Symposium EP01 : New Materials and Applications of Piezoelectric, Pyroelectric and Ferroelectric Materials

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.

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 (Bi₇Nb_2-xY_xO_15.5-x) [1] and gadolinium-doped ceria (Ce_1-xGd_xO_2-δ) (CGO) [2] 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(Mg_1/3Nb_2/3)O₃ (PMN) [4].
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.

References:
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.

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)

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.

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.

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.

References:
[1] M. H. Park et al., Advanced Materials 27, 1811 (2015)
[2] T. D. Huan et al., Physical Review B 90, 064111 (2014)
[3] R. Batra et al., Applied Physics Letters 108, 172902 (2016)
[4] R. Batra et al., Journal of Physical Chemistry C, 121, 4139 (2017)
[5] R. Batra et al., Chemistry of Materials, 29, 9102 (2017)

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.

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.

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 [1]. 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 [2]), photovoltaic cells based on photoferroic semiconductors (e.g. Pb and Sn halide perovskites [3]), 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)

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.

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.

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 (Hf_xZr_1-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 Si_0.3Ge_0.7 was grown from single solid mixture precursor and MOSCAPs were fabricated. Electrical analysis revealed low defect interface formation with D_it of <3x10¹² and low leakage current density of <1x10^-10 (A/cm²) similar to the control HfO₂ devices on SiGe_.

A major limitation of many high performance relaxor-based ferroelectric materials is their Curie temperatures. The recently developed Pb(In_1/2Nb_1/2)O₃-Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃ perovskite solid solution has a high rhombohedral to tetragonal phase transition temperature (T_rt) and Curie temperature (T_c), while also possessing a large piezoelectric charge coefficient (d₃₃), mechanical quality factor (Q_m), and coercive field (E_c). 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.

Ternary lead-based Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)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).

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.

References:
[1] Boscke et al, Applied Physics Letters 99, 102903(2011)
[2] CK. Lee et al., Phys. Rev. B 78, 12102(2008)
[3] P. Polakowski and J. Mueller Applied Physics Letters 106, 232905(2015)
[4] Ho et al. Applied Physics Letters 93, 1477(2003)
[5] Ohtaka et al. J. Am. Ceramm. Soc., 78 [1] 233-23(1995)
[6] Hoffmann et al. J. Applied Physics Letters 118, 072006(2015)

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.

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.

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 Hf_0.5Zr_0.5O₂ (HZO) thin films on (001)-oriented La_0.7Sr_0.3MnO₃/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.

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.

Aluminum nitride is a wide bandgap piezoelectric semiconductor that has superior hardness and thermal conductivity. It also has the electromechanical coupling factor and dielectric constant that fit perfectly for the band width needed in bulk acoustic wave (BAW) filters which serve at tremendous quantity in smart phones. Over the years, polycrystalline thin films of AlN have been used almost exclusively as the piezoelectric substrate in commercial BAW filter products. It is also a potentially outstanding candidate for commercial surface acoustic wave (SAW) devices. However, a major disadvantage for polycrystalline AlN used in SAW devices is the existence of grain boundaries which increase insertion loss and pass band ripple. The main stream SAW filters choose LiNb(Ni)O₃ single crystalline wafer as the piezo-substrate at present. To realize the full potential of AlN in SAW devices, it is necessary to refrain the negative influences of the grain boundaries, which make high quality single crystalline or epitaxial AlN highly desirable. This presentation will talk about the results of a study on the SAW devices fabricated on epitaxial AlN thin films. The films were grown on (0001) sapphire substrates by molecular beam epitaxy and the devices were fabricated by lift-off processes which use photolithography and sputtering to produce patterned electrodes. The epitaxial AlN thin films have good material quality showing by the relatively small full width of half maximum value of XRD (0002) rocking curve of around 100 arcsec and the small roughness of 1.8 nm. The SAW devices with center frequencies of 355 MHz and 714 MHz both exhibit much suppressed pass band ripples and improved out of band rejections comparing to those on polycrystalline thin films. It is therefore verified that the epitaxial form of AlN is better suited for high quality SAW devices.

Multiferroic thin films of GdFeO₃ are of significant importance due to their G-type antiferromagnetic behaviour and thus potential candidates for magnetic storage devices. Fabrication of single-phase GdFeO₃ films is challenging due to demixing into homometallic oxides and formation of thermodynamically preferred Gd₃Fe₅O₁₂. Herein we report the first selective synthesis of epitaxial GdFeO₃ perovskite films through atomic layer deposition of a bimetallic precursor [GdFe(O^tBu)₆(C₅H₅N)₂] on SrTiO_3. Based on the preformed Gd-Fe bonds in the molecule, phase pure GdFeO₃ films were accessible by atomic layer deposition experiments. The suppression of phase separation was validated by X-ray diffraction and X-ray photoelectron spectroscopy. Furthermore, magnetic properties of the material were determined by temperature dependent magnetization measurements and demonstrated comparable results as reported for thin films. Based on these results, the presented bimetallic precursor is suitable for the fabrication of high performance magnetic data storage devices.

Lead zirconate titanate Pb(Zr,Ti)O3 (PZT) is a ferroelectric material with excellent dielectric and piezoelectric properties. PZT films have been prepared using various fabrication techniques such as sol-gel process, metal organic decomposition, and pulsed laser deposition. Recently, hydrothermal method has attracted increased attention because it could produce homogeneous and uniform nano-sized particles with high crystallinity without high temperature crystallization process. In this study, PZT particles were synthesized by a surfactant-assisted hydrothermal method and the effect of the surfactant on the morphologies of the PZT particles was investigated.
PZT particles were synthesized from lead acetate trihydrate and water-soluble zirconium and titanium complex aqueous solution by hydrothermal method. A high-alkaline medium and surfactant were added into the PZT precursor solution to dissolve the precursor source in the aqueous solution and to control the crystal growth, respectively. After hydrothermal reaction with stirring, PZT particles were separated from liquid phase and followed by rinsing and drying several times.
When the surfactant was not added into the reaction solution, large-sized PZT particles with rough surfaces were synthesized. On the other hands, nano-sized PZT particles with facet were synthesized by adding the surfactant and it was confirmed that the added surfactant had the effect of inhibiting the growth of the certain faces of particles. The simple spot pattern in the electron diffraction pattern revealed that this nano-sized PZT particle synthesized by the surfactant-assisted hydrothermal method at 230°C had high crystallinity and it was a single crystal. Unlike conventional fabrication techniques with high temperature crystallization process above 600°C, PZT nanocrystals with high crystallinity were obtained by the surfactant-assisted hydrothermal synthesis.

HfO₂-based materials are fluorite-type oxide and were well known as multifunctional materials. In recent years, ferroelectricity has discovered just in HfO₂-based thin films with a metastable orthorhombic phase (O-phase), and these materials have attracted much attention as novel ferroelectrics. Especially, ZrO₂-HfO₂ solid solution thin films with O-phase showed the excellent ferroelectric properties and a wide process window.
CeO₂-HfO₂ solid solutions had been applied to various devices as high-k dielectrics. Therefore, from the viewpoint of multi-functionality, it is important to fabricate CeO₂-HfO₂ solid solution thin films with O-phase. In this study, we report on the successful growth of (001)-oriented epitaxial ferroelectric HfO₂-CeO₂ solid solution thin films.
20 nm-thick CeO₂-HfO₂ films were deposited on (100)YSZ substrates by ion-beam sputtering method. The deposition temperature and atmosphere were maintained at room temperature and Ar, respectively. After that, the deposited films were annealed at 900 °C for 10 min in N₂ atmosphere. Film composition was controlled by change the concentration of CeO₂ in sputtering target. The crystal structure of deposited films was investigated by XRD measurement and S/TEM observation, and the electrical properties were investigated using impedance analyzer and ferroelectric tester.
From XRD measurements, it was found that the obtained films were solid-solution because the diffraction peak derived from films shifted to low 2q angle with increasing CeO₂ concentration. In addition, all films were epitaxially grown on (100)YSZ substrates. STEM observation revealed that CeO₂-HfO₂ films had multidomain structures composed of O-phase and a stable monoclinic phase. In fact, P-E hysteresis loops caused by ferroelectricity were clearly observed at optimum CeO₂ concentration, and the maximum remanent polarization was about 10 μC/cm².

This research was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant No. 17J03160, 16K18231, 16K14378, 18H01701. And a part of this work was also supported by Nippon Sheet Glass Foundation for Materials Science and Engineering.

The engineering of nanoscale ferroelastic structures has attracted significant attention in the last few years. These nanostructures are reconfigurable and non-volatile, making them attractive for applications that harness the changes in electronic properties that arise at the ferroelectric-ferroelastic domain walls or novel (nano-)electromechanical devices based on ferroelastic switching. Here, we study the interplay between epitaxial strain, film thickness, and electric field in the creation, modification, and design of distinct ferroelectric-ferroelastic domain and superdomain architectures in the archetype ferroelectric PbTiO₃.
PbTiO₃ thin films, with thicknesses between 20 and 75 nm, were grown on SrTiO₃, DyScO₃, TbScO₃, and GdScO₃, SmScO₃ and PrScO₃ substrates by reactive molecular-beam epitaxy, spanning strains from -1.36% compressive to +1.54% tensile. X-ray diffraction, scanning transmission electron microscopy and piezoresponse force microscopy (PFM) were used to probe the evolution of the ferroelectric domains in PbTiO₃ as functions of both epitaxial strain and film thickness. In addition, the conducting PFM tip was used to apply a dc bias voltage to assess the reconfigurability of the ferroelastic structures and study their stability over time.
Our results show that for large compressive strain pure c-domains PbTiO₃ thin films are obtained. On reducing the compressive strain, a gradual increase in the population of a-domains embedded in a matrix of c-domains takes place, giving rise to a/c domain architectures; the density of domain walls increases on reducing compressive strain. For tensile strains a competing scenario of a/c and a₁/a₂ superdomains (ferroelastic structures formed by individual domains) is found; the ratio between both populations can be tuned by varying strain and thickness, enabling the superdomain architecture to be engineered at will. Furthermore, superdomains behave as independent entities; their size dramatically increases with thickness (reducing the superdomain wall density), in a similar fashion as individual ferroic domains behave in ferromagnetic, ferroelectric, and ferroelastic materials.
Applying an out-of-plane dc biased voltage to the domain and superdomain patterns reveals that the ferroelastic structures in PbTiO₃ are electrically very malleable, especially when a/c and a₁/a₂ superdomains coexist. For example, a vertical electric field fully converts the as-grown superdomain architecture into a/c superdomains. Moreover, depending on the ferroelectric switching of the individual c-domains, ordered a/c superdomains can be achieved. The stability, however, of the electrically written a/c superdomain structures strongly depends on strain: under low tensile strain they are stable for days, whereas at moderate tensile strains they rapidly convert into a₁/a₂ superdomains—the same equilibrium state as the as-grown films.

