Function Of Defect Density Research Articles

Carbon Vacancy Assisted Contact Resistance Engineering in Graphene FETs

Despite various remarkable properties, the state of the arts of graphene devices are still not up to the mark due to their high contact resistance. The contact resistance milestone has not been achieved yet, probably due to ambiguity in understanding graphene–metal contact properties. In this work, we did a systematic investigation of palladium–graphene contact properties using a density functional theory (DFT) and various process-based experimental approaches. Our study reveals significant interaction of palladium (Pd) with graphene. Their orbitals overlap leads to potential barrier lowering at the interface, which can be reduced further by bringing graphene closer to the bulk Pd using carbon vacancy engineering at the contacts. Thus, the carbon vacancy-assisted barrier modulation reduces contact resistance by increasing carrier transmission probabilities at the interface. The theoretical findings have been emulated experimentally by carbon vacancy engineering at the graphene field-effect transistors (FETs). Different contact-engineered graphene devices with Pd contacts show significant contact resistance reduction, measuring as low as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 78~\Omega ~\cdot ~\mu \text{m}$ </tex-math></inline-formula> at room temperature. The contact resistance shows a “V” shape curve as a function of defect density. Also, the optimum contact resistance achieved is significantly lower than their pristine counterpart, as predicted by the theoretical estimates. Due to contact engineering, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I_{{ \mathrm{\scriptscriptstyle ON}}}}$ </tex-math></inline-formula> improves by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 6\times $ </tex-math></inline-formula> , transconductance by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 8\times $ </tex-math></inline-formula> , and device mobilities by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 6\times $ </tex-math></inline-formula> in the device FETs. These investigations and understanding can help to boost the performance of graphene FETs, especially for high-frequency RF applications.

Influence of Nanosize Hole Defects and their Geometric Arrangements on the Superfluid Density in Atomically Thin Single Crystals of Indium Superconductor.

Using Indium sqrt[7]×sqrt[3] on Si(111) as an atomically thin superconductor platform, and by systematically controlling the density of nanohole defects (nanometer size voids), we reveal the impacts of defect density and defect geometric arrangements on superconductivity at macroscopic and microscopic length scales. When nanohole defects are uniformly dispersed in the atomic layer, the superfluid density monotonically decreases as a function of defect density (from 0.7% to 5% of the surface area) with minor change in the transition temperature T_{C}, measured both microscopically and macroscopically. With a slight increase in the defect density from 5% to 6%, these point defects are organized into defect chains that enclose individual two-dimensional patches. This new geometric arrangement of defects dramatically impacts the superconductivity, leading to the total disappearance of macroscopic superfluid density and the collapse of the microscopic superconducting gap. This study sheds new light on the understanding of how local defects and their geometric arrangements impact superconductivity in the two-dimensional limit.

Effect of Defective Microstructure and Film Thickness on the Reflective Structural Color of Self-Assembled Colloidal Crystals.

Structural color arises from geometric diffraction; it has potential applications in optical materials because it is more resistant to environmental degradation than coloration mechanisms that are of chemical origin. Structural color can be produced from self-assembled films of colloidal size particles. While the relationship between the crystal structure and structural color reflection peak wavelength is well studied, the connection between assembly quality and the degree of reflective structural color is less understood. Here, we study this connection by investigating the structural color reflection peak intensity and width as a function of defect density and film thickness using a combined experimental and computational approach. Polystyrene microspheres are self-assembled into defective colloidal crystals via solvent evaporation. Colloidal crystal growth via sedimentation is simulated with molecular dynamics, and the reflection spectra of simulated structures are calculated by using the finite-difference time-domain algorithm. We examine the impact of commonly observed defect types (vacancies, stacking fault tetrahedra, planar faults, and microcracks) on structural color peak intensity. We find that the reduction in peak intensity scales with increased defect density. The reduction is less sensitive to the type of defect than to its volume. In addition, the reflectance of structural color increases as a function of the crystal thickness, until a plateau is reached at thicknesses greater than about 9.0 μm. The maximum reflection is 78.8 ± 0.9%; this value is significantly less than the 100% reflectivity predicted for a fully crystalline, defect-free material. Furthermore, we find that colloidal crystal films with small quantities of defects may be approximated as multilayer reflective materials. These findings can guide the design of optical materials with variable structural color intensity.

