Variability of the electronic properties of 2D materials and ferroelectrics (FE) offers a wealth of fundamentally important physical phenomena and exciting technological opportunities for the hybrid 2D-FE heterostructures comprised of these materials. Among particularly promising aspects of these heterostructures is a coupling between the electrically-switchable polarization and electronic transport, which allows realization of advanced devices with enhanced functional characteristics. This talk will focus on recent advances in realization of these electronic devices. Specifically, we employ polarization reversal to modulate (1) the in-plane transport of the interfacial conducting channel in the ferroelectric field effect transistor (FE-FET) devices, and (2) the perpendicular-to-plane tunneling conductance across the ferroelectric barrier in the ferroelectric tunnel junction (FTJ) devices. We show that interface engineering in the 2D-FE systems provides a possibility of successfully addressing the most serious challenges relevant to device performance, such as ON/OFF ratio, lifetime, operation endurance and reliability.
(Invited) Non collinear antiferromagnetic surface on a ferromagnetic manganite
ABSTRACT.
María J. Calderón1, Z. Konstantinovic2, S. Valencia3, L. Brey1, Ll. Balcells4
1Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC) 28049 Madrid, Spain 2 Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, 11080 Belgrade, Serbia 3 Helmholtz-Zentrum-Berlin, 12489 Berlin, Germany 4Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Bellaterra 08193, Barcelona, Spain
Electronic reconstruction at interfaces between different oxides is giving rise to a plethora of new functionalities and novel phenomena. Complex oxides are characterized by a strong interplay of the different degrees of freedom (orbital, spin and lattice) which can be engineered and modified at interfaces and surfaces. In order to get experimental information on these modifications, different surface sensitive probes can be used. In particular, x-ray linear dichroism XLD can be used to get information on the symmetry of the electronic and magnetic properties on a manganite surface. The chosen manganite is the optimal doped La0.7Sr0.3MnO3, which is ferromagnetic in bulk. By studying the XLD results as a function of the angle between the applied magnetic field and the surface, an antiferromagnetic order, non collinear with the bulk ferromagnetic order, is inferred. Theoretical calculations reveal that the observed surface reconstruction is a result of the competition between spin-orbit coupling (enhanced at the interface) and the magnetic double exchange interaction.
Mixed-valence manganites have been a favorite of scientists interested in magnetism for sixty years. Their magnetic properties are particularly sensitive to structural and electronic reconstruction at surfaces and interfaces [1], due to the competing ferromagnetic and antiferromagnetic interactions that are associated with delocalized and localized manganese 3d electrons. There are some remarkable and unexplained differences between optimally-doped nano-LSMO in different forms. Nanoparticles of diameter 20 nm exhibit a magnetic dead layer 3.3 nm thick, whereas the dead layer in thin films is only 1-2 nm thick, depending on the substrate. There is also an electrical dead layer, in the sense that films thinner than 3-5 nm are not conducting. Here we compare the magnetization of films of various thicknesses grown by pulsed laser deposition on substrates of SrTiO3, LaAlO3 or LSAT that have lattice parameters of 390.5 pm, 379 pm and 387 pm, respectively, as compared with 388 pm for bulk LSMO. Nanoparticles are grown by several different methods. A problem with estimating the dead layer thickness of films on STO substrates is a temperature-independent anhysteretic contribution of order 10 Bohr magnetons/square nm, which is attributed to giant orbital paramagnetism associated with surface oxygen defects in STO [2]. An explanation has been proposed in terms of quantum fluctuations of the vacuum [3]. This signal may perhaps the explanation of the high Curie-temperatures LSMO films, reported by Boschker et al [4]. Ccmparing the magnetic dead volumes of unstrained 20 nm films and nanoparticles, we find a 'nonmagnetic' fraction of just 6% for the films, but 42% for the particles. These results are discussed in terms of a magnetic 'delocalization length' of order 50 nm.
[1] X. R. Wang et al, Science 249 716 (2015)
[2] J. M. D. Coey, M. Venkatesan and P. Stamenov, J. Physics Cond, Matter 28 485001(2016)
[3] J. M. D. Coey, K. Ackland, M. Venkatesan and S. Sen, Nature Physics 12 694 (2016)
[4] H Boschker et al, Phys. Rev. Letters 109 157207 (2012)
Disentangling low field Anisotropic Magnetoresistance in La0.7Sr0.3MnO3 Films
ABSTRACT.
