Kinds Of Point Defects Research Articles

An investigation into the effect of Gd on the optoelectronic properties of ZnO waveguide thin films by prism coupler

GdxZn1−xO (x=0, 0.005, 0.01, 0.02, 0.04) films were elaborated via spray pyrolysis on ordinary glass to explore GdZnO's potential as a waveguide. X-ray diffraction showed the incorporation of Gd in the wurtzite structure with decreasing crystallite size and a pronounced alignment along the [002] direction. UV-Vis measurements indicated a 0.02 eV increase in the optical gap energy, starting from 3.24 eV. Hall effect measurements showed a 104 decrease in charge carriers' density, mobility increase up to 209 cm2/Vs, and a 103 increase in magnetoresistance. Photoluminescence spectra of the films revealed the presence of different kinds of point defects with a slight tendency to promote Zni and VO defects. Atomic force microscopy assessments depicted randomly oriented and elongated grains over Gd doping and overall smoother films. The waveguide properties of the films were investigated using the prism coupler techniques. A slight and general decrease was observed in ordinary refractive indices going from 1.9745 in the ZnO film to 1.9713 in the Gd0.02Zn0.98O film while the extraordinary refractive index decreased from 1.9777 to 1.9692. The birefringence was inverted for all the doped films except for Gd0.04Zn0.96O film as it experienced no guiding modes at all. The simulated results were highly aligned with the prism coupler model of distinct interfaces and step refractive index.

Selective modulation of electronic transport in VO2 induced by 10 keV helium ion irradiation

Vanadium dioxide (VO2) manifests an abrupt metal–insulator transition (MIT) from monoclinic to rutile phases, with potential use for tunable electronic and optical properties and spiking neuromorphic devices. Understanding pathways to modulate electronic transport in VO2, as well as its response to irradiation (e.g., for space applications), is critical to better enable these applications. In this work, we investigate the selective modulation of electronic transport in VO2 films subject to different 10 keV helium ion (He+) fluences. Under these conditions, the resistivity in the individual monoclinic and rutile phases varied by 50%–200%, while the MIT transformation temperature remains constant within 4 °C independent of irradiation fluence. Importantly, different trends in the resistivity of the monoclinic and rutile phases were observed both as a function of total He fluence as well as in films grown on different substrates (amorphous SiO2/Si vs single crystal Al2O3). Through a combination of measurements including majority carrier sign via Seebeck, low frequency noise, and TEM, our investigation supports the presence of different kinds of point defects (V in; O in), which may arise due to grain boundary defect interactions. Our work suggests the utility of He irradiation for the selective modulation of VO2 transport properties for neuromorphic, in contrast to other established but non-selective methods, like doping.

High responsivity n-ZnO/p-CuO heterojunction thin film synthesised by low-cost SILAR method for photodiode applications

In this report, we have prepared n-ZnO/p-CuO heterojunction thin film by low-cost SILAR method and made a comparative study with physical properties of ZnO and CuO thin film photodetectors. The prepared heterojunction film is exhibit mixture of hexagonal structure of ZnO and monoclinic structure of CuO. All the films have uniform surface morphology without any voids and ensured that the presence of used elements like Zn, Cu, and O. The optical absorption spectrum declared that comparatively high photon absorption in the range of 2–6% occurred in 365 nm of UV light. The photoluminescence spectrum notices that different kind of point defects are present in the prepared heterojunction film. The photo sensing results announce that the ZnO/CuO heterojunction film exist a good photodiode characteristic behavior with good responsivity of 43.69 × 10−2 A/W.

