Spatial Distribution Of Defects Research Articles

Effect of spatial defect distribution on the electrical behavior of prominent vacancy point defects in swift-ion implanted Si

Samples of epitaxially grown $n$-type silicon have been implanted at room temperature with low doses $({10}^{6}--{10}^{9}\text{ }{\text{cm}}^{\ensuremath{-}2})$ of He, C, Si, and I ions using energies from 2.75 to 46 MeV. Deep level transient spectroscopy studies reveal that the generation of divacancy $({V}_{2})$ and vacancy-oxygen (VO) pairs has a distinct ion mass dependence. Especially, the doubly negative charge state of the divacancy, ${V}_{2}(=/\ensuremath{-})$, decreases in intensity with increasing ion mass compared to that of the singly negative charge state of the divacancy, ${V}_{2}(\ensuremath{-}/0)$. In addition, the measurements show also a decrease in the intensity of the level assigned to VO compared to that of ${V}_{2}(\ensuremath{-}/0)$ with increasing ion mass. Carrier capture cross-section measurements demonstrate a reduction in the electron capture rate with increasing ion mass for all the three levels ${V}_{2}(\ensuremath{-}/0)$, ${V}_{2}(=/\ensuremath{-})$, and VO; but a gradual recovery occurs with annealing. Concurrently, the strength of the ${V}_{2}(\ensuremath{-}/0)$ level decreases in a wide temperature range starting from below $200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, accompanied by an increase in the amplitudes of both the VO and ${V}_{2}(=/\ensuremath{-})$ peaks. In order to account for these results a model is introduced where local carrier compensation is a key feature and where two modes of ${V}_{2}$ are considered: (1) ${V}_{2}$ centers located in regions with a high defect density around the ion track $({V}_{2}^{\text{dense}})$ and (2) ${V}_{2}$ centers located in regions with a low defect density $({V}_{2}^{\text{dilute}})$. The ${V}_{2}^{\text{dense}}$ fraction does not give any contribution to the ${V}_{2}(=/\ensuremath{-})$ signal due to local carrier compensation, and the amplitude of the ${V}_{2}(=/\ensuremath{-})$ level is determined by the ${V}_{2}^{\text{dilute}}$ fraction only. The spatial distributions of defects generated by single-ion impacts were simulated by Monte Carlo calculations in the binary collision approximation, and to distinguish between the regions with ${V}_{2}^{\text{dense}}$ and ${V}_{2}^{\text{dilute}}$ a threshold for the defect generation rate was introduced. The model is shown to give good quantitative agreement with the experimentally observed ion mass dependence for the ratio between the amplitudes of the ${V}_{2}(=/\ensuremath{-})$ and ${V}_{2}(\ensuremath{-}/0)$ peaks. In particular, the threshold value for the defect generation rate remains constant $(\ensuremath{\sim}1.2\text{ }\text{vacancies}/\text{ion}/\text{\AA{}})$ irrespective of the type of ion used, which provides strong evidence for the validity of the model. Annealing at temperatures above $\ensuremath{\sim}300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ is found to reduce the spatial localization of the defects and migration of ${V}_{2}$ occurs with subsequent trapping by interstitial oxygen atoms and formation of divacancy-oxygen pairs.

Thermal effects in 10 keV Si PKA cascades in 3C–SiC

We present a molecular dynamics study of the influence of temperature on defect generation and evolution in irradiated cubic silicon carbide. We simulated 10 keV displacement cascades, with an emphasis on the quantification of the spatial distribution of defects, at six different temperatures from 0 K to 2000 K under identical primary knock-on atom conditions. By post-processing the simulation results we analyzed the temporal evolution of vacancies, interstitials, and antisite defects, the spatial distribution of vacancies, and the distribution of vacancy cluster sizes. The majority of vacancies were found to be isolated at all temperatures. We found evidence of temperature dependence in C and Si replacements and C Si antisite formation, as well as reduced damage generation behavior due to enhanced defect relaxation at 2000 K.

