Chang-Beom Eom, Duke Univ
Christopher Foster, Argonne National Laboratory
Marilyn Hawley, Los Alamos National Laboratory
Darrell Schlom, Pennsylvania State Univ
J. Speck, Univ of California-S Barbara
- Argonne National Laboratory
- Hewlett-Packard Laboratories
- Kurt J. Lesker Company
- Los Alamos National Laboratory
- National Science Foundation
- Oxford Instruments, Inc.
- Target Materials
1997 Spring Exhibitor
Proceedings published as Volume 474
of the Materials Research Society
Symposium Proceedings Series.
In sessions below "*" indicates an invited paper.
SESSION L1: EPITAXIAL FERROELECTRICS
Chairs: Lee R Hirsch and Randy Treese
Monday Morning, March 31, 1997
Golden Gate C1
8:30 AM L1.1
Pt/Pb(ZrTi)O/YBaCuO FERROELECTRIC FIELD EFFECT DEVICES, Zi-Wen Dong, Zoran Trajanovic, C. H. Chen, Vitaly Talyansky, R. P. Sharma, Univ of Maryland, Dept of Physics, College Park, MD; S. Mathews, Univ of Maryland, Dept of Physics & Matl & Nuclear Engr, Colege Park, MD; R. Ramesh, Univ of Maryland, Dept of Matls Science, College Park, MD; T. Venkatesan, Univ of Maryland, Dept of Physics & Electrical Engr, ColLege Park, MD.
The mechanism of conventional high T superconducting field effect devices made of Au/SrTiO/YBa (YBCO) has been proven to be associated with direct charge transfer by the electric field and the switching speed is limited by the time constant of circuitry. Large field effects were observed for ultrathin YBCO channels or YBCO grain boundary junction channels. In addition to suFETs and JoFETs, by replacing the gate SrTiO dielectric with a Pb(Zr (PZT) ferroelectric, nonvolatile memory devices can also be realized. For the studies of ferroelectric field affects in high T superconducting thin films or Josephson junctions, we have made Pt/PZT/YBCO heterostructures on (100) SrTiO substrates or 24 -SrTiO bicrystals by pulsed laser deposition. x-ray diffraction data, including scans, indicate that the PZT layer grows with -axis orientation epitaxially on top of the YBCO layers. Rocking curve and ion channeling angular scan of PZT/YBCO bilayers reveal good crystallinity of films. Polarization measurements at room temperature and at 77 K show singular hysteretic loops in the PZT layer. The field of remanent polarization of 30 C/cm can induce as many as 10/cm carriers at the interface of the bottom superconducting channel. By polarizing the PZT film, a 2-5 change in both the channel resistance and the critical current at 77-85 K was obtained for 100 nm thick YBCO channels without grain boundary (suFET). Such devices are operational up to 85 K and can be switched in MHz frequency range. However, for 100 nm thick YBCO channels with grain boundary (JoFET), much bigger effect is expected. Since the size of JoFET is much smaller than suFET's, much faster device operation also seems possible.
8:45 AM L1.2
EFFECTS OF INTERFACIAL STATES ON ASYMMETRIC POLARIZATION SWITCHINGS OF EPITAXIAL BiTiO THIN FILMS, Bae-ho Park, Sang-Jin Hyun, T. W. Noh, Seoul National Univ, Dept of Physics, Seoul, SOUTH KOREA; J. Lee, Sung Kyun Kwan Univ, Dept of Materials Engr, Suwon, SOUTH KOREA.
Epitaxial BiTiO (BTO) films with LaSrCoO (LSCO) or Pt bottom electrodes were grown on MgO(001) substrates by pulsed laser deposition. Surprisingly, a symmetric Pt/BTO/Pt capacitor showed a highly asymmetric polarization switching and an asymmetric Pt/BTO/LSCO capacitor revealed a nearly symmetric polarization switching. To understand this intriguing phenomenon, a capacitance-voltage (C-V) measurement was performed on the Pt/BTO/Pt capacitor. By fitting the C-V data with the back-to-back Schottky diode model, built-in voltages at the top and the bottom interfaces were determined to be 1.1 V and 3.2 V, respectively. The difference in the built-in voltages, i.e., 2.1 V, was found to be nearly the same as the voltage offset of the Pt/BTO/Pt capacitor. Therefore, the imprint failure of the Pt/BTO/Pt structure can be explained by existence of asymmetric interfacial states. To get further understanding on the high built-in voltage at the bottom interface, Auger electron spectroscopy and x-ray photoemission spectroscopy depth-profiles were used.
9:00 AM L1.3
CHEMICAL SOLUTION DEPOSITED BaTiO AND SrTiO THIN FILMS WITH COLUMNAR MICROSTRUCTURE, Susanne Hoffmann, Ulrich Hasenkox, Rainer Waser, RWTH Aachen, Inst fur Werkstoffe der Elektrotechnik, Aachen, GERMANY; C. L. Jia, K. Urban, Forschungszentrum Julich GmbH, Inst for Festkorperforschung, Julich, GERMANY.
Electroceramic thin films exhibit an increasing interest due to their opportunities of a possible integration in the silicon circuit technology. In particular, ferroelectric films of lead zirconate titanate (PZT) and strontium bismuth tantalate (SBT) have been investigated for nonvolatile memory applications. Nonferroelectric, high permittivity perovskite ceramic thin films find potential use in integrated capacitors and dynamic random access memories (DRAM). Typical materials discussed for these applications are BaTiO and SrTiO and solid solutions of both. Chemical solution deposition (CSD) is widely employed as a relatively easy and flexible technique for the preparation of ceramic thin films. While the CSD technique most probably is not suitable for submicron-size ultralarge-scale integration (ULSI) as required for future DRAM generations, integrating decoupling and filter capacitors remain an attractive issue. In contrast to the columnar structured PZT thin films, the CSD prepared BaTiO and SrTiO thin films deposited on platinum coated silicon wafers typically exhibit a polycrystalline structure. Different grain sizes and residual porosity limit the density, surface quality, and reproducibility of the CSD prepared earth alkaline titanate thin films. The differences in the microstructures of the CSD grown BaTiO and SrTiO thin films compared to the Pb(Zr,Ti)O thin films were attributed to the difference in the nucleation and growth kinetics of both types of films. As has been elucidated in recent studies, the PZT film formation starts with a nanocrystalline fluorite phase followed by a heterogeneous nucleation of the perovskite at the substrate interface or the surface. This results in the typically columnar structured PZT films. In contrast to this, a homogeneous nucleation takes place in the case of the BaTiO and SrTiO film formation, which is less controllable. In the present paper, we show that the CSD processing can be optimized in order to achieve columnar-structured BaTiO and SrTiO thin films. These columnar grained films were obtained from different precursor solutions, which emphasize the relevant impact of the processing conditions, in addition to the influence of the precursor chemistry. By controlling the film formation process, polycrystalline and columnar grained BaTiO and SrTiO thin films were grown on Pt coated Si substrates, at temperatures between 750C and 800C. The films were analyzed by glancing incidence x-ray diffraction and scanning electron microscopy. Detailed analyses on the thin films' microstructure was performed by means of transmission electron microscopy. From those data, the film formation process is discussed in the light of process control and precursor chemistry. Differences in the crystallization process of BaTiO thin films compared to SrTiO films are pointed out.
9:15 AM L1.4
GROWTH OF EPITAXIAL FERROELECTRIC AURIVILLIUS-TYPE PHASES ON METALLIC OXIDES BY PULSED LASER DEPOSITION, Christopher Curran, Stephan Senz, A. Pignolet, M. Alexe, Dietrich Hesse, Max-Planck-Inst, Dept of Microstructure Physics, Halle, GERMANY.
Bi-based layered perovskites, also called Aurivillius phases, are superior to normal perovskite rnaterials with regard to their ferroelectric long term stability. Another way to alleviate fatigue and aging problems in MFM heterostructures is to replace the bottom metallic electrode with a conductive oxide electrode. An attempt to combine the two approaches has been done to investigate whether a further irnprovement in stability can be achieved.
To promote an oriented growth of the ferroelectric films, epitaxial buffer layers (YSZ, CeO) and epitaxial electrodes of (LaSr)CoO (LSCO) have been subsequently deposited onto Si (100) single crystals. In the next step a ferroelectric thin fiim of the Aurivillius phases BiTiO or SrBi2Ta2O9 (FE) has been grown. Gold contacts were evaporated on top of it to allow electric characterization.
The as-growvn ferroelectric thin films show preferred a c-axis orientation which provides a reduced switching voltage if the ferroelectric thin film is used in a memory cell. X-ray texture analysis reveals an in-plane orientation of (100) FE 1 (110) LSCO. On SEM images, rectangularly shaped, parallel aligned patches with a typical dimension of 150 nm can be observed. Cross-section TEM images reveal a columnlike growth of the YSZ layer, while the other layers display a grainlike growth. Values of 1.5 C/cm for the remnant polarization are obtained.
10:00 AM *L1.5
OPTICAL CHARACTERIZATION OF LEAD-ZIRCONATE-TITANATE THIN FILMS, Michael B. Sinclair, Barret G. Potter, Duane Dimos, Sandia National Laboratories, Albuquerque, NM; Christopher M. Foster, Argonne National Laboratory, Matls Science Div, Argonne, IL.
Lead-zirconate-titanate thin films possess many attributes that make them attractive for use in a wide variety of electrical and optical applications. Accurate determination of the optical properties of these films is essential to assess their suitability for specific applications, as well as for device design. In addition, measurement of thin-film optical properties yields important information about microstructure, purity, and homogeneity, and thus provides an independent measure of overall film quality which can be useful in the development of better film fabrication strategies. This presentation will review the application of waveguide refractometry, spectrally resolved light scattering, and spatially resolved electrooptic measurements to characterize the optical properties of epitaxial and sol-gel derived PZT thin films. Waveguide refractometry allows for accurate determination of the refractive index, birefringence, thickness, and electrooptic properties of thin films, and has been extensively employed to characterize PZT thin films. Waveguide refractometry has also been used to measure the refractive index profile perpendicular to the plane of a PZT film. Spectrally resolved light scattering provides a quantitative measure of thin-film optical quality, and has been used to determine the relative contributions of surface scattering and volume scattering for sol-gel thin films. Finally, spatially resolved electrooptic measurements have revealed a wealth of interesting and hitherto unreported features, including spatially nonuniform polarization switching, optical bistability, and nonuniform ultraviolet induced imprint.
10:30 AM *L1.6
MOCVD OF COMPLEX OXIDES BY LIQUID DELIVERY, Jeffrey F. Roeder, S. M. Bilodeau, G. S. Stauf, Thomas H. Baum, P. C. van Buskirk, Advanced Technology Materials Inc, Danbury, CT; R. R. Woolcott, A. I. Kingon, North Carolina State Univ, Dept of MS&E, Raleigh, NC.
Perovskite and magnetic oxide thin films hold significant promise for enabling novel IC's. High density dynamic random access memories (DRAMs) are under development based on the high dielectric constant of (Ba,Sr)TiO (BST) as are nonvolatile ferroelectric random access memories (FeRAMs) based on the ferroelectric behavior of Pb(Zr,Ti)O (PZT). In addition to the perovskites, thin film magnetic oxides such as the inverse spinel NiFeO are being investigated for microwave device applications. For all of these materials, thin film deposition technology is a prime consideration for manufacturable processes and metalorganic chemical vapor deposition (MOCVD)) offers a unique combination of scalability to large areas, high uniformity, a high degree of conformality, and high deposition rates. Control of composition is another fundamental issue and control of microstructure is an increasingly recognized challenge. We have developed an MOCVD technique based on flash vaporization of metalorganic precursors which affords exceptional composition control. This method has been implemented for the deposition of BST, PZT, and related oxides. We will describe elements of the liquid delivery MOCVD approach common to these oxide systems and give specific examples of process-property relationships for perovskite and ferrite films.
11:00 AM L1.7
THE EFFECTS OF SUBSTRATE THERMAL MISMATCH ON THE DOMAIN STRUCTURE OF MOCVD-DERIVED POTASSIUM NIOBATE THIN FILMS, Michael J. Nystrom, Bruce W. Wessels, Northwestern Univ, Dept of MS&E, Evanston, IL.
The nonlinear optical properties of ferroelectric potassium niobate thin films are strongly dependent on the domain structure. Epitaxial potassium niobate films have been deposited by metalorganic chemical vapor deposition on a variety of oxide substrates. Domain stabilization was studied for substrates with different coefficients of thermal expansion. X-ray diffraction of KNbO films deposited on (100) spinel, (r) sapphire, and (100) YAlO substrates shows a mixed domain structure consisting of (110) and (001) domains. However, KNbO thin films deposited under identical conditions on (100) MgO, (100) SrTiO, and (100) LaAlO substrates exhibition only a single domain orientation. HREM results indicate the strain resulting from the lattice mismatch between the film and substrate is fully accommodated by dislocations at deposition temperatures. A direct correlation between (001) domain volume fraction in the as-deposited KNbO films and calculated strain resulting from thermal mismatch is observed. In addition, the thin film lattice parameter measured normal to the substrate is found to support the calculated state of elastic strain for the mixed domain films. The observed electro-optic response of the thin films was described by a model where by the effect was dependent on the volume fraction of the different domain types.
11:15 AM L1.8
HYDROTHERMAL SYNTHESIS OF Pb(ZrTi)O POWDERS AND HETEROEPITAXIAL THIN FILMS, Allen T. Chien, Y. S. Yoon, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA; J. S. Speck, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; F. F. Lange, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA.
Various perovskite heteroepitaxial thin films system have been successfully produced by hydrothermal routes at ambient pressure and low temperatures (T = 90-150 C). This processing method provides a simple low-temperature route for producing a wide compositional range of Pb(Zr,Ti)O powders and epitaxial thin films on SrTiO and LaAO single crystal substrates which has been applied to other ferroelectric materials. Growth mechanisms, structural development, and solution chemistry effects were studied by x ray diffraction (XRD), scanning electron microscopy (SEM), and zeta potential measurements. Nucleation studies reveal that faceted islands form along the surface steps on the SrTiO substrate, followed by growth and coalescence. EDS analysis of the Pb(Zr,Ti)O thin film shows that the Zr: Ti ratio is 44: 56, nearly identical to the molar ratio of the precursors. The aqueous solution chemistry is found to control interfacial characteristics and plays an important role in controlling faceting. Faceting can be changed from blocks to platelets by the introduction of additional cations (e.g., NMe during synthesis. Electrophoretic data shows that Pb is a potential determining counter-ion and adsorbs on SrTiO surfaces. Electrical properties will also be discussed in relation to the aqueous solution chemistry.
11:30 AM L1.9
PROCESSING OF PT AND PZT THIN FILMS BY NOVEL SINGLE-SOLID-SOURCE METALORGANIC CHEMICAL VAPOR DEPOSITION, Ping Lu, Hang Li, Shan Sun, New Mexico Inst of Mining & Tech, Dept of Matls Science, Socorro, NM; Bruce Tuttle, Sandia National Laboratories, Dept 1845, Albuquerque, NM.
Ferroelectric PbTiO3 (PT) and Pb(ZrxTi1-x)O3 (PZT) thin films have been deposited on (100) MgO and (111) Pt/SiO2/(100)Si substrates by using a novel single-solid-source metalorganic chemical vapor deposition (MOCVD) technique. The new technique, developed for processing of oxide as well as non-oxide thin films, uses a powder delivery system to deliver the mixed precursor powders directly into a hot vaporizer from room temperature, therefore, avoiding any problems associated with polymerization or decomposition of the precursors before evaporation. The technique simplifies MOCVD processing significantly and can greatly improve process reliability and reproducibility. The deposited PT and PZT films have a perovskite structure and are highly oriented with respect to the substrate. With improvement of process control, systematic studies of film evolution under various growth conditions have been carried out. Effects of substrate, substrate temperature, system vacuum, precursor ratios in the mixture, and other processing conditions on film microstructure and properties will be presented in this paper.
11:45 AM L1.10
IMPROVED DIELECTRIC RESPONSE OF STRONTIUM TITANATE THIN FILMS GROWN BY PULSED LASER ABLATION, Mark J. Dalberth, Charles T. Rogers, Univ of Colorado, Dept of Physics, Boulder, CO; David Galt, Superconducting Core Tech, Golden, CO.
We have grown strontium titanate films on lanthanum aluminate at a range of oxygen pressures and substrate temperatures and have performed low frequency capacitance measurements from room temperature to 4.2 K on a coplanar capacitor patterned on the film's surface. Using a conformal map to calculate the dielectric constant from capacitance data, we obtained dielectric constants, , as high as 4600 at 40K, a factor of 2 greater than previously reported for strontium titanate thin films and approaching the value observed for bulk STO. Applying a dc bias of 15 V across the 5 m coplanar gap at the temperature of maximum e yielded (V = 0)/(V = 15) = 3. Also, by increasing the amplitude of the measurement's ac drive signal, we saw an initial increase in both and tan () followed by saturation.
SESSION L2: HTS THIN FILMS
Chairs: Chang-Beom Eom and Tobin J. Marks
Monday Afternoon, March 31, 1997
Golden Gate C1
1:30 PM *L2.1
MOCVD PRECURSOR DELIVERY MONITORED AND CONTROLLED USING UV SPECTROSCOPY , Brian J. Rappoli, Naval Research Laboratory, Chemistry Div, Washington, DC; William J. DeSisto, Naval Research Laboratory, Electronic Science & Technology Div, Washington, DC; Tobin J. Marks, John A. Belot, Northwestern Univ, Dept of Chemistry, Evanston, IL.
The 2,2,6,6-tetramethyl-3,5-heptadionate (thd) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac) complexes of Ba are used as precursors in the MOCVD fabrication of HTSC thin films. The physical properties of the hfac containing precursors can be modified by the addition of glymeligands to the Ba complex. The gas phase transport properties of these compounds will be discussed. Gas phase concentration of the complexes as a function of purge time and bubbler temperature have been examined by in-situ UV spectroscopy. We will also report details of a UV spectrophotometric-based feedback control system designed to maintain constant gas phase concentration of thd precursors under MOCVD conditions.
