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2018 Fall Meeting



Epitaxial oxide films for electronic applications

Epitaxial oxide films enable a new generation of oxide electronic devices and energy applications. Materials such as Ga2O3, In2O3, and BaSnO3 open up new horizons in semiconductor electronics, while ferroelectric and multiferroic perovskites bring promise of a wide range of novel devices. This symposium covers topics ranging from theory, growth, alloying, and interface/surface properties to defect characterization, optical and electrical properties, and device fabrication.


Oxides are among the materials with the widest tunability of physical properties. Well-defined oxide structures of the highest material quality are particularly interesting for the next generation of electronic devices with tailored and/or unprecedented properties. Oxide semiconductors such as ZnO, Ga2O3, In2O3 and BaSnO3 are being investigated for their use in optoelectronics, power electronics, 2DEG electron transport, and device gating, whereas epitaxial layers with perovskite structure (e.g. KNbO3, SrTiO3, LaAlO3, BiFeO3, and others), along with their fascinating interfaces, pave the way to novel applications that exploit their ferroelectric, multiferroic, memristive and other outstanding functional properties. Despite their great promise, there are still many challenges associated with the technological development of epitaxial oxides for electronic applications. Preparation of tailored substrates and surfaces, precise control of growth, assessment of structural properties, understanding of the role of defects and interfaces, and correlating these with the optical and electronic properties – all constitute essential prerequisites for the successful deployment of novel oxide electronic devices. A more thorough understanding of the basic processes in these materials is thus required to control the various issues along the value chain and cross-fertilization between hitherto separated research fields will be needed to provide the necessary advancement. This symposium aims to provide the corresponding platform in this respect.

The symposium is dedicated to crystalline oxide materials, particularly transparent semiconducting oxides and oxides with the perovskite or related structures, of considerable structural quality that are designated for use in electronic devices and applications. It seeks to address all relevant topics in materials development, such as thermodynamic, material, and defect modelling, preparation of substrate materials, epitaxial thin film growth (e.g. MBE, MOCVD, PLD), doping, alloying, and defect formation, surfaces and interfaces, structural properties and defects, optical, electrical and thermal properties, contacts and structuring, and device design and preparation.

Hot topics to be covered by the symposium:

  • Ga2O3 and In2O3 homo- and heteroepitaxy
  • (Al,Ga,In)2O3 films and heterostructures
  • SrTiO3 and perovskite-based based thin film deposition
  • Ferroelectric thin films and heterostructures
  • Multiferroic oxides and devices
  • Approaches to memristive devices
  • BaSnO3 conducting layers
  • All-perovskite device concepts
  • Sensing with epitaxial oxide films (e.g. SAW)
  • Ga2O3 devices for power electronics
  • Ferroelectrics on semiconductors
  • Doping approaches during thin film growth
  • Surface and interface characterization
  • Correlation of structure/defects and properties
  • Spectroscopy and transport phenomena

Invited speakers (confirmed):

  • Ausrine Bartasyte, Univ. de Franche-Comté (FR)
  • Oliver Bierwagen, PDI Berlin (DE)
  • Ingrid Cañero Infante, Institut de Nanotechnologies de Lyon (FR)
  • Kookrin Char, Seoul National Univ. (KR)
  • Cristiana Di Valentin, Univ. Milano-Bicocca (IT)
  • Regina Dittmann , Forschungszentrum Jülich (DE)
  • Catherine Dubourdieu, Helmholtz-Zentrum Berlin (DE)
  • Saskia Fischer, HU Berlin (DE)
  • Marta Gibert, Univ. Zurich (CH)
  • Gervasi Herranz, Institut de Ciència de Materials de Barcelona (ES)
  • Marcel Himmerlich, TU Ilmenau (DE) and CERN (CH)
  • Felix Gunkel, RWTH Aachen (DE)
  • Brandon Howe, Air Force Research Lab (US)
  • Gregg Jessen, Air Force Research Lab (US)
  • Gertjan Koster, Univ. Twente (NL)
  • Lane Martin, UC Berkeley (US)
  • Hisashi Murakami, Tokyo Univ. of Agriculture and Technology (JP)
  • Tae Won Noh, Seoul National Univ. (KR)
  • Takayoshi Oshima, Saga Univ. (JP)
  • Amador Pérez-Tomas, Inst. Català de Nanociència (ES)
  • Lucian Pintilie, National Institute of Materials Physics (RO)
  • Fan Ren, Univ. Florida Gainesville (US)
  • Guus Rijnders, Univ. Twente (NL)
  • Sayeef Salahuddin, UC Berkeley (US)
  • Jacobo Santamaria, Univ. Madrid (ES)
  • Tony Schenk, namlab Dresden (DE) and LIST (LU)
  • Heidemarie Schmidt, TU Chemnitz (DE)
  • Uttam Singisetti, Univ. Buffalo (US)
  • James Speck, UC Santa Barbara (US)
  • Susanne Stemmer, UC Santa Barbara (US)
  • Christopher Sutton, Fritz Haber Institute, Berlin (DE)
  • Holger von Wenckstern, Univ. Leipzig (DE)
  • Man Hoi Wong, NICT (JP)
  • Chadwin Young, Univ. Texas Dallas (US)
  • Hongping Zhao, Ohio State Univ. (US)
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Ga2O3 Growth & Characterization I : Oliver Bierwagen
Authors : Takayoshi Oshima
Affiliations : Saga university

Resume : Recently, corundum structured ?-Ga2O3 has attracted attention as an ultra-wide-band-gap semiconductor for the future application of high-power and deep-UV devices. Among the studies, research and development on ?-(AlxGa1-x)2O3-based heterostructures have also gathered interest owing to a wide band-gap tuning range from 5.3 to 8.8 eV. In particular, strong quantum effects are expected in ?-Al2O3/Ga2O3 heterostructures because of the large difference in their band-gaps (Eg). However, fabrication of the heterostructures having coherent interfaces is challenging due to the large lattice mismatches. In this study, we therefore attempted to fabricate ?-Al2O3/Ga2O3 superlattices (SLs) on isostructural r-plane sapphire by molecular beam epitaxy and characterized them with various methods. By systematic variation of ?-Ga2O3 thickness and evaluation through X-ray reflectivity and diffraction measurements and scanning transmission electron microscopy, we verified that the ?-Al2O3/Ga2O3 SL with ?-Ga2O3 thickness up to ~ 1 nm had coherent interfaces without misfit dislocation in spite of the large lattice mismatches. The band alignment at the coherent ?-Ga2O3/Al2O3 heterointerface was also evaluated by analyzing X-ray photoemission spectra for an ?-Al2O3 homoepitaxial film and an ?-Ga2O3/Al2O3 SL. The extracted Eg of ?-Ga2O3 and ?-Al2O3 were 5.7 and 8.7 eV, and the conduction- and valence-band offsets were 2.7 and 0.3 eV, respectively. The smaller valence-band offset is a common feature of metal oxides that that the top of the valence bands in the oxides are mainly consisted of O 2p orbitals and so are less sensitive to changes in cations. Although the critical thickness of ?-Ga2O3 layer in the SL was small, it can be increased for ?-(AlxGa1-x)2O3 (0 < x < 1) alloy-based SLs. We believe our provided results will contribute to the further development of studies on ?-(AlxGa1-x)2O3-based heterostructures including SLs.

Authors : James S. Speck
Affiliations : Materials Department, University of California, Santa Barbara, CA 93106

Resume : β-Ga2O3 is a promising wide bandgap semiconductor for power electronics due to its ~4.8 eV bandgap, reasonable electron mobility, the availability of large area melt grown substrates, and the ability to form heterostructures by alloying on the group III site. In this presentation, we provide an overview of UCSB work on the development of plasma-assisted MBE growth of β-Ga2O3, systetmatic doping studies with Sn and Ge [1], etching and contact studies [2], and the development of the heterostructure system β-(AlxGa1-x)2O3/β-Ga2O3 [3,4]. Recently we realized modulation doping of β-(AlxGa1-x)2O3/β-Ga2O3 and demonstrated basic MODFET transistors with sheet charge densities as high as 1.2x1013 cm-2 [5]. We summarize the many opportunities and challenges in materials growth and development of a viable device technology. [1] E. Ahmadi et al. Semicond. Sci. Technol. (2017). [2] J. Hogan et al., Semicond. Sci. Technol. 31, 065006 (2016). [3] S.W. Kaun, et al. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 33, 41508 (2015). [4] Y. Oshima, et al. Appl. Phys. Express 9, 61102 (2016). [5] E. Ahmadi et al., Appl. Phys. Express 10, 071101 (2017).

Authors : H. Murakami1, K. Konishi1, K. Goto1,2,3, Q.-T. Thieu3, K. Sasaki2,3,4, A. Kuramata3, S. Yamakoshi2,3, M. Higashiwaki4, Y. Kumagai1
Affiliations : 1 Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan; 2 Tamura Corporation, Sayama, Saitama 350-1328, Japan; 3 Novel Crystal Technology, Inc., Sayama, Saitama 350-1328, Japan; 4 National Institute of Information and Communications Technology, Koganei, Tokyo 184-8795, Japan

Resume : Ga2O3 is an attractive material for deep ultraviolet optoelectronic device and next-generation power device applications due to large bandgap energy of 4.5~4.9 eV [1]. Especially for power device application, technology of growing a thick conductivity controlled Ga2O3 layer at a high rate is required. Our group has thermodynamically analyzed the halide vapor phase epitaxy (HVPE) growth of Ga2O3 and determined that the use of gallium chloride (GaCl) and oxygen (O2) precursors together with an inert carrier gas is suitable for Ga2O3 growth [2,3]. At present, high-speed growth of high-purity β-Ga2O3 homoepitaxial layers and control of n-type carrier concentration by intentionally doped silicon have been developed. In the presentation, an overview of HVPE grown β-Ga2O3 homoepitaxial layer on (001) β-Ga2O3 substrates prepared by edge-defined film-fed growth (EFG) will be reported. Precise control of n-type carrier concentration by intentional silicon doping by SiCl4 within the range of 1015 to 1018 cm-3 and the Ga2O3 vertical Schottky barrier diodes (SBDs) using the epitaxial wafers with Si-doped n−-Ga2O3 drift layers grown on n+-Ga2O3 (001) substrates by HVPE will be presented. Furthermore, HVPE growth of other polymorphs of Ga2O3 using GaCln (n=1~3) and O2 at various growth conditions will be also shown. [1] M. Higashiwaki et al., Appl. Phys. Lett. 100 (2012) 013504. [2] K. Nomura et al., J. Cryst. Growth 405 (2014) 19. [3] H. Murakami et al., Appl. Phys. Express 8 (2015) 0155003.

Authors : Max Kneiß, Daniel Splith, Holger von Wenckstern, Marius Grundmann
Affiliations : Universität Leipzig, Faculty of Physics and Earth Sciences, Felix Bloch Institute for Solid State Physics, Linnéstr. 5, 04103 Leipzig, Germany

Resume : In conventional pulsed laser deposition (PLD) a controlled variation of the alloy composition of laterally homogeneous ternary thin films under similar process conditions is not possible without using multiple single composition targets (one for each cation ratio). We therefore propose a technique to control the composition of ternary thin films using a single elliptically-segmented PLD-target with two regions of different composition. We control the Al/Ga ratio of our thin films by varying the radial position of the PLD laser spot on the target and thereby changing the ratio of the path lengths of the laser spot in the different regions. In analogy to our approach for lateral continuous composition spreads [1] (lateral CCS) and due to the possibility to further create vertical composition gradients with this technique, we call this method vertical CCS. We will show that we are able to control the composition of homogeneous (Alx,Ga1-x)2O3 thin films reproducibly. Therefore, homogeneous films with varying Al-contents (x = 0.07-0.28) were grown using the new technique on c-sapphire substrates. The Al-content in the films was determined by transmission spectroscopy and spectroscopic ellipsometry. XRD measurements reveal (-201)-oriented growth in the monoclinic β-phase, while AFM images confirm smooth surface morphologies without droplets. The structural and optical quality is similar to films grown by conventional PLD. [1] H. von Wenckstern et al., CrystEngComm 15, 10020 (2013)

2DEGs and conducting thin films : Saskia Fischer
Authors : Kookrin Char
Affiliations : Institute of Applied Physics, Dept. of Physics and Astronomy Seoul National Univ.

Resume : We have recently reported on the conducting LaInO3/BaSnO3 polar interface. [1] We have found that a small amount of doping in the BaSnO3 layer is critical for the conducting interface behavior, while the interface between the LaInO3 and the undoped BaSnO3 was not conducting. By way of series of experiments, we show that the conducting behavior is due to neither oxygen vacancies nor cation diffusion at the interface. One of such experiments is a modulation doped heterostructure, where an undoped BaSnO3 spacer layer is inserted between the LaInO3 and the slightly doped BaSnO3. [2] We will report on the electrical properties of such structures. The entire data can be well described by 1-dimensional Poisson-Schrodinger equation when an interface polarization is assumed. Our model for the conducting interface is based on the 2DEG-like structure created by the interface polarization of the LaInO3 and the Fermi level controlled by the doping in the BaSnO3. We will discuss the possible origin for such polarization and try to provide its evidences. Using such LaInO3/BaSnO3 conducting interfaces, all perovskite transparent FETs with excellent properties were made. [3] We believe that the stable oxygen stoichiometry and the ability to control the local doping level in LaInO3/BaSnO3 heterostructures will lead to more complete understanding of the 2DEG behavior, tuning the carrier density, and creating higher density 2DEG. [1] U. Kim, C. Park, Y. M. Kim, J. Shin, and K. Char, APL Mater. 4, 071102 (2016). [2] J. Shin, C. Park, Y. M. Kim, Y. Kim, and K. Char, preprint. [3] U. Kim, C. Park, T. Ha, Y. M. Kim, N. Kim, C. Ju, J. Park, J. Yu, J. H. Kim, and K. Char, APL Mater. 3, 036101 (2015).

Authors : K. Ridier 1,2, D. Aureau 2, B. Bérini 1, J. Vigneron 2, N. Keller 1, A. Etcheberry 2, B. Domengès 3, Y. Dumont 1, Arnaud Fouchet 1,*
Affiliations : 1 Groupe d’Étude de la Matière Condensée (GEMaC) UMR 8635 CNRS-UVSQ, Université Paris-Saclay, Versailles, France ; 2 Institut Lavoisier (ILV), UMR 8180 CNRS-UVSQ, Université Paris-Saclay, Versailles, France ; 3 LAMIPS - CRISMAT - NXP Semiconductors - Presto-Engineering Europe Joint Laboratory, CNRS-UMR6508, ENSICAEN, UCN, Presto-Engineering Europe, 2 rue de la Girafe, 14000 Caen, France * present adress: CRISMAT/ENSICAEN-CNRS UMR 6508, Caen, France

Resume : The quasi-2-dimensional electron gas (q-2-DEG) formed at the interface between the two band insulators LaAlO3 (LAO) and SrTiO3 (STO) [1] is one of the most fascinating systems in the field of oxide electronics. LAO/STO heterostructure is the subject of intensive research due to its multifunctional properties which open avenues for both fundamental and applied perspectives. Recently, a model based on first-principles calculations, [2] suggests that the various experimental observations (including critical thickness, carrier density, interface magnetism) originate from an intricate balance between surface oxygen vacancies in LAO and cation antisite defects thermodynamically induced by the polar discontinuity across the interface. In order to understand the underlying properties of this interface, we have studied the influence of a low energy ion beam irradiations using argon clusters on the LaAlO3 /SrTiO3 (LAO/STO) heterointerfaces by combining x-ray photoelectron spectroscopy (XPS) and electrical transport measurements. Due to its unique features, we demonstrate that a short-time cluster ion irradiation of the LAO surface induces significant modifications in the chemical and electronic properties of the buried STO substrate: (1) a lowering of Ti atoms oxidation states (from Ti4+ to Ti3+ and Ti2+) correlated to the formation of oxygen vacancies at the LAO surface (2) creation of new surface states for Sr atoms. (3) an increase of the electrical conductivity at the LAO/STO interface. Our XPS data also clearly reveal the existence of dynamical processes on the titanium and strontium atoms, which compete with the effect of the cluster ion beam irradiation. These relaxation effects are in part attributed to the diffusion of the ion-induced oxygen vacancies in the entire heterostructure since an increase of the interfacial metallicity is also evidenced far from the irradiated area. These results [3] highlights the possibility of tuning the electrical properties of LAO/STO interfaces by surface engineering, confirming experimentally the intimate connection between LAO chemistry and electronic properties of LAO/STO interfaces. ---------------------------- [1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [2] L. Yu and A. Zunger, Nat. Commun. 5, 5118 (2014). [3] K. Ridier, D. Aureau, B. Bérini, Y. Dumont, N. Keller, J. Vigneron, A. Etcheberry, B. Domengès, and A. Fouchet, Phys. Rev. B 97, 35146 (2018).

Authors : M. Lee,1,2 R. Arras,1 B. Warot-Fonrose,1 T. Hungria,3 M. Lippmaa,4 H. Daimon,2 and M. J. Casanove1
Affiliations : 1. Centre d’Elaboration des Matériaux et d’Etudes Structurales (CEMES), CNRS UPR 8011 and Université de Toulouse, 29 rue Jeanne Marvig, F-31055 Toulouse, France 2. Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma 630-0192, Japan 3. Centre de MicroCaractérisation Raimond Castaing, Université de Toulouse, 3 rue Caroline Aigle, F-31400 Toulouse, France 4. Institute for Solid State Physics, University of Tokyo, 277-8581 Chiba, Japan

Resume : Since the discovery of a two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3(001) interface [1], new interfacial properties (magnetism, superconductivity, tunable Rashba effect…) have been discovered, offering consequently new opportunities to investigate condensed-matter fundamental mechanisms and to propose alternatives concepts for future electronic devices. Doping the interface was later considered as a potential way to modify these properties either directly or via induced modifications in the atomic structure. We propose a study of LaAlO3/SrTiO3 interfaces doped with iridium or cobalt atoms. Cobalt was chosen for its role in developing dilute magnetic semiconductors, whereas novel properties arising from an interplay between strong spin-orbit interaction and electron correlations could be expected in the case of Ir doping. Besides, the 5d electronic structure of Ir could increase the carrier mobility compared to 3d electrons. We will report the evolution of the electronic and atomic structure of the interfaces as a function of the doping level [2], and its consequence on the transport properties. A special emphasis will be put on the location of the dopant atoms (preferred atomic site, cationic interdiffusion, strain state…) using a combination of experimental techniques and first-principles calculations. [1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [2] M. Lee et al., Phys. Chem. Chem. Phys. 19, 28676 (2017).

