T. Ohachi

A new theoretical approach to adsorption–desorption behavior of Ga on GaAs surfaces

Abstract We propose a new theoretical approach for studying adsorption–desorption behavior of atoms on semiconductor surfaces. The new theoretical approach based on the ab initio calculations incorporates the free energy of gas phase; therefore we can calculate how adsorption and desorption depends on growth temperature and beam equivalent pressure (BEP). The versatility of the new theoretical approach was confirmed by the calculation of Ga adsorption–desorption transition temperatures and transition BEPs on the GaAs (0 0 1) - (4×2) β2 Ga-rich surface. This new approach is feasible to predict how adsorption and desorption depend on the growth conditions.

Roughening transition for the ionic-electronic mixed superionic conductor α-Ag2S

Abstract Experimental observation of the equilibrium roughening transition of the high temperature phase of Ag 2 S (bcc structure) is reported. The roughening temperatures for {110} and {100}, which were measured for the vanishing of its facet, are 580 and 480°C respectively. The roughening transition in the heating process occurred continuously. The size of a facet decreased to zero during heating and was independent of the volume of a crystal was larger than a certain size. This special growth system with a solid/vapour reaction is only applicable to systems of ionic-electronic mixed superionic conductors, such as α - Ag 2 S.

Theoretical approach to influence of As2 pressure on GaAs growth kinetics

Abstract The newly developed first-principles calculation based computational method incorporating chemical potential of As 2 gas is applied to understand the influence of As 2 pressure on GaAs growth kinetics under the molecular beam epitaxy growth conditions with high As 2 pressures where the c(4×4) reconstructed structure appears on the surface. The calculated results suggest that the chemical potential of As 2 gas increases with As 2 pressure, which suppresses As 2 (As-dimer) desorption or extends As 2 surface lifetime. This induces the decrease of GaAs growth rate, because GaAs layer-by-layer growth does not proceed without As 2 desorption on the As-rich c(4×4) surface.

ac ionic conductivity of hollandite type compounds from 100 Hz to 37.0 GHz

Abstract The frequency- and temperature-dependences of a.c. ionic conductivity of one-dimensional super-ionic conductors K-priderites with a hollandite type structures were investigated from 100 Hz to 37.0 GHz. Four kinds of K-priderite, K 1.6 Mg 0.8 Ti 7.2 O 16 , (K 1.3 , Li 0.1 ) Mg 0.7 Ti 7.3 O 16 , K 1.6 Al 1.6 Ti 6.4 O 16 and (K 1.3 , Li 0.2 ) Al 1.5 Ti 6.5 O 16 , were studied. An equivalent circuit to combine the data of the complex conductivity at low and high frequencies was proposed. The data of complex conductivity at low frequencies can be analyzed in terms of the moving box model proposed by Beyeler et al. The transport of K + ions at low frequencies is characterized by the cooperative motion of the K + ions with various mobilities and is accompanied with the polarization of the K + ions in the channels. The ion transport across intrinsic barriers at or above microwave frequencies is characterized by the frequency-independent ionic conductivity and is interpreted by the configurational model proposed by Beyeler et al. The height of intrinsic barriers is related to the lattice constants of a crystal.

Ammonium sulfate regulation of morphology of Nd:Y2O3 precursor via urea precipitation method and its effect on the sintering properties of Nd:Y2O3 nanopowders

Ammonium sulfate has been widely used as a control agent in the preparation of yttrium-aluminium-garnet (YAG) transparent ceramics, however, research of its application in the preparation in transparent ceramic yttria has not been intensively studied. Neodymium–doped yttria (Nd:Y2O3) nanopowders with controlled morphology and size were synthesized via a urea precipitation method using ammonium sulfate as the additive. The effect of ammonium sulfate was intensively studied throughout the preparation process. Morphology of precursors was found to be evidently affected by the [(NH4)2SO4]/[Nd:Y2O3] ratio (measured by weight). Uniform spheres of Nd:Y2O3 precursor were obtained without the addition of ammonium sulfate. With increasing amounts of ammonium sulfate added, the scale of the Nd:Y2O3 precursors diminished which results in the aggregation of the Nd:Y2O3 precursor. Aggregates of coral like particles after precipitation and uniform well dispersed particles after calcinations were obtained as the dosage of ammonium sulfate reached 20 wt%. It was considered to be the optimum state for the preparation of highly sinterable Nd:Y2O3 nanopowders. Ammonium sulfate was proved to be a regulator that could mediate the nucleation and growth of the precursor as well as its decomposition behaviour. Results of this paper can contribute to the controllable synthesis of transparent ceramic yttria.

