Diffusion Of Point Defects Research Articles

Enhanced dynamic annealing in Ga+ ion-implanted GaN nanowires

Ga + ion implantation of chemical-vapor-deposited GaN nanowires (NWs) is studied using a 50-keV Ga+ focused ion beam. The role of dynamic annealing (defect-annihilation) is discussed with an emphasis on the fluence-dependent defect structure. Unlike heavy-ion-irradiated epitaxial GaN film, large-scale amorphization is suppressed until a very high fluence of 2×1016 ions cm−2. In contrast to extended-defects as reported for heavy-ion-irradiated epitaxial GaN film, point-defect clusters are identified as major component in irradiated NWs. Enhanced dynamic annealing induced by high diffusivity of mobile point-defects in the confined geometry of NWs is identified as the probable reason for observed differences.

Effects of Xe-ion irradiation at high temperature on single crystal rutile

Abstract Rutile (TiO 2 ) single crystals with (1 1 0) orientation were irradiated with 360 keV Xe 2+ ions at 923 K to fluences ranging from 7×10 14 to 1×10 16 Xe/cm 2 . Damage accumulation and evolution were analyzed using Rutherford backscattering spectroscopy combined with ion channeling analysis, and cross-sectional transmission electron microscopy. TiO 2 crystals were found to exhibit a layer-like damage structure after irradiation, with up to three characteristic layers: (1) a thin surface layer denuded of defects; (2) a second layer centered at the peak damage position, consisting of small voids about 1–3 nm in diameter; and (3) a third, deeper layer consisting of large defects such as dislocations and dislocation loops. The defect-denuded layer at the surface results from point defect diffusion and annealing. The voids in the second layer may be attributed to aggregation of vacancies. Point defect migration and precipitation is also responsible for the large defects observed in the deepest layer within the irradiated microstructure. Amorphization was not observed in the crystals irradiated at this high temperature.

Determination of the diffusivity of point defects in passive films on carbon steel

Mott–Schottky analysis is performed for passive films formed on carbon steel in borate solution. It is shown that the calculated donor density decreases exponentially with increasing film formation potential. The experimental data are interpreted in terms of the point defect model for passivity of carbon steel, assuming that the donors are oxygen vacancies and cation interstitials. The diffusivity of the donors D 0 is calculated to be in the range of 10 −16–10 −15 cm 2 s −1.

Simulation of Coupled Diffusion of Impurity Atoms and Point Defects under Nonequilibrium Conditions in Local Domain

A two-dimensional model of doping of the active regions in semiconductor devices by means of ion implantation and thermal annealing is stated and analyzed. Radiation enhanced diffusion of impurity atoms during high temperature implantation of hydrogen ions is also considered. An economic finite difference method for solving the obtained system of diffusion equations is developed and numerical computations of the diffusion processes are carried out.

The Nature and Diffusion of Intrinsic Point Defects in SiC

The Nature and Diffusion of Intrinsic Point Defects in SiC

Diffusion of topological solitons and dielectric αc relaxation in a polymeric crystal

A molecular dynamics simulation was performed to estimate the effective mass and the diffusion and friction coefficients of point defects in macromolecular chains of crystalline polyethylene. The results were compared with theoretical mass and kinetic coefficient predictions for topological solitons, with which these defects were identified. The results are used to discuss the soliton model of dielectric αc relaxation in weakly oxidized polyethylene.

Annealing effect and impurity doping effects on the defect generation in interstitial-rich Si crystals observed by infrared microscope

We have studied the extended defects in quenched Si crystals. Prismatic punched out dislocations (POD) have been found in the crystals. These dislocations disappeared depending on the holding time just after the growth. This phenomenon is closely related to the diffusion of point defects during the holding at high temperature. The defects have been observed by the infrared microscope after copper decoration. It should be noted that the prismatic dislocations density markedly decreased in boron doped crystals. The crystal becomes remarkably uniform. This means that the mechanical strength at high temperature greatly increases or the behavior of interstitial and/or oxygen atoms widely varied by doping with boron. The defect formation due to interstitials and the effect of boron will be discussed.

Phase-velocity spectra of background internal friction in polycrystalline solids

The phase-velocity spectrum of background internal friction has been shown to be a material property. The shape of such spectra seems to be different, in particular, for monophasic (e.g. pure metals) and multiphase materials (typically composites and rocks). An explanation for such difference, as yet not known, is proposed in the present work. All spectra are interpreted to be a combination of two or more simple relaxation sources, each emanating from point defect diffusion with a specific diffusion scale length and inducing dissipation into one another. The single dissipation sources are assumed to combine with the laws of linear circuits in parallel. In the case of monophasic metals a model with two sources of dissipation, one in the microstructure and the other in the lattice, fits the experimental spectra qualitatively well. For multiphase materials, including natural rocks and artificial conglomerates, which exhibit flat spectra, a continuous-source model is suggested.

