David Ginley, National Renewable Energy Laboratory
Shyam Dwaraknath, Lawrence Berkeley National Laboratory
Laura Schelhas, SLAC National Accelerator Laboratory
Abdelilah Slaoui, Laboratoire des Sciences de l’ingénieur, de l’Informatique et de l’Imagerie, iCUBE-CNRS
EP01.01: Computational Design of New Materials
Monday AM, November 26, 2018
Hynes, Level 1, Room 103
8:30 AM - EP01.01.01
Optimization of Si/ZnO/PEDOT:PSS Tri-Layer Heterojunction Photodetector by Piezo-Phototronic Effect Using Both Positive and Negative Piezoelectric Charges
Fangpei Li1,Wenbo Peng1,Zijian Pan1,Yongning He1
Xi'an Jiaotong University1Show Abstract
Piezo-phototronic effect has been extensively introduced to improve the performances of optoelectronic devices by utilizing external-strain-induced positive or negative piezoelectric charges (piezo-charges) to modulate the generation, separation, transportation, and recombination of charge carriers. However, in most cases till today, only the piezo-charges with one polarity (i.e., positive or negative) are effectively utilized. In this work, we fabricated an n-Si/n-ZnO/p-PEDOT:PSS tri-layer heterojunction photodetector (HPD) and systematically investigated the piezo-phototronic effect on its performances simultaneously utilizing both positive and negative piezo-charges for the first time.
In experiment, the photo-responses of the HPD to 405 nm and 648 nm laser illuminations under different externally applied compressive strains indicate the existence of an optimized compressive strain to achieve the maximized enhancements. For example, the photoresponsivities to 405 nm and 648 nm laser illuminations are gigantically improved, and reach 0.218 A/W (under -10.73‰ compressive strain) and 0.012 A/W(under -6.52‰ compressive strain), respectively. Compared to photoresponsivities under strain free condition, the enhancements achieve over 3000% and 1800%, respectively. Other figure of merits as a function of compressive strain, such as photocurrent and specific detectivity, also exhibit a similar optimizing tendency.
The optimizing phenomena are due to the positive and negative piezo-charges at n-Si/n-ZnO and n-ZnO/p-PEDOT:PSS interface, respectively, that introduce different adjustments to the local energy band diagrams which have either enhancing or weakening effects on the bahaviors of photo-generated carriers. Under a relatively small compressive strain, the enhancing influences play a dominant role so the photo-responses are improved. As strain rises, some weakening influences outgrow others, therefore the photo-responses are degraded. This competition mechanism is a combined result of both positive and negative piezo-charges, and eventually produces an optimized modulation to the photo-responses of the HPD. Theoretical validation is implemented by finite element analysis simulations and simulation results show that the strain-induced variations in energy band diagrams in the vicinity of the n-Si/n-ZnO and n-ZnO/p-PEDOT:PSS interfaces are both in good accordance with the proposed working mechanisms.
This work not only presents the utilization of both positive and negative piezo-charges to optimize the performances of the HPD by the piezo-phototronic effect, but also provides a deep understanding of how the piezo-charges of two opposite polarities work together in one optoelectronic device, hopefully proposing the idea of introducing the piezo-phototronic effect into three-/multi-layer devices in future applications.
8:45 AM - EP01.01.02
Defective Metal Oxides—New Generation of Electrostrictor Materials
Simone Santucci1,Simone Sanna1,Nini Pryds1,Vincenzo Esposito1
Technical University of Denmark1Show Abstract
Lead Zirconate Titanate (Pb(Zr,Ti)O3) (PZT) is the dominating electromechanically active functional material with a wide range of applications in electronics and micro-actuation, e.g. in MEMS. However, currently it is difficult to grow highly crystalline PZT directly on silicon due to the interfacial chemical reactions between the lead (Pb) and silicon at elevated temperatures required for the PZT crystallization. A possible solution to avoid interdiffusion is to grow PZT on insulating diffusion barrier layers such as ZrO2 or TiO2 that protect the silicon wafer substrate. This solution, however, brings complex processing steps and can result in an overall decreasing of the device electromechanical performances.
