Instability of the photoluminescence of porous silicon

Results of studies of the photoluminescence of porous silicon with different prehistories have revealed the mechanism and nature of the instability of the luminescence properties of freshly prepared samples. It was established that the initial quenching and subsequent rise of the photoluminescence is attributable to the intermediate formation of silicon monoxide (photoluminescence degradation) and subsequent additional oxidation to form SiO₂ (photoluminescence rise). Ultraviolet laser irradiation accelerates this process by a factor of 200–250 compared with passive storage of the samples in air. Plasma-chemical treatment in an oxygen environment merely results in a subsequent rise in the photoluminescence as a result of the formation of monoxide on the porous silicon surface. A kinetic model is proposed for this process. Zh. Tekh. Fiz. 69, 135–137 (June 1999)     

Spectroscopy of GaAs/AlGaAs microstructures with submicron spatial resolution using a near-field scanning optical microscope

A near-field scanning optical microscope (NSOM), which we built, is used to investigate 1–5-μm wide stripes with a 10-nm thick layer—a quantum well — on a GaAs surface. A map of the photoluminescence intensity is obtained synchronously with the topographic profile of the structures. The measured spatial distribution of the photoluminescence intensity is described satisfactorily in a model that takes into account carrier diffusion in the layer and the existence of a region with a short carrier lifetime near the side boundaries of the layer. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 523–527 (10 April 1996)     

Distinctive features of time-resolved photoluminescence of porous silicon coated with a layer of diamondlike carbon

Time-resolved photoluminescence from porous silicon coated with a diamondlike carbon film is investigated. The intensity of the photoluminescence from the carbon film is obserd to increase after deposition, and there is an accompanying change in the intensity and a shortwavelength shift of the photoluminescence band of porous silicon that depends on the porosity of its original layers. These changes are explained by the formation of carbon nanoclusters on the surface of the silicon filaments. Zh. Tekh. Fiz. 68, 83–87 (April 1998)     

Influence of physical and chemical surface treatment on the photoluminescence of porous silicon

It is shown that surface treatment of porous silicon in inorganic acids and solutions of metal chlorides leads to an increase in the intensity of photoluminescence of this material. In the case of chlorides, a short-wavelength shift of the photoluminescence maximum is also observed. The effect of a brief high-temperature anneal in vacuum on the photoluminescence of porous silicon is investigated. Such treatment is observed to cause partial degradation. Zh. Tekh. Fiz. 69, 133–134 (January 1999)     

Observation of two-step excitation of photoluminescence in silicon nanostructures

Efficient visible-range photoluminescence with photon energy higher than the photon energy of the exciting radiation is observed in nanostructures of porous silicon subjected to heat treatment in vacuum. The photoluminescence intensity is found to be virtually identical for cw and femtosecond excitation by Ti:sapphire laser radiation with the same average power. The results can be explained by a two-step cascade photoluminescence excitation process in which optical passivation of defects of the dangling silicon bond type occurs. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 732–736 (25 November 1998)     

Anomalous evolution of photoluminescence in porous silicon in an electric field

The change produced in the photoluminescence (PL) of n-type porous silicon (por-Si) by irradiation with ultraviolet laser radiation in the presence of an external electric field has been investigated. A field effect, consisting of a large change in the integrated PL intensity when the field is switched on or off, was observed. When the field intensity E exceeds a critical value, the change in the PL becomes anomalous — an alternating sawtooth signal. A kinetic model is proposed wherein the experimental results are explained by a change in the density of neutral acceptor and hole surface states formed by hydrogen, oxygen, and fluorine atoms adsorbed on the surfaces of pores. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 357–361 (10 September 1997)     

Self-organization of germanium nanoislands obtained in silicon by molecular-beam epitaxy

Nanometer germanium islands in epitaxial layers of silicon are obtained by molecular-beam epitaxy. The dimensions and shapes of the islands are determined in an atomic-force microscope. The photoluminescence spectra are found to contain lines that can be interpreted as quasidirect optical transitions in the islands. It is concluded on the basis of optical and microprobe measurements and theoretical calculations of the energies of electronic states that silicon is dissolved in the germanium islands. Values of the germanium and silicon contents in the solid solution are presented. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 46–50 (10 January 1998)     

Influence of successive electron and laser irradiation on the photoluminescence of porous silicon

