Self-interstitial Defects Research Articles

Light emission from ion-implanted SiGe quantum dots grown on Si substrates

We report on electroluminescence spectroscopy experiments demonstrating room-temperature light emission from heavily alloyed SiGe quantum dots, for which the light emission properties are enhanced by incorporated split-[110] self-interstitials. The quantum dots are formed during molecular beam epitaxy deposition of Si0.6Ge0.4 alloys on n-doped silicon-on-insulator substrates. To create the split-[110] self-interstitials the quantum dots were co-implantated in-situ using Si and Ge ions. The hybrid emitters were further embedded into the intrinsic region of a p-i-n diode structure to enable electrical pumping. Similar to previous theoretical results on unstrained Ge-based quantum dots containing these defects, radiative direct transitions are possible for these SiGe light emitters. However, in SiGe dots these transitions are not at the Brillouin zone center. Instead, first-principles calculations indicate that the presence of the split-[110] self-interstitial defect in strained and unstrained SiGe can lead to optically direct transitions in momentum space in the X-direction of the Brillouin zone.

A Model for the Temperature Distribution in a Rolled Joint in a CANDU Reactor Exploiting the Decomposition of the β-Zr Phase

A competing-rates model is presented to account for operational changes in the metastable β-Zr phase of the Zr-2.5Nb alloy used to make CANDU reactor pressure tubes and is used to predict temperature gradients at the outlet rolled joints using the decomposition of the β-Zr phase as a proxy for temperature. High temperatures decompose the β phase by enhancing the formation of small particles of ω and α phases. Fast neutron flux causes the ω and α phases to shrink. This process is assumed to depend on the total volume of the particles, because they are comparable to, or smaller than, the size of the neutron displacement cascades. The barrier energy for thermal growth was determined to be 2.43 eV, when an Arrhenius A factor of 1013/s was assumed. The cross section for (ω+α)-phase shrinkage is 24.5 barns for Zr-2.5Nb irradiated in CANDU reactors. Assuming that the shrinkage is dominated by the migration of self-interstitial point defects, a defect production efficiency of 1.4% was found.

Effect of temperature, pressure, crystal defect types, and densities on the mechanical behavior of tungsten under tensile deformation: A molecular dynamics simulation study

The mechanical behavior of tungsten under tensile deformation was investigated using molecular dynamics simulations, with a focus on the effects of temperature, pressure, crystal defect types, and densities. Single vacancies, spherical voids, and self-interstitials were studied at concentrations ranging from 0.1% to 2%. The Common neighbor analysis reveals that the crystal structure underwent a phase transition from the body-centered cubic to the face-centered cubic structure at increasing levels of applied strain, followed by the formation of a disordered phase. Increasing temperature reduced the mechanical properties of tungsten, including yield strength, yield strain, modulus of elasticity, and ultimate tensile stress, possibly due to the activation of dislocation nucleation and propagation, resulting in the formation of dislocation loops. Increasing pressure enhanced the mechanical properties of tungsten, with the formation of dislocation loops occurring at much lower strains at higher pressures. All mechanical parameters decreased with increasing defect density, with self-interstitial defects having a more severe effect than single vacancies and spherical voids. The number and length of dislocation loops also increased with increasing defect density. Temperature and pressure had varying effects on the stress–strain curve of tungsten with different defect types and densities, with increasing pressure generally enhancing its mechanical behavior and increasing temperature decreasing it. These findings provide valuable insights into the design and development of tungsten-based materials with enhanced mechanical properties.

Effect of solute Nb and Sn on self-interstitial atom defect in zirconium-based alloys by first-principles calculations

Zirconium based alloys are viewed as one of the most important fuel cladding materials, and alloy elements play a crucial role in improving its irradiation resistance, especially Nb and Sn. In present work, self-interstitial atom formation energy, binding energy between dopant Nb/Sn and the self-interstitial atom, and electronic structure are systematically investigated with the density functional theory. The diffusion barriers of the self-interstitial atom under the effect of Nb/Sn are calculated with the climbing image nudged elastic band (CI-NEB) method. Depending on the calculation results, the most stable configuration is BT and BS after doping Nb and Sn atom, respectively, different from BO for the Zr crystal without dopant. In the doped structure, the orbital hybridization of Nb atom and self-interstitial atom was obvious, while the orbital overlap of Sn atom with matrix atom and self-interstitial atom was less. Orbital hybridization of Nb atom and zirconium atoms mainly occurs between d orbits. After doping Nb, the diffusion barrier of the interstitial atoms increases, inhibits their diffusion or cluster growth. And the dopant Sn makes the formation of interstitial atom more difficult depending on the larger formation energy. The calculation results may explain the mechanism of improving its irradiation resistance by Nb and Sn doping.

