Charge States Of Point Defects Research Articles

Defect formation energy for various charge states of point defects in CdGa2S4

Defect formation energy for various charge states of point defects in CdGa2S4

Interplay of Anionic Quasi-Atoms and Interstitial Point Defects in Electrides: Abnormal Interstice Occupation and Colossal Charge State of Point Defects in Dense fcc-Lithium.

Electrides are an emerging class of materials with highly localized electrons in the interstices of a crystal that behave as anions. The presence of these unusual interstitial quasi-atom (ISQ) electrons leads to interesting physical and chemical properties and wide potential applications for this new class of materials. Crystal defects often have a crucial influence on the properties of materials. Introducing impurities has been proved to be an effective approach to improve the properties of a material and to expand its applications. However, the interactions between the anionic ISQs and the crystal defects in electrides are yet unknown. Here, dense fcc-Li was employed as an archetype to explore the interplay between anionic ISQs and interstitial impurity atoms in this electride. This work reveals strong coupling among the interstitial impurity atoms, the ISQs, and the matrix Li atoms near to the defects. This complex interplay and interaction mainly manifest as the unexpected tetrahedral interstitial occupation of impurity atoms and the enhancement of electron localization in the interstices. Moreover, the Be impurity occupying the octahedral interstice shows the highest negative charge state (Be8-) discovered thus far. These results demonstrate the rich chemistry and physics of this emerging material and provide a new basis for enriching their variants for a wide range of applications.

Charge states of point defects in plutonium oxide: A first-principles study

The stability of charged defects in PuO2 are systematically investigated by quasi-annealing simulations based on local spin density approximation (LSDA) + U calculations. The formation energies of intrinsic (Frenkel and Schottky pairs) and extrinsic (single vacancy or interstitial) defects at different charged states are calculated and presented. According to our results, the noninteracting oxygen Frenkel pairs made of the association of doubly charged defects are the most favorable form of intrinsic defects, the formation energy of which is in good agreement with experimental values. The negative charged oxygen interstitial (IO2−) and positive charged oxygen vacancy (VO2+) defects have the lowest formation energies at oxygen-rich and oxygen-poor environments, respectively, and thus are the most commonly seen extrinsic defects. Both the existences of oxygen vacancy and oxygen interstitial induce new electronic states in the band gap of stoichiometric PuO2, showing doping effects.

Charge states of point defects in uranium oxide calculated with a local hybrid functional for correlated electrons

The formation energies and charge states of point defects in uranium dioxide are calculated from first principles, using the local hybrid functional for correlated electrons, which offers a nice alternative to LDA$+$U calculations. The possible occurrence of multiple minima in such calculations is carefully taken into account. Point defects in UO${}_{2}$ are generally found to be charged with a \ensuremath{-}4 charge for uranium vacancies, \ensuremath{-}2 for oxygen interstitials, and a charge ranging from 0 to $+$2 for oxygen vacancies depending on the position of the Fermi level. The calculated formation energies of noninteracting oxygen Frenkel pairs and Schottky defects made of the association of charged defects are in very good agreement with experimental values. A Brouwer diagram based on a point-defect model for stoichiometry variations in UO${}_{2+x}$ is built. It fails to predict the dominant concentration of oxygen interstitials in the overstoichiometric oxide but predicts that oxygen vacancies are in a $+$1 charge state in the hypostoichiometric oxide. This charge state, which differs from the one assumed in the traditional (fully ionic) picture of UO${}_{2}$, is confirmed by the obtained variation of the deviation from stoichiometry with oxygen pressure, which is consistent with experiments.

Measuring the Charge State of Point Defects on MgO/Ag(001)

A detailed understanding of surface defects is highly desirable, e.g. to clarify their role as active sites in catalysis. Here localized defects on the surface of MgO films deposited on Ag(001) are investigated. Since the electronic structure of color centers depends on their local position, spectroscopic signals are highly convoluted and often difficult to disentangle. In this study we aimed to obtain morphological and spectroscopic information on single color centers at a microscopic level with frequency modulated dynamic force microscopy (FM-DFM) and scanning tunneling microscopy (STM) in an ultrahigh vacuum and at low temperature. Four of the major and in literature mostly discussed defect types on MgO have been characterized by their charge state and finally identified by the complementary application of FM-DFM and STM in combination with density functional theory results.

Energetics of charged point defects in rutile TiO 2 by density functional theory

The defect formation energies of all possible charge states of point defects in TiO 2, including titanium interstitials, titanium vacancies and oxygen vacancies, are calculated in the phase space of temperature, oxygen partial pressure and Fermi level by combining density functional theory (DFT) and thermodynamic calculations. The point defect phase diagram illustrates that fully charged defects dominate in most regimes. The calculations not only give reasonable defect formation energies compared with prior experimental measurements, but also predict n-type TiO 2 at high T and low P O 2 , and p-type TiO 2 at low T and high P O 2 , which agrees well with experimental data. In addition, we evaluate methods for correcting the effects of artificial electrostatic interactions caused by periodic boundary conditions in the DFT calculations, including the electrostatic potential alignment correction (Δ V correction) and the Makov–Payne correction.

