Takashi Hanada

Molecular beam epitaxy of Bi2Sr2CuOx and Bi2Sr2Ca0.85Sr0.15Cu2Ox ultra thin films at 300°C

Abstract Molecular beam epitaxy of ultra thin films of Bi2Sr2CuO6 (2201 phase) and Bi2Sr2(Ca0.85Sr0.15)Cu2O8 (2212 phase) i s realized on SrTiO3(100) at a substrate temperature of 300°C, using 10-5 Pa of NO2 as an oxidant. The structure of the ultra thin film formed is confirmed by X-ray diffraction. An in-situ RHEED (reflection high energy electron diffraction) study reveals that the film formed has the same in-plane lattice constant as that of the SrTiO3(100) substrate, namely the a and b axes are 0.39 nm. Because the a and b axes are lengthened approximately 3%, the c axis of the ultra thin 2201 film was 2.41 nm, shorter than the value for the bulk material. The crystallinity of the film strongly depended on the sequence of the oxidation process, revealing that the elementary unit of the epitaxial growth is the sub-unit of the perovskite structure. In the Bi cuprate series, Sr-Cu-Sr is the elementary unit. As a result, the sequential supply of Bi, Sr, Cu and the oxidant NO2 is necessary to realize low temperature epitaxy.

Study of successive phase transitions of the Si(001)-Bi surface by RHEED

Abstract Successive phase transitions of the 2 × n surface structures have been observed on a Si(001) surface with Bi overlayer. These structures are formed with about one monolayer of Bi. The coverage of Bi slightly decreases with increasing temperature. Since these phase transitions are accompanied by the desorption of Bi, transitions are irreversible. The dependence of n on the temperature has been measured using reflection high-energy electron diffraction (RHEED). It is revealed that n th order spots move away from fundamental spots continuously over the range from 1 13 to 1 5 as the temperature is elevated from 530 K to 820 K.

The reaction of copper and calcium dipivaloylmethanates (Cu(DPM)2 and Ca(DPM)2) with hydroxyls on oxide surface

Abstract Selective and stoichiometric reactions between surface hydroxyl groups (OH) on SiO 2 and calcium dipivaloylmethanate (Ca(DPM) 2 ) were investigated by infrared (IR) and photoelectron (XPS) spectroscopies. The stoichiometric ratio of initial surface OH and adsorbed Ca(DPM) 2 is estimated from IR absorbance to be ca. (2–3): 1. Introduction of water vapor at 673 K to this surface results in the removal of ligand DPM from the adsorbed Ca(DPM) 2 , leaving the Ca on the surface. The reactive property of Ca(DPM) 2 is identical to that of Cu(DPM) 2 , previously reported by us. A similar reaction on the surface of single crystal SrTiO 3 (100) with the Cu(DPM) 2 is also carried out.

Molecular beam epitaxial growth of superconducting Ba2DyCu3O6.5 thin films at 420 °C using NO2 as an oxidant

A superconducting Ba2DyCu3O6.5 (orthorhombic II phase) thin film is formed on a MgO(100) substrate at 420 °C by molecular beam epitaxial growth using NO2 as an oxidant. Metal elements Ba, Dy, and Cu are sequentially supplied from K cells onto MgO(100) at 420 °C under a flow of 5×10−5 Pa of NO2. The film gradually cooled in situ under the identical pressure of NO2 shows metallic behavior in the normal state and a Tczero of 50 K. The lattice constant of the c axis is observed at 11.75 A, which is known as an orthorhombic II phase of bulk Ba2DyCu3O6.5. Due to the slow deposition rate, the film thus formed is oriented with the c axis perpendicular to the surface in spite of the low‐temperature growth condition.