Fernando Ajejas1,2, Davide Maccariello1,2, Ruben Guerrero1, L. Méchin3, S. Flament3, J. Santamaria4, Julio Camarero1,2, Rodolfo Miranda1,2, and Paolo Perna1
1 IMDEA Nanoscience, c/ Faraday 7, Campus de Cantoblanco 28049 Madrid, Spain 2 DFMC, Universidad Autónoma de Madrid, 28049 Madrid, Spain & IFIMAC, Universidad Autónoma de Madrid, 28049 Madrid, Spain 3 GREYC (UMR6072) CNRS-ENSICAEN & Université de Caen Normandie, 6 Bd. de Maréchal Juin, 14050 Caen, France 4 GFMC, Departamento de Física de Materiales, Facultad de Física, Universidad Complutense de Madrid, Campus Moncloa, 28040 Madrid, Spain
Perovskite half-metallic manganites hold promise for spintronic applications, but are not yet available in today devices because of the lack of control of their magnetotransport properties. The reason for this relies on the complexity of the physical scenarios governing the interplay between a wide variety of coupled interactions. Different magnetoresistance (MR) contributions, such as colossal MR, Lorentz MR, spin-dependent scattering at grain-boundaries, domain-walls and other magnetic inhomogeneities, typically hide the switchable anisotropic magnetoresistance (AMR) that is more amenable for technological applications. In this work, we demonstrate the ability to engineer large anisotropic magnetoresistance in half-metallic La0.7Sr0.3MnO3 (LSMO) thin films by exploiting vicinal surfaces as substrate for the epitaxial PLD growth. These surfaces induce a surface symmetry-breaking, and hence specific magnetic anisotropy, which is responsible of the magnetization reversal pathways [1] and consequently of the magnetoresistance responses [2].
In order to correlate the magnetization reversal mechanisms to the magnetoresistive responses, we performed RT angular and field resolved vectorial-Kerr measurements [3] and acquired hysteresis loops of both in-plane components of the magnetization (parallel- and perpendicular-to the magnetic field) and the corresponding simultaneous resistance changes. This technique was earlier exploited to investigate single FM layer [4], FM/AFM bilayer [5] and spin-valve [6]. By combining simultaneous magnetization (vectorial Kerr) and transport measurements, we therefore disentangle the different contributions to the total MR response and demonstrate that AMR is in fact the dominant contribution [2]. In addition, we show that the artificially tailored film surfaces induce an extrinsic magnetic anisotropy that is responsible for the large AMR tunable, which can be tuned in sign and intensity by conveniently choosing the magnetization-current configuration [2].
[1] P. Perna, et al., J. Appl. Phys. 110, 13919 (2011); ibidem, J. Appl. Phys. 109, 07B107 (2011); P. Perna, et al., New J. Phys. 12, 103033 (2010).
[2] P. Perna, et al., (2017) submitted.
[3] E. Jiménez, et al., Rev. Sci. Instrum. 85, 053904 (2014); J. L. Cuñado, et al., Rev. Sci. Instrum. 86, 046109 (2015).
[4] P. Perna, et al., Appl. Phys. Lett. 104, 202407 (2014).
[5] P. Perna, et al., Phys. Rev. B 92, 220422(R) (2015); P. Perna, et al., AIP Advances 6, 055819 (2016) ; P. Perna et al., Proc. of SPIE Vol. 9931, 99312I (2016).
[6] P. Perna, et al., Phys. Rev. B 86, 024421 (2012).
Strain-induced selective occupancy of electronic orbitals in thin films of half-doped LSMO manganites probed by NMR.
ABSTRACT.
Marek Wójcik1, Ewa Jędryka1, David Pesquera2, Diego Gutiérrez2, Florencio Sánchez2 and Josep Fontcuberta2
1Institute of Physics, Polish Academy of Sciences, Aleja. Lotników 32/46, 02-668 Warsaw, Poland 2Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, 08193 Bellaterra, Catalonia, Spain
Electrical control of magnetism has become a subject of intensive research and giant electroresistance has recently emerged as an efficient realization of this idea. In order to increase the tunel electroresistance (TER) and to improve the performance of ferroelectric tunel junction (FTJ) it was proposed to incorporate, as a slave electrode, a material in which a metal-insulator transition is triggered by switching the polarization in the ferroelectric. Half-doped manganites like La0.5(Sr,Ca)0.5MnO3 seem ideal for this purpose, but epitaxial strain can profoundly modify their ground state. In order to explore the role of structural distortion on their magnetic and electric ground state we performed a thorough Nuclear Magnetic Resonance (NMR) study of LSMO-5 films under various strain conditions [1]. Epitaxial strain promotes tetragonal distortion of the octahedra causing Jahn–Teller effects on the Mn3+, which in turn has a strong impact on the orbital occupancy. We exploit the fact that NMR resonance frequency of Mnm+ is extremely sensitive not only to the total number of electrons in the 3d shell, but also to symmetry of the particular 3d orbit (x2-y2 or z2) that they occupy. Various oxide substrates with a different lattice mismatch to LSMO-05 were used in order to induce different strain effect (tensile or compressive strain). 55Mn NMR spin echo experiments were carried out at 4.2 K on films with thickness of 20 and 35 nm. The NMR spectra consist of two resonance lines at frequencies f0 = 376 MHz and f1 = 330 MHz, both assigned to Mn ions in a mixed valence state. Their respective frequencies reveal different configurations of 3d(eg) orbitals of Mn3+ ions. The line f0 corresponds to mixed valence state generated by Mn3+ ions with 3d(eg) orbitals fully degenerated, thus giving rise to the isotropic 3D conductivity. Orbital polarization, induced by the tensile strain, favors occupation of eg(x2-y2) orbitals and brings a non-zero dipolar contribution to the hyperfine field of Mn3+ ions, shifting down the resonance frequency and giving rise to the line f1 . This corresponds to the A-type antiferromagnetic configuration with double exchange interaction active only in the x-y plane, revealing the presence of a new phase with anisotropic conductivity. In conclusion, tensile strain induces phase separation into the antiferromagnetic A-type and the ferromagnetic phase, resulting not only in the decrease of film magnetization, but also in a strong reduction of the electrical conductivity perpendicular to the film plain, possibly affecting the TER.
