Different Types Of Point Defects Research Articles

First-principles prediction of point defect energies and concentrations in the tantalum and hafnium carbides

First-principles calculations are combined with a statistical–mechanical model to predict the equilibrium point-defect concentrations in the refractory carbides TaC and HfC as a function of temperature and chemical composition. Several different types of point defects (vacancies, interstitials, antisite atoms) and their clusters are treated in a unified manner. The defect concentrations either strictly follow or can be closely approximated by Arrhenius functions with parameters predicted by the model. The model is general and applicable to other carbides, nitrides, borides, or similar chemical compounds. Implications of this work for understanding the diffusion mechanisms in TaC and HfC are discussed.

Thermodynamics of native defects in V2O5 crystal: A first-principles method

The defect formation energy and its stability of native point defects in V2O5 are systematically investigated utilizing the DFT + U method and thermodynamic calculations over a range of temperatures, oxygen partial pressures and stoichiometries. The variation of the formation energy for different charged point defects with Fermi level is also discussed in great detail. The findings demonstrate that donors are deep-level defects and provide minimal contributions to an n-type conductivity. Acceptor exhibits shallow-level transition that is easily compensated from the donors and makes intrinsic p-type doping challenging. Furthermore, all the native point defects in V2O5 are extremely sensitive to temperature, oxygen partial pressure and Fermi level. The distribution of the most stable point defects that may exist in crystals in the three-dimensional space of temperature, oxygen partial pressure, and Fermi level has been obtained. The calculated results offer a clear picture for analyzing the different types of point defects that may appear in crystals under different conditions and serves as a guide for regulating the formation of crystal point defects.

Experimental evidence for spin–triplet states in Titanium implanted AlN film: An electron spin resonance study

Spin carrying defects in wide-bandgap semiconductors are promising candidates for the development of quantum information and quantum sensing technology. Here we report experimental evidence for the formation of paramagnetic defects in AlN films, subjected to high-energy hydrogen and titanium ion implantation. X-ray diffraction and Raman spectroscopy show that ion species and implantation process could alter the amount of local strain in AlN films. Electron spin resonance spectroscopy reveals resonance peaks with g-value higher than the one of a free electron (2.0023) for both H-implanted and Ti-implanted AlN films. The origin of these peaks is identified and attributed to different types of point defects. For the Ti-implanted film, in addition to the central peak, a half-field resonance peak has been detected and assigned to spin–triplet (S=1). These results provide new insight into the electronic spin state of point defects in AlN, which opens up interesting perspectives for applications to quantum devices.

Intrinsic Defects and the Inducing Conduction Mechanism of Langasite-Type High-Temperature Piezoelectric Crystals.

Increasing the crystal resistivity is critically important for enhancing the signal-to-noise ratio and improving the sensing capability of high-temperature piezoelectric sensors based on langasite-type crystals. The resistivity of structural ordered langasite-type crystals is much higher compared to that of the disordered crystals. Here, we selected structural ordered Ca3TaGa3Si2O14 (CTGS) and disordered La3Ga5SiO14 (LGS) as representatives to investigate the microscopic conduction mechanism and further reveal the origin of the different resistivities of the ordered and disordered langasite-type crystals at elevated temperatures. By combining first-principles calculations and experimental investigations, we found that the different conductivity behaviors of the ordered and disordered crystals originate from different types of point defects formed in the crystal and their different contributions to the conductivity. For the disordered LGS crystal, the oxygen vacancies are apt to be formed at high temperatures, promoting the transition of valence electrons and yielding high conductivity. For the ordered CTGS crystal, the dominant TaGa antisite defects can introduce an electron-hole recombination center in the electronic band gap, significantly shortening the carrier lifetime and thus reducing the conductivity. This provides effective guidance to improve the resistivity performance of langasite-type crystals at high temperatures by optimizing the experimental conditions, such as oxygen atmosphere treatment, antisite defect modification, etc.