Probing Electrochemical Structure-Property Relationships at Non-Porous Monolayer Electrodes of Exfoliated Graphene and MoS2 Single Layers

The electrochemically available surface area and porosity of 2D materials is difficult to control due to the strong aggregation and restacking phenomena that occur when single layers are processed into thin or thick film electrodes. Furthermore, the distributed resistance, the potential for pore-size related effects and the uncertainty in relating estimated surface area and porosity data to the actual electrochemically accessible surface area make measurements of the intrinsic electrochemical performance of these materials difficult to assess. Ideally, electrochemical measurements are carried out on flat, polished or atomically flat electrode surfaces for the evaluation of intrinsic double-layer capacitance and electrochemical rate constants. In this talk, we will discuss our efforts to create large-area monolayers of solution exfoliated 2D materials for electrochemical evaluation in aqueous and non-aqueous electrolytes. Using a modified Langmuir-Blodgett technique, we demonstrate the ability to create uniform and large area monolayers of graphene oxide, thermally reduced graphene oxide as well as 1T and 2H molybdenum disulfide which can be transferred to atomically flat electrode substrates such as highly oriented pyrolytic graphite. These films are found to be stable during electrochemical cycling in various electrolytes and allow us to probe electrochemical properties as a function of 2D material structure without porosity-related artifacts. This model system has been used to probe the changes in double-layer capacitance in both aqueous and non-aqueous electrolytes for graphene-based materials as a function of defect density. It has allowed us to estimate the theoretical limits of graphene-based supercapacitors as a function of graphene “type”. More recently, this analysis has been extended to probe the intrinsic capacitance of the metallic 1T and semiconducting 2H polymorphs of MoS2 as a function of layer number to probe the charging mechanisms at play. The system also offers the ability to probe the capacitance of graphene/MoS2 heterostructures. These and future studies are expected to provide valuable insight into how we might build more energy dense supercapacitors from the optimal combination of 2D materials.

Variation of optical bandwidth in defected ternary photonic crystal under different polarisation conditions

Photonic bandwidth of defected one-dimensional ternary photonic crystal is analytically calculated under both types of oblique incidences. Both P and S type polarised incidences on the structure are considered in order to compute transmittivity of the proposed Butterworth type bandpass filter, and passband spectrum is kept at 1,550 nm by suitably choosing structural parameters and defect density. Defect is kept within feasible limit, and ripple in the desired passband region is tailored by its controlled variation. Width of passband as function of defect density, angle of incidence and dimension of constituent layers is computed, and critical dimension for the sandwiched layer is predicted over which the structure will behave as equivalent binary one. For closely-spaced signal transmission, this narrow bandpass filter will work as excellent candidate to improve SNR.

Variation of optical bandwidth in defected ternary photonic crystal under different polarisation conditions

Photonic bandwidth of defected one-dimensional ternary photonic crystal is analytically calculated under both types of oblique incidences. Both P and S type polarised incidences on the structure are considered in order to compute transmittivity of the proposed Butterworth type bandpass filter, and passband spectrum is kept at 1,550 nm by suitably choosing structural parameters and defect density. Defect is kept within feasible limit, and ripple in the desired passband region is tailored by its controlled variation. Width of passband as function of defect density, angle of incidence and dimension of constituent layers is computed, and critical dimension for the sandwiched layer is predicted over which the structure will behave as equivalent binary one. For closely-spaced signal transmission, this narrow bandpass filter will work as excellent candidate to improve SNR.

Extended Tersoff potential for boron nitride: Energetics and elastic properties of pristine and defective h -BN

We present an extended Tersoff potential for boron nitride (BN-ExTeP) for application in large scale atomistic simulations. BN-ExTeP accurately describes the main low energy B, N, and BN structures and yields quantitatively correct trends in the bonding as a function of coordination. The proposed extension of the bond order, added to improve the dependence of bonding on the chemical environment, leads to an accurate description of point defects in hexagonal BN $(h$-BN) and cubic BN $(c$-BN). We have implemented this potential in the molecular dynamics LAMMPS code and used it to determine some basic properties of pristine 2D $h$-BN and the elastic properties of defective $h$-BN as a function of defect density at zero temperature. Our results show that there is a strong correlation between the size of the static corrugation induced by the defects and the weakening of the in-plane elastic moduli.

