Interstitial Configurations Research Articles

Acceptor complex signatures in oxygen-rich ZnO thin films implanted with chlorine ions

Spectroscopic identification of defects and impurities is crucial for understanding doping asymmetry issues in ZnO and, therefore, realization of true ZnO-based bipolar devices. Chlorine (Cl) is an amphoteric impurity in ZnO exhibiting acceptor behavior in the interstitial configuration and donor action once on substitutional oxygen sites (ClO). In its turn, the incorporation of Cl atoms depends on the material growth conditions and a ClO fraction should be suppressed in O-rich materials. In the present work, Cl ions were implanted into ZnO thin films synthesized under O-rich conditions. In contrast to a negligible effect of Cl incorporation to electrical conductivity, photoluminescence measurements revealed dramatic developments of optical properties with a strong acceptor-like spectral signature emerging after 900 °C anneals. We discuss the origins of a new excitonic I* line (3.355 eV) induced by Cl-implantation and propose two alternative defect models based on shallow acceptor and shallow donor complexes.

Fe-Doped g-C3N4: High-Performance Photocatalysts in Rhodamine B Decomposition.

Herein, Fe-doped C3N4 high-performance photocatalysts, synthesized by a facile and cost effective heat stirring method, were investigated systematically using powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron (XPS), UV–Vis diffusion reflectance (DRS) and photoluminescence (PL) spectroscopy. The results showed that Fe ions incorporated into a g-C3N4 nanosheet in both +3 and +2 oxidation states and in interstitial configuration. Absorption edge shifted slightly toward the red light along with an increase of absorbance in the wavelength range of 430–570 nm. Specific surface area increased with the incorporation of Fe into g-C3N4 lattice, reaching the highest value at the sample doped with 7 mol% Fe (FeCN7). A sharp decrease in PL intensity with increasing Fe content is an indirect evidence showing that electron-hole pair recombination rate decreased. Interestingly, Fe-doped g-C3N4 nanosheets present a superior photocatalytic activity compared to pure g-C3N4 in decomposing RhB solution. FeCN7 sample exhibits the highest photocatalytic efficiency, decomposing almost completely RhB 10 ppm solution after 30 min of xenon lamp illumination with a reaction rate approximately ten times greater than that of pure g-C3N4 nanosheet. This is in an agreement with the BET measurement and photoluminescence result which shows that FeCN7 possesses the largest specific surface area and low electron-hole recombination rate. The mechanism of photocatalytic enhancement is mainly explained through the charge transfer processes related to Fe2+/Fe3+ impurity in g-C3N4 crystal lattice.

High-Q Interstitial Square Coupled Microring Resonators Arrays

The properties of the square array of coupled Microring Resonators (MRRs) with interstitial rings are studied. Dispersion behavior of the interstitial square coupled MRRs is obtained through the transfer matrix method with the Floquet-Bloch periodic condition. Analytical formulas of the eigen wave vectors, band gaps and eigen mode vectors are derived for the special cases of the interstitial square coupled MRRs array with identical couplers and the regular square coupled MRRs array without the interstitial rings. Then, the eigen modes' field distribution are calculated for each of the four eigen wave vectors for a given frequency through the secular equation. Finally, numerical simulation is performed for an interstitial square coupled MRRs array with identical couplers and a regular square coupled MRRs array. The simulation result verifies the analytical analysis. Finally, the loaded quality factors of the interstitial 5-ring configuration, the regular 4-ring configuration and the 1-ring configuration are obtained. It is found that the loaded quality factor of the interstitial 5-ring configuration is up to 20 times and 8 times as high as those of the 1-ring configuration and the regular 4-ring configuration respectively, mainly due to the degenerated eigen modes at the resonant frequency. Thus, the interstitial square coupled MRRs array has the great potential to form high-quality integrated photonics components, including filters and resonance based sensing devices like the parity-time symmetric sensors.