Authors : L. Rigutti1*, E. Di Russo1, S. Moldovan1, J. Houard1, A. Normand1, I. Blum1, D. Blavette1, A. Jollivet2, M. Tchernycheva2, F. Julien2, M. Hugues3, J. M. Chauveau3
Affiliations : 1Normandie Univ., UNIROUEN, INSA Rouen, CNRS UMR 6634, GPM, 76000 Rouen, France 2 Centre de Nanosciences et de Nanotechnologies, CNRS UMR 9001, Univ. Paris-Sud, Université Paris-Saclay, C2N – Orsay, 91405 Orsay Cedex, France 3 Université Côte d’Azur, CNRS, CRHEA, 06560 Valbonne, France

Resume : The possibility of sequentially addressing single nanoscale objects by a combination of techniques such as Atom Probe Tomography (APT), Scanning Transmission Electron Microscopy (STEM) and micro-Photoluminescence (µ-PL) has recently disclosed interesting perspectives for the study of the relationship between functional and structural properties of nanoscale systems [1,2]. An example of this approach will be illustrated for the case of non-polar ZnO/(Zn,Mg)O multi-quantum wells (MQW), currently studied for perspective optoelectronic applications. These MQWs exhibit a V-groove grating profile when observed along the c-axis crystal direction. Analytical STEM and APT reveal that Zn rich and depleted a-planes are alternatively formed in (Zn,Mg)O barriers in correspondence of the grating edges. Both μ-PL and systematic APT investigations allow to determine the composition of (Zn,Mg)O barriers and indicate that carriers localize within the QW. Effective mass calculations of optical transition energies based on 3D structural data are in excellent agreement with the experimental µ-PL spectra and with the photo-induced absorption spectroscopy realized under UV irradiation on thin film samples. Finally, the first study of these structures by in-situ µPL within an APT instrument and the new perspectives opened by this instrument will be discussed. [1] E. Di Russo et al., Appl. Phys. Lett. 111 (2017), 032108. [2] L. Mancini et al., Nano Lett. 17.7 (2017), 4261.

Perovskite Heterostructures : Tae Won Noh
Authors : Gervasi Herranz
Affiliations : Institute for Materials Science of Barcelona (ICMAB-CSIC), Campus de la UAB, Bellaterra, Catalonia

Resume : The broad family of perovskites hosts an extraordinary range of functional properties. A clue to this astonishing versatility is the capability of perovskites to admit distortions from the ideal cubic structure, which leads to buckling, tilting, rotations or elongations of the octahedral structural units. In turn, these deformations modulate the ionic bonds in the lattice, fine-tuning the physical properties. Here we analyze some of the perovskite lattice instabilities and their interaction with electric fields. First we discuss this relation for antiferrodistortive (AFD) octahedral rotations and polar modes occurring in LaAlO3/SrTiO3 quantum wells (QWs) [1]. With some exceptions, these two order parameters rarely coexist in a bulk crystal. Interestingly, in the particular case of LaAlO3/SrTiO3 QWs, their competition leads to structural changes concomitant with a metal-insulator transition. Secondly, we discuss the role of a previously unreported antiferroelectric (AFE) mode in the dielectric properties of the canonical SrTiO3 perovskite [2]. In particular, we show that such properties can dominate the response of ferroelastic domains to external electric biases, overruling the response from polar domain walls. These observations illustrate how sensitive perovskite are to lattice distortions and provide insights towards nanoscale control of material properties. [1] Gazquez et al., Phys. Rev. Lett. 119, 106102 (2017) ; [2] B. Casals et al., Phys. Rev. Lett., in press

Authors : Marta Gibert
Affiliations : Physik-Institut, University of Zurich, Switzerland

Resume : Epitaxial heterostructures offer multiple approaches to manipulate the interplay between the different degrees of freedom in transition metal oxides. Strain and reduced dimensionalities are examples of successful strategies typically used to tune the functionalities of these materials and even allow access to novel electronic phases. The double-perovskite La2NiMnO6 is well studied in bulk due to its near room temperature ferromagnetic transition, accompanied by magnetoresistance and magnetoelectric effects [1]. Here, we tackle two different heterostructure strategies to investigate such double-perovskite. First, we investigate the electronic properties of La2NiMnO6 when grown as thin film, focusing on the thickness- and strain-dependence of the ferromagnetic behaviour. Second, we look at (111)pc-oriented LaNiO3/LaMnO3 superlattices, since they allow to reproduce the double-perovskite structure when periodicities of 1u.c./1u.c. are chosen. In this case, the electronic properties of the system are investigated as function of the superlattice periodicity. For 7-monolayer-thick-LaNiO3, low-temperature exchange bias phenomena followed by interlayer antiferromagnetic coupling between LaMnO3 layers is observed as function of temperature [2,3]. [1] N. S. Rogado et al., Adv. Mater. 17, 2225 (2005). [2] M. Gibert, et al., Nat. Mater. 11, 195 (2012). [3] M. Gibert et al. Nat. Commun. 7, 11227 (2016).

Authors : Hugo Meley 1 , Kareandeep 2 , L. Oberson 3 , J. de Bruijckere 1 , D. T. L. Alexander 3 , J.-M. Triscone 1 , Ph. Ghosez 2 and S. Gariglio 1
Affiliations : 1 DQMP, University of Geneva, 24 Quai E.-Ansermet, CH-1211 Geneva, Switzerland; 2 Physique Théorique des Matériaux, Université de Liège (B5), B-4000 Liège, Belgium; 3 Interdisciplinary Centre for Electron Microscopy (CIME), École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

Resume : The interplay between spin, charge, orbital and lattice degrees of freedom is extremely strong and at the origin of numerous phenomena in complex oxides [1]. The bulk 3d 2 AVO 3 (A be- ing a rare-earth) showcases an interesting phase diagram where the low temperature orbital and spin orderings are strongly dependent upon the A cation size [2]. The GdFeO 3 -type dis- tortions remove the t 2g orbital degeneracy to generate a Mott state. Above the Jahn-Teller transition temperature, orbitals fluctuate between the quasi non-degenerated t 2g energy lev- els [3]. For LaVO 3 , below the transition temperature (140 K), a spontaneous G-type orbital ordering concomitant with a C-type spin ordering arises along the long orthorhombic axis. We have explored the effect of biaxial strain in epitaxial thin films of LaVO 3 [4]. X-ray diffraction reveals that the layers accommodate the strain imposed by the substrate assum- ing different patterns of octahedral tilts and rotations. These structural changes have been predicted by ab-initio theory. We used temperature dependent X-ray diffraction, muon spec- troscopy and optical conductivity to investigate the strain effect on the orbital and magnetic transitions. [1] D. I. Khomskii, Transition metal compounds (Cambridge University Press, 2014). [2] Y. Ren et al., Nature (London) 396, 441 (1998); Phys. Rev. B 67, 014107 (2003). [3] M. De Raychaudhury, E. Pavarini, and O. Andersen, Phys. Rev. Lett. 99, 126402 (2007). [4] H. Meley et a., APL Materials 6, 046102 (2018).

Authors : D. Kumar[1], W. Prellier[1], A. David[1], A. Fouchet[1], O. Copie[2]
Affiliations : [1] Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, Normandie Universite, 6 Bd Marechal Juin, F-14050 Caen Cedex 4, France. [2] Institut Jean Lamour, UMR 7198, CNRS-Université de Lorraine, F-54506 Vandoeuvre-lès-Nancy, France.

Resume : The transition-metal oxides with ABO3 type perovskite structure show a strong coupling between spin-orbital and lattice degrees of freedom, implying the importance of lattice distortion in tuning the electronic and magnetic properties of ABO3 perovskites. With the recent experimental activities [O. Copie et. al. Adv. Mater. 1604112, (2017)] in mind, where authors reveal a pathway to tune the Neel temperature (TN) of PrVO3 (PVO) thin films in the range of 30 K via chemical strain engineering i.e. by monitoring the concentration of oxygen vacancies in PVO thin films. In here, we present an experimental study of magnetic response to the residual strain in PVO thin films by growing PVO films on a variety of single crystal substrates and by varying PVO film thickness on a substrate. PrVO3 in bulk, being a strong anti-ferromagnet shows TN ~ 130 K [1]. Our study reveals that the lattice distortion via misfit strain pave the way towards tuning the anti-ferromagnetic ordering temperature in PrVO3 thin films, resulting in a non-trivial evolution of the Neel temperature (TN) with substrate pseudo-cubic lattice parameter and PVO film thickness. We demonstrate that the strain produced via mechanical strain engineering, tune TN of PVO films in the range of 90 K (figure). Moreover, we also show that PVO thin films show low temperature ferromagnetic behavior along with the insulating properties irrespective of the substrate. [1] Sage M H et. al. Phys. Rev. B 76 195102 (2007). This work is supported by LABEX and Region Normandie.

Authors : Victor Haspot, Abdelmadjid Anane, Manuel Bibes, Agnès Barthélémy
Affiliations : Unité Mixte de Physique, CNRS, Thales, Univ. Paris-Sud, Université Paris-Saclay, 91767, Palaiseau, France

Resume : Among the numerous functionalities displayed by perovskite oxides such as superconductivity at the interface [1], multiferroism [2] or colossal magnetoresistance in manganites…, they may also be very useful materials for generating spin currents with weak relaxations. Moreover, thanks to the Rashba spin-orbit coupling appearing at interfaces, spin currents can generate charge currents and vice versa through the Edelstein effects [3]. In order to improve current interconversions, several pathways have been envisaged to improve the conversion rate between spin/charge current: (i) involve heavy elements to enhance spin-orbit coupling strength, (ii) study 2DEGs [4] or (iii) integrate barrier tunnels, etc. In our study, we focus on heterostructures based on La2/3Sr1/3MnO3 (LSMO), grown by pulsed laser deposition. We investigate the role of strain, thickness and temperature on the magnetization dynamics in order to estimate the ideal parameters to integrate LSMO as spin injector into heterostructures. We observe that the damping depends on the thickness of the layers and the strain imposed by the substrate. But above all, temperature-dependent damping show a peak at 25K indicates a relaxation of magnetic moments. From these measurements, it is possible for us to disentangle damping effects related to the surface and to the bulk. In addition, we looked at heterostructures including Pt and BiFeO3 -- heavy elements --, combined with thin layers of LSMO. We study the dependence of spin-mixing conductance g↑↓ for these materials with temperature and the thickness of the compounds. In any case, this whole preliminary study will allow us to estimate the best conditions for obtaining high spin/charge current conversion.

VO2 and Fe3O4 Heterostructures : Marta Gibert
Authors : Gertjan Koster 1, P.P.Le 1, Kevin Hofhuis 1, Abhi Rana 1, Mark Huijben 1, Hans Hilgenkamp 1, G. Rijnders 1, Prof. A. ten Elshof 1, Nicolas Gauquelin 2, G. Lumbeeck 2, Christian Schlüßler-Langeheine 3, Steef Smit 4, Xanthe H. Verbeek 4, Georgios Araizi-Kanoutas 4, Shrawan Mishra 4,5, Hermann A. Dürr 4,6, Mark S. Golden 4
Affiliations : 1 IMS, MESA+ UTwente; 2 Electron Microscopy for Materials Science (EMAT), University of Antwerp, 2020 Antwerp, Belgium; 3 Helmholtz-Zentrum Berlin für Materialien und Energie, BESSY II, Albert-Einstein-Str. 15 12489 Berlin, Germany; 4 Van der Waals-Zeeman Institute, Institute of Physics, Science Park 904, 1098 XH, Amsterdam, The Netherlands; 5 School of Material Science and Technology, Indian Institute of Technology (BHU), Varanasi 221005, India; 6 Department of Physics and Astronomy, Uppsala University, Box 516 751 20 Uppsala, Sweden

Resume : Vanadium dioxide (VO2) has been drawing attention since its metal-insulator transition (MIT) with several orders of magnitude resistivity change at 341 K was discovered. The MIT is accompanied with the abrupt first order structural phase transformation from a metallic tetragonal rutile (R) phase (P42/mnm), to an insulating monoclinic (M1) phase (P21/C). Up to now, most studies on epitaxial VO2 thin films used Al2O3 and TiO2 single crystal substrates to control film orientation. However, VO2-based devices are restricted to low cost and size-limited single crystal substrates, as well as on the integration compatibility with the current Si-based technology. Direct deposition of VO2 on glass or Si substrates with a native amorphous silicon dioxide layer leads to a polycrystalline film with predominant (011)M1 orientation, whereas VO2 is favorably grown (010)M1-oriented on a buffer layer of Pt(111) on Si substrate. The challenge is how to direct VO2 film orientation on Si or even arbitrary substrates at will. Oriented growth is not only of interest for the fundamental study of the MIT mechanisms, but also for potential applications for next-generation transistors, memory metamaterials, sensors, and novel hydrogen storage technology. Recently, various metal oxide films were grown by epitaxy on oxide nanosheets on glass and Si substrates. Oxide nanosheets span a wide range of crystal lattices and 2D symmetric structures, allowing for new possibilities to tailor the important structural parameters and properties of thin films. In the present work, Ti0.87O2 (TO) and NbWO6 (NWO) nanosheets were identified to direct the orientation of VO2 thin films.

Authors : Cristiana Di Valentin, Hongsheng Liu
Affiliations : Università di Milano Bicocca

Resume : Magnetite exhibits an interesting phase transition, called Verwey transition, at the critical temperature TV of about 120 K. [1] Though numerous efforts have been devoted to the understanding of this interesting transition, up to now, it is still under debate whether a charge ordering and a band gap exist in magnetite above TV. Besides the interesting Verwey transition, magnetite also shows wide applications in catalysis [2-4], spintronic devices [5], magnetic resonance imaging (MRI) and drug delivering [6]. In these applications, surfaces and their interaction with water play an important role. Here, we systematically investigate the electronic properties of cubic magnetite bulk and (001) surface using different methods based on density functional theory, which present some correction for the self-interaction error: DFT+U and hybrid functionals. The water adsorption thermodynamics on Fe3O4(001) surface was studied by hybrid functional calculations. Our results show that, for bulk magnetite, upon release of symmetry constraints on the electron density but not on the geometry, charge disproportionation (Fe2+/Fe3+) is observed, resulting in a band gap of around 0.2 eV at the Fermi level. This implies that the Verwey transition is probably a semiconductor-to-semiconductor transition and that the conductivity mechanism above TV is small polaron hopping. The (001) surface shows a band gap (about 0.6 eV) larger than that in the bulk. A mixed adsorption mode of water is favorable on Fe3O4(001) surfaces at a high coverage, indicating that the cooperative effects between adjacent water molecules is important in the dissociation reaction. Our results give a clear understanding of the electronic structure of magnetite bulk [7] and (001) surface [8], as well as the water adsorption behavior on the (001) surface, which is fundamental for Fe3O4 nanomaterials' applications. References [1] E. J. W. Verwey Nature 144 (1939) 327. [2] Ohe, K.; Tagai, Y.; Nakamura, S.; Oshima, T.; Baba, Y. J. Chem. Eng. Jpn. 38, 671-676 (2005). [3] Katsumata, H.; Kaneco, S.; Inomata, K.; Itoh, K.; Funasaka, K.; Masuyama, K.; Suzuki, T.; Ohta, K. J. Environ. Manage. 69, 187-191 (2003). [4] Martos, C.; Dufour, J.; Ruiz, A. Int. J. Hydrog. Energy 34, 4475-4481 (2009). [5] Eerenstein, W.; Palstra, T. T. M.; Saxena, S. S.; Hibma, T. Phys. Rev. Lett. 88, 247204 (2002). [6] Sun, C.; Lee, J. S.; Zhang, M. Adv. Drug Deliv. Rev. 60, 1252-1265 (2008). [7] Liu, H.; Di Valentin, C. J. Phys. Chem. C 121, 25736-25742 (2017). [8] Liu, H.; Di Valentin, C. Nanoscale 2018, DOI: 10.1039/c8nr02279h.

