Yoshiro Akagi

AES and XPS studies of surface of AlxGa1-xAs (110) treated by ammonium sulfide

We applied the technique of ammonium sulfide treatment to AlxGa1-xAs (110) surfaces and investigated the atomic composition of a surface by Auger electron spectroscopy and X-ray photoelectron spectroscopy. It was found that surface oxidation was significantly reduced by the sulfur treatment and that sulfur atoms were mainly bonded to Al atoms in an Al-enriched sample of AlxGa1-xAs (x=0.78). The surface compositions of the Al-enriched specimen (x=0.43, 0.78) were also found to be As-riched as a result of both sulfur treatment and the preferential etching of metallic atoms by an acidic solution, which prevents the initial oxidation of the surface of AlxGa1-xAs.

Improvement of high-power characteristics of 780-nm AlGaAs laser diode by (NH4)2S facet treatment

Coherent cw operation has been obtained with a 10x4 array of Grating Surface Emitting (GSE) lasers consisting of 40 lasers and 50 emitting sections. The array is a GaAs quantum well device, grown on an A1GaAs substrate, operating at 861 nm to which the substrate is transparent. It is mounted p-down to metallized traces on a BeO slab to provide isolated electrical contacts and thermal contact to a simple chilled-water cooler. A single spectral line 0.5 A wide indicates coupling of the 40 laser sections. More detailed measurements on a section of an array containing ten laterally coupled lasers, 20 outputs, show that it is operating at a junction temperature of 30°C, has a line width of 0.1 5A, and a measured coherence of >75%. The expected wavelength stabilization with temperature due to the DBR grating is found, with a value of =0.6A/°C. An array of 4 longitudinally coupled lasers produced a line width of 130MHz and evidence of high coherence.

Tunnelling molecular motion in glassy glycerol at very low temperatures as studied by 1H SQUID nuclear magnetic resonance

The 1H nuclear spin-lattice relaxation process in glycerol has been studied at temperatures from 3.5 K to 300 K over a very wide range of Larmor frequency between 236 kHz (0.00554 T) and 21.0 MHz (0.4932 T). A superconducting quantum interference device (SQUID) was used to detect the longitudinal component of magnetization of the proton at very low frequencies below 1.62 MHz. At sufficiently low temperatures the nuclear spin-lattice relaxation rate obeys a relation 1/T1∝(T2/ωβ)∫6/T0[(x dx)/sinh x], (with β around 0.9 below 25 K), implying that the relaxation rate is governed by an excitation of low-frequency disordered modes inherent to the glassy state of glycerol and becomes asymptotically 1/T1∝T2 below T = 3 K and 1/T1∝T above T = 3 K. The relaxation phenomena can be interpreted as the nuclear spin flipping associated with a Raman process which is induced by a coupling of thermally activated low-frequency disordered modes or low-frequency excitation (LFE) with a phonon bath. The LFE originates from a quantum-mechanical two-level system (TLS) reflecting an asymmetric-double-well (ASDW) potential which is formed by the hydrogen bonding configuration in the glassy state of glycerol. The maximum characteristic asymmetry of the double-well potential was found to be (3±1) K. This quantum-mechanical molecular motion dominates the other relaxation mechanisms at low temperatures, such as the dipolar relaxation due to molecular classical reorientation with distributed correlation times.

Preparation of highly oriented thin film of nonlinear optical material by vacuum deposition

Highly (001)‐oriented thin films of 4‐(N,N‐dimethylamino)‐3‐acetamidonitrobenzene were prepared on a thermally oxidized silicon substrate using vapor deposition. The degree of (001) orientation depends considerably on the substrate temperature (Ts) and deposition rate (R), which determines the kinetic energy (Ek) of the molecules in a flux. Under the optimized experimental parameters of Ts=30 °C and R=600 nm/min, we fabricated the (001)‐oriented thin film showing the full width at half‐maximum of x‐ray rocking curve of 1.6°. Surface flatness was also improved as the deposition rate increased. It became clear that kinetic energy (Ek) of the molecules in an evaporating flux strongly promoted (001) preferred orientation as could be seen by observing the surface morphology and a cross‐sectional view of the samples at their initial stage of growth.

