Effect of interfaces on quantum confinement in Ge dots grown on Si surfaces with a SiO2 coverage

We investigated the formation of Ge islands on Si(1 1 1) and Si(1 0 0) surfaces covered with ultrathin SiO2 films as a function of the growth temperature, the Ge deposition rate, and the amount of deposited Ge. The results showed that the mechanism of island formation corresponds to the growth model with the critical island size of i∗=0, at which an individual Ge adatom can create a stable nucleus by reacting with the surface. This mechanism enables the fabrication of Ge quantum-dot (QD) structures with dots approximately 6 nm in diameter and an extremely high dot density of 3×1012 cm−2 in each dot layer. The photoluminescence obtained in two ranges of the photon energy, around 0.8 eV and from 2 to 3 eV, suggests that our QD structures with weakly strained Ge dots had type-I Ge/Si heterojunctions and the radiative recombination involved heavy holes confined within the Ge dots and electrons confined in the conduction band minimum of Ge (in the low photon energy range), and electrons localized in the radiative defect states at the Ge-dot/SiO2 interfaces (in the high energy range). The recombination was supported by the migration of electrons and holes from Si spacer layers to the Ge dots.