The recent discovery of “non-classical” electrostriction in some defective metal oxides such as (Y, Nb)-Stabilized δ-Bi2O3 (Bi7Nb2-xYxO15.5-x)  and gadolinium-doped ceria (Ce1-xGdxO2-δ) (CGO)  drew a great interest as a promising candidate for the new generation of electromechanical micro devices. Particularly, CGO is not only an environmental friendly material but it is also highly compatible with silicon technology since cerium does not diffuse into silicon. Moreover, CGO shows better performances as compared to the best performing commercial lead based ceramics, e.g. the electrostrictive coefficient of CGO is in a range between 20-110 m4/C2 [1,2,3] vs 0,02 m4/C2 of Pb(Mg1/3Nb2/3)O3 (PMN) .
In this work, we demonstrate the great potential and some limitations of CGO by growing thin films directly on TiN/Si substrates, where a TiN deposition of 80 nm serves as bottom electrode for the CGO electrostrictor. The direct deposition yields impressive electrostrictive performances (50 m4/C2) and long term stability for GCO films of ca. 1 µm in thickness.
1. N. Yavo et al., Adv. Funct. Mater. 2016, 26, 1138–1142.
2. R. Korobko et al., Adv. Mater. 2012, 24, 5857–5861.
3. R. Korobko et al., Sensors and Actuators A 2013, 201, 73– 78.
4. J. Kuwata et al 1980, Jpn. J. Appl. Phys. 19 2099.
9:00 AM - EP01.01.03
First-Principles Studies of the Effects of Oxygen Vacancies on the HfO2-Based Ferroelectric Tunnel Junction
Jinho Byun1,Taewon Min1,Jaekwang Lee1
Pusan National University1Show Abstract
Owing to the recent advances in the oxide growth technology, ferroelectricity has been stabilized even in a few nm-thick films, which makes it possible to realize the oxides-based ferroelectric tunneling junction (FTJ) combining the quantum-mechanical tunneling phenomena and switchable spontaneous polarization into novel device functionality. Among various ferroelectric oxides, HfO2 is the most promising material for FTJ devices since it has the great advantage of complementary metal-oxide-semiconductor (CMOS) process compatibility. Despite this considerable attention, the influence of oxygen vacancies on the tunneling current has not been clearly understood yet. Here, using first-principles density functional theory calculations, we explored the role of interfacial oxygen vacancy on the tunneling current in the TiN/HfO2/metal devices at the atomic scale. We find that the tunneling current in defective HfO2 is enhanced by over three orders of magnitude compared to plain HfO2 thin film. Our results show that the modulation of electronic properties via interfacial oxygen vacancy has a significant impact on HfO2-based FTJ device performance.
This research was supported by the MOTIE (Ministry of Trade, Industry & Energy (#10080643) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.
This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2018R1A2B6004394)
9:15 AM - EP01.01.04
Dynamical Mean Field Theory Investigation of Piezoresistive Thin-Film Rare Earth Compounds Contacted to Metal Electrodes
Ivan Rungger1,Evgeny Plekhanov2,Debalina Banerjee2,Andrea Droghetti3,Dennis Newns4,Cedric Weber2,Glenn Martyna4
National Physical Laboratory1,Kings College London2,University of the Basque Country3,IBM Thomas J Watson Research Center4Show Abstract
The emerging piezoelectric transistor technology is based on heterostructures combining piezoelectric materials and piezoresistive thin films acting as ON/OFF switches and memories. Rare earth piezoresistive compounds such as SmS, SmSe and SmTe exhibit a reversible metal-insulator phase transition driven by either light, voltage or pressure, which can be applied by the piezoelectric. For device applications the contact with the metal electrodes critically affects switching behaviour in nanoscale piezoresistive materials, which has not been studied so far. Here we present first principles calculations to model these phase transitions both in the bulk and in nanoscale thin films used in transistor applications, and predict how switching can be induced by mechanical and electrical means in nanoscale devices. Importantly, density functional theory with semi-local exchange correlation functionals cannot correctly treat the strongly correlated electrons in the f-orbitals of Sm. We overcome this limitation by using our recent implementation of the dynamical mean field theory, and show good agreement with experimental data for the electrical and mechanical switching properties.
