Katsuhiko Inaba

X-ray diffraction reciprocal space mapping study of the thin film phase of pentacene

The structure of the thin film phase of pentacene was investigated using x-ray diffraction reciprocal space mapping (RSM). The crystal structure was found to be triclinic with the following lattice parameters: a=0.593nm, b=0.756nm, c=1.565nm, α=98.6°, β=93.3°, and γ=89.8°. Atomic positions were determined by comparing the observed RSM diffraction intensities with theoretical calculations.

Effects of an extremely thin buffer on heteroepitaxy with large lattice mismatch

We report on an experimental study of buffer mechanism in plasma-assisted molecular-beam epitaxy of ZnO on Al2O3(0001) with a MgO buffer. It lowers the surface energy and provides nucleation cores. As a result, lateral epitaxial growth of ZnO becomes favorable from the initial growth stage. The MgO buffer also affects the generation of dislocations in such a way that it reduces their density by introducing dislocation interactions. This study suggests that by employing an appropriate buffer to modify the initial nucleation environment, high quality heteroepitaxy is achievable even with large mismatch.

Crystal growth and structural characterization of new piezoelectric material La3Ta0.5Ga5.5O14

New piezoelectric La3Ta0.5Ga5.5O14 bulk single crystals were grown using the Czochralski technique. The crystal structure (space group P321, a=8.228(2), c=5.124(2) A) has been refined using single-crystal X-ray diffraction data with a precision corresponding to an R index of 0.04. Ta atoms were found to be ordered in the octahedral site coordinated by six oxygen atoms.

Ammonothermal Epitaxy of Thick GaN Film Using NH4Cl Mineralizer

Single-crystalline GaN films of ≤ 110 µm in thickness have been fabricated on hydride vapor phase-grown (0001) GaN substrates by employing a relatively low-pressure (≤170 MPa) ammonothermal growth method with NH4Cl as mineralizer. Metallic Ga and polycrystalline GaN has been the chosen precursor. An average growth speed on the (0001) face of the substrate of ≥5 µm/day was observed when using metallic Ga and about 7 µm/day for GaN. The maximum growth speed of 27.5 µm/day was achieved for the film grown at a supersaturation from a combined Ga/GaN precursor. The surface morphology is not affected by the nature of the precursor.

Lattice relaxation mechanism of ZnO thin films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy

We report on the lattice relaxation mechanism of ZnO films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy. The lattice relaxation of ZnO films with various thicknesses up to 2000nm is investigated by using both in situ time-resolved reflection high energy electron diffraction observation during the initial growth and absolute lattice constant measurements (Bond method) for grown films. The residual strain in the films is explained in terms of lattice misfit relaxation (compression) at the growth temperature and thermal stress (tension) due to the difference of growth and measurement temperatures. In thick films (>1μm), the residual tensile strain begins to relax by bending and microcrack formation.

Crystallite size effect on the hole mobility of uniaxially aligned copper phthalocyanine thin-film field-effect transistors

We fabricated copper phthalocyanine (CuPc) thin-film field-effect transistors (FETs) with three different orientations of CuPc with respect to channels. The substrate rubbing treatment induced uniaxial orientation in CuPc layers, yielding higher hole mobility (∼0.02cm2∕Vs) than that of untreated FETs. Although the rubbing treatment bore high-aspect-ratio (>10) CuPc domains oriented to rubbing direction, the mobility anisotropy of the film was only 1.4. This discrepancy was explained by analyzing grazing-incidence x-ray diffraction profiles, i.e.: (1) In-plane mean size of crystals was smaller than their appearance in atomic force microscopy, and (2) the crystallites were much shorter in the apparent long-hand direction (11nm) than the direction perpendicular to it (44nm).

Graphoepitaxy of sexithiophene on thermally oxidized silicon surface with artificial periodic grooves

Graphoepitaxial growth of a sexithiophene (6T) thin film was achieved on a thermally oxidized silicon surface with artificial periodic grooves. The surface structure was fabricated by electron beam lithography and the thin film was grown by molecular beam deposition. A well-pronounced, in-plane oriented component ([010]6T‖grooves) was identified by grazing incidence x-ray diffraction, though there also existed some randomly oriented 6T grains. Presence of the graphoepitaxial component was also confirmed by results of the orientational analysis of atomic force microscopy images. It was shown that the in-plane orientation control of organic semiconductors is possible using graphoepitaxy.

Grazing Incidence In-Plane X-Ray Diffraction Study on Oriented Copper Phthalocyanine Thin Films

Uniaxially aligned vacuum deposited films of copper phthalocyanine (CuPc) have been studied by means of angle-dispersive grazing-incidence X-ray diffraction. The lateral structure of the top surface of a 100-nm-thick film was successfully probed. It was revealed that the microcrystals near the surface have a finite in-plane distribution with a full-width at half maximum of 17.5°. In-plane diffraction profiles were transformed into a reciprocal space map that reproduced the electron diffraction pattern of a CuPc thin film reported previously.

Preparation and characterization of Yb:Y3Al5O12 fiber crystals

Abstract A micro-pulling-down process employing Ir crucibles and RF heating has been used to grow single crystal fibers of Yb-doped Y 3 Al 5 O 12 garnet (YAG). Single crystals ranging up to 550 mm in length have been produced. The fibers were about 1 mm in diameter and were seeded-grown in the direction. Results of X-ray rocking curve measurements and transmittance data were used to discuss the crystalline perfection of the fibers. The effective segregation coefficient of Yb 3+ in the process described is reported to be slightly less than 1.0 at a growth rate of about 5 mm/min.

Micro-pulling down growth studies of lead tungstate crystals: aspects of incongruent melt vaporization

The problem of increasing yellow coloration of PbWO4 crystal grown from the melt has been analyzed using a micro-pulling down technique. The ratio of free surface area to melt volume in μ-PD growth is noticeably higher in comparison with CZ method. The incongruent vaporization of PWO melt generates changes in stoichiometry and subsequently causes difficulties for crystal growth from tungsten-excess melt.