Risk Analysis of Structures in Presence of Stochastic Fields of Deterioration: Flowchart for Coupling Inspection Results and Structural Reliability

Inspection by non-destructive testing techniques of existing structures is not perfect and it has become a common practice to model their reliability in terms of receiver operating characteristic (ROC) curves. This paper suggests a method of building ROC curves in case of random fields of defects on a structure by using polynomial chaos decomposition. Knowledge of spatial distribution of defects allows a reliability analysis to be performed. When selecting stochastic finite element analysis to solve this problem, the format is the same as the one chosen for modelling inspections results. The paper shows how to link these quantities (ie. reliability and inspection results) in a risk analysis by using polynomial chaos decomposition as a common language.

Structure and electronic properties of imperfect oxides and nanooxides

Results of studies of the structure and physicochemical properties of perfect (defectless) and imperfect (containing point defects) nanoparticles of metal oxides by the methods of scanning tunnel microscopy (STM) and spectroscopy (STS) are summarized and reviewed. Using nanooxides of various metals (Al, Pt, W, and Ti), as examples, it was demonstrated that the modern STM and STS methods make it possible to measure the electronic spectra of perfect nanoparticles, detect individual and associated point defects, determine spatial distributions of defects, derive spatial and energetic distributions of electrons trapped on point defects (anionic vacancies, intercalation atoms, and adatoms), detect single electron spins (paramagnetic surface complexes), measure phonon and vibration spectra of individual nanoparticles and surface complexes, determine (from measured electronic and phonon spectra) the chemical composition and atomic structure of individual nanoparticles. It was emphasized that modern versions of the STM-STS methods can constitute the foundation of new analytical methods needed in nanometrology and nanodefectoscopy.

Uphill Drift of Vacancies and Self-Interstitials in Silicon Crystal Growth

Point defects, in a growing crystal, drift along the temperature gradient, G, at a velocity α G. The reported microdefect patterns in crystals grown in the vacancy mode, with a temporary halt, include an interstitial region located around the halted interface. The features of this region provide a clear evidence in favour of a strong uphill drift of both kinds of intrinsic point defects. The drift coefficient αV (for vacancies) and αI (for self-interstitials) have been deduced, by fitting the simulated defect profiles to the reported patterns, to be around 1.6x10-5 cm2/sK. The drift phenomenon is essential in establishing the point defect spatial distribution.

Effect of Substrates Thermal Etching on CVD Growth of Epitaxial Silicon Carbide Layers

The influence of in situ etching of Si-face n-4H-SiC wafers in H2 and propane on the surface morphology of the grown epi-layers were examined using differential interference contrast (DIC) optical microscopy and atomic force microscope (AFM). Two defect-selective etching techniques were applied in order to reveal the type and spatial distribution of defects in the substrates and epi-layers. It was found that for the flow applied in this experiment propane plays a significant role for the etching process. Depending on temperature and etching time we obtained completely different picture of substrate surface morphology. The propane etching was verified as a tool for substrate surface improvement.

Use of synchronous nonlinear interaction of flexural Lamb waves in NDT-application

Workability of synchronous nonlinear interaction of flexural Lamb waves in NDT-application is demonstrated. During experiments traveling and standing A0 waves were used. Lamb waves are dispersive, so to provide synchronous nonlinear mode for traveling waves, non-collinear interaction of two waves with frequency f were realized in aluminum alloy plate with defects. In the area of waves' nonlinear interaction, traveling wave of 2f frequency appeared. Standing Lamb waves were excited with amplitude-modulated signal in a cylindrical resonator of aluminum plate. Carrier frequency f and modulating frequency F≪f are equal to natural modes of the resonator. Spectrum of amplitude-modulated waves contained oscillations with frequencies f, f+F, and f-F. As a result of interaction, oscillation on heterodyne frequency F appeared in the plate and excited a natural mode. Amplitudes of traveling and standing waves on heterodyne frequencies depend on presence of defects in investigated area (non-classic nonlinearity). Spatial distribution of amplitudes of initial waves and ones on heterodyne frequency in samples were measured with laser vibrometer PolytecPSV300. In area containing heterogeneities, amplitude increase of waves on heterodyne frequencies was observed. The possibility to define the spatial distribution of defects using measurements of heterodyne wave amplitude distribution was shown experimentally. Work supported by RFBR.

