Symposium M
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Dr. T. Abe (Shin-Etsu Handotai, Japan) discussed the prevailing models of swirl formation in Si and the role impurity doping plays on their formation. B and C impurities give rise to swirl defects, while N significantly reduces swirl concentration. Al and Ga, because of their tendency to deoxidize Si through the formation of oxides, give rise to swirl-free crystals. Dr. Zhu Li-huei (Electronic Material Research Institute, China) described a reduced pressure (4-15 Torr Argon) Czochralski puller he has been using for growth of 3-4" diameter, 10-20 kg Si crystals. The design of the heating system and the controlled flow of argon over the surface of the melt before exiting resulted in improved control over thermal gradients (and thus convection). Crystals grown in this setup were swirl-free with reduced C contamination and a homogeneous distribution of oxygen. Dr. C. P. Khattak (Crystal Systems, Salem, MA) described how the Heat Exchange Method, used successfully for the growth of 8" diam. sapphire crystals, was adapted for the production of 45 kg, 32 cm2 Si crystals for photovoltaic applications. Crucial for this result was the solution of the SiO 2 crucible problem to produce crack-free crystals.
FRANK WANG (left) and JACK WERNICK
Dr. G. Jacob (CNET, Grenoble, France) argued that native defects in GaAs play the most important role regarding dislocation generation during crystal growth for he found no correlative influence of dopants on mechanical behavior and dislocation generation during growth. Dislocation free Ga 99In OiAs crystals were produced and he further argued that this material should be the one used technologically for producing GaAs-based devices. Dr. T. Fukuda (Optoelectronics Joint Research Laboratory, Japan) described an automated computer controlled 4" diameter GaAs Czochralski unit for growth of constant diameter crystals for high speed IC devices. Optical fibers are used for sensing and feedback for control. Two superconducting solenoids on the outside of the chamber generate a magnetic field normal to the growth axis. It was shown that the application of a magnetic field decreases the thermal oscillations at the melt-solid interface to near zero and extremely good chemical homogeniety is obtained; there is little evidence of growth striations. EPDs are = 3000/cm 2 over a major portion of the diameter. The EL2 trap concentration also decreased. A technique for melt purification prior to growth was also described. At the start of a run, GaAs is synthesized in-situ with excess As and B2O3 at a pressure of 70 atmospheres. The pressure is then reduced to 28 atmospheres followed by a further reduction to 1 atmosphere. "Boiling" or bubbling of excess As occurs and it appears that C as CO and Si as SiO is removed by this action. [Continued on Page 3l]
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