Raman study of the strain and H₂ preconditioning effect on self-assembled Ge island on Si (001)

Abstract

An investigation of the microscopic mechanisms of Ge self-assembling island growth is of great importance for future optoelectronic applications of quantum dot nanostructures. In this study, two sets of self-assembled germanium islands on Si (001) substrate, with and without preconditioning using a high-temperature hydrogenation step on their nucleation and subsequent temporal evolution, were grown by low-pressure chemical vapor deposition (LPCVD). The average germanium concentration, mean diameter of Ge crystalline regions and the strain inside the germanium quantum dots are characterized with high resolution micro-Raman spectroscopy (μRS). Both the intensity and peak position of the Si–Si vibration mode at about 520.07 cm⁻¹ in the Raman spectra have been used as a reference to separate the germanium Raman signal from the overlapping localized Si–Si optical phonon at ∼300 cm⁻¹.

In the absence of preconditioning, both the island size and germanium composition increase steadily as a function of deposition time. However, on the H₂ preconditioned surface, the nucleation and growth rates are greatly increased during the first stages and slow down significantly after deposition for 10 s.

Our results indicate that the compressive strain inside the islands acts as a barrier for Ge adatoms to diffuse from the wetting layer into the islands. For the growth times used in this study, for both sets of samples with and without H₂ preconditioning, the normalized rate of increase of the Ge concentration (%Δ [Ge]/Δ t) decreases by ∼0.13/s for a 1% compressive strain increase. The H₂ preconditioning can initially increase the density of island nucleation sites, but cannot accelerate the Ge island growth. It tends to lower %Δ [Ge]/Δ t by 0.015/s instead. The decreased strain due to surface roughing is the principal reason why the Ge islands grow so rapidly at the beginning on the H₂ preconditioned samples.