The photoelastic coefficient {P}_{12} of H+ implanted GaAs as a function of defect density

The photoelastic phenomenon has been widely investigated as a fundamental elastooptical property of solids. This effect has been applied extensively to study stress distribution in lattice-mismatched semiconductor heterostructures. GaAs based optoelectronic devices (e.g. solar cells, modulators, detectors, and diodes) used in space probes are subject to damage arising from energetic proton (H+) irradiation. For that reason, the effect of proton irradiation on photoelastic coefficients of GaAs is of primary importance to space applied optoelectronics. However, there yet remains a lack of systematic studies of energetic proton induced changes in the photoelastic properties of bulk GaAs. In this work, the H+ energy and fluence chosen for GaAs implantation are similar to that of protons originating from the radiation belts and solar flares. We present the depth-dependent photoelastic coefficient {P}_{12} profile in non-annealed H+ implanted GaAs obtained from the analysis of the time-domain Brillouin scattering spectra. The depth-dependent profiles are found to be broader than the defect distribution profiles predicted by Monte Carlo simulations. This fact indicates that the changes in photoelastic coefficient {P}_{12} depend nonlinearly on the defect concentrations created by the hydrogen implantation. These studies provide insight into the spatial extent to which defects influence photoelastic properties of GaAs.

Dependability of the Exemplary Technical System for Assumed Functions of Defect Density

Abstract The analysis of structural dependability of technical system, especially determining the change in dependability over time, requires knowledge on density function or the understanding of cumulative distribution function of components belonging to the structure. Based on previously registered data concerning component defect, it is relatively easy to establish the average uptime of component as well as the standard deviation for this time. However, defining distribution shape gives rise to some difficulties. Usually, we do not have the sufficient number of data at our disposal to verify the hypothesis regarding the distribution shape. Due to this fact, it is a common practice, depending on the case under consideration, to apply the function of defect density. However, the question arises: Does the incorrect determination of types of distributions of components leads to the big error of estimation results of dependability and system durability? This article will not respond to this question in whole, but one will conduct a comparison of calculation results for a few cases. The calculations were conducted for the exemplary technical system.

Increasing the active surface of titanium islands on graphene by nitrogen sputtering

Titanium-island formation on graphene as a function of defect density is investigated. When depositing titanium on pristine graphene, titanium atoms cluster and form islands with an average diameter of about 10 nm and an average height of a few atomic layers. We show that if defects are introduced in the graphene by ion bombardment, the mobility of the deposited titanium atoms is reduced and the average diameter of the islands decreases to 5 nm with monoatomic height. This results in an optimized coverage for hydrogen storage applications, since the actual titanium surface available per unit graphene area is significantly increased.

Conductivity enhancement of yttria doped ceria by spark plasma and ac assisted sintering methods

CeO2 doped with lower valence cations, such as Y3+, Sm3+, Gd3+, etc., is considered as electrolyte material for low temperature solid oxide fuel cells. Doping introduces additional oxygen vacancies and improves ionic conductivity. This work describes the preparation of yttria–calcia doped ceria precursor material, its sintering in DC and AC field methods, and characterisation of sintered electrolytes with respect to the ionic conductivity. Doped precursor materials were prepared by a controlled co-precipitation process. Sintering of prepared, pelletised particulates was carried out using three different methods such as: conventional, spark plasma sintering, and an alternating current field at 1125°C (AC assisted sintering). The conductivity was determined as a function of defect density, grain boundary area and phase stability. It was found that the applied field during sintering, either via pulsed DC (spark plasma), or AC, produced marginal effects on yttria doped ceria microstructure. However, it should be noted that because the sintering conditions were not optimised there is room for substantial conductivity improvements.On considère le CeO2 dopé avec des cations à valence plus faible, comme le Y3+, le Sm3+, le Gd3+, etc., comme matériau électrolyte pour les piles à combustible d’oxyde solide à basse température. Le dopage introduit des lacunes additionnelles d’oxygène et améliore la conductivité ionique. Ce travail décrit la préparation de matériau précurseur d’oxyde cérique dopé à l’oxyde d’yttrium-calcia, son frittage par des méthodes de champ CD et CA et la caractérisation des électrolytes frittés par rapport à la conductivité ionique. On a préparé les matériaux précurseurs dopés au moyen d’un procédé de co-précipitation contrôlée. On a effectué le frittage des particules préparées en boulette en utilisant trois méthodes différentes: conventionnelle, frittage au plasma à étincelles et un champ de courant alternatif à 1125°C (frittage assisté au CA). On a déterminé la conductivité en fonction de la densité de défaut, de la superficie de joint de grain et de la stabilité de phase. On a trouvé que le champ appliqué lors du frittage, soit par CD à impulsion (plasma à étincelles) ou par CA, produisait des effets marginaux sur la microstructure de l’oxyde cérique dopé à l’oxyde d’yttrium. Cependant, on doit noter que parce que les conditions de frittage n’étaient pas optimisées, il y a place pour des améliorations substantielles de la conductivité.

