Properties and application of ultrasonic Lamb waves in CdxHg₁₋xTe plates

Group and phase velocities of the lowest orders of Lamb waves in , directions for (100) Cd₀.₂Hg₀.₈Te plates are calculated. Frequency dispersion of a₀ and s₀ Lamb modes velocities were measured on (111)-plates in the range of frequencies from 0.2 to 10 MHz. The frequency dependencies of relative efficiency of the Lamb modes excitation using two geometrical versions: symmetrical - with two piezoelectric transducers, and antisymmetrical - with one piezoelectric transducer, have been studied. Possible applications of studied waves including ultrasonic treatment of semiconductors have been discussed

Effect of temperature variation on shift and broadening of exciton band in Cs₃Bi₂I₉ layered crystals

The exciton reflection spectra of Cs₃Bi₂I₉ layered crystals is investigated in the temperature region 4.2–300 K with light polarization E ⊥ c. It is estimated that the energy gap Eg equals 2.857 eV (T = 4.2 K) and the exciton binding energy Ry is 279 meV. A nontraditional temperature shift of Eg(T) for the layered substances is found for the first time. It is learned that this shift is described very well by the Varshni formula. A transition region in the temperature broadening of the half-width H(T) of the exciton band with the increase of temperature is registered in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. A surge of H(T) at the point of the ferroelastic phase transition (Tc = 220 K) is also observed

Influence of absorption level on mechanisms of Braggdiffracted x-ray beam formation in real silicon crystals

The methods of numerical calculations based on the formulae of the X-ray dynamic scattering theory by real crystals and of the Takagi-Topin equations were used for investigation of the basic regularities of inherent to the Bragg diffraction in conditions of a strong and weak absorption. The mechanisms of profile formation of a spatial intensity distribution of diffracted beams depending on an energy of radiation and on structural perfection parameters of crystals are discussed. The formulae for an analytical description of spatial intensity distribution profiles which take into account the dynamical corrections (coefficients of extinction) for coherent and incoherent components of the total reflectivity were used

X-ray diffraction investigation of GaN layers on Si(111) and Al₂O₃ (0001) substrates

Methodical approaches to the analysis of X-ray data for GaN films grown on various buffer layers and different substrates are presented in this work. Justification of dislocation structure investigation by various methods was analyzed and approaches for evaluation of deformation level and relaxation are discussed. Clarity and accuracy of obtained structure characteristics of nitride films are under discussion. Optimization methods for experimental data processing are shown. Structural properties were obtained using high resolution X-ray diffraction with two types of scans and reciprocal space maps. Microscopic nature of spatial inhomogeneities in these structures (deformations and dislocation density) and influence of the buffer layer thickness on properties of GaN layer were discussed with account of obtained results

X-ray diffraction study of deformation state in InGaN/GaN multilayered structures

High resolution X-ray diffractometry (HRXRD) was used to investigate InxGa₁₋xN/GaN multilayered structures grown by the metal-organic chemical vapor deposition (MOCVD) method. Deformation conditions in the superlattice (SL) and its separate layers, degree of relaxation in the structure layers, as well as the period of the SL, thicknesses of its layers and composition of InxGa₁₋x solid solution in active area were determined. It was found that SL was grown on the relaxed buffer layer. SL layers were grown practically coherent with slight relaxation of InGaN layer (about 1.5 %). The role of dislocations in relaxation processes was established. Analysis of experimental diffraction spectra in these multilayered structures within the frameworks of ParratSperiozu was adapted for hexagonal syngony structures

Photochromic properties of composite films of thermally irreversible diarylethenes and fulgimides in polytetrafluoroethylene matrix

Photochromic polymeric thin films have been prepared by vacuum codeposition of polytetrafluoroethylene (PTFE) and thermally irreversible photochromic compounds: the cyclopentene derivative of diarylethene and fulgimide. Their photochromic properties were studied using the spectral-kinetic method as compared with polymethyl methacrylate (PMMA) films filled with the same photochromic compounds prepared using the spin coating method. The films prepared using codeposition in vacuum are characterized by photochromic properties acceptable for recording media for 3D optical memor

Optical and acoustical phonon modes in superlattices with SiGe QDs

Multilayers with SiGe nanoislands grown in a broad temperature range (300-600 °C) are studied using Raman spectroscopy, HRXRD and AFM. It is shown that the islands are fully strained when obtained at 300 °C and gradually relax with the growth temperature increase. The main contribution to the Raman peaks caused by scattering on folded acoustic phonons in multilayers (n ≤ 10) with nanoislands is due to the islands themselves. The enhancement of the scattering intensity due to resonance of the exciting light with the electronic transitions inside the islands is shown to play a significant role

Structure perfection variations of Si crystals grown by Czochralski or floating zone methods after implantation of oxygen or neon atoms followed by annealing

Structure perfection of the silicon crystals grown by the Czochralski and floating zone methods after implantation with oxygen or neon fast iones followed by annealing at the temperatures T ~ 1050-1150 ⁰0C, when large SiOx precipitates were formed, was studied by means of various X-ray diffraction methods. Considerable increments of integral reflectivities for different Bragg reflections of such samples in comparison with those calculated for a perfect crystal were detected. Broadening of the spatial intensity distribution curves for the Bragg-diffracted beams taken by a single crystal spectrometer as well as the maps of the diffuse isointensity distribution near a reciprocal lattice point, registered by the Philips high-resolution diffractometer, are shown. All of these diffraction effects related to creation of the SiOx precipitates formed on structural damages caused by implantation of oxygen or neon ions and subsequent annealing. Contrary to FZSi, where the appearence of SiOx precipitates was discovered due to intensive diffuse scattering near the layer contained the implanted oxygen ions only, in the case of CZSi samples with larger concentration of oxygen (up to 1*10¹⁸ at/cm³) such defects were formed not only near the burried layer, created by ions of oxygen or neon (with energy E = 4 MeV, dose 10¹⁴cm⁻²) but in a bulk of a crystal. Annealing of the FZSi crystals implanted by oxygen (E ~ 200 keV, dose ~ 10¹⁶-10¹⁷ cm⁻²) at enhanced hydrostatic pressure, additionally stimulated SiOx precipitation close to the implanted layer

Anisotropy of ultrasonic waves propagation velocities in CdHgTe/CdTe

The complete set of elastic moduli of Cd₀.₂Hg₀.₈Te was obtained. Taking into account elastic moduli found for Cd₀.₂Hg₀.₈Te and appropriate literary data for CdTe the anisotropy of velocities of volume and Rayleigh waves propagating on (100) and (110) boundaries was calculated. Finally, peculiarities of the anisotropy of SAW propagating velocities for each of components of layered structures CdHgTe/CdTe and for the heteroepitaxial structure as a whole were analysed

Interface model of low temperature plasticity in high uniaxially strained monocrystalline semiconductors

The manifestation of the low temperature plasticity (LTP) in highly uniaxially strained Ge and Si single crystals was deduced from analysis of the both tensoeffect measurements data and defect-selective etching patterns of specimens. An appearance of additional tensoeffect mechanisms after the LTP display we attribute to the generation of electrical active defects of crystalline structure when the applied stress exceed some critical one. We found that under LTP conditions the generated dislocation pile-ups are directly concentrated in the phase-boundary field of some structural imperfections of crystalline lattice. The interface model of LTP phenomenon in monocrystalline semiconductors was proposed for acceptable explanation of the dislocation generation in the initially dislocation-free crystals