Structural phase changes in a titanium-silicon system modified by high-current electron beams and compression plasma flows

Structural phase changes in a titanium-silicon system treated by low-energy high-current electron beams (HCEBs) and compression plasma flows (CPFs) with the duration 100 μs and the energy density 12–15 J/cm² are studied. Scanning electron microscopy, X-ray diffraction and electron microprobe analysis are used in this work. The formation of a titanium-doped silicon layer 10–25 μm thick, titanium silicides (TiSi₂ under HCEBs and Ti₅Si₃ under CPF treatment), silicon dendrites, and needle-like eutectics (typical size of precipitates is about 50 nm) is revealed. It is shown via the results of numerical simulation that the thickness of the metal-doped layer is mainly controlled by the power density value and the surface nonuniformity of the heat flow over the target surface. The thermodynamic regularities of phase formation are discussed, taking into account heat transfer between the silicide nuclei and solid silicon.     

Melting and crystallization dynamics of single-crystal silicon exposed to compression plasma flows

The melting and crystallization of single-crystal silicon wafers exposed to compression plasma flows generated by quasi-stationary plasma accelerators are studied by numerical simulation. The results include the phase transition kinetics described on the basis of the Kolmogorov equation. The space-time characteristics of the melting and crystallization processes for variously shaped plasma pulses are discussed. Based on the experimental data and estimates, it is concluded that thermoelectric instability plays an essential role in the formation of three-dimensional periodic structures.     

Change in the relief of a target surface treated by compression plasma flows

The change in the surface relief of a steel-3 target (GOST 380) treated by compression plasma flows is experimentally studied. The energy density absorbed by the target varies in the range of 10–35 J/cm² and the pulse duration is 100 μs. It is shown experimentally and numerically that the development of KelvinHelmholtz instability strongly affects the formation of the target surface treated with compression plasma flows: a large-scale wave-like relief with characteristic sizes of 200 × 1000 μm is formed on the target surface and, as a result, the roughness of the surface increases. However, the microrelief at the scale of individual elements is smoothed to a maximum roughness of about 0.5 μm.     

Bulk periodic structures formation on monocrystalline silicon surface under the action of compression plasma flows

The results of numerical simulation of monocrystalline silicon melting and crystallization under the action of compression plasma flow generated by quasistationary plasma accelerators with regard to phase transformations based on Kolmogorov equation are presented. Temporal and spatial characteristics of melting and crystallization processes for pulses of various forms are discussed. Based on data received and estimates made, the conclusion on substantial role of thermoelectric instability in bulk periodic structures formation was made.     

Interaction between oppositely directed compression plasma flows

The dynamics of the interaction of two oppositely directed plasma flows generated by miniature gas-discharge magnetoplasma compressors is studied. The maximum plasma electron temperature and density in the region where the plasmas interact are found to be 4.5 eV and 1.4⋅10¹⁷ cm^–3. Exposure of samples to this kind of plasma near the interaction region leads to an energy flux at the surface of 2–8 J/cm², which is sufficient for modification of the surface properties of various materials.     

Experimental evidence of the vacancy-mediated silicon self-diffusion in single-crystalline silicon

We have determined silicon self-diffusivity at temperatures 735-875 degrees C based on the Raman shift of longitudinal optical phonon frequencies of diffusion annealed 28Si/30Si isotope superlattices. The activation enthalpy of 3.6 eV is obtained in such low temperature diffusion annealing. This value is significantly smaller than the previously reported 4.95 eV of the self-interstitial mechanism dominating the high temperature region T>855 degrees C and is in good agreement with the theoretical prediction for the vacancy-mediated diffusion. We present a model, containing both the self-interstitial and the vacancy terms, that quantitatively describes the experimentally obtained self-diffusivity between 735 and 1388 degrees C, with the clear crossover of the two diffusion mechanisms occurring around 900 degrees C.     

Electron-beam irradiation effects on luminescence properties in subsurface regions of single-crystalline sapphires treated with and without hydrogen plasma exposures

Electron irradiation effects on various insulating sapphires treated with and without hydrogen plasma have been investigated mainly by means of cathodoluminescence (CL) measurements. The samples examined included Be-diffusion-treated natural sapphire (BNS) and two types of synthetic sapphires grown by Verneuil and Czochralski methods. For all the samples examined, on one hand, their CL intensities of the F⁺-center-related emission peaked at ≈3.8 eV rapidly increased with increasing the fluences of keV electrons, and were represented roughly by exponentially saturating curves. There occurred slight blue-shifts of the F⁺-center luminescence other than the intensity increases for some of the electron-irradiated specimens, suggesting possible presence of two components for the F⁺-center luminescence. On the other hand, a hydrogen plasma exposure to these sapphires resulted in sample-dependent changes in the optical property and in the beam-irradiation effect on the F⁺-center CL emission. Such variations were induced most strongly in the BNS sample, whose color changed from orange to pink due to substantial decreases in the absorbance after the hydrogen plasma treatment. Furthermore, the energy positions of both the Cr³⁺-center luminescence peaked at ≈1.8 eV and its satellite peaks were found to slightly shift for the untreated and H-plasma-treated BNS samples after the electron beam irradiations. Possible origins of these observations are discussed.     

