Small Interstitial Clusters Research Articles

High-energy ion-implantation-induced gettering of copper in silicon beyond the projected ion range: The trans-projected-range effect

Five different species, namely B, Si, P, Ge, and As, were implanted at MeV energies into (100)-oriented n-type Czohralski Si, in order to form deep gettering layers during the subsequent annealing. Then the samples were contaminated with Cu by implanting the impurity on the backface and performing additional annealing. The resulting Cu depth distributions were measured by secondary ion mass spectrometry. Strong gettering of Cu atoms beyond the projected ion range RP and formation of a well-defined separate Cu gettering band therein is found for P and As implants. We call this phenomenon the “trans-RP effect.” It arises from the presence of a significant amount of defects in the regions much deeper than RP. Their gettering ability is higher than that of the extended defects around RP, as the amount of Cu atoms gettered beyond RP is, especially for the P implants, much greater than that in the implanted gettering layer at RP. These deep defects have not been detected by transmission electron microscopy, and we suggest that they are small interstitial clusters. A mechanism responsible for the migration of self-interstitials from RP into the trans-RP region and their clustering therein is proposed. An explanation is given of the possible reasons for the differences in the results for the P+ and As+ implants.

Transition from small interstitial clusters to extended {311} defects in ion-implanted Si

We have investigated the transition from small interstitial clusters to {311} defects in ion-implanted Si. Czochralski Si wafers were implanted with 1.2 MeV Si ions to fluences in the range 1012–5×1013/cm2 and annealed at temperatures of 600–750 °C for times as long as 15 h. Photoluminescence and transmission electron microscopy analyses allowed us to analyze the transition of small interstitial clusters, formed by the agglomeration of the excess interstitials introduced by the beam, into {311} defects. It is found that {311} defects form only at fluences ⩾1013/cm2 and at temperatures above 600 °C. When {311} are observed in transmission electron microscopy, the luminescence spectrum is dominated by a sharp signal at 1376 nm which has been correlated with optical transitions occurring at or close to these defects. At lower temperatures or at lower fluence, no extended defects are observed in transmission electron microscopy and the luminescence spectrum present two broad signatures arising from carrier recombination at interstitial clusters. These data strongly indicate that a severe structural transformation occurs in the evolution from small interstitial clusters to extended {311} defects.

Comparative study of radiation damage accumulation in Cu and Fe

Bcc and fcc metals exhibit significant differences in behavior when exposed to neutron or heavy ion irradiation. Transmission electron microscopy (TEM) observations reveal that damage in the form of stacking fault tetrahedra (SFT) is visible in copper irradiated to very low doses, but that no damage is visible in iron irradiated to the same total dose. In order to understand and quantify this difference in behavior, we have simulated damage production and accumulation in fcc Cu and bcc Fe. We use 20 keV primary knock-on atoms (PKAs) at a homologous temperature of 0.25 of the melting point. The primary damage state was calculated using molecular dynamics (MD) with empirical, embedded-atom interatomic potentials. Damage accumulation was modeled using a kinetic Monte Carlo (kMC) algorithm to follow the evolution of all defects produced in the cascades. The diffusivities and binding energies of defects are input data for this simulation and were either extracted from experiments, the literature, or calculated using MD. MD simulations reveal that vacancy clusters are produced within the cascade core in the case of copper. In iron, most of the vacancies do not cluster during cooling of the cascade core and are available for diffusion. In addition, self-interstitial atom (SIA) clusters are produced in copper cascades but those observed in iron are smaller in number and size. The combined MD/kMC simulations reveal that the visible cluster densities obtained as a function of dose are at least one order of magnitude lower in Fe than in Cu. We compare the results with experimental measurements of cluster density and find excellent agreement between the simulations and experiments when small interstitial clusters are considered to be mobile as suggested by recent MD simulations.

