Tadashi Tsukahara

In situ RHEED observation of CeO2 film growth on Si by laser ablation deposition in ultrahigh-vacuum

In a newly developed ultrahigh-vacuum (UHV) film preparation system equipped with in situ reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectrometer (XPS) analyzer, a sintered CeO2 target was ablated by ArF excimer laser to deposit films on Si(001), (111) and (110) substrates at temperatures ranging from 600 to 700°C. Tetravalence of the Ce in the films was confirmed by XPS, indicating that stoichiometric CeO2 was formed even in the UHV (≤10-6 Torr) condition. The orientation of deposited CeO2 films was strongly dependent on the surface state of Si substrates. The results are well explained in terms of preferential arrangement of the first plane of growing CeO2 by its interaction with substrate surface.

Heteroepitaxial Growth of CeO2(001) Films on Si(001) Substrates by Pulsed Laser Deposition in Ultrahigh Vacuum

Pulsed laser deposition in ultrahigh vacuum (UHV) was applied to the epitaxial growth of CeO2 film on Si(001). Although the direct deposition of CeO2(001) on Si(001) was unsuccessful, the desired epitaxy was achieved by inserting the growth of a SrTiO3 layer. The formation of a CeO2(001)//SrTiO3(001)//Si(001) layered structure was verified by reflection high-energy electron diffraction analysis of the growing surface at a temperature between 650 and 700°C in UHV. In addition to lattice matching, chemical interaction at the growing surface had a decisive effect on the epitaxy and orientation of growing ceramic lattices.

CERAMIC LAYER EPITAXY BY PULSED LASER DEPOSITION IN AN ULTRAHIGH VACUUM SYSTEM

Ceramic layer epitaxy, defined as the epitaxial growth of ceramic thin films with thicknesses regulated on an atomic scale, was verified by a new method using pulsed laser deposition in ultrahigh vacuum (UHV). The intensity oscillation of reflection high‐energy electron diffraction (RHEED) was observed in an ArF excimer laser deposition of CeO2 and SrTiO3 films on Si(111) and SrTiO3(001) substrates, respectively, at 650–750 °C under 5×10−7 Pa. The oscillation periodicities corresponded well to interplane distances of CeO2(111) and SrTiO3(001). This first observation of RHEED oscillation in laser deposition of ceramic films suggests that UHV laser deposition is a promising method for producing atomically regulated ceramic layers inclusive of possible new high Tc superconductors.

Growth Mechanism of Epitaxial Oxide Films by Laser MBE

Crystallographic structure and valence state of CeO 2 and Nd 2 -O 3 films deposited by laser MBE on Si substrates were investigated by reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). In contrast with epitaxial growth of CeO 2 (111) and Nd 2 O 3 (111) on Si (111), epitaxial films were not obtained on Si(001). Partially reduced mixed valence (4+ and 3+) Ce was indicated to exist by XPS in the cerium oxide film within 2nm from the interface with Si. Beyond this transition layer, two-dimensional growth of CeO 2 film on Si(111) was verified from in situ RHEED and as well as on Si(001) from XPS measurements.

Laser molecular beam epitaxy of single-crystal SrVO3−x films

Abstract Using ArF pulsed laser molecular beam epitaxy, SrVO 3− x epitaxial films were obtained on SrTiO 3 (001) substrates. During the deposition at 650°C, intensity oscillation of reflection high energy electron diffraction was clearly observed, suggesting that the film grew in a two-dimensional layer-by-layer mode with each layer thickness corresponding to the SrVO 3 perovskite unit cell. The as-grown films were highly conductive and their resistivity decreased to 10 −5 S −1 cm with a temperature decrease from 300 to 5 K.

Preparation of SrCuOy film in ultra-high vacuum system

Abstract SrCuO 2 crystal contains two-dimensional CuO 2 planes like those constituting supercurrent path in high T c superconducting cuprates. We have investigated on epitaxial SrCuO y film growth on SrTiO 3 (001) substrate by ArF pulsed laser deposition in ultra-high vacuum. In situ RHEED and XPS diagnoses of the film growth process under a very low NO 2 pressure indicated that there were slight fluctuations in surface and electronic states of the initial 1 nm thick film growth. Beyond this initial stage, the film growth gave streaky RHEED pattern with its intensity oscillated at a periodicity corresponding well to the c -axis length of SrCuO 2 , verifying the layer-by-layer growth. The as-grown film was electrically conductive.

Fabrication by Laser MBE and In Situ Characterization of Layered Cuprates

Epitaxial growth of (SrxCa1−x)CuOy layer, an essential constituent of high-T c superconductors, has been investigated by monitoring with RHEED the growing surface of pulsed laser deposition. The RHEED intensity oscillation was detected during, the deposition of (Sr0.5Ca0.5)CuOy with NO2 flushing at a pressure of 10−7 Torr. The deposited (SrxCa1−x)CuOy layer was analyzed by in situ XPS to evaluate the Cu valence and-oxygen sttoichiometry. The as-grown SrCuOy (x=1) film was stoichiometric (y≃2.0) and showed a metallic behavior of linearly decreasing resistivities with the temperature decrease from 300 to 4.2K.

Epitaxial Growth of Ceramic Layers by Laser MBE

The growth modes of oxide films on Si by uLtra-high vacuum (UHV) laser ablation was investigated by reflection high energy electron diffraction ( RHEED ) and X-ray photoelectron spectroscopy (XPS). Coulombic interaction between the film depositing speciLs and the atoms on the substrate surface is presumed to dominate the f i-1m orientation. Epitaxial layer-by-layer growth was achieved for CeO2(111) film on Si(111) substrate.

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.

Atomic Layer Control in Cuprate-Based Ceramic Superlattices

Two-dimensional epitaxial growth of SrCuO2-y and SrTiO3-z was examined using ArF pulsed laser MBE for the fabrication of cuprate-based superlattices. Throughout the film growth, streaky RHEED patterns were observed and their intensities oscillated with the same periodicities as the lattice spacing’s of SrCuO2 and SrTiO3; tetragonal SrCuO2-y containing infinite (CuO2)2- layers and SrTiO3-z were grown epitaxially on SrTiO3(001). The laser MBE deposition of epitaxial SrCuO2-y and SrTiO3-z films was iterated to prepare cuprate-based superlattice.