Phonons in single thin epitaxial layers of InAs on InP

We have used Raman spectroscopy to investigate the vibrational properties of a series of single thin layers of InAs grown on (100) InP substrate by molecular beam epitaxy with a layer thickness between 3 and 9 monolayers. Sensitivity to thin layers, down to 3 monolayers, has been successfully achieved by using incident photon energy resonant with the E1 gap of InAs. In addition to the LO and TO modes related to InP and InAs we have observed new structures assigned to the presence of an intermixed InAsxP1-x alloy at the interface region, and unusually intense features just below bulk optical phonon energies related to the lattice vibrations of interface bonds. We have attributed these modes to interface phonons and analyzed their frequencies by using a modulated dielectric model. This study demonstrates that Raman scattering is not only sensitive to a thin layer at the surface, but provides detailed quantitative information on the vibrational structure of complex stackings which takes place during the first stage of heteroepitaxy of semiconductors.