Small Self-interstitial Clusters Research Articles

Signatures of self-interstitials in laser-melted and regrown silicon

Photoluminescence spectroscopy investigates epitaxially regrown silicon single crystals after pulsed laser melting for atomic-level lattice defects. The measurements identify a transition from a regime free of defect-related spectral lines to a regime in which spectral lines appear originating from small self-interstitial clusters. This finding of self-interstitial clusters indicates supersaturated concentrations of self-interstitials within the regrown volume. Molecular dynamics simulations confirm that recrystallization velocities vre ≈ 1 m/s after laser melting lead to supersaturation of both self-interstitials and vacancies. Their concentrations ci and cv in the regrown volumes are ci ≈ cv ≈ 1017 cm−3. An analytical model based on time-dependent nucleation theory shows a very strong dependence of self-interstitial aggregation to clusters on the cooling rate after solidification. This model explains the transition identified by photoluminescence spectroscopy.

A deep-level transient spectroscopy study of p-type silicon Schottky barriers containing a Si-O superlattice

The presence of deep levels in a silicon-oxygen (Si-O) superlattice (SL) deposited on p-type silicon substrates has been investigated by deep-level transient spectroscopy (DLTS) on thermally evaporated Cr Schottky barriers (SBs). The SLs have been fabricated with different thicknesses of the silicon interlayers, formed by chemical vapor deposition. It is shown that a broad band of hole traps is present near the surface of the SB, which is associated with the SL. In addition, the activation energy corresponding with the peak maximum shifts to higher values with respect to the valence band and gives rise to a higher trap concentration with increasing silicon interlayer thickness. It is proposed that these states are associated with the structural defects found in similar SL structures, that is, with the epitaxial quality and not with the SiO bonds in the atomic layers. The change in the DLT-spectra with silicon thickness could be related with the transformation of the structural defects from small self-interstitial clusters to stacking faults.

Localised vibrational mode spectroscopy studies of self-interstitial clusters in neutron irradiated silicon

The evolution of self-interstitial clusters in silicon (Si), produced by fast neutron irradiation of silicon crystals followed by anneals up to 750 °C, is investigated using localised vibrational mode spectroscopy. A band at 582 cm−1 appears after irradiation and is stable up to 550 °C was attributed to small self-interstitial clusters (In, n ≤ 4), with the most probable candidate the I4 structure. Two bands at 713 and 758 cm−1 arising in the spectra upon annealing of the 582 cm−1 band and surviving up to ∼750 °C were correlated with larger interstitial clusters (In, 5 ≤ n ≤ 8), with the most probable candidate the I8 structure or/and with chainlike defects which are precursors of the {311} extended defects. The results illustrate the presence of different interstitial clusters In, at the various temperature intervals of the material, in the course of an isochronal anneal sequence. As the annealing temperature increases, they evolve from first-order structures with a small number of self-interstitials (In, n ≤ 4) for the temperatures 50 < T < 550 °C, to second order structures (In, 5 ≤ n ≤ 8) with a larger number of interstitials, for the temperatures 550 < T < 750 °C.

Small interstitials clusters migration in bcc metals: A Molybdenum model

The migration mechanisms of small self-interstitial clusters in two models for bcc Molybdenum are studied by computer simulation. The research is pertinent to the stages after collision cascade collapse of radiation damage, when defects diffuse to sinks and agglomerate into bigger units. Among the interaction models considered, one is a rather standard many-body central forces potential; the other one includes angle contributions to the energy, which in principle is a more consistent representation of a bcc transition metal. Also, two techniques are employed: Molecular Dynamics and Monomer. The former provides global parameters of the diffusion process; the latter is a static technique to search the potential energy surface for saddle configurations, thus providing detailed information on transition events. Reasonable agreement is found among the techniques, though the connection between the static parameters with the dynamic ones may not be straightforward. The two interaction models result in both quantitative and qualitative differences; notably in some cases when the former are small the underlying microscopic mechanisms can be nevertheless different.

