Vol 52, No 15 (2018)

The use of the spin-torque diode effect, which is generated upon the current-induced transfer of spin angular momentum in magnetic tunnel junctions, opens the prospect of considerably improving microwave sensitivity in the gigahertz frequency band over that of semiconductor Schottky diodes. In this work, the spin-diode effect of microwave signal rectification in a magnetic tunnel junction upon the resonant excitation of spin waves in the free magnetic layer as a result of the current-induced spin-transfer torque effect is analyzed theoretically. Frequency characteristics of the resonant response of a spin-torque diode to a microwave signal are calculated in the linear macrospin approximation as functions of the direction and value of the applied magnetic field and bias current. It is shown that with zero bias current, the maximum rectified voltage across a junction is obtained in the geometry of mutually perpendicular magnetizations of magnetic layers; as the resonant frequency of oscillations in a magnetic field increases, this voltage drops at the retained equilibrium orientation of spins in the layers. After the bias current is switched on, the resonance amplitude of forced oscillations of the free layer spins upon microwave excitation grows sharply at the critical point of the spin-torque diode’s loss of equilibrium state stability. The linewidth at this point is limited only by nonlinear effects. Improving spin-torque diode sensitivity is important for its use in microwave holographic vision systems.

Individual small particles consisting of an insulating core and a metal shell exhibit more intricate behavior when exposed to electromagnetic radiation than continuous metal particles. A composite medium containing a large number of such particles is therefore bound to have new optical properties. If the inclusions are small relative to the wavelength of electromagnetic radiation, the optical characteristics of an inhomogeneous medium can be estimated from its effective permittivity. On the basis of the generalized effective field approximation, a formula is derived to calculate the effective dielectric characteristics of a matrix composite containing spherical inclusions with shells. The formula can be considered a generalization of the classical Maxwell–Garnett formula for a matrix medium with inhomogeneous inclusions consisting of anisotropic cores and isotropic shells. Using the formula, the frequency dependences of the real and imaginary parts of the effective permittivity in the 0.282–0.855 μm range of wavelengths are calculated for a composite consisting of an α-quartz matrix and spherical nanoinclusions with α-quartz cores and silver shells. The dependences are obtained for different relative volume fractions of cores in the inclusions and of inclusions in the composite. The frequency dependences of the refractive index, the extinction coefficient of the composite, and the transmittance and reflectance of a thin composite film are calculated for the range of wavelengths indicated above. It is shown that inclusions with metal shells in a matrix composite result in an additional plasmon resonance, compared to a composite containing all-metal inclusions. With a matrix composite, the additional plasmon resonance is observed at a wavelength of 0.33–0.34 μm in the ultraviolet range and is much less intense than the principal plasmon resonance. The additional plasmon resonance is responsible for the very low transmittance of a composite film in the ultraviolet region. At a constant volume fraction of inclusions in the composite, an increase in the volume fraction of the cores in them shifts the principal plasmon resonance to longer wavelengths and lowers its intensity.

Knowledge of the effective mass of quasiparticles is needed for studying the band structure of semiconductors, simulating processes of conductivity, and developing actual semiconductor devices. The problem of determining the effective mass of carriers in a particular sample is therefore of great interest. Using the example of silicon, it is shown in this work that it is possible to determine the effective masses of conductivity and the density of states by treating a measured microwave reflection spectrum as a set of information parameters. The inverse problem is solved of finding the conditions for the minimum of the difference of squares of values corresponding to known theoretical and experimentally measured spectral dependences. Calculations and experiments are performed for the temperature range of 130–190 K, in which the highest accuracy of measurements is ensured. For Ga-doped p-silicon and Sb-doped n-silicon, values of the desired effective masses are obtained that coincide with ones given in the literature. The proposed contactless technique allows simultaneous determination of the effective masses of conductivity and density of states of charge carriers using conventional equipment. The approach can be used to measure the parameters of other types of semiconductors, including ones that are little studied.

The unique properties of carbon nanotubes (CNTs) make these systems promising for the production of composite electrode materials based on a combination of CNTs and transition metal oxides. The effect the parameters of the functionalization of vertically aligned CNT arrays in mixed argon–oxygen high-frequency plasma have on the structural characteristics of CNTs is studied by Raman spectroscopy. The effect the duration of treatment at higher partial argon consumption and low total gas consumption of the working mixture has on the distribution of nickel oxide over a CNT array deposited via SILAR is determined. Results from scanning electron microscopy show that lengthening the duration of functionalization from 30 to 1200 s allows the depth of coating a CNT surface with a nickel oxide layer to be increased from 300 nm to 2.5 μm. Composite structures produced in this work can be used as electrodes of supercapacitors.

