Yuko Inatomi

© 2014 Elsevier B.V. All rights reserved. Abstract A silicon germanium mixed crystal Si1-xGex (x~0.5) 10 mm in diameter and 9.2 mm in length was grown by the traveling liquidus-zone (TLZ) method in microgravity by suppressing convection in a melt. Ge concentration of 49.8±2.5 at% has been established for the whole of the grown crystal. Compared with the former space experiment, concentration variation in the axial direction increased from ±1.5 at% to ±2.5 at% although average Ge concentration reached to nearly 50 at%. Excellent radial Ge compositional uniformity 52±0.5 at% was established in the region of 7-9 mm growth length, where axial compositional uniformity was also excellent. The single crystalline region is about 5 mm in length. The interface shape change from convex to concave is implied from both experimental results and numerical analysis. The possible cause of increase in concentration variation and interface shape change and its relation to the two-dimensional growth model are discussed.

© 2015 AIP Publishing LLC. On the thermodynamic condition for forming a metastable phase from undercooled melt in a containerless state, we had proposed a criterion that crystals will preferentially form if they have a smaller entropy of fusion than the entropy of fusion of equilibrium crystals (Kuribayashi et al., Mater. Sci. Eng., A 449-451, 675 (2007)). This criterion is proposed for being applied to materials that exhibit a faceted interface, such as semiconductors and oxides. However, no experimental data that support this criterion have been obtained. From this point, we used an aerodynamic levitator as a tool for forming metastable phases from undercooled melt and verified the above-mentioned criterion using LnFeO3 (Ln: lanthanide and Y) as the model material. In addition, the condition for double recalescence, which corresponds to forming metastable phases and stable phases, was discussed in terms of competitive 2D isomorphic nucleation of the metastable phase and 3D polymorphic nucleation of the stable phase.

Compositionally homogeneous Ga-doped Si0.68Ge0.32 bulk crystals were grown with two different doping concentrations, i.e., 1 x 10(18) cm(-3) (GSG1) and 1 x 10(19) cm(-3) (GSG2), using a vertical gradient freezing method. The growth was carried out under a mild temperature gradient of 0.57 degrees C/mm using a sandwich structured sample, i.e., Si(seed)/Ga-doped Ge/Si(feed). The grown crystals were cut along the growth direction to study the compositional variations, etch pit densities (EPDs), and thermoelectric characteristics. Electron backscatter diffraction analysis indicated that the (111) orientation has a larger area compared with other orientations in the grown crystal. The electrical resistivity decreased along the growth direction, although the carrier concentrations and mobility of the crystals were unchanged, possibly because of the variation in EPDs. Moreover, the electrical resistivity was found to be large at the high EPD region of the crystal. The electrical resistivity of all the samples gradually increased with temperature. The maximum values of Seebeck coefficients in GSG1 and GSG2 samples were 466 mu V/K at 818 K and 459 mu V/K at 892 K, respectively. The calculated power factors of GSG1 and GSG2 were higher than previously reported values (1416 mu W m(-1) K-2) for Si0.81Ge0.19.

Space Utilization Research (January 24-25, 2015. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS)), Sagamihara, Kanagawa Japan

© 2015 Macmillan Publishers Limited. BACKGROUND: InxGa1 − xSb is an important material that has tunable properties in the infrared (IR) region and is suitable for IR-device applications. Since the quality of crystals relies on growth conditions, the growth process of alloy semiconductors can be examined better under microgravity (μG) conditions where convection is suppressed. AIMS: To investigate the dissolution and growth process of InxGa1 − xSb alloy semiconductors via a sandwiched structure of GaSb (seed)/InSb/GaSb(feed) under normal and μG conditions. METHODS: InxGa1 − xSb crystals were grown at the International Space Station (ISS) under μG conditions, and a similar experiment was conducted under terrestrial conditions (1G) using the vertical gradient freezing (VGF) method. The grown crystals were cut along the growth direction and its growth properties were studied. The indium composition and growth rate of grown crystals were calculated. RESULTS: The shape of the growth interface was nearly flat under μG, whereas under 1G, it was highly concave with the initial seed interface being nearly flat and having facets at the peripheries. The quality of the μG crystals was better than that of the 1G samples, as the etch pit density was low in the μG sample. The growth rate was higher under μG compared with 1G. Moreover, the growth started at the peripheries under 1G, whereas it started throughout the seed interface under μG. CONCLUSIONS: Kinetics played a dominant role under 1G. The suppressed convection under μG affected the dissolution and growth process of the InxGa1 − xSb alloy semiconductor.

