Hirotaka Yamaguchi

Rapid enlargement of SiC single crystal using a cone-shaped platform

We investigated the enlargement of SiC single crystal during physical vapor transport growth by modifying the shape of graphite lid. The single crystals grown on the cone-shaped platform were larger in diameter than those grown on the conventional one. The enlargement of ingot is discussed in terms of the dual role of polycrystals during crystal growth: (i) it provides a platform for single crystal and (ii) an obstacle to the lateral growth of ingot. The dependence of the broadening angle (β) of single crystal on the taper angle (θ) of the cone-shaped platform was also investigated and an optimum angle at a given growth condition found.

Polarized X-Ray Absorption Fine Structure of La2CuO4-y Single Crystal

Polarized Cu K X-ray absorption fine structure (XAFS) has been measured on a single crystal of undoped La 2 CuO 4- y for the first time, using a fluorescence detection technique. The polarized near-edge structure and EXAFS were strongly anisotropic, demonstrating the quasi-two-dimensional nature of electron states and local structure. The polarization dependence of the near-edge structure confirms that the Cu atoms take 2+ valence or d 9 configuration with a strong fourfold Cu–O1 bonds within the CuO 2 plane and weak twofold Cu–O2 bonds along the c -axis, which is consistent with the anisotropic local structure suggested by the Fourier transform of polarized EXAFS and a crystal structure.

Temperature dependent CuO distribution function of the superconducting CuO2 plane

The temperature-dependent pair distribution function of the in plane CuO pairs has been determined by in plane polarized (E | ab) Cu K-edge extended X-ray absorption fine structure (EXAFS) experiments on single crystal of La1.85Sr0.15CuO4 (LSCO) with Tc = 35 K. A splitting of the CuO (planar) distance larger than the correlated Debye-Waller factor appears for T < 100 K. The separation between the two distances decreases at Tc but increases again in the superconducting state (T < Tc). The comparison with diffraction data probing the uncorrelated Debye-Waller factors is discussed.

Analysis of contrasts and identifications of Burgers vectors for basal-plane dislocations and threading edge dislocations in 4H-SiC crystals observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry

Contrasts of dislocations in the sub-surface region of the Si-face of a 4H-SiC wafer were observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry. Basal-plane dislocations show very characteristic contrast depending on their Burgers vectors, running directions, and types of dislocations, whether they are screw dislocations, C-core edge dislocations, or Si-core edge dislocations. The rules for contrasts of basal-plane dislocations are summarized. It is shown that by observing those contrasts at fixed diffraction conditions, Burgers vectors of the basal-plane dislocation can be identified without performing a g · b analysis in some cases. Threading edge dislocations also have very characteristic contrasts depending on the angles between the projected g and their Burgers vectors. It is shown that Burgers vectors of threading edge dislocations can be determined uniquely by observing their characteristic contrasts without performing g · b analysis. Contrast mechanisms for these dislocations in grazing-incidence X-ray topography are discussed.

Temperature dependence of the local structure in superconducting and non-superconducting Tl2Ba2CuOy studied by extended x-ray absorption fine structure

Abstract The local structure of Tl 2 Ba 2 CuO y (Tl2201) has been investigated by extended X-ray absorption fine structure (EXAFS) on the Cu K-edge. The result for the high- T c sample with T c =70 K indicates a splitting of Cu-O bondlengths in the CuO 2 plane, which shows a strong temperature dependence over a wide temperature range well above T c , although the crystallographical symmetry of the unit cell is tetragonal. The splitting is evaluated from the filtered EXAFS by the beat-node method, and the maximum splitting distance is ≁0.16 A. The magnitude of the splitting abruptly decreases in the vicinity of T c by more than 10%. On the other hand, no splitting in the in-plane Cu-O bondlengths was observed in non-superconductor samples from room temperature down to 30 K. No apparent change of axial oxygen sites was observed. These facts strongly demonstrate that the local structural distortion in the CuO 2 plane may directly relate to the superconductivity.

X-ray Topography Used to Observe Dislocations in Epitaxially Grown Diamond Film

Semiconductor epitaxial chemical vapor deposited single-crystal diamond is considered as a potential material for power devices because of its unique characteristics. Its atomic purity and defect concentration have been considered in discussions on the relationship between crystal quality and device performance. In this paper, we propose a method that uses X-ray topography to experimentally analyze dislocations. The advantages of the proposed method make it suitable as a standard method for dislocation analysis. To demonstrate the method, we observe and analyze an edge dislocation and a mixed dislocation.

Bond Length Relaxation in Ultrathin InAs and InP0.4As0.6 Layers on InP(001)

Bond length relaxation in InAs and InP0.4As0.6 ultrathin layers on InP(001) substrates has been studied by the extended X-ray absorption fine structure using synchrotron radiation. The In–As bond lengths have been determined as a function of film thickness from 1000 A to 3 A. In ultrathin film, the In–As bond lengths in both InAs and InP0.4As0.6 alloys are shorter than the bulk value by the epitaxy-induced strain due to the lattice mismatch. It was found that on increasing the film thickness, the In–As bond lengths gradually approach the values in the bulk material. The relative variation of bond length is not dependent on the magnitude of lattice spacing, which suggests that the mechanism of relaxation is ascribed to the local strain (bond length mismatch) rather than the macroscopic average.

X-ray absorption fine structure studies of Fe-doped Ba2YCu3Oy

The effect of Fe-doping on the local structure and valence states of Ba 2 Y(Cu 1- x Fe x ) 3 O y ( x <0.06, y =6.9) have been investigated by X-ray absorption fine structure (XAFS) on the Cu K and Fe K edge. The results show that (1) Fe impurities exist as 3+ ions and (2) take fourfold Cu1 sites in the lightly doped region ( x <0.02); then with further doping, (3) the oxygen coordination number increases as the symmetry changes from orthorhombic to “tetragonal”, and (4) the orthorhombic ab-plane structure is retained on a microscopic scale. These results indicate that the orthorhombic-to-“tetragonal” phase transition is not induced by disordering of oxygen atoms. A possible structural model of the mechanism of doping is presented.

Contrast analysis of Shockley partial dislocations in 4H-SiC observed by synchrotron Berg–Barrett X-ray topography

Shockley partial dislocations in 4H-SiC were observed using monochromatic synchrotron X-ray topography with a grazing-incidence Bragg-case geometry, that is, Berg–Barrett topography. The contrast of partial dislocations at the edges of Shockley-type stacking faults is discussed in terms of whether they have C- or Si-core edge components, or screw components. The dissociated state of basal-plane dislocation is discussed on a basis of the stacking sequence for basal-planes in the 4H-SiC crystal structure. It is expected that the results obtained in this study will be useful for characterizing Shockley-type stacking faults in Berg–Barrett topography.

Anisotropic magnetic susceptibility of single crystal Ca2Y2Cu5O10 with edge-sharing CuO2 chain

Abstract We report a successful crystal growth of Ca2Y2Cu5O10 and anisotropic behavior of magnetic susceptibility associated with an antiferromagnetic phase transition. Single crystals were obtained by the floating-zone method using an infrared radiation furnace. Composition and structural characterization shows that the grown crystals are qualified for physical measurements. The magnetic susceptibility along each crystallographic axis shows a typical behavior of antiferromagnetic phase transition at TN=31 K, and the ordered magnetic moments are found to lie along the b-axis.