Tetsuo Ogama

NEXAFS spectroscopic studies of molecular orientation in α-sexithienyl evaporated thin films on metal films

Abstract The molecular orientation of evaporated α-sexithienyl (6T) films on gold (Au), silver (Ag) and copper (Cu) films at room temperature were studied by polarized near edge X-ray absorption fine structure (NEXAFS) spectroscopy and infrared reflection absorption spectroscopy (IR-RAS). The polarized NEXAFS spectra of 6T on Ag and Cu films exhibit a strong angular dependence of C1s → π∗ resonance intensity. Combining the results with theoretical simulations and auxiliary information from IR-RAS, it was found that 6T molecules in the film on Ag and Cu films are highly oriented with their molecular axes inclining by about 70° to the substrate surface. On the other hand, NEXAFS of 6T on Au film showed only a little dependence, indicating that 6T molecules are in nearly random orientation.

Structure of stacking faults formed in pairs in a ZnSe epitaxial layer on a GaAs(001) buffer layer

Structure of stacking faults formed in pairs in a ZnSe epitaxial layer grown by gas source molecular beam epitaxy on a GaAs(001) buffer layer was determined with transmission electron microscopy. Extrinsic type stacking faults were formed on (111) and (111) planes with the same polarity, which was determined by convergent‐beam electron diffraction. The two stacking faults meet at a point which is a few atomic layers away from the interface between ZnSe and GaAs. Partial dislocations at the edge of the stacking faults were found to be the Shockley type ones with a Burgers vector of 1/6〈211〉. Probable formation processes of the stacking faults have been discussed.

In-plane orientation and coincidence site lattice relation of (001) Bi2Sr2CaCu2Ox formed on (001) MgO

Abstract A (001) Bi 2 Sr 2 CaCu 2 O x thin film fabricated on a (001) MgO substrate has been found to consist of two kinds of domain with in-plane orientation of Bi 2 Sr 2 CaCu 2 O x [100]∥MgO [510] and Bi 2 Sr 2 CaCu 2 O x [100] ∥MgO [5 1 0]. It is shown that near coincidence site lattice theory supports the occurence of this in-plane orientation. The dependence of the in-plane orientation on the substrate temperature was also investigated.

(01n)-Oriented BiSrCaCuO Thin Films Formed on CeO2 Buffer Layers

With the use of RF magnetron sputtering, (01n)-oriented BiSrCaCuO superconducting films were first formed using MgO(110) substrates with CeO2(110) buffer layers. The film obtained using the just MgO(110) substrate had (01n)- and (0n)-preferred orientations with the c-axis tilted 45° against the surface of the CeO2 buffer layer. Moreover, with the use of an off-oriented MgO(110) substrate, a uniquely oriented BiSrCaCuO film with the c-axis not perpendicular to the substrate surface was obtained without twin boundaries.

BiSrCaCuO Thin Film Grown on SrTiO3 Substrate with Off-Oriented (110) Surface

A Bi2(Sr, Ca)3Cu2Ox thin film with the (117) orientation was formed on a slightly off-oriented (110) SrTiO3 substrate by single-target sputtering. The (110) substrate off-oriented by 5° is considered to have limited film growth in one direction. From RHEED and cross-sectional TEM observations, it was found that the c-axis of the film tilts against the substrate surface by 46-47° and the SrTiO3(110) plane by 41-42°. Also, the surface morphology observed by SEM has been much improved.

Study of α-Sexithienyl Thin Film by Polarized Near Edge X-ray Absorption Fine Structure

An ultrathin α-Sexithienyl (6T) film prepared by an organic molecular beam deposition method on a silver (Ag) film was studied by polarized near edge X-ray absorption fine structure (NEXAFS) spectroscopy using synchrotron radiation and IR-reflection absorption spectroscopy. The carbon K-edge NEXAFS spectrum of 6T was similar to those reported for poly-(3-methylthienylene) and thiophene. Polarized NEXAFS spectra of 6T exhibit strong angular dependence of 1 s→π* resonance intensity, showing that 6T molecules in the film deposited on the Ag film at room temperature had a highly oriented structure. To obtain quantitative information about molecular orientation, this dependence is analyzed by a comparison with theoretical calculation, indicating that the molecular axis is inclined by about 71° to the substrate surface. This angle is almost the same as that reported for 6T film on quartz.

45° grain boundary junctions in (001)-oriented BiSrCaCuO films

In this study, 45°-rotated [001] tilt boundary junctions were fabricated by controlling the in-plane epitaxy using an MgO seed layer grown on an SrTiO3(001) substrate. The in-plane preferential orientation of the BiSrCaCuO film (2223 composition) grown on the MgO seed layer on the SrTiO3(001) substrate of BiSrCaCuO [100], [010]//MgO[100]//SrTiO3[100] was found using X-ray diffraction. The 45°-rotated [001] tilt boundary junction in the BiSrCaCuO film exhibited RSJ-like behavior. With the use of these junctions, a dc SQUID was fabricated and operated at temperatures up to 65 K.

Artificial Grain Boundary Junctions in BiSrCaCuO Thin Films with (11n) and (001) Orientation

Artificial grain boundary junctions in sputtered BiSrCaCuO films on SrTiO3 (110) substrates have been fabricated by controlling the orientation of the films using a MgO buffer layer. A grain boundary with large twist misorientation is formed along the edge of the MgO buffer layer that is placed perpendicular to the [001] direction of the SrTiO3 substrate. The grain boundary junctions thus obtained exhibited clear Shapiro steps in response to microwave irradiation (15.7 GHz). This indicates that the artificial grain boundaries obtained by controlling the orientation of the BiSrCaCuO films behave as a weak link.

Microscopic Study on (11n)-Oriented BiSrCaCuO Films by Cross-Sectional Transmission Electron Microscopy

Microstructures of superconducting BiSrCaCuO films with (11n) orientation grown on SrTiO3(110) substrates by sputtering were first observed by cross-sectional transmission electron microscopy (TEM). Two kinds of films, which have 2212 and 2223 compositions, were studied here. Both films have two preferred orientations with the c-axes almost parallel to the [100] and [010] directions of the SrTiO3 substrate. High-resolution TEM observation of 2223-composition film showed that there is a transient region consisting of a few atomic layers at the interface.

Bi2(Sr, Ca)3Cu2Ox and Bi2(Sr, Ca)4Cu3Ox Thin Films with (11n) Orientation

As-grown Bi2(Sr, Ca)3Cu2Ox and Bi2(Sr, Ca)4Cu3Ox thin films with the (11n) orientation were formed on SrTiO3(110) substrates by consecutive sputtering. RHEED patterns and XRD analysis indicated that the c-axis of the Bi2(Sr, Ca)3 Cu2Ox and Bi2(Sr, Ca)4Cu3Ox films are tilted against the substrate surface by approximately 41° and 45°, respectively. The Bi2(Sr, Ca)3Cu2O3 film was found to have mainly a (117) orientation, whereas the Bi2(Sr, Ca)4Cu3Ox film is considered to have mainly a (1110) orientation.