Spatial light modulators (SLMs) are devices to control the amplitude, phase and polarization of light and are an important component of such as optical communication and optical computing systems. Magneto-optic SLM (MOSLM) using Faraday rotation can modulate light through the direction of magnetization with ultra-high speed and robustness. A voltage-driven MOSLM, which is composed of piezoelectric and magnetic materials, can be driven with relatively low power consumption. The structure in which columnar magnetic materials are embedded in the piezoelectric material is expected to increase the modulation of light easily by increasing the thickness. In order to apply this structure to MOSLM, simultaneous growth of the piezoelectric material such as PbZr_0.52Ti_0.48O₃ (PZT) and BaTiO₃ (BTO) and a magnetic rare earth iron garnet (Bi:RIG) is needed. In this study, we investigated the growth conditions for obtaining the aligned BTO film on nonmagnetic single crystalline Gd₃Ga₅O₁₂ (GGG) substrate and Bi:RIG/BTO composites.
At first, we checked several materials which can grow epitaxially on GGG substrate by pulsed laser deposition (PLD) method since it would be required to form columnar structure by simultaneous growth technique. As a result, we found that the (111) aligned CoFe₂O₄ (CFO) film could be grown on GGG (111) substrate. Furthermore, the BTO film preferentially oriented in the (111) plane could also be grown on CFO buffered GGG (111) substrate. Pole figure analysis of this sample revealed that the (111) oriented BTO film has two in-plane orientations in the plane. This means that this aligned film of BTO on CFO/GGG is not single crystalline feature but polycrystal with two specific crystal alignments. However this would not be a crucial issue to grow columnar structure by simultaneous growth; the relation between BTO and Bi:RIG crystal at the interface may be kept since the columnar size is usually several 10 to 100 nm order and would be smaller or comparable to the that of BTO grain. The detail about the composite films of BTO and Bi:RIG using this CFO buffered GGG substrate will be discussed. This work was supported in part by the Grants-in-Aid for Scientific Research (S) 26220902, (B) 16H04329 and Strategic international research network promotion program No. R2802

Low temperature chemical solution processing of nanocrystalline perovskite oxides can be attractive due to the ability to (i) enable precise control over stoichiometry and structure in the product and (ii) offer thin film integration options in device electronics for which high temperatures are not suitable. Ba(Ti,Fe)O₃ is a useful system for the exploration of multiferroic properties as a function of structure, based upon a model of intersubstitution of the B site cation. A series of iron-substituted barium titanate nanocrystals were synthesized using a hybrid sol-gel synthesis method, known as gel-collection, at 60°C. The as-prepared nanocrystals are fully crystalline, uniform in size (~8 nm) and dispersible in polar organic solvents. The synthesis method could effectively control Fe substitution over a full range of x = 0, 0.1, 0.2, 0.3, 0.5, 0.75 and 1.0, enabling a systematic study of the relative effect of Fe addition to the parent BaTiO₃ compound. In the case of x = 0.0-0.3, a model of Fe doping suffices with predictable trends in magnetic and dielectric behavior. In the case of x = 0.5 and up, Fe impacts the structure. Powder X-ray diffraction (XRD) initially indicated single phase nanocrystalline samples for x< 0.3. PDF analysis… size and morphology of the nanocrystals was analyzed by the transmission electron microscope (TEM) showing uniform shape and size (~8 nm) nanocrystals. Magnetic characterization (both magnetic hysteresis loops and zero field and field cooling measurements) was carried out on a magnetic properties measurement system (MPMS) and showed increased magnetization with increasing Fe ion concentration. Frequency dependent dielectric measurements were performed at room temperature on spin coated 0-3 nanocomposites of BFT and polyvinyl-pyrrolidone and show stable dielectric constants at 1 MHz of 27.0, 26.0, 24.6, 24.5, 23.6, and 22.2 for BFT samples with x = 0, 0.1, 0.2, 0.3, 0.5, and 0.75 concentrations respectively. The decrease in dielectric constant with increasing Fe concentration is due to the contribution of the electrons in the d orbital leading to a more leaky material.

Multiferroic devices, consisting of coupled ferromagnetic and ferroelectric phases, are of great importance in the drive towards creating faster, smaller and more energy efficient voltage control magnetic random access memory (MRAM) devices for information storage and communication technologies. Such devices require a strong magnetoelastic coupling between the ferromagnetic and the ferroelectric interface, this is obtained by using large magnetostriction materials as a ferromagnetic layer. Magnetostrictive Fe based amorphous alloys of Fe_70.2Co_7.8Si₁₂B₁₀ (FeCoSiB) and Fe₈₁Ga₁₉ (GdFe) amorphous alloys are of great interest for their ultrahigh saturation magnetization, low coercivity, and high magnetic permeability.
Wide varieties of the growth techniques are available for the fabrication of thin films, among the methods, pulsed laser deposition (PLD) is a thin film growth technique which has the advantage of stoichiometric transfer of the elements on to the substrate. By understanding the growth of amorphous thin film by PLD has the advantage of the in-situ growth of high quality epitaxial ferroelectric materials are grown at high temperature, and magnetostrictive materials can be are grown on top of the ferroelectric thin film.
In this present study, we have prepared the highly magnetostrictive FeCoSiB and GdFe thin films deposited on the Si substrate using PLD. The prepared films show the soft ferromagnetic property with coercivity of 25 Oe as given in the Fig. 1(a). Pulsed laser deposition induced uniaxial anisotropy in the GdFe thin shown in Fig. 1(b). The thickness dependent composition variation of the thin film was analysed using Auger electron spectroscopy. We also present the growth of this alloy on oxide substrates, magnetotransport characteristics and magnetic domain structure by Magnetic force microscopy (MFM).

(K, Na)NbO₃ has a relatively high piezoelectric property among lead-free piezoelectric materials with high environmental adaptability. Their films have been prepared by various methods. Hydrothermal method can prepare (K, Na)NbO₃ films at low temperature [1-3] and possible to control film composition that has been pointed out to be difficult for various preparation methods due to high deposition temperature and vapor pressure of K and Na elements. In this study, orientation-controlled (K,Na)NbO₃ films with 1-100 μm in thickness were prepared by hydrothermal method and their crystal structure, electrical properties, and piezoelectric properties were investigated.
{100}-oriented (K_0.8Na_0.2)NbO₃ thick films up to 17 mm in thickness in one batch was achieved at 240^oC on SrRuO₃-coated SrTiO₃ substrates and totally 100 mm-thick films were obtained by repeating this process. Well saturated hysteresis loops were obtained after the post annealing at 600 ^oC and the obtained remanent polarization and coercive field of 2 μm-thick films were 6 μC/cm² and 30 kV/cm, respectively. The effective piezoelectric coefficient measured using cantilever, e_31,f, was -9.2 C/m². This value is one of the largest valu for (K,Na)NbO₃ films deposited on single crystal substrates. This hydrothermal process also possible to direct prepare piezoelectric (K,Na)NbO₃ films at 120^oC on organic substrates.
This research was partially supported by Japan Science and Technology Agency (JST), Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP).
References
[1] Shiraishi et al., Mater. Res. Soc. Symp. Proc. 1494 (2013) DOI:10.1557/opl.2013.50.
[2] Shiraishi et al., J. Korean Phys. Soc., 62(7) (2013) 1055-1059.
[3] Shiraishi et al., Jpn. J. Appl. Phys., 53 (2014) 09PA10-1-4.

Multiferroic materials which exhibit both ferroelectric and ferromagnetic properties have attracted considerable attentions. GaFeO₃-type iron oxide is one of promising multiferroic materials due to the large spontaneous magnetization and polarization near room temperature. However, magnetic substitution is difficult due to instability of the substituted GaFeO₃. In this study, we successfully fabricated Ga_0.5Cr_0.5FeO₃ epitaxial thin films through epitaxial stabilization. These films simultaneously exhibit in-plane ferrimagnetism and out-of-plane ferroelectricity. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements of the Ga_0.5Cr_0.5FeO₃ film reveal that the valence states of the Fe and Cr ions are trivalent, and some Fe ions are located at the T_d Ga1 sites in the Ga_0.5Cr_0.5FeO₃ film. The Ga_0.5Cr_0.5FeO₃ film shows a unique temperature dependence of the magnetization behavior with a higher Curie temperature (240 K) as compared to the GaFeO₃ film. The effects of Cr substitution on the magnetic properties are strongly affected by the sites of the Fe³⁺ (3d⁵) and Cr³⁺ (3d³) ions. Furthermore, the films show ferroelectricity at room temperature. Interestingly, the change in the ferroelectric parameters via Cr substitution is very little, which disagrees with the previously proposed polarization switching mechanism. Our findings would be key to understand genuine polarization switching mechanism of the multiferroic GaFeO₃ system.
<References> T. Katayama et al., Chem. Mater. 30, 1436 (2018).

Polymer peizoelectric materials are wildly used in wearable smart devices as they are flexible lightweight, stretchable, environment-friendly and chemically stable, and poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)) is a representative piezoelectric polymer. To construct piezoelectric polymer fibers, electrospinning is a versatile technique. Comapred to randomly distributed electrospun fibers, aligned P(VDF-TrFE) fibers possess better electrical properties and larger response to mechanical stimuli. Here, we demonstrate a simple dynamical mechanical induced process which enables the formation of large-scale highly aligned electrospun P(VDF-TrFE) fibers obtained under low rotation speed. And the resultant fibers exhibit enhanced mechanical and piezoelectric properties and can be further used as wearable motion sensors.
We collect the initial P(VDF-TrFE) fiber using low speed rotating drum. The as-spun P(VDF-TrFE) fibers still contain residual solvent, thus they are easy to deform under applied external force. The electrospun P(VDF-TrFE) thin film is then peeled off the aluminum foil and mounted in the clamps of a linear travel stage driven by a controller for the following mechanical stretching process. As a result, the initial orientations can be globally unified, which leads to the realignment of a large amount of electrospun P(VDF-TrFE) into highly oriented fiber bundles.
The strain and stress curves are measured to investigate the mechanical properties. With the increasing of aligned fiber proportion, the electrospun P(VDF-TrFE) fibers can withstand larger external stress under the same strain, which means the shape change of better aligned electrospun P(VDF-TrFE) fibers is smaller than its random distributed counterpart under the same applied tension. This advantageous mechanical property makes highly aligned P(VDF-TrFE) fibers more suitable for wearable motion sensors. But electrospun P(VDF-TrFE) fibers with alignment proportion exceeding 90% show poorer mechanical endurance.
Then, the electrical property is characterized. The electrical respond first increases with the increase of aligned fiber proportion, which is agreed with the simulation study using COMSOL, but dropped when the fibers are highly paralleled (aligned fiber proportion > 80%). The highest output of 84.92 mV is achieved by P(VDF-TrFE) fibers with ~80% aligned proportion, about 266% of their randomly distributed counterpart (31.92 mV).
The ~80% aligned P(VDF-TrFE) fibers are further twisted into bundles and yarns and made into wearable sensors to moniter the bending angle of the elbow. The output signal for bending 45°, 90°, and 135° are 10.6 mv, 20.3 mV, and 42.5 mV, respectively. Furthermore, multiple P(VDF-TrFE) fiber bundles can be used in a combined way to moniter the direction of arm swing.

Throughout recent years, transition metal nitrides (TMNs) have garnered increased interest due to applications in optoelectronics and plasmonics due to high chemical and thermal stability, a wide range of material properties, the ability to influence film characteristics by varying deposition parameters, and the ability to alloy TMNs together to tune desired properties. One such material is aluminum nitride, a piezoelectric material with a high temperature stability and thermal conductivity but a somewhat lackluster piezoelectric coefficient. Nonetheless, by alloying AlN with scandium, one may increase the piezoelectric coefficient of AlN by a factor of 3.25. [1]. This increase is limited by the phase transition of Al_1-xSc_xN from hexagonal to cubic at scandium concentrations greater than x = 0.43 [1]. Growth of single crystal, epitaxial Al_1-xSc_xN thin films has proven challenging due to the tendency of AlN to not effectively wet to the Al₂O₃ substrate surface. One possible solution is to deposit a thin wetting layer between the AlN and the substrate. ScN was chosen as a possible wetting layer between the Al₂O₃ substrate and the Al_1-xSc_xN film due to previous success of single crystal, epitaxial growth of ScN (111) on Al₂O₃ (0001), the inclusion of Sc in the Al_1-xSc_xN film, the possible use of ScN as a bottom contact to the piezoelectric Al_1-xSc_xN film, and the similarity between lattice constants of ScN (111) and AlScN (0001). However, multiple domains of ScN can form due to the cubic nature of ScN films growing on the hexagonal Al₂O₃ substrate, possibly interfering with the desired growth of Al_1-xSc_xN. As such, a thickness suite of ScN thin films ranging from less than an nanometer to up to 10nm were grown through controllably unbalanced magnetron sputtering. These films were examined through x-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry in order to determine what thickness of ScN would produce the least amount of these domain variances. Preliminary XRD coupled scans show highly oriented ScN on Al₂O₃ with glancing angle scans showing no additional phases present in the ScN film. AFM surface analysis showed root mean square (RMS) roughness between 0.430nm and 0.911nm for films between 0.5nm and 10nm, corresponding to one to two monolayers of roughness. Additionally, thin films of Al_1-xSc_xN films were deposited both on bare substrate and with the additional ScN wetting layer, at similar thicknesses to the ScN thin films. These were also characterized using XRD, AFM, and spectroscopic ellipsometry to determine the effect of the ScN thickness and domains on the Al_1-xSc_xN film crystallinity and piezoelectric properties.

