Indirect, quasidirect, and direct optical transitions in the pseudomorphic (44)-monolayer Si-Ge strained-layer superlattice on Si(001)

Estimates are presented for lower-lying optical transitions in the pseudomorphic (4×{}4)-monolayer Si-Ge strained-layer superlattice on Si(001). The ten lowest-lying q=0 superlattice transitions tend to cluster into groups having energy centroids near 0.76, 1.03, 1.2, 1.6, and 2.3 eV, for light polarized in the plane of the interface. The transitions near 0.76 and 1.03 eV are indirect. Transitions near 1.2 and 1.6 eV arise from zone-folded bulklike bands and have a partially direct character, whereas the transitions near 2.3 eV are strictly direct. Surface steps and intervalley phonons would tend to relax the k? (in-plane) momentum conservation rule, and thus allow zone-folded quasidirect-character transitions near 0.76 eV.