T.V. Torchynska

Thermal activation of excitons in asymmetric InAs dots-in-a-well InxGa1−xAs∕GaAs structures

Photoluminescence, its temperature dependence, and photoluminescence excitation spectra of InAs quantum dots embedded in asymmetric InxGa1−xAs∕GaAs quantum wells [dots in a well (DWELL)] have been investigated as a function of the indium content x (x=0.10–0.25) in the capping InxGa1−xAs layer. The asymmetric DWELL structures were created with the aim to investigate the influence of different barrier values at the quantum dot (QD)/quantum well interface on the photoluminescence thermal quenching process. The set of rate equations for the two stage model for the capture and thermal escape of excitons in QDs are solved to analyze the nature of thermal activation energies for the QD photoluminescence quenching process. The two stage model for exciton thermal activation was confirmed experimentally in the investigated QD structures as well. The localization of nonradiative defects in InAs∕InGaAs DWELL structures is discussed on the base of comparison of theoretical and numerically calculated (fitting) results.

Scanning photoluminescence spectroscopy in InAs∕InGaAs quantum-dot structures

Spatially-resolved photoluminescence (PL) spectroscopy was performed at different temperatures on self-assembled InAs quantum dots embedded into MBE-grown In0.15Ga0.85As∕GaAs multiquantum-well heterostructures. Strong inhomogeneity of the PL intensity is observed by mapping samples with different In∕Ga composition of the InxGa1−xAs capping layers (0.1⩽x⩽0.2). Two different behaviors in the quantum-dot PL maps are observed: (1) a reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300K and “red” shift at 80K, and (2) PL intensity variation occurs at a stable peak position of the PL maximum. Two separate mechanisms are suggested to account for the observed intensity variation of the quantum-dot luminescence.

Some aspects of exciton thermal exchange in InAs quantum dots coupled with InGaAs/GaAs quantum wells

Photoluminescence (PL), its temperature and excitation power dependences, and PL excitation spectra have been investigated in InAs quantum dots (QDs) embedded in In0.15Ga0.85As/GaAs quantum wells (QWs) as a function of QD density. The QD density varied from 1.1×1011 down to 1.3×1010 cm−2 with the increase in QD growth temperature at the molecular beam epitaxy processing. A set of rate equations for exciton dynamics (relaxation into QWs and QDs, and thermal escape) has been solved to analyze the mechanism of PL thermal quenching in studied structures. Three stages have been revealed in thermal decay of the PL intensity of InAs QDs. Presented mathematical analysis provides the explanations of the mechanism of PL thermal decay as well as the peculiarities of PL excitation power dependences and PL excitation spectra. A variety of activation energies of PL thermal decay and the localization of nonradiative defects in InGaAs/GaAs QW structures with different InAs QD density are discussed as well.

Raman-scattering and structure investigations on porous SiC layers

Raman scattering spectroscopy, scanning electron microscopy, and scanning acoustic microscopy were studied on porous SiC layers prepared by different technological routes and subjected to reactive ion treatment. The Raman spectra revealed a number of features specific for nanocrystallite materials, which can be used for characterization and diagnostics of porous SiC layers for technological applications.

Thermal ionisation of ground and multiply excited states in InAs quantum dots embedded into InGaAs/GaAs MQW

The photoluminescence (PL) spectra of highly uniform self-assembled InAs quantum dots (QDs) embedded in In 0.15 Ga 0.85 As multi-quantum-well (MQW) heterostructures have been investigated at variable temperatures. This paper presents the PL bands, connected with ground (GS) and multi-excited states (ES) in QDs. Not equidistant optical transitions have been revealed. Spectral peak shifts and PL intensity variations in the temperature range 12-220 K for all PL bands are analyzed. The activation energy of the temperature quenching processes for GS and 4 ES optical transitions in InAs QDs are measured. The mechanism of these processes and the positions of the energy levels in QDs are discussed as well.

Mechanism of photoluminescence of silicon oxide films enriched by Si or Ge

Photoluminescence peculiarities of silicon oxide films enriched by Si or Ge have been investigated. Photoluminescence (PL) and Raman spectra were measured before and after thermal annealing at 800 °C. The dependences of PL peculiarities on the concentration of Si and Ge, as well as on the existence (or absence) of Si (Ge) quantum dots (QDs) in silicon oxide films are analyzed for the photoluminescence mechanism study in the above-mentioned systems.

Defect related photoluminescence in Si wires

Photoluminescence spectra and their dependence on the temperature have been used to study the peculiarities of the red photoluminescence in low-dimensional Si structures, such as porous silicon and silicon oxide films with an admixture of silicon. It has been shown that red photoluminescence band of Si wires is complex and can be decomposed into two elementary bands. Practically the same positions of photoluminescence bands are observed in silicon oxide films. Comparative investigation of photoluminescence temperature dependence in Si wires and silicon oxide indicates that oxide defect related mechanisms for photoluminescence bands are involved. The photoluminescence excitation mechanisms in both objects are discussed as well.

Three approaches to surface substance role investigation in porous silicon photoluminescence and its excitation

Photoluminescence (PL) and photoluminescence excitation spectra research, as well as secondary ion mass spectroscopy and infrared vibration spectra measurements, were used for the investigation of PL excitation mechanism of porous silicon. It is shown that there are two types of porous silicon PL excitation spectra: one that consists of visible and ultraviolet bands and one that contains only an ultraviolet one. The different dependencies of intensity of each excitation band upon anodization regimes, as well as ageing and thermal treatment, were observed. Two excitation channels have been shown in porous silicon. The visible PL excitation band at 300 K has been attributed to light absorption of some species on the Si wire surface. The nature of ultraviolet excitation band is also discussed.

Complex nature of the red photoluminescence band and peculiarities of its excitation in porous silicon

Abstract Photoluminescence and photoluminescence excitation spectroscopies, scanning electron microscopy, and atomic force microscopy were used to study the photoluminescence mechanism in porous silicon. The dependences of photoluminescence parameters on electrochemical etching regimes, excitation light wavelength, and vacuum ageing have been investigated. We show that intensive and broad “red” luminescence band (∼600–800 nm) is non-elementary, and can be decomposed into three elementary bands. The mechanisms of the elementary bands are discussed.

OH-related emitting centers in interface layer of porous silicon

Abstract Photoluminescence and excitation spectra measurements as well as SIMS and FTIR techniques were used to investigate the photoluminescence excitation mechanism of porous silicon. It is shown that there are two types of photoluminescence excitation spectra which consist either of two, visible and ultraviolet, or one, only ultraviolet, bands. The dependence of photoluminescence excitation spectra upon the various treatment (aging in vacuum, in air and in liquids) indicates that the excitation in the visible range occurs via light absorption of some species on the porous Si surface.