Displacement damage and transmutations in metals under neutron and proton irradiation

The knowledge of the defect and impurity generation rates, as well as the defect spatial distribution, is the corner stone for the understanding of the evolution of material properties under irradiation. This knowledge is also an essential element for comprehensive experimental simulations of the behavior of irradiated materials. In this article the interaction of neutron and proton irradiation with metals is discussed with respect to displacement damage production. Charged particle irradiation is also briefly illustrated. After discussion of the primary interaction of projectiles (neutrons, charged particles in general, and protons in particular) with target atoms/nuclei, we describe the interaction of a recoil atom with other target atoms resulting in the slowing down of the projectile, displacement damage, impurity atom production due to nuclear reactions, and the creation of atomic displacement cascades. Then the further evolution of defect structure is discussed. The next section, devoted to subcascade formation, is divided into two parts. The first experimental evidence of subcascade formation under neutron and charged particle irradiation is presented. Then the models of subcascade formation are described. Finally we review the models for the calculation of displacement damage and show how these models can be applied to displacement damage calculation under neutron irradiation with a demonstration of a real application of the methods discussed to several nuclear facilities. To cite this article: P. Vladimirov, S. Bouffard, C. R. Physique 9 (2008).

Spatial distribution of defects and the kinetics of nonequilibrium charge carriers in GaN wurtzite crystals doped with Sm, Eu, Er, Tm, and supplementary Zn impurities

By analyzing time-resolved and steady-state photoluminescence spectra, it is established that the spatial distribution of rare-earth ion dopants in wurtzite GaN crystals doped with Sm, Eu, Er, or Tm is governed by the type and concentration of defects in the initial semiconductor matrix as well as by the type of the impurity (its capacity for segregation). Doping with multicharged rare-earth impurities and additionally introduced Zn impurity leads to an intensification of emission. The effect of intensification of emission in the case of n-and p-GaN crystals is considered with the use of the model of isoelectronic traps.

Mechanical Properties of Carbon Nanotubes with Randomly Distributed Vacancy Defects

Carbon nanotubes (CNTs) o er very high mechanical properties with huge scatter. The scatter in the properties is believed to occur mainly due to the defects originated inherently during production. CNTs under a tensile load having randomly distributed vacancy defects are simulated to investigate the e ect of the spatial distribution of defects on the mechanical properties. A simple random unit generation method was used to allocate the defects randomly in the single-walled nanotube's structure. The simulation was carried out a using classical molecular dynamics (MD) simulation technique in atomic scale. Defect density of 1 % reduced the failure strength, the failure strain and Young's modulus of CNTs by as much as 42 %, 65 % and 2 %, respectively, while a defect density of 8 % lowered the properties by as much as 52 %, 71 % and 14 %, respectively. The scatter in the properties due to the random distribution of the defects was found to increase with increasing number of defects in the SWNTs. For example, for a defect density of 8 %, the standard deviations of the data for 20 sample simulations having di erent distributions normalized to the mean values calculated for the failure strength, the failure strain and Young's modulus were about 11 %, 20 % and 8 %, respectively. The defect arrangement in the SWNT's structure is one of the key factors in determining its mechanical properties and the population of defects.

Monte Carlo study of crystalline order and defects on weakly curved surfaces

We numerically study the ground states of particles interacting via a repulsive Yukawa potential on two rigid substrates shaped as isolated and periodically arranged bumps characterized by a spatially varying Gaussian curvature. Below a critical aspect ratio that describes the substrate deformation, the lattice is frustrated, but defect free. A further increase of the aspect ratio triggers defect unbinding transitions that lower the total potential energy by introducing dislocations either in isolation or within grain boundaries. In the presence of very strong deformations, isolated disclinations are nucleated. We show that the character and spatial distribution of defects observed in the ground state reflect the symmetries and periodicity of the two model surfaces investigated in this study.