Моделювання поля зсувних напружень в площині ковзання в твердих розчинах заміщення

The formation of stochastic shear stress field in the glide plane in the substitutional solid solution was investigated by computer simulation. If the atoms in the crystal lattice nodes of the substitutional solid solution are considered as a kind of point defects in the virtual solvent medium, the shear stress distribution in the glide plane can be calculated based on the interaction of edge dislocation and such defects. For concentrated solid solutions, the shear stress will be a normally distributed random value with zero mathematical expectation. The standard deviation of this distribution will be the greater the greater the effective distortion of crystalline lattice of the alloy. In the case of dilute solid solution, where one of the components has a predominant content, the simulation gives shear stress distribution in the glide plane, where large peaks are separated from each other by wide areas of near-zero stresses. Thus, there are separate discrete obstacles in the form of large stress peaks for the edge dislocation in the glide plane in dilute solid solution, and the space between the peaks is practically stress-free. The average distance between large peaks correlates with the average distance between the atoms of those components that are few in solution, if total atomic fraction of these components is considered. Thus, the proposed modeling gives a very realistic shear stress distribution in the glide plane for concentrated and dilute substitutional solid solutions with fcc and bcc structures. This can be useful in further modeling the yield strength in multicomponent alloys. Keywords: dislocation, distorsion, shear stresses.

Effects of mechanical and electronic properties for B2 FeAl modified by several kinds of point defects: First-principles study

Elastic, fracture and deformation behavior of B2 FeAl intermetallics modified by ternary additions as well as Al/Fe vacancy defects and anti-sites have been investigated based on density functional theory (DFT). Formation enthalpy indicates that ternary additions Sc, Y, Mo and W have a preference for Al site. While Fe site is more easily replaced by ternary Cu and Zn. Moreover, vacancy and antisite defects can be stable in FeAl. Pugh criteria ([Formula: see text] ratio), Poisson’s ratio [Formula: see text] and Cauchy pressure show that Sc, Y, Mo, W and Al vacancy (Al anti-site by Fe atom) can effectively improve the ductility of FeAl. Dislocation emission and micro-cracks propagation show that the improved ductility is due to the promoted dislocation emission but suppressed micro-cracks propagation. Bonding analyses reveal that the improved ductility is mainly own to the weakened the covalent interactions and strengthened the metallic interactions.

The effect of temperature and cascade collision on thermal conductivity of 3C-SiC: A molecular dynamics study

The cubic silicon carbide 3C-SiC, the significant nuclear material used to manufacture the nuclear fuel in High Temperature Gas-cooled Reactor (HTGR), operates under a high temperature and neutron flux condition. The effect of temperature and irradiation on thermal conductivity of 3C-SiC were investigated with the molecular dynamics method. The equilibrium and non-equilibrium methods were utilized to calculate the thermal conductivity of 3C-SiC and the results of tersoff/zbl and MEAM interatomic potential were compared. Exponential decay of thermal conductivity with increasing temperature was illustrated. The irradiation effect was simulated with a high-energy Si atom and subsequent cascade collision, a multiple-phase simulation procedure was employed and the number of four kinds of point defects was counted. Finally, a relatively smaller simulation cell was used to analyze the impact of the incident atom with various velocities on the thermal conductivity of 3C-SiC. The computed results showed the cascade collision, caused by a higher-energy atom, would induce the lower thermal conductivity of 3C-SiC.

Molecular Dynamics Simulation and Analysis the Primary Radiation Damage of 4H-Sic Crystal Material Induced by 10kev Si PKA Under Different Temperature

Based on the molecular dynamics method and the simulation toolkit LAMMPS, the distribution characteristics of PKA tracks and the different kinds of point defects in 4H-SiC crystal material induced by 10keV Si PKA with specific incident direction under different temperature conditions were simulated analyzed in this paper. The mechanism of the counts of the various point defects changed with time under a certain temperature (300K) condition is compared and analyzed. Simulation and analysis of the spatial distribution of point defects in the stable stage with PKA track and temperature changes. The results of this paper provide an important basis for further exploring the formation mechanism of 4H-SiC radiation displacement damage defects, predicting the thermodynamic properties of 4H-SiC matrix materials by displacement damage, and the electrical characteristics of 4H-SiC devices/detectors.

