NEW INSIGHTS ON SIGE GROWTH INSTABILITIES

In this work we investigate the influence of the Si substrate orientat ion on the growth instability of strained Si1-xGex heterostructures. T he work mainly consists in atomic force microscopy and grazing inciden ce x-ray diffraction analyses of the Si1-xGex layers deposited by gas source molecular beam epitaxy on vicinal Si substrates tilted from (00 1) to (111) surfaces. The major result is that the two- to three-dimen sional growth transition is dramatically affected by the orientation o f the substrate but also by the equilibrium shape of silicon. For inst ance, we evidence the layer by layer growth of Si1-xGex on Si (111) in contrast to the nucleation of three-dimensional islands on 2 degrees off Si (111) in the same experimental conditions. We systematically ve rify that the homoepitaxial growth of unstressed Si on vicinal Si (111 ) consists in a regular array of single steps. Therefore, we propose t hat the stress induced by the heteroepitaxial growth destabilizes the regular step train by reducing the repulsive elastic interaction betwe en steps, and induces step-bunching The presence of close-spaced steps and the metastability of the vicinal surfaces increase the tendency t owards instable growth and result to earlier development of bunching. Despite the accompanying increase of surface area, the development of low-energy facets balances the surface free energy excess. In all case s, step-bunching instability is a kinetic pathway towards the faceted equilibrium state. Long annealing treatment of the strained metastable Si1-xGex layers confirms this last point. (C) 1998 American Vacuum So ciety.