Critical Defect Density Research Articles

Investigations on antiferromagnetic skyrmion crystal generated by a staggered magnetic field

Antiferromagnetic skyrmions are promising candidates for future spintronic applications due to their particular robustness and ultrafast dynamics. In this work, we have used a staggered magnetic field to produce an antiferromagnetic (AFM) skyrmion crystal. The creation of such a skyrmion crystal is examined in two different situations: a regular clean system and a system containing impurity-like spin vacancies. We have found that defects decisively affect the AFM skyrmions creation, inducing skyrmions connections, the appearance of bimerons, and even the skyrmion lattice destruction at a critical defect density. In addition, we also explore the skyrmion dynamics in a hybrid racetrack which contains two distinct materials displayed in different areas (regions 1 and 2). A spin-polarized current can expel the skyrmions from region 1 (which sustains the skyrmion crystal) to region 2, initially empty of skyrmions. The organized collection of skyrmions of region 1 loses the traditional hexagonal symmetry when they penetrate the second region, composing a skyrmion gas. Nonetheless, this symmetry might be preserved if the spin-polarized current is high enough and reaches a determined value.

A graph based approach to model charge transport in semiconducting polymers

Charge transport in molecular solids, such as semiconducting polymers, is strongly affected by packing and structural order over several length scales. Conventional approaches to modeling these phenomena range from analytical models to numerical models using quantum mechanical calculations. While analytical approaches cannot account for detailed structural effects, numerical models are expensive for exhaustive (and statistically significant) analysis. Here, we report a computationally scalable methodology using graph theory to explore the influence of molecular ordering on charge mobility. This model accurately reproduces the analytical results for transport in nematic and isotropic systems, as well as experimental results of the dependence of the charge carrier mobility on orientation correlation length for polymers. We further model how defect distribution (correlated and uncorrelated) in semiconducting polymers can modify the mobility, predicting a critical defect density above which the mobility plummets. This work enables rapid (and computationally extensible) evaluation of charge mobility semiconducting polymer devices.

Generating large out-of-plane piezoelectric properties of atomically thin MoS2via defect engineering.

We calculated the piezoelectric properties of asymmetrically defected MoS2 using density functional theory. By creating uneven numbers of defects on either side of two-dimensional MoS2, the out-of-plane centrosymmetry of the charge distribution is clearly broken, and the out-of-plane piezoelectric response is induced. The largest out-of-plane piezoelectric response is associated with the highest defect ratio for MoS2 to be semiconducting. We calculated the critical defect density of the metal-insulator transition of the asymmetrically defected MoS2 to be 9.90 × 1014 cm-2 and chemical formula MoS1.22. The d33 of the multilayer of optimally defected MoS2 is found to be greater than those of AlN and ZnO, and in the same order of magnitude as lead zirconate titanate. All two-dimensional transition metal dichalcogenides can in principle be fabricated as piezoelectric with this approach. The required defect engineering is readily available with various types of ion irradiation or plasma treatment. By controlling the dose of the ion, the defect ratio and hence the piezoelectricity can be tuned. Such asymmetrically defected transition metal dichalcogenides can easily be integrated into two-dimensional transition metal dichalcogenide based devices, which is hard for conventional piezoelectric thin films to rival.

Characterizing interface structure between crystalline and ion bombarded silicon by transmission electron microscopy and molecular dynamics simulations

Application of ion beams in modern technology requires quantifying disorder induced by the collisions of energetic species and substrate atoms. For studying the structure of the transition region between crystalline silicon and amorphous layer Si(001) substrate was bombarded by gallium ions with the energy of 5 keV. The transition region width and amorphous layer thickness were evaluated using cross-correlation analysis of high- resolution electron microscopy (HRTEM) images taken from cross-section specimens. Performed molecular dynamics simulations allowed obtaining atom positions in amorphized material, which provided close agreement between the simulated and experimental HRTEM images. Analysis of calculated atomic arrangements realized by employing an introduced displacement order parameter enabled finding the interface transition region width and amorphous layer thickness which agreed satisfactorily with the values estimated from HRTEM images. This agreement gives evidence that MD simulation can be applied for reliable prediction and quantification of the disordered region extension in the ion bombarded material. The obtained amorphous layer thickness is also consistent with the results established utilizing the critical point defect density model applied to Monte Carlo simulations.