[1] Strain-Driven Orbital and Magnetic Orders and Phase Separation in Epitaxial Half-Doped Manganite Films for Tunneling Devices, D. Pesquera, A. Barla, M. Wojcik, E. Jedryka, F. Bondino, E. Magnano, S. Nappini, D. Gutiérrez, G. Radaelli, G. Herranz, F. Sánchez, and J. Fontcuberta, PHYSICAL REVIEW APPLIED 6, 034004 (2016).
Carbon fiber composites (CFC) are a fundamental class of materials in applications where both light weight and high mechanical features are requested (automotive, civil engineering, aerospace, etc.). The use of these materials in transportation leads to significant saving of fuel and reduction of emission of CO2, NOx, and noise. Nevertheless, because CFC intrinsic structure strongly depend on carbon fiber (CF) patch and resin arrangement, a precise predictive deformation and failure behavior is hard to model, which is one of the main causes limiting the use of these materials in aerospace technologies. Hence, on account of the very heavy loads the CFC structures are subject to, it is important to carry out real-time monitoring of deformations and vibrations. Nowadays, stress sensors in CFC, mainly based on piezoelectric PZT and optical fiber (Fiber Bragg Grating, FBG), present some drawback such as large size (compared to CF’s) and weight addition. Use of zinc oxide (ZnO) piezoelectric nanostructures can overcome these issues and lead to a real-time stress sensor completely embedded within CFC. In this work low cost, low temperature and low environmental impact synthesis of ZnO nanorods on CF is carried out, as well as piezoelectric characterization of such micro-composite sensor. A piezoelectric investigation of a nano-engineered transducer consisting of zinc oxide nanorods grown in-situ on carbon fibers has been performed by means of dynamic hysteresis and capacitance measurements. The device has been stimulated using both static and dynamic stress: the occurrence of characteristic current vs voltage polarization lobes of a ferroelectric material (stressed piezoelectric) and the corresponding saturation polarization of 290 μC/cm (at 2.4 V/μm electric field) have been recorded under static stress application. Under dynamic stress conditions a 400% capacitance increase has been measured with respect to the unstressed device. Noteworthy these results have been achieved using the carbon fiber as conductive element, without the need for external wiring, providing a true integration of the piezoelectric transducer into carbon fiber based materials. Then, the device has been successfully integrated into a CFC. The presented nano-engineered device acts as strain sensor and can actively react to the applied stress to hinder the deformation, enabling the design of smart carbon fiber based composites.
Robust and long-lasting security solutions have to be developed urgently to fight the more and more growing threats associated with counterfeiting, identity theft and misappropriation of data. Both in the real world of objects and in the virtual world of data, these solutions will have to guarantee inviolable traceability and authentication. The progressive entanglements of objects, people and data with the pervasiveness of connected objects will soon require to merge different functions, today developed with a limited application range, into a comprehensive solution. It is an absolute prerequisite for fraud resilient emerging progress associated with fields such as 3D-printing, Smart manufacturing or Internet of Things (IoT) and new business models based on certificate of ownership, citizen identity or VRM (Vendor Relation Management). In this direction, 3D-Oxides is developing unique unclonable tags consisting in multi-functional and multi-element oxide 3D patterned thin films. The device is achieved through an additive and combinatorial growth process using chemical gases in high vacuum conditions enabling both large area substrates (m2) and chemical/topographical thin film structuring with sub-micron resolution. Positioned on any kind of sensor able to measure at least one material property, it will provide a connected unclonable fingerprint. The production capability of the patented technology can address mass production (< billion tag/year/equipment) of all different tags at very competitive costs. The fingerprint of the tag, based on very different film properties patterns (optical, electrical, magnetic, etc…), will provide a secured and authenticated bijective relation between the physical and the virtual worlds with hardware embedded cyber-security and cryptography. This will provide in the long term, a robust and long term solution enabling convergence and interconnection of the different domains, favoring the emergence of a plethora of novel technological possibilities and business models. In the presentation, we will show main technological blocs and discuss most promising user-cases.
Dielectric properties of lithium niobate from mHz to optical frequencies
ABSTRACT.
C. Cochard1, T. Spielmann2, N. Balhawane1, A. Halpin3, and T. Granzow1
1MRT Department, Luxembourg Institute of Science and Technology (LIST), 5 Avenue des Hauts-Fourneaux, L-4362 Esch-sur-Alzette, Luxembourg 2IEE S.A., 11 rue Edmond Reuter, L-8326 Contern, Luxembourg 3Dutch Institute for Fundamental Energy Research (DIFFER), Eindhoven University of Technology, De Zaale 20, 5612 AJ Eindhoven, The Netherlands
One of the best-known ferroelectric materials is lithium niobate LiNbO3 (LN). LN is a uniaxial ferroelectric crystallizing in the R3c space group. It is renowned for its strong electro-optical and photorefractive properties. These properties have made LN a candidate of choice for applications in photorefractive devices, holographic memories and surface acoustic wave (SAW), etc. Today a lot of literature is available on the optical properties of LN in the visible and infrared spectral ranges. However, reports tend to focus on a limited frequency range and a discussion of the dielectric properties over an extended frequency range is today still missing.
