ORDERING IN SI-GE SUPERLATTICES

We study RH1 ordering in Si-Ge superlattices using empirical potentials for the Si-Ge system, one of which incorporates long-range interactions and accurately reproduces the phonon spectra as well as equilibrium structures. Our simulations show that the energy difference between the RH1 and the disordered phase remains small over a wide range of situations, which cannot account for the high transition temperature and the reversibility of the former. We consider experimental evidence of further frustration in the disordered phase only; even so we show that it fails to stabilize the RH1 phase up to the observed temperature. Experimentally, ordering is observed by means of transmission-electron diffraction done on thin samples cut in the (110) plane. Our simulations show that the reconstructions on the (110) surface may provide the missing energy necessary to stabilize RH1 ordering. © 1994 The American Physical Society.