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1. Optimized emitter-base interface cleaning for advanced Heterojunction Bipolar Transistors

Author

E. Brezza, F. Deprat, C. de Buttet, A. Gauthier, M. Gregoire, D. Guiheux, V. Guyader, M. Juhel, I. Berbezier, E. Assaf, L. Favre, P. Chevalier, C. Gaquière, N. Defrance, STMicroelectronics [Crolles] (ST-CROLLES), Institut d’Électronique, de Microélectronique et de Nanotechnologie - UMR 8520 (IEMN), Centrale Lille-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Université Polytechnique Hauts-de-France (UPHF)-JUNIA (JUNIA), Université catholique de Lille (UCL)-Université catholique de Lille (UCL), Puissance - IEMN (PUISSANCE - IEMN), Université catholique de Lille (UCL)-Université catholique de Lille (UCL)-Centrale Lille-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Université Polytechnique Hauts-de-France (UPHF)-JUNIA (JUNIA), Commissariat à l'énergie atomique et aux énergies alternatives - Laboratoire d'Electronique et de Technologie de l'Information (CEA-LETI), Direction de Recherche Technologique (CEA) (DRT (CEA)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA), Laboratoire Cycles Géochimiques et ressources (LCG), Géosciences Marines (GM), Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Institut des Matériaux, de Microélectronique et des Nanosciences de Provence (IM2NP), Aix Marseille Université (AMU)-Université de Toulon (UTLN)-Centre National de la Recherche Scientifique (CNRS), Hôpital Louis Pradel [CHU - HCL], Hospices Civils de Lyon (HCL), Declaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared toinfluence the work reported in this paper., NO FUNDING FOUND, and Laboratoire commun STMicroelectronics-IEMN T1

Subjects
[PHYS]Physics [physics], [SPI]Engineering Sciences [physics], Materials Chemistry, Heterojunction Bipolar Transistor, Thermal treatment, Electrical and Electronic Engineering, Interface, SiconiTM etching, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Hydrofluoric acid etching
Abstract

International audience; A B S T R A C THeterojunction Bipolar Transistors needed for high-frequency applications require precise dopant control. Insitu doped epitaxies used during device fabrication rely on surface preparation to obtain an optimized dopingprofile. Defects due to imperfect cleaning relates to high emitter resistance and low-frequency noise.Base epitaxy is followed by the fabrication of thin L-shaped oxide spacers and in-situ arsenic-doped emitterepitaxy by Low Pressure Chemical Vapor Deposition. A cleaning step between spacers formation and emitterepitaxy is mandatory to remove residual contamination. Hydrofluoric acid (HF) wet cleaning, in-situ remoteplasma cleaning (SiconiTM) and thermal treatments have been tested. A minimum etching budget is sought forlimiting spacer consumption while adequately cleaning the interface.Time Of Flight-Secondary Ion Mass Spectroscopy (TOF-SIMS) and High-Resolution Transmission ElectronMicroscopy (HR-TEM) are used to characterize the interface and measure levels of arsenic, oxygen, fluorineand carbon. Emitter resistance and base-emitter breakdown voltage measurements are used to show the impacton electrical figures of merit. Siconi targeting 10 Å of thermal oxide removal followed by a thermal treatment(800 ◦C, 60 s) in the deposition chamber results the best process among the tested ones.

Published
2023
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2. Climate-driven variability of the Southern Ocean CO 2 sink