Reconfiguration and Yield of a Hierarchical Torus Network

This paper presents the reconfiguration and yield of a new interconnection network, Hierarchical Torus Network (HTN). An HTN is a 2D-torus network of multiple basic modules, in which the basic modules are 3D-torus networks that are hierarchically interconnected for higher level networks. The static network performance and dynamic communication performance under dimension-order routing of the HTN have already been studied and network performances are good. However, the fault tolerance performance of the HTN has not yet been evaluated. The goal of this paper is to derive a theoretical estimate of system yield for the HTN as a function of defect density with a reconfiguration approach by hardware redundancy. Yield is the probability of obtaining a fault-free network. Despite the dramatic improvement in fault tolerance in recent years, it is still necessary to provide redundancy and fault circumvention to achieve efficient system yield for large multicomputer systems. Thus, we provide redundant hardware to reconfigure the faulty nodes, switches, and links to healthy nodes, switches, and links. The results indicate that with a 25% redundancy the yield of the HTN is satisfactory. We also discuss the 3D-WSI implementation issue and show that HTN permits efficient VLSI/ULSI/WSI realization due to the fewer numbers of vertical links between the silicon wafers. The longest wire has a length of 4.5 cm, which represents 4.20 times improvement over the planar implementation.

Influence of deep defects on device performance of thin-film polycrystalline silicon solar cells

Employing quantitative electron-paramagnetic resonance analysis and numerical simulations, we investigate the performance of thin-film polycrystalline silicon solar cells as a function of defect density. We find that the open-circuit voltage is correlated to the density of defects, which we assign to coordination defects at grain boundaries and in dislocation cores. Numerical device simulations confirm the observed correlation and indicate that the device performance is limited by deep defects in the absorber bulk. Analyzing the defect density as a function of grain size indicates a high concentration of intra-grain defects. For large grains (&amp;gt;2 μm), we find that intra-grain defects dominate over grain boundary defects and limit the solar cell performance.

On the origin of blue‐green emission from heteroepitaxial nonpolar a‐plane InGaN quantum wells

AbstractThe optical and structural properties of nonpolar a‐plane InGaN/GaN multiple quantum wells grown on r‐plane sapphire substrates were investigated as a function of defect density. The photoluminescence spectrum is characterised by a peak at 380 nm (3.24 eV) and a blue‐green emission band from 425‐550 nm (2.25‐3.10 eV). The near‐UV peak is assigned to carrier recombination in the quantum wells lying on the a‐plane. On the basis of microscopy, photoluminescence spectroscopy and cathodoluminescence imaging, the blue‐green emission band is attributed to emission from sidewall quantum wells formed on the various semipolar facets of small (∼100 nm) surface pits which form during quantum well growth at the lower temperatures and high NH3 flow in a nitrogen atmosphere. The density of the surface pits equals that of partial dislocations and threading dislocations in the quantum wells structure. Thus, the 300 K photoluminescence spectrum is dominated by the blue‐green emission band at a high defect density (&gt;1010 cm‐2) and its intensity is reduced with lower defect densities. For a spectrum with a single emission from the nonpolar a‐plane InGaN/GaN quantum wells the defect density needs to be below ∼109 cm‐2. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Quantum transport in chemically modified two-dimensional graphene: From minimal conductivity to Anderson localization

An efficient computational methodology is used to explore charge transport properties in chemically-modified (and randomly disordered) graphene-based materials. The Hamiltonians of various complex forms of graphene are constructed using tight-binding models enriched by first-principles calculations. These atomistic models are further implemented into a real-space order-N Kubo-Greenwood approach, giving access to the main transport length scales (mean free paths, localization lengths) as a function of defect density and charge carrier energy. An extensive investigation is performed for epoxide impurities with specific discussions on both the existence of a minimum semi-classical conductivity and a crossover between weak to strong localization regime. The 2D generalization of the Thouless relationship linking transport length scales is here illustrated based on a realistic disorder model.