He bubble growth in nickel simulated by object kinetic Monte Carlo

When Ni-based alloys are exposed to neutron irradiation, (n,α) transmutation introduces helium to the metal, leading to the formation of bubbles, which can severely affect the properties of the material. Identifying the key parameters controlling bubble growth can help us design materials with improved radiation tolerance. In this study, we parameterized an object kinetic Monte Carlo (OkMC) framework able to simulate the coalescence and growth of helium bubbles in pure Ni during and after helium ion implantation. The simulated bubbles size is consistent with phenomenological models based on past experimental studies. Our simulations indicate that the mean He bubble size is strongly correlated with temperature and inversely correlated with the implantation dose rate. The interactions between irradiation-induced defects and interfaces (sinks) are shown to play a key role in determining the size and stability of bubbles. Furthermore, our simulations suggest that the controlling reaction in swelling is the annihilation of self-interstitial defects at sinks, not the presence of He.

An atomic scale study on self-interstitial formation and diffusion behaviors in TiVTa and TiVTaNb concentrated solid-solution alloys

The formation and diffusion behaviors of self-interstitial defects in TiVTa and TiVTaNb concentrated solid-solution alloys (CSAs) are studied systematically by first-principles calculations and molecular dynamics simulations. It is found that [111] VV and TiV dumbbells are the most stable configurations in TiVTa-based CSAs, the formation energies of self-interstitials in TiVTaNb are higher than that in TiVTa CSAs. Local chemical environment can affect the formation of interstitials, where interstitials prefer V-rich environment. Larger local lattice distortion in TiVTa can lead to an irregular energy landscape and make it easier to form interstitials than in TiVTaNb. Compared with pure V, the diffusivity of interstitial is much lower than in TiVTa-based CSAs, which is related to the specific stability of interstitial configurations and lattice distortion. The preferential elemental diffusion is also observed which can be demonstrated by the interstitial formation preferences. Significant lattice distortion caused by atomic size mismatch in TiVTa-based CSAs can hinder the formation of interstitials, resulting in chemically biased diffusion and preferential diffusion pathways of interstitials.

Cost-effective calculation of defects in Si using hybrid density functional with downsampled reciprocal grids

A deep understanding of defects in terms of stability and optoelectronic properties is essential for optimizing semiconductor devices. For faster and more accurate investigation of defects, we examined whether the hybrid density functional theory calculation can be performed cost-effectively by downsampling the k-point meshes for Fock exchange potential. The P dopant and Si self-interstitial defects were used to check the convergence of the total energy. Our calculation shows that the defect properties like the formation energy and the charge transition levels can be cost-effectively calculated by using downsampled k-point meshes. We also investigated the intrinsic vacancy defects and obtained consistent results with previous studies.

Effects of annealing on the micro-internal stress induced by interstitial defects in aluminum crystal by molecular dynamics simulations

Micro-internal stress caused by self-interstitial defects in aluminum crystals was studied by using the molecular dynamics method. The effects of annealing on the lattice structure near the interstitial defects and the evolution of atoms near defects are analyzed. For octahedral, tetrahedral, and crowdion self-interstitial atoms, the atomic stress in the affected area after annealing decreases significantly compared with that before annealing. For dumbbell self-interstitial atoms, there are no obvious changes in atomic stresses in all regions before and after annealing. For four configurations of interstitial defects, the internal stress obviously decreased after annealing. Different concentrations of interstitial atoms have different effects on the internal stress and the size of the space region with internal stress. The size of the space region increases with the increase in concentration, and it can be reduced by annealing. When the concentration of interstitial atoms is within a certain range, annealing can effectively reduce the internal stress. When the concentration is low or high, annealing can only eliminate the internal stress in the local spatial regions and may increase the internal stress in other spatial regions.

Ultrabroadband and multiband infrared/terahertz photodetectors with high sensitivity

Broadband response is pursued in both infrared (IR) and terahertz (THz) detection technologies, which find their applications in both terrestrial and astronomical realms. Herein, we report an ultrabroadband and multiband IR/THz detector based on blocked-impurity-band detecting principle. The detectors are prepared by implanting phosphorus into germanium (Ge:P), where photoresponses with a P impurity band, a self-interstitial defect band, and a vacancy-P (V-P) pair defect band are realized simultaneously. The response spectra of the detectors show ultrabroad and dual response bands in a range of 3–28 μm (IR band) and 40–165 μm (THz band), respectively. Additionally, a tiny mid-IR (MIR) band within 3–4.2 μm is embedded in the IR band. The THz band arises from the P impurity band, whereas the IR and the MIR bands are ascribed to the two defect bands. At 150 mV and 4.5 K, the peak detectivities of the three bands are obtained as 2.9 × 10 12 Jones (at 3.9 μm), 6.8 × 10 12 Jones (at 16.3 μm), and 9.9 × 10 12 Jones (at 116.5 μm), respectively. The impressive coverage and sensitivity of the detectors are promising for applications in IR and THz detection technologies.