2:00 PM *L2.2
MECHANISM OF OXYGENATION OF YBaCuO THIN FILMS DURING IN SITU GROWTH BY CATHODIC SPUTTERING: THERMODYNAMIC AND KINETIC ASPECTS, J. Siejka, J. Garcia-Lopez, CNRS, Groupe de Physique des Soides des Univ, Paris, FRANCE; Y. Lemaitre, L. M. Mercandalli, Thomson CSF, LCR, Orsay, FRANCE.
The recent experimental results obtained for c-axis oriented YBaCuO thin films deposited at the optimized conditions for the microwave applications are presented and discussed.The emphasis is on the study of the relationship between oxygen profiles, crystallographic structure and electrical and physical properties of the films formed in situ at the same and optimized conditions of sample deposition and further submitted to the different conditions of sample cooling. The studies of the oxygen depth concentration profiles and of the oxygen contents are carried out using the recently developed Nuclear Reaction Analysis (NRA) . These results are correlated, on one hand, with the measurements of the atomic composition and structure by RBS, XRD and TEM and on the other hand by the measurements of the electrical and physical properties Tc, Jc and microwave surface resistance Rs. All results are consistent with the idea that the fully oxygenated films are formed during in situ growth at T 700C. For T< 300C, the oxygen interface transfer coefficient is very small and the oxygen content is practically preserved. However a large room temperature oxidation (corrosion like) takes place in the presence of water vapor. The fundamental and applied consequences of these findings for the mechanism of thin films growth and for the properties of the films in the presence of cathodic plasma or laser plume are analyzed .
2:30 PM L2.3
EPITAXIAL YBaCuO/SrRuO HETEROSTRUCTURES, Darrell G. Schlom, B. A. Merritt, Sriram Madhavan, Pennsylvania State Univ, Dept of MS&E, University Park, PA; Ying Liu, Pennsylvania State Univ, Dept of Physics, University Park, PA; Antke Dabkowski, Hania Dabkowska, McMaster Univ, Inst for Matls Research, Hamilton, CANADA.
SrRuO is the only known layered perovskite that is free of copper, yet superconducting (T 1.2 K in single crystals) . Its excellent lattice match with Yba make it an attractive candidate for use as a normal metal in Josephson junctions. The deposition conditions yielding Sr-phase films by pulsed laser deposition have been mapped out as a function of oxygen partial pressure and temperature for growth from Sr, Sr, and SrRuO targets. Single-domain axis oriented epitaxial films of Sr have been grown on (100) LaSrGaO substrates. Single-domain -axis oriented Sr films have been grown on (100) LaAlO, (110) NdGaO, and (100) SrTiO substrates. Heterostructures containing YBa have also been grown on single-domain axis oriented and -axis oriented Sr films. Resistivity versus temperature measurements reveal that the as-grown Sr films are metallic, but not superconducting.
3:30 PM *L2.5
MICROSTRUCTURE DEFECTS IN SrTi0 BICRYSTALS AND THEIR INFLUENDE ON YBaCuO GRAIN BOUNDARY JUNCTIONS, Eric B. McDaniel, Steve C. Gausepohl, Mark Lee, Julia W.P. Hsu, Univ of Virginia, Dept of Physics, Charlottesville, VA; Rajesh A. Rao, Chang-Beom Eom, Duke Univ, Dept of ME&MS, Durham, NC.
We use a near-field scanning optical microscope (NSOM) to investigate the microstructure defects near the fusion boundary on SrTiO bicrystal substrates. The optical transmission across the fusion boundary shows circular dark spots with diameters varying from 0.1 to 1 m distributed non-uniformly along the boundary. We attribute these optical features to near-surface voids at the boundary, found in TEM studies We show that stress-induced birefringence is associated with most defects. Stresses in the substrates will certainly influence YBCO growth. After careful characterization of Me substrate, a thin YBCO film (40 nm) was deposited on it and patterned to form 5 junctions across the fusion boundary. We show that the meandering of the YBCO boundary, which has been seen in TEM micrographs, can be traced to the presence of substrate defects. One junction, which happened to be grown on a part of substrate that included no defects, displayed the best electrical behavior: highest T, I, and a good modulation pattern in a perpendicular magnetic field.
4:00 PM *L2.6
CORRELATIONS BETWEEN YBCO THIN FILM MATERIALS CHARACTERISTICS AND RF DEVICE PERFORMANCE, Todd Kaplan, Ted Salazar, Chung Huang, Andy Barfknecht, Kookrin Char, Vladimir C. Matijasevic, Zihong Lu, Conductus Inc, Sunnyvale, CA.
Known relationships between YBCO's materials properties and RF device performance can lead to improved device yields and reduced process feedback times. However, these relationships are at present poorly understood. In order to establish correlations, a variety of materials tests were performed on over 30 YBCO microstrip resonators on lanthanum aluminate substrates. These materials parameters were then compared with the unloaded Q of the resonators at 77 K and 63 K. The best correlations were found to be room temperature resistivity, T, film roughness, thickness, buffer layer x-ray counts, critical current density, and the YBCO rocking curve FWHM.
4:30 PM L2.7
TRANSMISSION ELECTRON MICROSCOPE STUDY OF YBCO THIN FILM MULTILAYER DEVICES, Mark A. Barnett, Univ of Birmingham, School of Metallurgy and Matls, Edgbaston Birmingham, UNITED KINGDOM; J. S. Abell, Univ of Birmingham, School of Metallurgy & Matls, Birmingham, UNITED KINGDOM; Richard G. Humphreys, Nigel G. Chew, Philip J. Hirst, Defence Research Agency, Nanotechnology Div, Malvern, UNITED KINGDOM.
Reproducible deposition of fully epitaxial multilayer systems, including growth up steps, is a prerequisite for the development of a realistic high temperature superconducting (HTS) thin film device technology. Patterned thin film multilayer structures, consisting of superconducting YBa (YBCO) and insulating PrBa (PrBCO), deposited onto (001) MgO substrates by electron beam co evaporation, have been earned using Transmission Electron Microscopy (TEM) techniques. TEM investigations on YBCO/PrBCO bilayers, patterned wing conventional photolithographic techniques, show that PrBCO films grown over 30 steps in patterned YBCO layers are free of grain boundaries. The PrBCO is c-orientated everywhere and shows pronounced facetting on the steps. Where milling of the underlying YBCO track results in a low angle YBCO 'tail' defect contrast is observed in the PrBCO at the end of the tail. Similar TEM investigations of YBCO/PrBCO/YBCO trilayers have shown c orientated growth throughout, even in the region of YBCO/PrBCO steps. No grain boundaries are observed even for step angles up to 70. Films of YBCO grown over steps of either YBCO or PrBCO show deep holes in the step region. However, the holes do not penetrate through to the underlying layers, a result of particular significance for edge junction structures. Grain boundaries in YBCO or PrBCO layers are nucleated when depositing over small (0.1 m length, 0.04 m height) low angle (18) MgO steps formed during patterning of an YBCO/PrBCO layer down to the substrate. This reduces the critical current density in the overlayer by about one order of magnitude (Jc 5 x 10/cm @ 77K). The harmful effects of defects identified in this study can be eliminated by careful circuit design. For example, it is shown that grain boundaries associated with milled steps on MgO can be eliminated by the use of a PrBCO buffer layer beneath the superconducting base layer.
4:45 PM L2.8
NONLINEAR PROPERTIES OF SrTiO THIN FILMS AT MICROWAVE FREQUENCIES, Andrey B. Kozyrev, T. B. Samoilova, A. A. Golovkov, E. K. Holman, D. A. Kalinikos, V. E. Loginov, A. M. Prudan, O. I. Soldatenkov, Electrotechnical Univ, St Petersburg, RUSSIA; Tatiana Rivkin, Carl H. Mueller, G. A. Koepf, Superconducting Core Tech, Golden, CO.
Ferroelectric thin films are being developed for use in microwave devices such as tunable filters and phase shifters. SrTiO thin films are being studied because they have high dielectric tunabilities at cryogenic temperatures and are chemically compatible with Y%Ba. One of the important parameters of a material incorporated into a microwave circuit is its power handling capability. To our knowledge, these are the first reported measurements of (intermodulation distortion) IMD for microwave devices incorporating SrTiO thin films. The STO films were deposited by magnetron sputtering onto Al substrates with 200 thick CeO buffer layers. Film orientation and strain were characterized using X-ray Diffraction and interdiffusion between the various layers was measured using Surface Neutral Mass Spectroscopy. Planar capacitors, with 10 micron gap and 1 mm x 1 mm copper electrodes, were fabricated on the surface of the films. The capacitors were inserted into a resonant structure to perform the IMD measurements using two-tone standard technique. The results of the IMD measurements will be reported along with the data on the dielectric constant and loss tangent of the films at low (<1 MHz) and microwave frequencies (1-10 GHz). The variation of these parameters with a measurement temperature and bias voltage will also be reported.
SESSION L3: COLOSSAL MAGNETORESISTIVE OXIDE THIN FILMS
Chairs: Marilyn E. Hawley and T. Venkatesan
Tuesday Morning, April 1, 1997
Golden Gate C1
8:30 AM L3.1
NEW INSIGHTS INTO THE TRANSPORT AND FIELD ENHANCEMENT EFFECTS IN SOL-GEL DERIVED CMR THIN FILMS, Kannan M. Krishnan, H.-C. Sohn, C. Nelson, Lawrence Berkeley National Laboratory, Matls Sciences Div, Berkeley, CA; H. L. Ju, Lawrence Berkeley National Laboratory, Materials Sciences Div, Berkeley, CA.
La (where M = Sr, Ba, or Ca) thin films, exhibiting colossal magnetoresistance (CMR), have generated much recent scientific and technological interest. However, the transport mechanism underlying the CMR of such doped manganites is not yet understood and their technological applications are limited by the high fields (1T) required to obtain any significant MR.
Following the development of a polymeric chemical synthesis route, we have investigated the O unoccupied density of states in sol-gel derived La (O 0.7) thin films, grown epitaxially on LaAlO, by electron energy-loss spectroscopy at sub eV resolution. The spectra show a distinct prepeak in the OK edge at the Fermi level, the intensity of which correlates directly with the conductivity of the film. Similar correlation was also obtained for La annealed in vacuum to obtain well defined oxygen content (z) in the film. This is the first direct evidence to suggest that the majority charge carriers in the CMR manganites are oxygen 2p holes and that they may be responsible for their anomalous MR behavior.
In another set of experiments, we have studied room-temperature field amplification effects to enhance the low-field sensitivity of La films sandwiched between two thin rectangular slices of either -Fe or Mn-Zn ferrite. The soft ferromagnet produces a maximum amplified field, H, ranging from 3000 gauss (Mn-Zn Ferrite) to 7000 gauss ( Fe) which is proportional to its magnetization. This field amplification leads to an enhanced low-field MR value as high as 6 at an external field of 500 Oe which is 6 times the value observed without the amplification.
8:45 AM *L3.2
EFFECTS OF CONTROLLED LATTICE DISTORTION ON THE PHYSICAL PROPERTIES OF MAGNETORESISTIVE MANGANITES AND COBALTITES, Nai-Chang Yeh, California Inst of Technology, Dept of Physics, Pasadena, CA.
Studies of LaCaMnO epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: At high temperatures (; it T being the Curie temperature), the magneto transport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures ( , the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess carrier scattering. The effects of the controlled lattice distortion on the optical phonon modes is manifested by the significant phonon frequency shifts with the lattice strain in the Mn-O-Mn bending and the Mn-O stretching modes. In addition, the surface topography obtained from the scanning tunneling microscopy (STM) is found to be correlated with the film growth parameters and the magnetoresistance. Studies on another ferromagnetic perovskite, LaCaCoO films, exhibit much smaller negative magnetoresistance which may be attributed to the absence of Jahn-Teller coupling and the high electron mobility that prevents the formation of lattice polarons. The STM tunneling spectroscopy of both the manganites and cobaltites is carried out for the first time as a function of the temperature and doping level. The results are correlated with the density of states of these perovskites, and are dependent on the magnetic ordering, doping level, and the band structure. Similar studies of various physical properties on other manganites and cobaltites with different dopants are also conducted and will be reported.
9:15 AM *L3.3
COLOSSAL MAGNETORESISTANCE IN La-Ca-Mn-O, Sungho Jin, Bell Labs, Lucent Technologies, Murray Hill, NJ.
A colossal magnetoresistance effect with four to six orders of magnitude change in resistivity has been obtained in epitaxially grown La-Ca-Mn-O and La-Y-Ca-Mn-O thin films. The effect is negative and isotropic with respect to the field orientations. The magnetoresistance is strongly temperature-dependent, and exhibit a sharp peaks that can be shifted to near room temperature by adjusting processing parameters. The very large MR effect occurs only when the film thickness is less than about 2000 . Near-room-temperature R/R values of 1,300 at 260 K and 400 at 280 K have been observed. The orders of magnitude change in electrical resistivity could be useful for various magnetic and electric device applications.
10:15 AM *L3.4
HIGH Tc CUPRATES AND COLOSSAL MAGNETO-RESISTIVE MANGANITES: CONTRASTS AND SYNERGY, T. Venkatesan, Univ of Maryland, Dept of Physics, College Park, MD.
The perovskite family of the HTS cuprates and the CMR manganites offer a number of similarities and differences. The thin films of these systems are reasonably lattice-matched for heteroepitaxy and grow at comparable processing conditions. Both systems exhibit transitions in the transport properties: normal to superconducting for the cuprates and paramagnetic to ferromagnetic for the manganites as the temperature is lowered. While the Cu-O planes dominate the transport properties, the Mn-O-Mn bond acts as the transport pathway in the case of the manganites. In both systems, the core ions Cu 2+ and Mn 3+ are strong Jahn-Teller ions which tend to induce strong correlations between lattice distortions and the transport properties. Using ion channeling technique, a strong correlation between dynamic lattice distortions and the transport properties has been observed in both systems with the measured lattice displacements (most likely dynamic) of the order of 0.01 . From a technological point of view, novel possibilities exist for building devices utilizing both HTS and CMR materials. The ability of the C MR materials to convert magnetic signals to resistive modulations may be very useful in the design of "flux transformers'' where the changes of flux density in a single flux quantum (SFQ) logic device can be converted to electrical signal. The flexibility in the design of the impedance of the CMR sensor may be very useful for interfacing HTS and cooled CMOS logic families. In this talk, preliminary results on an HTS-based flux lens to enhance the sensitivity of a CMR detector for low magnetic fields will be discussed.
10:45 AM *L3.5
INTERPLAY BETWEEN ELECTRONIC TRANSPORT AND MAGNETIC ORDER IN FERROMAGNETIC MANGANITE THIN FILMS, Michael F. Hundley, Los Alamos National Laboratory, Condensed Matter & Thermal Physics Group, Los Alamos, NM; John J. Neumeier, Florida Atlantic Univ, Dept of Physics, Boca Raton, FL; Robert H. Heffner, Los Alamos National Laboratory, Condensed Matter & Thermal Physics Group, Los Alamos, NM; Quanxi X. Jia, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM; X. D. Wu, Los Alamos National Laboratory, Los Alamos, NM; Joe D. Thompson, Los Alamos National Laboratory, Condensed Matter & Thermal Physics Group, Los Alamos, NM.
The transition metal oxides LaAMnO (A = Ba, Ca, or Sr) order ferromagnetically with ordering temperatures ranging from as low as 50 K to well above room temperature. Magnetic order in these manganite compounds results in a concomitant metal-insulator transition, with small-polaron transport above T and metal-like transport below the ordering temperature. This is in stark contrast to conventional ferromagnetic metals wherein magnetic order has only a modest affect upon the electronic transport properties due to a small order-induced reduction of spin-disorder scattering. The feature displayed by the manganites that is most important technologically is the extremely large negative magnetoresistance that achieves its largest values near the ferromagnetic ordering temperature. An applied magnetic field of only a few tesla is sufficient to reduce the room-temperature resistivity by 65% in samples with magnetic ordering temperatures near 295 K. Qualitatively, this phenomena (known as colossal magnetoresistance or CMR) involves the suppression of the relatively sharp maximum in the resistivity that is centered at T. When considered collectively, the anomalous temperature-dependent transport properties, the colossal negative magnetoresistance, and the magnetically ordered ground state indicate that a novel interplay between magnetism and electronic transport occurs in the manganites. This interplay results in a key phenomenological correlation that is evident in (H,T) and M(H,T) data taken on both bulk samples and pulsed-laser deposited thin films. General features of the magnetic-field and temperature-dependent electrical resistivity and magnetization as exhibited by PLD-grown thin films will be presented. Particular emphasis will be placed on what these measurements tell us about the conduction process both above and below the magnetic ordering temperature.
11:15 AM L3.6
CHEMICAL SOLUTION DEPOSITION OF CMR EPITAXIAL THIN FILMS, Andrew D. Polli, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; Frederick F. Lange, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA; Markus Ahlskog, Reghu Menon, Anthony K. Cheetham, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA.
A simple process based on aqueous metal acetate solutions has been developed and employed to prepare (LaCa)MnO3 (LCMO) epitaxial thin films on (100) SrTiO and pseudocubic (100) LaAlO. The films first crystallize into a continuous, nanocrystalline network below 600C Upon further heat treatment, epitaxial grains grow and consume the nanocrystalline network. Heat treatment at 800C for 1 hour is enough to complete this nanocrystalline-to epitaxial transition, but surface roughness is eliminated only following treatment at elevated temperatures, 1000C. Four-circle x-ray diffraction shows that the epitaxial grains have a slight mosaic character (FWHM = 0.12-0.35). Temperature sweeps of electrical resistivity show that the conductivity and Curie temperature of the films increase with heat treatment temperature. Films fired at 600C and 800C for 1 hour show peak resistivities of 5.0 -cm and 0.3 -cm and Curie temperatures of 190 K and 270 K, respectively. Films fired at 1000C for 1 hour were found to have a Curie temperature near room temperature. For these films, the application of l and 8 tesla fields was found to drop the film's resistivity by 10 and 60, respectively, at room temperature.
