N. Korsunska

The effect of oxidation on the efficiency and spectrum of photoluminescence of porous silicon

The photoluminescence spectra of porous silicon and their temperature dependences and transformations on aging are studied. It is shown that the infrared band prevailing in the spectra of as-prepared samples is due to exciton recombination in silicon crystallites. On aging, a well-pronounced additional band is observed at shorter wavelengths of the spectra. It is assumed that this band is due to the recombination of carriers that are excited in silicon crystallites and recombine via some centers located in oxide. It is shown that the broad band commonly observable in oxidized porous silicon is a superposition of the above two bands. The dependences of the peak positions and integrated intensities of the bands on time and temperature are studied. The data on the distribution of oxide centers with depth in the porous layer are obtained.

Role of paramagnetic defects in light emission processes in Y-doped ZrO2 nanopowders

Luminescence and structural properties of pure and Y-doped ZrO2 nanopowders with different Y content synthesized by co-precipitation of Zr and Y salts were investigated by x-ray diffraction, transmission electron microscopy, electron paramagnetic resonance (EPR) and photoluminescence (PL) methods. It was found that at constant calcination temperature (700 °С), the increase of Y content stimulates the transformation of crystalline phase from monoclinic through the tetragonal to the cubic one. Generally, room temperature PL emission was found to be similar for the samples with different Y content, demonstrating the same overlapped PL components in visible spectral range under extrinsic excitation. The relative contribution of each PL component was found to be affected by calcination time. In EPR spectra of as-prepared samples no signals were observed. The annealing in N2 or H2 flow results in the appearance of the signal from surface Zr3+ defects. In the latter the signal assigned to F-center also arises. The anti-correlation observed between the PL intensity and the value of the Zr3+ EPR signal allows us to conclude that the Zr3+ center is the center of fast non-radiative recombination. At the same time, interrelation between the intensity of the EPR signal assigned to F-centers and observed PL bands was not found.

Si-rich Al2O3 films grown by RF magnetron sputtering: structural and photoluminescence properties versus annealing treatment

Silicon-rich Al2O3 films (Six(Al2O3)1−x) were co-sputtered from two separate silicon and alumina targets onto a long silicon oxide substrate. The effects of different annealing treatments on the structure and light emission of the films versus x were investigated by means of spectroscopic ellipsometry, X-ray diffraction, micro-Raman scattering, and micro-photoluminescence (PL) methods. The formation of amorphous Si clusters upon the deposition process was found for the films with x ≥ 0.38. The annealing treatment of the films at 1,050°C to 1,150°C results in formation of Si nanocrystallites (Si-ncs). It was observed that their size depends on the type of this treatment. The conventional annealing at 1,150°C for 30 min of the samples with x = 0.5 to 0.68 leads to the formation of Si-ncs with the mean size of about 14 nm, whereas rapid thermal annealing of similar samples at 1,050°C for 1 min showed the presence of Si-ncs with sizes of about 5 nm. Two main broad PL bands were observed in the 500- to 900-nm spectral range with peak positions at 575 to 600 nm and 700 to 750 nm accompanied by near-infrared tail. The low-temperature measurement revealed that the intensity of the main PL band did not change with cooling contrary to the behavior expected for quantum confined Si-ncs. Based on the analysis of PL spectrum, it is supposed that the near-infrared PL component originates from the exciton recombination in the Si-ncs. However, the most intense emission in the visible spectral range is due to either defects in matrix or electron states at the Si-nc/matrix interface.

Investigation of aging process of Si–SiOx structures with silicon quantum dots

In this work the aging processes of magnetron-sputtered Si–SiOx structures with silicon quantum dots are investigated by photoluminescence, electronic paramagnetic resonance, infrared absorption, and Raman-scattering methods. It is observed that oxidation of the silicon dots, change in the defect concentration in the oxide matrix, and oxidation of the silicon amorphous phase occur during storage in air at room temperature. A comparison of the variation of parameters of sputtered structures and porous silicon layers caused by the aging process is made. It is shown that the rate of oxidation of silicon dots and the decrease of their sizes in sputtered structures are essentially less than that in porous silicon. It is also shown that in Si–SiOx, layers in contrast to porous silicon, the intensity and spectral position of the emission band caused by exciton recombination in Si crystallites do not change practically during aging.

Effect of conjugation with biomolecules on photoluminescence and structural characteristics of CdSe/ZnS quantum dots

The photoluminescence and photoluminescence excitation spectra, the X-ray diffraction patterns, and the effect of conjugation with biomolecules upon these characteristics are studied for silanized CdSe/ZnS quantum dots. Along with the band of annihilating excitons in the quantum dots, the luminescence spectra exhibit emission associated with defects. It is established that the emission spectrum of defects involves at least two components. It is shown that the defects are located mainly at the small-sized quantum dots; the defects responsible for the long-wavelength component are located mainly at the quantum dots larger in size than the quantum dots, at which the defects responsible for the short-wavelength component are located. It is found that conjugation with biomolecules induces not only the blue shift of the excitonic band, but transformation of the emission spectra of defects and an increase in the contribution of defects to the luminescence spectrum as well. The changes observed in the emission spectrum of defects are attributed to the formation of certain emission centers. It is shown that, when conjugated with biomolecules, the quantum dots experience increasing compression strains. This effect is responsible for the blue shift of the luminescence band of the quantum dots.

