Point Defect Populations Research Articles

Mobile intrinsic point defects for conductive neutral domain walls in LiNbO3.

Conductive ferroelectric domain walls (DWs) hold great promise for neuromorphic nanoelectronics as they can contribute to realize multi-level diodes and nanoscale memristors. Point defects accumulating at DWs will change the local electrical transport properties. Hence, local, inter-switchable n- and p-type conductivity at DWs can be achieved through point defect population control. Here, we study the impact of point defects on the electronic structure at neutral domain walls in LiNbO3 by density functional theory (DFT). Segregation of Li and O vacancies was found to be energetically favourable at neutral DWs, implying that charge-compensating electrons or holes can give rise to n- or p-type conductivity. Changes in the electronic band gap and defect transition levels are discussed with respect to local property engineering, opening the pathway for reversible tuning between n- and p-type conduction at neutral ferroelectric DWs. Specifically, the high Curie temperature of LiNbO3 and the significant calculated mobility of O and Li vacancies suggest that thermal annealing and applied electric fields can be used experimentally to control point defect populations, and thus enable rewritable pn-junctions.

Dynamic and kinetic properties of point defects in γ U-10Mo: a molecular dynamics study

Controlled defects population and their movement constitute the foundation for describing micro-structural evolution in any material systems for nuclear applications. Molecular dynamics (MD) simulations were performed to study temperature (1200–1400 K) dependent displacement cascades with a U primary knock on atom (pka) carrying 2 keV kinetic energy in γ U-10Mo. We observed a strong impact of the temperature on the population of point defects. The impact of kinetic energy of the pka on the number of survived and maximum point defects is addressed carefully where an increase in the number of defects led to an increase in the energy. Nudged elastic band (NEB) calculations were involved to calculate the activation energies of self-interstitial Mo for different intra and inter-planar transitions. The transitions between 〈011〉 to 〈011〉, 〈001〉 to 〈011〉, 〈001〉 to 〈111〉, 〈111〉 to 〈001〉 and 〈111〉 to 〈110〉 Mo-Mo dumbbell configurations were explored. The impact of applied strains on the activation energy in each case of the transitions was investigated where irregular behavior in the energy with rise in the strain was observed.

A parallel discrete dislocation dynamics/kinetic Monte Carlo method to study non-conservative plastic processes

Non-conservative processes play a fundamental role in plasticity and are behind important macroscopic phenomena such as creep, dynamic strain aging, loop raft formation, etc. In the most general case, vacancy-induced dislocation climb is the operating unit mechanism. While dislocation/vacancy interactions have been modeled in the literature using a variety of methods, the approaches developed rely on continuum descriptions of both the vacancy population and its fluxes. However, there are numerous situations in physics where point defect populations display heterogeneous concentrations and/or non-smooth kinetics. Here, a kinetic Monte Carlo (kMC) approach for modeling vacancy transport in response to arbitrary stress fields is used. Vacancies are treated as point particles and are coupled to the dislocation substructure representing a deformed material via an advection term defined by the local stress gradients. The stress fields and the dislocation substructure are evolved using a discrete dislocation dynamics (DDD) module. To extend the coupled model to the treatment of large systems, we have implemented it in the massively-parallel DDD code ParaDiS. To avoid numerical incompatibilities associated with merging deterministic (DDD) and stochastic (kMC) integration algorithms, we cast the entire elasto-plastic-diffusive problem within a single stochastic framework, taking advantage of a parallel kMC algorithm to evolve the system as a single event-driven process. The large-scale implementation enables the study of the evolution of a variety of dislocation-defect scenarios governed by non-conservative transport kinetics. After carrying out an exhaustive numerical and computational analysis of our parallel algorithm, we show results that emphasize situations where inhomogeneous vacancy dynamics are of relevance, and compare discrete kinetics to continuum solutions for several cases.

Dose rate dependent cation & anion radiation enhanced diffusion in hematite

Irradiation induced non-equilibrium point defect populations influence mass transport in oxides, which in turn affects their stability and performance in hostile environments. In this study a strong dose rate dependence is observed.