Melt growth of benzophenone was in-situ observed under various cooling rate and temperature gradient. The melt growth experiments were performed for various cooling rate of melt and various temperature gradients. The movement of growth interface with time was measured for various growth faces such as (001), (101) and (011) and the growth rates of the respective faces were calculated. It was found that the growth rate of all the faces increased with cooling rate of the melt. Steady state growth was observed in the experiments with low temperature gradient while, the steep gradient leads to unstable growth of crystal. The observed growth rate variation was explained using the attachment energy model. © 2014 Elsevier B.V. All rights reserved.

Si dissolution into Ge melt, solute transport in the Si-Ge solution and crystal growth of SiGe alloys were in situ observed by X-ray penetration method. The rectangular shaped sandwich sample of Si (seed)/Ge/Si (feed) was used for the experiment. X-ray intensities penetrated through the sample, which was heated up to the growth temperature of 1200 C, were recorded by rectangular shaped CdTe line sensor as a function of time and temperature. The experimental results demonstrated that the dissolution of Si seed was larger compared to Si feed crystal although Si feed temperature was relatively higher than that of seed. Crystal growth of SiGe was observed at the feed interface as the growth interface was observed clearly by an abrupt change of penetrated X-ray intensity near the growth interface. Since the crystal grew with Si rich composition (at high temperature 1200 C), solution becomes Ge richer which causes penetrated X-ray intensity variation at the growth interface. The growth mechanism for the observed SiGe growth process was discussed based on the penetrated X-ray intensity profile and a growth model. The composition of the grown sample was measured by FE-EPMA analysis. © 2013 Elsevier B.V. All rights reserved.

The authors performed FACET (Investigation on Mechanism of Faceted Cellular Array Growth) experiments under long duration microgravity on the International Space Station (ISS) in 2010. The temperature and concentration distributions in the melt during the growth were precisely measured with high spatial resolution. Negative temperature gradient as well as negative concentration gradient ahead of the S/L interface can be expected to be the driving forces of the morphological instability. It is evident that the conventional model based on the frozen temperature approximation is insufficient to explain the growth mechanism of the faceted cellular array.

An alloy semiconductor Si1-xGex (x~0.5) crystal was grown by the TLZ method in microgravity. Ge concentration was 48.5±1.5 at% for the whole region of 10 mm diameter and 17.2 mm long crystal. Compositional uniformity was established but the average concentration was a little deviated from the expected 50 at%. For further improving compositional uniformity and for obtaining Si0.5Ge0.5 crystals in microgravity, growth conditions were refined based on the measured axial compositional profile. In determining new growth conditions, difference in temperature gradient in a melt, difference in freezing interface curvature, and difference in melt back length of a seed between microgravity and terrestrial growth were taken into consideration. © 2013 Elsevier Ltd. All rights reserved.

The thermal properties of InSb, GaSb and InxGa1-xSb, such as the viscosity, wetting property, and evaporation rate, were investigated in preparation for the crystal growth experiment on the International Space Station (ISS). The viscosity of InGaSb, which is an essential property for numerical modeling of crystal growth, was evaluated. In addition, the wetting properties between molten InxGa1-xSb and quartz, BN, graphite, and C-103 materials were investigated. The evaporation rate of molten InxGa1-xSb was measured to determine the affinity of different sample configurations. From the measurements, it was found that the viscosity of InxGa1-xSb was between that of InSb and GaSb. The degree of wetting reaction between molten InxGa1-xSb and the C-103 substrate was very high, whereas that between molten InxGa1-xSb and quartz, BN, and graphite substrates was very low. The results suggest that BN and graphite can be used as materials to cover InSb and GaSb samples inside a quartz ampoule during the microgravity experiments. In addition, the difference of the evaporation rate of molten InxGa1-xSb, GaSb, and InSb was small at low, and large at high temperature. (C) 2013 COSPAR. Published by Elsevier Ltd. All rights reserved.