Authors : Laura Rodríguez, Gustau Catalan, José Santiso, Felip Sandiumenge
Affiliations : Laura Rodríguez: Institut Catala de Nanociencia i Nanotecnologia (ICN2), CSIC and The Barcelona Institute of Nanoscience and Technology (BIST), Campus UAB, 08193 Barcelona, Spain.; Gustau Catalan: Institut Catala de Nanociencia i Nanotecnologia (ICN2), CSIC and The Barcelona Institute of Nanoscience and Technology (BIST), Campus UAB, 08193 Barcelona, Spain. ICREA-Institucio Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spain.; José Santiso: Institut Catala de Nanociencia i Nanotecnologia (ICN2), CSIC and The Barcelona Institute of Nanoscience and Technology (BIST), Campus UAB, 08193 Barcelona, Spain.; Felip Sandiumenge: Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, Bellaterra, Catalonia, Spain.;

Resume : VO2 is a widely known material by its first order metal-insulator transition (MIT) near room temperature (~ 68ºC in bulk) but on the other hand, widely controversial due to the non-well understood transition mechanisms. This MIT couples an electronic transition to a structural one: the insulating (semiconductor) state is associated to a monoclinic structural phase while the metallic state does it to a rutile (tetragonal) phase. Nevertheless, it may not occur this way under a certain strain and thickness conditions, which makes it even more difficult to interpret(1). Whatever it is, the interface separating these metallic and insulating domains is interesting by itself because it could be understood as a Schottky barrier, therefore a diode. But if we want to explore this possibility we firstly need to stabilize such a transitory state. How? Are there techniques that allow us to see these phase boundaries in-situ? How do they look like? This ongoing work tries to put some light on all these questions. Since we know that MIT in VO2 is extremely sensitive to its strain state we wanted to cover the whole strain span, so we grew epitaxial VO2 thin films between 10 and 120 nm on (001)-oriented TiO2 substrates by pulsed laser deposition (PLD). To have a general understanding of the results many characterization techniques have to be taken into account as a whole, all together. Here we want to show that optical microscopy, x-ray diffraction (XRD), and several atomic force microscopy (AFM) modes could be the most clarifying ones. (1) Yang, M.; Yang, Y.; Hong, B.; Wang, L.; Hu, K.; Dong, Y.; Xu, H.; Huang, H.; Zhao, J.; Chen, H. & others Suppression of structural phase transition in VO 2 by epitaxial strain in vicinity of metal-insulator transition Scientific reports, Nature Publishing Group, 2016, 6, 23119

Authors : Gongin Lee1, Dooyong Lee1,2, Jiwoong Kim1, Sehwan Song1, Donghyuk Yang1, Sungkyun Park1,*
Affiliations : Department of Physics, Pusan National University, Busan 46241, Korea; Advanced Nano Surface Research Group, Korea Basic Science Institute, Daejeon 34133, Korea;

Resume : Recently, there have been many studies on a hydrogen incorporation into the vanadium dioxide (VO2) to lower the insulator-metal-transition (IMT) temperature of VO2 to near room temperature. However, since, the incorporated hydrogen in VO2 not only changes the local structure but also affects the electronic structure of VO2, it is difficult to understand a cause of IMT characteristics change by hydrogen incorporation. In this presentation, we show the change in structure and electronic structure of VO2 films by hydrogen incorporation. The films grown on Al2O3(0001) using RF-magnetron sputtering were annealed at 400 oC under N2, N2:H2 and H2 environments to control the amounts of hydrogen incorporation. As a result, the IMT temperatures decreased with increasing the amounts of hydrogen in VO2 film. X-ray diffraction and Raman spectroscopy showed the volume expansion and monoclinic phase of hydrogen doped VO2 films. In addition, X-ray absorption spectroscopy showed an increase in octahedral symmetry. Temperature-dependent X-ray photoelectron spectroscopy reveals that the V4+ state change to the V3+ state and the V 3d level shifted to the Fermi level as the amounts of hydrogen increased. These results provide an insight to understand the role of hydrogen incorporation in VO2 films. This work is supported in part by NRF-Korea (NRF-2015R1D1A1A01058672 and NRF-2017K1A3A7A09016305).

Authors : Rosalía Cid Barreno, Juan Rubio Zuazo, Eduardo Salas Colera, Germán Rafael Castro Castro
Affiliations : SpLine BM25 Beamline, European Synchrotron Radiation Facility, 38000 Grenoble, France; Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas (ICMM-CSIC), 28049 Madrid, Spain

Resume : Highly spin-polarized materials as Fe3O4 (magnetite) and La0.7Ca0.3MnO3 (mixed-valence manganites) are of special interest for the design of new spintronic devices and magnetic random access memories. The coupling of magnetic, spin and lattice degrees of freedom makes tailoring their performance in engineered thin-film heterostructures a major challenge. Bulk Fe3O4 experiences a metal-to-insulator transition at Tv=120 K, while bulk La0.7Ca0.3MnO3 shows an insulator-to-metal transition close to room temperature (Tc). Three different coupling states are foreseen in a Fe3O4/LCMO heterostructure (insulator/metal coupling above Tc, metal/metal between Tc and Tv and metal/insulator below Tv). The ability to tune the conductivity coupling of the bilayer gives the unique possibility to design transport based devices working simultaneously in perpendicular and parallel configurations. We report the epitaxial growth of thin Fe3O4/LCMO bilayers by PLD on SrTiO3 (001) and (111) substrates. LCMO was crystallized at 800C under 1 bar of flowing oxygen, while Fe3O4 was deposited at different temperatures to artificially tune its morphology from homogeneous to nanostructured. The experimental conductivity of the heterostructures reveals the overimposed transitions occurring in magnetite and manganite individual layers, identifying the three coupling states. Magneto-optical Kerr Effect is used to elucidate the magnetic coupling between the layers at the different thermal regimes. Detailed grazing incidence x-ray diffraction, x-ray reflectivity and X-ray absorption spectroscopy has been performed revealing the growth of fully relaxed single phase single oriented Fe3O4 layer, while single phase single oriented but slightly strained by epitaxy with the substrate is found for the LCMO layer. A correlation between the lattice (structure and morphology), transport and magnetic coupling between layers will be presented.

Poster Session : Matthias Bickermann, Pavlo Zubko
Authors : A. Muslimov 1, A.Butashin 1, V.Kanevsky 1, E.Muslimova 2.
Affiliations : 1 FSRC “Crystallography and Photonics” Moscow, Russia 2 High School №1 Golitcino, Russia

Resume : We performed a structural, electrical, and optical characterization of β-Ga2O3–In2O3 thin films with different ratios of the initial components in the mixture, grown on sapphire substrates of different orientations. It was shown that, regardless of the sapphire substrate orientation, upon annealing the initial gallium–indium metallic film (with a component ratio of 30.33/69.67 at %), the solid solution with the In2O3 cubic structure and a fraction of the monoclinic β-Ga2O3 phase is formed. The samples exhibit an instantaneous rise in conductivity by 5–30% under UV illumination in the solar-blind region, with a conductivity relaxation time shorter than 1 s, which is comparable with the best results for β-Ga2O3 films. At a gallium–indium ratio of 44.24/55.76 at % in the initial metallic film, the solid solution with the monoclinic β-Ga2O3 structure and insignificant fraction of the In2O3 cubic phase is formed. These samples exhibit a high conductivity (up to 70 µA at 1 V), which depends also on the sapphire substrate orientation. In the samples fabricated on the А sapphire substrates by annealing at 1100°C, the UV illumination in the solar-blind region induces an instantaneous rise in conductivity by about 10–50%, depending on the sample; the relaxation time, however, becomes as long as several tens of minutes. The UV photocurrent maxima observed in the samples are apparently related to the electronic transitions from the oxygen 2p orbitals to the gallium 4s orbitals.

Authors : Wen-Yi Tong, Eric Bousquet, and Philippe Ghosez
Affiliations : Theoretical Materials Physics, Q-MAT, CESAM, Université de Liège (B5), B-4000 Liège, Belgium

Resume : The epitaxial deposition of oxides on silicon opens the possibility of incorporating their diverse properties into silicon-device technology. Growth of SrTiO3 has received particular attention because of its large dielectric constant, thermodynamical stability in contact with Si, and potential ferroelectricity. Although deposition of SrTiO3 on Si was first reported more than a decade ago, the exact atomic structure of the SrTiO3/Si interface is still under debate. Moreover, definitive explanation of the influence of interfacial structure on electronic properties is still poorly understood. In this work, we systematically investigate SrTiO3/Si interfaces from first-principles modeling in order to contribute to a better understanding of these issues. The best-known method for growing epitaxial oxides on silicon requires the deposition of monolayer of an alkaline earth metal, usually Sr, as the initial step. However, the details, as well as the impact of defects on this process are not well understood. Using first-principles density functional theory calculations,we firstly analyze the electronic states of the (1×2) reconstructed Si(001) surface with half monolayer Sr coverage. Various types of defects, including arrays of Sr adatoms and rows of Sr vacancies, are taken into account. Their scanning tunneling microscopy (STM) images are presented and compared with recent experimental results. Based on the thorough understanding of Sr/Si(001) surface, we further focus on the SrTiO3/Si heterostructures. As we know, for the clean (001) surface of silicon, owing to a lack of upper bonding partners, pairs of silicon atoms dimerize, using up one dangling bond per atom to form the dimer bond. By depositing SrTiO3, the structural complexity of both the ferroelectric SrTiO3 itself and the buffer layers offers the possibility to break the ’dimer-row’ reconstruction of silicon atoms. The driving force has been analyzed in details. Especially, in our first-principles calculations, the electrostatic coupling between the surface and the interface is taken into account. This work helps to a fundamental understanding of the important role played by interfacial structures in the SrTiO3/Si heterostructures, provides theoretical support for the interpretation of experimental reports, and may suggest guidelines for the future design of coupled functional oxide-semiconductor devices.

Authors : Seung Han Lee(1), Tae Cheol Kim(1), Hyun Kyu Jung(1), and Dong Hun Kim(1)*
Affiliations : Department of Materials Science and Engineering, Myongji University, Yongin, Republic of Korea

Resume : Self-assembled nanocomposite thin films, in which a ferrimagnetic CoFe2O4 (CFO) phase grows epitaxially as pillars within an immiscible ferroelectric BiFeO3 (BFO) phase are promising for next generation memory devices. In these BFO-CFO nanocomposite thin films, it is essential to find a suitable conductive layer to apply electric filed or to read the ferroelectric signal. The conductive layer also has to possess similar lattice parameters to CFO and BFO phases to form epitaxial nanocomposites. So far, single crystal Nb-doped SrTiO3 (Nb:STO) substrates have been almost exclusively grown due to the epitaxial growth but they are very expensive which limits their utility in microelectronic devices. Therefore, the integration of nanocomposites on an epitaxial conductive layer is critical towards large scale and low cost devices. Here, we report on the epitaxial growth, structure, magnetic and electric properties of La0.7Sr0.3MnO3 (LSMO) thin films grown by radio frequency magnetron sputtering. Both films were conductive enough to measure the electrical and ferroelectric properties and imposed different strain to the BFO and CFO phases. Then the structure, ferroelectric and magnetic properties of epitaxial BFO-CFO nanocomposites on LSMO layers were integrated using scanning electron microscope (SEM), X-ray diffraction (XRD), piezoelectric force microscopy (PFM), and vibrating sample magnetometer (VSM). The isotropic magnetic nature of nanocomposites on LSMO layers were explained using strain effects.

Authors : Ho-Yeol Choi, Chan-Hwa Hong, young-jin kwack, Jun-min Lee, Young-Hoi Kim, and Woo-Seok Cheong
Affiliations : Electronics and Telecommunications Research Institute

Resume : Oxide thin film transistor (oxide-TFT) have a lot of advantage compared with the amorphous-Si TFT and polycrystalline Si TFT. They have a high carrier mobility, high transmittance, low process temperature and fabrication cost. However, the most of them are n-type transistor because most of the oxide semiconductors are n-type. For CMOS technology, study of p-type oxide semiconductor was essential. Among the p-type oxide semiconductors, SnO is a good candidate because of the high hole mobility. Nevertheless, p-type SnO TFT has a low mobility than n-type oxide TFT. To overcome this problem, many researchers study about the p-type SnO TFTs and they also study about enhancement of electrical performance. We fabricated SnO TFTs with doping process to improve the performance of p-type TFT. We use co-sputtering system to fabricate Cu dopped-SnO TFT. In the film, Sn4 is substituted by Cu2 because the Cu2 ionic radius (73pm) is similar to Sn4 (71pm). Because of this atomic substitution, Cu play a role of acceptor and carrier is increased. As a result, we confirm the p-type electrical performance of SnO TFT was successfully improved by cu-doping.

Authors : K. Maksimova, O.Dikaya, A.Kozlov, A.Goikhman
Affiliations : REC "Functional Nanomaterials", Immanuel Kant Baltic Federal University, Kaliningrad, Russia

Resume : Pulsed Laser Deposition (PLD) has recently emerged as an excellent tool to grow epitaxial oxide heterostructures, particularly comprising ultrathin heteroepitaxial FE BaTiO3 (BTO) and SrTiO3 (STO) layers. The BTO films have been already utilized as a storage medium in the FE tunnel junctions [1,2], and exhibited giant bulk photovoltaic effect [3]. In this work, we report on the effect of PLD growth conditions of BTO and STO on MgO(100) with different bottom electrode (Pt,Ir or SrRuO3) on their structural and functional properties. The structure of 3-100 nm PLD grown layers was monitored in situ by RHEED, and further analyzed ex situ by XRD, XRR, AFM and TEM methods. We show that the structural properties of the oxide layer can be actively controlled via the partial O2 pressure in the growth chamber, substrate temperature and laser ablation parameters. The ferroelectric properties of the films were investigated by piezoresponse force microscopy. It was shown, that BTO and STO thin films grown at the optimized conditions are monodomain ferroelectrics with the polarization direction perpendicular to the substrate. The resistive switching effect was demonstrated in Cr/BTO(STO)/Ir/Pt//MgO(100) stacks. [1] A. Zenkevich, M. Minnekaev, Yu.Matveev, Yu.Lebedinskii, K. Bulach, A.Chouprik, A.Baturin, K.Maksimova, S.Tiess, W.Drube, “Electronic band alignment and electron transport in Cr/BaTiO3/Pt ferroelectric tunnel junctions”, Appl. Phys. Lett. 102, 062907 (2013) [2] M.Minnekaev, K.Bulakh, A.Chouprik, W.Drube, P.Ershov, Y.Lebedinskii, K.Maksimova, A.Zenkevich, “Structural, ferroelectric, electronic and transport properties of BaTiO3/Pt heterostructures grown on MgO(100)”, Microelectronic Engineering, V.109, p. 227-231 (2013) [3] A. Zenkevich, Yu. Matveyev, K. Maksimova, R. Gaynutdinov, A. Tolstikhina, and V. Fridkin, “Giant bulk photovoltaic effect in thin ferroelectric BaTiO3 films”, Phys. Rev. B, 161409(R), (2014)

Affiliations : 1 Department of Physics, Faculty of Science, Gazi University, Teknikokullar, 06500, Ankara, Turkey 2 Department of Energy Systems Engineering, Faculty of Engineering and Natural Sciences, Ankara Yıldırım Beyazıt University, 06010 Ankara, Turkey

Resume : ZnO nanostructures have been attracted over the past years because of many useful properties such as wide band gap, a large exciton binding energy. Different growth methods are used to obtain quality ZnO nanostructures. In this study, ZnO nanostructures were grown by Ultrasonic Spray Chemical Vapor Deposition (USCVD) method on SiO2 substrates. Structural and surface properties of grown ZnO nanostructures were characterized by X-Ray diffraction (XRD), Atomic force microscope (AFM) and Scanning electron microscope (SEM) measurements. ZnO nanostructures had wurtzite crystal structure was obtained according to XRD results and Zn2SiO4 which is called Willemite as a phosphorescent material was also observed in XRD results. ZnO nanostructures with hexagonal and triangular were successfully grown like nanoplates according to as a result of AFM and SEM measurements.

Affiliations : 1 Department of Physics, Faculty of Science, Gazi University, Teknikokullar, 06500, Ankara, Turkey 2 Department of Energy Systems Engineering, Faculty of Engineering and Natural Sciences, Ankara Yıldırım Beyazıt University, 06010 Ankara, Turkey

Resume : Over the past years, ZnO nanostructures are grown by chemical vapor deposition (CVD) method owing to high deposition rate, to produce high-quality crystals etc. In this study, the growth behavior and the structural properties of ZnO nanostructures have been investigated. ZnO nanostructures were grown by Ultrasonic Spray CVD method on SiO2 substrate. To compare the effect of growth at different lower temperatures, ZnO nanostructures were grown at 350 and 450 °C and then annealed at 900 °C under Ar and O2 atmospheres. The surface and structural properties were analyzed by X-ray diffraction (XRD), Atomic force microscope (AFM), Scanning electron microscope (SEM). Although ZnO nanostructures deposited at 450 °C have been observed at a low peak intensity of (0002) growing direction, others have been mostly oriented at (0002) growing direction with higher peak intensity. ZnO nanostructures grown at 350 °C show lower surface roughness than other growth temperature. It is shown that ZnO nanostructures have become nanostructures with hexagonal edges and non-sharp edges after annealing of grown samples. Moreover, the nanostructures on the surface have (0002) growing directions according to SEM images. The lattice and the surface parameters of grown samples have been calculated as well.

Affiliations : 1Department of Energy Systems Engineering, Faculty of Engineering and Natural Sciences, Ankara Yıldırım Beyazıt University, 06010, Ankara, Turkey 2Department of Physics, Faculty of Science, Gazi University, Teknikokullar, 06500, Ankara, Turkey

Resume : In this study, MgxZn1-xO nanostructures with different Mg content (X) have been grown by Ultrasonic Spray Chemical Vapor Deposition (USCVD) method at atmospheric pressure on the soda lime substrates. The structural and optical properties of grown MgxZn1-xO nanostructures have been investigated in detail. As a result of XRD measurements, Mg mole fraction in MgZnO nanostructures has been calculated. The surface properties of the grown samples have been determined with Atomic Force Microscopy (AFM). Optical band gap values have been calculated using absorption obtained from UV-visible spectrophotometer measurement. It is shown that as increase Mg mole fraction, the absorption band edge has shifted to the lower wavelength as expected.