Anelasticity of the martensitic phase of Ni55Fe18Ga27 single crystals in hyperstabilized and nonstabilized states

Apparent elastic and anelastic properties of the martensitic phase of Ni55Fe18Ga27 single crystals have been studied after different heat treatments over wide ranges of temperatures (13–370 K) and strain amplitudes (5 × 10-8–10-5) at frequencies near 100 kHz. These properties depend crucially on the heat treatment because of strong pinning of twin boundaries by quenched-in point defects. Two stages are revealed in the temperature dependence of the effective Young’s modulus and anelastic properties of the alloy that can be associated with pinning/depinning of twin boundaries by/from different types of point defects, most likely vacancies and divacancies. Experimental data evidence that diffusion of the vacancy-type defects in the temperature range under study is predominantly defect-assisted, enhanced by twin boundaries.

Imaging Nonradiative Point Defects Buried in Quantum Wells Using Cathodoluminescence.

Crystallographic point defects (PDs) can dramatically decrease the efficiency of optoelectronic semiconductor devices, many of which are based on quantum well (QW) heterostructures. However, spatially resolving individual nonradiative PDs buried in such QWs has so far not been demonstrated. Here, using high-resolution cathodoluminescence (CL) and a specific sample design, we spatially resolve, image, and analyze nonradiative PDs in InGaN/GaN QWs at the nanoscale. We identify two different types of PDs by their contrasting behavior with temperature and measure their densities from 1014 cm-3 to as high as 1016 cm-3. Our CL images clearly illustrate the interplay between PDs and carrier dynamics in the well: increasing PD concentration severely limits carrier diffusion lengths, while a higher carrier density suppresses the nonradiative behavior of PDs. The results in this study are readily interpreted directly from CL images and represent a significant advancement in nanoscale PD analysis.

Point Defects in a Two-Dimensional ZnSnN2 Nanosheet: A First-Principles Study on the Electronic and Magnetic Properties

The reduction of dimensionality is a very effective way to achieve appealing properties in two-dimensional materials (2DMs). First-principles calculations can greatly facilitate the prediction of 2DM properties and find possible approaches to enhance their performance. We employed first-principles calculations to gain insight into the impact of different types of point defects (vacancies and substitutional dopants) on the electronic and magnetic properties of a ZnSnN2 (ZSN) monolayer. We show that Zn, Sn, and N + Zn vacancy-defected structures are p-type conducting, while the defected ZSN with a N vacancy is n-type conducting. For substitutional dopants, we found that all doped structures are thermally and energetically stable. The most stable structure is found to be B-doping at the Zn site. The highest work function value (5.0 eV) has been obtained for Be substitution at the Sn site. Li-doping (at the Zn site) and Be-doping (at the Sn site) are p-type conducting, while B-doping (at the Zn site) is n-type conducting. We found that the considered ZSN monolayer-based structures with point defects are magnetic, except those with the N vacancy defects and Be-doped structures. The ab initio molecular dynamics simulations confirm that all substitutionally doped and defected structures are thermally stable. Thus, our results highlight the possibility of tuning the magnetism in ZnSnN2 monolayers through defect engineering.

Tunable electronic and magnetic properties of MoSi2N4 monolayer via vacancy defects, atomic adsorption and atomic doping

The two dimensional MoSi2N4 (MSN) monolayer exhibiting rich physical and chemical properties was synthesized for the first time last year. We have used the spin-polarized density functional theory to study the effect of different types of point defects on the structural, electronic, and magnetic properties of the MSN monolayer. Adsorbed, substitutionally doped (at different lattice sites), and some kind of vacancies have been considered as point defects. The computational results show all defects studied decrease the MSN monolayer band gap. We found out the H-, O-, and P-doped MSN are n-type conductors. The arsenic-doped MSN, and MSN with vacancy defects have a magnetic moment. The MSN with a Si vacancy defect is a half-metallic which is favorable for spintronic applications, while the MSN with a single N vacancy or double vacancy (N + S) defects are metallic, i.e., beneficial as spin filters and chemical sensors.