The viscosities of molten InSb, GaSb, and Inx Ga1-xSb (x = 0.2 and 0.4) were measured as a function of temperature using an oscillating viscometer for fundamental understanding of the physical properties for fabricating high quality InGaSb multicomponent semiconductor crystals. The measured values showed good Arrhenius linearity for InSb, GaSb, and In x Ga1-x samples. The absolute values of the viscosity for InSb and GaSb agreed with a previous study. Also, it is suggested that the absolute values of the viscosity among the compounds are quite similar, and the results can be associated with their crystal structures. © 2014 Springer Science+Business Media New York.

© 2014 Elsevier B.V. Recently, a Si1-xGex(approximately x=0.5) crystal has been grown by the traveling liquidus-zone (TLZ) method under microgravity condition in the International Space Station (ISS). In this work, a mathematical model of the TLZ crystal growth has been developed to investigate details of the transport and solidification phenomena occurred during the TLZ growth of SiGe crystals performed in the ISS. Using this model, the experimental Ge concentration distributions in the grown SiGe crystal is explained, and the emissivity variation of the metal cartridge surface due to oxidation during the crystal growth is revealed to strongly affect the Ge concentration distribution in the grown crystal. In addition, a strategy for growing SiGe crystals, which are more homogeneous than those obtained in the current experiment, is proposed on the basis of the numerical results.

Crystal growth of alloy semiconductor has been under investigation at the International Space Station (ISS) to investigate growth kinetics at the solid-liquid interface, because microgravity can suppress the natural convection. In this study, we focused on InGaSb which is one of the promising ternary alloy semiconductors. As a preliminary experiment, wetting angle between the InGaSb and the materials of the ampoule and cartridge, such as quartz, carbon sheet, BN, and C-103 alloy, were measured to check their affinity of the configuration. The InGaSb exhibited much higher wetting ability on the C-103 substrate than that on the other substrates (quartz, BN, and graphite), and this result suggested C-103 can prevent leakage of the InGaSb samples in case the ampoule collapses in the cartridge. On the other hand, BN and graphite exhibited low wetting abilities with InGaSb; therefore, they are suitable materials for the ampoule because they do not significantly affect the InGaSb crystal growth. In another preliminary experiment, concentration of Te dopant, which was added to make striation in the grown crystal, was measured using inductively coupled plasma mass spectrometry (ICP-MS). It is confirmed that Te concentrations were relatively higher at the striation area.

Viscosity of molten InSb, GaSb and In_xGa_1-xSb (x = 0.2 and 0.4) were measured as a function of temperature using an oscillating viscometer for fundamental understanding of the physical property for growing high quality of InGaSb multi-component semiconductor crystal. The measured values showed good Arrhenius linearity for InSb, GaSb and In_xGa_1-xSb samples. The absolute values of viscosity for InSb and GaSb agreed with previous study of other group. Also, it is suggested that absolute value of viscosity among InSb, GaSb and In_xGa_1-xSb were quite similar, and the value of In_xGa_1-xSb was between InSb and GaSb. The results are applied to the parameter of numerical simulations on InGaSb crystal growth under microgravity condition on the International Space Station.

The review reported the microgravity experiments performed using space shuttle, drop tower and Chinese recovery satellite. In addition, the experimental plan for microgravity experiment in the International space station was introduced. From the space shuttle experiment, it was found that the space-grown sample with free melt surface was almost spherical and Marangoni convection enhanced the melt mixing. Whereas the melt mixing without free melt surface was controlled by diffusion. Formation of spherical projections on the surface of InGaSb was in-situ observed using a high speed CCD camera in the drop experiment. Microgravity studies on the dissolution and crystallization of In_xGa_1-xSb was carried out having the sandwich combination of GaSb(111)A/InSb/GaSb(111)B using the Chinese recoverable satellite. It was demonstrated clearly that the shape of the solid/liquid interface and composition profiles in the solution was significantly affected by gravity.

