Yu. A. Pusep

Structural and optical analysis of GaAsP/GaP core-shell nanowires

The structural and optical properties of GaAsP/GaP core-shell nanowires grown by gas source molecular beam epitaxy were investigated by transmission electron microscopy, Raman spectroscopy, photoluminescence (PL), and magneto-PL. The effects of surface depletion and compositional variations in the ternary alloy manifested as a redshift in GaAsP PL upon surface passivation, and a decrease in redshift in PL in the presence of a magnetic field due to spatial confinement of carriers.

Capacitance spectroscopy of InAs self-assembled quantum dots embedded in a GaAs/AlAs superlattice

The characteristics of the InAs self-assembled quantum dots embedded both in a GaAs bulk matrix and in a GaAs/AlAs superlattice were investigated. Evidences of electrons confinement inside the InAs quantum dots were obtained using both capacitance–voltage measurements and Raman spectroscopy. A much stronger electron localization was detected for the quantum dots embedded in the superlattice in comparison with those embedded in bulk GaAs. As a consequence, the electrical characteristics of the structures with quantum dots grown in superlattices were found to be significantly thermo-stabilized. The origins of these effects are discussed in connection with the differences between the electronic features of the two kinds of structures.

CHARACTERIZATION OF GAAS WIRE CRYSTALS GROWN ON POROUS SILICON BY RAMAN SCATTERING

We measured the Raman spectra of GaAs wirelike crystals grown on porous silicon (PS) using two different excitation radiations which probe the near surface and the bulk. The transverse optic and longitudinal optic vibrational bands appear redshifted and broadened when compared to bulk GaAs, and with shoulders on their low frequency sides. These features are attributed to a disorder-induced relaxation in the selection rules for Raman activity arising from point defects. In addition, the bands show a strong frequency and bandwidth dependence on excitation energy. Differences in penetration depth together with a gradient in defect density are invoked to account for the observations. This analysis yields a picture in which there is a predominantly amorphous GaAs region buried at the PS–GaAs interface followed by microcrystalline GaAs cylindrical structures that become less defective as they grow farther from the interface. The near surface tends to approach the low defect density of crystalline GaAs.

Parallel conductivity of random GaAs/AlGaAs superlattices in regime of controlled vertical disorder

The transport properties of intentionally disordered short-period doped GaAs/AlGaAs superlattices were explored. We found that a redistribution of the electron density in a random potential significantly influences the electron properties of the disordered superlattices resulting in a suppression of the vertical disorder and in the increase of the in-plane mobility of the layers with increasing disorder.

Raman study of the topology of InAs/GaAs self-assembled quantum dots

The topology of self-assembled InAs/GaAs quantum dots was studied by resonant Raman scattering caused by the interface modes localized near the edges of the dots. Evidences were found that on both sides of the InAs layer containing the dots, their topologies show some resemblances. In addition, in the multilayered systems the evidence of the coalescence of the dots (which form vertical columns) in neighbor layers separated by the distance smaller than 25 monolayers was obtained.

The role of quantum confinement and crystalline structure on excitonic lifetimes in silicon nanoclusters

The emission energy dependence of the photoluminescence (PL) decay rate at room temperature has been studied in Si nanoclusters (Si-ncl) embedded in Si oxide matrices obtained by thermal annealing of substoichiometric Si oxide layers SiyO1−y, y=(0.36,0.39,0.42), at various annealing temperatures (Ta) and gas atmospheres. Raman scattering measurements give evidence for the formation of amorphous Si-ncl at Ta=900 °C and of crystalline Si-ncl for Ta=1000 °C and 1100 °C. For Ta=1100 °C, the energy dispersion of the PL decay rate does not depend on sample fabrication conditions and follows previously reported behavior. For lower Ta, the rate becomes dependent on fabrication conditions and less energy dispersive. The effects are attributed to exciton localization and decoherence leading to the suppression of quantum confinement and the enhancement of nonradiative recombination in disordered and amorphous Si-ncl.

A study of the vertical transport of electrons in (GaAs)n(AlAs)m superlattices by Fourier transform infrared spectroscopy

The longitudinal optical vibrational modes have been studied in (GaAs)n(AlAs)m superlattices by means of FTIR spectroscopy. In the undoped samples, confined LO phonons have been observed; the dispersion of LO phonons obtained by FTIR spectroscopy was in good agreement with the Raman data. In the doped (GaAs)n(AlAs)m superlattices, coupling of confined vibrational LO modes with superlattice plasmons has been found. Analysis of the experimental results showed that the frequency of superlattice plasmons in the superlattices studied is mostly determined by the population of the miniband formed by the Gamma -like conduction band states. The fitting of the calculated reflection spectra to the experimental spectra allowed us to measure the population of the minibands, the effective mass and the vertical mobility of electrons.

Raman study of collective plasmon-longitudinal optical phonon excitations in cubic GaN and AlxGa1−xN epitaxial layers

The plasmon-longitudinal optical (LO) phonon collective excitations were studied by Raman scattering in cubic GaN doped with Si and in intrinsically doped cubic AlxGa1−xN alloys. The specific asymmetry of the corresponding Raman lines, associated with the dispersion of the coupled excitations was clearly detected in GaN. The spatial coherence of the relevant coupled oscillations was determined from the comparison of the experimental Raman spectra with the calculated ones. A broad line, which was previously attributed to the overdamped plasmon-LO phonon oscillations, was also observed in the GaN and AlxGa1−xN samples in the range between the transversal optical and LO phonon frequencies.

Carrier confinement in an ultrathin barrier GaAs/AlAs superlattice probed by capacitance–voltage measurements

Abstract The capacitance–voltage characteristics of (GaAs)m/(AlAs)n superlattices and of a GaAs/AlGaAs multiple quantum well system were used as a tool to probe the homogeneity of the studied samples. Evidences of a strong electron localization in the superlattices even with the presence of minibands were found. We interpret this result taking into account the presence of local inhomogeneities in the superlattices which causes the breakdown of the coherence of the miniband transport and therefore, give rise to the electron localization. In order to support this conclusion we numerically calculate the capacitance of the superlattices assuming a localization center near the region where electron confinement takes place and the results were found in good agreement with the measured capacitance.

High resolution x-ray diffraction analysis of InGaAs∕InP superlattices

The interfacial properties of lattice-matched InGaAs∕InP superlattice (SL) structures grown by gas source molecular beam epitaxy were investigated by high resolution x-ray diffraction (HRXRD). SLs with various periods were grown to determine the contributions of the interface layers to the structural properties of the SLs. The HRXRD curves exhibited a number of features indicative of interfacial layers, including weak even-order satellite peaks, and a zero-order diffraction peak that shifted toward lower diffraction angles with decreasing SL period. A detailed structural model is proposed to explain these observations, consisting of strained InAsP and InGaAsP monolayers due to the group-V gas switching and atomic exchange at the SL interfaces.