Affiliations : 1Department of Energy Systems Engineering, Faculty of Engineering and Natural Sciences, Ankara Yıldırım Beyazıt University, 06010, Ankara, Turkey 2Department of Physics, Faculty of Science, Gazi University, Teknikokullar, 06500, Ankara, Turkey

Resume : Zinc oxide (ZnO) is one of the most extensively studied semiconducting materials for use in various types of applications due to it’s a direct wide band gap and high exciton binding energy. In this study, ZnO nanostructures on a microscope slide and soda-lime substrates were grown using Ultrasonic Spray Chemical Vapor Deposition (USCVD) method. In order to understand the effect of annealing atmospheres on grown samples, ZnO samples were annealed under O2 or Ar atmospheres at 500 oC temperature. XRD (X-ray diffraction), AFM (Atomic Force Microscope) and UV-visible measurements of all the samples have been carried out. The (0002) peak of ZnO has become a stronger peak after the annealing of grown samples. It is obtained that annealed samples show better optical properties. Therefore, the annealing at O2 atmosphere has a substantial effect on optical and structural properties of the grown samples.

Authors : Max Kneiß, Anna Werner, Daniel Splith, Holger von Wenckstern, Marius Grundmann
Affiliations : Universität Leipzig, Faculty of Physics and Earth Sciences, Felix Bloch Institute for Solid State Physics, Linnéstr. 5, 04103 Leipzig, Germany

Resume : Ga2O3 in the metastable ε-modification has recently gained remarkable interest. This is due to its high bandgap of ~ 5 eV [1] and the possibility of alloying with Al2O3 or In2O3 to change the bandgap for the use in heterostructures. While this is also true i.e. for the β-phase, ε-Ga2O3 is expected to possess additionally a high spontaneous electrical polarization along its c-direction [2]. Polarization differences at heterointerfaces can therefore be utilized to achieve high sheet densities of electrons in a two-dimensional electron gas located at the interface. To create high quality heterostructures, epitaxial growth of the ε-Ga2O3 with high crystalline quality and smooth surfaces is necessary. We show the growth of c-oriented ε-Ga2O3 with well-defined in-plane epitaxial relationships on c-sapphire, STO (111), YSZ (111) and MgO (111) substrates using pulsed laser deposition (PLD). A Sn-doped Ga2O3 PLD-target was used to catalyze the growth in the ε-phase, an effect that was recently reported for MBE-grown thin films [3]. The ε-phase, the epitaxial relationships and a high crystalline quality were measured by XRD, while AFM measurements reveal smooth surface morphologies with a minimum RMS-roughness of ~ 1 nm. The optical bandgap was further determined by spectroscopic ellipsometry or transmission spectroscopy. [1] Furthmüller et al., Phys. Rev. B 93, 115204 (2016), [2] Maccioni et al., Appl. Phys. Expr. 9, 041102 (2016), [3] Kracht et al., Phys. Rev. Appl. 8, 054002 (2017)

Authors : C. Villalobos, C. Ferrater, M.C. Polo, M. Varela
Affiliations : Departament de Física Aplicada, Universitat de Barcelona, Spain

Resume : CuO is a multiferroic material in which ferroelectricity emerges from a spin-spiral magnetic structure, and, therefore, the multiferroic ordering temperature corresponds with the magnetic ordering (< 230K), probably still too low for most potentially useful applications. However, it has been shown that a relatively small pressure produces an increase of the magnetic interaction by about a 50%, suggesting that the Néel temperature can be pushed up to room temperature under suitable pressure. In CuO thin films this pressure can be induced by the film-substrate epitaxial strain. In this work we study the epitaxial growth of CuO films on substrates with different lattice mismatch and, as consequence, different strain. The final aim is to map the magnetic and dielectric properties of CuO films as a function of epitaxial strain. The films have been deposited by pulsed laser deposition (PLD) on (100) SrTiO3, (001) (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) and (100) LaAlO3 substrates. We study the effect of the thickness and substrate temperature on the film epitaxial strain for the three types of substrate. The samples have been analysed by X-ray diffractometry (XRD), High resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM).

Authors : C. Villalobos, C. Ferrater, M.C. Polo, J. Bertomeu, J.M. Asensi, J. Andreu, M. Varela
Affiliations : Departament de Física Aplicada, Universitat de Barcelona, Spain

Resume : SrVO3 it is emerging material which present a clear interest as a transparent conductor in photovoltaics, with optical and electrical properties outreaching those of the most used material indium tin oxide (ITO) [1,2]. The growth of thin films based on metal oxides with perovskite structure and high crystalline quality requires the use of suitable substrates and high deposition temperatures. These requirements are not compatible with silicon-based photovoltaic applications and consequently the integration of these materials constitutes one of the most important challenges in the development of new conductive and transparent electrodes. The purpose of this work is to study the effect of crystallinity of the deposited SrVO3 thin films on the optical and electrical properties. The films have been deposited by pulsed laser deposition (PLD) on (001) (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) substrates, on which high-quality growth was demonstrated before [1], and (100) SrTiO3 and (100) Si substrates. The control of the temperature of the substrate has allowed to obtain layers with different crystalline quality. The samples have been analysed by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). In addition conductive and optical measurements were performed in order to study the influence of the structure on the functional properties. [1] L. Zhang, Y. Zhou, L. Guo, W. Zhao, A. Barnes, H.-T. Zhang, C. Eaton, Y. Zheng, M. Brahlek, H. F. Haneef et al., Nat. Mater. 15, 204 (2015). [2] A. Boileau, A. Cheikh, A. Fouchet, A. David, R. Escobar-Galindo, C. Labbé, P. Marie, F. Gourbilleau, and U. Lüders. Applied Physics Letters 112, 021905 (2018).

Authors : I. Spasojevic,1 E. Pach,1 K. Cordero-Edwards, J.Santiso,1 G. Catalan,1A.Verdaguer1,2 N. Domingo1
Affiliations : 1Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain 2 Institut de Ciència de Materials de Barcelona, 08193 Bellaterra, Spain

Resume : Ferroelectric materials show strong electric fields at the surface. These electric fields can be screened by different mechanisms, namely intrinsic, such as charge carriers or defects, or extrinsic, mainly adsorbates, that play a crucial role in the stabilization of polarization domains. In this sense, when considering a ferroelectric material, the electrostatic interactions between the surface and adsorbates are a critical aspect for the polarization dynamics: screening and atomistic processes at the surface is a key to control low-dimensional ferroelectricity as it has been proven in different Piezoresponse Force Microscopy (PFM) experiments [1]. Among the adsorbates in ambient conditions, water molecules due to its ubiquitous presence and its polar nature play a critical role. Despite its well known influence little is known about the water induced electrochemical reactions at the surface of ferroelectric materials for different environmental conditions. Ambient Pressure X-ray photoelectron spectroscopy (AP-XPS) has proved as a powerful tool to study the interface of oxide surfaces in the past [2] such as SrTiO3 and also ferroelectric surfaces. Using this technique, we studied the composition of the surface of ferroelectric single crystals such as LiNbO3 [3] at the line CIRCE in ALBA. In this contribution we will show our measurements on BaTiO3 thin films and single crystals in different controlled water atmospheres. We were able to identify the presence and measure the thickness of molecularly absorbed water, OH groups and the formation of carbonates for different conditions of pressure (humidity), temperature and polarization state. Results can be used to explain PFM results in the literature when working in ambient conditions. Moreover, we will show how the modification of the surface through the use of different molecules is correlated with ferroelectric response. References [1] J.J. Segura, N. Domingo, J. Fraxedas, A. Verdaguer J. Appl. Phys. 113, 187213 (2013). [2] N.Domingo, E. Pach,, K. Cordero-Edwards, V. Pérez-Dieste, C. Escudero, A. Verdaguer (manuscript under revisión J.Phys. Chem C) [3] K. Cordero-Edwards, L. Rodríguez, A. Calo, M. J. Esplandiu, V. Peŕez-Dieste, C. Escudero, N. Domingo and A.Verdaguer J. Phys. Chem. C 120, 24048 (2016).

Authors : Rosalía Cid Barreno, Juan Rubio Zuazo, Germán Rafael Castro Castro
Affiliations : SpLine BM25 Beamline, European Synchrotron Radiation Facility, 38000 Grenoble, France; Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas (ICMM-CSIC), 28049 Madrid, Spain

Resume : The growth of perovskite oxides in uncommon crystallographic directions is mandatory to open routes toward new functionalities based on different electronic and topological phases. For example, the multiferroicity or active control of quantum phases in thin-film devices require an effective tuning of the octahedral configurations via the strain and interface engineering. Controlled growth of perovskite oxides on [001] oriented interfaces is becoming increasingly routine. However, coherent film epitaxy on other directions is needed in order to provide opportunities of realizing new electronic and topological phases. In this scenario, we have grown differently doped mixed-valent manganites La1-xCaxMnO3 thin films by pulsed laser deposition along the [111] crystallographic direction and studied their phase-diagram. We demonstrate their epitaxial single phase single oriented growth as a trigonal lattice along the SrTiO3-111 substrate axis, studied in detail by grazing incidence x-ray diffraction, and obtain their valent state by XANES. Moreover, we discuss their magnetic and transport properties and the role of strain.

Authors : Kohei Ono, Takeshi Ishiyama
Affiliations : Toyohashi University of Technology

Resume : Zinc oxide (ZnO) is an interesting II–VI semiconductor with various electrical, optical, chemical properties because of its wide direct bandgap of 3.3 eV at room temperature. Therefore, it is applied to electrodes for display devices and solar cells. It is expected as an ultraviolet blue light-emitting diodes. However, the research of such optoelectronic devices has been impeded because fabrication of p n homojunctions in ZnO have been extremely difficult. Most of the II-VI semiconductors have a Wurzite structure, and they shows n-type conductivity. Therefore, it is difficult to achieve p-type doping because of the “self-compensation”. In this study, we aimed to fabricate p- and n-type ZnO films by spray pyrolysis deposition (SPD) method. SPD method has the simple and inexpensive characteristics as compared with sputter deposition and vacuum evaporation. First, n-type ZnO films were fabricated with different Ga-doping rations. The resistivity was significantly improved by thermal treatment at low temperature. The minimum resistivity was 6.35 × 10-3 Ω cm in annealed sample. Next, p-type ZnO films were fabricated with different Li-doping rations. P-type conductivity was achieved by accurately optimizing the Li concentration, annealing temperature, and annealing atmosphere to control defects. Especially, high temperature annealing treatment of 700 to 800 degrees was effective in order for decrease defects. The average transmittance in visible right region was above 80% in all the samples.

Authors : Ahalapitiya H Jayatissa and Bharat Pant
Affiliations : Department of Mechanical, Industrial, and Manufacturing Engineering (MIME), The University of Toledo, Toledo, OH 43606, USA

Resume : Gallium oxide (Ga2O3) is well known as a transparent semiconductor compound. Many techniques have been employed to prepare the Ga2O3 thin films, including sol-gel methods, spray pyrolysis, magnetron sputtering, metal-organic chemical vapor deposition, and plasma-assisted molecular beam epitaxy In this research, preparation of nanocrystalline Ga2O3 by a sol-gel process was investigated. The investigations are focused to understand the UV light detection capability of nanocrystalline gallium oxides. The microstructure, electronic properties and photonic properties of Ga2O3 thin films were investigated as a function of film thickness. The current-voltage characteristics of Ga2O3 were investigated at the different regions of the spectrum to understand the spectral response of this material. In order to understand the defect density of photonic characteristics, the films were treated with oxygen plasma conditions. The properties of plasma treated and untreated films were investigated to understand the controllability of grain boundary effect on photonic characteristics of Ga2O3.

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Ga2O3 Growth & Characterization II : Hisashi Murakami
Authors : Brandon Howe
Affiliations : Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton, OH, U.S.A.

Resume : Revolutionary advances in next-generation electronics rely on the development of truly disruptive and robust electronic materials with novel and/or enhanced physical properties. Recently, we have built up a state-of-the-art, high-throughput physical vapor epitaxy (PVE) suite capable of synthesizing extremely high quality epitaxial layers, using custom computer-controlled systems to rapidly assess and identify novel materials with enhanced physical properties. This talk will focus on the growth of high quality oxide heterostructures through unconventional PVE routes. I will show how these synthesis tools are creating exceptionally high quality and novel magnetic oxides with the unique combination of record high magnetostriction and microwave performance, and new UWBG materials such as β-Ga2O3 that can be doped with silicon and alloyed with Al2O3 to control electronic properties. We found that at 0.2 wt % Si, a carrier density and mobility of 7.6 x 1019 cm-3 and 52.2 cm2/Vs (respectively) was observed. As the silicon concentration increased, the carrier density and mobility decreased. Epitaxial films of β-(AlxGa1-x)2O3 were deposited using targets with Al composition ranging from 0.175 to 0.5. X-ray diffraction confirmed both homoepitaxial Si-doped β-Ga2O3 and β-(AlxGa1-x)2O3 films. Hence, we demonstrate that pulsed laser deposition as a viable technique to grow both Si-doped homoepitaxial β-Ga2O3 and β-(AlxGa1-x)2O3/Ga2O3 heterostructures for wide bandgap electronic devices. Finally, I will give an update on our recent acquisition of a molecular beam epitaxy tool for β-Ga2O3 and share any initial results on homoepitaxial layers achieved to date.

Authors : Oliver Bierwagen, Piero Mazzolini, Patrick Vogt; Robert Schewski, Martin Albrecht
Affiliations : Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5--7, 10117 Berlin, Germany; Leibniz-Institut fur Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany

Resume : Due to its wide band gap of ≈4.7 eV the n-type semiconducting oxide Ga2O3 has recently been recognized as a promising material for power electronics applications and UV detectors. These advanced applications typically require single crystalline films with well-defined thickness and doping, which can be readily realized by molecular beam epitaxy (MBE). During the MBE of Ga2O3 and In2O3 the desorption of their suboxides Ga2O and In2O has been determined as growth-rate-limiting step [1,2]. We explain this behavior quantitatively by a two-step growth model that involves the intermediate suboxide formation [3]. Through metal-exchange catalysis [5], the kinetic advantage for the In2O3 formation [2] combined with the thermodynamic advantage for the Ga2O3 formation [4] lead to a strong enhancement of the Ga2O3 growth rate under an additional In-flux. For the device-application relevant homoepitaxy of Ga2O3(010) under Ga2O-desorption limited growth conditions the formation of {110} facets and a strongly enhanced growth rate by metal-exchange catalysis are demonstrated.[6] This work was performed in the framework of GraFOx, a Leibniz-ScienceCampus partially funded by the Leibniz association. [1] P. Vogt et al., Appl. Phys. Lett. 108 (2016) 072101 [2] P. Vogt et al., Appl. Phys. Lett. 109 (2016) 062103 [3] P. Vogt et al., (submitted) (2018) [4] P. Vogt et al., APL Materials 4 (2016) 086112 [5] P. Vogt et al., Phys. Rev. Lett. 119 (2017) 196001 [6] P. Mazzolini et al., in preparation (2018)

Authors : Hongping Zhao
Affiliations : Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH, USA, 43210

Resume : Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic beta-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases. The breakdown field of beta-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make beta-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal beta-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Different from the molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) growth techniques, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality beta-Ga2O3 thin films on both native Ga2O3 and sapphire substrates with controllable doping and fast growth rates up to 10 um/hr. In this talk, we present the growth, material characterization and device demonstration of beta-Ga2O3 thin films grown via LPCVD. The beta-Ga2O3 thin films were grown on native Ga2O3 (010), (001) and (-201) substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. Fundamental material properties will be discussed.

Authors : S. Bin Anooz1, A. Popp1, M. Schmidbauer1, C. Wouters1, A. Fiedler1, A. Kwasniewski1, M. Ramsteiner2, M. Albrecht1, K. Irmscher1 and G. Wagner1
Affiliations : 1Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany 2Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, Berlin, D-10117, Germany

Resume : Homoepitaxial β-(In,Ga)2O3 thin films were successfully grown on (100) β-Ga2O3 substrates by metal organic vapor phase epitaxy (MOVPE) using triethylgallium (TEG) and triethylindium (TEIn). Growth temperatures from 650 to 825 °C and pressures from 5 to 20 mbar have been investigated. The growth rate decreased linearly with increasing growth temperature and decreased exponentially with increasing pressure. The resulting films were investigated by atomic force microscopy (AFM), high resolution X-ray diffraction (HR-XRD), Raman spectroscopy, transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDX). AFM images of the β-(In,G)a2O3 with different Indium contents showed smooth surfaces having a surface roughness of about 1 nm which slightly increased with decreasing growth temperature. HR-XRD analysis revealed that the out-of-plane lattice parameter of β-(In,Ga)2O3 thin film increases with increasing Indium content and the incorporation of indium is slightly sensitive to the growth temperature and pressure. In the Raman spectra of the -(In,Ga)2O3 films no phonon modes of additional phases, such as In2O3, could be detected. As a signature of the In incorporation, a slight shift of the Raman peaks was observed. TEM was used to examine the structural details of the grown films and the average Indium contents were verified by HR-XRD and EDX.

Authors : Andreas Popp, Saud Bin Anooz, Andreas Fiedler, Charlotte Wouters, Martin Albrecht and Günter Wagner
Affiliations : Andreas Popp, Saud Bin Anooz, Andreas Fiedler, Charlotte Wouters, Martin Albrecht and Günter Wagner Leibnitz-Institute of Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, Germany

Resume : In this contribution we report about the growth of homoepitaxial β-Ga2O3 layers by MOVPE on different oriented substrates. In the focus of our work is the deposition of delta doped layers structures. We investigated the beginning of growth for the (100) direction and we were able to prove a layer by layer growth. For this type the growth takes place by the formation of 2D islands nucleate on the surface, leading to defects and stacking faults. Therefore we used (100) substrates with a miscut of 6° in c-direction to generate a step flow growth resulting in a suppression of the 2D island nucleation. To compare the growth of ultra-thin layers in relation to the defect generation and the shape of the interfaces we used as an alternative (010) substrates. We realized stacking layers with the following shape: 1) 175 nm LD (low doped) layer 2) 3-4 nm highly doped (HD) ẟ-layer 3) 20 nm LD layer on (010) and (100) 6° off β-Ga2O3 substrates by MOVPE. As precursors we used triethylgallium (TEGa), oxygen as oxidant and tetraethylorthosilicate (TEOS) as source for silicon as dopant. The morphology was analyzed by AFM and SEM. To determine the layer thickness ellipsometry measurements were performed. The depth profile and the silicon concentration of the multilayer stack was measured by using secondary ion mass spectroscopy (SIMS). The shape of the interfaces between the layers and the structural perfection were analyzed by SIMS and high resolution transmission electron microscopy (HRTEM).