Critical density of topological defects upon a continuous phase transition

Using extensive Monte Carlo simulations, we test the hypothesis that the density of corresponding topological defects has a universal value at the temperature of a continuous phase transition. We consider several simple two-dimensional models where domain walls, vortices, so-called Z2 vortices or their combinations are presented. These topological defects are relevant correspondingly to an Ising second-order phase transition, a Berezinskii–Kosterlitz–Thouless transition and an explicit crossover. We compare results for square and triangular lattices as well as for the complicated situation when two types of defects are presented and two transitions occur separated in temperature. All considered cases demonstrate consentient results confirming the hypothesis.

Reliability Prediction of Highly Scaled MOSFET Devices via Fractal Structure of Spatial Defects

The needs for continuous size reduction of metal-oxide-semiconductor field effect transistor (MOSFET) devices can cause serious reliability concerns. In particular, gate oxide breakdown is a key mechanism concerning the lifetimes of MOSFET devices. In this paper, several spatial point processes are employed to represent general patterns of defect generation in gate oxide. By defining oxide breakdown as a creation of conduction path connecting two oxide interfaces by overlapped defects, percolation models are discussed to predict reliability of MOSFET devices in terms of critical defect density. In the final, we proposed a method to evaluate lifetimes of area-scaled gate oxides in MOSFET devices mainly through their fractal structure. The method suggests an easy way to predict the lifetimes of the devices with area-scaled gate oxides by examining their fractal structure through a fractal dimension without involving breakdown distributions of gate oxides with different areas.

Charge transport model to predict dielectric breakdown as a function of voltage, temperature, and thickness

As the minimum pitch in interconnects continues to shrink, dielectric breakdown is becoming increasingly more difficult to qualify for each new technology node. Standard voltage-acceleration models provide quick, but general, assessments of the dielectric quality. Instead, a one-dimensional charge transport model has been developed as a tool to investigate the process of the dielectric breakdown and why it occurs. The model couples Poisson's equation with constitutive equations for mobile electrons, trapped electrons, and defects in the dielectric. Bonds in the dielectric matrix are weakened by the electric field, and broken by energetic electrons, creating defects. Failure occurs when a critical defect density is reached, causing trap-to-trap tunneling and an abrupt increase in the current.The model successfully replicates electrical data for leakage current and dielectric failure as a function of voltage, temperature, and thickness. The activation energy for dielectric failure is shown to increase as the electric field decreases, resulting in much higher activation energies at operating conditions compared to testing conditions. The dielectric strength also increases for decreasing thickness based on a previous theory for planar dielectrics, and is shown to cause the failure vs. field slope to increase for thinner dielectrics.

