically-active, impacting free carrier density and mobility. Their segregation toward surfaces and interfaces can alter space charge regions, tunneling, and contact rectification. We used depth-resolved cathodoluminescence spectroscopy coupled with hyperspectral imaging to measure defect redistribution in three dimensions on a nanoscale for oxides including ZnO, MgZnO, SrTiO3, and BaSrTiO3 as well as conventional semiconductors including SiC and GaN. These results help assess the relative importance of physical mechanisms such as electrical or chemical potentials, intrinsic or externally applied, that can drive interface segregation and their effect on Schottky barriers and ohmic contacts. Defect formation and segregation in polycrystalline ZnO during flash sintering reveals nanoscale defect redistribution that may provide insights into dielectric breakdown phenomena in general. The magnitude of defect segregation at metal-oxide diodes on ZnO reveals a combination of carrier density changes, tunneling, and trap-assisted hopping that account for major differences in Zn- vs. O-polar Schottky barriers with the same metal. Extending these measurements from bulk oxides to nanostructures, we found that native point defects are present inside, not only on the surfaces of nano- and microwires of ZnO as conventionally viewed.[1] Nanoscale localized cathodoluminescence spectroscopy shows the nature of spatial distribution of these defects to be growth-dependent.[2] Three-dimensional measurement and imaging on a nanoscale reveals defect segregation that is electrically-active and that can extend hundreds of nanometers into the “bulk,” introducing new donors or acceptors that respectively decrease or increase depletion widths, altering conducting channel volumes and metal-ZnO nano-contact rectification. Using a combination of electron and energy-dependent ion beams, we are able to alter defect distributions and create rectifying, ohmic, or blocking contacts with the same Pd metal on the same nanowire. Besides demonstrating how a wide range of ultrahigh vacuum contacts can be obtained within a focused ion beam system without breaking vacuum, this work demonstrates the interplay between the nature of native point defects, the intrinsic doping, and the physical dimensions of the nanostructure itself in determining the electronic properties of the oxide interface. These results also suggest new avenues for controlling native point defects in oxides and their effect on oxide electronics. This work supported by NSF Grant DMR-1305193.
1. W.T. Ruane, K. Leedy, D.C. Look, G. Farlow, H. von Wenckstern, M. Grundmann, and L.J. Brillson, Nanoscale 8, 7631 (2016).
2. L. J. Brillson, W.T. Ruane, H. Gao, Y. Zhang, J. Luo, H. von Wenckstern, and M. Grundmann, Mater. Sci. Semicond. Process. 57, 197 (2017).
9:00 AM - EM05.01.02
The Role of Defect Distribution on Space Charge Formation in Nanoscale Magnesium Aluminate Spinel
Mahdi Halabi 1 2 , Amit Kohn 3 , Shmuel Hayun 1 2
1 Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva Israel, 2 , Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva Israel, 3 Department of Materials Science and Engineering, Tel Aviv University, Ramat Aviv, Tel Aviv Israel
Show AbstractCharge distribution in magnesium aluminate spinel (MAS) results in the formation of a space-charge region that plays a critical role in assigning functional properties. Explanation of this phenomenon are reported though quantitative experimental evidence for nano-scale granular MAS is indirect. In this talk, the effect of composition, grain size and applied electric field on the space-charge potential in nanoscale MAS is presented.
The electrostatic potential distribution in nonstoichiometric grains was measured by off-axis electron holography and compared to the distribution of cations and defects as measured by electron energy loss spectroscopy.
We demonstrated quantitatively that regardless of grain size, excess Mg+2 or Al+3 cations resides in the vicinity of grain-boundaries of Mg- and Al-rich MAS, respectively. Such variations in cation and defect distribution should enable to calculate the space-charge-potential (SCP). However, the wide range of reported values for defect formation energies (0.82-8.78eV) prevents estimating the SCP. Consequently, we applied electron holography to measure directly the SCP. Furthermore, by applying a moderate electric field (~150 V/cm) during thermal annealing, structural ordering was observed.
9:15 AM - EM05.01.03
Atomic-Scale Identification of Space-Charge-Driven Solute Segregation in a Perovskite Oxide
Sung-Yoon Chung 1
1 Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon Korea (the Republic of)
Show AbstractSince the first prediction by Frenkel, many follow-up studies have been carried out to show the presence of subsurface space-charge layers having the opposite sign to that of the excess charge at the surface for overall neutrality in ionic crystals. Early theoretical works largely dealt with a free surface rather than other types of interfaces, and suggested a plausible distribution of charged defects and dopants near the surface. However, no precise experimental evidence demonstrating how the aliovalent solutes segregate in the space charge region beneath the surface has been provided over the past several decades. In this work, we elucidate the characteristics of space-charge-driven solute segregation, which is completely distinct from the segregation driven by the misfit elastic strain between the solute and solvent atoms. By utilizing both atomic-scale physical imaging and chemical probing (S.-Y. Chung et al., Angew. Chem. Int. Ed. 55, 9680 (2016)), we precisely determine the origin of the surface excess charge and the position of segregated solutes and thereby discriminate the grain-boundary core and the top-most surface from the space-charge region at atomic resolution. The findings in this work highlight the impact of the space-charge contribution to the solute distribution near the surface in oxide crystals, and thus information from the surface and the subsurface space-charge layer is necessary to fully understand the surface region.
9:30 AM - EM05.01.04
Bayesian Calibration of a Poisson-Cahn Model Predicting Co-Accumulation of Cationic and Anionic Defects at Grain Boundaries in Highly-Doped Ceria
Xiaorui Tong 1 , David Mebane 1 , David Diercks 2 , Brian Gorman 2
1 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia, United States, 2 Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States
Show AbstractThe presence of grain boundaries leads to defect redistributions in doped ceria, resulting in important material property changes. Atomistic simulations and atom probe tomography (APT) of grain boundaries in doped ceria have shown co-accumulation of oxygen vacancies and acceptor dopant cations. Dilute-case space-charge models are not able to replicate this phenomenon. Because atomistic models are inherently restricted to short length scales, a continuum theory is desirable. The Poisson-Cahn thermodynamic framework, which incorporates defect interactions and gradient energy contributions to the free energy of the inhomogeneous system, has been introduced to treat doped systems of all concentrations. Building on the results of a study that employed a kinetic Poisson-Cahn model to predict the kinetics of dopant segregation at surfaces and interfaces in acceptor doped fluorites, a two-step equilibration model was employed to replicate co-accumulation phenomena in Nd-doped ceria as observed by APT. Bayesian calibration of the adopted model has identified parameter spaces that can successfully predict co-enrichment of dopant species and oxygen vacancies near the grain boundaries.
EM05.02: Point Defects I
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 1, Room 109
10:15 AM - EM05.02.01
Imaging of Single La Vacancies in LaMnO3
Jie Feng 1 , Alex Kvit 1 , Chenyu Zhang 1 , Jason Hoffman 2 , Anand Bhattacharya 2 , Dane Morgan 1 , Paul Voyles 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractVacancies can control a wide variety of properties and processes in materials, including electronic and ionic conductivity, catalytic ability and luminescence. Therefore, it is necessary to control the type, concentration, and spatial distribution of vacancies to realize new materials’ functions. Three-dimensional (3D) characterization of vacancies in is an essential step in achieving control.
Here, we report an approach for 3D imaging of single cation vacancies in LaMnO3 using high precision quantitative high-angle annular dark-field Z-contrast scanning transmission electron microscopy. Vacancies are identified by both the reduction in scattered intensity created by the missing atom and the distortion of the surrounding atom positions. Vacancy positions are determined laterally to a unique lattice site in the image and in depth to within one of two lattice sites by dynamical diffraction effects. 35 single La vacancies are identified in images of a LaMnO3 thin film sample. The vacancies are randomly distributed in depth and correspond to a La vacancy concentration of 0.79%, which is consistent with the level of control of cation stoichiometry within our synthesis process (~1%) and with the equilibrium concentration of La vacancies under the film growth conditions [1].
The approach presented here is generalizable to cation vacancies in other oxides and to complexes of cation and anion vacancies. This work demonstrates an approach to characterize low concentrations of vacancies with high spatial resolution, which will accelerate our ability to use control over point defects to create new materials functionality.
Reference:
[1] Mizusaki, J. et al. Solid State Ionics 129, 163–177 (2000).
10:30 AM - *EM05.02.02
First Principles Informed Defect Chemistry in Ceramic Grade SrTiO3
Douglas Irving 1 , Jonathon Baker 1 , Preston Bowes 1 , Joshua Harris 1 , Yifeng Wu 1 , Daniel Long 1 , Nicole Creange 1 , Elizabeth Dickey 1
1 , North Carolina State University, Raleigh, North Carolina, United States
Show AbstractCeramic grade strontium titanate (STO) is an important material used for many technological applications, including capacitors and varistors. However, ceramic grade STO contains a substantial number of high concentration background impurities. The influence of these impurities on the bulk properties of the materials is not widely studied. Practically, the presence of these defects is often "mitigated" through the addition of a higher concentration of a dominant defect to control electrical properties. Here, we present results from our first principles calculations of point defect properties in the grand canonical formalism. These results are used together with charge and mass balance solutions to deduce the role of defects in optical and electronic properties of ceramic STO as a function of processing conditions. We will also discuss results of extending these methods to investigate influence of metal-dielectric interfaces on the concentrations of the point defects in the vicinity of the interface.
11:00 AM - EM05.02.03
Enhanced Electron Mobility in Nb-Doped SrTiO3 Films with Lattice Defect-Induced Strain Fields
Shunsuke Kobayashi 1 , Tsuyoshi Ohnishi 2 , Naoya Shibata 3 , Yuichi Ikuhara 1 3 , Takahisa Yamamoto 1 4
1 Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya Japan, 2 Global Research Center for Environment and Energy based on Nanomaterials Science, National Institute for Materials Science, Tsukuba Japan, 3 Institute of Engineering Innovation, The University of Tokyo, Tokyo Japan, 4 Department of Quantum Engineering, Nagoya University, Nagoya Japan
Show AbstractElectron-doped SrTiO3 film systems exhibit myriad interesting properties. To exploit these properties in useful applications, larger values of the electron mobility are required. One route to achieving high electron mobility is growth of high quality SrTiO3 films with low defect concentrations. Another approach for mobility enhancement is applying a strain to the crystal. In this study, we demonstrate a unique crystal engineering approach to alter the strain in Nb-doped SrTiO3 films based on the deliberate introduction of lattice defects or Sr vacancy clusters [1].
Nb-doped SrTiO3 films were prepared using pulsed laser deposition. Nb-doped (0.1 at. %) and undoped SrTiO3 single crystals were used for the target and substrate, respectively. The defect structure of each film was investigated using aberration-corrected scanning transmission electron microscopy (STEM). Resistivity and Hall coefficient measurements were carried out using a standard four point method and van der Pauw geometry, respectively.
Enhancement of the electron mobility in Nb-doped SrTiO3 films was achieved by introducing a unique defect structure, the rod-type Sr vacancy cluster, into their crystals through careful control of the composition and synthesis conditions. We found that rod-type Sr vacancy clusters create a compressive strain field in the SrTiO3 crystal. A LAADF imaging in STEM are very sensitive to the strain field around lattice defects. The appearance of bright contrast around the Sr vacancy cluster in LAADF STEM image indicates the presence of localized strain field. Moreover, STEM EELS analysis provide the information of compressive strain filed induced by Sr vacancy clusters. A chemical shift of the Ti-L2,3 edge toward higher energy loss and the split peak in the t2g and eg of the Ti-L2 edge were observed due to the compressive strain field. These results strongly suggest that compressive strain fields induced by Sr vacancy clusters altered the band structure in the SrTiO3 crystal. The compressive strain field results in a high electron mobility at low (including room) temperatures (e.g., over 53,000 cm2 V-1s-1 at 2 K). This mobility is about seven times larger than that of the single crystal used as a target even though there exist lattice defects in the crystal. The method we have outlined for enhancing mobility using a defect-induced strain field, however, should also be applicable to other perovskite materials, and represents a new strategy for crystal engineering of nanoelectronic materials [1].
References
[1] S. Kobayashi et al., ACS Nano 9, 10769-10777, (2015).
[2] Part of this work was supported by Grant-in-Aid for Young Scientists B (Grant No. 17K14119) from the Japan Society for the Promotion of Science (JSPS) and Kazato Research Foundation, Japan.
11:15 AM - EM05.02.04
Universality of Electron Mobility in LaAlO3/SrTiO3 and Bulk SrTiO3
Felix Trier 1 , K Reich 2 4 , Dennis Christensen 3 , Yu Zhang 3 , Harry Tuller 5 , Yunzhong Chen 3 , B. Shklovskii 2 , Nini Pryds 3
1 , Unité Mixte de Physique CNRS, Thales, University Paris-Sud, Université Paris-Saclay, Palaiseau France, 2 , Fine Theoretical Physics Institute, University of Minnesota, Minneapolis, Minnesota, United States, 4 , Ioffe Institute, St Petersburg Russian Federation, 3 DTU Energy, Technical University of Denmark, Roskilde Denmark, 5 , Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMetallic LaAlO3/SrTiO3 (LAO/STO) interfaces have attracted enormous attention, but the relationship between the electron mobility and the sheet electron density, ns, is poorly understood. Here we derive a simple expression for the three-dimensional electron density near the interface, n3D, as a function of ns and find that the mobility for LAO/STO-based interfaces depends on n3D in the same way as it does for bulk doped STO. It is known that undoped bulk STO is strongly compensated with N ≈ 5×1018 cm-3 background donors and acceptors. In intentionally doped bulk STO with a concentration of electrons n3D < N background impurities determine the electron scattering. Thus, when n3D < N it is natural to see in LAO/STO the same mobility as in the bulk. On the other hand, in the bulk samples with n3D > N the mobility reduces drastically because scattering happens on n3D intentionally introduced donors. For LAO/STO, though the polar catastrophe provides electrons, it is not supposed to provide equal number of random donors and the electron mobility should consequently be larger. The fact that the mobility for LAO/STO is still the same as for bulk doped STO implies that the polar catastrophe model should be modified.
11:30 AM - EM05.02.05
The Role of Cation Vacancies and Cation Transport in Oxide Materials
Tor Grande 1
1 , Norwegian University of Science and Technology (NTNU), Trondheim Norway
Show AbstractOxygen vacancies and migration of oxygen vacancies are well documented in oxide materials due to the importance of oxygen ion conductivity in energy technology. In relation to point and electronic defects in oxide materials, oxygen vacancies and their mobility are traditionally the only point defects considered and the role of cation vacancies and cation mobility are often neglected. We have recently reported on experimental and computational studies of cation diffusion in AIIBIVO3 perovskites. 96Zr and 134Ba tracer diffusion in polycrystalline AZrO3 (A = Ba, Sr, Ca) and BaBO3 (B = Zr, Ti, Ce) ceramics were studied by SIMS [1,2]. Systematic trends in the mobility of cations were observed with respect to cation size on the A and B site in the perovskite lattice. Grain boundary diffusion was observed to be several orders of magnitude higher than lattice diffusion. The data will be discussed in comparison to similar diffusion data on cation diffusion in AIIIBIIIO3 perovskites. Diffusion of cations in perovskite oxides is related to the Schottky point defect equilibrium and the time and length scale of the equilibration of this point defect equilibrium will be discussed in relation to possible implications for the distribution of point and electronic defects in oxide materials. We have recently also reported on surface diffusion in similar oxide materials [3], and ionic diffusion in oxide materials will finally be discussed with respect to lattice, grain boundary and surface diffusion.
1 R. Sazinas, I. Sakaguchi, I. Hasle, J. M. Polfus, R. Haugsrud, M.-A. Einarsrud and T. Grande, Tracer diffusion of 96Zr and 134Ba in polycrystalline BaZrO3, submitted.
2. R. Sazinas, I. Sakaguchi, and T. Grande, in preparation.
3. M. G. Sahini, M.-A. Einarsrud, T. Grande, Surface diffusion of oxygen transport membrane materials studied by grain boundary grooving, submitted J Am. Ceram. Soc. 100 (2017) 354.
11:45 AM - EM05.02.06
Tuning the Magnetic Ordering in EuTiO3 Bulk and Heterostructures
Anderson Janotti 1 , Zhigang Gui 1
1 , University of Delaware, Newark, Delaware, United States
Show AbstractEuTiO3 (ETO) features a strong spin-lattice coupling, large magnetoelectric effects, and undergoes a series of structural and magnetic phase transitions under pressure or epitaxial strain. Isostructural to SrTiO3, ETO displays an antiferrodistortive tetragonal structure with a G-type antiferromagnetic (AFM) ordering at very low temperatures. Several approaches have been presented to tune the magnetic ordering from the G-type antiferromagnetism to ferromagnetism, often relying on external pressure or epitaxial strain. Doping through substitution of trivalent species on the europium site or creation of oxygen vacancies have also been proposed to lead to ferromagnetism. However, the fundamental mechanism by which excess electrons from impurities or defects lead to ferromagnetic ordering is yet to be understood. Here, we explore the effects of doping on the magnetic ordering in EuTiO3 through first-principles calculations. In special, we discuss how ferromagnetic ordering can be stabilized by means of charge transfer across the interface in polar/nonpolar complex-oxide heterostructures.
EM05.03: Mixed Conductors and Memristors
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 1, Room 109
1:30 PM - EM05.03.01
Self-Regulation of Surface Chemistry in Oxygen Electrodes for Solid Oxide Fuel Cells
Clement Nicollet 1 , Dmitri Kalaev 1 , Chang Sub Kim 1 , Thomas Defferriere 1 , Harry Tuller 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractSolid oxide fuel cells (SOFCs) offer direct conversion of hydrogen or hydrocarbon fuels to electricity with high energy conversion efficiency (>50%). In reverse electrolysis operation, powered by wind or solar generated electricity, the same devices produce H2 for later consumption, thus serving as a valuable means of energy storage. As the electrolyte membranes are becoming thinner, thanks to progress in materials processing, the electrode polarization related to electrochemical reactions at the gas/solid interface of the cathode becomes the dominant flux limiting mechanism. Accumulating surface impurities (e.g. Si, Na, Al, Cr, and Sr) block active reaction sites at the gas/solid interface and lead in reductions in long term durability [1]. Identifying means for overcoming the detrimental impact of surface impurity accumulation on oxygen surface exchange kinetics can be expected to result in high payoffs in both improved performance and extended operating life.
Pr-doped ceria (PCO), a mixed ionic and electronic conductor with a high electrocatalytic activity toward the oxygen reduction reaction, has a strong advantage over state-of-the-art materials such as La0.6Sr0.4CoO3-δ and La0.6Sr0.4Co0.2Fe0.8O3-δ by not containing strontium, known to readily segregate to the solid gas interface, thereby limiting catalytic activity. Silicon contamination, however, remains a problem even with Sr-free materials, given external sources such as glass sealants, or as an impurity in precursor materials used to fabricate the devices.
This work focuses on Si contamination of PCO cathodes, observed to reduce electrode kinetics by as much as several orders of magnitude. By controlling the levels of Si contaminant applied to PCO thin films by wet chemical methods, it becomes possible to systematically study its impact on the oxygen reduction reaction by an in-situ optical transmission relaxation method [2]. We also further investigate the feasibility of using lanthanum as an efficient Si getter [3] by studying its exsolution from La substituted PCO specimens, potentially leading to a self-cleaning electrode material, more resistant to silicon poisoning.
[1] S.R. Bishop, J. Druce, J.J. Kim, J.A. Kilner, H.L. Tuller, ECS Trans. 50 (2013) 35–38.
[2] J.J. Kim. S.R. Bishop, N.J. Thompson, D. Chen, H.L. Tuller, Chem. Mater. 2014, 26, 1374−1379
[3] L. Zhao, N.H. Perry, T. Daio, K. Sasaki, S.R. Bishop, Chem. Mat. 27 (2015) 3065-3070
1:45 PM - *EM05.03.02
Investigation of Oxygen Reduction Reaction on Mixed-Conducting Oxide Cathodes by Using Operando X-Ray Absorption Spectroscopy
Koji Amezawa 1 , Yoshinobu Fujimaki 1 , Keita Mizuno 1 , Yuta Kimura 1 , Takashi Nakamura 1 , Kiyofumi Nitta 2 , Yasuko Terada 2 , Keiji Yashiro 1 , Fumitada Iguchi 1 , Hiroo Yugami 1 , Tatsuya Kawada 1
1 , Tohoku University, Sendai Japan, 2 , JASRI, Sayo Japan
Show AbstractIn a solid oxide fuel cell cathode using a mixed ionic and electronic conductor (MIEC), it is thought the electrochemical reduction of oxygen gas takes place on oxide surfaces (double phase boundaries) as well as at triple phase boundaries. In order to directly observe the reaction in the SOFC MIEC cathode, we applied operando X-ray absorption spectroscopy measurements. For the quantitative investigation of the reaction in the SOFC MIEC cathode while eliminating the influence of the electrode microstructures, a patterned thin film model electrode, which is a kind of a columnar electrode having a very simple structure, was fabricated. LaCoO3-based cathodes on the Ce0.9Gd0.1O1.95 electrolyte were chosen as model systems. From the results of operando XAS measurements of the model patterned thin film model electrode, the contribution of the triple phase boundary reaction to the total reaction was discussed.
