Polar oxides: synthesis, science and applications

Resume : The present study concerns the microwave dielectric characterization of new layered-type perovskite ferroelectric materials in the form of dense ceramics. They correspond to (Sr2Ta2O7)100-x(La2Ti2O7)x solid solutions with compositions in the range x = 0 - 5, tailored to attain Curie temperatures (TC) close to ambient temperature. At 10 kHz and ambient temperature, dielectric measurements, by means of a standard metal-insulator-metal structure and RLC equipment, show permittivities in the range ’ = 110 - 375. P-E measurements pointed out a ferroelectric behavior for composition in the range x = 1.65 – 3. At high frequencies, dielectric measurements were performed using a dielectric coaxial probe and a cylindrical resonant cavity. These measurements exhibit nearly the same evolution of the permittivity than the one observed at low frequencies, with a maximum attained for the composition x = 2. Low dielectric losses are observed on these materials; they are in the range 5.10-3 to 7.10-2. In both frequency ranges, losses are shown to increase when the materials are in their ferroelectric state at ambient temperature. These results are promising and point a potential use of the STLTO ceramics in microwave antennas, for example as dielectric resonator antenna, for compositions x ≤ 1 where moderate permittivities (~ 100) and very low losses (< 8.10-3) have been measured.

Resume : The bulk photovoltaic effect arising from shift-current mechanism in ferroelectric oxides has attracted immense attention ever since strategies to overcome the biggest hurdle-– inherent high bandgap – have been employed in [KNbO3]1-x [BaNi1/2Nb1/2O3-δ]x [1]. Ab intio studies conducted with PbTiO3 system suggest coupling between cation and oxygen vacancy essential for this purpose. One route to achieve and stabilize such coupling involves nanolayering mediated by cation ordering which poses a severe challenge for experimentalists. [2] Inspired by this challenge and the preceding work, we deposited single crystalline thin films of PbTiO3 doped with Bi, Ni, Nb epitaxially on single crystal SrTiO3, LaAlO3 and scandate substrates by pulsed laser deposition using a single ceramic target. The structural properties of the resulting films were investigated by high-resolution X-ray diffraction and transmission electron microscopy. Besides high degrees of phase purity and crystallinity, a long range structural ordering along the [001]-direction was observed, corresponding to a self-assembled layered superstructure with a period length of a few nanometers. The photoexcitation processes in these novel structures were investigated by conducting transport measurements over a broad range of temperature and in different charge collection geometries. [1]: Grinberg, I. et al. (2013). Nature, 503(7477), 509–512 [2]: Wang, F. et al. (2016). Nature Communications, 7, 10419

Resume : Quantum criticality [1] occurs in SrTiO3, where the quantumly driven ferroelectric transition would be at 0 K. We aim to expand quantum criticality into new crystal structures. K3Li2Ta5O15 (KLT) with stuffed TTB structure, has a Curie temperature of 7 K [2]. Literature reports [2,3] both orthorhombic and tetragonal space groups. We show that increasing the K and Li when synthesising, enlarges the orthorhombic distortion as K and Li are volatility at high temperature. Newer neutron data shows extra peaks, possibly supercells, in 3 orthorhombic and 1 tetragonal samples. We reported [4] that “stoichiometric” orthorhombic KLT has relaxor type behaviour with 2 peaks below 110 K. Extrapolation with the Vogel-Fulcher obtains some freezing temperatures below 0 K. This suggests a degenerate glassy ground state exists at 0 K, required for quantum criticality. Also, other techniques suggest multiple possible transitions that will be described. Pyrochlore, Pb2Nb2O7 is controversial [4] with a possible antiferroelectric transition around 14 K, but no specific heat anomaly. From literature, we suggest [4] that relaxor type behaviour exists which may freeze below 0 K as required for quantum criticality. We will report experiments to investigate whether quantum criticality occurs. [1] S. E. Rowley et al., Nat. Phys. 10, 367 (2014). [2] T. Fukuda, Jpn. J. Appl. Phys. 9, 599 (1970). [3] M.-S. Kim et al., Integr. Ferroelectr. 69, 11 (2005). [4] R. M. Smith et al., Phys. Rev. Mater. 2, 084409 (2018).

Resume : Recently Zubko et al.1 reported a negative capacitance (NC) effect in PbTiO3/SrTiO3 superlattices. This NC was shown to be strongly related to 180° stripe domain structure produced to minimize the electrostatic energy, arising from the depolarization field. It is gaining a great interest since it could provide a way to reduce the power consumption of field-effect transistors. Here we evidence for the first time the 180° stripe domains on a series of Pb(Zr0.2Ti0.8)O3 /SrTiO3 (PZT/STO) superlattices by X-ray diffraction. Compared to PbTiO3, leakage currents are less problematic in PZT. The electrostatic effect is confirmed by a noticeable reduction of the Curie temperature. The temperature dependent dielectric susceptibility measurements, between 300K and 600K, give the indication that we are close to the NC threshold. These experimental results are analyzed in a Ginzburg-Landau framework. 1 P. Zubko et al., Nature, 2016, 534, 524–528.

Resume : The development of spherical aberration correctors for electromagnetic established a major improvement in the new generation of electron microscopes. The growing desire to control materials at an atomic level requires the capability to image and analyse material with atomic resolution. This work focus on the application of state of the art electron microscopy to ferroelectric oxide materials. Using aberration-corrected scanning transmission electron microscopy (STEM), we analysed in detail the domain structure of PbTiO3/(La,Sr)MnO3 ferroelectric capacitors with ultra-thin active ferroelectric layers. Annular Bright Field (ABF) imaging was used to directly visualise the both heavy and light elements, measuring the relative displacement and dipole distribution unit cell by unit cell. For a given system with a set lattice strain, the depolarization field increases with as film thickness decreases. Polarisation maps of different PbTiO3 thin films revealed a clear influence of the asymmetric screening of the depolarizing field on the equilibrium domain pattern. The dipole distribution reveals the evolution from conventional 180° Kittel type domains to flux closure and vortex type domain configurations with reduced film thickness. Additionally, effects such as polarisation and octahedral tilt suppression can be observed immediately adjacent to the interface. We show how the different polarisation orientations interact with the LSMO at the interface. This provides insight into how such devices may be designed and tuned to achieve the desired performance.

Resume : The lead-free piezoelectric materials are intensively investigated for their great interests for many applications. For a few years now, the KxNa1-xNbO3 (KNN) perovskite oxide has been presented as the alternative for the largely used PbxZr1-xTiO3 (PZT). Its high dielectric permittivity that can be driven by an external DC electric field combined with a high Curie temperature (near 400°C) makes it a promising candidate for miniaturized devices. In this work, KNN thin films have been deposited by pulsed laser deposition on R-cut sapphire and (100) MgO substrates. Both substrates exhibit closed dielectric properties suitable for microwave applications. Whereas sapphire has a great chemical stability and mechanical properties making it easier to insert into a device, MgO presents a better lattice mismatch with KNN (close to 5%) favorable for thin films orientation. The influence of the substrate on the structural properties and therefore on the dielectric behavior has been investigated, in particular by the use of in- and out-of-plane X-ray diffraction. Dielectric properties of the thin films have been measured at microwaves from coplanar waveguide devices (stub resonators and transmission lines). The results have been compared with those retrieved from a TE modes cavity method using a resonant system at 12 GHz. A correlation between the dielectric and the structural properties of the thin films has been established.

Resume : BaTiO3-CaTiO3-BaZrO3 (BCTZ) ternary phase diagram displays as bulk ceramics a rich variety of classical-to-relaxor ferroelectric behaviors along with a Phase Convergence Region between polar phases (PCR) that lead to enhanced polarizability dependence to external stresses. In this work, the attention is focused on the tunable properties of BCTZ films, i.e. the control their dielectric permittivity by external electrical field. First, BCTZ phase diagram is explored over a wide range of compositions using Combinatorial Pulsed Laser Deposition (CPLD), allowing the deposition of gradients of composition on a single sample. A series of 8 samples were analyzed, each corresponding to one line in the ternary phase diagram and containing 480 capacitors (40 different compositions and 12 capacitors per composition). Room temperature dielectric analyses revealed a Zone Of Interest (ZOI) for displaying the best compromise between high tunability and low dielectric loss. Second, thin films with various compositions crossing the ZOI along the line Ba1-xCaxTi1-xZrxO3 (0  x  0.18) were deposited by RF magnetron co-sputtering. Their dielectric properties, measured as a function of temperature and frequency, showed a continuous crossover from classical-to-relaxor ferroelectric behavior as x increases. A PCR was evidenced for compositions close to x = 0.12. At this point, all of the dielectric parameters underwent specific variations with extreme dielectric non-linearities.

