Distribution Of Point Defects Research Articles

Equilibrium Point Defect and Electronic Carrier Distributions near Interfaces in Acceptor‐Doped Strontium Titanate

A mathematical model based on Frenkel's theory for charged defect segregation at interfaces is used to calculate the equilibrium grain boundary depletion layer widths and conductivity profiles in acceptor‐doped SrTiO3 ceramics. The calculations examine the effect of oxygen vacancy equilibration during annealing at moderate temperatures (∼1000 K) on the development of interfacial charge that influences grain boundary electrical properties at lower temperatures. Good agreement is demonstrated between the model predictions and experimental results reported in the literature.

Influence of Czochralski silicon crystal growth on wafer quality: an extensive investigation using traditional and new characterization techniques

In this paper results on the relationship between Czochralski silicon crystal growth and wafer quality characteristics are presented. Several crystals grown with different oxygen content and growth parameters are examined. The growth parameters are varied in order to obtain different types and distributions of intrinsic point defects, namely vacancies and interstitials. Traditional and more recent characterization techniques are applied in this analysis, such as gate oxide integrity and light point defects evaluation, or high resolution lifetime mapping. In this way, a comprehensive picture is generated of the properties of a silicon wafer and how it is impacted by the crystal growth process.

Simulation of point defect distributions in silicon crystals during melt-growth

Simulations of the point defect diffusion during the crystal growth process are reported for the investigation of the relationship between the distribution of the oxygen precipitation and point defect concentration in CZ silicon crystals. Distribution of the oxygen precipitation is strongly affected by the point defect concentrations during the growth process. There have been many proposed models for the point defect diffusion during the crystal growth process, and the correspondence between point defect concentrations and grown-in defects has been extensively investigated. In this paper, we have compared the distribution of the oxygen precipitation and the ratio of super-saturation between vacancies and self-interstitials. The experimental results agreed with the results obtained by calculation.

The primary damage in Fe revisited by Molecular Dynamics and its binary collision approximation

ABSTRACTMolecular Dynamics (MD) is a very powerful tool for studying displacement cascades initiated by the neutrons when they interact with matter and thus evaluate the primary damage. The mean number of point defects created can be obtained with a fair standard error with a reasonable number of cascade simulations (10 to 20 [1]), however other cascades characteristics (spatial distribution, size and amount of defect clusters …) display a huge variability. Therefore, they may need to be studied using faster methods such as the Binary Collision Approximation (BCA) which is several order of magnitude less time consuming. We have investigated the point defect distributions subsequent to atomic collision cascades by both MD (using EAM potentials for Fe) and its BCA. MD and its BCA lead to comparable point defect predictions. The significant similarities and differences are discussed.

Analytical Approximations for the Distributions of Intrinsic Point Defects in Grown Silicon Crystals

Analytical approximations for the distributions of vacancies and self-interstitials in Si crystals are derived in a one-dimensional point defect model. In the case of an unlimited high recombination rate all results depend only on the growth parameter P = V/G with V being the growth rate and G the temperature gradient at the melt/solid interface. For the critical growth parameter P crit , which separates the vacancy-dominated region from the interstitial-dominated region, the well-known relation of Voronkov is derived. This relation and also the first correction to P crit are in excellent agreement with recent experimental results published in literature. In the case of a limited recombination rate the transport equations were solved first of all by neglecting diffusion. This solution is completed by small correction terms using a special series expansion, which depend on the diffusivities. With this approximation important features such as the influence of the recombination and cooling rate on the distributions of intrinsic point defects can be analyzed.

Nitrogen addition during flame deposition of diamond: a study of nitrogen-enhanced growth, texturing and luminescence

Abstract The effect of nitrogen addition during flame deposition of diamond on the growth rate, morphology and spatial distribution of luminescent point defects in the layers has been studied. For this, well-defined amounts of nitrogen were added to the acetylene and oxygen source gases. The samples were grown under conditions that are known to give diamond deposits with a central area which is unaffected by in-diffusion of nitrogen from the ambient. Consequently, the observed features in this region must be totally ascribed to the deliberately added nitrogen. Upon nitrogen addition, the diamond growth rate in the central area of the deposits initially increases from 54 to 125 μm/h, while at the same time a {001} texture develops. With further increasing nitrogen concentrations, the growth rate drops again while the diamond crystallites in the layer start to deteriorate and become increasingly separated from each other. This results in a morphology which is usually observed in an annulus around the central area of flame deposited diamond. Photoluminescence spectroscopy and cathodoluminescence topography reveal an increasing incorporation of nitrogen as nitrogen–vacancy pairs and a loss of homogeneity in the central area of the diamond layers with increasing nitrogen addition. Based on the results of the present study, a recipe is given for reproducible flame deposition by the use of a commercially available welding torch and 99.6% pure acetylene.