As a preliminary experiment for the growth of InGaSb alloy crystals under microgravity at International Space Station (ISS), bulk crystal was grown under terrestrial condition using the same gradient heating furnace (GHF). Czochralski grown GaSb <111>B single crystal was used as a seed and feed crystals for the growth of InGaSb bulk crystals. During the growth, heat pulses were intentionally introduced periodically to create the growth striations. From the striations, the growth rate of the grown crystal was estimated. The results show that the growth rate was gradually increased from the beginning of the growth and became stable. On the other hand the In composition of the grown crystal decreased along the growth direction. From the In composition, the temperature gradient in the solution was estimated and it was almost the same of that fixed during the growth. © (2012) Trans Tech Publications.

An in situ observation experiment of faceted cellular growth was carried out using transparent organic alloy, salol - t-butyl alcohol, in microgravity conditions on the International Space Station. The temperature and solute concentration fields in the vicinity of the solid-liquid and the growth rate were simultaneously measured by microscopic interferometers. © (2012) Trans Tech Publications.

The purpose of "Alloy Semiconductor" crystal growth project is to make clear the factors for crystal growth of a high-quality bulk alloy semiconductor by investigating (1) solute transport in liquid and (2) surface orientation dependence of growth kinetics under microgravity and terrestrial conditions. The temperature gradient furnace Gradient Heating Furnace (GHF) onboard "Kibo" is used for the growth of an In_xGa_1-xSb bulk crystal which is a potential substrate material of optoelectronic devices such as thermo-photo-voltaic cells and gas sensors, since the band gap and the lattice constant of the crystals are tuned by adjusting the composition. The current status of the space experiment project will be reported in the presentation.

There are numerous amounts of nanoparticles, which have been called cosmic dust, in universe. Since cosmic dust is a building block of planetary system and works as a substrate for molecular formation, the estimation of their characters such as the composition, size and number density is very important subject in the field of planetary and astronomical sciences. Cosmic dust forms via homogeneous nucleation from ejecta gas of a dying star. To understand the formation condition, nucleation theory and homogeneous nucleation experiment is required. Recent our experiment under gravity indicated that semi-phenomenological nucleation theory is able to explain a result of homogeneous nucleation experiment. We expect that a gas evaporation experiment under microgravity will give a chance to reproduce wider formation condition and better view of formation process of cosmic dust. In addition to the microgravity experiment, we are taking account of characteristic physical properties of nanoparticles to the cosmic dust, because the size of cosmic dust is nanometer scale. Here, we will show our recent results of direct observation of homogeneous nucleation in vapor phase.

Homogeneous polycrystalline Si1-xGex were grown using a Si(seed)/Ge/Si(feed) sandwich structure under the low temperature gradient less than 0.4 degrees C/mm. It was found that the composition of the Si1-xGex was controlled by the growth temperature. The homogeneous Mg2Si1-xGex was synthesized by heat treatment of the homogeneous Si1-xGex powders under Mg vapor. The Mg2Si1-xGex sample with the relative density of 95% was synthesized by spark plasma sintering technique. The resistivity and the Seebeck coefficient of the Si, Ge, Si1-xGex and Mg2Si1-xGex samples were evaluated as a function of temperature. It indicated that Seebeck coefficients of the Si1-xGex and Mg2Si1-xGex samples were higher than those of Si and Ge. Moreover, the Seebeck coefficient of Mg2Si0.7Ge0.3 sample was higher than that of Mg2Si0.5Ge0.5 and Si0.5Ge0.5 samples. (C) 2011 Elsevier B. V. All rights reserved.