The influence of electron irradiation on the light-emitting properties of p-and n-type porous silicon prepared by electrochemical etching is investigated. The dose and energy dependences of the electron-stimulated quenching of the photoluminescence (PL) are determined. It is shown that electron treatment of a porous silicon surface followed by prolonged storage in air can be used to stabilize the PL. The excitation of photoluminescence by a UV laser acting on sections of porous silicon samples subjected to preliminary electron treatment is discovered for the first time. The influence of the electron energy and the power of the laser beam on this process is investigated. The results presented are attributed to variation in the number of radiative recombination centers as a result of the dissociation and restoration of hydrogen-containing groups on the pore surface. Zh. Tekh. Fiz. 68, 58–63 (March 1998)     

Spectroscopy of electroreflection, the electron band structure, and the mechanism of visible photoluminescence of anisotropically etched silicon

We present the results of studies of electroreflection in the 1.1–4.4 eV spectral range, of electron Auger spectroscopy, and of electron diffraction involving the photoluminescent Si-SiO₂ system prepared via anisotropic chemical etching of the Si(100) surface. These results are explained on the basis of a four-layer model of the band structure and energy transition diagram for a system with a quantum well at the silicon surface, surface electron states at the boundary, and a gradient of the band potential in the transition layer. We find that light-emitting silicon remains an indirect-gap semiconductor and that the visible photoluminescence is due to direct recombinations of the light-excited electrons and holes in the quantum well at the center of the Brillouin zone with the participation of the band of deep localized states, which is due to the presence of oxygen at the silicon surface. Zh. éksp. Teor. Fiz. 116, 1750–1761 (November 1999)     

Enhanced photoluminescence in grooved silicon microstructures

Grooved silicon structures formed by anisotropic chemical etching of crystalline silicon (c-Si) wafers in alkaline solution and composed by c-Si walls and voids (grooves) with thicknesses of several micrometers were found to exhibit efficient photoluminescence after excitation with laser radiation at 1.06 μm. The photoluminescence emission which originates from the interband radiative recombination of charge carriers in c-Si walls was represented by a broad spectral band centered at 1.1 eV. Independently on the polarization direction of the excitation light the photoluminescence of grooved silicon structures was partially linear-polarized with the polarization degree of 0.15–0.24 along c-Si walls and the photoluminescence intensity was strongly enhanced in comparison with that of c-Si substrate. These experimental observations are explained by considering an enhancement of the photoluminescence excitation due to both partial light localization in c-Si walls and a low rate of the non-radiative recombination at surface defects on c-Si walls. The defect density could be modified by additional chemical treatment or thermal annealing, which resulted in significant changes of the photoluminescence intensity of the grooved Si structures. The obtained results are discussed in view of possible applications of grooved Si in optoelectronics and molecular sensorics.     

Magnetic phase diagrams of ferrimagnets having two magnetically unstable subsystems

The molecular field approximation is used to construct magnetic phase diagrams along with magnetization curves for ferrimagnets having two magnetically unstable subsystems. Diverse sequences of phase transitions of various kinds—first order (metamagnetic) and second order (via a noncollinear phase)—are realized in the course of magnetizing such ferrimagnets. Analysis reveals how the magnetization curves depend on the relative values of parameters of the two subsystems, e.g., sublattice magnetizations in weak and strong states, critical fields, and values of the exchange interaction between sublattices. Fiz. Tverd. Tela (St. Petersburg) 40, 280–284 (February 1998)     

Possibility of the appearance of spin autowaves in a model of a ferromagnet with nonuniform dissipation

A model of a ferromagnet with nonuniform dissipation is introduced for the Landau-Lifshitz equations. It is shown that in this model a ferromagnet can be regarded as an oscillating active medium where the formation of autowave structures — spin autowaves, pacemakers, and spiral waves — is possible. Their wave characteristics, expressed in terms of the parameters of the medium, are found for a special case. Fiz. Tverd. Tela (St. Petersburg) 39, 513–515 (March 1997)     

Luminescence of porous silicon films with an europium-containing complex

We obtained porous silicon films modified at room temperature by an Eu³⁺-containing polymer complex. The most intense photoluminescence of Eu³⁺ implanted in the porous silicon was observed at the wavelengths of 611, 618, 691, and 704 nm. In this case, the intensity of the intrinsic photoluminescence of strongly irradiated specimens of porous silicon decreased, while the intensity of weakly emitting films multiply increased. An investigation of the photoexcitation spectra made it possible to establish the effect of Eu³⁺-containing complexes on the mechanism underlying the excitation of photoluminescence of porous silicon. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 499–501, July–August, 1997.     