Authors : R. Schewski, A.Fiedler, K. Irmscher, K. Lion, S. Levchenko, M. Scheffler C. Draxl, T.Schulz, M. Schmidbauer, A. Popp, M. Baldini, G. Wagner, Z. Galazka, M. Albrecht
Affiliations : Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany;Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Institut of physics, Humboldt-University zu Berlin, Newtonstraße 15, D-12489 Berlin, Germany; Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany; Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany; Institut of physics, Humboldt-University zu Berlin, Newtonstraße 15, D-12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany; Leibniz Institute for Crystal Growth (IKZ), 12489 Berlin, Germany

Resume : In this contribution, we investigate the influence of the stepdown direction, namely the -c and c direction of the monoclinic lattice on the structural and electronic properties of homoepitaxial layers. We will show that for substrates with a stepdown direction prepared 6° towards -c, perfect step flow growth is achieved. Such layers exhibit excellent structural and electronical properties. TEM investigations reveal no structural defects within the layer and Hall-Effect measurements showed high carrier motilities between 60 -100 cm²V-1s-1 at doping concentrations in the range of 1 - 2 x 1018 cm-³. On contrary, for substrates prepared towards the c direction the layer is characterized by a high amount of twin lamellae generated at the interface between the substrate and the layer. These twin defects generate incoherent twin boundaries that penetrate through the whole layer. As shown previously8,9 such incoherent twin boundaries act as barriers for carrier transport and cause a significantly reduced charge carrier mobility of around 10 cm²V-1s-1 in these layers. To identify the reason for the different growth behaviours, we performed high-resolution scanning transmission electron microscopic (STEM) investigations at surface of the grown layer. Our data reveals that the step down direction is always -c, independent whether the substrate miscut was towards c or -c. This means that, in the case of growth on substrates with a miscut direction towards the c direction, the whole layer growth in an orientation that can be described by a c/2 glide reflection in respect to the substrate, which is the typical twin relation as described in our previous studies. Moreover, we find that the surface is characterized by (100) terraces with (2 ?01) step edges, which also accounts for both substrate miscut directions c or -c. This suggests that these facets are energetically very favourable and thus prevailing during growth To verify this assumption, density functional theory (DFT) calculations were carried out evaluating the surface free energies of different ideal crystallographic planes of b-Ga2O3. The DFT results confirm that both - the (100) and the (2 ?01) facets - are lowest in energy thus supporting our observed growth results.

Ga2O3 Transport Properties : Gregg Jessen
Authors : R. Ahrling1, J. Boy1, M. Handwerg1, R. Mitdank1, G. Wagner2, Z. Galazka2, and S. F. Fischer1
Affiliations : 1 Novel Materials Group, Humboldt-Universität zu Berlin, Newtonstraße 15, 12489 Berlin, Germany;2 Leibniz Institute for Crystal Growth, Max-Born-Straße 2, 12489 Berlin, Germany

Resume : Thin films of the wide band gap semiconductor ß-Ga2O3 have promising applications in transparent electronics and high power devices. However, to date the transport, thermal and thermo-electrical properties in thin films remain unknown. Here, the electrical and thermal conductivities, Hall densities and mobilities and Seebeck coefficients were measured for various thin films. In thin homoepitaxially MOVPE grown (100)-orientated ß-Ga2O3 films the room temperature mobilities in thicker films (225 nm) were similar to that of bulk of about 115 cm2/Vs , in the thinnest film (28 nm) it was reduced by about 5.5 cm2/Vs which is discussed by finite size effects. Seebeck coefficients in the range of a few hundreds µV/K have been measured at room temperature and below. The thermal conductivity was investigated of thin MOVPE grown polycristalline ß-Ga2O3 films on sapphire-substrates and single-crystalline, electrically conductive ß-Ga2O3 films on insulating Mg-doped Czochralski-grown ß-Ga2O3 substrates. In order to measure the thermal conductivity of films and substrates, we used the different techniques (3-omega and 2-omega approaches). We observe a reduction of the bulk thermal conductivity in dependence of the crystallinity and film thickness which can be explained with the reduced mean free path of the phonons due to the film thickness and grain sizes.

Authors : C.D. Young1, M.S.L. Narayanan1, X. Qin1, P. Zhao1, A. Padovani2, M.I. Pintor-Monroy1, Jesus J. Alcantar Peña1, P. Bolshakov1, L. Larcher3, O. Auciello1, M. Quevedo-Lopez1, and R.M. Wallace1
Affiliations : 1University of Texas at Dallas, 800 W. Campbell Rd., Richardson, TX 75080, USA; 2MDLSoft Inc., Santa Clara, CA, USA; 3University of Modena and Reggio Emilia, Modena, Italy

Resume : The wide bandgap material (WBM), single crystal beta phase gallium oxide has emerged as a semiconductor for use in several applications which may exceed the performance of current WBM semiconductors (e.g., SiC, GaN, and diamond) [1,2]. Some WBM power/RF devices are typically configured as high electron mobility transistors (HEMTs), which can be impacted by parasitic leakage current at the Schottky junction that serves as the gate – especially at elevated temperatures. In addition, a particular challenge of beta-Ga2O3 is its low thermal conductivity. Therefore, continued research and innovation are required to improve understanding and device performance. One approach is integrating a gate dielectric to significantly reduce this leakage effect while simultaneously increasing the breakdown field. Furthermore, amorphous Ga2O3 films demonstrate some of the same characteristics with the added advantage of the deposition process being carried out at room temperature [3,4]. This enables the integration of thin films in flexible electronics along with easier thickness and carrier concentration control, thereby establishing a variety of devices with no degradation of charge transport when compared with polycrystalline films. We will evaluate single crystal, metal-oxide-semiconductor capacitors, along with amorphous un-doped Ga2O3 UV detectors, Ga2O3–based TFTs, and all-oxide NiO/Ga2O3 pn junctions. [1] K. Irmscher, et al., JAP, 110, p. 063720, 2011. [2] M. Higashiwaki, et al., APL, 100, p. 013504, 2012. [3] M. Heinemann, et al., APL, 108, p. 022107, 2016. [4] S. Cui, et al., Adv. Opt. Mat., 2017.

Authors : Uttam Singisetti, Krishnendu Ghosh, Ke Zeng, Abhishek Vaidya, Ankit Sharma, Avinash Kumar
Affiliations : University at Buffalo, 230 H Davis Hall, Buffalo, NY, 14260, USA

Resume : Beta-Ga2O3 has recently attracted a lot of attention as a novel ultra-widebandgap semiconductor for next generation power devices. First, we will discuss the fundamental transport studies on Ga2O3. The low symmetry monoclinic crystal of Ga2O3 and presence of multiple IR active optical phonon modes are a challenge in these studies as the popular methods developed for symmetric crystals cannot be used in this material system. Both low field and high field transport calculations in Ga2O3 will be discussed with implications on device performance. Next, we will discuss our recent work on atomic layer deposited SiO2 dielectrics on Ga2O3. The interface electrical characterization shows low interface state densities making it suitable both as a gate and passivation dielectric. Both depletion mode and enhancement mode devices with high breakdown voltage were demonstrated using SiO2 as a gate dielectric. A low cost and effective spin-on-doping technology for low resistance source/drain contacts was developed and integrated into Ga2O3 devices. Temperature dependent characterization of these devices shows the potential of Ga2O3 for extreme environment applications.

Authors : Andreas Fiedler, Manfred Ramsteiner, Zbigniew Galazka, Klaus Irmscher
Affiliations : Leibniz-Institut für Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany; Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5-7, 10117 Berlin, Germany; Leibniz-Institut für Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany; Leibniz-Institut für Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany

Resume : Monoclinic gallium sesquioxide (beta-Ga2O3) belongs to the material class of transparent semiconducting oxides. It is distinguished by its large band gap of about 4.7 eV, which is the reason for a transparency range extending deep into the ultraviolet and for a high electrical breakdown field estimated at 8 MV/cm. Combined with the feasibility of n-type doping by Sn or Si, beta-Ga2O3 has great potential as a material for solar-blind photo detection and for power electronics where it might outperform GaN and SiC. For such applications, doping control is of outermost importance. Here we report on a Raman spectroscopic investigation on highly n-type doped beta-Ga2O3 single crystals obtained by the Czochralski method. For degenerate material (? > 3 × 1018 cm-3), we observe Raman lines at 256 cm-1 and 282 cm-1, forming a double peak, and at 563 cm-1 (about twice the wavenumber of the second peak), which cannot be assigned to first-order scattering by phonons. These lines exhibit only a weak temperature dependence and are essentially independent of the shallow donor species (Sn or Si). We attribute the doping induced Raman features to electronic Raman scattering caused by excitation of electrons from an effective-mass like donor impurity band into the conduction band. This assignment is based on the facts that the high-energy edge of the double peak coincides with the ionization energy of effective-mass like donors (36 meV), and that the Raman signals only appear for doping concentrations exceeding the Mott criterion. Consequently, the high-frequency Raman line at 563 cm-1 (= 2 × 282 cm-1) is explained by second-order electronic Raman scattering.

Ferroelectric Heterostructures I : Pavlo Zubko
Authors : Lane W. Martin
Affiliations : Department of Materials Science and Engineering, University of California, Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory

Resume : Complex-oxide materials possess a range of interesting properties and phenomena that make them candidates for next-generation devices and applications. But before these materials can be integrated into state-of-the-art devices, it is important to understand how to control and engineer the response of these materials in a deterministic manner. Here, we will discuss the science and engineering of emergent ferroic order and phenomena. In particular we will explore the role of epitaxial thin-film growth processes and the use of new types of lattice mismatch strain to engineer classic ferroelectric systems. In recent years, the use of epitaxial strain has enabled the production of model versions of these complicated materials and the subsequent deterministic study of field-dependent response. Here, we will investigate how new manifestations of epitaxial constraint can allow us to expand beyond bistable switching responses. In this regard we will explore the use of epitaxial film orientation to produce exotic domain structures and emulate neuron-like function in materials and defect-based routes to engineer local switching behaviors so as to produce multi-state stability in ferroelectric devices. The discussion will range from the development of a fundamental understanding of the physics that lies at the heart of the observed effects, to an illustration of routes to manipulate and control these effects, to the demonstration of solid-state devices based on these materials.

Authors : Tae Won Noh
Affiliations : 1 Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul, Republic of Korea 2 Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea

Resume : Magnetic skyrmion is a topologically protected whirling spin texture in nanoscale. Its small size, topologically-protected stability, and solitonic characteristics together hold great promises for future spintronics applications. To translate such compelling features into practical spintronic devices, a key challenge lies in achieving effective control of skyrmion properties, such as size, density, and thermodynamic stability. Here, we report the discovery of ferroelectrically tunable skyrmions in ultrathin BaTiO3/SrRuO3 bilayer heterostructures. The ferroelectric proximity effect at the BaTiO3/SrRuO3 heterointerface can trigger a sizable Dzyaloshinskii-Moriya interaction, thus stabilizing robust high-density skyrmions. The skyrmion size in this system can be minimized down to approximately 10 nm. Moreover, by manipulating the ferroelectric polarization of the BaTiO3 layer, we achieve local, switchable and nonvolatile tunability of both skyrmion density and thermodynamic stability. Such ferroelectric control of skyrmion properties heralds a novel approach to simultaneously enhance in the integratability and addressability of skyrmion-based functional devices.

Authors : Xiaoyan Li 1, Qiuxiang Zhu2*, , Lorenzo Vistoli 2, Manuel Bibes 2, Agnès Barthélémy 2, Stéphane Fusil 2, Vincent Garcia 2, Alexandre Gloter 1
Affiliations : 1 Laboratoire de Physique des Solides, CNRS UMR 8502, Université Paris Sud XI, 91405 Orsay, France 2 Unité Mixte de Physique CNRS/Thales, Univ. Paris-Sud, Université Paris-Saclay, Palaiseau, France

Resume : Controlling the functional properties of strongly-correlated system by ferroelectric field effect (Fe-FET) that allows faster and lower-power operations has sparked a surge of research activities in the last decades. Here, we report on perovskite heterostructures combining (Ca,Ce)MnO3 and BiFeO3 in its ‘‘supertetragonal’’ phase on YAlO3(001) substrates. The rich phase diagram of (Ca,Ce)MnO3 offers great potential for electrostatically induced metal-insulator transitions and the large polarization of BiFeO3 qualifies it as a suitable ferroelectric gate. BiFeO3/(Ca,Ce)MnO3 bilayers with different Ce contents are epitaxially grown by pulsed laser deposition. High quality is assessed by X Ray Diffraction, Piezoresponse Force Microscopy (PFM) and Scanning Transmission Electron Microscopy (STEM). For the Fe-FET studies, these BiFeO3/(Ca,Ce)MnO3 bilayers are patterned into Hall bars in which the polarization of the BiFeO3 gate can be switched back and forth with the PFM. The influence of the BiFeO3 polarization direction on the resistance and the carrier concentration in (Ca,Ce)MnO3 is checked by transport measurements. Nevertheless, no prominent electronic responses are observed. We carried out depth resolved mapping of ferroelectric switching by STEM. Indeed, the ferroelectric field effect is very sensitive to the interfacial state and STEM combined with Electron Energy Loss Spectroscopy (EELS) can be used to probe the interfacial electronic and structural properties between the ferroelectric and the Mott insulator. Surprisingly, atomic-resolved STEM images reveal that the polarization switching is incomplete with 31% to 91% of the volume population being effectively reversed from down to up. The switched domains seem to nucleate from the top surface of BiFeO3 and propagate toward the BiFeO3/(Ca,Ce)MnO3 interface. However, only 19% of the interface is successfully switched. While the small fraction of switched polarization at the interface causes the deterioration of the field effect at the micron scale, the charge transfer onto the adjacent (Ca,Ce)MnO3 can be clearly observed in nanometric areas which are successfully switched down to the interface. The electrostatic carrier modulation is revealed by the EELS near-edge fine structure of Mn L3 at interfaces with opposite polarization. Close to the BiFeO3/(Ca,Ce)MnO3 interface, the excess of electrons is lowered for the upward polarization with an effective charge modulation of 2.38 e- corresponding to a sheet carrier density variation of 1x1015 cm-2. This is within the range of the expected modulation for BiFeO3 with a polarization of the order of 100 μ, showing the expected ferroelectric field-effect at the local scale.

Authors : Matjaž Spreitzer1, Daniel Diaz1, Danilo Suvorov1
Affiliations : 1 Advanced Materials Department, Jožef Stefan Institute, Jamova 39, Ljubljana, Slovenia

Resume : Ag(Nb1−xTax)O3 ceramics and thin films have received a great deal of attention over the years because of their interesting dielectric/ferroelectric properties and complex phase transitions. Additionally, the microwave dielectric properties as another research hotspot of Ag(Nb1−xTax)O3 system have also been investigated due to combination of a high dielectric constant, a medium Qf value and a low temperature coefficient of resonant frequency, and can be thus integrated with various passive dielectric components like band-pass filters and resonators. In the present work we investigated the growth and dielectric properties of Ag(Nb0.5Ta0.5)O3 (ANT) thin film grown on (0001) Al2O3 single-crystalline substrates by pulsed laser deposition. Samples were characterized by reflection high-energy electron diffraction, X-ray diffraction (XRD), Rutherford backscattering spectroscopy, electron microscopy, and high-frequency dielectric measurements. The effect of laser fluence and oxygen background pressure on plume size and ANT composition were determined, together with the effect of target-to-substrate distance, deposition temperatures, and the laser frequency. Optimized deposition conditions enabled us to grow a pure-phase polycrystalline Ag(Nb0.5Ta0.5)O3 thin film with the thickness of 300 nm. For as-prepared thin film electrical properties in broad frequency range were determined and correlated with characteristics of ceramics counterparts.

Authors : N.D. SCARISOREANU (1); F. ANDREI (1); V. ION (1); R. BIRJEGA (1); M. DINESCU (1); V. TEODORESCU (2)
Affiliations : 1) National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor , Magurele, Romania; 2) National Institute of Materials Physics, 405 A Atomistilor, Magurele, Romania

Resume : Due the efficient ferroelectric polarization-driven carrier separation perovskitic thin films are increasingly being studied for applications in solar energy. The multiferoics materials, with a small band gap are potential candidates for green energy production. In this category, bismuth ferrite (BiFeO3-BFO) can have a high impact. Due the chemical stability and high leakage issues, BFO application in photovoltaics still pends. Having the possibility of tailoring the value of band gap of BFO by rare earth elements doping, this material is intensively studied including for photocatalitic apllications such as water splitting or degradation of organic pollutants. In this work we report the functional properties BiFeO3/LaFeO3 heterostructures obtained by Pulsed Laser Deposition on different substrates (Nb doped SrTiO3, SrTiO3, GdScO3 substrates). The purpose of the study is to benefit from the strong ferroelectricity of BFO in combination with the excellent chemical stability of LaFeO3 (LFO) for obtaining superior photocatalytic behavior. A parametric study on the influence of deposition parameters on the properties of the BiFeO3/LaFeO3 heterostructures was carried out. Crystalline properties and topography of surface of thin films were studied using X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM) and atomic force microscopy (AFM). The optical properties were determined by spectroscopic ellipsometry (SE) and the values of band gap (Eg) where determined from Tauc plot. We have obtained high photocurrent densities (J photocurent> 700 µA/cm2) of the heterotructures for a limited interval of the monolayer's thickness, by photoelectrochemical analysis, showing a non-monotonic dependence of the photocatalytic activity with induced structural strain.

