The work presents and describes various mechanisms for the formation of a conductive channel in bipolar memristors of the vacancy and ionic types: in the first case, due to the generation and growth of conductive threads, and in the second, due to segregation processes with the formation of spatial associations of ions from the material of the active electrode. The current—voltage characteristics for both types of memristors are presented and a comparison of the design features of these memristors is conducted.

Experimental observation of the enhanced terahertz (THz) emission in a large-area photoconductive antenna-emitter (LAE), boosted by an array of cylindrical sapphire-fiber-based microlenses, is reported. The observed enhancement is achieved, thanks to the sharp focusing of a pump laser beam near the semiconductor surface, for which the high-refractive-index sapphire lenses are used. We predict numerically and confirm experimentally a considerable enhancement in the emitted THz spectral power for such a sapphire-fiber-coupled LAE, as compared to an ordinary one with an equal electrode topology. In fact, a ≃8.5-fold THz power boost is achieved, resulting in a +9.3 dB increase in the dynamic range. The results of our findings can be used to improve the performance of large-area THz devices, aimed at meeting the demands of rapidly developed THz spectroscopy, imaging, sensing, and exposure technologies.

The paper shows and describes various mechanisms for the formation of a conductive channel in bipolar memristors of the vacancy and ion types: in the first case, due to the nucleation and growth of conductive filaments, and in the second, due to segregation processes with the formation of spatial associations of ions from the active electrode material. The volt-ampere characteristics for memristors of both types are given and a comparison is made of the design features of these memristors

Objectives. The study aims to improve the efficiency of a large-area photoconductive terahertz (THz) emitter based on an optical-to-terahertz converter (OTC) having a radiating area of 0.3 × 0.3 mm2 for generating high-power THz radiation by using an array of close-packed profiled sapphire fibers having a diameter in the range of 100–300 μm as focusing optics. Methods. As a photoconductive substrate, we used a semi-infinite LT-GaAs layer (low-temperature grown GaAs; GaAs layer grown by molecular beam epitaxy at a low growth temperature). Additional Si3N4 and Al2O3 layers are intended for reducing leakage currents in the OTC and reducing the reflection of the laser pump pulse from the air/semiconductor interface (Fresnel losses), respectively, at a gap width of 10 μm. For forming the antenna electrodes and feed strips, the Ti/Au metal system was used. The simulation was carried out by the finite element method in the COMSOL Multiphysics environment. Results. The use of a profiled sapphire fiber whose diameter has been optimized with respect to the gap parameters to significantly increase the concentration of charge carriers in the immediate vicinity of the electrodes of an OTC is demonstrated. The integrated efficiency of a large-area photoconductive THz emitter was determined taking into account the microstrip topology of the array with a characteristic size of feed strips proportional to the gap width in the OTC and with the upper (masking) metal layer. The maximum localization of the electromagnetic field in close proximity to the edges of electrodes at the “fiber–semiconductor” interface is achieved with a profiled sapphire fiber diameter of 220 μm. Conclusions. By optimizing the diameter of the sapphire fiber, the possibility of improving the localization of incident electromagnetic waves in close proximity to the edges of the OTC electrodes by ~40 times compared to the case without fiber, as well as increasing the overall efficiency of a large-area emitter by up to ~7–10 times, was demonstrated.

Experimental data on measurement of the resistance and electrical conductivity in the low-resistance mode of operation of a germanium-selenide-based memristor with a self-forming conductive channel in the form of a silver filament are obtained in the range of operating frequencies and temperatures. In the frequency experiment, the influence of the switching frequency is tested at room temperature in the range of 1–10 000 Hz. The main result of the experiment is the identification of a linear relationship between the electrical conductivity and the memristor operating cycle time in semilogarithmic coordinates, which made it possible to introduce a temperature-dependent kinetic constant. This experimental fact made it possible to establish the main parameter affecting the shape of the current–voltage characteristics, namely the thickness of the conductive channel.

We report a seminal approach for localization of photocarriers in a photoconductive antenna (PCA)-emitter via a focusing element comprising the sapphire fiber. Using numerical simulation, we showed that at a certain ratio between the fiber diameter and antenna gap size (d/g~ 22.5), one can attain a ~ 35-fold enhancement of laser irradiation in the vicinity of the PCA electrodes. This provides the formation of subwavelength electromagnetic wave caustics located at the edges of the PCA electrodes, which potentially promotes an increase in the optical-to-terahertz conversion efficiency. Keywords: terahertz frequency, terahertz emitters, semiconductors, photoconductive antenna, IR radiation.

We report on a sapphire-fiber-based lens that can be used to enhance the emitted THz power of a large-area photoconductive antenna (PCA). Using numerical simulations, we demonstrate that the lens provides a spatial redistribution of the photocarriers density in the PCA's gap. By optimizing the diameter of the sapphire-fiber, one could reach efficient confinement of the photocarriers in the vicinity of the PCA electrodes with a 10-μm gap size for a 220-μm-thick sapphire-fiber. This allows enhancing the coupling of the incident electromagnetic waves at the interface between the sapphire fiber and the semiconductor with the antenna terminals by ∼40 times for a single PCA element, as well as boosting the total efficiency of the large-area PCA-emitter up to ∼7-10 times. To validate our approach, we propose a step-by-step process that can be used for the precise and controllable placement of the sapphire-fiber on the surface of a single PCA.