[1] O. Zywitzki, T. Modes, S. Barth, H. Bartzsch, P. Frach. Effect of scandium content on structure and piezoelectric properties of AlScN films deposited by reactive pulse magnetron sputtering. Surf. Coat. Technol., 309 (2017), pp. 417-422.

Thin films of lead zirconate titanate (PbZr_xTi_1-xO₃ - PZT) are of considerable interest for a range of applications, including piezoelectric MEMS (with x~0.52) and pyroelectric thermal IR sensing (with x~0.30 [1]). Thin films of PZT (x=0.15 to 0.42) have in the past been grown onto platinized Si substrates by sputtering from multiple metal targets [2], but there are considerable technological benefits to deposition from a single ceramic target [3].
This paper will discuss the process of magnetron sputter deposition of PZT thin films from a single ceramic target. We used Monte-Carlo simulation method based on the algorithm, presented in [4], to describe the sputter atoms transport process and their delivery on the substrate. The modeling was carried out taking into account the geometry and dimensions of the deposition system, and the sputter target erosion zone. A complementary SIMS analysis of the PZT sputter target was carried out to identify the type of sputtered particles (e.g. single atoms, binary compound, clusters), which were further used in the simulation process.
Finally, we will present the structural and electrical properties of the sputtered PZT films (e.g. crystal structure, stoichiometry, dielectric permittivity, loss, pyroelectric coefficient etc.) and discuss their dependence on the existence of particular species in the gas phase during the sputtering process.


Acknowledgements
This work was supported by Innovate UK under Project “Advanced manufacturable sputtering of high performance pyroelectric thin films (HiPer-Spy)”, Ref No: 103525.

References
[1] Q. Zhang and R. W. Whatmore, Integr. Ferroelectr., 41, 1695-1702, (2001)
[2] R. Bruchhaus, H. Huber, D. Pitzer, and W. Wersing, Ferroelectrics, 127, 137-142, (1992)
[3] A. Mazzalai, M. Kratzer, R. Matloub, C. Sandu, and P. Muralt, MRS Online Proceedings Library Archive, 1674, (2014)
[4] P.K. Petrov, V.A. Volpyas, and R.A. Chakalov, Vacuum, 52, 427-434 (1999)

Fabrication of ferroelectric polymers at lower temperatures, thickness control and film conformality in polymeric multilayer structures are required for full integration of the ferroelectric polymers to the modern integrated devices. Also, designing of multilayer ferroelectric films requires precise control over the deposition parameters.
We use Initiated Chemical Vapor Deposition (iCVD) method to deposit 10-30 nm ferroelectric layers and fabricate multilayer poly [(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)] thin films. We control the thickness of the layers and control the chemical composition in each layer along the thickness.
Also, the change in dielectric constant and dielectric loss at moderately low and high frequencies for each multilayer configuration will be discussed. Hence, we report the frequency dependence of dielectric constant, loss tangent, imaginary electric modulus of multilayered thin films.

Control over the electron behaviors is essential for the quest of the unusual coexistence of seemingly conflicting physical properties in condensed matter science. Although the coexistence of ferroelectricity, conductivity and magnetism in a single-phase material has attracted considerable attention due to the fundamental interest and tremendous technological promise, the mutually exclusive mechanisms among them hinder the discovery of multifunctional conducting multiferroics. Here, we propose a novel material design approach for electron engineering, by which we realize an unusual coexistence of these conflicting properties. We demonstrate from first-principles that the appropriate mechanical strain turns the excess electrons in doped BaTiO₃ from free carrier configuration to localized polaronic state through the modulation of electron-phonon coupling. The resulting localized spin-polarized electron survives the host ferroelectricity and consequently manifests itself as multiferroic polaron. The multiferroic properties coexist with the electronic conductivity arising from high hopping mobility of polaron, and thus enable the doped epitaxial BaTiO₃ to act as multiferroic conducting material. This mechanical control over electron configuration opens up a new possibility for unusual coexisting properties and new technologies.

LiNbO₃ structure, belongs to the space group of R3c, is frequently referred as “strained perovskite structure”. Recently, many compounds have found to be possible to form LiNbO3-type structure under the high-pressure condition, and some of them were quenchable phase. By systematic first-principles prediction of piezoelectricity and phase stability of possible combination of A-site and B-site ions, we have found new Al-based LiNbO₃-type piezoelectric materials. Dynamical stability analysis on AlScO₃, AlInO₃ and AlTlO₃ showed no unstable phonon mode. Formation energies of each compound were predicted to show negative value at high pressure. Predicted piezoelectric constants e₃₃ and d₃₃ of each compound were larger than those of LiNbO₃. Especially, e₃₃ and d₃₃ values of AlTlO₃ were anomalously large to be 10.7 C/m² and 56.6 pC/N. By utilizing the formalism of density functional perturbation theory, piezoelectric constants of each compound were decomposed into elastic and dielectric contribution from each atom. As a result, large piezoelectricity of Al(Sc,In,Tl)O₃ was found to be originated in the large strain-displacement coupling.

Piezoelectric wurtzite materials such as ZnO and GaN have recently received a lot of attention as piezotronics and piezo-phototronics device materials. The wurtzite-type piezoelectric materials, especially AlN, have another advantage of applicability in high-temperature environments such as sensors in automobile engines, because their noncentrosymmetric crystal structures are thermodynamically stable even at high temperatures. However, the piezoelectric constants of wurtzite-type materials are generally much smaller than those of the perovskite-based materials such as Pb(Zr_xTi_1-x)O₃ by a few orders. It remains a challenge to explore better piezoelectric wurtzite materials, and there have been many reports aiming to enhance piezoelectricity by element doping into parent materials. Among the wurtzite materials, the highest piezoelectricity has been experimentally discovered for Sc_xAl_1-xN (about 25 pC/N for x ~ 0.5). Novel low-cost materials, which are superior to Sc_xAl_1-xN, have not been synthesized yet as there are no clear and general materials-design criteria practically usable for enhancing the piezoelectricity of wurtzite materials.
In this study, we calculate longitudinal piezoelectric constants (e₃₃) of more than a dozen binary wurtzite materials, which are listed in a structure database, by using the first-principles methods, and we investigate possible correlations between the piezoelectric constants and several material parameters using the statistical-learning methods [1]. As a result, it is theoretically shown that wurtzite materials with high e₃₃ generally have small lattice constant ratios (c/a) almost independent of constituent elements, and approximately expressed as e₃₃ ∝ c/a - (c/a)₀ with ideal lattice constant ratio (c/a)₀. We find that this relation also holds for highly-piezoelectric ternary materials such as the calculated e₃₃ values of Sc_xAl_1-xN [2]. Therefore, this material-design criterion can be applicable to the case in doped ternary materials. We have conducted a computational search for high-piezoelectric wurtzite materials by identifying materials with smaller c/a values. It is theoretically proposed that the piezoelectricity of ZnO can be significantly enhanced by partial substitutions of Zn with Ca. Though the calculated value of e₃₃ of Ca_xZn_1-xO is still smaller than that of Sc_xAl_1-xN, we expect that Ca_xZn_1-xO is at a definite advantage in materials cost and natural abundance of constituent elements.
References:
[1] H. Momida and T. Oguchi, Appl. Phys. Express 11, 041201 (2018).
[2] H. Momida, A. Teshigahara, and T. Oguchi, AIP Advances 6, 065006 (2016).

Since the seminal work by Ferenc Preisach in 1935, the so-called Preisach model, in which a real, non-ideal ferroic material is described as the sum of a distribution of ideal ‘hysterons’, has been a cornerstone in the fields of ferromagnetism and ferroelectricity. However, the physical reality of the model in ferroelectrics has been hard to establish, limiting its further applicability and utility. Here, we show how an experimental Preisach distribution-based analysis can quantify the energetic disorder and elucidate the concomitant dispersive polarization switching kinetics common for different classes of ferroelectrics.

We experimentally determine the Preisach (hysteron) distribution for two differently structured ferroelectric systems, the liquid crystalline benzenetricarboxamide (BTA) and the polycrystalline copolymer P(VDF-TrFE). For BTA a broad circular distribution is found, in contrast to the narrow elliptical distribution for P(VDF-TrFE). We explain how this broadening can be directly related to the materials’ morphology: in BTA the ferroelectric domains consist of strongly interacting columns, while P(VDF-TrFE) consists of non-interacting crystallites. Our explanation is supported by simulations using a simple electrostatic model. The offered insight in the shape of the Preisach distribution is especially relevant for ferroelectric multi-bit data storage applications.

The model also provides an explanation for the dispersive switching kinetics observed in most ferroelectrics, and the underlying distribution in switching times. By measuring the switching kinetics of discrete parts of the Preisach plane, we can directly extract this distribution. The combination of the Preisach model, the thermally-activated nucleation-limited switching formalism and the adapted Kolmogorov-Avrami-Ishibashi theory provides a full and consistent description of the measured macroscopic switching kinetics in terms of device morphology and energetic disorder.

Our results reveal that the in principle mathematical construct of the Preisach model has a strong physical basis and is a powerful tool to explain polarization switching processes of different types of ferroelectrics.

Recently, the emergence of pre-perovskite PbTiO₃ nanowires have attracted increasing research interests. In this work, systematic first-principles calculations were performed to investigate the lattice dynamics and thermodynamic properties of the new pre-perovskite PbTiO₃ phase. The stability of pre-perovskite PbTiO₃ at finite-temperature was analyzed in terms of the lattice contribution, and its thermodynamic properties were obtained and compared with those of cubic and tetragonal PbTiO₃. The pressure-temperature phase diagram for these three types of PbTiO₃ was established, indicating that pre-perovskite PbTiO₃ can be stable under negative pressure. These theoretical insights are useful for understanding the origin of phase transitions among pre-perovskite, traditional cubic and tetragonal PbTiO₃ phases, hence providing meaningful guidance for future experimental study and potential applications of pre-perovskite PbTiO₃.

This newly developed modelling framework for the simulation of electric current flow in ZnO varistors is based on an equivalent circuit representation of the varistor microstructure where the grain boundaries are represented by nonlinear resistors in the circuit. The present approach extends on similar models introduced earlier by including the effect of mechanical stress on the grain boundary conductivity. This effect is based on the coupling between the semiconducting and the piezoelectric properties of ZnO. The stress-induced piezoelectric polarization modifies the interface charge at the grain boundaries. This changes the effective potential barrier and therefore leads to a stress induced modification of the current voltage characteristics of the grain boundary.
The model used for the calculation of single grain boundary conductivities is based on the theory of Blatter et al. and Verghese et al.. It includes a self-consistent solution for the interface charge and for the potential barrier of the boundary, taking into account the local stress in the grain.
Using the above model, the grain boundary potential barriers are parametrized with respect to voltage and piezoelectric charge density. Such tabulated data can be easily incorporated in the modeling of larger varistor structures. 2D and 3D varistor models are constructed using appropriate Voronoi tessellations as well as measurement data obtained by EBSD scans. The mechanical stress distribution within the material is calculated by FEM. The electrical resistance of each grain boundary is then determined according to the local voltage and piezoelectric polarization charge. Finally, the electric current flow patterns within the microstructure and the corresponding current-voltage characteristic of the bulk material are obtained by solving the nonlinear circuit equations for each applied voltage and mechanical stress condition of the sample. The simulated characteristics reveal a significant sensitivity of the bulk electrical conductivity to mechanical stress. Furthermore, the simulations demonstrate the current concentration effect in the voltage breakdown region.
Further topics of interest, which have been addressed by the modeling, include the influence of microstructural inhomogeneities, the investigation of the properties of purposely tailored microstructures (such as sandwiched polycrystalline layers) and the influence of sintering temperature on residual stresses and varistor characteristics.

Piezoelectric MEMS devices are well established in the field of filters, e.g. RF filters for communication applications. The modal behavior and thus the filter characteristics are strongly affected by the in-plane and out-of-plane geometry and are therefore controlled by the design. A post-fabrication adjustment is difficult and commonly made with irreversible processes far away from application conditions, for example modifying the residual stress in the resonating structure by annealing processes.

