Step Junction Research Articles

A comprehensive study of InGaAs based linearly graded channel DL-TFET and its single event transient effect

In this paper, the DC, RF and single-event transient effect studies are carried out for investigating the performance and reliability of hetero-junction InGaAs DL-TFET having linearly graded channel region. The mole-fraction of source side In1−xGaxAs is varied for obtaining the higher ION i.e. obtained at x = 0.40; this also impact the RF simulation studies by increasing the gain-bandwidth product range to 52.4 GHz. To reduce Iambipolar, a step junction is introduced for first time in the channel region at channel-drain interface which reduces the Iambipolar by 93.27% to the magnitude of 0.867 μA μm−1. Further reduction in ambipolarity is achieved at TG = 3.9 eV to the level of 0.269 fA μm−1, ION increases to 0.558 mA μm−1, sub-threshold swing is 23.8 mV/decade and shows highest cut-off frequency upto 155 GHz. Hence, the optimized device structure is investigated under the influence of single-event transient; therefore, the effect of heavy ion is investigated on optimized linearly graded structure for different LETs’ (Linear energy transfer). It shows increase in the drain current and charge collected with increase in LET. The maximum IDS is 0.618 mA μm−1 in OFF state, and IDS of 0.819 mA μm−1 in ON state at VDS = 1.0 V, which is higher than its ION at 10 MeV.cm2/mg (i.e. 0.11 pC μm−1). By simulating heavy ion single event effect on different positions, it is found that channel-drain interface is more sensitive to irradiation. This gives an insight for radiation hardened applications in space for TFET based circuit designs.

Controlled Excitation of Waveguide High-Order Modes for a Simple and Accurate Monopulse Tracking System Test Bench

Monopulse tracking systems require a calibration procedure, which is typically performed using a calibration tower or a known satellite, leading to high costs, antenna downtime, and potential inaccuracies. This work proposes a simple circular waveguide component, based on a step junction, able to excite high-order modes (HOMs) in a controlled way, so as to mimic the modes generated by the tracking antenna. This allows testing in laboratory the tracking system, avoiding the drawbacks of the traditional techniques. The theoretical working principle of the step-waveguide junction is discussed and an analytical model for the quick calculation of the offset between the two waveguides to achieve a desired HOM excitation coefficient is derived. Then, a Ku -band mode exciter is designed and its effectiveness is proved in conjunction with a designed TE21 tracking coupler. Finally, the fabrication and experimental validation of a prototype is reported, demonstrating the claimed performance.

Charge-Plasma-Based Doping-Less Vertical Thyristor for Highly Integrated Volatile Memory-Cell

Conventional dynamic random-access memory (DRAM) has been facing a severe challenge to scale down to 10 nm size. Since the cell capacitor should be able to store sufficient charges of 25~30 fF/cell, high aspect ratio of cell capacitor is inevitable, resulting in capacitors leaning into each other. To overcome this issue coming from capacitor, the two-terminal vertical thyristor-based capacitorless memory was proposed as a candidate to replace current DRAM, which consists of n++-emitter/p+-base/n+-base/p++-emitter vertical structure using conventional Si technology. The two-terminal vertical thyristor-based capacitorless memory cell having n++-emitter/p+-base/n+-base/p++-emitter structure utilizes latch-up and -down features showing bi-stable current-voltage (I-V) characteristics, so that data 0 (D0) and data 1 (D1) states can be distinguished. Nevertheless, in two-terminal vertical thyristor-based capacitorless memory, in-situ doped epitaxial growth process of silicon layers is essential. As is well known, the major disadvantage of epitaxial growth process is low throughput ratio. Besides, it is difficult to form step junctions since the dopants diffuse out from their original position during the serial epi-processes due to the high process temperature. Diffusion of dopants near the junctions is unavoidable in conventional doped thyristor memory cell fabricated with epitaxial growth, as shown in SIMS profile of Fig. 1(a). By applying the concept of charge plasma, the novel doping-less vertical thyristor as volatile memory is proposed, as shown in Fig. 1(b). In contrast with conventional doped thyristor memory cells, doping-less thyristor memory cells have step junction, as shown in carrier concentration profile of Fig. 1(c). Consequently, the vertical energy band diagram for doping-less thyristor memory cells differ from that of doped thyristor memory cells, as shown in Fig. 1(d). In addition, the conventional thyristor memory cells have been suffered from off-state leakage current because of the misfit dislocations at the highly doped junctions. On the other hand, the doping-less thyristor memory cells are misfit dislocation-free as their junctions are formed by the charge plasma, not the dopants. Moreover, the coulomb scattering can be dramatically reduced in doping-less thyristor memory cells owing to the absence of dopant and space charge, leading to the higher carrier mobility. As a result, the doping-less thyristor memory cells have lower off-state leakage current and higher on-state current, as shown in the I-V curves of Fig. 1(e) and (f). In this work, the effect of gap oxide thickness, which decides the intrinsic layer thickness, on latch-up voltage was investigated as well. In addition, the impact of p- and n-base metal work-function on the latch-up voltage of doping-less thyristor memory cell was investigated. Finally, the optimized metal work-function and gap oxide thickness for the memory cell were discussed. Acknowledgment* This research was supported by Brain Korea 21 PLUS Program in 2020, the MOTIE (Ministry of Trade, Industry & Energy 10069063) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device. Figure 1

