Alexander G. Cherkov

Ge Nanoclusters in GeO2: Synthesis and Optical Properties

Ge nanocrystals (NCs) in GeO2 films obtained with the use of two methods were studied. The first method is a film deposition from supersaturated GeO vapor with subsequent dissociation of metastable GeO on heterophase system Ge:GeO2. The second method is growth of anomalous thick native germanium oxide layers with chemical composition GeOx(H2O) during catalytically enhanced Ge oxidation. The obtained films were studied with the use of photoluminescence (PL), Raman scattering spectroscopy, high-resolution electron microscopy (HREM). Strong PL signals were detected in GeO2 films with Ge-NCs at room temperature. “Blue-shift” of PL maximum was observed with reducing of Ge excess in anomalous thick native germanium oxide films. Also a correlation between reducing of the NC sizes (estimated from position of Raman peaks) and PL “blue-shift” was observed. The Ge NCs presence was confirmed by HREM data. The optical gap in Ge-NCs was calculated with taking into account quantum size effects and compared with the position of the experimental PL peaks. It can be concluded that a Ge-NC in GeO2 matrix is a quantum dot of type I.

Synthesis, Crystal Structure, and Liquid Exfoliationof Layered Lanthanide Sulfides KLn 2 CuS 6 (Ln= La, Ce, Pr, Nd, Sm)

Among the great amount of known lanthanide nanoparticles, reports devoted to chalcogenide ones are deficient. The properties of such nanoparticles remain almost unknown due to the lack of simple and proper synthetic methods avoiding hydrolysis and allowing preparation of oxygen-free lanthanide nanoparticles. A liquid exfoliation method was used to select the optimum strategy for the preparation of quaternary lanthanide sulfide nanoparticles. Bulk KLn2CuS6 (Ln = La-Sm) materials were obtained via a reactive flux method. The crystal structures of three new members of the KLn2CuS6 series were determined for Pr, Nd, and Sm as well as for known KLa2CuS6. KLn2CuS6 (Ln = La, Pr, Nd) compounds crystallize in the monoclinic C2 /c space group, whereas KSm2CuS6 crystallizes in the orthorhombic Fddd space group. The analysis of their electronic structures confirms that the main bonding interactions occur within the anionic {Ln2CuS6}- layers. Due to their layered structure, exfoliation of these compounds is possible using ultrasonic treatment in appropriate solvents with the formation of colloidal solutions. Colloidal particles show a plate-like morphology with a lateral size of 100-200 nm and a thickness of 2-10 nm. Highly negative or positive charges found in isopropanol and acetonitrile dispersions, respectively, are associated with high stability and concentration of the dispersions.

Specific features of the ion-beam synthesis of Ge nanocrystals in SiO2 thin films

The systematic features of the formation of Ge nanocrystals in SiO2 thin films implanted with Ge+ ions and then subjected to high-temperature annealing (1130°C) are studied in relation to hydrostatic pressure. It is established that annealing at atmospheric pressure is accompanied by the diffusion of Ge atoms from the implantation region to the Si substrate and does not induce the formation of Ge nanocrystals. An increase in pressure during annealing yields a deceleration in the diffusion of germanium into silicon and is accompanied by the formation of twinned lamellae at the Si/SiO2 interface (at pressures of ~103 bar) or by the nucleation and growth of Ge nanocrystals (at pressures of ~104 bar) in the SiO2 film. The results are discussed on the basis of the concept of a change in the activation volume of the formation and migration of point defects under conditions of compression.

Lorentz's Electron Microscopy of Thin Films with Frustrated Magnetic Structure

The unique electrical and magnetic properties Fe86Mn13C alloy (Hadfields steel) are due to special modulated structure. The alloy is widely used in mechanical engineering. Complex studies of the structure and properties of the alloy both in bulk samples and thin-films are suggested to describe the formation of a deformation martensite structure in Fe86Mn13C alloy in the form of self-assembled clusters. It is shown that combinations of anti-ferromagnetic austenite and ferrimagnetic martensite of deformation create unique electric and magnetic properties of Fe86Mn13C alloy both in bulk and in a thin-film state. We seek to explain the reason for the change of the sign of thermoelectric effect depending on temperature. Our task is to investigate thin Fe86Mn13C films as a possible solution to the problems of spintronics.

The Reason of Sign-Variable Thermoelectric Effect in Fe86Mn13C Alloy

The unique electrical and magnetic properties Fe86Mn13C alloy (Hadfilds steel) are due to special modulated structure. The alloy is widely used in mechanical engineering. Complex studies of the structure and properties of the alloy both in bulk samples and thin-films are suggested to describe the creation of a deformation martensite structure in Fe86Mn13C alloy in the form of self-assembled clusters. It is shown that combinations of anti-ferromagnetic austenite and ferrimagnetic martensite of deformation create unique electric and magnetic properties of Fe86Mn13C alloy both in bulk and in a thin-film state. We seek to explain the reason for the change of the sign of thermoelectric effect depending on temperature. Our task is to investigate thin Fe86Mn13C films as a possible solution to the problems of spintronics.

