Role of surface segregation in formation of abrupt interfaces in Si/Si1−x Gex heterocompositions grown by molecular-beam epitaxy with combined sources

The coefficients of segregation of germanium atoms were measured for the Si_1?x Ge_x system grown by molecular-beam epitaxy with combined Si-GeH₄ sources under the conditions of efficient filling of surface bonds by the products of the decomposition of hydrides. In their turn, these measurements made it possible to determine for the first time the ratio between the coefficients of the incorporation of Si and Ge atoms into the growing Si_1?x Ge_x layer using the developed kinetic model of growth. For the Si-Si_1?x Ge_x structures grown by molecular-beam epitaxy with the Si-GeH₄ combined sources, the role of various mechanisms (pyrolysis, segregation, etc.) in the formation of the profile of metallurgical layer interfaces was compared for a wide range of technological parameters.     

Analysis of acoustic radiation of circular piezoelectric plates enclosed in a flat rigid shield

Acoustic radiation of circular piezoelectric plates encased in a flat rigid shield is analyzed on the basis of a combined numerical-analytical method. It is shown that a plate made of piezoelectric ceramics of the second type with a higher dynamic coefficient of electromechanical coupling ensures a higher sensitivity in the transmission mode and a wider band of radiated frequencies. In addition, a plate of this type of piezoceramics has a lower level of side lobes of the directivity characteristic because of the more uniform normal-displacement distribution on its flat surface.     

On some paradoxes in the analysis of pulsed operation mode of acoustic transducers by the method of integral transformations

Reasons for the appearance of “precursors” and “tails” in the analysis of pulsed operation mode of acoustic transducers by the method of integral transformations are considered. It is emphasized that the reason is the impossibility to make an accurate allowance for the actual dependence of the complex wave number on frequency over the entire frequency range.     

Analysis of acoustic radiation of circular piezoelectric plates with complex shapes in a flat rigid shield

Acoustic radiation from circular piezoelectric plates with complex shapes in a flat rigid shield is analyzed on the basis of a combined numerical-analytical method. It is shown that such piezoelectric plates of the first-type ceramics ((??????C-3) provide a higher sensitivity in the transmission mode in comparison to a standard plate. In addition, piezoelectric plates of this type have a lower level of the side lobes of the directivity characteristic because of a more uniform distribution of the normal displacement on a flat radiating surface.     

Characteristic Vibrational Modes of Piezoceramic Plates with Variable Thicknesses

The paper considers characteristic oscillations of finite round piezoceramic plates of variable thicknesses, with convex and concave surfaces. Characteristic oscillation spectra of these plates were studied in the regimes of resonance and antiresonance, and the dynamic coefficient of electro-mechanical coupling was calculated as a function of plate dimensions and displacement distribution over the surface. Improvements in the characteristics of thickness oscillations in the plates (the possibility of increasing the coefficient of electro-mechanical coupling and smoothing the distribution of the normal displacement component over the plate surface) owing to variable thicknesses of piezoceramic plates have been analyzed. This can be achieved only in plates with convex conical surfaces, whereas concave conical surfaces of plates lead to inferior characteristics of thickness oscillations. Improvements in characteristics of thickness oscillations in piezoelectric plates fabricated from ceramics with various compositions will be discussed.     

Analysis of Natural Vibrations of Round Bimorph Piezoelectric Ceramic Plates of Arbitrary Dimensions. I. Simply Supported Piezoelectric Plate

The problem of natural vibrations of a round bimorph simply supported piezoelectric ceramic plate (piezoplate) of an arbitrary thickness with arbitrary axisymmetric electrodes is solved in the exact formulation using the method of finite elements. The natural frequency spectra are analyzed in the resonance and antiresonance regimes. The displacement distribution over the piezoplate surfaces (vibration modes) is also analyzed and the dynamic electromechanical coupling coefficient (DCC) is investigated as a function of the relative plate thickness for various piezoelectric ceramic compositions. Round bimorph piezoplates with partial electrodes have been also considered. The results obtained make it possible to estimate the limits of applicability of an approximate one-dimensional theory and choose the optimum geometric dimensions of the plate and electrodes that ensure the maximum DCC.     