Intrinsic defects and non-stoichiometry in undoped cadmium silicate hosts

Cadmium silicates, mainly the CdSiO3 phase, are interesting materials due to their persistent and intrinsic luminescence, making them possible candidates for a number of applications. Although many of the luminescence properties of these materials are known from the experimental point of view, the current problem that still remains is to understand the origin of the intrinsic luminescence. Different types of point defects were considered in the literature and, most of them are contradictory and/or controversial and lack direct evidence for the assumptions. The aim of the present work is to investigate in depth the possible origin of the luminescence properties of cadmium silicates, focusing on the CdSiO3 phase, considering not only the possibilities reported in the literature but all other mechanisms involving intrinsic defects in these materials. Our results predicted that CdSiO3 tends to show stoichiometric deviation due to CdO pseudo-Schottky defects and the simulated structure for the deficient matrix is much closer to the experimental X-ray diffraction pattern than the stoichiometric one. This feature indicates that most of the available CdSiO3 phases are truly CdO deficient. Additionally, it was found that this defect induces the formation Si–Si bonds that are responsible for the intrinsic luminescence in this matrix. The CdO deficiency also accounts for the presence of additional cadmium silicate phases, like Cd2SiO4 or Cd3SiO5, during the synthesis of CdSiO3, a common feature found in many works in the literature. These results strongly indicate that CdO pseudo-Schottky defects, that trigger the CdO deficiency naturally present in cadmium silicate structures, should be considered not only in all luminescent models proposed, including intrinsic and extrinsic matrix, but also for the structural properties of these materials.

DFT Study of Cs/Sr/Ag Adsorption on Defective Matrix Graphite

The geometries, adsorption energies, and electronic structures of Cs, Sr, and Ag atoms on matrix graphite surface with point defects were calculated and analyzed using the density functional theory (DFT) and the Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA). Three different types of point defects, i.e., single vacancy and “bridge” and “spiro” interstitials are considered using approximate van der Waals (vdW) correction methods. The results of adsorption energies show that the metal fission products of Cs, Sr, and Ag are more stable on single vacancy defects than “bridge” or “spiro” interstitial defects. This is further confirmed by the analysis of electronic structures, such as charge density difference (CDD) and density of state (DOS). All these results indicate that dangling bonds play an important role in the adsorption behaviors of metallic fission products on matrix graphite.

Energetic, structural and electronic features of Sn-, Ga-, O-based defect complexes in cubic In2O3

Defect energy formation, lattice distortions and electronic structure of cubic In2O3 with Sn, Ga and O impurities were theoretically investigated using density functional theory. Different types of point defects, consisting of 1–4 atoms of Sn, Ga and O in both substitutional and interstitial (structural vacancy) positions, were examined. It was demonstrated, that formation of substitutional Ga and Sn defects are spontaneous, while formation of interstitial defects requires an activation energy. The donor-like behavior of interstitial Ga defects with splitting of conduction band into two subbands with light and heavy electrons, respectively, was revealed. Contrarily, interstitial O defects demonstrate acceptor-like behavior with the formation of acceptor levels or subbands inside the band gap. The obtained results are important for an accurate description of transport phenomena in In2O3 with substitutional and interstitial defects.

Effect of A-Site Nonstoichiometry on Defect Chemistry and Electrical Conductivity of Undoped and Y-Doped SrZrO3.

The effect of Sr-nonstoichiometry on phase composition, microstructure, defect chemistry and electrical conductivity of SrxZrO3−δ and SrxZr0.95Y0.05O3−δ ceramics (SZx and SZYx, respectively; x = 0.94–1.02) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy followed by distribution of relaxation times analysis of impedance data. It was shown that at low Sr deficiency (x > 0.96 and 0.98 for SZx and SZYx, respectively) a solid solution of strontium vacancies in strontium zirconate crystal structure forms, whereas at higher Sr deficiency the secondary phase, zirconium oxide or yttrium zirconium oxide, is precipitated. Yttrium solubility limit in strontium zirconate was found to be close to 2 mol%. Y-doped strontium zirconates possess up to two orders of magnitude higher total conductivity than SZx samples. A-site nonstoichiometry was shown to have a significant effect on the electrical conductivity of SZx and SZYx. The highest total and bulk conductivity were observed at x = 0.98 for both systems. Increasing the conductivity with a rise in humidity indicates that proton conduction appears in the oxides in wet conditions. A defect model based on consideration of different types of point defects, such as strontium vacancies, substitutional defects and oxygen vacancies, and assumption of Y ions partitioning over Zr and Sr sites was elaborated. The proposed model consistently describes the obtained data on conductivity.

