Beschreibung:
<jats:p>The thermodynamical stability of ultrathin semiconductor layers embedded in isovalent bulk materials has been investigated by ab initio pseudopotential techniques. The AlAs/GaAs and GaAs/GaP structures have been chosen as prototypical examples for this study. We show that independently of the thin layers thickness: (i) At 0 K, the lattice-matched structures are thermodynamically more stable in the ideal configuration than in the case in which some Al and Ga atoms are allowed to diffuse forming a mixed plane; it is the finite-temperature entropy which finally favors the mixed configuration. Conversely, (ii) the lattice-mismatched structures are thermodynamically more stable in the mixed configuration than in the ideal one. In the latter case, it is mainly the chemical mixing and not the relaxation effects which favors diffusion.</jats:p>