Spatial Distribution Of Defects Research Articles

In situ and tomographic characterization of damage and dislocation processes in irradiated metallic alloys by transmission electron microscopy

Abstract

Model for transport and reaction of defects and carriers within displacement cascades in gallium arsenide

A model is presented for recombination of charge carriers at evolving displacement damage in gallium arsenide, which includes clustering of the defects in atomic displacement cascades produced by neutron or ion irradiation. The carrier recombination model is based on an atomistic description of capture and emission of carriers by the defects with time evolution resulting from the migration and reaction of the defects. The physics and equations on which the model is based are presented, along with the details of the numerical methods used for their solution. The model uses a continuum description of diffusion, field-drift and reaction of carriers, and defects within a representative spherically symmetric cluster of defects. The initial radial defect profiles within the cluster were determined through pair-correlation-function analysis of the spatial distribution of defects obtained from the binary-collision code MARLOWE, using recoil energies for fission neutrons. Properties of the defects are discussed and values for their parameters are given, many of which were obtained from density functional theory. The model provides a basis for predicting the transient response of III-V heterojunction bipolar transistors to displacement damage from energetic particle irradiation.

Radiation stability of iron nanoparticles irradiated with accelerated iron ions

In the present work the dynamic processes occurring in a nanoscale iron particle exposed to irradiation with iron ions of different energies are studied in detailed. It is shown that the elastic and thermoelastic crystal lattice responses to irradiation form force factors affecting the evolution of defect-impurity system, which, in turn, leads to a decrease in the number of structural defects. Quantitative estimations of the spatial distribution of defects resulting in their migration to the surface were obtained. Such self-organization of nanoparticles exposed to ionizing radiation can be used as a basis for the production of radiation-resistant nanostructured materials capable of sustaining a long-term radiation influence.

Half-metallicity in single-layered manganese dioxide nanosheets by defect engineering.

Defect engineering is considered as one of the most efficient strategies to regulate the electronic structure of materials and involves the manipulation of the types, concentrations, and spatial distributions of defects, resulting in unprecedented properties. It is shown that a single-layered MnO2 nanosheet with vacancies is a robust half-metal, which was confirmed by theoretical calculations, whereas vacancy-free single-layered MnO2 is a typical semiconductor. The half-metallicity of the single-layered MnO2 nanosheet can be observed for a wide range of vacancy concentrations and even in the co-presence of Mn and O vacancies. This work enables the development of half-metals by defect engineering of well-established low-dimensional materials, which may be used for the design of next-generation paper-like spintronics.

Half‐Metallicity in Single‐Layered Manganese Dioxide Nanosheets by Defect Engineering

AbstractDefect engineering is considered as one of the most efficient strategies to regulate the electronic structure of materials and involves the manipulation of the types, concentrations, and spatial distributions of defects, resulting in unprecedented properties. It is shown that a single‐layered MnO2 nanosheet with vacancies is a robust half‐metal, which was confirmed by theoretical calculations, whereas vacancy‐free single‐layered MnO2 is a typical semiconductor. The half‐metallicity of the single‐layered MnO2 nanosheet can be observed for a wide range of vacancy concentrations and even in the co‐presence of Mn and O vacancies. This work enables the development of half‐metals by defect engineering of well‐established low‐dimensional materials, which may be used for the design of next‐generation paper‐like spintronics.

Calculation of the spatial distribution of defects and cascade- probability functions in the materials

In this article we carried out the calculations of the depth distribution of implanted ions of arsenic and indium, loss of energy and cascade-probability functions in silicon. Comparison of the calculations with the experimental data is in the satisfactory agreement. The computer simulation and analysis of the characteristics of ions depending on the depth of penetration and the number of interactions were carried out.

Simulation of TL Mechanisms in Materials with Using Fractal Rate Kinetics

The thermoluminescence glow curves were simulated in terms of the fractal rate kinetics. It was analyzed how effective spectral dimensions of systems of traps and recombination centers influenced geometrical parameters of peaks. It was shown that high values of the form-factors could be interpreted in fractional kinetics terms taking into account an inhomogeneous spatial distribution of defects in material.

