Step Structure Research Articles

Influence of the growth conditions on the formation of macro-steps on the growth interface of SiC-Crystals

Several SiC crystals were grown in the same temperature field with a variation of the gas phase and the resulting step morphologies on the surface are analyzed using Laser Scanning Microscopy (LSM). It is shown that the step morphology changes due to doping, but also due to different C/Si ratios in the gas phase. By the addition of graphite into the source powder a reduction of Si-excess was achieved and an ordered step structure with an average height of 0.014 µm and width of 5.0 µm was formed. In the n-type doped case (n = 5.9∙1018 cm−3) the morphology on the facet differs strongly. Bunched steps with heights of 0.17 µm and widths of 152 µm are found. Between the diverse large macro-steps, a very ordered structure of unit cell height steps with a step spacing of 13.3 µm is found by Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). A different change in the step morphology occurs over the whole crystal surface when elemental silicon is added into the source material. In contrast to the doping induced case, the bunched steps are more closely spaced and exhibit average heights of 0.20 µm and widths of 22 µm. The steps are noticeably retained at defects like micropipes or threading dislocations. The N2 induced step bunching is related to step kinetics on the main facet but the formation of macro-steps in the Si-rich case is assigned to the formation of stable surface steps. The variation of the inert gas pressure in the growth cell also changes the C/Si ratio in the gas phase as the diffusion from source to seed of the reactive gas species is affected differently. With a low inert gas pressure of 0.2 mbar a larger C/Si ratio in the gas phase is formed leading to steps with heights too small for analysis in the LSM, while the growth in 40 mbar promotes a smaller C/Si ratio leading to the formation of bunched steps with heights of 0.10–0.20 µm and with an average distance of 4.3 µm between them. The crystals grown in a gas phase comprising a large C/Si ratio show a better polytype stability than the crystals grown in Si-rich conditions. The instabilities can partly be attributed to the pile up of the macro-steps at defects exposing large 0001¯ terraces, where 2D-nucleation of foreign polytypes can occur.

Recombination Rate and Internal Field Efficiency of Staggered InGaN/GaN Light-Emitting Diodes with Left and Right Steps

In this study, the recombination rate and internal field efficiency of staggered InGaN/GaN quantum well lightemitting diodes with left and right steps were investigated using the multiband effective mass theory. The left step quantum well structure shows a larger optical matrix element than that of the right step quantum well structure. In addition, the heavy-hole effective mass around the uppermost valence band for the left step quantum well structure is slightly larger than that for the right step quantum well structure. Consequently, the staggered left step quantum well structure shows a much larger radiative recombination rate and internal quantum efficiency than that of the right step structure.

CMOS-MEMS Accelerometer With Stepped Suspended Gate FET Array: Design & Analysis

This article presents a novel stepped suspended gate field-effect transistor (SSGFET) array-based <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${z}$ </tex-math></inline-formula> -axis accelerometer with an enhanced detection range. The stepped gate electrode structure of SGFET aids in extending the stable driving range beyond 33.33% of the initial air gap. The stable driving range is extended to 50% of the initial air gap with ~90% increase in the pull-in voltage. Mechanical, electrical, and electromechanical analytical models are developed. These models are validated through microelectromechanical systems (MEMS) simulations using CoventorMP and transistor simulations in Synopsys TCAD. An SSGFET-based common source (CS) amplifier with diode-connected p-MOSFET load is designed and simulated in Cadence Virtuoso using the lookup table (LUT) approach. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -axis accelerometer exhibits a sensitivity of 38 (mV/g) with a supply voltage of 3.3 V for a dynamic range (DR) of ±6 g with the nonlinearity of about 5.3% comparedtoSGFETswith a planar gate electrode,whichcan detect up to ±4 g with the same sensitivity. The 3-dB bandwidth of the accelerometer is 412 Hz with a noise-limited resolution of 109.31 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$ </tex-math></inline-formula> g/(Hz) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . This article also presents a detailed analysis of the relation between the number of gate fragments, the pull-in voltage, stable driving range, and the DR along with a feasible fabrication integration plan.