Features of the Initial Stage of the Heteroepitaxy of Silicon Layers on Germanium When Grown from Silicon Hydrides

Semiconductors - Data on the dependence of the growth rate of Si layers deposited onto Ge(111) by the hydride method on their thickness at the initial heteroepitaxy stage are reported. The effect...     

Features of the two-component decomposition of monosilane molecules on a silicon surface under epitaxial-process conditions

Using the results of technological experiments based on a two-component kinetic model (SiH₄ → SiH₃ + SiH), the range of characteristic decomposition frequencies of silicon-hydride molecule radicals adsorbed by the layer growth surface in the temperature range of 450–700°C is determined; the degree of silicon surface coverage by SiH radicals under epitaxial-growth conditions is estimated. The behavior of the temperature dependences of the factor of surface filling by individual molecule fragments and the rate of their decomposition on the silicon surface are determined under various conditions corresponding to a constant concentration ratio of monosilane radicals (SiH = gSiH₃) or constant rates of their decomposition \(\left( {v_{SiH_3 } = \xi v_{SiH} } \right)\) in the entire temperature range. It is shown that the observed shape of the temperature dependence of the molecule decomposition rate on the growth surface is not described by simple activation-type curves, which is associated with features of the interaction of the molecular hydride beam with the Si surface under conditions of low and high levels of surface-bond saturation with hydrogen. The effect of the mechanism of the adsorption of hydrogen atoms and various conditions of their transfer from the molecule to the growth surface on the temperature dependences is considered.     

Analysis of Natural Vibrations of Round Flexing Membrane-Type Piezoelectric Transducers with an Arbitrary Dimension Ratio

The problem of natural vibrations of a round exing membrane-type piezoelectric transducer (piezoelectric plate) with an arbitrary dimension ratio is solved in the exact formulation using the finite element method for two variants of fastening the membrane (simply supported and rigidly fixed plate). The natural frequency spectra are analyzed in the resonance and antiresonance regimes. The displacement distributions over the plate surfaces are also analyzed and the dependences of the dynamic electromechanical coupling coefficient are investigated as functions of the geometric dimensions of the piezoelectric transducer, the plate material, and the piezoelectric ceramic composition. The optimum geometric dimensions of the transducer which ensure the maximum dynamic electromechanical coupling coefficient are determined and the value of the coefficient is estimated.     

Unexpected effect of the vehicle (grain ethanol) of homeopathic FSH on the in vitro survival and development of isolated ovine preantral follicles

The aims of this study were to investigate the effects of medium replacement system (experiment I) and of FSH presentations (homeopathic – FSH 6cH and allopathic FSH – rFSH; experiment II) on the in vitro development, hormone production and gene expression of isolated ovine preantral follicles cultured for 6 days. In experiment I, secondary follicles were cultured in the α‐MEM⁺ supplemented with FSH 6cH (0.05 fg/ml) or recombinant bovine FSH (100 ng/ml) without/with daily medium addition. The homeopathic FSH treatments with/without medium addition improved (p < .05) follicular development compared to rFSH100 treatment without addition. FSH 6cH with addition showed the highest (p < .05) estradiol production. To verify whether the effects of homeopathic FSH were not due to its vehicle, experiment II was performed. The α‐MEM⁺ was supplemented or not with alcohol (0.2% grain ethanol, v/v), FSH 6cH or rFSH100 with daily medium addition. Surprisingly, we found that all treatments improved follicular development compared to the α‐MEM⁺ (p < .05). Moreover, homeopathic FSH was similar to the other treatments including its vehicle. In conclusion, its vehicle (ethanol) causes the effect of homeopathic FSH on in vitro development of isolated ovine preantral follicles.