Cathodoluminescence Microcharacterization of Radiative Recombination Centers in Lifetime-Controlled Insulated Gate Bipolar Transistors

Cross-sectional cathodoluminescence (CL) measurements were applied to the study of electron-irradiated punch-through insulated gate bipolar transistors (IGBTs) to investigate the relationship between radiative recombination centers and electrical characteristics. IGBTs were additionally annealed at temperatures of 200–400 °C for 1 h. As annealing temperature rose, collector–emitter saturation voltage (VCES) decreased and current fall time (tf) increased. The cross-sectional CL measurements showed sharp luminescent peaks at 1018 meV (W or I1), 1040 meV (X or I3), and 790 meV (C) and a broad band at approximately 0.90–1.05 eV. As annealing temperature rose, the intensity of the W line decreased and that of the X line increased, suggesting that small self-interstitial clusters agglomerate and form stable, large self-interstitial clusters reducing the total number of self-interstitial clusters. The C line, which originated from an interstitial oxygen and carbon complex, showed no significant change. We consider that self-interstitial clusters play important roles in the electrical characteristics of lifetime-controlled IGBTs.

Nucleation and first-stage growth processes of extrinsic defects in GaAs triggered by self-interstitials

The growth process of small self-interstitial clusters ${I}_{n}$ $(n\ensuremath{\le}7)$ in crystalline GaAs has been addressed by semi-empirical tight-binding molecular-dynamics technique. The ${I}_{n}$ ground-state structures have been found among many possible choices of topological properties and stoichiometric compositions. The stable structure have been fully characterised concerning the structural, electronic, energetic, and elastic properties; some remarkable findings emerged concerning, among the others, the stability scenario of the ground-state structures, the possible low-energy reaction paths involved in the growth process, the electrostatic and the elastic capture volumes and the Fermi-level pinning. It is demonstrated that compact geometries are no longer energetically favoured for $n\ensuremath{\ge}5$ and that the ${I}_{n}$ growth proceeds via capture processes involving either isolated interstitials or di-interstitials. An extended pentainterstitial $({I}_{5})$ ground-state structure has been identified as the possible core-basic structure of extrinsic linear defects along the {111} direction of the GaAs lattice.

Growth and shape transition of small silicon self-interstitial clusters

The growth behavior of small self-interstitial clusters in crystalline Si is presented based on extensive combined Metropolis Monte Carlo, tight-binding molecular dynamics, and density-functional theory calculations. New stable structures for small interstitial clusters $({I}_{n},5\ensuremath{\le}n\ensuremath{\le}16)$ are determined, showing that the compact geometry appears favored when the cluster size is smaller than 10 atoms $(nl10)$. The fourfold-coordinated dodecainterstitial $({I}_{12})$ structure with ${C}_{2h}$ symmetry is identified to serve as an effective nucleation center for larger extended defects. This work provides the first theoretical support for earlier experiments which suggest a shape transition from compact to elongated structures around $n=10$.

Structure and Stability of Small Self-interstitial Clusters in Si: Prediction by First Principles-based Monte Carlo Simulations

We present an efficient computational method for determining the ground-state configurations of self-interstitial clusters in Si, based on a combination of continuous random network model, Metropolis Monte Carlo sampling, and Keating-like valence bond model. Using the new approach, we have determined the structure and stability of small interstitial clusters (In, 3 < n < 12). The results are in good agreement with first principles results.

Diffusion of small self-interstitial clusters in silicon: Temperature-accelerated tight-binding molecular dynamics simulations

Temperature-accelerated tight-binding molecular dynamics simulations show that self-interstitial clusters formed from two and three defects are mobile species. In particular, the di-interstitial $({I}_{2})$ cluster is found to diffuse nearly as fast as the single self-interstitial (dumbbell) over a wide temperature range. In particular, at room temperature ${I}_{2}$ is found to diffuse at a rate similar to the dumbbell, thus making an important contribution to silicon self-diffusion at temperatures relevant for silicon bulk processing. The simulations also reveal the atomistic mechanisms responsible for the defects' mobility, showing that the ${I}_{2}$ cluster must be promoted to a metastable state in which it executes several diffusive events before decaying to a new ground-state configuration equivalent to the initial one.

