Michitomo Iiyama

Low-energy ion scattering spectroscopy observations of c-axis-oriented YBa2Cu3O7-x thin films : effects of in-vacuum annealing

The atomic structure of (001) surfaces of c‐axis‐oriented YBa2Cu3O7−x (YBCO) thin films was studied by low‐energy ion scattering spectroscopy (ISS) after annealing in ultrahigh vacuum. This is the first report on the surface analysis of superconducting oxide evaluated by ISS. A clean surface of YBCO (001) 1×1 is produced by annealing at around 500 °C. The surface structure is discussed in terms of the blocking and the nonblocking effects caused by the desorption of O(4) atoms. We conclude that the (001) surfaces are stabilized by Cu(1)‐O chains.

In situ surface characterization of SrTiO3(100) substrates and homoepitaxial SrTiO3 thin films grown by molecular beam epitaxy and pulsed laser deposition

The topmost atomic layer of SrTiO3 (STO) substrates was investigated by in situ low-energy ion scattering spectroscopy (LEISS). After mechanochemical polishing, the topmost layer of the STO substrate consisted of SrO and TiO2 planes. It was dominantly stabilized with TiO2 planes after the STO substrate was treated with a pH-controlled NH4F–HF(BHF) solution. STO thin films were deposited on the BHF-treated STO substrates by the molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) methods. STO films were confirmed to have topmost layers with TiO2 planes by the MBE method, and SrO planes by the PLD method. We also investigated the effects of the deposition conditions and surface treatments, and confirmed that the TiO2 plane was more stable on the STO homoepitaxial film surface. Furthermore, the BHF-treated STO substrates greatly improved the thickness dependence of Tc of heteroepitaxial YBa2Cu3Ox (YBCO) ultra-thin films.

Dielectric Properties of SrTiO3 Thin Films Grown by Ozone-Assisted Molecular Beam Epitaxy

Crystalline SrTiO3 thin films were fabricated by an ozone-assisted molecular beam epitaxy (MBE) method at a relatively low deposition temperature of around 300° C. These films had high crystallinity, smooth surface and insulating properties with increasing deposition temperature. The relative dielectric constant for a 250-nm-thick SrTiO3 film deposited at 480° C was 825 at 35 K. This value was comparable to that of SrTiO3 films, which were deposited at over 650° C by pulsed laser deposition. This film had similar temperature and applied electric field dependence to bulk SrTiO3. The electric field-effect evaluation on SrTiO3 films in the Ag/SrTiO3/YBa2Cu3O7-x configuration indicated suppression of the SrTiO3/YBa2Cu3O7-x interface layer.

All in situ deposition and characterization of YBa2Cu3O7-x thin films by low-energy electron diffraction and low-energy ion scattering spectroscopy

In an ultrahigh vacuum apparatus, c‐axis oriented YBa2Cu3O7−x (YBCO) thin films were deposited by an ozone‐assisted reactive coevaporation method and characterized by low‐energy electron diffraction (LEED) and low‐energy ion scattering spectroscopy (LEISS). A clean surface with crystal perfection within the top two monolayers was confirmed by LEED for the first time. LEISS analysis showed that this clean surface is terminated by Cu(1)‐O chains. These experiments also indicate that the oxygen pressure and the vacuum condition are one of the essential factors in keeping an outermost surface clean.

Surface Analysis of C-Axis-Oriented YBa2Cu3O7-X Thin Films by QMS, XPS and LEED: Effects of In-Vacuum Annealing

The effects of in-vacuum annealing on the crystallinity and superconductivity in the surface region of YBa2Cu3O7-X thin films were investigated by QMS, XPS and LEED. Contaminated and disordered layers, which were nonsuperconducting phase, covered the film surface after being exposed to air. Surface contaminants are removed by annealing at 350°C in ultra high vacuum. The orthorhombic YBa2Cu3O7-X(001) structure and superconducting phase are obtained at the outermost surface after annealing at 350-400°C.

