Y. Hayakawa

The measurement of deep level states caused by misfit dislocations in InGaAs/GaAs grown on patterned GaAs substrates

Rectangular Schottky diodes were fabricated on In0.06Ga0.94As grown by organometallic vapor phase epitaxy on GaAs substrates patterned with mesas. The density of α and β misfit dislocations at the strained‐layer interface changed with the size of the rectangular mesas. Since all mesas (four sizes and two orientations) are processed simultaneously, all other defect concentrations are expected to remain constant in each diode. Scanning cathodoluminescence showed that the misfit dislocation density varied linearly with rectangle size. Deep‐level transient spectroscopy showed that an n‐type majority‐carrier trap is present at 0.58 eV below the conduction band with a concentration that increases with increasing α‐type misfit dislocation density. The β misfit dislocation density had no influence the deep level spectra, indicating that this trap is related to the cores of only α‐type misfit dislocations. The capture rate trend corroborates the view that the trap is associated with the dislocation cores and not w...

Improved thermoelectric performance of hot pressed nanostructured n-type SiGe bulk alloys

Silicon germanium alloys (Si80Ge20) have been used in thermoelectric generators for deep space missions to convert radioisotope heat into electricity. This work demonstrates the highest value of thermoelectric figure-of-merit (ZT) ∼1.84 at 1073 K for n-type SiGe nanostructured bulk alloys, which is 34% higher than the reported record value for n-type SiGe alloys. The optimized samples exhibit a Seebeck coefficient of ∼284 μV K−1, resistivity of ∼45 μΩ m and thermal conductivity of ∼0.93 W m−1 K−1 at 1073 K. The main contributing factor for the enhanced ZT is very low and almost temperature independent thermal conductivity, which overcomes the low power factor of the material. Significant reduction of the thermal conductivity is caused by the scattering of low, medium and high wavelength phonons by atomic size defects, dislocations, and grain boundaries that are present due to the formation of nanocrystalline grains in the bulk material.

Experimental and numerical investigations on dissolution and recrystallization processes of GaSb/InSb/GaSb under microgravity and terrestrial conditions

The effects of gravity and crystal orientation on the dissolution of GaSb into InSb melt and the recrystallization of InGaSb were investigated under microgravity condition using a Chinese recoverable satellite and under normal gravity condition on earth. To investigate the effect of gravity on the solid/liquid interface and compositional profiles. a numerical simulation was carried out. The InSb crystal melted at 525 degrees C and then a part of GaSb dissolved into the InSb melt during heating to 706 degrees C and this process led to the formation of InGaSb solution. InGaSb solidified during the cooling process. The experimental and calculation results clearly show that the shape of the solid/liquid interface and compositional profiles in the solution were significantly affected by gravity. Under microgravity, as the Ga compositional profiles were uniform in the radial direction. the interfaces were almost parallel. On the contrary, for normal gravity condition, as large amounts of Ga moved up in the upper region due to buoyancy, the dissolved zone broadened towards gravitational direction. Also. during the cooling process, needle crystals of InGaSb started appearing and the value of x of InxGa1-xSb crystals increased with the decrease of temperature. The GaSb with the (111)B plane dissolved into the InSb melt much more than that of the (111)A plane

Effect of Ultrasonic Vibrations on InSb Pulled Crystals

InSb single crystals were pulled from the melt by the Czochralski method. During the growth of the crystals, ultrasonic vibrations were introduced into the melt through the carbon crucible. The facet region at the center of the pulled crystals shifted to the periphery of the crystals and simultaneously decreased in size, because the ultrasonic vibrations stirred the melt and raised its temperature. Furthermore, the difference in the values of the spreading resistance between the facet and the off-facet regions also decreased. These results indicate that inhomogeneous distributions of impurity concentration in the crystals were improved by the introduction of ultrasonic vibrations.

High thermoelectric performance of (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites having all-scale natural hierarchical architectures

Recent studies have shown that thermoelectric materials exhibit a high figure-of-merit if it consists of hierarchically organized microstructures that significantly lower the lattice thermal conductivity without any appreciable change in the power factor. Here, we report a new class of thermoelectric (AgCrSe2)0.5(CuCrSe2)0.5 nano-composites synthesized via the vacuum hot pressing of a mixture of the constituents, which naturally consists of phonon scattering centers in a multiscale hierarchical fashion, i.e. atomic scale disorder, nanoscale amorphous structure, natural grain boundaries due to layered structure and mesoscale grain boundaries/interfaces. The presence of a natural hierarchical architecture of different length scales in the composite samples is confirmed by scanning electron and transmission electron microscopy. Detailed characterization reveals that in the composite samples there is a slight migration of Cu into the Ag site. Composite samples exhibit extremely low thermal conductivity ∼2 mW cm−1 K−1 at 773 K, which is nearly one third of the pure AgCrSe2 and CuCrSe2. The composite samples exhibit a high ZT ∼ 1.4 at 773 K, which is attributed to the scattering of heat carrying phonons of all wavelengths via the natural hierarchical architecture of the material. The ease of synthesis of such high performance (AgCrSe2)0.5(CuCrSe2)0.5 nanocomposites with a natural hierarchical architecture offers a promise for replacing conventional tellurides.