Here we present the study of a piezoelectric resonator, whose modal characteristics can be tuned using additional piezoelectric elements. This concept allows a flexible and reversible adjustment of the modal behavior in the application under operation conditions. The design is made of three parallelly-oriented mechanically-coupled fixed-fixed unimorph cantilevers. This allows using the outer cantilevers as actuators to vary the stress on the central unimorph structure. The resulting piezoelectric resonator exhibits a modal behavior which can be tuned by a DC voltage on the two additional piezoelectric elements. Devices have been fabricated using silicon technology on 8-inch wafers. A sputter deposited 1 µm thick AlN layer is used as active material. The suspended cantilevers have been realized from a poly-Si layer using backside release.

While the principle is generally applicable to achieve tunable piezoelectric MEMS filters, this study focuses on the use as magnetoelectric sensors for weak magnetic signals, e.g. for biomagnetic applications [1]. To achieve the sensitivity to a magnetic field, a magnetostrictive FeCoSiB layer is added on the unimorph structure. The piezoelectric and magnetostrictive layers build a magnetoelectric composite capable of converting a magnetic signal into an electrical signal via the mechanical coupling. The resonance of the cantilever structure is employed to enhance the mechanical response for certain frequencies and thus the electrical signal. The fabricated devices are used to study the fundamental relationship between sensitivity and stress in the resonator structure.

Several benefits of the presented tunable magnetoelectric sensor are anticipated. The frequency can be adjusted precisely to the magnetic signal in the measurement to get the highest benefit from the resonance effect. In addition, different measurements with varying signal frequencies can be addressed without having to change the sensor. Superimposed magnetic signals at different frequencies can be measured with a single sensor in a serial manner, due to the filter effect of the narrow-bandwidth resonance amplification.

Funding by the DFG via the Collaborative Research Center SFB 1261 is gratefully acknowledged.

[1] J. Reermann, P. Durdaut, S. Salzer, T. Demming, A. Piorra, E. Quandt, N. Frey, M. Höft, G.Schmidt: “Evaluation of magnetoelectric sensor systems for cardiological applications”, Measurement 116 (2018), pp. 230-238

The emergent discovery of two-dimensional electron gas (2DEG) at the LaAlO₃/SrTiO₃ (LAO/STO) heterostructure with n-type interface has attracted considerable attention over the past decade. Despite several mechanisms such as polar catastrophe, oxygen vacancy and cation intermixing have been suggested, the origin of 2DEG remains still unclear. Recently, unlike LAO/STO (001) interface, a wide 2DEG distribution have been reported at the LAO/STO (111) interface. According to the polar catastrophe, although the p-type 2DEG is expected at the LAO/STO (111) interface consisting of [Ti]⁴⁺/[LaO₃]^3- layers, the n-type 2DEG has been experimentally observed. Here, using a first-principles density functional theory calculations, we explore the origin of wide n-type 2DEG at LAO/STO (111) interface at the atomic scale. Compared with LAO/STO (001) interface, we find that the oxygen adsorption on the [Al]³⁺-terminated LAO (111) surface and following surface reconstruction play a key role in forming n-type 2DEG and the existence of critical thickness.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2018R1A2B6004394). This work also 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.

Piezoelectric ceramics are used for sensors and actuators as they have high piezoelectric coefficient. However, due to the brittle nature of ceramics coming from the fact that the size of flaw or crack determines the ultimate strength and elastic strain limit, it is still a challenge to use them for flexible or stretchable devices. In this study, we introduce ZnO truss nanostructure which shows high piezoelectric coefficient confirmed by local piezoresponse map and high elastic limit measured by nano-indenter.
The photoresist SU8 was used for the template of ZnO truss structure via 3D photolithography. We used low temperature atomic layer deposition to coat the SU8 truss template with conformal ZnO thin film. The piezoelectric characteristics of the ZnO/SU8 composite truss were analyzed by Dual AC Resonance Tracking Piezoresponse Force Microscopy (DART-PFM) where the effective piezoresponse was 37.8 pm/V. This is almost three times larger than the piezoelectric coefficient reported for bulk ZnO (8 – 10 pm/V). The stress vs strain curve was measured by nano-indenter, and showed linear behavior up to ~4%, which is more than 10~20 times the strain limit of bulk ZnO. We expect that our novel ZnO truss nanostructure can serve as basic materials component for future haptic enhanced applications.

The broken inversion symmetry in ferroelectric semiconductors causes the bulk photovoltaic effect. The same symmetry considerations predict a non-equivalence between electrical currents flowing parallel and anti-parallel to the polarization direction. While ferroelectrically-switchable current rectification in metal-ferroelectric-metal diodes has been observed due to interfacial phenomena like injection or tunneling barrier modulation, a coupling between bulk conductivity and polarization has not been observed. Here, we present a class of molecular ferroelectrics that show a polarization-dependent bulk conductivity.¹
We have synthesized disc-like semiconducting organic molecules that are functionalized with strong dipolar side groups.² These materials self-assemble into supramolecular polymers, which provides long-range polar order that supports collective ferroelectric behavior of the side groups, as well as charge transport through the stacked semiconducting cores.
We find that the ferroelectric polarization couples to the charge transport and leads to a bulk conductivity that is both switchable and rectifying. When sweeping the applied electric field, the conductivity is switched from a high to low state at the ferroelectric coercive field. Detailed analysis of the current-voltage curves shows that the current is a combination of Ohmic and space-charge-limited currents. This demonstrates that it truly is the bulk conductivity that is modulated by the ferroelectric polarization.
A simple quasi-1D hopping model is developed to investigate the effect of the asymmetric potential caused by the polarization. This model reproduces the experimental on/off ratio using reasonable parameters.

References:
1. Gorbunov, A. V. et al. Ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity. Sci. Adv. 3, e1701017 (2017).
2. García-Iglesias, M. et al. A Versatile Method for the Preparation of Ferroelectric Supramolecular Materials via Radical End-Functionalization of Vinylidene Fluoride Oligomers. J. Am. Chem. Soc. 138, 6217–6223 (2016).

We have studied the photo-induced phenomena using strongly correlated YMnO₃ ferroelectric thin films. Unipolar material YMnO₃ are suitable for studying the effect of the ferroelectric polarization on the photo-induced current. The clear relationship between the direction of the polarization and the photo-induced current was recognized using (0001) YMnO₃ epitaxial films. The current switching corresponding to the polarization switching is also observed under the illumination of white light. To study the origin of the photo-induced current originated from the photo-induced carrier generation, the light energy dependence of the photo-induced current was investigated. The small peak at 1.75 eV and broad peak at around 2.5 eV are observed at room temperature. The peak at 1.75 eV corresponds to the optical absorption at 1.7 eV that generated by the electron transition between Mn 3d (xy,x²-y²)(e_2g state)/O 2p hybridized band and upper Mn 3d (3z²-r²)(a_1g state) orbital [1]. The broad peak of photo-induced current corresponds to the broad photoluminescence excitation spectrum at around 2.5 eV, which is never observed in absorption measurement but reported as the hidden optical channel.
After the introduction of the origin of photo-induced current of YMnO₃ is discussed including associated with the carrier generation and the emission process, ultrafast dynamics of coherent optical phonon correlated with the antiferromagnetic transition in a hexagonal YMnO₃ epitaxial film is discussed. The observations of the coherent optical phonon using a reflection-type pump-probe technique at various temperatures, excitation powers and energies were carried out. We detected an oscillatory structure with a frequency of ~5.1 THz, which is assigned to the coherent optical phonon with A1 symmetry, in a time-domain signal. It was found that the coherent optical phonon can be observed at temperatures from 10 K to room temperature, while the oscillation amplitude markedly decreases with an increase in temperature around ~70 K corresponding to the Néel temperature. The temperature dependence of the oscillation amplitude indicates that the coherent optical phonon is sensitive to the spin-lattice coupling connected with the antiferromagnetic transition [2].

[1] M. Nakayama and N. Fujimura et al., Appl. Phys. Express, 7, 023002 (2014)
[2] T. Hasegawa and N. Fujimura et al., Appl. Phys. Letters, 111, 192901 (2017)

The bulk photovoltaic effect (BPVE) is the generation of photocurrents in the bulk of a single-phase material. It holds advantages over traditional photovoltaics based on p-n junctions, such as above-band gap photovoltages, and current generation in the bulk without the need for interface engineering. Despite numerous theoretical and experimental research efforts into the BPVE, there has been no systematic investigation into its maximum magnitude attainable in solid-state materials. In this talk, we present an upper bound on the dominant microscopic mechanism of BPVE: the shift current response. We show that this bound depends on the band gap, band width, and geometrical properties of the material in question. As a proof of principle, we perform first-principles calculations of the response tensors of a wide variety of materials, finding that the materials in our database do not yet saturate the upper bound. This suggests that new large BPVE materials will likely be discovered by future materials research guided by the factors mentioned in this work.
These results imply that small band gap materials can potentially host large BPVE. As examples, we propose materials which are tuned across a band-gap-closing phase transition from a normal semiconductor into a topological insulating phase. This class includes some inorganic layered semiconductors, such as BiTeI, and inorganic halide perovskites, such as CsPbI₃. We show that this results in a dramatic enhancement of photocurrent as well as an abrupt reversal in its direction. Using first-principles calculations, we show that that this effect is robust across different materials systems as long as such a transition into a topologically insulating phase is present.

In this work we study the chemical substitution of the transition metal in BiFeO₃ by Co-ions to explore the potential to judiciously engineer the optical band gap and examine its impact on the ferroelectric properties and the photoresponse. Nonetheless, the stabilization of BiFe_1-xCo_xO₃ (BFCO) phases shows a rather narrow growth window requiring high pressure synthetic conditions.Here by using low-cost chemical solution deposition we have been able to stabilize by epitaxial growth the perovskite BFCO phase modulating the band gap from 2.7 to 2.4 eV while preserving robust ferroelectricity (P_r= 60 μC/cm²). Photoresponse measurements performed at 520 nm and 1.5 W/cm² on 100 nm BFCO films show a clear enhancement of the current density compared to pristine BFO films. Also, we observe that the magnitude of the current can be modulated by applying a voltage of a particular polarity and this effect is stronger in cobalt substituted films. Towards an all-oxide device, the use of selective layers and transparent conducting oxides are also assessed to further improve the incident photon to charge charrier efficiency of these devices. With this comprehensive study we demonstrate the complexity but also the richness of this system for future light harvesting applications.

We discuss a novel manifestation of quantum mechanical current flow in solids upon photoexcitation. From old days, bulk photovoltaic effect has been known to exist in non-centrosymmetric crystals such as poled ferroelectrics [1]. Naive explanation was that the drift current flows due to electric field uncompensated by insufficient formation of electric double layer on the surfaces of polar crystals. Now, it is proposed and confirmed that a quantum mechanical effect, described by the Berry’s connection of Floquet bands, drives photocurrent called “shift current” as a second order optical process [2, 3]. We present experimental observations of photovoltaic effect in such polar materials systems as LaFeO3/SrTiO3 interfaces [4], a ferroelectric organic TTF-CA [5], and a polar semiconductor SbSI [6]. Ultrafast THz spectroscopy [7] and device physics [8] studies have elucidated interesting features of the shift current.
[1] W. T. H. Koch et al., Ferroelectrics 13, 305 (1976).
[2] S. M. Young, M. Rappe et al. Phys. Rev. Lett. 109,116601 (2012).
[3] T. Morimoto, N. Nagaosa Science Advances 2, e1501524 (2016).
[4] M. Nakamura et al. Phys. Rev. Lett. 116, 156801 (2016).
[5] M. Nakamura et al. Nature Commun. 8, 281 (2017).
[6] N. Ogawa et al. Phys. Rev. B (R) 91, 241203 (2017).
[7] M. Sotome, N. Ogawa, et al. arXiv:1801.10297
[8] M. Nakamura et al. submitted.

