Molecular beam epitaxial growth of high-Tc Bi-Sr-Ca-Cu-O Films

Molecular beam epitaxy (MBE) has been used to grow high-temperature superconducting Bi-Sr-Ca-Cu-O films with adequate control over growth of number of unit layers. Oxide sources of Sr and Ca used for electron beam evaporation have been found to be useful for epitaxial growth of films. Deposited films show superconducting properties comparable to films deposited by using pure metals with a complicatedin situ oxidation technique. Optimum deposition and annealing conditions have been obtained to growc-axis-oriented 2212 phase BSCCO film.In situ reflection high-energy electron diffraction (RHEED) study of the films has revealed the growth of epitaxial films with atomically smooth surfaces.     

Effect of mechanical deformation and annealing on the evolution of c-axis texture of Bi1.6Pb0.4Sr2Ca2Cu3O z superconductor hot-isostatic-press cladded onto a Ag substrate

A systematic study was undertaken of the mechanical deformation and annealing effects on the c-axis texture evolution of a Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O _z (BSCCO) superconductor cladded onto a Ag substrate. As the amount of cold-rolling reduction (%) increased, a tighter clustering of the (001 l4) poles around the surface normal indicated that randomly oriented grains from the initial hot-isostatic-press (HIP) cladded surface are progressively oriented nearly perpendicularly to the plane of the tape. Conventional X-ray diffraction (XRD), and X-ray pole-figure studies support a basal-plane sliding mechanism of plastic deformation. In samples annealed for 100 h, the superconductor (BSCCO) material near the BSCCO/Ag interface appears to undergo incipient melting, and there is a layer-like growth (c-axis texture), which extends macroscopically from the Ag interface. The sample-orientation distribution of short-term (5 h) annealed samples showed a strong c-axis texture, with the c-axis aligned nearly perpendicularly to the plane of the tape with no preferred alignment of the a- and b-axes.     

Cation off-stoichiometric SrMnO<Subscript>3−δ</Subscript> thin film grown by pulsed laser deposition

The laser energy density (laser fluence) dependency of the Sr/Mn ratio was investigated for SrMnO_3−δ (SMO) thin films grown by pulsed laser deposition (PLD). It was found that the Sr/Mn ratio showed a steep increase followed by a gradual increase as the laser fluence was increased. However, the Sr/Mn ratio always showed Mn-excess under the present laser fluence condition as long as stoichiometric SrMnO₃ targets were used. In order to obtain cation stoichiometric SMO films, it was necessary to use Sr-excess SrMnO₃ targets in addition with laser fluence tuning. The crystal quality of the SMO thin film was found to vary with the Sr/Mn ratio. In stoichiometric or Sr-excess SMO thin films, epitaxial thin films could be obtained, whereas Mn-excess thin films showed very low crystallinity. Sr-excess films were also found to have some extra SrO planes. In addition, they exhibited out-of-plane lattice expansion which electron energy loss spectroscopy analysis revealed was due to Mn vacancies. The variation of film growth was closely related to point defects due to excess cations included in growing thin films.     

Processing and characterization of high temperature superconductor films formed by spin-on coating and rapid thermal annealing

A simple method has been developed to fabricate oxide high temperature superconductor (HTS) films of Y---Ba---Cu---O (YBCO) and Bi---Sr---Ca---Cu---O (BSCCO) compounds. Using spin-on coating and rapid thermal annealing (RTA), high T_c films on MgO and copper substrates have been demonstrated. Of the various solvents employed, we have achieved the best results utilizing nitric acid. The use of this solvent coupled with annealing in oxygen at 960–990 °C for 30–60 s by RTA has produced polycrystalline films with grain sizes of 5–10 μm, as indicated by scanning electron microscopy. Auger electron spectroscopy of the films shows that the stoichiometric ratio has been maintained during the transition from precursor to film. X-ray diffraction patterns of YBCO films show that the films mainly contain 123 phase. Superconductivity with zero resistance temperatures above 77 K was observed from both YBCO and BSCCO films. When copper substrates are used, a buffer layer of silver is coated between the substrate and the film so as to prevent interface reaction. The simple spin-on method combined with the short and intense heating of the RTA process may prove to be a simple and rapid method of manufacturing HTS films for large-scale electrical applications.     

