Phonon behavior and interfacial stress in the strained (InAs)m/(GaAs)n ultrathin superlattices

Phonon behavior in the strained (InAs)_m/(GaAs)_n ultrathin superlattices grown by molecular beam epitaxy has been investigated by means of Raman scattering spectroscopy. The phonon frequency in the GaAs layers shifts toward lower energy with increasing InAs layer thickness under fixed thickness of GaAs layers. The frequency in the InAs layers does not change significantly, as deduced from the behavior of the InAs-like mode in In_xGa_1−xAs alloys. These observed results are phenomenologically discussed on the basis of the combined effect of phonon confinement in the respective layer and stress at the alternating interfaces. Furthermore, a large softening of the phonon confined in the GaAs layers decreases the frequency gap, resulting in traveling of the optical phonons confined in both layers. The strain at the interfaces is estimated by an empirical method, i.e., by comparing the frequency in the superlattice and in the alloy of equivalent composition. In the In_xGa_1−xAs alloys, the composition dependence of the mode frequency is considered as being due to the local strain. The group III elements are considered to be in the local strain state from an extended X-ray absorption fine structure (EXAFS) analysis.