Low-temperature electrical conduction of plasma-treated bilayer MoS2

Abstract

Probabilistic mechanical properties and reliability of carbon nanotubes

Carbon nanotubes (CNTs) and their products such as polymer nanocomposite (PNC) are an undeniable part of future materials. To use such future materials, it is necessary to have an accurate evaluation of their properties. Several uncertainties such as structural defects and their distributions cause change in the properties of CNTs that could be considered probabilistic variables. A novel procedure is presented for evaluating CNTs’ probabilistic fracture properties and structural reliability using stochastic finite element methods. By employing two dimensionless parameters, both types of Stone–Wales 5-7-7-5 defects are randomly applied to CNTs. Section defect density and critical section defect density are defined and used to manage the distribution and geometrical configuration of CNTs’ structural defects. A probabilistic method is used to evaluate the effect of defects’ distribution on Young's modulus, ultimate strain, and ultimate stress. It has been observed that normal and Weibull distribution functions are suitable for describing Young's modulus distribution and ultimate stress distribution, respectively. Defect density ratio is defined and, using this parameter, the effect of aggregated defects on mechanical properties is evaluated. It is demonstrated that the defects out of critical section have an unavoidable effect on Young's modulus and ultimate strain; but they have an insignificant effect on ultimate stress. A reliability analysis is performed on armchair (15,15) CNTs and it is investigated that the reliability of CNTs depends on critical defect density significantly. In addition, the reliability is equal to one for the stress of less than 50GPa and this value is equal to zero for the stress of higher than 100GPa, independent from the changes of critical defect density. Eventually, a procedure is described to estimate the reliability of armchair CNTs using critical defect density and the results’ accuracy is discussed and evaluated.

Dielectric/metal/dielectric alternative transparent electrode: observations on stability/degradation

The use of indium-free transparent conductive electrodes is of great interest for organic optoelectronic devices. Among the possible replacements for ITO, dielectric/metal/dielectric (D/M/D) multilayer structures have already proven to be quite efficient. One issue with organic devices is their lifetime, which depends not only on the organic molecules used but also on the electrodes. Therefore we study the variation, with elapsed time, of the electrical and optical properties of different D/M/D structures, with M = Ag or Cu/Ag. Six years after realization, it has been shown that if some structures retained an acceptable conductivity, some others became non-conductive. For a sample which remains conductive, in the case of a PET/MoO3/Ag/MoO3 multilayer structure, the sheet resistance changes from 5 Ω/sq–17 Ω/sq after six years. This evolution can be compared to that of a PET/ITO electrode that varies from 25 Ω/sq–900 Ω/sq after six years. It means that not only are the PET/MoO3/Ag/MoO3 multilayer structures more flexible than PET/ITO, but they can also be more stable. Nevertheless, if some PET/MoO3/Ag/MoO3 multilayer structures are quite stable, some others are not. This possible degradation appears to be caused primarily by the physical agglomeration of Ag, which can result in Ag film disruption. This Ag diffusion seems to be caused by humidity-induced degradation in these Ag-based D/M/D structures. Initially, defects begin to grow at a ‘nucleus’, usually a microscopic particle (or pinhole, etc), and then they spread radially outward to form a nearly circular pattern. For a critical density of such defects, the structure becomes non-conductive. Moreover the effect of humidity promotes Ag electrochemical reactions that produce Ag+ ions and enhances surface diffusivity with AgCl formation.

Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halide

Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electron–hole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 1025/m3. The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics.

Amorphization in Nanoparticles

Nanoparticles have more propensities for amorphization than their bulk counterparts, opening the opportunity to achieve the amorphous states in well-known poor glass-former compounds. Classical size effects are often invoked to explain such phenomenon. However, this argument is not sufficient to cover all experimental results such as functionalization effect. In this work, Y2O3 nanoparticles of 7 nm diameter are investigated under pressure. Special care is taken on the surface state of the particles by comparing the pressure-induced transformation of nanoparticles stored under different conditions (atmospheric vs argon environment). A clear difference is reported as one batch shows a transformation to an amorphous state, whereas the second undergoes a crystal-to-crystal transition. These results are discussed in terms of interface energy taking into account not only the usual surface energy but also the surface state contribution. The link between different types of amorphization (pressure-, mechanical-, and radiation-induced amorphization) in nanoparticles is underlined as a critical defect density is required to achieve the crystal-to-amorphous transformation. Defect creation may arise from multiple sources: irradiation, functionalization, and reconstructive transition.