Computer simulation of fundamental behaviors of interstitial clusters in Fe and Ni

Atomistic features of small interstitial clusters in Fe and Ni have been investigated by computer simulation. The gradual change from an interstitial cluster to a dislocation loop was observed in detail from three points of view, (i) strain distribution along crowdion axis for each crowdion in a cluster, (ii) distribution of Burgers vector of peripheral dislocation line of a cluster, and (iii) recombination behavior of a vacancy of with a cluster. It is found that strain distribution was spread into two opposite directions on a crowdion axis with the increase of a cluster size. The irregularity on the curve of the distribution of Burgers vector of a smaller loop gradually disappears with the increase of a loop size. Recombination between a crowdion and a vacancy occurs on the peripheral position of smaller clusters, but this does not occur in larger clusters. These aspects show that the gradual change from an interstitial cluster to a dislocation loop occurs at a certain width of cluster size. Dynamic behavior was also investigated under cylindrical shear stress and the Peierls stress was obtained as a function of loop size. The results show that the Peierls stress decreases with increasing loop size down to the value of a straight edge dislocation. Activation energies of one atomic jump of these small dislocation loops were also calculated and small values of about 0.2 eV were found for loops of about 200 crowdions for both Fe and Ni.

Electrically active defects in surface pre-amorphized Si under rapid thermal anneal and their removal by concurrent titanium silicidation

The interstitial-type nature of electrically active defects observed in surface pre-amorphized and subsequently annealed p-type Si is established by comparing the thermal evolution of electrically active defects for rapid thermal anneals (RTA) with/without titanium films. Detailed analyses of the deep level transient spectroscopy (DLTS) and transmission electron microscopy results suggest that some of these defects are small interstitial clusters. The results of this study also suggest that the release of self-interstitials from EOR extended defects and their subsequent diffusion into bulk are involved in the formation of the hole trap levels observed after high temperature RTA anneals. The observation of the complete elimination of the implantation-induced electrically active defects by concurrent RTA Ti-silicidation should be of interest to silicon microelectronic technology since it demonstrates the possibility of eliminating implantation induced defects using low thermal budget.

Impurity gettering by high-energy ion implantation in silicon beyond the projected range

Deep gettering layers have been formed in Si wafers by MeV implantation of Si+ and P+ ions, followed by annealing. Samples have been subsequently contaminated with Cu. Secondary ion mass spectrometry analysis reveals for P implants gettering of Cu atoms in regions significantly deeper than the projected ion range RP and formation of a separate Cu gettering band there. We call this phenomenon the “trans-RP effect.” The results obtained indicate the presence of an appreciable amount of defects in the region beyond RP. Their gettering ability is much higher than that of the implanted gettering layer at RP. The size of these deep defects is below the resolution limit of transmission electron microscopy. We suggest that they are interstitials and/or small interstitial clusters. An explanation of the mechanism responsible for their migration from RP into the trans-RP region and their clustering is proposed.

The primary damage state and its evolution over multiple length and time scales: Recent atomic-scale computer simulation studies

During his long and illustrious career, Professor Kiritani made many of the most significant and revealing observations regarding the nature of the primary damage state and the fate of the produced defects in irradiated metals and semiconductors. We present a review of recent results of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations of defect production and annealing in irradiated metals and semiconductors. The MD simulations describe the primary damage state in two prototypical elemental metals and in one-elemental semiconductor, namely Fe, Au, and Si. These materials were all thoroughly investigated by Prof. Kiritani and his colleagues using neutron irradiation followed by TEM observation, and here we attempt to provide some further understanding of the experimental observations by using atomic-scale computer simulation tools. We describe the production of interstitial and vacancy clusters in the cascades and highlight the differences among the various materials. In particular, we discuss how covalent bonding in Si effects defect production and amorphization resulting in a very different primary damage state from the metals. We also use MD simulations to extract prefactors and activation energies for migration of point defects, as well as to investigate the energetics, geometry and diffusivity of small vacancy and interstitial clusters. We show that in the metals, small interstitial clusters are highly mobile and glide in one dimension along the direction of the Burgers vector. The results for the primary damage state and for the defect energetics and kinetics are then combined and used in a KMC simulation to investigate the escape efficiency of defects from their nascent cascade in metals. We show that in fcc metals Au and Pb at or above stage V the escape probability is approximately 40% for 30 keV recoils so that the freely migrating defect fraction is approximately 10% of the displacement per atom (dpa) standard.