2:15 PM - EM05.03.03
La2NiO4+δ -Based Memristive Devices—Role of the Electrodes and Oxygen Stoichiometry Tuning
Klaasjan Maas 1 , Quentin Rafhay 2 , Michel Boudard 1 , Carmen Jimenez 1 , Herve Roussel 1 , Laetitia Rapenne 1 , Monica Burriel 1
1 , LMGP, CNRS, Grenoble-INP, Grenoble France, 2 , Univ. Grenoble Alpes, CNRS, IMEP-LAHC, Grenoble France
Show AbstractThe strive to increase control over the resistance change, as well as improving reliability and reproducibility of memristive devices has led to a multitude of creative ideas to engineer the materials composing the MIM – metal/insulator (or semiconductor)/metal - stacks. The choice of the materials used as electrodes is crucial when trying to optimize the electrical response of any MIM device. The sole change in work function of the metal will influence the charge injection and thus the resistance at the electrode/semiconductor junction [1]. When the sandwiched material is an oxide, the electrode can also undergo an oxidation process at the electrode/oxide interface under operation (i.e. when an external bias is applied to one of the electrodes). This effect is particularly important when using metals with high oxygen affinity such as Al [2] or Ti. The creation of this new interfacial layer influences the electrical characteristics and the overall resistance of the device.
The present work focuses on how the memristive response in M/La2NiO4+δ/M’ heterostructures can be tuned by changing the electrode materials or the oxygen content (δ) of the sandwiched film. Highly-oriented and dense La2NiO4±δ thin films (50 to 80 nm thick) were grown on SrTiO3 single crystal substrates by pulsed-injection metal-organic chemical vapor deposition (PI-MOCVD). The structural and microstructural characterization revealed the high crystal-quality of the films, which presented a columnar microstructure. The oxygen content in the films was varied ex-situ by annealing the samples both in oxidizing (O2) and reducing (Ar and 6%H2:Ar mixture) atmospheres. To construct the final device, metallic top electrodes were deposited by standard lithography using e-beam evaporation in cleanroom facilities. Noble metals with different work functions (Pt and Ag) as well as a metal with a high oxygen affinity (Ti) were used to optimize the switching characteristics of the device. Special emphasis was given to the electrical characteristics of the samples with the use of both I(V) sweeps and pulsed measurements to operate the device. The key role played by the oxygen content of the films on the initial resistance and on the operation window of the M/La2NiO4+δ/M’ heterostructure will be discussed. What is more, pulsed measurements showed a continuous change in resistance for a defined voltage range when using Ti as active top electrode, giving rise to multilevel programming capabilities in these nickelate-based devices.
2:30 PM - EM05.03.04
Atomic-Scale Modelling of a HfO2-Based Memristor
Antonio Claudio Padilha 1 , Keith McKenna 1
1 , University of York, York United Kingdom
Show AbstractThe memristor, also referred to as resistance switching device, is a promissing candidate for the next generation of resistive random access memories (RRAM). Its simple 2-terminal architecture - a thin film of insulating material between two electrodes - and fast read-write as well as long retention times has drawn much attention [1]. In particular, oxide-based devices have been extensively studied and oxygen-deficient conducting filaments inside the oxide matrix have been reported to be formed and dissolved during the switching process [2-4]. This is believed to happen due to the migration of oxygen ions under electric fields and temperature gradients in the insulating layer which reversibly oxidises and reduces the filament [5-7]. However, very little is known about the nature of the oxygen deficient phase, its interface with the host oxide or the associated inter-diffusion of oxygen presenting a challenge to understanding the switching process.
To address this problem we investigate the structure and electronic properties of an interface between an oxygen deficient phase of hafnium oxide (HfO) and HfO2 using first principles simulations. HfO has been identified as a stable and conductive phase of hafnium oxide and serves as a model for the material forming the conductive filament in RRAM devices [8]. We construct a supercell containing an interface between HfO and HfO2 and calculate the band alignment between the valence bands of both materials using methods we have previously applied to TiO2 [9]. We find a valance band offset of 4.26 eV and a type-II alignment indicating that HfO can in principle act as a conducting channel inside the HfO2 matrix.
Next, we characterise the electronic structure of HfO on insertion of additional oxygen ions (mimicking the switching process). We show that the insertion of oxygen atoms in this material causes the disruption of the conducting behaviour, which is in line with the models reported in the literature. Finally, we estimate the activation energy for the migration of one oxygen atom through the interface, from HfO2 to an interstitial position in HfO to be ~ 1 eV, showing that a reasonable amount of energy should be provided for the system to properly switch via the proposed mechanism. These results provide invaluable insights into prospective switching mechanism for HfO2 based RRAM devices.
[1] Pan, F., et al., Mat. Sci. Eng. R Rep., 83, 1–59 (2014)
[2] Kwon, D-H., et al., Nature Nano., 5(2), 148–53 (2010)
[3] Strachan, J. P., et al., Adv. Mat., 22, 3573-3577 (2010)
[4] Kim, G. H., et al., Appl. Phys. Lett., 98(26), 262901 (2011)
[5] Waser, R., Aono, M., Nature Materials, 6, 833-840 (2007)
[6] Hildebrandt, E., et al., J. Appl. Phys., 112, 114112 (2012)
[7] Choi, J.H., et al., Mat. Sci. Eng. R, 72, 97-137 (2011)
[8] Zhang, J., Oganov, A. R., et al., Phys. Rev. B, 92, 184104 (2015)
[9] Padilha, A. C. M., et al., Phys. Rev. Appl., 3, 024009 (2015)
2:45 PM - EM05.03.05
Effect of Interfacial Carbon Layer in Oxide ReRAM Devices
Alexander Schönhals 1 , Carlor Rosário 1 2 , Susanne Hoffmann-Eifert 3 , Rainer Waser 1 3 , Stephan Menzel 3 , Dirk Wouters 1
1 Institute of Materials in Electrical Engineering and Information Technology II, RWTH Aachen University, Aachen Germany, 2 Departamento de Física and I3N, Universidade de Aveiro, Aveiro Portugal, 3 Peter Grünberg Institut, Forschungszentrum Jülich GmbH, Jülich Germany
Show AbstractBipolar resistive switching (BRS) metal-oxide ReRAM shows outstanding potential due to high reliability, extreme scalability, and excellent CMOS compatibility. However it suffers from a low resistance ratio RHRS/RLRS of typically one to two orders of magnitude. This limitation is usually caused by a comparably low RHRS which is obtained during BRS RESET.
In order to improve the accessible RHRS/RLRS ratio in Ta2O5 based ReRAM devices, the RESET process was studied. An additional process involving oxygen exchange with the active electrode was identified as a good candidate to explain observed RESET limitation in these devices.
Based on the proposed physical model a suppression of the additional limiting process could be achieved by a material engineering approach. Therefore an oxygen blocking thin carbon layer was introduced at the interface between the metal-oxide and the active electrode in order to inhibit oxygen transport through this interface. The resulted device shows a significantly increased RHRS/RLRS ratio therefore supporting the assumption the observed additional process limits BRS RESET.
These new aspects allow for a better general understanding of the BRS RESET process and therefore for further improvement of ReRAM device properties and reliability.
EM05.04: Defects in Ferroics
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 1, Room 109
3:30 PM - *EM05.04.01
Defect-Based Routes to Control Order and Properties in Ferroelectric Thin Films
Lane Martin 1
1 , University of California, Berkeley, Berkeley, California, United States
Show AbstractModern approaches to epitaxial thin-film growth have enabled unprecedented control of ferroelectric materials including the realization of enhanced PS and TC, production of ordered-domain structures, and improved properties. Today we are looking beyond simple lattice mismatch control for new ways to manipulate and control ferroic response and to produce unexpected or emergent effects. In this talk, we will investigate a number of observations of such emergent or unexpected properties in epitaxial thin films made possible via innovative synthesis and processing methodologies. In particular, we will explore recent examples of how synthesis, defects, and epitaxial constraint can be combined to produce exotic effects in ferroic systems. Potential topics for coverage include, but are not limited to, the in situ and ex situ production of defects with ion bombardment to control defect-induced electronic states that can drive dramatic changes in leakage currents and impact ferroelectric response in materials like BaTiO3, PbTiO3, BiFeO3, etc. In turn, we will highlight specifics about the routes to produce defect-engineered ferroelectric thin films, will explore approaches to characterize and study the nature of defects that are produced – including application of techniques like deep-level transient spectroscopy, and will examine the implication of such defect structures for dielectric and ferroelectric properties – including studies of defect-based effects on switching processes and kinetics. We will end with an exploration of what further growth of defect-engineering approaches might enable in the way of novel function and applications in these materials.
4:00 PM - EM05.04.02
Ferroionic States—Coupling Between Electrochemistry of Surfaces and Interfaces with Bulk Ferroelectricity
Sergei Kalinin 1 , Sang Mo Yang 1 , Rama Vasudevan 1 , Eugene Eliseev 2 , Anna Morozovska 2
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Ukrainian Academy of Sciences, Kiev Ukraine
Show AbstractFerroelectricity on the nanoscale has remained a subject of much fascination in condensed matter physics for the last several decades. It is well-recognized that stability of the ferroelectric state necessitates effective polarization screening, and hence screening mechanism and screening charge dynamics become strongly coupled to ferroelectric phase stability and domain behavior. Previously, the role of the screening charge in macroscopic ferroelectrics was observed in phenomena such as potential retention above Curie temperature, back switching of ferroelectric domains, and chaos and intermittency during domain switching. In the last several years, multiple reports claiming ferroelectricity in ultrathin ferroelectrics and devices combining ferroelectrics with graphene and other 2D materials, based on formation of remanent polarization states, local hysteresis loops, and pressure induced switching were made. However, similar phenomena were reported for traditionally non-ferroelectric materials, creating significant level of uncertainty in the field. We pose that in the nanoscale systems, the ferroelectric state is fundamentally inseparable from electrochemical state of the surface or interface, leading to emergence of coupled electrochemical-ferroelectric states. I will present the results of experimental and theoretical work exploring the basic mechanisms of emergence of these coupled states including the basic theory and phase-field formulation for domain evolution. I further discuss the thermodynamics and thickness evolution of this state, and demonstrate the experimental pathway to establish its presence based on spectroscopic version of piezoresponse force microscopy. Finally, the role of chemical screening on domain dynamics is explored using phase-field modelling. This analysis reconciles multiple prior studies, and set forward the predictive pathways for new generations of ferroelectric devices and applications, including surface based devices and devices with electrodes with finite density of states.
This research was sponsored by the Division of Materials Sciences and Engineering, BES, DOE, and was conducted at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division.
4:15 PM - EM05.04.03
Chemical Phenomena of Local Polarization Reversal in Ferroelectric Thin Films
Anton Ievlev 1 , Chance Brown 1 , Petro Maksymovych 1 , Sergei Kalinin 1 , Olga Ovchinnikova 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractFunctionality of ferroelectric materials is defined by the number of physical and chemical phenomena on the nanoscale. Therefore, ferroelectric studies require nanoscale investigative techniques. Atomic Force Microscopy (AFM) is one of such techniques, which is widely used for characterization of the ferroelectric functional properties with nanometer spatial resolution. However, chemical contribution in the ferroelectric phenomena in the most studies are ignored. These studies can be carried out by combination of AFM with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), allowing nanoscale local investigations of the chemical properties on the sample surface and in the sample bulk.
Here, we utilize combined AFM/ToF-SIMS approach for correlated investigations of the functional and chemical phenomena in PbZr0.2Ti0.8O3 thin film. In the studies, we used AFM to induce local ferroelectric switching by the electric field of the biased tip and study associated local changes in the chemistry of the film by ToF-SIMS. Investigations showed 3-5% change in the spatial concentrations of the base chemical elements close to the sample surface. Those changes were localized within 3.5-nm surface layer are caused by the screening process minimizing depolarization electric fields produced by the boundary charges. Observed phenomenon was also found to be reversible by application of the electric field of opposite polarity, which confirms its ferroelectric origin. Results of the study shed light on the chemical phenomena associated with ferroelectric properties and are important for their fundamentals investigations and practical applications.
This work was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility.
4:30 PM - EM05.04.04
Chemical Studies of Ferroelectric Fatigue by Approach Combining Atomic Force Microscopy with Time-of-Flight Secondary Ion Mass Spectrometry
Chance Brown 1 2 , Anton Ievlev 2 , Rama Vasudevan 2 , Yunseok Kim 3 , Xiaoli Lu 4 , Marin Alexe 5 , Sergei Kalinin 2 , Olga Ovchinnikova 2
1 , University of Tennessee - Knoxville, Knoxville, Tennessee, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 , Sungkyunkwan University Advanced Institute of NanoTechnology, Suwon Korea (the Republic of), 4 , Xidian University, Xi'an China, 5 , University of Warwick, Coventry United Kingdom
Show AbstractFerroelectric materials have become a cornerstone material for development of novel electronic devices microelectromechanical (MEMS) devices, sensors and data storage devices [1]. These applications are based on the unique set of properties of the ferroelectric materials (piezoelectricity, pyroelectricity, high non-linear activity, etc.). These properties can be mediated through the ferroelectric domain structure, which can be switched (or locally modified) by the application of external electric field. However, multiple cycles of the polarization switching can lead to changes in composition and crystallography of the ferroelectric medium. This phenomenon is called is called ferroelectric fatigue [3]. Its physical origin has been thoroughly studied in past few decades by number of local characterization techniques [4,5]. However, chemical implications into fatigue process remain poorly studied. Using a novel system that combines the chemical sensitivity of mass spectrometry with the functional probing Atomic Force Microscopy (AFM) system can overcome this limitation.
In this work, we used a combined Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to investigate fatigue phenomenon in lead zirconate titanate (PZT) ferroelectric thin film through the evolution of chemical profiles. PZT thin films with continuous bottom electrode and 500 nm circular copper electrodes on the top surface were probed as a function of bias from sub-switching to switching and high voltages. The AFM tip was used to cycle the nanocapacitors formed by bottom electrode and top pads. Subsequent measurements of the cycled nanocapacitors ferroelectric and chemical response was probed using co-registered Piezoreponse Force Microscopy (PFM) ToF-SIMS measurements. In particular, these investigations revealed penetration of the copper ions form electrode into the PZT bulk, which confirms chemical modifications of the cycled ferroelectric and explained observed degradation in switching current.
This work was funded by the Laboratory Directed Research and Development Program and conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility.
[1] W. Heywang, K. Lubitz, W. Wersing (Eds.), Piezoelectricity – Evolution and Future of a Technology, Springer Series in Materials Science, vol. 114 (2008)
[2] Binning G.K, Phys. Scr., T19A (1987), p.53
[3] K. H. Hardtl, Ferroelectrics 12, 9 (1976)
[4] V. Shvartsman, A. Kholkin, C. Verdier, D. Lupascu, J. Appl. Phys. 98, 094109 (2005)
[5] V. Shvartsman, A. Kholkin, C. Verdier, Z. Yong, D. Lupascu, J. Eur. Ceram. Soc. 25, 2559 (2005)
4:45 PM - EM05.04.05
Coupling and Competition between Ferroelectricity, Magnetism, Strain and Oxygen Vacancies in AMnO3 Perovskites
Astrid Marthinsen 1 , Carina Faber 2 , Ulrich Aschauer 2 3 , Nicola Spaldin 2 , Sinéad Griffin 4 5 , Magnus Moreau 6 , Tor Grande 1 , Thomas Tybell 6 , Sverre M. Selbach 1
1 Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Trondheim Norway, 2 Materials Theory, ETH Zürich, Zurich Switzerland, 3 Department of Chemistry and Biochemistry, University of Bern, Bern Switzerland, 4 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkely, California, United States, 5 Department of Physics, US Berkeley, Berkeley, California, United States, 6 Department of Electronic Systems, NTNU - Norwegian University of Science and Technology, Trondheim Norway
Show AbstractThe interplay between oxygen vacancies, A-site cation size, epitaxial strain in the (001) plane, ferroelectricity and magnetism in the perovskite manganite series, AMnO3 (A=Ca2+, Sr2+, Ba2+) is investigated by first principles calculations1. Increasing the unit cell volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies1,2. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsion associated with cation off-centering is reduced. Ferroelectric polarization favors ferromagnetism over antiferromagnetism as the polar distortion bends the Mn-O-Mn bond angles away from the optimal 180o. Polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for ferromagnetic than antiferromagnetic order. Finally, we explore the structural and magnetic strain dependence and oxygen vacancy stability when (111) strain in employed in SrMnO3. The structural response to (111) strain is fundamentally different to (001) strain, yielding rotationally invariant Goldstone-like phonon modes with a Mexican hat shaped energy surface. (111)-strained SrMnO3 could thus serve as a potential new system to emulate condensed matter Higgs modes. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure and magnetism can be modified using pO2, epitxial strain, and electric or magnetic fields.
1. Marthinsen, A., Faber, C., Aschauer, U., Spaldin, N. A. & Selbach, S. M. Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO3 perovskites. MRS Commun. 6, 182–191 (2016).
2. Aschauer, U., Pfenninger, R., Selbach, S. M., Grande, T. & Spaldin, N. A. Strain-controlled oxygen vacancy formation and ordering in CaMnO3. Phys. Rev. B 88, 54111 (2013).
Symposium Organizers
Elizabeth Dickey, North Carolina State University
Ulrich Aschauer, University of Bern
Nicole Benedek, Cornell University
Sakyo Hirose, Murata Manufacturing Co Ltd
EM05.05: Domain Walls
Session Chairs
Tuesday AM, November 28, 2017
Hynes, Level 1, Room 109
8:30 AM - *EM05.05.01
Domain Walls, Polar Regions and Defects in Ferroelectric Oxides
Dragan Damjanovic 1
1 , Swiss Federal Institute of Technology at Lausanne, Lausanne Switzerland
Show AbstractDomain walls and polar regions (or regions with a different polarity than the surrounding material) in ferroelectrics have a dominant effect on materials' dielectric, elastic and piezoelectric properties. Domain walls may either modify locally the material between the neighbouring walls or may affect the dynamic response of the material to external fields by their displacement. In both cases, interaction of domain walls with point and extended defects is of a critical importance. The dynamics of polar regions is less understood than that of domain walls in part because the structure of polar regions is not well known. The effect of mesoscopic features on the macroscopic properties of materials is difficult to predict and this presents the next important challenge in the theory and experimental characterization of ferroelectric materials.
In this presentation, some recent results of the studies of domain walls and their interaction with defects in lead zirconate titanate (Pb(Zr, Ti)O3), barium titanate (BaTiO3) and bismuth ferrite (BiFeO3) are discussed. The domain walls and their relationship with defects have been investigated in collaboration with several groups by Density Functional Theory calculations, measurements of macroscopic properties, High Resolution Transmission Electron Microscopy and by Hard X-ray Microscopy. The studies reveal why donor- and acceptor-doped Pb(Zr,Ti)O3 behave differently, give evidence of large concentrations of specific charged point defects within the domain walls in BiFeO3 ceramics, demonstrate a long-range effect of domain walls on strain in the surrounding material in BaTiO3, and give atomic-scale evidence of polar regions and their structure in (Ba,Sr)TiO3. The macroscopic manifestation of the dynamics of polar regions in a paraelectric phase is compared with that of domain walls in a ferroelectric phase.
9:00 AM - EM05.05.02
From Conductivity to Chemistry with Ferroelectric Domain Walls
Petro Maksymovych 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe main source of uncertainty in the electronic conduction of ferroelectric domain walls stems from the contact problem. Indeed, despite almost ten years since the original report on conducting domain walls in BiFeO3, hardly any values for the actual domain wall conductivity, the definitive assignment of conduction mechanism, and most importantly electronic devices incorporating domain walls – have been reported or demonstrated to date.
We recently investigated domain wall conductivity in ultrathin BiFeO3 films, below 10 nm. where the film thickness approaches the anticipated width of the Schottky barrier [1]. We found robust apparent electronic conductivity of domain walls in these samples. Detailed data analysis has however revealed a different regime of domain wall conduction, wherein the domain wall modifies the band alignment at the contacts. The wall itself is therefore not a conductor in this case, because its gating effect is limited to the interface. However, our results bear striking similarity to most previous microscopic measurements of domain wall conductance, calling into question their prior interpretation.
The problem of the electronic contact to the domain wall may be intrinsic to the polarization topology, and may not be resolved, for example by doping ferroelectric films. We therefore explored an alternative regime of domain wall conductance, at high AC frequencies – which can be free of contact effects. To this end, we combined piezoresponse force microscopy with microwave microscopy, enabling the probe of local dielectric tunability and conductance at high frequencies and with ~50 nm resolution. Domain walls in both BiFeO3 and Pb(ZrxTiy)O3 appear to be conducting at 3 GHz, with a remarkably large estimated conductivity of 6 S/m. Using phase field modeling and analytical solutions of the Ginzburg-Landau-Devonshire model for a ferroelectric semiconductor, we conclude that nominally straight domain walls can be charged by either defect-induced roughening or an intrinsic flexoelectric effect. The non-destructive electronic read-out of ferroelectric domain walls at microwave frequenciess opens new opportunities for device integration and fundamental understanding of the carrier-density and dielectric effects at the domain walls.
Finally, I will present our recent results on manipulation of defects in ferroelectric thin films, that is, in part, enabled by the electronic conductance of inclined ferroelectric domain walls naturally occurring in ferroelectric nanodomains. In particular, this approach appears to be an efficient way to controllably inject oxygen vacancies into perovskite lattice at room temperature.
Support was provided by the U.S. Department of Energy, Basic Energy Sciences, Materials Science and Technology Division. Microscopy experiments were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
9:15 AM - EM05.05.03
A First Principles Study of Charged Domain Walls in Improper Ferroelectric YMnO3, InMnO3 and YGaO3
Didrik René Småbråten 1 , Quintin Meier 3 , Sandra Helen Skjærvø 1 , Tsuyoshi Miyazaki 2 , Dennis Meier 1 , Sverre M. Selbach 1
1 , Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Trondheim Norway, 3 , Materials Theory, ETH Zurich, Zürich Switzerland, 2 , Internaional Center of Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Ibaraki Japan
Show AbstractWe study charged head-to-head and tail-to-tail ferroelectric domain walls in isostructural improper ferroelectric hexagonal YMnO3, InMnO3 and YGaO3 by first principles calculations. The energy landscape for different domain wall configurations in YMnO3 is mapped, and compared to experimental results. We study the local crystal structure at the domain walls, and report asymmetric crystal structure behavior for head-to-head and tail-to-tail domain walls. This is attributed to the intrinsic difference in local chemical environment at these two interfaces. From the evolution of crystal structure across the domain walls we find a trend in domain wall width with the energy difference between bulk and the structure at the domain wall centre. The local electronic structure at the domain walls is calculated and discussed with respect to cell size and electrostatic potential across the simulation cells. We suggest general rules for when the domain walls show n- or p-type conductivity based on domain size, saturation polarization and band gap of the host material.