This presentation aims at filling some part of this gap by presenting the characterization of the dielectric permittivity depending on the crystal orientation over a broad frequency range: 1 mHz to 1 PHz (λ = 300 nm) using impedance measurement, quasi-optical free-space characterization, THz time domain spectroscopy (THz-TDS) and optical ellipsometry. Three different frequency ranges, separated by well visible resonances, are observed: low frequency “free-piezoelectric” response, intermediate frequency “clamped-ionic” response and high frequency “electronic” response. All of these features are discussed with an emphasis on the role of the crystallographic structure and piezoelectric response.
Large-scale and flexible energy harvester based on ZnO conical nanostructures by nano-imprint lithography and atomic layer deposition
ABSTRACT.
D.Spirito1,2, E.Defay1, K.Menguelti1, J.Kreisel1,2, and D.Lenoble1
MRT Department, Luxembourg Institute of Science and Technology (LIST), 5 Avenue des Hauts-Fourneaux, L-4362 Esch-sur-Alzette, Luxembourg
University of Luxembourg, 2 Avenue de l’université, L-4365 Esch-sur-Alzette, Luxembourg
Mechanical energy harvesting still remains an emerging technology since the late 90’s. The development of lead-free piezoelectric materials during the past few years and their now reliable and high properties make them a natural choice for the design of environmentally friendly energy sources. In this work, we report both the processing of conical shape ZnO nanostructures as well as their characterizations under low electric field and large mechanical field stimuli. The large scale fabrication of an efficient piezoelectric nanogenerator is hence demonstrated. We propose here an original top-down approach while bottom-up processes are usually favoured for the growth of ZnO nanowires. The fabrication combines Nano-Imprint Lithography (NIL) and low-temperature (<80°C) Atomic Layer Deposition (ALD). The NIL process leads to a PMMA regular array of truncated conical holes of 2 μm in depth. The conical shape of the stamp allows for reaching a high aspect ratio of 10. These holes are then filled with 150 nm-thick polycrystalline ZnO film by low-temperature ALD. A p-n junction is created by adding 100 nm-thick PEDOT by plasma radicals assisted polymerization via chemical vapour deposition. Using this method, we demonstrate the possibility to grow functional device on a flexible substrate with an active surface from 5x5 mm2 to 50x50 mm2. Voltage output, effective transverse piezoelectric coefficient, generated power were measured using electrical and mechanical stimuli. We measured an output voltage of 200 mV for the 5x5mm2 device corresponding to an effective transverse piezoelectric coefficient e31eff of -0.45 C/m2.
The two-dimensional electron system at the interface between LaAlO3 and SrTiO3 has several unique properties that can be tuned by an externally applied gate voltage. This opens opportunities for future electronic applications. However, the fundamental mechanisms underlying this tunability are still not fully understood, as well as the factors that determine the effective band structure at the interface.
In our magnetotransport experiments on top-gated Hall bars, the low gate leakage current enables exploration of a larger carrier density range than before. Our data reveal a dyz,xz Lifshitz transition at a carrier density of 2.9 x 1013 cm-2 and a surprising reduction of dxy-type carrier density with gate voltage above this transition. These observations indicate a gate-tunable band structure, which is controlled by the electrostatic confinement. Self-consistent Schrödinger-Poisson calculations support this conclusion: they reproduce the observed reduction of dxy-type charge carrier density by including interband electronic correlations.
In combination with back-gating, we show that the top-gated dyz,xz Lifshitz transition can be tuned by a back-gate voltage, establishing full electrostatic control of the band structure and confinement of the system. The expected effect of either a top- or back-gate voltage on the boundary conditions of the Schrödinger-Poisson model is confirmed by the experimental results.
As a first step to study the implications of the gate-tunable band structure, we investigated the top- and back-gate dependence of the superconducting critical temperature Tc. We observe a striking asymmetry in the effect of both gate voltages. Subsequent magnetotransport studies above Tc showed perfect gate reversibility. This enables us to compare the evolution of Tc with the band filling, from which we conclude that the gate-tunability of superconductivity cannot be explained by changing the carrier density alone.
Superconductivity and high mobility transport in an oxygen-vacancy-engineered two-dimensional electron gas on SrTiO3
ABSTRACT.