Author

N. Mayot, C. Le Quéré, C. Rödenbeck, R. Bernardello, L. Bopp, L. M. Djeutchouang, M. Gehlen, L. Gregor, N. Gruber, J. Hauck, Y. Iida, T. Ilyina, R. F. Keeling, P. Landschützer, A. C. Manning, L. Patara, L. Resplandy, J. Schwinger, R. Séférian, A. J. Watson, R. M. Wright, J. Zeng, Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL), Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette] (LSCE), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Modelling the Earth Response to Multiple Anthropogenic Interactions and Dynamics (MERMAID), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Centre national de recherches météorologiques (CNRM), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), National Oceanic and Atmospheric Administration, NOAA: NA20OAR4320278, OPP-1922922, Horizon 2020 Framework Programme, H2020: 820989, 821003, Natural Environment Research Council, NERC: NE/P021417/1, Royal Society: RPR1191063, University of East Anglia, UEA, European Commission, EC, Deutsche Forschungsgemeinschaft, DFG: PA 3075/2-1, Eidgenössische Technische Hochschule Zürich, ETH: 101003536, Norges Forskningsråd: 270061, Helmholtz Association: VH-NG-1301, Neurosciences Foundation, NSF, N.M., C.L.Q., R.B., N.G., L.G. and A.C.M. acknowledge the funding from the European Commission through the H2020 project 4C (grant no. 821003). J.H., L.G., M.G. and N.G. acknowledge the funding from the European Commission through the H2020 project COMFORT project (grant no. 820989). C.L.Q. was funded by the UK Royal Society (grant no. RPR1191063). N.M. and R.M.W. were funded by UK's Natural Environment Research Council (SONATA: grant no. NE/P021417/1). J.S. received funding from the Research Council of Norway through project INES (grant no. 270061) and HPC resources provided by the National Infrastructure for HPC and Data Storage in Norway, UNINETT Sigma2 (grant no. nn/ns2980k). L.R. acknowledges the Princeton University Catalysis Initiative. N.G. and L.G. acknowledge funding from ETH Zürich. M.G. and R.S. acknowledge the ESM2025 project under the grant agreement number 101003536. M.G. also acknowledges funding from the European Union's Horizon 2020 Blue Growth research and innovation programme under grant agreement number 862923 (project AtlantECO). J.H. acknowledges support by the Initiative and Networking Fund of the Helmholtz Association (Helmholtz Young Investigator Group Marine Carbon and Ecosystem Feedbacks in the Earth System [MarESys], grant number VH-NG-1301). The integration of the ORCA025-GEOMAR experiment was performed at the North German Supercomputing Alliance (HLRN) and was financially supported by the German Research Foundation (project PA 3075/2-1). The APO measurements were supported by a series of grants to the Scripps Institution of Oceanography from the US NSF and NOAA, most recently OPP-1922922 and NA20OAR4320278. Acknowledgements, and Thanks to Erik Buitenhuis and David Willis for their work on the NEMO-PlankTOM12 model development. The research presented in this paper was carried out on the High Performance Computing Cluster supported by the Research and Specialist Computing Support service at the University of East Anglia.

Subjects
[PHYS]Physics [physics], carbon sink, General Mathematics, General Engineering, General Physics and Astronomy, Southern ocean, Carbon sink, Climate, Oxygen, Interannual, interannual, Southern Ocean, decadal, climate, oxygen
Abstract

The Southern Ocean is a major sink of atmospheric CO2, but the nature and magnitude of its variability remains uncertain and debated. Estimates based on observations suggest substantial variability that is not reproduced by process-based ocean models, with increasingly divergent estimates over the past decade. We examine potential constraints on the nature and magnitude of climate-driven variability of the Southern Ocean CO2 sink from observation-based air-sea O2 fluxes. On interannual time scales, the variability in the air-sea fluxes of CO2 and O2 estimated from observations is consistent across the two species and positively correlated with the variability simulated by ocean models. Our analysis suggests that variations in ocean ventilation related to the Southern Annular Mode are responsible for this interannual variability. On decadal time scales, the existence of significant variability in the air-sea CO2 flux estimated from observations also tends to be supported by observation-based estimates of O2 flux variability. However, the large decadal variability in air-sea CO2 flux is absent from ocean models. Our analysis suggests that issues in representing the balance between the thermal and non-thermal components of the CO2 sink and/or insufficient variability in mode water formation might contribute to the lack of decadal variability in the current generation of ocean models. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'., Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 381 (2249), ISSN:1364-503X, ISSN:1471-2962

Published
2023
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3. A SPHERE survey of self-shadowed planet-forming disks