Calculations of the Electronic and Atomic Structure and Diffusion of Point Defects in KNbO3 Perovskite Crystals and Relevant KTN Solid Solutions

AbstractIn this paper we review our recent achievements in large scale computer simulations of point defects in advanced perovskite crystals. We have calculated the defect migration energies in the KNbO3 cubic phase using quantum chemical method of the Intermediate Neglect of Differential Overlap (INDO) and classical shell model (SM). The migration energies for the O vacancy obtained by means of these two quite different methods are reasonably close (0.68 eV and 0.79 eV, respectively) and also agree with the only experimental estimate available (ca. 1 eV). Atomic relaxations calculated by these two methods also agree quite well. We used INDO method for a large-scale modeling of the atomic and electronic structure of KNbxTa1-xO3 (KTN) perovskite solid solutions. Results for periodic defect model (large unit cell) of 40 and 320 atoms are compared with 135-atom INDO cluster calculations. Periodic Nb impurities in KTaO3 reveal clear off-center displacement beginning with the smallest calculated concentrations, so does an isolated Nb impurity in a cluster INDO calculation. The magnitude of this displacement is close to the EXAFS observation (0.27 a.u.).

Dopant diffusion control by adding carbon into Si and SiGe: principles and device application

The incorporation of low concentrations of carbon (<10 20 cm −3) into the SiGe region of a heterojunction bipolar transistor (HBT) can significantly suppress boron outdiffusion caused by subsequent processing steps. This effect can be described by coupled diffusion of carbon atoms and Si point defects. We discuss the increase in performance and process margins in SiGe heterojunction bipolar technology by adding carbon. SiGe:C HBTs demonstrate excellent static parameters, exceeding the performance of state-of-the-art SiGe HBTs. Carbon also enhances the high frequency performance, because it allows one to use a high B doping level in a very thin SiGe base layer without outdiffusion from SiGe, even if applying post-epitaxial implants and anneals. Finally, we demonstrate the first modular integration of SiGe:C HBTs into a 0.25 μm, epi-free, dual-gate CMOS platform.

Diffusion of point defects in two-dimensional colloidal crystals.

Uniform colloidal microspheres dispersed in a solvent will, under appropriate conditions, self-assemble into ordered crystalline structures. Using these colloidal crystals as a model system, a great variety of problems of interest to materials science, physical chemistry, and condensed-matter physics have been investigated during the past two decades. Recently, it has been demonstrated that point defects can be created in two-dimensional colloidal crystals by manipulating individual particles with optical tweezers. Direct imaging of these defects verified that their stable configurations have lower symmetry than the underlying triangular lattice, as predicted by numerical simulations for a number of two-dimensional systems. It was also observed that point defects can dissociate into pairs of well-separated dislocations, a topological excitation especially important in two dimensions. Here we use a similar experimental system to study the dynamics of mono- and di-vacancies in two-dimensional colloidal crystals. We see evidence that the excitation of point defects into dislocation pairs enhances the diffusion of di-vacancies. Moreover, the hopping of the defects does not follow a pure random walk, but exhibits surprising memory effects. We expect the results presented in this work to be relevant for explaining the dynamics of other two-dimensional systems.

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.

Ultrasonic dislocation dynamics in Al (0.2 at.% Zn) after elastic loading

Ultrasonic resonance measurements at frequencies near 1 MHz were performed as a function of time and temperature on polycrystalline Al (0.2 at.% Zn) subjected to uniaxial loading below the plastic regime. The application of loads induced a decrease in the resonant frequency ( f) and an increase in the damping ( Q − 1) that fully recovered nonexponentially with time when the load was released. The relative magnitude of the changes in f and Q −1 cannot be explained by a single anelastic relaxation process, considering that no significant frequency dependence was observed. The changes in f may be dominated by a purely elastic effect or a second anelastic effect that is relatively rapid. Analysis of the measurements of f after loading is performed under the assumption that the changes are elastic and associated with the diffusion of point defects to dislocations. Data at three temperatures are fit to a theoretical expression for the time-dependent number of point defects bound to a dislocation, yielding an activation energy for diffusion of 0.69 eV. This activation energy is in the range of values reported for vacancies in aluminum.

Interaction processes between dislocations and particles in the ODS nickel-base superalloy INCONEL MA 754 studied by means of in situ straining in an HVEM

In situ straining experiments in a high-voltage electron microscope (HVEM) have been performed on the oxide-dispersion strengthened (ODS) nickel-base superalloy INCONEL MA 754 at 1020 and 1065 K. The results can be interpreted by interfacial pinning of dislocations at the dispersoids and, in some cases, by the Orowan mechanism. In addition to the thermally activated detachment of dislocations from particles, a viscous friction mechanism possibly due to point defect diffusion in the dislocation cores and Taylor hardening contribute to the flow stress.