Shamashis Sengupta,1 Florence Linez,2 Miguel Monteverde,3 Emilie Tisserond,3 Anil Murani,3 Tobias Rodel,1 Sophie Guéron,3 Hélène Bouchiat,3 Philippe Lecoeur,2 Thomas Maroutian,2 Claire Marrache-Kikuchi,1 Andrés Santander-Syro,1 and Franck Fortuna1
1 Centre de Sciences Nucléaires et de Sciences de la Matière, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, 91405 Orsay, France 2 Institut d’Electronique Fondamentale, Univ. Paris-Sud, CNRS, Université Paris-Saclay, 91405 Orsay, France 3 Laboratoire de Physique des Solides, Univ. Paris-Sud, CNRS, Université Paris-Saclay, 91405 Orsay, France
It is known that a two-dimensional electron gas (2DEG) can be realized at the surface of the band insulator SrTiO3 and it exhibits very different conduction behaviour (either insulating, metallic or superconducting) as a function of the carrier density. These systems are usually realized by developing complex heterostructures in which a layer of binary (e.g. Al2O3) or ternary (e.g. LaAlO3) oxide is grown on top of SrTiO3. We have demonstrated an alternative method [1] for creating a similar 2DEG that is easier and more cost-effective than conventional hetero-epitaxial techniques. This involves the deposition of an atomically thin layer of pure Al at the surface of SrTiO3. By a simple redox reaction, Al pumps oxygen from the surface of the underlying oxide. Al is converted into insulating AlOx and oxygen vacancies are created at the surface of SrTiO3. At such an AlOx/SrTiO3 interface, a homogeneous quantum-confined 2DEG develops due to doping from the oxygen vacancies in the underlying substrate. Our experiments show that by adjusting the annealing and deposition temperatures, we can access various regimes of carrier concentrations corresponding to different types of electronic transport behaviour. At low carrier densities, the 2DEG shows superconductivity with a critical temperature of 400 mK while at high carrier densities, it remains metallic down to low temperatures with a mobility greater than 105 cm2/V.s.
[1] Universal Fabrication of 2D Electron Systems in Functional Oxides, T. C. Rodel, F. Fortuna, S. Sengupta, E. Frantzeskakis, Patrick Le Fevre, F. Bertran, B. Mercey, S. Matzen, G. Agnus, T. Maroutian, P. Lecoeur, and A. F. Santander-Syro. Advanced Materials, 28:1976–1980, 2016.
Intrinsic inhomogeneities in LXO/STO oxide iterfaces
ABSTRACT.
M. Grilli1, N. Bergeal2, J. Biscaras3, S. Caprara1, S. Hurand2, C. Feuillet-Pama2, J. Lesueur2, R. Raimondi4, N. Scopigno1, G. Seibold5
1Dipartimento di Fisica, Università di Roma "Sapienza", Rome, Italy 2LPEM UMR8213/CNRS—ESPCI ParisTech, Paris, France 3Institut de Minèralogie, de Physique des Matèriaux et de Cosmochimie, Sorbonne, Paris, France 4Dipartimento di Matematica e Fisica, Università Roma Tre, Rome, Italy 5Institut fur Physik, BTU Cottbus-Senftenberg, Cottbus, Germany
Several experiments in oxide interfaces like LaAlO3/SrTiO3 or LaTiO3/SrTiO3 (LXO/STO) heterostructures, clearly indicate that the 2D electron gas and the resulting superconducting state at the interface is strongly inhomogeneous on the nanoscopic scale [1]. The self-consistent electrostatic electron confinement at the interface has recently been proposed as a possible mechanism of electronic instability [2] (possibly cooperating with the strong density-dependent Rashba spin-orbit coupling (RSOC) observed in these systems, [3]). This leads to an electronic phase separation (EPS) establishing a possible intrinsic origin for the inhomogeneous character of LAO/STO or LTO/STO superconductors. The inhomogeneous character of the 2DEG, accompanied by an inhomogeneous RSOC opens the way to two interesting classes of phenomena: i) a novel superconducting quantum criticality, and ii) inhomogeneous spintronics.
i) The unusual quantum critical behaviour of superconductivity in LXO/STO [4-6] has been investigated by tuning temperature, gating, and/or magnetic field finding a novel type of SC-to-metal quantum criticality related to the vanishing of the critical temperature of the EPS [5,6], where the critical superconducting fluctuations are coupled to and driven by the strong dynamical density fluctuations. Indications of a Griffith phase have also been found. ii) The softness of the 2DEG allows an easy designing of specific structures where the RSOC can be modulated at the submicrometric scale. This opens the way to a variety of 2DEG structures, where spin-Hall effect [7], (inverse) Edelstein effect, and Majorana Fermions [8], could be obtained.
[1] S. Caprara, et al., Phys. Rev. B (Rapid Communications) 88, 020504(R) (2013); S. Caprara, D. Bucheli, N. Scopigno, N. Bergeal, J. Biscaras, S. Hurand, J. Lesueur, and M. Grilli, Superc. Sc. and Tech. 28, 014002 (2015); D. Bucheli, S. Caprara, and M. Grilli, Superc. Sc. and Tech. 28, 045004 (2015).
[2] N. Scopigno, D. Bucheli, S. Caprara, J. Biscaras, N. Bergeal, J. Lesueur, and M. Grilli, Phys. Rev. Lett. 116, 026804 (2016).
[3] S. Caprara, F. Peronaci, and M. Grilli, Phys. Rev. Lett. 109, 196401 (2012); D. Bucheli, M. Grilli, F. Peronaci, G. Seibold, and S. Caprara, Phys. Rev. B 89, 195448 (2014).
[4] J. Biscaras, N. Bergeal, S. Hurand, C. Feuillet-Palma, A.Rastogi, R. C. Budhani, M. Grilli, S. Caprara, J. Lesueur, Nature Materials 12, 542 (2013).