Vacancy-driven variations in the phonon density of states of fast neutron irradiated nuclear graphite

Research examining radiation damage and its effects in graphite began in the 1940’s during the development of moderated nuclear reactors. Interest in this topic is expanding because of emerging applications associated with fullerenes and carbon nanostructures as well as its long-standing use as a fission moderator. In this work, we report the measurements of the full phonon density of states of irradiated nuclear graphite using inelastic neutron scattering experiments at room temperature. The samples were previously exposed to different levels of neutron doses and irradiation temperatures. The phonon density of states of perfect and defected graphite supercells including different configurations of vacancies and interstitials were calculated using first-principles direct method. Despite high neutron irradiation doses, the scattering measurements show that the induced damage is localized, and the layered structure is preserved. A comparison of the measured phonon densities of states of irradiated samples with those calculated of defected supercells indicates that the main changes observed are mainly attributed to formation of vacancies.

Influence of hydrogen implantation on emission from the silicon vacancy in 4H-SiC

The silicon vacancy (VSi) in 4H-SiC is a room temperature single-photon emitter with a controllable high-spin ground state and is a promising candidate for future quantum technologies. However, controlled defect formation remains a challenge, and, recently, it was shown that common formation methods such as proton irradiation may, in fact, lower the intensity of photoluminescence (PL) emission from VSi as compared to other ion species. Herein, we combine hybrid density functional calculations and PL studies of the proton-irradiated n-type 4H-SiC material to explore the energetics and stability of hydrogen-related defects, situated both interstitially and in defect complexes with VSi, and confirm the stability of hydrogen in different interstitial and substitutional configurations. Indeed, VSi-H is energetically favorable if VSi is already present in the material, e.g., following irradiation or ion implantation. We demonstrate that hydrogen has a significant impact on electrical and optical properties of VSi, by altering the charge states suitable for quantum technology applications, and provide an estimate for the shift in thermodynamic transition levels. Furthermore, by correlating the theoretical predictions with PL measurements of 4H-SiC samples irradiated by protons at high (400°C) and room temperatures, we associate the observed quenching of VSi emission in the case of high-temperature and high-fluence proton irradiation with the increased mobility of Hi, which may initiate VSi-H complex formation at temperatures above 400°C. The important implication of hydrogen being present is that it obstructs the formation of reliable and efficient single-photon emitters based on silicon vacancy defects in 4H-SiC.

Controlling the strength of Zr (10 [formula omitted] 2) grain boundary by nonmetallic impurities doping: A DFT study

Impurity segregation even small amounts, can drastically change the cohesive properties of the grain boundaries (GB), eventually leading to intergranular embrittlement and failure of the materials, thereby effectively controlling the types and the concentrations of the impurity is very important. In this work, the nonmetallic impurities (C, H, O, N) segregation and their effects on the strength of Zr (10 1¯ 2) GB were thoroughly investigated using first-principles calculations based on density functional theory. A comprehensive analysis of the interstitial configurations and the relative site energies indicating that C, N and O overwhelmingly prefer the octahedral sites, only H, prefers to reside in the tetrahedral sites. Moreover, the strengthening/embrittlement potency of impurity atoms on the GB was estimated using both the Rice-Wang model and first-principles tensile test calculations. The results show that all impurities, exhibit a strong segregation tendency near the GB region. The segregation of C, N and O has a remarkable strengthening effect on strength of the GB, whereas the presence of impurity H weaken the GB. Most importantly, the underlying mechanism of the strength change of the GBs due to the segregation of impurities was profoundly discussed by charge density and the bond lengths analyses, revealing that the strengthening effect especially for C-doped GB, mainly comes from an enhancement of the charge density across the GB plane. In the end, we expect that our results will be certainly useful for future theoretical and experimental investigations on Zr and its alloys.

