V. V. Ushakov

Ion implantation of porous gallium phosphide

The effect of irradiation by Ar ions and thermal annealing on the properties of porous gallium phosphide (por-GaP) obtained by electrolytic methods is investigated. It is shown on the basis of Raman scattering and photoluminescence data that, in contrast with porous silicon, por-GaP does not have high radiation hardness, and that thermal annealing of defects in layers amorphized by ion implantation is impeded by the absence of a good crystal base for solid-state epitaxial recrystallization processes. Data on radiation-induced defect formation and from probing of the material with a rare-earth “luminescence probe” are consistent with a mesoporous structure of the material.

Microphotoluminescence Spectra of Cadmium Telluride Grown under Nonequilibrium Conditions

Microphotoluminescence spectroscopy and imaging were used to study impurities and defects in CdTe crystals grown by nonequilibrium techniques. The growth procedure includes low-temperature synthesis and purification of the material via congruent sublimation, with subsequent deposition under the conditions of gas-dynamic vapor flow and high-rate low-temperature condensation. Although the growth conditions are highly nonequilibrium, the obtained polycrystalline material with a grain size of 1–2 mm exhibits strong low-temperature exciton luminescence, whose intensity is nearly uniform over the bulk of the ingots. At the same time, it is found that residual impurities and defects have a tendency to accumulate to form clusters within certain areas which are a hundred micrometers in size; the density of some impurities in these clusters is sufficiently high.

Effect of grain boundaries on the properties of cadmium telluride grown under nonequilibrium conditions

Microphotoluminescence spectroscopy and imaging were used to study the effect of grain boundaries on the properties of textured CdTe polycrystals with a single-crystal grain size of 1–2 mm grown under nonequilibrium conditions. The technological procedure included low-temperature synthesis and purification of the material via congruent sublimation, with subsequent deposition under conditions of gas-dynamic vapor flow and high-rate low-temperature condensation. Microluminescence probing revealed that most of the grain boundaries are decorated with local centers emitting in the 1.4-eV band of donor-acceptor recombination involving shallow donors and A-center acceptors. The boundary regions are, to some extent, free of nonradiative recombination centers active at room temperature. Gettering activity of the grain boundaries could be detected at the distances up to 100 µm, which reflects the specific features of the nonequilibrium crystallization conditions of the material under study.

Radiation hardness of porous silicon

The effect of irradiation by 300-keV Ar+ ions on the properties of electrochemically produced porous silicon is studied at doses of 5×1014–1×1016 cm−2. Raman scattering and photoluminescence data are used to show that the radiation hardness of porous silicon layers is substantially greater than that of single crystal silicon.

Microphotoluminescence of undoped cadmium telluride grown by nonequilibrium direct synthesis from the flow of components' vapors

Properties of undoped CdTe obtained by nonequilibrium direct synthesis from the flow of components’ vapors are studied by the method of microphotoluminescent spectral analysis and imaging. In spite of an appreciable increase in the crystallization rate, the high intensity of the edge band with resolution of spectral features within the contour of this band at temperatures ∼ 100 K is indicative of the high quality of the crystalline material with the sizes of single-crystal grains as large as 0.5 mm. The form of luminescence maps of the samples under study resembled that of the crystals grown by quasi-equilibrium methods. However, the mechanism of segregation of impurities at the intergrain boundaries in the crystal structures under study was not related to diffusion; rather, it had another nature.

Y and Z Luminescence of Polycrystalline Cadmium Telluride

The properties of polycrystalline CdTe with a grain size of 5–30 μm have been investigated using the microphotoluminescence methods of spectral analysis and topography. This material has been prepared by direct synthesis in a vapor flow of components at a low temperature. The dominance of the Y and Z bands in the spectra reflects a nonequilibrium character of the crystallization processes. The superlinear dependences of the luminescence intensity on the level of the band-to-band excitation indicate the exciton nature of the corresponding transitions. The activation energies for temperature quenching of luminescence in the temperature range T = 100–150 K are found to be 120 meV for the Y luminescence and 180 meV for the Z luminescence, which correspond to the dissociation of excitons bound to defects with the transition of charge carriers to the conduction and valence bands. The monochromatic topography data indicate that Y and Z defects have different material bases.

Y and Z luminescence of polycrystalline cadmium telluride grown by a nonequilibrium reaction of the direct synthesis of components

Microphotoluminescence methods of spectral analysis and topography are used to study the properties of polycrystalline CdTe with grain size 10–20 μm; the polycrystals were grown by a nonequilibrium reaction of direct synthesis in the vapor flow of components at a relatively low temperature. The presence of the Y and Z bands in the luminescence spectra was indicative of clearly nonequilibrium types of crystallization processes. Superlinear dependences of the intensities of these bands on the level of the band-to-band excitation, unusually weak electron-phonon interaction, and also data of monochromatic microluminescence topography correspond to the model of radiation of excitons bound by delocalized potential at various extended defects (elements of dislocations, lamellas twins, and packing faults) generated in the studied samples by the concentration and thermal fluctuations at growth surface as a result of dynamic instabilities in the vapor phase in the crystallization zone.

Radiative d–d transitions at tungsten centers in II–VI semiconductors

The luminescence spectra of W impurity centers in II–VI semiconductors, specifically, ZnSe, CdS, and CdSe, are studied. It is found that, if the electron system of 5d (W) centers is considered instead of the electron system of 3d (Cr) centers, the spectral characteristics of the impurity radiation are substantially changed. The electron transitions are identified in accordance with Tanabe–Sugano diagrams of crystal field theory. With consideration for the specific features of the spectra, it is established that, in the crystals under study, radiative transitions at 5d W centers occur between levels with different spins in the region of a weak crystal field.

Ion implantation of erbium into polycrystalline cadmium telluride

The specific features of the ion implantation of polycrystalline cadmium telluride with grains 20–1000 μm in dimensions are studied. The choice of erbium is motivated by the possibility of using rare-earth elements as luminescent “probes” in studies of the defect and impurity composition of materials and modification of the composition by various technological treatments. From the microphotoluminescence data, it is found that, with decreasing crystal-grain dimensions, the degree of radiation stability of the material is increased. Microphotoluminescence topography of the samples shows the efficiency of the rare-earth probe in detecting regions with higher impurity and defect concentrations, including regions of intergrain boundaries.

Microphotoluminescence of undoped single-crystal zinc telluride produced by nonequilibrium vapor-phase growth techniques

The properties of undoped bulk ZnTe crystals grown under nonequilibrium crystallization conditions by chemical synthesis from a vapor phase have been examined using microphotoluminescence spectral analysis and imaging. In spite of a considerable increase in the crystallization rate, the samples under study compare well, in terms of the concentration of different types of residual defects, with high-quality single crystals grown from a vapor phase under quasi-equilibrium conditions at much higher temperatures. At the same time, the absence of a luminescence contrast at the grain boundaries and the inefficiency of thermal annealing indicate that the main nonradiative growth defects in the materials obtained are low-mobility thermally stable complexes formed due to the association of stoichiometric defects and, possibly, background impurities.