[5] S. Caprara, N. Bergeal, J. Lesueur, and M. Grilli, Phys.: Condens. Matter 27 425701 (2015)
[6] S. Hurand, J. Biscaras, N. Bergeal, C. Feuillet-Palma, G. Singh, A. Jouan, A. Rastogi, A. Dogra, P. Kumar, R. C. Budhani, N. Scopigno, S. Caprara, M. Grilli, J. Lesueur, arXiv:1506.06874
[7] G. Seibold, S. Caprara, M. Grilli and R. Raimondi EPL, 112 17004 (2015). [8] M. V. Mazziotti, N. Scopigno, M. Grilli, and S. Caprara, preprint.
Electronic structure and fabrication of 2DESs beyond SrTiO3
ABSTRACT.
T. C. Rödel1,2,3, F. Fortuna2, E. Frantzeskakis2, P. Le Fèvre3, F. Bertran3, T. Maroutian4, P. Lecoeur4, A.F. Santander-Syro2
1LPV, Physics and Material Science, University of Luxembourg, L-4422 Belvaux, Luxembourg 2CSNSM, Université Paris-Sud and CNRS/IN2P3, Bâtiments 104 et 108, 91405 Orsay cedex, France 3Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France 4Institut d’Electronique Fondamentale, Université Paris-Sud CNRS, Bâtiment 220, 91405 Orsay, France
Two-dimensional electron systems (2DESs) in transition metal oxides are currently a field of intense research in the quest of novel functionalities in materials showing competing ground states. The 2DESs in SrTiO3-based interfaces have been the cornerstone of such research [1]. To go further, it is essential to create new types of oxide 2DESs in a technically easy way.
Here we show, using angle-resolved photoemission spectroscopy (ARPES) that the deposition of atomically-thin layers of an elementary reducing agent (e.g. aluminum) in UHV results in the creation of 2DESs at the interface of several oxides [2], such as the ferroelectric BaTiO3, CaTiO3, TiO2 and ZnO. The study of the electronic structure of the 2DESs by ARPES gives, among others, insights in many body phenomena (e.g. electron-phonon coupling) and demonstrates how different lattice distortions in perovskite titanates ATiO3 (A=Ca, Sr, Ba) affect the 2DES. This technique can be adapted for transport studies and opens up the possibility to study these various 2DESs beyond UHV.
Based on this generalization of the creation of the 2DESs, the question naturally arises which other oxide-based 2DESs could be interesting from the perspectives of fundamental or applied science?
The electron gas at the interface between LaAlO3 and SrTiO3 has been deeply investigated since more than a decade [1]. Various transport models have been developed which may explain the occurrence of the conducting layer [e.g. 2,3]. The resistance of this electron gas typically drops monotonically with temperature and R/T curves during cooling and warm-up look identical for large area structures. We have been able to fabricate nanostructures [4] which show a surprising temperature dependence. If the LAO/STO is laterally restricted by nanopatterning the resistance exhibits a temperature anomaly and warming up nanostructures from low temperatures leads to one or two pronounced resistance peaks between 50 and 100 K not observed for larger dimensions. This results can be explained by the structural phase transitions which take place in the STO substrate during coold own and warm up. During cool-down current filaments emerge at the domain walls that form during the phase transition. This effect had already been observed by scanning SQUID microscopy [5]. During warm-up, however, the reverse phase transition can interrupt filaments before the sheet conductivity which dominates at higher temperature is reestablished. It appears that the area between the domain walls becomes completely insulating and due to the limited number of filaments in a nanostructure this process can result in a complete loss of conductance. As a consequence of these findings the transport physics extracted from experiments in small and large area LAO/STO structures may need to be reconsidered.
[1] Ohtomo, A. & Hwang, H.. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423–426 (2004).
[2] Thiel, S., Hammerl, G., Schmehl, A., Schneider, C. W. & Mannhart, J. Tunable Quasi-Two-Dimensional Electron Gases in Oxide Heterostructures. Science. 313, 1942–1945 (2006).
[3] Siemons, W. et al. Origin of Charge Density at LaAlO3 on SrTiO3 Heterointerfaces: Possibility of Intrinsic Doping. Phys. Rev. Lett. 98, 196802 (2007).
[4] Minhas, M. Z. Blaschek, H. H. Heyroth, F. & Schmidt, G. Sidewall depletion in nano-patterned LAO/STO heterostructures. AIP Adv. 6, 035002 (2016).
[5] Frenkel, Y. et al. Anisotropic Transport at the LaAlO3/SrTiO3 Interface Explained by Microscopic Imaging of Channel-Flow over SrTiO3 Domains. ACS Appl Mater and Interfaces 8, 12514 (2016)
Reducing the production costs is of upmost importance in modern electronic industry. The existing technology is mainly based on physical vapor deposition of functional layers and patterning the discrete structures by etching, which can be difficult and may damage the layers. Inkjet printing has emerged as an alternative due to its unique features, such as cost- and time- efficient deposition of material without the need of any patterning steps. Yet, a high complexity of the printing process—extending from the jetting concerns to the issues connected with the morphology of dried deposit—has impaired the fabrication of electronic devices using inkjet printing. Here we report the inkjet printing of conductive lanthanum nickelate (LNO)—a promising electrode material for applications in ferroelectric devices. The LNO inks consisted of lanthanum nitrate and nickel acetate dissolved in a mixture of organic solvents with different surface tensions and boiling points. Our results show that the morphology of dried deposits is critically dependent on the ink’s solvent composition, wetting of the substrate, and the drying temperature. These parameters appear to be interrelated in the drying process; well-defined structures with a flat thickness profile can be printed only when all three parameters are optimized. We discuss the observed morphologies of dried features by considering the evaporation of multi-solvent ink and related variations in the solvent composition caused by different volatilities of the solvents present. Variations in the solvent composition induce changes in physical properties, which results in the evaporation-driven internal liquid flows and sometimes also in the wetting transition. The internal liquid flows have already been suggested to strongly influence the deposit morphology, while the wetting transition (and the associated three-phase contact line movement) is generally unwanted and rarely reported in inkjet printing. We demonstrate how the wetting transitions can be exploited to boost the printing resolution beyond the limits set by the volume of printed drop. The inks, which were designed to promote the inward contact line movement enabled a deposition of small electrodes with a diameter of only few micrometers. The electrical characterization of ferroelectric capacitors with inkjet-printed LNO electrodes on top of a Si/SiO2/LNO/PZT stack revealed their good and reliable performance, thus proving a high potential of inkjet printing in future production technology.
