Simulation of Silicon Carbide Sputtering by a Focused Gallium Ion Beam

Semiconductors - The precision fabrication of microstructures and nanostructures by the focused-ion-beam method is significantly simplified by the simulation of sputtering of materials by...     

Interaction of spatially modulated pump and FIR waves in Raman laser

The propagation of spatially modulated IR pump and FIR generated waves interacting in the active gas medium of Raman laser is simulated numerically. Stationary nonlinear diffraction of monochromatic beams is studied for the isolated homogeneously broadened three-level system without any limitations on the values of field strength and frequency offset. It is shown that FIR radiation is trapped in the active wave guide induced owing to the nonlinear refractive index mechanism and gain inhomogeneity across the pump wave beam. Nonlinear focus positions and on-axis focal intensities for both fields are calculated.     

Spontaneous and stimulated emission from magnetron-deposited ZnO-SiO2-Si thin-film nanocavities

The spontaneous and stimulated emission spectra from optically pumped zinc oxide nanolayers on oxidized silicon were studied. Single-mode lasing with a wavelength of 397 nm was observed for a coupled longitudinal mode of the ZnO film at room temperature. For the thin-film nanocavities of ZnO-SiO₂-Si, optimal parameters that allow the lowering of the threshold and the attainment of single-mode lasing are determined.     

Influence of the order-disorder transition in the crystal electron subsystem on the electron density at lattice sites

The temperature dependence of the Mössbauer spectrum centroid S of ⁶⁷Zn²⁺ impurity atoms at the copper and yttrium sites of YBa₂Cu₃O_6.9, YBa₂Cu₃O_6.6, YBa₂Cu₄O₈, Nd_1.85Ce_0.15CuO₄, La_1.85Sr_0.15CuO₄, HgBa₂CuO₄, HgBa₂CaCu₂O₆, Bi₂Sr₂CaCu₂O₈, and Tl₂Ba₂CaCu₂O₈ compounds at temperatures T>T _c (T _c is the superconducting transition temperature) is controlled by a second-order Doppler shift. The value of S in the temperature range T<T _c is affected by the energy-band mechanism associated with the formation of Cooper pairs and their Bose condensation. A relationship between the electron density at the metal site of the crystal and its superconducting transition temperature is found. In the case of compounds containing two structurally nonequivalent sites for copper atoms, a change in the electron density caused by the Bose condensate of Cooper pairs is shown to be different for these sites. The experimental temperature dependence of the superconducting electron fraction conforms to a similar dependence following from the Bardeen-Cooper-Schrieffer theory for all the sites under study.     

Mode structure of laser emission from ZnO Nanorods with one metal mirror

The effect of the length of ZnO nanorods (500 nm in diameter) on the mode structure of their lasing in the ultraviolet spectral region is studied by optical luminescence microscopy. It is shown that separate nanorods with a metal mirror at one of the end faces exhibit only two or three laser modes at small nanoresonator lengths (8–20 μm). The different optical losses of longitudinal and transverse waveguide modes are established for nanorods lying on a glass substrate. An increase in the optical Q factor and a decrease in the lasing thresholds can be attributed to optical reflection from the metal mirror at the end face of the nanorod.     

Infrared and submillimeter spectroscopy of grooved silicon structures

Transmission of grooved silicon structures with 4-and 7-μm periods is studied by polarization-sensitive IR and submillimeter spectroscopy in a wide spectral range. The experimental results obtained in the range 1–10-μm are explained in terms of geometrical optics taking into account the scattering of radiation. In the far-IR (20–2000 μm) range, the structures exhibit a strong birefringence, which can be described in terms of the effective medium model in the electrostatic approximation. An influence of photoexcitation on the optical transmission and its anisotropy is observed; this effect can be explained in terms of the effective medium model taking into account the interaction of radiation with free carriers.     

Optimization of the parameters of HEMT GaN/AlN/AlGaN heterostructures for microwave transistors using numerical simulation

The numerical simulation, and theoretical and experimental optimization of field-effect microwave high-electron-mobility transistors (HEMTs) based on GaN/AlN/AlGaN heterostructures are performed. The results of the study showed that the optimal thicknesses and compositions of the heterostructure layers, allowing high microwave power implementation, are in relatively narrow ranges. It is shown that numerical simulation can be efficiently applied to the development of microwave HEMTs, taking into account basic physical phenomena and features of actual device structures.     

Development and study of robast control system for dc thyristor electric drive using polynomial techniques

Possibilities of polynomial techniques for synthesizing and analyzing a robast control system of dc thyristor electric drivers are considered. It is shown that the use of the suggested techniques makes it possible to decrease the sensitivity of a controlled electric drive to parametric, structural, and external disturbances by about an order of magnitude. A technique for optimizing the parametric sensitivity of the quality factors is suggested.     

Study of accessible free volume and transport properties of TFPS-co-TMSP copolymer

Copolymers of 1-trimethylsilyl-1-propyne (TMSP) with 1-(3,3,3-trifluoropropyldimethylsilyl)1-propyne (TFPS) as part of a systematic series with the TFPS content of the final polymer in the range of 0 to 46 mol % have been synthesized. The resulting samples have been characterized by different instrumental methods including NMR, IR, and DSC techniques. It has been shown that the accessible free volume of the polymer decreases from 30.4 to 19.1% with an increase in the proportion of TFPS units. Moreover, the gas transport properties of the membrane materials are reduced by factors of 30 (oxygen), 50 (nitrogen), and 15 (carbon dioxide), although the selectivity for the O₂/N₂ and CO₂/N₂ gas pairs increases from 1.6 to 2.5 and 2.5 to 11.5, respectively. A study of nanofiltration of organic media has shown that the ethanol permeability through the TFPS-co-TMSP copolymer material is reduced by no more than 30%, a value that is comparable with the relative decrease in the fractional free volume of the polymer. An increase in the TFPS content has improved the stability of the membrane material in ethanol (swelling ratio decreased from 61 to 39%), thereby resulting in an increase in the retention of the Orange II anionic dye (350 g/mol) from 89 to 94%.