Improvement of the Output Mode Purity of a Complex-Cavity Resonator for a Frequency-Tunable Sub-THz Gyrotron

One way to improve the mode selection in terahertz gyrotrons is the use of stepped cavity resonators that satisfy the resonance condition for two modes with the same azimuthal but different radial indices. Such sectioned resonators, or complex cavities, were analyzed in detail by several authors, recently and in the past. One common problem for such designs is the excitation of unwanted modes by both the stepped cavity structure and the cavity input and output tapers. Mode conversion becomes especially a severe problem for higher-order resonant modes in highly oversized resonators. In this article, we introduce ways to minimize the unwanted mode conversion in both the radial step of the cavity as well as in the tapered sections of the cavity and the up-taper to the collector and output window. Frequency tuning between 391.5 and 393.5 GHz has been achieved by operation at different axial mode numbers of the mode pair TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8,4</sub> /TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8,5</sub> . Due to their intrinsic higher output mode purity, the so-called antiphase modes of the complex cavity have been chosen for this output mode purity study.

Structural characteristics of pools in headstream areas

Pools play an important role among stream bed morphologies in mountain streams. They provide habitat and contribute to energy dissipation. However, they have received only marginal attention compared with step structures, and their forms and driving factors, such as stream-channel characteristics and structural characteristics of upstream morphologies, remain unclear. Therefore, the purpose of this study was to determine the structural characteristics of pools, including morphology, size, and deposit conditions, and the factors controlling them. Based on the results of a field survey for a channel length of 9524 m, with 428 reaches and 1302 pools, the correlations between the pool spacing, morphology, size, and deposit conditions of the pool, and controlling factors were investigated for three rock types (granite, rhyolite, and sandstone). The average pool spacing calculated by dividing the interval length of the pool by the channel width was 8.80, 12.87, 4.56, and 16.14 for fall-pool, cascade, step-pool and pool-riffle respectively. This is due to the fact that the target streams are first-order streams with small widths as well as their regional characteristics, that is, few log jams and forced pools. In addition, the depositional depth of the pools varies depending on the geology. The maximum percentage of sediment deposition in the pool was 78%, 33%, and 34% for granite, rhyolite, and sandstone, respectively. Thus, the sedimentation in granite is significantly greater than in the rhyolite and sandstone headstream areas. This is probably due to the discontinuous weathering of granite during which it is broken down into large-sized core stones and fine grains. The difference in the depositional environment of the pools depending on the geology indicates that the biota that use those pools as a habitat may also differ. However, research on the microhabitats of pools is lacked, and research regarding the future perspectives of ecosystems is required.

Atomic structure of a single step and dynamics of Sn adatoms on the Si (111)−3×3− Sn surface

The atomic structure of well-ordered single steps on the Si(111)-\sqrt{3}\times\sqrt{3}-Sn surface and the dynamics of Sn adatoms in the vicinity of these steps was studied. The work was performed using low-temperature scanning tunneling microscopy (STM) and ab initio calculations based on the density-functional theory. The STM tip was used to record the tunneling current versus time on top of oscillating adatoms, keeping the feedback loop turned off. The dynamics of adatoms, detected as the telegraph noise present in the tunneling current, was registered near the steps at 80 K. The atomic structure model of the single steps consisting of Sn atomic chains along the steps was developed. This structure leads to the formation of potential double wells near the steps acting as traps for Sn atoms and explains the fluctuating current recorded in these areas.

Indicators of cosmonaut locomotor functions stability: A new method for ground-reaction forces analysis

The paper introduces a new method for ground-reaction forces (GRF) analysis for the evaluation of cosmonaut locomotor functions stability after a long-duration spaceflight (LDSF). The method is based on the analysis of GRF profiles obtained before and after LDSF by standard БД-2 treadmill reaction force data readings. Following probabilistic indicators were introduced: distributions of step and step-phase duration probabilities, their extremums, standard deviations and inverse coefficients of variety; step structure variability; the non-stationarity index for step durations arrays; GRF pattern, its width and maximum. Every indicator of one cosmonaut pilot study has changed drastically after LDSF. The new statistics provided appeared to be promising indicators of locomotor function state. A further trial of the new method may provide new knowledge of human movement strategies alterations after LDSF and may establish it as a standard analytical procedure for personalized flight assistance after its reliability verification.

New pull-in voltage modelling of step structure RF MEMS switch

In this paper, pull in voltage in closed form model is derived from the capacitance generated in RF MEMS switches during ON and OFF conditions. The proposed model contains the ligament efficiency term and can be applicable for all perforated RF MEMS switches. The proposed model is validated by computing error percentage between simulated and modelled values. The proposed switch with the non-uniform meander with dimensions tb=0.5 μm, td=0.1 μm, airgap=2 μm, Wb=120 μm, silicon nitride as a dielectric and gold as a beam material shows a very low error percentage of 3.9% and chosen as the best dimensions for proposed model of step structure RF MEMS switch.