Structural transformations of the cumulene form of amorphous carbyne at high pressure

Structural transformations of the cumulene form of amorphous carbyne which are induced by heating at high pressure (7.7 GPa) are investigated. These can be described by the sequence amorphous phase — crystal — amorphous phase — disordered graphite. Raman scattering shows that predominately the chain structure of carbyne remains at the first three stages. It was found that the intermediate crystalline phase is an unknown modification of carbon whose structure is identified as cubic (a=3.145 Å). A mechanism of structural transformations in carbyne that involves the formation of new covalent bonds between chains is discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 237–242 (25 August 1997)     

Luminescent properties of semiconductor composite systems composed of erbium triphthalocyanine molecules and a silicon slot structure in the near-infrared region

Photoluminescence spectra of organic semiconductors based on mono-, bis-, and triphthalocyanine containing erbium as a complexing agent have been obtained in the range of 1–1.8 μm. Comparison of the spectral characteristics has shown that erbium triphthalocyanine has the highest photoluminescence quantum yield at a wavelength of 1.5 μm. To enhance this effect, composite materials based on erbium triphthalocyanine and a silicon slot structure have been synthesized, in which an additional increase in the photoluminescence signal near 1.14 μm has been observed. At the same time, no photoluminescence signal has been observed near the wavelength of 1.5 μm. This can be explained by taking into account the interaction of the erbium triphthalocyanine molecules with the adsorption centers of the silicon matrix.     

Luminescence of amorphous silicon nanoclusters

Films of amorphous silicon suboxide α-SiO _x containing amorphous silicon nanoclusters have been grown by direct current magnetron sputtering. It has been found that two radiation bands are observed in the photoluminescence spectra of relatively large amorphous nanoclusters (a size of ∼2 nm) unlike one photoluminescence band of silicon nanocrystals of the same size. The form of the spectra upon the change in the nanocluster sizes agrees with that predicted in theoretical works, in which the energy spectrum of amorphous nanoclusters has been calculated taking into account quantum confinement of delocalized, weakly localized, and strongly localized states.     

Dynamics of localized excitons in a superlattice grown by molecular-beam epitaxy with submonolayers of CdSe

The low-temperature photoluminescence spectrum and the recombination dynamics of localized excitons have been studied in a short-period superlattice of CdSe/ZnSe submonolayers. As distinct from structures with isolated submonolayers, which exhibit one narrow photoluminescence peak, the photoluminescence and photoluminescence excitation spectra of a superlattice have two peaks, separated by ∼50 meV. The amount of splitting, as well as the temporal characteristics of the damping of the photoluminescence, are interpreted in terms of a model of a disordered superlattice of extended islands, sited randomly in the submonolayers making up the superlattice. Fiz. Tverd. Tela (St. Petersburg) 40, 837–838 (May 1998)     

Molecular layering of 2D films and superlattices based on II–VI compounds

The method of molecular layering is used to prepare CdS thin films and CdS/ZnS and CdS/CdSe superlattices. The dependence of the exciton photoluminescence on film thickness is studied, and the role of internal strains is examined. The effect of the excitation intensity on the superlattice photoluminescence spectra is examined, manifested in a shift of the emission maximum toward shorter wavelengths when this intensity is increased. Fiz. Tverd. Tela (St. Petersburg) 40, 820–821 (May 1998)     

Observation of a single laser-excited center at the point of a crystalline needle

The points of lithium fluoride needles are investigated by laser photoelectronic projection microscopy. A situation in which a single atomic-size source of electrons — an F ₂ color center — is observed in the region near the point is realized. As a result of the good fluorescence properties of these centers, these needles can be used as the active element of a scanning fluorescence microscope employing resonance transfer of electronic excitation energy. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 441–444 (25 March 1997)     

Nonelectrical method of pumping solid-state lasers

An alternative, nonelectrical method for obtaining a dense radiating plasma and the possibilities of using this method to pump solid-state lasers are investigated. The plasma was obtained experimentally by heating the working gas in a two-stage ballistic plasmatron. A new device — a vortex chamber — is proposed for transferring energy into the plasmatron-laser system. Zh. Tekh. Fiz. 68, 67–70 (September 1998)