Role of dislocation blocking in limiting strain relaxation in heteroepitaxial films

Abstract

The residual strain following relaxation in a variety of Si1−xGex heteroepitaxial films grown on (001) Si wafers has been compared with the values of residual strain predicted by the theory based on the incremental movements of isolated threading dislocation segments. It is found that for very thin films (40–500 nm) the measured residual strains after relaxation are significantly higher than the values predicted by this theory. For thicker films, the residual strains are very close to the predicted values. The effect of the interactions of parallel dislocations on the residual strain are investigated using the model developed by Willis, Jain, and Bullough [Philos. Mag. A 62, 115 (1990)] for a two-dimensional array of dislocations. It is found that the experimental data cannot be explained by this model since it predicts even lower values of residual strain than the model based on isolated threading segments. The residual strains are also compared with predictions based on Freund’s treatment of the blocking of a moving threading segment by an orthogonal misfit dislocation present in its path [J. Appl. Phys. 68, 2073 (1990)]. It is found that Freund’s blocking criterion gives a very good account of the residual strain in Si1−xGex films. Blocking of threading dislocations by other misfit dislocations appears to play an important role in the late stage of strain relaxation in these structures and thus may limit the possibility of obtaining fully relaxed films with low threading dislocation densities.