9:30 AM - EM05.05.04
Domain Walls in Oxide Thin Films for Resistive Switching
Wilson Román Acevedo 2 , Mart Salverda 1 , Diego Rubi 2 , Zarief Hasrat 1 , Saeedeh Farokhipoor 1 , Miguel Rengifo 2 , Beatriz Noheda 1
2 , CNEA and INN, Buenos Aires Argentina, 1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
Show AbstractFerroelastic domain walls in thin films of some complex oxides are more conducting than the domains [1][3]. It has been proposed that this effect originates from the accumulation of oxygen vacancies at the domain walls due to the presence of strain gradients [2]. This reduces the Schottky barrier present at the interface with metallic electrodes [3]. So far, conductivity has been measured locally in the out-of-plane direction by conductive AFM[1][3] and also in-plane, by macroscopic electrical measurements for both as-grown [4] and written [1] domain walls. It has also been shown that domain walls, when probed between top and bottom electrodes (out-of-plane direction), can show memristive behaviour[5]. However, the intrinsic conduction mechanisms of domain walls are still unknown.
In this work, we investigate the out of plane electrical properties of metal/TbMnO3/Nb:SrTiO3 structures with different oxide thicknesses. The TbMnO3 films present a pattern of as-grown domain walls with different chemistry and magnetic properties with respect to the bulk compound [6]. Our aim is to elucidate the nature of the electrical transport properties of these domain walls and to investigate the “electrical plasticity” in order to explore their application in neuromorphic computing.
References
[1] J. Seidel et al., Nature Materials 8, 229 (2009)
[2] E. Salje and H. Zhang, Phase Transitions 82, 452 (2009)
[3] S. Farokhipoor and B. Noheda, Physical Review Letters 107, 127604 (2011)
[4] Q. He et al., Physical Review Letters 108, 067203 (2012)
[5] P. Maksymovych et al., Nano Letters 11, 1906 (2011)
[6] S. Farokhipoor et al., Nature 515, 379 (2014)
EM05.06: Metal-Insulator Transitions and Interfaces
Session Chairs
Tuesday PM, November 28, 2017
Hynes, Level 1, Room 109
10:15 AM - EM05.06.01
Collapse and Recovery of Strong Correlations in Vanadium Dioxide Interfaces by Defect Engineering
Zhen Zhang 1 , Helen Park 2 , Thomas Larrabee 3 , Sharka Prokes 4 , Cory Cress 4 , Shriram Ramanathan 1
1 , School of Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 2 , ASEE Postdoctoral Fellow at the U.S. Naval Research Lab, Washington, District of Columbia, United States, 3 , Sotera Defense Solutions, Herndon, Virginia, United States, 4 , U.S. Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractDefects control the optical, electrical, and magnetic properties of strongly correlated oxides due to the interaction between disorder and lattice, electron, spin, orbital degree of freedom. Vanadium dioxide is a prototypical correlated oxide with broad interest in electronics and photonics exploiting the metal-insulator transition. In this work, oxygen vacancies are created in vanadium dioxide by a reversible manner using an unique extractive metallurgy technique involving solid Mg buffers. Vanadium dioxide thin-films of 50 nm were grown by reactive sputtering and atomic layer depositions methods. The vanadium dioxide thin films are annealed under extremely low oxygen partial pressures to create large amounts of oxygen vacancies. This oxygen starvation environment is obtained by a Mg-buffering technique and no CO and H2 gases are required. It is found that a non-volatile metallic state emerges at ambient conditions after annealing and is stable down to 2 K. Moreover, this emergent state can recover back to the original insulating state by oxygen annealing. The emergent non-volatile metallic state and its smooth recovery process results from the generation and annihilation of oxygen vacancies in the lattices. The oxygen vacancies introduce charge-filling and lattice distortions which can collapse the strongly correlated insulating state of vanadium dioxide at ambient conditions. Our work introduces a new platform to tune strong correlations in oxides via extreme defect concentrations and can be widely adapted to a host of ionic liquid gated, solid electrolyte gated and environmental oxide devices.
10:30 AM - *EM05.06.02
Interfacial Oxygen Vacancy Formation and Colossal Ionic Conductivity in a Fluorite-Bixbyite Superlattice Nanobrush
Ho Nyung Lee 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show Abstract
The atomistic response and charge modulation at the interface between mixed valence materials have been the subject of intense studies in electronic and ionic materials. However, little is known for the formation and ionic activities of anionic defects, such as oxygen vacancies. Here, we have constructed interfacial oxide superlattices composed of the fluorite CeO2 and the bixbyite Y2O3 within a vertically aligned nanobrush architecture. Interestingly, such nanosuperlattices revealed at least three orders of magnitude higher ionic conductivity as compared to undoped CeO2. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) investigation found that the CeO2-Y2O3 interface is ionically reconstructed, forming spontaneously a space charge region to compensate the valence difference between Ce4+ and Y3+ or to match the oxygen coordination between Ce4+ and Y3+. Moreover, low-angle annular dark-field imaging by STEM and EELS investigation revealed that the space charge region (~few nm in thickness) contains a large number of oxygen vacancies that are formed only within the CeO2 layer, providing the fast ion conduction pathway. In addition, we will also present a completely new synthesis route to the formation of micron-long single-crystalline oxide nanobrushes. Single crystalline nanobrush architectures of CeO2, Y2O3 and CeO2/Y2O3 superlattices, with a large porosity (up to ~50 %), were fabricated by pulsed laser epitaxy enabling a direct and template-free synthesis. This directed synthesis was made possible by growing oxide nanostructures under kinetically and thermodynamically balanced non-equilibrium conditions, far from the conventional growth conditions optimized for 2D thin films. This new synthesis approach enabled us to synthesize completely new oxide nanomaterials, in which the interfaces were engineered to achieve highly improved ionic conductivity ever accomplished.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.
11:00 AM - EM05.06.03
Investigations on Oxygen Scavenging Effect at Metal/Oxide Interfaces for Reliable Memory Applications
Ryosuke Nakagawa 1 , Atsushi Tsurumaki-Fukuchi 1 , Masashi Arita 1 , Yasuo Takahashi 1
1 , Hokkaido University, Sapporo Japan
Show AbstractRedox reactions and resultant carrier doping phenomena in metal oxides have attracted increasing attentions because of their potential usage in novel electronic devices. Efforts have been devoted for controlling the formation and migration of oxygen vacancies in oxides, especially in recent developments of redox-based resistive switching memories. A new method to create high-concentration oxygen defects has been proposed in these studies based on insertion of a thin metal layer at electrode/oxide interface [1, 2]. The interfacial layer (so-called "scavenging" layer) has been believed to electrochemically react with oxides and serve to improve the stability of resistive switching. However, although it is widely used in recent devices, the detailed mechanism of oxygen scavenging has not yet been elucidated.
To clarify the actual role of the scavenging layer and establish its design concept, we investigated formation processes of oxygen vacancies in devices with various scavenging layers by electrical measurements and transmission electron microscopy (TEM) analysis. We fabricated test structures of Pt(100 nm)/M(t nm)/amorphous SrTiO3(a-STO, 100 nm)/Pt(100 nm) on SiO2/Si substrates by sputtering method. The influences of the thickness (t) and material (M) of the scavenging layer were investigated by changing them as t = 0–50 nm and M = Al, Zr, Ti, and Ta, respectively. In the current–time (I–t) characteristics, we found that the resistive switching voltages and times are significantly reduced in devices with Ta layers as compared with other more reactive metals (Al. Zr, and Ti). This suggests that the performance of scavenging layer does not depend on the bulk reactivity of the materials.
In the TEM analysis, we observed that Ta scavenging layer has a very uniform structure with no grain boundaries and forms flat interfaces with a-STO. Energy dispersive x-ray spectroscopy analysis showed that oxygen distributions in scavenging layers are strongly dependent on the materials, and uniform distributions are possible in Ta layers. The high homogeneity of Ta layer can be considered as the origin of the active interfacial reactions and smooth oxygen scavenging, and is attributable to the "adaptive" crystal structure of TaOx [3]. These results provide important insights into the future developments of resistive switching devices, and will expand the applications of oxygen scavenging phenomena at metal/oxide interfaces.
Part of this work was supported by JSPS KAKENHI (Nos. 16K18073, 16H04339, 15H01706), Nippon Sheet Glass Foundation for Materials Science and Engineering, and the Scholar Project of Toyota Physical and Chemical Research Institute. Experiments were partly performed under the Nanotechnology Platform Program (Hokkaido Univ.) organized by MEXT, Japan.
[1] Y. Guo and J. Robertson, Appl. Phys. Lett. 105, 223516 (2014).
[2] X. Zhong et al., Phys. Chem. Chem. Phys. 18, 7502 (2016).
[3] H. Jiang and D. A. Stewart, J. Appl. Phys. 119, 134502 (2016).
11:15 AM - EM05.06.04
Tunnel Transport across the Crystalline BaTiO3-Germanium Interface
Yichen Jia 1 , Charles Ahn 1 , Fred Walker 1
1 , Yale University, New Haven, Connecticut, United States
Show AbstractThe integration of epitaxial functional complex oxides on semiconductors opens new opportunities for coupling their unique properties to conventional semiconductor devices. Understanding the conduction mechanism at the interface between the ferroelectric oxide BaTiO3 and the elemental semiconductor Ge is a key step to developing CMOS-compatible, non-volatile ferroelectric devices based on BaTiO3. Here we investigate vertical transport in epitaxial BaTiO3-Ge devices. BaTiO3 is grown on p-type Ge via molecular beam epitaxy (MBE) and patterned into devices using electron-beam lithography. Vertical transport measurements are carried out from room temperature to 2 Kelvin to determine the nature of carrier transport from different metal electrodes into the Ge. Through the application of an external bias, the metal Fermi level is tuned through the Ge band gap. For biases with the metal Fermi level above the Ge conduction band, we observe a temperature-independent conduction below 250 K, indicative of Fowler-Nordheim tunneling transport by minority carriers. This finding paves the way for tunnel transport devices based on BaTiO3-Ge heterostructures.
11:30 AM - EM05.06.05
X-Ray Photoemission Study for Evaluation of Electronic States at Metal/Oxide Interfaces
Takeo Ohsawa 1 , Sakyo Hirose 3 1 , Shigenori Ueda 1 , Naoki Ohashi 1 2
1 , National Institute of Materials Science, Tsukuba Japan, 3 , Murata Manufacturing Co., Ltd., Nagaoka-kyo Japan, 2 , Tokyo Institute of Technology, Yokohama Japan
Show AbstractThe interfacial electronic structure at the metal/oxide interfaces has been investigated by hard-x-ray photoemission. In particular, observed photoemission spectra were used to extract interfacial potential profiles. The junctions, such as Au/SrTiO3:Nb, Pt/SrTiO3:Nb, Pt/BaTiO3, were formed by sputtering metal contacts on the surface of those oxide single crystals and the core-level spectra of those oxides beneeth the contacts were measured by hard-x-ray photoemission utilizing 6-keV X-rays generated with a synchrotron beam line, BL15-XU at SPring-8, Japan. In this study, the effect of electrode deposition conditions, such as sputtering power and substrate-target distance, were varied to see to effect of damages caused by sputtering deposotion was studied. As a esult, it was suggested that the dielectric permittivity of the oxide around the interface chaged by sputtering conditions. Detail of the results will be presented.
Reference:
Ohsawa et al., DOI:10.1063/1.4972849.
Hirose et al., DOI:10.1149/2.0051407jss
Hirose et al., DOI:10.1063/1.4921092
Ohashi et al., DOI:10.1063/1.4772628
11:45 AM - EM05.06.06
Band Alignment of Metal/Amorphous-Oxide Interface Using Atomic Orbitals Projection of Plane-Wave—A First Principle Study at the Al/a-SiO2 Interface
Jianqiu Huang 1 , Fei Lin 1 , Celine Hin 1
1 , Virginia Polytechnic Institute and State University, Blacksburg, Virginia, United States
Show AbstractAmorphous insulating oxides play a significant role in the contemporary electronic industry. Understanding the band alignment of heterogeneous interfaces containing amorphous structures helps to better control the carrier transport property at the interface. Classical band offset methods developed previously line-up eigenlevels with respect to an ideal bulk reference or vacuum level. However, the local disorder of amorphous structures makes the bulk reference ambiguous. Therefore, classical methods cannot be applied. In this study, we introduce a new approach based on the Linear Combination of Atomic Orbital (LCAO) projection of wave-function to line-up bands at metal/oxide interfaces. The LCAO projection of wave-function accounts for all metal/oxide interface effects, such as built-in voltage, interface dipole, virtual oxide thinning, barrier deformation, etc. Therefore, it provides accurate band alignments. Calculations performed at an Al/amorphous-SiO2 exhibit a good agreement between existing experiments and simulation data. We Also observed a space charge region at the interface resulting in non-linear band bending in the oxide, which virtually decreases its thickness, hence lowering the dielectric strength.
EM05.07: Oxide Interfaces and Heterostructures I
Session Chairs
Tuesday PM, November 28, 2017
Hynes, Level 1, Room 109
1:30 PM - EM05.07.01
Strain Control of Oxygen Kinetics in Ruddlesden-Popper Oxides
Tricia Meyer 1 , Ryan Jacobs 2 , Dongkyu Lee 1 , John Freeland 3 , Changhee Sohn 1 , Dane Morgan 2 , Ho Nyung Lee 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 , Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractOxygen defect control has long been considered an influential tuning knob for producing various property responses in complex oxide films. However, the nature of oxygen defects in thin films is often not investigated beyond basic redox chemistry. One of the model examples for oxygen defect studies is the layered Ruddlesden-Popper (RP) phase La2−xSrxCuO4−δ (LSCO), in which the superconducting transition temperature (Tc) of epitaxial thin films was previously found to be highly sensitive to epitaxial strain. However, previous observations on strain-superconductivity coupling in LSCO thin films were mainly understood in terms of elastic contributions to mechanical buckling, with minimal consideration of kinetic or thermodynamic factors. Here, we report that the oxygen nonstoichiometry that is commonly reported for these strained cuprates is mediated by the strain-modified surface exchange kinetics, rather than reduced thermodynamic oxygen formation energies for one strain state versus another. Both of these are highly correlated with the variability of the superconducting transition in LSCO. Remarkably, tensile-strained LSCO shows nearly an order of magnitude faster oxygen exchange rate than a compressively strained film, revealing a strong contrast in the time scales required to modify oxygen stoichiometry. Since strain engineering is a very powerful tool for controlling oxygen surface exchange kinetics in Ruddlesden-Popper oxides, it is an enticing strategy for developing high-performance ionotronic materials and devices in addition to the controlling superconductivity.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (experiment) and by the NSF Software Infrastructure for Sustained Innovation (SI2) award No. 1148011 (theory).
1:45 PM - *EM05.07.02
Beyond Electrostatic Effects at Oxide Hetero-Interfaces—Strong Electric Fields, Electrochemical Phase Change and Elastic Strain
Bilge Yildiz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractTransition metal oxide hetero-interfaces are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity and magnetism. These interfaces are the source for local heterogeneities in composition, atomic structure and electronic structure. Classically, defect redistribution is quantified at the continuum level by concurrent solution of Poisson’s equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. It is possible to inform this level of modeling with first principles calculations of band off-sets, and defect formation and segregation energies at thermodynamically relevant conditions. This approach had numerous successful implementations, including the quantification of charge transport properties at surfaces and grain boundaries. In this talk, I will discuss three phenomena that also need to be considered in a broader framework of defect structures and distributions at oxide hetero-interfaces. 1) Presence of strong electric fields that can cause polarization of defective systems and affect the defect abundance and structure. We have assessed this effect on neutral oxygen vacancies in simple binary oxides from first principles calculations. 2) Phase change under the effect of local electrostatic potential because of a change in the electrochemical potential of oxygen. We have assessed the ability to trigger phase change electrochemically in two classes of oxides, SrCoOx and VOx, and have quantified the phases and the corresponding distinctly different electronic properties by combining in operando x-ray diffraction and x-ray photoelectron and absorption spectroscopy. The results have implications both for oxide hetero-interfaces and for oxide electronic devices that aim to control properties electrically. 3) Elastic strain, that affect the stability and mobility of defects. In this recent work, we have focused on the stability of electronic defects, specifically the electron polarons versus free electrons SrTiO3, as a function of temperature and hydrostatic stress, by combining first principles calculations and quasi harmonic approximation. Our results demonstrate that it is possible to control the type of electronic defect, and so the transport properties, by means of electro-chemo-mechanics.
2:15 PM - EM05.07.03
Strain-Stabilization of Novel Magnetic Orderings in A2B2O5 Brownmillerite Oxides
Yongjin Shin 1 , James Rondinelli 1
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractBrownmillerite oxides with the chemical formula A2B2O5 are derived from stoichiometric ABO3 perovskite with ordered oxygen vacancies. The ordered arrangements enable unique equilibrium A2B2O5 structures and functional properties. The presence of ordered vacancies transforms the octahedral BO6 units in perovskite into different BO6-x polyhedral units, which leads to large changes in the crystal field split orbital structure. The magnetic order of transition metal oxides is governed by the electron occupancy among these orbitals. Ferrite brownmillerite (Sr2Fe2O5; SFO) is relatively insensitive to the crystal field splitting as its d5 electronic configuration yields AFM-G type order following Goodenough-Kanamori rule regardless of polyhedral types. However, ferromagnetism (FM) was predicted to occur in SFO under high hydrostatic pressure when the equilibrium structure transitions from exhibiting a combination of octahedra and tetrahedra to square pyramidal network [1]. Here, we search for the critical epitaxial strain value at which this magnetic transition can be activated in (001) thin films using density functional calculations. As epitaxial strain produces anisotropic stresses, we observe different magnetic transitions depending on the relative orientation of the vacancy order with respect to the biaxial epitaxial strain. We conclude by describing these transitions in terms of the softness of the magnetic states arising from strain-dependent variations in the exchange interactions.
2:30 PM - EM05.07.04
Electronic Defect Study in SrTiO3 under Biaxial Strain Using DFT+U and QHA
Yen-Ting Chi 1 , Bilge Yildiz 1 , Krystyn Van Vliet 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractElectronic defects strongly modulate performance of oxides used in energy conversion. The type and mobility of electronic defects can also couple directly to mechanical deformation of these materials. Our recent work has established that hydrostatic stress exerted on oxide crystals such as SrTiO3 (STO) can increase electron mobility by modulating electronic structure, but can also decrease electron mobility if small polarons are stabilized at sufficient mechanical pressure and temperature. Here, we predict and experimentally validate the effect of biaxial strain on electronic defects in STO. This strain state can be induced via biaxial growth, varies predictably with distance from the film-substrate interface and is more relevant to thin-film electronic and electrochemical devices. Specifically, we first predict the effect of biaxial strain ranging -5% to +5% over a range of temperatures 0 - 1000 K on the thermodynamic stability of free electrons and small polarons in single crystal STO, using density functional theory with Hubbard U (DFT + U) and the quasiharmonic approximation. We construct an electronic defect predominance diagram by computing the self-trapping energy, i.e. the energy difference between the small polaron and free electron. Compared with predictions of hydrostatic strain effects on STO, when considering Gibbs free energy of polaron trapping, we find that a lower stress is required to stabilize the small polaron with respect to the free electron. This difference originates from the relaxation along the z-axis under biaxial stress in contrast to hydrostatic stress, and the contracted lattice parameter of small polaron along the z-axis, which eliminates the energy penalty at low temperature. We then validate this prediction by growing STO epitaxial films with film-substrate lattice mismatch to induce biaxial strains, and confirm reduced STO conductivity when the small polaron is the dominant electronic defect. These findings relate directly to the design of functional oxides for energy conversion and memory storage applications, whereby mechanical strain can modulate electronic conductivity.
2:45 PM - EM05.07.05
Dynamic Rearrangement of Substrate Surface Termination in LaFeO3/SrTiO3 Heterostructures
Peter Sushko 1 , Steven Spurgeon 1 , Ryan Comes 1 2 , Scott Chambers 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Department of Physics, Auburn University, Auburn, Alabama, United States
Show AbstractPredictive understanding of non-equilibrium processes that take place during materials synthesis underpins design of the synthetic routes and control of the properties and functions of the resulting materials. Owing to the recent advances in deposition and characterization techniques, epitaxial thin films and hetero-structures of complex oxides are uniquely suited for the analysis of defects and disorder associated with internal interfaces. In addition, judicious selection of the deposition protocols allows one to design non-equilibrium structures in a controlled fashion.
Here we apply molecular beam epitaxy, STEM-HAADF imaging, and ab initio computational modeling to investigate transformation pathways of non-equilibrium structures at the polar-nonpolar interfaces in LaFeO3/SrTiO3 (LFO/STO). To this end we prepare TiO2- and SrO- terminated surfaces of STO and deposit LFO in a shuttered growth mode. The resulting structures show a well-defined, although partially intermixed LaO/TiO2 interface, while the FeO2/SrO interface is all but dissolved. Computational modelling of these systems confirms that the positive (LaO/TiO2) interface is more stable than the negative (FeO2/SrO) interface. We then quantify the effect of cation intermixing on the interface stability for both STO terminations and each of the cation species. Finally, we propose atomic scale mechanisms that facilitate stabilization of LaO/TiO2 interface and degradation of FeO2/SrO interface via dynamical rearrangement of the near-surface region at the initial stages of LFO deposition. These mechanisms are related to the growth conditions. This analysis suggests how the details of interfacial structures can be controlled using pre-designed substrate termination and deposition protocols.