Yield prediction via spatial modeling of clustered defect counts across a wafer map

In this paper we propose spatial modeling approaches for clustered defects observed using an Integrated Circuit (IC) wafer map. We use the spatial location of each IC chip on the wafer as a covariate for the corresponding defect count listed in the wafer map. Our models are based on a Poisson regression, a negative binomial regression, and Zero-Inflated Poisson (ZIP) regression. Analysis results indicate that yield prediction can be greatly improved by capturing the spatial distribution of defects across the wafer map. In particular, the ZIP model with spatial covariates shows considerable promise as a yield model since it additionally models zero-defective chips. The modeling procedures are tested using a practical example.

Simulated positron lifetime (SimPL): a Monte Carlo simulator for positron diffusion, trapping, and lifetime spectra

AbstractA JAVA‐based Monte Carlo simulation program (SimPL) has been created which generates realistic positron lifetime (PL) spectra. The program achieves this by simulating the diffusion, trapping, and annihilation, of positrons in solids. Users can define: defect structures (type, size, concentration, and spatial distribution); behaviour of the positrons in each trap state (trapping rate, lifetime, and effective range of interaction); and spectrometer performance (time resolution, background). SimPL is shown to be very useful for assessing the accuracy of fitted parameter (lifetimes, intensities) values extracted from PL spectra using, e.g. PATFIT. It is also shown to be useful for quantifying the effect that defect spatial distributions have on the positron trapping fraction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modelling spatial distribution of defects and estimation of electrical degradation of silicon detectors in radiation fields at high luminosity

The irradiation represents a useful tool for determining the characteristics of defects in semiconductors as well as a method to evaluate their degradation, a fact with important technological consequences. In this contribution, starting from available data on the degradation of silicon detector characteristics in radiation fields, these effects are explained in the frame of a model that supposes also the production of the Si FFCD defect due to irradiation. The displacement threshold energies different for different crystallographic axes, considered as parameters of the model, are established and the results obtained could contribute to clarify these controversial aspects. Predictions of the degradation of electrical parameters (leakage current, effective carrier concentration and effective trapping probabilities for electrons and holes) of DOFZ silicon detectors in the hadron background of the LHC accelerator, supposing operation at −10 °C, are done. The non-uniformity of the rate of production of primary defects and of complexes, as a function of depth, for incident particles with low kinetic energy was obtained by simulations using some particular and very simplifying assumptions, suggesting the possible important contribution of the low-energy component of the background spectra to detector degradation.

Laser-energy-dependent Raman scattering studies of graphitic foams

In this work, we present a detailed Raman spectroscopy study of graphitic foams probing the spatial and laser excitation energy dependences of the double resonance Raman peaks. We have observed a spatial dependence of the $D$ to $G$ band intensity ratio $({I}_{D}∕{I}_{G})$ and of the relative contribution from the two-dimensional (2D) and three-dimensional (3D) graphite regions to the ${G}^{\ensuremath{'}}$ band in the Raman spectra. The $D$ band integrated intensity was found to decrease linearly with increasing laser energy $({E}_{L})$, in contrast with recent experiments on nanographite, which showed an ${E}_{L}^{\ensuremath{-}4}$ dependence for the ${I}_{D}∕{I}_{G}$ ratio. The calculation of the skewness of the ${G}^{\ensuremath{'}}$ band was found to be a good qualitative measure of the relative density of 2D and 3D graphite in a given region of the sample. The direct comparison between the spatial distribution of defects, given by the ${I}_{D}∕{I}_{G}$ ratio, and the presence of the 2D graphite phase, given by the skewness of the ${G}^{\ensuremath{'}}$ band, suggests a correlation between the presence of defects and the high density of 2D graphite.