Analysis of grain growth in solution-derived Ba0.5Sr0.5TiO3 thin films upon isothermal multi-step annealing
ABSTRACT.
Tanja Pečnik1, Hana Uršič1, Sebastjan Glinšek2, Barbara Malič1
1Electronic Ceramics Department, Jožef Stefan Institute, Ljubljana, Slovenia 2Materials Research & Technology Department, Luxembourg Institute of Science and Technology, Belvaux, Luxembourg
Barium strontium titanate BaxSr1-xTiO3 (x=0–1) solid solution based thin films with the compositions in the paraelectric phase exhibit high dielectric permittivity, low dielectric losses and high voltage-tunability at microwave frequencies and are suitable for tunable microwave applications. But to prepare thin films by Chemical Solution Deposition with the above mentioned properties different processing-related characteristics, such as the grain size and shape, porosity, thermal stresses, etc. should be considered. In our previous work we prepared the Ba0.5Sr0.5TiO3 (BST) films with columnar microstructure by multi-step annealing at 900 °C. By increasing the film thickness from 90 nm to 240 nm the average lateral grain size increased from 45 nm to 90 nm, which contributed to increased kHz-range dielectric permittivity from 650 to 1350, respectively [1]. In the present work we focus on the study of the grain growth in BST films. The BST solutions with CM = 0.25 M were synthesized from alkaline-earth acetates and Ti-butoxide dissolved in acetic acid/2-methoxyethanol solvents. After spin-coating on polycrystalline alumina substrates each deposited layer was dried, pyrolyzed and rapid-thermally annealed at 900 °C, i.e., by multi-step annealing. These deposition–drying–pyrolysis–annealing steps were repeated in order to prepare the films with the thicknesses around 240 nm. The annealing time of the first deposit (15 min) and next five deposits (5 min) was constant for all BST films, while it varied between 1 s and 60 min for the seventh deposited layer. According to X-ray diffraction analysis all BST films crystallized in pseudo-cubic perovskite phase. The cross-section and surface microstructures of the films were analyzed by Field-Emission Electron Microscope (FE-SEM) and Atomic Force Microscope (AFM). The microstructure of the film consisting of six layers, which served as the template for the last deposited layer, consisted of predominantly columnar grains with the average lateral grain size ~60 nm. The surface microstructure of the film where the last, 7th, layer was annealed for 1 s, mainly consisted of 10-20 nm large grains. This last about 40 nm thick layer consisted of a few equiaxed grains in cross-section. As the annealing time of the last layer increased to 1 min the surface grain size increased to ~50 nm, but also much finer grains located on the boundaries of larger grains could be discerned. As the annealing time increased to 60 min the average surface grain size increased to ~90 nm. In parallel, the vertical root-mean square roughness increased from 1.9 nm to 5.6 nm for 1 s and 60 min annealing times, respectively, evidencing the evolution of pronounced surface grooving. In the contribution we discuss the grain growth of BST films upon multi-step annealing which proceeds via crystallization of fine, equiaxed grains within individual layers on top of preexisting columnar grains.
[1] T. Pečnik, S. Glinšek, B. Kmet, B. Malič, J. Alloys Compd. 646 (2015), 766–772.
Designing ferroelectric and ferromagnetic multifunctional nanocomposites thin films
ABSTRACT.