9:30 AM - EP01.01.05
Three-Dimensional Interconnected Piezoelectric Ceramic Foam Based Composites as Flexible, High-Performance Piezo/Pyroelectric Materials for Concurrent Mechanical and Thermal Energy Harvesting
Sulin Zhang1,Qing Wang1,Guangzu Zhang2,Peng Zhao1
The Pennsylvania State University1,Huazhong University of Science and Technology2Show Abstract
Flexible Piezoelectric (PZT)-polymer composites with superior piezoelectric effect have received much attention for a wide range of applications, particularly in energy harvesting. However, classical PZT-polymer composites with low-dimensional ceramic fillers suffer from low piezoelectricity, owing to the poor load-transfer efficiency from the polymer matrix to the active ceramic fillers. The fundamental mechanics is that the load-transfer efficiency for these composites scales with the ratio of the stiffness of the polymer matrix to that of the ceramic fillers, a value typically on the order of 10-5. Here we introduce a cost-effectively producible ceramic-polymer composite consisting of three-dimensional (3-D) interconnected piezoelectric microfoams in polydimethylsiloxane (PDMS) matrix. The resulting composite breaks the conventional scaling law of the load-transfer efficiency, and enables continuous strain and heat transfer, giving rise to exceptionally improved piezo and pyroelectric effects as compared to those based on low-dimensional ceramic fillers. The 3-D composite is also mechanically flexible, robust, and durable, able to sustain thousands of thermomechanical cycles without noticeable degradation, while yielding stable piezo/pyroelectrical signals. We further demonstrate that combining the piezo and pyroelectric effects of the 3-D composites enable concurrent mechanical and thermal energy harvesting. These attributes, along with the scalable production, make the 3-D composite attractive to a wide range of applications in soft robotics, wearable electronics, and artificial muscles and skins, etc.
9:45 AM - EP01.01.06
Potential Ferroelectric Binary Oxides Beyond Hafnia
Rohit Batra1,2,Huan Tran1,Brienne Johnson3,George Rossetti1,Jacob Jones3,Rampi Ramprasad2
University of Connecticut1,Georgia Institute of Technology2,North Carolina State University3Show Abstract
In the past couple of years, there have been extensive empirical and theoretical efforts to elucidate the surprising phenomenon of ferroelectricity recently discovered in hafnia (HfO2) thin films (<30 nm) [1-5]. While the origin of this unexpected ferroelectric (FE) behavior is associated with the formation of the metastable orthorhombic Pca21 phase owing to unusual thermodynamic or processing conditions [2,4], the most critical lesson to be learned from the example of hafnia is that even binary oxides can be FE if low-lying metastable (or stable) polar phases are present. Thus, in this contribution, we extend the findings from the case of hafnia to discover new FE binary oxides, as opposed to the traditionally explored class of perovskite-structured oxides, using computations. We employed a combination of structural search methods, first principles computations and group-theoretical considerations to find at least six simple oxides as potential ferroelectric candidates. Among them, a previously unexplored candidate, CaO2, was successfully synthesized in the polar Pna21 phase, in accordance with our theoretical predictions. Furthermore, the high occurrence (~40 %) of low-energy polar phases among the oxides considered in this work strongly advocates the possibility of discovery or engineering ferroelectricity in many more simple oxides beyond hafnia.
 M. H. Park et al., Advanced Materials 27, 1811 (2015)
 T. D. Huan et al., Physical Review B 90, 064111 (2014)
 R. Batra et al., Applied Physics Letters 108, 172902 (2016)
 R. Batra et al., Journal of Physical Chemistry C, 121, 4139 (2017)
 R. Batra et al., Chemistry of Materials, 29, 9102 (2017)
10:30 AM - *EP01.01.07
Accelerated Materials Design of Novel Polar Materials
University of California, Berkeley1,Lawrence Berkeley National Laboratory2Show Abstract
Novel polar multifunctional materials are needed for next generation sensors, energy converters, and high-performance computing. Leveraging the growing body of computational resources, from the prediction of novel materials with target properties to their characterization and synthesis, it is possible to accelerate the pace of discovery. In this talk we will utilize the data and analysis resources of the Materials Project (www.materialsproject.org) which is harnessing the power of supercomputing coupled with a sophisticated software infrastructure that carries out, organizes and disseminates 100-1000s of calculations per week – enabling effective screening, prediction and tandem exploration together with experimental teams. We will survey the available methods and data, exemplified through a recent realization of a novel metastable piezoelectric material, from prediction to successful synthesis and testing. Finally, we will comment on future directions in this exciting field, in particular the need for predictive synthesis.