Combined EXAFS and MD study to elucidate defect behaviors and stage IV recovery mechanism in heavy ion irradiated molybdenum

The microstructural behaviors of materials control material properties and the recovery mechanism is a key to understanding the microstructural changes induced by irradiation. But the recovery mechanism of many metal materials is not fully understood through traditional research methods. By using Extended X-ray absorption fine-structure (EXAFS) measurements combined with Molecular Dynamics (MD) simulations, we studied the stage IV recovery process through investigating the behaviors of irradiation induced defects (caused by 104 MeV xenon ion beam irradiation) at an elevated temperature (~350 °C) in single crystal Molybdenum (Mo). We distinctly characterized self-interstitial dumbbell defect configurations and simulated their behaviors at this elevated temperature. EXAFS measurements, together with MD simulations and TEM observations, revealed that the mechanism of IV stage recovery is the migration of interstitial type defects (including dumbbells) rather than vacancy type defects. We also discussed the possible states of dumbbells after the stage IV recovery. These results further revealed the nature of the irradiated BCC structure (Mo).

Photoluminescence spectra of intrinsic interstitials in Ⅱa diamond after near threshold energy irradiation

In this work, the intrinsic optical defects are created by near threshold energy electron irradiation and then the low temperature micro-photoluminescence (PL) spectroscopy is employed to investigate the dependences of laser power and measurement temperature. The optical centers include 1.673 eV, 2.091 eV and 2.253 eV. Compared with the emissions of 2.091 eV and 2.253 eV, the 1.673 eV emission has an obviously higher growth rate in intensity with the increase of laser power, a larger lattice relaxation, the stronger electron-phonon coupling strength, indicating that they are associated with the different defects. These results show that the 2.091 eV and 2.253 eV emissions are attributed to the self-interstitial defects.

Experimental and Theoretical Study of Oxygen Precipitation and the Resulting Limitation of Silicon Solar Cell Wafers

Commercial silicon is prone to form silicon oxide precipitates during high-temperature treatments typical for solar cell production. Oxide precipitates can cause severe efficiency degradation in solar cells. We have developed a model describing the nucleation and growth of oxide precipitates that considers silicon self-interstitial defects and surface effects influencing the precipitate growth in ∼150 μm thick wafers during the solar cell processing. This kinetic model is calibrated with experiments that cause a well-defined and strong precipitate growth to give a prediction of the carrier lifetime limitation because of the oxide precipitates. We test the oxide precipitate model with scanning Fourier-transform infrared spectroscopy, selective etching, and lifetime measurements on typical Cz solar cell wafers before and after solar cell processes. Despite the relatively rough saw damaged etched surfaces and the thin wafers, we observe recurring ring patterns in the measurements of interstitial oxygen reductions, oxide precipitate etch pit density, and recombination activity by photoluminescence imaging. The concentration of precipitated oxygen correlates with the recombination activity and with the initial interstitial oxygen concentration. However, we found lifetime measurements to be a more sensitive technique to study oxide precipitates and using these we find smaller precipitates not detected by selective etching are very recombination active too. The measured concentrations of precipitated oxygen and lifetime agree fairly well with the predictions of the model.

Reconfigurable photo-induced doping of two-dimensional van der Waals semiconductors using different photon energies

Two-dimensional semiconductors have a range of electronic and optical properties that can be used in the development of advanced electronic devices. However, unlike conventional silicon semiconductors, simple doping methods to monolithically assemble n- and p-type channels on a single two-dimensional semiconductor are lacking, which makes the fabrication of integrated circuitry challenging. Here we report the reversible photo-induced doping of few-layer molybdenum ditelluride and tungsten diselenide, where the channel polarity can be reconfigured from n-type to p-type, and vice versa, with laser light at different frequencies. This reconfigurable doping is attributed to selective light–lattice interactions, such as the formation of tellurium self-interstitial defects under ultraviolet illumination and the incorporation of substitutional oxygen in tellurium and molybdenum vacancies under visible illumination. Using this approach, we create a complementary metal–oxide–semiconductor (CMOS) device on a single channel, where the circuit functions can be dynamically reset from a CMOS inverter to a CMOS switch using pulses of different light frequencies. Few-layer molybdenum ditelluride and tungsten diselenide field-effect transistors can be reversibly doped with different carrier types and concentrations using pulses of ultraviolet and visible light, allowing reconfigurable complementary metal–oxide–semiconductor circuits to be created.

