M. D. Efremov

Effect of quantum confinement on optical properties of Ge nanocrystals in GeO2 films

Germanium dioxide films containing Ge nanocrystals are studied. The films have been prepared by two methods: (i) deposition from supersaturated GeO vapors with subsequent decomposition of metastable germanium monoxide into a heterophase Ge:GeO2 system, and (ii) formation of anomalously thick native germanium oxides with a GeO2(H2O) chemical composition by a catalyst-accelerated oxidation of germanium. The films, which have been prepared on various substrates, are studied using the photoluminescence technique, Raman spectroscopy, spectral ellipsometry, and high-resolution electron microscopy. In the GeO2 films with built-in Ge nanoclusters, intense photoluminescence is detected at room temperature. The nanocluster sizes are estimated from the position of the Raman peak related to localized optical phonons. The correlation between a decrease in the nanocluster size and the shift of the photoluminescence peaks to the blue spectral region as the relative Ge content decreases is revealed. The presence of nanoclusters is confirmed by the data obtained from high-resolution electron microscopy. The correlation of the optical gap calculated taking into account the quantum confinement of electrons and holes in the nanoclusters with the experimentally observed luminescence peak is established. It can be concluded from the data obtained that the Ge nanoclusters constructed in the GeO2 matrix represent type I quantum dots.

Determination of the composition and stresses in GexSi(1−x) heterostructures from Raman spectroscopy data: Refinement of model parameters

Raman spectroscopy is used to monitor the composition and strains of GexSi1−x alloy films with 0.17 ≤ x ≤ 1.0. The composition and strains in the films were also determined from the X-ray diffraction data. Both the position and intensity of the Raman peaks related to vibrations of the Ge-Ge, Ge-Si, and Si-Si bonds were analyzed. This analysis provided substantial refinement of certain model parameters for calculation of the composition and strains in GexSi1−x/Si(100) heterostructures on the basis of Raman spectroscopy data.

Electrical properties and photoluminescence of SiOx layers with Si nanocrystals in relation to the SiOx composition

The photoluminescence and electrical properties are compared for silicon-oxide layers containing Si nanocrystals and having different Si content. The oxide was deposited by co-sputtering of silicon dioxide and silicon with the subsequent annealing for the formation of nanocrystals. Excess Si content in the layer varies along the sample from 6 to 74 vol %. It is found that a charge magnitude determined from the flat-band voltage has a pronounced peak for the excess Si content of about 26%, the largest charge correlating with the highest photoluminescence intensity. The further increase in the excess Si content in oxide leads to a decrease in both the oxide charge and the photoluminescence intensity and to the appearance of percolation conductivity.

Modification of quantum dots in Ge/Si nanostructures by pulsed laser irradiation

The goal of this study was the development of a method for the modification of a quantum dot (QD) structure in Ge/Si nanostructures by pulsed laser irradiation. The GexSi1−x QD structures were analyzed using data furnished by Raman spectroscopy. Frequency-dependent admittance measurements were used to study the energy spectrum of holes in the Ge/Si heterostructures with GexSi1−x QDs before and after the laser treatment. The obtained experimental data show that laser treatment makes it possible to reduce the sheet density of QDs, modify their composition, and increase the average size. The most important result is that the QD parameters become more uniform after the treatment with nanosecond laser pulses. In a sample with ODs of 8-nm average lateral size (six monolayers of Ge), the scatter of energy levels in the QD array is reduced by half after the treatment with 10 laser pulses.

Photoluminescence of GeO2 films containing germanium nanocrystals

Germanium nanocrystals were formed in a GeO2 film during the process of germanium monoxide gas-phase deposition onto a sapphire substrate and studied by photoluminescence (PL) and Raman scattering spectroscopy. A PL peak in this heterosystem was observed in the visible region at room temperature. The sizes of Ge nanocrystals were estimated from the position of a Raman peak corresponding to scattering by localized optical phonons in germanium. The PL peak position calculated with allowance for the electron and hole size quantization in Ge nanocrystals coincides well with the experimentally observed position of this peak.

