Raman scattering of Ge/Si dot superlattices under hydrostatic pressure

Abstract

We have studied the self-organized Ge/Si quantum dots (QD's) by Raman scattering under hydrostatic pressure near resonance and off resonance with confined Ge-like E1 transition. The Raman spectra of Ge-Ge, Si-Ge, and the Si acoustic phonon (Si-2TA) modes were obtained as function of pressures in the range 1-70 kbar. Our results show that the Ge-Ge and Si-2TA modes can be easily resolved at low pressure due to a high degree of compressive built-in strain in the Ge layers. 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 the corresponding quantity in bulk Ge. Normalized Raman intensity profiles of Ge-Ge mode exhibit a resonance enhancement peak at ∼32 kbar. The pressure coefficient a of this resonating electronic transition thus obtained is ∼5±1 meV/kbar. This value is smaller than the pressure shift of the E1 transition in bulk Ge.