Distribution Of Point Defects Research Articles

Vacancy and interstitial depth profiles in ion-implanted silicon

An experimental method of studying shifts between concentration-versus-depth profiles of vacancy- and interstitial-type defects in ion-implanted silicon is demonstrated. The concept is based on deep level transient spectroscopy measurements utilizing the filling pulse variation technique. The vacancy profile, represented by the vacancy–oxygen center, and the interstitial profile, represented by the interstitial carbon–substitutional carbon pair, are obtained at the same sample temperature by varying the duration of the filling pulse. The effect of the capture in the Debye tail has been extensively studied and taken into account. Thus, the two profiles can be recorded with a high relative depth resolution. Using low doses, point defects have been introduced in lightly doped float zone n-type silicon by implantation with 6.8 MeV boron ions and 680 keV and 1.3 MeV protons at room temperature. The effect of the angle of ion incidence has also been investigated. For all implantation conditions the peak of the interstitial profile is displaced towards larger depths compared to that of the vacancy profile. The amplitude of this displacement increases as the width of the initial point defect distribution increases. This behavior is explained by a simple model where the preferential forward momentum of recoiling silicon atoms and the highly efficient direct recombination of primary point defects are taken into account.

Interactions of point defects with dislocations in n-type silicon-dopedGaAs

Raman scattering and cathodoluminescence experiments have been performed toinvestigate the effect of dislocations on the spatial distribution of pointdefects and on the free electron concentration in n-type GaAs:Si. Anexperimentally extended increase of the free electron and (SiGaVGa)2−complex concentrations from the matrix to the dislocation isexplained as resulting from the formation of arsenic precipitatesaround the dislocation by means of computer simulations based on adiffusion–aggregation model.

Systematic study of the influence of the Czochralski hot zone design on the point defect distribution with respect to a “perfect” crystal

In the present paper the influence of the hot zone design on the quality and yield of silicon crystals with 100 mm and 300 mm diameters is numerically studied using the software package CrysVUn. Based on the numerical results solutions for the design of the heat shields are identified which allow nominally to grow the crystals at higher growth velocities, and at the same time with improved quality in terms of smaller point defect concentrations and improved radial uniformity, compared to conventional hot zones.

Silicon melt convection in large size Czochralski crucibles

The melt flow in large diameter crucibles during the growth of silicon (Si) single crystals of 300 mm diameter is characterized by turbulent large-scale velocity and temperature fluctuations. Strong efforts of the crystal growth industry are dedicated to the control of the interface shape and the related point defect distribution in the crystal, of the oxygen and particle transport in the melt, of the crucible overheating and of the conditions for dislocation free growth. Static and time-dependent electromagnetic fields offer new possibilities to meet the continuously increasing demands on crystal quality and yield improvement. Numerical simulation helps to investigate a wide range of possible process conditions, and to reduce experimental costs and time to market significantly. Depending on the required accuracy and available computation resources, two-dimensional (2D) or three-dimensional (3D) models are used. Temperature measurements in the melt, carried out during crystal growth and in model facilities, provide data for the verification of the numerical models, for direct process optimization and for a better understanding of the heat and mass transport behavior.

In situ measurements of the channeling dependence of ion-beam-induced recrystallization in silicon

The crystallization rate of surface amorphous silicon layers irradiated with 7 MeV Au 4+ ions has been studied for random and channeling incident beams. The movement of the amorphous/crystalline interfaces was monitored by in situ time resolved reflectivity and ex situ Rutherford backscattering spectrometry. Our experimental results reveal a clear channeling effect on the crystallization rates. Comparison of our data with calculations of the point defect distributions performed with the MARLOWE code suggests that defects produced at the amorphous crystalline interface are responsible for ion-beam-induced epitaxial crystallization.