11:00 AM - EP01.01.08
Kinetic Monte Carlo Simulations of Organic Ferroelectrics
Tim Cornelissen1,Indre Urbanaviciute1,Martijn Kemerink1
Linköping University1Show Abstract
Organic ferroelectric materials are emerging as a class of materials that may find application in a broad range of fields; for instance, they might solve the ‘missing memory’ problem in printed electronics. However, a full understanding of their switching kinetics on all length and time scales is still lacking. A variety of computational models have been employed to tackle this problem and to study different aspects of organic ferroelectrics. However, these are usually restricted to idealized morphologies or short time scales.
In contrast, we have developed an electrostatic model that, when used in kinetic Monte Carlo simulations, can reproduce the ferroelectric properties and kinetics on experimental time scales and for realistic 3D morphologies. We apply this model on the prototype small molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA).
We simulate hysteresis loops and depolarization curves and find a good agreement with experiments. Like the experiments, the dependence on frequency and temperature of our model results can be interpreted in the framework of thermally activated nucleation limited switching. Specifically, we find two different modes of switching, each associated with their own kinetics and energetics. One mode corresponds to a full rotation of the dipoles, while the other mode only flips the component along the polarization axis. The existence of these two modes is confirmed by molecular dynamics simulations. Both simulation methods find that the second mode has a lower coercive field and thus is the one occurring in polarization switching experiments.
We also investigate the effect of structural disorder on the ferroelectric properties. When the disorder in the system is increased, the retention time decreases dramatically, while the coercive field remains mostly unchanged. For device applications a high retention time and moderate coercive field is required. Aside from providing a detailed insight into polarization switching processes on experimental length and time scales, our model thus is also able to provide guidance in improving the performance of ferroelectric devices.
11:15 AM - *EP01.01.09
Lone Pair Engineering for Multi-Functional Polar Semiconductors
Imperial College London1Show Abstract
Beyond the group oxidation state (N), post-transition metals can adopt a lower (N-2) oxidation state, which is associated with a metal s2 lone electron pair. Solid-state lone pairs, as found in the compounds formed of ions such as In(I), Sn(II), Sb(III), and Te(IV), are linked to the formation of asymmetric local coordination environments and non-centrosymmetric crystal structures . Lone pairs underpin the physical properties of many piezoelectric, pyroelectric and ferroelectric materials.
I will discuss progress in the understanding of structure and reactivity of lone pair containing compounds, including the driving force for structural distortions and how they can be controlled to enable novel functionality. Applications areas to be discussed will include thermoelectric devices that incorporate high levels of phonon anharmonicity (e.g. SnSe ), photovoltaic cells based on photoferroic semiconductors (e.g. Pb and Sn halide perovskites ), as well as new classes of ternary V-VI-VII semiconductors based on Bi and Sb chalcohalides [4,5] that encompass photocatalysts, Rashba semiconductors, and topological insulators.
1. Stereochemistry of post-transition metal oxides: revision of the classical lone pair model. Chem. Soc. Rev. 40, 4455 (2011)
2. Anharmonicity in the high-temperature Cmcm phase of SnSe: soft modes and three-phonon interactions. Phys. Rev. Lett. 117, 075502 (2016)
3. Spontaneous octahedral tilting in the cubic inorganic caesium halide perovskites CsSnX3 and CsPbX3 (X = F, Cl, Br, I). J. Phys. Chem. Lett. 8, 4720 (2017)
4. Quasi-particle electronic band structure and alignment of the V-VI-VII semiconductors SbSI, SbSBr, and SbSeI for solar cells. Appl. Phys. Lett. 108, 112103 (2016)
5. Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity. Chem. Sci. 7, 4832 (2016)
EP01.02: Experimental Realization of Predicted Materials Including Synthesizability
Monday PM, November 26, 2018
Hynes, Level 1, Room 103
1:30 PM - EP01.02.01
Enhancement of Ferroelectricity in Perovskite Oxides by Sulfurization
Muhammad Sheeraz1,Ill Won Kim1,Chang Won Ahn1,Tae Heon Kim1
University of Ulsan1Show Abstract
Sulfurization, an anion substitution to oxide materials is considered a progressive route for designing new multi-functional materials artificially and realization of unusual physical properties which do not exist in nature. Sulfur among the other anions has got major attraction due to its isoelectronic nature and large ionic radius compared to oxygen. However, the sulfurization to polycrystalline perovskite other than bulk single crystal perovskite oxides is rarely reported due to the synthetic limitation. Despite this an alternative feasible synthetic route is developed to better understand the structural and physical properties sulfur is doped quantitatively at atomic level. Sulfur doped ferroelectric perovskite [Pb(Zr,Ti)O3] is grown epitaxially by employing the thiourea (CH4N2S) solution at various mole ratio using sol gel method. Microscopic analyses of electronic and crystal structures reveal that oxygen ions are substituted by sulfur atoms with tetragonal distortion. In response to this structural phase transition, macroscopic ferroelectric polarization is enhanced, although a band gap is reduced. More details of theoretical calculations and experimental results will be presented in conjunction with a discussion about the potential usage of our synthetic technique in aspect of novel material design.