Resume : The integration of single crystalline lead-free piezo-(ferro-)electric barium titanate (BaTiO3, BTO) films on semiconductor substrates (Si or Ge) is of great interests for the achievement of high performance “on-chip” functional devices like actuators, sensors and the micro-electro-mechanical systems and optical modulators etc.. The lattice constants of BTO and Ge are almost entirely matched and thus Ge represents a good substrate to realize high quality BTO films. However, such BTO/Ge integration is quite complex and generally includes the Ge surface passivation (using Ba) and the subtle interface control in a strict ultra-high vacuum ambiance (>10-8 Torr) due to the facile oxidation of Ge surfaces. Meanwhile, future society featuring “internet of things” requires more and more wearable devices and thus high quality BTO films grown on flexible substrates attract increasing attention. Nevertheless, melting temperatures of flexible substrates are normally much lower than the crystalline temperature (~600-700°C) for high quality epitaxial BTO films. In this report, we will demonstrate a “proof of concept” study for the remote epitaxy of single crystalline BTO on Ge by simply using a graphene monolayer as an intermediate buffer. Such idea of remote epitaxy was shown recently by Kim et al. for semiconductor hetersystems[1-3] and we clearly show here that it can be achieved as well in even highly heterogeneous epitaxial systems of complex oxides on semiconductors. It will be shown that the crystallization of the BTO film can be realized due to the impact of the chemical potential of the underlying Ge substrate, even with a distance of several angstrom created by the graphene monolayer. The growth mode, interface, crystallization and piezoelectric properties of BTO films with difference thicknesses on graphene/Ge have been studied. More interestingly, we will show that, using a Cr stressor layer, the BTO film can be easily exfoliated and transferred onto arbitrary substrates including flexible substrates, thanks to the weak van der Waals force between graphene and the substrate. Our results show a novel and simple approach to achieve single crystalline BTO on Ge and open a pathway, using remote epitaxy, to realize high quality epitaxial BTO films on arbitrary substrates, which are of great significance for wearable devices for the future “internet of things” society.

Resume : Spatial inhomogeneity in composition, defect distribution, and strain, which is found in various functional oxides, can cause modulation of the local electronic structure as well as crystal structure of materials and induces many intriguing phenomena. The appearance of morphotropic phase boundary and relaxor phase in perovskite-type ferroelectric system are typical examples of using such spatial inhomogeneity for inducing improved functionality in the system. In other cases, degradation of the material’s functionality is also caused by such inhomogeneity. Oxide thin film systems have attracted much attention because epitaxial strain induced by substrate can modulate the material’s functional properties. In this study, we exploit coupled ferroelectric capacitors made of epitaxial PZT thin film to model a ferroelectric system having a spatial inhomogeneity, which corresponds to a simple macroscopic analogy to the microscopic spatial inhomogeneity. Each capacitor in the coupled-capacitors had a self-polarization, showing strongly asymmetric polarization-voltage hysteresis loop shifted toward positive voltage, i.e. the existence of built-in internal field to out-of-plane direction of the film. Self-polarization of epitaxial PZT film should be induced by either interface effect or flexoelectric effect in the film. The polarization switching characteristics of the coupled-capacitors in series connection showed symmetric hysteresis loop with the same remnant polarization as that of a single capacitor. In the coupled-capacitors system, fatigue and rejuvenation characteristics could be easily and clearly demonstrated by applying different voltages. The switching characteristics of the coupled-capacitors can be explained by an instantaneous field transfer to each capacitor at different switching cycles. The coupled-capacitor system provides an intuitive, simple, and practical model of ferroelectric fatigue and rejuvenation process, which might be important for the performance improvement of ferroelectric non-volatile memory devices.

Resume : Piezoelectric thin films for microsystems have recently met some noticeable industrial success such as AlN for acoustic RF filters and Pb(Zr,Ti)O¬3¬ (PZT) for inkjet print-heads actuators. In the case of perovskite oxides such as PZT, the sol gel technique has become the standard deposition means thanks to the high quality of the grown films on silicon. This approach gives nowadays the opportunity to think of deposition techniques that are more versatile such as inkjet printing (IJP). This leads to low cost and reliable fabrication techniques with which neither lithography nor etching are required anymore. Moreover, one can think of printing piezoelectric thin films on alternative substrates such as steel, glass or even polymers. At LIST, we study Chemical Solution Deposition (CSD) for functional oxides and more specifically for piezoelectric films. In this talk, I will present our latest advances regarding CSD and IJP deposition of piezoelectric films on silicon and glass substrates [Godard et al., Adv. Mat.Technol, 1800168 (2018)]. The key parameters of CSD and IJP will be discussed together with a comparison of the piezoelectric properties films on silicon and glass. Finally, two applications will be detailed, namely 1) an all-printed energy harvester and 2) fully transparent ultrasonic transducers, embedding transparent electrodes.

Resume : Epitaxial PbZr0.2Ti0.8O3 capacitors were manufactured on single crystal SrTiO3 substrates with bottom and top SrRuO3 electrodes. Electrical measurements, namely polarization-voltage (P-V or hysteresis) and capacitance-voltage (C-V), were performed to evidence ferroelectricity. A modified C-V measurements was also developed to obtain the capacitance value in static conditions, with pre-set polarization state. The main conclusions of this study are: the experimental P-V and C-V characteristics are not in agreement with the theory predicting that the second is the derivative of the first one; the static C-V measurements reveal the fact that, after polarization switching and saturation, the capacitance value is practically constant, with negligible dependence on the applied voltage; the capacitance value calculated from static C-V at different thicknesses of the PZT layer extrapolates towards optical dielectric constant at thickness limit for stable ferroelectricity (about 1.2 nm); the derivative of P-V loop reveal the presence of a negative capacitance effect in the voltage range around coercive voltage, thus around zero value of polarization. It is suggested that, in almost perfect epitaxial PZT films, as it is the case for very thin films, the volume is completely depleted of free carriers, all being attracted to the electrode interfaces to compensate polarization charges. This may explain the low value of dielectric constant and the negative capacitance effect around P=0.

Resume : Thin films of Pb(Mg1/3Nb2/3)O3?PbTiO3 (PMN-PT) have been shown to exhibit attractive properties, which can be exploited for energy harvesting (EH), energy storage and electrocalorics. While most of the research and applications of thin-film piezoelectric EHs are based on the d31 mode, the d33 mode offers a higher potential figure of merit. We found that PMN-PT grown directly on SrTiO3 (STO) is extremely sensitive to the deposition conditions, easily leading to the nucleation of the undesired Pb-deficient pyrochlore phase. Contrarily, the window for growing phase-pure PMN-PT on STO, covered with a LaNiO3 bottom electrode, is considerably broader. In order to find the origin of the phase stabilization, we grew PMN-PT on other buffer layers such as SrO, SrRuO3 and PbZr0.52Ti0.48. The main reason for the enhanced stability of the perovskite phase was found to be the increased interface roughness, which enables stronger binding of Pb-based species. Thus, we modified the surface of the STO substrates by depositing a homoepitaxial layer prepared with a specifically designed process that ensures the growth of a stoichiometric and simultaneously rough STO layer. This led to a significantly improved phase purity of PMN-PT on STO, which represents an important step in the fabrication of EHs that operate in the d33 mode. Additionally, applying the same principle, other rough buffer layers can be designed to improve the integration of different Pb-based functional oxides on Si.

Resume : The perovskite KxNa1-xNbO3 lead-free material is ever more investigated for its promising piezoelectric and ferroelectric properties. In the same K-Na-Nb-O (KNN) system, a tetragonal tungsten bronze (TTB) phase was reported but never fully investigated until now. This type of structure is known to exhibit piezoelectric properties in other systems such as Li-Na-Nb-O chemically close to KNN. However the synthesis and the identification of pure TTB phase are sensitive due to growth competition with numerous phases in these chemical systems. Therefore, the growth conditions using the pulsed laser deposition were optimized to obtain pure TTB KNN thin films. To identify the TTB phase with certitude, X-ray diffraction and electron transmission microscopy with electron diffraction techniques have been used. Raman spectroscopy as well as Energy Dispersion X-ray Spectroscopy and Scanning Electron Microscopy were also carried out to properly characterize the thin films. Pure TTB thin films have been deposited on (100) and (110) SrTiO3 substrates in order to investigate the oriented growth of the distinctive TTB nanorods. The low frequency (1 kHz - 1 MHz) dielectric characteristics (permittivity and loss tangent) have been retrieved from a thin film grown on (111)Pt/(001)Si substrate, whereas a thin film on C-cut sapphire substrate was used for measurements and characterizations at microwaves (1GHz - 40 GHz). A quite high dielectric permittivity value between 80 and 150 was measured.

Resume : Research on novel lead-free pyroelectric thin films is triggered by their promising integrability in CMOS processes and application in environmental friendly infra-red sensors. High precision characterization of such materials is a key challenge in development of new lead-free pyroelectric materials. In this study, we report on the characterization of pyroelectric thin films and piezo-composites, in particular analysis of the pyroelectric response and micro- and nanoscale thermal analysis, by using an advanced laser-based stimulation technique. We identified wide band gap semiconductor thin films fabricated by PVD and CSD processes, including Aluminum nitride and Scandium doped Aluminum nitride, as model materials for pyroelectric sensors due to their excellent dielectric properties and CMOS compatibility. By high frequency stimulation of a quantum cascade infrared-laser with selectable wavelength we measured through thickness polarization and thermal flux of exposed and integrated samples similar to the Laser Intensity Modulation Method. Parasitic piezoelectric contributions are accounted and modeled. These results are complemented by thermal simulation, and microscopic and spectroscopic techniques for fundamental thermal analysis down to the nanoscale. Our novel laser-based measurement system is an essential step forward for precise material characterization in research, and quality control on industrial scales for optimized device reliability.

