Optical phonons and resonant Raman scattering in II–VI spheroidal quantum dots

Abstract

PACS 63. 22.+m, 78.30.Fs First order and multiphonon resonant Raman scattering processes from confined and interface polar optical phonons in II-VI nanocrystallites are analyzed. Fraehlich interaction between excitons and optical phonons has been considered and general selection rules for the exciton-phonon matrix elements of the scattering processes in the case of spheroidal quantum dots are given. A comparison is made between the Raman selection rules and non-linear hyper-Raman selection rules for spherical dots. It is shown that the relative intensities between phonon overtones in II-VI spherical quantum dots can be correctly described in the framework of excitonic model and the optical polar vibrational continuum model satisfying both the mechanical and electrostatic matching conditions at the interface. Detailed discussions of the surface optical phonons are provided in the case of a spherical Quantum-Dot/Quantum-Well (QD/QW) heterostructure. Their frequency dependence on the geometry, material parameters, as well as the electron-phonon interaction Hamiltonian are also reported. In addition, the interface optical phonons and the corresponding electron-phonon interaction in nanostructures with prolate and oblate spheroidal geometries and their contribution to the first order scattering process have been studied. This surface-optical phonon modes are strongly dependent on the nanocrystal geometry and new Raman selection rules are obtained given a plausible explanation for the observed low frequency shoulders present in the spectrum of several II-VI semiconductor nanostructures. [1] and reference therein). How do the optical phonon vibrations and their symmetry properties look like as the dimensions of the system change? Which is the behavior of the electron-optical phonon Hamiltonian as a function of the geometry and nanocristal size? These and other questions have attracted much attention in the last years due to their fundamental issues and the potential applications of the new generation of semiconductors. Several theoretical approaches have been considered to describe the confined and surface modes of colloidal nanostructures (see Refs. [1, 2] and reference therein). Weakly polar semiconductor compounds, in the form of slabs and spheres, were considered in pioneering works on the subject. Present developments of sophisticated growth techniques made possible to obtain size selected II-VI semiconductor quantum dots (QDs) of nanoscale dimensions with good size distributions [3,4]. On the other hand, resonant Raman technique allows to study the optical vibrations modes and the characteristic of the electron-phonon interaction at an almost individual level [5][6][7]. The present contribution on the field is organized as follows: In Sec. 2, a review of the optical longitudinal phonons and Raman selection rules in spherical quantum dots for the first and higher order is presented. Also, the non-linear hyper-Raman effect is discussed. The evolution of the surface optical

Оптичні Дослідження Надмалих Колоїдних Наночастинок Напівпровідників A2B6 Та Гетерочастинок На Їх Основі

“…Оскiльки НЧ складають лише незначну частку (близько 1%) розсiюючого об'єму зразка, то для отримання сигналу КРС достатньої iнтенсивностi необхiдно забезпечити резонанснi умови збудження. Для даної довжини хвилi збуджуючого лазерного випромiнювання, перерiз розсiяння на одиницю просторового кута Ω s та одиницю частоти ω s для ансамблю НЧ з певним розкидом за розмiрами визначається таким виразом [31]:…”

Optical properties of Quantum Disks: Real density matrix approach

“…In the present paper we consider a quantum dot of cylindrical shape, with the symmetry axis z, and with infinite hard wall potentials for electrons and holes located in the xy plane at the radius R. Such a quantum dot is also called a Quantum Disk (QDisk) [9][10][11][12][13][14][15][16][17]. Mostly the lateral extension in a QDisk is much greater then the vertical one: the latter is usually smaller than the respective effective exciton radius.…”

Magnetooptical properties of quantum disks in the Faraday configuration

“…In the present paper we consider a quantum dot of cylindrical shape, with the symmetry axis z, and with infinite hard wall potentials for electrons and holes located in the xy plane at the radius R. Such a quantum dot is also called quantum disk (QDisk) [6], [9]- [17]. Mostly the lateral extension in a QDisk is much greater then the vertical one: the latter is usually smaller than the respective effective exciton radius.…”