1:45 PM - EP01.02.02
Anisotropy Control for Enhanced Performance Magnetoelectric Nanocomposites
Jennifer Andrew1,Matthew Bauer1,Austin Kubart1
Univ of Florida1Show Abstract
Nanostructured composite materials have the potential to overcome challenges in many areas of materials research, which cannot be addressed by more conventional single-phase materials. The unique properties of these composite materials often arise due to unique phenomena that occur at the interface between the phases being coupled. An additional control is the anisotropy of the individual phases and the resultant composite, which can be used to control the magnitude and direction of composite properties. For example, ferroelectric and ferromagnetic materials can be combined to form composites with enhanced multiferroic or exchange coupling properties. Here, I will present on these composite materials prepared using the electrospinning technique, generating materials with controllable anisotropy and resultant properties. Specifically, Janus type nanofibers, where two phases are coupled longitudinally, are used to create an anisotropic building block that allow access to both surface and bulk properties of each phase. This novel architecture is linked to an anisotropic interface between the coupled phases, and a model is developed relating fiber composition to interfacial area and resulting functional properties. Applications of these composites as zero-power magnetic field sensors will also be presented.
2:00 PM - EP01.02.03
WITHDRAWAL: 11/26/18 (EP01.02.03) Ferroelectric HfO2 Growth from HfCl4 –ZrO4 Solid Solution for Stress/Strain Induced Grain Formation and Defect Control at Oxide-Semiconductor Interface
Mahmut Sami Kavrik1,Evgueni Chagarov1,Michael Katz2,Norman Stanford2,Albert Davydov2,Min-Hung Lee3,Andrew Kummel1
University of California, San Diego1,National Institute of Standards and Technology2,National Taiwan University3Show Abstract
Recent findings in ferroelectric HfO2 and discovery of negative capacitance may provide unexpected improvements in CMOS due to scalability of HfO2 and ease of integration. Ferroelectricity can be induced into thin film HfO2 via doping (Al, Y, Gd, Si), but the composition window for each dopant is narrow, sometime only +/-2%. Conversely, Zr doped HfO2 has a broad stoichiometry window (+/- ~15%) in which ferroelectricity can be stabilized. The mechanism for stability of the phases of HfO2, ZrO2, and HZO (HfxZr1-xO2) were investigated with DFT-MD to determine the origin of larger process window of ferroelectric phase for the binary HfZrO oxides. It was shown that for all three oxides although the bulk states of the monoclinic phase (“m”) are more stable than either the orthorhombic ferroelectric (“f”) phase or tetragonal (“t”) phases and even the surface free energy does not favor f-phase formation. Instead, the higher surface area per unit cell induced by the stress/strain due to post annealing of the amorphous oxide with a crystalline capping layer such as TiN can favor the orthorhombic f-phase since it has a larger area per unit cell than the monoclinic phase; the only requirement is that epitaxial crystallization occurs over at least 5 unit cells of the capping layer. Consistent with this hypothesis, high resolution TEM images of TiN/HZO/Si gate stacks shows regions of epitaxial alignment between HZO and TiN. To improve the consistency of HZO ferroelectric gates, a new method of deposition was developed. The conventional method of Zr doping into HfO2 employs consecutive ALD cycles of ZrO2 and HfO2 in a nanolaminate structure from separate precursors. This process may limit intermixing of the Hf and Zr when the oxide is scaled to 1.5 nm as required for commercial CMOS devices. Furthermore, this process can limit the defect control at the oxide semiconductor interface due to necessity of the precise control of the oxidant between dosing of each precursors to maintain precise stoichiometry. Control of oxidant for growth of HZO on SiGe is particularly challenging since oxidant dosing must be differentially controlled at the interface to avoid GeOx formation; for example O3 intermittent dosing during growth of HZO on SiGe has been show to lower the interface defect density. In this work, an alternative method was investigated in which HfCl4 and ZrCl4 solid mixture was employed; this relies upon the vapor phase composition being a function of the solid-state composition. Ferroelectric Hf-ZrO2 was grown in Ni/HZO/TiN/Si structure from single solid mixture precursor abd >25uC/cm2 polarization was observed in 6 nm HfZrO2 grown from single solid mixture. In second step, ferroelectric Hf-ZrO2 on Si0.3Ge0.7 was grown from single solid mixture precursor and MOSCAPs were fabricated. Electrical analysis revealed low defect interface formation with Dit of <3x1012 and low leakage current density of <1x10-10 (A/cm2) similar to the control HfO2 devices on SiGe.