Modeling and simulation of polycrystalline ZnO thin-film transistors

Thin-film transistors (TFTs) made of transparent channel semiconductors such as ZnO are of great technological importance because their insensitivity to visible light makes device structures simple. In fact, there have been several demonstrations of ZnO TFTs achieving reasonably good field effect mobilities of 1–10 cm2/V s, but the overall performance of ZnO TFTs has not been satisfactory, probably due to the presence of dense grain boundaries. We modeled grain boundaries in ZnO TFTs and performed simulation of a ZnO TFT by using a two-dimensional device simulator in order to determine the grain boundary effects on device performance. Polycrystalline ZnO TFT modeling was started by considering a single grain boundary in the middle of the TFT channel, formulated with a Gaussian defect distribution localized in the grain boundary. A double Schottky barrier was formed in the grain boundary, and its barrier height was analyzed as a function of defect density and gate bias. The simulation was extended to TFTs with many grain boundaries to quantitatively analyze the potential profiles that developed along the channel. One of the main differences between a polycrystalline ZnO TFT and a polycrystalline Si TFT is that the much smaller nanoscaled grains in a polycrystalline ZnO TFT induces a strong overlap of the double Schottky barriers with a higher activation energy in the crystallite and a lower barrier potential in the grain boundary at subthreshold or off-state region of its transfer characteristics. Through the simulation, we were able to estimate the density of total trap states localized in the grain boundaries for polycrystalline ZnO TFT by determining the apparent mobility and grain size in the device.

Nanoscale anodization of an amorphous silicon surface with an atomic force microscope

Nanoscale anodization was performed on the surface of amorphous silicon thin films by means of an atomic force microscope. The anodization mechanism was different from that previously reported on metal thin films. We found that the anodization was a function of defect density and current through the sample. The optical properties of the anodized area were measured by means of micro-photoluminescence, in which photoluminescence intensity decreases with oxidation. We concluded that both defect reaction and creation processes are important during the nanoscale anodization of amorphous material.

Burn-in effect on yield

By removing infant mortalities, burn-in of semiconductor devices improves reliability. However, burn-in may affect the yield of semiconductor devices since defects grow during burn-in and some of them end up with yield loss. The amount of yield loss depends upon burn-in environments. Another burn-in effect is the yield gain. Since yield is a function of defect density, if some defects are detected and removed during burn-in, the yield of the post-burn-in process can be expected to increase. The amount of yield gain depends upon the number of defects removed during burn-in. In this paper we present yield loss and gain expressions and relate them with the reliability projection of semiconductor devices in order to determine burn-in time.

Stability of supercurrents in a BiSrCaCuO (Bi-2212) high- Tcsuperconductor with artificially created defects

Recent results in a systematic study of the stability of supercurrents in Bi-2212 tapes with randomly oriented, highly splayed columnar defects are presented as a function of defect density. The defects were artificially created by fission fragments of bismuth nuclei, fissioned by irradiation with energetic protons (∼0.8 GeV). Significant enhancements in the persistent current density Jpare observed at all temperatures and fields. Also, a marked shift of the irreversibility line towards higher fields and temperatures improves considerably the capacity of the material for practical applications. Moreover, a significant decrease in the logarithmic decay rate S=dln (Jp) /dln(t) indicates a strong stabilization of the persistent currents. All features point to high effectiveness of this artificial pinning mechanism. However, the optimal proton fluence needs yet to be established.

Excimer laser processing of magneto-optic garnet films

A 248-nm KrF excimer laser was used to introduce defects into single-crystal GGG substrates and magneto-optic bismuth garnet films. The number, size, and distribution of defects can be controlled accurately in the 1–100-μm range. Defect introduction raises the coercivity of the films, controls their switching behavior, and modifies the hysteresis loop. Present theories do not explain the coercivity data as a function of defect density.