We report the seminal approach for localization of photocarriers in a photoconductive antenna (PCA)-emitter via the focusing element comprising the sapphire fiber. Using numerical simulation, we showed that at the certain ratio between the fiber diameter and the gap size, i.e. d/g = 22.5, one can attain a 35-fold enhancement of the laser irradiation in vicinity of the PCA electrodes. This provides the formation of subwavelength EM wave caustics located at the edges of the PCA electrodes which potentially gives rise to an increase of optical-to-terahertz conversion efficiency.

In this paper, we develop a finite element model that allows calculating the current-voltage characteristic of a bipolar memristor based on Pt/HfO2/TiN hafnium oxide, which reflects both the high- and low-resistance states of the memristor. Maxwell’s equations for the stationary case served as the mathematical basis of the model. The model allows evaluating the relation between the properties of materials included in the structure of the memristor and its operating current.

A finite element model has been developed that allows calculating the current-voltage characteristic of a bipolar memristor based on hafnium oxide Pt/HfO2/TiN, which reflects both the high-resistance and low-resistance states of the memristor. The mathematical basis of the model was Maxwell's equations for the stationary case. The model allows us to evaluate the relationship between the properties of materials included in the structure of the memristor and its operating current.

Была разработана конечно-элементная модель, позволяющая рассчитывать вольт-амперную характеристику биполярного мемристора на основе оксида гафния Pt/ HfO2 / TiN, которая отражает как высокоомное, так и низкоомное состояние мемристора. Модель позволяет оценивать связь свойств материалов, входящих в конструкцию мемристора, с его рабочим током, а также учитывает изменение свойств материалов вследствие нагрева.

We report on two approaches aimed at increasing the performance of a photoconductive antenna (PCA) due to enhanced optical light confinement in a PCA's gap. The first refers to the use of a high-aspect ratio metal grating maintaining plasmonic guided modes while the second implies an asymmetric dielectric particle combined with a prism on a metal surface to provide a sub-wavelength focusing of a laser beam. We reached a 3000-fold increase in the emitted THz power and a 25-fold THz photocurrent enhancement in a plasmon-assisted PCA; the further increase in both parameters might be reached by optimizing the geometry of the grating. We propose a new curved beam - the photonic hook plasmon that can be realized using the in-plane focusing of the surface plasmon-polariton wave through a dielectric particle. We demonstrate a potential possibility to reach predominantly a sub-wavelength focused and high-intensive beam obtained from the opposite side of the dielectric particle.

Аннотация -В работе представлены результаты моделирования распределения тепловых потоков в монолитных интегральных схемах (МИС), в основе которых лежат HEMT-транзисторы с гетероструктурами AlGaN/GaN.Представлена методика моделирования, учитывающая профиль распределения тепловыделения в гетероструктуре HEMT-транзистора

We report on experimental study of the photoconductive antennas (PCA)-emitters featuring conventional topology as well as metallic metasurface with plasmonic grating, based on the InGaAs/InAlAs superlattices. We measure and determine the photocurrents and the emission spectra of the PCAs, as well as the energy characteristics of the terahertz (THz) radiation, and the efficiency of the optical-to-THz conversion for different bias voltages and average laser excitation power. The integral THz emission power of 10 μW as well as conversion efficiency of 0.2% are demonstrated for the PCA with the metasurface, which are not reachable for the conventional due to thermal breakdown of the antenna. Therefore, the PCAs with the metasurface can be considered to be the attractive THz emitters that can potentially become among the elemental base for modern THz spectroscopic systems for biomedical applications.

Рассмотрены механизмы формирования токопроводящих каналов в мемристорах на базе оксидов переходных металлов и твердых электролитов, в основе которых лежат процессы переноса заряженных кислородных вакансий (в оксидах переходных металлов) или ионов активного электрода (в твердых электролитах) в условиях, определяемых сильными электрическими полями, обусловленными наноразмерными эффектами.

AbstractThe fitting of θ/2θ and ω peaks in X-ray diffraction curves is shown to be most accurate in the case of using an inverse fourth-degree polynomial or probability density function with Student’s distribution (Pearson type VII function). These functions describe well both the highest-intensity central part of the experimental peak and its low-intensity broadened base caused by X-ray diffuse scattering. The mean microdeformation ε and mean vertical domain size D are determined by the Williamson–Hall method for layers of GaN (ε ≈ 0.00006, D ≈ 200 nm) and Al_0.32Ga_0.68N (ε = 0.0032 ± 0.0005, D = 24 ± 7 nm). The D value obtained for the Al_0.32Ga_0.68N layer is most likely to result from the nominal thickness of this layer, which is 11 nm.