Role of native and impurity defects in optical absorption and luminescence of Li2MoO4 scintillation crystals

The Li2MoO4 crystals have been considered as perspective cryogenic scintillation materials to be used in elementary particle physics experiments related to dark matter search and properties of neutrino. Since the crystals may be used as cryogenic scintillators, the native vacancies, uncontrolled impurities and other defects play the key role in worsening of their scintillation characteristics. The paper presents complex experimental and computational studies of the electronic structure and optical properties of Li2MoO4 crystals. The studies are aimed to explain peculiarities of the optical (luminescence) properties of Li2MoO4 using the electronic structure results obtained for a wide range of point defects and their combinations. For investigations, several samples of Li2MoO4 with different defectiveness were grown using Czohralski method and characterized by SEM and ICP-OES technique. Luminescence emission spectra of the samples under X-ray and laser excitations, optical and IR absorption spectra, TSL characteristics of the samples were recorded and analyzed. Ability to incorporate some gases was examined by measuring absorption spectra after one-year storage of the samples at ambient conditions. The electronic structures of several kinds of point defects (natural vacancies, iso- and aliovalent cationic and anionic substitutions, interstitial ions and molecular groups) in Li2MoO4 were calculated using the DFT-based plane-wave pseudopotential method. Geometries of lattice relaxations around defects, partial densities of states and energies of defect formation were calculated and analyzed together with experimental results. Performed analysis allowed making several conclusions regarding the origin of spectral components of intrinsic and defect-related luminescence, formation of defect-related bands in the optical and IR absorption spectra, dependence of TSL characteristics on sample defectiveness. It was found that isovalent cationic substitutions on lithium positions are presented in substantial concentrations in the Li2MoO4 samples and such defects form shallow traps which can affect scintillation characteristics in cryogenic experiments. Substantial effect of gases inclusions on the optical characteristics of crystals can be related to the peculiarity of the crystal lattice structure - the presence of empty "channels", which can incorporate molecular objects like H2O, -CH3 or -OH.

First-principles study of electronic structure and optical properties of monolayer defective tellurene

Monolayer tellurene is a novel two-dimensional semiconductor with excellent intrinsic properties. It is helpful in understanding doping and scattering mechanism to study the electronic structure of defective tellurene, thus it is important for the application of tellurene in electronic and photo-electronic devices. Using first-principles calculation based on the density functional theory, we investigate the effects of commonly seen point defects on the electronic structure and optical properties of monolayer <i>β</i>-Te. Seven kinds of point defects that may be present in <i>β</i>-Te are designed according to the lattice symmetry, including two single vacancies (SV-1, SV-2), two double vacancies (DV-1, DV-2) and three Stone-Wales (SW) defects (SW-1, SW-2, SW-3). It is found that the defect formation energies of these defects are 0.83–2.06 eV, which are lower than that in graphene, silicene, phosphorene and arsenene, suggesting that they are easy to introduce into monolayer <i>β</i>-Te. The two most stable defects are SV-2 and SW-1 where no dangling bond emerges after optimization. The calculated band structures show that all seven defects have little effect on the band gap width of monolayer <i>β</i>-Te, but they can introduce different numbers of impurity energy levels into the forbidden band. Among them, the SV-1, SV-2, DV-1 and SW-2 each act as deep level impurities which can be recombination centers and scattering centers of carriers, SW-1 acts as a shallow level impurity, DV-2 and SW-3 act as both deep level impurity and shallow level impurity. Besides, SW-1, SW-2 and DV-1 can change the band gap of monolayer <i>β</i>-Te from direct band gap to indirect band gap, which may result in the increase of the lifetime of carriers and decrease of photoluminescence of monolayer <i>β</i>-Te. The optical properties of monolayer <i>β</i>-Te, which are sensitive to the change in band structure, are also affected by the presence of defects. New peaks are found in the complex dielectric function and the absorption coefficient of defective monolayer <i>β</i>-Te in an energy range of 0–3 eV, of which the number and the position are dependent on the type of defect. The SV-1, DV-1, DV-2 and SW-2 can enhance the light response, polarization ability and light absorption in the low energy region of monolayer <i>β</i>-Te. This research can provide useful guidance for the applications of <i>β</i>-Te in the electronic and optoelectronic devices.