Electronic structures, mechanical properties and defect formation energies of U3Si5 from density functional theory calculations

The uranium silicide U3Si5 has been utilized as the second phase in the UN-U3Si5 composite fuel. However, there have thus far been few theoretical investigations on its microscopic structure and mechanical behaviors. In this work, the electronic structures, elastic mechanical properties, Debye temperature and defect formation energies of U3Si5 are systematically studied by density functional theory. The crystalline structure of U3Si5 is determined to be a defective β-USi2 with a distorted lattice, in good agreement with the experimental observations. The theoretical results indicate that the U3Si5 is a metallic and brittle material and the chemical bonds in U3Si5 are found similar with those of U3Si2. We have also calculated formation energies of different types of point defects: vacancies, interstitials, and Frenkel pairs in U3Si5. The smaller Si atoms exhibit lower defect formation energies than U atoms with an isolated Si or U atom as the reference. Besides, the silicon vacancies are not prone to cluster. The volumes of supercells with interstitial and Frenkel pair defects are found to increase but single vacancies cause an opposite trend. This work may provide theoretical insights into the behavior of uranium silicide materials under irradiation.

Antisite Pairs Suppress the Thermal Conductivity of BAs.

BAs was predicted to have an unusually high thermal conductivity with a room temperature value of 2000 W m^{-1} K^{-1}, comparable to that of diamond. However, the experimentally measured thermal conductivity of BAs single crystals is still lower than this value. To identify the origin of this large inconsistency, we investigate the lattice structure and potential defects in BAs single crystals at the atomic scale using aberration-corrected scanning transmission electron microscopy (STEM). Rather than finding a large concentration of As vacancies (V_{As}), as widely thought to dominate the thermal resistance in BAs, our STEM results show an enhanced intensity of some B columns and a reduced intensity of some As columns, suggesting the presence of antisite defects with As_{B} (As atom on a B site) and B_{As} (B atom on an As site). Additional calculations show that the antisite pair with As_{B} next to B_{As} is preferred energetically among the different types of point defects investigated and confirm that such defects lower the thermal conductivity for BAs. Using a concentration of 1.8(8)% (6.6±3.0×10^{20} cm^{-3} in density) for the antisite pairs estimated from STEM images, the thermal conductivity is estimated to be 65-100 W m^{-1} K^{-1}, in reasonable agreement with our measured value. Our study suggests that As_{B}-B_{As} antisite pairs are the primary lattice defects suppressing thermal conductivity of BAs. Possible approaches are proposed for the growth of high-quality crystals or films with high thermal conductivity. Employing a combination of state-of-the-art synthesis, STEM characterization, theory, and physical insight, this work models a path toward identifying and understanding defect-limited material functionality.

Effect of Defects on the Spatial Localization of Nonlinear Acoustic Waves

A self-consistent mathematical model that includes equations of elasticity theory and kinetic equations for the density of different types of point defects is reduced to a nonlinear equation of evolution that combines the familiar Korteweg–de Vries–Burgers and Klein–Gordon equations of wave dynamics. Exact analytical solutions for this equation are found and analyzed.

Local Electron Interaction with Point Defects in Sphalerite Zinc Selenide: Calculation from First Principles

The present article deals with the description of electron scattering on the different types of point defects in zinc blende ZnSe on the basis of short-range principles. The electron interaction with polar and nonpolar optical phonons, piezoelectric and acoustic phonons, neutral and ionized impurities and static strain centers is considered. The electron transition probabilities and, respectively, the kinetic coefficients in zinc selenide, were calculated using the numerical eigenfunction and self-consistent potential obtained within the ab initio density functional theory. The latter were evaluated using the projector augmented waves formalism as implemented in the ABINIT software suite. We investigated ZnSe samples with defect concentration 4.7 × 1015–1.08 × 1017 cm−3 , then calculated temperature dependencies of electron mobility and Hall factors in the range of 20–400 K. It is shown that the theoretical curves obtained in the framework of short-range scattering models much better coincide with experimental data than the curves calculated on the basis of long-range scattering models.