Frequency-independent ionic conductivity of hollandite type compounds

Abstract The frequency- and temperature-dependences of a.c. ionic conductivity of one-dimensional superionic conductors K-priderites, K 1.6 Mg 0.8 Ti 7.2 O 16 (KMTO) and K 1.6 Al 1.6 Ti 6.4 O 16 (KATO), which were grown by using a flux with high grade purity, were investigated from 100 Hz to 10 MHz. The complex ionic conductivity along a c-axis was analyzed by using an equivalent circuit constructed by considering the frequency-dependent ionic conductivity σ mb and the frequency-independent ionic conductivity σ dc which was not observed for samples grown by using a flux with normal grade purity. The σ dc was thermally activated and the activation energies for KMTO and KATO were 0.26 and 0.30 eV respectively. The lateral conductivity was very small.

The contribution of anion disorder to ionic conductivity on single crystals of β-PbF2

Abstract The effect of the different cooling processes on the disorder of flourine ions and ionic conductivity in β-PbF 2 has been studied by X-ray method and ionic conductivity measurements on single crystals below the transition temperature T c . The spike-like diffuse scattering was observed along the ∗ directions around the Bragg reflections. The activation energies for the conduction process are 0.40 eV for the sample quenched from 970 K and 0.54 eV for the one from 720 K. The higher the quenching temperature is, the higher the conductivity and the lower the activation energy become. The dependence of conductivity on the different cooling processes is more evident in single crystals than in polycrystalline samples. The contribution of the different cooling processes to ionic conductivity can be quantitatively explained by the extent of ordering of mobile fluorine ions. Time dependence of ionic conductivity has not been observed.

Controlled filamentary growth of silver from silver compounds

Abstract Growth of filamentary silver was controlled using a solid-state-ionics technique. The controlled growth process was recorded on a 16 mm film, “High-speed growth of silver filaments”. The morphology of filaments was examined using a scanning electron microscope. The growth representing a variety of silver compounds is classified into three groups according to their ionic and electronic conductivity, σ1 and σe respectively. (1) Growth from α-Ag2X (X = S, Se, or Te) (σ1: large, σe: large); the maximum growth rate is about 30 μm/sec and corresponds to the maximum nucleation rate. The filament is fluted and thicker at its root and appears to consist of a group of smaller filaments. This growth process is reversible, that is the grown filaments can re-dissolve in the compounds.(2) Growth from α-Agl (σ1: large, σe: small). The maximum growth rate is about 103 μm/sec which is also the maximum nucleation rate. This high growth rate may be due to electronic breakdown in α-Agl. A bunch of filaments, which are fin-like and have a orientation, is cotton-like in appearance. This process is also reversible. (3) Growth from AgBr or AgCl (σ1: small, σe: small); whiskers and dendrites grow very slowly owing to the small ionic conductivity. The dendrites have many steps.

Phase transition and cluster formation in silver selenide

Abstract Kinetics of polymorphic transition of silver selenide is presented. Silver selenides formed at both low temperature by β-Ag 2 Se (coprecipitation samples) and at high temperature by α-Ag 2 Se (single crystals) were investigated to show a Ag-trapping phenomenon i.e. the formation of Ag clusters at Ag atomic traps. DSC (differential scanning calorimetry) signals and resistivity data explain the formation of Ag clusters in the cooling process and their dissolution in the heating sequences. The trapping phenomenon of the coprecipitation samples, which were prepared at the β-phase, was affected by phase transition. The Ag trapping phenomenon at the β-phase of a single crystal, quenched from a phase with excess Ag, was also observed. The cooling process relates to the kinetics of the phase transition itself and the cluster formation which is affected by lattice defects and the concentration of non-stoichiometric Ag atoms.

Arsenic vapor pressure dependence of surface morphology and silicon doping in molecular beam epitaxial grown GaAs (n11)A (n=1–4) substrates

Surface morphology of undoped AlGaAs/GaAs asymmetric double quantum wells (ADQWs) and Si-doped single epitaxial GaAs layers were studied by atomic force microscopy. Samples were prepared on GaAs(n11)A (n=1–4) substrates in molecular beam epitaxy at different As pressures and the same substrate temperature and Ga flux. The surfaces of (n11)A ADQWs change from rough to smooth and featureless with increasing As pressures, while that of (100) sample shows an inverse change according to the atomic force microscopy results. Triangular pyramidal facets were observed at lower As pressure for (111)A and (211)A orientations. The interface roughness of ADQWs was improved at higher As pressure showing by a transmission electron microscope observation. The conduction type of (n11)A (n<=4) was p-type for lower As pressure and n-type for higher As pressure.