Raman determination of uniformity of multilayer Si/Ge structures with Ge quantum dots.

Spectra of Raman scattering by optical phonons of multilayer Si/Ge structures with Ge quantum dots (QDs) grown by means of low-temperature (T(s) = 250 degrees C) molecular beam epitaxy are studied. Two types of Ge islands are observed: pseudomorphic to a Si matrix, and those in which the strains are relaxed non-uniformly. Application of polarization measurements allows us to separate the phonon lines corresponding to these two components of a QD array. A method for the quantitative estimation of the degree of uniformity of the QD array, determining the integrated electron-hole spectrum of the structure, is proposed.

An electron-hole spectrum of Ge/Si structures with Ge quantum dots: photoconductivity measurements.

The lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots (QDs) is investigated. Photoresponse in the range of 1.2-0.3 eV related to the optical transitions between QD hole levels and Si electron states is observed. It is shown that the main contribution to the lateral photoconductivity is made by the electron states localized in the Si band bending region. Application of a 'quantum box' model for the description of QD hole levels allows us to clear up the nature of peaks observed in the photoconductivity spectrum. A detailed energy scheme of the Ge/Si structures with Ge QDs is built up.

Raman resonance in the strained Ge quantum dot array.

We study the Raman resonance of a Ge quantum dot (QD) array grown pseudomorphically to a Si matrix using low-temperature molecular-beam epitaxy. A change of the resonance energy and the shape of the resonance curve in comparison with bulk Ge are observed. These features are shown to be explained by taking into account QD strain and the quasistationary character of the electronic states responsible for the observed resonance. Application of a model of the two-dimensional critical point of the interband density of states allows us to estimate the damping parameter and localization size of these states. It is shown that the observed enhancement of the resonance amplitude in a QD array as compared to the bulk case is related to transformation of the interband density of states into the δ-function due to quantization of the electron-hole spectrum.