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A Thin Film Approach to Engineering Functionality Into Oxides J. Am. Ceram. Soc., 91; [8] 2429–2454 (2008) DOI: 10.1111/j.1551-2916.2008.02556.x r 2008 The American Ceramic Society Journal A Thin Film Approach to Engineering Functionality into Oxides Darrell G. Schlom,w,ww,z Long-Qing Chen,z Xiaoqing Pan,y Andreas Schmehl,z,z and Mark A. ZurbuchenJ zDepartment of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802-5005 yDepartment of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 z Experimentalphysik VI, Elektronische Korrelationen und Magnetismus, Institut fu¨ r Physik, Universita¨ t Augsburg, D- 86159 Augsburg, Germany JThe Aerospace Corporation, Microelectronics Technology Department, El Segundo, California 90245 The broad spectrum of electronic and optical properties exhibited But as every ceramist knows, oxides are an exciting class of by oxides offers tremendous opportunities for microelectronic electronic materials in their own right. Oxides exhibit the full devices, especially when a combination of properties in a single spectrum of electronic, optical, and magnetic behavior: insulat- device is desired. Here we describe the use of reactive molecular- ing, semiconducting, metallic, superconducting, ferroelectric, beam epitaxy and pulsed-laser deposition to synthesize functional pyroelectric, piezoelectric, ferromagnetic, multiferroic, and oxides, including ferroelectrics, ferromagnets, and materials that nonlinear optical effects are all possessed by structurally com- arebothatthesametime.Owingtothedependenceofproperties patible oxides. The unparalleled variety of physical properties of on direction, it is often optimal to grow functional oxides in oxides holds tremendous promise for electronic applications. particular directions to maximize their properties for a specific Analogous to today’s semiconductor device structures, many application. But these thin film techniques offer more than orien- device concepts utilizing oxides will likely use alternately layered tation control; customization of the film structure down to structures where dimensions are minute enough to observe the atomic-layer level is possible. Numerous examples of the quantum size effects (nanometer-scale thicknesses). controlled epitaxial growth of oxides with perovskite and perovs- The physical behavior of oxides confined to quantum size di- kite-related structures, including superlattices and metastable mensions is not well established, and an understanding of the effect phases, are shown. In addition to integrating functional oxides of such confinement (reduced dimensionality) on the physical with conventional semiconductors, standard semiconductor properties of these structures cannot be achieved without the practices involving epitaxial strain, confined thickness, and controlled preparation of well-ordered crystalline samples. Equally modulation doping can also be applied to oxide thin films. exciting are the new functionalities that can emerge at oxide inter- Results of fundamental scientific importance as well as results re- faces. For instance, the interface between appropriate antiferro- vealing the tremendous potential of utilizing functional oxide thin magnetic materials has been shown to be ferromagnetic.1–7 films to create devices with enhanced performance are described. Similarly, charge transfer at the interface between appropriate nonmagnetic insulators8 can give rise to a magnetic9 or supercon- ducting10 electron gas. These examples offer a glimpse of the new or enhanced functionalities that can be achieved by nanoengineer- I. Oxides Beyond SiO2 ing oxide heterostructures with atomic layer precision. As we show NTIL recently, the word ‘‘oxide’’ could only mean one thing in this article, it is possible to structurally engineer crystalline Uto anyone working in the semiconductor industry—SiO2. oxides with a precision that rivals the structural control achieved in today’s most advanced semiconductor structures. Examples of the functional properties of specific oxides are D. Green—contributing editor listed in Table I. The oxides chosen were those with exceptional properties for each category. These include the oxide with the 11 highest known electron mobility (SrTiO3), change in resistance at a metal–insulator transition (EuO),12 superconducting transition Manuscript No. 24429. Received March 17, 2008; approved June 1, 2008. 13 This work was financially supported by the National Science Foundation under grants temperature (HgBa2Ca2Cu3O81x), switchable spontaneous po- DMR-0507146 and DMR-0213623, the Office of Naval Research under grant N00014-04-1- 14 15–18 larization (PbZr0.2Ti0.8O3 and BiFeO3 ), piezoelectric coeffi- 0426 monitored by Dr. Colin Wood, and the U.S. Department of Energy through grants 19 20 DE-FG02-03ER46063 and DE-FG02-07ER46417. This work was supported by The Aero- cient (PbZn1/3Nb2/3O3–PbTiO3), magnetization (EuO), space Corporation’s Independent Research and Development Program. 