Ferroelectric (FE) materials are under intense scrutiny for photovoltaic applications (PV), following the demonstration of above 8% conversion efficiency in FE-based solar cells [1]. In these cells, there is no need for a p-n junction because the electric polarization from ferroelectricity is responsible for the current flow. The key issue for the development of oxide absorbers for PV is their bandgap that is generally above 3 eV.
In this work, we produced Bi₂FeCrO₆ (BFCO) oxide materials by pulsed laser deposition (PLD). The structural, optical and electrical properties are presented. High quality epitaxial growth and phase-pure films are demonstrated by X-ray diffraction. We have studied the evolution of parameters such as the bandgap versus the growth conditions, proving that it can be adjusted from 1.9 to 2.6 eV [2]. The ferroelectric properties are investigated by piezoresponse force microscopy (PFM). We observe that light influences the state of polarization of BFCO. Finally, devices based on BFCO are fabricated and their photovoltaic properties are analysed.

References:
[1] R. Nechache, C. Harnagea, S. Li, L. Cardenas, W. Huang, J. Chakrabartty, F. Rosei, Nature Photonics, 2015, 9, 61
[2] A. Quattropani, D. Stoeffler, T. Fix, G. Schmerber, M. Lenertz, G. Versini, J. L. Rehspringer, A. Slaoui, A. Dinia and S. Colis, Journal of Physical Chemistry C 122, 1070 (2018)

Layered van der Waals crystals like ferroelectric CuInP₂S₆ (CIPS) provide a range of intriguing functional properties. They open a straightforward path to ultrathin ferroic structures through exfoliation and avoiding dangling bonds. They enable electric-field tunable interfaces with 2d materials like graphene and transition-metal dichalcogenides (TMDs). And they exhibiting an intermediate optical band gap while maintaining ferroelectric or antiferroelectric properties. Recently, we measured giant negative electrostriction of CIPS that leads to very large out-of-plane piezoelectric coefficients despite a small polarization of only a few μC/cm². Here we demonstrate the effect of large negative electrostriction below and above the Curie temperature using scanning-probe-force microscopy and quantify piezoelectric and electrostrictive tensor elements using density-functional theory and Landau-Ginzburg-Devonshire calculations [1]. The piezoelectric tensor element d₃₃ is temperature dependent and can exhibit values up to -100 pm/V in the ferroelectric state, which is larger than values reported for polyvinylidene fluoride. At 100°C, which is above the Curie temperature, the relative permittivity of CIPS is dependent on the applied electric field, allowing to tune the electrostrictive response, which opens up further opportunities for applications. Finally, we established atomistic origin of electrostriction in this material rooted in the multiwell potential of the Cu ions and pointing to new approaches to enhance piezoelectric performance of these materials.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy.

[1] Neumayer et al, ”Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric”, arXiv:1803.08142 (2018)

Epitaxial growth of thin piezoelectric oxide films is of great importance for the minimization of devices applied in sensors and actors at high-temperatures (HT) and in oxidizing atmospheres. The latter prevents e.g. the application of aluminum nitride. Langasite (La₃Ga₅SiO₁₄, LGS) is of special interest as it is an oxide crystal that is piezoelectrically excitable up to its melting point at 1473 °C. To achieve crystalline LGS films, a growth process must be developed. A key challenge is the evaporation of Ga suboxides occurring during film preparation under high-vacuum conditions and at elevated substrate temperatures.
In this work, the homo- and hetero-epitaxial deposition of LGS thin films on LGS single crystals, Si and SiO₂ substrates by pulsed laser deposition (PLD) is presented. For preparation of targets the oxides La₂O₃, Ga₂O₃ and SiO₂ are mixed. PLD is performed at a substrate temperature of several hundred °C. To counterbalance the Ga deficit during deposition two strategies are discussed. First, an enhanced Ga content in the PLD target is applied by using off-stoichiometric targets. The second approach focuses on an increased oxygen partial pressure (up to 10^-2 mbar) during deposition (typical base pressure 10^-6 mbar). Combining these adaptions allows for the growth of stoichiometric LGS thin films.
Films deposited on LGS substrates do not show any additional X-ray diffraction reflexes nor broadening of the peaks of the single crystalline substrates. Therefore, the homoepitaxial approach can be considered as successfully performed. The deposition on Si and SiO₂ under the same conditions leads to the formation of polycrystalline films. However, post-annealing at 800 °C increases the crystallinity. The stoichiometry and homogeneity of the cations La, Ga, Si is characterized by secondary neutral mass spectrometry. The composition remains constant with film thickness, representing stable process parameters. However, as expected, strong dependence is seen on the deposition parameters.
An application based on these efforts is the preparation of monolithic electrodes for resonators. Typically, LGS resonators are equipped with metallic electrodes which suffer under degradation (oxidation, evaporation etc.) at extremely high temperatures. Oxide electrodes with matched thermal expansion promise to show better long-term stability. For this purpose the conductivity of the LGS films is increased by partial replacement of La₂O₃ with SrO₂ as Sr doping increases the concentration of oxygen vacancies. For 33 % Sr at the original La site a conductivity enhancement of 2 orders of magnitude is observed. The resonance frequency and the inverse Q-factor of resonators with such monolithic electrodes is discussed as a function of temperature (30-1000 °C) and electrode thickness. They are compared with traditional resonators with Pt electrodes.

Multiferroic materials have attracted a great deal of attention because of their ferroelectric, ferromagnetic and ferroelastic properties in a single material. They enable to control electrical polarization under the application of magnetic field, or magnetization under the application of electric field. Due to its promising feature, they have gained a remarkable usage area in non-volatile information storage, spintronics, multiple state memories and sensors. Among multiferroics, BiFeO₃ (BFO) is one of the possible candidate for room temperature multiferroic materials. There are several methods to synthesize BFO varying from conventional solid state reaction to sol-gel technique.

In this study, one of the multiferroic material, BiFeO₃ (BFO) was investigated. It was hereby with this work proposed a hydrothermal method to synthesize BFO powders. Well- crystallized BFO and Yb-doped BFO (Bi_1-xYb_xFeO₃: x = 0, 0.01, 0.03, 0.05, 0.1) particles have been synthesized successfully for the first time with NaOH as a mineralizer. XRD patterns confirmed that almost all of peaks were indexed to BFO (ICDD 00-020-0169) with small amount of secondary phases. Rietveld analysis via GSAS program showed rhombohedral structure for all cases and indexed as R3c space group. SEM images of all samples were displayed spherical morphology with a diameter of various size at 30 μm. Undoped BFO displayed Neel temperature at around 370°C and Curie temperature nearly 850°C attained by simultaneously taken DSC/DTA/TGA analysis. %64.3 weight percent loss was mainly ascribed to decomposition of nitrate and evaporation of water. Those were also proven by in-situ XRD in which crystallization of BFO completed itself at almost 600°C and transformation of crystal structure from rhombohedral to monoclinic was attained between 800°C - 850°C corresponding to its Curie temperature. In comparison with un-doped BFO, Yb-doped BFOs exhibit small reduction from its T_C as a result of substitution of Yb³⁺ for Bi³⁺ ions. XPS results demonstrated Fe-O & Bi-O & Yb-O bonds along with presence of Bi³⁺, Yb³⁺ and Fe³⁺ rather than Fe²⁺. This proved that Yb has been successfully doped in BFO without forming any secondary phases. The change in the polarization and remnant polarization values were measured for the crystallites and hysteresis curves were observed. The maximum polarization values were achieved above 0.5 μC/cm² at around 50 kV/cm. Lastly, VSM technique revealed remanent magnetization (M_r) nearly 0.02 (emu/g) at 2T. In conclusion, all these results proposed the outstanding extrinsic ferroelectric and magnetic behavior of Yb-doped BiFeO₃ in a single phase.

κ-Al₂O₃, same as ε-Fe₂O₃, GaFeO₃ structures, structured materials whose space group is Pna2₁, are one of very attractive multiferroics because of coexistence of ferroelectric and ferrimagnetic properties at room temperature. Ferroelectric property of this material has been investigated using single crystal and epitaxial thin films.[1] However, measurement of ferroelectricity is prevented by very large leakage current in GaFeO₃ single crystal. Moreover, this structured single crystal, except to GaFeO₃, is difficult to prepare due to metastable phase. On the other hand, the measurement of ferroelectric property was achieved by formation of high quality epitaxial thin film[2.3]. However their measured polarization values were one order smaller than calculated one[4]. We resulted that this issue was originated to three-fold structural variant which is formed on (111)SrTiO₃ single crystal. Therefore, we have tried to prepare GaFeO₃ single crystal and then prepare single crystal k-Al₂O₃ structured thin films on GaFeO₃ single crystal substrate. GaFeO₃ single crystal was prepared by floating zone method using 10 atom oxygen pressure. Then we cut and polished this single crystal for preparation of thin films using substrate. Sc_0.5Fe_1.5O₃ epitaxial thin films were fabricated on (001)GaFeO₃ single crystal by pulsed laser deposition method. Growth temperature and oxygen pressure for deposition condition were 800^oC and 300 mTorr, respectively. Laue image of prepared GaFeO₃ single crystal measured along 001 zone axis is in good agreement with simulated one. From X-ray diffraction(XRD) 2θ-θ patterns of Sc_0.5Fe_1.5O₃/(001)GaFeO₃ thin films and (001)GaFeO₃ single crystal substrate, κ-Al₂O₃-type structured Sc_0.5Fe_1.5O₃ thin film was grown along 001 direction. XRD phi scan at {013}Sc_0.5Fe_1.5O₃ and {013}GaFeO₃ shows two-fold peaks at same phi degree. This result indicates that prepared Sc_0.5Fe_1.5O₃ thin films is single domain epitaxial thin films without structural variant. We will report ferroelectric, dielectric, leakage, magnetic properties of GaFeO₃ single crystal and Sc_0.5Fe_1.5O₃ single domain epitaxial thin film. [1]T. Arima et al., Phy. Rev. B 70, 064426 (2004). [2] M. Gich et al., Adv. Mater. 26, 4645 (2014). [3] T. Katayama et al., Adv. Funct. Mater. 28, 1704789 (2018). [4]D. Stoeffler, J. Phys.: Condens. Matter 24, 185502 (2012).

Two-dimensional materials are of scientific and technological interest as they run the gamut of electronic classifications from metal, to semiconductor, to insulator while being able to be easily joined to other materials with controlled number of layers. Metal thiophosphates offer a rich class of 2D materials, comprising metal ions occupying octahedrally coordinated sites in the with [P₂S₆]^4- triangular lattice, leading to a variety of magnetic, structurally correlated and polar ground states. Copper indium thiophosphate (CuInP₂S₆) is one such member of this family and has been noted for its ferrielectric characteristic. In this talk, we present the results of density functional theory calculations that explore the effect of strain on both the structure and polarization of CuInP₂S₆. We show CuInP₂S₆ exhibits a negative longitudinal piezoelectric coefficient, and a strain-induced phase transition between two phases, differentiated by the relative displacement of Cu within the individual layers. The existence of two phases may also explain the experimentally observed inhomogeneity of piezoresponse observed experimentally. More generally, these calculations reveal the crucial role played by the van-der-Waals gap in defining their functional properties.

Voltage-tunable radio frequency (RF) electronics are critical for fifth-generation (5G) millimeter-wave telecommunications; applications include frequency-agile filters and tunable phase shifters. For the former, these materials allow handsets to communicate on many frequencies. For the latter, they enable phased-array antennas for beam-steering, which mitigates atmospheric attenuation and interference. State-of-the-art tunable materials are too lossy for these applications because the 5G network requires the use of high carrier frequencies to meet capacity demands. This problem requires new, highly-tunable, low-loss compounds that can support 5G.

Combinatorial methods provide an opportunity to rapidly screen many candidate materials, and develop better models of materials behavior as a function of composition, frequency, and applied DC voltage. These models inform the search for materials with optimal properties. The primary advantage of combinatorial experiments is that full materials systems are synthesized simultaneously, maintaining constant experimental conditions over the full sample space.

