Aspects of Ge/Si self-assembled quantum dots

We have investigated the growth and the optical properties of vertically-ordered Ge/Si self-assembled quantum dots. The quantum dots are grown by ultrahigh-vacuum chemical vapor deposition. We first investigate the morphological evolution of the islands following the overgrowth by a thin silicon film. A shape transition (domes to pyramids) is observed along with a significant flattening of the islands and a modification of the density. The vertical alignment of the islands is then investigated. We show that the vertical correlation can be monitored by the decrease of the critical thickness from layer to layer. The three different regimes governing the correlation between the islands are analyzed in terms of the shape transition of the islands. The density of states in the Ge/Si quantum dots is finally investigated. A strong dependence of the photoluminescence of the dots is observed as a function of the excitation power density. This dependence is interpreted in terms of state filling and recombination from the confined excited hole states in the dots. The photoluminescence data are correlated to the density of states as calculated by solving the three-dimensional (3-D) Schrِdinger equation in these islands with a lateral size of the order of 100 nm.