Effects of point defect distribution on arsenic precipitation in low-temperature grown III–V arsenides

The behavior of As precipitation in low-temperature grown III–V arsenides is investigated and correlated with the doping level, crystal bond strength, and dislocation density. Experimental results reveal that the doping level affects the concentration of charged defects, such as vacancy and antisite point defects, and hence leads to the selective precipitation of excess As in homojunctions. For heterostructures, As precipitates tend to condense in materials with a lower bond strength due to differences in point defect concentrations between the materials. In addition, dislocations are found to be a vacancy source that facilitates As precipitation around them. These results indicate that column III vacancies play an important role in As precipitation of low-temperature grown III–V arsenides.

Point defect distributions in ZnSe crystals: effects of gravity vector orientation during physical vapor transport growth

ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) configurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the grown ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [ V Zn]. Annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photoluminescence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [ V Zn] were observed along the gravity vector direction whereas in the vertically stabilized grown crystal the segregation of these point defects was radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth configurations were interpreted based on the possible buoyancy-driven convection in the vapor phase.

Factors determining the saturation of point defects in growing silicon crystals

An analytical calculation on the behavior of point-defects in growing silicon crystals was performed. The calculation concluded that the saturation of point-defect is determined by only two factors, which are the ratio of the normalized temperature gradient to the growth rate ( T′/ TV) and the ratio of the second-order differential of the temperature to the product of the temperature gradient and the growth rate ( T″/ T′ V). The former is same as Voronkov's conclusion except that the temperature gradient is normalized and the latter is newly found in the current calculation. In case of zero or near to zero growth rate, the saturation is determined by only one factor, which the ratio of the second-order differential of the temperature to the square of the temperature gradient ( T″/( T′) 2). In addition, diffusion equations where the heat of transfer takes minus values of the formation enthalpy, which were adopted by most of the models proposed so far, are also discussed. It was concluded that those equations do not show any dependency of point-defect distributions on the growth conditions without assuming such large diffusivity that the transport of point-defects due to Fickian diffusion is not negligible compared with that due to the crystal pulling.

Destination of point defects and microstructural evolution under collision cascade damage

Microstructural evolution strongly depends on the annihilation sites of point defects and the time sequence for point defect reactions. Factors to determine the destination of point defects in collision cascade damage are categorized and the mechanism to control the destination is examined. Major factors that determine the destination are the initial distribution of point defects, the type and distribution of annihilation sites, and the style of migration of point defects and their mobility. The initial distribution of point defects varies with primary recoil energy, materials parameters and damage rate. Major annihilation sites for point defects are point defects, point defect clusters and permanent or fixed sinks such as dislocations, grain boundaries, surfaces, etc. Microstructural evolution strongly depends on the geometrical relationship between annihilation sites and cascades. The mobility and direction for migration of point defects are changed by the irradiation temperature and the strain fields from various sources, respectively. The consequence of the different mode of point defect migration is also discussed.

On the Nonlinear Mechanics of Bravais Crystals with Continuous Distributions of Defects

Basic ideas in the mechanics of Bravais crystals with continuous distributions of point and line defects are revisited from a perspective that emphasizes the role of configurational forces. In addition, the creation, destruction, and transport of defects are presented in detail. The formulation of the balance laws is given over time-dependent spatial control volumes; furthermore, independent configurational and deformational force balances are postulated. Such a formulation is more versatile than one with standard fixed control volumes and provides the correct Eshelby relation from simple invariance arguments independent of constitutive relations. Two alternative but equivalent constitutive models are proposed, and the corresponding restrictions following from a dissipation inequality are determined. Relations to some previous formulations of continuous distributions of dislocations and vacancies are discussed.

Studying interfaces on a nm scale by BEEM

Ballistic electron emission microscopy is a technique derived from scanning tunnelling microscopy and allows the investigation of the electrical and structural properties of metal/semiconductor and semiconductor/semiconductor interfaces on a small scale. In our experiments on thin (2–3 nm) metallic CoSi 2 films grown epitaxially on Si(111), we have studied the distribution of point defects at the interface. They tend to accumulate within the core of partial dislocations. The lowered density of point defects found close to dislocations is explained by diffusion and subsequent trapping during an annealing step. Ballistic electron emission spectroscopy performed on a single point defect was used to identify the contrast mechanism. A current contribution of k ‖ conserving processes with a delayed onset is much less affected by the presence of a point defect than a current component due to electrons scattered at the interface, which shows a strong enhancement.

