Oleg Korotchenkov

Doped ZnS:Mn nanoparticles obtained by sonochemical synthesis.

A study of sonochemically synthesized ZnS:Mn nanoparticles is presented. The particles prepared at low rf power (about 20 W) and room temperature coalesce to form morphologically amorphous large species (30-100 nm in diameter). As the power is increased in the range from 20 to 70 W, and the solution temperature is raised to 60 to 80 degrees C, finer particles are produced with the size ranging from 2 to 20 nm and improved crystallinity. The results indicate the dispersion of the Mn(2+) ions at near-surface sites in the particles. It is shown that the sonochemically fabricated particles approach the quality of the ones obtained by a standard chemical route and show a reasonable luminescence performance.

Photovoltage transients at fullerene-metal interfaces

Photovoltage (PV) transients are studied in C60–Pb and C60–Au thin films. The morphology of the C60 layers is characterized by x-ray diffraction and atomic force microscopy, which evidence the formation of a nanocrystalline C60 layer on polycrystalline Pb and Au underlayers. In contrast to Au substrate, Pb crystallites with a (111) texture are predominantly formed. The signs of the PV signals developed at the C60–Pb and C60–Au interfaces are found to be opposite due to very different workfunction values of the two metals. The evolution of the PV rise and decay curves with increasing light illumination intensity is completely different at the C60–Pb and C60–Au interfaces. The rise for the C60–Pb interface speeds up considerably with the increase in intensity, which is markedly different from the behavior at C60–Au, which exhibits nearly unchanged curve shapes. The PV decay time for C60–Au is also only weakly affected by varying light intensity. In contrast, increasing the illumination intensity causes the ...

Sonoluminescence and acoustically driven optical phenomena in solids and solid–gas interfaces

Abstract During the last decade, significant progress has been achieved in our understanding of the generation of light in acoustic fields, a research area which is known as sonoluminescence (SL). Some of the data obtained, including SL effects in water, have previously been reviewed in the literature. This article takes a broader view and reports on experimental evidence of SL phenomena in solids and solid–gas interfaces as well as on interpretations and potential applications. It is shown that the understanding of SL is facilitated when couched in the language of moving dislocations which produce vacancy–interstitial pairs of host atoms. Radiative transitions in defect pairs would then constitute the SL effect in solids. It is further shown that the occurrence of electric fields due to the generated point defects and charged dislocations produces a number of interesting phenomena. These fields are particularly important for the occurrence of SL at solid–gas interfaces which has been suggested to be initiated by gas discharges due to strong electric fields of charged dislocations. The appearance of acoustically driven internal electric fields is shown to lead to remarkable effects with regard to exciton lifetimes. The storage of photogenerated electron–hole pairs in the moving piezoelectric potential of acoustic waves allows prolonged exciton recombination times of μs in InGaAs/GaAs quantum well structures. The intertwining of acoustically driven long-range electric fields and microfields occurring at the exciton sites turns out to be a prerequisite for attaining the lifetime tuning of the bound excitons in CdS crystals. The review is concluded by discussing sonoluminescence effects in granular systems. Implications for the relevance of this effect to the dynamical behavior of granular media are outlined.

Radiation defects manipulation by ultrasound in ionic crystals

The radiation-induced optical absorption in ionic crystals is remarkably removed by a room-temperature ultrasonic treatment of the crystals. It is shown that the effect can be explained by defect migration processes occurring in ultrasonic fields. The experimental results display features consistent with a recent theory of the quantum diffusion. This new method of a cold annealing of radiation defects in solids can be termed as ultrasonic defect manipulation

Ultrasonically Recovered Performance of

The MHz-frequency ultrasound treatment is shown to offer a recovery tool in the current-voltage (I-V ) characteristics of the γ-irradiated metal-silicon structures. Experimental observations of the ultrasound treatment effect on the carrier transport and photocurrent transient parameters are highlighted. It is shown that up to 30% of the Schottky diode currents and free carrier lifetimes worsen by the irradiation could be recovered in the stress field of ultrasound. The likely scenario behind the treatment effect is outlined, implying the involvement of the vacancies released from the E-centers and subsequently trapped at the Si- SiO2 interface. It is shown that the technique enables near-room temperature modification of electronic properties of metal-semiconductor Schottky diodes and metal-oxide-semiconductor devices. The density of the electrically active bulk and interface traps can be controllably tuned using this technique. Potential relevance of the processing approach in the context of applications in electronics and detector technologies is pointed out.