R. Comes, S. Chambers, Interface Structure, Band Alignment, and Built-In Potentials at LaFeO3/n-SrTiO3 Heterojunctions, Phys. Rev. Lett. 117, 226802 (2016).
S. R. Spurgeon, P. V. Sushko, R. B. Comes, S. A. Chambers, Dynamic Rearrangement of Substrate Surface Termination in LaFeO3/SrTiO3 heterostructures (under review).
EM05.08: Point Defects II
Session Chairs
Tuesday PM, November 28, 2017
Hynes, Level 1, Room 109
3:30 PM - *EM05.08.01
How Defects Influence Schottky Barriers of Semiconducting Oxides
Andreas Klein 1
1 , TU-Darmstadt, Darmstadt Germany
Show AbstractThe electrical and functional properties of semiconducting oxides are strongly influenced by defect concentrations, which can reach values of several percent. Due to the contribution of charged defects to the space charge layer formation, defects can also influence barrier formation at interfaces of semiconducting oxides. Defects might (i) be present before interface formation, (ii) be induced during interface formation, and (iii) migrate to the interface after completion of interface formation if they are sufficiently mobile. Apart from oxygen vacancies, also dopants, impurities and multivalent cations can contribute to barrier formation. The presentation gives an overview on how the different influences of defects on Schottky barrier formation of semiconducting oxides can be studied using photoelectron spectroscopy. The effects are illustrated using transparent conductors, dielectrics and ferroelectrics as examples.
4:00 PM - EM05.08.02
A Unifying Perspective on Oxygen Vacancies in Wide Band Gap Oxides
Christopher Linderälv 1 , Anders Lindman 1 , Paul Erhart 1
1 , Chalmers University of Technology, Gothenburg Sweden
Show AbstractWide band gap oxides are versatile materials with numerous applications in research and technology. Many properties of these materials are intimately related to defects with the most important defect being the oxygen vacancy. Here, using electronic structure calculations, we show that the charge transition level (CTL) and the eigenstates associated with oxygen vacancies, which to a large extent determine their electronic properties, are confined to a rather narrow energy range, even while band gap and the electronic structure of the conduction band vary substantially. The vacancies are classified according to their character (deep and shallow), which shows that the alignment of electronic eigenenergies and CTL can be understood in terms of the transition between cavity-like localized levels in the large band gap limit and strong coupling between conduction band and vacancy states for small to medium band gaps. We consider a semi-local as well as a hybrid functional and, complementing earlier work, demonstrate that the former yields results in very good agreement with the latter provided that band edge alignment is taken into account.
4:15 PM - EM05.08.03
Strain and Electronic Effects on Charged Defects in Oxides
Yanwen Zhang 1 2 , Jianqi Xi 2 , Benjamin Petersen 2 , Dilpuneet Aidhy 3 , Bin Liu 4 , William Weber 2 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , The University of Tennessee, Knoxville, Tennessee, United States, 3 , University of Wyoming, Laramie, Wyoming, United States, 4 , School of Materials Science and Engineering, Shanghai University, Shanghai China
Show AbstractExtreme energy deposition and dissipation due to energetic particle irradiation are non-equilibrium thermodynamic processes. Understanding such processes at the level of electrons and atoms is fundamental to a wide range of research and applications. Among many important research topics, strain and electronic effects on charged defects are investigated at grain boundaries and interfaces in a few model oxides with rock salt, fluorite, perovskite and pyrochlore structures. Based on ab initio molecular dynamics, our studies on low-energy recoil events have shown that the recoil events are partial-charge transfer assisted processes. In an effort to better understand experimental observations of defect annealing and enhanced grain growth at cryogenic temperatures in cubic ZrO2, density functional theory has been used to investigate the stability and migration energies of neutral and charged oxygen vacancies and interstitials, as well as reveal insights on the role of charged defects on enhanced kinetics. To understand strain-induced phase and oxygen-vacancy stability, the effect of hetero-interfaces, CeO2|ZrO2 and ThO2|ZrO2, are investigated. The tensile strain from interfacing of ZrO2 to other crystals with larger lattice parameters can lower the oxygen vacancy formation energy. Modifying oxygen kinetics and dynamics near grain boundaries or interfaces provides a control to tune material functionality.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.
4:30 PM - EM05.08.04
Nanoscale Charge Mapping for Interfaces in Dielectrics
Thomas Moran 1 , James Steffes 1 , Tadasu Hosokura 2 , Keigo Suzuki 2 , Takafumi Okamoto 2 , Koji Murayama 2 , Nobuhiko Tanaka 2 , Bryan Huey 1
1 , University of Connecticut, Storrs, Connecticut, United States, 2 , Murata Manufacturing Co., Ltd., Yasu Japan
Show AbstractDielectric properties are a challenge to measure for advanced capacitors, necessitating macroscopic electrodes, pinhole-free films, and homogeneous specimens. Nanoscale dimensions and interfaces are increasingly relevant, however, motivating the development of an AFM-based approach for mapping capacitive charges and the associated local dielectric properties and ferroelectric polarization. This method is based on depositing charges with a known bias, followed by sequential mapping of the surface potential as it decays over time due to various dissipation processes. Comparing results in air and in vacuum, for specimens engineered with distinct interface densities, reveals insight into charge compensation from the environment, trap states at interfaces, and mobile charges within a specimen. Most importantly, this technique also can yield maps of the surface charge and local dielectric constant. The results are demonstrated for BaTiO3 thin films of various thicknesses and microstructures.
4:45 PM - EM05.08.05
Hydrogenation of the Buried Interface as a Stable Nano-Doping Technique to Oxide Semiconductors
Takeaki Yajima 1 , Tomonori Nishimura 1 , Akira Toriumi 1
1 , Univ of Tokyo, Tokyo Japan
Show AbstractDoping to semiconductors has often been performed by ion implantation, which enables nano-scale control of dopant distribution in the device structure. However, ion implantation poses substantial challenges in low-temperature processes, doping to 3D device structures, as well as alleviating the implantation damages. In this sense, ion implantation is not always applicable to oxide semiconductors, which are utilized for applications such as displays, sensors, solar cells, etc. For oxide semiconductors, an alternative technique is needed, which can perform room-temperature, isotropic, and damageless doping. Here, we demonstrate electrochemical doping to thin films of archetypical oxide semiconductor TiO2, and confirm hydrogen can be doped as donor impurity at room temperature without any detectable damages. Interestingly, the majority of hydrogen stay at the buried interface with the substrate rather than in the TiO2 thin film, exhibiting excellent stability and long-term electric conductivity. Besides, optical lithography is exploited for successfully patterning hydrogen distribution in the buried interface. This doping technique to buried interfaces suggest a unique doping technique for oxide electronics, and also facilitate nanoscale manipulation of hydrogen for next-generation electronic-ionic devices. We acknoledge experimental supports from Prof. S. Yamaguchi and Dr. S. Miyoshi in The Univ. of Tokyo.
EM05.09: Poster Session
Session Chairs
Wednesday AM, November 29, 2017
Hynes, Level 1, Hall B
8:00 PM - EM05.09.01
The Enhancement of Electronic Phase Switching Efficiency in VO2 Freestanding Nanowires
Yoshiyuki Higuchi 1 , Teruo Kanki 1 , Hidekazu Tanaka 1
1 , Osaka University, Ibaraki Mihogaoka Japan
Show AbstractVanadium dioxide (VO2) as a prototypical strongly correlated electron material exhibits the dramatic changes in conductivity owing to insulator-metal transition (IMT) induced by a variety of external stimuli, such as thermal variation, a lattice strain and an electric field. Thermally driven IMT due to local Joule-heating in two terminal VO2 devices has attracted attention for being applied to exotic electronics devices by their ultra-fast and abruptly resistivity modulation. From the view point of power consumption, 3D device architecture and size play important factors to realize the operation with smaller power consumption. Actually, we have indicated lower power-driven IMT in micro-scaled VO2 freestanding structures than that in the clamped structures [1]. Despite device size should impact on the PC, on the other hand, size dependent on the PC values has not been enough investigated owing to difficulty in realization of freestanding nanostructures, bringing changeover of fabrication process from photolithography and intense of capillary force in nano scale.
In this research, we realized construction of several hundred nanometer-scaled freestanding VO2 nanostructure and demonstrated smaller power-driven IMT and higher efficient switching [2]. In this symposium, we will show the details of their transport property, and offer new guidelines to modify the PC in two terminal VO2 phase switching devices.
[1] S.Yamasaki et al, Applied Physics Express 7, 023201(2014).
[2] H. Higuchi et al., Applied Physics Express 10, 033201(2017).
8:00 PM - EM05.09.02
Two-Dimensional Ruddlesden-Popper Faults and Oxygen Electrocatalysis in LaNiO3 Thin Films
Jumi Bak 1 , Hyung Bin Bae 2 , Sung-Yoon Chung 1
1 , Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 , KAIST Analysis Center, Daejeon Korea (the Republic of)
Show AbstractAtomic-scale direct probing of active sites and subsequent elucidation of the structure-activity relationship are important issues for oxide-based electrocatalysts to increase electrochemical conversion efficiency. By simple control of the cation nonstoichiometry, we demonstrate that two-dimensional homologous Ruddlesden-Popper (RP) faults are generated in LaNiO3 thin films as well as strong tetragonal distortion of [NiO6] octahedra is induced by more than 20% elongation of Ni-O bonds in the faults. The elongation is directly visualized by scanning transmission electron microscopy. In addition, we identify that the distorted [NiO6] octahedra in the faults show considerably higher electrocatalytic activities than other surface sites during the electrochemical oxygen evolution reaction. This unequivocal evidence of the octahedral distortion and its impact on electrocatalysis in LaNiO3 suggests that the formation of RP-type faults can provide an efficient way to control the octahedral geometry and thereby remarkably enhance the oxygen catalytic performance of perovskite oxides.
8:00 PM - EM05.09.03
Characterization of Room-Temperature Processed BZN (Bi1.5Zn1Nb1.5O7) Thin-Film Capacitors under Curvature
Elizabeth Schell 1 , Andy Shih 1 , Akintunde Akinwande 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractOrganic thin film transistors (TFTs) have been of great interest lately, because of their potential applications in flexible systems. High-k dielectrics, such as BZN (Bi1.5Zn1Nb1.5O7), have the potential to improve this technology, because they allow for a thicker film without decreasing capacitive coupling in the device. Thicker oxide layers help prevent tunneling and are less likely to be stretched thin when flexed, therefore leading to lower leakage currents. In previous works, TFTs have been made and characterized with BZN on conventional, non-flexible substrates, however there is little to no research into how the crystal structure of BZN is affected when flexed at a macroscale and how this will affect the operating characteristics of any devices made with BZN.
MIM capacitors were fabricated with BZN on flexible Kapton substrates. The use of Kapton substrates limited the maximum processing temperature, and therefore the sputtered BZN could not be annealed at the usual temperatures of 500-700 C. This amorphous BZN was examined to find the hardness, elastic modulus, dielectric constant, tunability of dielectric constant with voltage, and change in dielectric constant with the application of stress in the form of flexing the sample to different radii of curvature. The hardness and elastic modulus found were similar to values for silicon dioxide. While the dielectric constant for amorphous BZN is not as high as that for crystalline BZN, it was shown that room-temperature processed BZN devices could still have a dielectric constant an order of magnitude higher than silicon dioxide. A model was developed for the change in dielectric constant of BZN with curvature based on experimental data.
8:00 PM - EM05.09.04
Insulting-to-Conducting Behavior and Band Profile across the La0.9Ba0.1MnO3/Nb:SrTiO3 Interface
Weiwei Li 1 , Josée E. Kleibeuker 1 , Rui Wu 1 , Hongliang Zhang 1 , Chao Yun 1 , Judith MacManus-Driscoll 1
1 , Department of Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom
Show AbstractHetero-interfaces of perovskite oxides provide a fertile new ground for creating novel electronic states driven by the coupling among lattice, charge, spin and orbital degrees of freedom. Applications of these ultrathin oxide interfaces in the fields of electronics, photon harvesting, photovoltatics and photocatalysis strongly depend on the energy bands alignment at the interface.
In the bulk, La0.9Ba0.1MnO3 is an insulating ferromagnet with a Curie temperature (TC) of 185 K, but, in the film form, it can exhibit metallicity with a higher TC. In this work, we combine electrical and in-situ x-ray photoelectron spectroscopy (XPS) measurements to understand the thickness-dependent electronic structure and interface band profile of La0.9Ba0.1MnO3/Nb:SrTiO3 heterojunctions. High-quality ultrathin La0.9Ba0.1MnO3 films were fabricated by pulsed laser deposition with an atomic layer precision monitored by in-situ RHEED via a layer-by-layer growth mode. We found the films to be insulating at 5 unit cells and conducting at 40 unit cells, which can be explained by the determined interface band diagram of La0.9Ba0.1MnO3/Nb:SrTiO3. A type II heterojunction was formed at the interface of La0.9Ba0.1MnO3/Nb:SrTiO3.
8:00 PM - EM05.09.05
Electrical and Structural Phase Transition Correlations in VO2 Thin Films
Adele Moatti 1 , Ritesh Sachan 1 , John Prater 1 , Jagdish Narayan 1
1 , North Carolina State Univ, Raleigh, North Carolina, United States
Show AbstractUnstrained VO2 single crystals undergo structural (from elevated temperature tetragonal to low-temperature monoclinic phase) and electronic transitions, simultaneously. In thin films, however, in the presence of epitaxial strains, structural (Peierls) and electronic (Mott) transitions are affected differently, and are separated. In this study, we have used in situ X-Ray diffraction measurements to study the structural (Peierls) transition and employed resistance measurements to investigate the electrical (Mott) transition in epitaxially grown vanadium dioxide films on c-sapphire substrates. The structural transition shift is discussed using a combined kinetic and thermodynamic approach, where the velocity of phase transformation is controlled largely by kinetics, and the formation of intermediate phases is governed by thermodynamic considerations. The electrical transition shift is explained by d-orbital occupancy and changes in the bandgap. A delay is found between the onset of structural and electrical transitions in the presence of strains, which is explained by our model. With this study, we suggest that the control of structural and electrical transitions is possible by varying the transition activation barrier and bandgap through strain engineering. This control and tuning will impact the design of VO2 smart sensors.
Ref: Adele. Moatti, Ritesh. Sachan, John. Prater, Jagdish. Narayan, "Control of structural and electrical transitions of VO2 thin films", ACS Applied Materials & Interfaces, In press.
8:00 PM - EM05.09.06
Dependence of Electrical Transport Properties of CaO(CaMnO3)m(m=1, 2, 3, ∞) Thermoelectric Oxides on Lattice Periodicity
Andrei Baranovskiy 1 , Yaron Amouyal 1
1 Materials Science and Engineering, Technion, Haifa Israel
Show AbstractThe electrical transport properties of CaO(CaMnO3)m (m=1, 2, 3, ∞) compounds are studied applying the density functional theory (DFT) in terms of band structure at the vicinity of the Fermi level (EF). It is shown that the total density of states (DOS) values at EF increase with increase in the m-values, which implies an increase in the electrical conductivity, σ, with increasing m-values,
in full accordance with experimental results. Additionally, the calculated values of the relative
slopes of the DOS at EF correlate with the experimentally measured Seebeck coefficients. The electrical
conductivity and Seebeck coefficients were calculated in the framework of the Boltzmann
transport theory applying the constant relaxation time approximation. By the analysis of experimental
and calculated σ(T) dependences, the electronic relaxation time and mean free path values
were estimated. It is shown that the electrical transport is dominated by electron scattering on the
boundaries between perovskite (CaMnO3) and Ca oxide (CaO) layers inside the crystal lattice.
References
1. A. Baranovskiy, Y. Amouyal, Dependence of electrical transport properties of CaO(CaMnO3)m
(m=1, 2, 3, ∞) thermoelectric oxides on lattice periodicity, J. Appl. Phys. 121(6) (2017) 065103.
2. A. Baranovskiy, Y. Amouyal, Structural stability of calcium-manganate based CaO(CaMnO3)m
(m=1, 2, 3, ∞) compounds for thermoelectric applications, Journal of Alloys and Compounds 687 (2016) 562 - 569.
8:00 PM - EM05.09.07
The Impact of Oxide Hetero-Interface on the Oxygen Reduction Reactivity
Huijun Chen 1 , Yapeng Zhang 1 , Zheng Guo 2 , Fei Li 1 , Yu Chen 3 , Xiaoqiang Wang 1 , Xinwei Wang 2 , Chenghao Yang 1 , Jiang Liu 1 , Meilin Liu 3 , Yan Chen 1
1 , South China University of Technology, Guangzhou, Guangdong, China, 2 Shenzhen Graduate School, Peking University, School of Advanced Materials, Shenzhen, Guangdong, China, 3 , Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractSolid Oxide Fuel Cells (SOFCs) have attracted great attention due to their high efficiency, low emission and excellent fuel flexibility. The large scale implementation of SOFCs, however, is limited by the lack of high performance electrode materials. The sluggish oxygen reduction kinetics on the cathode surface is normally considered to dominate the performance of SOFCs. Cathode materials with high oxygen reduction reaction (ORR) activity and long-term thermal stability, therefore, is crucial to the commercialization of SOFCs. Decorating or covering existing cathode materials surface with a secondary phase to form a hetero-structure has been widely studied as a promising approach to enhance electrode performance. Such secondary phases include alkaline earth metal oxides (e.g. SrOx, BaO or CaO), transition metal oxides, perovskite-based oxides, etc. Despite of many successful examples, contradicting results were reported in literature. A detail understanding of the correlation between the hetero-structure and the electro-catalytic activity and stability of the electrodes is vital to achieving the rational design of more efficient SOCs electrode materials with excellent durability.
In this study, widely studied cathode materials La0.6Sr0.4Co0.2Fe0.8O3(LSCF) thin films were used as model systems. PrxCe1-xOy (PCO, x=0, 0.2, 1) with controllable thickness was deposited onto LSCF surface using pulsed laser deposition (PLD). The ORR activity of PCO decorated LSCF films were evaluated by electrochemical impedance spectroscopy (EIS) at temperature from 500 oC to 600oC. The structure and composition of PCO decorated LSCF surface was characterized by Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM). EIS results showed that the ORR activity and stability of LSCF were greatly enhanced after PCO decoration. Based on SEM, XPS and TEM measurements, we believe such enhancement is partly due to the suppression of Sr segregation on LSCF surface by the top PCO layer at elevated temperature. By measuring the dependence of impedance on the oxygen partial pressure, we found that the surface decoration also changed the rate limiting step of ORR on the surface.
8:00 PM - EM05.09.08
Variation in the Conductance Enhancement at Polar Interfaces Formed between BaSnO3 and LaInxGa1-xO3
Young Mo Kim 1 , Juyeon Shin 1 , Char Kookrin 1
1 Physics, Seoul National University, Seoul Korea (the Republic of)
Show AbstractLaInO3 (LIO)/BaxLa1-xSnO3 (BLSO) interface is an interface between a polar perovskite LIO and a nonpolar perovskite BSO doped with La. It was reported that LIO/BLSO interfaces show conductance enhancement by 4 orders of magnitude while both materials are band insulators [1, 2]. To identify the origin of the conductance enhancement, we investigated the interface between BSO and another polar material, LaGaO3 (LGO). LGO shares the same orthorhombic perovskite structure with LIO but LGO has smaller band gap and lattice constant compared with LIO. We examined the epitaxial growth of LaIn1-xGaxO3 (LIGO) on BLSO by x-ray diffraction measurement and transmission electron microscopy. Conductance variation of LIGO/BLSO interface before and after the LIGO layer deposition was also measured while varying the Ga ratio in LIGO. From the measured structural and electronic properties of the interfaces, we will discuss the relationship between the conductance enhancement and the crystal structure at the interfaces.
[1] U. Kim et al., APL Mat. 3, 036101 (2015).
[2] U. Kim et al., APL Mat. 4, 071102 (2016).
8:00 PM - EM05.09.09
Effects of La-Doped BaSnO3 Epitaxial Electrode on the Ferroelectric Properties of Pb(Zr,Ti)O3
Hahoon Lee 1 , Char Kookrin 1
1 Physics, Seoul National University, Seoul Korea (the Republic of)
Show AbstractIn order to integrate the newly discovered high-mobility perovskite semiconductor BaSnO3 [1] with a ferroelectric perovskite, we have grown epitaxial ferroelectric Pb(Zr,Ti)O3 (PZT) of various Zr/(Zr+Ti) ratio by pulsed laser deposition technique on top of the 4 % La-doped BaSnO3 (BLSO). X-ray diffraction measurement suggests that the PZT films on top of BLSO electrode are epitaxially grown and the lattice constants of PZT depend on Zr/(Zr+Ti) ratio in PZT. An all epitaxial sandwich structure of BLSO/PZT/BLSO was fabricated in order to investigate the relation between Zr/(Zr+Ti) ratio of PZT and the strain effect on the ferroelectric properties of PZT. The polarization-electric field (P-E) hysteresis curve will be discussed from the viewpoint of the strain effect. In addition, the breakdown field will be measured to evaluate the potential of PZT for a gate oxide of ferroelectric field-effect transistor on top of BLSO.
[1] H. J. Kim, U. Kim et al., Appl. Phys. Express 5, 061102 (2012).
8:00 PM - EM05.09.10
Effects of Electrochemical Potential on Aliovalent Dopant Segregation on Perovskite Oxides
Dongha Kim 1 , Roland Bliem 1 , Gulin Vardar 1 , William Bowman 1 , Bilge Yildiz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractLa-based perovskite oxides such as (La,Sr)CoO3 (LSC), (La,Sr)MnO3 (LSM), and (La,Sr)FeO3 have been extensively studied as cathodes in solid oxide fuel cells (SOFCs) due to their high oxygen reduction activity and good compatibility with electrolyte material. However, cathode performance is still regarded as one of the main bottlenecks in SOFC operation. In the case of perovskite cathodes, the activity for oxygen reduction and incorporation is impeded by the surface segregation of aliovalent dopants such as Sr.1 This dopant segregation to the oxide surface often leads to the formation of insulating phases that hinder the oxygen reduction reaction on the cathode surface.