The electronic structures and properties for carbon doped aluminum phosphide

Based on the first-principle method, the effects of carbon dopants on AlP are studied. Our investigations indicate carbons substituting aluminum (CAl) or phosphorus (CP) cannot induce the AlP to be magnetic system. The AlP with CAl defects shows n-type metallic characteristic. As the carbons forming the interstitial doped configurations (Cint), a few percent of Cint could induce the AlP to be a half-metallic ferromagnet, and the half-metallic gaps could reach 0.45∼0.48 eV. Mean field theory predicts the Curie temperature for Cint doped configurations would be above room temperature. Formation energies indicate that Cint, CAl and CP defects could be realized experimentally. Positive pair binding energies indicate that carbon would form homogenous distribution in AlP host. This research suggests carbon is a promising dopant, and could be utilized to functionalize the AlP's properties for versatile applications.

Vacancies and interstitials in yttrium

The paper deals with the first principles simulation of formation energies and migration barriers of self point defects, including vacancies, di-vacancies and single interstitial atoms, in metallic yttrium. The vacancy formation energy in yttrium is shown to be relatively high (~1.8 eV), whereas the migration barriers are very similar for the jumps inside the basal planes and between basal planes, being equal to ~0.65 eV. The sum of these numbers reasonably reproduces the experimental values of the self-diffusion activation barriers. The vacancy pairs at the first nearest neighbor separation (divacancies) have binding energy of ~0.2 eV, which is only weakly sensitive to the divacancy orientation in the lattice, whereas vacancy pairs at the second and third nearest-neighbor separations are energetically unfavorable, suppressing the dissociation of divacancies. Together with the noticeably lower divacancy migration barriers with respect to single vacancies, this makes divacancies efficient mediators for mass transfer in Y. Among multiple possible configurations of a single interstitial, only the basal octahedral one is found to be the true energy minimum, while all the other considered possibilities are either unstable, or saddle points on the potential energy surface. This is in contrast to other hcp metals, where several metastable interstitial configurations often coexist. The lowest migration barriers for single interstitial diffusion along the basal plane and between planes are practically equal, ~0.4 eV, implying isotropic diffusion of interstitials in yttrium. Overall, the predicted properties of point defects in yttrium are in line with the general trends for hcp metals with the c/a ratio below .

Atomistic simulations of temperature and direction dependent threshold displacement energies in [formula omitted]- and [formula omitted]-uranium

We performed a systematic study of the threshold displacement energy (Ed) in metallic uranium as a function of both the recoil direction and temperature using Molecular Dynamics simulations. We developed a novel orientation sampling scheme that utilizes crystallographic symmetrical geodesic grids to select directions from the orientation fundamental zone to study the directional dependency. Additionally, we studied the temperature dependency by considering both the α-uranium phase, corresponding to the ground state for temperatures ranging from 0 K to 600 K, and the γ-uranium phase, corresponding to high-temperature state for temperatures above 900 K. In this study, we compared several definitions of the threshold energy: a direction-specific threshold displacement energy (Edθ,ϕ), an angle-averaged threshold energy (Edave), a production probability threshold displacement energy (Edpp), and a defect count threshold displacement energy (Eddc). The direction-specific threshold displacement energies showed large angular anisotropy and variations in accordance with crystallographic considerations. Specifically, preferred defect channeling directions were observed in the [120],[12¯0],[11¯1] directions for the α-uranium, and [001],[111] directions for the γ-uranium. The production probability threshold displacement energy (Edpp) is calculated as approximately 99.2659 eV at 10 K (α-U), 103.4980 eV at 300 K (α-U), 76.0915 eV at 600 K (α-U), and 42.9929 eV at 900 K (γ-U). With exception of those calculated at 10 K, threshold displacement energies decrease with increasing temperature. Analyses of the stable defect structures showed that the most commonly observed interstitial configuration in α-uranium consists of a 〈010〉 dumbbell-like interstitial; while in γ-uranium no preferential defect configuration could be identified due to thermally-induced lattice instabilities at the elevated temperatures.