Author

A. Garufi, C. Dominik, C. Ginski, M. Benisty, R. G. van Holstein, Th. Henning, N. Pawellek, C. Pinte, H. Avenhaus, S. Facchini, R. Galicher, R. Gratton, F. Ménard, G. Muro-Arena, J. Milli, T. Stolker, A. Vigan, M. Villenave, T. Moulin, A. Origne, F. Rigal, J.-F. Sauvage, L. Weber, INAF - Osservatorio Astrofisico di Arcetri (OAA), Istituto Nazionale di Astrofisica (INAF), Astronomical Institute Anton Pannekoek (AI PANNEKOEK), University of Amsterdam [Amsterdam] (UvA), Laboratoire Franco-Chilien d'Astronomie (LFCA), Universidad de Chile = University of Chile [Santiago] (UCHILE)-Pontificia Universidad Católica de Chile (UC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Universidad de Concepción - University of Concepcion [Chile], Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Centre National d'Études Spatiales [Toulouse] (CNES)-Observatoire des Sciences de l'Univers de Grenoble (OSUG ), Institut national des sciences de l'Univers (INSU - CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Université Grenoble Alpes (UGA)-Météo-France -Institut national des sciences de l'Univers (INSU - CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Université Grenoble Alpes (UGA)-Météo-France, Leiden Observatory [Leiden], Universiteit Leiden, Lakeside Labs [Klagenfurt], University of Cambridge [UK] (CAM), Università degli Studi di Milano = University of Milan (UNIMI), European Southern Observatory (ESO), Observatoire de Paris, Université Paris sciences et lettres (PSL), INAF - Osservatorio Astronomico di Padova (OAPD), Institute for Particle Physics and Astrophysics [ETH Zürich] (IPA), Department of Physics [ETH Zürich] (D-PHYS), Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich] (ETH Zürich)- Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich] (ETH Zürich), Laboratoire d'Astrophysique de Marseille (LAM), Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS), Jet Propulsion Laboratory (JPL), NASA-California Institute of Technology (CALTECH), DOTA, ONERA, Université Paris Saclay [Palaiseau], ONERA-Université Paris-Saclay, Geneva Observatory, Université de Genève = University of Geneva (UNIGE), SPHERE was funded by ESO, with additional contributions from CNRS (France), MPIA (Germany), INAF (Italy), FINES (Switzerland) and NOVA (Netherlands). SPHERE also received funding from the European Commission Sixth and Seventh Framework Programmes as part of the Optical Infrared Coordination Network for Astronomy (OPTICON) under grant number RII3-Ct-2004-001566 for FP6 (2004–2008), grant number 226604 for FP7 (2009–2012) and grant number 312430 for FP7 (2013–2016). We also acknowledge financial support from the Programme National de Planètologie (PNP) and the Programme National de Physique Stellaire (PNPS) of CNRS-INSU in France. This paper also makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.01600.S and 2015.1.00847.S. ALMAis a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.This work was supported by the PRIN-INAF 2016 'The Cradle of Life – GENESIS-SKA (General Conditions in Early Planetary Systems for the rise of life with SKA)', the project PRIN-INAF-MAIN-STREAM 2017 'Protoplanetary disks seen through the eyes of new-generation instruments', the PRIN-INAF 2019 'Planetary systems at young ages (PLATEA)', the program PRIN-MIUR 2015 STARS in the CAOS – Simulation Tools for Astrochemical Reactivity and Spectroscopy in the Cyberinfrastructure for Astrochemical Organic Species (2015F59J3R, MIUR Ministero dell’Istruzione, dell’Università, della Ricerca e della Scuola Normale Superiore), the European Marie Sklodowska-Curie Actions under the European Union’s Horizon 2020 research and innovation program through the Project 'AstroChemistry Origins' (ACO), Grant No 811312, INAF/Frontiera (Fostering high ResolutiON Technology and Innovation for Exoplanets and Research in Astrophysics) through the 'Progetti Premiali' funding scheme of the Italian Ministry of Education, University, and Research, as well as NSF grants AST- 1514670 and NASA NNX16AB48G, the Italian Ministero dell’Istruzione, Università e Ricerca through the grant Progetti Premiali 2012 – iALMA (CUP C52I13000140001).C.D. and C.G. acknowledge support from the NWO TOP grant 'Herbig Ae/Be stars, Rosetta stones for understanding the formation of planetary systems', project number 614.001.552. T.H. acknowledges support from the European Research Council under the Horizon 2020 Framework Program via the ERC Advanced Grant Origins 83 24 28., European Project: 811312, European Project: 832428, and Low Energy Astrophysics (API, FNWI)