Point Defects in Two-Dimensional γ-Phosphorus Carbide.

Defects are inevitably present in two-dimensional (2D) materials and usually govern their various properties. Here, a comprehensive density functional theory-based investigation of seven kinds of point defects in a recently produced γ allotrope of 2D phosphorus carbide (γ-PC) is conducted. The defects, such as antisites, single C or P, and double C and P and C and C vacancies, are found to be stable in γ-PC, while the Stone-Wales defect is not presented in γ-PC due to its transition-metal dichalcogenides-like structure. The formation energies, stability, and surface density of the considered defect species as well as their influence on the electronic structure of γ-PC is systematically identified. The formation of point defects in γ-PC is found to be less energetically favorable than in graphene, phosphorene, and MoS2. Meanwhile, defects can significantly modulate the electronic structure of γ-PC by inducing hole/electron doping. The predicted scanning tunneling microscopy images suggest that most of the point defects are easy to distinguish from each other and that they can be easily recognized in experiments.

Tunable point defects in hydroxyl fused silica enabled by ultrashort laser pulses: photostimulated luminescence and functional module fabrication

Three kinds of point defects, SiE’ center (≡ Si·), unrelaxed oxygen deficiency center (ODC (II)) and non-bridging oxygen hole center (≡ Si−O·, NBOHC), have been generated in hydroxyl fused silica by ultrashort pulsed laser irradiation. Hydroxyl is proved to be a decisive component for defect producing: NBOHC originates directly from hydroxyl; Hydroxyl facilitates the generation of SiE’ in an indirect way; No obvious relevance could be built between ODC (II) and hydroxyl. By improving hydroxyl content to 1000 ppm, NBOHC becomes the dominant defect species and its red luminescence is hence discernible to naked eye. Intended for application, high hydroxyl fused silica is screened out as the desired candidate, and NBOHC becomes the final interested defect. NBOHC’s intrinsic features of lifetime and temperature stability and extrinsic properties of laser condition dependence are specifically and systematically investigated. Prospective use of defect manipulation and fabrication in one-chip exploited for anti-counterfeiting and lab-on-a-chip is also discussed.

First Principles Study on the Thermodynamic and Elastic Mechanical Stability of Mg2X (X = Si,Ge) Intermetallics with (anti) Vacancy Point Defects

In this paper, based on the density functional theory, through thermodynamic and mechanical stability criteria, the crystal cell model of intermetallic compounds with vacancy and anti-site point defects is constructed and the lattice constant, formation heat, binding energy, elastic constant, and elastic modulus of Mg2X (X = Si, Ge) intermetallics with or without point defects are calculated. The results show that the difference in the atomic radius leads to the instability and distortion of crystal cells with point defects; Mg2X are easier to form vacancy defects than anti-site defects on the X (X = Si, Ge) lattice site, and form anti-site defects on the Mg lattice site. Generally, the point defect is more likely to appear at the Mg position than at the Si or Ge position. Among the four kinds of point defects, the anti-site defect x M g is the easiest to form. The structure of intermetallics without defects is more stable than that with defects, and the structure of the intermetallics with point defects at the Mg position is more stable than that at the Si/Ge position. The anti-site and vacancy defects will reduce the material’s resistance to volume deformation shear strain, and positive elastic deformation, and increase the mechanical instability of the elastic deformation of the material. Compared with the anti-site point defect, the void point defect can lead to the mechanical instability of the transverse deformation of the material and improve the plasticity of the material. The research in this paper is helpful for the analysis of the mechanical stability of the elastic deformation of Mg2X (X = Si, Ge) intermetallics under the service condition that it is easy to produce vacancy and anti-site defects.

