Evolution of coherent islands in Si1-xGex/Si(001)

The evolution of strain driven coherent islands is examined using sensitive real time stress measurements during heteroepitaxial growth of Si1-xGex/Si(001), combined with ex situ microscopy. We show that the sequence of morphological transitions at low mismatch strain is qualitatively identical to that for pure Ge heteroepitaxy on Si(001). In particular, films with strains less than 1% undergo Stranski-Krastanov-like island-on-layer growth, followed by an extended regime of [501]-faceted hut clusters that eventually transform into higher aspect ratio dome clusters. The hut and dome islands are fully coherently strained and do not exhibit lateral composition modulation. Quantitatively, the relevant island length scales are significantly increased at low strain. Scaling of the morphological transitions with strain is directly demonstrated using the real time stress data. We further show that the apparent formation of a ripplelike surface morphology at low strain is actually a consequence of kinetic limitations on adatom diffusion, and does not necessarily signify the presence of a surface instability. [S0163-1829(99)01704-X].