Structural changes in nanocrystalline silicon deposited by rf-magnetron sputtering

We have measured structural and optical properties for nanocrystalline silicon thin films deposited by rf-magnetron sputtering and characterized the microstructure of the films with the methods of scanning electron microscopy, X-ray diffraction, and Raman scattering. The measurements suggest that the level of reactive gases, leaving residue gas molecules that remain inactive for the crystal growth, affect the formation of nanocrystals. They also identify the columns microstructually appeared in the nanocrystalline silicon to the amorphous networks. PACS 78.30.Am; 78.67.Bf; 81.07.Bc     

Structural analysis of silicon dioxide and silicon oxynitride filmsproduced using an oxygen plasma

Plasma grown silicon dioxide and oxynitride layers are shown to represent, for microelectronic applications, a good alternative method to conventional thermally grown layers. Fast growth rates, together with good electrical properties are demonstrated, at low process temperatures. Growth kinetics of SiO₂ layers synthesized both in RF and microwave plasma anodization systems are presented for a wide range of substrate temperatures in the range (90-560°C). Structural properties of the films can be affected during preparation, due to radiation from the plasma and particle bombardment. For the SiO₂ layers obtained by RF anodization at 300°C, these surface structural features were investigated by scanning electron microscopy; bulk and interface dielectric properties of the layers were analyzed by spectroellipsometry. The results were correlated with electrical properties and data coming from the growth kinetics. It was found that the properties of the layers, both structural and electrical, are strongly dependent on the growth regime (linear or parabolic). Silicon oxynitride films produced by plasma anodization of silicon nitride layers are investigated by spectroellipsometry and Auger electron spectroscopy. These results are correlated with electrical measurements and used to explain the changes in film properties     

Crack detection in single-crystalline silicon wafers using impact testing

This paper presents acoustic measurements obtained by mechanically exciting vibratory modes in single-crystalline silicon wafers with hairline periphery cracks of different type and location. The data presented shows a dependence of natural frequencies, peak amplitudes and damping levels of four audio vibration modes in the frequency range up to 1000 Hz on crack type and crack location. Data from defective wafers exhibit lower natural frequencies, higher damping levels, and lower peak amplitudes. The results suggest an impact test method may be useful for solar cell crack detection and quality control.     

Structural influence of erbium centers on silicon nanocrystal phase transitions

Two different types of erbium-doped silicon nanocrystals, along with undoped, oxide-capped Si dots, are employed to probe the impact of the impurity center location on phase transition pressure. Using a combination of high pressure optical absorption, micro-Raman, and x-ray diffraction measurements in a diamond anvil cell, it is demonstrated that the magnitude of this phase transition elevation is strongly dictated by the average spatial location of impurity centers introduced into the nanocrystal along with the interfacial quality of the surrounding oxide.     

Nitriding of steel and titanium surface layers under the action of compression plasma flows

The elemental and phase compositions of St3 steel and VT1-0 titanium surface layers nitrided by the action of compression plasma flows (CPFs) have been investigated. The plasma flow parameters are shown to be correlated with the modified-layer nitrogen content. The basic mechanism by which the steel and titanium surface layers are saturated with nitrogen has been revealed. The performed experiments indicate that an increase in the absorbed energy density leads to a decrease in the nitrogen concentration because a shock-compressed layer is formed in the near-surface region, impeding nitrogen diffusion into the sample. The higher nitrogen concentration of surface layers treated by CPFs is achieved by increasing the pressure of the residual nitrogen atmosphere. It has been established that γ_N-Fe nitrous austenite, α″-Fe(N) and α′-Ti(N) martensitic phases, and γ′-Fe₄N and δ-TiN _x nitrides can be produced by nitriding the surface layers of St3 steel and VT1-0 titanium.     

