Influence of surface reconstructions and epitaxial-growth conditions on long-range order in Si1-xGex alloys

In this paper we show unambiguously that ordering in Si1-xGex is an entirely kinetic phenomenon governed completely by growth conditions and surface reconstructions, and not by bulk thermodynamic equilibrium. We describe a set of experiments involving the use of annealing, surface modification through molecular-beam-epitaxial adlayers, ultrahigh-vacuum chemical vapor-deposition growth, and growth on Si substrates with different orientations which indicate that both low-temperature growth and a 2×1 reconstructed surface are individually necessary but not sufficient conditions to observe long-range order in Si1-xGex. The experimentally observed ordered phase, which consists of bilayers of Si and Ge along all four 111 directions, is a metastable phase that does not correspond to the lowest-energy phase of the alloy and is irreversibly destroyed by annealing. Our experiments also emphasize that ordering in Si1-xGex occurs even in the absence of strain in Si1-xGex films. Finally, we demonstrate that long-range order in Si1-xGex occurs due to local segregation induced by stresses at the growing surface. This mechanism successfully explains all experimental findings to date. © 1992 The American Physical Society.