Strain relaxation in epitaxial overlayers

Abstract

The misfit between an epilayer and a substrate may be accommodated by misfit dislocations (MDs) or misfit strain (MS) or both. In a small misfit system the misfit is accommodated by MS alone up to a critical thickness whereafter the residual MS decreases as the thickness increases. The main objective of this paper is to review theoretical work aimed at understanding MS relief in a growing epilayer by the introduction of MDs, with the view of resolving the discrepancies between predicted and observed critical thickness and residual MS after onset of MS relief. Since the predictions are based on equilibrium principles, equilibrium theories for monolayers (MLs) in the Frenkel-Kontorowa model, and for thickening epilayers (growing ML-by-ML) in the Volterra model, are briefly summarized, including some consideration of the conditions for ML-by-ML growth. Since equilibration can be drastically retarded by barriers to nucleation and motion of dislocations the observed quantities are most often non-equilibrium values. Calculations show (i) that MD sources are normally needed to the onset of MS relief, (ii) that threading dislocations are the “softest” sources, (iii) that even with threading dislocation sources MS relief may lag behind equilibrium predictions and (iv) that Peierls friction may lead to an infinitely large critical thickness.