Microstructural investigation of Bi–Sr–Ca–Cu–oxide thick films on alumina substrates

The microstructure of Bi–Sr–Ca–Cu–oxide (BSCCO) thick films on alumina substrates has been characterized using a combination of X-ray diffractometry, scanning electron microscopy, transmission electron microscopy of sections across the film/substrate interface and energy-dispersive X-ray spectrometry. A reaction layer formed between the BSCCO films and the alumina substrates. This chemical interaction is largely responsible for off-stoichiometry of the films and is more significant after partial melting of the films. A new phase with f c c structure, lattice parameter a = 2.45 nm and approximate composition Al₃Sr₂CaBi₂CuO _x has been identified as reaction product between BSCCO and Al₂O₃. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characterization of compositional oscillations in InGaAs films induced by MBE cell configuration and substrate rotation

We examine compositional non-uniformities in InGaAs films grown on InP substrates by molecular beam epitaxy (MBE). Transmission electron microscope (TEM) images of samples cut near the edge of the wafer show periodic bands of contrast typical of a superlattice. Flux variations across the wafer lead to mole fraction oscillations that are dependent on the growth rate and substrate rotation speed. Without careful analysis, this film morphology could be mischaracterized as spontaneous ordering due to strain effects.     

Growth dynamics of GaAs, AlAs and (Al, Ga)As on GaAs (110) and (111)A substrates during molecular beam epitaxy

The growth dynamics of GaAs, AlAs and (Al, Ga)As films grown by molecular beam epitaxy (MBE) on GaAs(110) and (111)A substrates have been studied using reflection high energy electron diffraction (RHEED) intensity oscillations and scanning tunnelling microscopy (STM). In contrast to growth on (001) oriented substrates, the period of the RHEED intensity oscillation does not in general provide a measure of the growth rate. This is explained by the very different surface chemistry involved, since the short lifetime of arsenic molecules (As₂ or As₄) on non-(001) surfaces results in cation-stable surface conditions, which generate arsenic (anion)- induced intensity oscillations, whereas on (001) surfaces they are cationinduced under all normal growth conditions. The effects of this behaviour on surface morphology are illustrated, as are the relative influences of Ga and Al. STM images obtained during the first few monolayers of growth provide a detailed indication of the growth mode and in particular explain in a simple manner the origin of bilayer period RHEED intensity oscillations obtained during growth on GaAs (110).     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.     

Cathodoluminescence microscopy of high T c superconducting YBCO and BSCCO materials

Spectroscopic and imaging cathodoluminescence (CL) microscopy has been used to study high T _c YBCO and BSCCO superconducting thin films and pellets in the scanning electron microscope. The effects of beam parameters, such as voltage, current, and diameter, have been investigated with a view to optimizing the CL signal intensity whilst preventing sample damage. Limiting the CL signal generation volume to within the thin film is important in eliminating any substrate contribution. Areas of strong luminescence have been observed in YBCO and BSCCO pellets as well as BSCCO thin films. At low beam energies, there is some correlation between the CL and secondary electron images. The CL spectra of the strongly luminescent spots differed from those of the poorly luminescencing superconducting phases. CL was able to identify a copper-rich impurity phase in the BSCCO material, and a barium cuprate phase in the YBCO material. It is proposed that the quality of a thin film, with respect to impurities, can be monitored using CL.     

Layer-by-layer growth of thin epitaxial Fe3Si films on GaAs (001)

Molecular beam epitaxy of Fe₃Si films on GaAs (001) is studied in situ by grazing incidence X-ray diffraction. Fe₃Si grows layer-by-layer. During deposition the growth front roughens as indicated by the damping of the X-ray oscillations and corresponding atomic force micrographs. The X-ray oscillations are modified during growth at substrate temperatures of 180 °C and below.     