Ferroelectrics on Semiconductors : Lane Martin
Authors : Guus Rijnders
Affiliations : MESA+ Institute for Nanotechnology, University of Twente, POBox 217, 7500AE, Enschede, the Netherlands

Resume : In recent years, it has been shown that novel functionalities can be achieved in oxide heterostructures in which the interfaces are atomically controlled, in terms of atomic stacking as well as in terms of the local symmetry. In this contribution, I will highlight the recent developments in atomic controlled growth of epitaxial oxides by pulsed laser deposition, with a focus on heterostructures showing manipulated magnetic and electronic properties. Integration of epitaxial complex oxides in Si and III-V technologies has recently attracted a lot of attention in science and industry. Such complex oxides include, amongst others, ferro- and piezo-electrics and materials for resistive switching devices. In this presentation I will focus on the integration of Pb(Zr,Ti)O3 (PZT) with Si and III-V semiconductors. The epitaxial integration of PZT with Si and for instance GaN is hampered by the difference in crystal structure and large lattice mismatch. Using epitaxial buffer layers and optimized growth with pulsed laser deposition, we are able to obtain epitaxial growth of PZT on, for instance, MgO-buffered GaN. The thickness of the MgO can be lowered down to single monolayers while maintaining the high quality and good properties of epitaxial PZT films, which enable practical applications for high power FET’s and non-volatile ferroelectric controlled electronics devices. I will furthermore highlight some recent new insights in the “physics” of pulsed laser deposition of complex oxides, focusing on the influence of oxygen pressure on the deposited species during growth as well as the large-scale growth of epitaxial oxides on wafers up to 200 mm in diameter.

Authors : C. Dubourdieu1, L. Mazet2, M. M. Frank3, E. A. Cartier3, J. Bruley3, V. Narayanan3, S. Schamm-Chardon4, S.W. Schmitt1, S. Yang5, R.K. Vasudevan5, S.V. Kalinin5
Affiliations : 1. Helmholtz Zentrum Berlin für Materialien und Energie, Berlin, Germany; 2. Institut des Nanotechnologies de Lyon, CNRS, Ecole Centrale de Lyon, Ecully, France; 3. IBM T.J. Watson Research Center, Yorktown Heights, NY, USA; 4. CEMES-CNRS, Université de Toulouse, Toulouse, France; 5. CNMS, Oak Ridge National Laboratory, Oak Ridge, TN, USA

Resume : Ferroelectric polarization switching has been shown to persist down to about three or four monolayers of BaTiO3 when it is inserted between two metal electrodes. When it comes to a metal-insulator-semiconductor capacitive structure, the stabilization of a ferroelectric polarization in the insulator phase raises specific questions due to the inability of the semiconductor to provide free carriers to screen the surface charges. We studied the growth, structural and ferroelectric properties of BaTiO3 grown on silicon either in the form of epitaxial ultrathin films (1.6 – 4 nm) or in the form of composite material consisting of an amorphous matrix and of nanocrystallites. The crystalline structure studied by X-ray diffraction and high resolution scanning electron microscopy as well as the composition will be presented. Difference in terms of dielectric permittivity, leakage currents and ferroelectric polarization will be discussed on the basis of current-voltage measurements and investigations by scanning probe microscopy in different electrical modes.

Authors : Amador Perez-Tomas, Carlos Rubio, Pablo Vales, Jose Santiso, Gustau Catalan, Monica Lira-Cantu, Ekaterine Chikoidze, Yves Dumont, Michael R Jennings, Stephen AO Russell, Tamar Tchelidze, Ferechteh H Teherani, Philippe Bove, Eric V Sandana, David J Rogers
Affiliations : (1) Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Barcelona, Spain; (2) Groupe d’Etude de la Matière Condensée (GEMaC), Université de Versailles Saint Quentin en Y. – CNRS, Université Paris-Saclay, 45 Av. des Etats-Unis, 78035, Versailles Cedex, France; (3) Faculty of Science, University of Warwick, Coventry, CV4 7AL, UK; (4) Faculty of Exact and Natural Science, Department of Physics, Ivane Javakhishvili Tbilisi State University, 3 Av. Tchavtchavadze, 0179, Tbilisi, Georgia (5) Nanovation, 8 route de Chevreuse, 78117, Châteaufort, France.

Resume : Oxides are among the semiconductor materials with the widest tunability of physical properties and have been investigated for a long time as a candidate platform for novel electronic devices. In practice, however, silicon (ubiquitously) and other compound semiconductors (in particular, III-V (GaN (power 0,6-1,2 kV), InGaN (opto), GaAs (RF)) and IV (SiC (power > 1,2 kV)) are already well-established commercial semiconductors in their respective application domains. In many cases, therefore, a strong case for adopting an oxide semiconductor is still required. Our paper will discuss some novel and recently identified opportunities that are being opened up by wide bandgap oxides. The emerging property set of Ga2O3 is one of these opportunities. Owing to a number of advances including commercial wide area single crystal wafer production using cheap and scalable methods, good control of shallow donor doping and (our) recent demonstration of p-type conductivity, this material now presents unrivalled material properties for some power electronic applications. A key underlying reason for this is that Ga2O3’s critical electric field is ~2-3 times larger than current competitors (SiC and GaN) and probably presents a better forward/reverse trade-off despite its modest electron mobility. A new kind of oxide based device called “Solaristors” is another of these emerging opportunities. Classic perovskite oxides (in particular Pb(Zr,Ti)O3) are wide bandgap transparent oxides and the class of materials where ferroelectricity is strongest. An often-overlooked property of a ferroelectric semiconductor is that the switchable bipolar surface arrangement also implies a switchable surface band-bending. In a heterojunction with other more efficient light harvesters, we have engineered this property to define cheap and large area compact self-powered two-terminal solar transistors (or “solaristors”) for the first time. Acknowledgements. This work has been partially financially supported by Agencia Estatal de investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) under contract ENE2015-74275-JIN. [1] E. Chikoidze, A. Fellous, A. Pérez-Tomás, G. Sauthier, T. Tchelidze, C. Ton-That, T. T. Huynh, M. Phillips, S. Russell, M. Jennings, B. Berini, F. Jomard, Y. Dumont, “P-type β-gallium oxide: A new perspective for power and optoelectronic devices”, Materials Today Physics 3, 118-126 (2017). [2] A. Pérez‐Tomás, A. Lima, Q. Billon, I. Shirley, G. Catalan, M. Lira‐Cantú, “A Solar Transistor and Photoferroelectric Memory”, Adv. Funct. Mater. 28, 1707099 (2018).

Authors : C. Merckling (a), M. Korytov (a), U. Celano (a), M. Hsu (b), S. de Gendt (a, c)
Affiliations : (a) Imec, Kapeldreef 75, 3001 Leuven, Belgium (b) TSMC – Belgium, Kapeldreef 75, 3001 Leuven, Belgium (c) KULeuven - Belgium, Celestijnelijn 200, 3001 Leuven, Belgium

Resume : Perovskite oxides with the ABO3 chemical formula have a very wide range of interesting intrinsic properties such as metal-insulator transition, ferroelectricity, pyroelectricity, piezoelectricity, ferromagnetic and superconductivity. For the integration of such oxides, it is of great interest to combine their properties with traditional electronic, memory and optical devices on the same silicon-based platform. In this work, we propose to analyze in-depth the relaxation mechanism and defects generation in functional oxides (such as BaTiO3 or PZT) grown by molecular beam epitaxy or pulsed laser deposition on SrTiO3/Si(001) pseudo-substrates. Based on various physical, optical and electrical characterization techniques, including reflection high-energy electrons diffraction (RHEED), X-ray diffraction (XRD) and high resolution spectroscopic transmission electron microscopy (HR-STEM), we could reveal the critical thicknesses of the different epitaxial systems. Finally, we propose to use strain mediated superlattice-based heterostructures to reduce the defect density in the structures and improve the polarization properties as confirmed by Piezo-Force Microscopy (PFM) characterization technique.

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Defects and Resistive Switching I : Gervasi Herranz
Authors : Heidemarie Schmidt1,2,3, Nan Du1, Danilo Bürger1, Ilona Skorupa,4, Ramona Ecke1, Stefan E. Schulz1
Affiliations : 1Fraunhofer-Institut für Elektronische Nanosysteme, Abteilung Back-End of Line, Technologie-Campus 3, 09126 Chemnitz, Germany; 2Faculty of physics, Friedrich-Schiller University of Jena Max-Wien-Platz 1, 07743 Jena, Germany; 3Leibniz-Institut für Photonische Technologien e.V. (IPHT), Albert-Einstein-Str. 9, 07745 Jena, Germany; 4Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany

Resume : The thriving of memristive oxide devices is arousing interest in the field-enhanced hopping transport of oxygen vacancies not only because in many n-type oxides oxygen vacancies effectively act as double donors, but also, more importantly, because memristive oxide devices exhibit ultra-nonlinear switching dynamics [1]. Optimized performance requires resistive switching (SET and RESET) within tens of nanoseconds upon the application of a writing bias, and the ON and OFF resistance states should remain stable for up to ten years. Field-accelerated ion mobility constitutes one such source of ultra-nonlinearity [2]. The main problems of RS cells which need further improvements include mainly the issue of electroforming step, reproducibility in RS characteristics and/or RS material, so-called “voltage-time” dilemma and sneak paths. Electroforming-free bipolar BiFeO3 [3] and unipolar YMnO3 [4] RS have been realized. [1] N. Du, M. Kiani, C. Mayr, T. You, D. Bürger, I. Skorupa, O. G. Schmidt, and H. Schmidt, Front. Neurosci. 9, 227 (2015). [2] S. Menzel, M. Waters, A. Marchewka, U. Böttger, R. Dittmann, and R. Waser, Adv. Funct. Mater. 21, 4487 (2011). [3] T. You, S. Yao; W. Luo, N. Du, D. Bürger, I. Skorupa, R. Hübner, S. Henker, C. Mayr, R. Schüffny, T. Mikolajick, O.G. Schmidt, H. Schmidt, Adv. Funct. Mat. 24 (2014) 3357-3365 [4] A. Bogusz, A. D. Müller, D. Blaschke, I. Skorupa, D. Bürger, A. Scholz, O. G. Schmidt, H. Schmidt, AIP Advances 4 (2014) 107135.

Authors : J. Santamaria1, J. Tornos1, G. Sanchez-Santolino2, D. Hernandez-Martin1, F. Gallego1, G. Orfila1, M. Cabero1, A. Perez-Muñoz1, J. I. Beltran1, C. Munuera2, A. Rivera-Calzada1, Z. Sefrioui1, F. Mompean2, M. Garcia-Hernandez2, S. J. Pennycook3 , M. Varela1, M. C. Muñoz2, S. Valencia4, Y. H. Liu5,6, V. Lauter5, R. Abrudan4, C. Luo4, R. Hanjo4, F. Radu4 Q. Wang6, S. G. E. te Velthuis6, C. Leon1
Affiliations : 1GFMC, Departamento de Física de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain; 2 Instituto de Ciencia de Materiales de Madrid ICMM-CSIC; 3 Department of Mat. Sci.& Engineering, Natnl. Univ.of Singapore, Singapore 117575; 4Hemholtz-Zentrum Berlin für Materialen und Energie, Albert-Einstein-Str.15, Berlin; 5 Quantum Condensed Matter Division., Oak Ridge National Laboratory, Oak Ridge, TN37831, USA; 6 Materials Science Division, Argonne Natnl. Lab. Argonne, Illinois 60439, USA

Resume : The electronic reconstruction occurring at oxide interfaces may be the source of interesting device concepts for future oxide electronics. Among oxide devices, multiferroic tunnel junctions are being actively investigated for the possibility of independently controlling the switching of the magnetization of the electrodes and of the ferroelectric polarization of the barrier. Here, we exploit the dynamic control of the vacancy profile in the nanometer thick barrier of a ferroelectric tunnel junction to demonstrate the interplay between resistive (oxygen vacancy) and ferroelectric switching. We find a strong interplay between resistive and ferroelectric switching which highlights the coupling between electrochemical and electronic degrees of freedom triggered by the known coexistence of multiple valence states in transition metal oxides. In this talk we show that the spin reconstruction at the interfaces of a La0.7Sr0.3MnO3 /BaTiO3 / La0.7Sr0.3MnO3 multiferroic tunnel junction is the origin of a spin filtering functionality which can be turned on and off by reversing the ferroelectric polarization. The ferroelectrically controlled interface spin filter enables a giant electrical modulation of the tunneling magnetoresistance between values of 10% and 1000%, that could inspire device concepts in oxides based low dissipation spintronics. [1] Nature Nanotechnology 12, 655 (2017)

Authors : Toni Markurt, Laura Bogula, Jos E. Boschker, Jutta Schwarzkopf, Julian Stöver, Klaus Irmscher, and Martin Albrecht
Affiliations : Leibniz Institute for Crystal Growth, Max Born Str. 2, 12489 Berlin, Germany

Resume : It is commonly believed that variation of the oxygen pressure during PLD growth is a direct way to control formation of oxygen vacancies (VO). Recent reports, however, address this point more critically [1],[2]. In our work we systematically studied formation of point defects in homoepitaxial SrTiO3 PLD films. For growth at chamber pressures ≤ 10-2 mbar in an oxygen flow we obtain 3 findings: (1) HRTEM reveals local lattice distortions in the film not present in the substrate. (2) These lattice distortions exhibit fluctuations on a time scale of seconds. (3) The STEM LAADF intensity is increased for such films. The image contrast between film/substrate could, however, not be reproduced by STEM image simulations even for defect concentrations of several percent. All 3 mentioned features are less pronounced for higher growth temperature. Samples grown in a pure Argon atmosphere but otherwise identical conditions showed surprisingly the same results. Consequently there is no simple relationship between oxygen partial pressure used during PLD growth and formation of VO. Our results suggests that the observed point defects originate from bombardment of the film with highly energetic species typical for PLD. An increase of the chamber pressure, irrespectively of the used gas, reduces the kinetic energy and leads to less point defects. Higher growth temperatures enable an in-situ annealing of the damaged film. The nature of the temporal fluctuations will be discussed in the paper. [1] Lee et al., Sci. Rep. 6, 19941 (2016) [2] Keeble et al. PRB 87, 195409 (2013)

Authors : Yooun Heo, Daisuke Kan, Yuichi Shimakawa
Affiliations : Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan

Resume : We investigate local electrical conduction in oxygen-deficient strontium iron oxide SrFeO3-δ (SFO) epitaxial thin films by conductive atomic-force microscope (C-AFM). The brownmillerite-structured insulating SrFeO2.5 epitaxial thin films were first grown on 0.5 wt% Nb-doped SrTiO3 substrates by pulsed laser deposition. No electrical conduction was detected in the as-grown films (Figure 1b). On the other hand, oxidizing the as-grown films by post annealing treatment in air results in a drastic change in conduction behavior. The film after annealing at 600 ºC shows spatially inhomogeneous conduction across terraces across step edges. Interestingly, with further annealing (or oxidizing), this inhomogeneous conduction disappears, resulting in a uniformly high conduction state. These observations imply that oxygen ions are preferentially incorporated near the step edge regions in the initial stage of the oxidizing process of the SrFeO2.5 films. We also reveal that such local conduction can be tuned by redox reactions under an electric field due to bias dependent local movement of incorporated oxygen ions. These results reveal the strong influence of the local oxygen content (or local oxidation state) on the electrical conduction of SrFeO3-δ.

Authors : Patrick Salg 1, Lukas Zeinar 1, Dominik Walk 2, Aldin Radetinac 1, Holger Maune 2, Rolf Jacoby 2, Philipp Komissinskiy 1, Lambert Alff 1
Affiliations : 1 Institute of Materials Science, TU Darmstadt, 64287 Darmstadt, Germany; 2 Institute for Microwave Engineering and Photonics, TU Darmstadt, 64283 Darmstadt, Germany

Resume : The transition metal perovskite oxide SrMoO3 (SMO) with Mo4 in a 4d2 electronic configuration has a room-temperature resistivity of 5.1 µΩcm as single crystal and, thus, can be considered as a promising conducting electrode material for all-oxide microelectronic devices. SMO thin films with resistivity values below 20 µΩcm at room temperature have been reported. However, stabilizing the unfavorable Mo4 valence state in SMO films requires reductive growth conditions incompatible with a highly oxidative environment as required for other functional oxides such as dielectrics. In order to overcome the constrictions of contradicting conditions during thin film growth, we have investigated oxygen diffusion in epitaxial oxide heterostructures comprised of SMO films in combination with several thin perovskite oxides SrTiO3, BaTiO3, Ba0.5Sr0.5TiO3 (BST), and SrZrO3. The valence states of the transition metal cations as Ti, Zr, and Mo were investigated in these materials at different oxidation conditions using X-ray photoelectron spectroscopy. Among the investigated materials, the lowest oxygen diffusion has been observed in BST. A BST film with a thickness of only 5 unit cells preserves the Mo4 oxidation state in the SMO underlayer at the used oxygen partial pressure of 3 mTorr and a temperature of 630 °C. Thus, SMO thin films covered with atomically layered thin BST remain conducting in an oxygen environment and can be integrated into all-oxide thin-film device heterostructures.

Defects and Resistive Switching II : Heidemarie Schmidt
Authors : F. Hensling, C. Bäumer*, Th. Heisig, N. Raab, R. Dittmann
Affiliations : Peter Grünberg Institute 7 and JARA-FIT, Forschungszentrum Jülich, 52425 Jülich, Germany

Resume : Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to its well know defect chemistry and the absence of grain boundaries. We studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process and the switching performance. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values and an increased memory window with respect to stoichiometric thin films. We investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Whereas stoichiometric thin films form a single filament, Sr rich thin films tend to form multiple filaments with increasing current compliance. We could prove explicitely, that stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. These results provide a pathway to a rational design of resistive switching devices STO with improved reliability.