Our previous work assessed the role of temperature and oxygen pressure on segregation of aliovalent dopants on perovskite oxide surfaces.2 The present study systematically investigates the effect of polarization on dopant segregation with surface-sensitive analytical techniques. We applied a potential gradient parallel to the surface of LSM thin-film model cathodes, following a design by Huber et al.3 Thus, we changed the effective oxygen chemical potential, μO2, across the surface, which modified the local oxygen vacancy concentration and therefore affected the electrostatic driving force of segregation. The cation composition was measured at five points with different effective μO2 by ambient pressure X-ray photoelectron spectroscopy (APXPS). The amount of segregated Sr was calculated by deconvoluting the Sr 3d peak into two doublet components: a surface component and a lattice component, corresponding to Sr in the surface species and in the original perovskite phase, respectively. We observed a clear trend towards stronger segregation as cathodic polarization was increased across the surface. The studied potential ranged from 0 mV to -500 mV. The portion of surface component gradually increased with respect to the lattice component. Furthermore, the resonant photoemission of Mn was performed to analyze valence band spectra in detail. Both off-resonance and resonant spectra indicate a significant change in oxidation state under polarization, following the potential gradient. These results indicate that cathodic potential promotes segregation of Sr out of perovskite lattice.
Acknowledgements
We acknowledge support from the US Airforce Office of Scientific Research.
References
[1] Lee, W. et al., Journal of the American Chemical Society, 135 (2013) 7909-7925.
[2] Nikolai T. et al., Nature Materials, 15 (2016) 1010-1016.
[3] Huber et al., Journal of the Electrochemical Society, 164 (2017) F809-F814.
8:00 PM - EM05.09.11
Effect of Crystal Orientation on the Segregation of Dopant Cations on Perovskite Oxides—The Case of La0.6Sr0.4CoO3
Fatih Piskin 2 1 , Roland Bliem 1 , Bilge Yildiz 1
2 Metallurgical and Materials Engineering, Middle East Technical University, Ankara Turkey, 1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractPerovskite oxides with mixed electronic and ionic conductivities have been studied widely as catalysts for applications such as oxygen permeation membranes, metal-air batteries and solid oxide fuel cells (SOFCs). Especially in SOFCs, La-based ABO3 perovskite-type oxides, such as (La,Sr)CoO3 (LSC), (La,Sr)FeO3 (LSF), and (La,Sr)MnO3 (LSM), find application as state-of-the-art cathode materials, offering high oxygen reduction activity and oxygen ionic conductivity. However, the cathodes in question suffer from degradation of surface chemistry and oxygen reduction reaction kinetics under operational conditions. This surface degradation is often attributed to dopant cation enrichment or formation of dopant rich insulating phases on the cathode surface. While the cation enrichment leaves a dopant-poor subsurface region, the formation of insulating phases blocks the electron transfer and oxygen exchange pathways on the cathode surface, and thus, hinders the oxygen reduction reactions [1]. This dopant segregation can be driven by strain and the electrostatic attraction of aliovalent dopants by surface oxygen vacancies [2]. It is expected that the surface energy, which is affected by the atomic arrangements, alters the oxygen vacancy concentration of the perovskite surface and the interface energy between the possible secondary phases.
The current study systematically investigates the crystal orientation effect on the dopant cation segregation on LSC with complementary surface-sensitive analytical techniques. Polycrystalline LSC pellets were produced and then annealed at 800 °C in air for several hours (2h, 5h and 10h) to induce the surface cation segregation. The correlation between the crystal orientation and the segregation tendency were investigated via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). In the analysis, 60x70 µm2 area of three sample surfaces, experienced different annealing times, 2h, 5h and 10h, were examined grain by grain in terms of respective area coverage and the number density of the segregated particles. Then the orientation of each grain was confirmed by EBSD. The results show statistically that the orientations close to {001} are more prone to cation segregation than the orientations close to {101} and {111}. This orientation dependency indicates that the exposed facet has a significant impact on the cation segregation. The LSC surfaces were also investigated with XPS and AES before and after annealing procedures to reveal the chemical state of the segregated particles for each condition. The present study aims to probe the importance of the atomic arrangements on the cathode surface stability and the cation segregation tendency of LSC material.
References
[1] Tsvetkov, N. et al., Nature Materials, 15, 1010-1016 (2016)
[2] Lee, W. et al., Journal of American Chemical Society, 135, 7909-7925 (2013)
8:00 PM - EM05.09.12
Defect Dynamics Under Electric Field in Orthorhombic Zirconia Model System
Minh Dinh 1 2 , Mostafa Youssef 1 2 , Bilge Yildiz 1 2 3
1 Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Laboratory for Electrochemical Interfaces, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractUnderstanding the response of insulators and semiconductors to applied external electric fields is of fundamental and practical interest. Growing interest in understanding the effect of large electric fields on the polarization, thermodynamics and kinetics of defects in insulating oxides is driven by emerging technologies including resistive switching memories, electrocaloric refrigeration, field assisted ceramic sintering, and controlling nanowire growth. Point defects, particularly oxygen vacancies, play a prominent role in creating interfacial electric fields and dictating the functional properties of these metal oxides. The polarization response and thermodynamics of a defect-free insulating crystal under high electric field is reasonably well formulated. However, the analogous high field effect on a defective crystal remained challenging to address. Applying a homogeneous electric field to an insulating crystal bends its electronic bands linearly, and polarizes the crystal uniformly. Thermodynamically, the former effect augments the differential of the internal energy, dU, of the crystal by a charge transfer or electrochemical work. The second effect extends dU by what is known as the polarization work, and invokes electric Gibbs free energy. There is no detailed and quantitative analysis for the impact of polarization work on a realistic insulator that contains point defects. In particular, we seek a thorough analysis that spans from the global effects of electric field on the overall abundance of defects, down to the local effects on the single defect site. Here we demonstrate a thermodynamic formulation to quantify the defect formation energetics in an insulator under high electric field. As a model system, we apply this approach on investigation of thermodynamics of different charge states of oxygen vacancies in the ferroelectric phase of ZrO2. In this study, we reported the discrepancy between results of classical, electrochemical treatment of point defects and that of modern theory of polarization. Results indicate enrichment of doubly charged oxygen vacancies, and depletion of triplet 4-fold oxygen vacancy, because of work of polarization. This result is justified by examining the dependence of defects’ dipole moments on the applied field. It was found that doubly charged vacancies is enriched due to their high intrinsic dipole moment and high polarizability. In the case of triplet neutral oxygen vacancy at 4-fold site, delocalization of the electron pair and the subsequent relaxation leads to the stiffening of the structure. It is found that, when electric Gibbs free energy is used to determine stability instead of electrochemical effect alone, electric field favors the formation of one enantimorphous over the other leading to switching in polarization direction when field switches. The approach formulated in this study can be used to study the defect dynamics on different model systems under the effect of applied electric field.
8:00 PM - EM05.09.13
Oxide/ Silicon Heterojunctions for Photodiode and Photovoltaic Applications
James Masi 1 , Kyle Curley 1 , Jacob Littlefield 1
1 , Univ of Southern Maine, Gorham, Maine, United States
Show AbstractZnO, In2O3/SnO2, and BaTiO3, wide bandgap oxide semiconductors are widely applied and have useful properties as active window layers in a heterojunction photovoltaic and photoconductive diode structures. They are considered among other transparent and electrically conducting and semiconducting films as having the potential for high conductivity (with proper treatment), low optical absorption and low sensitivity for damage from high energy radiation. They open the possibility of having extended spectral response in the near ultraviolet region of the spectrum as well as enhanced x-ray sensitivity.They have wide band gaps ranging from ~3.3 to 4 eV at 300 K and a large exciton binding energy. Epitaxial growth of these films on both p- and n-type silicon is performed by methods such as magnetron sputtering, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), spray pyrolysis, sol gel, as well as low temperature thermal oxidation of polymeric precursors. The work described herein included: Growth of these films by low temperature precursor oxidation and by spray pyrolysis (with subsequent heat treatment/annealing) and measurement of their thicknesses; Doping by inclusion of dopant(s) in the precursor; Measurement of optical properties, such as index of refraction, absorption, and transmittance; Measurement of fluorescence in these films; Measurement of electrical/electronic properties, such as resistivity, mobility, and photoconductivity; and Measurement of I-V, C-V, lnJ vs 1/T (determination of Jo, etc.), band structure, and temporal and spectral response of the optically active heterojunction detectors.Results, conclusions, and further work are discussed.
8:00 PM - EM05.09.14
Direct Observation of Trapping of Implanted Deuterium in Poly-Si/Al2O3/HfxSi1-xO2/SiO2 High-k Stacks
Yuan Tu 1 , Bin Han 1 , Yasuo Shimizu 1 , Masao Inoue 2 , Yorinobu Kunimune 3 , Yasuhiro Shimada 3 , Toshiharu Katayama 3 , Takashi Ide 3 , Shinji Nagata 4 , Koji Inoue 1 , Yasuyoshi Nagai 1
1 The Oarai Center, Institute for Materials Research, Tohoku University, Oarai, Ibaraki, Japan, 2 Device Technology Division, Renesas Electronics Corporation, Hitachinaka, Ibaraki, Japan, 3 Technology Division, Renesas Semiconductor Manufacturing Co., Ltd., Hitachinaka, Ibaraki, Japan, 4 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan
Show AbstractHigh permittivity (high-k) dielectrics based on hafnium oxide (HfO2) has been reported as a charge-storage layer in non-volatile memory devices with an aluminum oxide (Al2O3) blocking layer, in consideration of the scaling of equivalent oxide thickness1. In general, H2 sintering in BEOL may be also required for high-k stacks to acquire high-quality interface with channel. Al2O3 film, however, is known to be a barrier to prevent H attack during the back-end-of-line (BEOL) which can degrade the performance and reliability in memory devices such as ferroelectric random access memories2. Thus, it is of great interest to examine H distribution in Al2O3/HfxSi1-xO2/SiO2 (AHO) high-k stacks.
Atom probe tomography (APT) is a powerful technique for its capability of obtaining an elemental distribution map with nearly atomic scale resolution3. Thus, APT is suitable to intuitively examine the H stopping location, i.e. surface, bulk, or interface. However, the observation and quantitative analysis of H is difficult due to the absorption of H atoms from the residual gas in analysis chamber during APT measurement4. Extensive efforts were made to charge deuterium (D), H isotope, for studying the behavior of H-species5. However, the charged D may easily diffuse out of the samples, thus the specimens need to be preserved in cryogenic conditions before APT measurement. Whereas, Takamizawa et al. attempted to introduce D into Si by ion implantation, and demonstrated that D was partly trapped in the defects induced by ion implantation, and partly bonded with Si, thus the sample could be kept even in room temperature6.
In this work, we fabricated AHO high-k stacks with poly-Si capping and then D ions were implanted into the top poly-Si layer to a high dose to make it distinguished from H from the residual gas. By annealing the samples, D atoms are expected to diffuse toward the AHO region and the H trapping ability of Al2O3 can be examined. It clearly demonstrated that D atoms are trapped at the interface of poly-Si and Al2O3 after annealing at 627 °C, which proved that Al2O3 is likely to prevent H2 diffusion into the high-k dielectrics during the H2 annealing process in current fabrication technology.
This work was supported in part by JSPS KAKENHI Grant No. 15H05413.
1 M. Inoue et al., Jpn. J. Appl. Phys. 55, 08PB03 (2016).
2 A. Chowdhury et al., in Non-Volatile Mem. Technol. Symp. (2007), pp. 49–52.
3 T.F. Kelly and M.K. Miller, Rev. Sci. Instrum. 78, 031101 (2007).
4 Y. Kunimune et al., AIP Adv. 6, 045121 (2016).
5 Y. Chen et al., Science 355, 1196 (2017).
6 H. Takamizawa et al., Appl. Phys. Express 6, 066602 (2013).
8:00 PM - EM05.09.15
Hydrogen Incorporation Effect in Phosphorus-Doped P-Type ZnO Thin Films Grown by Radio-Frequency Magnetron Sputtering
Min-suk Oh 1 , Young Gon Kim 1 , Jae-Won Lee 2 , R. Navamathavan 3
1 , Korea Institute of Industrial Technology, Gwangju Korea (the Republic of), 2 , Pohang Institute of Metal Industry Advancement, Pohang Korea (the Republic of), 3 , VIT University Chennai, Chennai India
Show AbstractWe report on the influence of hydrogen incorporation on the conductivity of phosphorous (P) doped ZnO thin films grown by using radio-frequency (RF) magnetron sputtering. The P dopant is an oxide form of P2O5, which is introduced into the ZnO thin films using RF plasma with oxygen ambient to suppress the generation of O vacancies. The resultant P-doped ZnO thin films were analyzed by means of field-emission scanning electron microcopy (FE-SEM), atomic force microscopy (AFM), secondary ion mass spectroscopy (SIMS), Fourier transform infrared (FT-IR) spectroscopy, photoluminescence and Hall effect measurements. It was observed that the P2O5-doped ZnO thin films annealed at 800 °C exhibited the best electrical property with p-type behavior. Hydrogen atoms in ZnO thin films play an unusual role since it acts as a shallow donor and it may control the n-type conductivity in undoped material. Measurements revealed that the hydrogen atoms can be easily incorporated from the P-doped ZnO sputtering target as the natural hydrogen incorporation in P-doped ZnO thin films during magnetron sputtering. The role of hydrogen atoms incorporated in ZnO thin films is investigated by means of SIMS analysis.We report on the influence of hydrogen incorporation on the conductivity of phosphorous (P) doped ZnO thin films grown by using radio-frequency (RF) magnetron sputtering. The P dopant is an oxide form of P2O5, which is introduced into the ZnO thin films using RF plasma with oxygen ambient to suppress the generation of O vacancies. The resultant P-doped ZnO thin films were analyzed by means of field-emission scanning electron microcopy (FE-SEM), atomic force microscopy (AFM), secondary ion mass spectroscopy (SIMS), Fourier transform infrared (FT-IR) spectroscopy, photoluminescence and Hall effect measurements. It was observed that the P2O5-doped ZnO thin films annealed at 800 °C exhibited the best electrical property with p-type behavior. Hydrogen atoms in ZnO thin films play an unusual role since it acts as a shallow donor and it may control the n-type conductivity in undoped material. Measurements revealed that the hydrogen atoms can be easily incorporated from the P-doped ZnO sputtering target as the natural hydrogen incorporation in P-doped ZnO thin films during magnetron sputtering. The role of hydrogen atoms incorporated in ZnO thin films is investigated by means of SIMS analysis.
8:00 PM - EM05.09.16
The Chemical Tendency of Metal Atom Stability in Amorphous-SiO2
Takeshi Miyajima 1 , Hiroki Shirakawa 1 , Masaaki Araidai 1 2 , Kenji Shiraishi 1 2
1 , Graduate School of Engineering, Nagoya University, Nagoya Japan, 2 , IMaSS, Nagoya University, Nagoya Japan
Show AbstractMetallic contamination was one of the most serious issues for fabrication of MOSFETs. The contamination is caused by an incorporation of metal atoms into a silicon wafer or an insulating film. The incorporated metals trap electrons/holes. Furthermore, incorporated metals become ion, and migrating in insulating films when gate voltage is applied. The trapped charges and ions migration lead to the degradation, such as threshold voltage shift or dielectric breakdown.[S. I. Raider, et al., J. Electrochem. (1973)],[ SM Sze, KK Ng – 2006][S. Yokogawa, et al., Microelec. Reliability (2001)]. However, devices utilizing the incorporated metals in insulating films have been recently invented. One of the devices is the electret made of potassium-ion-doped SiO2. [G. Hashiguchi, et al., AIP Advances (2016)]. The electret can store charges by the doped potassium in SiO2 generating negative fixed charges. Therefore, it is important to clarify the chemical trends of metal species in the insulator for the development of the new devices and the improvement of the devices.
In this study, we aim to clarify the chemical tendency of metal atom stability in amorphous-SiO2. To investigate the stability, we defined the formation energy that the metal atoms incorporate into amorphous-SiO2. The formation energy is as follows: ΔE=E(SiO2 with metal)-{E(αquartz)+μ(metal)}.E(SiO2 with metal), E(αquartz), and μ(metal) are corresponding to the total energy of an amorphous-SiO2 including a metal, an α-quartz, and a metal atom in vacuum. The total energies are calculated using the VASP (Vienna ab initio simulation package) code [G. Kresse and J.Hafner, Phys. Rev. (1993)]. We calculated the formation energy of Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag.
The calculated formation energies of Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag ware -9.66eV, -6.53eV, -4.33eV, -3.85eV, -1.20eV, -4.01eV, -2.98eV, -1.87eV, respectively. The smaller formation energy indicates stronger bonding with SiO2. To investigate chemical bonding around metal atoms in detail, we used COHP analysis. As the result of COHP analysis, we found that the bonding state between metal atom and SiO2 rely on the d-orbital occupancy of metal atoms. The metal with the d-orbitals of less than half occupancy, such as Zr, Nb, Mo, and Tc tend to combine with O atoms, while the metal with the d-orbitals of more than half occupancy, such as Ru and Rh combine with Si and O atoms in equal proportion. On the other hand, the metal with the closed d-orbitals, such as Pd and Ag did not combine with Si nor O atoms. This indicates that the interaction between Pd/Ag and SiO2 is very small.
Next, we investigated how the structure of SiO2 is affected by the metal contamination. We found that Zr, Nb, Mo and Tc gather many oxygen atoms around the metal, leading to generate Si-Si bond in SiO2. On the other hand, Pd and Ag with the closed d-orbitals does not affect the structure of SiO2. This is because the interaction between Pd/Ag and SiO2 is very small.
8:00 PM - EM05.09.17
Strain-Engineered Nano-Composite BaTiO3 Films from Aqueous Chemical Solution Deposition
Trygve Ræder 1 , Mari-Ann Einarsrud 1 , Tor Grande 1
1 Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Trondheim Norway
Show AbstractBaTiO3 is one of the prototype ferroelectric materials used in capacitors due to its superior dielectric properties, but the low Curie temperature limits the use of BaTiO3 for many applications. Strain engineering has been demonstrated to increase the Curie temperature of BaTiO3, both by substrate induced strain[1][2] in epitaxial films and in nano-composites with a secondary material introducing strain at the heterophase interfaces[3]. These previous examples of strain engineering have relied on physical deposition techniques such as MBE and PLD, while chemical deposition techniques has to lesser extent been used to induce strain in nano-materials. In this work, nano-composite BaTiO3:Sm2O3 thin-films are fabricated by aqueous chemical solution deposition using spin-coating on single crystal SrTiO3 substrates. An aqueous precursor solution containing the Ba, Ti and Sm cation precursors was developed. The synthesis platform is confirmed to yield epitaxial or textured pure BaTiO3 and BaTiO3:Sm2O3 nano-composites with varying volume ratio of the two phases. The thermal processing of the pure BaTiO3 as well as the composites were optimized to control the crystallization and microstructure of the nano-materials. It is shown that the nucleation and growth of BaTiO3 is strongly influenced by the volume fraction of Sm2O3 in the composite. Out of plane tensile strain in BaTiO3 is confirmed by X-ray diffraction. The nano-composites are characterized by a combination of X-ray diffraction, transmission electron microscopy, dielectric spectroscopy and ferroelectric characterization. The structural and physical properties will be discussed in light of the observed microstructure and strain induced from the substrate and the interface between the two phases in the nano-composite.
[1] Schlom, Darrell G., et al. "Strain tuning of ferroelectric thin films." Annu. Rev. Mater. Res. 37 (2007): 589-626.
[2] Schlom, Darrell G., et al. "Elastic strain engineering of ferroic oxides." Mrs Bulletin 39.02 (2014): 118-130.
[3] Harrington, Sophie A., et al. "Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain." Nature nanotechnology 6.8 (2011): 491-495.
8:00 PM - EM05.09.18
Optimising the Device Performance of SiOx-Based RRAM by Tailoring the Oxide-Electrode Interface Roughness
Wing Ng 1 , Adnan Mehonic 1 , Manveer Munde 1 , Mark Buckwell 1 , Michel Bosman 2 , Anthony Kenyon 1
1 , University College London, London United Kingdom, 2 Institute of Materials Research and Engineering, A*STAR, Innovis Singapore
Show AbstractSilicon oxide based resistance switching random access memories (RRAM) have attracted much attention in recent years due to their potential as next generation memory devices, and suitability for advanced applications such as neuromorphic computing. Here we present an investigation into how the oxide-electrode interface roughness affects the electrical characteristics of SiOx-based RRAM devices. Our results show that interface roughness promotes lower electroforming voltages and improves the consistency of set and reset voltages.
Our metal-insulator-metal based RRAM devices comprised a molybdenum bottom electrode, a sputtered SiOx switching layer, and a titanium/gold top electrode. The interface roughness between the Mo and SiOx layers was controlled by varying the sputtering deposition parameters of the Mo: substrate temperature, deposition power, and pressure.
The roughness of the Mo surface was characterised by atomic force microscopy (AFM); results showed our Mo surfaces had root mean squared roughness varying from 0.9nm to 1.5nm. Subsequent current-voltage measurement shows there was a significant reduction in electroforming voltages for rougher surfaces.
These results are consistent with our separate studies showing that enhanced columnar structure in the oxide layer promotes switching. This suggests that the rough electrode interface may template the formation of oxide columnar microstructure, in addition to providing some degree of electric field enhancement within the oxide.
