Results of this period
(1) Invention and modeling of the traveling liquidus-zone (TLZ) method
Full paper: "Quantitative modeling of traveling liquidus-zone method" in PDF file (File Size: 658 KB)
Combining ideas of the graded solute concentration method and partial melting method, we have invented a new crystal growth method and named it the traveling liquidus-zone (abbreviated as TLZ) method after the crystal growth mechanism. We have developed the modeling of the TLZ method. The principle of the TLZ method is schematically shown in Fig. 1. When a feed with graded In/Ga ratios is heated up to around 1020 degrees centigrade at the interface with a seed and at a relatively low temperature gradient such as 20 K/cm, part of high InAs content is melted and a narrow melt zone is formed as shown in Fig. 1a. Because both sides of the melt are solids with lower InAs concentration, reaction between the melt and solids occurs and the melt composition becomes the equilibrium one at given temperatures at both interfaces. Since the equilibrium InAs concentration is lower than that in the melt, diffusion of InAs towards both sides of the melt occurs until a saturated liquidus-zone is established by widening the melt zone. Since the solubility depends on temperature, concentration gradient of InAs is set in the zone by the imposed temperature gradient (Fig. 1b). At this stage, diffusion from the seed side towards the feed side occurs due to the concentration gradient and InAs content decreases at the seed side, resulting in the crystal growth on the seed. Then, InAs is piled up at the freezing interface due to segregation and the piled-up InAs is transported towards the opposite side of the zone by diffusion and part of the feed is dissolved. Thus, the zone spontaneously travels along the liquidus towards the feed by diffusion. Long homogeneous crystals can be grown when the sample is translated according as the zone traveling rate. Details are described in "Quantitative modeling of traveling liquidus-zone method".
(2) Experimental studies for growing large homogeneous single crystals
Full paper: "Experimental study of InxGa1-xAs homogeneous single crystal growth by the traveling liquidus-zone (TLZ) method" in PDF file (File Size: 253 KB)
Conditions for homogeneous crystal growth were investigated experimentally as well as theoretically. As predicted theoretically, growth rates for producing homogeneous crystals depend on the temperature gradient in the zone and importance of accurate measurements of temperature gradients are revealed. Seeding conditions were also examined for growing larger diameter single crystals. Technique of enlarging a small seed to a large single crystal was not applicable form viewpoints of both mass balance at the seeding position and prevention of poly-crystallization. Single crystal growth became difficult with increasing crystal diameter and constitutional supercooling in the liquidus-zone due to convection was implied as an origin of poly-crystallization.
(3) Numerical analyses of InxGa1-xAs crystal growth conditions
Full paper: "Effect of gravity-induced convection on crystal growth process" in PDF file (File Size: 253 KB)
Fluid flow in the liquidus-zone was analyzed by varying zone dimensions, gravity level, and so on. Transportation of small nuclei by convection in the zone was simulated as parameters of particle size and gravity level for investigating the TLZ growth related poly-crystallization mechanism since solid and liquid coexisting zone is formed in the TLZ method. Two-dimensional calculation code for simulating the TLZ growth in which the solid-liquid moving interface can be treated has been developed. Results show that constitutional supercooling due to convection increases with increasing zone diameter, which may explain poly-crystallization of large diameter crystals grown by the TLZ method. Transportation of nuclei by convection may not cause poly-crystallization because buoyancy force prevents light nuclei from approaching the growth interface which locates at the bottom of the zone. In the International Space Station, convective flow is reduced to less than 1/400 of that on the ground and this is beneficial to the TLZ growth as well. Simulated growth rates for growing homogeneous crystals agree well with those obtained by one dimensional modeling of the TLZ growth.
(4) Preparation of feeds with concentration gradient of InAs
Full paper: "Preparation of InGaAs starting materials having the gradient InAs concentration" in PDF file (File Size: 1,087 KB)
Since feeds with graded InAs concentration are used in the TLZ method as shown in Fig. 1a, preparation method of feeds having well defined compositional profile and dimensions without cracks has been studied. Cracks due to lattice mismatching caused by the compositional variation were avoided by preparing small grained ingots by the directional solidification of InxGa1-xAs melts with x = 0.3, 0.5 and 0.7. Samples for investigating surface smoothness required for non-destructive characterization of compositional profile using Raman Scattering were prepared.
(5) InxGa1-xAs seed preparation by the multi-component zone melting method
Full paper: "In0.3Ga0.7As SEED CRYSTAL PREPARATION USING THE MULTI-COMPONENT ZONE MELTING METHOD (III)" in PDF file (File Size: 980 KB)
Multi-component zone melting method has been improved. At the initial stage of the growth, an InxGa1-xAs crystal with increased x value from 0.03 to 0.3 was grown by the gradient freezing of a melt zone and then an In0.3Ga0.7As crystal was grown in combination with sample translation and lowering furnace temperature. Improvement in inner surface treatments of a crucible and lowering the growth rate for avoiding constitutional supercooling increased reproducibility of single crystal growth up to more than 50 %.
(6) Thermal analysis of sample devices
Full paper: "Numerical Analysis of a Cartridge in the Traveling Liquidus-Zone Method under Various Gravity Conditions" in PDF file (File Size: 1,593 KB)
Temperature distribution in the growth ampoule and the cartridge has been analyzed so as to obtain convex solid-liquid interface curvature, which is important for growing large single crystals. To evaluate the effect of convection in the melt on temperature distribution of the growth ampoule and the cartridge, non-steady calculation is required and analytical code has been developed for more stable analysis of heat and mass flow under 1 g conditions. Growth and dissolving interfaces characteristic to the TLZ method are also treated in the newly developed code.
(7) Characterization of InxGa1-xAs feeds using Raman scattering
Full paper: "Raman characterization of cylindrical polycrystalline InxGa1-xAs starting material" in PDF file (File Size: 386 KB)
Non-destructive analysis of compositional profile of feeds is important in the TLZ method because it determines liquidus-zone width and hence compositional homogeneity of grown crystals (Fig. 1). Micro-Raman scattering method, which has been proved one of the best non-destructive methods by our research team, was applied. Precision and accuracy of the measurement were investigated as a function of surface roughness of cylindrical feeds. Slight polishing of the sample surface (about 50 mm depth and 1 mm width) after grinding showed enough accuracy and precision to evaluate compositional profile of the feed.
(8) Application to the Microgravity Science Research International Announcement of Opportunity
Proposal Document in PDF file (File Size: 927 KB)
Necessity of microgravity experiments on the TLZ method has been made clear in the course of theoretical and experimental studies and we applied to the 1st International Announcement of Opportunity. Objectives of microgravity experiments are; (1) further development of modeling of the TLZ method and (2) analysis of poly-crystallization mechanism of the TLZ method. The TLZ growth model that we have been developing is one-dimensional model. However, three-dimensional convection occurs on the ground and results obtained by terrestrial experiments cannot be directly compared with those derived from the model. Since convection is suppressed in micogravity, results of microgravity experiments are directly compared with those of the model. We found that single crystal growth became difficult with increasing crystal diameter in the terrestrial experiments. Combined with results of fluid flow analysis, constitutional supercooling in the liquidus-zone caused by convection is considered to be an origin of poly-crystallization. If this is true, poly-crystallization will be suppressed in microgravity and large homogeneous single crystal will be grown in microgravity.