2:15 PM - EP01.02.04
ZnO-Activated Low Temperature Reactive Sintering of High Coercive Field Lead Zinc Niobate Based Piezoelectrics
Michael Brova1,Beecher Watson1,Elizabeth Kupp1,Mark Fanton1,Richard Meyer1,Gary Messing1
The Pennsylvania State University1Show Abstract
A major limitation of many high performance relaxor-based ferroelectric materials is their Curie temperatures. The recently developed Pb(In1/2Nb1/2)O3-Pb(Zn1/3Nb2/3)O3-PbTiO3 perovskite solid solution has a high rhombohedral to tetragonal phase transition temperature (Trt) and Curie temperature (Tc), while also possessing a large piezoelectric charge coefficient (d33), mechanical quality factor (Qm), and coercive field (Ec). In order to lower the sintering temperature and minimize number of heat treatments necessary to fabricate PIN-PZN-PT ceramics, we investigated reactive sintering of ZnO-doped PIN-PZN-PT. Reactive sintering reduced the required processing temperature from 1150°C to 800°C when compared to traditional sintering. ZnO-doping stabilized the perovskite phase, reduced the sintering temperature, and significantly increased the reaction and densification. This effect is attributed to a modification in the defect chemistry of PIN-PZN-PT perovskite and an intermediate pyrochlore phase. Electromechanical properties of reactively sintered ZnO-doped PIN-PZN-PT ceramics are compared with those synthesized by conventional sintering.
3:00 PM - EP01.02.05
Low Temperature Reactive Sintering and Reactive Templated Grain Growth of CuO-Doped Lead-Based Piezoelectric Ceramics
Beecher Watson1,Michael Brova1,Scott Misture2,Mark Fanton1,Richard Meyer1,Gary Messing1
The Pennsylvania State University1,Alfred University2Show Abstract
Ternary lead-based Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) ferroelectric ceramics are leading candidates for next-generation textured piezoelectrics. Fabrication of those bulk textured ceramics requires high sintering temperatures of ~1200°C-1250°C to initiate epitaxial growth and long hold times to achieve full texture development. Holding at high sintering temperatures presents a significant challenge because of the volatility of certain constituents (e.g. PbO) and the limitation of developing multilayered actuators, requiring the use of platinum electrodes. In this work, we explore new doping strategies (such as CuO) to reduce the sintering temperature and through reactive sintering to initiate epitaxy at lower temperatures. The effects of CuO doping on the kinetics of perovskite phase formation and reactive sintering were studied using in situ x-ray diffraction as well as diffraction analysis on samples heated under isothermal conditions. Reactive sintering conditions of CuO-doped PIN-PMN-PT ceramics were explored by isothermally treating ceramic green bodies at temperatures below 900°C, with a relative density of 95-97% achieved at remarkably low temperatures of 790°C for 6.7 h. Using a reactive sintering approach, we adapted a reactive templated grain growth (RTGG) system using BaTiO3 microcrystal platelets to seed the phase transformation of the PIN-PMN-PT perovskite at much lower temperatures than previously demonstrated in the TGG process (~1200-1250°C).