MD study of the dynamic behavior of small interstitial clusters in Fe

Dynamic behavior of small interstitial clusters (dislocation loops), such as the interaction with a self-interstitial atom (SIA) in Fe and the interaction between a SIA and an edge dislocation in Fe have been studied by means of molecular dynamics (MD) method in order to clarify their role in the evolution of damage structure during irradiation, especially in the so-called production bias effect through one-dimensional migration to sinks and in the so-called dislocation bias. Model crystal was constructed by using N-body potentials and small interstitial clusters, i.e., bundle of crowdions, a SIA and an edge dislocation were inserted. It was found that a SIA migrates to the edge dislocation core and finally the direction of the crowdion is converted to the direction parallel to the Burgers vector of the edge dislocation. Dynamic behavior of the loop, e.g., the interaction with a crowdion on a central 〈1 1 1〉 loop axis was also investigated as a function of time. It was shown that small initial cluster, e.g., I 19 in Fe is very mobile under the interaction with a crowdion, which shows that this interstitial cluster I 19 has already property of a dislocation loop of edge character and low Peierls potential for the motion of this loop, which is consistent with the straight edge dislocation in Fe.

Molecular dynamics calculations of properties of the self-interstitials in copper and nickel

Molecular dynamics calculations are performed to study the properties of self-interstitials in copper and nickel using semi-empirical, many-body potentials based on Embedded Atom Method. A set of values of formation, binding and migration energy calculated for single, di-, tri-, tetra-, penta- and hexa-interstitial in copper and nickel are reported. The stable configurations and migration mechanism of them are also presented. The results show that the small interstitial clusters have a significant lower migration energy than the single interstitials and move in the form of one-dimensional crowdion migrations. The clusters in copper are very mobile and the tri-, penta-interstitial in nickel are less mobile because they are difficult to relax to the crowdion dumbbell configurations. The results are also compared with available experimental investigations and previous interstitial calculations using pair potentials.

Atomic structure and dynamic behavior of small interstitial clusters in Fe and Ni

Atomic structure and dynamic behavior of small interstitial clusters (dislocation loops) in Fe and Ni have been studied by means of both the static relaxation method and the molecular dynamics (MD) method in order to clarify their role in the evolution of damage structure during irradiation, especially in the so-called production bias effect through one-dimensional migration and sink efficiency to dislocations. Model crystals were constructed by using N-body potentials and stable atomic structures of small interstitial clusters, i.e., bundle of crowdions were obtained. It was found that each crowdion in the cluster has a split structure when the number of crowdions in the cluster is larger than 10. Dynamic behavior of the loop, e.g., the interaction with a crowdion on a 〈1 1 1〉 loop axis was also investigated as a function of time by the MD method. It was shown that a small interstitial cluster, e.g., I 19 in Fe is very mobile under the interaction with a crowdion, which shows that this interstitial cluster I 19 has already the property of a dislocation loop of edge character and low Peierls potential for the motion of this loop, which is consistent with the straight edge dislocation in Fe.

