Distribution Of Point Defects Research Articles

Mechanisms of irradiation growth of alpha-zirconium alloys

Irradiation growth is thought to occur when there is an anisotropic distribution of sinks receiving a net flux of vacancies and this anisotropy is different from that of the distribution of sinks receiving a net flux of self-interstitials. For a complete understanding of irradiation growth one must identify the possible sinks, identify the source of the anisotropy and rationalize the distribution of point defects amongst the sinks. Early theoretical models for growth attempted to explain what appeared to be relatively straightforward behaviour with simple models and microstructures. However, experimental observations in the last few years have shown that the range of irradiation growth behaviour of alpha-zirconium alloys is more varied, that a wider variety of sinks must be considered, and that there are more potential sources of anisotropy than was previously recognized. The important new experimental observations which influence our perception of the growth phenomenon in zirconium alloys include the growth of single crystals, accelerating growth in annealed material with the coincident appearance of vacancy loops on the basal planes, the occurrence of “negative” growth, i.e., contractions along prism directions, the absence of a pronounced effect of grain size on the long term growth rate at low temperatures, and the presence of intergranular constraints prior to irradiation. With the greater complexity of behaviour now being observed, it is necessary to apply new theoretical concepts to assist in understanding growth, e.g., the potential role of anisotropic diffusion in segregating point defects to different sinks and “growth” caused by the anisotropic relaxation of intergranular constrains. These can be combined with earlier ideas to predict a variety of growth behaviours, including “negative growth”. Because the most important physical information required for theoretical treatments of growth, i.e, the characteristics of vacancies and self interstitial atoms, are still poorly understood, it is almost impossible to test rigorously any particular theoretical concept and a complete picture of growth has yet to emerge.

Photoluminescence tomography as a method to image point-defect distributions in crystals: Nitrogen-vacancy pairs in synthetic diamonds

The distribution of grown-in nitrogen-vacancy pairs in synthetic Ib diamond crystals has been imaged by means of a new photoluminescence tomographic technique. Using this method, local changes in luminescence intensity introduced by these defect centers were mapped by means of a laser-scanning microtomographic arrangement. It was found that the spatial variation in nitrogen-vacancy pair concentrations within a crystal qualitatively correlates with the nitrogen impurity distribution.

On the distribution of point defects in large sized bismuth germanate crystals

The spatial distribution of UV-blue absorbing point defects in Bridgman grown bismuth germanate (Bi 4(GeO 4) 3) crystals has been imaged by means of UV absorption topography. From the occurence and position of growth bands and sector boundaries it was concluded that in all conditions the crystals only grow via the crystallographic non-equivalent {112} and {ovbar|112} facets. The exclusiveness and the F-type character of the (slower growing) {112} and (faster growing) {ovbar|112} faces were confirmed by sphere growth experiments and ex-situ surface microtopography. Within many crystals it was found that the {112} sectors show a higher UV-blue absorption and thus an enhanced impurity segregation than the adjacent {ovbar|112} sectors. This point to kinetically (i.e. non-equilibrium) determined segregation. Imaging of crystals by the Schlieren method showed a higher optical density for the {ovbar|112} growth sectors. Laser light scattering disclosed a preferential occurence of inclusions near sector boundaries.

Two-dimensional modeling of ion implantation induced point defects

An analytical model for the description of ion-implantation-induced damage profiles is presented. The model is based on extensive Monte Carlo simulations of B-, P-, As-, and Sb-implantations in Si. One-dimensional profiles are described by a Gaussian function and an exponential function joined together continuously with continuous first derivatives. The two-dimensional model has previously been developed by the authors for dopant profiles and is demonstrated to apply well to point defect distributions. Parameters have been obtained for the four ions by fitting the model to the Monte Carlo results, and they are provided in the form of tables for the energy range of 10-300 keV (for the 1D model 1-300 keV). The Monte Carlo simulations are based on the binary collision approximation, the assumption of a random target, and the validity of the linear collision cascade theory. The importance of energy transport by recoils is pointed out.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Boron Precipitation and misfit dislocation structure in Si(Ge,B) buried epitaxial layers

