CONSIDERATIONS ABOUT THE CRITICAL THICKNESS FOR PSEUDOMORPHIC SI1-XGEX GROWTH ON SI(001)

Thin epitaxial Si1-xGex layers were grown on Si(001) substrates and an alysed with transmission electron microscopy. We show that the critica l thickness for perfect two-dimensional pseudomorphic Si1-xGex layer g rowth depends on two different mechanisms for strain relief. For low G e concentrations (x < 50%) the critical thickness is determined by the formation of misfit dislocations, whereas for high Ge concentrations it is rather governed by the formation of Stranski-Krastanov islands. In the concentration range between 50% and 60% both mechanisms become equally likely. We show that the critical Stranski-Krastanov thickness increases with decreasing lattice mismatch, in agreement with recent theoretical predictions.