Evolution of pressurized xenon bubble and response of uranium dioxide matrix: A molecular dynamics study

To explore the microstructure evolution of uranium dioxide (UO2) due to accumulation of xenon (Xe) atoms at high burnup, the growth of highly pressurized Xe bubbles has been simulated in molecular dynamics (MD) by sequentially inserting Xe into a pre-existing equilibrium Xe bubble in UO2 using five different interatomic potentials at 1500 K. The results reveal that the characteristics of Xe bubbles and the microstructure evolution of UO2 due to the bubble growth can be divided into two stages in terms of the maximum pressure (Pmax) of bubbles. Before the bubble reaches Pmax (stage I), the characteristics of bubbles and the microstructure evolution of UO2 are relatively independent of the interatomic potentials used. The bubble volume and pressure increase with inserting additional Xe atoms, and the Xe atoms in the bubble evolve from an initial gaseous state to a glassy/amorphous state, a nearly fcc solid structure and subsequently a high density amorphous or glassy state. Moreover, the high density, over-pressurized bubbles displace U and O atoms, creating a larger volume to accommodate Xe atoms and correspondingly produce self-interstitial atoms (SIAs) with increasing Xe atoms. Most of the SIAs within stage I are distributed at the center of six {100} facets around the Xe bubble. However, after the bubble pressure reaches Pmax (stage II), the bubble characteristics predicted by MD simulations depend on both the UO2 interatomic potential and the Xe-UO2 potential. However, the microstructure evolution of UO2 is mainly determined by the UO2 potential since it determines the mobility of self-interstitial defects and the stability of the UO2 fluorite structure. ½<110> dislocations (loops) or UO2 phase transitions along with incipient boundaries (or interfaces) appear due to the accumulation of Xe in over-pressurized bubbles.

Migration behavior of self-interstitial defects in tungsten and iron

Based on the basic characteristic of self-interstitial defect, we propose a new method for locating self-interstitial defect in body centered cubic transition metals, with which the migration behavior of self-interstitial defects with up to three self-interstitial atoms in W and Fe is investigated over a wide range of temperatures by molecular dynamics simulations. With increasing temperature, the migration of self-interstitial defects in W exhibits a change in mechanism from one-dimensional to three-dimensional. The one-dimensional diffusion coefficients rise linearly with temperature. For self-interstitial defects in Fe, the three-dimensional migration is observed, and the obtained migration energy of the tri-interstitial is lower than that of the single- and di-interstitial. During the tri-interstitial migration, several different <111> configurations are found with increasing temperature in both W and Fe.

Re-examining the silicon self-interstitial charge states and defect levels: A density functional theory and bounds analysis study

The self-interstitial atom (SIA) is one of two fundamental point defects in bulk Si. Isolated Si SIAs are extremely difficult to observe experimentally. Even at very low temperatures, they anneal before typical experiments can be performed. Given the challenges associated with experimental characterization, accurate theoretical calculations provide valuable information necessary to elucidate the properties of these defects. Previous studies have applied Kohn–Sham density functional theory (DFT) to the Si SIA, using either the local density approximation or the generalized gradient approximation to the exchange-correlation (XC) energy. The consensus of these studies indicates that a Si SIA may exist in five charge states ranging from −2 to +2 with the defect structure being dependent on the charge state. This study aims to re-examine the existence of these charge states in light of recently derived “approximate bounds” on the defect levels obtained from finite-size supercell calculations and new DFT calculations using both semi-local and hybrid XC approximations. We conclude that only the neutral and +2 charge states are directly supported by DFT as localized charge states of the Si SIA. Within the current accuracy of DFT, our results indicate that the +1 charge state likely consists of an electron in a conduction-band-like state that is coulombically bound to a +2 SIA. Furthermore, the −1 and −2 charge states likely consist of a neutral SIA with one and two additional electrons in the conduction band, respectively.

