Si adatom surface migration biasing by elastic strain gradients during capping of Ge or Si1-xGex hut islands

Hut cluster formation during Ge or Si1-xGex solid source molecular beam epitaxial growth on Si(001) is a well-known kinetic pathway for partial strain relief. It results in undulated morphologies with {105} facets allowing a(parallel to) lattice parameter relaxation on the island apexes. Here, we show how subsequent Si coverages, grown at 500 degrees C, avoid being tensile strained and impede further increase of stored elastic strain energy. Dominant inhomogeneous Si surface diffusions take place as proven by a Ge marker technique able to provide transmission electron microscopy or high-resolution transmission electron microscopy images of the initial Si morphology stages and by reflection high-energy electron diffraction examinations. This mechanism prevails for high enough Si growth rates, able to quench lateral Ge diffusion and limit chemical strain relief. Mediated by stress variations on the noncapped island curvatures, Si is depleted from the top of the islands and accumulates in the troughs of the ripples where it accommodates mostly unstrained. By this selective Si coverage, the surface undergoes a rapid smoothing and a(parallel to) recovery toward the Si bulk value. When the Ge containing islands are completely buried, their strain, dictated by the Si buffer and cap layers, ends by being mainly along the growth direction or tetragonal (Delta a(parallel to) = 0). (C) 1998 American Institute of Physics. [S0003-6951(98)01834-8].