Resume : The study of II-VI semiconductor has attracted much attention due to their potential applications in several fields. They are excellent candidates for optoelectronic devices due to their large direct band gap, chemical stability and large exciton binding energy. Among the II-VI semiconductors, ZnO is the most studied and applied in the fabrication of devices. The doping of ZnO with Yttrium has extended its use in gas sensors, solar cells or LEDs, as this dopant improves the optical and electrical properties of the zinc oxide. In this work, the material has been studied in two cases. Y-doped ZnO ceramics have been sintered, using a mixture of ZnO or ZnS and Y2O3 as precursor. Besides, doped micro and nanostructures have been grown using a vapor-solid method, using ZnS and Y2O3 as precursor. In both cases, the starting Yttrium content ranges from 0.4 to 3.6 at.%. Different characterization techniques have been applied to understand the differences between the ceramics and nanostructures properties. The characterization of the Y-doped ZnO has been performed by scanning electron microscopy based techniques: emissive mode, cathodoluminescence (CL) and X-ray microanalysis. Also, micro-Raman and X-Ray Diffraction measurements have been made. Special attention has been focused on the influence of the dopant in the CL emission and in the morphology of the micro- and nanostructures grown. As a proof of concepts, the behaviour as a gas sensor of the treated material have been studied.

Resume : Despite their simple structure, the family of perovskite oxides present a rich range of functionalities, such as superconductivity, insulator-metal transition, ferroelectricity and ferromagnetism. The rich functionality is often closely coupled to the rotation of corner-connected oxygen octahedron; the angle and pattern of oxygen octahedral rotations (OORs) have a direct influence on electrical conductivity, magnetic superexchange interaction, dielectric properties and so on. While this makes the OOR act as a fundamental parameter for understanding functional perovskites, most perovskite bulks adopt the orthorhombic (Pnma) structure, corresponding to the a–a–c+ OOR pattern in Glazer notation. The predominance of the a–a–c+ OOR pattern prevents full utilization of functional perovskites, but it also implies a potential tunability of physical phenomena and functions via OOR pattern engineering in perovskites. In particular, the most common a–a–c+ OOR pattern has turned out to be competitive with ferroelectric distortion. This situation has motivated recent experiments to control the OOR, e.g., via artificial heteroepitaxy. In this talk, I will introduce our recent effort on predictive design and stabilization of a functional OOR pattern, distinct from the original a–a–c+ pattern and cooperative with ferroelectricity. Our work reveals hidden but functional OOR patterns, and will open avenues for designing novel multifunctional perovskites.

Resume : Several studies have demonstrated the role of the electrode interface for the resultant photovoltaic effect in ferroelectric materials.[1] This effect has been utilized to characterize the out of plane polarization.[2] In this work, however, the focus is on the bulk photovoltaic (BPV) effect which is, alike ferroelectricity, a symmetry driven property. The photoresponse of bismuth ferrite thin films with different domain configurations were investigated using an in-plane electrode configuration. The BPV characteristics, viz. short circuit current as a function of the orientation of the linearly polarized light, are surprisingly sensitive to the change of the domain configuration after applying electric fields. This can be attributed to the analogous form of the BPV and piezoelectric tensor. Analysis with previously developed mathematical relations manifested in a model that is capable of estimating the proportion of switched and un-switched regions. The results unravel the potential utility of BPV effect as means to trace the rotation of polarization vectors in areas much larger than can be accommodated in probe-based techniques like piezo-force microscopy.[3] References: [1] Pintilie et al., Journal of Applied Physics 2007, 101, 64109. [2] Yi et al., Advanced materials 2011, 23, 3403. [3] Knoche et al., submitted 2019, adma.201900312.

Resume : Exotic polarization states have been experimentally observed at the nanoscale recently [1]. Some have been suggested as candidates for its exploitation in memory devices, that would be able to achieve a much larger information density storage. Here we report the experimental observation of rotational polarization nanotopologies in a set of n-BaTiO3/n-SrTiO3 superlattices, by using Aberration-Corrected Scanning Transmission Electron Microscopy (STEM). STEM allows imaging with sub-Angstrom resolution in real space, making possible to map the polar distortions with atomic resolution. More specifically, we observe how the collective arrangement of dipoles evolves from the standard polarization monodomain state for short period superlattices (periods of n=2 and n=4 unit cells), to the exotic nanodomains for the longest period superlattice (n=10 unit cells) [2]. The nanodomains reveal different kinds of topologies, such as continuous flux closure rotations, with pairs of vortices, and others topologies like disclinations, not observed before in BaTiO3. We compare it to similar topologies recently observed in PbTiO3/SrTiO3 SPLs [3] and discuss how the different balance of energies coming from the electrostatic conditions imposed by the SPLs period may lead to the formation of the different polar configurations. [1] Yadav et al., Nature 530, 198 (2016) [2] Khestanova et al., Ad. Funct. Mater. 26, 6446 (2016) [3] Lu Lu et al., Physical Review Letters, 177601 (2018)

Resume : In order to fulfil the promise of plasmonics, it has been viewed that one of the most disruptive technologies will be the “all dielectric plasmonics”. This, by itself, contradicts the definition of plasmonics (interaction of light with collective charge oscillations at metallic nanostructures). However, some interesting approaches can be exploited. In that respect, polar dielectrics can be used to couple an electromagnetic field to collective lattice oscillations, namely optical phonons. Those polar crystals can support optical modes that are confined either to the surface of the material or can be highly confined in the vicinity of sub-wavelength geometries, resulting in strong field enhancement. Similar to their metallic counterparts these oscillations are only supported when the real permittivity is negative and in the case of the polar dielectrics this happens at the so-called Reststrahlen band. Naturally, the extent of this band defines the spectral range of operation. In this work we focus our attention to a very important technological polar dielectric perovskite, namely SrTiO3, which has been failed to be spotted as an active nano-photonic element. We present its exceptional capabilities, in terms of its bulk optical constants, which we have measured with an IR Spectroscopic Ellipsometer and which we compare with the most prominent polar dielectrics. We then calculate, the localised modes on subwavelength structures and we: (i) clarify the fundamental nature of the modes and (ii) we demonstrate the extraordinary tunability, that SrTiO3 offers, up to the THz regime.

Resume : KxNa1-xNbO3 is a promising material for, e.g., surface acoustic wave (SAW) sensors based on ferroelectric thin films. For these systems a strong enhancement of SAW transmission coefficients in the vicinity of phase transition temperatures has been reported, recently.[1] In order to exploit such elevated piezoelectric properties, phase transitions have to be deliberately shifted to the operating temperatures of intended applications. In this regard, the understanding of the relationship between phase formation, transition temperature and incorporated lattice strain is one key to optimize functional material properties. Here, we present a study of phase transitions in compressively strained KxNa1-xNbO3 thin films grown by liquid-delivery metal-organic vapor phase epitaxy (MOVPE). We have identified a transition from the ferroelectric monoclinic MC phase to a ferroelectric, orthorhombic c phase. Furthermore, our results reveal a continuous shift of this transition by up to 400 K upon variation of strain in the range of -0.9% to 0%, which is altered by choice of different (110) oriented rare-earth scandate substrates and variation of the potassium to sodium ratio in the film. The phase transition was investigated in detail by temperature dependent piezoresponse force microscopy (PFM), X-ray diffraction (XRD) and laser interferometry. [1] S. Liang et al., Appl. Phys. Lett 113, 052901, (2018).

Resume : The abnormal photovoltaic effect in ferroelectrics, known as bulk photovoltaic effect (BPVE), has attracted immense attention. The origin of BPVE is related to the non-centrosymmetric structure of ferroelectrics, and exhibits open-circuit voltages that are larger than the band gap with light polarization dependent short-circuit current. Another interesting aspect is the potential to enhance the polarization, indicative of stronger ferroelectric character, by incorporating the ferroelectric within oxide superlattice structures [1]. Superlattices comprising of few unit cell layers of ferroelectric sandwiched between layers of paraelectric materials have been explored for this purpose. However, there are only few reports about the photovoltaic effect in superlattice structure. In this work, BaTiO3/CaTiO3/SrTiO3 superlattice structures were fabricated on STO (001) substrate with different number of unit cells. High crystallinity and abrupt interfaces were confirmed by X-ray diffraction and transmission electron microcopy. We investigated the ferroelectricity in the superlattice structures, and analyzed the results with single crystalline BaTiO3 thin films. The photovoltaic response was observed as a function of temperature, wavelength and light polarization.

Resume : Pb(Zr,Ti)O3 (PZT) is the most widely used ferroelectric and piezoelectric material and finds application in a variety of sensors and actuators. As a result, several studies have been dedicated to understand and manipulate the associated ferroelectric character. However, most of the pathways that have been explored in this regard, such as post-growth cooling regimes, strain control of a/c domain population and purity starting materials, can also have a direct impact on the photoelectronic processes. Here we show that by tuning the growth related parameters, it is possible to control the resulting photo-electronic and sub-bandgap properties of Pb(Zr0.2Ti0.8)O3 thin films. X-ray diffraction (XRD) results helps us in understanding the difference in the crystal structure, obtained by controlling growth related aspects. Piezo-response force microscopy (PFM) helps in characterizing the domain structure of Pb(Zr0.2Ti0.8)O3 thin films. Temperature resolved transport measurements help in understanding the existence of sub-bandgap levels and associated activation energies. The role of embedded domain walls (90 and 180o domain walls) on the resulting photo-electronic properties will be discussed. Eventually, efforts will be made to investigate the prerequisite conditions for the manifestation of photovoltaic effect in these films.