Structural, electronic, and energetic properties of small self-interstitial clusters in GaAs by tight-binding molecular dynamics

Intrinsic interstitials in GaAs are characterized by a remarkable formation energy that makes them unlikely to be present in as-grown materials and therefore commonly neglected. However, the role of interstitials must be considered in implanted GaAs, where collision cascades by energetic ions produce a large amount of these point defects. This paper reports on semiempirical tight-binding molecular dynamics simulations of interstitial defects in GaAs. The adopted parametrization has been initially applied to the simulation of isolated self-interstitials, on the basis of previous state of the art density-functional theory results, showing good performances. Then di-interstitial properties have been addressed, showing that self-interstitials have a remarkable tendency to aggregate in bigger structures. The binding energy of these clusters has been calculated on the basis of a simple algebraic model that does not require the calculation of the formation energy of the compound. This work will lay the ground work for a subsequent study of larger aggregates.

A physically based model for the spatial and temporal evolution of self-interstitial agglomerates in ion-implanted silicon

A physically motivated model that accounts for the spatial and temporal evolution of self-interstitial agglomerates in ion-implanted Si is presented. For the calibration of the model, a genetic algorithm is used to find the optimum set of physical parameters from experimental data. Mean-size evolution of {113} defects obtained by transmission electron microscopy and self-interstitial oversaturation results measured in the vicinity of extended defects are combined in the same fitting procedure. The calibration of parameters shows that binding energies of small self-interstitial clusters exhibit strong maxima, as reported in other investigations. Results of the calibrated model are compared to experimental data obtained in complementary investigations. It is demonstrated that the model is able to predict a wide variety of physical phenomena, from the oversaturation of self-interstitials via the mean-size evolution of {113} defects to the depth distribution of the density of the latter.

Modeling of extrinsic extended defect evolution in ion-implanted silicon upon thermal annealing

A physically motivated model that accounts for the spatial and temporal evolution of extended defect distribution in ion-implanted Si is presented. Free physical parameters are extracted from experimental data and by means of a genetic algorithm (GA). Transmission electron microscopy (TEM) data and self-interstitial oversaturation measurements are combined in the same fitting procedure to eliminate unphysical solutions and find the optimum set of parameters. The calibration of parameters shows that binding energies of small self-interstitial clusters exhibit strong minima, as reported in other investigations. It is demonstrated that the calibrated model we propose is able to predict a wide variety of physical phenomena, from the oversaturation of self-interstitials via the mean-size distribution of {1 1 3} defects to the depth distribution of the density of the latter.

Multiscale modeling of radiation damage in Fe-based alloys in the fusion environment

Ferritic alloys represent a technologically important class of candidate materials for fusion first wall and blanket structures. A detailed understanding of the mechanisms of defect accumulation and microstructure evolution, and the corresponding effects on mechanical properties is required to predict their in-service structural performance limits. The physical processes involved in radiation damage, and its effects on mechanical properties, are inherently multiscale and hierarchical, spanning length and time scales from the atomic nucleus to meters and picosecond to decades. In this paper, we present a multiscale modeling methodology to describe radiation effects within the fusion energy environment. Selected results from atomic scale investigation are presented, focusing on (i) the mechanisms of self-interstitial dislocation loop formation with Burgers vector of a〈1 0 0〉 in iron relative to vanadium, (ii) helium transport and (iii) the interaction between helium and small self-interstitial clusters in iron, and (iv) dislocation-helium bubble interactions in fcc aluminum.

Structure, energetics, and extrinsic levels of small self-interstitial clusters in silicon

Native defects in Si are of obvious importance in microelectronic device processing. Self-interstitials in particular are known to mediate, in many cases, anomalous impurity diffusion. Here we study the energetics and electronic structure of single, double, and triple self-interstitial clusters in crystalline Si in various charge states and geometries, providing extrinsic levels and binding-association-dissociation energies. We discuss the comparison of our results with some experimental data on self-implanted Si. We also reconsider the migration energetics of single interstitials in the light of recent experiments, and point out the possible role of cluster nucleation in the electrical activation of acceptor-implanted samples.