3:15 PM - EP01.02.06
Influence of Anneal Parameters on Strained TiN Layers in Ferroelectric HfO2 Capacitors
Teresa Buttner1,Patrick Polakowski1,Konrad Seidel1,Joachim Metzger2,Robert Binder2
Fraunhofer Institute for Photonic Microsystems1,Globalfoundries2Show Abstract
The ferroelectric (FE) behavior of HfO2 strongly depends on the crystalline structure and is observed when the high symmetrical non-centrosymmetric orthorhombic phase is dominant. Therefore, it is necessary that controlled crystallization positively influences the crystal phases of HfO2 and reducing the stability of unfavored structures like tetragonal or monoclinic crystal phases.1 Numerous studies investigated various process parameters and proved that various dopants, film thickness or annealing conditions have an impact on FE properties and ferroelectric phase stability.2-4 Early work on HfO2 ceramics identified mechanical stress as further parameter to induce orthorhombic phase in hafnium oxide.5 In further studies on undoped HfO23 and silicon doped6 thin films indicated that the crystalline phase is mechanically influenced by capping layers.
In this work we investigate differently strained TiN electrodes and their influence on ferroelectric films in metal-ferroelectric-metal (MFM) stacks to identify ideal stress conditions for enhanced ferroelectricity in HfO2 thin films. First electrical results confirmed FE behavior for all MFM samples with differently strained TiN Top Electrodes (TE) ranging from -4 GPa to 0 GPa, but showed marginal difference in ferroelectric performance. These results are in contradiction to fundamental research of ceramic hafnium oxide. Thus, this leads to the assumption that the different applied stress levels were equalized during subsequent processing. To examine the influence of TiN film properties in respect to process parameters applied in the MFM flow, the strained TiN layers were investigated individually on blanket wafers by using the same TiN film conditions. The samples were annealed with different temperatures and gas ambient conditions (Ar, N2 and NH3) and with/without a-Si and poly-Si encapsulation. Finally, we did a full material analysis, characterized the wafers regarding change in stress, composition, sheet resistivity, structure and thickness for pre and post anneal treatment. The results confirmed the first assumption, that due to a subsequent thermal treatment the films stress has been equalized. The tensile stress was relaxed or changed into compressive stress after annealing, which resulted in equal stress data for all used TiN conditions.
Possible solutions to overcome these findings and to maintain stress levels after thermal processing will be discussed. Further MFM experiments were conducted by lowering anneal temperatures, removing of TiN TE and subsequent TE deposition or deposition of stressed TiN on annealed relaxed TiN films.
 Boscke et al, Applied Physics Letters 99, 102903(2011)
 CK. Lee et al., Phys. Rev. B 78, 12102(2008)
 P. Polakowski and J. Mueller Applied Physics Letters 106, 232905(2015)
 Ho et al. Applied Physics Letters 93, 1477(2003)
 Ohtaka et al. J. Am. Ceramm. Soc., 78  233-23(1995)
 Hoffmann et al. J. Applied Physics Letters 118, 072006(2015)
3:45 PM - *EP01.02.08
Applying Chemistry to Make Today’s Best Tunable Millimeter Wave Dielectric Even Better
Darrell Schlom1,Natalie Dawley1,Eric Marksz2,Aaron Hagerstrom2,Megan Holtz1,Gerhard Olsen1,J. Zhang1,Christian Long2,James Booth2,Craig Fennie1,David Muller1,3,Nathan Orloff2
Cornell University1,National Institute of Standards and Technology2,Kavli Institute at Cornell for Nanoscale Science3Show Abstract
Tunable dielectrics are key constituents of emerging high-frequency devices in telecommunications—including tunable filters, phase shifters, and baluns—and for miniaturizing frequency-agile microwave and millimeter-wave components. Today’s tunable dielectric with the highest figure of merit at room temperature is strained films of (SrTiO3)6SrO. The low loss at frequencies up to 125 GHz comes from the defect mitigating nature of the (SrTiO3)nSrO Ruddlesden-Popper structure; the tunability arises from imposing strain to induce a ferroelectric instability. Unfortunately the necessity for strain limits the film thickness to around 50 nm, which reduces the device tuning that can be achieved. In this talk we describe a chemical alternative to strain to induce a ferroelectric instability—the introduction of barium into this Ruddlesden-Popper titanate. No barium-containing Ruddlesden-Popper titanates are known, but this atomically engineered superlattice material can be made thicker and we demonstrate a 300% improvement in the figure of merit of this this new, metastable (SrTiO3)n−m(BaTiO3)mSrO tunable dielectric over its predecessor, (SrTiO3)6SrO.