Enhancement of critical current density in a superconducting NbSe2 step junction.

We investigate the transport properties of a NbSe2 nanodevice consisting of a thin region, a thick region and a step junction. The superconducting critical current density of each region of the nanodevice has been studied as a function of temperature and magnetic field. We find that the critical current density has similar values for both the thin and thick regions away from the junction, while the critical current density of the thin region of the junction increases to approximately 1.8 times as compared with the values obtained for the other regions. We attribute such an enhancement of critical current density to the vortex pinning at the surface step. Our study verifies the enhancement of the critical current density by the geometrical-type pinning and sheds light on the application of 2D superconductors.

Surface Morphology of the Interface Junction of CVD Mosaic Single-Crystal Diamond.

The diamond mosaic grown on the single-crystal diamond substrates by the microwave plasma chemical vapor deposition (MPCVD) method has been studied. The average growth rate was about 16–17 μm/h during 48 hours’ growth. The surface morphologies of the as-grown diamond layer were observed. It was found that the step flow was able to move across the substrates and cover the junction interface. Raman spectroscopic mapping in the central area of the junction revealed the high stress region movement across the junction interface from one substrate to the other for about 200–400 μm. High-resolution X-ray diffractometry (HRXRD) results proved that the surface step flow movement direction had nothing to do with the off-axis directions of the original substrates. It was found that the surface height difference of substrate was the main driving force for the step flow movement, junction combination and surface morphology changing. The mechanism of the mosaic interface junction combination and step flow transformation on the mosaic surface was proposed.

Mode Matching Analysis of Waveguide Components Exploiting the Variational Meshless Method

This letter presents the analysis of waveguide components comprising a cascade of step junctions by mode matching in conjunction with the variational meshless method (VMM). In particular, a large number of modal fields of the waveguides are calculated very efficiently by the VMM. Moreover, the collocation points are randomly distributed in each waveguide cross section, and the coupling coefficients are obtained regardless of their mutual distribution. The presented technique is validated through the analysis of two junctions reported in the literature and by comparison with an finite element method (FEM) full-wave software.

Unidirectional Self-Driving Liquid Droplet Transport on a Monolayer Graphene-Covered Textured Substrate.

Controllable directional transport of liquid droplets on a functionalized surface has been a challenge in the field of microfluidics because it does not require energy supply, and the physical mechanism of such self-driving transport exhibits extraordinary contribution to fundamental understanding of some biological processes and the design of microfluidic apparatus. In this paper, we report a novel design of a surface microstructure that can realize unidirectional self-driving liquid mercury (Hg) droplet transport on a graphene-covered copper (Cu) substrate with a three-dimensional surface microstructure. We have demonstrated that a liquid Hg droplet spontaneously propagates on a grooved Cu substrate covered by a monolayer graphene without any external force fields. Classical molecular dynamics results provide a profound insight on the self-driving process of Hg droplets. It shows that the Hg droplet undergoes acceleration, deceleration, and return stages successively from the narrow to wide ends of the gradient groove. Intriguingly, Hg droplets can move continuously and unidirectionally on the three-dimensional graphene-covered surface microstructure when they accumulate enough kinetic energy from the gradient groove to break the energy barrier at the step junctions between the two neighboring unit cells. The design of the zigzag textured surface covered by a monolayer graphene artfully uses the facts; (1) the monolayer graphene can effectively reduce the droplet pinning on the textured surface, (2) the hydrophobic graphene layer reduces the friction between Hg droplets and the substrate, and (3) the textured surface can permeably interact with the droplets through the monolayer graphene to achieve a continuous self-driving process. The findings reported here open a door to explore the graphene-covered functional surface to directional transport of liquid droplets and provide an in-depth understanding of the self-driving mechanism for liquid droplets on graphene-covered textured substrates.

