Effects of hydrostatic pressure on Raman scattering in Ge quantum dot superlattices

Self-organized Ge/Si quantum dots (QD's) in strained Si/Ge short-period superlattices are studied by Raman scattering under hydrostatic pressure excited in-resonance and off-resonance with the confined Ge-like E₁ transition of Ge using 488 and 514.5 nm lines from an argon-ion laser and 632.8 nm line from a He-Ne laser. The Raman spectra of Ge-Ge, Si-Ge and Si-2TA modes of the QD's were obtained as a function of pressure in the range of 1-70 kbar. Our results show that the mode Grüneisen parameter of the Ge-Ge phonon mode in QD's is found to be γ=0.81±0.01, which is smaller than that of the bulk Ge. We observe resonance effects with the confined Ge-like E₁ transition and the pressure coefficient of this resonating electronic transition obtained is ∼5±1 meV kbar⁻¹.