In Situ Surface Characterization of SrTiO3 (100) Substrates for Well-defined SrTiO3 and YBa2Cu3O7-x Thin Film Growth

The topmost atomic layer of SrTiO3 (STO) substrates was investigated using in situ low-energy ion scattering spectroscopy (LEISS) for ultra-thin YBa2Cu3O7-x (YBCO) film growth. The topmost layer of a STO substrate after mechanochemical polishing consisted of SrO and TiO2 planes. The topmost layer was dominantly stabilized with a TiO2 plane after the STO substrate was treated with a pH-controlled NH4F–HF(BHF) solution. It was confirmed that the topmost layer of STO thin films deposited on STO substrates was affected by the surface structure of the STO substrates. Furthermore, the dependence of T c of heteroepitaxial YBCO ultra-thin films on thickness was better for BHF-treated STO substrates than for other substrates.

Characterization of Interdiffusion in SrTiO3/YBa2Cu3O7-x Multilayers by In Situ Auger Electron Spectroscopy

In situ deposition and surface characterization by Auger electron spectroscopy enabled precise surface composition analysis. The annealing temperature dependence of the surface composition in SrTiO3(2.8 nm)/YBa2Cu3O7-x and YBa2Cu3O7-x (2.7 nm)/SrTiO3 multilayers revealed the diffusion of each of the constituent atoms. The interdiffusion is negligible below 600° C. Ba atom diffusion into SrTiO3 was dominant in the temperature range from 600° C to 660° C. The activation energy was calculated to be 2.4 eV. Ti atom diffusion into YBa2Cu3O7-x became distinctive above 660° C. The activation energy of Ti was 4.4 eV.

Electric field effect of SrTiO/sub 3//YBa/sub 2/Cu/sub 3/O/sub 7-x/ multilayers grown by all-MBE

Epitaxial SrTiO/sub 3//YBa/sub 2/Cu/sub 3/O/sub 7-x/ (STO/YBCO) multilayers for a superconducting field-effect transistor (SuFET) were fabricated by an ozone assisted Molecular Beam Epitaxy (MBE) method. To eliminate interdiffusion at the interface, we deposited high quality STO films at around 500/spl deg/C on YBCO films with clean surfaces. At this temperature, interdiffusion was negligible through in-situ Auger Electron Spectroscopy (AES). The electric field effect in a Ag/STO/YBCO device configuration also indicated the suppression of the interfacial layer between the STO and YBCO film.<<ETX>>

Electric field effect of SrTiO/sub 3//YBa/sub 2/Cu/sub 3/O/sub 7-x/ bi-layer grown by MBE and PLD

Oxygen content and interface properties in SrTiO/sub 3//YBa/sub 2/Cu/sub 3/O/sub 7-x/(STO/YBCO) bi-layer definitely affect electric properties of superconducting field effect transistor (SuFET). Good interface can be obtained by molecular beam epitaxy (MBE). Deposition pressure of MBE is too low to oxygenate the ultra-thin YBCO through STO. The oxygen pressure of pulsed laser deposition (PLD) is high enough to oxygenate the YBCO but YBCO film surface is degraded by impurity gases because of its high oxygen pressure. We successfully combined MBE and PLD methods to obtain STO/YBCO bilayers with both good interface and electrical properties. The transconductance of SuFET was 5mS/cm, which is higher than SuFET grown by all-MBE and all-PLD.

Electric Field Effect in SrTiO3/YBa2Cu3O7-x Multilayers Grown by All-MBE

SrTiO3/YBa2Cu3O7-x(STO/YBCO) multilayers were grown on STO(100) substrates by an ozone-assisted MBE method, which enables us to grow thin films with smooth surfaces at relatively low substrate temperature. Film surfaces were evaluated by RHEED, LEED, XPS and ISS. These observations showed the YBCO surfaces were smooth, clean and crystalline and terminated by Cu-O chains, and STO films grew epitaxially onto these YBCO(001) surfaces. Furthermore, we made use of a selective epitaxial growth technique to fabricated device configurations in course of film depositions. Applying this process to a FET fabrication, we observed field effect modulations of drain current both in normal and in superconducting states.