Subjects
Physics, Earth and Planetary Astrophysics (astro-ph.EP), Protoplanetary disks, [PHYS]Physics [physics], Polarimetric, Astronomy, FOS: Physical sciences, Astronomy and Astrophysics, Astrophysics, Astrophysics::Cosmology and Extragalactic Astrophysics, Astrophysics - Astrophysics of Galaxies, [SPI]Engineering Sciences [physics], Astrophysics - Solar and Stellar Astrophysics, Space and Planetary Science, Planet, Astrophysics of Galaxies (astro-ph.GA), Techniques: Polarimetric, Astrophysics::Earth and Planetary Astrophysics, Solar and Stellar Astrophysics (astro-ph.SR), Astrophysics::Galaxy Astrophysics, Astrophysics - Earth and Planetary Astrophysics
Abstract

To date, nearly two hundred planet-forming disks have been imaged with high resolution. Our propensity to study bright and extended objects is however biasing our view of the disk demography. In this work, we contribute to alleviate this bias by analyzing fifteen disks targeted with VLT/SPHERE that look faint in scattered light. Sources were selected based on a low far-IR excess from the spectral energy distribution. The comparison with the ALMA images available for a few sources shows that the scattered light surveyed by these datasets is only detected from a small portion of the disk extent. The mild anti-correlation between the disk brightness and the near-IR excess demonstrates that these disks are self-shadowed: the inner disk rim intercepts much starlight and leaves the outer disk in penumbra. Based on the uniform distribution of the disk brightness in scattered light across all spectral types, self-shadowing would act similarly for inner rims at a different distance from the star. We discuss how the illumination pattern of the outer disk may evolve with time. Some objects in the sample are proposed to be at an intermediate stage toward bright disks from the literature with either no shadow or with sign of azimuthally confined shadows., Accepted for publication in A\&A

Published
2021
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4. Numerical Investigations on the Rotating Stall in an Axial Compressor and Its Control by Flow Injection at Casing

Author

Marty, Julien, Castillon, Lionel, Joseph, Pierric, DAAA, ONERA, Université Paris Saclay [Meudon], ONERA-Université Paris-Saclay, Laboratoire de Mécanique des Fluides de Lille – Kampé de Fériet - UMR 9014 (LMFL), Centrale Lille-ONERA-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Arts et Métiers Sciences et Technologies, HESAM Université - Communauté d'universités et d'établissements Hautes écoles Sorbonne Arts et métiers université (HESAM)-HESAM Université - Communauté d'universités et d'établissements Hautes écoles Sorbonne Arts et métiers université (HESAM), and This paper is funded by European Union's Horizon 2020 research and innovation program under grant agreement No 886352, project ACONIT.

Subjects
[PHYS]Physics [physics], CONTROLE ACTIF D'ECOULEMENT, Mechanical Engineering, Active flow, air injection, INJECTION D'AIR, rotating stall, [SPI.MECA.GEME]Engineering Sciences [physics]/Mechanics [physics.med-ph]/Mechanical engineering [physics.class-ph], [SPI.MECA.MEFL]Engineering Sciences [physics]/Mechanics [physics.med-ph]/Fluids mechanics [physics.class-ph], [SPI.AUTO]Engineering Sciences [physics]/Automatic, [SPI]Engineering Sciences [physics], COMPRESSEUR, compressor, Active flow control, Mécanique: Mécanique des fluides [Sciences de l'ingénieur], DECOLLEMENT TOURNANT
Abstract