Resume : Ferroelectrics materials are intensely used for multiple applications. From this class of materials, Pb(ZrxTi1-x)O3 (PZT) is one of the most studied. Following the tendency of miniaturization of technologies, these materials are studied in thin film structures. The high quality epitaxial thin films structures are used for miniaturization of different extrinsic contribution. Usually, epitaxial PZT thin films are obtained by deposition on perovskites single crystal substrates with similar lattice in-plane parameters as for example SrTiO3 (STO). The integration of epitaxial PZT in semiconductor technology for different applications implies building of the ferroelectric structure on Si substrate. In these studies, we evaluate the ferroelectric and dielectric properties of thin films of PZT, epitaxial deposited by pulsed laser deposition (PLD) on Si substrate with STO buffer layer and SRO bottom and top electrodes. These properties are compared with the ones of similar ferroelectric structure deposited on STO substrate and the results are collaborated with different structural properties resulting from X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, a thin film multilayered structure based on PZT/STO/PZT deposited on Si is study for obtaining multiple polarization states. The multi-layered structure deposited on Si presents a fast response, with the possibility of being operational at higher frequencies and at lower voltages.

Resume : BaTiO3 thin films (60 nm-thick) grown on SrTiO3/Si templates have been characterized for their structural and electrical properties. The epitaxy of the BTO film on silicon was confirmed by X-ray diffraction with a good crystallinity. The temperature-dependance structural properties were checked by infrared spectroscopy in the absorption mode. The films were found to remain in a single ferroelectric phase over a temperature range from 5 to 385 K. Low-temperature orthorhombic-rhombohedral phase transitions characteristic of bulk BaTiO3 are absent in the films due to the clamping effect from the Si substrate. The electrical characterizations of the BaTiO3 films using MFIS structures show that samples shows a memory window with hysteresis loops which thus enable BaTiO3 possibly being applied to the non-volatile memory application.

Resume : The properties of BaTiO3 (BTO) thin films deposited on different substrates by RF magnetron sputtering were investigated. Two representative substrates were selected and different heterostructures were studied. (i) SrTiO3 (STO) single crystals as a bulk oxide reference material, and (ii) silicon as a semiconductor. SrRuO3 (SRO) and Pt bottom electrodes were deposited on the silicon substrate. The BTO structural characterizations show that the films have (001) crystallographic orientation. We have compared the electrical properties of the different BTO layers and the same dielectric constant and polarization values were obtained whatever the substrate.

Resume : BaTiO3 (BTO) was the first ferroelectric material wherein abnormal photovoltaic effect was observed. Subsequent investigations studied the relation between photoresponse, symmetry and ferroelectric character, which led to the term bulk photovoltaic effect. Doping in BTO has been known to affect the ferroelectricity and symmetry. However, its impact on the photoreponse and conductivity is still under deliberation. In this work, BTO thin films with different percent of lanthanum were fabricated by pulsed Laser Deposition (PLD) and characterized by piezo force microscopy (PFM) and X-Ray Diffraction (XRD). Electrical measurements were conducted in vertical and planar geometries. Properties such as photo conductivity and short circuit current were measured and analyzed in respect to La-doping. In conjunction, the states within the band gap were investigated by temperature dependent measurements. The intricate co-relations highlight the impact of doping on the photo electronic processes.

Resume : HfO2-based materials have already been used in the high-k metal gate(HKMG) technology [1]. The ferroelectricity of HfO2 materials was first reported in 2011 by doping ZrO2 into the HfO2 films [2]. Molybdenum disulfide (MoS2) have gained substantial attention because of their high carrier mobility and optical properties [3,4]. In this study, we fabricated back-gate field-effect transistors (FETs) which exhibited excellent performance in ferroelectric polarization switching with high ON/OFF ratio and little degradation in its retention properties as measured up to 5000 s. Our research suggests MoS2- HZO ferroelectric field effect transistors can be very promising alternative for next generation nonvolatile memories. Keywords: Molybdenum disulfide, Ferroelectric Field-Effect Transistor References: [1] T. S. Böscke, et al, International Electron Devices Meeting, 24 (2011) 5.1-5.4. [2] J. Müller, et al, Nano Letters, 12 (2012) 4318-4323. [3] B. Radisavljevic, et al, Nature Nanotechnology, 6 (2011),147-150. [4] Y. Yoon, et al, Nano Letters, 11 (2011) 3768-3773.

Resume : Strontium titanate SrTiO3 is considered to be incipient ferroelectric due to the dielectric plateau occurrence in the law-temperature region. Quantum paraelectric SrTiO3 can be passed into the proper ferroelectric state via Sr-substitution by isovalent ions resulting to the suppression of the abnormal Ti-quantum motion. Hence, the ferroelectric state arises at nonzero temperatures. In the present work, we performed Raman study of the Pb-doped SrTiO3 ceramics in wide temperature range 12-300 K with the aim to understand the effect of the dopant concentration on the ferroelectric phase formation. Strontium titanate ceramics doped by Pb (x = 0, 1, 2, 3, 4, 10 %) were synthesized by the solid state method using stoichiometric mixture of SrO2, TiO2 and PbO. At room temperature the Raman spectra of all samples consist of only second-order bands caused by phonon coupling. On cooling narrow lines at ~170 and ~540 cm-1 appear on the wide complex background. Note, that their frequencies do not depend on temperature. The extrapolation of the temperature dependence of the intensity of these modes allows to suppose that the increasing of the dopant concentration leads to the phase transition temperature rising. The Raman spectra law-frequency region of the Pb-doped SrTiO3 is of particular interest. On cooling, the two narrow lines appear at low frequency range and their frequencies dramatically change with temperature variation. Emergence of these modes may be attributed to ferroelectric phase formation in doped quantum paraelectric SrTiO3.

Resume : We study the local crystallization produced by focused irradiation of silica based glasses with femtosecond lasers. We have pointed out that the following glass 33Li2O-33Nb2O5-34SiO2 exhibits the potentiality to precipitate LiNbO3 crystals that are optically non-linear active (e.g. second harmonic generation), when the laser pulse repetition rate is above 100-200kHz. We have shown that when the crystal is nanosized, it is possible to orient them with the laser polarization. During the presentation, I will make a short review on previous published results that used CW laser and the progress introduced by using femtosecond laser. I will suggest mechanisms showing the control of the microstructure in the same time (phase separation distribution). The important fundamental aspect here is that light polarization appears to be a new “button” that we can adjust in solid phase in order to control and orient a transformation, in short the immaterial acts on the material. In the field of applications (integrated optics), the creation and control of the direction of the birefringence and of the second order optical properties open unprecedented possibilities.

Resume : Combining density functional theory and experimental investigations we elucidate a unique tetragonal ground state structure of MnTi2O4 [1]. On lowering the temperature, cubic MnTi2O4 enters into a chiral polar tetragonal structure accompanied with orbital ordering among Ti 3d orbitals. Further lowering of temperature results into a paramagnetic to ferrimagnetic transition in MnTi2O4, in which the Mn and Ti spins are antiferromagnetically coupled. We shall compare the obtained results with previous investigations on MnTi2O4, which had proposed a cubic [2], spin-singlet ground state [3] and also for another related compound, MgTi2O4, in which the ground state tetragonal structure comprises of spin-singlet dimers among Ti spins [4]. References [1] A. Rahaman et al. (Manuscript to be submitted). [2] Y. Huang et al. J. Magn. Magn. Mater. 324, 2075 (2012). [3] T. Sonehara et al. Phys. Rev. B 74, 104424 (2006). [4] M. Schmidt et al. Phys. Rev. Lett. 92, 056402 (2004).

Resume : The structure and electrical properties of BaTi0.5(Fe0.33Mo0.17)O3 ceramic prepared by the standard solid-state reaction method were investigated. The X-ray powder diffraction data for this compound were refined using the Rietveld method. It was identified as a h-BaTiO3-type hexagonal perovskite with the space group of P63/mmc. The electrical properties analysed by impedance and electrical modulus spectroscopies showed that the grains play a major role in electrical properties, compared with the grain boundaries. The co-existence of the overlapping and separation between the frequency dependence plots of the normalized imaginary part ofimpedance (Z/Zmax) and electric modulus (M/Mmax) peaks is observed.

