Temperature and Composition Dependent Structures of Si_xGe₁-_x/Si and SLGeiv/Ge Superlattices

We have investigated the equilibrium structures of thin-layer Si_xGe_1-x/Si and Si_xGe_1-x/Ge superlattices as a function of composition and temperature using a Monte Carlo simulation technique combined with a many-body empirical potential. In general, the alloy layers are found to adopt an overall random structure at equilibrium. Some segregation of Ge to the interface is found to occur at room temperature in superlattices containing pure Si layers and high Ge content in the alloy. The alloy lattice is highly sensitive to the degree of strain imposed by the Si or Ge substrate: We observe a significant expansion or compression of the alloy lattice perpendicular to the interface as a means of relieving the imposed strain.