11:30 AM L3.7
GROWTH-DEPENDENT MICROSTRUCTURE AND MAGNETIC PROPERTIES OF LaSrMnO CMR FILMS, Geoffrey W. Brown, Los Alamos National Laboratory, Center for Matls Science, Los Alamos, NM; Marilyn E. Hawley, Los Alamos National Laboratory, Los Alamos, NM; Quanxi X. Jia, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM.
The preponderance of CMR growth work on doped LaMnO films has focused on the Ca based materials. However, since the 33% Sr-based CMR films are ferromagnetic at room temperature, they make idea candidates for dual growth-magnetic structure RT scanning probe microscopy studies. In this study interest was focused on those processing details, including deposition parameters, substrate, post-deposition anneal, and thickness, which were expected to impact on the structure-property relationships. With that in mind, two different thickness LSMO films were grown by pulsed laser deposition from a stoichiometric target on both LaAlOand SrTiO substrates at temperatures in the 500 C° to 800° C range. The microstructure, crystal quality, epitaxy, and magnetic and electrical properties of these films were characterized by scanning tunneling microscopy, magnetic force microscopy (MFM), x-ray diffraction, temperature-dependent magnetization and four-point probe transport measurements, respectively. The trends in T-dependent growth-controlled structure of the LSMO films followed that previously reported for the Ca-based films, island sizes increasing with T coalescing into extended layers after annealing at high T. Although topographic contributions to MFM images complicates extraction of unambiguous magnetic information, local magnetic structure was often associated with defects and film coercivity varied with growth. Strain induced magnetic structure was not seen to have a simple lattice-mismatch dependence.
11:45 AM L3.8
TEM STUDIES OF LATTICE DISTORTIONS IN PLD La(Ca,Sr)MnO CMR THIN FILMS, Rhonda M. Stroud, Naval Research Laboratory, Washington, DC; Michael S. Osofsky, Naval Research Laboratory, Code 6344, Washington, DC; Jeff M. Byers, Naval Research Laboratory, Dept of Matls Physics, Washington, DC; Michael L. Wilson, Clemson Univ, Dept of Physics & Astronomy, Clemson, SC; James S. Horwitz, Naval Research Laboratory, Code 6673, Washington, DC; Kenneth S. Grabowski, Naval Research Laboratory, Washington, DC; Douglas B. Chrisey, Naval Research Laboratory, Code 6673, Washington, DC.
The results from transmission electron microscopy (TEM) studies of the defect structure in pulsed laser deposited (PLD) La thin films are presented. The goal of these studies is to quantify the effects of oxygen vacancies, cation disorder and substrate strain on the colossal magnetoresistance (CMR) of La thin films. Thin films of approximately 1500 were deposited by PLD on LaAlO and SrTiO substrates, at temperatures ranging from 700C to 850C, in 200 to 400 mTorr O. Oxygen vacancies were removed and substrate strain minimized in the as-deposited films by annealing in oxygen for 24 hours at 1000C. The absorption of oxygen produced a decrease in the lattice constant by approximately 1.5, as measured by x-ray diffraction. After annealing, defects were controllably reintroduced by charged particle irradiation. A defect density of 0.01 displacements per atom, as estimated by TRIM, was obtained using 6 MeV Si ions. Irradiation with 200 KeV electrons was used to selectively displace oxygen atoms. Local structural distortions in the as-deposited, annealed and irradiated thin films were characterized by TEM, and correlated with the CMR behavior.
SESSION L4: CMR AND MAGNETIC OXIDES
Chairs: Chang-Beom Eom and Susan Trolier-McKinstry
Tuesday Afternoon, April 1, 1997
Golden Gate C1
1:30 PM *L4.1
TRANSPORT PROPERTIES OF SOME CMR PEROVSKITES, Marcelo Jaime, Univ of Illinois-Urbana, Dept of Physics, Urbana, IL.
In recent years considerable attention has been paid to a long ignored branch of the perovskite family, the manganites ABMnO (A=La, B=Ca,Pr,Pb,Sr). One of the reasons lies in the potential applications of the now well-established colossal negative magnetoresistance effect (CMR). However, there is not yet a satisfactory explanation for the responsible mechanisms. We have measured the transport properties, including the resistivity, thermopower and Hall effect for temperatures between 4K and 500K and magnetic fields up to 18 Tesla, in ceramic polycrystalline samples, PLD films, MBE films and single crystals of various compositions. The results of these experiments were interpreted as the first evidence of small polaronic conduction in CMR manganites.  In fact, the thermoelectric power is highly sensitive to the semiconductor-to-metal phase transition, and its values and temperature dependence at high temperatures indicate conduction by positive small polarons. The Hall coefficient, measured in films, is negative and has an exponential temperature dependence with an activation energy equal to 2/3 that of the conductivity in excellent agreement with theoretical predictions for small polaronic systems. In addition we will discuss how oxygen vacancies can be controlled to produce changes in the effective doping level and Tc. Finally, it was found that the low field magnetoresistance in the ferromagnetic state can be substantially enhanced by control of the grain size in ceramic samples. Together, granularity and oxygen content control could be the way to fine tune the physical properties of CMR materials for applications.
2:00 PM *L4.2
OXYGEN-PLASMA ASSISTED MBE GROWTH OF THE STABLE OXIDES OF IRON, S. A. Chambers, Yufei Gao, Yong Joo Kim, Pacific Northwest National Laboratory, Environmental Molecular Sciences Lab, Richland, WA.
The need to synthesize well-defined, single-crystal surfaces of the less common oxides will become increasingly more critical as the surface science of these materials matures. Magnetite (FeO) and a- and y- hematite (FeO) are among the most important oxides for fundamental studies of molecular geochemistry, but are also of interest as magnetic materials. We have used oxygen-plasma assisted MBE to synthesize pure, crystallographically well-defined surfaces of these three materials. -FeO has been grown on AlO r- c-, and -plane, and FeO and -FeO were grown on MgO(100), MgO(110), and -AlO(0001). The resulting epitaxial films and surfaces have been characterized by RHEED, LEED, XPS, XPD, XRD, AFM, and STM. In general, we find that lattice matching, minimization of interface chemistry, matching of surface free energies, oxygen sublattice continuity across the interface, and careful flux control during growth are extremely important for successful epitaxy. Successful FeO growth requires oxygen-rich conditions whereas more iron-rich conditions are required for the growth of high-quality FeO. In this talk, we discuss recent progress in our laboratory in the growth and detailed characterization of these important oxide surfaces.
2:30 PM L4.3
DEPOSITION OF DOPED-LANTHANUM MANGANITE THIN FILMS BY MOCVD, Klaus Hermann M. Dahmen, Michael Carris, Eduard Gillman, Florida State Univ, Dept of Chemistry, Tallahasee, FL; Hamid Garmestani, Florida State Univ, Dept of Mechanical Engr, Tallahassee, FL.
Alkaline earth doped lanthanum manganites, LaMMnO, (M = Ca, Sr) have recently attracted a great deal of attention for their magnetic properties, in particular their giant or colossal magnetoresistance. We have investigated the deposition of these materials as thin films by MOCVD, using a commercial MOCVD system adapted with a liquid precursor delivery apparatus. Precursors, growth conditions as well as the charaterization and transport properties of these epitaxial films on substrates such as LaAlO3, MgO will be discussed.
3:15 PM *L4.4
THE (001) SURFACE OF FeO GROWN EPITAXIALLY ON MGO, CHARACTERIZED BY SCANNING TUNNELING MICROSCOPY, J. M. Gaines, P.J.H. Bloemen, R. M. Wolf, A. Reinders, Reiner M. Jungblut, J.T.W.M. van Eemeren, Fred Roozeboom, Philips Research Laboratories, Eindhoven, NETHERLANDS; Juergen T. Kohlhepp, P.A.A. van der Heijden, Wim J.M. de Jonge, Eindhoven Univ of Technology, Dept of Physics, Eindhoven, NETHERLANDS; R. J. G. Elfrink, Philips Research Laboratories, Eindhoven, NETHERLANDS.
Spinel Fecontains two sites for iron: tetrahedrally coordinated sites containing Fe ions and octahedrally coordinated sites containing a mixture of Fe and Fe ions. Scanning tunneling microscopy performed on the (001) surface of Fe, grown epitaxially on MgO, shows localized charge density (individual ' 'atoms'') at the tetrahedral sites. The images show that the p(11) surface reconstruction (also observed during molecular beam epitaxy of Fe) is produced by a displacement of the tetrahedrally coordinated Fe ions from their bulk positions along  directions. The octahedral Fe ions are imaged as extended rows with no resolution of the individual atoms. The slight corrugation of electron charge along the octahedral sites is consistent with the original conjectures explaining the high electrical conductivity in bulk Fe: electrons move by hopping between the Fe and Fe atoms along the octahedral rows of Fe ions.
Larger area scans show that the surface contains terraces that are typically several hundred Angstroms broad, separated by steps that are 2.1 high, corresponding to the distance between adjacent oxygen planes. The arrangement of the unit cells rotates by 90 upon crossing a step edge, consistent with the bulk structure.
Stacking faults, originating from the symmetry difference between the substrate and the epitaxial layer, are apparent in the STM images. They appear in the images as disordered bands separating two regions having orientations of the surface unit cells that are inconsistent with a continuous bulk structure.
3:45 PM L4.5
STRAIN STABILIZED METAL-INSULATOR TRANSITION IN EPITAXIAL THIN FILMS OF METALLIC OXIDE CaRuO, Chang-Beom Eom, Rajesh A. Rao, Qing Gan, Duke Univ, Dept of ME&MS, Durham, NC; Robert J. Cava, Princeton Univ, Princeton Matls Inst, Princeton, NJ; Yuri Suzuki, Bell Labs, Lucent Technologies, Murray Hill, NJ; Steve C. Gausepohl, Mark Lee, Univ of Virginia, Dept of Physics, Charlottesville, VA.
We have observed both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO deposited under identical conditions on (100) SrTiO substrates. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single crystal SrTiO substrates, metallic films with lattice parameters close to the bulk material grew on poor crystalline quality SrTiO substrates. The films deposited on (100) LaAlO substrates consistently showed metallic behavior. Atomic force microscope images suggest that the semiconducting films had a coherent layer-by-layer type growth. In contrast, an island-like incoherent growth was seen in the metallic films. It is believed that in the coherent films a strain induced substitution of the small Ru cations by the larger Ca cations occurs, breaking the conduction pathway within the three dimensional network of the RuO octahedra and leading to a metal-insulator transition. This unique phenomenon - which is not observed in bulk material - can be significant in technologically important epitaxial perovskite oxide heterostructures.
4:00 PM L4.6
THE ROLE OF STRAIN IN MAGNETIC ANISOTROPY OF MANGANITE THIN FILMS, Yuri Suzuki, Cornell Univ, Dept of MS&E, Ithaca, NY; H. Y. Hwang, S.-W. Cheong, Bertram Batlogg, R. B. van Dover, Bell Labs, Lucent Technologies, Murray Hill, NJ.
Recently bulk studies of the manganite materials have revealed dramatic changes in magnetic properties, including magnetoresistance and Curie temperature, in response to lattice distortions imposed by chemical or hydrostatic pressure. In epitaxial manganite films, the substrate imposes a strain on the film which affects not only Curie temperature and magnetoresistance properties but also the magnetic anisotropy. In this study we show the importance of the role of strain in these materials by revealing the dominance of strain anisotropy effects over magnetocrystalline anisotropy effects in the magnetic anisotropy of these films. We have fabricated (001) and (110) La thin films on (001) and (110) SrTiO substrates with excellent structural quality as observed from x-ray diffraction and Rutherford backscattering spectroscopy analysis. Magnetization measurements reveal square hysteresis loops along the easy axis in the plane of the films and Curie temperatures of 370380 K. In order to measure magnetic anisotropy in the plane of the films, remnant magnetization was measured as a function of in-plane angle. The (110) La films reveal a twofold symmetry while the (100) films have fourfold symmetry. Such symmetries can only be explained by strain anisotropy contributions in the plane of the film. In light of recent theoretical and experimental studies that have revealed the importance of lattice distortions on the magnetic properties in these materials, it is not surprising that in these materials magnetostriction is significant and that strains of less than 1 between the film and the substrate result in significant strain anisotropy contributions.
4:15 PM L4.7
MBE-GROWTH OF IRON AND IRON OXIDE THIN FILMS ON MgO(001) WITH VARIOUS OXIDIZING AGENTS, Frans Christiaan Voogt, Tjipke Hibma, Univ of Groningen, Dept of Chemical Physics, Groningen, NETHERLANDS; Peter J.M. Smulders, Lambertus Niesen, Univ of Groningen, Dept of Nuclear Solid State Physics, Groningen, NETHERLANDS.
We have investigated the use of NO, NO and NO as alternative oxidizing agents in the MBE-growth of iron oxide thin films. From previous studies, it is known that the conventional oxygen source O is only capable of forming stoichiometric and slightly oxidized magnetite FeO thin films on MgO(001) substrates. The highest degree of oxidation that can be reached is FeO with 0.06. We find that NO is a much more efficient oxidizing agent. Not only both the FeO and FeO phases can be obtained, but also all solid solutions Fe in between, with 0 0.33. The stoichiometry of the films has been determined by applying Mossbauer spectroscopy on Fe probe layers, sandwiched between layers with identical composition, but grown with natural iron. When small doses of NO are used, new wuestite FeO and Fe phases are being formed. In contrast to the FeO films, these phases contain nitrogen incorporated into the crystal lattice. XPS measurements show that the content increases nearly linearly with decreasing NO flux. Similar compounds can be obtained if NO is used as the source of oxygen, which gives insight into the oxidation mechanism of NO. The use of NO does not lead to the formation of iron oxides. Interestingly, however, it does alter the growth mode of iron on MgO(001). Whereas Fe deposited under UHV conditions forms 3D islands, the NO acts as a surfactant and induces 2D layer by-layer growth.
4:30 PM L4.8
STRUCTURE PROPERTY RELATIONSHIPS IN EPITAXIALLY DEPOSITED SrRuO THIN FILMS, Jon-Paul Maria, Susan Trolier-McKinstry, Darrell G. Schlom, Pennsylvania State Univ, Dept of MS&E, University Park, PA; Ram S. Katiyar, Univ of Puerto Rico, Dept of Physics, Rio Piedras, PR.
Given its close lattice match with many oxide perovskite crystals and metallic conductivity, SrRuO is a promising electrode material for heterostructure devices incorporating ferroelectric, dielectric, and superconducting oxides. The quality and reliability of such devices are dependent upon the condition of the electrode; thus an investigation of the relationships between structure, processing, and material properties has been performed.
Epitaxial SrRuO films have been prepared on SrTiO substrates by pulsed laser deposition. The film structure has been characterized by both 4-circle x-ray diffraction and Raman scattering spectroscopy, while the electrical behavior was studied by temperature dependent resistivity measurements. With variations in the deposition conditions, the room temperature resistivity value ranged from 180 -cm to 1900 -cm. In addition, a slope change in the temperature dependence of the resistivity indicated the Curie transition. The Curie transition was found to shift towards lower temperatures as the film structure became more defective. Strong variations in the crystal structure and the Raman scattering spectra accompanied the changes in resistivity. The variations in the structure and properties were attributed to energetic bombardment during deposition.
4:45 PM L4.9
MAGNETIC AND CRYSTALLINE MICROSTRUCTURE OF SrRuO THIN FILMS, Ann F. Marshall, Stanford Univ, Stanford, CA; L. Klein, C. Ahn, S. Dodge, J. Reiner, Stanford Univ, Dept of Applied Physics, Stanford, CA; Laurent Mieville, Stanford Univ, E L Ginzton Lab, Stanford, CA; A. Kapitulnik, Theodore H. Geballe, Malcolm R. Beasley, Stanford Univ, Dept of Applied Physics, Stanford, CA.
The magnetic domain structure of ferromagnetic SrRuO thin films has been studied by Lorentz transmission electron microscopy as the films are cooled through T (about 150 K). For zero-field-cooled films, we observe a regularly spaced, striped magnetic domain wall structure in orientations which are consistent with its uniaxial magnetic anisotropy. Since SrRuO has a perovskite-based, orthorhombic structure, it grows on cubic perovskite substrates (such as SrTiO) in six different orientations; thus in the studied films which contain all six domains, we could fully establish the correlation between the crystallographic and the magnetic domain microstructure. We also study domain wall motion in an applied magnetic field. The intradomain walls annihilate systematically with orientation as the field is increased to about 1-2 KOe corresponding to a large, irreversible change in resistivity vs. field. The remaining magnetic microstructure persists up to fields of about 20 KOe, as expected from the high magnetocrystalline anisotropy. Not only do our results provide important confirmation to magnetization and transport measurements of specially grown, single domain films, but they also provide missing quantitative information such as the spacing of the domain walls and the variations between the different domains.
SESSION L5: POSTER SESSION:
Chairs: Christopher M. Foster and J. S. Speck
Tuesday Evening, April 1, 1997
STRUCTURAL CHARACTERIZATION OF SrBiTaO THIN FILMS, Anthony Reynes-Figueroa, Eleicer Ching-Prado, Univ of Puerto Rico, Dept of Physics, Rio Piedras, PR; Amar S. Bhalla, Pennsylvania State Univ, Materials Research Lab, University Park, PA; Ram S. Katiyar, Univ of Puerto Rico, Dept of Physics, Rio Piedras, PR; William Perez, Univ of Puerto Rico, Dept of Physics, Rio Piedras, PR; D. Ramachandran, Pennsylvania State Univ, Materials Research Lab, University Park, PA.
Thin films of SrBiTaO (SBT) were deposited on Si by sol-gel technique. The thickness of the films are 200 nm (A) and 400 nm (B), respectively. EDX analysts indicates that the relative atomic ratio of the Ta/Bi and Ta/Sr are 1.16 and 1.77, respectively, which are close to the expected values. However, the reactive atomic ratio of Bi/Sr is 1.52, which indicates a possible excess of Sr. SEM studies show isolated grains in both SBT films with a grain size distribution between 0.1 and 0.5 . But, most of the grain size in the film is smaller than 0.1 . FTIR measurements of the samples show bands around 773, 610, and 511 cm, which correspond to the bands at 753, 631, and 550 cm found in SBT powder. The band at 550 cm corresponds to the well known TaOoctahedron vibrational motion found in perovskite structure. The comparison between the frequency position of this vibrational mode and the one in the ceramic suggests a different Ta-O bond in the SBT films. Micro Raman spectra shows inhomogeneities in the SBT films. Bands corresponding to the SBT materials were found, but frequency shifts and broadening were observed in almost every band. Particularly, the band around 818 cm-1, which corresponds to the A mode due to the stretching of the TaO octahedron, was found to change from 818 cm to 827 cm. This change seems to be related to the Ta-O bond length. In addition, we discuss the frequency shift and the half width of the Raman bands with respect to those in the SBT ceramics. Also, x-ray diffraction is used for structural characterization of the SBT thin films.
