The Role of Interdiffusion and Spatial Confinement in the Formation of Resonant Raman Spectra of Ge/Si(100) Heterostructures with Quantum-Dot Arrays

TL;DR

This study investigates how interdiffusion and spatial confinement influence the Raman spectra of Ge/Si quantum-dot heterostructures, revealing exciton-enhanced vibrational modes and size-dependent phonon frequency shifts.

Contribution

It demonstrates the enhancement of vibrational modes via exciton excitation and identifies the optimal Ge layer thickness for minimal quantum dot size dispersion.

Findings

Enhanced Ge-Ge and Si-Ge vibrational modes upon exciton excitation.

Ge-Ge phonon frequency decreases with reduced Ge layer thickness.

Optimal Ge layer thickness minimizes quantum dot size dispersion.

Abstract

The phonon modes of self-assembled Ge/Si quantum dots grown by molecular-beam epitaxy in an apparatus integrated with a chamber of the scanning tunneling microscope into a single high-vacuum system are investigated using Raman spectroscopy. It is revealed that the Ge-Ge and Si-Ge vibrational modes are considerably enhanced upon excitation of excitons between the valence band $\Lambda_3$ and the conduction band $\Lambda_1$ (the E1 and E1 + $\Delta_1$ transitions). This makes it possible to observe the Raman spectrum of very small amounts of germanium, such as one layer of quantum dots with a germanium layer thickness of 10 \r{A}. The enhancement of these modes suggests a strong electron-phonon interaction of the vibrational modes with the E1 and E1 + $\Delta_1$ excitons in the quantum dot. It is demonstrated that the frequency of the Ge-Ge mode decreases by 10 cm^-1 with a decrease in the thickness of the Ge layer from 10 to 6 \r{A} due to the spatial-confinement effect. The optimum thickness of the Ge layer, for which the size dispersion of quantum dots is minimum, is determined.