THERMAL STRESS ESTIMATION FOR LARGE DIAMETER SILICON SINGLE CRYSTAL USING MATERIAL PROPERTIES OBTAINED FROM MOLECULAR DYNAMICS METHOD

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The following are results of the present research.(1) A molecular dynamics code was developed to calculate thermal expansion coefficient and elastic constants of single crystals. These values for alpha-Fe were successfully obtained up to intermediate temperatures using the molecular dynamics code. At elevated temperatures, stable solutions cannot be obtained, because thermal oscillation becomes large with temperature increase. To obtain thermal expansion coefficient and elastic constants at the elevated temperatures such as the melting point remains unsolved as a future problem.(2) A computer code was developed for simulations of dislocation density in semiconductor bulk crystals during growth process. The Haasen-Sumino model was used as a constitutive equation for single crystals at elevated temperatures. Dislocation density simulations were perfomed using this computer code for Si, GaAs and InP to show the effectiveness of the computer code. The results indicate W-type dislocation density distributions across the diameter in GaAs and InP single crvstals that can be observed in practical Czochralski (CZ) growth of GaAs and InP.This computer code can be used as a powerful tool for research and development of high quality bulk single crystal.(3) The Young's modulus of Si single crystal near the melting point was estimated in accordance with the fact that dislocation-free bulk single crystals with 8-inch diameter are produced commonly by meana of the practical CZ growth. When the Young's modulus obtained from the experimental relation between the critical resolved shear stress and the corresponding strain for Si single crystal was used in the dislocation density simulation of a 8-inch diameter Si single crystal, dislocation-free single crystal was obtained as a result.Therefore, the use of this Young's modulus is recommended for dislocation density simulations for large diameter CZ silicon crystals such as 16-inch diameter crystals.