Step Bunches, Nanowires and Other Vicinal “Creatures”—Ehrlich–Schwoebel Effect by Cellular Automata

Different patterns can be created on the surface of growing crystals, among which the step bunches and/or step meanders are two of the most studied. The Ehrlich–Schwoebel effect at the surface steps is considered one of the “usual suspects” of such patterning. A direct step barrier is when it is easier to attach a particle to the step from the lower terrace than from the upper terrace. Thus, during the process of crystal growth leads to the formation of meanders, while an inverse barrier leads to step bunching. Based on our vicinal Cellular Automaton model, but this time in (2 + 1)D, we show that the combination of a direct and inverse step barrier and the proper selection of the potential of the well between them leads to the formation of bunched step structures. Following this is the formation of anti-bands. In addition, changing the height of the direct step barrier leads to the growth of nanocolumns, nanowires, and nanopyramids or meanders, in the same system.

Flexible Wearable Composite Antennas for Global Wireless Communication Systems.

Although wearable antennas have made great progress in recent years, how to design high-performance antennas suitable for most wireless communication systems has always been the direction of RF workers. In this paper, a new approach for the design and manufacture of a compact, low-profile, broadband, omni-directional and conformal antenna is presented, including the use of a customized flexible dielectric substrate with high permittivity and low loss tangent to realize the compact sensing antenna. Poly-di-methyl-siloxane (PDMS) is doped a certain proportion of aluminum trioxide (Al2O3) and Poly-tetra-fluoro-ethylene (PTFE) to investigate the effect of dielectric constant and loss tangent. Through a large number of comparative experiments, data on different doping ratios show that the new doped materials are flexible enough to increase dielectric constant, reduce loss tangent and significantly improve the load resistance capacity. The antenna is configured with a multisection microstrip stepped impedance resonator structure (SIR) to expand the bandwidth. The measured reflection return loss (S11) showed an operating frequency band from 0.99 to 9.41 GHz, with a band ratio of 146%. The antenna covers two important frequency bands, 1.71–2.484 GHz (personal communication system and wireless body area network (WBAN) systems) and 5.15–5.825 GHz (wireless local area network-WLAN)]. It also passed the SAR test for human safety. Therefore, the proposed antenna offers a good chance for full coverage of WLAN and large-scale development of wearable products. It also has potential applications in communication systems, wireless energy acquisition systems and other wireless systems.

Design and optimization of stepped compound horn based on finite element method

A stepped composite horn structure with column holes at the front end is proposed for use in power ultrasonic fields. Based on the finite element method, the vibration characteristics and amplitude magnification of the horn at resonance frequency are calculated, Analyzed the influence of different structural parameters on its performance parameters, analyzed the influence of the aperture size on the resonance frequency of the horn, Optimize the design of the proposed composite horn to obtain the maximum amplitude magnification, Perform simulation analysis on the horn through ANSYS Workbench, extract the seventh-order modal results, and perform modal analysis. The simulation results show that: compared with the traditional horn, the maximum amplitude of the step compound-shaped horn is increased by 72%, The frequency of longitudinal vibration of this open-hole horn is 26.92KHZ. The performance parameters of the optimized horn have been significantly improved.

Microstructure Study and Morphology of Ni / Nano Al2O3 Composite Coating Synthesied by Electroless Plating

The electroless deposition method was used to prepare Ni-based nanocomposite coatings, this approach represents an alternate way of having coatings on the different substrates. Most of the previous literatures on this subject and work deals with a study the effect of the process conditions and, Bath composition on the microstructure at the macroscale of Ni-Al2O3 composite coatings. Though, effect on the step structure of the composition of the bath and microstructure are little, the present work aims to study the effect of hard ceramic Al2O3 nanoparticles at different concentrations (0.0, 0.5, 1.0, 2, 4) g/L, on the phase structure, microstructure and morphology of Al2O3 nanocomposite coating, in order to enhance the mechanical, plysical and chemical properties of nanocomposite coating. In this paper, An X-Ray diffraction method, spectroscopy (EDS), energy dispersive, and scanning electron microscope (SEM) were studied in the phase structure, chemical composition and morphological nanocomposition coatings. In the present paper, is evident from EDS study that the composite coating consists of Ni and nanoparticles of Al2O3. The micrograph study of the EDS showed that A flat and smooth surface is present in the deposited nanocomposite coating. Uniform distribution of nanoparticles of alumina within Ni-Matrix. And the XRD study showed the crystalline structure of the Ni- Al2O3 nanocomposite coating.