Chromium and Lanthanum on Transition Alumina Surfaces: The Role of Bulk Point-Defect Distributions on Catalytic Activity

ABSTRACTWe employ a combination of Z-contrast scanning transmission electron microscopy (Z-STEM) and first-principles density-functional calculations to investigate the interaction between metal atoms and cubic alumina catalytic supports. We show that there are two observed La sites at the porous γ- alumina surface, and that single La atoms do not exhibit a tendency to cluster. Cr behaves very differently from La - it has a tendency to cover the alumina surface in ordered, periodic raft-like patches. The degradation of the chromia/alumina catalyst is related to the possibility for the chromium to move away from the surface into the bulk, and the activation barrier for such a process is higher in η- alumina than in γ- alumina, i.e., η- alumina is a more durable support for chromium catalyst.

Raman spectra of synthetic sapphire

We have studied two samples of synthetic sapphire. Sample 1 was used as a window in a cryostat and was subjected to liquid nitrogen temperature for a long time (two years). Sample 2 was stored at room temperature. Under identical conditions, three types of Raman spectra were measured on Sample 1 in the wave number range 100–800cm−1. In the spectra from the central part of the sample, seven bands were observed, which are in good agreement with a spectrum of natural sapphire. In the spectra off the center, a subtle band was observed at a wave number of 724.6cm−1 which was related to the distribution of point defects. On the boundary of the sample, just a fluorescent background was measured. The measured Raman spectra resemble the spectra of γ-Al2O3 phase. On Sample 2, the spectra from the border were similar to those from the middle of the sample. The observed results indicate possible conversion of α-Al2O3 into γ-Al2O3 due to prolonged exposure of synthetic sapphire to low temperatures.

The stability of homogeneous microstructure development under cascade-damage conditions

The development of microstructure, under cascade-damage conditions, in regions far away from any major sink is considered. Within the mean-field theory, a homogeneous distribution of point defects and their clusters is a pre-imposed artificial constraint on the kinetic system. The resulting excessive recombination of the vacancies and interstitials at a high density of accumulated point-defect clusters dictates a low rate of void growth. Considerations beyond the mean-field theory, by taking into account the concentration fluctuations of both the point defects and their clusters, relax the restriction of the homogeneous distribution. In this paper, we consider a system without pre-existing sinks, except the void nuclei, in which vacancies, interstitials and their clusters are continuously produced. Taking into account the mobility of small clusters and the stochastic fluctuations of the point-defect fluxes, a kinetic theory is formulated from first principles. It is rigorously shown that through the stochastic fluctuations, and the positive-feedback action of the mobility of the small clusters on the interstitial concentration, the homogeneous interstitial distribution is unstable at temperatures above stage V, leading to the formation of a spatially heterogeneous microstructure in pure metals at low irradiation doses. The characteristics of the microstructure evolution and void swelling, predicted from the theory, are found to be in good agreement with the experimental results.

Atomic scale simulation of extended defects formation under high energy electron irradiation: space distribution

Atomic scale simulation (ASS) of the diffusion and agglomeration of point defects under high energy electron irradiation has been treated by means of a personal computer. The physical model has been developed by using the Monte Carlo technique. We have assumed that vacancy–interstitial pairs are created by particle impact. The diffusion of point defects through the solid and their association, forms a nucleus for a new dislocation loop and the incorporation of interstitials into already existing dislocation loops leads to their growth. We have determined the concentration of point defects and extended defects and size of these latters, and compared them with the results of the chemical reaction rate theory (CRRT) and to experimental data. Some comparisons are also made with the results performed on parallel computers (J. Japanese Psychosom. Soc. 37 (1998) 2703) and the raisons for lack of quantitative and qualitative agreements are discussed. The spatial distribution of extended defects and point defects has been studied. We found that the spatial distribution of vacancies around of extended defects reduces their further growth. This type of information is totally occulted in the CRRT.

Study of growth hillocks in GaN:Si films by electron beam induced current imaging

Remote electron beam induced current (REBIC) measurements have been carried out to investigate electrically active regions in Si doped GaN films. REBIC bright–dark contrast has been observed in the border of growth, round or pyramidal, hillocks, while pyramidal hillocks also show bright contrast at the center. The results are explained by the inhomogeneous distribution of charged point defects and impurities at the hillocks.

