ELASTIC STRAIN-ENERGY OF GRADED SI1-XGEX BUFFER LAYERS

A partly or completely relaxed Si1-xGex buffer layer is used for strain adjustment of strained Si1-xGex/Si structures on silicon substrates. Recently, it was reported that with rather thick (greater than 1 mum) graded Si1-xGex buffer layers grown at rather higher temperatures (750-900-degrees-C) the surface morphology and crystalline quality could be improved considerably. The residual strain in the buffer layers is thought to be the driving force for morphology and crystal quality problems. We calculated the misfit dislocation arrangement and the residual strain in linearly graded buffer layers capped by a constant composition layer. The treatment follows closely the equilibrium energy approach of van der Merwe and van der Berg. An equilibrium approach is justified for Si1-xGex growth temperatures above 750-degrees-C. The evolution of strain during growth and the share of elastic energy in the total energy is calculated for a 1.3% lattice mismatch configuration. The increase in strain energy with decreasing buffer thickness t(B) is shown for the t(B) range from 2 to 0.5 mum.