Formation, evolution and annihilation of interstitial clusters in ion implanted Si

Formation, thermal evolution and annealing kinetics of self-interstitial clusters in ion implanted Si have been investigated. Deep level transient spectroscopy measurements performed on epitaxial and Czochralski Si samples implanted with Si ions at energies of 145 keV or 1.2 MeV reveal that these clusters are formed for fluences above 10 12/cm 2 and annealing temperatures higher than 550°C. Interstitial clusters introduce seven well defined levels in the Si band gap at E V + 0.33 eV, E V + 0.52 eV, E C − 0.58 eV, E C − 0.50 eV, E C − 0.37 eV, E C − 0.29 eV and E C − 0.14 eV. Analysis of the annealing kinetics at temperatures in the range 550–700°C reveal that the clusters undergo Ostwald ripening and anneal out with a characteristic dissociation energy of ∼2.3 eV for a Si fluence of 1 × 10 12/cm 2. Furthermore, their annealing temperature increases with implantation fluence. These results indicate that small interstitial clusters act as the source of interstitial supersaturation that drive transient enhanced dopant diffusion in the absence of extended defects.

A comparison of the effect of electron irradiation and of thermal aging on the hardness of FeCu binary alloys

The hardening of binary FeCu alloys under 2.5 MeV electron irradiation between 175°C and 360°C was investigated. We show that an extra-hardening component which cannot be ascribed to copper precipitation is induced during irradiation with electrons. We discuss the possible nature of the objects responsible for this extra-hardening component. They have to be very small since they are not visible by transmission electron microscopy. We come to the conclusion that they most likely are very small interstitial clusters nucleated by random encounter of interstitials. An important issue from the technological point of view raised by this work is to know whether it is reasonable to totally ignore any hardening component of the pressure vessel steels attributed to the clustering of freely migrating point defect escaping from the core of the displacement cascades or created between them.

A comparison between the irradiation damage response of spinel and zirconia due to Xe ion bombardment

Abstract The mechanical properties of Xe-implanted spinel and cubic zirconia surfaces, as determined by nano-indentation measurements, are distinct and the differences can be related to their microstructures. Upon Xe 2+ ion irradiation to high dose at cryogenic temperature (120 K), the Young’s modulus of irradiated spinel falls dramatically until the modulus is only about 3/4 the un-irradiated value. The maximum modulus occurs concurrent with the formation of a metastable crystalline phase of spinel. The subsequent elastic softening at higher Xe 2+ doses is an indication of the onset of amorphization of the spinel. Xe-implanted zirconia surfaces behave differently, in all cases showing almost no change in elastic modulus with increasing Xe 2+ ion dose. This is consistent with microstructural observations of Xe-implanted zirconia crystals which, unlike spinel, show no change in crystal structure with increasing ion dose. The hardness of both spinel and zirconia increases slightly for low Xe 2+ ion doses. At higher doses, zirconia shows little change in hardness, while the hardness of the implanted spinel falls by more than a factor of two. The initial increase in hardness of both spinel and zirconia is consistent with point defect accumulation and the precipitation of small interstitial clusters, while the drop in hardness of spinel at high Xe 2+ ion doses is due to the formation of an amorphous phase.

Swelling modification by one-dimensional diffusion of cascade-produced small interstitial clusters

The paper deals with the influence of one-dimensional diffusion of small interstitial clusters produced in collision cascades on the swelling of irradiated structural materials. It is demonstrated that the resulting swelling modification is very sensitive to microstructural parameters of irradiated materials. Qualitative trends of swelling modification in well annealed materials correlate with experimental observations; however for cold-worked materials the model predicts noticeable decrease in swelling rate by one-dimensionally diffusing clusters, which is in contrast with the observed material behaviour.

Molecular Dynamics Simulations of the Behavior of Small Interstitial Clusters in Copper

Molecular dynamics (MD) simulations are carried out to study the behavior of small interstitial clusters in copper using a semi-empirical many-body potential of the embedded atom method (EAM). In this paper, a set of values of formation, binding and migration energy calculated for single, di-, tri-, tetra-, penta- and hexa-interstitial are reported. Their stable configurations and migration mechanisms are also presented. The results show that the small interstitial clusters have a significantly low migration energy and move in the form of one-dimensional crowdion migration, which indicates that they have a high mobility relative to the single interstitial. The results are also compared with available experimental investigations and previous interstitial calculations using pair potentials.