Design and Application of Waveguide Microwave Electron Synthesis Power Amplifier Using Er3+−Yb3+ Co-Doped Nanocrystalline

The solid-state power device has the advantages of small size, light weight and low working voltage, but the output power of the device is limited and cannot meet the engineering needs of power synthesis. Based on the characteristics of erbium (Er3+)-doped optical waveguide amplifier (EDWA), Er3+-ytterbium (YB3+) nanocrystalline were introduced into the design of solid-state power devices, and then nanoelectronic devices were designed to improve the output power of the device. First, NaYF4: 8%Er3+, 16%Yb3+ nanocrystalline were prepared. SU-8 UV glue was used to prepare NaYF4: Er3+. Yb3+ nanocrystalline doped polymer waveguide amplifier was designed and used to observe the waveguide morphology and its performance was tested. Then, the power synthesis amplification technology was studied, and the power distribution-synthesis network based on waveguide E–T junction was designed. The waveguide-microstrip transition structure adopted ridge waveguide transition. On this basis, the 4-stage waveguide E-plane stepped structure was introduced and its structure was simulated. In the design of the power synthesis amplifier, the signal gain of the synthesis amplifier was tested by using the nanocrystal-doped polymer waveguide amplifier as the power unit device. The experimental results show that the output signal light of the nanocrystal-doped polymer waveguide amplifier can be received by an infrared detector. When the pump light power at 980 nm is 200 mW and the signal light power is only 0.1 mW, the maximum relative gain is 4.7 dB in a 1.2 cm long waveguide. The saturated output power of the power unit device is 19.3 dB~19.8 dB, which is close to the saturated output power required by the chip manual. The gain curve of the power synthesis amplifier is relatively flat, that is, the use of nanocrystal-doped polymer waveguide electronic devices effectively improves the synthetic signal gain and reduces the adverse impact of power distribution-synthetic passive network loss.

Analytical breakdown voltage model for a partial SOI-LDMOS transistor with a buried oxide step structure

We have developed a simple physics-based two-dimensional analytical off-state breakdown voltage model of a partial buried oxide step structure (PBOSS) silicon-on-insulator laterally diffused metal oxide semiconductor (SOI-LDMOS) transistor. The analytical model includes the expressions of surface potential and electric field distributions in the drift region by solving the 2D Poisson equation. The electric field at the Si–SiO2 surface is modulated by additional electric field peaks developed due to the presence of the PBOSS structure. The uniformly distributed electric field results in improved breakdown voltage. Further, the breakdown voltage is analytically obtained by means of the critical electric field concept, to determine the breakdown characteristics. The model reveals the impact of the critical device design parameters such as thickness and length of the PBOSS structure, doping, and thickness of the drift region on the surface electric field and the breakdown voltage. The proposed model is verified by ATLAS two-dimensional simulations. The analytical model will be useful to design high-voltage SOI-LDMOS transistors for power switching applications.

Комплексные исследования фазосмещающих свойств ступенчатой структуры класса II на связанных линиях передачи с несогласованными нагрузками

Комплексные исследования фазосмещающих свойств ступенчатой структуры класса II на связанных линиях передачи с несогласованными нагрузками

Theoretical and experimental study on epitaxial growth of antiphase boundary free GaAs on hydrogenated on-axis Si(001) surfaces

The formation of double-layer atomic steps on Si(001) surfaces is an efficient way to eliminate the antiphase boundaries (APBs) on GaAs/Si(001) interfaces. The surface energy of on-axis Si(001) surfaces with different atomic step structures was calculated and analyzed from the first principles. An optimal hydrogen-annealing process condition, the hydrogen pressure of 800 mbar and the annealing temperature of 800 °C for 10 min, was obtained experimentally. Under this annealing condition, a 420 nm APB-free GaAs epitaxial layer grown on on-axis Si(001) substrates was achieved by metal-organic chemical vapor deposition. The effect of the annealing temperature on the APB density of the GaAs/Si(001) samples was explained from the aspects of thermodynamics and kinetics. It is of great significance to optimize the growth conditions of APB-free III–V epitaxial materials on on-axis Si(001) surfaces for the large-scale silicon monolithic integration of optoelectronic devices.

