Resonant Raman Scattering by Strained and Relaxed Ge Quantum Dots

Abstract

Fundamental vibrations in Ge/Si structures with strained and relaxed Ge quantum dots (QDs) grown by molecular beam epitaxy were investigated using resonant Raman spectroscopy. Transmission electron microscopy experiments show that the strained Ge QDs are typical “hut clusters” with base size of 15nm and a height of 2nm. A two mode distribution in size (100–200nm and 3–6nm) is found for relaxed QDs. The Raman efficiencies of the Ge optical phonons as a function of excitation energy reveal maxima at 2.35–2.41eV attributed to the E0 resonance in Ge QDs due to electronic confinement. The frequency positions of optical phonons localized in Ge “hut clusters” under non-resonant conditions correspond to fully strained Ge QDs while the frequency position of optical phonons in relaxed Ge QDs corresponds to the value in bulk Ge. With increasing excitation energy (2.5–2.7eV) the position of the Ge optical phonons shifts downwards due to size-confinement effect of optical phonons in strained and relaxed Ge QDs, indicating the presence of a QD size distribution in Ge dot structures.

Phonons in InAs quantum dot structures

“…As it was shown in [3] optical phonons localized in the QDs are shifted towards higher (lower) energies with respect to their bulk values. This manifests compressive (tensile) strain in InAs (AlAs) QDs in the plane of the layers.…”

“…However, the vibrational spectrum which contains information on the structural properties of QDs (such as size dispersion, shape [2,3] and mechanical strain [4,5]) even in InAs/Ga(Al)As QDs is much less investigated. The knowledge and the possibility to control the phonon spectrum are especially important in potential optoelectronic devices based on QDs, since the phonons influence the relaxation of electronic excitations [1] and the mobility of charge carriers.…”

“…Several publications [2][3][4][5][7][8][9][10] devoted to experimental (mostly by Raman scattering) and theoretical investigation of optical and interface phonons in InAs/Al(Ga)As QDs have occured recently. As it was shown in [3] optical phonons localized in the QDs are shifted towards higher (lower) energies with respect to their bulk values.…”

“…A relaxation of the elastic strain in multilayered QD structures with decreasing interlayer distance was also observed [7]. The small size of QDs (typically of pyramidal shape with a base size of 10-20 nm with a height of 2-4 nm) leads to confinement of optical phonons in QDs and, therefore, to lowering their phonon energies [3]. The spectrum of interface phonons in the QD structures differs significantly from that of multilayered planar structures and depends on the QD shape [11,12].…”

Raman study of interface phonons in InAs quantum dot structures

“…It was shown that optical phonons observed in the Raman spectra can effectively be used as a sensitive probe to study the alloy composition and the residual strain in the islands [9][10][11][12]. Recently, we reported a study of acoustic and optical phonons in structures with InAs QDs embedded in an AlAs matrix, as well as GaAs and AlAs nanosize islands ("antidots") in an InAs matrix [13,14]. In the low frequency region, Raman scattering in Ge/Si and InAs/InP QD structures was observed even for a single QD layer [15,16].…”