CONTROL OF STRUCTURAL TEXTURE IN PZT/RuO HETEROSTRUCTURES ON SiO/Si(001), G.-R. Bai, Argonne National Laboratory, Dept of Matls Science, Argonne, IL; A. Wang, J. Vetrone, Christopher M. Foster, Argonne National Laboratory, Matls Science Div, Argonne, IL.
There has been considerable interest in the deposition of PZT/RuO heterostructures on SiO/Si(001) substrates with controllable orientation. In this talk, the methods used to control the orientations of PZT/RuO will be presented and the difference in the morphology, phase purity, ferroelectric properties, and fatigue behavior between PZT(001)/RuO(101) and PZT(110) /RuO(110) heterostructures will be discussed. For growth temperatures in the range of 275C to 475C, highly conductive RuO thin films with either (110)- or (101)-textured orientations were grown by metal-organic chemical vapor deposition (MOCVD) on SiO/Si(001) substrates. Using these two different textured orientations of RuO electrodes as structural templates, we were able to template the structure of PZT grown by MOCVD. The PZT films were also grown at low temperature (525C). The films exhibited good ferroelectric hysteresis without any post deposition treatment. However, PZT films grown on (101)-textured RuO2 showed substantially higher P (40 C/cm) than films grown on (110)-textured RuO (22 C/cm). In contrast, PZT films grown on (101)-textured RuOshowed substantial fatigue after 10 cycles, whereas films grown on (110)-textured RuO were fatigue free to cycles.
REGROWTH OF LOW TEMPERATURE ARGON ION AMORPHIZED LITHIUM TANTALATE (LiTaO) SINGLE CRYSTAL, Z. Zhang, L. A. Rosakova, J. Wilson, Wei-Kan Chu, Univ of Houston, Texas Cntr for Superconductivity, Houston, TX.
C+ and C- faces of LiTaO single crystals were amorphized by 200 keV Ar ions at liquid nitrogen temperature. Chemical etching was used to identify the C+ and C- faces. RBS-Channeling, optical measurement and Transmission Electron Microscopy (TEM) were used to investigate the radiation damage and the regrowth of the amorphized layers. Optical measurement showed that, in UV and visible light region, no optical absorption center had been created by low temperature Ar ion bombardment. Thermal annealing was carried out in oxygen ambient. RBS Channeling showed that complete regrowth had been achieved after annealed at 550C for 30 minutes. For 450C annealing, regrowth rate was linear from 30 minutes to 60 minutes. After 60 minutes, regrowth rate reduced gradually. After 4 Hours, the regrowth nearly stopped. The primarily results indicated that the regrowth of C- faces was faster than that of C+ faces. The microstructure of the regrown layers was investigated by TEM.
MICROSTRUCTURES AND EPITAXIAL GROWTH MECHANISM OF LEAD TITANATE THIN FILMS DERIVED BY SOL-GEL PROCESS, Qifa Zhou, Hong Kong Polytechnic Univ, Dept of Applied Physics, Kowloon, HONG KONG; X. G. Tang, Zhongshan Univ, Dept of Physics, Guangzhou, CHINA; Q. Q. Zhang, Hong Kong Polytechnic Univ, Dept of Applied Physics, Kowloon, HONG KONG; J. X. Zhang, Zhongshan Univ, Dept of Physics, Guangzhou, CHINA.
Ferroelectric lead titanate (PbTiO) thin films have been epitaxially grown on miscut (001)SrTiO substrate by sol-gel process from lead acetate trihydrate and titanium butoxide. The influence of processing of the hydrolysis-polymerization condition on the thin films has been investigated. The electron microscope and x-ray diffraction analysis suggested the evidence of epitaxial growth of (001) PbTiO/(001) SrTiO along c-axis direction. Another, the microstructure, with emphasis on surface morphology and formation of growths, was studied by using atomic force microscopy (AFM) and Raman spectra, the epitaxial growth mechanism of perovskite PbTiO thin film growing will be discussed, the dielectric and ferroelectric properties were measured.
AUTOSTOCHIOMETRIC CHEMICAL VAPOR DEPOSITION OF FERROELECTRICS - STOICHIOMETRY OF CO-EVAPORATIONS, R. C. Zhang, Ren Xu, Univ of Utah, Dept of MS&E, Salt Lake City, UT.
The stoichiometry control in MOCVD of multicomponent oxides is studied by a comprehensive approach. For depositions of high melting point oxides, such as LiNbO and LiTaO, the stoichiometry of the film depends critically on (i) the stoichiometry of precursor co-evaporation, whether controlled mechanically or chemically, and (ii) the stoichiometry of pertinent deposition reactions. We report and compare experimental results on the stoichiometry of co-evaporation which are controlled mechanically and chemically. Double alkoxides LiNb(OC1)6 and LiTa(OC1)6, where n = 2-4, in their associative and dissociative forms are subjected to evaporation under carefully controlled temperature and pressure conditions. The stoichiometry of evaporation processes for the double alkoxides under various conditions is studied by controlled sublimation in combination with composition analysis of both the sublimate and the residual. It is found that the stoichiometry of co evaporation depends on the relative partial pressures of the single alkoxides at the respective evaporation temperature, as well as the stability of the double alkoxides. The ultimate accuracy of the chemical control of stoichiometry by double alkoxide evaporation is reported and compared with mechanically controlled co-evaporation. The results are generally applicable to other associative metalorganic precursors.
GROWTH, DIELECTRIC AND PIEZOELECTRIC PROPERTIES OF NaKNbO THIN FILMS, Xin Wang, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN; Lars-David Wernlund, Staffan Rudner, FOA Defence Research Est, Linkoping, SWEDEN; Lynnette D. Madsen, Ulf Helmersson, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN.
The perovskite material Na has been grown in thin film form using rf magnetron sputtering. To compensate for an observed loss of Na and K during growth, the target was enriched with these elements by a factor of three. The films were grown on polycrystalline Pt and (001) LaAlO. X-ray diffraction (XRD) showed that the films were single phase and from Rutherford Backscattering the relationship between the Na and K (the x-value) was estimated to be 0.43 < x < 0.60. On Pt, the films grew in a polycrystalline form with either a (001) or (101) preferred orientation depending on growth conditions. On LaAlO, the film grew epitaxially (001) with a slight increase in the XRD peak broadening as compared with the substrate peaks. The width of the (002) peak of the film is 00.4 as compare with 0.02 for the substrate peak. The dielectric properties were estimated from measurements on parallel plate capacitors for the films grown on Pt and using interdigital finger capacitors on the films grown on LaAlO. In both cases, the dielectric constant of the films were measured to be 700 at room temperature. For the finger capacitor, the dielectric constant was considerably reduced when a bias potential of 300 V was applied giving a ''tunability'' of 38. Preliminary measurements of the piezoelectric constant d, for films grown on Pt, gave a value of >100 pC/N.
TEXTURE CONTROL OF LSCO/PZT/LSCO FERROELECTRIC CAPACITORS FOR NONVOLATILE MEMORY APPLICATIONS, Gerd J. Norga, Dirk J. Wouters, Herman E. Maes, IMEC, Dept of MAP, Leuven, BELGIUM; Ria Nouwen, Lucien Vanpoucke, Limburgs Univ Centrum, Dept of Chemistry, Diepenbeek, BELGIUM.
The use of conductive metal oxide electrodes to improve the fatigue characteristics of lead zirconate titanate (PZT) based capacitor structures for next generation nonvolatile RAMs has attracted much research interest in recent years. Because of its high conductivity and good lattice match to PZT, lanthanum strontium cobalt oxide (LSCO) is particularly suited for heteroepitaxial growth of PZT ferroelectric capacitors (FECAPs) on single crystalline substrates such as MgO and LaAlO3. Excellent fatigue properties have also been demonstrated for (100)-textured polycrystalline LSCO/PLZT/LSCO FECAPs fabricated on Pt/SiO2/Si .
Because a (111) texture results in superior hysteresis properties for PZT FECAPs fabricated on silicon substrates , we have investigated the possibility of growing (111) LSCO/PZT/LSCO epitaxially on (111) MgO and on SiO2/Si using sputtered (111) Pt as a template layer. LSCO films were grown by pulsed laser deposition, PZT by sol-gel. On MgO(111) and Pt(111)/SiO2/Si substrates, PZT grows (111), while a mixture of (111) and (100) orientations is observed for LSCO. A pure (111) texture for LSCO could only be achieved by growth on (111) PZT. LSCO/PZT/LSCO FECAPs grown on (111) PZT/Pt/Ti/SiO2/Si show excellent hysteresis characteristics.
MICROSTRUCTURE DEVELOPMENT AND DEFECT ANALYSIS OF BARIUM TITANATE THIN FILMS PREPARED BY HYDROTHERMAL SYNTHESIS, Lijie Zhao, Allen T. Chien, F. F. Lange, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA; J. S. Speck, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA.
The hydrothermal growth of epitaxial BaTiO thin films on single crystal SrTiO (001) substrates were studied by plan view and cross section TEM. The microscopy studies demonstrated that the early growth takes place by incoherent islanding, followed by island coalescence (confirming early SEM and AFM studies). The initial islands have pyramidal shape with facet planes. Electron diffraction shows that both the islands and the fully formed film have mosaic character with a cube on-cube epitaxial relationship with the substrate. A several monolayer thick interfacial layer, presumably BaO, was observed between the SrTiO substrate and the BaTiO film for early stages of growth. At this point, no misfit dislocations were observed at the film substrate interface. Rather, only Moire fringes were seen, thus demonstrating that the films have incoherent interface with the substrate. At later stages, the interfacial layer disappears and the misfit dislocation network forms. Defects analysis shows that the misfit dislocations have pure edge character with a <100> line direction and <010> Burgers vector. The results of this work are discussed in the framework of specific barium adsorption on the SrTiO substrate prior to BaTiO film growth and subsequent dissolution at late stages in the film growth.
DOMAIN STRUCTURE IN MOCVD (001) RHOMBOHEDRAL PZT FERROELECTRIC THIN FILMS, Matthew LeFevre, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; A. E. Romanov, A.F. Ioffe Phys-Technical Inst, St Petersburg, RUSSIA; Lijie Zhao, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA; Christopher M. Foster, Argonne National Laboratory, Matls Science Div, Argonne, IL; J. S. Speck, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA.
The micromechanics of domain patterns for epitaxial tetragonal ferroelectric or ferroelastic films grown on (001) substrates have been developed previously. In the present study, domain formation in rhombohedral ferroelectric films is investigated employing the same defect theory framework. Imaging of -fringes at the domain boundary interfaces in plan view TEM tilting experiments showed the (001) and (010) pseudocubic planes to be the predominant twin boundaries in the rhombohedral films. On cooling below the Curie temperature, the cubic paraelectric films transform to the ferroelectric rhombohedral structure, with a <111> polar displacement. This leads to four different structural variants and eight possible polarization vectors. The polarization vectors align themselves in a head-to-tail fashion across the domain boundaries to preserve electrical neutrality. Therefore, there is no net polarization normal to the film, only in the plane of the film. The resulting films adopt a lamellar domain structure, where the lamellae are parallel to the two in-plane pseudocube directions. A complete energetic analysis shows that there is no critical thickness for forming the lamellar domain patterns in epitaxial rhombohedral (001) ferroelectric films. The analytical framework permits the calculation of the dependence of the domain size on film thickness and response to externally applied electric fields.
GROWTH OF LITHIUM NIOBATE THIN FILMS FOR OPTICAL WAVEGUIDE APPLICATIONS, Song Yun Lee, Robert S. Feigelson, Stanford Univ, Dept of MS&E, Stanford, CA.
High crystalline quality lithium niobate (LiNbO) epitaxial thin films have been grown for waveguiding applications on c-plane sapphire and (111) silicon substrates by solid-source MOCVD. Tetramethylheptanedionate sources, (Li(thd) and Nb(thd)) were used. For these applications, films with the low optical propagation losses (less than 2 dB/cm) are required. Since the major source of optical loss is scattering due to rough film surfaces, we have studied the mechanisms involved in film roughening and its relationship with the initial growth behavior of LiNbO on sapphire. According to our studies, film roughening occurs with large grain size and the deep grain grooves formed between them. Increasing the nucleation density during the initial stage of growth clearly decreases grain growth and therefore reduces roughness even at high growth temperatures. Based on these results, a 2-step growth process has been developed in which the nucleation and growth stages are treated separately. The optimum conditions for nucleation are designed to provide a high nucleation density (at low temperature) while those for growth to maintain high crystalline quality. Using the new growth process, film roughness was decreased to less than 2 nm rms even at high growth temperatures. These few also had good crystallinity and reduced optical loss. To date we have succeeded to measure an optical loss of 1.8 dB/cm for TE mode with 632.8 nm wavelength. It is expected that by refining this growth process, we can reduce the optical losses to less that 1 dB/cm Silicon (111) has also been used as substrate. Since the index of refraction of silicon is much larger (3.42) than that of LiNbO (2.29), SiO was used as a cladding layer. The results show that epitaxial films with c-axis normal to the substrates were successfully deposited at low growth temperatures at fast growth rate. The preferred orientation was changed to the (012) direction as the temperature was increased above 650C. Detailed results on film crystallinity and optical properties will be discussed.
THE PROPERTIES OF ORIENTED AND EPITAXIAL DIELECTRIC AND FERROELECTRIC THIN FILMS, Tingkai Li, Pete Zawadzki, Richard A. Stall, EMCORE Corp, Somerset, NJ; Chien-Hsiung Peng, Sharp Microelectronics Tech, Dept of Process Technology, Camas, WA; Yongfei Zhu, Seshu B. Desu, Virginia Polytechnic Inst, Dept of MS&E, Blacksburg, VA; Shaohua Liang, Rutgers Univ, Dept of Electrical & Computer Engr, Piscataway, NJ; Yicheng Lu, Rutgers Univ, Dept of Electrical Engr, Piscataway, NJ.
Oriented and epitaxial dielectric BaSrTiO (BST) films and ferroelectric SrBiTaO (SBT) and PbZrTiO (PZT) films have been prepared by MOCVD processes. The orientation, phase formation, interface and lattice structure, and properties of the oxide films were measured. The degree of orientation in the oxide film was systematically varied from highly oriented to random polycrystalline by varying the growth conditions and bottom metal electrode material. We have identified a novel device structure in which the oxide thin films can be crystallographically oriented along different axes relative to the underlying capacitor electrode. It was found that if the oxide lattice structures are highly anisotropic, such as the layered structure SBT material, the dielectric and ferroelectric properties (spontaneous polarization, coercive field, dielectric constant, etc.) are strongly dependent on the orientation of the films with respect to the underlying substrate materials. For oxide films whose lattice structures are isotropic, such as BST and PZT, as the film thickness decreased, the epitaxial films showed much better properties than the oxide films having random orientation due to smaller interface mismatch between the epitaxial film and the substrate.
EPITAXIAL GROWTH AND OPTICAL PROPERTIES OF SOL-GEL (Pb,La)TiO3 THIN FILMS FOR WAVEGUIDES, Byeong-Soo Bae, Junmo Koo, Kwang-Soo No, Changho Lee, KAIST, Dept of MS&E, Taejon, SOUTH KOREA.
Epitaxial lead lanthanum titanate thin films were fabricated on various single crystal substrates with varing La content using the spin coating of metallo-organic solutions. The influences of heat treatment, substrate material on PLT thin film, and the composition of PLT for epitaxial growth have been researched.
SESSION L6: POSTER SESSION:
Chairs: Christopher M. Foster and J. S. Speck
Tuesday Evening, April 1, 1997
FLUX-ENHANCED MAGNETIC SENSOR MADE OF YBaCuO/NdSrMnO HETEROSTRUCTURES, Zi-Wen Dong, Univ of Maryland, Dept of Physics, College Park, MD; T. Boettcher, Univ Bremen, Dept of Physics, Bremen, GERMANY; C. H. Chen, M. Rajeswari, R. P. Sharma, Univ of Maryland, Dept of Physics, College Park, MD; T. Venkatesan, Univ of Maryland, Dept of Physics & Electrical Engr, ColLege Park, MD.
There is great interest in obtaining practical magnetic sensors with the Colossal Magnetoresistive (CMR) materials. Large magnetoresistance (MR) of up to 99.99 has been reported at low temperatures in the presence of a few Tesla fields. In order to explore low field applications in magnetic sensors, we have developed a new technique to integrate CMR Nd (NSMO) resistors with high T superconducting YBa (YBCO) flux-focusing devices. YBCO/NSMO bilayers were deposited on (100) LaAlO substrates by pulsed laser deposition. The influence of the deposition conditions on the crystallinity of NSMO and YBCO, the MR effect, and the peak resistance temperature () in YBCO/NSMO heterostructures has been carefully studied. To optimally utilize both superconductivity and magnetoresistivity in the same temperature range, the should be lower than the superconducting transition temperature () of YBCO. When magnetic field is applied, magnetic flux is expelled from patterned YBCO regions and thus focused onto the NSMO resistors. A considerable enhancement of magnetoresistance in NSMO has been observed with such a magnetic ''lens''. Up to 800 enhancement in magnetoresistance at 77 K was observed, which corresponds to more than 7 change of MR in NSMO resistor in the presence of one hundred Gauss field. A low field response of 0.075/G is high enough for achieving sensitive and low cost magnetic sensors operated under a bias magnetic field.
THERMODYNAMICS AND KINETICS OF THE YBaCuO THIN FILMS GROWTH BY PULSED LASER DEPOSITION, Gun Yong Sung, Joeng Dae Suh, ETRI, Dept of Research, Taejon, SOUTH KOREA.