In situ resistivity measurements of RAFM base alloys at cryogenic temperatures: The effect of proton irradiation

A four-probe technique for measurement of electrical resistance on low-temperature ion-irradiated metallic sheets is described. The design, temperature control system, preparation method of samples and the resistivity measurements are described in detail. The resistivity recovery (RR) curve has been measured on a Fe–5%Cr model alloy irradiated with 5MeV protons. The procedure to obtain the RR derivative curve is outlined and experimental errors are identified and quantified. Special care has been taken to use a sample with very low impurity content and low dislocation density (1.2×108cm−2). Thus, effects in recovery spectrum of the Fe–5%Cr alloy are only due to the presence of Cr and irradiation defects, which will be mainly Frenkel Pairs (FPs) given that the mean energy of the Primary Knock-on Atoms (PKA) is close to 0.35keV. The results obtained for the Fe–5%Cr under 5MeV proton irradiation are found to be in overall agreement with previous experimental measurements performed under electron irradiation although some differences appear probably due to the different spatial distribution of the created defects and the higher temperature resolution of annealing steps. The RR spectrum obtained reveals the appearance of the structure of stages I and II and also a partial suppression of the stage III peak with respect to previous results obtained after electron irradiation. The stage III suppression is explained as a superposition of vacancy recombination effects and short-range ordering (SRO) effects which are apparently dependent on the spatial distribution of defects created during irradiation. Moreover, recombination phenomena are observed beyond stage III up to 500K.

Numerical simulation of the influence of small-scale defects on the true-triaxial strength of rock samples

A numerical model is used to investigate the influence of randomly located ‘defects’ on the strength of rock samples with a Mogi–Coulomb three-dimensional failure criterion that are subjected to polyaxial states of stress. A methodology is presented to compute ‘equivalent’ Mohr–Coulomb strength parameters as a function of the ‘instantaneous’ stress, and sensitivity analyses are employed to study the influence of the stress state, of the total percentage of defects, and of the type of spatial distribution of defects within the sample. Results indicate that increments of strength due to the intermediate principal stress are less relevant for samples with defects, hence suggesting that the influence of polyaxial stress states is less significant for rock masses (that incorporate ‘defects’ or ‘discontinuities’ into their structure) than for intact rock. Results also indicate that the strength of rock with defects can be modelled as a ‘homogeneous’ material with reduced strength properties. The reduction factors are a linear function of the percentage of defects, and they tend to be greater for “lumped” defects than for “isolated” defects.

Effects of substitutional He atoms on the displacement cascades in α-Fe

Molecular dynamics method has been applied to investigate the effects of substitutional helium atoms on the displacement cascades in α-Fe. It is found that the defects production decreases at low temperature and increases at high temperature with the increase of substitutional helium concentration. The clusters sizes and spatial distribution of defects depend on the concentration of helium. Large He–Vacancy clusters can be observed and different defects distribute in different zones, which is different from pure iron under the same radiation conditions.

Analyzing the distribution of threshold voltage degradation in nanoscale transistors by using reaction-diffusion and percolation theory

Continued scaling of transistors into the nanoscale regime has led to large device-to-device variation in transistor characteristics. These variations reflect differences in substrate doping, channel length, interface and/or oxide defects, etc. among various transistors. In this paper, we develop a theory for the statistical distribution of threshold voltage degradation (ΔV T ) due to the Negative Bias Temperature Instability (NBTI). First, we model the time dynamics of interface defects within the Reaction-Diffusion (R-D) framework and calculate the statistics of interface defect using Markov Chain Monte-Carlo method. We show that the generation and annealing of interface defects are strongly correlated and that the statistics of interface defect at a given stress time (N IT @t STS ) follows a skew-normal distribution. Second, we explore the differential effect of the spatial distribution of interface defects in nanoscale transistors pre-populated with a discrete number of randomly placed substrate dopants. We model the effect of spatial distribution of defects using a percolative network and demonstrate that the distribution of threshold voltage degradation for a single additional interface defect, i.e., ΔV T @ΔN IT =1, is exponential, with a fraction of transistors having ΔV T ~0. Finally, we obtain the statistics of ΔV T @t STS by convolving the statistics of N IT @t STS with that of ΔV T @ΔN IT =1. The resultant statistics of ΔV T @t STS compares favorably with a broad range of experiments reported in the NBTI literature.

