Nanometer-scale mapping of the strain and Ge content of Ge/Si quantum dots using enhanced Raman scattering by the tip of an atomic force microscope

Abstract

Tip-enhanced Raman spectra of as-grown self-assembled Ge/Si quantum dots have been obtained with nanoscale spatial resolution. It is found that Ge-Ge and Si-Ge modes in the Raman spectra are significantly enhanced only when the atomic force microscope tip with a gold nanoparticle is positioned on the dots. It is also found that the Si substrate peak at 520 cm${}^{-1}$ is shifted considerably in the neighborhood of the dots. This implies that a large stress acts on the Si substrate around the dots. The Ge content reduces at the center of the dots. This result suggests that the dots consist of a Si-rich core and a Ge-rich shell. The Ge content decreases and the strain in the Si substrate increases with an increase in the dot height.