Surprising phase transformation of a-Si:H films under femtosecond laser impact

Thin (90 nm) a-Si:H films have been crystallized on Corning 7059 glass substrates by 120 fs pulses of Ti:sapphire laser. Initial films were deposited using low-temperature plasma enhanced deposition technique. The pulses with wavelength of 800 nm, pulse energy up to 0.8 mJ, and repetition rate of 1 kHz were employed. The focused to 280 micron laser beam was raster scanned, using x-y sample translation by computer-controlled motors. The structural properties of the films were characterized by the spectroscopy of Raman scattering, excited by the argon laser (line 514,5 nm). The ablation threshold was found to be of about 65 mJ/cm2. When pulse energy density was lower than ~30 mJ/cm2 no structural changes were observed. In optimal regimes the films were found to be fully crystallized with fine grain structure, according to the Raman scattering data. Numerical estimates show the pulse energy density was lower than the Si melting threshold, so non-thermal explosive impacts may play some role. The possibility of the femtosecond laser pulses to crystallize Si films on glass substrates is shown for the first time. The results obtained are of great importance for manufacturing of polysilicon layers on non-refractory substrates for thin film microelectronics.

Silicon-Germanium Heterostructure on Insulator Formed by Ge + Ion Implantation and Hydrogen Transfer

Using bulk silicon can be ended for 32 nm technological nodes. New type of substrates needs for further scaling in CMOS microelectronics. We speculate that these new type of materials will be semiconductor heterostructure on insulator (SHI) compatible with current silicon planar CMOS technology. Joint semiconductor material stack placed on cheap amorphous dioxide is presented. First of all thin film SiGe heterostructure properties is considered. It was obtain using Ge ion implantation in dioxide with followed Ge segregation to the directly bonded interface between silicon and silicon dioxide wafers. The method is also compatible with A3B5 thin film formation. Background Transfer to giga-scale integration with the CMOS transistors channel length, lower than 20 nm and based on silicon, is difficult due to decreasing a carrier mobility, increasing leakage currents, and respective growth of heat-generation caused by the low dimensions. Last few years, a number of the structures, that promote an increase in the carrier mobility in the channel, were proposed. For example, these are stressed siliconon-insulator film and pure germanium instead of silicon. Integration of few nanometer thick germanium or semiconductor compounds with silicon and silicon dioxide promises overcoming the limitation of silicon based CMOS technology in volume CMOS production. Recent publications (1-4) shown, that SiGe quantum well (QW) device structure and its drain current gate voltage characteristics have great advantages relative to the volume silicon. SiGe thin film dual channel (TF DC) MOSFETs may provide even higher parameters, that needs for current technological 45 nm node (2). Other important things are connected with the stresses in such layers. As was published in recent review (4), right tuning of stresses for SiGe TF DC MOSFETs provides five times lower delay time for inverter with symmetrical electron and hole mobilities in comparison with pure silicon MOSFET inverter. Such tuning should be more easer in the case of TF DC SiGe on insulator that in the case of buffer layer with stepped Ge distribution on bulk silicon. As was observed in our earlier work (5) Ge accumulation and thin pure Ge layer formation take place at the Si/SiO2 interface after Ge implantation into thermal dioxide on silicon wafer and followed thermal treatment. We suggest to use hydrogen induced Si layer transfer on Ge implanted and bonded wafer to produce semiconductor heterostructure on insulator (SHI) using ion syntheses and transferred layer as matrix for Ge epitaxy. In the present work silicon-germanium (Si-Ge) heterostructures were produced by epitaxial growth of nanometer thick Ge layer at the bonding interface during annealing of the Ge ion implanted silicon dioxide on Si substrate, directly bonded with hydrogen ion implanted silicon wafer. The idea of Ion Cut And Synthesis (ICAS) is also compatible with A3B5 heterostructure formation.

Raman and HRTEM investigations of Ge nanocrystals produced by Ge+-ion implantation of SiO2 films and subsequent high-pressure annealing

The properties of Ge nanocrystals formed in ion-implanted SiO2 films during annealing under elevated (12 kbar) hydrostatic pressure have been studied. It has been found that high-pressure heat treatment produces the nanocrystals with uniformly strained Ge-Ge bonds. Its average dimensions are larger than these obtained from unstrained nanocrystals produced during annealing at atmospheric pressure. The degree of deformation of the Ge nanocrystals has been determined using the Raman shift in the optical phonon frequency. The energy shift E1, E1 + Δ1 in the Raman resonance corresponds to quantization of the energy of the two-dimensional excitons M1 ground state of the Ge critical point. It has been established that the formation of strained nanocrystals correlates with the appearance of green PL band centered at 520 nm. Comparing the PL data with HRTEM results allows to suggest that green PL originates from stressed Ge nanocrystals of a radius of less than 6 nm.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.