Point defect equilibrium in strontium titanate thin films

Equilibrium calculations of point defect and electronic carrier distributions in acceptor impurity-containing SrTiO3 thin films were performed. The simulation methodology was based on previous calculations of near-grain boundary point defect equilibrium in acceptor-doped SrTiO3 ceramics. Attention was focused on the effects of temperature and the presence of Pt electrode interfaces on the conditions for defect equilibrium. The local electrostatic potential and the distribution of oxygen vacancies across films of varying thickness were predicted for various simulated equilibration conditions. The effects of hydrogen doping were analyzed, and simulated equilibrium potential distributions were compared to the results of reported photoelectron spectroscopy studies of Pt/SrTiO3 and Pt/(Ba, Sr)TiO3 thin film structures.

Electrochemical study of the passive behaviour of Ni–Cr alloys in a borate solution—a mixed-conduction model approach

This paper discusses the application of the recently introduced mixed-conduction model (MCM) to describe the electrochemical behaviour of anodic films formed on Ni–Cr alloys. The MCM emphasises the coupling between the spatial distribution of ionic point defects in the film and the electronic conductivity of the film. The basic concepts of the MCM are introduced, and equations for the concentration profiles of ionic defects, for the conductivity and for the ac response of the film are derived. The experimental part of this work consist of characterising the anodic behaviour of Ni–Cr alloys, pure Ni and pure Cr in 0.1 M Na 2B 4O 7 solution (pH 9.3) at ambient temperature using rotating disc and cyclic voltammetry, contact electric resistance (CER) and electrochemical impedance spectroscopic (EIS) techniques. The experiments indicate that the film on a Ni–20%Cr alloy resembles that on pure Cr, whereas similarities can be found between the film on Ni–10%Cr alloy and pure Ni. The conductivity of the passive film formed on Ni–Cr alloys can be concluded to be based on the spatial and potential distribution of the valence states of Ni and Cr in the film. In spite of several simplifying assumptions used in the simulations, a qualitative agreement between the theory and experimental data was achieved.

A comparative study of vacancies produced by proton implantation of silicon using positron annihilation and deep level transient spectroscopy

Proton irradiation at an energy of 1 MeV has been used to create an approximately even distribution of simple point defects in both Cz and FZ, n-type silicon to a depth of greater than 10 μ m. The implanted dose ranged from 1×10 11 to 1×10 16 cm −2 . The vacancy component of the defect concentration has been quantitatively measured using positron annihilation and deep level transient spectroscopy. Through careful experimental design it has been possible to meaningfully compare the concentrations independently determined using the two approaches. Good agreement is found between the two techniques for the FZ material adding credence to the methods through which defect concentrations are obtained from the primary data. Somewhat less reasonable agreement is found for the Cz material for which one possible explanation is offered.

Formation and distribution of point defects on a disclination line near a free nematic interface.

Point defects of opposite signs can alternately nucleate on the -1/2 disclination line that forms near the free surface of a confined nematic liquid crystal. We show the existence of metastable configurations consisting of periodic repetitions of such defects. These configurations are characterized by a minimal interdefect spacing that is seen to depend on sample thickness and on an applied electric field. The time evolution of the defect distribution suggests that the defects attract at small distances and repel at large distances.

Numerical analysis for the dynamics of the oxidation-induced stacking fault in czochralski-grown silicon crystals

The continuum model of point defect dynamics to predict the concentration of interstitial and vacancy is established by estimating expressions for the thermophysical properties of point defects and the point defect distribution in a silicon crystal and the position of oxidation-induced stacking fault ring (R-OiSF) created during the cooling of crystals in Czochralski silicon growth process are calculated by using the finite element analysis. Temperature distributions in the silicon crystal in an industrial Czochralski growth configuration are measured and compared with finite volume simulation results. These temperature fields obtained from finite volume analysis are used as input data for the calculation of point defect distribution and R-OiSF position. Calculations of continuum point defect distributions predict the transition between vacancy and interstitial dominated precipitations of microdefects as a function of crystal pull rate (V). The dependence of the radius of R-OiSF (ROiSF) on the crystal length with fixed growth rate for a given hot zone configuration is examined. The ROiSF is increased with the increase of crystal length. These predictions from point defect dynamics are well agreed with experiments and empirical V/G correlation qualitatively, where G. is the axial temperature gradient at the melt/crystal interface.