Modeling of Integrated Circuit Yield Using a Spatial Nonhomogeneous Poisson Process

This paper proposes a new yield model for integrated circuits using a spatial point process. The defect density variation by location on a wafer is modeled by a spatial nonhomogeneous Poisson process. The intensity function of the yield model describes the defect pattern on wafers. As a result, the model differs from the existing compound Poisson yield models in its capability to describe the spatial defect distribution. Using model-based clustering, the defect clusters from assignable causes, which contain the information about the variations of manufacturing processes, can be classified from others. The proposed model considers the defect density variation among wafers and the impact of defect size on the probability of a defect causing the circuit failure. The performance of the new yield model is verified using simulated data and real data. Simulation results show that the new yield model performs better than a compound Poisson yield model.

Measurement of local critical currents in TFA-MOD processed coated conductors by use of scanning Hall-probe microscopy

We have carried out 2-dimensional (2D) measurement of local critical current in a Trifluoroacetates-Metal Organic Deposition (TFA-MOD) processed YBCO coated conductor using scanning Hall-probe microscopy. Recently, remarkable R&D accomplishments on the fabrication processes of coated conductors have been conducted extensively and reported. The TFA-MOD process has been expected as an attractive process to produce coated conductors with high performance at a low production cost due to a simple process using non-vacuum equipments. On the other hand, enhancement of critical currents and homogenization of the critical current distribution in the coated conductors are definitely very important for practical applications. According to our measurements, we can detect positions and spatial distribution of defects in the conductor. This kind of information will be very helpful for the improvement of the TFA-MOD process and for the design of the conductor intended for practical electric power device applications.

Dynamical simulations of radiation damage induced by 10 keV energetic recoils in UO 2

We have performed classical molecular dynamics simulations to simulate the primary damage state induced by 10 keV energetic recoils in UO 2. The numbers versus time and the distance distributions for the displaced uranium and oxygen atoms were investigated with the energetic recoils accelerated along four different directions. The simulations suggest that the direction of the primary knock-on atom (PKA) has no effect on the final primary damage state. In addition, it was found that atomic displacement events consisted of replacement collision sequences in addition to the production of Frenkel pairs. The spatial distribution of defects introduced by 10 keV collision cascades was also presented and the results were similar to that of energetic recoils with lower energy.

Amorphous silicon under mechanical shear deformations: Shear velocity and temperature effects

Mechanical shear deformations lead, in some cases, to effects similar to those resulting from ion irradiation. Here we characterize the effects of shear velocity and temperature on amorphous silicon ($a$-Si) modeled using classical molecular-dynamics simulations based on the empirical environment-dependent interatomic potential (EDIP). With increasing shear velocity at low temperature, we find a systematic increase in the internal strain leading to the rapid appearance of structural defects (fivefold-coordinated atoms). The impacts of externally applied strain can be almost fully compensated by increasing the temperature, allowing the system to respond more rapidly to the deformation. In particular, we find opposite power-law relations between the temperature and the shear velocity and the deformation energy. The spatial distribution of defects is also found to depend strongly on temperature and strain velocity. For low temperature or high shear velocity, defects are concentrated in a few atomic layers near the center of the cell, while with increasing temperature or decreasing shear velocity, they spread slowly throughout the full simulation cell. This complex behavior can be related to the structure of the energy landscape and the existence of a continuous energy-barrier distribution.

