坂東 弘之

A new method of InP substrate cleaning is presented for molecular beam epitaxy (MBE). It is based on the congruent sublimation of InP at a temperature of 380 ± 50 ° C and is phosphorous-beam free, so that the substrate cleaning can be carried out in the preparation chamber with no phosphorous K-cell provided. This is important to keep the growth chamber free from degassing during substrate preparation. InGaAs-InAlAs MQW epilayers grown on InP substrates cleaned by the present method were characterized crystallographically, optically and electronically, demonstrating that the quality is as good as that of those grown on a substrate cleaned by the conventional method. © 2005 Elsevier B.V. All rights reserved.

Misfit dislocation was investigated by observing crosshatch over surfaces of InxGa1-xAs-In0.52Al0.48As strained multiple quantum wells (MQWs) and InxGa1-xAs bulk-like thin epifilms grown on InP substrates. It was found that no crosshatch was observed on surfaces of non-doped InxGa1-xAs-In0.52Al0.48As strained MQW over an In composition range of 0.32 < x < 0.78 and that Be-doped InxGa1-xAs bulk-like thin films were also crosshatch free over a wide range of 0.42 < x ≤ 1.0, which clearly exceeds the critical thickness limit. Surface and cross-sectional diagnostics and a preliminary result on the electron mobility of the crosshatch-free InAs bulk-like thin films were presented. © 2005 Elsevier B.V. All rights reserved.

Optical absorption saturation density I-s was measured for InxGa1-xAs-In0.52Al0.48As multiple quantum well structures grown on InP (100) substrates by molecular beam epitaxy. Indium composition x was varied from 0.32 to 0.70 so that the quantum well layer was under tensile strain (x < 0.53), lattice matched (x = 0.53), or compressive strain (x > 0.53). Optical measurement was carried out using femtosecond light pulses from the optical parametric amplifier of a mode-locked Ti-sapphire laser amplifier. The density I-s in a sample changed as a function of photon energy and exhibited a minimum value at the band-edge exciton energy. This minimum, showed the smallest value at x = 0.46 (under tensile strain) of all the samples with different x. This was in marked contrast to the results reported in the literature. Degenerate pump-probe measurement was also carried out, and the results showed that the relative transmission change Delta T/T measured at zero delay between the pump and probe pulses exhibited the largest value at x = 0.46, confirming the result of I-s measurement.

Optical absorption saturation intensity Is was measured by using femtosecond light pulse for InxGa1-xAs-In(0.52)Al(0.48)AS MQW grown on InP (100) substrates. Is showed minimum at x=0.46 (under tensile strain). This was in marked contrast to the previous results.

Thin films of crystalline B/C/N materials were deposited on nickel(111) single crystal by low pressure chemical vapor deposition (LPCVD) using acrylonitrile and boron trichloride as starting materials at temperatures between 823-1123 K under the pressure of 10(3) Pa. The WON thin film obtained at 823 K was gold in color. RHEED analysis indicates that the film obtained at 823 K was polycrystalline with a random orientation, while films obtained at 923-1123 K were formed of single crystals having a graphite-like structure. ESCA spectra suggested that in the films boron, carbon, and nitrogen atoms were bound in a layered structure. The results of RHEED and ESCA suggest that Ni(111) could play the decisive role of a substrate for hetero-epitaxial growth as well as a catalyst for the formation of crystalline B/C/N film. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

In order to obtain ultrafast responses with no additional slow component, we have investigated optical nonlinearity in molecular-beam-epitaxy(MBE)-grown GaAs at various excitation wavelengths L The slow response component decreases in magnitude and almost disappears when undoped GaAs is excited deep inside the energy band of GaAs. For beryllium-doped low-temperature-grown GaAs, fast responses of less than I ps response time with a negligible slow component are obtained from the bandedge to 230 meV or more above it. Measurement of saturation energy density I, of the optical absorption shows that the increase in I-s at lambda = 780 nm as compared to I, at the bandedge is only by a factor of 3-4 for undoped GaAs and it is very small for beryllium-doped low-temperature MBE-grown GaAs.

The effect of doping with Hf atoms on the nearly commensurate (NC) charge-density-wave (CDW) structures of 1T-TaS2 was investigated using scanning tunnelling microscopy (STM) at room temperature, for levels of substitution of Hf for Ta up to 6.9%. It was found that the Hf atom introduces a point defect to the CDW superlattice at the CDW domain boundary for lower contents. For Hf contents higher than 1%, the CDW superlattice melted and a distorted structure was observed instead of the usual CDW superlattice. The change of the CDW structure observed by STM was compared with the change of the rho-T characteristics.

Tunnelling spectroscopy measurements were carried out on the mixed crystal TiSe2-xSx (0 less than or equal to x less than or equal to 0.55) to investigate the change in the density of states in the CDW state. The temperature dependences of the CDW energy gap widths were obtained. Experiments showed that the Battening of the peak in the rho-T curve with increasing x corresponds to the decrease in the CDW energy gap. A model was proposed in which the anomalous rho-T characteristics in this system were explained in terms of the competition of the two conduction mechanisms associated with the metallic carriers and the 'excitonic' carriers.

The cow energy gap structure of TiSe2 has been observed by tunnelling spectroscopy and its temperature dependence obtained for 77 K less than or equal to T less than or equal to 295 K. In addition to the energy gap associated with the cow with T-c = 201 K, a narrower gap structure was observed to persist at higher temperature, which explains the negative temperature coefficient of the rho-T curve above T-c.