Simulation of atomic displacement cascades in the binary alloy Zr-1%Nb near symmetrical tilt grain boundaries by molecular dynamics method

ABSTRACTThe atomic displacement cascades simulation near symmetrical tilt grain boundaries (GBs) in binary alloy Zr-1%Nb was considered in this paper. Three symmetrical tilt GBs: , with the axis of rotation and with the axis of rotation were considered. Further defect structure analysis in modeling crystallite was conducted. For atomic displacement cascades simulation, we used molecular dynamics method and two different interatomic potentials. A tendency of the point defects produced in the cascade to accumulate near the GB plane, which was an obstacle to the spread of the cascade, was discovered. The results of the point defects’ clustering produced in the cascade were obtained. Clusters of vacancies and self-interstitial atoms were represented mainly by single point defects for both potentials. Vacancies formed clusters of a large size (more than 20 vacancies per cluster) for both potentials. The maximal size of the interstitial cluster was less than 20 point defects and was formed only in one potential. Interstitial atom clusters were mainly small-sized. Significant Nb atoms’ proportion in interstitial configuration (single defects, interstitial dimmers and interstitial clusters) was established. It means than Nb atoms actively went out from crystallite matrix in interstitial configurations.

Theoretical study of bismuth-doped CdTe

The electronic structure and thermodynamic properties of bismuth impurities in cadmium telluride (CdTe) are studied by means density functional theory calculations. The exchange and correlation are treated using the hybrid functional HSE, where the range-separation parameter is tuned to fit the CdTe band gap. The spin-orbit coupling is included in the calculations, showing a strong effect on the Bi-related energy levels. The band diagram and the formation energies are obtained for substitutional and interstitial configurations. It is shown that simple substitutional impurities can generate in-gap bands, which can act as intermediate levels in two-photon absorption and can be appropriate for intermediate-band solar cells.

The electronic and magnetic properties of V-doped ZnSe at substitutional/interstitial sites

The spin polarised density functional theory along with self-consistent plane wave pseudopotential is used to investigate the electronic and magnetic properties of V-doped ZnSe at Se and interstitial sites. The generalized gradient approximation is used for the exchange-correlation potential. The equilibrium lattice constants, bulk modulus and its pressure derivatives are calculated. It is found that the lattice parameter enhances while V is at the Se and interstitial sites. The ZnV0.125Se0.875 and ZnV0.25Se with V at the body centre interstitial sites configurations are ferromagnets while ZnV0.25Se0.75 and ZnV0.25Se with V at the edge interstitial sites are antiferromagnets. The antiferromagnetic phase with V at the edge interstitial sites is the most stable. The configuration ZnV0.125Se0.875 is half-metallic with band gap of 0.18 eV while the configurations ZnV0.25Se0.75 and ZnV0.25Se with V at the body centre and edge interstitial sites are metallic.

First principle calculation of helium in La2Zr2O7: Effects on structural, electronic properties and radiation tolerance

First principle calculations based on density functional theory have been employed to study structural effects of trapping helium in La2Zr2O7 pyrochlore. Lattice swelling and the distortion of unit cell have been found in He-La2Zr2O7 systems. By analyzing the electronic structures and chemical bonding of He-La2Zr2O7 systems, weak repulsive and attractive chemical interactions of helium in La2Zr2O7 pyrochlore have been observed. The formation energies have been calculated to assess the relative stability of various helium interstitial configurations and the results show that the octahedral interstitial site is the most stable structure. The cation antisite defect formation energies and the x positional parameter for 48f-site oxygen are calculated to predict the radiation resistance of He-La2Zr2O7 systems. The results indicate that the presence of low concentration of He interstitials may increase the radiation resistance of La2Zr2O7 pyrochlore.