Extended 'Five-Stream' Model for Diffusion of Implanted Dopants in Silicon during Ultra-Shallow Junction Formation in VLSI Circuits

Ion implantation of different dopants (donors and acceptors) into crystalline silicon with subsequent thermal annealing is used for the formation of ultra-shallow p-n junctions in VLSI technology. The experimentally observed phenomenon of transient enhanced diffusion (TED) during annealing hinders further downscaling of advanced VLSI circuits. However, modern mathematical models of dopant diffusion, which are based on the so-called “five-stream” approach, and software packages such as SUPREM4 encounter difficulties in describing TED. In this work, an extended five-stream model for diffusion in silicon is developed, which takes into account all the possible charge states of point defects (vacancies and silicon self-interstitials) and diffusing pairs “dopant atom–vacancy” and “dopant atom–silicon self-interstitial”. The model includes diffusion and drift of differently charged point defects and pairs in the internal electric field and the kinetics of interaction between unlike species. The expressions for diffusion fluxes and sink/source terms that appear in the non-linear, non-steady-state reaction-diffusion equations are derived for both donor and acceptor dopants accounting for multiple charge states of the diffusing species.

Modeling charged defects, dopant diffusion and activation mechanisms for TCAD simulations using kinetic Monte Carlo

This work will show how the kinetic Monte Carlo (KMC) technique is able to successfully model the defects and diffusion of dopants in Si-based materials for advanced microelectronic devices, especially for non-equilibrium conditions. Charge states of point defects and paired dopants are also simulated, including the dependency of the diffusivities on the Fermi level and charged particle drift coming from the electric field. The KMC method is used to simulate the diffusion of the point defects, and formation and dissolution of extended defects, whereas a quasi-atomistic approach is used to take into account the carrier densities. The simulated mechanisms include the kick-out diffusion mechanism, extended defect formation and the activation/deactivation of dopants through the formation of impurity clusters. Damage accumulation and amorphization are also taken into account. Solid phase epitaxy regrowth is included, and also the dopants redistribution during recrystallization of the amorphized regions. Regarding the charged defects, the model considers the dependencies of charge reactions, electric bias, pairing and break-up reactions according to the local Fermi level. Some aspects of the basic physical mechanisms have also been taken into consideration: how to smooth out the atomistic dopant point charge distribution, avoiding very abrupt and unphysical charge profiles and how to implement the drift of charged particles into the existing electric field. The work will also discuss the efficiency, accuracy and relevance of the method, together with its implementation in a technology computer aided design process simulator.

Determination of effective charge states for point radiation defects in fusion ceramic materials

Point defects in fusion ceramic materials under irradiation can have an effective charge due to ionization and covalent chemical bonding. The kinetics of charged point defects in these materials is determined by their effective charge and this kinetics is completely different from non-charged point defects in metals. The experimental investigation of point defect charge states in ceramic materials is rather difficult. In this paper, a new method is proposed for the investigation of charge states of point defects in irradiated ceramic materials. This method is based on a new theoretical model and experimental observations of defect cluster formation near permanent sinks, such as grain boundaries. Previous experimental investigations show that a denuded zone is formed in irradiated ceramic materials near free surfaces and grain boundaries. The effect of an applied electric field on the formation of interstitial-type point defect clusters (dislocation loops) near grain boundaries in irradiated ceramic materials is investigated. For this purpose a new theoretical model is suggested, which takes into account the effect of an applied electric field on the denuded zone formation. It is shown that the denuded zone size depends on point defect charge and direction of applied electric field. The obtained theoretical results are compared with experimental data.

Type and charge states of point defects in heavily As- and B-doped silicon

In order to determine the type and charge states of dominant point defects in As- and B-doped silicon, self-diffusion in doped natural Si/ 30Si/natural Si double-heterostructures has been investigated under oxidizing ambient. Enhanced self-diffusion is observed in B-doped Si, while retarded self-diffusion is observed in As-doped Si. These results indicate that single-charged Si self-interstitals (I + donors) are dominant in B-doped Si, while excess single-charged vacancies (V − acceptors) exist in As-doped Si.

Growth and instability of charged dislocation loops under irradiation in ceramic materials

We have investigated the physical mechanisms of the growth and stability of charged dislocation loops in ceramic materials with very strong different mass of atoms (stabilized cubic zirconia) under different energies and types of irradiation conditions: 100–1000 keV electrons, 100 keV He + and 300 keV O + ions. The anomalous formation of extended defect clusters (charged dislocation loops) has been observed by TEM under electron irradiation subsequent to ion irradiation. It is demonstrated that very strong strain field (contrast) near charged dislocation loops is formed. The dislocation loops grow up to a critical size and after then become unstable. The instability of the charged dislocation loop leads to the multiplication of dislocation loops and the formation of dislocation network near the charged dislocation loops. A theoretical model is suggested for the explanation of the growth and stability of the charged dislocation loop, taking the charge state of point defects. The calculated distribution of the modified strain field by the electrical field around the charged dislocation loops is stronger than that of noncharged dislocation loops. The obtained theoretical results for the modified strain field contrast and the critical radius of unstable charged dislocation loops are compared with observed experimental data.

Dislocation motion in semiconducting crystals under the influence ofelectronic perturbations

The radiation enhanced dislocation glide effect was confirmed in h-ZnO aswell as in other semiconductors, which proved further the ubiquitous nature of this effect insemiconductors. Analysis of the radiation enhanced dislocation vibrationsobserved in the same h-ZnO led us to a tentative model that the effect isbrought about by fluctuations of the charge state of point defects which exertelectrostatic forces on the charged dislocations. The hydrogen-plasma enhanceddislocation glide was studied in Ge, SiGe and GaAs in addition to the previousreports on Si. The absence of the hydrogen effect in α-dislocations inn-GaAs and in thin films of SiGe is interpreted in terms of the soliton model,the essence of which is a strong binding between kinks and hydrogens.