On the structure of (111) twist grain boundaries in diamond: atomistic simulations with Tersoff-type interatomic potentials

In nanocrystalline diamond thin films, the high grain boundary density and the associated grain boundary properties may dominate the overall material behavior. In the present work, we systematically investigate (111) twist grain boundaries in diamond carbon by using atomistic simulations with Tersoff-type potential. Our work reveals the relation between atomic scale grain boundary structure and bicrystallography, bond deformation, point defect population. A comparative study with diamond silicon highlights general trends in the grain boundary energy as a function of the misorientation. We predict a transition between glide-plane grain boundary position and shuffle-plane grain boundary with increasing misorientation in diamond cubic materials with a (111) twist grain boundary.

Implications of doping on microstructure, processing, and thermoelectric performance: The case of PbSe

In this work, we highlight the often-overlooked effects of doping on the microstructure and performance of bulk thermoelectric materials to offer a broader perspective on how dopants interact with their parent material. Using PbSe doped with Na, Ag, and K as a model material system, we combine original computational, experimental, and microscopy data with established trends in material behavior, to provide an in-depth discussion of the relationship between dopants, processing, and microstructure, and their effects on thermoelectric efficiency and thermal stability. Notable observations include differences in the microstructure and mass loss of thermally treated samples of Na- and Ag-doped PbSe, as well as findings that Na and K cations exist predominantly as substitutional point defects while Ag also occupies interstitial sites and exhibits lower solubility. We discuss how these differences in point defect populations are known to affect a dopants’ ability to alter carrier concentration and how they may affect the mechanical properties of PbSe during processing.Graphic

Radiation-Enhanced Anion Transport in Hematite

The influence of radiation-induced (1 MeV energy H+ to ∼0.1 displacements per atom (dpa) at 450 °C), nonequilibrium point defect populations on mass transport is studied with an integrated campaign...

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells.

A Preliminary Study on Helium and Sulfur Ion-Irradiated BCC Iron: In Situ Tensile Testing Using a Push-To-Pull Device

Under energetic radiation exposure, significant changes in material behavior occur because of the substantial increase in the point defect population and other higher order defects. Here, we report preliminary results of a study in which an in situ transmission electron microscopy (TEM)-based push-to-pull (PTP) tensile test technique was utilized to understand the microstructure and its impact on mechanical and fracture behavior in irradiated BCC iron. Density functional theory-based calculations predict a strong segregation tendency of helium and sulfur to grain boundaries. Pure iron specimens were separately implanted with energetic helium and sulfur ions at 550°C to examine the effect of ion irradiation on the microstructure using TEM, and the mechanical response in terms of strength and embrittlement behavior using the PTP technique.

Role of electronic and magnetic interactions in defect formation and anomalous diffusion in δ-Pu

Previous experimental work has shown self-irradiation in Pu solids produces point defect populations that correlate with increases in local disorder, long-range structural changes, induced magnetic moments, and other thermo-physical property changes. Thermally activated kinetic processes drive these defects to diffuse and interact toward either damage evolution or lattice recovery. Using DFT and cNEB, as implemented in VASP, barriers for mono-vacancy and split-interstitial diffusion and Frenkel pair recombination were calculated in fcc δ-Pu. The results indicate the migration barrier of a monoclinic mono-vacancy is lower when compared to the migration barrier of a tetragonal split-interstitial in δ-Pu, contrary to typical fcc metal point defect migration. This fundamentally different diffusion mechanism is a result of local symmetry breaking induced by electronic and magnetic interactions leading to the development of Pu–Pu short bonds (<3.0 Å) within a many-atom complex defect forming and migrating. The migration of the monoclinic mono-vacancy maintains short bonds with anti-parallel spins throughout the transition; whereas, during the migration transition state for the tetragonal split-interstitial, formation of short bonds with parallel spins and a spin-flip of the migrating Pu interstitial occurs. The associated energy cost is reflected in an increase in the migration barrier energy. Frenkel pair recombination is not spontaneous at 0K, but correlates with magnetic moment interactions, leading to an energy barrier for recombination. From these results, it is concluded that migration of defects in unalloyed δ-Pu are highly dependent on the electronic and magnetic interactions that induce associated low-symmetry structures and consequently influence the diffusional properties. Typical fcc defect diffusion mechanisms do not apply to the monoclinic mono-vacancy and tetragonal split-interstitial in the complex 5f δ-Pu system suggesting that the experimental observation of radiation damage induced localized magnetic moments and anomalous diffusion properties measured in δ-Pu could be understood in terms of defect kinetics and interactions.