Scratch-concerned yield modeling for IC manufacturing involved with a chemical mechanical polishing process

In existing integrated circuit (IC) fabrication methods, the yield is typically limited by defects generated in the manufacturing process. In fact, the yield often shows a good correlation with the type and density of the defect. As a result, an accurate defect limited yield model is essential for accurate correlation analysis and yield prediction. Since real defects exhibit a great variety of shapes, to ensure the accuracy of yield prediction, it is necessary to select the most appropriate defect model and to extract the critical area based on the defect model. Considering the realistic outline of scratches introduced by the chemical mechanical polishing (CMP) process, we propose a novel scratch-concerned yield model. A linear model is introduced to model scratches. Based on the linear model, the related critical area extraction algorithm and defect density distribution are discussed. Owing to higher correspondence with the realistic outline of scratches, the linear defect model enables a more accurate yield prediction caused by scratches and results in a more accurate total product yield prediction as compared to the traditional circular model.

Permeation measurements and modeling of highly defective Al2O3 thin films grown by atomic layer deposition on polymers

We measured the thickness and temperature dependence of moisture permeation in highly defective (10–20 at. % hydrogen) Al2O3 thin films grown by atomic layer deposition on polymer substrates. We found that when films were grown at higher temperature or were thicker, independent of growth temperature, they were better moisture barriers. We determined the threshold thickness for measurement-limited barrier performance to be 7.5 nm for growth at 100 °C compared to 9.6 nm at 50 °C. We explained the permeability of these highly defective films with a new model, which relates moisture permeability to a critical density of defects and not due to pinholes.

Influence of Composition and Roughness on Localized Corrosion of Al-Mg-Si Alloys Characterized by Microelectrochemistry

The influence of surface roughness on the pitting potential using the electrochemical microcell technique is reported. Two Al-Mg-Si model alloys of varying manganese content were investigated. With potentiodynamic anodic polarization in 0.1 M NaCI solutions and with different sizes of exposed area it is shown, that a critical defect density is present which controls the pitting potential at small polarization potentials. This defect density is related to the crevice distance. With smoother polished surfaces a larger crevice distance is measured. The pitting potential measured on an exposed area smaller than the crevice distance reveals that small effects of the alloying element Mn become visible. For the fine polished surface it is shown that the addition of 0.5 wt% Mn to the model alloy lowers the pitting potential.

Effect of stress voltages on voltage acceleration and lifetime projections for ultra-thin gate oxides

This paper investigates the effect of NFET (N+ poly gate, N+ diffusion of FET) stress voltage conditions, for ultra-thin gate oxides, on the voltage acceleration, and lifetime projections to use conditions. This work employs the model relating the critical defect density ( N BD) to the charge-to-breakdown and the defect generation probability ( P g). The models for N BD and P g were adjusted for effects at voltages between 2 V and 3 V, and oxide thickness less than 2.7 nm. For N BD, a model is proposed that is supported by published data and provides a gradual transition to a plateau for oxide thickness less than 2.7 nm. For P g, a stronger dependency of Log( P g) in the range of 2–3 V is employed to give a better fit to published data. This adjusted P g is also used below 2 V to show trend of projection. In the direct tunneling range below 3 V, there is an increase of the voltage acceleration factor (AF) with decreasing voltage. Also, below 3 V, AF shows a decrease as the oxide thickness is reduced from 2.0 nm to 1.2 nm, and this trend becomes stronger as the gate voltage is reduced. Above a gate stress voltage of 3 V, in the range of 3–4 V, AF is almost constant, and there is a slight decrease of AF with decreasing oxide thickness in the range of 2.0–1.2 nm. A voltage power-law fit for the range above 3 V shows a decreasing power index with decreasing oxide thickness.