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We present analytical equations based on Green’s function formalism, which are used to calculate elastic fields produced by a quantum dot buried in a semi-infinite matrix with a rigid surface. The dot of an arbitrary shape as well as quantum dot arrays can be very efficiently carried out within the presented technique. These equations can be exploited for matrices with weak electromechanical coupling, making the computations much faster than other approaches based on Green’s functions.

-Irradiated Metal-Silicon Structures

An enhanced photovoltage is reported to occur in Ge/Si structures with a SiOx layer having a thickness of 0.5-2 nm and placed between a Si substrate and Ge nanoislands. The effect is interpreted in terms of an increased separation distance for photoexcited electrons and holes occurring in the stress fields generated in the oxidized Ge/SiOx/Si structure. The electron-hole separation is modeled utilizing finite-element method techniques, and a good agreement between the experimentally observed enhancement and the computationally increased inter-charge distance is obtained. It is also found that insertion of the oxide layer accelerates the photovoltage decay. This result is interpreted in terms of competing processes, involving the direct recombination of the separated electrons and holes and multi-trapping behavior typical of disordered systems caused by Ge islands.

Elastic fields of quantum dots in semi-infinite matrices: Green’s function analytical analysis

We report on the time decays of surface photovoltage (SPV) and SPV spectra for Si nanocrystals (nc-Si) embedded into a SiO2 matrix. After precipitation at 1150 °C anneal in Ar the SPV increases by a factor of ≈30 compared with the value observed in an oxidized Si substrate. An increase in the signal is accompanied by longer time decays in the SPV transients (roughly from tens to hundreds of microseconds). The separation of photoexcited electrons and holes at the nc-Si/SiO2 interface is expected to play a major role in increasing the SPV signal. We emphasize that annealing of nc-Si at 450 °C in either N2 + O2 or H2 results in a remarkable increase (up to 10-fold) in photoluminescence intensity, which is accompanied by a concomitant decrease in the SPV signal and modification of the SPV decay transients. Anneal in N2 + O2 ambient slightly accelerates the SPV decay, whereas anneal in H2 dramatically speeds it up. Employment of Fourier transform infrared absorption and x-ray photoelectron spectroscopy techniq...

Photovoltage Performance of Ge/Si Nanostructures Grown on Intermediate Ultrathin SiOX Layers

We report on a novel method of charge particle transport. It is based on the application of a moving electrostatic potential and an oscillating friction force to the particle which occur due to acoustic waves travelling in a piezoelectric medium. ZnS grains placed onto the surface of a Y-cut, Z-propagating LiNbO3 plate experience a sequence of jumps in the forward and in the backward directions with respect to the phase velocity of the plate wave. The jump probability appears to change with the Lamb mode supported by the plate and the sign of the grain charge. We present computed spatial profiles of the piezoelectric potential and the displacement components on the surface of the plate for the two lowest resonant modes of Lamb waves. The occurrence of the grain motions is then explained by the presence of the electrical and friction forces, and good qualitative correspondence of the theory and experiment is found.

Effects of low temperature anneals on the photovoltage in Si nanocrystals

The purpose of this paper is to present new acousto-optical effects in semiconductors. Here we discuss acoustically driven carrier diffusion length and photovoltaic effect in Si and SiGe heterostructures, acoustically driven radiative recombination kinetics in ZnSe/ZnS quantum wells and acousto-photo-reflectance from GaAs epitaxial layers. We argue that acoustic driving can significantly affect the carrier dynamics in bulk and low-dimensional semiconductors.