Glancing angle dependence of the X-ray emission measured under total reflection angle X-ray spectroscopy (TRAXS) condition during reflection high energy electron diffraction observation

We measured the glancing angle (θg) dependence of the X-ray emission from Si(111)-√3×√3-Ag and α-√3×√3-Au surfaces during Reflection High Energy Electron Diffraction observation under the Total Reflection Angle X-ray Spectroscopy condition. The characteristic X-rays AgL and AuM decreased according to 1/sin θg. The function 1/sin θg is easily understood in terms of Ag and Au atoms located at the top layer of the surface. The SiK and the bremsstrahlung showed broad peaks around 8°. These trends of the curves are explained by an analysis using Monte Carlo electron trajectory simulation. By measuring the glancing angle dependence we can easily distinguish whether or not a specific kind of atom is confined at the top layer of the surface.

New Models for the 7 ×7, 5 ×5, 2 ×8 Structures on Si(111) and Ge(111) Surfaces

On the basis of a detailed analysis of RHEED intensity distribution along reciprocal lattice rods taken from the Si(111)7 ×7 structure, a new structure model has been obtained. The structure consists of twelve raised atoms and a vacancy in the unit mesh. The raised atoms are arranged in regular triangular forms with side 2 a , a being the size of the bulk unit mesh and they can be classified into four kinds of atoms A, B, C and D. Among their heights of the displacements Z A , Z B , Z C and Z D , an inequality Z A >Z B >Z C >Z D has been consequently obtained. The Ge(111)7 ×7-Sn surface has an identical structure with the Si(111)7 ×7. Similarly the Ge(111)2 ×8 structure consists of two of them, A and B, and the Ga(111)5 ×5-Sn and Si(111)5 ×5-Ge three of them, C, D and a vacancy.

Reaction between copper dipivaloylmethanate Cu(DPM)2 and H2O adsorbed on SrTiO3(100)

Abstract Selective reaction of copper dipivaloylmethanate (Cu(DPM) 2 , β-diketonate complex) with H 2 O adsorbed on the SrTiO 3 (100) surface is investigated. Adsorption of H 2 O on SrTiO 3 (100) occurs on Ti 3+ sites produced by prior Ar + ion bombardment. It is found that the β-diketonate complex, Cu(DPM) 2 , reacted selectively with the adsorbed H 2 O at room temperature. Further reaction of the adsorbed Cu(DPM) 2 with H 2 O vapor results in the removal of the ligand, DPM, leaving elemental copper on the surface.

Structure and growth mechanism of ultrathin films of Bi cuprates grown by low-temperature MBE

The structure and growth mechanism of a Bi cuprate formed by low-temperature MBE are discussed. Examination of the X-ray diffraction of the ultrathin films revealed that the lattice constants of an ultrathin film of Bi 2201 are affected by the lattice constants of the substrate. The key process of the low-temperature epitaxial growth realized by the usage of NO 2 is also discussed in correlation with the oxidation mechanism of deposited metals with NO 2 examined by X-ray photoelectron spectroscopy.

Selective surface reaction between metal compounds and surface functional groups

Reactivity of copper dipivaloylmethanate (Cu(DPM) 2 ) adsorbed selectively on the H 2 O on a SrTiO 3 (1 0 0) surface is discussed. Reaction of the adsorbed (Cu(DPM) 2 with H 2 O vapor at 473 K results in the removal of the ligand, DPM, leaving the highly dispersed copper atoms in the +1 oxidation state on the surface. Dispersed copper is found to aggregate to form particulates at 673 K.

In situ RHEED and XPS studies on ceramic layer epitaxy in UHV system

Ceramic films relating to high‐Tc superconductors were grown in a UHV deposition‐analysis system. Against our intuition that large oxygen deficiency is inevitable in the growth of oxide films in UHV, required oxidation state was achieved either by using such a highly oxidative gas as NO2 or by choosing thermodynamically stable oxides. Two‐dimensional growth of ceramics were verified in MBE growth of Bi‐Sr‐Cu‐O as well as in laser MBE growth of various ceramic thin films. The growth mode and the oxidation state of the films were investigated by in situ RHEED and XPS analyses. Discussion is extended to the possibility of our method for constructing new ceramics layers which may exhibit higher Tc.