The processing parameters, i.e.,the deposition temperature, oxygen partial pressure, laser energy density, laser repetition rate, target-substrate distance, substrate materials, substrate surface treatment and morphology are well known to relate with the nucleation and growth of thin film growth by pulsed laser deposition.
We investigated the effect of laser energy density and repetition rate on the growth direction of YBCO thin films on LaSrGaO4 (100) substrate by PLD. The x-ray diffraction patterns showed that an a-axis oriented growth is preferred at the conditions of high deposition rate and energy density. YBCO thin films deposited at high deposition rate of 16.7 nm/s and 720C showed an a-axis oriented growth, whereas thin films deposited at 0.3 nm/s and 700C showed a c-axis oriented growth. This means that the faster the deposition rate, the higher the a axis oriented growth temperature. The dependence of the film orientation with the laser parameters will be explained in terms of thermodynamic and kinetic models, such as supersaturation and adatom surface diffusion.
SUPPRESSION OF SUPERCONDUCTIVITY BY INJECTION OF SPIN-POLARIZED CURRENT, Michael S. Osofsky, Naval Research Laboratory, Code 6344, Washington, DC.
We present results demonstrating a new type of nonequilibrium superconductivity based on the injection of a spin-polarized current from a ferromagnet into a superconductor. The effect occurs in both low- (Sn) and high- () superconductors when the ferromagnetic material, permalloy (), is used. When unpolarized current from a nonmagnetic metal was injected into the superconductor, a smaller change in the transport properties resulted. A phenomenological model in which the singlet ground state of the superconductor is strongly perturbed by the presence of a spin-polarized population of quasiparticles is presented.
NOISE AND OPERATIONAL CHARACTERISTICS OF MAGNETOMETERS MADE FROM SUPERCONDUCTING-NORMAL-SUPERCONDUCTING JOSEPHSON JUNCTIONS, David W. Reagor, C. Mombourquette, J. Decker, Y. Fan, Marilyn E. Hawley, R. Houlton, Los Alamos National Laboratory, Los Alamos, NM; Quanxi X. Jia, Los Alamos National Laboratory, Superconductivity Tech Ctr, Los Alamos, NM.
We have fabricated numerous magnetometers using our high temperature superconducting Josephson junctions. These Josephson junctions are fabricated in a superconducting-normal-superconducting ramp edge configuration  with silver doped YBa for the superconducting electrodes and PrBa for the normal layer. Small inductance quantum interference devices made from this junction technology have a transfer function exceeding 150 microvolts per flux quanta and a flux noise of 5x10 flux quanta per root hertz. In addition, we have established that these junctions have identical electrical characteristics after either a year of storage or repeated thermal cycling. We have also examined the tradeoff of 1/f noise versus thermal noise that is obtained as we vary the critical current of the devices. The magnetometers were made using galvanically coupled input coils. These devices exhibit excellent operational characteristics in the geomagnetic field. They functioned in an unshielded environment for more than 24 hours and operated in a moving dewar (without any feedback fields to compensate the changing applied field), both without flux tapping. Noise characteristics under these conditions will be reported.
COMPARATIVE STUDY OF ELECTICAL TRANSPORT ACROSS INTERFACES BETWEEN WIDE GAP SEMICONDUCTOR AND METAL OXIDES, Vitaly Talyansky, R. D. Vispute, R. P. Sharma, S. Chupoon, M. Downes, T. Venkatesan, Univ of Maryland, Dept of Physics, College Park, MD; Agis A. Iliadis, Univ of Maryland, Dept of Electrical Engr, College Park, MD; M. C. Wood, R. T. Lareau, K. A. Jones, U.S. Army Research Laboratory, Fort Monmouth, NJ.
Recent developments in the field of III-V nitride wide gap semiconductors have attracted considerable attention from the optoelectronic community. A clear need to integrate nitrides and metal oxides into the next generation of circuit assemblies has arisen. The technological challenges being faced while growing oxide/nitride heterostructures include material decomposition at high temperatures, interdiffusion, and formation of various undesirable compounds at the interface. We have investigated the structural and electrical properties of the heterostructure interfaces formed by either p-type YBa (YBCO) or n-type Pt (PCCO) deposited on n-type GaN by laser ablation. High resolution transmission electron microscopy (TEM) and Rutherford backscattering (RBS) studies revealed the formation of an interdiffusion layer of a deposition temperature dependent thickness at the oxide/nitride interfaces. The heterojunctions made with the p-type YBCO exhibited a far stronger rectifying behavior than the ones made with the n-type PCCO. We will compare the C-V measurements as well as the temperature dependent I-V transport studies of the two types of heterojunctions. We will argue that the drastic difference in the results obtained from these heterojunctions may be explained on the basis of the hole or electron conduction in the oxides.
EDGE AND SCREW DISLOCATION FORMATION IN EPITAXIAL THIN FILMS, V. L. Svetchnikov, Vladimir M. Pan, Inst for Metal Physics, Kiev, UKRAINE.
Dislocations are able to significantly affect the properties of thin films (e.g., dislocations having their cores normal to substrate are responsible for high-critical current density in thin superconducting films ). We considered the formation of dislocations in the very first film layer grown in registry with mismatched substrate (in pseudomorphic growth mode). We obtained both edge and screw dislocations in case the film had small (1) azimuthal misorientation with respect to the substrate. The cores (dislocation lines) of edge dislocations (their density is about 10 cm) were perpendicular to substrate; the screw dislocation lines were in interface plane. When the film reaches the thickness just above some h, film structure transforms from strained (like in psuedomorphic film) to relaxed one, and screw dislocations (with the density of 10cm) emerge normal to the substrate.
Our TEM observations of dislocations in this superconducting films revealed (by using technique of the Moire contract fringes together with their optical diffraction patterns analysis) that edge dislocation density and arrangement correspond to the predictions of our model. STM and TEM results on screw dislocations in thin superconducting films fit the model as well.
PREPARATION OF THE DOUBLE-SIDED YBaCuO THIN FILM BY HOT-WALL TYPE MOCVD, Yoshiaki Ito, Satoshi Taniguchi, Yutaka Yoshida, Izumi Hirabayashi, Shoji Tanaka, ISTEC, Nagoya, JAPAN.
Double-sided coated superconducting film is indispensable for high Q. low loss microwave circuits. For this purpose, we have developed a hot wall type MOCVD apparatus to deposit YBCO films using liquid state metalorganic sources. The heating method is indirect, that is, a infrared lamp heated a SiC tube around the quartz reactor tube. Then the heated SiC tube heats the substrate in the quartz reactor tube.
By improving the reactor shape and gas flow of the source materials, we succeeded in fabricating double-sided YBCO films. The films on the SrTiO (100) substrate showed the c-axis oriented single phase for both sides. They also indicated biaxial alignment in the a/b plane and Tc (zero) was 86 K. The results of the experiments including microwave properties on various substrates will be discussed. This work was supported by NEDO.
MEASUREMENT OF STRAIN ASSOCIATED WITH DEFECTS IN SrTiO BICRYSTAL SUBSTRATES USING NEAR FIELD SCANNING OPTICAL MICROSCOPY, Eric B. McDaniel, Julia W.P. Hsu, Univ of Virginia, Dept of Physics, Charlottesville, VA.
Thin film high-T grain boundary junctions are often made on SrTiO bicrystal substrates. We use a near-field scanning optical microscope (NSOM) to examine defects along the bicrystal boundary. We observe non-uniformly distributed dark circular defects which vary in diameter from 100 nm to 1 m and are usually accompanied by no topographic features. Since most of the defects leave no topographic signature on the substrate, they must influence the film growth by another mechanism if they contribute to device non-uniformity. The presence of strain fields would provide such a mechanism. By incorporating polarimetry into our NSOM, we image and quantitatively measure stress-induced birefringence patterns associated with these defects with spatial resolution exceeding the diffraction limit. The birefringence patterns are larger than the defects themselves and range in size from 1 m to 2.5 m. The patterns are predominantly spiral shaped. Quantitative analysis of the patterns yields retardances of 0.02 to 0.13 radians at 670 nm, corresponding to strain levels of 910 to 6 10.
SESSION L7: POSTER SESSION:
Chairs: Christopher M. Foster and J. S. Speck
Tuesday Evening, April 1, 1997
A STUDY OF THE GIANT MAGNETORESISTIVE THIN FILMS OF (La,Pr)(Ca, Sr)MnO PREPARED BY AEROSOL MOCVD, Oleg Yu Gorbenko, Sergey Samoylenkov, A. R. Kaul, Moscow State Univ, Dept of Chemistry, Moscow, RUSSIA; N. A. Babushkina, L. M. Belova, Kurchatov Inst, Moscow, RUSSIA.
Epitaxial thin films of RMMnO were successfully prepared by aerosol MOCVD at 750C from volatile thd-complexes. The subsequent annealing in oxygen at 750C was necessary to stabilize the oxygen content of the films. XRD patterns of the films were pseudocubic with an apparent lattice parameter linear in the average ionic radius of R and M. Evolution of RMMnO films morphology with an increase of the film thickness was studied. The formation of hillocky surface began at the thickness > 2000. The electrical properties of LaCaMnO and LaSrMnO correlate with those reported in literature for bulk and thin film materials. A substitution of Pr for LaCrMnO thin films reduces maximum resistivity temperature Ip nonlinearly. LaCaMnO thin films reveal shift of T downward in the case of substrate materials with positive lattice mismatch with the films. LaPrCaMnO demonstrated very complex temperature dependence of resistivity described using a conceptual T-H phase diagram of RMMnO. Amazing GMR effect was observed for LaPrCaMnO below 21 K (10) and about 70 K even in the field 1 T. The irreversible resistivity variation during cooling and heating found in  for PrCaMnO at H = 2.5 T appeared in LaPrCaMnO in zero field.
CHARACTERIZATION OF OFF-AXIS SPUTTERED LaCaMnO FILMS, Phillip R. Broussard, Naval Research Laboratory, Materials Science Div, Washington, DC; Syed Qadri, Naval Research Laboratory, Condensed Matter & Radiation Science Div, Washington, DC; Victor C. Cestone, Naval Research Laboratory, Materials Science Div, Washington, DC.
Thin films of the colossal magnetoresistive material LaCaMnO (LCMO) have been grown by off-axis sputtering from a single target of the LCMO material onto various substrates and studied by electrical transport, x-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). XRD in the plane of the samples shows them to be epitaxial. We find magnetoresistances at 6 T of 70%, and we observe a scaling between the temperature of the resistivity peak (T) and the relative change in resistivity between room temperature and the resistance peak for T varying from 90 to 220 K. This scaling is consistent with an activated resistivity above the peak, with an activation temperature of 942 K. XPS, in conjunction with SEM, indicates that after annealing LCMO films grown on SrTiO in oxygen, there is a reaction between the substrate and the film, resulting in Sr and Ti being introduced into the film, as well as a reduction in the Mn signal level. There is however no change in the Mn-O valence band spectrum. Subsequent anneals in UHV dramatically increase the Ca signal level. Films of LCMO grown on thin (50 Å) YBaCuO layers still exhibit a resistance peak, but do not obey the scaling behavior seen for pure LCMO films.
MAGNETORESISTIVE La-Sr-Mn-O POWDERS AND FILMS BY DAAS TECHNIQUE, S. Yang, Chhiu-Tsu Lin, Northern Illinois Univ, Dept of Chemistry, DeKalb, IL; Krzysztof Rogacki, Bogdan M. Dabrowski, Northern Illinois Univ, Dept of Physics, DeKalb, IL; P. M. Adams, Donna Maria Speckman, Aerospace Corp, Los Angeles, CA.
Magnetic perovskites of the general form LaSr have successfully been synthesized using deposition by aqueous acetate solution (DAAS). This novel technique, which involves the preparation of appropriate metal-acetate precursor mixtures, followed by the subsequent firing of these mixtures for relatively short periods of time, was found to produce crystalline lanthanum-strontium-manganate (LSMO) materials as determined by x-ray diffraction (XRD), fourier transform infrared (FTIR) spectroscopy, and magnetic field dependent resistivity measurements. Both LSMO powders and LSMO thin films on SrTiO, sapphire, and silicon substrates have been prepared using this technique. Unlike conventional solid state routes for preparing these manganates, the DAAS process can produce crystalline LSMO under very mild conditions, with firing temperatures as low as 600C and firing times as low as 100 minutes; however, higher firing temperatures appear to produce the best quality materials. For manganates with a nominal composition of LaSrMnO, varying the firing temperature from 600C to 900C to 1200C (while maintaining a constant firing time of 100 minutes) results in an increase in product crystallite size (from to ), a sharpening of the metallic transition temperature, a decrease in the product resistivity measured at 350 K (by an order of magnitude for each temperature studied), and a change in the XRD 2 peaks consistent with a material phase transition. Lattice parameters refined for the 1200 -fired LSMO samples were comparable to those of materials prepared by much harsher, conventional methods. Lattice parameters, magnetic dependent resistivities, metallic and ferrometallic transition temperatures, and magnetization values will be given for a variety of LSMO powders and thin films prepared using DAAS. Comparisons of these materials with LSMO perovskites prepared by other methods will be presented. Details of the DAAS process and LSMO growth chemistry will be presented, and the advantages of using the DAAS technique over other solid-state and wet-chemical methods will be discussed.
GIANT FERROMAGNETIC HALL COEFFICIENT IN LaCaCoO EPITAXIAL FILMS, Arkadii V. Samoilov, Nai-Chang Yeh, California Inst of Technology, Dept of Physics, Pasadena, CA; Richard P. Vasquez, Jet Propulsion Laboratory, Pasadena, CA.
We report a giant ferromagnetic Hall coefficient, , in a perovskite system, LaCaCoO epitaxial films. The value of significantly exceeds that of other known single-phase ferromagnetic materials. Unlike in weakly magnetic metals, the Hall resistivity in ferromagnets is proportional to the magnetization M rather than to the magnetic induction B, with the coefficient of proportionality called ferromagnetic, or anomalous, Hall coefficient . Although existing theory can correctly describe the temperature dependence of the anomalous Hall coefficient both in the ferromagnetic and paramagnetic states, it fails to account for the absolute value by seven orders of magnitude. We suggest that the coexistence of high- and low-spin configurations in the perovskite cobaltites, which gives rise to the magnetic percolation behavioR, may be responsible for the giant.
MAGNETORESISTIVE PROPERTIES OF RSrMnO (R=La, Nd, Sm) THIN FILMS FABRICATED BY PULSED-LASER DEPOSITION METHOD, Yoshinori Konishi, Masahiro Kasai, Hideki Kuwahara, Yasuhide Tomioka, Yoshinori Tokura, Joint Res Ctr for Atom Technology, Ibaraki, JAPAN.
We prepared RSrMnO (R=La, Nd, = Sm) thin films by pulsed-laser deposition (PLD) method and investigated their magnetotransport properties. For the fabrication of films, oxygen gas of 200mTorr was introduced into a growth chamber during the deposition, and SrTiO(100) substrates were heated at 700C. For a LaSrMnO film, the residual resistivity is as low as 1-210cm and the Curie temperature (T) reaches 370K. These values are comparable to those of single crystals. We also fabricated R=Nd and Sm films with a fixed hole concentration x=0.4. For the NdSrMnO film the T was lowered to about 200K, while the R=Sm film (x=0.4) is insulating over all temperature ranges. These results indicate that the conduction electron bandwidth is narrowed by substitution of La with smaller ionic-radius cations. Filling dependence of magnetotransport was investigated in SmSrMnO (x=0.40.6) films. For x=0.4 and 0.6 the resistivity remains insulating even in a magnetic field up to 7T. On the contrary, the films x=0.450.55 showed the colossal magnetoresistance (CMR) effect accompanying a hysteretic change in resistivity exceeding five orders of magnitude as well as a metamagnetic behavior in the magnetization. The origin of the CMR observed x0.5 in the thin films with narrowed bandwidth is likely to be a collapse of the charge-ordered state under an external magnetic field, as observed for the corresponding bulk crystals. This work, partly supported by NEDO, was performed in the JRCAT under the joint research agreement between the NAIR and the ATP.
THE ROLE OF EPITAXIAL THIN FILMS IN SPIN-POLARIZED NONEQUILIBRIUM SUPERCONDUCTING DEVICES, Jeff M. Byers, Naval Research Laboratory, Dept of Matls Physics, Washington, DC.
The recent discovery of spin-polarized nonequilibrium superconductivity in high superconductors strongly depends on the epitaxial growth of thin films. Quasiparticles injected into a superconducting film can either thermalize and suppress the order parameter or recombine into pairs and emit a phonon. The behavior of any device is dependent upon whether the emitted phonon leaves the material via the substrate or breaks a pair in the condensate of the superconductor. The so-called ``phonon bottleneck'' effect occurs when pair-breaking phonons are trapped in the film. A phenomenological discussion is presented on the importance of epitaxially-matched materials in the control of heating effects and reduction of the ``phonon bottleneck'' effect in spin-polarized nonequilibrium superconductivity. A comparison is made to experiments with both epitaxial thin films and non-epitaxial metallic films. The Use of the CMR manganites as spin-polarized current injectors into high superconductors will also be discussed.
LASER FLUENCE DEPENDENCE OF COMPOSITION VARIATION IN La-Ca-Mn-O FILMS GROWN BY PULSED LASER DEPOSITION, Wei Zhang, Univ College London, Dept of Electronics & Electonic Engr, London, UNITED KINGDOM; Ian W. Boyd, Univ College London, Dept of E&EE, London, UNITED KINGDOM.
Recent studies have shown that hole-doped manganese oxides exhibit a complex magnetic phase, which depends crucially on the ratio of the Mn/Mn in affecting by the substitution of trivalent ions with divalent ions and oxygen stoichiometry. It is becoming clear that the stoichiometry of each element in these doped perovskite-type manganese oxides plays an important role in deciding the nature of the ferromagnetic state and the electronic transport.