Structural and electronic properties of rock salt phase of ZnO under compression

Structural and electronic properties of rock salt phase of ZnO under high pressure have been reinvestigated in the light of some recent experimental results. Behavior of direct and indirect energy band gap under increasing pressure is analyzed on account of overlapping of p (O) and d (Zn) orbitals and the results are compared with other theoretical studies. An empirical relation involving elemental electronegativity is suggested to estimate the change in band gap under increasing pressure. Furthermore, phase transformation of ZnO into other possible structures is also discussed and their structural and electronic properties analyzed.     

Structural changes within the plastic phase of diamantane

Structural changes within the pre-melting plastic phase of diamantane have been revealed from high-temperature X-ray powder diffraction data. A gradual transition to a more disordered state is suggested, and hexagonal symmetry ($\text{a = 14.1}\text{A}\text{?}\text{, c = 11.0}\text{A}\text{?}$) for the plastic phase is firmly established.     

Structural changes in α → γ phase transformations of polycaprolactam

The transition from the to the modification of polycaprolactam caused by treatment with iodine was studied by means of X-ray diffraction. The measurements were carried out on foils with a plane texture. During treatment with aqueous solutions of LiI₃, NaI₃, KI₃ and RbI₃ stable iodine complexes of polycaprolactam were produced. These complexes have a three-dimensional, not quite ordered structure and are similar to one another. Their unit cell has the dimensionsa=15,5 å,b=4,4 å,c=24,7 å; the edges of the unit cell are perpendicular to one another. The iodine forms in the complexes columns that are parallel to the polycaprolactam chains which are arranged into planes parallel to the plane of the foil. The action of water causes the complexes to disintegrate and when the iodine is completely eliminated from the foil the modification of polycaprolactam, which has an equally perfect texture as the initial modification, is formed.In conclusion, the author would like to thank Dr. K. Toman, Dr.Sc., for guiding the work and Ing. B. Sedláek, Dr.Sc., for his interest.     

Structural and magnetic properties of single-crystalline Co-doped barium titanate nanoparticles

Undoped and Co-doped BaTiO₃ nanoparticles were synthesized by a one-step sol-precipitation method. For all the samples, X-ray diffraction showed characteristic diffraction lines for BaTiO₃ without the indication of secondary phases. High-resolution transition electron microscopy images showed that BaTiO₃ nanoparticles exhibit the nature of single-crystal. Magnetometry revealed that all the Co-doped BaTiO₃ samples show paramagnetic behaviors and Co ions in BaTiO₃ are present as isolated paramagnetic centers. This is contrasted to several reported cases of ferromagnetism in Co-doped BaTiO₃.     

Void fraction prediction in two-phase flows independent of the liquid phase density changes

Gamma-ray densitometry is a frequently used non-invasive method to determine void fraction in two-phase gas liquid pipe flows. Performance of flow meters using gamma-ray attenuation depends strongly on the fluid properties. Variations of the fluid properties such as density in situations where temperature and pressure fluctuate would cause significant errors in determination of the void fraction in two-phase flows. A conventional solution overcoming such an obstacle is periodical recalibration which is a difficult task. This paper presents a method based on dual modality densitometry using Artificial Neural Network (ANN), which offers the advantage of measuring the void fraction independent of the liquid phase changes. An experimental setup was implemented to generate the required input data for training the network.ANNs were trained on the registered counts of the transmission and scattering detectors in different liquid phase densities and void fractions. Void fractions were predicted by ANNs with mean relative error of less than 0.45% in density variations range of 0.735 up to 0.98 gcm⁻³. Applying this method would improve the performance of two-phase flow meters and eliminates the necessity of periodical recalibration.     

Influence of electron diffraction on measured energy-resolved momentum densities in single-crystalline silicon

Electron momentum spectroscopy is used to determine the spectral function of silicon single crystals. In these experiments 50 keV electrons impinge on a self-supporting thin silicon film and scattered and ejected electrons emerging from this sample with energies near 25 keV are detected in coincidence. Diffraction effects are present that give rise to additional structures in the measured spectral momentum densities. Spectra for a specific momentum value can be obtained at different orientations of the crystal relative to the analysers. By comparing these spectra for which the measured momentum density is the same, but the diffraction conditions of the incoming and outgoing electron trajectories differ, one can distinguish between features due to diffraction of the incoming and/or outgoing electrons, and those due to the electronic structure of the target itself.     

Surface phase transitions of silicon in the presence of a dense electron-hole plasma

A thermodynamic equation is given, that relates the surface phase transitions with phonon-defect- electron interactions. It permits the calculation of the variation of the phase transition temperatures, T_c, with the density of an electron-hole plasma in silicon. Under conditions met in short-pulse laser irradiation, it is shown that T_c might differ from thermal values. Also, temperature calculations derived from LEED experiments might be in error, due to electron-phonon couplings.