8:00 PM - EM05.09.19
Leakage-Current Reduction in Atomic-Layer-Deposition Al2O3 Films by Al gate/Al2O3 Interface Engineering Using O3 Treatment
Satoshi Okubo 1 , Daisuke Matsumura 1 , Atsushi Hiraiwa 1 2 , Hiroshi Kawarada 1 3
1 , Waseda University, Tokyo Japan, 2 , IMaSS, Nagoya University, Nagoya Japan, 3 , The Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Tokyo Japan
Show AbstractWide-bandgap semiconductors (WBGSs) have recently been attracting attention from power device engineers as a substitute for Si because of their high blocking capability. A challenge of WBGS metal-insulator-semiconductor field-effect transistors (MISFETs) is high-performance, high-reliability gate insulation, because the thermal SiO2 with proven performance and reliability is not available there. A promising solution to the challenge is atomic-layer-deposition (ALD) Al2O3 films, due to their wide bandgap (7eV), high dielectric constant (9), and good reproducibility [1]. A remaining task for the films is to achieve good electric insulation. To secure a good Al2O3/semiconductor interface, we need to adopt H2O as oxidant, but the H2O-grown films allow a larger current compared to O3-grown films [2]. Although it has traditionally been believed to be determined by the bulk properties of the films, the current in this study is, for the first time, reduced by modifying the gate electrode/Al2O3 interface properties.
We first formed the ALD-Al2O3 films using H2O oxidant and then exposed them in-situ to an O3 atmosphere. Finally, Al gate electrodes were formed by thermal evaporation. For negative bias, the current in the films decreased with O3 exposure time, whereas, for positive bias, the O3 treatment only had a negligible effect. We also formed samples of which the O3-treated Al2O3 films were wet-chemically etched prior to the gate formation. The current reduction effect of the O3 treatment decreased with the film etching, being lost for the etching of more than 4 nm. These results clearly show that the O3 treatment only modifies the Al2O3 films within 4 nm from the surface. The space-charge-controlled field emission analysis [3] revealed that the electron affinity (EA) of the Al2O3 films near the Al/Al2O3 interface is effectively decreased by the O3 treatment, hence enhancing the potential barrier against the electron emission, as modeled below. Because of its strong reducing capability, Al at the Al/Al2O3 interface takes oxygen out of the Al2O3 film and, therefore, is negatively charged due to the large electronegativity of O atoms, whereas the surficial Al2O3 film is positively charged due to the O vacancies. This pair of negative and positive charges forms a sheet of dipoles, decreasing the potential energy in the Al2O3 film, i.e., increasing the effective EA. The O3 treatment is supposed to effectively suppress the formation of the positive charges in the Al2O3 film by providing ample O in the film, hence reducing the effective EA. Additionally, we confirmed that the O3 treatment had a negligible effect on the Al2O3/semiconductor interface properties. Therefore, the O3 treatment is a powerful tool for realizing high-performance, high-reliability gate insulation in non-Si MISFETs.
[1] T. Suntola, Mater. Sci. Rep. 4, 261 (1989).
[2] A. Hiraiwa, et al., to be presented at this meeting.
[3] A. Hiraiwa, et al., J. Appl. Phys. 119, 064505 (2016).
8:00 PM - EM05.09.20
Enhancement of Electronic Transport Modulation in Single Crystalline VO2 Nanowire-Based Solid State-Field-Effect Transistor
Teruo Kanki 1 , Tingting Wei 1 , Hidekazu Tanaka 1
1 , Osaka University, Osaka Japan
Show AbstractResearch on electrostatic carrier doping utilizing correlated oxide-FETs have developed as new avenue to not only realize Beyond-CMOS, but also probe underlying complicated physics in condensed matters. Among many correlated oxides, vanadium dioxide (VO2) as a prospective candidate, possessing a dramatic resistance change with metal-insulator phase transition (MIT) around 340 K, has attracted much attention. Although VO2 FETs are focused and have been prepared by conventional gate schemes such as SiO2, Al2O3 and HfO2, atomic mixture at the interface and/or the defects of channel interface generates high trap density, inducing slow and inefficient switches. Meanwhile, recent researches on MIT induced by electric double layer transistor (EDLT) have indeed been demonstrated, while an open question about the existence of chemical reaction still remains. Therefore, it is necessary to develop robust dielectrics for gate layer to realize not only lower trap density but also trigger huge sheet carrier density. In this study, we prepared VO2 FETs using a hybrid gate insulator using organic polymer polychloro-p-xylylene (parylene-C) and high-k material Ta2O5 (εr=26). The parylene-C has a role as reduction of interface trap density and the hybrid gate with high-k material keeps high dielectric constant. In this symposium, we will report on the observation of reversible and immediate conductive switching by the hybrid gate [1] and on the achievement of over 8 % modulation of resistance change rate defined as (Roff-Ron)/Roff x 100 % in VO2 nanowire channels. The results in this research provide a possibility for clarifying the origin of metal-insulator transition in VO2 through the electrostatic field-induced transport modulation.
[1] T. Wei, T. Kanki and H. Tanaka et al., Appl. Phys. Lett. 108, 053503 (2016).
8:00 PM - EM05.09.21
Large Photocurrent in the Sandwiched SrRuO3 Monolayer Oxide Heterostructure
Heng-Jui Liu 1 , Jing-Ching Wang 2 , Deok-Yong Cho 3 , Kang-Ting Ho 4 , Jheng-Cyuan Lin 5 , Bo-Chao Huang 5 , Yue-Wen Fang 6 7 , Yuan-Min Zhu 8 9 , Qian Zhan 8 , Lin Xie 10 11 , Xiaoqing Pan 11 12 , Ya-Ping Chiu 2 5 13 , Chun-Gang Duan 6 , Jr-Hau He 4 , Ying-Hao Chu 5 14
1 Department of Materials Science and Engineering, National Chung Hsing University, Taichung Taiwan, 2 Department of Physics, National Sun Yat-sen University, Kaohsiung Taiwan, 3 IPIT and Department of Physics, Chonbuk National University, Jeonju-si Korea (the Republic of), 4 Computer, Electrical and Mathematical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal Saudi Arabia, 5 Institute of Physics, Academia Sinica, Taipei Taiwan, 6 Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai China, 7 Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya Japan, 8 Department of Material Physics and Chemistry, University of Science and Technology Beijing, Beijing China, 9 National Center for Electron Microscopy in Beijing, School of Materials Science and Engineering, Tsinghua University, Beijing China, 10 National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing China, 11 Department of Chemical Engineering and Materials Science, University of California, Irvine, Irvine, California, United States, 12 Department of Physics and Astronomy, University of California, Irvine, Irvine, California, United States, 13 Department of Physics, National Taiwan University, Taipei Taiwan, 14 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan
Show AbstractOver the past decades, low dimensional materials have attracted tremendous interest in the applications of novel optoelectronics. Greatly enhanced photocurrent due to the generation of multi-exciton has been observed in zero-dimensional (0D) colloidal quantum-dot[1,2] and one-dimensional (1D) carbon nanotube systems[3]. Recently, two-dimensional (2D) materials such as graphene and transition-metal dichalcogenide (TMD) monolayers, also become another fast-growing branch in this research field due to their extraordinary electrical and optical properties.[4,5] The key concepts to make these systems exhibit superior light-dependent functionalities than their bulk counterparts are the quantum confinement effect and adequate bandgap engineering. Compared to these well-developed systems, complex oxides also exhibits huge potential because their diversity in application. Relying on the mature thin film process, to fabricate artificial oxide monolayers and apply them in the optoelectronics become feasible. In this work, a 2-dimensional SrRuO3 monolayer inserted in the complex oxide heterostructure has been fabricated by reflection high energy electron diffraction (RHEED) assisted pulsed laser deposition (PLD). By reducing the dimension of the metallic SrRuO3 into the scale of the monolayer, an expected opening of band gap with a semiconducting behavior can be observed. In addition, the electronic and lattice states of this SrRuO3 monolayer can be further modulated by applying different boundary conditions (SrTiO3/SrRuO3/SrTiO3 and LaAlO3/SrRuO3/SrTiO3). Through the cross-section scanning tunneling microscopy (X-STM), the variation of band structure of the SrRuO3 monolayer under illumination of visible light has been unveiled, which can be correlated to the corresponding great enhancement of photocurrent (>300% at 500 mW/cm2) observed in this study. This work paves a new route to understand the confinement of dimensionality and explore new intriguing phenomena in complex oxide heterostructures.
References:
[1] V. Sukhovatkin, S. Hinds, L. Brzozowski, E. H. Sargent, Science 2009, 324, 1542.
[2] E. Rabani, R. Baer, Chem. Phys. Lett. 2010, 496, 227.
[3] N. M. Gabor, Z. Zhong, K. Bosnick, J. Park, P. L. McEuen, Science 2009, 325, 1367.
[4] F. Bonaccorso, Z. Sun, T. Hasan, A. C. Ferrari, Nat. Photon. 2010, 4, 611.
[5] Z. Sun, A. Martinez, F. Wang, Nat. Photon. 2016, 10, 227.
8:00 PM - EM05.09.22
A Computational Framework for Simulation of Degradation of Electrode Materials in High-Temperature Fuel Cells
Franziska Hess 1 , Bilge Yildiz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractDoped perovskite oxides are widely applied in solid oxide fuel cells (SOFCs) as electrode materials with high electronic conductivity and good compatibility with common electrolyte materials. It has been known for several decades that aliovalently doped perovskites, such as La1-xSrxMnO3 (LSM), form segregation layers capping the electrode surface during operating conditions that are detrimental for electrode activity, thus degrading the material over time. Reduction of strain energy due to cation size mismatch and electrostatic interaction with a space charge zone in the near-surface region have been proposed as the key aspects leading to cation segregation. The segregation is affected by external parameters, such as p(O2), T and applied potential. Because the interplay between the external conditions and material properties (cation size mismatch, oxygen vacancy concentration…) are not well understood on the atomistic level, it is hardly possible to stabilize these surfaces using knowledge-based approaches.
This project addresses this issue by uniting the material properties and thermodynamic factors into a single Monte-Carlo model of the near-surface region that takes into account explicitly the distribution of oxygen vacancies (giving rise to the proposed space charge zone) in order to predict the distribution of La and Sr cations in LSM. The formation energies of oxygen vacancies and SrLa’ defects close to the surface are computed by density function theory calculations and analyzed in terms of a cluster expansion in order to obtain effective interaction energies between the vacancies that make it possible to predict the vacancy formation energy of arbitrary configurations. Monte Carlo simulations based on these parameters within the near-surface region are used to obtain the distribution of oxygen vacancies in the near-surface region (resulting in the aforementioned space charge zone).
8:00 PM - EM05.09.23
Defect Chemistry of Mesoporous Oxides with Well-Defined Architecture and High Surface Area
Matthias Elm 1 2 3 , Kathrin Michel 2 1 , Christian Reitz 4 , Juergen Janek 2 1 , Torsten Brezesinski 4
1 Center for Materials Research, Justus Liebig University, Giessen Germany, 2 Institute of Physical Chemistry, Justus Liebig University, Giessen Germany, 3 Institute of Experimental Physics I, Justus Liebig University, Giessen Germany, 4 Institute of Nanotechnology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen Germany
Show AbstractOxygen ion conductors, such as ceria and yttria-stabilized zirconia, are of great interest for various applications such as ionic membranes, solid oxide fuel cells or gas sensors. The performance of these devices strongly depends on the ability of the oxide to exchange oxygen with the surrounding atmosphere making a detailed knowledge of the surface properties necessary. To study the influence of the surface properties, in particularly the concentration and type of electronic and ionic defects as well as their transport properties, well-defined model systems are required, where the surface dominates the bulk behavior. Perfect candidates are mesoporous materials, which exhibit a high surface area due to their regular pore structure surrounded by a closed packed, interconnected 3D architecture of nanocrystallites. Here, we present the investigations of the electrical properties of mesoporous CexZr1-xO2 (CZO) and CexPr1-xO2 (CPO) thin films prepared from common salt precursors and an amphiphilic diblock copolymer by using an evaporation induced self-assembly process. The structural quality of the films was analyzed via SEM, WAXD, XRD, XPS and Raman spectroscopy, confirming the successful synthesis of a mesoporous material of high structural quality with a surface area of about 150 m2/g. The electrical properties were investigated as a function of temperature and for varying oxygen partial pressures using electrochemical impedance spectroscopy. Comparing the results with transport properties of single crystals reveals the profound effect of the mesoporous morphology on the electrical transport properties.
8:00 PM - EM05.09.24
Design of Nanostructured Solid Ionic Hydrogen Barrier Coatings—Engineering Defect Chemistry and Space-Charges
William Bowman 1 , Bilge Yildiz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractRecent research from Iceland’s Krafla geothermal area [1] and other similar studies indicate that hydrogen embrittlement of metallic components is the leading cause for the corrosion failure of geothermal systems, whose components operate in extreme environments. Hydrogen is a byproduct of corrosion in water- or H2S-containing environments, such as geothermal fluid—a commonly used heat transfer medium employed in high-temperature geothermal heat exchange systems. And while protons should ideally be discharged from the solid-liquid interface into the liquid phase as gaseous H2, some atomic and/or ionic H is absorbed by the solid as point defects, ultimately inducing embrittlement and making the metal increasingly susceptible to fracture. For H uptake to occur, it must penetrate a synthetic or native surface layer (e.g. oxide, sulfide, carbonate), which involves interfacial and bulk solid-state ionic processes, including surface adsorption and absorption, as well as bulk diffusion. Hence, there is a significant opportunity to elucidate these processes, and to engineer solid ionic barrier coatings that mitigate H uptake and embrittlement in geothermal systems and metal components employed elsewhere.
Here we aim to develop a solid ionic H barrier coating by employing design and engineering approaches to (i) minimize the H point defect solubility and mobility in the coating material, and (ii) enhance electron transfer from the coating surface to adsorbed protons to accelerate the evolution of H2. We employ a doping strategy for monoclinic ZrO2 based on a recently-developed theoretical framework that predicts, for various dopant species, the predominant H point defects and their solubility in this oxide [2]. Based on this framework, promising dopant candidates for exploring phenomena (i) and (ii) above include Cr, Fe, Nb, Ta, W, and P. Thermal desorption spectroscopy is employed to characterize low-concentration H solubility in these doped oxides, and found Cr-doping as an effective means to minimize hydrogen solubility in ZrO2. Alternatively, we attempt space-charge engineering using solid-solid heterointerfaces with the intention of enhancing surface activity towards the proton reduction and H2 evolution reactions at the surface, thus reducing proton uptake in the coating. These processes are investigated on model thin film samples electrochemically via e.g. impedance spectroscopy, and/or spectroscopy via e.g. X-ray photoemission spectroscopy.
Acknowledgements
We gratefully acknowledge the MIT Energy Initiative and Statoil Inc. for financial support.
References
[1] S.N. Karlsdottir, H Embrittlement and Corrosion in High Temperature Geothermal Well, NACE (2012).
[2] M. Youssef, M. Yang, B. Yildiz. Phys. Rev. Applied, 5 (2016) 014008-16.
8:00 PM - EM05.09.25
Design of Doped α-Alumina Thin Films for use as Hydrogen Barrier Coatings Using First Principles Methods
Vrindaa Somjit 1 , Bilge Yildiz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWith the twin challenges of meeting escalating energy demands and the increasing emphasis on clean energy sources, hydrogen is touted to be the fuel of choice for an energy-secure future. This necessitates the development of an especially robust hydrogen storage and distribution system, given the unique complications posed by the gas: its extremely low density, high diffusivity and high flammability. A wide range of coatings have been studied (Fe3O4, Al2O3, TiN, Si3N4, to name a few) for use as diffusion barriers on steel to prevent hydrogen embrittlement. Of these, Al2O3[1], specifically α-Al2O3[2], is found to be the most promising, given its low hydrogen diffusion coefficient and good adherence to steel. However, the studies insofar conducted only examine the hydrogen permeation characteristics at low pressures (<1 MPa) and have been largely empirical. Thus, there is a need to systematically understand the mechanism behind the barrier characteristics of alumina under the operating conditions of H2 fuel transport, i.e., at high pressures (~70 MPa) and ~298 K. To obtain more effective coatings against hydrogen entry, we aim to tailor the point defect chemistry of the material and utilize space-charge effects to trap protons, slow down proton mobility, and aid in H2 evolution at the surface. H-incorporation, proton-dopant association, and proton solubilities in the presence of various acceptor (Mg, Fe, Co) and donor (Ti, Si, Y, Zr) dopants in α-Al2O3 have been quantified using density functional theory. Tools of first-principles methods will further be used to understand defect segregation in Al2O3 bulk and grain boundaries. A 1-D continuum approach will be used to predict the space-charge potential and defect concentrations in the grain boundary core. Optimization of the bulk and grain boundary structure will be done such that the activation energies for OH- reorientation and migration of lone H+, the two steps in long range proton conduction, are both substantially increased. The results will aid in the better understanding of proton transport in the oxide, and lead to the design of superior surfaces against hydrogen entry into steels in hydrogen infrastructure.
References:
[1] G.W. Hollenberg et al., Tritium/hydrogen barrier development, Fusion Engineering and Design 28 (1995) 190-208.
[2] D. Levchuk et al., Deuterium permeation through Eurofer and α-Al2O3 coated Eurofer, Journal of Nuclear Materials, 328 (2004) 103-106.
8:00 PM - EM05.09.26
Materials Study of Transition Metal Oxides for Memristive Switching Devices
Christian Pedersen 1 , Juan Maria García Lastra 1 , Tejs Vegge 1
1 , Technical University of Denmark, Kongens Lyngby Denmark
Show AbstractMemristor-based Resistive Random Access Memory shows great promise as the next generation of nonvolatile memory technology. The fundamental electronics unit is a memristive switching device, which relies on manipulation of oxygen vacancy defects in transition-metal oxides for switching between resistive states. In this regard, the most common and well-studied oxide is rutile TiO2; however, tantalum oxides have emerged as a high-performance alternative [1]. Much still remains to be learned about tantalum oxide, especially conditions related to oxygen vacancy formation. To this end, this project employs first-principles simulations to investigate wide ranges of oxygen-vacancy distributions in rutile TiO2 and lambda-phase Ta2O5. For each distribution, the total energies as well as atomic arrangements are obtained. Comparisons are drawn to corresponding oxygen-depleted stoichiometries, i.e. Magnéli-phase Ti4O7 and rutile TaO2. The object is to determine if and when structural phase changes occur, as well as how they influence device performance.
Another important aspect, related to device arrays, is the elimination of sneak current paths. Arising from linear IV profiles, sneak currents lead to increased power consumption and may even create read errors by decreasing ON/OFF current ratios. Mitigation of sneak currents may be done by inclusion of a selector layer, e.g. a nanometer-thin layer of aluminum oxide, which works as a tunneling barrier to increase non-linearity in IV profiles [2]. We investigate materials for selector layers by means of Non-Equilibrium Green’s Functions within the DFT framework. We compute potential profiles and transmission spectra with the goal of achieving a qualitative description of the selector material.
1. Ho, P. W. C. and Hatem, F. O. and Almurib, H. A. F. and Nandha Kumar, T. "Comparison on Pt/TiO2/Pt and Pt/TaO-x/TaO-y/Pt Based Bipolar Resistive Switching Devices." IEEE Int. Conf. Electron. Des. 249–254 (2014). doi:10.1109/ICED.2014.7015808
2. Aluguri, R. and Tseng, T. Y. "Overview of Selector Devices for 3-D Stackable Cross Point RRAM Arrays." IEEE J. Electron Devices Soc. 4, 294–306 (2016). doi: 10.1109/JEDS.2016.2594190
8:00 PM - EM05.09.27
Probing the Valence Band Density of States of NiO—Understanding the Impact of Defect States on Transport, Charge Injection and Recombination
Taylor Moot 1 , Lesheng Li 1 , Shannon McCullough 1 , Youske Kanai 1 , James Cahoon 1
1 , University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show Abstract
Nickel oxide (NiO) is one of the most common wide-bandgap p-type semiconductors used in solar energy devices because it can function as a hole-transporting layer or a dye sensitized photocathode. Despite its ubiquitous use, NiO is plagued with a large trap-state density that turns the material black, gives rise to quick electron-hole recombination, and lowers solar-cell efficiency. The effect of traps are of particular concern for the use of NiO as a high surface area photocathode, which is necessary for p-type dye sensitized solar cells (p-DSSCs) and tandem solar fuel devices. Although the problems associated with trap states in NiO have been anecdotally known for years, the exact character and density of trap states has been debated.
The NiO trap state density extends substantially above the valence band edge, as characterized by cyclic voltammetry. Experimental characterization through thermogravimetric analysis and Raman spectroscopy suggest there is residual Ni(OH)2/NiOOH like character left within NiO, as typically prepared. First-principles calculations based on DFT+U methodology were performed to examine potential impurities/defects based on these species. It was determined that defects arise from a combination of Ni vacancies and vacancy sites occupied with OH- or H2O, in agreement with experimental characterization. Upon the identification and characterization of the trap states, their effect on charge injection, transport and recombination was studied. We consider the potential role of the Ni vacancies and Ni vacancies occupied by OH- or H2O in charge carrier injection and recombination processes. In addition, we investigate the role of defects in charge transport through the films. This in depth analysis of the character and role of trap states in NiO devices gives insight into many of the long standing questions about the poor performance of NiO devices.