Heavily boron-doped silicon buried epitaxial layers are becoming increasingly important in the fabrication of thin membranes, three dimensional structures in silicon and latch-up free CMOS circuits. Si(Ge,B) co-doping has been utilized to compensate the B-induced lattice contraction in Si and hence buried high conducting layers which are strain-free and lattice matched to the Si substrate have been realized. The utilization of isoelectronic Ge also alters the point defect distribution in silicon resulting in reduced dopant diffusion which is an added advantage in realizing shallow junctions and reduced interfacial transition width in epitaxial layers. This contribution addresses the evolution of misfit dislocation structure and B precipitation behavior in heavily B-doped buried Si epitaxial layers. In addition, the effect of Ge co-doping on B solubility in Si will be discussed.Silicon epitaxial layers were grown at 1080°C by chemical vapor deposition on 4-inch diameter p-type (100) substrates (10 and 0.04 Ω-cm) employing the SiH2Cl2-B2H6-GeH4-H2 chemical system. Single 5 μm thick epilayers and 2 μm buried layers with 4 μm intrinsic cap layers (10 Ω-cm) were grown.

Ion bombardment effect on Si homoepitaxial growth from ion molecular beams

Abstract The effect of ion bombardment with energy of 0.5–5 keV on the process of Si film epitaxial growth on a silicon substrate has been studied. It was shown experimentally that single crystal films are formed at an accelerating potential of 2 keV if the degree of vapour current ionization is 0.1%. In this case the epitaxial temperature is two times lower. Spatial distributions of thermal energy being released during Si+ and Ar+ ion implantation according to ion energy and oxide layer thickness and the distribution of point defects have been obtained by computer simulation based on the Monte-Carlo method. SO2 film destruction by ion bombardment has been considered. The density of growing centres has been calculated and it is in a good agreement with the experimental value. The mechanism of ion irradiation effect on the process of Si film epitaxial growth is discussed.

Simulation of retarded diffusion of antimony and enhanced diffusion of phosphorus in silicon

The paper is concerned with the influence of point defects on dopant diffusion in silicon. This influence is analysed by means of comparing the experimental results of Mizuo and Higuchi with our simulation. The experiments deal with the oxidation retarded diffusion (ORD) of Sb and the oxidation enhanced diffusion (OED) ofP in FZ silicon at 1100°C. The simulation is carried out by using a physical model without simplifying assumptions. It is shown that simplifying assumptions are not admissible. A reasonable set of parameters is deduced from this analysis. As each parameter represents a physical effect, information about the importance of bulk and surface recombination of point defects and about the equilibrium concentration values and diffusion coefficients of diffusing species are obtained. It turns out that the influence of the surface is decisive for the distribution of point defects.

A NEW MODEL FOR THE DETERMINATION OF POINT DEFECT EQUILIBRIUM CONCENTRATIONS IN SILICON

In this paper the boundary conditions for point defect distributions in monocrystalline silicon are investigated. These boundary conditions are established by simple thermodynamic considerations. A circle process is used including vacancy, interstitial and Frenkel pair generation which yields a simple relationship between the vacancy and interstitial equilibrium concentrations at the surface. A new OED model is also presented which explains the t−1/4 behaviour of the interstitial supersaturation. This model is used to simulate experiments of Mizuo and Higuchi. In this way values for the equilibrium concentrations and the diffusion coefficients of vacancies and interstitials are obtained.

Ordered defects in the oxides of uranium

Like some other oxides of the actinide elements, uranium dioxide has a fluorite structure and a propensity towards deviation from simple stoichiometry. Because oxygen can be transferred with ease between the UO 2 lattice and the ambient gas phase, gross departures from stoichio­metry occur readily. Diffraction data have shown that structural models of the hyperstoichiometric phases U 4 O 9 and U 3 O 7 cannot be based on a random distribution of point defects. Instead, interaction between interstitial defects produces microscopic substructures or clusters intro­ducing specific local properties. Spectroscopic evidence is presented together with the alternative cluster arrangements which are currently proposed as the basic structural units occurring in the hyperstoichiometric oxides of uranium. From this base a new relationship for the structure of the uranium-oxygen system has been developed for the compositional range UO 2 -UO 3 .

