The general review of the history and present-day situation of diluted magnetic semiconductor investigation in Ukraine is given by S. Ryabchenko. Some noteworthy results of diluted magnetic semiconductor investigation obtained in Ukraine are pointed out. The main features of the present day situation are mentioned also. As an example of last diluted magnetic semiconductor investigations, the new result obtained by Abramishvili, Komarov, Ryabchenko, Semenov, Kyrychenko and Dubowski for Cd_{1-x}Mn_{x}Te/CdTe/Cd_{1-x}Mn_{x}Te quantum well structures grown by laser ablation method are presented. A weak additional line was observed in the reflectivity spectra of a 27 Å wide quantum well with x= 0.11 in the barrier. Such additional line has not been observed in spectra of similar molecular beam epitaxy grown structures. Based on the theoretical computations of the energies and the relation of intensities of the main and additional lines we conclude that this line might be associated with hh2 → e1 transitions, which ceases to be forbidden in the presence of technologically caused asymmetry of quantum well potential profile.
Oscillations of photoluminescence properties in external magnetic fields are investigated in CdTe modulation doped quantum wells. The oscillatory behaviour of the luminescence intensity, the line width and the g factor is due to many-body effects in the 2-dimensional electron gas. The oscillation of photoluminescence intensity can be easily used as optically detected Shubnikov de Haas effect to determine the electron concentration in quantum wells without contacts.
We study the exciton localization in the semimagnetic semiconductor Cd_{1-x}Mn_{x}Te by selective excitation of the exciton photoluminescence. We show that the energy position of the effective mobility edge for excitons is subject to the competition between nonmagnetic and magnetic localization due to the magnetic polaron formation. External magnetic fields affect this competition by suppressing the polaron formation, which shifts the mobility edge.
Wire patterns (80-1000 nm) of molecular beam epitaxy grown Cd_{0.2}Zn_{0.8}Se/ZnSe quantum well were fabricated by a CH_{4}/H_{2} reactive ion etching technique. Photoluminescence emission shows with decreasing lateral size a broadening of line shape and a spectral red shift. Calculations for the change of the band gap due to strain relaxation show that this shift of the photoluminescence emission for narrow Cd_{0.2}Zn_{0.8}Se/ZnSe structures (lattice mismatch of 1.34%) can be explained by a partial elastic strain relaxation of the biaxially compressively strained Cd_{0.2}Zn_{0.8}Se quantum well after the patterning process.
X-ray photoelectron spectroscopy was employed to characterize the surface chemistry and electronic properties of the Zn_{1-x}Cd_{x}O semiconductor systems obtained at the different growth conditions. The effect of the growth conditions on the core and valence band spectra as well as room-temperature photoluminescence of the Zn_{1-x}Cd_{x}O films was investigated and discussed. Behavior of the X-ray photoelectron spectroscopy peaks indicated an increase of the cadmium and a depletion of the oxygen concentrations upon changing the Ar/O_2 gas ratio and dc power.
We present an attempt to control the properties of CdTe/ZnTe self-assembled quantum dots during their formation in the process of molecular beam epitaxy. Namely, the structures were in situ annealed at various temperatures and annealing times after the formation of quantum dots, before the deposition of a capping layer. Depending on the annealing parameters, the dots exhibit different optical properties which were studied by means of spatially resolved photoluminescence. From the analysis of these results, the information about relative changes of the average size and sheet density of quantum dots was extracted.
The characteristics of ZnSe-based electron beam pumped semiconductor lasers are presented in detail. The laser structures consist of a 0.6 μm thick superlattice waveguide centered with ten equidistantly placed CdSe/ZnSe quantum dot active layers. The maximum light output pulse power of 12 W per facet at room temperature along with an extremely high quantum efficiency of ≈8.5% were obtained at an electron beam pumping energy of 23 keV (the laser wavelength is of 542 nm). The calculations of a spatial distribution of non-equilibrium carrier concentration within the semiconductor structures under electron beam pumping are presented. The possible ways of further improvement of laser efficiency are discussed.
Measurements of steady state and transient stimulation and quenching of photocurrent and luminescence were performed in ZnSe single crystals doped with Cu which exhibit marked change in the photoconductivity and luminescence when illuminated by infrared light simultaneously with shorter wavelength radiation. A model which might explain all IR induced phenomena on the basis of donor-acceptor pair recombination mechanism including releasing holes from deep center by IR light and energy redistribution between different recombination centers is presented.
ZnO nanostructures doped with a high concentration of Gallium (Ga) were synthesized on a Si substrate by thermal evaporation. Subsequent heat treatments of selected nanostructures was done at 600°C, 700°C, 800°C and 900°C. Scanning electron microscope (SEM), X-Ray diffraction (XRD) and photoluminance (PL) studies was performed after every heat treatment. Systematic scanning electron microscope (SEM) studies suggest significant sublimation at 800°C. XRD results show that crystal quality was improved by annealing and phase separation may occur after high temperatures annealing. Ultraviolet (UV) and visible emission depends strongly on the annealing temperatures and luminescent efficiency of UV emission is enhanced significantly with heat treatment.
Halogen transport method was applied to grow the crystals of solid solutions of ZnSe and transition metals at the temperature far below the melting point and phase transition temperature. The large crystals of ZnMnSe, ZnFeSe, ZnNiSe and ZnFeSSe were obtained. The technological parameters and shape of the quartz reactor were chosen for growth of a large crystal by self-nucleation; the transparent quartz furnace enabled the control of nucleation by visual observation. The parameters of crystal growth were determined. The crystal quality was estimated by X-ray diffraction method. The composition of crystals was determined by electron microprobe analysis and energy dispersive X-ray fluorescence analysis.
