Folded Acoustic Phonon Modes in Ge/Si Quantum Dot Superlattices With Different Periods

摘要

We present low-frequency Raman scattering measurements on self-assembled Ge/Si quantum dot superlattice samples grown by molecular beam epitaxy. The samples had different growth parameters, such as the number of periods, nominal thicknesses of Ge layers, and growth temperatures. Raman scattering peaks were observed in the low-frequency region (< 30 cm{sup}(-1)), which were attributed to the folded acoustic phonon modes related to the periodicity of the Ge/Si quantum dot superlattices. We applied Rytov's model to calculate the frequencies of the folded acoustic phonons in the Ge/Si quantum dot superlattices. The calculated frequencies were in good accordance with the experimental results among most of the samples. It was observed from the experiments that the Raman peak intensities decreased with the increased order of the folded acoustic phonons. Raman peak intensities from different order folded acoustic phonons were explained by photoelastic effect theory. It was also found that the intensities of Raman peaks were related to the Ge layer thickness and the periods of the Ge/Si quantum dot superlattices: the thinner the Ge layer thickness, the lower the intensity; the smaller the periods, the lower the intensity of the Raman peaks.