Functional oxides in photonic integrated devices

[EN] At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore¿s law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community.Gervasi Herranz acknowledges financial support from MAT2014-56063-C2-1-R and Severo Ochoa SEV-2015-0496 Projects, and the Generalitat de Catalunya (2014 SGR 734Project). Pablo Sanchis acknowledges financial support from TEC2016-76849 and FP7-ICT-2013-11-619456 SITOGA.Herranz, G.; Sanchis Kilders, P. (2019). Functional oxides in photonic integrated devices. Applied Surface Science. 482:52-55. https://doi.org/10.1016/j.apsusc.2019.03.312525548

Conducting interfaces between amorphous oxide layers and SrTiO3(110) and SrTiO3(111)

Interfaces between (110) and (111)SrTiO3 (STO) single crystalline substrates and amorphous oxide layers, LaAlO3 (a-LAO), Y:ZrO2 (a-YSZ), and SrTiO3 (a-STO) become conducting above a critical thickness tc. Here we show that tc for a-LAO is not depending on the substrate orientation, i.e. tc (a-LAO/(110)STO) ~ tc(a-LAO/(111)STO) interfaces, whereas it strongly depends on the composition of the amorphous oxide: tc(a-LAO/(110)STO) < tc(a-YSZ/(110)STO) < tc(a-STO/(110)STO). It is concluded that the formation of oxygen vacancies in amorphous-type interfaces is mainly determined by the oxygen affinity of the deposited metal ions, rather than orientational-dependent enthalpy vacancy formation and diffusion. Scanning transmission microscopy characterization of amorphous and crystalline LAO/STO(110) interfaces shows much higher amount of oxygen vacancies in the former, providing experimental evidence of the distinct mechanism of conduction in these interfaces

Magnetoelastic coupling in La2/3Sr1/3MnO3 thin films on SrTiO3

Clamping of epitaxial La2/3Sr1/3MnO3 (LSMO) magnetic thin films on SrTiO3 (STO) substrates is shown to promote a clear modification of their magnetic properties at the STO cubic-tetragonal transition. Two distinct mechanisms triggered by the STO transition, namely magnetic domain pattern reconstruction and creation of regions within the magnetically soft LSMO with enhanced magnetic anisotropy, are proposed to be behind the observed anomalous magnetic responses at low ac-magnetic field and at high dc-field, respectively. The persistence of these anomalies in LSMO films as thick as 220 nm shines new light into the magnetoelastic coupling mechanisms across interfaces

High ferroelectric polarization in c-oriented BaTiO3 epitaxial thin films on SrTiO3/Si(001)

Scigaj, M. et al.The integration of epitaxial BaTiO3 films on silicon, combining c-orientation, surface flatness, and high ferroelectric polarization is of main interest towards its use in memory devices. This combination of properties has been only achieved so far by using yttria-stabilized zirconia buffer layers. Here, the all-perovskite BaTiO3/LaNiO3/SrTiO3 heterostructure is grown monolithically on Si(001). The BaTiO3 films are epitaxial and c-oriented and present low surface roughness and high remnant ferroelectric polarization around 6 μC/cm2. This result paves the way towards the fabrication of lead-free BaTiO3 ferroelectric memories on silicon platforms.ICMAB-CSIC authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496) and the MAT2014-56063-C2-1-R project, and from Generalitat de Catalunya (2014 SGR 734). Work at Oak Ridge National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. C. H. Chao acknowledges the NSC-CSIC 2014 Summer Program in Spain for Taiwanese PhD students. I. Fina acknowledges Juan de la Cierva – Incorporación postdoctoral fellowship (IJCI-2014-19102) from the Spanish Ministry of Economy and Competitiveness of Spanish Government. INL gratefully acknowledges the European commission and the national French research agency (ANR) for funding, through the projects SITOGA (FP7-ICT-2013-11-619456), TIPS (H2020‐ICT‐02-2014‐1-644453), ANR HIRIS and ANR DIAMWAFEL. INL also acknowledges P. Regreny, C. Botella and J.-B. Goure for MBE technical assistance.Peer reviewe

Untangling the contributions of cerium and iron to the magnetism of Ce-doped yttrium iron garnet

