Og. Schmidt et al., Strain and band-edge alignment in single and multiple layers of self-assembled Ge/Si and GeSi/Si islands, PHYS REV B, 62(24), 2000, pp. 16715-16720
Strain in coherently embedded Ge/Si islands significantly modifies the band
-edge alignment in and around the nanostructures. Our calculations on embed
ded flat truncated Ge pyramids show that tensile strain in the surrounding
Si causes a splitting of the sixfold-degenerate Delta valleys into the four
fold-degenerate Delta (4) and twofold-degenerate Delta (2) valleys. This st
rain-induced splitting energy can be larger than 400 meV in stacked Ge/Si i
slands. The Delta (2) valleys in the Si constitute the conduction-band mini
mum, and the heavy hole in the island constitutes the valence-band maximum.
The band gap in the Si above and below the Ge island is smaller than for b
ulk Si, in perfect agreement with recent experiments. Relevant energies are
worked out as a function of Si interlayer thickness, number of islands, an
d Ge concentration in the islands. We compare our calculations of the band-
gap energy with photoluminescence experiments on embedded Ge islands, yield
ing an average Ge fraction in the nanostructures of 60%.