International audience; The operating range of a compressor is limited by surge or rotating stall line, among others. Numerical simulations must accurately predict these phenomena. This study is based on the experimental compressor CME2, which is a low-subsonic axial compressor. This compressor is tip-critical as the rotor tip is responsible for the rotating stall. This paper shows that the rotating stall onset flow rate is well captured by CFD, compared to experiments. After ten revolutions, all cells are merged and only one cell remains, as in experiments. Active flow control improves compressor performance and extends the stable operating range. In the present configuration, flow injection is performed at the casing. In the simulation, the insertion of the actuators is carried out through hybrid meshes: structured mesh for blade passages and unstructured mesh for each actuator. For the some stalled operating points of baseline configuration, there is no rotating cells in the controlled configuration. Thus, the rotating stall is delayed at lower flow rate, as expected by the use of active flow control and is in agreement with the experiments.; Le domaine de fonctionnement d'un compresseur est limité par la ligne de pompage ou de décollement tournant, entre autres. Les simulations numériques doivent être capables de prévoir cette limite. Cette étude repose sur le compresseur expérimental CME2 (compresseur axial faible-vitesse). Ce compresseur est critique en tête car le jeu du rotor est responsable du décrochage tournant. Ce papier montre que le décollement tournant est bien prévu par les simulations numériques, par comparaison aux essais. Après dix révolutions, toutes les cellules de décrochage ont fusionné et il ne reste qu'une seule cellule, comme dans les essais. Le contrôle actif par injection de fluide améliore la performance du compresseur et étend sa plage de fonctionnement stable. Dans les simulations numériques, les injecteurs sont pris en compte à l'aide de maillages hybrides : maillage structuré pour la veine et non-structuré pour chaque actionneur. Pour certains points initialement décrochés, le décollement tournant n'apparait plus. La limite de décrochage est ainsi retardée vers des débits plus faibles.

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5. Interplay between Ca- and Ti-driven ferroelectric distortions in (Ba, Ca)TiO 3 solid solutions from first-principles calculations

Author

Andres Cano, Danila Amoroso, Philippe Ghosez, Université de Liège, Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB), Université de Bordeaux (UB)-Institut Polytechnique de Bordeaux-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), Théorie de la Matière Condensée (TMC ), Institut Néel (NEEL), Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019]), and D.A. is grateful to S. Picozzi (CNR-SPIN) for the provided time needed for writing this paper and to B. Dkhil (Centrale-Supléc) for enlightening discussions. Ph.G. acknowledges the support of the F.R.S-FNRS Hit4Fit project. This work was supported by the European project EJD-FunMat 2015 and program H2020-MSCA-ITN-2014 under Grant Agreement No. 641640. Calculations have been performed on the Céci facilities funded by F.R.S-FNRS (Grant No. 2.5020.1) and on the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles funded by the Walloon Region (Grant No. 1117545).

Subjects
010302 applied physics, Steric effects, [PHYS]Physics [physics], Phase boundary, Materials science, Physics and Astronomy (miscellaneous), 02 engineering and technology, [CHIM.MATE]Chemical Sciences/Material chemistry, Partial substitution, 021001 nanoscience & nanotechnology, 01 natural sciences, Ferroelectricity, Piezoelectricity, Chemical physics, 0103 physical sciences, [PHYS.COND.CM-MS]Physics [physics]/Condensed Matter [cond-mat]/Materials Science [cond-mat.mtrl-sci], [PHYS.COND]Physics [physics]/Condensed Matter [cond-mat], 0210 nano-technology, Polarization (electrochemistry), Solid solution, Perovskite (structure)
Abstract

(Ba,Ca)(Ti,Zr)O3 solid solutions are promising lead-free piezoelectrics near their polymorphic phase boundary, which is believed to be linked to the interplay between B-site driven ferroelectricity and A-site driven ferroelectricity. Focusing on (Ba,Ca)TiO3, we support this picture from first-principles calculations. In particular, we show how steric effects related to the partial substitution of Ba by Ca largely enhance the Ca-driven ferroelectricity, already virtually allowed in the parent CaTiO3. The emergent interplay between the Ca-driven and Ti-driven mechanisms lowers the energy barrier between different polar states, which eventually results in a quasi-isotropic polarization under substitution of a small concentration of Ba by Ca. A sizeable enhancement of the piezoelectric response directly results from these features.(Ba,Ca)(Ti,Zr)O3 solid solutions are promising lead-free piezoelectrics near their polymorphic phase boundary, which is believed to be linked to the interplay between B-site driven ferroelectricity and A-site driven ferroelectricity. Focusing on (Ba,Ca)TiO3, we support this picture from first-principles calculations. In particular, we show how steric effects related to the partial substitution of Ba by Ca largely enhance the Ca-driven ferroelectricity, already virtually allowed in the parent CaTiO3. The emergent interplay between the Ca-driven and Ti-driven mechanisms lowers the energy barrier between different polar states, which eventually results in a quasi-isotropic polarization under substitution of a small concentration of Ba by Ca. A sizeable enhancement of the piezoelectric response directly results from these features.

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