Thermal conductivity variation of Bi2Te3 nanofilm with interfacial defects using molecular dynamics

In this study, the thermal conductivity and thermal boundary resistance (TBR) of Bi2Te3 nanofilms with different interfacial defects were investigated using nonequilibrium molecular dynamics simulations, and the effects of temperature, defects (step junctions and grooves), and interfaces (amorphous and telluride) were assessed. The results show a strong temperature dependence of the thermal conductivity for Bi2Te3 nanofilms with an ideal structure; moreover, as the height of step-junction defects increased, the thermal conductivity decreased, exhibiting a linear dependency. In addition, the thermal conductivity gradually decreased by 36%–40% as the width of the interface defects increased. We also verified the self-assembly mechanism for nanoscale Bi2Te3 and found that the Bi2Te3–Te interface induces strong phonon scattering. In addition, the TBR decreased as the width of the amorphous or Te interface increased. Thus, interfacial defects in Bi2Te3 nanofilms affect the thermal conductivity and TBR. The results of this study may be useful for optimizing Bi2Te3 thermoelectric devices in the future.

Thermal transport across graphene step junctions

Step junctions are often present in layered materials, i.e. where single-layer regions meet multilayer regions, yet their effect on thermal transport is not understood to date. Here, we measure heat flow across graphene junctions (GJs) from monolayer-to-bilayer graphene, as well as bilayer to four-layer graphene for the first time, in both heat flow directions. The thermal conductance of the monolayer-bilayer GJ ranges from ~0.5 to 9.1 × 108 W m−2 K−1 between 50 K to 300 K. Atomistic simulations of such a GJ device reveal that graphene layers are relatively decoupled, and the low thermal conductance of the device is determined by the resistance between the two distinct graphene layers. In these conditions the junction plays a negligible effect. To prove that the decoupling between layers controls thermal transport in the junction, the heat flow in both directions was measured, showing no evidence of thermal asymmetry or rectification, within experimental error bars. For large-area graphene applications, this signifies that small bilayer (or multilayer) islands have little or no contribution to overall thermal transport.

Modeling of Graphene Coplanar Waveguide and its Discontinuities for THz Integrated Circuits Applications

Graphene, having extraordinary electronic and optical properties at THz frequencies, can be used in the design and development of miniaturized components, devices, and integrated circuits. Model of graphene-based transmission lines and their discontinuities play a crucial role in the realization of THz integrated circuits, which is not readily available in the literature. This paper reports the characteristics of graphene film deposited over a SiO2/Si wafer. Transmission line characteristics of graphene-based plasmonic coplanar waveguide (GPCPW) on SiO2/Si wafer have been determined from the electromagnetic modeling of the structure. Discontinuities in the GPCPW such as open end, series gap, and step junction have also been analyzed to obtain their lumped element electrical equivalent. Closed-form expressions are developed so that it can be used in the circuit design at later stage. It has been observed that GPCPW transmission lines have very high characteristic impedance values (∼1000–2500 Ω) which are not possible in non-graphene transmission line structures.

Suppression of Andreev conductance in a topological insulator–superconductor nanostep junction**Project supported by the National Natural Science Foundation of China (Grant Nos. 11204065 and 11474085) and the Natural Science Foundation of Hebei Province, China (Grant Nos. A2013205168 and A2014205005).

When two three-dimensional topological insulators (TIs) are brought close to each other with their surfaces aligned, the surfaces form a line junction. Similarly, three TI surfaces, not lying in a single plane, can form an atomic-scale nanostep junction. In this paper, Andreev reflection in a TI–TI–superconductor nanostep junction is investigated theoretically. Because of the existence of edge states along each line junction, the conductance for a nanostep junction is suppressed. When the incident energy (ɛ) of an electron is larger than the superconductor gap (Δ), the Andreev conductance in a step junction is less than unity while for a plane junction it is unity. The Andreev conductance is found to depend on the height of the step junction. The Andreev conductance exhibits oscillatory behavior as a function of the junction height with the amplitude of the oscillations remaining unchanged when ɛ = 0, but decreasing for ɛ = Δ, which is different from the case of the plane junction. The height of the step is therefore an important parameter for Andreev reflection in nanostep junctions, and plays a role similar to that of the delta potential barrier in normal metal–superconductor plane junctions.

