MISFIT STRAIN RELAXATION IN GEXSI1-X SI HETEROSTRUCTURES - THE STRUCTURAL STABILITY OF BURIED STRAINED LAYERS AND STRAINED-LAYER SUPERLATTICES

The critical thickness-strain relationships for buried strained layers and strained-layer superlattices (SLSs) are derived using an energy balance model. Relaxation of the entire heterostructure and individual strained layers by both 60°type a/2〈011〉 and pure edge dislocations is considered. Ge xSi1-x/Si heterostructures designed to investigate the stability regimes predicted by the model were grown by molecular-beam epitaxy. The extent of relaxation and the detailed dislocation structure were assessed in annealed structures by x-ray rocking curve analysis, transmission electron microscopy, and Nomarski microscopy of etched samples. Comparison of metastable as-grown and post-growth annealed microstructures revealed the evolution of misfit dislocation structure as equilibrium was approached on annealing in the temperature range 600-900°C. The predominant relaxation mechanism for most GexSi1-x/Si heterostructures was by 60°a/2 〈011〉 misfit dislocations at the first strained-layer/substrate interface. However, for SLSs with strained layers exceeding their individual critical thickness, pairs of dislocations and prismatic loops on (011) planes were observed entrained within the GexSi1-x layers. The agreement between experimentally observed relaxation behavior and the critical thickness versus strain predictions of the energy balance model is remarkable. Superlattices and buried strained layers were found to be only slightly more stable than an uncapped strained layer of the equivalent strained-layer thickness. Interdiffusion was observed at annealing temperatures above 800°C; annealing at 900°C for 30 min was sufficient to eliminate the strain and composition modulation in a superlattice with a 55-nm period and x=0.25. The relative influence of the various strain relaxation mechanisms is discussed for the alloy range 0<x<1 and for geometries with strained-layer dimensions varying from 0.8 to ∼500 nm.