Amorphous zone evolution in Si during elevated temperature ion bombardment

Evolution of amorphous zones (a-zones) in Si during elevated temperature ion/electron bombardment is considered. Consideration is based on a point defect diffusion model for the ion-beam-induced amorphous–crystalline (a/c) phase transformation in Si. Direct thermal annealing and an additional interfacial driving force for crystallization are taken into account. The problem of a-zone size distribution during ion bombardment is addressed and solved for a particular annealing situation. Possible experiments necessary to test this theoretical treatment are proposed. This formalism can also be applied to describe damage build-up in Si during ion bombardment.

1/ f Noise and intermittency due to diffusion of point defects in a semiconductor material

Diffusion of point defects is investigated as a possible origin of 1/ f noise in a semiconductor; as an example, diffusion of donor atoms in a strongly extrinsic semiconductor is dealt with. Due to diffusion of donor atoms, the generation–recombination ( g– r) process at a certain site may be regarded as an intermittent stochastic process; consequently, the spectral patterns of this process are established. The time of intermission is equivalent to the time a donor atom takes to return to a certain site. Simulating the random walk of donor atoms on a simple cubic lattice this return time is found to be distributed like a power law and can be the source of 1/ f noise in semiconductor with inhomogeneous current distribution.

Ion beam induced amorphous–crystalline phase transition in Si: Quantitative approach

A point defect diffusion model for ion beam induced crystallization and amorphization at the amorphous/crystalline interface in Si is presented and shown to be successful in providing a quantitative explanation for the phase transition kinetics. The model takes into account a minimum set of the most plausible elementary processes of point defects in crystalline Si as well as the formation and dynamic annealing of disordered regions during ion bombardment. Two mechanisms for the phase transition are proposed and shown to lead to nearly identical mathematical formulations. The first mechanism assumes that the recombination of interstitial atoms at the phase boundary leads to crystallization, while the same process for vacancies causes the growth of the amorphous layer. Based on the second mechanism, the recombination of a Frenkel pair at the amorphous/crystalline interface produces the recrystallization of a small volume, whereas excess of the vacancies provokes the process of amorphization. Contribution to amorphization from disordered regions, vacancies, and divacancies at the phase boundary is also taken into account in the model. The defect processes are described by nonlinear differential equations. The defect reaction parameters are identified based on experimental data. The results indicate that the transition from amorphization to crystallization regime with increase in the substrate temperature is mainly due to strong reduction of vacancy concentration and a weak increase in the concentration of interstitial atoms. A number of experiments are simulated with the calculated parameters to test the developed model.

The effect of stress on point-defect diffusion in hep metals and irradiation creep

Abstract Crystalline materials exposed to irradiation by high-energy particles produce defects such as lattice vacancies and interstitials. If the migration properties of the vacancy and interstitial are different, their annihilation behaviours at different sinks are biased. The resulting microstructure changes lead to macroscopic deformation. In hcp metals, anisotropic diffusion of the point defects is an intrinsic property related to the structure of the crystal lattice. The diffusional anisotropy is changed when a stress is applied to the crystal. The intrinsic anisotropy produces irradiation growth, while the stress-induced change in the diffusional anisotropy causes a deformation proportional to the applied stress, and contributes to irradiation creep. Irradiation creep due to the stress-induced change of the diffusion anisotropy has been investigated extensively in the cubic but not the hcp metals. In this paper, the elastodiffusion tensor of point defects in hcp metals is derived and applied to cal...

The effect of stress on point-defect diffusion in hcp metals and irradiation creep

The effect of stress on point-defect diffusion in hcp metals and irradiation creep

Study of ion beam induced epitaxial crystallization of SrTiO3

Amorphous SrTiO3 on single crystal SrTiO3 (100) has been crystallized by He+, Ne+, or Ar+ ions with energy of 200 keV–2 MeV at a substrate temperature of 100–250 °C. Rutherford backscattering spectrometry in channeling geometry and x-ray diffraction were used to evaluate the crystallization. Ion-beam-induced epitaxial crystallization (IBIEC) of SrTiO3 was confirmed and the activation energy of IBIEC observed was about 0.1–0.3 eV, a value about 1/10 relative to thermal solid phase epitaxial crystallization. The observed IBIEC seems to be consistent with previously proposed models in which IBIEC is dominated by point defects produced by ion irradiation and their migration to the amorphous/crystal (a/c) interface. The IBIEC mechanism and point defect behavior are discussed by the use of simple models taking into account the rate limiting processes of IBIEC for both point defect diffusion and atomic rearrangement at a/c interface.