Direct observation of dislocation emission from crack tips in silicon at high temperatures

Abstract

The emission of dislocations from crack tips in silicon under combined conditions of temperature and stress is observed in situ at 550-750°C, using transmission electron microscopy. The generation and motion of 1 2[11̄0] shear dislocation loops on (111) planes, corresponding to those having the maximum Schmid factor, occurs in both mode I and mode III cases. Estimation of the emission condition is made based on the observed dislocation mobility and resultant distribution of dislocations around the crack tip. The difference in the values of K1 = 0.94 ± 0.06 MPa m 1 2 and Km = 0.17± 0.03 MPa m 1 2 for dislocation emission is attributed to Schmid factor differences and ledge energy effects. Substituting the estimated stress intensity, resolved on to the dislocation emission glide plane, into the Rice-Thomson energy analysis indicates that spontaneous dislocation emission occurs without a nucleation barrier. A value for the lattice friction stress of ~ 110 MPa is deduced from the positions around the crack tips that the dislocations adopt after emission. In a modification of the Rice-Thomson energy analysis, the lattice friction stress is shown to influence the nucleation and growth to a stable size of a dislocation loop emitted from a crack tip. © 1989.