Relaxation of strained silicon on Si0.5Ge0.5 virtual substrates

High resolution X-ray diffraction was used to measure the relaxation of strained silicon layers on 50% virtual substrates in the thickness range of 10 to 70 nm, many times the critical thickness of 4 nm. Relaxation was observed to reach only 2% at a thickness of 30 nm, this stability arising from dislocation pinning. Transmission electron microscope studies show relaxation occurs by the glide of pre-existing 60deg dislocations which become dissociated into stacking faults. At 7 nm, the nucleation of 90deg Shockley partial dislocations to form microtwins was observed, which increases relaxation to 14%. Annealing did not increase the relaxation of layers thinner than 30 nm, but in the 70 nm layer a significant increase in relaxation was observed due to the possible onset of the modified Frank-Read multiplication mechanism.