In this paper the composition of pulsed-laser-deposited (PLD) La-Ca-Mn-O films was investigated as a function of laser fluence by electron probe microanalysis (EPMA). The constituents and kinetic energy distributions of ions in plume induced by PLD are also examined using energy-dispersive mass spectrometry (ED-MS). It is found that the composition of the films is strongly dependent on laser fluence. The films are depleted of calcium at low laser fluence used, while a stoichiometry film can be obtained at a higher laser fluence. A mechanism in association with the preferential evaporation occurred in the target induced by laser ablation and a preferential resputter in the deposited film driven by high energy ions irradiation is proposed to explain the observed phenomenon.
SESSION L8: POSTER SESSION:
CMR AND MAGNETIC OXIDE FILMS
Chairs: Christopher M. Foster and J. S. Speck
Tuesday Evening, April 1, 1997
DOMAIN STRUCTURE CONTROL OF EPITAXIAL SrRuO THIN FILMS ON VICINAL (100) SrTiO SUBSTRATES, Qing Gan, Rajesh A. Rao, Chang-Beom Eom, Duke Univ, Dept of ME&MS, Durham, NC.
The effect of substrate miscut on the domain structure and growth of epitaxial SrRuO thin films has been investigated. The thin films were deposited on vicinal (100) SrTiO substrates with varying miscut angle and miscut direction. X-ray diffraction analysis indicated that the films grown on vicinal substrates display a significant improvement in crystalline quality and epitaxial alignment as compared to the films deposited on exact (100) SrTiO substrates. Single domain (100) SrRuO thin films (miller indices based on perovskite structure) were obtained on substrates with miscut angle, 1.9 and miscut direction < 13 from the in-plane  axis. Decreasing the miscut angle and changing the miscut direction to close to the  axis on the substrate resulted in increasing the amount of 90 misoriented grains in the films. The mechanisms for the variation in domain structure with miscut angle and direction will be discussed.
SURFACE STRUCTURE AND MORPHOLOGY OF Mg-SEGREGATED, EPITAXIAL FeO THIN FILMS ON MgO(001), J. F. Anderson, Tulane Univ, Dept of Physics, New Orleans, LA; U. Diebold, Tulane Univ, Dept of Physics, New Orleans, LA; K. A. Shaw, Eric Lochner, D. M. Lind, Florida State Univ, Dept of Physics, Tallahassee, FL.
The thermal stability of epitaxially grown thin films on single-crystalline is of great importance in technological applications. We have investigated the structural and compositional changes that are induced by the segregation of substrate Mg to the surface of 1m-thick FeO films on MgO(001). The thin films have been grown with plasma-assisted MBE, and characterization with RHEED (reflection high-energy electron diffraction), x-ray diffraction, and Superconducting Quantum Interference Device (SQUID) magnetometry show slightly strained, single-crystalline FeO films. For the surface studies, we have combined X-ray Photoelectron Spectroscopy (XPS), Low-Energy Electron Diffraction (LEED), and Scanning Tunneling Spectroscopy (STM). The surfaces of the MBE-grown samples are flat and show a reconstruction with respect to the FeO surface unit cell. We observe onset of Mg segregation to the surface occurs around C, with long, narrow extensions of terraces being observed growing along the  and  directions. Upon prolonged heating at C, massive Mg segregation to the surface is observed. Heating in an oxygen atmosphere induces a 1x4 surface reconstruction, and extremely long (), wide terraces. We attribute this annealing stage to the formation of an MgFeO surface phase, exhibiting highly anisotropic surface diffusion and step formation energy.
MSSBAUER STUDY OF NON-STOICHIOMETRIC MAGNETITE LAYERS GROWN ON MgO(100), Lambertus Niesen, Univ of Groningen, Dept of Nuclear Solid State Physics, Groningen, NETHERLANDS; Frans Christiaan Voogt, Tjipke Hibma, Univ of Groningen, Dept of Chemical Physics, Groningen, NETHERLANDS; Peter J.M. Smulders, Gerben H. Wijnja, Univ of Groningen, Dept of Nuclear Solid State Physics, Groningen, NETHERLANDS.
Epitaxial layers of FeO (0<<1/3) were grown on MgO(100) substrates at 250C by means of NO assisted MBE. The layers were 150 ML thick and consisted of 100 ML enriched in Fe, sandwiched between 25 ML iron oxide layers WIth natural abundance. RHEED oscillations were observed during growth, demonstrating that the growth proceeded layer by layer. Conversion electron Mssbauer spectroscopy (CEMS) was used to measure the composition and the magnetic properties of the "bulk" of the layers. The spectra showed that the whole range of stoichiometries could be covered by adjusting the NO-flow at the sample position. They further revealed that the direction of local magnetization is random or along <111> for pure magnetite, but the spins turn gradually to the plane of the layer for increasing . Surprisingly, the Fe spins turn largely out of plane at the moment that pure magnetite ( FeO) is formed. Channeling studies have shown that the layers are pseudomorphic, i.e., in the plane of the layer they adopt the lattice constant of the underlying substrate.
STRUCTURAL AND ELECTRICAL PROPERTIES OF EPITAXIAL FeO/MgO SUPERLATTICES, C. L. Chang, Tamkang Univ, Dept of Physics, Tamsui, TAIWAN; G. Chern, Y. R. Chean, Chung-Cheng Univ, Dept of Physics, Chia-Yi, TAIWAN.
A series of epitaxial FeO/MgO superlattices have been grown by oxygen plasma assisted molecular beam epitaxy where the MgO thickness is held to 20 while the FeO thickness is varied from 10 to 80 . Modulation coherency and sharp interface of the superlattices are characterized by reflection high energy electron diffraction (RHEED) and x-ray diffraction (XRD). Different growth modes of the constituents, in which MgO proceeds with a step propagation and FeO a unit-cell-by-unit-cell mode, are revealed by RHEED intensity oscillation study. The interlayer lattice mismatch is found to be proportional to the modulation wavelength and that FeO relaxes within the first unit cell along the perpendicular direction. Electrical conductivity is characterized by a series of curves taken in various temperatures. These measurements show strong nonlinearity and anisotropy which are correlated to the modulated structure.
EPITAXIAL CaRuO - THIN FILMS PREPARED BY BAND FLASH EVAPORATION MOCVD, Alexander Molodyk, Moscow State Univ, Dept of Inorganic Chemistry, Moscow, RUSSIA; M. A. Novozhilov, Moscow State Univ, High School of Matls Science, Moscow, RUSSIA; Oleg Yu Gorbenko, A. R. Kaul, Moscow State Univ, Dept of Chemistry, Moscow, RUSSIA.
Perovskites possessing metallic conductivity in wide temperature range are promising materials for numerous electronic applications in heterostructures with superconducting and ferroelectric layers. CaRuO thin films and heterostructures containing CaRuO layers were grown by single source band flash evaporation MOCVD technique on various substrates. With band flash evaporation MOCVD technique used complex multilayer multicomponent oxide heterostructures could be easily grown continuously .
XRD studies of CaRuO films grown on (1102) AlO (001) MgO and (001) YSZ revealed two types of reflections for perovskite material: (110) and (110) (in pseudocubic indices). For CaRuO on YSZ, two types of texture were found by -scans. High quality epitaxial (110) CaRuO films were obtained on coherent (110) SrTiO substrates. To obtain epitaxial CaRuOfilms on noncoherent substrates, appropriate buffer layers improving structural match were used: PrO and CeO on YSZ and CeO on AlO. Epitaxial heterostructures (001) CaRuO / (110) PrO / (001) YSZ, (001) CaRuO / (001)CeO / (001) YSZ, (001) CaRuO / (001) CeO / (1102) AlO were grown successfully, each in continuous deposition run. Layered structures obtained were studies by SNMS depth profiling.
CaRuO films demonstrated metal-type temperature dependence of resistivity in 77-1000 K range. Layers were consequently deposited on CaRuO material prepared. Ferroelectric properties of PbTiO (tan() and ) were measured.
REACTIVE GROWTH OF EPITAXIAL Fe-Mg-O SPINEL FILMS ON (100) MgO, Stephan Senz, A. Graff, M. Zimnol, Max-Planck-Inst, Dept of Microstructure Physics, Halle, GERMANY; H. Sieber, Max-Planck-Inst, Microstructure Physics, Halle/Saale, ; C. Teichert, S. K. De, H. P. Oepen, D. Hesse, Max-Planck-Inst, Dept of Microstructure Physics, Halle/Saale, GERMANY; J. Kirschner, Max-Planck-Inst, Dept of Microstructure, Halle/Saale, GERMANY; U. M. Gosele, Max-Planck-Inst, Dept of Microstructure Physics, Halle/Saale, GERMANY.
Epitaxial MgFeO thin films were prepared by solid state reactions between MgO(100) substrates and thin films or vapours of iron oxide. Iron or iron oxide was deposited by electron beam evaporation in an oxygen background pressure onto a heated MgO crystal. The formation of MgFeO proceeds via cation counterdiffusion in the fixed oxygen sublattice. Depending on substrate temperature, oxygen partial pressure and target material, epitaxial spinel films of different composition and magnetic properties were obtained. Crystal structure, composition and sub-micron morphology were investigated by XRD and TEM (SAED, EDX, EELS). Magnetic hysteresis loops were recorded at room temperature using the magnetooptical Kerr effect.
Thickness interference fringes observed by XRD confirm the growth of very flat films. The main feature found in TEM plane view samples is a network of cation antiphase boundaries. EDX linescans show a single phase MgFeO spinel for a substrate temperature of 1123 K, without changes of composition with depth. At substrate temperatures higher than 1073 K Fe is dissolved in the MgO substrate, forming FeMgO.
Ti- AND Ca- SUBSTITUTION IN SrRuO THIN FILMS BY SEQUENTIAL DEPOSITION PROCESS AND ITS APPLICATIONS IN JOSEPHSON JUNCTIONS AND SPIN VALVES, Laurent Mieville, Louis Antognazza, Stanford Univ, E L Ginzton Lab, Stanford, CA ; Kookrin Char, Conductus Inc, Sunnyvale, CA; Theodore H. Geballe, Stanford Univ, Dept of Applied Physics, Stanford, CA.
We report on the growth and properties of (Sr)RuO and Sr(Ru)O thin films obtained by pulse laser deposition. A sequential deposition of submonolayers from SrTiO and CaRuO end members has been successfully used to substitute Ca- and Ti- in the SrRuO perovskite structure. Magnetization measurements as well as transport properties exhibit a very different behavior for each type of substitutions. In the case of Ca-substituted samples, the resistivity remains metallic and is consistent with the expected behavior for intermediate compositions. A progressive reduction of the Curie temperature with increasing doping is reported. In the case of Ti-substituted samples, we observe a much stronger reduction of the Curie temperature and remnant magnetization even for small Ti substitution. Resistivity as a function of temperature shows a crossover from metallic to semiconducting behavior with variable range hopping process for high level of Ti doping and points out the clear differences between the two substitutions sites. In both cases, the observed reduction of the magnetization with increasing doping concentration can be well described by assuming a random distribution of substituted sites in the perovskite structure. Applications of this deposition technique to barriers materials with variable magnetic properties in step-edge Josephson junctions and to spin valves based on substituted SrRuO multilayers devices will also be reviewed.
SESSION L9: POSTER SESSION:
EPITAXIAL OXIDE FILM GROWTH
Chairs: Christopher M. Foster and J. S. Speck
Tuesday Evening, April 1, 1997
ORIGIN OF THE (110) ORIENTATION OF YO AND CeO FILMS GROWN ON (100) SILICON, Robert L. Goettler, Darrell G. Schlom, Pennsylvania State Univ, Dept of MS&E, University Park, PA.
A perplexing issue in the growth of epitaxial oxide films on (100) silicon is the observed (110) orientation of Y (bixbyite structure) and CeO (fluorite structure) despite the (100) orientation having a lower lattice mismatch. On the other hand, YSZ, also with a fluorite structure, grows with a (100) orientation despite a worse lattice mismatch than Y and CeO. This may be explained by supposing that a metal silicide layer forms initially during growth, before the oxidizing ambient is introduced. Calculation of the ensuing lattice mismatch between the oxide and silicide layers and multiplicity () of the near coincident-site lattice of the oxide lattice shows that the (110) orientation is better matched than (100) for Y (2.3 x -2.4, = 1, vs. -2.41 x 3.45, = 4) and CeO (2.34 x -3.62, = 5, vs. 2.27 x 2.27, = 5) and that for YSZ (100) is better matched than (110) (-0.82 x -1.632, = 1, vs. -1.1 x 4.82, = 3). In each case the in-plane rotation yielding the lowest mismatch with the silicide layer maintains the in-plane orientation observed between the oxide film and substrate. Furthermore, the commonly observed rotational twinning in the oxide film can be accounted for by the expected orthogonal domain multipositioning in both the silicide and oxide layers.
FEATURES OF THE MBE GROWTH OF THE CERIUM DIOXIDE THIN FILMS ON SILICON, Oleg V. Smolsky, Dmitry V. Denisov, Nelly F. Kartenko , Vladimir V. Mamutin, A.F. Ioffe Phys-Technical Inst, Quantum-size Heterostructures Lab, St Petersburg, RUSSIA.
Cerium dioxide (CeO2) has prospective applications, primarily as a buffer layer between YBCO high-temperature superconductor film and silicon . In this paper we report the results of an investigation of the epitaxial growth of CeO2 films on Si by MBE technique. In situ x-ray photoelectron spectroscopy (XPS) study of CeO2 growth in its initial stages allowed us to investigate a formation of the Si/CeO2 interface in wide range of the growth temperature. It has been observed that when the CeO2 is deposited on Si heated above 500 C, the CeO2 molecules are completely decomposed into Ce2O3 along with the formation of an incompletely oxidized layer of silicon near the interface . The formation of such an amorphous interlayer during deposition is the main cause of the disruption of the oriented CeO2 growth. Taking these results into account, we modified the growth process with the aim to obtain Si/CeO2 structures with an abrupt interface. The first monolayers (1-2)nm were deposited at room temperature, then the subsequent growth of CeO2 film was done in a flow of oxygen at a higher substrate temperature. The structures grown by such two-step method have been studied by using the x-ray structural phase analysis. The obtained results enable us the conclusion that this method is preferable for producing the high quality CeO2 thin films on Si substrates.
EPITAXIAL CeO ON SILICON SUBSTRATES AND THE POTENTIAL OF Si/CeO/Si FOR SOI STRUCTURES, A. Morshed, S. X. Liu, R. T. Leonard, F. G. McIntosh, North Carolina State Univ, Dept of E&CE, Raleigh, NC; N. A. El-Masry, S. M. Bedair, North Carolina State Univ, Dept of MS&E, Raleigh, NC.
CeO is nearly lattice matched to Si and has the CaF cubic structure; thus it offers the potential for the epitaxial growth of insulating film on Si. Laser ablation of CeO target in an ultrahigh vacuum system was used for the deposition of single crystal CeO. The effect of post growth thermal and rapid thermal annealing in O, N, and Ar atmosphere were found to have pronounced effects on the electrical properties measured by C-V and the optical properties measured by photoluminescence. Violet and blue emission from the heat treated CeO films were obtained. We will also report on our initial results for the growth of epitaxial Si on the deposited CeO using low pressure CVD. Both RHEED and TEM showed that single crystal epitaxial Si was deposited on CeO2. The details of the Si deposition on CeO films and the potential of Si/CeO/Si for SOI structure will be discussed.
EPITAXIAL GROWTH OF YO FILMS ON Si(100) BY IONIZED CLUSTER BEAM DEPOSITION, Mann Ho Cho, Chung Nam Whang, Sang Woo Whangbo, Yonsei Univ, Dept of Physics, Seoul, SOUTH KOREA; San Bom Kang, Moon Yong Lee, Samsung Electronics Co Ltd, Suwon, SOUTH KOREA; Sung Chang Choi, Korea Inst of Machinery & Metals, Ceramics Div, Seoul, SOUTH KOREA; Sang-In Lee, Samsung Electronics Co Ltd, Semiconductor R&D Center, Suwon, SOUTH KOREA.
In this study, the Y films on p-Si(100) have been fabricated by UHV reactive ionized cluster beam deposition (r-ICBD) system. The crystallinity of the films was investigated by glancing x-ray diffraction (GXRD) and in-situ reflection of high energy electron diffraction (RHEED) analyses. The results show that the preferentially oriented crystallinity of the films was increased with increasing acceleration voltage and substrate temperature. Especially, under the condition of the substrate temperature 700C and the acceleration voltage 5 kV, the Y films are found to be grown epitaxially in direction of Y(110)//Si(100). The characteristics of Al/Y/Si MIS structure with varying acceleration voltage and substrate temperature were obtained by C-V and I-V measurements. The breakdown field strength of the epitaxially growing films increases up to 2 MV/cm without any interface silicon oxide layer and the dielectric constant is found to be = 15.6. These results have demonstrated the potential of using yttrium oxide for future VLSI/ULSI gate insulator applications.
HIGH CURRENT DENSITY ITO/NiO THIN FILM DIODES, Wen Y. Lee, IBM Almaden Research Center, San Jose, CA; D. Mauri, C. Hwang, IBM, Storage System Div, San Jose, CA.
High current density diodes were fabricated from junctions between p type NiO and n-type ITO thin films, sputter-deposited at ambient temperature from a Ni and (In target, respectively, in an Ar and O gas mixture. Both unpatterned and lithographically patterned diodes ranging from 0.2 to 4x10 in size were prepared on glass or N58 (Al-TiC composite) substrates up to 5'' in diameter. For the diodes fabricated from 250 NiO and 1200 ITO thin films with a resistivity of 0.5 and20 cm, respectively, a turn-on voltage of 0.3-0.5 V and a current density exceeding 10 /cm at an applied voltage of 1 V were observed. The current-voltage characteristics of these diodes degrade at higher deposition temperatures and with higher residual water content in the film. These results are correlated to the carrier concentration and mobility obtained from Hall effect and resistivity measurements of single layer NiO and ITO thin films, and can be interpreted based on an ideal p-n junction diode with a high density of injected carriers.
ROLE OF Ag IN THE EPITAXIAL GROWTH OF OXIDE THIN FILMS, C. B. Lee, North Carolina A&T State Univ, Dept of Electrical Engr, Greensboro, NC; Derrick Mathis, Donovan Moxey, North Carolina A&T State Univ, Dept of Elect Engr, Greensboro, NC; D. Kumar, Rajiv K. Singh, Univ of Florida, Dept of MS&E, Gainesville, FL.