Substrate rotation effects on β-FeSi2 epitaxial film growth using MBE

This study showed that substrate rotation plays an important role in the growth of high-quality β-FeSi₂ epitaxial film on hydrogen terminated Si (1 1 1) substrate using molecular beam epitaxy (MBE). The present work elucidated the substrate rotation effects on morphology, thickness, and purity. Results verified that substrate rotation is essential to grow thicker epilayers with better morphology and compositional uniformity. In addition, purity analyses indicated that substrate rotation increases the concentration of non-metallic impurities (H, C, and O), but does not further introduce metallic impurities into β-FeSi₂ films.     

Electrochemical processing of high-T c Bi(Pb)-Sr-Ca-CuO thin films

Superconducting thin films of Bi(Pb)-Sr-Ca-CuO system were prepared by depositing the film onto silver substrate by d.c. electrodeposition technique with dimethyl sulphoxide bath in order to examine the effect of Pb addition to the BSCCO system. The films were deposited at the potential of -0.8 V vs saturated calomel electrode (SCE) onto the silver substrate. The different preparative parameters such as deposition potential, deposition time were studied and optimized. These films were then oxidized electro-chemically at room temperature in an alkaline (1 N KOH) solution, and also at 600°C temperature in an oxygen atmosphere. The films showed the superconducting behaviour, with T_c values ranging between 85 K and 96 K, respectively.     

Andreev Bound-State Tunneling and ESR Spectroscopy of High-Temperature Superconductors and Observations of Broken Time-Reversal Symmetry

An emphasis on reliable materials growth and development of new fabrication techniques has allowed us to investigate the electronic structure of high-temperature superconductors by planar quasiparticle tunneling and electron paramagnetic resonance (EPR) spectroscopies. The quasiparticle (QP) density of states (DoS) is investigated by tunneling into oriented thin films of Y₁Ba₂Cu₃O₇ (YBCO) and single crystals of Ba₂Sr₂Ca₁Cu₂O₈ (BSCCO). Data are obtained as a function of crystallographic orientation, temperature, doping, damage, and applied magnetic field. These data demonstrate that the observed zero-bias conductance peak (ZBCP) is composed of Andreev bound states (ABS) which intrinsically form at a symmetry-breaking interface of an unconventional superconductor, for example, a (110)-surface of d-wave YBCO. Tunneling into doped or ion-damaged YBCO provides a measure of the QP scattering rate below T _c. An applied field causes Doppler shift of the ABS, arising from the scalar product between the QP velocity and the superfluid momentum, v _F·P _s, observed as a splitting in the ZBCP. Magnetic hysteresis of the splitting is consistent with the effects of strong vortex pinning near the interface. The directional field dependence shows that the ABS is highly anisotropic in its transport. These results, plus in-plane crystallographic orientational dependence on single-crystal BSCCO, demonstrate the d-wave symmetry of this superconductor. Below 8 K and in zero applied field, the ZBCP splits, indicating a transition into a superconducting state with spontaneously broken time-reversal symmetry (BTRS). EPR experiments are used to detect directly the spontaneous formation of the magnetic moments in the BTRS state.     

Electrostatic bonding of high-Tcsuperconducting films to any substrate

Complete integration of high-T _c thin-film superconducting technology with other electronic technologies requires hybrid structures with both technologies on the same substrate. This is difficult to do with direct growth of the superconductor on certain substrates (GaAs, InP) because of the high temperatures required for formation of the superconductor. A method is proposed to circumvent this problem by using electrostatic forces and appropriate thin-film materials to bond superconducting films to any substrate at 300°C. The same principle can be applied to the bonding of other devices on other substrates.     

Electrostatic bonding of high-T c superconducting films to any substrate

Complete integration of high-T _c thin-film superconducting technology with other electronic technologies requires hybrid structures with both technologies on the same substrate. This is difficult to do with direct growth of the superconductor on certain substrates (GaAs, InP) because of the high temperatures required for formation of the superconductor. A method is proposed to circumvent this problem by using electrostatic forces and appropriate thin-film materials to bond superconducting films to any substrate at 300°C. The same principle can be applied to the bonding of other devices on other substrates.     