We investigated the dissolution process of GaSb into InSb melt by numerical simulations using the finite volume method. In addition, the dissolution process was in-situ observed by the X-ray penetration method. Rectangular shaped GaSb (seed)/InSb/GaSb (feed) sandwich structure of sample was considered for the numerical analysis and the same structure of sample was used for the X-ray penetration experiment. The numerical and experimental results were comparatively analysed. From the results, it was found that the quantity of the dissolved GaSb seed (at the low temperature region) was larger than that of the feed (at the high temperature region). The numerical simulation results supported the experimental results well. Both the experiment and the simulation provide deep insight into the dissolution process and composition profile in the solution during the dissolution process of ternary alloy semiconductor crystal growth. © 2011 Elsevier B.V.

Compositionally homogeneous Si0.52Ge0.48 bulk crystals were grown under a mild temperature gradient using a Si(seed)/Ge/Si(feed) sandwich structure for thermoelectric (TE) applications. The furnace temperature was kept constant for 300 h for the growth of homogeneous Si1-xGex bulk crystal with various temperature gradients. The temperature gradient of 0.4 °C/mm resulted in homogeneous Si0.52Ge0.48 bulk crystals of 22 mm length with a Ge compositional fluctuation of about 0.0023/mm. The compositions of the grown crystals were determined by means of Electron Probe MicroAnalysis (EPMA). Electrical resistivity and Seebeck coefficient of the prepared samples were measured using in-house built measurement system. It was found that the compositional fluctuation of Ge was decreased as the temperature gradient of the furnace decreased. The prepared Si0.52Ge0.48 sample showed high Seebeck coefficient compared to that of pure Si and Ge crystals. © 2010 Elsevier B.V. All rights reserved.

Homogeneous SiGe crystal growth experiments will be performed on board the ISS "Kibo" using a gradient heating furnace (GHF). A new crystal growth method invented for growing homogeneous mixed crystals named "travelling liquidus-zone (TLZ) method" is evaluated by the growth of Si0.5Ge0.5 crystals in space. We have already succeeded in growing homogeneous 2mm diameter Si0.5Ge0.5 crystals on the ground but large diameter homogeneous crystals are difficult to be grown due to convection in a melt. In microgravity, larger diameter crystals can be grown with suppressing convection. Radial concentration profiles as well as axial profiles in microgravity grown crystals will be measured and will be compared with our two-dimensional TLZ growth model equation and compositional variation is analyzed. Results are beneficial for growing large diameter mixed crystals by the TLZ method on the ground. Here, we report on the principle of the TLZ method for homogeneous crystal growth, results of preparatory experiments on the ground and plan for microgravity experiments.

Dissolution process of GaSb into InSb melt was observed by an X-ray penetration method. The intensity of X-rays penetrated through the rectangular shaped GaSb (seed)/InSb/GaSb (feed) sandwich sample was recorded by the CdTe line sensor detector. The penetrated X-ray intensities and images of the sample were obtained as a function of time and temperature. The gallium (Ga) composition profile of the sample was calculated as a function of time by making the calibration line with the penetrated X-ray intensities of GaSb and InSb standard samples. The calculated Ga composition profile of the grown sample agreed well with the data measured by energy dispersive X-ray spectroscopy analysis. The result suggested that lower GaSb seed dissolved faster than upper GaSb feed despite of the low temperature at the lower GaSb seed. It clearly indicates that the solutal transport induced by gravity strongly affects the dissolution process. © 2010 Elsevier B.V. All rights reserved.

In this study, a real-time optical system was developed to observe crystallization in a small spherical melt droplet (few millimeters in diameter) by containerless processing. This system can be used to simultaneously observe the inside and the surface of a transparent melt droplet, as well as its ambient gas atmosphere at high temperatures. A silicate melt with a diameter of similar to 2 mm and a composition of MgO: SiO(2)=48:52 was levitated using a gas-jet levitation system, and its crystallization process was successfully observed from 2385 K in real time with good contrast using the developed optical setup. (C) 2010 American Institute of Physics. [doi:10.1063/1.3462968]

