Study of phonons in semiconductor superlattices by Raman scattering spectroscopy and microscopic model calculation

Raman spectroscopy is used to study phonons in a series of thin (AlAs)m/(GaAs)n superlattices (SLs) grown by molecular beam epitaxy (MBE). The influence of buffer layer type on the interface roughness of heterostructures is carefully evaluated. The accuracy of optical phonons and the degree of peak sharpness of GaAs-like confined modes are examined via off-resonance Raman spectroscopy. Theoretical calculations of phonons in thin (AlAs)m/(GaAs)n superlattices (i.e. samples with m, n ≤ 12) are reported for various directions of propagation by using a rigid-ion model. Optical phonons acquire significant dispersive character when the wavevector q forming an angle θ with the growth axis of the superlattice is changed from θ = 0 to π/2, i.e. from [001] to [100]. The frequency gaps in the angular dispersions due to mode anti-crossing behavior observed recently by Zunke et al. using micro-Raman spectroscopy and studied by a continuum model are found to be in reasonably good agreement with our lattice dynamical model calculations.