Ken-ichi Shiramine

Tip artifact in atomic force microscopy observations of InAs quantum dots grown in Stranski–Krastanow mode

The tip artifact in atomic force microscopy (AFM) observations of InAs islands was evaluated quantitatively. The islands were grown in the Stranski–Krastanow mode of molecular beam epitaxy. The width and height of the islands were determined using transmission electron microscopy (TEM) and AFM. The average [1¯10] in-plane width and height determined using TEM excluding native oxide were 22 and 7nm, respectively; those determined using AFM including the oxide were 35 and 8nm, respectively. The difference in width was due to the oxide and the tip artifact. The sizes including the oxide were deduced from TEM observations to be a width of 27nm and a height of 6nm with correction for the thickness of the oxide. The residual difference of 8nm between the width determined using AFM and that determined using TEM including the oxide was ascribed to the tip artifact. The results enable us to determine the actual size of the islands from their AFM images.

Size, density, and shape of InAs quantum dots in closely stacked multilayers grown by the Stranski-Krastanow mode

Closely stacked multilayer structures of InAs islands with intermediate-layer thicknesses d of 3, 6, 10, and 20 nm were grown by the Stranski–Krastanow mode of molecular beam epitaxy and were observed using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The multilayers consisted of five InAs layers each of a thickness of 1.8 monolayers and four GaAs layers each of a thickness d. Columns of coherent islands were observed by cross-sectional TEM. Changes in the size and density of the islands with d, determined by AFM, could be explained in terms of (i) change in the vertical pairing probability of islands, (ii) detachment of In from the top of the island, and (iii) surface segregation of In. The observed AFM images of the islands were elliptical. Their major axis was in the [110] direction, and the length of the minor axis was 80% of that of the major axis.

Critical cluster size of InAs quantum dots formed by Stranski-Krastanow mode

The number of In atoms in a critical cluster, i*, in Stranski–Krastanow (S–K) mode of InAs islands was determined to be 1–10. The i* was determined using an activation energy EA of 2.0 eV determined from an Arrhenius plot of the saturated density of InAs islands formed on a GaAs (001) surface by S–K mode of molecular beam epitaxy [K. Shiramine et al., J. Cryst. Growth 242, 332 (2002)], and an activation energy of 1.6 eV for migration (surface diffusion) of In adatoms, inferred from other references. The common value ∼2.0 eV of EA in S–K mode was ascribed to the small i*.

Quantum gates using tunneling electron spins of a quantum-dot chain

Abstract Quantum gates using tunneling qubits: tunneling of electrons whose spins work as qubits is described by the Hamiltonian using both electron spins and pseudo-spins representing the photon-assisted tunneling. The phase factor during the controlled-NOT operation is also discussed. Preliminary data on a triple quantum dot system realized by closely stacking technique of molecular beam epitaxy are also shown.

Volume distributions of InAs/GaAs self-assembled quantum dots by Stranski–Krastanow mode of molecular beam epitaxy

Abstract We studied the volume distributions of dislocation-free InAs/GaAs self-assembled quantum dots obtained by Stranski–Krastanow mode of molecular beam epitaxy. The same scaling function was obtained over a wide range of dot densities. The scaling function indicated that the cluster size fluctuation normalized by the average size is constant for all the quantum dot densities we studied. The resemblance of the scaling function to that of the submonolayer homoepitaxial growth implies that the strain is not the essential factor determining the cluster size distribution of quantum dots. We also describe the prospects for their device applications.

Volume distributions of InAs/GaAs self-assembled quantum dots by Stranski–Krastanow mode

Abstract We studied the volume distributions of dislocation-free InAs/GaAs self-assembled quantum dots obtained by Stranski–Krastanow mode of molecular beam epitaxy, with and without in situ annealing. The same scaling function was obtained over a wide range of dot densities. The scaling function indicated that the cluster size fluctuation normalized by the average size is constant for all the quantum dot densities we studied. The resemblance of the scaling function to that of the submonolayer homoepitaxial growth implies that the strain is not the essential factor determining the cluster size distribution of quantum dots.

Anisotropy of lattice vibration in uniaxially aligned high temperature superconductor Ba2EuCu3O7 observed by Mössbauer spectroscopy

Abstract An anisotropy of the lattice vibration of Eu in high-Tc superconductor Ba 2 EuCu 3 O y was detected by Mossbauer effect. It was shown that the vibration along c-axis( θ D∥ = 230 K) is softer than that in the plane normal to c-axis( θ D⊥ = 276 K). The anomalies in the line shift were observed between Tc and 140 K, and 220–230 K. It was suggested that the anomaly in low temperature region is attributed to the anomalous volume change and that the anomaly at 220 K–230 K is related to a transition in the neighborhood of europium.

Anisotropy of the lattice vibration of Eu and Cu in highTc superconductor Ba2EuCu3O7 analyzed by151Eu and57Fe Mössbauer effects

The anisotropy of superconductivity in highTc superconductors indicates that it is important to clarify the anisotropies of microscopic properties in these materials. In this work, we observed the anisotropy of the lattice vibration ofEu andCu inBa2EuCu3O7 by151Eu and57Fe Mössbauer effect. Thec-axis aligned samples were prepared by healing the mixture of epoxy andBa2EuCu3O7 powder in a magnetic field. For57Fe Mössbauer measurement the sample doped with57Fe,Ba2Eu(Cu0.99Fe0.01)3O7, was used. The temperature dependence of Mössbauer spectrum was measured in two cases,c-axis ‖ γ-ray andc-axis ⊥ γ-ray. The Debye temperature ofEu alongc-axis (230 K) was lower than that inab-plane (265 K). The Debye temperature of57Fe substituted forCu(2) was lower (378 K) alongc-axis than inab-plane (417 K). The Debye temperature of57Fe substituted forCu(1) having no oxygen atom ina-axis was 435 K alongc-axis and 416 K inab-plane. These results are interpreted by the oxygen configuration around the Mössbauer atoms.

Possibility of Substitution of Sn for Cu in High-Tc Superconductors - Solubility of Sn in CuO

Possibility of substitution of Sn for Cu oxide is investigated by X-ray diffraction and TEM. By firing of mixtures of CuO and SnO2 below 1080 °C, it was clarified that few Sn atoms were substituted for Cu in CuO by conventional method (firing at 900 °C). Above 1080 °C, Sn-solubility in CuO suddenly increased and Sn atoms were substituted for Cu of 0.45% in Cu oxide at 1090 °C.

Lattice Vibration in Aligned EuBa2(57Fe0.0.1Cu0.99)3O7 Observed by Mössbauer Spectroscopy

57Fe Mossbauer spectra of c axis aligned EuBa2(57Fe0.0.1 Cu0. 99)3O7 were measured at 80–330K. From the temprature dependence of the total line area of three doublets, the Debye temperature, Θ D, was estimated. There was no significant difference between Θ D in the lattice vibration of iron along the c axis and that in the basal plane. However this result does not mean the absence of the lattice vibration anisotropy of copper in EuBa2Cu3O7.