Effects of the thermal annealing on the thermoelectric properties of Ga-doped ZnO thin films

We deposited successfully Ga-doped ZnO (GZO) thin films by using magnetron Dcsputtering technique, followed by annealing. The effects of the thermal annealing on thermoelectric properties of GZO films were investigated. The obtained results showed that due to annealing, the thermoelectric properties of the GZO films were significantly enhanced: (1) power factor increased with an increase of electron mobility due to high film crystallinity; (2) The figure of merit ZT values of the GZO film annealed at 500 oC (ZT = 0.114) was one order higher the asdeposited GZO film (ZT = 0.012). The room temperature photoluminescence (PL) spectra depicted various kinds of point defects which controlled thermoelectric properties and both oxygen vacancies VO and zinc interstitial Zni played an important role.

Atomic scale characterization of point and extended defects in niobate thin films

Niobium-based oxides have a wide range of applications owing to their rich crystal and electronic structures. Defects at the atomic scale are always unavoidable and will affect their functionalities, especially when in the form of thin films. Here, atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy have been performed on various defects (point, line, planar defects and segregated phases) in alkaline and alkaline-earth niobate thin films: CaZrO3 modified (K, Na)NbO3 and strontium niobate (SNO), respectively. In CaZrO3 modified (K,Na)NbO3 thin films, a tetragonal tungsten bronze phase was found, with a sharp boundary with the perovskite phase. In SNO thin films, several kinds of point defects and antiphase boundaries are commonly observed. In addition, a strongly Sr deficient phase, SrNb2O6, precipitates inside the SrNbO3 phase with a coherent interface. The different oxidation states of Nb in SrNbO3 and SrNb2O6 were revealed from the O K edge. Our characterization of the point defects and extended defects in niobate thin films offers practical guidelines for thin film deposition or discovery of defect-based novel functionalities.

Formation energy and diffusion barrier of point defects in three uranium compounds as nuclear fuels

During fission neutron irradiation, point defects are generated in uranium fuel, and the point defects inevitably have some effects on the properties of the uranium fuel. In particular, the point defects accumulate through diffusion, resulting in formation of body defects that degrade the properties of the fuel. Using the first principles calculation method, based on the density functional theory, we calculated the elastic properties of three kinds of uranium compounds (UO2, UN, and UC). The results are in agreement with calculated and experimental results of others. On the basis of verifying the accuracy of the calculation model, we investigated formation energy and diffusion barrier of three kinds of point defects (vacancy, self-interstitial atom, and hydrogen/helium (H/He) atoms) in the three kinds of uranium compounds. The formation energy of the U vacancy is lower than that of the O, N, and C vacancy in the three kinds of uranium compounds, whereas the interstitial U atom has a higher formation energy than the interstitial O, N, and C atoms. The formation energy of the interstitial H atom is much lower than that of the interstitial He atom in the three kinds of uranium compounds, and the H atom prefers to occupy the O, N, and C vacancy while the interstitial He atom prefers to occupy the U vacancy. The diffusion of the interstitial H atom in UO2 is easier than that of the interstitial He atom, whereas the diffusion of the interstitial He atom in UN and UC is easier than that of the interstitial H atom. The diffusion of the interstitial self-atom in UO2 is more difficult than that of the self-atom in UN and UC. The work provides a fundamental understanding of the point defects formation and diffusion to develop uranium fuel by controlling defect formation.

Effect of symmetrical and asymmetrical tilt grain boundaries on radiation-induced defects in zirconium

In this article, molecular-dynamics-based simulations were used to study the effect of grain boundaries (GBs) on the formation and spatial distribution of radiation-induced point defects. In order to perform this study, two sets of symmetrical and asymmetrical tilt grain boundaries were constructed along [0 0 0 1] and [0 −1 1 0] as the tilt axis, respectively. Vacancy, interstitial and Frenkel pair formation energies were estimated as a function of the distance from the GB core for both symmetrical as well as asymmetrical tilt GBs. The trend obtained between GB energies and point defect formation energies helps explain the biased absorption of interstitials over vacancies in most cases, as well as the equal absorption of both kinds of point defects in a few of them. It has already been reported from the experimental work that [0 0 0 1] GB structures closely resemble the polycrystalline texture of hcp materials, which motivates us to study the effect of irradiation on these GBs.

