Interaction of point defects with impurities in the Si-SiO2 system and its influence on the properties of the interface

The results of investigations of the point defect generation, redistribution and interaction with impurities in the Si-SiO₂ system during the process of its formation by means of electron paramagnetic resonance (EPR) and nucleus magnetic resonance (NMR) techniques are presented. The type and density of the point defects that are generated in the Si surface layer during thermal oxidation depend on the oxidation condition: temperature, cooling rate, oxidation time, and impurity content. The interaction between the point defects with extended defects and impurities affects the properties of the Si-SiO₂ interface. The influence of the point defects may be diminished and the interface properties improved by an appropriate choice of the oxidation conditions. The difference between the interface properties of n- and p-type wafers may be connected with the different position of the Fermi level at the interface and different point defects density in the volume near the interface.     

An Empirical Indicator of Product Appearance for Process Control

An innovative approach based on multiattribute utility theory and group processes was used to directly monitor and control cosmetic characteristics affecting product appearance to improve the quality of a product. The main advantage of this approach is its ability to provide a direct and reliable measure of visual appearance for products for which this is the critical quality characteristic. The paper is divided into two main parts. The first part briefly presents a systematic approach to develop an empirical indicator of product appearance, called the index of visual condition (IOVC). The second part presents a study of the applicability of the IOVC during process control to directly monitor and improve product appearance. This study was performed during industrial production of a ceramic product for a period of seven months. Several Shewhart charts were developed, including the first exploratory X-bar and R charts for product appearance (IOVC charts) and c-charts for number of defects. The performance of these charts was evaluated by their ability to detect special causes of variation and improve the product. The case study indicates that these two approaches complement each other. While the c-chart allows one to monitor the number of defects that impair product functionality, the IOVC chart brings a new level of capability, providing the ability to directly monitor product appearance considering defects that do not affect functionality. Both approaches were useful for process control and improvement.     

An analysis of defect size evolution

The progression of structural material towards failure may be thought of in terms of the evolution of the population of defects that it contains. An important statistical characteristic of the defect population is its size distribution. In the present paper we review the results of experimental observations of defect size evolution during damage accumulation due to different mechanisms and at different scales, i.e. creep bubbles, pores and cavities. We consider the probability density functions and cumulative density functions for defect size distributions, and identify several types of evolution observed. We also focus on the appearance of a `tail' of large size defects, and discuss whether it could be used as an indicator of the transition to a new structural level or scale.     

Diffusion processes in the Au(111)/Ag(111) thin film system

The diffusion mechanisms of silver through Au(111) films 360 Å thick for different temperatures and heating times are studied. By means of quantitative Auger electron spectroscopy and in-depth profiling techniques the composition of the first monolayer and the composition profiles of diffused silver against time and heating temperature are determined. After elimination of instrumental effects the bulk silver interdiffusion coefficients for different temperatures and the activation energy of diffusion are obtained. The extrapolation stages allows the time required for the diffused onto the surface in the first diffusion stages allows the time required for the first appearance to be determined. Comparison with two standard expressions gives the corresponding grain-boundary diffusion coefficients and the activation energy. A numerical formalism, which includes the variation of composition at the interface but makes no allowance for grain boundary-grain diffusion, is tested esing the calculated coefficients. The agreement between the experimental results and the theoretical surface compositions is satisfactory within the limitations of the proposed model.     

An analysis of defect size evolution

The progression of structural material towards failure may be thought of in terms of the evolution of the population of defects that it contains. An important statistical characteristic of the defect population is its size distribution. In the present paper we review the results of experimental observations of defect size evolution during damage accumulation due to different mechanisms and at different scales, i.e. creep bubbles, pores and cavities. We consider the probability density functions and cumulative density functions for defect size distributions, and identify several types of evolution observed. We also focus on the appearance of a `tail' of large size defects, and discuss whether it could be used as an indicator of the transition to a new structural level or scale.     