A swelling model for stoichiometric SiC at temperatures below 1000°C under neutron irradiation

A simple phenomenological model for the saturation swelling below 1000°C of neutron-irradiated silicon carbide (SiC) is presented in this paper. Under fast neutron irradiation, SiC is known to undergo volumetric expansion (swelling) which quickly saturates at a fast fluence of approximately 10 25 n/m 2 for irradiation temperatures below 1000°C. A previous model due to Balarin attributes swelling to lattice dilation as a result of single point defects. We show in this paper that the experimentally observed linear temperature dependence of saturation swelling can be explained in terms of the formation and growth of small interstitial clusters, resulting directly from collision cascades initiated by energetic neutrons. These loops grow by absorption of mobile carbon interstitials and their composition is subject to stoichiometry constraints, requiring absorption of slower silicon interstitials. Because of cascade re-solution events, the density of loops decreases sharply with temperature as a result of overlap of cascades with larger size loops at higher temperatures. The average radius of these loops increases with temperature. Volumetric swelling is shown to obey a linear temperature dependence as a consequence of the strong decrease in density and the simultaneous increase in average radius, and to saturate with fluence. The model is shown to be consistent with experimental observations. In the temperature range below 500–600°C, swelling seems to be dominated by single point defects, or defect clusters containing only a few atoms, in accordance with the explanation offered by Balarin.

Defect production and annealing kinetics in elemental metals and semiconductors

We present a review of recent results of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations of defect production and annealing in irradiated metals and semiconductors. The MD simulations describe the primary damage state in elemental metals Fe, V and Au, and in an elemental semiconductor Si. We describe the production of interstitial and vacancy clusters in the cascades and highlight the differences among the various materials. In particular, we discuss how covalent bonding in Si affects defect production and amorphization resulting in a very different primary damage state from the metals. We also use MD simulations to extract prefactors and activation energies for migration of point defects, as well as to investigate the energetics, geometry and diffusivity of small vacancy and interstitial clusters. We show that, in the metals, small interstitial clusters are highly mobile and glide in one dimension along the direction of the Burger's vector. In silicon, we show that, in contrast to the metals, the neutral vacancy diffuses faster than the neutral self-interstitial. The results for the primary damage state and for the defect energetics and kinetics are then combined and used in a kinetic Monte Carlo simulation to investigate the escape efficiency of defects from their nascent cascade in metals, and the effect of dose rate on damage accumulation and amorphization in silicon. We show that in fee metals Au and Pb at or above stage V the escape probability is approximately 40% for 30 keV recoils so that the freely migrating defect fraction is approximately 10% of the dpa standard. In silicon, we show that damage accumulation at room temperature during light ion implantation can be significantly reduced by decreasing the dose rate. The results are compared to scanning tunneling microscopy experiments.

The influence of dynamical structural relaxation of point defect clusters on void formation in irradiated copper