Room temperature study of a strain-induced electronic superstructure on a magnetite (111) surface

A magnetite (Fe3O4) single crystal (111) surface has been studied at various oxygen-iron surface stoichiometries. The stoichiometry was modified by controlling the in-situ sample anneal conditions. We have found the conditions that lead to the formation of an oxygen-rich surface that forms a quasi-hexagonal superstructure with a 42A periodicity. The superstructure is highly regular and was observed by both LEED and STM. The superstructure consists of three regions, two of which have identical atomic scale structures with a periodicity of 2.8A, and a third having a periodicity that is about 10% larger (3.1A). The subtle difference in the atomic periodicities between the three areas results from the modulation of intrinsic strain developed along the surface. The superstructure results from electronic effects rather than being a mosaic of different iron oxide terminations. The onset of the superstructure is sensitive to the surface stoichiometry. From our results we could estimate the critical density of defects leading to the disappearance of the superstructure. We have modelled the experimental results and calculated the electron density using a DFT algorithm. The model clearly shows the development of strain along the surface.

Reliability Characteristics of poly Si-gated High Quality Chemical Vapor Deposition Hafnium Oxide Gate Dielectric

In this paper, reliability characteristics of chemical vapor deposition (CVD) HfO2 gate stacks with n+-poly-Si gate electrode are investigated systematically, along with long-term lifetime projection. The area dependence and critical defect density of CVD HfO2 gate stacks have been investigated and compared to that of SiO2. Results show that in addition to the significant reduction of tunneling leakage current by a factor of 103–104, a comparable Weibull slope factor and critical defect density are obtained from a high quality CVD HfO2 mainly due to the thicker physical thickness, compare to SiO2. Considering the cumulative impact of temperature acceleration at 150°C, scaling of an effective gate oxide area of 0.1 cm2 and a maximum allowed fraction of failures of 0.01%, the maximum allowed operating voltage is projected to be ∼0.85 V for HfO2/poly-Si gate stack with equivalent oxide thickness (EOT) = 14.5 Å.

TDDB and Polarity-Dependent Reliability of High-Quality, Ultrathin CVD>tex<$hbox HfO_2$>/tex<Gate Stack With TaN Gate Electrode

In this letter, we present a comprehensive study on longterm reliability of ultrathin TaN-gated chemical vapor deposition gate stack with EOT=8.5-10.5. It is found that, due to the asymmetric band structure of HfO/sub 2/ gate stack with an interfacial layer, the HfO/sub 2/ gate stack shows polarity-dependent leakage current, critical defect density, and defect generation rate, under gate and substrate injection. However, no such polarity dependence of time-to-breakdown (T/sub BD/) is observed when T/sub BD/ is plotted as a function of gate voltage. The 10-year lifetime of an HfO/sub 2/ gate stack is projected to be Vg=-1.63 V for the equivalent oxide thickness (EOT) =8.6 and Vg=-1.88 V for EOT=10.6 at 25/spl deg/C. These excellent reliability characteristics are attributed to reduced leakage current of HfO/sub 2/ gate stack with physically thicker films that result in larger critical defect density and Weibull slope to that of SiO/sub 2/ for the same EOT. However, at 150/spl deg/C, and with area scaling to 0.1 cm/sup 2/ and low percentile of 0.01%, the maximum allowed voltages are projected to Vg=-0.6 V and -0.75 V for EOT of 8.6, and 10.6, respectively.

Experimental evidence of T/sub BD/ power-law for voltage dependence of oxide breakdown

In this paper, we present experimental evidence on the voltage-dependence of the voltage acceleration factors observed on ultrathin oxides from 5 nm down to /spl sim/1 nm over a wide range of voltages from /spl sim/2 V to 6 V. Two independent experimental approaches, area scaling method and long-term stress, are used to investigate this phenomenon. We show the exponential law with a constant voltage-acceleration factor violates the widely accepted fundamental breakdown property of Poisson random statistics while the voltage-dependent voltage acceleration described by an empirical power-law relation preserves this well-known property. The apparent thickness-dependence of voltage acceleration factors measured in different voltage ranges can be nicely understood and unified with these independent experimental results in the scenario of a voltage-driven breakdown. In the framework of the critical defect density and defect generation rate for charge-to-breakdown, we explore the possible explanation of increasing voltage acceleration factors at reduced voltage by assuming a geometric model for the critical defect density.