Resume : BaTiO3 (BTO) is an important ferroelectric. It is stable, does not constitute a health hazard and relatively cheap. The tetragonal ferroelectric phase exists between -5 and 120oC. Charged surfaces are inherently unstable and induced screening, which in turn can strongly influence the FE properties[1]. Amongst the possible screening mechanisms are surface reconstruction and relaxation. Theory has shown that the BTO(001) surface is rumpled with a relative displacement of the oxygen atoms with respect to the cations[2-4]. The atomic structure is expected to be polarization dependent.Spatially resolved X-ray photoelectron diffraction (XPD) is therefore an ideal tool to probe element specific local distortions at the surface of single ferroelectric domains, with a precision of 0.02 ºA[5]. In the BTO(001) single crystal spontaneous domain ordering polarized P+ and P-, parallel and antiparallel with respect to the surface normal, gives rise to micron sized poled domains at the surface. We have made use of BTO thin film grow on a STO(001) substrate to study the the XPD signal from individual domains, with a view to identifying the polarization dependence of the atomic surface structure of BTO(001). The measurements were done under ultra high vacuum (P=2.0.10-10 mBar), in the XPD set-up of the SGM beamline at LNLS. Area averaged XPD was also acquired for comparison with the single domain XPD. The BTO(001) has a (1x1) structure (Fig 1a). XPD was then carried on the Ba 4d, Ti 2p and O 1s core levels at two distinct regions, as well as on the C1s level expected due to remnant surface contamination. The experimental results are compared with first simulations using multiple scattering calculations installed on a Linux cluster at Unicamp in Brazil. [1]G. Geneste, B. Dkhil, Phys. Rev. B 79, 235420 (2009) [2]J. Padilla, D. Vanderbilt, Phys. Rev. B 56 1625 (1998) [3]C.H. Ahn et al., Science 303, 488 (2004) [4]M. Dawber et al., Rev. Mod. Phys. 77 1083 (2005) [5]L.Despont, C. Koitzsch, F. Clerc, M.G. Garnier, P. Aebi, C. Lichtensteiger, J.-M. Triscone, F.J. Garcia de Abajo, E. Bousquet, Ph. Ghosez, Phys. Rev. B 73 094110 (2006)

Resume : Lacunar spinels combine clusters of transition metal oxides, whose orbitals are thought to hybridize strongly into molecular orbitals, that give rise to peculiar structural distortions and magnetism. Here we combine Landau theory and ab-initio calculations to study the relationship between structure and magnetism in these compounds.

Resume : HfO2 based solid state light emission devices (SSI-LEDs) with the conductive filaments (CFs) related lighting mechanism are promising candidates for future white light luminescence applications [1,2]. SSI-LEDs are simple metal-oxide-semiconductor (MOS) structure devices from which the white light can be emitted. This is related to the thermal excitation of the current flowing through conductive filaments (CFs) consisting of oxygen vacancies in HfO2. Considering its CFs-related lighting nature, a thermal emission relation (i.e. an impact by substrate temperature) of the light emission of SSI-LEDs has to be taken into account. Furthermore, the Si substrate takes a significant role in the performance of this device because Si can diffuse into the oxygen vacancies thus impacting the formation CFs. It is of interest to explore the impact of substrate patterning on the light emission performance of SSI-LEDs. In this report, we will show the temperature impact on electrical and light emission properties of SSI-LEDs. Under different substrate temperatures, the electron transportation mechanism before the formation of CFs is different. At the low substrate temperature and the low external bias, the current is small and follows the Schottky emission mechanism. With the increase of the external voltage, the electron emission follows not only the Schottky emission, but also Frenkle-Poole (F-P) emission. When the applied bias is further increased to the F-P voltage (VFP), the F-P emission is the dominating reason for the leakage current. The emission current increases with the temperature raising in the Schottky emission while get closer in the F-P emission. This is because that the Schottky emission is a thermal assisted electron emission, otherwise, the F-P emission is additionally affected by the external bias and defect density. Because the defect assisted the electron emission, a lower VFP suggests easier transportation of electrons through the vacancies. Furthermore, the turn-on voltage (VBD) is also decreased with the increase of the substrate temperature. It is attributed to the improved transportation of electrons via filaments under the high temperature. The decreases of VFP and VBD with the increasing temperature provide clear evidence for CFs assisted breakdown process. However, the working current and the lighting intensity are lower in the high substrate temperature. The explanation for this difference to former results is the resistivity of the Si wafer, which has a positive proportional relationship with the temperature ranging from RT to 250 ℃. These temperature-influenced properties provide clear evidence for the CFs assisted light emission mechanism of SSI-LEDs. By understanding the significant role of the Si substrate, one natural approach to improve the performance of the devices is to use patterned wafers to locally enhance the electrical field. We use the nano-stripe patterned Si wafer to replace the planar Si wafer as the substrate of SSI-LEDs. The geometric confinement of the electrical field at the tip of the nano-stripe electrode can be numerically simulated using Computer Simulation Technology (CST) on a 2D slice. This E enhancement attributes to the formation of CFs at the tip of the nano-stripe. By comparing the electrical and optical properties of the two devices, we found that the nano-stripe patterned substrate can lower the VBD and enhance the lighting intensity of SSI-LEDs at the same time. The lower working voltage and the brighter light mean that the nano-stripe device has a higher luminous efficiency and lower energy consumption than the conventional SSI-LEDs. In summary, we explored significant effects of the substrate temperature on the electron transportation and performances of SSI-LEDs verifying the CFs assisted current thermal excitation light emission mechanism. The temperature can also influence the device performance by changing the resistivity of the Si substrate. The nano-stripe structure patterned on the Si substrate is utilized to geometrically confine the electric field and it is proved to be an effective method to enhance the device lighting performance. Our results not only explain the light emission mechanism of SSI-LEDs, but also offer an effective solution to get a high-performance device, which is of great importance for the future applications of SSI-LED devices. [1] Y Kuo and C C Lin 2013 Ecs. Solid-State Lett. 2 Q59. [2] Y Kuo and C C Lin 2013 Appl. Phys. Lett. 102 1687.

Resume : The interest to Mn4+ activated oxides is motivated by their potential application in warm white LEDs as cost-effective alternative to rare-earth doped red phosphors. However, the ability of manganese to change charge state under interaction with intrinsic defects and impurities as well as due to incorporation in different lattice sites can limit the intensity of Mn4+ red photoluminescence (PL). Here we present the results of study of manganese charge state in Mn doped MgTiO3 phosphors synthesized by a solid-state reaction method under different MgO to TiO2 ratios. The phosphors produced under stoichiometric ratio demonstrated very weak red PL peaked at 701 nm due to spin forbidden transition of Mn4+ ions substituted Ti4+ in crystal lattice. In turn, the electron spin resonance (ESR) spectra showed intense signal ascribed to Mn2+ ions apparently located on Mg2+ site. The phosphors fabricated with an excess of MgO got noticeable pink color caused by optical absorption of Mn4+ ion and demonstrated more than ten times increased Mn4+ red PL, while the ESR spectra testified to decreased concentration of Mn2+ ions in MgTiO3 phase. It is concluded that an excess MgO stimulates substitution of Mn on Ti site and is necessary to produce efficient red phosphor.

Resume : FeRh is a martensitic alloy showing an antiferromagnetic-ferromagnetic first order phase transition near above room temperature accompanied by a large change on its lattice parameters resulting in a large magnetostrictive coefficient. Therefore, FeRh is a perfect candidate to show large magnetoelectric coupling in combination with a piezoelectric material. However, piezoelectricity can not dictate the sign/direction of magnetization in a FeRh/piezoelectric system and it is interesting to study strategies to circumvent this fundamental issue. I will show that micro and macro magnetoelectric characterization reveals that magnetic memory effects present in the antiferromagnetic to ferromagnetic transition of FeRh can help to overcome the aforementioned fundamental issue. With this purpose, we have characterized a PMN-PT/FeRh structure. First, I will show that FeRh phase transition shows thermal memory effect: After the FeRh magnetization is oriented in the ferromagnetic phase by an external magnetic field and FeRh is brought to the antiferromagnetic phase by cooling it, if one measures the orientation of the magnetization again in the ferromagnetic phase, it partially recovers its initial state. Secondly, I also show that by the application of low electric field, we can isothermally manipulate magnetization. Concomitantly, large magnetoelectric coupling is observed [1-3]. [1] J. Clarkson, I. Fina, et al., Scientific reports 7, 15460 (2017). [2] I. Fina, et al., ACS Appl. Mater. Interfaces 9, 15577 (2017). [3] I. Fina, et al., Appl. Phys. Lett. 113, 152901 (2018).

Resume : The local polarization reversal (LPR) under the action of mechanical load performed by Berkovich-type prism indentation in La doped BiFeO3 ceramics was studied. Nanoscale domain structure and polar-to-nonpolar phase transition was observed in the area of plastic deformation. In vicinity of this stress-induced ferroelastic domains appeared driving by conditions of mechanical energy minimization. Indirect action of grain clamping by neighboring grain resulted in shrinkage of existed ferroelastic domains and nucleation of new domains near the grain boundaries. Area of switching was found dependent linearly on the load force. The value of the critical stress σc =14.6 MPa for indentation induced LPR was evaluated from fitting that is close to values obtained earlier in the other ferroelectric materials. Obtained experimental results are significant for understanding mechanical stress mediated depolarization effects in ferroelectric ceramics. The study was funded by RFBR (grant No. 17-52-04074) and BRFFR (grant No. F17RM-036). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 778070.