Fe3Si nanodots epitaxially grown on Si(111) substrates using ultrathin SiO2 film technique

Ultrahigh density (> 10¹² cm⁻²) Fe₃Si nanodots (NDs) are epitaxially grown on Si(111) substrates by codeposition of Fe and Si on the ultrathin SiO₂ films with ultrahigh density nanovoids. We used two kinds of methods for epitaxial growth: molecular beam epitaxy (MBE) and solid phase epitaxy. For MBE, low temperature (< 300 °C) growth of the Fe₃Si NDs is needed to suppress the interdiffusion between Fe atoms deposited on the surfaces and Si atoms in the substrate. These epitaxial NDs exhibited the ferromagnetism at low temperatures, which were expected in terms of the application to the magnetic memory device materials.     

MBE growth of (1 1 0) refractory metals on a-plane sapphire

The initial stages of growth of epitaxial (1 1 0) niobium, tantalum, and molybdenum films on (1 1 2 0) sapphire by molecular beam epitaxy, MBE, are analyzed in situ by reflection high-energy electron diffraction (RHEED), and ex situ by high-resolution electron microscopy (HREM) and X-ray diffraction (XRD) techniques. The RHEED analysis shows that both Nb and Ta initially deposit (for approximately the first two monolayers in the case of Nb and the first four monolayers for Ta) with an hexagonal surface symmetry that is not consistent with the normal bcc structure of these metals, in agreement with the previous result of Oderno et al. [1] for Nb on sapphire. On further deposition, the films are observed to relax into the normal bcc (1 1 0) structure. Intensity oscillations of the RHEED specular beam are observed during both stages of growth. The RHEED oscillations are damped out after approximately 20 monolayers of Nb (30 monolayers of Ta) as a steady-state surface roughness is reached. HREM analysis reveals that the initial hexagonal structure has transformed completely to the normal bcc structure in the thicker films. The epitaxial relationships at each stage of growth are identified and a model is suggested for the development of the initial hexagonal structure based on Nb–O bonding at the interface. As the thickness of the bcc film increases, strain relief is observed by the formation of misfit dislocations. The growth mode of Mo is found to be different from that of Nb and Ta. The initial deposit grows in a three-dimensional mode with poor atomic order; the RHEED patterns are not sufficiently distinct to identify whether the hexagonal structure is formed. However, the surface becomes progressively smoother as growth proceeds and thicker films of Mo are comparable in crystalline quality, as measured by X-ray rocking curves, to Nb and Ta. The different growth mode of Mo is attributed to greater mismatch with the sapphire substrate and possibly different M–O ionic bonding at the substrate interface.     

Thermodynamics for formation of each stable single phase in BSCCO thin films

High quality BSCCO thin films have been fabricated by means of an ion beam sputtering at various substrate temperatures, T_sub, and ozone gas pressures, PO₃. The correlation diagrams of the BSCCO phases appeared against T_sub and PO₃ are established in the 2212 and 2223 compositional films. In spite of 2212 compositional sputtering, Bi2201 and Bi2223 phases as well as Bi2212 one come out as stable phases depending on T_sub and PO₃. From these results, the thermodynamic evaluations of H and S, which are related with Gibbs' free energy change for single Bi2212 or Bi2223 phase, are performed.     

Controlled Modifications of HTSC Thin-Film Properties by Tailoring Substrate Surfaces

This chapter introduces the use of substrates for high-temperature superconductor (HTSC) thin-film deposition beyond their usual purpose as chemically inert, lattice-matched support for the films. Substrates are used as functional elements in order to controllably modify the growth mode of YBa₂Cu₃O_7–x thin films in the case of vicinal-cut SrTiO₃ single crystals or to locally modify the surface morphology and film/substrate lattice mismatch in the case of ion implanted SrTiO₃. Furthermore, the use of biaxial epitaxial strain is briefly reviewed for HTSC thin films, and the application of this concept to tailor the properties of perovskite thin films in general is shown.     

Effect of growth conditions on the flatness of micro-crystal terraces of CdTe thin film

An atomic force microscopy study of the surface roughness of CdTe thin films growth by the close-space vapor transport technique on Corning glass substrate under different growth conditions (substrate temperature, Cd overpressure, and annealing) is presented. The roughness measurements show that under certain growth conditions the surface of the micro-crystals is flatter—on the micrometer scale—than in the case of CdTe layers grown by molecular beam epitaxy (MBE) on a CdTe substrate. ©1999 Kluwer Academic Publishers