Authors : Felix Gunkel
Affiliations : Institute of Electronic Materials, RWTH Aachen University, Germany

Resume : Perovskite oxides exhibit a plethora of fascinating electronic material properties covering an exceptionally wide range of phenomena in solid state and surface physics. This has led to tremendous efforts to functionalize these materials in applications for energy technology, gas sensing, and electronics. Layered in an atomically defined epitaxial heterostructures and superlattices, diverse properties of perovskites can be combined on the nanoscale level. In such structures, even new functionality can arise at interfaces of layered materials, exhibiting properties that are absent in the bare bulk materials. In our approach, we utilize atomically-defined layer growth to obtain desired material properties. However, on top of that, we employ thermodynamic engineering of crystal defects as a unique approach to functionalize material properties at surfaces and interfaces: Even at material synthesis conditions close to perfection, device properties are often determined by imperfection, hence, by lattice disorder and crystal defects. As we discuss, we can intentionally control defect structure in nanoscale devices, by developing and utilizing thermodynamic routes to trigger surface and interface reactions in confined systems. While historically defects were seen as something to be avoided, a change of paradigm is required in the field of complex oxides today: In these materials, we can promote functionality, such as metallicity in nominally insulating compounds, by atomic defect-management. Therefore, rather than avoiding defect formation, it is an essential necessity to control and to utilize defect formation in oxides on the nanoscale. Here, we discuss fundamental aspects of lattice disorder effects in bulk oxides, and elaborate the special character of defect formation in thin films, surfaces and interfaces. Focusing on SrTiO3 as a perovskite model system, we will crosslink fundamental perspectives on lattice disorder to actual applications, addressing different examples, such as resistive switching memories, high-mobility electron gases and induced magnetism, oxygen sensors, and electro-catalysts.

Authors : Toni Markurt1, Christo Guguschev1, Dirk Kok1, Mutong Niu2, Klaus Irmscher1, Franz Kamutzki1, and Martin Albrecht1
Affiliations : 1 Institute for Crystal Growth, Max Born Str. 2, 12489 Berlin, Germany; 2 Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215125, P. R. China

Resume : Although a number of studies address the topic of intrinsic atomic defects in SrTiO3 [1-3], though with contradictory results, little is known about the dynamics of point defect in SrTiO3 including diffusion and clustering. By means of TEM we found nanometer sized voids in SrTiO3 bulk crystals grown in oxygen containing atmospheres, irrespectively of the used growth method. In contrast to that, crystals grown in a nitrogen atmosphere are free of voids. The total void volume increases with the oxygen partial pressure during the growth and the size of voids increases with decreasing cooling rates after the growth. Annealing of crystals at high temperatures leads to an increase of the void size. We suggest a vacancy diffusion and clustering process to be responsible for formation of voids: Initially oxygen and metal vacancies are incorporated into the growing crystal. Their total equilibrium concentration is expected to be highest for slightly oxygen rich conditions [1]. During cool down after growth, the solubility of vacancies in the crystal decreases and eventually they become supersaturated. Since oxygen vacancies, shallow donors, and compensating metal vacancies are oppositely charged [1] and Coulomb attractive they cluster and form nuclei. Further diffusion of vacancies leads to growth of these nuclei resulting in the observed nanometer sized voids. Based on our results we give an estimation for formation energies and diffusion coefficients of vacancies in SrTiO3 crystals. [1] Varley et al., Phys. Rev. B 89, 075202 (2014) [2] Liu et al., Phys. Chem. Chem. Phys. 16, 15590 (2014) [3] Tanaka et al., Phys. Rev. B 68, 205213 (2003)

Authors : J. Stöver, L. Bogula, J. E. Boscher, T. Markurt, J. Schwarzkopf, M. Albrecht, K. Irmscher
Affiliations : Leibniz-Institut für Kristallzüchtung im Forschungsverbund Berlin e.V.

Resume : Electric field induced oxygen vacancy drift in SrTiO3 between two Schottky-type platinum electrodes is a widely used model to describe the resistive switching behaviour of the material. The permittivity of SrTiO3 is strongly dependent on the electric field strength and the temperature. Since high electric fields can occur in SrTiO3 Schottky diodes, profound knowledge on the field dependence of the permittivity is indispensable for a correct interpretation of switching experiments. Here we present an investigation of the temperature dependence of the permittivity in homogeneous high electric fields. Insulating SrTiO3 bulk crystals were used as dielectric for thick plate capacitor structures. For high electric field experiments, insulating SrTiO3 thin-films were epitaxially grown on highly conductive, niobium doped SrTiO3 substrates. Capacitance-voltage and current-voltage measurements were performed at temperatures between 15 K and 300 K and electric field strengths up to 1 MV/cm. High electric field strengths of up to 1 MV/cm can suppress the increase of the permittivity with decreasing temperatures. At fields above 400 kV/cm, the increasing permittivity is inverted to a decreasing permittivity with decreasing temperatures. The impact of the field dependence on the capacitance-voltage characteristics of Schottky diodes is visible by a deviation of the 1/C² ~ V relation. In consequence, the drift of oxygen vacancies in resistive switching devices might be influenced.

Authors : Mohit Rameshchandra Kulkarni,Nidhi Tiwari,Anh Chien Nguyen,and Nripan Mathews
Affiliations : School of Materials Science and Engineering and Energy Research Institute@NTU (ERI@N), Nanyang Technological University, 637553 Singapore

Resume : Amorphous metal oxide semiconductor provides high performance and transparency compared to amorphous silicon, properties required for the transparent electronics. However, the high processing temperature limits the choice of substrates. Polymer substrates cannot handle high temperatures. One of the factors affecting the activation of the thin film transistor (TFT) and its properties, is the oxygen vacancies present in the oxide. Previously the oxygen vacancies were controlled by the oxygen partial pressure while deposition and post-annealing. It limits the control over transistor behavior. This work tries to address the issue by using an ionic liquid as a gate dielectric. With high electrostatic field due to ionic liquid gating, the reversible oxygen vacancies variation was achieved. This was supported by the controlled modulation of the threshold voltage and on voltage of transistor using a predefined gate biasing. This is useful to correct the defects occurring while the formation of the oxide thin film and making fabrication process more repeatable without post-annealing.

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Ferroelectrics for Devices : Catherine Dubourdieu
Authors : Sayeef Salahuddin
Affiliations : EECS, University of California Berkeley

Resume : Negative capacitance in a ferroelectric material is characterized by a state of the polarization where the polarization charge and the net electric field oppose each other. In a prototypical ferroelectric this situation occurs where the polarization is significantly suppressed. Conventionally, this is the region where the material experiences a polarization catastrophe; in other words left all by itself in this state, the material will spontaneously polarize. As a result, in an isolated ferroelectric material, this state can only be accessed in a time-dependent manner. In the first part of my presentation, I shall discuss how one could go beyond this and stabilize the ferroelectric material at a state of negative capacitance in equilibrium, i.e., at the steady state, with the help of another dielectric material placed in a series connection. Interestingly, this series connection emulates what happens at the gate of a transistor where the gate insulator and the semiconductor capacitors are connected in series. Therefore if the gate oxide is replaced by an appropriate ferroelectric, this series combination can stabilize the ferroelectric material at a state of negative capacitance. At this state, the total capacitance of the series combination is enhanced, leading to more charge at the channel at the same voltage. The boost of charge, in turn, leads to larger current at the same voltage. In fact, this boost makes it possible to reduce supply voltage of transistors below the traditional Boltzmann limit --- often termed as the Boltzmann tyranny. In the recent years, many groups around the world, both in academy and in the industry, have demonstrated the fundamental effect and the Negative Capacitance Transistors. In the second part of this presentation, I shall discuss the principle of operation and potential routes for optimization of the Negative Capacitance Transistors.

Authors : Tony Schenk1,2,*, Everett D. Grimley3, Abinash Kumar3, James M. LeBeau3, Thomas Mikolajick1, Uwe Schroeder1
Affiliations : 1 NaMLab gGmbH/TU Dresden, Noethnitzer Str. 64, D-01187 Dresden, Germany; 2 Materials Research and Technology Department, Luxembourg Institute of Science and Technology (LIST), 41 Rue du Brill, Belvaux, Luxembourg; 3 Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695–7907, USA

Resume : The discovery of ferroelectricity in 10 nm thin polycrystalline HfO2 has revived the research on ferroelectric memories due to hafnia’s excellent compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication[2,3]. Meanwhile, also the potential for sensors and actuators is being explored.[4,5] As film quality and dominant types of defects within the ferroelectric and at its interfaces are crucial, the polycrystallinity is a challenge. We show different phases within single grains and identify preferential interphase boundaries that form despite mismatches of > 4 %.[6] Moreover, we found an orientation relationship between HfO2 and the TiN electrodes. When annealing the amorphously deposited HfO2 films, some TiN grains serve as seeds for the crystallization of the HfO2. Both the interphase boundaries and the partial templating effect of the TiN can be explained by looking at the unit cells. Phase coexistence and preferential orientation are expected to be key elements for future devices in both the fields of nanoelectronics and microelectromechanical systems. [1] T. S. Böscke et al., Appl. Phys. Lett. 99, 102903 (2011). [2] J. Müller et al., IEEE Symposium on VLSI Technology (VLSIT) 2012. [3] P. Polakowski et al., Proc. IEEE International Memory Workshop (IMW) 2014, pp. 1-4. [4] C. Mart et al., Appl. Phys. Lett. 112, 052905 (2018). [5] S. Starschich et al., Appl. Phys. Lett. 110, 182905 (2017). [6] E. D. Grimley et al., Adv. Mater. Interfaces, 5, 5, 1701258 (2018).

Authors : Ausrine Bartasyte*, Stefania Oliveri, Vincent Astié, Thomas Baron, Samuel Margueron, Jean-Manuel Decams
Affiliations : Femto-ST Institute, University of Bourgogne Franche-Comté, ENSMM, CNRS UMR 6174, 15B Avenue des Montboucons, Besancon, F-25030, France; Laboratoire Matériaux Optiques, Photoniques et Systèmes, Université de Lorraine et Centrale Supèlec, EA 4423, 2 rue Eduard Belin, Metz, F-57070, France; ANNEALSYS, 139 rue des Walkyries, 34000 Montpellier, France

Resume : Over the past five decades, LiNbO3 single crystals and thin films have been studied intensively for their exceptional acoustic, electro-optical, pyroelectric and ferroelectric properties [1]. Today, LiNbO3 single crystals are key materials in electro-optics and RF acoustic filters. LiNbO3 thin films are needed urgently for the development of the high-frequency and/or wide-band RF filters adapted to the 5G communications. The integration of LiNbO3 films in guided nanophotonic devices will allow higher operational frequencies, wider bandwidth, and miniaturized optical devices in line with improved electronic conversion. The challenges and the achievements in the epitaxial growth of LiNbO3 films and their integration with Si technology and to acoustic and nanophotonic devices will be discussed in detail. In the case of SAW and optical waveguides, proper propagation axis is demanded. Thus, particular effort was done to achieve the growth of films with single orientation and nearly stoichiometric Li2O composition. Preliminary results on the micro-structuring of LiNbO3 films for the fabrication of waveguides, and acoustical/optical properties will be presented. Future prospects of potential applications and the expected performances of thin film devices are overviewed, as well. [1] A. Bartasyte et al., Adv. Mater. Interfaces 1600998, 2017. [2] A. Bartasyte et al., Mat. Chem. Phys. 149-150, 622-631 (2015). [3] A. Bartasyte et al., J. Phys. Condens. Matter 25, 205901 (2013).

Authors : P. Komissinskiy (1), A. Radetinac (1), P. Salg (1), D. Walk (2), L. Zeinar (1), A. Zintler (1), R. Egoavil (3), G. Van Tendeloo (3), J. Verbeeck (3), L. Molina-Luna (1), R. Jakoby (2), H. Maune (2), L. Alff (1)
Affiliations : (1) Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany; (2) Institute for Microwave Engineering and Photonics, Technische Universität Darmstadt, 64283 Darmstadt, Germany; (3) EMAT, University of Antwerp, 2020 Antwerp, Belgium.

Resume : The exemplary story of the CMOS microelectronic technology shows that modern electronic devices require various functional layers of a thickness up to a few micrometers, combined with atomically engineered interfaces controlled at the unit cell level. Considering realization of oxide electronic devices for high-frequency applications such as low-loss tunable filters and antennas, the desired thickness of the conducting oxide electrode is determined by the electromagnetic skin depth which increases with resistivity of the electrode material and has a value of the order of several micrometers in the frequency range of a few GHz. Among the known conductive oxide perovskites, the smallest skin depth is achieved in single crystal of SrMoO3 due to its low resistivity of 5 cm at room-temperature outperforming even Pt (10.6 muOhmcm), thus, making SrMoO3 a prominent conducting electrode material for high-frequency microelectronic devices. The used low-resistive SrMoO3 thin-film oxide electrode leads to low losses and high quality factor Q of the varactors. The layer-by-layer grown epitaxial Ba0.5Sr0.5TiO3 results an unprecedentedly high capacitance (C) tunability of n (3.7 V) = C (0) / C (3.7 V) = 3.1 at the Li-ion battery voltage level of 3.7 V. Thus, the obtained high aggregate performance of the varactors expressed in the high values of the “standard” commutation quality factor CQF (1 GHz) ~ 3700 and the more relevant voltage performance factor VPF (3.7 V, 1 GHz) = n (3.7 V) Q (0V, 1 GHz) / 3.7 V ~ 65 stands out in the gigahertz frequency range of interest and is comparable.

Ferroelectric Heterostructures II : Ausrine Bartasyte
Authors : Andra Georgia Boni, Cristina Chirila, Lucian Dragos Filip, Lucian Trupina, Raluca Negrea, Lucian Pintilie
Affiliations : National Institute of Materials Physics, Atomistilor 405A, Magurele, Romania

Resume : Ferroelectric multi-layers were prepared by pulsed laser deposition (PLD) method on single crystal SrTiO3 (STO) substrates with bottom SrRuO3 (SRO) electrodes. Most of the studies were performed on symmetric three-layers structures of PZT-interlayer-PZT type (Zr/Ti ratio of 20/80 for PZT). The interlayer was varied from completely conductive (SRO) to insulating (STO), with BaTiO3 (BTO) and CoFe2O4 (CFO) as materials with intermediate electrical conductivity. The structural analysis performed by XRD and TEM has revealed that the multi-layers are epitaxial, with some strain gradient from the bottom PZT layer to the top one. Hysteresis loop measurements performed in capacitor geometry, with top SRO contacts of 100x100 µm2, have revealed that the shape of the hysteresis changes with the material used for interlayer separating the two PZT films. The most interesting behavior was obtained on PZT-STO-PZT structures, for which 4 distinct polarization states were obtained, each of them accessible by applying an external voltage of suitable value. The study performed on PZT-STO-PZT structures having the same thickness for PZT layers (150 nm) and different thickness for the STO interlayer has shown that there is an optimum thickness range for STO for which the 4 polarization states can be obtained. If the thickness is too small then the entire structure behaves as a normal ferroelectric layer with only 2 polarization states, and if the thickness is too high the 4 polarization states start to be blurred and un-stable. Therefore, the optimum thickness range for STO was found to be between about 10 nm and 50 nm. Further on, a five layer PZT-STO-PZT-STO-PZT was grown, with 10 nm for STO and 100 nm for PZT layers. Up to 8 distinct polarization states were found from hysteresis measurements. These results suggest that multi-layer ferroelectric-insulator structures can be used as memory cells with multiple polarization states (up to 2n states, where n is the number of ferroelectric layers). The finding is explained by successive switching of polarization in the component ferroelectric layers, predicted also theoretically by applying thermodynamic theory to these structures. The same structures were found to have distinctive capacitance values, depending on polarization orientation in the component layers, offering in this way the possibility of non-destructive reading. However, the number of distinctive states varies as 2n-1, with n the number of ferroelectric layers. It is also shown that such structures can behave not only as memory cells but also as logic gates, allowing logic operations like AND/NAND or OR/NOR if a certain sequence of voltage pulses is applied on the structure.