Resume : Recently, fly ash (FA) generated from coal supplied Thermal Power Plants is considered as raw material instead of waste. Therefore, different ways are being developed for its proper utilization. Among them is the synthesis of zeolites with application in waste water treatment, gas cleaning and self-catalytic systems. FA has a predominantly aluminosilicate composition that determines the possibility of its zeolitization, but it has been found that the cohesive macro components, mostly iron and calcium oxides, play a significant role in the characteristics of the resulting material. In the present study, FA with а significant content of calcite and calcium hydroxide was studied with respect to its zeolitization extent. Fly ash zeolites (FAZ) from high calcium were prepared by ultrasound-assisted hydrothermal and double-stage fusion hydrothermal syntheses using sodium hydroxide as an alkaline activator. The products were characterized for their phase composition, morphology and surface properties. Obtained FAZ are attributed to zeolite Faujasite type, they are nanocrystalline with high specific surface above 400 m2/g. It is expected that FAZ will have a high adsorption capacity of pollutants from gas and liquids. The presence of Ca2+ into Faujasite except of Na+ favors the removal of ammonia and phosphates from waters. Acknowledgements: The financial support of Bulgarian National Science Fund (BNSF) under the projects DN 17/18 and DNTS/Slovakia 01/6 is greatly appreciated.

Resume : Dielectric materials are ubiquitously applied in electronics as capacitors, frequency filters, and so on. An optical control of dielectric response is thus expected to provide a new route to the development of innovative photo-electronic devices. A change of dielectric permittivity due to photo-irradiation (photo-dielectric effect: PDE) has been reported to date in several sulfides and oxides. Almost all PDE are, however, recognized as an extrinsic effect due to the photoconduction, which fatally deteriorate an insulating property of the material. In the present study, an intrinsic PDE has been demonstrated in wide bandgap aluminates including modified LaAlO3 and BaAl2O4 with dielectric measurements under the photo-irradiation. An origin of the intrinsic PDE is suggested to be the dielectric response of strongly localized photo-excited carriers trapped in weakly-dispersive in-gap states due to oxygen-vacancies.

Resume : Although thin-film oxide materials have been extensively studied for their potential in resistive random access memory (ReRAM), the switching properties of nanocrystal assemblies remain underexplored due to difficulties in fabricating ordered structures. Here, we employ a simple flow coating method to deposit HfO2 nanocrystals (synthesized in benzyl alcohol [1]) as highly ordered ribbon-like assemblies. The resistive switching character of the nanoribbons is directly correlated with the organic ligands on the surface of the constituting HfO2 nanocrystals. First, oleic acid, dodecanoic acid, and undecenoic acid are used as model nanocrystal ligands.[2] Through a systematic comparison of the forming process, operating set/reset voltages, and resistance states, we establish the trend that shorter ligands provided lower set and reset voltages (and thus lower power consumption). However, ligands shorter than undecenoic acid result in unstable nanocrystals, thus hampering solution processing. Second, we synthesize very short phosphonic acid ligands with branched alkyl chains.[3] We find that 2-ethylhexyl phosphonic acid is a superior ligand, providing both a high colloidal stability and a compact ligand shell. This results in a record-low operating voltage (1 V) that is promising for application in flexible electronics. [1] De Roo, J. et al. Nat. Mat. 2016, 15, 517. [2] Nonnenmann, S. et al. ACS Appl. Mater. Interfaces. 2018, 10, 4824 [3] De Roo, J. et al. Chem. Mater. 2018, 30, 8034

Resume : Perovskite-like compounds are widely known as model systems for studying the relationships between crystal structure and physical properties. Among them, magnetically-ordered ferroelectric oxides have attracted much attention in recent years. Such materials combine spin and electric dipole ordering in the same phase, thus providing the technologically important possibility to control magnetism with an electric field. While BiFeO3 is the most thoroughly studied magnetic ferroelectric compound, the properties of its solid solutions remain a matter of intensive debate. In this work we show how variation in the chemical composition of Bi1-xAExFe1-xTixO3 (AE= Ca, Sr, Ba; x≤0.2) perovskites affects their crystal structure and magnetic behavior. In particular, our research demonstrates that Ca/Ti and Sr/Ti substitutions suppress the cycloidal antiferromagnetic structure specific to the parent compound, thus stabilizing a weak ferromagnetic and ferroelectric state. The Ba/Ti-doped solid solutions retain the magnetic behavior characteristic of the pure BiFeO3. The composition-driven changes in the magnetic properties of the Bi1-xAExFe1-xTixO3 compounds correlate with the structural evolution, thus confirming the existence of a tight coupling between the magnetic and electric dipole order.

Resume : Robust ferroelectricity at room temperature was recently discovered in a metastable orthorhombic phase of doped HfO2. The full compatibility of this oxide with CMOS processes opens opportunities for non-volatile ferroelectric memories. The metastable phase, coexisting with non-ferroelectric phases, was found to be stabilized in polycrystalline films. The polycrystallinity challenges the understanding of the role of interfaces and crystal orientation on the ferroelectric properties. Epitaxial ferroelectric HfO2 films, more convenient for this purpose, have been scarcely reported and little is still known about the epitaxial growth process, and the structural and ferroelectric properties of the films. Here, we have grown Hf0.5Zr0.5O2 films by pulsed laser deposition. We have studied the influence of substrate temperature, oxygen pressure during deposition, and film thickness on the epitaxial growth of the ferroelectric phase. Suitable deposition parameters permit epitaxial stabilization of Hf0.5Zr0.5O2(111) films on both perovskite SrTiO3(001) substrates and YSZ buffered Si(001) wafers. X-ray diffractometry reveals small amount of non-ferroelectric phases and atomic force microscopy shows extremely flat films deposited under optimized growth conditions. The films are ferroelectric at room temperature without significant wake up effect and present remnant polarization up to around 30 μC/cm2. Endurance up to around 108 cycles and retention time longer than 10 years are measured.

Resume : Ferroelectric (FE) gate oxides have been proved to be promising materials in field effect transistors (FETs), which enable low voltage operation for logic devices and thus reduce power consumption [1]. HfO2, which is a typical gate oxide, shows ferroelectric behavior that arises from a non-symmetric orthorhombic lattice structure with various dopants such as Si, Y, Al and Zr. Meanwhile, the doped HfO2 is prior to traditional ferroelectric materials due to its advantages like lead-free and complementary-metal-oxide-semiconductor (CMOS) compatible features. Moreover, recently two-dimensional materials like graphene and transition metal dichalcogenides (TMDs), have attracted intensive attention due to their remarkable electronic and optoelectronic properties. [2] The combination of graphene and FE oxides could thus be a promising concept to improve the performance of FETs. In this report, we will show the realization of ferroelectric (Hf,Zr)O2 (HZO) thin films with an orthorhombic structure on TiN/Si substrates using the RF-sputtering method. Structural properties were studied with various methods. Piezoelectric force microscopy (PFM) and Polarization field measurements were then carried out to further confirm the ferroelectricity of HZO. P-E loop was measured with Ag applied on both sides of the sample as electrodes and the ferroelectricity was proved exist in the core layer of HZO. It was essential to grow the skin layer TiN, in order to form orthorhombic phase in HZO. Such bottom electrode could influence the growth and orientation of grains during thermal annealing process. It is noticed that different annealing condition could affect the condition of the electrodes, leading to difference in electrical behavior of HZO thin film. Ferroelectric behavior of HZO was dependent on thickness and the grain size of the film as well. Electrical and ferroelectric properties of the HZO changed with graphene transferred on top of the HZO. Response time of the structure differed from bare HZO compared to that of graphene/HZO structure due to the high electron mobility of graphene. Other properties including the ferroelectricity and the interface of the structure was further studied. Our results offer an opportunity for a novel FETs based on FE HZO with graphene applied as the channel material, which is a promising prospect for high-performance logic devices with low power consumption. References [1] Salahuddin, S., & Datta, S. (2008). Use of negative capacitance to provide voltage amplification for low power nanoscale devices. Nano letters, 8(2), 405-410. [2] Di Bartolomeo, A. (2016). Graphene Schottky diodes: An experimental review of the rectifying graphene/semiconductor heterojunction. Physics Reports, 606, 1-58.