Deformation and fracture in directionally solidified Co-CoAl eutectic

The effect of growth defects known as lamellar terminations on the yielding and fracture behaviour of Co-CoAl eutectic single crystals was studied using tensile tests and finite-element modelling. The yield strength and strain to fracture were found to decrease with increasing termination density. Observations of deformed surfaces and serial sectioning experiments on fractured tensile specimens revealed that crack initiation during the fracture process was enhanced by the presence of lamellar terminations. The fracture surfaces were found to have a staircase-type appearance, which indicated that the final fracture process was discontinuous with a step-wise propagation from one CoAl lamella to adjacent CoAl lamellae. A computer simulation was conducted to determine the stress distributions about lamellar terminations in model microstructures, since the experimental results suggested that the lamellar terminations behaved as stress concentrations in the microstructure. The finite-element calculation confirmed that lamellar terminations can influence the yielding process; the stress at which the first slip system was activated was found to decrease with increasing termination density.     

Metastabilities in the electrical characteristics of CIGS devices: Experimental results vs theoretical predictions

Recent theoretical calculations have traced an origin of light- and voltage bias-induced metastabilities in Cu(In,Ga)Se₂-based solar cells to negative-U properties of the V_Se-V_Cu complex. In this paper we compare experimental findings with theoretically predicted properties of these defects and calculated values of parameters characteristic for transitions between their different states. Profiles of net acceptor concentrations in the relaxed and metastable states obtained by capacitance profiling have been studied, as well as annealing kinetics of the persistent defect distributions by thermally stimulated capacitance and conductivity. Good qualitative and quantitative agreement are found between theory of V_Se-related defects and experimental results. The consequences from the point of view of photovoltaic efficiency of the Cu(In,Ga)Se₂-based solar cells are discussed.     

Understanding nanoparticle formation by a wire explosion process through experimental and modelling studies

A wire explosion process (WEP) has been used to produce nano aluminium powder in nitrogen, argon and helium atmospheres. The impact of energy deposited into the exploding conductor on the size and shape of the particles was analysed using TEM analysis, which forms the first part of the study. It is observed that the higher the energy deposited, the smaller the particles formed. In the second part, modelling studies were carried out by solving the general dynamic equation through the nodal approach, and the particle size distributions were predicted. It is realized that, at the point of high saturation ratio and nucleation rate, the size of the critical nucleus formed is low. The particle size distribution predicted by the model correlates well with the experimental results. Time-series analysis of particle formation indicates that particles of lower dimensions form and, in the process of coagulation, larger particles are formed. It is realized that the plasma formed during the explosion plays a major role in the particle formation, and the modelling studies confirm that particle formation is not an instantaneous process but requires a certain time period to form stable sizes and shapes.     

Quantitative investigation into the nucleation mechanism of lead and silver films vacuum deposited onto SiO,carbon and mica substrates

The origin of the occurrence of nuclei was quantitatively investigated using a microbalance and an electron microscope. The completion time of preferred nucleation and the time variation of the number density of stable nuclei were expressed in terms of measurable quantities such as the sticking coefficient of the incident vapour beam and the surface diffusion distance of adatoms. An analysis of the experimental results showed that the sum of the calculated values of the completion time of preferred nucleation and the appearance time of nuclei ranged from 2 to 60 s, whereas the time at which nucleus saturation occurred was found to range from 100 to 6000 s. It was concluded that the observed increase in the density of nuclei is caused by random nucleation. The occurrence of nucleus saturation can also be explained on the basis of random nucleation and growth coalescence.     

Development and validation of a probabilistic model for notch fatigue strength prediction of tool steels based on surface defects

A new model for the high cycle notch fatigue strength prediction of tool steels subjected to axial loading is proposed, based on previous literatures studies and experimental tests carried out on six different tool steels, including rotating bending fatigue tests on notched specimens, fractographic analyses, hardness, residual stress, and roughness measurements. The novelty is the assumption that surface defects are the main cause of notch fatigue failures of such steels. A probabilistic approach was implemented by modeling size distributions of defects, resulting in the prediction of normal distributions of fatigue strength. Like to other previous models, the effect of steel hardness, surface residual stress, notch severity, and specimen size was also taken into account. Model calibration and validation were performed using the data collected by the experimental activity. Model behavior was investigated by performing a sensitivity analysis, aiming to verify the response to variations of the considered input variables. Prediction errors of only 1.3% (on average) and 3.1% (maximum) resulted from the comparison between model-predicted and experimental notch fatigue strength.     