Point defect populations of forsterite revealed by two-stage metastable hydroxylation experiments

Hydroxylation is a method that allows “decoration” of the pre-existing point defect structure of nominally anhydrous minerals, such as olivine. We tested this method on synthetic forsterite (Fo: $${\text{Mg}}_{2} {\text{SiO}}_{4}$$ ) crystals. To control starting point defect structures, Fo crystals were pre-annealed at different temperatures ( $$1100{-}1500\,{^\circ }{\text{C}}$$ ), silica activity conditions (forsterite–enstatite Fo–En and forsterite–periclase Fo–Per) and oxygen fugacity (0.21 and $$10^{-6}$$ bars). Then low-temperature hydroxylation (900 °C, 1.5 GPa) of the crystals successfully allowed the decoration with protons of pre-existing point defect structures, as subsequently revealed by infrared spectroscopy. Protons are arranged in three different point defect stoichiometries in Fo, related to Mg and Si vacancies ([Mg] and [Si], respectively) as well as to a trivalent cation-associated substitution mechanism ([triv]). Over the timescale and equilibrium conditions studied, hydroxylation does not reset the point defect structure inherited from pre-anneal. The data further show that the concentrations of [Mg]-, [Si]- and [triv]-hydrated defects are function of pre-anneal silica activity and temperature. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of the crystals revealed diffusion of Al and Fe into the crystals during the pre-annealing, a phenomenon clearly promoted at high $${\text{a}}_{{{\text{SiO}}_{2} }}$$ . The data confirm a very fast mechanism of Al diffusion in Fo during pre-annealing, and suggest a strong coupling between $${\text{H}}^{+}$$ and $${\text{Al}}^{3+}$$ during hydroxylation. Overall, they show the strong importance of $${\text{a}}_{{{\text{SiO}}_{2} }}$$ and temperature in the incorporation of trace cations in forsterite, and the subsequent effects of incorporation of trace cations on Mg- and Si-related point defects in Fo. The dry point defect population of Fo is determined by interactions between the trace trivalent cations and dry Si and Mg vacancies. Without trace elements, T only has a limited effect on Mg- and Si-related point defect populations. Finally, approaching or potentially slightly exceeding the Fo–En solidus leads to strong changes in the trace element concentration and point defect population in Fo, which may be related to either partial melting or pre-melting effects.

Defect evolution in ultralow energy, high dose helium implants of silicon performed at elevated temperatures

There is a growing interest in using high dose helium implants to alter point defect populations in silicon. Previous reports have shown that the interaction between helium and vacancies leads to the formation of cavities for medium energy (e.g., 20–100 keV) implants. However, the role of certain factors, such as the proximity of the surface, the damage created by the implant, and the effect of the implant temperature, is not well understood for low energy implants. This study explored a new regime of ultralow energy, elevated temperature implants in order to offer an insight into the effect of these parameters. Transmission electron microscopy (TEM) showed that cavity formation was avoided for 0.5 keV, 450 °C implants up to a dose of 8 × 1016 cm−2. However, extended defects in the form of {311} ribbon-like defects and stacking faults were observed. Quantitative TEM showed that the number of interstitials in these defects was less than 0.2% of the implant dose. In addition, thermal helium desorption spectrometry suggested that only 2% of the implanted He dose was retained in interstitial He and HemVn complexes. A first-order dissociation kinetic model was applied to assess desorption from HemVn, which closely matched energies predicted by density functional theory. This population of excess vacancies and excess interstitials was possibly formed because of incomplete Frenkel pair recombination. Raman spectroscopy showed that the stress from the implant was dominated by the stress from the interstitial-type defects. The evolution of the stress and defects was also explored as a function of post-implant annealing.

