Anomalous first-order Raman scattering in III-V quantum dots: Optical deformation potential interaction

Abstract

In contrast to the most commonly studied nanocrystals of II-VI materials, resonant Raman spectra of colloidal III-V quantum dots (QDs) show two almost equally intense peaks centered approximately at the longitudinal and transverse optical (TO) bulk phonon frequencies. The ``anomalous'' spectra of III-V QDs are explained in the framework of a microscopic theory for the first-order resonant Raman scattering, which takes into account the optical deformation potential (ODP) and Fr\ohlich exciton-phonon interactions---valid for spherical nanoparticles. It is obtained that: (i) the ``anomalous'' TO peak is mostly due to confined phonon modes with the angular momentum ${l}_{p}=3$; (ii) Raman intensity depends on the QD radius $(R)$ as ${R}^{\ensuremath{-}3}$ for the ODP mechanism, while for the Fr\ohlich one it is proportional to ${R}^{\ensuremath{-}1}$; and (iii) the relative intensity ${I}_{\text{TO}}/{I}_{\text{LO}}$ ratio value is higher in backscattering configuration for cross polarization than for parallel one. Raman spectra calculated within the Luttinger-Kohn Hamiltonian for the electronic states and a phenomenological theory of optical vibrations including rigorously both the mechanical and electrostatic matching boundary conditions explain the experimental data for InP QDs using bulk phonon parameters and ODP constant.