Depth distribution of point defects in Si bombarded by high-energy N5+ and Si5+ ions

Electron spin resonance has been used to study the depth distribution of point defects in Si samples bombarded by N5+ (E=16 MeV) and Si5+ (E=26.8 MeV) ions at 175 and 300 K in the dose range (4–8)×1015 cm−2. It was established that unlike the implantation of moderate-energy Si ions (E ∼ 100 keV), the depth distributions of planar tetravacancies in samples bombarded by ions at 300 K under these conditions have two maxima. The experimental results indicate that the tetravacancy density maximum closer to the surface is formed as a result of secondary defect formation processes. No continuous amorphous layer was observed in the bulk of any of the Si samples. This experimental observation is evidence of defect annealing which takes place when high-energy ions are implanted in Si.

Luminescence from growth topographic features in GaN:Si films

Cathodoluminescence (CL) in the scanning electron microscope is used to investigate the nature of defects responsible for the luminescence associated with round and hexagonal-like topographic features of GaN:Si films. Round hillocks of the size of a few microns, which sometimes have a nanopipe related central hole, do not influence the luminescence emission of the film. Hillocks with sizes of several tens of microns show a marked CL contrast at the center and at the border. The origin of the observed contrast is attributed to a growth induced inhomogeneous distribution of point defects and impurities. Radiation with the electron beam of the scanning microscope causes a decrease of the CL intensity without spectral changes.

Kinematics of edge dislocations. I. Involutive distributions of local slip planes

The geometry of continuous distributions of dislocations and secondary point defects created by these distributions is considered. Particularly, the dependence of a distribution of dislocations on the existence of secondary point defects is modeled by treating dislocations as those located in a time-dependent Riemannian material space describing, in a continuous limit, the influence of these point defects on metric properties of a crystal structure. The notions of local glide systems and involutive distributions of local slip planes are introduced in order to describe, in terms of differential geometry, some aspects of the kinematics of the motion of edge dislocations. The analysis leads, among others, to the definition of a class of distributions of dislocations with a distinguished involutive distribution of local slip planes and such that a formula of mesoscale character describing the influence of edge dislocations on the mean curvature of glide surfaces is valid.

Analysis of light emitting polymer electrochemical cells

Polymer electrochemical cells with ion blocking electrodes were reported to emit light under applied voltage. This work analyzes the current-voltage relations, internal electric fields, and point defect distribution in the polymer. The polymer is regarded as a mixed-ionic-electronic conductor. Two relevant defect models are investigated. A good fit is obtained between experimental data and theory which also takes into consideration electrode overpotentials.

A study of grain-boundary structure in non-stoichiometric NiAl by atomistic simulation and electron microscopy

Abstract The atomic structure of a Σ =5,(310)[001] grain boundary in NiAl has been determined by a synergistic approach using high-resolution electron microscopy (HREM) and atomistic structure calculations. The effect and distribution of point defects at the grain boundary were determined using molecular statics calculations employing N-body empirical potentials to calculate the relaxed grain-boundary structures and energies resulting from various initial structures and stoichiometries. Monte Carlo calculations confirm the stability of the lowest-energy structure, which contains Ni antisite defects adjacent to the grain-boundary plane. Multislice image simulations of this structure are in good agreement with the experimental HREM image. This is the first combined application of experimental and theoretical structure determinations to an intermetallic grain boundary.

Formation and growth process of defect clusters in magnesia under ion irradiation

In order to understand the effect of damage cascades on the formation and growth process of dislocation loops in MgO under ion irradiation, in-situ observations of microstructural evolution have been performed with the use of a 400 keV TEM combined with two sets of ion accelerators. In-situ observations under 1000 keV electrons have been also done with the use of high voltage electron microscopy. The 1 2〈110〉{110} type of interstitial dislocation loops are formed under irradiation with various kinds of particles. The minimum necessary dose at which the loops are able to be observed in TEM and their formation and growth rates, strongly depends on the irradiation particles. Such dependences of between kinetic processes on irradiation particles are considered to be influenced not only by the distribution of point defects but also by their stability, that is, the higher density of point defects inside damage cascades or at their periphery results in suppressing the growth of loops due to the enhancement of loop nucleation.

Study of Interfacial Point Defects by Ballistic Electron Emission Microscopy

Ballistic electron emission microscopy has been used to study individual point defects, which are located at the ${\mathrm{CoSi}}_{2}/\mathrm{Si}(111)$ interface of thin ( $\ensuremath{\sim}30\AA{}$) silicide films grown epitaxially on silicon substrates by molecular beam epitaxy. Clear evidence for trapping of point defects at dislocations is presented. The lateral distribution of the interfacial point defects is explained in terms of diffusion during an annealing step in the growth process.

Reconstruction of recombination properties of extended defects in Si

Electron beam induced current (EBIC) profiles of extended defects in plastically deformed Si are studied. Dependences of profile width and contrast on excitation level are found to be different for the defects with different impurity atmosphere. Numerical simulations of the EBIC profiles for dislocations and slip planes show that experimental data can only be described by taking into account recombination at point defects surrounding the extended defects. The characteristic length of point defect distribution is found to be about 1 μm.