A Study on Mass Irregularity Limits of RC Structures for Various Seismic Zones of India

The performance of any structure under lateral loads depends on its structural configuration. Most of the codes worldwide mentioned the provisions for the lateral load analysis for regular structures. Structures may be irregular in practice due to uneven distributions in mass, stiffness and strength. In India, most common type of multi-storied apartments constructed varies from 5 to 15 storeys and many apartments are constructed with penthouse which creates mass irregularity in the structures. The present study investigates the performance of mass irregular structures which are created by providing penthouse and also modeled as a stepped setback structure. Total four structures, 5, 7, 11 and 16 storied with an aspect ratio (height to width of the structure) of 0.64, 0.88, 1.36 and 1.96 are considered. For each structure response spectrum analysis is carried and the response parameters like storey displacements, storey drift and base shear variation in 4 seismic zones of India is studied. The main focus of the investigation is to understand the acceptance of mass irregularity with respect to the aspect ratio of the structure. The acceptance limit of the mass irregularity is also investigated as per 4 seismic zones of India. The results are compared with Indian standard code limits and concluded the acceptance limit as per seismic zones of India.

Impact of rainfall spatiotemporal variability and model structures on flood simulation in semi-arid regions

The spatiotemporal heterogeneity in precipitation and underlying surfaces and hybrid runoff generation mechanism make hydrological modelling and forecasting in semi-arid regions becoming a challenging work. Therefore, to provide a reference for the development of hydrological models in such regions, two nested hydrological experimental watersheds in semi-arid regions were selected for attribution analysis. Based on the concept of large-sample hydrology, large-scale numerical simulation experiments were performed by constructing different spatial and temporal scale rainfall schemes and combining three hydrological models with different runoff generation mechanisms. Finally, the influences of the time step, station density, and model structure on the flood simulations in semi-arid regions were evaluated. The spatial interpolation technique was used simultaneously to describe the high-dimensional complicated nonlinear relationships between the influencing factors and simulation results. The results show the following: (1) the flood simulation accuracy was more sensitive to the time step than the spatial station density of the rainfall schemes and was highly dependent on the time step of the original observation data, and (2) compared with the accuracy of the rainfall schemes, the model structure plays a dominant role in flood simulation accuracy. Thus, the hybrid model has significant potential for flood forecasting in semi-arid regions by combining different runoff generation mechanisms. (3) The spatial interpolation technique based on the k-nearest neighbour algorithm can construct a high-dimensional distribution between the influencing factors and model simulation accuracy, and describe the complicated relationships among the time step, station density, model structure, and flood simulations.

How Does the Step on Graphite Surface Impact Ice Nucleation?

Heterogeneous ice nucleation has always been a hot research topic because of its fundamental importance to a variety of research areas, from climate to biology and from aviation to the energy industry. The formation of ice in the earth’s atmosphere depends on aerosols of various sources. As the main component of atmospheric aerosols, carbon particles composed of graphite have attracted much attention in heterogeneous ice nucleation. There is great variability in the freezing efficiency of ice induced by surface nanostructures, and the presence of surface defects, such as steps, can lead to the complex behavior of ice nucleation. In this work, molecular dynamics (MD) simulation was employed to investigate ice nucleation on graphite surfaces with the step structure. It was found that the graphite step with a real atomic structure reduced the freezing efficiency of surfaces, which was attributed to the large interfacial free energy between ice and the side face of the graphite steps. It was demonstrated by further investigations that the effect of nanogrooves consisting of step edges on ice nucleation was not only determined by the matching of groove width and ice lattice constant but also by the atomic structure of nanogrooves, shedding light on the study of control strategy of ice nucleation by surface nanostructures.

Carrier-envelope phase dependence of high-order above-threshold ionization by few-cycle laser pulses

With a stereo above-threshold ionization (ATI) apparatus, we systematically investigate the carrier-envelope phase (CEP) dependence of the photoelectron kinetic energy spectra from high-order ATI of Xe atom subject to a strong few-cycle laser field. Our measurements reveal a prominent step structure and a maximum in the photoelectron kinetic energy dependence of the yield asymmetry. The observed structures can be well reproduced by the numerical solution of time-dependent Schrödinger equation (TDSE) and the improved strong field approximation (ISFA) theory. According to our analysis, the step structure can be ascribed to the interference of long and short orbits, while the maximum is closely related to the distinct kinetic energy distributions of photoelectrons which elastically scatter on the core at odd and even returns. Our work provides a more comprehensive understanding of the CEP effects on high energy photoelectron emissions in a few-cycle laser field.