Identification of defective two dimensional semiconductors by multifractal analysis: The single-layer [formula omitted] case study

Two dimensional semiconductor such as single-layer transition metal dichalcogenides (SL-TMD) have attracted most attentions as an atomically thin layer semiconductor materials. Typically, lattice point defects (sulfur vacancy) created by physical/chemical method during growth stages, have disadvantages on electronic properties. However, photoluminescence (PL) spectroscopy is conventionally used to characterize single-layer films but until now it has not been used to show the presence of defects or estimate their population due to overall similarity of general feature PL spectra. To find a feasible and robust method to determine the presence of point defects on single layer MoS2 without changing the experimental setup, Multifractal Detrended Fluctuation Analysis (MF-DFA) and Multifractal Detrended Moving Average Analysis (MF-DMA) are applied on the PL spectrum of single layer MoS2. We compare the scaling behavior of PL spectrum of pristine and defective single layer MoS2 determined by MF-DFA and MF-DMA. Our results reveal that PL spectrum has multifractal nature and different various population of point defects (sulfur vacancy) on single layer MoS2 change dramatically multifractality characteristics (Hurst, Hölder exponents) of photoluminescence spectrum. It is exhibited creating more lattice point leads to smaller fluctuations in luminescent light that it can help to design special defect structure for light emitted devices. The relative populations of point defects are almost elucidated without utilizing expensive characterization instruments such as scanning tunneling microscopy (STM) and high resolution transmission electron microscopy (HR-TEM).

Controlling metastable native point-defect populations in Cu(In,Ga)Se2 and Cu2ZnSnSe4 materials and solar cells through voltage-bias annealing

This contribution describes the influence of low-temperature annealing with and without applied voltage bias on thin-film Cu2ZnSnSe4 (CZTSe), Cu(In,Ga)Se2 (CIGS), and CdS material properties and solar cell performance. To quantify the effects of cation disorder on CZTSe device performance, completed devices were annealed under open-circuit conditions at various temperatures from 110 °C to 215 °C and subsequently quenched. Measurements on these devices document systematic, reversible changes in solar-cell performance consistent with a reduction in CZTSe band tails at lower annealing temperatures. CIGS and CZTSe solar cells were also annealed at various temperatures (200 °C for CIGS and 110 °C–215 °C for CZTSe) and subsequently quenched with continuously applied voltage bias to explore the effects of non-equilibrium annealing conditions. For both absorbers, large reversible changes in device characteristics correlated with the magnitude and sign of the applied voltage bias were observed. For CZTSe devices, the voltage-bias annealing (VBA) produced reversible changes in open-circuit voltage (VOC) from 289 meV to 446 meV. For CIGS solar cells, even larger changes were observed in device performance: photovoltaic (PV) conversion efficiency of the CIGS device varied from below 3% to above 15%, with corresponding changes in CIGS hole density of about three orders of magnitude. Findings from these VBA experiments are interpreted in terms of changes to the metastable point-defect populations that control key properties in the absorber layers, and in the CdS buffer layer. Computational device modeling was performed to assess the impacts of cation disorder on the CZTSe VOC deficit, and to elucidate the effects of VBA treatments on metastable point defect populations in CZTSe, CIGS, and CdS. Results indicate that band tails impose important limitations on CZTSe device performance. Device modeling results also indicate that non-equilibrium processing conditions including the effects of voltage bias can dramatically alter point-defect-mediated opto-electronic properties of semiconductors. Implications for optimization of PV materials and connections to long-term stability of PV devices are discussed.

Amorphous intergranular films act as ultra-efficient point defect sinks during collision cascades

Atomistic simulations are used to explore the effect of interfacial structure on residual radiation damage. Specifically, an ordered grain boundary is compared to a disordered amorphous intergranular film, to investigate how interface thickness and free volume impacts point defect recombination. Collision cascades are simulated and residual point defect populations are analyzed as a function of boundary type and primary knock on atom energy. While ordered grain boundaries easily absorb interstitials, these interfaces are inefficient vacancy sinks. Alternatively, amorphous intergranular films act as ultra-efficient, unbiased defect sinks, providing a path for the creation of radiation-tolerant materials.

Si Dopant Behavior in InGaAs

Si Dopant Behavior in InGaAs

Point-Defect Populations As Induced by Cation/Anion Substitution in β-Si3N4Lattice. A Cathodoluminescence Study

Point-defect characterizations in bulk β-silicon nitride polycrystalline ceramics were performed by means of cathodoluminescence (CL) spectroscopy. Three kinds of samples were compared: undoped Si3N4, SiO2-doped Si3N4, and Si3N4 doped with Al2O3 and Y2O3. Intrinsic defects and the effect of anion and cation substitutions (i.e., O for N and Al(Y) for Si) on the spectroscopic behavior of defect emissions were investigated with the aim of assigning the physical origin of the observed CL spectra. Upon deconvoluting the retrieved CL spectra into individual sub-bands, emissions originating from different defects stored in the neighborhood of the surface of these materials could be classified. A clear dependence of defect type and content on the presence of dopants could be found, despite the well-known notion that most of the dopant ions reside at grain boundaries rather than being assimilated into the hexagonal structure of β-silicon nitride.

