To prepare a silicon-germanium bulk crystal with uniform composition distribution, it is necessary to supply solute component to the growth interface and to control the growth interface temperature. The multicomponent zone melting method with the feedback system of the growth interface temperature which we established fulfills these necessary conditions. The homogeneous bulk crystal with the length of 20 mm over was successfully grown by this method. This paper provides an overview of this growth method and related in-situ techniques to measure position and temperature of growth interface during the crystal growth. To grow high quality crystals, it is also necessary to control supercooling and supersaturation in the solution. We also introduce an in-situ observation technique for supersaturation.
We have invented a new crystal growth method for growing homogeneous In_<0.3>Ga_<0.7>AS crystals. In this method, a liquidus zone is formed by heating a feed with graded or step solute concentration at relatively low temperature gradient i,e., 10 to 20℃/cm. The liquidus-zone travels spontaneously by diffusion due to concentration gradient of solute in the zone and segregation at the freezing interface. When the sample is translated at the rate of the spontaneous zone traveling rate, the solute concentration and the temperature at the freezing interface are kept constant and long homogeneous mixed crystals can be grown. We, therefore, named the new method the traveling liquidus-zone (abbreviated as TLZ) method. The principle of the TLZ method and some experimental results are described. Merits of the TLZ method in microgravity are also discussed.
Highly pure Si was undercooled by an electromagnetic levitator combined with a laser heating unit. The crystal growth velocity was measured as a function of undercooling and the appearance of the solid-liquid interface was observed by a high-speed camera. The result was compared with the predicted value based on the dendrite growth theory. The growth behaviors of Si were found to be classified into three categories of plate-like growth (region I) , isolated dendrite growth (II) , and closer dendrite growth (III) at low, moderate, and high undercooling values, respectively. The transition undercoolings for the classifications were 100 and 210 K. In region I, a thin plate crystal was split to several plates at the undercooling of over 50 K. A novel stability criterion for the transition was derived from the relationship between the plate thickness and the tip radius of the solidification front.
Marangoni convection, which is one of mechanisms of heat and mass transfer during crystal growth, was investigated by using a liquid-bridge configuration under microgravity and on earth. Using microgravity is a convenient way to study Marangoni convection, because buoyancy flow can be suppressed so that only Marangoni flow can be distinguished. In the liquid-bridge configuration, which corresponds to floating-zone growth, flow instability and its three-dimensional structure were investigated through measurement of temperature-oscillation, flow visualization, optical pyrometry of the melt surface, observation of oscillation of the melt/crystal interface, and observation of surface oscillation by phase-shift interferometry. Azimuthal wave number m for instability structure depends on the aspect ratio of the bridge, T, which is defined as the ratio of height h to radius r. Marangoni flow was found to be affected by oxygen partial pressure of the ambient atmosphere, which corresponds to concentration of oxygen in Si melt. This is very important finding, because for the Czochralski growth system, oxygen dissolves into melt from a crucible wall made of SiO_2 It was also found that surface tension and its temperature coefficient strongly depend on oxygen partial pressure. Above the equilibrium oxygen partial pressure for SiO_2, where the total droplet system behaves as a liqund but the melt surface is coated with a SiO_2 film, surface stress and its temperaTokyo Institute of Technology ture coeflicient can be measured. Previously reported smalltemperature coefficients of surface tension, ∂σ/∂T<0.2×10^<-3> N・m_<-1>・K^<-1> would correspond to this unique behavior of Si melt above the equilibrium oxygen partial pressure for the SiO_2 phase, because these measurements have been carried out without taking account of the effect of oxygen partial pressure. A cellular pattern was observed at a surface of 20 cm deep Czochralski melt, whereas we found a hydrothermal wave at a surface of 8-mm-thick thin melt. Observed patterns are discussed in light of driving force of surface-tension-driven flow in the Czochralski melt.
SnO_2 thin films were deposited by thermal Chemical Vapor Deposition (CVD) under micro-gravity (μg) utilizing parabolic flight of airplane and compared with films deposited under normal gravity (1g) environment. The films were deposited on Si substrates at the substrate temperature of 500℃ for 13seconds from the reactant gas of SnCl_4 and O_2 transported by Ar carrier gas It is found that SnO_2 thin film thickness deposited under μg is flatter than that under lg. The results were discussed with the carrier gas flow pattern obtained from the visualization experiment using a drop tower as well as the simulation calculation results by SIMPLE approximation.
The three dimensional structure of protein molecule is essential for the comprehension of numerous life phenomena and the industrial utilization of biological systems. It is important to determine the three dimensional structure of biological macromolecule components by certain techniques, X-ray crystallographic structure analysis, neutron crystallographic structure analysis or NMR. Both X-ray and neutron crystallographic techniques require high-quality crystals, which can provide well defined diffractions for precise structure analysis. Microgravity is expected to give a well condition for protein crystal growth. Physical approach to for the protein crystal growth under the microgravity is strongly believed to be important for the systematic crystal design. Cytochromes c are a class of c-type cytochrome, which have been found in several photosynthetic bacteria and denitrifying bacteria. Cytochrome c' from denytrifying bacteria is a positively charged protein containing a heme prothetic group covalently bound to the protein backborn through two thioether linkages as well as cytochrome c. In the space experiment STS-107, cytochrome c' from Achromobacter xylosoxifdans NCIMB 11015 will be used for the crystal growth experiments based on the crystal growth mechanisms.
The growth dynamics of colloidal crystallization was evaluated under sedimentation free conditions using sounding rocket and Brownian Dynamics (BD) simulation. The Bragg's reflections of colloidal crystals were measured during microgravity flight and average sizes of crystallites were obtained by the Sherrer's method. Results showed a power-law relationship between size and time, L∞t^α where L is the size of crystallites and t is time. The obtained as were 0.33±0.03 in microgravity and 0.25±0.02 in normal gravity, respectively. Browninan Dynamics (BD) simulation showed the time evolution of ordered domains that consisted of connected structures of crystalline clusters. The power law relationship n∞t^<0.5> in nucleation period was confirmed between the number of particles (n) in clusters and time(t). The calculated power was related to a using the fractal dimension of crystalline clusters and α=0.31 was obtained. The value was matched well with that of the microgravity experiment.