Chemical deposition from vapors of metalorganic compounds and volatile hydrides (metalorganic vapor phase epitaxy) is the main way of growing devices’ heteroepitaxial structures based on gallium arsenide (GaAs) and solid solutions of it (Al_xGa_{1 – x}As and In_yGa_1 – yAs) on an industrial scale. Electrically active impurities deposited uncontrollably on epitaxial layers during growth worsen the electrophysical parameters of devices’ structures. Sources of deposited background impurities include the initial material. In this work, ways of synthesizing trimethylgallium (TMG) are compared from the viewpoint of impurities, especially electrically active GaAs, being incorporated into the final product. Ways of preparing TMG that include the exchange reaction between gallium trichloride and trimethylaluminum (TMA) and magnesium–organic syntheses in which metallic magnesium is used as the initial reagent are selected as the objects of study. The behavior of electrically active impurities during the purification of TMG via rectification is studied. The investigations are performed by means of spectral analysis and functional control over the growth of electrophysical parameters of GaAs epitaxial layers from TMG and arsine. It is established that the qualitative and quantitative composition of impurities in TMG depends on their content in the initial reagents. TMG prepared using the exchange reaction between gallium trichloride and TMA is the source of n-type impurities in GaAs. These are mainly Group IV elements (silicon, tin, and lead), with silicon impurities predominating. TMG fabricated using metallic magnesium is a source of both p- (preferentially zinc) and n-type (preferentially silicon) impurities. Irrespective of the quality of the initial raw TMG, the use of reduced-pressure rectification allows the fabrication of TMG with low impurity contents. Epitaxial GaAs layers grown using purified TMG (TMG rectificate) have n-type conduction with a low level of background doping (0.7–4) × 10¹⁴ cm^–3 and high carrier mobilities of 7300–8500 cm²/(V s) at 300 K and 90 000–156 000 cm²/(V s) at 77 K. These values correspond to purest samples made with TMG and arsine using MOC-hydride epitaxy. Based on the data on functional control, the content of impurities that display electrical activity in GaAs is on the level of 10⁻⁷–10⁻⁶ at % in fabricated TMG-rectificate samples.

Films of silicon nitride SiN_x, obtained by plasma-enhanced chemical vapor deposition from the monosilane SiH₄ and ammonia NH₃ gases, are widely used in microelectronics and micro- and nanoelectromechanical systems. Residual mechanical stresses and film composition are important characteristics for many applications. The properties of SiN_x films, particularly mechanical stresses and composition, depend largely on the conditions of production, e.g., the ratio of the reacting gas flow rates, the composition of the gas mixture, the power and frequency of the plasma generator, and the temperature and pressure during deposition. Despite the great volume of works on the subject, data regarding the dependence of the properties and composition of SiN_x films on the conditions of production remain sparse. This work considers the effect the ratio of the reacting gas flow rates has on the mechanical stresses and composition of silicon nitride films SiN_x obtained by plasma-enhanced chemical vapor deposition from gaseous mixtures of SiH₄ monosilane and NH₃ ammonia using low-frequency plasma. It is found that when the ratio of the gas flow rates of SiH₄ and NH₃ is raised from 0.016 to 0.25, the compressive mechanical stresses are reduced by 31%, the stoichiometric coefficient falls from 1.40 to 1.20, the refractive index rises from 1.91 to 2.08, the concentration of N–H bonds is reduced by a factor of 7.4, the concentration of Si–H bonds grows by a factor of 8.7, and the concentration of hydrogen atoms is reduced by a factor of 1.5. These results can be used for the controlled production of SiN_x films with such specified characteristics as residual mechanical stresses, refractive index, stoichiometric coefficient, and the concentration of hydrogen-containing bonds.

Centrifugation is used in fabricating, e.g., films with large areas and/or thicknesses of several micrometers. However, it has yet to be widely employed for chalcogenide compounds, due to their relatively weak solubility in most solvents. Determining the optimum conditions for preparing solutions of chalcogenide compounds and obtaining films via centrifugation is therefore of great interest. Specific features of amorphous arsenic sulfide (As₂S₃) films prepared via the centrifugation of solutions in n-butylamine have been studied. These films were characterized by means of X-ray diffraction analysis, IR spectroscopy, atomic-force microscopy and Raman spectroscopy. It was shown that amorphous As₂S₃ films have a greater elasticity modulus than those of analogous composition produced via thermal evaporation in vacuum, or As₂S₃ glass. A structural model based on arsenic sulfide clusters whose surfaces are bound by negatively and positively charged ions is used to explain the experimental results obtained in this work. DC measurements show that the amorphous films exhibit semiconductor-type conductivity. Their room temperature conductivity is ~10⁻¹⁵ S/cm, which indicates good dielectric properties. The films are optically transparent starting from the yellow spectral range, making them promising functional materials for engineering applications in optics and photonics.

Studies in the field of the development of new structures of Hall sensor with improved characteristics, in particular with increased magnetic sensitivity are in high demand. A physical model is proposed, which explains features of the Hall–hate characteristic and the formation of the increased magnetic sensitivity region in the SOI field-effect Hall sensor (SOI FEHS). The results of simulation in the Synopsys TCAD system confirm the proposed physical model of SOI FEHS functioning. The model explains features of the Hall–gate characteristic of the SOI FEHS in a wide range of gate voltages and allows the conclusion that the SOI FEHS has two operating regions and their choice is dictated by specific conditions of the sensor application. To achieve the maximum magnetic sensitivity, regions of partial depletion and enhancement should be chosen. To provide high noise immunity, it is reasonable to choose the full enhancement modes.

Magnetoresistive random access memory (MRAM) has some advantages over other types of memory. However, MRAM has one substantial drawback: the current density and magnetic field that must be applied to switch the spin-valve free layer inside an MRAM cell are too high. The dependence of the current density and switching magnetic field on the magnetic parameters of the material from which the ferromagnetic layers of a spin valve are fabricated is therefore analyzed. A comparison of the critical characteristics of a spin valve with longitudinal anisotropy shows that cobalt, iron, and alloys of them; cobalt ferroborates; and alloys of cobalt with gadolinium, are promising materials for fabricating spin valves. Bifurcation diagrams of equations that describe the valve switching process are presented and analyzed. The four optimum switching modes of a valve are selected, based on an investigation of the dynamics of the magnetization vector. The magnitudes of the external magnetic field and controlling injection current that correspond to stable MRAM cell switching are compared for a variety of materials. The switching time of an MRAM cell is estimated numerically, and the conditions for its optimum speed are determined. It is established that the most promising materials for spin-valve fabrication are Fe₄₀Co₄₀B₂₀ and Co₈₀Gd₂₀, annealed at 300 and 200°C, respectively.