Influence of Preferred Orientation in Indium Tin Oxide.

We investigated the electrical and optical properties, the chemical composition, the surface morphology and the crystallinity of sputtered Sn-doped In 2 O 3 films deposited on different substrate positions. Both the electric conductivity and the optical transparency are related to the free carrier density and moreover the preferred crystal orientation. The former relation is attributed to the Burstein-Moss effect and the latter suggests the Sn doping mechanism.

Aes and Xps Studies of Sulfur-Treated AL x GA 1−x as Surfaces

Passivation of Al x Ga 1−x As surfaces by ammonium sulfide treatment has been investigated. Enhancement of photoluminescence intensity and dramatic reduction of oxide peaks in spectra acquired by X-ray photoelectron spectroscopy on Al x Ga 1−x .As surfaces prepared by this treatment lead to the reduction of surface state density. It was found that sulfur atoms were mainly bonded to Al atoms in an Al-enriched specimen of Al x Ga 1−x As(x=0.78). The surface compositions of Al-enriched specimen(x=0.43, 0.78) were also found to be As-rich as a result of both sulfur treatment and the preferential etching of metallic atoms by an acidic solution, which prevents the initial oxidation of the surface of A1 x Ga 1−x As.

Effect of anisotropic current of the order parameter on InxGa1-xN compositional fluctuation caused by phase separation

We studied details of the anisotropic spatial fluctuation in In compositions numerically as described by an early stage in the time evolution of InGaN phase separation using the cell dynamical system (CDS) approach. We extended the previous isotropic CDS approach to the anisotropic case of current of the order parameter corresponding to the two-dimensional anisotropic diffusion of the constituents. The results of this numerical-based work indicate that the anisotropic current of the order parameter, which consists of a reduction of the current in one direction and a nonreduction in the opposite direction, suppresses the In compositional fluctuation. This suppression arises from the unidirectional current of the order parameter, which relates to the enhancement of anisotropic diffusion of the constituents. Thus we substantiate the homogeneous growth of the InGaN film by using the surface of an off-axis substrate consisting of uniformly spaced steps with high density.

Diffusion coefficient of the order parameter in the early stages of the time evolution of InGaN phase separation

We have carried out a study involving a model for the diffusion mechanism of group III atoms in an InGaN alloy semiconductor that uses a cell dynamical system (CDS) approach. In this CDS analysis, we focused on gathering information concerning the In-rich region with a dot-like structure that is formed at an early stage in the time evolution of phase separation of this alloy. A similar dot-like structure is known to appear in the active layer of InGaN-based multiquantum well laser diodes. Thus, we can determine the diffusion coefficient of group III atoms in InGaN by comparing the dot-like structures observed by cross-sectional transmission electron microscopy with those obtained by the CDS numerical approach. In order to perform this comparison quantitatively such that we could use it to calculate the diffusion coefficient, we derived a relationship between the rule of mapping used in CDS, and the discretization of the Cahn–Hilliard–Cook equation, which describes the dynamics of phase separation with a c...

Effects of Nitrogen Doping in the Insulational Character of Anodically Oxidized Films of Tantalum

The effects of nitrogen doping as a terminator in an anodically oxidized film of tantalum have been investigated. In the oxide film of nitrogen-free tantalum, the electric leakage current abruptly increased with the applied voltage. It is well-known as the Poole-Frenkel effect [1] [2]. After annealing at 623K in a hydrogen atmosphere, the leakage current increased. On the other hand, in the oxidized films of nitrogen-doped tantalum the leakage current increased in proportion to the applied voltage, but it was very small as compared with the nitrogen-free oxide. Moreover the leakage current decreased after annealing. The decrement strongly depended on the amount of doped nitrogen.

EPR Studies of Cu2+ and Gd3+ in High-Tc Superconductors Y1Ba2Cu3O6+x

EPR of Cu2+ and Gd3+ in Y1Ba2Cu3O6+x have been studied between about 85K and 300K. Experiments reveal that EPR linewidths are temperature independent for both ions. This indicates that the magnetic interaction between these ions and conduction electrons is vanishingly small. Experiments also indicate that phonon instability such as softening may not occur in the YBaCuO system in the temperature range.