Defect studies of ZnSe nanowires

During the synthesis of ZnSe nanowires various point and extended defects can form, leading to observed stacking faults and twinning defects, and strong defect related emission in photoluminescence spectra. In this paper, we report on the development of a simple thermodynamic model for estimating the defect concentration in ZnSe nanowires grown under varying Se vapour pressure and for explaining the results of our experimental findings. Positron annihilation spectroscopy was used successfully for the first time for nanowires and the results support predictions from the defect model as well as agreeing well with our structural and optical characterization results. Under very high Se vapour pressure, Se nodules were observed to form on the sidewalls of the nanowire, indicating that beyond a limit, excess Se will begin to precipitate out of the liquid alloy droplet in the vapour-liquid-solid growth of nanowires.     

An analysis of transport phenomena for multi-solid particle systems at higher Reynolds numbers by a 5th order polynomial Karman-Pohlhausen method

Using the concentric spheres free surface model and a 5th order polynomial Karman-Pohlhausen method of the laminar boundary layer theory, the dimensionless tangential stress distributions, the dimensionless pressure distributions around a solid sphere in a swarm and the viscous, form and total drag coefficients for multi-solid sphere systems were numerically computed at higher Reynolds numbers, based on the first assumption that the pressure distribution equals that of potential flow between concentric spheres up to the separation point, and behind the separation point in the wake region the pressure does not recover and keeps constant, and on the second assumption that the pressure distribution varies according to the measurement of Flachsbart.The theoretical drag coefficient of single solid spheres in an infinite medium based on the second assumption agreed with the experimental data in the range of the Reynolds numbers from $\text{3 × 10}{}^2\text{to 10}{}^5$.The friction factor for multi-solid particle systems based on the first assumption is almost the same as that on the 4th order polynomial and agreed with the experimental data of packed and distended beds.The void functions obtained from the drag coefficients for multi-solid particle systems based on both first and second assumptions were almost the same as the one on the 4th order polynomial.Using the velocity profiles based on concentric spheres free surface model and a 5th order polynomial Karman-Pohlhausen method of the laminar boundary layer obtained previously, the diffusion equation was solved numerically at higher Reynolds numbers on the first assumption of the pressure distribution around a solid sphere in a swarm equals that of potential flow between concentric spheres from the frontal stagnation point to the separation one, and the pressure does not recover, but keeps constant behind the separation point in the wake region. The mass transfer rate for multi-solid particle systems so computed was almost the same as that on the 4th order polynominal and agreed with the experimental data of single Solid spheres, and packed and particulate fluidized beds at higher Reynolds numbers.     

Rotational invariance approach for the evaluation of multiple phases in interferometry in the presence of nonsinusoidal waveforms and noise

Incorporation of two phase-shifting devices in a holographic moiré configuration not only renders the interferometer compatible with automated measurements but also allows for simultaneous measurement of multiple phase information in the interferometer. However, simultaneous handling of multiple phase steps and subsequent simultaneous determination of multiple phase distributions requires the introduction of novel tools in phase-shifting interferometry. In this context, the aim of this paper is to propose a subspace invariance approach to address these issues. This approach takes advantage of the rotational invariance of signal subspaces spanned by two temporally displaced data sets formed from the intensity fringes recorded temporally on pixels of the CCD camera. The method first identifies the arbitrary phase steps imparted to the piezoactuator devices. The estimated phase steps are subsequently applied in the linear Vandermonde system of equations to determine the phase distributions. The method also allows for handling nonsinusoidal wavefronts. Since the phase steps are extracted at every point on the interferogram, the method is applicable to configurations that use spherical beams. The robustness of the method is investigated by adding white Gaussian noise during the simulations.     

Anisotropy characterization in amorphous ribbons from the stressdependence of compliance

The authors describe the theoretical basis and procedure for a method to characterize anisotropy that makes use of the observation that the reorientation of spontaneous magnetostriction with applied tensile stress is the source of a magnetoelastic component of the mechanical compliance of a ferromagnet. Features of the appearance and magnitude of this component and the manner in which it varies with increasing stress can be correlated with specific distributions of domain orientations in the unstressed state. Salient aspects of the expected apparition of these features are catalogued against systematically varied, ad hoc assumptions of initial domain orientations and distributions. Identification of catalogued features in the experimentally determined stress dependence of compliance of a real sample is interpreted to imply the existence of the correlated anisotropy. Since the experimental arrangement is comparatively simple, particularly in regard to sample preparation, such correlation allows for rapid characterization of the anisotropy and permits real-time observation of its variation with thermal, magnetic, and other treatments     

Point defects in metals

This review surveys present knowledge of the nature and behaviour of point defects, especially vacancies. The general theoretical concepts and various experimental observations connected with the formation and mobility of vacancies and their complexes, their effect on the physical properties of metals, are described. The formation of secondary defects by vacancy condensation, the interaction between vacancies and solute atoms and some general ideas about point defects, are also discussed in detail.     