The interest in epitaxial oxide thin films continues to flourish due to their significant potential in electronic and optical device applications. The current technology favors the in-situ growth of oxide thin films because of multiple advantages of in-situ growth over ex-situ growth. In the present paper we would be reporting the growth (in-situ) and characterization of of high temperature superconducting YBa, colossal magnetoresistive La, and phosphor Y2O3 thin films. The films were grown using pulsed laser deposition (PLD) techniques and characterized as per the requirement using x-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, four probe, Squid magnetometer and optical emission measurements. The focus of our work is on the realization of significant improvement in microstructural and physical properties of these films by the addition of a common material (silver) to the films during their in-situ formation. The mechanisms by which silver improves the quality of these films would be discussed.
PULSED LASER DEPOSITON FOR COMBINATORIAL SYNTHESIS OF NOVEL METAL OXIDE THIN FILMS, Chen Gao, Lawrence Berkeley National Laboratory, Inst of Molecular Design, Berkeley, CA; Kai-An Wang, Lawrence Berkeley National Laboratory, Molecular Design Institute, Berkeley, CA; Ichiro Takeuchi, Lawrence Berkeley National Laboratory, Materials Science Division, Berkeley, CA; Hauyee Chang, Lawrence Berkeley National Laboratory, Molecular Design Inst, Berkeley, CA; Jingsong Wang, Lawrence Berkeley National Laboratory, Berkeley, CA; X.-D. Xiang, Lawrence Berkeley National Laboratory, Inst of Molecular Design, Berkeley, CA; Peter G. Schultz, Univ of California-Berkeley, Dept of Chemistry, Berkeley, CA.
In order to address the materials issues of increasingly complex functional metal oxides, we have developed a high-vacuum pulsed laser deposition system for combinatorial synthesis of thin films. Combinatorial libraries can be used to effectively screen out multi elemental compounds with different compositions for various physical properties. Using automated in-situ metal shutters and a target carousel control, up to 10 sites of different compositions can be fabricated on substrates of 2" x 2" area. At each site, a multilayer combination of precursor materials are deposited in vacuum at up to 300 C. Subsequently, the libraries are thermally processed ex-situ to induce controlled crystal growth of the materials. Various techniques are used to measure the properties of samples in the libraries. We have fabricated libraries of dielectric and ferroelectric metal oxides. Issues involving materials designs and characterization of the libraries will be discussed.
SESSION L10: POSTER SESSION:
SUBSTRATES FOR OXIDE EPITAXY
Chairs: Christopher M. Foster and J. S. Speck
Tuesday Evening, April 1, 1997
GROWTH OF ZnO THIN FILMS ON SAPPHIRE SUBSTRATES BY ECR-ASSISTED MBE, Hee-Bog Kang, Kiyoshi Nakamura, Kazuo Ishikawa, Tohoku Univ, Dept of Elect Communications, Sendai, JAPAN.
High quality single crystalline ZnO films arc required for high frequency piezoelectric resonators, surface acoustic devices, and acousto-optic devices. This work was performed to obtain appropriate process conditions to get such high quality ZnO films on sapphire substrates by molecular beam epitaxy (MBE). In this method Zn provided by a knudsen cell reacts with oxygen activated in an electron cyclotron resonance (ECR) source on the surface of the substrate. The films were deposited on c plane sapphire substrates at oxygen pressures of 0.5 5 x 10 Torr and at temperatures between 200 and 400C.
The structural properties, such as crystallinity, microstructure, and surface roughness of ZnO films, were investigated as a function of growth temperature and molecular beam flux rates using x-ray diffractometry, reflection high energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The full width at half-maximum (FWHM) of the x-ray rocking curves of the (002) peak was as narrow as 0.58 for a 0.8 m-thick ZnO film grown at 350C.
INTRINSIC STRESS IN FILMS GROWN BY PULSED LASER DEPOSITION, Ed Hachfeld, E. J. Tarsa, F. Quinlan, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA; J. S. Speck, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA.
The lattice parameter of homoepitaxial SrTiO films grown by pulsed laser deposition was found to depend sensitively on the ambient oxygen pressure during growth. By increasing the oxygen ambient from 0.01 to 150 mTorr, the lattice parameter of the film varied from 3.935 to 3.917 . The films were found to be fully coherent with the substrate (no misfit dislocations) and in a state of biaxial compression with film stresses ranging from 2 to 0.2 GPa. A model is proposed in which the intrinsic stress in homoepitaxial SrTiO films results from continuous shallow implantation of the energetic species produces during the ablation process. The observed variation in stress with oxygen pressure during growth is found to be consistent with scattering and thermalization of the ablated species with the ambient gas molecules. The implications of intrinsic implantation in electronic materials grown by pulsed laser deposition is discussed.
EFFECT OF ZnO BUFFER LAYER ON THE GROWTH OF GALLIUM NITRIDE THIN FILMS, R. F. Xiao, Hong Kong Univ Sci & Tech, Dept of Physics, Kowloon, HONG KONG; X. W. Sun, Hoi Sing Kwok, Hong Kong Univ Sci & Tech, Dept of Electronic & Electrical Engr, Kowloon, HONG KONG.
Crystalline gallium nitride (GaN) thin films have been grown by a liquid target pulsed laser deposition technique in the presence of ammonia (NH) gas at a deposition temperature as low as 550C - 600C. It was shown by XRD and TEM that high quality single c axis oriented GaN columnar films can be formed on many different kinds of substrates (including fused silica) if a thin (less than 500 ) zinc oxide (ZnO) film was first grown as an alignment (buffer) layer. Without this ZnO layer, the obtained films were found polycrystalline with randomly oriented grains. It was also found by SEM and UV/VIS that the surface morphology and optical transparency of these GaN films could be greatly improved by the ZnO layer.
STRUCTURE AND MORPHOLOGY EVOLUTION OF EPITAXIAL (001)- AND (111)-ORIENTED CeO FILMS, Sissel N. Jacobsen, Ulf Helmersson, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN.
The surface morphology and microstructure of epitaxial CeO thin films grown on Al (102) (R-cut) and on MgO (111) substrates have been investigated by atomic force microscopy and x-ray diffraction (XRD) and compared to CeO films grown on amorphous fused silica (SiO) substrates. The R-cut Al substrate surface has a nearly cubic lattice structure on which the CeO films grow epitaxially with a (100)-orientation. The top surfaces of these films are rough and have a faceted structure where all facets are terminated by (111) planes. In thin films (10 nm) the crystallites are randomly oriented in the plane of the substrate surface and a small amount of (111)-oriented material could be detected by XRD. In the thicker films they are rectangularly shaped, oriented mainly in two directions, orthogonal to each other and only the (100)-orientation could be detected. Grown on MgO (111), the CeO films are epitaxial, but with a (111) orientation. The top surfaces of these films are characterized by fairly flat areas, exposing the (111) surface of the CeO crystal, interrupted by a few grains showing the same facets as observed in the films grown on Al. CeO films grown on amorphous fused silica are clearly multi-oriented with a preferred orientation which shifts from (111)- for thin films (50 nm) to a (001) orientation for thicker films (100) nm. The equivalent change in the surface morphology shows the same tendency as for films grown on MgO and Al, respectively. This implies that as the films grow thicker, competitive growth favors the (100) orientation, giving rise to a faceted top surface rather than the (111)-orientation giving a smooth (111)-surface. The combined results are discussed in view of surface and interfacial energies.
AND HARDNESS OF EPITAXIAL ALUMINA THIN FILMS CONTAINING NANOSIZED ZIRCONIA INCLUSIONS, Frank J. McNally, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA; Frederick F. Lange, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA.
Single crystal homoepitaxial alpha-alumina films containing nanoscale zirconia inclusions were grown on (006) sapphire substrates. These films were prepared from mixtures of aluminum nitrate and zirconium acetate aqueous precursors by spin coating and subsequent heat treatment at 1200C. Powders were also prepared from these precursors by spray pyrolysis and subsequent heat treatment. XRD and TEM studies on both powders and films confirm the expected nonequilibrium phase evolution from amorphous inorganic to gamma-alumina to gamma-alumina + tetragonal zirconia to alpha-alumina + tetragonal zirconia. Cross section TEM of films confirm homoepitaxy and random spatial distribution of spherical inclusions approximately 20nm in diameter. Nanoindention studies show that these inclusions at 2, 5 and 15v% do not significantly affect room temperature plasticity of alumina.
EPITAXIAL GROWTH OF OXIDE THIN FILMS ON (100) METAL SURFACES USING PULSED-LASER DEPOSITION, David P. Norton, Chan Park, Bernd Saffian, John D. Budai, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Amit Goyal, Oak Ridge National Laboratory, Metals & Ceramics Div, Oak Ridge, TN; David K. Christen, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN.
The formation of epitaxial metal/metal oxide heterostructures is potentially important for many applications involving superconducting and ferroelectric oxide materials. However, the interfaces created in these structures often present special challenges including the formation of native oxides, chemical reactivity, and thermal expansion mismatch. In this paper, we will address the epitaxial growth and properties of various oxides on both noble and oxidizing metal surfaces. Specifically, we will describe the epitaxial growth and properties of ceria, yttria-stabilized zirconia, and strontium titanate on (100) Ag, Pd, and Ni surfaces using pulsed-laser deposition. The control of the epitaxial relationships between the oriented oxides films and the underlying noble and oxidizing metal surfaces will be addressed. In addition, the structural properties of the oxides will be discussed in detail. Emphasis will be given to the recent use of these heterostructures in the epitaxial growth of high temperature superconducting films on oriented metal substrates.
EQUILIBRIUM DOMAIN STRUCTURE OF PEROVSKITE FERROELECTRIC EPITAXIAL THIN FILMS, S. Pamir Alpay, Alexander L. Roytburd, Univ of Maryland, Dept of Matls & Nuclear Engr, College Park, MD.
Possible equilibium domain structures due to elastic interactions between the film and the substrate as a result of cubic-tetragonal transformation observed in perovskite epitaxial films are analyzed. The formation of two-and three-domain states is taken into account. A three- dimensional map of misfit strain, tetragonality and temperature illustrating domain stability regions is developed. Barium titanate and PZT films on different substrates are considered as examples. Effect of electrical field on domain structure is examined. Results are compared with previous work by J.S. Speck and group.
SESSION L11: SUBSTRATES FOR OXIDE EPITAXY
Chairs: Jason R. Heffelfinger and Darrell G. Schlom
Wednesday Morning, April 2, 1997
Golden Gate C1
8:30 AM L11.1
THE CHARACTERIZATION AND REACTIVITY OF A DEFECTIVE TiO(110) SURFACE: AN ATOMIC-SCALE STM STUDY, M. Kuhn, J. F. Anderson, Tulane Univ, Dept of Physics, New Orleans, LA; U. Diebold, Tulane Univ, Dept of Physics, New Orleans, LA.
The effect of molecular oxygen on prepared substrates is of considerable interest in oxide thin film deposition processes. The interaction of molecular oxygen, at room temperature, with a defective TiO(110) surface has been studied, in situ, by scanning tunneling microscopy. Oxygen vacancies were created on a clean (1x1) TiO(110) surface by annealing in ultra-high vacuum and subsequently imaged on the atomic scale. The appearance of these defect sites was found to be highly dependent upon the tunneling gap voltage, indicating a highly localized density of states.
These defect sites were stable under ultra-high vacuum conditions. The ease with which defect sites could be created depended upon sample history. Upon exposure to molecular oxygen up to five Langmuirs, the number of defect sites was reduced and an excess oxygen species could be found on top of the titanium rows. This excess oxygen species possessed considerable mobility in the time frame of the scanning process. The oxygen exposure of the scanning tip was found to have profound effects on the tunneling behavior and consequently the appearance of the resultant images. These observations shed light on an ongoing controversy on whether the scanning tunneling process probes the salient bridging oxygen atoms or the unsaturated in-plane titanium atoms.
8:45 AM L11.2
MgO SUBSTRATE SURFACE PREPARATION FOR THE DEPOSITION OF SUPERCONDUCTING YBCO FILMS, Zihong Lu, Vladimir C. Matijasevic, Todd Kaplan, Chung Huang, Dawei Zhang, Conductus Inc, Sunnyvale, CA.
Due to its low dielectric loss, magnesium oxide (MgO) has been used extensively as a substrate for growth of superconducting YBCO films for microwave applications. However, the MgO substrate surface is notorious for its surface damage and hydroscopic deterioration, which greatly affect the microwave properties of top YBCO films. Using atomic force microscopy and other surface analytical tools, we systematically studied the substrate surface conditions resulting from various preparation methods, including furnace annealing with and without oxygen, in-situ predeposition annealing, phosphoric acid etching, and homoepitaxial MgO layer growth. Physical and structural properties of YBCO films grown on these substrates were studied and the influence of substrate preparation on microwave performance was evaluated.
9:00 AM *L11.3
OXIDE EPITAXIAL LIFT-OFF (OELO), Michael M. Eddy, Robert Hanson, Muralidhar R. Rao, Betty Zuck, Superconductor Technologies Inc, Santa Barbara, CA; J. S. Speck, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; E. J. Tarsa, Univ of California-S Barbara, Dept of Matls, Santa Barbara, CA.
Over the past 10 years there has been increasing emphasis on epitaxial oxide research because of the wide range of possible applications, such as ferroelectric thin films for memory and tuning applications, high dielectric constant films for capacitors, CMR for magnetic recording heads, superconducting thin films for digital and passive microwave devices and nonlinear optic films for wave guides and frequency modulation. However, inability to reproduce the bulk properties in thin film form and incompatibility of high temperature growth with many of the low cost ''application'' substrates has limited applicability of epitaxial oxides. Presented in this paper is an approach for separating the growth substrate from the application substrate similar to that used in GaAs epitaxial lift-off. Thus, the film is grown on the most suitable substrate for epitaxy, thermal expansion match, defect reduction and interdiffusion minimization, using a release layer to allow subsequent separation of the epitaxial film from the substrate.
In this paper the problems associated with selection of an appropriate release layer, multilayer oxide growth, stress minimization-from thermal expansion mismatch and growth stresses-and counterbalancing those stresses with support layers will be discussed. Finally, separation of the epitaxial oxide layer and transfer via a film support to different substrate materials, such as glass and silicon, will be demonstrated. Use of this technique opens up a wide variety of possible applications which have thus far been excluded because of epitaxial growth constraints.
10:00 AM *L11.4
GROWTH AND EVALUATION OF HTS SUBSTRATE MATERIALS, Charles D. Brandle, G. W. Berkstresser, V. J. Fratello, A. J. Valentino, A. J. ven Graitis, Bell Labs, Lucent Technologies, Murray Hill, NJ.
Today, one of the best substrate materials for HTS films is LaAlO. However, the existence of a phase transition at approximately 500C and the resulting twin formation places severe limitations on the operating device. In an effort to identify other possible perovskite substrate materials, we have prepared numerous compounds with the general formula (ABO), were n ranges from 1 to 4, as well as many solid solutions of various types. As a result of this effort, we have identified several compounds that are possible candidate materials.
Of those identified in the above-mentioned work, the most promising material to date is a solid solution of LaAlO and SrAlTaO. This material (LSAT) is cubic and shows no phase transitions as observed in pure LaAlO and other similar perovskite materials. Single crystals capable of producing 2 diameter fabricated wafers have been grown by the Czochralski technique. Epitaxial films of HTS materials, e.g., YBCO that have been deposited on this substrate material have shown excellent morphology and good superconducting properties.
10:30 AM *L11.5
ATOMIC SCALE ENGINEERING OF SrTiO SINGLE CRYSTAL SURFACES AND BICRYSTAL BOUNDARY FOR EPITAXIAL GROWTH OF OXIDE THIN FILMS , Q. D. Jiang, C. W. Chu, Univ of Houston, Houston, TX; J. Zegenhagen, Max-Planck-Inst, Cardona Abteilung, Stuttgart, GERMANY.
We introduce a new annealing procedure to prepare well defined surfaces of SrTiO single crystal and bicrystal substrates. Aneealing the SrTiO (001) substrates in oxygen and then in ultra high vacuum (UHV) produces a uniformly TiO terminated, atomically flat and globally ordered SrTiO (001) surfaces, as revealed by the studies of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution scanning tunneling microscopy (STM). Annealing a slightly off-cut (1.2) SrTiO (001) surfaces leads to a strong faceting effect, a fact that is used to tailor the surface and bicrystal boundary down to an atomic level. For example, there is a possibility to tune the SrTiO (001) surfaces with surface step sizes to be dominated by single, double, or multiple unit-cell height (n0.3905 nm, n=1, 2, 3, ...). Results on the growth of ultra-thin (down to a single unit-cell) GdBCO films, as well as pseudomorphic and spiral-free SmBCO films on SrTiO (001) surfaces, investigated with LEED/AES, and UHV-STM are presented. Possible mechanisms which lead to the enhancement of critical thicknesses for 2-dimensional growth and pseudomorphic growth, the suppression of spiral growth of SmBCO films are discussed.
11:00 AM L11.6
HETEROEPITAXIAL GROWTH OF ZnO FILMS ON SAPPHIRE BY PLD, R. D. Vispute, Vitaly Talyansky, R. P. Sharma, S. Chupoon, M. Downes, T. Venkatesan, Univ of Maryland, Dept of Physics, College Park, MD; Y. X. Li, Lourdes Salamanca-Riba, Univ of Maryland, Dept of Matls & Nuclear Engr, College Park, MD; M. C. Wood, R. T. Lareau, K. A. Jones, U.S. Army Research Laboratory, Fort Monmouth, NJ.
Here we present our recent work on the fabrication of high optical quality ZnO films on sapphire(001) by pulsed laser deposition. The influence of deposition parameters such as the substrate temperature, oxygen pressure, laser fluency and pulse repetition rate on the optical and crystalline quality of ZnO layers has been studied. The films deposited at 750C and oxygen pressure ranging from 0.1 to 20 mTorr were found to be c-axis oriented normal to sapphire (001). The -rocking curve FWHM of the ZnO(002) peak was 0.2. The XRD- scans and TEM studies revealed that the films were epitaxial with a 30 rotation of the unit cell with respect to the sapphire to achieve a low energy configuration for epitaxial growth. The high degree of epitaxial quality was further confirmed by the RBS ion channeling with a minimum channeling yield was found to be 2. The optical absorption edge measured by UV-visible spectroscopy was sharp at 383 nm. The smooth crystalline PLD ZnO films with sharp interfaces and desirable electrical and optical properties are useful for optoelectronic devices. This work was supported with an MRCP Army Grant, DAAL019523530.
