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.