Strain Evolution in Coherent Ge/Si Islands

Strain evolution of coherent Ge islands on Si(001) is measured using a newly developed transmission electron microscopy technique based on two-beam dark-field strain imaging. The strain measurements show that a metaslable Ge island shape is involved in the shape transition between pyramids and domes; this shape is more readily observed for growth at 550 than 600 °C because of the slower rate at which islands cross the kinetic barrier between shapes. The strain relaxation changes discontinuously between pyramids and domes, indicating that the underlying shape transition is first order.