11:15 AM L11.7
IN-SITU CAICISS STUDY OF DYNAMIC PROCESS OF OXYGEN DESORPTION ON TiO-TERMINATED SrTiO(001) SURFACE, Takaharu Nishihara, Osamu Ishiyama, Shigeki Hayashi, Makoto Shinohara, Shimadzu Corporation, Dept of Surface Analysis & Semicond Equip, Kanagawa, JAPAN; Mamoru Yoshimoto, Takeshi Ohnishi, Hideomi Koinuma, Tokyo Inst of Technology, Dept of Matls & Structures, Kanagawa, JAPAN.
It is important to control atomically the terminating surface structure on SrTiO (STO) substrate under the growth condition of high T oxide thin films. In order to examine oxygen desorption process on STO(001) substrate surface at the deposition temperature quantitatively, we employed in-situ coaxial impact collision ion scattering spectroscopy (CAICISS). This method enables us to observe only the topmost layer.
The STO substrate, which was treated by pH-controlled NH-HF solution, was subjected to CAICISS measurements. In-situ CAICISS observations were carried out in ultra high vacuum with 1 x 10 Pa and He ion with 3 keV was used as a probe ion. The time-of-flight (TOF) spectra of STO surface were taken at the temperatures between 150C and 800C.
Both TOF spectra at the incident angle of 35.3 along  and of 45 along  revealed Ti and O peaks and no Sr peak at 150C, which means that the topmost layer at 150C is terminated by TiO-plane, completely. On the other hand, as increasing the substrate temperature, Sr peak began to appear above 400C. This Sr peak intensity drastically increased with elevating the substrate temperature. This indicates that the topmost O atoms desorb from the surface at the higher substrate temperatures. The ratio of Sr/Ti corresponds to the amount of the topmost oxygen vacancy due to the desorption. We found that 40 % of the topmost oxygen atoms desorb from TiO-terminated STO(001) surface at 800°C.
11:30 AM L11.8
THE EFFECT OF SURFACE STRUCTURE ON THE INITIAL STAGES OF OXIDE THIN-FILM GROWTH, Jason R. Heffelfinger, Univ of Minnesota, Dept of Chem Engr & Matls Sci, Minneapolis, MN; Matthew T. Johnson, Univ of Minnesota, Dept of Chemical Engr & Matls Science, Minneapolis, MN; C. Barry Carter, Univ of Minnesota, Dept of Chem Engr & Matls Sci, Minneapolis, MN.
The structure of substrate surfaces can positively or adversely affect the morphology, epitaxy and microstructure of oxide thin films. To illustrate these effects, thin films of CuO, NiO, cubic-ZrO, Y, Fe and Cr were grown by pulsed-laser deposition (PLD) onto ceramic monocrystalline substrates. The initial stages of thin film growth were studied in order to understand the effect that surface steps, surface facets and surface terminations have on film growth. Substrate materials of (0001) and -Al and MgO provided a range of surface microstructures that are representative of ceramic substrates. Substrates are prepared by either polishing and annealing, or cleaving for Al and MgO, respectively. Thin oxide films, ranging in thickness from 10 nm to 100 nm, were deposited on these surfaces. These films were then characterized by atomic-force microscopy and electron microscopy techniques. The surface structure of these monocrystalline substrates can affect thin film growth in different ways. For example, when thin films of cubic ZrO were grown on the (0001) Al surface, alignment between the thin film and the substrate was found to depend on whether growth occurred on the (0001) terraces or on/near surface steps. For growth of a Cu2O film on (0001) Al, the morphology of CuO grains was found to depend on the terrace on which growth occurred. Several other observations of the effects of surface structure on oxide thin-film epitaxy, morphology and microstructure will be discussed.
11:45 AM L11.9
INITIAL STAGES OF EPITAXIAL GROWTH OF METASTABLE ZrO AND WO FILMS ON SAPPHIRE, Robert J. Lad, Scott C. Moulzolf, Oliver D Greenwood, L. Jay LeGore, David J. Frankel, Univ of Maine, LASST, Orono, ME.
We have used ECR oxygen-plasma-assisted molecular beam epitaxy to grow high quality epitaxial films of ZrO and WO on sapphire substrates. To grow the films, the ECR oxygen plasma was operated during e-beam evaporation of Zr or WO species. studies of film structural evolution in the 1-30 mn thickness regime over a range of temperatures were carried out using a differentially pumped RHEED system. We find that an epitaxial metastable cubic phase of ZrO can be stabilized on the sapphire substrate without the addition of intentionally added alloy stabilizers when the film growth rate is below 2 /sec. At higher deposition rates, the equilibrium monoclinic phase is stable. In the WO system, we also achieve growth of an epitaxial metastable phase An epitaxial tetragonal WO phase is stabilized by lattice match to the sapphire substrate, although in some cases, additional monoclinic and/or orthorhombic phases are produced. The initial stages of growth depend on the structure and stoichiometry of the sapphire substrates. We will discuss several experiments in which we used Al buffer layers and various sapphire substrate treatments to influence initial growth stages.
SESSION L12: OXIDE STRUCTURE
Chairs: Tomoyasu Inoue and Masashi Kawasaki
Wednesday Afternoon, April 2, 1997
Golden Gate C1
1:30 PM L12.1
HIGH QUALITY EPITAXIAL VO THIN FILMS: A MATERIAL FOR INFRARED SWITCHING APPLICATIONS, Heiko Schuler, Siegfried Horn, Serguei Grigoriev, Univ Augsburg, Inst fuer Physik, Augsburg, GERMANY.
The compound VO undergoes a metal-insulator- transition (MIT) at T=150K with an optical gap in the insulating phase of 0.7eV. Therefore epitaxial thin films of this compound are of interest for infrared applications. We present a systematic study of the growth conditions of such films utilizing electronic transport and infrared transmission measurements in addition to studies of crystal and micro structure.
Epitaxial VO thin films were grown in reactive oxygen atmosphere on c-axis oriented sapphire substrates by e-beam evaporation. While VO single crystals show the typical MIT in a extremely narrow temperature region of T<1K, epitaxial films show a broadening of the transition width due to defects and stress induced by film-substrate interaction.
As a quality criterion for the films we used the transition width as measured by electrical resistivity, the absolute change of resistivity at the transition and the infrared contrast between the metallic and insulating phase. We found the most important parameter for a good quality films is the growth temperature. Only in a narrow region around 550 the growth of high quality films is observed. The oxygen pressure during evaporation, however, can be varied over a range from 10 to 10 mbar. The introduction of epitaxial CrO buffer layers improves the film quality due to reduced lattice mismatch between film and substrate and a concommitant reduction of stress in the film. Good quality films show a transition width between 5 and 15K, jumps in the electrical resistance up to 5 orders in magnitude and changes in their infrared transmission from 10% to 80%.
1:45 PM L12.2
EPITAXIAL MBE GROWN PbTiO ON (100) LaAlO AND (100) SrTiO, Christopher D. Theis, Darrell G. Schlom, Pennsylvania State Univ, Dept of MS&E, University Park, PA.
Epitaxial PbTiO films have been grown by reactive molecular beam epitaxy (MBE) on (100) LaAlO and (100) SrTiO substrates. Lead is supplied from a conventional effusion cell. Titanium is sublimated from a Ti-Ball, and oxygen is supplied in the form of purified ozone. Atomic layer-by-layer composition control is obtained using real-time atomic absorption spectroscopy (AA) feedback. The titanium flux is controlled with a shutter directly coupled to the titanium AA feedback to deliver a burst corresponding to one monolayer of titanic Similarly, lead is monitored in situ using an atomic absorption signal. An adsorption controlled growth mechanism leads to the use of a lead overpressure to insure films with proper stoichiometries. Film structure is studied during growth using in-situ RHEED, and after growth using SEM. 4-circle x-ray diffraction analysis indicates that films grown on LaAlO are epitaxial and are mixed - and -axis oriented. Films grown on SrTiO are purely -axis oriented and have a FWHM of 0.4 for the 002 reflection.
2:00 PM *L12.3
MAGNETIC STRUCTURE OF TRANSITION-METAL OXIDE SUPERLATTICES, Julie A. Borchers, NIST, Reactor Div 856, Gaithersburg, MD; Ross W. Erwin, NIST, Reactor Div, Gaithersburg, MD; Y. Ijiri, NIST, Gaithersburg, MD; D. M. Lind, Scott D. Berry, Eric Lochner, K. A. Shaw, P. Stoyanov, Florida State Univ, Dept of Physics, Tallahassee, FL; Pieter J. van der Zaag, R. M. Wolf, Philips Research Laboratories, Eindhoven, NETHERLANDS.
In NiO/FeO and CoO/FeO superlattices, the hysteresis loop of the ferrimagnetic FeO is shifted when field cooled through the Neel temperature of the antiferromagnetic constituent. This exchange biasing originates from local exchange interactions at the superlattice interfaces, which are sensitive to the layer thicknesses and growth morphology. To probe the magnetic structures of the individual layers, we have performed neutron diffraction studies on a series of superlattices grown by molecular beam epitaxy. The antiferromagnetic order is long-range despite the contrasting symmetries of the NiO and CoO rock salt and FeO spinel unit cells. The magnetic and structural coherence lengths of the FeO, however, are less than the width of a single bilayer () due to interfacial stacking-faults. The magnetic correlation lengths for the antiferromagnetic and ferrimagnetic layers also differ in the growth plane, suggestive of magnetic frustration. The Neel temperatures for the NiO and CoO are larger than their bulk values of 520 K and 290 K, respectively, and approach T = 858 K for FeO as the relative thickness of the antiferromagnetic layer is decreased. In contrast, as determined by the shift in the hysteresis loop, the biasing temperature of CoO/FeO superlattices decreases as the CoO thickness is reduced. The interlayer exchange coupling also gives rise to a field-induced reorientation of the NiO moments in NiO/FeO superlattices with thin NiO interlayers (), an effect not apparent for a 1 m NiO film.
2:30 PM *L12.4
OXIDE QUANTUM STRUCTURES AND SUPERFUNCTIONS, Masashi Kawasaki, Tokyo Inst of Technology, Ceramics Matls & Structure Lab, Yokohama, JAPAN.
We show our recent development on the epitaxial growth of oxide thin films by Laser MBE toward a possible new field of electrophotonics based on metal-oxide quantum structures. Examples of quantum structures include superlattices and tunnel junctions (2-dimensional), superconducting quantum wire arrays (1-dimensional), and quantum dots for photonic application (0-dimensional). For fabricating such structures, it is vitally important to prepare the substrate surface and to understand the growth dynamics. My talk will be partly devoted to our challenges to such novel devices as high T superconducting Josephson tunnel junction and excitonic UV-laser operating at room temperature making use of self-organized hexagonal quantum dot array of ZnO.
3:30 PM *L12.5
ARTIFICIAL DOPING IN EPITAXIAL SrCuO/Pb(ZrTi)O AND Pb(ZrTi)O/SrRuO HETERO-STRUCTURES USING A FERROELECTRIC FIELD EFFECT, C. H. Ahn, T. Tybell, L. Antognazza, M. Dccroux, Univ of Geneva, Geneva, SWITZERLAND; O. Fischer, J.-M. Triscone, Univ de Geneve, DPMC, Geneve, SWITZERLAND; Robert H. Hammond, Theodore H. Geballe, Malcolm R. Beasley, Stanford Univ, Dept of Applied Physics, Stanford, CA; Kookrin Char, Conductus Inc, Sunnyvale, CA.
In this talk, we will discuss the possibility of using the nonvolatile polarization field present in ferroelectrics to artificially dope thin film complex oxides. In particular, we will report on the ferroelectric field effect in ultrathin layers of SrCuO and SrRuO using SrCuO/Pb(ZrTi)O and Pb(ZrTi)O/SrRuO epitaxial heterostructures. Upon reversing the polarization of the Pb(ZrTi)O ferroelectric layer, we measured a nonvolatile change in the resistivity of 30 Åthick layers of SrCuO and SrRuO. This effect, both reversible and nonvolatile, is attributed to the electric field-induced charge at the interface between SrCuO (SrRuO) and Pb(ZrTi)O. In SrRuO a 9 resistivity change has been observed at room temperature. This change was nonvolatile for a period of several days, and measurements taken between 4.2 K and 300 K suggest n-type conductivity throughout this temperature range. Hall effect measurements also yield n-type conductivity, with 2 x 10 electrons/cm at low temperature, allowing a quantitative understanding of the magnitude of the observed resistivity change.
4:00 PM *L12.6
ALL-MBE GROWTH OF HETEROSTRUCTURES INVOLVING CUPRATES, MANGANITES, AND TITANATES, Ivan Bozovic, Varian Research Ctr, Palo Alto, CA; James N. Eckstein, Varian Associates Inc, Ion Implant Systems, Palo Alto, CA.
We have used atomic layer-by-layer molecular beam epitaxy (ALL-MBE) to synthesize single-crystal thin films of cuprate superconductors, titanates, and manganites, as well as a variety of multilayers and heterostructures. For example, we have grown an artificial (metastable) Bi-1278 compound in which the central Ca layer is doped by Dy. This provides, within a single unit cell, the bottom superconducting electrode, an insulating barrier layer, and the top superconducting electrode. In these way we fabricated the first HTS tunnel (SIS) junctions, which showed very sharp quasi-particle tunneling I-V characteristics. Next, we have grown lateral superlattices with alternating nanowires of LaCaMnO and LaSrMnO, which showed substantial in-plane anisotropy in transport properties, and a fivefold increase in low-field sensitivity. Finally, we will report successful growth of heterostructures and superlattices involving manganites and cuprates.
4:30 PM L12.7
EFFECT OF ELECTRON INCIDENCE IN EPITAXIAL GROWTH OF CeO(110) LAYERS ON Si(100) USING BIAS EVAPORATION, Tomoyasu Inoue, Iwaki-Meisei Univ, Dept of Electronic Engr, Fukushima, JAPAN; Yasuhiro Yamamoto, Hosei Univ, Electronic Informatics, Tokyo, JAPAN; Masataka Satoh, Hosei Univ, Research Ctr of Ion Beam, Tokyo, JAPAN.
Epitaxial growth of CeO(110) layers on Si(100) substrates requires a substrate temperature of 820C. In order to reduce the growth temperature and improve the crystalline quality of the CeO(110)/Si(100) structure, we have verified the effectiveness of bias evaporation, where a bias voltage is applied to substrates during electron beam evaporation. It is expected that bias-voltage-applied substrates attract charged particles included in evaporating materials, which give additional energy for migration needed for the epitaxial growth to atoms adsorbed on the surface. We report successful results of epitaxial temperature reduction using bias evaporation in conjunction with electrons incident onto the surface.
The sample current during bias evaporation was measured to be a few A (cations) under negative bias and a few tenth mA under positive bias (45 is electronic current). Both negative bias of -60 V and positive bias above +30 V proved to be effective for the epitaxial temperature reduction by 5070C. The positive bias is more effective than negative one for the epitaxial temperature reduction, wherein the higher bias leads to the more effect. It is experimentally clarified that species which promote epitaxy is not anions but electrons. Investigation on the effect of assistance by an additional electron beam is now in progress.
4:45 PM L12.8
EPITAXIAL GROWTH OF THIN FILM Si/AlO/Si HETEROSTRUCTURES, Jaroslaw T. Zborowski, GEM Inc, Houston, TX; Rebecca L. Forrest, Terry D. Golding, Univ of Houston, Dept of Physics, Houston, TX; Denes Marton, Univ of Houston, Dept of Chemistry, Houston, TX.
We have performed an investigation into the synthesis of Si/AlO/Si heterostructures grown by gas source molecular beam epitaxy. The study has been motivated by the potential of the aluminum oxide as a barrier material for silicon, enabling Si-based tunneling resonant diode, as well as the possibility for a coherent SOI material with low dislocation density.
The films have been grown on Si (100) and (111) orientations, in a gas source molecular beam epitaxy chamber equipped with silane, nitrous oxide gas sources and solid state aluminum cell. The growth temperature was varied between 700C and 900C. A silicon thin film has been grown above 950C with the source material obtained from cracking of silane gas at the growth surface.
The samples were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, Auger electron spectroscopy and scanning electron microscopy. The films growth rate has been found to be very sensitive to the growth temperature and aluminum flux decreasing rapidly above 700C. The films grown on Si (111) substrate are -AlO, a deficient spinel structure, and are oriented in the (311) direction, which allows the lattice constant matching between surface nets of silicon and aluminum oxide. The small full width at half maximum of the peaks in the x-ray diffraction spectra indicates small mosaic spread and a large coherence lengths confirming good crystalline quality.
5:00 PM L12.9
AFM STUDIES ON THE GROWTH OF EPITAXIAL METALLIC OXIDE SrRuO THIN FILMS ON VICINAL SrTiO SUBSTRATES, Rajesh A. Rao, Qing Gan, Chang-Beom Eom, Duke Univ, Dept of ME&MS, Durham, NC.
The growth mechanism and surface morphology of epitaxial metallic oxide SrRuO thin films deposited on vicinal (100) SrTiO substrates has been studied. Vicinal substrates with miscut angle, , up to 4 from (100) and miscut direction, , up to 42 away from the in-plane (010) direction were used. The film grown on exact (100) SrTiO substrate had a growth mechanism involving two dimensional nucleation and growth. In contrast, the films deposited on vicinal substrates had a step flow growth. These films displayed characteristic step patterns with the average step height and average terrace width on the films being larger than the corresponding values on the substrate. Furthermore, the average step height of the films was found to increase with the miscut angle, . The films deposited on (100) LaAlO substrates showed surface features typical of a three dimensional island type growth. The correlation of growth mechanisms with the crystalline structure and electrical transport properties of the thin films will be discussed.