Using lifetime of point defects for dislocation bias in bcc Fe

The interaction between dislocations and point defects is key to deformation processes and microstructural evolution of structural materials. In this work, we compute the lifetime of point defects to describe their interaction with dislocations. This approach can accurately account for the effects of the dislocation core and anisotropic defect dynamics to accumulatively determine the capture efficiency, sink strength, and dislocation bias at different temperatures and dislocation densities. Particularly, the absorption of point defects by straight screw and edge dislocations in a model bcc iron system is studied. The maximum swelling rates based on the obtained bias factors are in close agreement with a variety of experimental measurements, including both neutron and ion-irradiation data, especially when considering the survival fraction for point defects from displacement cascades. This approach applies to many other processes and sinks, such as dislocation loops and interfaces, providing a powerful means to develop fundamental insights critical for improving radiation resistance and mechanical properties of structural materials through controlling defect interaction and evolution.     

Variational Convolutional Autoencoders for Anomaly Detection in Scanning Transmission Electron Microscopy

Identifying point defects and other structural anomalies using scanning transmission electron microscopy (STEM) is important to understand a material's properties caused by the disruption of the regular pattern of crystal lattice. Due to improvements in instrumentation stability and electron optics, atomic-resolution images with a field of view of several hundred nanometers can now be routinely acquired at 1–10 Hz frame rates and such data, which often contain thousands of atomic columns, need to be analyzed. To date, image analysis is performed largely manually, but recent developments in computer vision (CV) and machine learning (ML) now enable automated analysis of atomic structures and associated defects. Here, the authors report on how a Convolutional Variational Autoencoder (CVAE) can be utilized to detect structural anomalies in atomic-resolution STEM images. Specifically, the training set is limited to perfect crystal images , and the performance of a CVAE in differentiating between single-crystal bulk data or point defects is demonstrated. It is found that the CVAE can reproduce the perfect crystal data but not the defect input data. The disagreesments between the CVAE-predicted data for defects allows for a clear and automatic distinction and differentiation of several point defect types.     

Determination of breakdown rates and defect densities in SiO2

Breakdowns in SiO₂ have been classified as defect related, due to wear-out and intrinsic. However, techniques to ascertain defect densities and breakdown rates at defects have not been available, nor has the distinction between wear-out and defect- related or intrinsic breakdowns been clearly demonstrated. A particular problem has been the inability to distinguish defect types, i.e. defects having different breakdown rates. Another source of confusion has been the tacit assumption that breakdown field histograms obtained from ramp breakdown tests are independent of the ramp rate, which cannot be valid for finite breakdown rates. We obtained relationships specifying the statistics of breakdown, including the effect of defects. These actually derive from results describing a Markov death process and depend on the time integrals of breakdown rates in defect-free regions and at defects and on parameters describing the defect distributions. For Poisson distributions of the defect, these parameters are the mean number of defects per device for each defect type. Any breakdown test is described by the same relations since the nature of the test enters only through the time integral of the breakdown rates. If a wear-out mechanism is operative, then the breakdown rates will depend on the time explicitly, i.e. not only via the time dependences of the applied field and temperature. procedures for obtaining defect densities and breakdown rates follow from the derived dependence on these quantities of the expectation value of the fraction of devices broken down. Ramp tests at various ramp rates are advantageous for this purpose. The field dependence of the breakdown rates can be extracted directly from the experimental data and no a priori form for this dependence need be assumed. Experimental results obtained from multiple ramp breakdown tests will be presented. The field dependence of the breakdown rates is found to vary significantly from a simple exponential dependence. Following Klein, the effect of fluctuations on the breakdown rates will be considered qualitatively to rationalize their observed field dependence. No explicit time dependence of the breakdown rates is indicated over the range of field covered by the data, implying the absence of wear-out.