INFLUENCE OF ALLOY DISORDER ON THE VIBRATIONAL PROPERTIES OF SI/GE SUPERLATTICES

A systematic study of the influence of disordered layers on the vibrational properties of Si/Ge superlattices is presented. The formalism used allows us to calculate the phonon Green function in the mixed representation for a superlattice with a general stacking of monolayers (presently along the 001 axis). The disordered layers are described within the coherent-potential approximation. Two arrangements of disordered layers in the superlattice are studied: thin interfacial alloy layers representing a diffuse interface and a superlattice whose one component is the Si-Ge alloy. Drastic suppression of superlattice quantum effects and the formation of modes induced by the alloy layers are predicted in both cases. The results show the sensitivity of the phonon spectrum of the superlattices to a long-range coherence of their chemical composition and to the perfection of their preparation. © 1990 The American Physical Society.