We have measured the temperature-dependent onset of strain relief in metastable ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ strained layers grown on Ge substrates. On the basis of these measurements, and physical arguments, we propose that strained-layer breakdown is most directly determined not by thickness and lattice mismatch, but rather by (1) an "excess" stress (the difference between that due to misfit strain and that due to dislocation line tension) and (2) temperature. With use of these parameters, observed regimes of stability and metastability are shown to be described within a simple, unified framework.

• Received 6 August 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.2455