Two-dimensional phosphorus diffusion for soft drains in silicon MOS transistors

Phosphorus has a considerably less steep concentration profile than arsenic. Therefore phosphorus is considered as an alternative dopand for soft drain concepts in future MOS devices. In-diffusion of phosphorus starting from a high surface concentration generatesexcess point defects which diffuse into the depth of the crystal and lead to a tail in the phosphorus concentration profile by considerably enhancing the phosphorus diffusion in this region. It is also well known that the interface between silicon and a non growing oxide acts as a sink for excess point defects. Since source/drain areas of MOS transistors are surrounded by gate and isolation oxides, the question arises how the resulting excess point defect distribution may influence the lateral and vertical diffusion profile of phosphorus and hence the channel length and the junction depth of the source/drain region in a MOS device. We extended the one-dimensional Fair-Tsai model of phosphorus diffusion into two dimensions and incorporated that the interface between silicon and a gate oxide acts as a sink for excess point defects and modifies their distribution. The appropriate code was implemented in the two-dimensional process simulation program LADIS. Based on this extended model two-dimensional simulations of phosphorus drains have been performed and compared to experimental results and to results from other numerical models. It turns out that the presence of the gate oxide reduces the tail in the phosphorus concentration profile, considerably in lateral direction and less pronounced in vertical direction. Limitations of the model will be discussed in detail.

Characterisation of the β"-Al2O3 solid electrolyte with thermoluminescent techniques

The current study has examined the thermoluminescent properties of polycrystalline beta "-Al2O3. The material is utilised as a solid electrolyte in the Na-S cell. The work has examined the thermoluminescence technique with regard to its ability to assess changes in the point defect distribution within the solid electrolyte. beta "-Al2O3 fabricated with the incorporation of various divalent cation additives has been characterised with thermoluminescence. The additives employed have been those used to stabilise the beta "-phase of the alumina structure. The effect that cell operation has upon the thermoluminescent properties has also been investigated. General conclusions are drawn with regard to how both additives and cell operation effect the point defect distribution within the solid.

Spatial Distribution of Point Defects and Complexes in Semi-Insulating LEC and Si-Doped GF Grown GaAs Crystals

Spatial Distribution of Point Defects and Complexes in Semi-Insulating LEC and Si-Doped GF Grown GaAs Crystals

A Model for Predicting the Production Yield of Integrated Circuits

In the production of integrated circuits (I.C.s), the ability to estimate the yield of working devices, or die, is economically important. This yield depends on the distribution of material and processing defects across the surface of the wafer of devices.A number of models have been proposed to explain the distribution of point defects on I.C.s. Most of these models are based on the Poisson distribution for describing the number of defects expected on a die. These models do not inherently allow for dependence between the number of defects expected on two adjacent die.This paper develops a model for generating an I.C. defect distribution that allows for dependence in the number of defects on die that are near one another. The correlation matrix and yields of observed data are found to compare favourably with theoretical results of the proposed dependent model. Simulation results are used to compare the new dependent model to a previously proposed independent model. These dependent-model simulations show defective die clustered near each other on the wafer, a property that has been observed on real wafers. Methods of parameter and yield estimation in the dependent model are discussed.

Introducing dependency into IC yield models

Many models have been proposed to explain the distribution of point defects on integrated circuit (IC) chips. The most popular of these models are based on distributions derived from the Poisson distribution for describing the number of defects expected on a chip. These models do not inherently allow for dependence between the number of defects expected on two adjacent chips. This paper introduces a model for generating an IC defect distribution that allows for dependence in the number of defects on chips that are near one another. Simulations are done to compare the new dependent model to a previously proposed independent model. These simulations with the new model show defective chips clustered near each other on the water. This clustering property has been observed on real wafers. Methods of parameter and yield estimation in the dependent model are discussed.

Radiation defects in Te-implanted germanium Electron microscopy and computer simulation studies

Abstract Direct observation of radiation damage induced by heavy ion implantation in crystalline germanium by means of high-resolution electron microscopy is reported. The dark-field lattice imaging mode is used, under conditions suitable for object like imaging. Conventional TEM is used for estimating the efficiency of creating visibly damaged regions. Heavy ion damage clusters with three types of inner structure are observed: with near-perfect crystalline cores, and with metastable and stable amorphous cores. The MARLOWE computer code is used to simulate the atomic collision cascades and to obtain the lateral spread distributions of point defects created. A comparison of high-resolution electron microscopy (HREM) with computer simulation results shows encouraging agreement for the average cluster dimensions and for the lateral spread of vacancies and interstitials.