The influence of mask area ratio on GaN regrowth by epitaxial lateral overgrowth

The optical performance of blue-light emitting GaN is greatly influenced by the density and distribution of threading dislocations (TDs), which can be reduced and controlled by varying the ratio of mask-to-window width during epitaxial lateral overgrowth (ELOG). Here we study the evolution of microstructure and optical property along with the spatial distribution of defects in ELOG GaN films by varying the fill factor (FF), defined as mask-to-window width ratio, which was varied from one to five while the window width was maintained at 5 μm. We employed a two-step ELOG method for this study by way of metal organic chemical vapor deposition, where SiO 2 was used as the mask and grown on a low-temperature grown GaN layer, which acted as the buffer and seed layer on a sapphire substrate. The patterns were comprised of circular dot arrays arranged along the 〈1 1¯ 0 0〉 directions of the underlying GaN and developed by the conventional photolithography method. X-ray rocking curves show that all the resulting films are of excellent epitaxial quality, of which the sample with FF=5 exhibits the best quality from the narrowest peak broadening. Cathodoluminescence spectra show the near bandgap UV emission as well as a yellow emission, which could have arisen from TDs. Transmission electron microscopy results show that the TDs emerging through the windows can be effectively impeded by the mask and bent toward the faceted planes of GaN pyramid islands.

Self-ordering of random intercalates in thin films of cuprate superconductors: Growth model and x-ray diffraction diagnosis

We propose a simple model for the nucleation of random intercalates during the growth of high-temperature superconductor (HTSC) films by pulsed laser deposition (PLD). The model predicts a very particular spatial distribution of defects: a Markovian-like sequence of displacements along the growth direction ($c$ axis), as well as a two-component in-plane correlation function, characteristic of self-organized intercalates. A model for x-ray diffraction (XRD) on such structures is also developed and accounts for both $c$-axis and in-plane anomalies observed in XRD experiments. The method presented in this work constitutes a useful characterization tool in the optimization of deposition parameters for the growth of HTSC films.

Photoluminescence study of the 1.3–1.55 eV defect band in CdTe

We report on the photoluminescence (PL) study of defect levels in bulk CdTe. The PL of CdTe was measured in the 1.3–1.55 eV spectral range and at temperatures from 9 to 300 K. The PL spectra were analyzed numerically and various parameters (peak positions, Huang–Rhys factors, etc.) determined. Numerical modeling of low-temperature spectra was performed, and it was shown that the defect band consists of three distinct transitions. Also, we performed hyper-spectral imaging of PL at 77 K and analyzed the spatial distribution of defects and calculated the level of spatial correlation between the defects.

Effects of Spatial Distribution of Defects on Bending Deformation and Critical Current in Bi2223/Ag Superconducting Composite Tapes

The strain dependence of the critical current, Ic, of Bi2223/Ag/Ag-alloy composite superconducting tapes has been studied both experimentally and analytically under bending deformation for two types of tape used in the VAMAS bending round-robin program (classified as VAM1 and 3). Our former analysis showed that the experimentally obtained Ic values were between the calculated ones based on a damage-free initial state and a case where delamination occupied the full width of the tape mid-plane. The experimentally obtained Ic values were explained by the delamination occupying partial width of the tape mid-plane. However, the microscopic observation indicated that the delamination location in the thickness direction was not limited to the mid-plane. In the present study, the analysis was modified to incorporate the movement of the delamination location in the thickness direction. The calculated Ic values with delamination increased when the delamination location moved to compressive side of the tape, and decreased when that moved to tensile side of the tape. Finally, the experimental Ic values can be understood by the distribution of delamination in both width and thickness direction.

Surface topology using laser backscattering and photoluminescence on Cu- and In-rich CuInSe2 thin films

An easy-to-arrange imaging system was fabricated and used to probe surface topology of polycrystalline CuInSe2 thin films used for solar cell fabrication. Surface images developed using laser backscattering and photoluminescence (PL) showed correlation with AFM images. Imaging by laser backscattering was faster compared to PL imaging. The backscattered image was a profile of the air–film interface, which represented more closely the topology obtained using atomic force microscopy (AFM) analysis. Surface roughness calculated using laser backscattering was found to be agreeable with the value obtained using a stylus probe. PL topology was of lower resolution probably because of the infrared emission wavelength used for the imaging. Cu-rich films possessed hills and valleys on their surfaces while In-rich films were smoother with rounded features. The spatial distribution of PL intensity was correlated with the spatial defect distribution.