How planar optical waves can be made to climb dielectric steps.

We show how to optically connect guiding layers at different elevations in a 3-D integrated photonic circuit. Transfer of optical power carried by planar, semi-guided waves is possible without reflections or radiation losses, and over large vertical distances. This functionality is realized through simple step-like folds of high-contrast dielectric slab waveguides, in combination with oblique wave incidence, and fulfilling a resonance condition. Radiation losses vanish, and polarization conversion is suppressed for TE wave incidence beyond certain critical angles. This can be understood by fundamental arguments resting on a version of Snell's law. The two 90° corners of a step act as identical partial reflectors in a Fabry-Perot-like resonator setup. By selecting the step height, i.e., the distance between the reflectors, one realizes resonant states with full transmission. Rigorous quasi-analytical simulations for typical silicon/silica parameters demonstrate the functioning. Combinations of several step junctions can lead to other types of optical on-chip connects, e.g., U-turn- or bridge-like configurations.

New improved capacitance–voltage model for symmetrical step junction: a way to a unified model for realistic junctions

Based on the mass-action law, a new capacitance---voltage (C---V) model for symmetrical step junction is presented. Furthermore, we propose a unified C---V model for realistic junction and for any applied-voltage.

Josephson effect in normal and ferromagnetic topological-insulator junctions: Planar, step, and edge geometries

We investigate Josephson junctions on the surface of a three-dimensional topological insulator in planar, step, and edge geometries. The elliptical nature of the Dirac cone representing the side surface states of the topological insulator results in a scaling factor in the Josephson current in a step junction as compared to the planar junction. In edge junctions, the contribution of the Andreev bound states to the Josephson current vanishes due to spin-momentum locking of the surface states. Furthermore, we consider a junction with a ferromagnetic insulator between the superconducting regions. In these ferromagnetic junctions, we find an anomalous finite Josephson current at zero phase difference if the magnetization is pointing along the junction (and perpendicular to the Josephson current). An out-of-plane magnetization with respect to the central region of the junction opens up an exchange gap and leads to a non-monotonic behavior of the critical Josephson current for sufficiently large magnetization as the chemical potential increases.

Direct-current sputter epitaxy of Si and its application to fabricate n+-emitters for crystalline-Si solar cells

Direct-current sputter epitaxy of Si on a Si(001) substrate was realized at 175 °C at a growth rate of 3.3 nm·s−1 and was applied to form n+-emitters of crystalline-Si solar cells. A solar cell with a 50-nm-thick n+-emitter exhibited a short current density of 23.8 mA·cm−2 owing to an increased internal quantum efficiency at wavelengths between 400 and 600 nm. The improved efficiency was due to the step junction characteristics of the epitaxially grown n+-emitter exhibiting a better response at short wavelengths, a performance better than that of the graded junction formed by thermal diffusion.

A Working of Eddy Current Transducers under Control of Ferromagnetic Pipes in Condition of Insufficient Magnetic Saturation

The numerical method of calculation of electromagnetic field in a control sample at a time of work of encircling eddy current transducer is offered. The method is based on a numerical solution of the two Maxwell equations, which connect a change of electrical and magnetic fields. It allows to make calculations taking into account the actual value of magnetic inductivity of metal and to get results in any form, convenient for further interpretations. For the calculation of the encircling eddy current transducer the equivalent circuit of defect as step junction is offered. The numerical calculation shows, that the greatest sensitivity of the transducer is achieved when the value of magnetic permeability of the object of control is approximately 10-30. Therefore, it is not necessary to lead the material of the object of control up to satiety, as it considered before.

Compact High-Precision Models for Silicon p-n Step Junction Avalanche-Breakdown Voltages

Advanced silicon impact-ionization models are linked with simple accurate 1-D breakdown-voltage formulas for p-n step junctions. The models are useful for ab initio calculations and improve the accuracy of older models due to a new parameterization based on a numerical simulation. New parameters for effective impact-ionization coefficients are presented for Chynoweth and Fulop formulations. The parameter sets are based on impact-ionization models of Van Overstraeten and De Man and of the Universita di Bologna, including its recent updates. They allow quick computing of silicon p-n junction breakdown voltages from 10 V to 10 kV with maximum deviation from numerical data of ±5%. The new breakdown-voltage models account for the junction temperature and the depletion on both sides of the p-n junction.