The Formation of Amorphous Silicon by Light Ion Damage

ABSTRACTA model for formation of amorphous silicon by light ion implantation is proposed. It is suggested that accumulation of point defects and/or complexes is required at the initial stage of the amorphization process. Amorphous zones can only form at the end of incoming light ion tracks when the pre-accumulated concentration of point defects reaches a critical value. Depending on the uniformity of the point defect distribution, two possibilities for the second stage of amorphization are suggested when ion implantation is performed at different temperatures.Silicon wafers implanted with boron ions below and above the critical amorphization dose at various temperatures have been investigated using cross section specimens in high resolution TEM. Complementary analyses of these specimens by Electron Paramagnetic Resonance have revealed the presence of dangling bonds in amorphous zones and point defect clusters. Extrinsic stacking faults with 1/3 &lt;111&gt; displacements and other smaller distortions with 1/x&lt;111&gt; displacements were also found to result from the amorphization process. Liquid nitrogen temperature was found to be necessary to cause complete amorphization of silicon by boron ion implantation.

Strain scattering of electrons in piezoelectric semiconductors

Static strains in piezoelectric semiconductors give rise to an electric field or potential which can have an effect on the electrical properties of the material. We have calculated the electric potential due to the strain field arising from a random distribution of point defects. This potential contributes a term to the mobility that is proportioned to T1/2 and a Hall factor of 1.10. Crude estimates of strain strengths indicate that this scattering mechanism may contribute significantly to the mobility of electrically rather pure III-V semiconductors below room temperature when neutral impurity concentrations are greater than 1018 cm−3. The mechanism may also constitute a dominant one in the mobility of some III-V alloys at fairly low temperatures. The existence of strain induced electric potentials also provides at least a possible mechanism whereby different donors can have different line shapes as measured in photoconductivity experiments.

Preservation of Point Defect Distribution in FZ Silicon

Evidences for the preservation of point defect distribution in FZ Si crystals have been observed by two experimental methods. One is divided selective oxidation and the other is divided backside selective oxidation with Si3N4 as the oxidation resistant films. In both experiments, the selective oxidation is divided into several oxidation steps, and the effects of oxidation division on the range of oxidation effects on impurity diffusion is investigated. The results show that, both laterally and vertically from the oxidizing Si–SiO2 interface, the effect of oxidation on impurity diffusion is determined only by total oxidation time. This suggests that point defect distribution is preserved after heat treatment.

Effects of temperature in binary collision simulations of high-energy displacement cascades

Several hundred cascades ranging from 1 to 500 keV were generated using the binary collision code MARLOWE for primary knock-on atoms (PKAs) with randomly chosen directions in both a non-thermal copper lattice and one having atomic displacements representative of room temperature. To simulate the recombination occurring during localized quenching of the highly excited cascade region, an effective spontaneous recombination radius was applied to reduce the number of defect pairs to be consistent with values extracted from resistivity measurements at 4 K. At room temperature fewer widely separated pairs are produced, thus the recombination radius is smaller; however, the recombination radii were found to be independent of energy over the entire energy range investigated for both the cold and room temperature cases. The sizes and other features of the point defect distributions were determined as a function of energy. Differences between cold and room temperature cascade dimensions are small. The room temperature cascades tend to have a greater number of distinct damage regions per cascade, but about the same frequency of widely separated sub-cascades.

Coordination model for the defect structure of hyperstoichiometric UO2+x and U4O9

Non-stoichiometric compounds or solids in which the ratios of the amounts of elements present are not integral may be characterized by the formula MXx where x is non-integral and, in many crystalline compounds, may vary over quite a wide range without change of structure type. Ideally the thermodynamic stability and physical properties of such compounds should be related to the same atomic and electronic model but the ways in which various simple structures adapt to departures from ideal composition have not been adequately explored. Models based on a random distribution of point defects have proved successful but selected area electron diffraction and ultra high resolution electron microscopy have shown that their distribution is not simple. Interaction between defects produces microscopic sub-structures or clusters introducing specific local properties. Although our knowledge of the principal morphological features of sub-structures has advanced, their precise interatomic arrangements are generally unknown. Here we derive a model for non-stoichiometric oxides of uranium.