Visible photoluminescence from silicon nanopowders produced by silicon evaporation in a high-power electron beam

Silicon nanopowders produced by electron-beam-induced evaporation of a bulk silicon sample in an argon atmosphere are studied by the photoluminescence technique and Raman scattering spectroscopy. A photoluminescence peak in the visible region of the spectrum has been detected at room temperature in powders consisting of silicon nanocrystals. The strong short-wavelength shift of the photoluminescence peak can be attributed to the quantum size effect of electrons and holes in small silicon nanocrystals (about 2 nm). The size of silicon nanocrystals is determined by analyzing Raman spectra, and it is consistent with estimates obtained from photoluminescence data.

Ge/Si waveguide photodiodes with built-in layers of Ge quantum dots for fiber-optic communication lines

The results of research aimed at the development of high-efficiency Ge/Si-based photodetectors for fiber-optic communication applications are reported. The photodetectors are designed as vertical p-i-n diodes on silicon-on-insulator substrates in combination with waveguide lateral geometry and contain Ge quantum-dot layers. The layer density of quantum dots is 1×1012 cm−2; the dot size in the plane of growth is ∼8 nm. Unprecedentedly high quantum efficiency suitable for the range of telecommunication wavelengths is attained; specifically, in the waveguides illuminated from the end side, the efficiency was as high as 21 and 16% at 1.3 and 1.55 µm, respectively.

Peculiarities of immunocorrective effects of the bone marrow regulatory peptides (myelopeptides)

Myelopeptides (MPs) are low-molecular-weight immunoregulatory peptides of bone marrow origin. The peculiarities of their immunoregulatory effects are demonstrated with two of the six synthesized MPs, MP-1 (Phe-Leu-Gly-Phe-Pro-Thr) and MP-2 (Leu-Val-Val-Tyr-Pro-Trp). It is shown that MP action is directed to the damaged links of immunity. MP-1 enhances a decreased level of antibody production in cyclophosphamide (Cy)-treated mice, but does not influence the antibody formation in normal animals. MP-2 inhibits the tumor growth more in a tumor-bearing organism as the tumor size gets larger, insofar as MP-2 antitumor effect is concerned, by its ability to recover functional activity of T lymphocytes suppressed by tumor products. Selective immunocorrective effects of MPs are based on ligand-receptor interactions. Using FITC-labeled MP-1 and [3H]-labeled MP-2, specific binding of these peptides with appropriate cell populations is shown. The cytofluorimetric analysis revealed a target cell for MP-1--CD4+ T lymphocyte (T helper). The data obtained suggest that MPs are endogenic immunoregulators which participate in the maintenance of immune homeostasis.

High Volume Synthesis of Silicon Nanopowder by Electron Beam Ablation of Silicon Ingot at Atmospheric Pressure

The evaporation of high purity silicon ingot was performed in Ar, N2, and air atmospheres using a power electron accelerator. The obtained powders with primary particle sizes of 10–500 nm were investigated using Brunauer–Emmett–Teller analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence measurement, and Raman spectroscopy. The structure and photoluminescence properties of Si(Ar) nanopowder obtained at a large quenching rate differ substantially from those of Si(Ar) and Si(N2) obtained at a smaller quenching rate. Photoluminescence peaks in the visible region of the spectrum are detected at room temperature for the Si(Ar) nanopowders.

Determination of the composition and strains in GexSi1−x -based nanostructures from Raman spectroscopy data with consideration of the contribution of the heterointerface

The procedure of determining the composition and mechanical strains in GexSi1−x quantum dots with the use of Raman spectroscopy is substantially refined. The parameter x characterizing the composition is determined by analyzing the intensity of the peaks in the Raman spectra that correspond to the Ge-Ge and Ge-Si bond vibrations, taking into account the Ge-Si bonds at the heterointerface. The strains in the quantum dots are determined by analyzing the position of the Raman peaks, taking into account the data obtained for the composition of the quantum dots.