New type of eigenoscillations and total-transmission resonance through an iris with below-cutoff hole in a rectangular waveguide

The total-transmission phenomenon of the main mode of a rectangular waveguide through a below-cutoff hole in the waveguide shorting before the point of occurrence of the higher-order waveguide modes is found. The effect is due to generation of high-Q eigenoscillations at the evanescent E11 mode and is originated from a specific oscillation formed near a step junction of two waveguides, namely a larger single-mode waveguide and a smaller below-cutoff one. The iris composed of such two junctions is longitudinally symmetric. Thus, a pair of resonances occurs in accordance with the existence of a pair of symmetric and antisymmetric field distributions at the iris. Analogy with the recently found effects in double-periodic gratings with small holes is revealed.

MODAL SOLUTIONS FOR JUNCTION PARAMETERS OF DISCONTINUITY PROBLEMS IN DIELECTRIC RECTANGULAR WAVEGUIDES

In this paper the discontinuity problems of the junction of two different dielectric rectangular waveguides has been studied, both for two dimensional (2D) and three dimensional (3D) cases. The technique used, is to obtain expressions for the z-directed complex power due to all modes (propagating and non propagating) present at the step junction for a normalized incident field. The expressions for junction parameters like admittance and susceptance have been derived for the structures with step junctions in x direction, in y direction and for the 3D case where the step is both x and y directed. The numerical results have then been computed for different step ratios of these three cases.

Calculation of cut-in profile cutters using KOMPAS 3D V8 graphic software

Owing to progress in continuous path machining of profiled parts on lathe machines with numerical program control, application of profile cutters for cutting parts of the rotary-body type has shrunk. However, there is no alternative to such cutters when cutting parts with a complex profile (step junctions with kinks and short segments of straight lines, circular arcs, and other curves) that cannot be produced by continuous path machining. Profile cutters have a number of additional advantages: a long service life, which is possible thanks to a large number of resharpening cycles; the exact profile of a part is ensured for the same initial setting with a resharpened cutter during its entire service life; and the possibility to use one cutter instead of several sequentially operating cutters machining segments of a complex profile that require coordinated setting during initial configuring, resharpening, and replacement, thus, resulting in a reduced number of changeovers and toolheads required for automatic lathe machines. All of these advantages can only be realized if cutters are automatically designed by a computer program removing the problems related to the selection of a cutter’s design that correspond to the specific features of a part’s profile surface. The variety of profile-cutter designs is explained by the following factors: the different location of the profiled surface on a part (internal, external, end face), the different location of the cutter with respect to a part owing not only to the previous reason but also due to additional turns and tilts needed to increase the back rake angle, the different shapes of the profiled flank surface of cutters (prismatic, spiral, or body of rotation), and the different positions of a cutter’s face surface (below profile cutters with a flat face surface are considered). The latter feature ensures obtainment of optimal front rake angles and easy installation of the cutter on a machine. When designing a cutter, to diminish the spread of the back rake angle, it is desirable to have not only those points of the cutting edge (CE) that are located on the height of a part’s axis and below it (but not higher) but also to ensure a possibly small range of the heights of the CE points. In addition, when machining conic segments of a profile for a part where strict linearity of the generatrix is required, the cutter face has to cross the surface of such a segment along the generatrix. In a standard approach, for each such specific case, an algorithm for calculation of the cutter profile is created (which is called the adjustment calculation); however, such algorithms are not always available, so a designer often has to develop the required algorithm independently. The solution to this problem usually consists of obtaining a sequence of formulas derived on the basis of geometric relations. If a more universal calculation program integrated in modern graphic packages is developed, method [1] is more convenient. This method, which is called the coordinate one, includes the following steps. 1. The position of a cutter’s front face is set in the part’s coordinate frame Σ p ( X p , Y p , Z p ) (Fig. 1) using a reference point, a reference line (through which the front plane passes), and the face angle set in the plane perpendicular to the cutter’s base 1 or in the part’s faceend plane (for prismatic cutters with back slope of the frame), i.e., the cutter face is set in the standard way. The cutter’s front face is oriented by the designer (program user) regardless of the cutter’s position.