Decentral distribution of helium in β-SiC: Studied by density functional theory

β-SiC is an excellent material as a fuel cladding material in next generation fission reactors and structural components in the fusion reaction. The introduction of helium (He) atoms from the deuterium-tritium reaction or (n,α) reaction will inevitably affect the properties of nuclear materials. It is imperative to explore the behavior of He in β-SiC, and thus, the influence on the physical and chemical properties of materials on the atomic scale. In this work, the possible configurations of He interstitials existing in β-SiC were investigated using density functional theory (DFT). It was found that the He atom prefers to occupy Si tetrahedron interstitial (Tsi) site, surrounded by four Si atoms rather than the C tetrahedron interstitial (Tc) site, surrounded by four C atoms. The detailed analysis concerned with chemical bonding of host atoms with He has been carried out using partial density of states (PDOS) and an orbital-based descriptor (projected crystal orbital Hamilton population, pCOHP). In addition, the calculation of binding energy implies that in the absence of vacancies, He atoms tend to decentralize themselves in separated Tsi positions, and are hard to cluster through exothermal processes. This is due to the fact that in β-SiC, tetrahedral interstitials, surrounded by strong SiC bonds, act as firm cages to make helium atoms confined separately. Our results are in agreement with the experimental observation that no He bubbles are generated in a single crystal.

Influence of PE‐ALD of GaP on the Silicon Wafers Quality

An attractive method of low‐temperature plasma‐enhanced atomic layer deposition (PE‐ALD) of GaP on silicon wafer was recently proposed. In the present paper, the influence of the growth process on the quality of silicon wafers is explored by space charge capacitance techniques, C–V profiling and deep level transient spectroscopy (DLTS). No DLTS peak is observed for PE‐ALD GaP deposited onto n‐type wafer, meaning that the defect concentration is very low (less than 1 × 1012 cm−3) and that the growth process does not affect the properties of the n‐Si wafer. For boron‐doped p‐type silicon, C–V profiling shows that there is no deactivation of boron doping after the PE‐ALD process, as could have been expected from the presence of hydrogen in the plasma. Measurements on the reference Schottky diodes formed on the p‐type Si wafer reveal the presence of the well‐known Fe interstitial defects at the position EV + 0.38 eV with a concentration of 3 × 1013 cm−3. PE‐ALD of GaP leads to a modification of the response of this defect and to the appearance of another response in the low temperature range, possibly related to changes in the Fe interstitial defect environment or configuration. However, deep‐levels were not detected in p‐Si after PE‐ALD, meaning that the quality of p‐Si does not degrade.

Bulk vs. Surface Structure of 3d Metal Impurities in Topological Insulator Bi2Te3

Topological insulators have become one of the most prominent research topics in materials science in recent years. Specifically, Bi2Te3 is one of the most promising for technological applications due to its conductive surface states and insulating bulk properties. Herein, we contrast the bulk and surface structural environments of dopant ions Cr, Mn, Fe, Co, Ni, and Cu in Bi2Te3 thin films in order to further elucidate this compound. Our measurements show the preferred oxidation state and surrounding crystal environment of each 3d-metal atomic species, and how they are incorporated into Bi2Te3. We show that in each case there is a unique interplay between structural environments, and that it is highly dependant on the dopant atom. Mn impurities in Bi2Te3 purely substitute into Bi sites in a 2+ oxidation state. Cr atoms seem only to reside on the surface and are effectively not able to be absorbed into the bulk. Whereas for Co and Ni, an array of substitutional, interstitial, and metallic configurations occur. Considering the relatively heavy Cu atoms, metallic clusters are highly favourable. The situation with Fe is even more complex, displaying a mix of oxidation states that differ greatly between the surface and bulk environments.

Defect levels and hyperfine constants of hydrogen in beryllium oxide from hybrid-functional calculations and muonium spectroscopy