8:00 PM - EM05.09.28
Magnetic and Structural Deviation from Rule of Mixtures in Entropy Stabilized Oxides
Peter Meisenheimer 1 , John Heron 1
1 Materials Science and Engineering, University of Michigan–Ann Arbor, Ann Arbor, Michigan, United States
Show AbstractEntropy-stabilized materials are stabilized by the configurational entropy of the constituents, rather than the enthalpy of formation of the compound. These materials have attracted significant interest due to the apparent deviations from Gibbs phase rule and desirable mechanical properties1,2. Despite the discovery of high entropy crystals nearly 15 years ago3, reported investigations outside transition metal alloys have just recently been extended to ionic crystals, particularly oxides4, a class of materials which can demonstrate useful and dynamic functional properties such as ferroelectricity5, magnetoelectricity6,7, thermoelectricity8, and superconductivity9. As the magnetic and electronic properties of oxides are strongly correlated to their chemistry and electronic structure, the concept of entropy stabilization could lead to interesting and novel properties. Though known entropy-stabilized oxides contain magnetic constituents, the magnetic properties of the multi-component oxide have yet to be investigated. Here we examine the role of entropy and composition on the exchange coupling and magnetic anisotropy of permalloy/(Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O thin film heterostructures. We observe a strong exchange field and an apparent deviation from the rule of mixtures in the structural and magnetic parameters. This result demonstrates that entropy stabilized oxides can be engineered to show concerted magnetic properties that are dependent on constituent species, yet differ from a simple weighed average of the components and can result in unexpected phenomena.
1 M.-H. Tsai and J.-W. Yeh, Mater. Res. Lett. 2, 107 (2014).
2 F. Otto, Y. Yang, H. Bei, and E.P. George, Acta Mater. 61, 2628 (2013).
3 J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, and S.Y. Chang, Adv. Eng. Mater. 6, 299 (2004).
4 C.M. Rost, E. Sachet, T. Borman, A. Moballegh, E.C. Dickey, D. Hou, J.L. Jones, S. Curtarolo, and J.-P. Maria, Nat. Commun. 6, 8485 (2015).
5 D.G. Schlom and others, Annu Rev Mater Res 37, 589 (2007).
6 T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M. Barry, A. Doran, M.P. Cruz, Y.H. Chu, C. Ederer, N.A. Spaldin, R.R. Das, D.M. Kim, S.H. Baek, C.B. Eom, and R. Ramesh, Nat. Mater. 5, 823 (2006).
7 P. Borisov, A. Hochstrat, X. Chen, W. Kleemann, and C. Binek, Phys. Rev. Lett. 94, 117203 (2005).
8 A. Weidenkaff, R. Robert, M. Aguirre, L. Bocher, T. Lippert, and S. Canulescu, Renew. Energy 33, 342 (2008).
9 W.E. Pickett, Rev. Mod. Phys. 61, 433 (1989).
8:00 PM - EM05.09.29
Exchange Bias in Oxide Heterostructures with a Hidden Interfacial Layer
Aiping Chen 1 , Qiang Wang 3 , Michael Fitzsimmons 2 4 , Erik Enriquez 1 , Judith MacManus-Driscoll 5 , Quanxi Jia 6
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 , West Virginia University, Morgantown, West Virginia, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 4 , University of Tennessee, Knoxville, Knoxville, Tennessee, United States, 5 , University of Cambridge, Cambridge United Kingdom, 6 , State University of New York at Buffalo, Buffalo, New York, United States
Show AbstractInterfacial layers in oxide heterostructures have often been observed in many different systems. However, the role of such an interfacial layer in controlling functionalities has not been fully explored. In this talk, we will discuss the role of buried interfaces on exchange couplings in single-phase manganite thin films with nominally uniform chemical composition across the interfaces. Specifically we show compelling evidence for exchange bias (EB) in heterostructures. Interestingly, the sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically degraded layer and the ferromagnetic region of the film’s interior. The sign of EB is related to whether or not antiferromagnetic coupling is frustrated across the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.
8:00 PM - EM05.09.30
Theoretical Study on Interface Orientation Dependence of Si Thermal Oxidation
Takuya Nagura 1 , Shingo Kawachi 1 , Kenta Chokawa 1 , Hiroki Shirakawa 1 , Masaaki Araidai 1 5 2 , Hiroyuki Kageshima 3 2 , Tetsuo Endoh 4 6 2 , Kenji Shiraishi 1 5 2
1 Graduate School of Engineering, Nagoya University, Nagoya Japan, 5 , IMaSS, Nagoya University, Nagoya Japan, 2 , JST-ACCEL, Sendai Japan, 3 , Interdisciplinary Graduate School of Science and Engineering, Shimane University, Matsue Japan, 4 , Graduate School of Engineering, Tohoku University, Sendai Japan, 6 , CIES, Tohoku University, Sendai Japan
Show AbstractDownscaling MOSFETs has led to an increase in off-state leakage current. It is expected that the all around gate structure of Vertical Body Channel MOSFETs (V-MOSFETs) will enable the leakage current to be reduced. [1] To fabricate these devices, it is necessary to oxidize Si pillars. However, the structure of these pillars cannot sometimes be maintained during thermal oxidation due to the phenomenon of missing Si atoms. [2] In previous work, it was shown that Si atoms are spontaneously emitted to release the strain induced by thermal oxidation. [3] We deduced that this emission is the origin of the missing Si atoms. In a Si pillar the Si/SiO2 interface has various orientations. Thus, we investigated the dependence of the thermal oxidation on the orientation of Si based on the Si emission model.
In this study, we used first-principles calculations based on density functional theory. The calculations were performed using VASP code. First, we simulated the oxidation process by inserting O atoms into the Si-Si bonds at interfaces between SiO2 and Si with (100), (110), (111), and (310) orientations. We calculated the change in total energy after the emission of a Si atom.
The results show that the energy decreases as the number of O atoms increases for (100), (110), and (310) orientations. A decrease in the energy corresponds to an increase in the number of Si atoms emitted. Thus, the strain at the Si/SiO2 interface increases as O atoms are inserted. However, Si(111) is different from the others because there are two types of Si-Si bond at the Si(111)/SiO2 interface. The first is where the Si-Si bonds are perpendicular to the interface (type-A), and in the other the bonds are parallel to the interface (type-B). We found that oxidation of type-B-bonds does not occur until all of the type-A bonds have been oxidized. zIn addition, we found that no Si emission occurs during type-A bond oxidation, since the Si emission is energetically unfavorable. After type-B bond oxidation begins, Si emission occurs with the formation of defects after three O atoms have been inserted. This result agrees with XPS observations of the oxidation of Si(111) made in a previous study. [4] Thus, the emission rate is in the following order: (111)typeB > (110) > > (310) > (100) > (111)typeA. Missing Si atoms occurs more easily in V-MOSFETs than planar MOSFETs, since the strain due to oxidation in V-MOSFETS is larger than in planar MOSFETs, which have a Si(100)/SiO2 interface.
This work has been supported by a grant from “Three-Dimensional Integrated Circuit Technology Based on Vertical BC-MOSFETs and Exploration of their Advanced Applications” (Research Director: Prof: Tetsuo Endoh, Program Manager: Dr. Toru Masaoka) of ACCEL under JST (JPMJAC1301).
[1] T. Endoh, et al, IEICE Trans. 2010
[2] H. Kageshima, K. Shiraishi , and T. Endoh, Jpn. J. Appl. Phys. 2016
[3] H. Kageshima and K. Shiraishi, Phys. Rev. Lett. 1998
[4] K. Ohishi and T. Hattori, Jpn. J. Appl. Phys. 1994
8:00 PM - EM05.09.31
Monomer Derived Poly furfuryl/BaTiO3 0-3 Nancomposite Capacitors—Maximization of the Effective Permittivity through Control at the Interface
Frederick Pearsall 1 2 3 , Julien Lombardi 1 2 3 , Stephen O'Brien 1 2 3
1 Chemistry, City College of New York, New York, New York, United States, 2 Chemistry, The Graduate Center of the City University of New York, New York, New York, United States, 3 , The CUNY Energy Institute, New York, New York, United States
Show Abstract0-3 nanocomposites were prepared using barium titanate nanoparticles, suspended in a poly (furfuryl alcohol) matrix, resulting in a stable, high permittivity, low loss dielectric. Cubic barium titanate nanocrystals 8 nm in diameter were synthesized using a solvothermal method which results in a characteristic gel-rod, facilitating suspension of the ceramic in organic solvents. Nanocrystal composition and structure were verified with EDS and XRD respectively. These nanocrystals were then suspended in furfuryl alcohol inside a uniquely prepared mold, where thickness was controlled accordingly. Polymerization of the matrix occurred in situ at temperatures varying from 70-90oC resulting in large reductions in void space and increasing the effective permittivity of the nanocomposites, with less than 0.1 dissipation factor. The increased permittivity is attributed to the covalent attachment of the poly(furfuryl alcohol) matrix to the surface of the barium titanate nanocrystals, homogenizing the particle-matrix interface, reducing void space. Further cross-linking of the polymer creates a 3D covalent network. This as well as the hydrophobicity of the polymer reduces the amount of water inclusions present within the dielectric, further stabilizing the permittivity at lower frequencies. XPS and FTIR were used to probe the nature of nanocrystal interactions with the polymer matrix and confirm strong interfacial interaction between matrix and nanocrystals. Effective medium approximations were used to compare with similar nanocomposite systems, finding the large effective permittivity cannot be attributed to the high permittivity inclusions alone.
8:00 PM - EM05.09.32
Electronic Structure and Elastic Properties of Metal/Amorphous-Oxide Interface—A First-Principle Study of Oxygen Vacancy at the Al/a-SiO2 Interface
Jianqiu Huang 1 , Fei Lin 1 , Celine Hin 1
1 , Virginia Tech, Blacksburg, Virginia, United States
Show AbstractAmorphous oxides have been widely used in the electronic industry due to their isotropic physical properties and low fabrication cost. Billions of transistors in the integrated circuit apply amorphous oxides in the gate metal interface. In this study, we perform first-principle calculations based on the density functional theory to investigate the relation between electronic structure and elastic properties at the metal/amorphous-oxide interface. We introduce an oxygen vacancy at the interface and find a linear relation between the strain applied to the supercell and the oxygen vacancy formation energy. The electronic study also showed that the strain also varies the built-in dipole and alters the eigenstate. In addition, no significant net charge transfer across the interface or interfacial bonding geometry changes observed due to the introduction of mechanical strain. We further apply the quantum formulation of the microscopic stress density integrating the local pressure to study the variation of oxygen vacancy formation energy as a function of the local pressure.
8:00 PM - EM05.09.33
Development of High-Performance P-Type Thin-Film Transistors Using Aluminum-Doped Tin Oxides
Jeong-jin Park 2 1 , Ho-Yeol Choi 2 , Chan-Hwa Hong 2 , Chang-Ho Lee 2 , Joon-Min Lee 2 , Byeong-Kwon Ju 1 , Woo-Seok Cheong 2
2 , Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of), 1 Display and Nanosystem Laboratory College of Engineering, Korea University, Korea University, Seoul Korea (the Republic of)
Show AbstractThere have been many researches on the instability of the SnO phase in p-type SnO TFTs. The p-type transport in tin monoxide occurs only in a very narrow window of deposition process condition. In this study, we tried to widen the process window and enhance the p-type performance by doping aluminum. Al-doping in SnO TFTs could improve the electrical properties such as field effect mobility, subthreshold swing, and threshold voltage.
8:00 PM - EM05.09.34
High Mobility Thin Film Field Effect Transistor Based on SnO2-x
Yeaju Jang 1 , Hahoon Lee 1 , Kookrin Char 1 , Joon Seok Park 2
1 , Seoul National University, Seoul Korea (the Republic of), 2 Research and Development Center, Samsung Display, Yongin Korea (the Republic of)
Show AbstractWe report on high mobility thin film field effect transistors (TFT) based on SnO2-x. The TFTs show high mobility values over 200 cm2/Vs and high Ion/Ioff ratios larger than 105. The SnO2 channel layer was deposited by reactive sputtering and was annealed in a tube furnace to crystalize and enhance the electrical properties. HfO2 was used as the gate insulator and deposited by atomic layer deposition method. Using pulsed laser deposition (PLD) method, ITO or Sb-doped SnO2 were deposited as the source, the drain, and the gate electrodes. All transparent layers were deposited on glass substrates. In order to demonstrate reliable TFT properties and understand the mechanism behind the high mobility, we further studied the effects of the post-ALD annealing temperature and the electrode materials on the TFT properties.
Symposium Organizers
Elizabeth Dickey, North Carolina State University
Ulrich Aschauer, University of Bern
Nicole Benedek, Cornell University
Sakyo Hirose, Murata Manufacturing Co Ltd
EM05.10: Oxide Interfaces and Heterostructures II
Session Chairs
Wednesday AM, November 29, 2017
Hynes, Level 1, Room 109
8:30 AM - *EM05.10.01
Donor-Doped SrTiO3 as a Model Oxide—Fundamentals and Functionalities
Rainer Waser 1 2 , Roger De Souza 3 , Alexander Zurhelle 2 , Felix Gunkel 1
1 JARA-FIT & PGI-7, Forschungszentrum Julich, Julich Germany, 2 IWE2, RWTH Aachen University, Aachen Germany, 3 IPC, RWTH Aachen University, Aachen Germany
Show AbstractPerovskite-type oxides exhibit a plethora of fascinating electronic material properties covering an exceptionally wide range of phenomena in solid state and surface physics. The prototypical perovskite SrTiO3 (STO), its isostructural cousin BaTiO3 (BTO), and their solid solutions belong to the most widely studied oxides because of their diverse functionalities.
The electrical properties of donor-doped SrTiO3 (n-STO) are profoundly affected by an oxidation induced metal-insulator transition (MIT). In this seminar, we present dynamical numerical simulations to examine the high-temperature MIT of n-STO over a large range of time and length scales. The simulations are based on the Nernst–Planck equations, the continuity equations, and the Poisson equation, in combination with surface lattice disorder equilibria as time-dependent boundary conditions. The simulations reveal that n-STO, upon oxidation, develops a kinetic space charge region (SCR) in the near-surface region. The surface concentrations of the variously mobile defects (electrons, Sr vacancies, and O vacancies) are found to vary over time and to differ considerably from the values of the new equilibrium. The simulation results will be compared to experimental diffusion and conduction data.
n-STO, n-BTO, and their solid solutions are commercially used as ceramics for thermistors and grain-boundary layer capacitors for many decades. Furthermore, n-STO is explored for functionalities such as thermoelectricity, memristive switching, and photoelectrolysis. We discuss implications of our findings for the electrical conductivity of n-STO crystals used as substrates for epitaxial oxide thin films, of n-STO thin films and interfaces, as well as of polycrystalline n-STO with various functionalities.
9:00 AM - EM05.10.02
Modified Mixed-Ionic Electronic Conductivity in PCO/STO Heterostructures
George Harrington 1 2 3 , Nicola Perry 4 2 , Bilge Yildiz 5 , Kazunari Sasaki 1 3 , Harry Tuller 2 4
1 Center for Co-Evolutional Social Systems, Kyushu University, Fukuoka Japan, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Next-Generation Fuel Cell Research Centre, Kyushu University, Fukuoka Japan, 4 International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka Japan, 5 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractPr substituted CeO2 (PCO) is an excellent model mixed ionic-electronic conductor (MIEC) for fundamental studies and has potential applications in intermediate temperature electrochemical devices. In high pO2 conditions (1 to 10-6 atm O2) vacancy formation is accompanied by the reduction of Pr4+ to Pr3+, and at much lower pO2 conditions (<10-15 atm O2) additional oxygen vacancies are created accompanied by the reduction of Ce4+ to Ce3+. In these regions the material displays MIEC behaviour via oxygen vacancy hopping and small polaron hopping between the valence-active cations.
PCO has been extensively studied, and the defect chemistry, chemical expansion, and transport properties are well described in the bulk material. This makes it an excellent choice for studying the interplay of electro-chemo-mechanical effects at heterogeneous interfaces, including their impact on transport properties.
We have fabricated multilayer films of alternating Pr0.1Ce0.9O2-d and SrTiO3 (STO) layers using pulsed laser deposition. The nanostructures have been characterised in detail using X-ray diffraction, Raman spectroscopy, and advanced transmission electron microscopy. Electron energy loss spectroscopy studies reveal that the PCO is substantially reduced in the vicinity of the interfaces, with the majority of the cerium cations in the Ce3+ valence state even at 1 atm O2. The conductivity of the layers shows a dramatic weakening of the pO2 dependence as the density of the interfaces is increased, which is suggestive of a lowering of the enthalpy for Pr reduction. This work represents an excellent example of the significant potential to modify the ionic and electronic transport at oxide interfaces.
9:15 AM - EM05.10.03
Spin States Control in LaCoO3-Based Heterostructures
Sangjae Lee 1 , Ankit Disa 1 , Alexandru Georgescu 1 , Gilberto Fabbris 2 , Yichen Jia 1 , Mark Dean 2 , Sohrab Ismail-Beigi 1 , Fred Walker 1 , Charles Ahn 1
1 , Yale University, New Haven, Connecticut, United States, 2 , Brookhaven National Laboratory, Upton, New York, United States
Show AbstractThe cobalt cation in perovskite oxides displays competing electronic states that have a low-spin, intermediate-spin, or high-spin configuration due to an interplay between the crystal field, exchange interaction, and Hund’s coupling. The crystal field of the cobalt cation can be tuned in heterostructures designed using first principles theory to enable access to ground states with different orbital polarizations and macroscopic properties. We investigate (LaCoO3)n/(LaTiO3)n heterostructures by growing epitaxial films with varying layer thicknesses n. This two-component superlattice is designed using first principles theory to induce interfacial electron transfer from the filled Ti t2g to the empty Co eg states driven by a difference in electronegativity of Ti and Co. The consequences of interfacial charge transfer are a structural distortion of the CoO6-octahedra, breaking the degeneracy of the Co eg manifold and inducing orbital polarization. Measurements of magnetization demonstrate the suppression of ferromagnetic order. Changes are also observed in measurements of x-ray absorption spectroscopy as the layer thickness of the LaCoO3, n, is varied from 1 to 5 unit cells.
9:30 AM - EM05.10.04
Method for Measuring Ionic Mobility and Activation Energy in Mixed-Ionic-Electronic-Conductor Thin Films
Dmitri Kalaev 1 2 , Harry Tuller 1 , Ilan Riess 2
1 , MIT, Cambridge, Massachusetts, United States, 2 , Technion–Israel Institute of Technology, Haifa Israel
Show AbstractFinding the ionic properties of ion conducting solids, in particular ion mobility, is of increasing interest for electronic, energy and catalysis applications. However, measurement of these properties is challenging due to typically low ionic conductivities, requiring experiments at elevated temperatures, with high impedance analyzers and long resistivity transients. Most of these techniques are not applicable for measurements of thin films. Moreover, in mixed-ionic-electronic-conductors (MIECs), the ionic conductivity is typically dominated by electronic conductivity, characterized by much higher carrier mobilities.
We present a method for measuring ionic conductivity in nano-dimensioned thin films, that allows one to overcome the above-mentioned limitations. First, measurement time is reduced in selected materials, due to the faster ionic response of thin film to applied electrical stimuli. That reduces the required measurement temperatures to near ambient conditions, as demonstrated experimentally. Second, the effect of the redistribution of ionic defects on the nano-scale has a major impact on the electronic conductivity of nano-dimensioned thin films.
The method described here relies on measuring the nonlinear I-V response of MIEC thin films to linear voltage sweeps of high amplitude (relative to kBT) as function of sweep rate. In the MIEC thin films the I-V relations have characteristic shapes that can be fitted by a previously developed model, enabling the to extraction of key thin film transport parameters. There is no requirement for small signals or redox reactions to take place during the voltage sweeps, key differences from other techniques.
The modelling results, relying on solution of coupled electron-ion transport equations, are applied to experimental results obtained on thin films of non-stoichiometric molybdenum trioxide and other MIEC materials. While the near ambient temperature ionic defect mobility in molybdenum trioxide was found to be seven orders of magnitude lower than the corresponding electronic one, it could nevertheless be well characterized.
EM05.11: Defects at Surfaces
Session Chairs
Wednesday PM, November 29, 2017
Hynes, Level 1, Room 109
10:15 AM - EM05.11.01
Defect Engineering in Ultrathin CaMnO3 Films
Ravini Chandrasena 1 , Weibing Yang 1 , Qingyu Lei 1 , Maryam Golalikhani 1 , Bruce Davidson 1 , Elke Arenholz 2 , Keisuke Kobayashi 3 , Masaaki Kobata 3 , Mario Delgado-Jaime 4 , Frank de Groot 4 , Ulrich Aschauer 5 , Nicola Spaldin 6 , Xiaoxing Xi 1 , Alexander Gray 1
1 Department of Physics, Temple University, Philadelphia, Pennsylvania, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Quantum Beam Science Center, Japan Atomic Energy Agency, Sayo-cho, Hyogo, Japan, 4 Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Utrecht Netherlands, 5 Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, 6 Materials Theory, ETH Zürich, CH-8093 Zürich Switzerland
Show AbstractA fundamental understanding of the energetics and strain-control of active ionic defects in oxides is essential for achieving technical feasibility and efficient performance of future electronic devices relying in these properties [1]. Here, we utilize atomic layer-by-layer pulsed laser deposition from two separate targets to synthesize coherently strained epitaxial CaMnO3 films and a combination of x-ray spectroscopies (high-resolution soft x-ray absorption spectroscopy and bulk-sensitive hard x-ray photoemission spectroscopy) to experimentally observe and confirm the systematic increase of the oxygen vacancy content in CaMnO3 thin films as a function of applied in-plane strain. In conjunction with the x-ray spectroscopies, the relevant defect states in the densities of states are identified and the vacancy content is quantified with state-of-the art theoretical calculations and core–hole multiplet calculations. We also show that the vacancies partially diffuse out of the film when exposed to ambient atmosphere, owing to their high mobility [2], thus requiring an in-situ-grown capping layer to preserve the original strain-induced oxygen-vacancy content. Our findings [3] suggests a robust characterization platform for detecting and quantifying oxygen vacancy content at buried depths and open the door for designing strongly-correlated transition-metal oxides with tunable ionic defect content via strain-induced oxygen-vacancy formation .
[1] Kalinin, S.V. et al. Functional Ion Defects in Transition Metal Oxides, Science 341, 858 (2013).