4:15 PM - EP01.02.09
Lift-off of Ultrathin Single-Crystalline Layers for Flexible Ferroelectric Tunneling Memristors
Zhengdong Luo1,Dae-Sung Park2,Marin Alexe1
University of Warwick1,Martin-Luther-University Halle-Wittenberg2Show Abstract
Recently, there has been an upsurge in pursuing the flexible devices for next generation of smart electronics. Among a large number of reported devices, integration of single-crystal functional oxides films onto the flexible substrates is rare because of the challenging fabrication process. Functional oxides, showing a rich variety of complex emergent properties including memristive effects, photovoltaics, multiferroic effects and so on, have become promising high-tech functional materials beyond their traditional role as dielectrics. Therefore, integration of functional oxides thin films and flexible substrates would add a wide range of exciting applications to the flexible electronics library. Here, we demonstrate a successful fabrication of single-crystal form ferroelectric oxides films on PET flexible substrates. We will discuss the lift-off of the ferroelectric films from the growth substrate by etching a water-soluble sacrifice layer Sr3Al2O6 layer, the transfer of those 2D single-crystal membranes onto the PET substrates and the functional electronic properties of the resulting ferroelectric thin film/PET flexible memristors. Moreover, we will show the transfer of other oxides films using the same lift-off method and discuss some potential interesting applications like flexible bulk photovoltaic devices and so on.
4:30 PM - EP01.02.10
A Rhombohedral Ferroelectric Phase in Epitaxially-Strained Hf0.5Zr0.5O2 Thin Films
Yingfen Wei1,Pavan Nukala1,2,Mart Salverda1,Sylvia Matzen2,Hongjian Zhao3,Jamo Momand1,Arnoud Everhardt1,Graeme Blake1,Philippe Lecoeur2,Bart Kooi1,Jorge Íñiguez3,Brahim Dkhil2,Beatriz Noheda1
University of Groningen1,Université Paris-Saclay2,Luxembourg Institute of Science and Technology3Show Abstract
After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 substrates. The films, which are under epitaxial compressive strain and are (111)-oriented, display large FE polarization values up to 34 μC/cm2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.
EP01.03: Poster Session I: Growth and Characterization of Piezoelectric, Pyroelectric and Ferroelectric Materials
Monday PM, November 26, 2018
Hynes, Level 1, Hall B
8:00 PM - EP01.03.02
Effects of Post-Draw Processing on the Structure and Functional Properties of Electrospun PVDF-HFP Nanofibers
Adriano Conte1,Khosro Shirvani1,Wei Xue1,Xiao Hu1,Vince Beachley1
Rowan University1Show Abstract
The current surge in wearable electronics has initiated a need for alternative energy sources. Energy harvesters that employ piezoelectric materials are capable of harnessing the mechanical energy from muscular contractions to power portable devices. The present study examined the properties of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) nanofibers fabricated from conventional electrospinning, and an automated track collector system that contains a post-drawing component. The polymer solution was originally processed by means of the traditional technique, flat-plate electrospinning, which produced a fiber arrangement with random orientations. When carrying out mechano-electrical testing and analysis these fibers yielded negligible voltage. The solution was subsequently processed employing a post-drawing electrospinning procedure, exclusive to our research laboratory, that permitted fiber alignment and individual nanofiber post-drawing immediately upon collection, prior to total solvent evaporation. Fibers that endured post-drawing displayed an increase in crystal alignment in the direction of the fiber axis (verified by polarized FTIR), and resulted in higher voltages than undrawn fibers and fibers from the traditional electrospinning method. It was examined that fibers produced by means of the post-drawing technique, with varying draw ratios (DR=Final length/Initial length) including DR-2 and DR-3, exhibited improved piezoelectric characteristics. Mechanical properties of the nanofibers were also enhanced as a result of post-drawing . This investigation suggests that the post-drawing practice results in PVDF-HFP nanofibers that are more suitable for piezoelectric applications than conventionally electrospun nanofibers.
8:00 PM - EP01.03.03
Epitaxial AlN Thin Film for High Performance Surface Acoustic Wave Devices