Theoretical study of the interaction between metallic fission products and defective graphite

The adsorption and diffusion behaviors of the metallic fission products of Cs, Sr, Ag on the defective graphite surface with double vacancy and Stone–Wales defect are theoretically studied by using density functional theory. Our results show that the Cs, Sr and Ag adatoms are adsorbed preferably at the defect areas of graphite surface. Electronic structure analysis indicates that the defects enhance the adsorption of metallic fission products. Further study on the diffusion properties also proves that the adatoms prefer to diffuse toward the defective area. These processes can readily occur due to zero or ignorable diffusion energy barriers of different kinds of point defects, indicating that point defects can serve as the trap sites for the metallic fission products.

The role of point defects in the swelling and elastic modulus of irradiated cubic silicon carbide

A molecular dynamics study has been performed to investigate the role of point defects in volumetric swelling and elastic modulus of irradiated 3C-SiC in the low and intermediate temperature regime. It is found that different kinds of point defects have distinctive effects on the swelling and Young’s modulus. The vacancies have the negligible influence on volumetric swelling while significant on the Young’s modulus. However, the value of volumetric swelling and the change in Young’s modulus due to the formation of other point defects vary with defect concentration exponentially which we are interested. Furthermore, it is indicated that the relation of the swelling of 3C-SiC and the contribution of point defects is independent on the temperature. During the tensile test, however, the role of carbon interstitial in Young’s modulus would be enhanced by temperature. Finally, an approximate model to estimate the antisite defect concentrations in irradiated 3C-SiC is presented.

Quantitative study on structural evolutions and associated energetics in polysilazane-derived amorphous silicon carbonitride ceramics

Several important structural changes and their energetics during high-temperature annealing of polysilazane-derived amorphous silicon carbonitride ceramics were quantitatively studied. A 29Si solid-state NMR study indicated that the structural transition in the Si-containing area can be described by an equilibrium reaction, 4SiCN3=SiC4+3SiN4. The enthalpy and entropy for the reaction were calculated to be positive. Raman and electron paramagnetic resonance (EPR) studies revealed that the structural evolution within the free carbon area includes the graphitization of amorphous carbon and the lateral growth of nanographite, accompanied by a decrease in the point defect concentration. EPR results also suggested that the materials contain two kinds of point defects: carbon-dangling bonds at the edge and in the interior of the nanographite. It was found that the lateral growth of the nanographite followed a 2-D grain growth process, and that the decrease in the defect concentration was mainly due to the growth of the nanographite. The energetics of the structural changes was rationalized according to a simple structural model, and the effects of these changes on the stability of the materials were discussed.

Recovery behavior of point defects after low-dose neutron irradiation at ∼423 K of sintered 6H–SiC by lattice parameter and macroscopic length measurements

To evaluate neutron-irradiation-induced crystalline defects and its thermal stability, α-SiC sintered bodies consisting of mainly 6H polytype were neutron irradiated using the Japan Materials Testing Reactor up to 1.9×1023n/m2 (E>0.1MeV) at a low temperature of ∼423K. Due to very low dose irradiation at low temperature, expected defects induced into crystalline lattice should only be simple point defects. Changes in the lattice parameters and macroscopic lengths resulting from post-irradiation isothermal annealing up to 6h between room temperature and 1673K were measured. Macroscopic volume swelling of α-SiC specimens coincided well to those of unit cell volume changes, indicating the presence of only simple defects. The recovery behavior of the macroscopic length and lattice parameters showed almost the same tendencies, although residual changes in the c-axis length slightly exceeded that of the a-axis length at temperatures lower than 1273K. Difference in these axes lengths diminished at temperatures above 1273K. Calculation of activation energies, obtained from precise length measurement using a dilatometer during each isothermal annealing step, revealed that length recovery behavior between 473 and 1573K could almost be completely explained by a first-order reaction, and three stages with activation energies of 0.14, 0.26, and 1.13eV. Changes in activation energy were related to the behavior of lattice parameter changes. The results indicated the presence of several kinds of point defects, some of which induced anisotropic lattice expansion.