Homogeneity of ultrafine-grained copper deformed by high-pressure torsion characterized by positron annihilation and microhardness

Spatially resolved positron annihilation was employed for mapping lateral defect distribution in Cu deformed by high-pressure torsion. Parameters describing Doppler broadening of the annihilation peak were used for mapping the spatial distribution of defects. In comparison with microhardness showing a clear development towards homogeneity, positron annihilation revealed that spatial distribution of defects was far from uniform, even after 25 turns. This was mainly due to a higher concentration of deformation-induced vacancies at the periphery compared with the center.

Nanoscale depth-resolved electronic properties of SiO2/SiOx/SiO2 for device-tolerant electronics

We have used nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS) of device-tolerant oxides to measure the energy levels and spatial distribution of defects within nanocrystalline grains in an α-SiO2 matrix that counteract hole trapping and reduce CMOS transistor degradation due to ionizing radiation. DRCLS of 58 nm SiO2/SiOx/SiO2 oxide structures distinguishes spatially and electronically between the intrinsic SiO2 and SiOx-related defects. Annealing the α-SiO2 oxide produces nanocrystalline Si grain formation that increases the concentration of defects 0.3 eV above the Si valence band that can trap electrons and block radiolytic proton transport. This study validates a carrier trapping mechanism for phase-separated device-tolerant SiOx oxide layers.

Control of Materials and Interfaces in μc-Si:H-based Solar Cells Grown at High Rate

ABSTRACTGrowth process of microcrystalline silicon (μc-Si:H) using plasma-enhanced chemicalvapor- deposition method under high-rate-growth condition has been studied for the control of optoelectronic properties in the resulting materials. We have found two important things for the spatial-defect distribution in the resulting μc-Si:H through a precise dangling-bond-density measurement, e. g., (1) dangling-bond defects are uniformly distributed in the bulk region of μc- Si:H films independent of their crystallite size and (2) large number of dangling bonds are located at the surface of μc-Si:H especially when the film is deposited at high growth rate. Starting procedure of film growth has been investigated as an important process to control the dangling-bond-defect density in the bulk region of resulting μc-Si:H through the change in the electron temperature by the presence of particulates produced at the starting period of the plasma. Deposition of Si-compress thin layer on μc-Si:H grown at high rate followed by thermal annealing has been proposed as an effective method to reduce the defect density at the surface of resulting μc-Si:H. Utilizing the starting-procedure-controlling method and the compress-layerdeposition method together with several interface-controlling methods, we have demonstrated the fabrication of high conversion-efficiency (9.27%) substrate-type (n-i-p) μc-Si:H solar cells whose intrinsic μc-Si:H layer is deposited at high growth rate of 2.3 nm/sec.

Near-field diffraction of gratings with surface defects

Diffraction gratings produce self-images in the near field. Defects on the surface of the grating may occur due to the manufacturing process. These devices are often placed in dirty industrial environments. Dust particles or drops of liquid can be deposited over their surface. In this work, we analyze the effect of surface defects placed over the grating on the self-imaging process. We analytically show how the self-images gradually recover as we separate from the grating when one defect is present. Also a random distribution of surface defects over the grating is analyzed. In particular, we focus on how the contrast of the self-images decreases in terms of the density of the defects. Analytical expressions for the near field are derived, considering a stochastic description of the spatial distribution of defects. In addition, numerical simulations based on the Rayleigh-Sommerfeld formulation are performed to validate the analytical results.

Variogram analysis of charge-carrier effective lifetime topograms in mc-Si materials

All relevant physical properties of semiconductors show spatial fluctuations. It is possible to analyze this spatial variation statistically using variogam analysis. The present paper gives a first example for the application of this method, for the case of charge-carrier effective lifetime, which was measured with the microwave-detected photoconductivity (MDP) system. It can be shown that different types of spatial distribution of defects in wafers are well-characterized by different variograms. Furthermore, there exist different spatial correlation ranges and variances of charge-carrier effective lifetime in wafers from different height positions in a brick.