THE INFLUENCE OF SB AS A SURFACTANT ON THE STRAIN RELAXATION OF GE/SI(001)

We have investigated by the use of surface x-ray diffraction the initial strain relaxation of Ge on Si(001) when a "surfactant" layer of Sb [0.7 monolayers (ML)] is present. Due to the high sensitivity of the technique to lateral strain in the overlayer, we have been able to observe directly the onset of strain relaxation at a coverage of 9-10 ML. This strain relief proceeds gradually as a function of coverage, but unlike the case without an Sb surfactant, it was not possible to relax the overlayer fully. No bulklike Ge was seen even up to a coverage of almost-equal-to 55 ML. Concurrent specular reflectivity measurements also showed that the overlayer formed in a layer-by-layer mode up to the same level. These results differ quite markedly from those obtained without the surfactant layer, where several stages of island formation and strain relaxation are seen. They show dramatically how the modification of surface energies by the presence of a surfactant can affect the morphology of, and strain in, a lattice mismatched overlayer. In addition, they are direct confirmation that the initial strain relaxation mechanism in operation when a surfactant is not present is the formation of coherent Ge islands, rather than that of dislocations.