Effect of annealing on point defect population in cold-drawn pearlitic steel wires

Cold-drawn pearlitic steel wires were subjected to differential scanning calorimetry and resistivity measurements during heating. Both techniques yield two peaks, one at 500–600 K, the other at 400–450 K. Whereas the peak at 500–600 K can be ascribed to voids consisting of about 10 vacancies as confirmed by positron annihilation spectroscopy, the low-temperature peak is ascribed to V1C2 and/or V2C4 complexes.

Hydration of mantle olivine under variable water and oxygen fugacity conditions

The incorporation of H into olivine is influenced by a significant number of thermodynamic variables (pressure, temperature, oxygen fugacity, etc.). Given the strong influence that H has on the solidus temperature and rheological behavior of mantle peridotite, it is necessary to determine its solubility in olivine over the range of conditions found in the upper mantle. This study presents results from hydration experiments carried out to determine the effects of pressure, temperature, and the fugacities of H2O and O2 on H solubility in San Carlos olivine at upper mantle conditions. Experiments were carried out at 1–2 GPa and 1,200 °C using a piston-cylinder device. The fugacity of O2 was controlled at the Fe0–FeO, FeO–Fe3O4, or Ni0–NiO buffer. Variable duration experiments indicate that equilibration is achieved within 6 h. Hydrogen contents of the experimental products were measured by secondary ion mass spectrometry, and relative changes to the point defect populations were investigated using Fourier transform infrared spectroscopy. Results from our experiments demonstrate that H solubility in San Carlos olivine is sensitive to pressure, the activity of SiO2, and the fugacities of H2O and O2. Of these variables, the fugacity of H2O has the strongest influence. The solubility of H in olivine increases with increasing SiO2 activity, indicating incorporation into vacancies on octahedral lattice sites. The forsterite content of the olivine has no discernible effect on H solubility between 88.17 and 91.41, and there is no correlation between the concentrations of Ti and H. Further, in all but one of our experimentally hydrated olivines, the concentration of Ti is too low for H to be incorporated dominantly as a Ti-clinohumite-like defect. Our experimentally hydrated olivines are characterized by strong infrared absorption peaks at wavenumbers of 3,330, 3,356, 3,525, and 3,572 cm−1. The heights of peaks at 3,330 and 3,356 cm−1 correlate positively with O2 fugacity, while those at 3,525 and 3,572 cm−1 correlate with H2O fugacity.

Anisotropic Thermal and Chemical Expansion in Sr-Substituted LaMnO3+δ: Implications for Chemical Strain Relaxation

The anisotropic thermal and chemical expansion of rhombohedral (R3̅c) La1–xSrxMnO3+δ (x = 0.2, 0.3) was investigated by in situ high temperature X-ray diffraction of submicrometer size powders in pure oxygen and nitrogen (inert) atmospheres. The thermal expansion of the long axis c was found to be close to twice as high as the thermal expansion of the short axis a. The large thermal expansion of the c-axis is caused by rectification of the antiferrodistortive tilting and decompression of the MnO6/2 octahedra. The unit cell parameters were shown to be strongly dependent on the partial pressure of oxygen, which was attributed to chemical expansion/contraction due to reduction/oxidation of Mn. Anisotropic chemical expansion/contraction was more pronounced for the unit cell parameter a than for c, and the chemical expansion/contraction was inferred to reflect the size of the MnO6/2 octahedra. The onset of chemical expansion in nitrogen and contraction in oxygen atmosphere during heating was discussed in terms of a gradual transformation from a nonequilibrium to an equilibrium point defect population in La1–xSrxMnO3+δ. Possible implications of slow relaxation of chemically induced stresses at the nanoscale and in epitaxial thin films are addressed. Finally, a second order phase transition from the ferroelastic (R3̅c) to paraelastic state (Pm3̅m) is reported for La0.7Sr0.3MnO3+δ at 850 ± 25 °C. The temperature of the phase transition decreases with increasing Sr content in La1–xSrxMnO3+δ.