The atomistic and electronic structures of isolated hydrogen states in BeO were studied by ab initio calculations and muonium spectroscopy (SR). Whereas standard density-functional theory with a semi-local GGA functional led to a detailed probing of all possible minimum-energy configurations of hydrogen further calculations with the hybrid HSE06 functional provided improved properties avoiding band-gap and self-interaction errors. Similarly to earlier findings for the other wide-gap alkaline-earth oxide, MgO, hydrogen in BeO is also predicted to be an amphoteric defect with the pinning level, E(), positioned in the mid-gap region. Both donor and acceptor levels were found too deep in the gap to allow for hydrogen to act as a source of free carriers. Whereas, hydrogen in its positively-charged state, , adopts exclusively hydroxide-bond OH configurations, and instead show a preference to occupy cage-like interstitial sites in the lattice. in particular displays a multitude of minimum-energy configurations: its lowest-energy ground state resembles closely a trapped-atom state with a nearly spherical spin-density profile. In contrast to the strongly ionic MgO, in BeO was further found to stabilise in additional higher-energy elongated-bond OH configurations whose existence should be traced to a partial covalent character of the Be–O bonding. Calculations of the proton-electron hyperfine coupling for all states showed that the ground-state interstitial configuration is dominated by an isotropic hyperfine interaction with a magnitude very close to the vacuum value, a finding corroborated by the SR-spectroscopy data.

Primary radiation damage of an FeCr alloy under pressure: Atomistic simulation

The primary radiation damage of a binary FeCr alloy deformed by applied mechanical loading is studied by an atomistic molecular dynamics simulation. Loading is simulated by specifying an applied pressure of 0.25, 1.0, and 2.5 GPa of both signs. Hydrostatic and uniaxial loading is considered along the [001], [111], [112], and [210] directions. The influence of loading on the energy of point defect formation and the threshold atomic displacement energy in single-component bcc iron is investigated. The 10-keV atomic displacement cascades in a “random” binary Fe–9 at % Cr alloy are simulated at an initial temperature of 300 K. The number of the point defects generated in a cascade is estimated, and the clustering of point defects and the spatial orientation of interstitial configurations are analyzed. Our results agree with the results of other researchers and supplement them.

Dynamics of Small Mobile Helium Clusters Near a Symmetric Tilt Grain Boundary of Plasma-Exposed Tungsten

We report the results of a systematic atomic-scale analysis of small helium cluster dynamics near a Σ3<111>{121} symmetric tilt grain boundary (GB) in tungsten based on molecular-dynamics simulations according to a reliable interatomic interaction potential. We find that small, mobile helium clusters (Hen, 1 ≤ n ≤ 7) in the near-GB region are attracted to the GB due to an elastic cluster-GB interaction force. Moreover, as the clusters drift toward the GB, cluster trap mutation (TM) reactions in the near-GB region are activated at rates much higher than those in the bulk of the material's grains. This near-GB cluster dynamics has significant effects on the near-GB defect structures and the amount of helium retained in the material upon plasma exposure. Each TM reaction generates a tungsten vacancy, which traps helium by forming an immobile helium-vacancy complex, and an interstitial tungsten atom in the form of an extended tungsten interstitial complex on the GB. This interstitial configuration is characterized by mobility that depends on the location where the TM reaction occurs: It is immobile when the vacancy produced by the TM reaction is located a few lattice planes away from the GB plane and highly mobile along a specific direction when the produced vacancy is located on the GB. The latter mechanism initiates a potentially fast migration path for W atoms along the GB toward a free surface, which may influence significantly the surface morphology of plasma-exposed tungsten.

Interatomic potential to study the formation of NiCr clusters in high Cr ferritic steels

Under irradiation NiSiPCr clusters are formed in high-Cr ferritic martensitic steels as well as in FeCr model alloys. In the literature little is known about the origin and contribution to the hardening of these clusters. In this work we performed density functional theory (DFT) calculations to study the stability of small substitutional NiCr-vacancy clusters and interstitial configurations in bcc Fe. Based on DFT data and experimental considerations a ternary potential for the ferritic FeNiCr system was developed. The potential was applied to study the thermodynamic stability of NiCr clusters by means of Metropolis Monte Carlo (MMC) simulations. The results of our simulations show that Cr and Ni precipitate as separate fractions and suggest only a limited synergetic effect between Ni and Cr. Therefore our results suggest that the NiCrSiP clusters observed in experiments must be the result of other mechanisms than the synergy of Cr and Ni at thermal equilibrium.