Resume : Recently discovered, doped HfO2 [1] is a promising candidate for several ferroelectric applications. Among them, the most addressed are Negative-Capacitance Field Effect Transistor (NC-FET) and Ferroelectric Random Access Memory (FRAM). Compared to other materials, HfO2 processes have not only the advantage of being already integrated in Si CMOS industry, but also its ferroelectric properties are adequate for using oxide layer thinner than 10nm. Nevertheless, some of its properties should be improved for industrial applications. During cycling, the main issues are the limitation of the wake-up effect and the imprint, the fatigue and the endurance of the material. The ferroelectric phase (f-phase) has often been studied by Atomic Layer Deposition (ALD) [2]. However, most of the time the electrodes are made by Physical Vapor Deposition (PVD) processes, and industrial processes to separate Hf and Zr are usually expensive. It could be interesting to have only one process where the separation of Hf and Zr is not needed and where all depositions are made in the same machine. Furthermore, one of the most important benefit of sputtering is the deposition time which can be until 120 times faster than ALD depositions for the oxide layer. In this presentation, details will be given on how we succeeded in the deposition of ferroelectric hafnium/zirconium oxide (HZO) solid solutions by changing the pressure in the magnetron sputtering chamber in order to realize Metal/Insulator/Metal (MIM) capacitors. X-rays Diffraction (XRD), Transmission Electron Microscopy (TEM) and advanced electrical characterizations have been led to understand the difference between ferroelectric and non-ferroelectric samples. Two targets will be compared: a metallic Hf/Zr (56/44) target (reactive sputtering) and an oxide ZrO2/HfO2 (50/50) target (non-reactive sputtering). On these two targets, stoichiometry issues will be described and different fabrication tests have been led, such as: annealing at different temperatures, changing the size of the ferroelectric insulator layer, changing the size and the materials of the bottom and top electrodes, changing the oxygen flows, annealing with and without top electrodes. In each one of the studies, our understanding of the phenomena involved will be given. References: [1] T.S. Böscke, J. Müller, D. Bräuhaus, U. Schröder, U. Böttger, Ferroelectricity in hafnium oxide thin films, Appl. Phys. Lett. (2011). [2] M.H. Park, Y.H. Lee, H.J. Kim, Y.J. Kim, T. Moon, K. Do Kim, J. Müller, A. Kersch, U. Schroeder, T. Mikolajick, C.S. Hwang, Ferroelectricity and Antiferroelectricity of Doped Thin HfO2-Based Films, Adv. Mater. (2015)

Resume : We developed Ge (p-type) MFS capacitors based on ferroelectric (FE) ZrxHf1-xO2 (ZHO). The ZHO and TiN are produced in one growth step at 225 C and RT, respectively, by plasma enhanced molecular beam deposition, followed by rapid thermal annealing at 750 C for crystallization and ferroelectric phase formation. Ge, as a low gap semiconductor has a large concentration of free carriers which effectively screen the polarization charges, stabilizing the FE domains. As a result, we obtain robust, symmetric hysteresis characteristics without wake-up effects. Also, Ge has one of the lowest thermal expansion coefficients, so during cooling down from crystallization temperature, a large thermal tensile strain builds in the film which could favor the formation of the FE orthorhombic (o) phase against the non-polar monoclinic one. This is supported by electron diffraction pattern in TEM, where the (111)o diffraction is clearly detected while the monoclinic phase is absent. The formation of the predominately (o) phase produces a large remnant polarization Pr =34.4 μC/cm^2, for nearly stoichiometric material (x=0.58), which is among the largest reported on Ge substrates [1,2]. Moreover, fatigue measurements show good endurance up to 10^5 cycles at cycling field of 2.3 MV/cm. It is considered that the sharp ZHO/Ge interfaces observed in our TEM could improve endurance and memory window in Ge FeFETs. Work funded by EU project 3eFERRO-780302 [1] X. Tian et al., APL. 112, 102902 (18) [2] Y. Goh et al., Nanotech. 29, 335201 (18)

Resume : 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 ground-breaking discovery by the Namlab team exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 substrates. The films, which are under epitaxial compressive strain and are (111)-oriented, display large FE polarization values up to 34 μC/cm2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

Resume : Ferroelectric HfO2-based thin films receive extensive research interest due to their large spontaneous polarization, scalability, and CMOS compatible manufacturing. As in ferroelectrics, the remnant polarization exhibits a temperature dependence, one can observe a strong pyroelectric response in such films. Recently, the pyroelectric effect of doped HfO2 films has been observed [1]. The electrocaloric effect is closely related to it, as describing a temperature change of the material due to the application of an electric field. First published results indicate rather large electrocaloric coefficients, making doped HfO2 a promising candidate for on-chip solid-state cooling [2]. In this work, a specialized test structure is used to directly assess the strength of the electrocaloric effect in a 20 nm Si-doped HfO2 nano-laminate. A thin-film temperature sensor is formed on the metal-ferroelectric-metal structure, enabling excitation frequencies of up to 60 kHz. Measurement with respect to an electric bias field provides insight into the nature of thermal-electric energy conversion in HfO2 thin films. Additionally, bias dependent pyroelectric measurements are employed to assess the role of defect dipoles, which may have important implications for electrocaloric applications. [1] C. Mart et al., Appl. Phys. Lett. 112 (5), 052905 (2018) [2] S. Pandya et al., Phys. Rev. Materials 2, 124405 (2018)

Resume : Ferroelectric HfO2-based thin films are one of the most promising candidates for emerging high-density embedded non-volatile memory technologies thanks to their compatibility with silicon technology and the possibility of 3D integration. The electrode-ferroelectric interface may play an important role in such memory cells. We investigate the interface between a 5 nm TiN top electrode and a 10 nm thick Hf0.5Zr0.5O2 (HZO) films in a metal-ferroelectric-metal (MFM) stack with X-ray photoelectron spectroscopy. Partial oxidation of the electrode and presence of reduced Hf near the interface are identified. Careful attention was paid to the removal of the fully oxidized Ti at the surface of the electrode using Ar+ sputtering to avoid any interference with the suboxide interface signal. The protocol was validated by photoemission electron microscopy and conductive atomic force microscopy. A 0.8 nm interface layer (TiOxNy) is formed and the oxygen vacancy concentration near the interface was estimated to be 4.5%. This redox reaction or n-type doping at the interface is believed to happen during the crystallization annealing process. Such phenomena was suggested to be the reason of the pinned polarization observed during the first cycles screened by mobile charges accumulated at the interface leading to a weak switching response. Our results provide the physical chemistry of the n-type doping and the accumulation of defective charges at the top interface.

Resume : Over the last ten years or so, it has become evident that domain walls in ferroelectric and mutiferroic oxides can have functionality that is completely different from the bulk. Of particular note are systems in which the bulk of the material is insulating, while the domain walls conduct. The fact that domain walls can be moved, created and destroyed immediately prompts the notion that conducting walls could be used as dynamic connectors in new kinds of agile nanocircuits. Recently this paradigm has developed further, as it has become clear that conducting domain walls can support both p and n-type semiconducting behaviour; moreover, intersections between domain walls can act as 1D p-n junctions, which themselves can be moved, formed and annihilated, such that devices AND interconnects could be ephemeral in nature. In this talk, progress that has been made to date in the investigation of conducting domain walls which converge to form p-n domain wall junctions will be presented, as will the characteristics of the p-n junctions they create. The primary material system in which the research has been done so far is ErMnO3 although junction points in Cu-Cl boracite will also be considered.

Resume : Identifying materials with significantly tunable thermal conductivity is a key unsatisfied ingredient to achieve logic operations using “phonon currents” and, thus, to boost the emergent technology of phononics. In this regard, ferroelectric materials with intrinsic interfaces -domain walls- that i) can act as effective phonon scattering centres and ii) can be extensively engineered by controlling their type and distribution, are excellent candidates to be harnessed as heat modulators. Here we use epitaxial strain engineering, covering a wide range of strain values from -1.36% to +0.71%, and electrostatic boundary conditions to design a variety of ferroelectric domain configurations in PbTiO3 thin films: c-domain structures, various a/c domain architectures, diverse patterns formed by a/c and a1/a2 superdomains, and even a no-domain-wall scenario. We prove that the phonon transport strongly couples with the density and the types of domain walls of these engineered ferroelectric structures and, consequently, the thermal conductivity can be modified at will by up to 70% at room temperature. This large modulation in our PbTiO3 films suggests a potential direction for engineering phononic devices based on epitaxially-grown ferroelectric thin films.

Resume : There is currently a vigorous research effort on the functional properties of ferroelectric domain walls. An important part of their appeal is that domain walls possess functional properties distinct from the host material, such as conductivity in the walls of insulators, ferromagnetism in the walls of antiferromagnets, or polarization in the walls of ferroelastics. Their distinct functionality foment new concepts in electronic nanodevices where domain walls act as mobile two-dimensional elements. Among the many properties of domain walls, mechanical response appears to have been largely neglected. In this presentation, we will show our experimental measurements on stiffness of domain walls. In particular, we have used Atomic Force Microscopy to investigate the difference in stiffness between domains and purely ferroelectric (non-ferroelastic) domain walls separating antiparallels 180o domains. Surprisingly, they have a distinct mechanical response, markedly softer than that of the domains they separate. Theoretical calculations, based also in flexoelectric effects, enhance our experimental results. However, flexoelectricity has a unique intrinsic effect, related to the fact that strain gradients are size-dependent property that scales in inverse proportion to size, therefore flexoelectricity can be huge at the nanoscale, affecting the stiffness of domain walls. Therefore, we also focus on this size effect comparing softness of domain walls in thin films and bulk materials.

Resume : The non-equilibrium creep dynamics of elastics objects under small external forces is phenomenologically described by a 3D potential landscape with multiple minima, leading to metastable states. Quantitative studies of creep on nanoscopic systems with pinning potential are needed and ferroelectric domain walls (DWs), separating regions of uniform polarization, are of special interest. Ferroelectric DWs are typically stationary because of pinning. However, new technological developments which rely on the controlled motion of DWs are emerging. Indeed, DW nano-electronics, where the wall rather than the domain is the functional element, is attracting a lot of attention. Given their small size and their controllable location, DW may play an important role in future electronic devices in fast high-density non-volatile data storage and logic devices with low power consumption. Hence, understanding the basic mechanism of domain wall motion in ferroelectrics is much needed for technological applications. Nanoscale domain and DW dynamics at BaTiO3 single crystal surfaces were investigated using photoelectron emission microscopy . The sample surface quality was ascertained by photoemission survey spectra. Threshold imaging allowing mapping of local work function variations linked to surface polarization. In this way DW creep over several days can be followed. Measurement of DW velocity versus temperature under the Curie temperature reveal a disordered creep process.

