Principles of strain relaxation in heteroepitaxial films growing on compliant substrates

In spite of various reports that the density of misfit dislocations threading a growing heteroepitaxial thin film can be considerably reduced by means of using a "compliant substrate," the underlying physical mechanisms are not well understood. The common theoretical models suppose that the growing film can preferably relax in an elastic way by slipping on this kind of substrate. This idea, however, requires us to suppose macroscopic slip displacements. Such displacements are disregarded in the common theories and not reported to occur experimentally. This very doubtful free-slipping hypothesis has been used to establish a force balance ("strain partitioning") between the growing film and the template layer below it and, consequently, to theoretically derive an enhanced critical thickness for the onset of slip of misfit dislocations. In the present article, more realistic mechanisms are discussed, including early plastic relaxation at a low film thickness where multiplicative interaction of dislocations hardly occurs. Possibilities for an enhanced elastic relaxation of the film in the case of Stranski-Krastanow island growth are discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)09119-2].