Influence of low thermal budget pre-anneals on the high temperature redistribution of low energy boron implants in silicon

ABSTRACTIt is now well established that the transient enhanced diffusion (TED) of ion implanted boron in silicon limits the formation of the ultra-shallow junctions required for the extreme deep sub- micron devices. It is also known that this TED is linked to the fate (elimination and agglomeration) of ion implantation related excess self-interstitials. Thus it can be expected that the final high temperature redistribution is at least partly governed by the effective initial point defect distribution at the onset of the high temperature plateau.In this contribution we present the experimental evidence that low thermal pre-anneals, by affecting the initial self-interstitials distribution, affects boron redistribution during a subsequent high temperature anneal. Samples implanted with high dose boron at 3 keV were first annealed at 700°C for various durations. These samples, as well as reference samples without the pre-anneal, were then RTA annealed at various high temperatures around 1000°C. The resulting B profiles were measured by SIMS. It is found that the pre-annealed samples exhibits a clear reduction of the TED as compared with the reference ones.

Surface reconstruction and the difference in surface acidity between gamma- and eta-alumina.

The surfaces of the gamma and eta forms of alumina are well known to differ significantly in Lewis acidity. Surface reconstructions observed on gamma- and eta-alumina are also qualitatively different, despite the close similarity of the bulk structures. Here we demonstrate through first-principles calculations that subtle differences in the bulk point defect distribution between these two forms of alumina give rise to the major differences in the mode of surface reconstruction and correlate with the different levels of Lewis acidity.

Structural and dimensional control in micromachined integrated solid state gas sensors

The application of electron microscopy for quality control in the fabrication of solid state integrated tin oxide gas sensors is described. Among the parameters that influence the performance and mechanical stability of these devices are the thickness, grain size, porosity and point defect (oxygen vacancy) distribution of the active SnO 2 layer and the intrinsic stresses and interfacial adhesion in the support layers. The sputtered SnO 2 films were highly crystalline with a columnar growth structure and equiaxed grains of mean diameter ∼10 nm. HREM studies revealed evidence of CSP defects, which act as traps for free carriers and can therefore be expected to influence the conductivity of the oxide layer. Increasing the thickness of the film from 300 to 600 nm lead to an increase in the sensitivity to low concentrations of NO 2 (∼1 ppm) by up to a factor of two. Sensitivity is also affected by surface roughness and film porosity, which increase the effective area on which gas molecules can be adsorbed.

On the geometric origin of Orowan-type kinematic relations and the Schmid yield criterion

It is shown that the Orowan-type kinematic relations as well as the Schmid yield criterion can be derived basing oneself on a formula defining the mean curvature of glide surfaces for a principal congruence of edge dislocation lines accompanied with a particular distribution of secondary point defects. Moreover, it appears that this mean curvature has the physical meaning of a mesoscopic material parameter defining a relation between the evolution of the dislocation state and plastic deformation. It is pointed out that the existence of Orowan-type relations puts kinematic constraints, dependent on the isometry group of glide surfaces, on the dislocation density tensor.

Solute Atom Segregation to Yttria-Stabilized Cubic Zirconia Grain Boundaries

Abstract Grain boundaries in yttria-stabilized cubic zirconia (YSZ) are believed to have positively charged grain boundaries with compensating negative space charge layers at ambient temperatures and pressures [1]. The space charge region is formed by equilibrium segregation of point defects to the vicinity of the grain boundary to establish local charge neutrality. The point defect distributions, calculated from analytical space charge models [1] show that the negative space charge region is formed by a depletion oxygen vacancies and an excess of substitutional Y atoms, indicated as'VO” and YZr’ in standard Kröger-Vink notation (see fig. 1). The grain boundary segregation, in turn, has important ramifications for the mechanical and electrical behavior of the material [1,2]. The present work addresses space-charge segregation in YSZ, specifically addressing variations in segregation with grain boundary misorientation. We have examined a range of symmetric tilt [001] YSZ grain boundaries since they allow us to systematically vary the misorientation while providing the opportunity for atomic scale analysis.