Authors : H. Volkova [1], P. Gemeiner [1], P. Nukala [1], B. Dkhil [1], G. Geneste [2], C. Frontera [3], I. Fina [3], F. Sanchez [3], J. Guillot [4], D. Lenoble [4], N. Chauvin [5], C. Botella [5], G. Grenet [5], X. Bai [5], S. Brottet [5], D. Albertini [5], N. Baboux [5], B. Gautier [5], I. C. Infante [5]
Affiliations : [1] SPMS CentraleSupélec CNRS-UMR8580 Université Paris-Saclay; [2] CEA DAM DIF; [3] Institut de Ciència de Materials de Barcelona CSIC; [4] Luxembourg Institute of Science and Technology - Materials Research and Technology Department; [5] Institut des Nanotechnologies de Lyon CNRS UMR5270 ECL INSA UCBL CPE Université de Lyon

Resume : Understanding photoinduced properties in ferroelectric solid solutions and gaining control over them could enable design of new, more efficient photovoltaic devices or photocatalysts. In our work, we have focused on studying the optical properties of BaSnxTi1-xO3 solid solutions, a BaTiO3-ferroelectric-based system where the Sn-5s-5p electronic states at the conduction band are expected to provide increased electron mobility. We will show from our study in the bulk compounds the close relationship of the optical and electronic properties in this system, results obtained from dielectric measurements and ultraviolet-visible-near-infrared, Raman, X-ray photoelectron and photoluminescence spectroscopies, together with density functional theory calculations. We observe a significant nonlinear increase of the direct band gap (>0.3 eV) for BaSn0.8Ti0.2O3 compared to the parent compounds values, whereas for the BaSn0.1Ti0.9O3 compound, our synchrotron radiation diffraction experiments indicate that this composition is not cubic at room temperature. In view of these investigations in the bulk system, we have selected the BaSn0.1Ti0.9O3 composition as a potential candidate composition presenting room temperature ferroelectricity and enhanced photo(ferro)conduction performances. Therefore, we have grown epitaxial thin film heterostructures of BaSn0.1Ti0.9O3 and studied their structure, chemical, surface, electronic and optical properties together with photo(ferro)voltaic responses. We will discuss these preliminary results in the film heterostructures in the framework of the bulk BaSnxTi1-xO3 compounds results and of future applications. This project has received funding from the EU‐H2020 research and innovation program under grant agreement N 654360 having benefited from the access provided by ICMAB-CSIC in Cerdanyola del Vallès within the framework of the NFFA-Europe Transnational Access Activity (project ID 524)

Authors : J. Schwarzkopf1, L. von Helden1, S. Liang2, R. Wördenweber2, M. Schmidbauer1
Affiliations : 1Leibniz Institute for Crystal Growth, Max-Born-Str. 2, Berlin, Germany; 2Peter Grünberg Institute, Forschungszentrum Jülich, Jülich, Germany

Resume : Ferroelectric phases with monoclinic symmetry are expected to exhibit enhanced piezoelectric properties. Together with high coupling coefficients this can be exploited for thin film surface acoustic wave (SAW) applications. In this regard, KxNa1-xNbO3 represents a suitable material system that offers both high coupling coefficients and the possibility to stabilize monoclinic phases by incorporating anisotropic epitaxial strain. Thereby, their formation crucially depends on the incorporated lattice strain provided by the epitaxial growth on a lattice mismatched substrate. In this study, we have grown K0.7Na0.3NbO3 epitaxial films by metal-organic chemical vapor deposition on different (110) oriented rare-earth scandate substrates. For a thickness of 30 nm, all films on TbScO3, GdScO3 and SmScO3 exhibit pseudocubic (001)pc surface orientation, while the average strain state changes from compressive to slightly tensile. Piezoresponse force microscopy and x-ray diffraction data reveal periodically arranged ferroelectric stripe domains and domains walls that contain the [1-12] or [-112] in-plane directions of the orthorhombic substrates. With increasing thickness, elastic lattice relaxation set in by the additional formation of a (100)pc oriented monoclinic phase eventually forming a complicated herringbone domain pattern. Propagation of SAWs is demonstrated for K0.7Na0.3NbO3 thin films on all substrates, whereby, strength and propagation direction depend on the epitaxial strain.

Authors : C. Kwamen1,2, M.Rössle1, M.Reinhardt1, W.Leitenberger2, F.Zamponi2, M.Alexe3, P. Rojo-Romeo4, B. Vilquin4, C. Dubourdieu1,5, M. Bargheer2,1
Affiliations : 1) Helmholtz-Zentrum Berlin, Berlin, Germany; 2) University of Potsdam, Potsdam, Germany; 3) University of Warwick, Warwick, UK; 4) INL, CNRS, Ecole Centrale de Lyon, Université de Lyon, Ecully, France; 5) Freie Universität Berlin, Berlin, Germany

Resume : The behavior of ferroelectric materials under an applied electric field has been the subject of investigation in order to understand the effects of both intrinsic and extrinsic contributions during device operation. Time-resolved characterization techniques allow for the investigation of structural and electrical changes associated with different loading conditions, suitable for different applications. Also, the permanent quest for energy efficient technologies drives investigations on making a ferroelectric operational under lowest switching voltage. Here we present a simultaneous study of the electrical and structural responses of lead-zirconate-titanate-based capacitor heterostructures during charging, discharging, and polarization reversal, using time-resolved X-ray diffraction. Concomitant with the ferroelectric current peak, we observe that the switching is characterized by a transient disorder evidenced by a decrease of the Bragg peak intensity. A peak width increase reveals the domain dynamics during the reversal. Our investigations show how the incomplete screening of the depolarization charges affect the piezoelectric response, measured via the Bragg peak position. We examine the interplay between charge flow and atomic motion in real time during device operation. We investigate how the device time constant can tremendously modify both the piezoelectric strain and macroscopic coercive field.

Authors : Charlotte Wouters (1), Toni Markurt (1), Christopher Sutton (2), Holger Von Wenckstern (3), Oliver Bierwagen (4), Martin Albrecht (1)
Affiliations : (1) Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany; (2) Fritz Haber Institute of the Max Planck Society, Faradayweg 4, 14195 Berlin, Germany; (3) Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany; (4) University of Leipzig, Felix-Bloch-Institute for Solid State Physics, Linnéstr. 5, 04103 Leipzig, Germany

Resume : The potential of Ga2O3 and In2O3 as transparent conducting oxides, could be greatly increased by alloying them to tune their properties - especially the band-gap - over a wide range. Alloying into (InxGa1-x)2O3 is however not straightforward due to the different thermodynamically stable structures of the binaries (monoclinic β-Ga2O3 and cubic bixbyite In2O3). Recent calculations of the phase diagram show rather narrow stability windows with low solubility limits for the above-mentioned phases and an additional window for the hexagonal InGaO3(II)-phase close to x=0.5. TEM and XRD investigations of thin PLD grown (InxGa1-x)2O3 layers on c-sapphire show however that we can expand these stability windows. Under certain growth conditions, the β-phase is stable up to x=0.5 and the hexagonal phase can contain In contents up to x≈0.7. In a dedicated in-situ experiment where amorphous (InxGa1-x)2O3 layers are crystallized by annealing, we observe even the stabilization of a Ga-rich cubic phase. Additionally to the shrinkage of the miscibility gaps, it is possible to stabilize an ε-(InxGa1-x)2O3 phase at low indium contents which is characterized with Pna21 space group symmetry. The formation of the ε-phase is enabled by introducing Sn atoms during the PLD growth, and it is also enhanced by lowering the growth temperature. The existence of metastable composition windows is explored to explain these experimental observations, but they surely open up new perspectives for applications.

Other Conducting Oxides : Matthias Bickermann
Authors : Christopher Sutton,1 Robert J. N Baldock,2 Luca M. Ghiringhelli,1 Matthias Scheffler1
Affiliations : 1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany 2 École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

Resume : Computational high-throughput screening has generated much interest in materials science because of the possibility to guide the development of functional materials along with the ability to enhance the understanding of fundamental properties. Identification of stable crystalline materials for a mixture of two components requires examination of the lowest free energy of approximately 2N configurations, where N is the number of atoms in the unit cell. Cluster expansion-based energy functions offer a numerically efficient approach for estimating the stability of new potential alloys. Combining this approach with the nested sampling algorithm, which is a Bayesian Markov chain Monte Carlo method, allows for a one-shot calculation of the phase diagram as a function of composition and temperature. Our results for stable ternary and quaternary mixtures in various crystalline symmetries of group-III oxides with the formula (InxGayAlz)2O3 where x+y+z=1 will be presented. A new stable InGaO3 compound is reported as a potential transparent conductor material. In addition, the key aspects that determine the stability of these materials will be discussed.

Authors : Holger von Wenckstern
Affiliations : Universtiät Leipzig, Felix-Bloch-Institut für Festkörperphysik, Linnéstrasse 5, 04103 Leipzig, Germany

Resume : Wide band gap semiconductors have applications in fields like power electronics, UV-dosimetry, transparent electronics, flexible circuitry and so on. Depending on the application appropriate doping and/or alloying is used to tailor the material properties. We have developed a novel approach for combinatorial material exploration by pulsed-laser deposition (PLD) allowing growth of thin films with defined lateral variation of doping and/or the alloy composition [1]. This approach was used to utilize wavelength-selective UV and deep UV photo-detectors. Narrow bandwidth down to 7 nm (29 meV) was achieved for MgZnO-based detectors [2]. Next we demonstrate room temperature deposition of the ternary amorphous oxide zinc tin oxide (ZTO) [3] focusing on the dependence of material properties and that of simple devices on the cation composition. Further the influence of oxygen deficiency on the properties of ZTO-based MESFETs, JFETs and inverters is discussed. [1] H. von Wenckstern, Z. Zhang, F. Schmidt, J. Lenzner, H. Hochmuth, M. Grundmann, CrystEngComm 15, 10020 (2013). [2] Z. Zhang, H. v. Wenckstern, M. Grundmann, IEEE Journal of Selected Topics in Quantum Electronics 20, 106 (2014). [3] S. Bitter, P. Schlupp, M. Bonholzer, H. v. Wenckstern, M. Grundmann, ACS Comb. Sci. 18, 188 (2016).

Authors : Theresa Berthold, Jonas Michel, Simeon Katzer, Stefan Krischok, Marcel Himmerlich
Affiliations : Institut für Physik & IMN Macro Nano, Technische Universität Ilmenau, Germany; Institut für Physik & IMN Macro Nano, Technische Universität Ilmenau, Germany; Institut für Physik & IMN Macro Nano, Technische Universität Ilmenau, Germany; Institut für Physik & IMN Macro Nano, Technische Universität Ilmenau, Germany; CERN, European Organization for Nuclear Research, 1211 Geneva 23, Switzerland

Resume : A high electron concentration is typically observed at In2O3 surfaces. It is a drawback for electronic devices since fabrication of rectifying metal contacts is hindered. However, variation of the electron accumulation layer can be implemented as sensing principle in gases or liquids. The influence of surface adsorption processes, plasma oxidation and doping on the electronic surface properties is studied by in-vacuo photoelectron spectroscopy and 4-point probe resistivity measurements. Using model gas adsorption experiments, a clear correlation is observed between adsorbate-semiconductor charge transfer, density of surface charge carriers, and the conductivity of the material. In general, a modification of the surface electron density by electronegative (acceptor-type) oxygen adatoms is a key factor that increases the film resistivity. Furthermore, acceptor doping by Mg and Ni also reduces the surface electron concentration after growth, but both approaches are not capable to fully deplete the surface electron channel. On the other hand, an oxygen plasma treatment induces upward band bending even for highly n-doped films, generating an energy barrier applicable to produce Pt-based rectifying contacts. The oxygen-rich Pt/In2O3 interface is critical in stability and oxidation of Pt contacts is beneficial to enhance electronic barriers. Combining interface oxygen plasma modification and fabrication of PtOx contacts enables a significant improvement of In2O3 Schottky diodes.

Authors : Tamar Tchelidze1, Ekaterine Chikoidze2, Amador Perez-Tomas3, Yves Dumont2, David J. Rogers 4
Affiliations : 1 . Faculty of Exact and Natural Science, Department of Physics, Ivane Javakhishvili Tbilisi State University, 3 Av. Tchavtchavadze, 0179 Tbilisi, Georgia, 2. Groupe d’Etude de la Matière Condensée (GEMaC), Université de Versailles Saint Quentin en Y. – CNRS, Université Paris-Saclay, 45 Av. des Etats-Unis, 78035 Versailles Cedex, France, 3. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Barcelona, Spain, 4 . Nanovation, 8 route de Chevreuse, 78117 Châteaufort, France

Resume : Wide-bandgap metal oxide semiconductors are an important class of electronic materials in emerging applications such as transparent and power electronics. The perspectives of miniaturization for such applications depend critically on the ability of tuning the optoelectronic properties, which, in turn, depend on controlling the defect and dopant composition. The main goal of this study is to investigate the effect of the metal oxide dimensionality on the defect and free carrier concentration in equilibrium. The current work is focused on two of the most relevant oxide semiconductors nowadays: ZnO and Ga2O3. The thermodynamic analyses of defect and free carrier concentration equilibrium are performed using the Kroger method of quasi-chemical equations for the thermodynamic system: oxide crystal – gas of one component. In this way the concentration of free carriers and defects can be ascertained as functions of gas pressure and temperature from few nanolayers to bulk crystal. The n- and/or p-type dopability is mainly conditioned by the electronic structure of the system: band gap, ionization energies of donors, acceptors and their compensating centers; as well as by the formation enthalpies of defects. These quantities may undergo substantial changes when dimensionality is reduced and reveal strong size dependence. Ionization energies of donors and acceptors, which are needed for thermodynamic analyses, are calculated from few monolayers to a bulk crystal. The variation of defect creation energies in low dimensional systems is also considered.

Ga2O3 Devices : Chadwin Young
Authors : 1. F. Ren and Jiancheng Yang 2. S. J. Pearton 3. Marko Tadjer 4. A. Kuramata
Affiliations : 1. Department of Chemical Engineering, University of Florida, Gainesville FL 32611, USA 2. Department of Materials Science and Engineering, University of Florida, Gainesville FL 32611 3. Naval Research Laboratory, Washington DC 20375 4. Tamura Corporation and Novel Crystal Technology, Inc., Sayama Saitama 350-1328, Japan

Resume : β-Gallium Oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells and sensors with capabilities beyond existing technologies due to its large bandgap and the availability of high quality, large diameter bulk crystals and epitaxial layers of Ga2O3 with a range of controllable n-type doping levels by edge-defined film-fed, Czochralski, Bridgman or float zone. The high energy bandgap of Ga2O3, ∼4.9 eV, yields a very high theoretical breakdown electric field (∼8 MV/cm). For power electronics, the Baliga figure-of-merit proportional carrier mobility, critical electric field and breakdown voltage, is almost four times higher for Ga2O3 than for GaN. In this work, we report demonstrations high currents or high breakdown voltages of Ga2O3 rectifiers, diode area up to 0.2 cm2, with and without edge termination fabricated on a Si-doped n-Ga2O3 drift layer grown by halide vapor phase epitaxy on a Sn-doped n+ Ga2O3 (001) substrate. For high current diodes, current level of 1 to 2 A were achieved with the breakdown voltages ranging 150 to 645 V. For high breakdown voltage diodes, breakdown voltage ranging from 1600 to 2300 V were demonstrated.

Authors : Man Hoi Wong 1, Yoshiaki Nakata 1, Chia-Hung Lin 1, Kohei Sasaki 2, Yoji Morikawa 3, Ken Goto 2,4, Akinori Takeyama 5, Takahiro Makino 5, Takeshi Ohshima 5, Akito Kuramata 2, Shigenobu Yamakoshi 2, Hisashi Murakami 4, Yoshinao Kumagai 4, Masataka Higashiwaki 1
Affiliations : 1 National Institute of Information and Communications Technology, Koganei, Tokyo 184-8795, Japan; 2 Tamura Corporation, Sayama, Saitama 350-1328, Japan; 3 Silvaco Japan Co., Ltd., Yokohama, Kanagawa 220-8136, Japan; 4 Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan; 5 National Institutes for Quantum and Radiological Science and Technology, Takasaki, Gunma 370-1292, Japan

Resume : The pursuit of Ga2O3 as an ultra-wide-bandgap (4.5–4.9 eV) semiconductor for next-generation power-switching and harsh-environment electronics has catalyzed the rapid development of Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) in recent years [1]. Field-plated lateral depletion-mode devices demonstrated a high off-state breakdown voltage of 755 V, a large on/off current ratio of over nine orders of magnitude, dispersion-free output characteristics, stable high temperature operation, and strong gamma-ray tolerance [2,3]. Enhancement-mode operation with a six-order-of-magnitude on/off current ratio was enabled by an unintentionally-doped epitaxial Ga2O3 channel that was fully depleted at zero gate bias due to a low background carrier density [4]. Planar-gate vertical Ga2O3 MOSFETs, wherein a current blocking layer provided electrical isolation between source and drain except at an aperture opening through which drain current was conducted, demonstrated successful transistor action [5]. Advanced transistor architectures, notably normally-off vertical devices, will further enhance the impact of Ga2O3 power electronics. [1] M. Higashiwaki et al., Appl. Phys. Lett. 112, 060401 (2018). [2] M. H. Wong et al., IEEE Electron Device Lett. 37, 212 (2016). [3] M. H. Wong et al., Appl. Phys. Lett. 112, 023503 (2018). [4] M. H. Wong et al., Appl. Phys. Express 10, 041101 (2017). [5] M. H. Wong et al., Proc. IEEE Device Res. Conf. (2017).

Authors : Gregg Jessen, Kelson Chabak, Andrew Green, Kevin Leedy, Antonio Crespo, Steve Tetlak, Dennis Walker Jr., Eric Heller
Affiliations : Air Force Research Laboratory

Resume : As an electronic material, β-Ga2O3 has received significant attention in the last several years due to a combination of unique material properties, namely high-critical electric field strength (Ecrit), large range of controllable n-type doping, and ability to grow large area crystals from a melt. The large Ecrit offers substantial advantages in terms of power switching losses over existing commercial solid state technologies for both dc conduction losses and dynamic switching. The large range of controllable n-type doping allows operation over a large range of voltages, and the large area substrate availability offers significant cost advantages that are compounded by the high energy density capabilities of the material enabling more parts to be fabricated in a smaller area. The scalability enabled by the large Ecrit also carries over into the RF operation space in terms of increased power×frequency product. The focus of this presentation will be the evolution of lateral β-Ga2O3 technology and critical enabling material and process developments that have occurred since the first MOSFET demonstrations to include increased mobility, lower ohmic contact resistance, gate recess, and short gate-length. Emphasis will be placed on more recent efforts by AFRL to characterize high-speed switch loss metrics for β-Ga2O3 devices, which are now beyond the theoretical limit of Si, as well as pushing the RF performance of the devices beyond maximum frequency of operation of 20 GHz.


Symposium organizers
Debdeep JENACornell University

Materials and ECE Departments, Ithaca, NY 14853 USA
Matthias BICKERMANNLeibniz Institute for Crystal Growth (IKZ)

Max-Born-Str. 2, 12489 Berlin, Germany
Pavlo ZUBKOUniversity College London

London Centre for Nanotechnology, 17-19 Gordon Street, London WC1H 0AH, UK
Ulrike DIEBOLDTU Vienna

Surface Physics - Institute for Applied Physics, Wiedner Hauptstr. 8-10/134, 1040 Vienna, Austria