Small self-interstitial clusters in GaAs

Recent studies have shown evidence for the aggregation of self-interstitials inion-implanted Si, resulting in nanoscopic damage structures. Similarly,self-interstitial atoms are expected to play an important role for defect clusteringin ion-implanted GaAs. Accurate first-principles total-energy calculations arereported for different As and Ga self-interstitial configurations. These results areused to study by first-principles total-energy calculations the structural andelectronic properties of small complexes involving self-interstitials and/orantisites.

The fascinating dynamics of defects in silicon

Ab initio molecular-dynamics (MD) simulations are increasingly being used to study defects in silicon. Simulated quenching and/or conjugate gradient calculations are now common tools to explore the minima of complicated potential energy surfaces. However, the real dynamics are in the constant-temperature runs. These highly computer-intensive simulations are still limited to real times of the order of picoseconds. However, they provide a fantastic window into processes that are beyond the reach of static ( T=0 K) calculations. In this paper, the challenges of constant-temperature ab initio MD simulations are discussed and examples given: the formation of H 2 *, the diffusion of small self-interstitial clusters, the restless interstitial H 2 molecule, and the dynamic calculation of vibrational frequencies from the velocity–velocity autocorrelation function. The results are obtained using methods based on Sankey's ‘ab initio tight-binding’ approach, with atomic-like basis sets rather than plane waves.

Unusual features in trap emission characteristics ofheavily damaged silicon induced by MeV ion implantation

We have investigated the electrical characteristicsof defects in heavily damaged silicon induced by MeV ionimplantation at high doses, by extending the scope of depletionlayer capacitance transient techniques. The heavily damagedlayer is embedded in the depletion layer of a Schottky diode andhigh-frequency capacitance measurements are carried out toevaluate charge relaxation kinetics of defects specific to high-dose implantation. Deep-level transient spectroscopy ofas-implanted silicon shows presence of the divacancy trap(V2) and relatively high concentration of a damage-relatedtrap (D1) with unusual spectral lineshape. Thermally stimulatedcapacitance spectra show large capacitance step even withoutapplication of a trap-filling pulse. Constant-capacitance time-analysed transient spectroscopy studies of the D1 peakreveal that the skewed peakshape is due to premature termination of thetransient signal during trap emission. Strong temperature dependence ofspectral lineshape, ranging from broad to narrow peak (stronger thanthat expected from exponential transient), and trap occupancypoints to a dynamic interdependence of trap occupancy andquasi-Fermi level. Unusual features in spectral lineshape aresimulated by introducing a time-dependent capture term into therate equations for trapping dynamics for a single trap leveland provide strong support for our model on the dynamicinterdependence of quasi-Fermi level and trap occupancy. Thedefect parameter is found to be sensitive to implantation doseand low-temperature annealing. The D1 trap is ascribed to smallself-interstitial clusters.

Energetics of formation and migration of self-interstitials and self-interstitial clusters in α-iron

Energetic primary recoil atoms from fast neutron irradiation generate both isolated point defects and clusters of vacancies and interstitials. Self-interstitial mobility as well as defect cluster stability and mobility play key roles in the subsequent fate of defects and, hence, in the overall microstructural evolution under irradiation. Self-interstitials and two, three and four-member self-interstitial clusters are highly mobile at low temperatures as observed in molecular-dynamics simulations and high mobility probably also extends to larger clusters. In this study, the morphology, energetics and mobility of self-interstitials and small self-interstitial clusters in α-iron are studied by molecular-statics and molecular-dynamics simulations using a Finnis-Sinclair many-body interatomic potential. Self-interstitial migration is found to be a two-step process consisting of a rotation out of the 〈110〉 split-dumbbell configuration into the 〈111〉 split-dumbbell configuration and 〈111〉 translational jumps through the crowdion configuration before returning to the 〈110〉 dumbbell configuration. Self-interstitial clusters of 〈111〉 type split-interstitials assembled on adjacent {110} planes migrate along 〈111〉 directions in an amoeba-like fashion by sequential local dissociation and re-association processes.