In this presentation, we will explain a novel approach to extract the frequency-dependent complex permittivity and voltage-tuning behavior of combinatorial thin-film composition spreads from DC – 110 GHz. We tested our approach on the well-studied Ba-doped SrTiO₃ (BSTO) system, emphasizing 5G frequency bands, and developed a unified model of the frequency dependence, voltage tunability and composition. Such models assist in the design of 5G RF electronics, and are related to thermodynamic quantities. This relationship points to a potential path connecting optimal RF performance to materials through first-principles theory.

Ferroelectricity 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. Similarly, atomic scale defects can strongly affect polarization stability and affect wall pinning and nucleation and give rise to relaxor states. In this presentation, I will illustrate several recent results on ferroelectric and ferroic – chemical coupling on mesoscopic and atomic scales. In the nanoscale systems, the ferroelectric state is fundamentally inseparable from electrochemical state of the surface, leading to emergence of coupled electrochemical-ferroelectric states. These considerations further stimulate the development of the novel SPM modalities addressing time-dependent dynamics and chemical changes during SPM imaging, and studying th emechnisms fo rhtese transformations from atomically-resolved data. I will further delineate the applications of in-situ SPM – time of flight secondary ion mass spectrometry (ToF SIMS) to map the changes in surface chemistry during tribological and local electrochemical experiments, including ferroelectric polarization switching and pressure-induced resistance changes in oxides. On the atomic scales, significant inroads in local ferroelectric behaviors can be obtained from atomically-resolved studies of ferroelectric materials that allow direct visualization of materials structures and order parameter fields. These approaches further necessitate analysis and data mining of large volumes of information, and first examples of deep learning analysis on STEM data to infer local materials behavior and kinetics of point-defect reactions will be illustrated

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.

Theory tools have advanced to the point that we can now rapidly predict new multifunctional piezoelectric materials. Here, first-principles density functional perturbation theory tools within the Materials Project are used to identify promising new lead-free piezoelectric materials. One of these phases is P4mm SrHfO₃, which is predicted to be energetically accessible (within 50 meV of the convex hull), have a high piezoelectric response (for a lead-free compound), and be ferroelectric at room temperature. Combining computationally optimized substrate selection and synthesis conditions allowed us epitaxial stabilize the novel P4mm phase of SrHfO₃. The films were found to be structurally consistent with the theory predictions. A large signal effective converse piezoelectric response of 5.2 pm V^-1 for a 35 nm film is observed. These films also exhibit ferroelectricity, with a moderate coercive field and polarization, and a high breakdown strength. Thus, we show that the coupled theory-experimental approach developed here provides a route to discover and realize other new lead-free piezoelectric polymorphs. Building upon this approach, multiple properties were targeted simultaneously, identifying other candidate narrow bandgap piezoelectrics for bulk photovoltaic effect solar cells and piezotronic (pieozelectric-electronic) applications.

Electroceramics are needed for diverse energy converters (1,2). The prerequisite for a durable active material is the constant regeneration of the structure under thermochemical and heating cooling cycles. Perovskite-type ceramics as well as their nanocomposites are prospective candidates for multifunctional high temperature energy converters. Their good performance relies on their flexible crystal structure being able to accommodate defects during thermal redox processes. The design of our materials is based on theoretical predictions and a deep knowledge on composition-structure-property relationship. The perovskite structure allows diverse substitution reactions to tune the band structure, charge carrier density and mobility as well as thermal and ionic transport. The electronic mobility can become high while the thermal conductivity can be low. Strongly correlated electronic systems are employed as additional design elements for a targeted materials design (3).
The resulting high temperature oxide materials as well as low temperature intermetallic (half-Heusler) and chalcogenite phases are characterized and tested in in-situ high temperature applications to improve the efficiency and energy density of energy conversion devices.
1.)Saucke, G., Populoh, S., Thiel,P., Xie, W., Funahashi, R. and Weidenkaff, A., Journal of Applied Physics, 118, (2015) 035106.
2.)Thiel, P., et al, J. Phys. Chem. C 119(38) (2015) 21860-21867.
3.)Xiao, X., et al,, Phys.Chem.Chem.Phys., 19, (2017) 13469-13480.

The piezo-phototronic effect has been confirmed as a promising methodology to optimize the performances of optoelectronic devices. However, not only positive effects but also negative effects may be produced in some types of photodiodes (PDs) by the piezo-phototronic effect, resulting in the restriction of the PDs’ photoresponse performance enhancement. In order to obtain as large photoresponse performance enhancement as possible, it is essential to investigate how the piezo-phototronic effect influences the photoresponse performance of PDs with different device configurations and structures. Here, we have thoroughly investigated the piezo-phototronic effect on the photoresponse performance enhancement of anisotype (p-Si/n-ZnO) and isotype (n-Si/n-ZnO) heterojunction PDs. The experimental results show that distinct photoresponse performance enhancement of the p-Si/n-ZnO and the n-Si/n-ZnO heterojunction PDs are both achieved by the piezo-phototronic effect. The photoresponsivity enhancement can reach maximized values of 151.06% and 54.95% for the p-Si/n-ZnO and the n-Si/n-ZnO heterojunction PDs, respectively, under -1.0% externally applied compressive strain condition and 6.00 × 10^-3 W 405 nm laser illumination, additionally indicating that the magnitude of the photoresponse performance improvement of the p-Si/n-ZnO heterojunction PD is much larger than that of the n-Si/n-ZnO heterojunction PD. The fundamental working mechanisms of how the piezo-phototronic effect influences the photoresponse performances of the p-Si/n-ZnO and the n-Si/n-ZnO heterojunction PDs are systematically investigated by carefully analyzing their different energy band diagrams under a series of externally applied compressive strain conditions, to explore the in-depth physics beneath the experimental phenomenon. Our proposed mechanisms together with the finite element analysis theoretical simulation results reveal that, two positive effects are introduced to and contribute to the photoresponse performance improvement of the p-Si/n-ZnO heterojunction PD, whereas one positive effect and two negative effect are introduced to the photoresponse performance improvement of the n-Si/n-ZnO heterojunction PD. These three effects compete with each other and finally lead to a weakened photoresponse performance improvement compared with the case of the p-Si/n-ZnO heterojunction PD. This work not only presents in-depth understandings about the piezo-phototronic effect on the photoresponse performances of PDs with different device configurations and structures, but also provides methodology guidance to achieve optimized photoresponse performances of optoelectronic devices by the piezo-phototronic effect.

We regret that, due to ongoing patent applications, we are as of now still unable to discuss aspects that would reveal the composition of the new ferroelectric compound (such as its crystal structure) and the intended applications or related literature. We are however certain that we will be able to share all relevant information in the end of August the latest. For the same reasons, this work is still pending journal publication.

The drive towards miniaturization of piezoelectric sensors and actuators as well as the introduction of ferroelectric functionality into integrated circuit (IC) technology have led to substantial scientific and commercial interest in ferroelectric thin-films. Many of the more important ferroelectrics are perovskite oxides, with typical disadvantages such as low paraelectric transition temperatures, non-linear displacement or compatibility issues with complementary metal-oxide-semiconductor (CMOS) technology. Here, we report experimental results of a first material of what can be expected to be a new group of CMOS compatible ferroelectrics with remarkable properties: Solid-solutions based on a technologically significant subgroup of the III-V compound semiconductors.
Virtually leakage current free polarization hysteresis loops with large remnant values of up to 110 µC/cm² are obtained on polycrystalline thin-films grown by sputter-deposition. Untypical for a polycrystalline ferroelectric, sharp switching events give rise to an almost perfectly square hysteresis. Systematic tuning of the coercitive fields from 1.8 MV/cm to 5 MV/cm was achieved by varying the composition of the solid solution and, independently, via permanent process induced lateral straining of the films. Both mechanisms, as the occurrence of ferroelectricity itself, can be related to a continuous distortion of the initial III-V crystal structure with increased alloying or strain. The inverse piezoelectric effect reveals highly linear strain regimes over a wide range from -0.3% to 0.4% - a direct result of the narrow polarization switching events. Moreover, polarization inversion appears to be complete, as direct and inverse piezoelectric coefficient measurements result in largely identical absolute coefficient values for both polarization states. Measurements of the direct piezoelectric effect after annealing at up to 600°C revealed only a slight decline of the piezoelectric performance without subsequent repolarization, therefore setting a high lower limit of 600°C for the paraelectric transition temperature of the material.
We are confident that these findings could make a valuable contribution towards a more extensive implementation of ferroelectric functionality in thin-film technology.

Functional tactile sensing device is mandatoryfornext-generation robotics and human-machine interfacessince the emulationof touching requires large-scale pressuresensor arrays with high-spatialresolution,high-sensitivity, and fast-response[1]. Some tactile sensors fabricated with organic transistors or micro-structured rubber layer pressure sensor arrays have been reported[2]. While with a resolution at the orderof millimeter, these devices have not yet met the requirements ofartificial skins whose spatial resolution is near 50 μm. Our group have demonstrated pressure sensor array base on piezotronic and piezo-phototroniceffects[3]. An ultra-high resolution of 2.7 μm was derived from piezo-phototronic pressure sensor array usingZnO nanowire (NW)/p-GaN LEDs array[4]. These devices provide stable, fast response, as well as parallel-reading detections of spatial pressure distributions. However, the lacking of flexibility with a rigid sapphire substrate prevents the NW-LEDs array device from applications as smart skin; andthe pressure measuring range of the device is in a relatively high pressure region. Therefore, a flexible pressure mapping system with moderate spatial-resolution become necessary and may find numerous potential applications in human-machine interfaces.
Recently, we designed and fabricated a flexible LED array composed of PEDOT:PSS and patterned ZnO NWs with a spatial resolution of 7 μmfor mapping of spatial pressure distributionsby using the piezo-phototronic effect.These devices possess a wide range of pressure measurements from 40 MPa to 100 MPa depending on the growth conditions of ZnONWs.Furthermore, a LED array composed of PEDOT:PSS and CdSnanorods had been demonstrated for mapping spatial pressure distributions. The emission intensity of which depends on the local strain owing to the piezo-phototronic effect. Therefore, pressure distribution is obtained by parallel-reading the illumination intensities of LED arrays based on electroluminescence working mechanism.The spatial resolution is achieved as high as 1.5μm. Flexible LED device array has been prepared by CdSnanorod array on Au/Cr/Kapton substrate.
The flexibility and stability of these LED arrays mapping system was studied. The outstanding flexibility, high resolution and controllability of these pressure mapping sensors provide promising technologies for future applications in biological sciences, human-machine interfacing, smartsensor and processorsystems, and even defense technology.

Reference
[1] S. C. B. Mannsfeld, B. C. K. Tee, R. M. Stoltenberg, C. V. H. H. Chen, S. Barman, B. V. O. Muir, A. N. Sokolov, C. Reese, Z. Bao, Nature Materials2010, 9, 859;
[2] B. C. K. Tee, A. Chortos, R. R. Dunn, G. Schwartz, E. Eason, Z. A. Bao, Advanced Functional Materials 2014, 24, 5427.
[3] W. Z. Wu, X. N. Wen, Z. L. Wang, Science 2013, 340, 952.
[4] C. F. Pan, L. Dong, G. Zhu, S. M. Niu, R. M. Yu, Q. Yang, Y. Liu, Z. L. Wang, Nat. Photonics 2013, 7, 752;

Single crystals of BaTiO₃(BTO) that have been doped at the titanium site with Fe³⁺or Mn³⁺have previously been shown to demonstrate large and recoverable electrostrain of up to 0.8 % that is thought to be due to the alignment of defects (i.e. O^2-vacancies) with the crystallographic symmetry in the ferroelectric state when the samples are aged.[1,2] This results in a restoring force where the ferroelectric domains favour alignment with the defect dipoles, giving rise to a large reversible strain due to repeated non-180^odomain rotation. There is also the possibility that the incorporation of a magnetic ion could give rise to a magnetic signature and even possibly multiferroic coupling in these doped samples, the possibility of which has not been previously investigated. In this presentation, results from magnetic measurements and polarization measurements with bias magnetic fields will be discussed for a 0.5% Fe doped BTO crystal. Impact of repeated cycling at different electric fields and the recoverability of this large strain will also be presented.