The remarkable magnetic properties of yttrium iron garnets (YIGs) underpin the use of these materials in a broad scope of spintronic and photonic applications. In particular, the addition of rare earth metals in the structure enhances to a great extent the magneto-optical activity, which is beneficial for the development of nonreciprocal optical devices. Exploiting the wavelength selectivity of magneto-optics, we have identified a range of frequencies at which one can unravel the individual contributions to the magnetism and gyrotropic response arising from cerium and iron. We envision that this outcome may pave the way to further experiments to assess quantitatively the effect on the optical properties of rare earth incorporation into YIG.This work was supported by the Spanish Government by the MAT2011-29269-C03, MAT2014-56063-C2-1-R, and the Severo Ochoa SEV-2015-0496 Projects and the Generalitat de Catalunya (2014 SGR 734 Project). B.C. acknowledges his grant FPI BES-2012-059023 and R.C. acknowledges his fellowship from CNPq - Brazil. S.G., M.O., and R.G. thank T. Brenninger and E. Zamburg for technical support. We also acknowledge the contribution of C. Rubio in preliminary structural and magnetic characterization of thin films as well as Dr. F. Sanchez for scientific guiding and discussions on materials issues.Peer Reviewe

Electron energy loss spectroscopy determination of Ti oxidation state at the (001) LaAlO3/SrTiO3 interface as a function of LaAlO3 growth conditions

At the (001) interface between the two band-insulators LaAlO3 and SrTiO3, a high-mobility electron gas may appear, which has been the object of numerous works over the last four years. Its origin is a subject of debate between the interface polarity and unintended doping. Here we use electron energy loss 'spectrum images', recorded in cross-section in a scanning transmission electron microscope, to analyse the Ti3+ ratio, characteristic of extra electrons. We find an interface concentration of Ti3+ that depends on growth conditions.Comment: 6 page

Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films

The electron occupancy of 3d-orbitals determines the properties of transition metal oxides. This can be achieved, for example, through thin-film heterostructure engineering of ABO(3) oxides, enabling emerging properties at interfaces. Interestingly, epitaxial strain may break the degeneracy of 3d-e(g) and t(2g) orbitals, thus favoring a particular orbital filling with consequences for functional properties. Here we disclose the effects of symmetry breaking at free surfaces of ABO(3) perovskite epitaxial films and show that it can be combined with substrate-induced epitaxial strain to tailor at will the electron occupancy of in-plane and out-of-plane surface electronic orbitals. We use X-ray linear dichroism to monitor the relative contributions of surface, strain and atomic terminations to the occupancy of 3z(2)-r(2) and x(2)-y(2) orbitals in La(2/3)Sr(1/3)MnO(3) films. These findings open the possibility of an active tuning of surface electronic and magnetic properties as well as chemical properties (catalytic reactivity, wettability and so on)

Multiple strain-induced phase transitions in LaNiO3 thin films

Weber, M.C. et al.Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully-relaxed films, appear under both compressive and tensile strains, indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain which may impact the electronic orbital hierarchy. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations.J.K., M.W., M.G., and J.I. acknowledge support from the National Research Fund, Luxembourg through a Pearl grant (Grant No. FNR/P12/4853155). ICMAB-CSIC authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015- 0496) and the MAT2014-56063-C2-1-R and MAT2013- 40581-P projects, and from Generalitat de Catalunya (2014 SGR 734). UB authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, project MAT2013-41506-P and from Generalitat de Catalunya (2014 SGR 672).Peer reviewe

Untangling Electrostatic and Strain Effects on the Polarization of Ferroelectric Superlattices

Khestanova, Ekaterina et al.The polarization of ferroelectric superlattices is determined by both electrical boundary conditions at the ferroelectric/paraelectric interfaces and lattice strain. The combined infl uence of both factors offers new opportunities to tune ferroelectricity. However, the experimental investigation of their individual impact has been elusive because of their complex interplay. Here, a simple growth strategy has permitted to disentangle both contributions by an independent control of strain in symmetric superlattices. It is found that fully strained short-period superlattices display a large polarization whereas a pronounced reduction is observed for longer multilayer periods. This observation indicates that the electrostatic boundary mainly governs the ferroelectric properties of the multilayers whereas the effects of strain are relatively minor.Financial support by the Spanish Government [Projects MAT2014- 56063-C2-1-R and MAT2013-41506 ] and Generalitat de Catalunya ( 2014-SGR-734 and 2014-SGR-672 ) is acknowledged. ICMAB-CSIC authors acknowledge fi nancial support from the Spanish Ministry of Economy and Competitiveness , through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496 ). I.F. acknowledges Juan de la Cierva – Incorporación postdoctoral fellowship (IJCI-2014- 19102) from the Spanish Ministry of Economy and Competitiveness. The transmission electron microscopy works were conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragón (Universidad de Zaragoza). The authors acknowledge the LMA-INA for offering access to their instruments and expertise. The authors thank Massimiliano Stengel for useful discussions.Peer reviewe