DISLOCATION REDUCTION IN GAAS CRYSTALS GROWN FROM THE CZOCHRALSKI PROCESS

The dislocation density in the gallium arsenide (GaAs) crystal is gene rated by excessive thermal stresses during the Czochralski (CZ) growth process. A constitutive equation which couples the dislocation densit y with the plastic deformation is employed to simulate the dislocation density in the crystal. The temperature distribution in the crystal d uring the growth process is obtained by solving the quasi-steady-state (QSS) heat-transfer equation. The thermal stresses induced by the tem perature distribution are calculated using the finite-element method. The crystal is assumed to be an axisymmetrical ingot. The resolved she ar stress (RSS) in each slip system is obtained by stress transformati on. The RSS in each slip system is no longer axisymmetric. The disloca tion motion and multiplication in each slip system are simulated using the constitutive equation. The total dislocation density in the cryst al is obtained by summing the dislocation densities in ail of the slip systems. Since the thermal stresses are sensitive to the temperature gradients and the dislocations move faster at a higher temperature, th e dislocation densities are generated most near to the solid-melt inte rface. The dislocation density is also found to be affected by the gro wth orientation, growth speed, ambient temperature and the radius of t he crystal. The dislocation density in GaAs crystals grown with the di fferent growth orientation, growth speed, and crystal radius at variou s ambient temperatures has been calculated so that the influence of th ese growth parameters on the dislocation density can be understood. Co nsequently, the growth parameters can be controlled to reduce the disl ocation density generated in the crystal during the CZ growth process.