The commonly accepted Stranski-Krastanow model, according to which island formation occurs on top of a wetting layer (WL) of a certain thickness, predicts for the morphological evolution an increasing island aspect ratio with volume. We report on an apparent violation of this thermodynamic understanding of island growth with deposition. In order to investigate the actual onset of three-dimensional islanding and the critical WL thickness in the Ge/Si(001) system, a key issue is controlling the Ge deposition with extremely high resolution [0.025 monolayer (ML)]. Atomic force microscopy and photoluminescence measurements on samples covering the deposition range 1.75–6.1 ML, taken along a Ge deposition gradient on 4 in. Si substrates and at different growth temperatures $\left(T_{\text{g}}\right)$, surprisingly reveal that for $T_{\text{g}}>675°\text{C}$ steeper multifaceted domes apparently nucleate prior to shallow {105}-faceted pyramids, in a narrow commonly overlooked deposition range. The puzzling experimental findings are explained by a quantitative modeling of the total energy with deposition. We accurately matched ab initio calculations of layer and surface energies to finite-element method simulations of the elastic energy in islands, in order to compare the thermodynamic stability of different island shapes with respect to an increasing WL thickness. Close agreement between modeling and experiments is found, pointing out that the sizeable progressive lowering of the surface energy in the first few MLs of the WL reverts the common understanding of the SK growth onset. Strong similarities between islanding in SiGe and III/V systems are highlighted.

Unimodal SiGe islands with dominant $\{111\}$ facets were grown coherently on pit-patterned Si (001) substrates by molecular beam epitaxy. With increasing Ge deposition, the $\{111\}$ pyramids evolve from dome-shaped islands, reaching significantly larger volumes than are coherently possible on flat substrates. Finite element calculations and molecular dynamics simulations show that SiGe islands in pits can have less misfit strain with respect to islands of the same shape on flat substrates. The injection of dislocations is thus delayed, allowing for the observed development of coherent islands with a very high aspect ratio.

We compare elastic relaxation and Si-Ge distribution in epitaxial islands grown on both pit-patterned and flat Si(001) substrates. Anomalous x-ray diffraction yields that nucleation in the pits provides a higher relaxation. Using an innovative, model-free fitting procedure based on self-consistent solutions of the elastic problem, we provide compositional and elastic-energy maps. Islands grown on flat substrates exhibit stronger composition gradients and do not show a monotonic decrease of elastic energy with height. Both phenomena are explained using both thermodynamic and kinetic arguments.