With the rise of Internet of Things, flexible and transparent electronic devices are expected to fulfill rising technical requirements which silicon-based electronics cannot achieve. As known to everyone, the advancement of transistors is most close to the development of technology. However, the performance of present flexible and transparent transistors have been restricted due to the poor crystallinity. In order to make a high quality nonvolatile transistor with full transparency and lower energy consuming, here, we demonstrated a transparent ferroelectric field effect transistor (TFeFET) on muscovite substrate. With a high quality aluminum-doped zinc oxide as active channel layer and high transparency lead lanthanum zirconate titanate as ferroelectric layer, this heteroepitaxy performs excellent electrical properties. Moreover, this flexible TFeFET not only shows high transparency and high thermal stability, but also exhibits promising stability against to mechanical strain during a series of bending tests. Our study demonstrates an unusual concept to achieve flexible transparent nonvolatile transistor for development of next-generation smart devices.

The performance of current information processors are predominantly based on complementary metal-oxide-semiconductor (CMOS) transistors. However, CMOS scaling have started to face significant challenges and besides the physical limits, the conventional computing paradigm based on binary logic and Von Neumann architecture is becoming increasingly inefficient with onset of big data revolution and growing complexity of computation. Neuromorphic computing is the state-of-the-art research trend in the field of memory and logic devices where the goal is to build a versatile computer that is efficient in terms of energy and space, homogeneously scalable to large networks of neurons and synapses, and flexible enough to run complex behavioral models of the neocortex as well as networks inspired by neural architectures. Memristors, with their gradually modified conductivity level can mimic the biological synapses. Low energy consumption, ultrafast operation and small dimensions are the most essential requirements for a memristor to perform tasks similar to a synapse and become as efficient as human brain. A ferroelectric tunnel junction (FTJ), where gradual modulation of conductance can be achieved by controlled rotation of ferroelectric domains can act very efficiently as a synapse. Also the non-volatility of the stored information in the ferroelectric memories make them even more attractive as potential candidates for future neuromorphic computing building blocks. Here, we report on the performance of FTJs with a spin-coated organic ferroelectric P(VDF-TrFE) tunnel barrier. We have measured up to 10⁷% tunneling electroresistance (TER) effect in these FTJs on a semiconducting Nb-doped STO bottom electrode at room temperature that persists until the ferroelectric Curie point of P(VDF-TrFE). ¹ Also these junctions show very clear and reproducible memristive behavior based on variable amplitude and duration of the applied voltage pulses, fast switching, long data retention of the high, low and different intermediate states, short and long-term potentiation (STP & LTP) and depression and spike-time-dependent-plasticity (STDP) which is extremely promising for neuromorphic applications.² Our recent experiments suggest based on the morphology of the ferroelectric film and the top electrode material, the available number of computational states in these devices can be significantly modified that can bring advantages for the synaptic computational devices.³


References
[1] S. Majumdar, et al., Adv. Func. Mater. 28, 1703273 (2018). https://doi.org/10.1002/adfm.201703273
[2] S. Majumdar et al. Nat. Electron. (Submitted).
[3] S. Majumdar et al. . (Manuscript under preparation).

Nowadays piezoelectric systems have been studied and used in interesting daily applications, such as smartphone cameras, accelerometers and microphones [1]. Another interesting area for piezoelectric materials is energy harvesting, as transducers capable of convert vibrations, electromagnetic waves or even wind/water flow into electric potential difference [2]; this might lead to improve our actual collection of renewable power sources, and will let us explore other energy solutions. Even though these materials are promising with their applications, there are some limitations such as energy dissipation through Joule heating [3]. In addition, the most common piezoelectric materials are based in ceramic materials, hence, they are susceptible to wear fatigue. An interesting option is to use instead piezoelectric polymers, these materials are flexible, fatigue resistant and biochemical resistant due to their chemical composition. One of the most renown materials of this type is PVDF (Poly vinylidene fluoride), a semi-crystalline fluoropolymer stable to harsh thermal, chemical and UV environments [4]. Its β phase (β-PVDF) has demonstrated the best piezoelectric response from all the other phases. Therefore, β-PVDF is an excellent option to be used as a piezoelectric material for renewable energies and sensors. In addition, this material, owing to its chemical endurance might be used also as a wearable biosensor/actuator that tracks in real time changes on blood pressure. Nevertheless, the real challenge is to obtain a predominant β-PVDF phase; this is because PVDF, as a semi-crystalline polymer, still has an entropic tendency to rearrange its molecules in lower energy state. We solved this problem by doping the polymer with ytterbium ions (Yb³⁺) in different concentrations below 10% in weight. As the electronic density of Yb³⁺ interacts with the polymer chains, a fixed order is promoted into the PVDF structure with a tendency of a b structure. In addition, an unexpected optical result was found; when β-PVDF:Yb is exposed to sunlight, it presents a photochromic response that corelates to the interaction absorption frequencies of the Yb³⁺. Thus, we consider that a piezoelectric polymer with photochromic characteristics, it is an interesting system to be further explored for technological applications in relevant fields.

References:
[1] G. Lesieutre, «Vibration damping and control using shunted piezoelectric materials,» Shock Vib., 30, 88-95 (1998)
[2] H. A. Sodano, D. J. Inman, G. Park, «Generation and storage of electricity from power harvesting devices,» J. Intell. Mater. Syst. Struct., 16, 67-75 (2005)
[3] G. A. Lesieutre, G. K. Ottman, H. F. Hofmann, «Damping as a result of piezoelectric energy harvesting,» J. Sound Vib., 269, 991-1001 (2004)
[4] T. P. I. Foundation, «PAGEV,» 2018. [Online]. Available: https://www.pagev.org/pvdf-en. [Last access: 13 06 2018].

Chemical expansion of a solid refers to dimensional change due to change in stoichiometry. Dimensional change due to charged defects redistribution in an electric field has been termed the electro-chemo-mechanical (ECM) effect. Such instability is clearly deleterious for batteries or fuel cells, but, as recently suggested, has potential for use in actuation[1]. A typical ECM actuator scheme includes: electrode1\WB1\solid-electrolyte(SE)\WB2\electrode2, where WB denotes working-body solids with large chemical expansion coefficient. The main advantage of ECM is that it can deliver simultaneously large strain and large stress, which is difficult to achieve with other actuation mechanisms. We have constructed a room temperature ECM nanocrystalline membrane actuator (2mm diameter; ≈2µm thick) with Gd-doped ceria as SE. We tested two alternatives for WB’s: (1) metal/(metal oxide) or (2) ceria/metal nanocrystalline composite. Electrical and electromechanical measurements demonstrated that actuator response with metal/metal oxide WB is limited by the rate of oxygen diffusion from the solid electrolyte to the metal surface. Actuators with ceria/metal composite WB provide faster response time (≈20sec) and larger vertical displacement (>3.5µm). Our findings suggest that ECM may become a viable actuation mechanism.

[1] J. G. Swallow, J. J. Kim, J. M. Maloney, D. Chen, J. F. Smith, S. R. Bishop, H. L. Tuller, K. J. Van Vliet, Nat. Mater. 2017, 16, 749.

The demand for ultra low-powered high-speed devices has pushed scientists and engineers to consider new approaches that involve many aspects, such as materials engineering, device architectures, power management, etc., for the next generation electronics. Ferroelectrics offer a promising route toward a nonvolatile and low power consumption per bit operation (~10 aJ/bit) if one can switch the ferroelectric polarization by 100 mV. Here we demonstrate a reliable pathway to achieve 100 mV switching by the heterostructure: SrRuO₃/La-doped BiFeO₃/SrRuO₃. BiFeO₃ exhibits a robust ferroelectricity at room temperature and possesses a large polarization ~ 80 μC/cm², which can be a burden during switching. Substituting Bi with La enables BiFeO₃ to be switched at a lower voltage due to the suppressing of rhombohedral distortion and resulting in a reduced polarization down to ~ 40 μC/cm² and a lower Curie temperature as well. Moreover, in order to further reduce the coercive voltage, the thickness of the ferroelectric layer also needs to be scaled. However, thinner ferroelectric films generally face multiple issues such as leakage, and depolarization effect, which will lead to an unmeasurable or degraded ferroelectricity. A detailed chemical analysis revealed a limited interdiffusion, which limits the leakage current as well, at the interface between the metal and ferroelectric layer by cross-sectional TEM/EDX. We also explore several oxide metal electrode materials, such as SrRuO₃, LaNiO₃, and La_0.7Sr_0.3MnO₃, to minimize the depolarization effect and the contact potential difference. By carefully controlling the interfaces, film growth, and La doping concentration, the coercive voltage of ~100 mV can be achieved in a 20 nm La_xBi_1-xFeO₃ film. Our results not only provide a profound understanding of low-voltage ferroelectric switching as well as pave the way to the low-power information storage/processing technology.

Sound waves flowing in a peizoelectric crystal could be exploited to excite a ferromagnetic resonance. Here we shall discuss our recent work that aims to exploit such ferromagnetic resonace as a way to couple to nearby defect centers. Specifically, we have studied the nitrogen vacancy centers in diamond. We find that it is indeed possible to couple to these NV centers efficiently, even at zero external magntic field. These findings may allow drive defect centers purely electrically.

Computer clock speeds have not significantly increased since 2003, creating a challenge to invent a successor to CMOS technology able to resume the improvement in clock speed and power performance. The key requirements for a viable alternative are scalability to nanoscale dimensions - following Moore’s Law - and simultaneous reduction of line voltage in order to limit switching power. Achieving these two aims for both transistors and memory allows clock speed to again increase with dimensional scaling, a result that would have great impact across the IT industry. PETMEM is a European partnership amongst Universities, Research Institutions, SMEs and a large company that will focus on the development of new materials and characterization tools to enable the fabrication of an entirely new low-voltage, memory element. This element makes use of internal transduction in which a voltage state external to the device is converted to an internal acoustic signal that drives an insulator-metal transition. Modelling based on the properties of known materials at device dimensions on the 15 nm scale predicts that this mechanism enables device operation at voltages an order of magnitude lower than CMOS technology (power is reduced two orders) while achieving 10GHz operating speed. In this presentation the first two years results will be summarised with a focus on new piezoelectric and new piezoresistive materials development, and some performance properties of our first demonstrator device will be discussed.

Arrays of ferroelctric vortices formed in ferroelectric / paraelectric thin film multilayers with a predominant Néel-type rotational character [1] and emergent chirality [2] are an enticing foray into topological complexity that is typically the purview of magnetic systems. The nanometer length scale and direct electrical field manipulation makes ferroelectric polarization texture an attractive counterpart to spin systems wherever parity exists. Moreover, electric field control of vortex array blocks has been demonstrated by scanning surface probe [3] in geometries where the vortex structure is degenerate with classic a1/a2 domains [4]. In this work using in situ TEM we present the electric field response of these ferroelectric vortices length scales concomitant with the vortex structure (nm). In geometries where the vortex structure is highly stable, applied electric fields induce vortex asymmetry within the PTO layer manifesting as shifts of the rotation centers. In this manner the vortex structure adapts to applied fields via short range small domain wall translations without need of nucleation events. In geometries degenerate with a1/a2 domains, deterministic switching between vortex and a1/a2 structures can be achieved as in bulk [3].
[1] A Yadav, et al., Nature 530 (2016) p. 198.
[2] P Shafer, et al., PNAS (2018), DOI: 10.1073/pnas.1711652115.
[3] A Damodaran, et al., Nat. Mater. 16 (2017), p. 1003.
[4] Z Hong, et al., Nano Lett. 17 (2017), p. 2246.
[5] Authors acknowledge support by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract number DE-AC05-00OR22725

Ferroelectric nanodomains are inevitably created upon polarization reversal. They provide a natural setting to explore conductive properties of ferroelectrics, because the repolarization nuclei are decorated by weakly charged domain walls, and because they can be created and subsequently tuned on demand by appropriately chosen electric field. As such, nanodomains are a model system to probe presently open questions surrounding domain wall conductance, such as pathways to increase conductance (through carrier density and possibly mobility), understanding the stability of conductive walls and the origin of the screening charge.

