INVESTIGATION OF THE REVERSIBILITY OF DEFORMATION IN SILICON SHEETS

We have investigated the degree to which plastic deformation is reversible in silicon by bending and re-flattening initially defect-free single-crystal Czochralski silicon samples using four-point bending to simulate the deformation experienced during ribbon crystal growth. Optical and scanning electron microscopy of etched sample cross-sections after single bending and bending and re-flattening at 1100° C and 1200° C revealed that the dislocation densities in the re-flattened samples were lower than in singly-bent samples by 60-90%, indicating that dislocations are either being annihilated within the silicon bulk or are exiting the silicon at the free surfaces. There was little evidence of dislocation interaction in the singly-bent single-crystal samples investigated with transmission electron microscopy, so the latter mechanism is more likely. Although the re-flattened specimens have a lower dislocation density, there is little improvement in the minority-carrier diffusion length, measured by electron-beam induced current, which in all cases ranged from 10-20 μm. Since the minority-carrier diffusion length changed little, even with a change in dislocation density from 106 dislocations/cm2-108 dislocations/cm2, other defects must be controlling the diffusion length. There is some correlation between dislocation density and minority-carrier diffusion length within a given sample, but this may be due to indirect effects such as generation of point defects by moving dislocations. © 1990 AIME.