Features of ZnS-powder doping with a Mn impurity during synthesis and subsequent annealing

Luminescence, electron spin resonance, and X-ray diffraction (XRD) methods were used to investigate the features of ZnS-powder doped by Mn impurity during self-propagating high-temperature synthesis and subsequent annealing. The obtained powder consists of ZnS microcrystals with mainly hexagonal phase (80 ± 5)%. It was found, that after synthesis Mn presents not only in the form of non-uniformly distributed microscopic impurities in ZnS, but also in the form of Mn metal nanocrystals. Thermal annealing at 800°C leads to the additional doping of ZnS from metallic Mn, to the redistribution of the embedded Mn in the volume of microcrystals, and to the ZnS oxidation. At the same time, the ratio between the cubic and hexagonal phases does not change. It was shown that annealing causes a decrease in the concentration of the defects responsible for the luminescence-excitation bands, which correspond to transitions from the ground to the excited states of the Mn2+ ion. As a result of annealing, there is also a change in XRD coherent domain size. Simultaneously, the intensity of peaks in the luminescence-excitation spectrum with wavelengths of 375 and 395 nm was changed. The causes of these changes and the nature of the corresponding bands are discussed.

New insight on the interaction of self-activated and Mn-related emission centers in ZnS

The photoluminescence (PL) and PL excitation (PLE) spectra of undoped and thermally doped with Mn ZnS single crystals are studied. In the PL spectra, the bands caused by Mn-related and self-activated (SA) emission centers were observed. A number of narrow peaks whose intensity enhanced with increasing Mn content were found in the PLE spectra of SA emission. The same peaks were present in the PLE spectra of the Mn-related emission band. Some of these peaks were previously observed in the absorption spectra and attributed to Mn2+ ions. The appearance of Mn-related peaks in the PLE spectra of SA emission is explained by excitation transfer from the Mn2+ ions to SA emission centers. The conditions required for this transfer and possible mechanisms of the process are discussed.

Impurity-Governed Modification of Optical and Structural Properties of ZrO2-Based Composites Doped with Cu and Y

The influence of calcination temperature on copper spatial localization in Y-stabilized ZrO2 powders was studied by attenuated total reflection, diffuse reflectance, electron paramagnetic resonance, transmission electron microscopy, electron energy loss, and energy-dispersive X-ray spectroscopies. It was found that calcination temperature rise in the range of 500–700xa0°C caused the increase of copper concentration in the volume of ZrO2 nanocrystals. This increase was due to Cu in-diffusion from surface complexes that contained copper ions linked with either water molecules or OH groups. This copper in-diffusion led also to an enhancement of absorption band peaked at ~270xa0nm that was ascribed to the formation of additional oxygen vacancies in nanocrystal volume. Further increasing of calcination temperature from 800 up to 1000xa0°C resulted in outward Cu diffusion accompanied by a decrease of the intensity of the 270-nm absorption band (i.e., oxygen vacancies’ number), the transformation of ZrO2 tetragonal (cubic) phase to monoclinic one as well as the enhancement of absorption band of dispersed and crystalline CuO in the 600–900xa0nm range.

Structural and optical properties of ZnS:Mn micro-powders, synthesized from the charge with a different Zn/S ratio

The influence of Zn/S ratio in the charge on structural and optical properties of ZnS:Mn powders produced by high-temperature self-propagated synthesis was investigated. The samples was shown to consist of mixed-polytypes ZnS crystallites with hexagonal (2H) and cubic (3C) phases, the contribution of the latter increases with the sulfur content in the charge. The most homogeneous size distribution were found at stoichiometric Zn/S ratio. The Zn/S relation affects the Mn incorporation into ZnS lattice. The highest quantity of incorporated Mn is observed at stoichiometric Zn/S relation while lowest one is realized at Zn excess. Besides, the distribution of manganese ions in the blocks, which compose the crystallites, was found to be inhomogeneous, their concentration decreases from crystallites surface to the depth. Mn ions are nearer to the surface in ZnS:Mn synthesized with Zn excess. At Mn concentration in the charge of 1xa0wt% the shift of ZnS band edge to low energy side is observed, that is ascribed to formation of solid solution ZnS–MnS with lower band gap value.

Mechanisms of the degradation of Schottky-barrier photodiodes based on ZnS single crystals

The effect of ultraviolet (UV) illumination on the electrical and spectral characteristics of Schottky-barrier photodiodes based on ZnS single crystals is studied. It is found that irradiation deteriorates their photosensitivity and changes the current–voltage and capacitance–voltage characteristics and the surface profile of the blocking electrode. It is shown that the main reason for a decrease in the photosensitivity of the diodes is the photoinduced drift of mobile donors in the electric field of the barrier. This drift depends on the crystallographic orientation of the surface being irradiated. Another photoinduced process observed in the diodes is photolysis of the ZnS crystal. This process mainly determines the change in the electrical characteristics of the diodes and in the surface profile of the electrode at an insignificant change in the photosensitivity.