21 w magnetoresistance (Pr0.7Sr0.04Ca0.26MnO3Àd), magnetostrictive Author to whom correspondence should be addressed. e-mail: [email protected] 22 23 ww coefficient (Co0.8Fe2.2O4), Verdet constant (EuO) —a measure Current Address: Department of Materials Science and Engineering, Cornell Univer- 24,25 sity, Ithaca, NY 14853-1501 of the strength of the Faraday effect, spin polarization (CrO2), Feature 2430 Journal of the American Ceramic Society—Schlom et al. Vol. 91, No. 8 Table I. Examples of the Properties of Oxides Property Value Oxide material References 2 À1 11 Mobility m 5 22 000 cm Á (V Á s) (2 K) SrTiO3 Tufte and Chapman 13 12 Metal–insulator transition DR=RTlow > 10 EuO Petrich et al. 13 Superconductivity Tc 5 135 K HgBa2Ca2Cu3O81x Schilling et al. 2 14 Ferroelectricity Ps 5 105 mC/cm PbZr0.2Ti0.8O3 Vrejoiu et al. 2 15 16 Ps 5 100 mC/cm BiFeO3 Wang et al., Li et al. Das et al.,17 Dho et al.18 19 Piezoelectricity d33 5 2500 pC/N PbZn1/3Nb2/3O3–PbTiO3 Park et al. 20 Ferromagnetism Ms 5 6.9 mB/Eu EuO Matthias et al. 11 21 Colossal magnetoresistance DR/RH410 (5 T) Pr0.7Sr0.04Ca0.26MnO3Àd Maignan et al. À6 22 Magnetostriction l100 5 À590 Â 10 Co0.8Fe2.2O4 Bozorth et al. Faraday effect n 5 4 Â 105 1 Á (T Á cm)À1 EuO Ahn and Shafer23 24 25 Spin polarization P498% CrO2 Soulen et al., Anguelouch et al. 29 Ferromagnetic TC 5 105 K BiMnO3 Hill and Rabe, Moreira dos and Santos et al.,28 Sharan et al.,30 ferroelectric Baettig et al.31 27 simultaneously TC 5 250 K LuFe2O4,FeTiO3 Ikeda et al., Fennie and the ferromagnetic ferroelectric with the highest reported26 belong to the same structural family—perovskite. The perovs- 27 (LuFe2O4)orpredicted (FeTiO3) transition temperature. A chal- kite ABO3 structure can accommodate with 100% substitution lenge with materials that are simultaneously ferromagnetic and some 30 elements on the A site and over half the periodic table ferroelectric is that they are often too conductive for the fabrication on the B site, as shown in Fig. 2.33 Given the chemical and of conventional ferroelectric switching devices. The ferromagnetic structural compatibility between many perovskites, this mallea- ferroelectric with the highest transition temperature on which a ble structural host offers an opportunity to customize electronic, conventional polarization-electric field hysteresis measurement has magnetic, and optical properties in thin films in ways not 28 been reported is BiMnO3 (TCB105 K). possible with conventional semiconductors. The electroceramics industry currently utilizes the diverse electrical properties of oxides in separate components made (1) Perovskites primarily by bulk synthesis and thick-film methods for capaci- The crystal structures of the oxides with the outstanding prop- tors, sensors, actuators, night vision, and other applications. erties listed in Table I are shown in Fig. 1.32 Half of the oxides A significant opportunity exists, however, to combine these Fig. 1. The crystal structures of the oxides with the exceptional properties described in Table I. Two equivalent representations of these crystal struc- tures are shown: the atomic positions (above) and the coordination polyhedra (below). The oxygen atoms occupy the vertices of the coordination polyhedra. Color is used to distinguish the two types of oxygen coordination polyhedra in Co0.8Fe2.2O4, octahedra (orange), and tetrahedra (green). The pseudocubic subcell of the perovskites PbZr0.2Ti0.8O3,BiFeO3,PbZn1/3Nb2/3O3–PbTiO3,Pr0.7Sr0.04Ca0.26MnO3Àd, and BiMnO3 is shown for clarity. The relative sizes of the atoms reflect their relative ionic radii as given by Shannon.32 August 2008 A Thin Film Approach to Engineering Functionality into Oxides 2431 Fig. 3. The n 5 1(Sr2RuO4), n 5 2(Sr3Ru2O7), n 5 3(Sr4Ru3O10), n 5 4(Sr5Ru4O13), n 5 5(Sr6Ru5O16), and n 5 N (SrRuO3)members of the homologous Ruddlesden–Popper series of compounds Fig. 2. A compilation of elements of the periodic table that can occupy 41 Srn11RunO3n11.Ru lie at the center of oxygen coordination polyhe- the three sites (A, B,andX) of the perovskite crystal structure with 100% 21 occupancy (based on the data in Landolt-Boernstein33). dra (octahedra). The filled circles represent Sr ions (reprinted from Haeni et al.,80 with permission; r2001 American Institute of Physics). properties together in oxide heterostructures where multiple case are sandwiched between Bi O layers. The structures shown properties can be utilized to yield functional integrated devices. 2 2 include the layered n 5 2 Aurivillius compound SrBi Ta O Integration of epitaxial stacks of oxide crystals
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