Localized and nonlocalized states in thin Ge layers grown by molecular-beam epitaxy at low temperatures

Symmetry-allowed and -forbidden resonant and nonresonant Raman scattering from the Ge-Si and the Ge-Ge optical phonons of thin Ge layers grown epitaxially at low temperatures by molecular-beam epitaxy on Si(001) have been studied to characterize the spatial localization of these modes and the perfection of the Ge layers. Both the large wave-vector Ge layer phonons and the Ge-Si interface LO phonons in nanometer thickness films strongly mix with the Si host phonons producing significant perturbations to the simple picture of confined phonon modes in heterostructures. We identify the presence of alloylike regions in these samples from the resonant behavior of the Si-Ge optical phonons, demonstrating that the interfaces between the Si and Ge are not atomicly abrupt. The symmetry properties of the Ge-derived Raman scattering show the layered character of the Ge regions and also indicate that these layers are not perfect. © 1992 The American Physical Society.