We study experimentally and theoretically excitonic recombination processes in CdTe/ZnTe quantum dots. The single quantum dot photoluminescence spectrum was observed and emission lines from X, X^-, X^+ and 2X excitonic states were identified. Experimental results were analysed in the theoretical model based on the effective mass approximation. Numerical calculations of energy positions and recombination probabilities of X, X^-, X^+ and 2X were performed. Computed results reproduce correctly the order and relative positions of emission lines and ratios of radiative lifetimes.
In this paper we review the results of our effort to grow layers and low-dimensional structures containing Cd_{1-x}Μn_{x}Te diluted magnetic semiconductor with relatively high values of Mn molar fraction x. A high quality of the structures grown so far is demonstrated by making use of results of several selected experiments. In the case of the epilayers having bulk-like thickness with x ≥ 0.7 we discuss, in particular, the magnetic phase diagram as well as we report on collective spin excitation (magnons) observed in Raman scattering experiments. The discussion of the growth of different quantum wells, including rectangular, digital, parabolic and wedge quantum wells, is accompanied by a brief overview of their optical and magnetooptical properties. These results include first measurements concerning magnetic polarons in quantum wells embedded in Cd_{1-x}Mn_{x}Te barriers with 0.4 ≤ x ≤ 0.8. Finally, we report on the present status of the search for dimensional effects in the spin-glass phase performed with the use of our specially designed superlattice structures.
We discuss possible mechanisms of quantum dot population control. A set of experiments, including time-resolved photoluminescence, single photon correlations, excitation correlation, and photoluminescence excitation is used to determine the actual mechanism under non-resonant and quasi-resonant regime.
The interfaces between nonmagnetic CdTe quantum wells and semimagnetic barriers of Cd_{1-x}Mn_{x}Te were investigated for several well widths by low temperature photoluminescence and photoluminescence excitation spectroscopy. Specially designed Cd_{1-x}Mn_{x}Te/CdTe/Cd_{1-y}Mg_{y}Te structures enable us to distinguish the quality of the semimagnetic normal and inverted interfaces. The normal interface shows to have a better structural quality than the inverted interface.
The photoluminescence studies in CdTe/CdMnTe quantum wells are reported in the temperature range 10-300 K. The MnTe concentration in the barriers is x = 0.3, 0.5, 0.63 and 0.68. Thus the potential wells in our samples are very deep, of the order of ≈ 800 meV in the conduction band and ≈ 200 meV in the valence band in the case of the x = 0.68 sample. In spite of the large lattice mismatch (related to high x value) between the wells and the barriers the observed line widths are as narrow as 2 meV in the case of 100 Å. Clear manifestations of internal strain are observed. In particular, the temperature coefficient of the luminescence energies shows strong dependence on the width of wells.
We report on the growth and optical studies of II-VI semiconductor parabolic quantum wells made of Cd_{1-x}Mn_{x}Te for a broad range of quantum well widths and Mn molar fractions x. Photoluminescence excitation spectra revealed several series of peaks equidistant in energy associated with interband optical transitions between harmonic oscillator levels. From the analysis of the spectra the valence band offset Q_{hh} = 0.44±0.1 was determined for the CdTe/Cd_{1-x}Mn_{x}Te system.
Unreconstructed CdTe(100) surface prepared by ion bombardment and annealing is investigated by angle-resolved photoemission. The experimental band structure E(k_{∥}) is determined along high-symmetry lines of the surface Brillouin zone by measuring energy-distribution curves of photoelectrons. Different criteria were applied to separate surface and bulk related spectral features, e.g. calculating the position of bulk-derived emissions in the frame of the free-electron final state approximation assuming k conservation. In this way, most dispersing features could be explained. All remaining features were compared with theoretical surface band structures for different polar surface terminations which were calculated within a layer doubling procedure on the basis of an EHT-fit to the bulk band structure. The investigated CdTe(100)-(1×1) surface could be identified as Cd terminated. Two surface bands were observed, one located above the valence-band edge and the second in the open pocket of the projected bulk band structure along the Γ̅K̅ direction. At 4.6 eV binding energy an additional weakly dispersing band was found, which contains mixed surface and bulk character. The high density of bulk states associated with this edge of the heteropolar gap is also expected to contribute to this feature.
The spin-glass transition in Cd_{1-x}Mn_{x}Te epitaxial layers and bulk samples with 0.24 ≤ x ≤ 0.43 and in quantum well structures on the basis of Cd_{1-x}Mn_{x}Te were investigated by means of optical spectroscopy. Reduction of dimensionality of Cd_{1-x}Mn_{x}Te layers down to the quasi-two-dimensional case realized in Cd_{1-x}Mn_{x}Te/Cd_{1- y}Mn_{y}Te heterostructures frustrates the spin-glass formation, which is in agreement with theoretical predictions. The spin-glass formation is also frustrated in the vicinity of interfaces between semimagnetic and nonmagnetic semiconductors in CdTe/Cd_{1-x}Mn_{x}Te quantum wells.
Reflectivity spectra for mixed crystals Cd_{1-x}Fe_{x}Te with 0.00 ≤ x ≤0.05 were investigated. The measurements were carried out in the energy range 6.0-30.0 eV at 300 K. The influence of Fe^{2+}(3d^{6}) states on the changes in the energy positions of the reflectivity maxima E'_{1} , E'_{1} + ∆, c, d, e, f, h and i is analysed. The obtained results confirm the band structure diagrams published previously.
We present studies of resonant excitation of self-assembled CdTe/ZnTe quantum dots. Photoluminescence excitation measurements revealed existence of sharp resonances, common for photoluminescence lines attributed to different quantum dot charge states. We conclude from the ensemble of photoluminescence and photoluminescence excitation results that we observe energy transfer in coupled quantum dot pairs.
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