Resume : Motivated by experimental findings that Bi vacancies in BiFeO3 accumulate at ferroelectric domain walls and create holes localized on nearby Fe atoms, we performed density-funcional theory calculations for such domain walls, and investigated the localization behavior of holes and electron-hole pairs in the presence of Bi vacancies near the domain walls. We find, in agreement with experiment, that Bi vacancies trap holes on nearby Fe atoms. We also find that Bi vacancies change the localization of electron-hole pairs at domain walls, compared to pristine domain walls.

Resume : Here we address a long-standing question in the fundamental physics of perovskite ferroics: What is the critical thickness for a spontaneous lattice distortion to occur in a thin film? It sounds like a déjà vu to anybody that is familiar with the field: The quest for ultrathin ferroelectric capacitors (i.e. as thin as they can be made without sacrificing a switchable spontaneous polarization) has occupied materials scientists for decades. Yet, polar ferroelectric modes are just one small example of the many lattice (and electronic) degrees of freedom that make perovskites so attractive. Antiferrodistortive (AFD) tilts have received much less attention than polar modes as they do not couple linearly to an applied field. In this context, we explored whether quantum confinement could control, in an arbitrary perovskite film, the competition between AFD tilts and polar modes at will. We bring a positive answer to this question by using LaAlO3/SrTiO3 system as a test case, in which quantum confinement is indeed a viable tool to shift the balance between AFD and polar modes and selectively stabilize one of the two phases. By combining scanning transmission electron microscopy and first-principles based models, we find a crossover between a bulk-like LaAlO3 structure to a strongly polar state with no AFD tilts at a thickness of approximately three-unit cells. The phenomena here described are completely general and applicable to a broad range of physical systems.

Resume : Piezoelectric thin films are becoming more and more important for the development of Micro Electromechanical Systems (MEMS) for sensing and actuation. However, thin films often suffer from clamping which describes the mechanical boundary condition imposed by the substrate on the films. The piezoelectric response can be reduced by one to several orders of magnitude compared to bulk material. In this report, we show that by patterning the film, we allow the defined structures to move freely, which leads to a partial declamping and an enhanced piezoelectric response. PZT (Pb[Zr0.52Ti0.48]O3) thin films of 1 μm thickness were deposited by Chemical Solution Deposition on platinized silicon. Ion Beam Etching was used to define pillars of micron and submicron lateral dimensions. The actuators were contacted, covered with a SiO2 cladding which was planarized down to the pillar’s top electrode. The process flow was designed to leave an air gap between the cladding and the pillar to permit its free motion. Finally, a freestanding metal bridge was fabricated to access the top electrode contact. The partially declamped d33 coefficient was measured using a micro-spot Double Beam Laser Interferometer. Piezoelectric d33 values above 200 pm/V are measured for actuators down to 750 x 750 nm2. On larger structures, the increased declamping effect near the edges can be observed and is supported by FEM analysis. The reported d33 values are amongst the highest values reported in literature. This robust process flow is specially designed to facilitate the integration of different kind of devices on top of the actuator. It paves the way for the fabrication of novel Micro and Nano Electromechanical Systems.

Resume : Ferroelectric thin films are intensively studied for the design of tunable devices at microwaves since their dielectric permittivity can be tuned under an external DC electric field. KTa0.5Nb0.5O3 (KTN) ferroelectric oxide exhibits one of the highest dielectric constant (permittivity= 400 at 15 GHz) and is thereby a promising candidate for reconfigurable and miniaturized devices. In this study we investigate the microwave performance of a KTN coplanar waveguide leaky-wave antenna made of 6 periodic slots and operating at 17 GHz. A 600 nm-thick KTa0.5Nb0.5O3 film was deposited by Pulsed Laser Deposition on R-plane sapphire substrate. XRD measurements evidence pure KTN perovskite phase with textured growth and preferential orientation. SEM observations show quite smooth surface. The leaky-wave antenna has been fabricated (metallization by RF sputtering + wet etching process) on the KTN film after its confinement in 6 active regions by laser microetching with micrometer accuracy. This patterning process allows reducing the total insertion loss by lowering the impact of the KTN intrinsic loss (loss tangent=0.3 at 15 GHz). Measurements show a variation of the reflection coefficient from 17.0 GHz without biasing to 17.3 GHz under Ebias≈ 85 kV/cm, corresponding to a frequency tunability of 2%. Gains of 6.7 and 7.3 dBi measured at 17 GHz without and under biasing respectively, are in good accordance with the numerical results.

Resume : Lithium Tantalate (LiTaO3 or LTO) is a material with unique piezoelectric properties and high potential for application in Surface Acoustic Wave (SAW) filters. The combination of thin LTO layers with Si/SiO2 substrates allows the increase of SAW Q factors as well as the decrease of frequency temperature coefficient. One promising option to manufacture such structures is to transfer the LTO layer by using the Smart CutTM technology, which is based on Hydrogen ion implantation and annealing. How such a polar material reacts to H implantation is expected to be very complex. Also, the way its piezoelectric and structural properties are affected is not known. We studied the effect of H ions fluence (from 1e16 to 9e16 H /cm2) on strain generation within the implanted regions of initially single crystalline LTO (oriented off the piezoelectric axis [0001]). By combining high-resolution XRD measurements with elasticity theory we developed for this system, we extracted depth profiles of 3D full strain tensor components. Hydrogen depth profiles measured by SIMS and TEM cross-sectional imaging supported this investigation. We highlighted the appearance of strong shear strain in such modified regions and estimate strain-induced piezoelectric polarization distribution inside the layer. Finally, the relation between H concentration and strain will be discussed, as well as the impact of ion implantation on piezoelectric properties.

Resume : The ferroelectric materials have as a distinct property the presence of spontaneous polarization of which direction can be switched by applying an external electric field. These materials are used for conventional random access memory in multiple applications. The studying of new functionalities of the ferroelectric materials in thin films multi-layered structures for building devices for the cutting-edge technologies as memcomputing or neuromorphic circuits is timely and with many perspectives. In the present study we examine the ferroelectric properties of symmetric ferroelectric (FE)/interlayer/ ferroelectric (FE) multilayers thin film structure. For isolator material interlayers a sequential change of polarization direction in FE constituent layers is obtained during switching. The multiple polarization states can be separately access and are stable in time with very good memory characteristics, opening the way towards the design of multi-bit ferroelectric memories. We prove that different configuration of the polarizations in FE layers determines different capacitance/losses values. These results can be used to build FE non-destructive capacitive reading memory based on simple capacitor. Furthermore, binary logic function can be operated on this capacitor test structure with the possibility of storing the results on the same unit, by designing different algorithms of pulse sequence and reading process. These results are in accordance with the memcomputation philosophy.

Resume : The recent progress on epitaxial stabilization of ferroelectric Hf0.5Zr0.5O2 thin films on single crystalline substrates has boosted a strong research activity aiming the exploitation of this material in advanced electronic applications and data storage. Whereas hafnia films have been much investigated in resistive switching application, the stabilization of its ferroelectric phase opens new perspectives for data storage and computing. Here we explore the resistive switching observed in ferroelectric Hf0.5Zr0.5O2 thin films (about 4nm) grown on SrTiO3 (001) using La2/3Sr1/3MnO3 and Pt as electrodes. We aim to disentangle polarization from non-polarization related mechanisms, potentially contributing to the electroresistance (ER) of tunnel barriers. We measured polarization loops and confirm the ferroelectric switching at coercive voltage around 2 V. Analysis of the I-V curves are consistent with tunnel transport across the barrier. Interestingly, ER is only observed for writing voltage pulses larger than 4V and reaching values as large as 900%. A modulation of the barrier height and width is observed depending on the writing pulse amplitude and duration. Therefore, ER does not appear to be ruled by polarization reversal. Attempts to enhance the asymmetry of the potential profile across the devices by using different electrodes will be also reported.

Resume : Ferroelectric junctions in thin film form can be used to store information owing to their availability to display different spontaneous surface charge states at remanence (named remanent polarization) depending on the electric history of the device. If the ferroelectric junction is thin enough, it can allow tunneling current, which is known to depend on polarization state. Thus the polarization state, in this case, can be probed by means of the measurement of the device resistance. Any resistive switching memory element (where resistance state depends on the electric history) suffers two main issues: i) in a crossbar array, when several elements are in low resistance state, if current is used to probe a particular element, current can leak (sneak current) across adjacent elements if these are in low resistance state, and ii) the presence of low resistance states implies the presence of large currents, with the concomitant undesired Joule dissipation. Antiserial connection of resistive switching elements can be used to solve these two important issues. The resulting device is named complementary resistive switching device. In our work, we present a nanometric complementary resistive switching device based on ferroelectric tunneling elements [1]. It shows particular double-switching mechanism, which we will describe in detail and compare with an equivalent device based on non-tunneling ferroelectric thick films [2]. [1] M. Qian, I. Fina et al., submitted.; [2] F. Liu, I. Fina et al., Sci. Rep. 6, 25028 (2016)