The present work is concerned with the real time in situ visualization of crystallization processes inside strongly supercooled silicate melts using optical projection technique. The crystallization experiments are carried out for forsterite composition under container-less conditions. Starting material is heated above its liquidus temperature (2169 K) using a high power CO2 laser and crystallization is initiated following rapid cooling. Three different values of supercooling (Delta T approximate to 320, 400, and 500 K, calculated with respect to the liquidus temperature of forsterite composition as reference) are independently employed to initiate the nucleation process by adjusting the output power of CO2 laser. Primary findings of the study show that a suitably designed optical system is capable of imaging melt convection at temperatures as high as near liquidus and presents a novel approach for the prediction of resultant crystallization textures in real time nondestructively. Using the developed optical arrangement, formation of porphyritic-like textures and parallel-barred structures could be successfully visualized during the crystallization process. The results also reveal that for very large values of supercooling, it is possible to initiate nucleation from inside the melt droplet. The in situ predictions of resultant crystalline textures are compared with the textures revealed by photomicrographs of the corresponding thin sections and a good agreement is seen between the two observations. (C) 2010 American Institute of Physics. [doi:10.1063/1.3406149]

Microgravity studies on the dissolution and crystallization of InxGa1-xSb have been done using a sandwich combination of InSb and GaSb as the starting material using the Chinese recoverable satellite. The same type of experiment was performed under 1G gravity condition for comparison. From these experiments and the numerical simulation, it is found that the shape of the solid/liquid interface and composition profile in the solution was found to be significantly affected by gravity. GaSb seed was dissolved faster than GaSb feed even though the GaSb feed temperature was higher than that of GaSb seed temperature. These results clearly indicate that solute transport due to gravity affects dissolution and growth processes of alloy semiconductor bulk crystals.

Homogeneous semiconductor alloy crystals of Si0.5Ge0.5 will be grown aboard the International Space Station (ISS). We are preparing for microgravity experiments. We first invented a new growth method named the traveling liquidus-zone (TLZ) method for growing compositionally uniform alloy crystals. We determined growth conditions for the TLZ method on the ground. In this paper, we describe the current state of preparation for microgravity experiments.

The new phase field model for strongly an Isotropic systems proposed by Torabi et al(1)) was employed to simulate the faceted cellular growth in three dimensions Simulation reveals the whole formation process of the faceted cellular clearly Simulation results also show that a linear relation of undercooling and growth velocity when the shape selection is completed at the late stage of the evolution but a nonlinear relation holds during the shape selection stage During the facet cellular formation the crystal-melt interface is kept isothermal although the interface is sawtooth and areas with negative temperature gradient appear in the melts particularly at the ravine bottom of the sawtooth interface

Homogeneous semiconductor alloy crystals of S0.5Ge0.5 will be grown aboard the International Space Station (ISS). We are preparing for microgravity experiments. We first invented a new growth method named the traveling liquidus-zone (TLZ) method for growing compositionally uniform alloy crystals. We determined growth conditions for the TLZ method on the ground. In this paper, we describe the current state of preparation for microgravity experiments.

To provide long duration and good quality of micro-gravity environment with moderate cost, we proposed and have been developed an experiment system that is released from a high altitude balloon. The experiment system has a double-shell drag-free structure and it is controlled not to collide with the inner shell to realize good quality of micro-gravity environment. This paper shows the configuration of the experiment system and summarizes its five-year development including three flight test results. The fist stage of the development was successfully completed this year. The next step is micro-gravity fall with engine for longer duration of experiment. Another direction of the development is real operation of the system for micro-gravity scientists. Those future plans are also described.

The second flight of microgravity experiment system using a free fall capsule from a high altitude balloon was conducted in May 2007. Using a drag free control, around 10^-4G gravity conditions were obtained for 30 seconds. Results of a combustion experiment with Japanese sparker conducted inside the microgravity experimental unit were also reported.

Rocket-shaped vehicle is developed to conduct microgravity experiment by dropping from the high-altitude balloon. Its design strategy and development status is introduced. Also, the result of its 2nd flight test is summarized to show the feasibility of the balloon-based microgravity experiment.