Effects of gas flow rate on the structure and elemental composition of tin oxide thin films deposited by RF sputtering

Photovoltaic technology is one of the key answers for a better sustainable future. An important layer in the structure of common photovoltaic cells is the transparent conductive oxide. A widely applied transparent conductive oxide is tin oxide (SnO2). The advantage of using tin oxide comes from its high stability and low cost in processing. In our study, we investigate effects of working gas flow rate and oxygen content in radio frequency (RF)-sputtering system on the growth of intrinsic SnO2 (i-SnO2) layers. X-ray diffraction results showed that amorphous-like with nano-crystallite structure, and the surface roughness varied from 1.715 to 3.936 nm. X-Ray photoelectron spectroscopy analysis showed different types of point defects, such as tin interstitials and oxygen vacancies, in deposited i-SnO2 films.

RETRACTED: First-Principles Study of Advanced Nuclear Materials: Defect Behavior and Fission Products in U-Si System

Uranium silicides are envisaged as potential nuclear materials for the next generation. U3Si is featured by the high actinide density and the better thermal conductivity relative to UO2. To properly and safely utilize nuclear materials, it is crucial to understand their chemical and physical properties. First-principles in theory is mostly used to analyze the point defect structures for uranium silicides nuclear fuels. The lattice parameters of U3Si and USi2 are calculated and the stability of different types of point defects are predicted by their formation energies. The results show that silicon vacancies are more prone to be produced than uranium vacancies in β-USi2 matrix. The most favorable sites of fission products are determined in this work as well. According to the current data, rare earth elements cerium and neodymium are found to be more stable than alkaline earth metals strontium and barium in a given nuclear matrix. It is also determined that in USi2 crystal lattice fission products tend to be stabilized in uranium substitution sites, while they are likely to form precipitates from the U3Si matrix. It is expected that this work may provide new insight into the mechanism for structural evolutions of silicide nuclear materials in a reactor as well as to provide valuable clues for fuel designers.

Defect Physics of BiOI as High Efficient Photocatalyst Driven by Visible Light

Point defects are very important for semiconductor photocatalyst: promoting visible‐light absorption, tailoring energy bands, trapping photogenerated carriers, etc. As promising high efficient photocatalyst, BiOI could absorb most solar energy, and has high oxidization activity. However, its conduction band edge is lower than the reduction potential of H+/H2, thus BiOI could not achieve overall photocatalytic water splitting. In the present work, the crystal structure and electronic structure of BiOI with three different types of point defects are systematically investigated by density functional theory calculations, to find suitable strategy to solve above issue. Based on detailed analysis of calculated results, it is found that antisite defect of I@Bi (iodine occupies the bismuth site) and oxygen vacancy defect in BiOI could achieve above requirement: their conduction band and valence band edge positions are straddle the redox potential of water, resulting in as the promising photocatalyst candidates for overall water splitting driven by visible light. The findings could provide reasonable explanations for the reported experiments, and are beneficial to the development of novel bismuth oxyhalogenide‐based photocatalyst.

Research on the influences of point defects on the thermal conductivity of carbon nanotube by simulation with orthogonal array testing strategy

In the preparation process of carbon nanotubes, various point defects inevitably come into being in the lattice structures. The defects strongly affect the thermal transport properties of carbon nanotubes. Thermal conduction in carbon nanotube is simulated by using nonequilibrium molecular dynamics method with reactive bond order (REBO) potential. Thermal conductivities of carbon nanotubes with and without defects are calculated for comparison. An orthogonal array testing strategy is employed. In the calculation it greatly saves the experimental effort and identifies the degrees of influence of such structural factors as defect type, tube length, tube radius, etc. on thermal conductivity of tube. The effects of three types of point defects: vacancy, doping and adsorption are primarily studied, and the ambient temperature factor is also analyzed. Simulation results show that the thermal conductivity of carbon nanotubes with defects decreases significantly due to point defects compared with that of perfect carbon nanotubes. The defect type has the first greatest influence on the decrease of thermal conductivity, and hvae the second third greatest infuluences respeetively the radius and the length of carbon nanotubes. The degrees of influence of the above types of point defect are in the order of vacancydopingadsorption. Different types of point defects have different effects on tubes at different ambient temperatures.