We have measured conductance of two different kinds of ferroelectric nanodomains, aiming to maximize polarization charge in the ferroelectric volume. In the first case, a radially symmetric electric field is applied to a ferroelectric with substantial component of in-plane polarization – in our case the 100-oriented film of BiFeO₃. Such nanodomains are intentionally unstable but arguably achieve the largest possible polar discontinuity. Indeed, we observe near-record high local conductivity for ferroelectric as well as metastability in applied electric field, producing an electronic function of a volatile resistive switch. However, the net conductance is not metallic in this case. Phase-field modeling reveals localization of polarization charge to near-electrode region, effectively screening applied electric field. We anticipate that conductance will be dramatically enhanced in the ultrathin limit, where the volume of polar discontinuity becomes comparable to the overall film thickness. On the other hand, we have investigated the signatures of inclined domain walls in lead zirconate titanate at the instance of ferroelectric switching by microwave probe, which is sensitive to the bulk volume of the film. We have again observed the largest microwave conductance among accessible polarization configurations, as well as profound metastability of nandomains in a relatively broad range of applied fields. An inspection of the dielectric properties of domain walls at and above their depinning field was carried out to separate the contributions of domain wall motion from nanodomain hysteresis. This analysis provided further evidence for electronic (rather than displacive) origin of microwave conductance for ferroelectric structures created by localized electric fields. Finally, we will comment on the stability of the charged configurations based on detailed analytical modeling of charged domain walls in various screening scenarios. Charged domain walls appear to be generally unstable for polarization exceeding ~10 microC/cm², even with efficient supply of the screening carriers. Support provided by the U.S. Department of Energy, Basic Energy Sciences, Materials Science and Technology Division. Microscopy experiments performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Polarization switching in ferroelectric materials underpins a broad gamut of applications ranging from random access memory, tunneling barriers, data storage, and ferroelectric ceramics. Classically, the polarization switches due to a co-existence of energetically equivalent crystallographic states, that can be altered with an external electric field. To stabilize polarization, charge discontinuity at surfaces and interfaces requires compensation, or screening, to avoid long-range electrostatic fields that destabilize the ferroelectric phase. Most studies consider polarization screening to be chemically inert; leaving the composition of the ferroelectric intact. However, analysis of extant ferroelectric phenomena suggests higher complexity. It is well known that multiple polarization switching cycles can accumulate damage at interfaces, dubbed “ferroelectric fatigue.” Typically, tens or hundreds of thousands switching events are required, and the exact mechanisms remain controversial. Furthermore, polarization-dependent photovoltaic effects in perovskites suggest that even under optimal screening conditions a considerable electric field remains in the material. Thus, switching is associated with high fields, which can chemically alter material composition.
Here we utilize multimodal approach combining time of flight secondary ion mass spectrometry (ToF-SIMS) with atomic force microscopy (AFM) to explore the structure property interplay of ferroelectrics during polarization switching in lead zirconate titanate (PZT, PbZr_0.2Ti_0.8O₃) thin films. Using this multimodal imaging platform, we demonstrated that chemical phenomena plays significant role in ferroelectric switching process. Specifically, we found that local ferroelectric switching by the AFM tip, significantly alters the chemical composition in the 3-nm-thick surface layer of the sample, forming reversible concentration wave, of Pb⁺ ions. Furthermore, investigations of the polarization cycling in the PZT sample with copper electrodes, showed penetration of the copper cations into the structure of PZT. This explains ferroelectric fatigue phenomenon, leading to decrease in spontaneous polarization with sample cycling.
Altogether, explored chemical phenomena associated with ferroelectric switching will enhance fundamental understanding of ferroelectric phenomena and aid in the practical application of ferroelectrics in devices.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility, and using instrumentation within ORNL's Materials Characterization Core provided by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy.

Lead zirconate titanate (PbZr_0.52Ti_0.48O₃, or PZT) films are of interest for piezoelectric microelectromechanical systems as actuators, e.g. in inkjet printers, adjustable optics, micromirrors, and ultrasound transducer arrays. In many cases, these actuators are driven at higher electric fields than would be characteristic of bulk ceramic actuators. Thus, understanding the factors that control the electrical and mechanical reliability of these films under aggressive conditions for electric fields and strains is critical. To address this, acceptor (1-4% Mn) and donor (1-4%) doped PZT films were grown. Thermally stimulated depolarization current (TSDC) measurements in Mn doped PZT films showed one depolarization peak with an activation energy of 0.6-0.8 eV, associated with ionic space charge presumably due to ionic migration of oxygen vacancies. The magnitude of the depolarization current peak increases with increasing degradation times, suggesting the dissociation of defect dipoles during electrical degradation. A similar depolarization current peak attributed to existence of mobile oxygen vacancies was also observed for undoped and Nb doped PZT films; the magnitude of this peak increases on lowering Nb or PbO contents. An additional TSDC peak, associated with trapped charges was found in both Nb doped PZT films and undoped PZT films annealed under low PbO partial pressure. The trap depth is estimated to be 1.1±0.03 eV, which is attributed to trapped electronic charge carriers at reduced Ti on the B site. Electron energy loss spectroscopy studies of degraded Nb doped samples confirmed localized Ti reduction near the cathode. A model describing the failure mechanisms will be presented.

Technological applications of ferroelectric materials often depend on their tuning under an applied electric field. In recent years, polarization switching processes have attracted interest for their role in transient negative capacitance, which could be used increase transistor energy efficiency. Switching processes also govern how quickly a microwave-frequency device based on ferroelectric materials can be reconfigured. Despite the technological motivations to study switching speed, high-frequency measurements remain difficult. In particular, separating the behavior of the measurement circuit from the behavior of the material under test is increasingly difficult with increasing frequency. Interpretation of measurements often requires complicated models of both the material and the measurement circuit. In this talk, we describe a new method for quantifying switching dynamics through nonlinear mixing products up to 40 GHz. From our measurement technique, we are able to empirically describe the dynamical switching behavior of the material under small signals in the tens of GHz without making any physical assumptions about the material itself. We apply this method to Ba_0.5Sr_0.5TiO₃ as a proof of concept, and show that our frequency-dependent results agree with a physical model derived from Landau-Ginzberg-Devonshire (LGD) theory.

Sr₂RuO₄is the n=1 member of the Sr_n+1Ru_nO_3n+1Ruddlesden-Popper family and a well-known metallic oxide that becomes superconducting below 1K. Since it is thermodynamically the most stable phase in this ruthenate family, it can be grown at very high temperatures compared to several other metallic oxides such as Fe₃O₄, SrRuO₃, or (La,Sr)MnO₃. Moreover, the oxygen pressure window is wider than for many other oxides, notably SrRuO₃, that are commonly used as metallic electrodes in oxide device structures. We present the results of a study on the thermal stability of Sr₂RuO₄thin film electrodes and demonstrate the usefulness of this electrode layer material for high-temperature growth of ferroelectric BaTiO₃films¹and photocatalytic Rh:SrTiO₃²films.
The Sr₂RuO₄electrode films were prepared on BHF-treated SrTiO₃(001) substrates by pulsed laser deposition. Atomic force microscopy revealed atomically smooth surfaces for 20-nm-thick Sr₂RuO₄films. To investigate the thermal stability, ferroelectric BaTiO₃thin films were deposited at 700-1000^oC on the Sr₂RuO₄electrode layer. Pyroelectric hysteresis loop measurements were used to verify that the BaTiO₃films grown on Sr₂RuO₄electrodes were ferroelectric, implying that the Sr₂RuO₄electrode layers were thermally stable even during high-temperature deposition at 1000°C in 100 mTorr of oxygen. The wide oxygen pressure window of Sr₂RuO₄electrode was investigated by the deposition of Rh:SrTiO₃thin films for photoelectrochemical water splitting electrodes. At growth temperature of around 800°C in 10^-6Torr of oxygen, the Sr₂RuO₄electrode formed an atomically sharp interface with the film even at high growth temperatures and low oxygen pressures, yielding atomically flat Rh:SrTiO₃photocathode films.

1. R. Takahashi et al, ACS Appl. Mater. Interfaces 9, 21314 (2017).
2. S. Kawasaki et al, Appl. Phys. Lett. 101, 033910 (2012)

With the advancement catered by the use of lead-free piezoelectric nanogenerators (PENGs) for flexible electronics in energy harvesting, the challenge is to design an efficient system which is high power-producing. For this study, structural engineering approach was implemented to improve the electromechanical response of PENGs. Effects of varying patterns of the one-dimensional, vertically-arrayed BaTiO₃ nanotubes used in PENG devices, theoretically baselined with concepts on pile patterning and geometries in building foundations, on their output power were observed. Different patterns of vertically-arrayed, tetragonal phase BaTiO₃ nanotubes were synthesized via in situ conversion of selectively-anodized TiO₂ nanotubes on Ti substrates using hydrothermal process. Selective anodization which established the patterning of the BaTiO₃ was achieved through photolithography using a negative photoresist dry mask. The patterns of two sets vary in the diameter (1 mm and 1.5 mm), and the arrangement (linear and staggered arrays) of the circles printed on the mask. The methodology produced highly crystalline BaTiO₃ nanotubes based on the obtained X-ray diffractogram and EDX analysis. SEM images showed that the synthesized nanotubes had an average length of 66 μm and inner diameter of 67 nm. In addition to this, the study established that selective anodization using photoresist dry film mask can be utilized in creating patterned BaTiO₃ without significant loss in accuracy of pattern. Using this material, PENG devices were fabricated. The PENGs comprised a sandwich structure of Ti- BaTiO₃ nanotube-graphite-Ti and were further made flexible by encapsulating the structure with polydimethylsiloxane. The cantilever-type PENG devices were subjected to repeated bending stresses using a rotating motor to determine the effect of different BaTiO₃ patterns on the output voltages of the devices under constant cyclical stress. It was observed that pile characteristics such as pile diameter, pile arrangement, and pile spacing which was brought about by the varied diameter and arrangement parameters, affect the output voltage and voltage behavior of the PENG devices. Decrease in both BaTiO₃ nanotube array spacing and pattern diameter, increases the lateral displacement of the piezoelectric material and decreases the pile stiffness, respectively; all conditions consequently leading to an increase in the output voltage of the device. It was observed that the voltage behavior is dependent on the pile-matrix-pile interaction which is affected largely by adjacent pile spacing. Furthermore, the piezoelectric test showed that the highest peak to peak output voltage generated by the unpoled devices reached up to 1.9 V using the pattern with linear arrays of smaller circle diameter. The research, overall, is majorly a proof of concept study wherein the aim was to see the effect of patterning the piezoelectric material on the output voltage values of the fabricated PENG devices.

We synthesized modified Barium zirconate titanate electro ceramics by a conventional solid-state reaction method with stoichiometric formula Ba_1-xLa_2x/3Zr_0.30Ti_(0.70-3y/4) Fe_y O₃ (BLZTF), where y = 0.01 & 0.0≤ x≤ 0.06 & investigated its structural, microstructural, dielectric, electrical, ferroelectric and magnetic properties. X-ray diffractometry was used to probe the phase purity and to derive the crystallographic parameters. A uniform distribution of grains on the surface of the sample was observed from scanning electron micrographs (SEM) recorded on pellets. The stoichiometry of the chemical compositions was examined using energy dispersive x-ray (EDS) analysis method. We carried out dielectric measurements on Ag/PLZTS/Ag metal-ferroelectric-metal capacitors using impedance analyzer as a function of temperature (100-600 K) and frequency (10²-10⁶ Hz). We observed enhanced dielectric constant in doped BZT. The room temperature magnetic measurements (M-H) were obtained using a vibrating sample magnetometer. Additionally, we observed thin PE hysteresis loop, suggesting that synthesized materials is relaxor multiferroics and promising materials for high energy storage applications.

Spin capacitors have the potential to store both the electronic charge and magnetic spin that can produce conventional electric current and spin polarized current. The time evolution of spin polarized electrons injected into the piezoelectric material can be used for accurate sensing of magnetoelectric fields. To further study the application of multiferroic spin capacitors for future use in memory applications, Ferromagnetic/Ferroelectric/Ferromagnetic tri-layer artificial multiferroelectric structures in spin capacitor configuration were fabricated by sputtering ferromagnetic Nickel (Ni) electrodes on lead zirconate titan