[2] Aschauer, U. et al. Strain-controlled oxygen vacancy formation and ordering in CaMnO3. Phys. Rev. B 88, 054111 (2013).
[3] Chandrasena, R.U. et al. Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films. Nano Lett. 17 (2), 794 (2017).
10:30 AM - *EM05.11.02
Mechanisms of Oxygen Exchange at (La,Sr)CoO3 Interfaces
Dane Morgan 1 , Yipeng Cao 1 , Milind Gadre 1 , Anh Ngo 2 , Stuart Adler 3
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Chemical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractMaterials with the ability to rapidly exchange oxygen with the environment play a critical role in many existing and developing technologies, e.g., solid oxide fuel cells (SOFCs), gas separation membranes, oxygen sensors, chemical looping devices, and memristors. In particular, reducing the working temperature of SOFCs, which is critical to their increased commercialization, is inhibited by the slow oxygen exchange kinetics at the cathode, thereby limiting the overall rate of the oxygen reduction reaction (ORR). We use ab-initio methods to develop an elementary reaction model of oxygen exchange in a representative SOFC cathode material, La0.5Sr0.5CoO3-δ, and predict that under SOFC operating conditions, the rate limiting step for oxygen incorporation from O2 gas at the stable, (001)-SrO surface is O adatom – surface vacancy recombination. We predict that a high vacancy concentration on the metastable CoO2 termination enables a vacancy-assisted O2 dissociation that is about 100 times faster than O2 dissociation on the Sr-rich (La,Sr)O termination, suggesting mechanisms by which surface termination and Sr segregation can influence oxygen exchange. This result implies that dramatically enhanced oxygen exchange performance could be obtained by suppressing the (La,Sr)O layer that is known to form on many commercial cathodes under fabrication and in-situ conditions. We further use the kinetic theory of O2 adsorption to set an upper limit for the oxygen exchange rate and rate coefficient, providing an assessment of the remaining opportunities available for enhanced oxygen exchange.
11:00 AM - EM05.11.03
Improving Oxygen Reduction Activity via Sub-Surface Layering—A Combined Molecular-Beam Epitaxy, Ambient Pressure X-Ray Photoelectron Spectroscopy, Electrochemistry Study
Chuhyon Eom 3 , Ding-Yuan Kuo 3 , Carolina Adamo 1 , Eun Ju Moon 2 , Steven May 2 , Ethan Crumlin 4 , Darrell Schlom 3 , Jin Suntivich 3
3 , Cornell University, Ithaca, New York, United States, 1 , Stanford University, Palo Alto, California, United States, 2 , Drexel University, Philadelphia, Pennsylvania, United States, 4 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractActive and cost-effective oxygen reduction reaction (ORR) catalyst play an enabling role in air-breathing electrochemical energy technologies. This presentation demonstrates a strategy to increase the ORR activity on La2/3Sr1/3MnO3 by customizing its surface and sub-surface layering. Previous studies of the material have focused on examining the role of bulk composition on the ORR. We push the analysis further by examining how the arrangement of the atoms, specifically La and Sr layers, affects the ORR while maintaining identical bulk compositions. Our study shows that, even between samples with identical bulk compositions, placing the Sr layer in the sub-surface increases the activity over placing the Sr layer on the surface. Ambient pressure X-ray photoelectron spectroscopy reveals that surface Sr oxidizes to form surface species upon contact with oxygen, but not when placed in the sub-surface or deeper. We therefore attribute the enhancement to the increased surface site availability. Our result demonstrates that independent control of surface and sub-surface stoichiometry with atomic precision is an opportunity to enable an improved design of new catalysts beyond controlling only for bulk compositions.
11:15 AM - EM05.11.04
The Role of Adsorbed Metal Cations in the Surface Stability of Perovskite Oxide Cathodes
Roland Bliem 1 , Dongha Kim 1 , Bilge Yildiz 1 2
1 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMaterials for solid oxide fuel cell (SOFC) cathodes cover a broad range of highly active doped ternary oxides, including perovskites. Under operating conditions, however, segregation of dopant cations at the surface is a common problem of state-of-the-art cathodes, such as the Sr-doped La-based transition metal perovskites (La,Sr)CoO3 (LSC), (La,Sr)FeO3 (LSF), and (La,Sr)MnO3 (LSM). In these cases Sr segregation driven by strain and electrostatics1 leads to the formation of inactive dopant oxide layers at the surface blocking the oxygen reduction reaction. As a means to tackle this issue, our group recently proposed a strategy of depositing metal cations of low reducibility (Zr, Ti, Hf) to minimize the surface oxygen vacancy concentration and thus reduce the electrostatic driving force. This approach has proven successful in stabilizing the surface of LSC thin films2.
In the present study we investigate the relation between surface properties and electrochemical performance upon the deposition of metal species. We attempt to achieve an in-depth understanding of the surface and adsorption properties underlying the enhanced surface stability observed for LSC due to metals of low reducibility. In parallel we attempt to generalize the observed effects to include LSM, an intrinsically different type of cathode material. Using x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we analyze the structure and chemistry of LSM surfaces and compare the results before and after metal deposition. Electrochemical stability measurements show how the admetals affect the surface stability of cathode materials. In an experiment to confirm the close relation between surface oxygen vacancies and Sr segregation we apply a polarization gradient parallel to the surface to modify the local oxygen vacancy concentration, a property that is expected to be modified by admetals. Spatially resolved XPS measurements along this potential gradient across the surface demonstrate that Sr enrichment is potential-dependent, and also demonstrate the effect of metal deposition compared to pristine samples. This study is an important step to generalize our strategy of metal deposition for enhancing surface stability under operating conditions.
Acknowledgements
We acknowledge support from the US Airforce Office of Scientific Research.
References
[1] Lee, W.; Han, J. W.; Chen, Y.; Cai, Z. & Yildiz, B. Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites. Journal of the American Chemical Society 135, 7909-7925 (2013).
[2] Tsvetkov, N., Lu, Q., Sun, L., Crumlin, E.J., & Yildiz, B. Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface. Nat. Mater. 15, 1010-1016 (2016).
11:30 AM - *EM05.11.05
Optical and Impedance Spectroscopic Investigation of Factors Impacting Oxygen Surface Exchange Kinetics in Mixed Conducting Perovskite Films
Nicola Perry 1 2 , Ting Chen 1 3 , Namhoon Kim 4 , Elif Ertekin 4 , George Harrington 5 1 2 , Kazunari Sasaki 5 3 1 , Harry Tuller 2 1
1 I2CNER, Kyushu University, Fukuoka Japan, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Hydrogen Energy Systems, Kyushu University, Fukuoka Japan, 4 , University of Illinois at Urbana-Champaign, Urbana-Champaign, Illinois, United States, 5 NEXT-FC, Kyushu University, Fukuoka Japan
Show AbstractThe efficiency of many high temperature electrochemical devices is dependent on the ability of their mixed conducting oxide electrodes to catalyze the oxygen incorporation/evolution process at their gas-solid interfaces. Insights are needed into factors controlling the oxygen surface exchange kinetics in order to rationally engineer more efficient electrodes. While typical electrodes exhibit a complex porous structure, where overall kinetics may be influenced by limitations in bulk or gas diffusion, dense thin films offer ideal geometries in which to isolate and study the surface exchange kinetics and their dependence on various structural features. For example, by varying film growth conditions during pulsed laser deposition, differences in crystallinity, crystalline quality, grain boundary density, orientation, oxidation, and surface chemistry have been obtained. Compositions within the mixed conducting (Sr,La)(Ti,Fe,Co)O3-x perovskite system were studied by this approach in order to better understand the roles of point defect chemistry, electronic structure, and microstructural features in controlling the surface exchange coefficient (k) and its degradation over time. In situ optical transmission relaxation and impedance spectroscopy have been applied to measure these kinetics continuously, with techniques including angle-resolved X-ray photoelectron spectroscopy applied to probe corresponding surface chemistry. Factors found to significantly affect the magnitude of k were crystallinity and thermal history (as it relates to changes in surface chemistry). Factors not found to significantly impact the magnitude of k were grain boundary density, film orientation, and choice of multivalent B-site cation (Fe vs. Co). The activation energy for k decreased as the Fermi level increased via donor doping and did not change significantly, despite surface chemical changes, over time. Implications for optimizing such electrodes for rapid oxygen surface exchange will be discussed.
EM05.12/PM03.09: Joint Session: Interfaces in Oxides
Session Chairs
Tor Grande
David Rowenhorst
Wednesday PM, November 29, 2017
Sheraton, 3rd Floor, Commonwealth
1:30 PM - EM05.12.01/PM03.09.01
Metal Diffusion along the Metal-Ceramic Interface in Partially Dewetted Thin Films
Hagit Barda 4 , Dor Amram 2 , Eugen Rabkin 1
4 , Technion–Israel Institute of Technology, Haifa Israel, 2 , Massachusetts Institute of Technology, Boston, Massachusetts, United States, 1 , Technion, Haifa Israel
Show AbstractThe established hierarchy of diffusion paths in crystalline solids, in the order of increasing diffusivities, is bulk diffusion, dislocation core diffusion, grain boundary diffusion, and surface diffusion. The position of diffusion along the interfaces between dissimilar materials in this hierarchy is largely unknown. In this work, we aimed at filling this gap by studying the metal hetero-diffusion along the metal-ceramic interface.
In a recent work [1], an indirect evidence for fast self-diffusion of Ni along the Ni-sapphire interface has been obtained. Based on these results, we propose a new method of measuring the metal heterodiffusion along the Ni-sapphire interface. We prepared a series of samples, starting from a partially dewetted 40 nm-thick Ni film, which consists of holes surrounded by a bi-crystalline film, and covered by a 4 nm thick Au deposited film. Afterwards, a diffusion annealing was performed at the temperature range of 450°C - 600°C, at which the morphology of the Ni film is highly stable. Gold atoms diffused from the edge of the holes along the Ni-sapphire interface, and the concentration decay in the direction from the hole edge to the unperturbed film was quantitatively characterized by high resolution transmission electron microscopy. Based on the suggested method, diffusion coefficient of Au along a film-substrate interface was determined.
[1] D. Amram, L. Klinger, N. Gazit, H. Gluska, E. Rabkin. ”Grain boundary grooving in thin films
revisited: the role of interface diffusion”, Acta mater. 2014; 69:386.
1:45 PM - EM05.12.02/PM03.09.02
Computational Investigations on the Interface Properties to Improve the Stability of Ag Thin Film on Oxide Substrates in Low-Emissivity Glasses
Mingfei Zhang 1 , Liang Qi 1
1 , Univ of Michigan, Ann Arbor, Michigan, United States
Show AbstractTo improve the energy efficiency of windows and buildings, a multilayer thin-film of various metal oxides and a low-emissivity material (such as silver) are often deposited on the architectural glass. However, the interfaces between the Ag thin film (thickness ~ 10 nm ) and its adjacent oxides, usually ZnO or Al-doped ZnO, have relatively weak interfacial bonding strength and often result in dewetting and agglomeration of Ag thin film on the oxide substrate during the thin film processing procedures and the long-time service. Furthermore, hydrogen and other impurities can penetrate the multilayer and intensify the above detrimental effects when the glass is exposed to the external environment. We plan to solve this problem from two aspects by computational modelling. First, first-principles calculations will be applied to explore the possible chemical doping into the oxide substrates to enhance the bonding strength between Ag thin film and the substrates under the co-existence of interfacial impurities. Second, since the agglomeration of Ag thin film usually starts from the grain boundary grooving in the Ag polycrystalline thin film, atomistic simulations and numerical models will be performed to investigate the kinetics of grain boundary grooving in Ag nano thin film and explore the possible strategies to impede the grooving kinetics. The results will provide physical insights on the design of multilayer thin-film processing procedures to improve Ag thin film wettability and stability on oxide substrate to produce low-emissivity glasses with enhanced and enduring performance.
2:00 PM - EM05.12.03/PM03.09.03
Atomistic Simulation of Interface-Driven Self-Alignment of Si-SiO2 Nanostructures
Thomas Pruefer 1 , Karl-Heinz Heinig 1 , W. Moeller 1
1 , Helmholtz Zentrum Dresden Rossendorf, Dresden Germany
Show AbstractSi nanostructures are very promising candidates for optical and electrical applications. Charged nanocluster can be used for data storage [2]; their discrete energy levels can be used for logic operations; sponge nanostructures can be used as the ion conductor in fuel cells. The size-dependency of their energy levels makes them interesting for application in colour displays.
Among a lot of other methods to synthesize nanoclusters or sponges we present an approach which allows a self-alignment of nanostructures at an interface. The basic idea is to bring together Si, SiO2 and SiOx and anneal it to cause phase separation of SiOx. The interfaces between Si/SiOx and SiOx/SiO2 act as driving forces for the self-alignment of the separated Si and SiO2. To create SiOx we consider 2 processes: (i) Deposition of SiOx films by PVD or CVD and (ii) Ion beam Mixing of Si/SiO2 interfaces.
By PVD it’s possible to create arbitrary shapes of Si/SiO2/SiOx layerstacks. The subsequent annealing causes different effects at the interface. Mainly depending on the structure of the layerstack, but also on the annealing time, different reaction pathways can be observed. The system can end up with different numbers of cluster layers or sponge structures, aligned parallel to the interface. Here we show how and why it is possible to control the sizes, densities and distances of these structures.
The ion irradiation through a Si/SiO2 interface causes mixing of both phases and transforms the interface into SiOx. This method is not that flexible as PVD, but it’s easier to be implemented into common industrial technologies, like the production of CMOS compatible devices. The reformation of the Si/SiO2 interface during heat treatment is again acting as a driving force for the self-alignment and forms a zone between the interface and the resulting nanostructures which is denuded of excess Si. In this case, sizes and density can be controlled by irradiation and annealing parameters.
Earlier studies [1] have proven the reliability of dot formations using ion beam mixing technologies for application as memories [2]. Here, we show simulation results for the formation of Si nanostructures at interfaces in layerstacks of Si, SiOx, SiO2 and basic principles of the driving forces for this kind of self-alignment. Computer simulations using the binary collision approximation (TRIDYN [3]) and the kinetic monte carlo method [4] are employed to subsequently describe the ion irradiation and annealing processes, respectively.
This part of the work is being funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No 688072 (Project IONS4SET).
[1] T. Müller et al., Appl. Phys. Lett. 81 (2002) 3049; ibid. 85 (2004) 2373.
[2] K.H. Heinig et al., Appl. Phys. A77 (2003)17.
[3] W. Möller, W. Eckstein, Nucl. Instr. and Meth. in Phys. Res. B2 (1984) 814
[4] M. Strobel et al., Phys. Rev. B64 (2001)245422.
2:15 PM - EM05.12.04/PM03.09.04
Phase Field Modeling of Grain Boundary Evolution in Porous Oxides—Grain Growth and Pore Mobility Effects
Anter El-Azab 1 , Karim Ahmed 2
1 , Purdue University, West Lafayette, Indiana, United States, 2 , Idaho National Laboratory, Idaho Falls, Idaho, United States
Show AbstractWe present a phase field model for investigating grain boundary evolution in porous oxides with applications to UO2 and CeO2. The model takes into account the interactions between pores and grain boundaries as well as the pore mobility effects. Using a formal asymptotic analysis, the phase field model was matched to its sharp-interface counterpart and all model parameters were uniquely determined. Therefore, the model is able to obtain accurate growth rates that can be compared with experiments. The model was used to reveal various growth regimes in porous oxides and sort the boundary-controlled versus pore-controlled growth kinetic regimes. The model results showed that the pore breakaway phenomenon can only be observed in 3D simulations. The important features of the model and results will be presented.
2:30 PM - EM05.12/PM03.09
BREAK
3:30 PM - *EM05.12.05/PM03.09.05
Probing the Electrical Potential Distribution at SrTiO3 Hetero-Interfaces through Transient Transport Measurements
Roger De Souza 1
1 Institute of Physical Chemistry, RWTH Aachen University, Aachen Germany
Show AbstractThe combination of 18O/16O exchange and Secondary Ion Mass Spectrometry (SIMS) analysis constitutes a powerful tool for probing the behaviour of oxygen vacancies in oxides. In this contribution, I demonstrate the application of this method to investigating the behaviour of oxygen vacancies in SrTiO3 and at its hetero-interfaces. The diffusion kinetics in single-crystal SrTiO3 from 50 < θ / °C < 1400 will be presented. Non-uniform equilibrium distributions of oxygen vacancies at three hetero-interfaces will be discussed: the O2(g)|SrTiO3 interface, the SrRuO3|SrTiO3 interface and the LaAlO3|SrTiO3 interface. The variation of the electrostatic potential across such interfaces will be extracted and compared with complementary measurements.
4:00 PM - *EM05.12.06/PM03.09.06
The Electrochemical Interface—Progress toward a Unified Continuum Theory for Dilute and Concentrated Systems
David Mebane 1
1 , West Virginia Univ, Morgantown, West Virginia, United States
Show AbstractInterfaces, surfaces and other extended defects often control the properties of ion conducting materials. Interfacial properties are often governed by charge separation. The dominant continuum models for space charge phenomena are decades old and are strictly valid only in the dilute limit; atomistic models are constrained by the inherent multi-scale nature of space charge, wherein a surface is in equilibrium with a macroscopically distant bulk phase. The development of quantitative models for observed phenomena such as co-accumulation of oppositely charged defects in concentrated systems thus relies on the emergence of a continuum theory (or a combination of atomistic and continuum approaches) for space charge in the concentrated case. This talk will detail the development over the past couple of years of the Poisson-Cahn space charge theory, including its success in replicating experiment in both dilute and concentrated systems, and ongoing efforts to define and validate a standard theoretical formalism.
4:30 PM - EM05.12.07/PM03.09.07
Differences in Space-Charge Formation at Grain Boundaries in BaZrO3 and BaCeO3
Anders Lindman 1 , Edit Helgee 1 , Goran Wahnstrom 1
1 , Chalmers University of Technology, Gothenburg Sweden
Show AbstractProton conducting ceramics as electrolytes in solid oxide electrochemical devices are a promising alternative for reducing the operating temperature to the intermediate regime (400-700 °C). Among the best proton conductors are acceptor-doped BaZrO3 and BaCeO3, which both exhibit considerable bulk proton conductivity in humid atmospheres, and a lot of attention has been devoted towards BaZrO3-BaCeO3 solid solutions such as BaZr0.7Ce0.2Y0.1O3-d. In these polycrystalline materials, however, grain boundaries (GBs) display high proton resistivity, which severely limits the overall proton transport. This has been attributed to the presence of space-charges at the GBs that cause a depletion of protonic carriers. This effect is less prominent in BaCeO3, but in contrast to BaZrO3, BaCeO3 has problems with chemical stability as it reacts with CO2. The more resistive nature of GBs in BaZrO3 compared with BaCeO3 is evident in the activation energy for the proton GB conductivity (cf. 0.99 eV with 0.79 eV), while the bulk conductivities are similar (cf. 0.46 eV and 0.45 eV).
We have performed a density-functional theory based computational study in order to elucidate the origin to the differences in space-charge formation at GBs in BaZrO3 and BaCeO3, caused by segregation of charged oxygen vacancies and protons. We consider formation of these defects both in bulk and at GBs, for which a proper comparison of the two materials requires both aligned electronic structures and appropriate GB configurations. For the latter, it is essential that we compare GBs in BaZrO3 and BaCeO3 that are as structurally similar as possible and to this end we have selected four symmetric tilt GBs in BaCeO3 that are analogous to the (111)[-110] and (112)[-110] GBs in BaZrO3.
We find that the oxygen vacancy formation and segregation energy is quite similar in BaZrO3 and BaCeO3. For protons, on the other hand, the segregation energy is systematically more negative in BaZrO3 by about 0.3 to 0.5 eV. This difference is found not to be related to the GBs, where protons have similar formation energies in the two materials, but to the bulk phases where protons are less stable in the cubic environment of BaZrO3. The reason for this is found to be related to hydrogen bond formation, which is facilitated in the distorted lattice structures of the GBs and the orthorhombic bulk phase of BaCeO3. Segregation energies are evaluated in a thermodynamic space-charge model, which yields space-charge potentials that are 0.2-0.3 V lower in BaCeO3 compared with BaZrO3. We conclude that proton segregation is consistently more favourable in BaZrO3 compared with BaCeO3 due to differences in proton stability in the bulk phases, and this may be the reason for the less severe GB effect in BaCeO3.
4:45 PM - EM05.12.08/PM03.09.08
Interfacial La Vacancies Ordering and Stress Relaxation Mechanism of LaMnO3 /DyScO3 System
Alexander Kvit 1 , Jie Feng 1 , Chenyu Zhang 1 , Dane Morgan 1 , Paul Voyles 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractWe report detailed structural analysis of thin LaMnO3 (LMO) films grown on DyScO3 (DSO) substrate. The DSO substrate has nearly perfect epitaxial matching with LMO film, but the difference in coefficients of thermal expansion between LMO and DSO and the onset of the Jahn-Teller distortion in LMO below the growth temperature leads to a quite complex stress relaxation at the interface. The LMO film consists of alternating domains rotated by 90° with respect to each other and the substrate. We observe no interface misfit dislocations. We utilize high precision quantitative STEM imaging with superior signal to noise ratio via non-rigid registration and averaging of a series of STEM images. High-angle annular dark field (HAADF) images obtained by this method reveal periodic changes of the La column intensity at LMO/DSO interface. These images are consistent with ordered La vacancies at the LMO/DSO interface. Atomic-scale chemical imaging of composition and bonding of LMO/DSO interface by electron energy loss spectroscopy (EELS) mapping excludes the possibility of antisite defects formation or residual impurity accumulation at the interface. Using the sub-picometer precision in locating atomic column positions from the high precision STEM images, we can map atomic column shifts near the interface and the stress relaxation inside individual domains in real space. The effect of La vacancy ordering on LMO/DSO interface and individual La vacancies in LMO film on residual strain relaxation will be discussed.