Solute diffusion by self-interstitial defects and radiation-induced segregation in ferritic Fe–X (X=Cr, Cu, Mn, Ni, P, Si) dilute alloys

This work investigates solute transport due to self-interstitial defects and radiation induced segregation tendencies in dilute ferritic alloys, by computing the transport coefficients of each system based on ab initio calculations of binding energies, migration rates, as well as formation and migration vibrational entropies. The implementation of the self-consistent mean field method in the KineCluE code allows for the calculation of transport coefficients extended to arbitrary interaction ranges, crystal structures, and diffusion mechanisms. In addition, the code gives access to the diffusion and dissociation rates of small solute-defect clusters – in this case, vacancy- and dumbbell-solute pairs. The results show that the diffusivity of P, Mn, and Cr solute atoms is dominated by the dumbbell mechanism, that of Cu by vacancies, while the two mechanisms might be in competition for Ni and Si, despite the fact that the corresponding mixed dumbbells are not stable. Systematic positive radiation-induced segregation (RIS) at defect sinks is expected for P and Mn solutes due to dumbbell diffusion, and for Si due mainly to vacancy drag. Vacancy drag is also responsible for Cu and Ni enrichment at sinks below 1085 K. The RIS behavior of Cr is the outcome of a fine balance between enrichment due to the dumbbell diffusion mechanism and depletion due to the vacancy one. Therefore, for dilute Cr concentrations global enrichment occurs below 540 K, and depletion above. This threshold temperature grows with solute concentration. The findings are in qualitative agreement with experimental observations of RIS and clustering phenomena, and confirm that solute-defect kinetic coupling plays an important role in the formation of solute clusters in reactor pressure vessel steels and other alloys.

The study of the properties of point defects in pure-Zr and Zr-1%Nb alloy using density-functional theory and atomic simulation

Crystal defects play an important role in the material strength and its mechanical properties. The Zr-1%Nb alloy, because of its low cross-section for thermal-neutron capture, corrosion resistance in water and suitable mechanical properties, is widely used in nuclear reactors. This alloy has an HCP structure at low temperatures and for low concentrations of Nb impurity. In this work, using the first-principles density-functional theory calculations, as well as molecular dynamics calculations with interatomic potentials, we have investigated the properties of vacancy and self-interstitial point defects in pure zirconium. The formation energy and formation volume are calculated; the results show a good agreement with the experimental values. These quantities are calculated for the Zr-1%Nb alloy as well; the results do not show any significant differences with those of the pure Zr. In addition, the interaction between two vacancies is investigated; by the calculation of the binding energies for di-vacancy clusters in different configurations, it is shown that only those clusters are stable for which the vacancies are in the first neighbor positions. Finally, the displacement energy of a vacancy in the basal plane is calculated, showing a good agreement with experiment.

Effect of stress on vacancy formation and migration in body-centered-cubic metals

Vacancy formation and migration control self-diffusion in pure crystalline materials, whereas irradiation produces high concentrations of vacancy and self-interstitial atom defects, exceeding by many orders of magnitude the thermal equilibrium concentrations. The defects themselves, and the extended dislocation microstructure formed under irradiation, generate strongly spatially fluctuating strain and stress fields. These fields alter the local formation and migration enthalpies of defects, and give rise to the anisotropy of diffusion even if in the absence of stress the diffusion tensor is isotropic. We have performed ab initio calculations of formation and migration energies of vacancies in all the commonly occurring body-centered-cubic (bcc) metals, including alkaline, alkaline-earth and transition metals, and computed elastic dipole and relaxation volume tensors of vacancies at the equilibrium lattice positions and along the vacancy migration pathways. We find that in all the bcc metals the dipole tensor of a migrating vacancy at a saddle point exhibits an anticrowdion character. Applied external stresses or the local stresses generated by dislocations may enhance or suppress anisotropic diffusion by altering the energy barriers with respect to the direction of migration of a defect.

Symmetry-broken self-interstitial defects in chromium, molybdenum, and tungsten

Since the nineteen-seventies, it was believed that the results of Huang x-ray diffuse scattering from radiation defects proved that a self-interstitial atom (SIA) defect in Mo adopted a $\ensuremath{\langle}110\ensuremath{\rangle}$ dumbbell configuration. However, resistivity recovery experiments performed using the same irradiated materials suggested a different, highly mobile, $\ensuremath{\langle}111\ensuremath{\rangle}$ SIA defect structure. Using density functional theory calculations, the authors have discovered that an SIA adopts a symmetry-broken $\ensuremath{\langle}11\ensuremath{\xi}\ensuremath{\rangle}$ configuration in chromium, molybdenum and tungsten, where $\ensuremath{\xi}$ is an irrational number. A $\ensuremath{\langle}11\ensuremath{\xi}\ensuremath{\rangle}$ defect migrates on average one-dimensionally through a sequence of three-dimensional nonplanar transitions, well correlated with the observed defect migration temperatures. Direct simulations of Huang diffuse scattering patterns from $\ensuremath{\langle}11\ensuremath{\xi}\ensuremath{\rangle}$ defect structures agree with observations, fully resolving the problem of the structure of defects in Mo and similar metals.