Bias and temperature dependent charge transport in high mobility cobalt-phthalocyanine thin films

The temperature dependent current-voltage (J-V) characteristics of highly-oriented cobalt phthalocyanine films (rocking-curve width=0.11°) deposited on (001) LaAlO3 substrates are investigated. In the temperature range 300–100 K, charge transport is governed by bulk-limited processes with a bias dependent crossover from Ohmic (J∼V) to trap-free space-charge-limited conduction (J∼V2). The mobility (μ) at 300 K has a value of ∼7 cm2 V−1 s−1 and obeys Arrhenius-type (ln μ∼1/T) behavior. However, at temperatures <100 K, the charge transport is electrode-limited, which undergoes a bias dependent transition from Schottky (ln J∼V1/2) to multistep-tunneling (conductivity varying exponentially on the inverse of the square-root of electric field).

CuCrSe2: a high performance phonon glass and electron crystal thermoelectric material

The efficient conversion of heat into electricity using a thermoelectric approach requires high performance materials with the thermoelectric figure of merit ZT ≥ 1. Here we report on bulk CuCrSe2, which exhibits a very high ZT ∼ 1 at 773 K. The titled compound exhibits an electrical resistivity of ∼2.8 mΩ cm, a Seebeck coefficient of ∼160 μV K−1, together with very low thermal conductivity ∼7 mW cm−1 K−1 at 773 K. The very low thermal conductivity of bulk CuCrSe2 is attributed to phonon scattering by various sources such as (i) superionic Cu ions between the CrSe2 layers, (ii) nanoscale precipitates in the bulk and (iii) natural grain boundaries due to the layered structure of the material. This unusual combination of thermoelectric properties for CuCrSe2 suggests that it is an ideal example of the phonon glass and electron crystal approach.

Single-step synthesis and catalytic activity of structure-controlled nickel sulfide nanoparticles

Nanoparticle single-phase nickel sulfides such as NiS, NiS2, Ni3S4, and Ni7S6 were prepared from elemental sulfur and nickel nitrate hexahydrate, using a temperature-controlled precursor injection method. The initial ratio of the concentrations of the sources was used to control the size and phase of the final product. Phase control was confirmed using X-ray diffraction and transmission electron microscopy. The synthesized nickel sulfide phases, which had metallic characteristics, were used to study the catalytic reduction of 4-nitrophenol. The results showed that the catalytic activity of the NiS nanoparticles in the reduction of 4-nitrophenol to 4-aminophenol was higher than those of the other nickel sulfide phases. In addition, the nanocrystals showed good separation ability and reusability for reduction of 4-nitrophenol.

Melt Mixing of the 0.3In/0.7GaSb/0.3Sb Solid Combination by Diffusion under Microgravity

In order to investigate the effects of diffusion and convection on the melt mixing of semiconductors, experiments under microgravity in space and 1-g on earth were conducted. Sandwich combinations of In/GaSb/Sb solids closed in a BN cylinder were heated up to 733° C in space and 744° C on earth, and they were then cooled rapidly. In both samples, many needle crystals were distributed in the whole area. It was observed that the melt mixing in space was controlled by diffusion which was represented with an error function, and the diffusion coefficient of indium was given by a value of 2.4×10-4 cm2/ s. In the earth sample, however, the indium concentration distribution followed an exponential curve. This indicated that both factors, diffusion and thermal convection, have contributed to the mixing of semiconductor melts.

The effect of the substitution of Sm for Ba on the superconductor

Characteristic changes occurring in superconductors on substituting Sm for Ba have been investigated. An appropriate amount of Sm substitution (, in the form of ) promoted grain growth; the grain size became larger than that in the non-substituted sample and had a maximum value at x = 0.07. Single-phase samples were obtained for . The critical temperature increased by about 1.5 K for concentrations up to x = 0.02, while it decreased rapidly beyond this amount. Samples did not show superconductivity above 15 K for . The crystal structure transformed from orthorhombic to tetragonal at x = 0.3. The average valency of the Cu and the oxygen content decreased linearly with increase of x in the ranges of and , while they both increased linearly for the range .