Paula Ferreira1, Alichandra Castro1, Jacobo Morère2, Albertina Cabañas2, Liliana P. Ferreira3,4, Margarida Godinho3, Paula M. Vilarinho1
1CICECO – Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal 2Departamento de Química-Física I, Universidad Complutense de Madrid, 28040 Madrid, Spain 3Biosystems and Integrative Sciences Institute (BioISI), Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal 4Department of Physics, University of Coimbra, 3004-516 Coimbra, Portugal
Oxide-based porous films prepared by easy and low cost chemical methods may be seen as “bottom-up” platforms for nanotechnology, finding potentially applications at the microelectronics and photonics fields. These methodologies allow the nanofabrication of functional ordered porous patterns with lateral thickness below 100 nm. Moreover, the free pores can be further functionalized with molecules or nanoparticles. In this work,1 a new concept to prepare nanocomposite thin films is explored. Two chemical-based bottom-up steps are used to design functional films including: i) block copolymer-assisted self-assembly of a porous matrix; and ii) impregnation of nanoparticles from a ferroic phase within the pores by supercritical CO2 deposition. Porous nanopatterned BaTiO3 thin films with ca. 17 nm of thickness are prepared using a cost-effective sol-gel solution containing a block copolymer and evaporation-induced self-assembly methodology. Hexagonal-arranged pores with diameter of ca. 95 nm, running perpendicularly to the substrate are filled with Ni nanoparticles using the supercritical fluid deposition technique from reduction of hydrated nickel nitrate in a supercritical CO2-ethanol mixture at 250 ºC. Small Ni nanoparticles with 21 ± 5 nm nm are selectively deposited inside the pores of the matrix. Although through the MFM measurements it was impossible to confirm the magnetic behaviour due to the parallel orientation of the domains, SQUID measurements undoubtedly prove that a magnetic material was successfully deposited in the porous films of BaTiO3. The chemical composition of the porous matrix and of the nanoparticles can be modified by the use of different inorganic precursors. This novel strategy of functionalization based in supercritical fluid deposition to create nanocomposite materials is very versatile and can open perspectives for the application of these materials as nanopatterned media (magnetic data storage devices) as well as multiferroic materials.
[1] Castro et al. J. Mater. Chem. C, 2016, DOI: 10.1039/C6TC04232E.
Acknowledgements: Project CICECO-Aveiro Institute of Materials POCI-01-0145-FEDER-007679 (Ref. FCT UID/CTM/50011/2013) (FCT/ME, FEDER and PT2020 Partnership Agreement). FCT and POPH/FSE, for doctoral SFRH/BD/67121/2009 and investigator IF/00327/2013 grants. This work was also supported by BioISI (Ref. UID/MULTI/04046/2013) from FCT/MCTES/PIDDAC, and by MINECO (project CTQ2013-41781 and J. Morère grant.
The fabrication costs of integrated piezoelectric devices are mainly determined by the manufacturing process itself. Inkjet printing recently emerged as an efficient lithography-free technique. Patterned structures can be printed on various substrates under ambient atmospheric conditions using only minimal amounts of precursors, thus significantly reducing economic and ecological footprints. In the present contribution we will describe a strategy for printing a sol-gel-based lead zirconate titanate (PZT) ink on platinized silicon.
The employed PZT ink has near-morphotropic-phase-boundary composition (MPB). A mixture of dehydrated lead(II) acetate, zirconium(IV) butoxide and titanium(IV) isopropoxide in 2-methoxyethanol with 10% excess lead is heated at reflux during two hours to ensure homogenization and stabilization of alkoxide species via ligand exchange. The resulting PZT sol is then diluted to 0.3 M with ethylene glycol and bis(2-ethoxyethyl) ether to adjust ink viscosity and surface tension for efficient droplet ejection using Dimatix cartridges.
Platinum has very high surface energy (~ 1 J/m2), which makes direct printing of organic solvent-based inks difficult because of extreme wetting. Self-assembled monolayers (SAMs) have been used to modify the platinum surface and constrain ink spreading. Then 2×2 mm2 PZT squares were printed on the substrate. After drying, pyrolysis is performed at 350 °C and crystallization at 700 °C. The obtained 100 nm-thick structures show a typical Raman signature of PZT in MPB composition and exhibit ferroelectric switching. We will describe in detail the optimization of inkjet printing from ink formulation to deposition and the influence of the processing parameters on microstructure and functional behavior of the PZT films.
Piezoelectric thin films have been extensively deployed in MEMS components. Recent works have demonstrated the fabrication of lead zirconate titanate (PZT) thin films on glass substrates, exhibiting well-crystallized perovskite phase and transmittance in the visible range higher than 60 %. In this work, we report the development of the large area ultrasonic transducers based on sol-gel-deposited piezoelectric films that are integrated on glass. We designed transparent circular interdigital electrodes (IDE) to optimize the volume of the active piezoelectric material.
Near-morphotropic-phase-boundary PZT films were deposited on fused silica substrates covered with a 10-nm-thick TiOx buffer layer. The IDE structures, with width of lateral gaps and electrodes in the range between 2 and 20 µm, were patterned by lift-off lithography. Indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO) were tested as transparent electrodes, while Pt and Au-based patterns were used for the reference. The mechanical resonance was characterized with laser vibrometer.
The XRD and SEM characterization of PZT films revealed well crystallized perovskite phase, dense microstructure and no cracks. Light transmittance, measured in the visible range, is higher than 60 %. Piezoelectric measurements on a cantilever-like structure with 200-nm-thick PZT films yield piezoelectric coefficient d33 of 140 pm/V.
The Au-based circular actuators, with diameter of 8 mm, exhibit capacitance of 1.1 nF, which corresponds to the capacitance per unit surface of 2.2 nF/cm². Dielectric losses are below 5 %. When actuated with a 10 Vpeak-peak sinusoidal voltage at resonant frequency of 104 kHz, the peak-to-peak displacement is 32 nm, corresponding to a quality factor of 109. At lower frequencies the actuators withstands up to 100 V, which should allow hundreds of nanometers displacements at the resonance. The results demonstrate the potential of large area transparent piezo-actuators for ultrasonic and haptic applications.
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