Yu. A. Stepanov

Optical and structural properties of thallium halide microcrystals in a porous matrix

Thallium halide microcrystals were grown from water solutions in a porous matrix obtained from alkali borosilicate glass by etching, and their absorption and luminescence spectra were studied. Constraining the crystal size was found to affect the stability of some crystal modifications. The exciton radii were determined, and the dimensions of microcrystals in which size quantization effects become observable were estimated from the absorption spectra of bulk TlBr and TlCl crystals.

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ∼105 W/cm2, excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.

Photoluminescence of anodized silicon carbide

The luminescence of single-crystalline 6H-SiC plates after electrochemical etching has been investigated. The photoluminescence spectrum was found to change strongly after etching; the decay times of separate bands were determined. Just as in the case of silicon, the change in the photoluminescence could be due to the formation of nanostructures.

Exciton Luminescence in Tin Dioxide Single Crystals

The exciton spectra of SnO 2 single crystals with the low and high concentrations of the oxygen vacancies (OV) are investigated. The series of luminescence lines adjacent to the lowest free exciton states (the excitons localized by the isolated OV and OV clusters) increases superlinearly with the optical excitation level. It is shown that the luminescence spectra of SnO 2 can be described as the zero-phonon and one LO-phonon assisted anihilation of the free and localized excitons. The shape of LO-phonon replicas of the free exciton luminescence corresponds to Maxwellian distribution of the excitons which have the effective temperature significantly higher than the lattice temperature in the case of pulse excitation level 10 6 –10 7 W/cm 2 .

Radiative recombination in Zn{sub 1-x}Mn{sub x}Te/Zn{sub 0.59}Mg{sub 0.41}Te quantum well structures: Exciton emission and intracenter luminescence

The luminescence spectra controlled by excitons and intracenter 3d emission of Mn2+ ions are studied for a series of Zn1-xMnxTe/Zn0.59Mg0.41 Te quantum well (QW) structures that differ in manganese content and QW width. It is shown that the relative intensities of exciton emission of the QWs and barriers and the dependences of the intensities on the optical excitation level are controlled mainly by the manganese content in the QWs that affect the efficiency of excitons transfer of to the 3d shell of the Mn2+ ions. The effects of QW width and manganese content on the decay kinetics of the intracenter luminescence of the Mn2+ ions are studied.

Effect of contact with air on the photoluminescence spectrum of porous silicon

A study has been made of the transformation of photoluminescence (PL) spectra of porous silicon (PS) induced by its ageing, including the early stages of contact with air. The sample was prepared under conditions that minimized this contact, and spectral measurements were carried out in a high vacuum or in liquid nitrogen. The PS PL spectra obtained under continuous measurement in high vacuum are always dominated by one emission band of PS nanoelements, which shifts toward shorter wavelengths with ageing by 150 nm. At 80 K, the band intensity is considerably higher than at 300 K, and this difference grows with ageing. Exposure of a sample to air for a few tens of seconds is long enough to strongly transform its time-resolved PL spectra, which is evidence of a change in the sample surface. The effect of immersion of PS samples in liquid nitrogen on PL spectra is associated not only with their cooling, but also with the field of adsorbed nitrogen molecules, whose influence becomes weaker with increasing thickness of the oxidized near-surface layer. The variation of the spectral properties and kinetics of the long-wavelength PS PL band with temperature, medium (liquid nitrogen or vacuum), and exposure time suggest that these factors affect carrier migration between silicon nanoelements.

Radiative recombination in the CdMgTe matrix with ultrathin narrow-gap CdMnTe layers

Exitonic emission from three structures consisting of CdMnTe layers separated by thick wide-gap CdMgTe layers are studied in the temperature range from 13 to 300 K. The thickness of the CdMnTe layers corresponds to the 0.5, 1.5, and 3.0 monolayers. To calculate the excitonic density-of-state spectra in these structures, a simple model that takes into account variations in the elemental composition of the alloy inside the exciton sphere under variations in its position with respect to the narrow-gap layer is used. It is shown that the energy extent of the emission and density-of-state spectra and the positions of the characteristic features of the spectra are in agreement. On low-intensity excitation (1–10 W/cm2), heating of the samples from 13 K yields a relative decrease in the emission intensity of excitions localized in the CdMgTe layers, so that this emission at 80 K is much lower in intensity than the emission of excitions bound at the narrow-gap layers. In the conditions of high-intensity excitation (103–105 W/cm2) at 80 K, the excitonic emission from the CdMgTe layers prevails; however, at room temperature, the emission from the narrow-gap layers dominates over the emission from the wide-gap layers. The intracenter luminescence of the Mn2+ ions in the sample with three monolayers CdMnTe presents a doublet structure, with the components corresponding to the internal and external layers of the three-layer planar inclusions containing manganese.

Optical properties of a polymer host matrix containing mercuric iodide microcrystals

Microcrystals of mercuric iodide have been grown in a polymer matrix with pores 200 nm in diameter. The absorption and luminescence spectra indicate the formation of microcrystals of two (red and orange) modifications in the host matrix. The temperature dependence of the emission spectra and the evolution of absorption and emission spectra in the course of sample ageing have been studied. The evolution of the spectra is governed by the transition from the orange to red modification and the increase in size of microcrystals.

Effect of γ irradiation on the photoluminescence kinetics of porous silicon

The effect of γ irradiation on the photoluminescence decay dynamics in porous silicon is investigated. Growth of the photoluminescence intensity and decrease of the decay time in irradiated porous silicon are explained by a lowering of the barriers to recombination of spatially separated electrons and holes via tunneling. The γ irradiation of porous silicon leads to a greater dispersion of the decay time.