In the neutron-irradiation experiment with a temperature controlled capsule at JMTR, residual-gas-free copper was irradiated at 200°C and 300°C together with as-received copper. The fluences were 5 × 10 18 n/cm 2 (the low fluence) to 1 × 10 20 n/cm 2 (the high fluence). TEM observation of the irradiated specimens showed that interstitial clusters form a colony at the low fluence which develops into a dislocation structure at the high fluence. Between the colonies only vacancy clusters in the form of voids and stacking fault tetrahedra (sft) were observed. There are no effects of residual gas atoms on the formation of voids at the low fluence although the effects become appreciable at the high fluence. The number of vacancies which are accumulated in a void is 350 times larger than that in a sft at the low fluence. The number density of voids decreased with increasing neutron fluence while the number density of sft increased. The voids form uniformly in copper irradiated to the low fluence while they were observed along dislocations at the high fluence. Computer simulations by molecular dynamics show that small interstitial clusters relax to a bundle of 〈110〉 crowdions and move long distances in response to small strain fields. Interstitial clusters move along a 〈110〉 direction and can switch to other 〈110〉 directions, and form groups of clusters. At high temperature, a dense colony of the clusters forms and develops into a dislocation structure. It is shown that small vacancy clusters relax to movable structures at high temperature. The structure consists of vacancies which are connected in a curved string shape. Along the vacancy strings, many relaxations of a tri-vacancy of Damask- Dienes-Weizer type (3v-sft) were observed. Such a relaxation to the 3v-sft type makes it difficult for a single vacancy evaporation. Small vacancy clusters move and coalesce into larger vacancy clusters. The linkage of the results of experiments and computer-simulations suggests that voids nucleate as a metastable defects at coalesced vacancy clusters at high temperature. The nucleation of voids is not affected by the influence of gas atoms dissolved in the material. Micro-voids migrate in the specimens after their nucleation. During their movement, gas atoms are trapped in the voids. The trapping of a larger number of gas atoms limits the movement of voids. This leads to a higher number density of voids in the as-received copper than in residual-gas-free specimens at the high fluence. Voids form uniformly in specimens at the low flunce and they migrate to dislocation lines. Dislocations are also trapped at voids during climbing by absorbing interstitial clusters. These finally lead to the preferential formation of voids along dislocation lines.

Point defects and their clusters in f.c.c. metals studied by computer simulations

Extensive theoretical and experimental studies of vacancies and interstitials in f.c.c. metals have been carried out at the University of Illinois since 1950. In the present work, the studies of molecular dynamics are carried out by computer simulation in order to explore the detailed atomistic process of dynamical movement of point defects of vacancies and interstitials, and their clustering in f.c.c. metals. An isotropic EAM potential due to Daw and Baskes ([itPhys. Rev. B, 29 (1984) 6443) was utilized to calculate the force which acts on atoms in a crystal. Calculations were carried out for Cu, Ag, Au, Ni and Al. The first part of the results of this paper describes the vacancies and their clusters. The importance of the migration of small vacancy clusters is emphasized. The atomistic growth step of vacancy clusters is also described. In the second part, the results of interstitials and their clusters are summarized. The small interstitial clusters relax to a bundle of parallel 〈110〉 crowdions which moves with the very small migration energy of 0.001 eV. The simulation of the movement of interstitials in Au at very low temperature was carried out. The movement of interstitial clusters is sensitively responsive on the strain field.

Evolution from point to extended defects in ion implanted silicon

We present a quantitative study of the evolution of point defects into clusters and extended defects in ion-implanted Si. Deep level transient spectroscopy (DLTS) measurements are used to identify and count the electrically active defects in the damaged region produced by Si ion implantation at energies of 145 keV–2 MeV, and fluences from 1×108 to 5×1013 Si/cm2. Analyses of silicon annealed in the temperature range 100–680 °C allow us to monitor the transition from simple point defects to defect clusters and extended defects that occur upon increasing the ion fluence and the annealing temperature. At low doses, <1010 Si/cm2, only about 2% of the Frenkel pairs generated by the ion beam escape recombination and are stored into an equal number of interstitial and vacancy-type point defects. Thermal treatments produce a concomitant annealing of interstitial and vacancy-type defects until, at temperatures above 350 °C, only two to three interstitial-type defects per ion are left, and the DLTS spectra contain signatures of second-order point defects. Interstitial clusters at Ev+0.29 and Ev+0.48 eV are found to dominate the residual damage of silicon implanted at higher fluences, 1×1012–7×1013 Si/cm2, and at annealing temperatures, T⩾600 °C. These interstitial clusters have point defect capture kinetics and are not observable in transmission electron microscopy (TEM), suggesting that they are smaller than ≈50 Å. Finally, for silicon implanted at higher Si doses, ⩾5×1013 Si/cm2, thermal treatments at 680 °C result in a strong decrease in the concentration of the interstitial cluster signatures and in the introduction of a different DLTS signal, Ev+0.50 eV, which exhibits logarithmic rather than exponential carrier capture kinetics, a feature typical of an extended defect. Comparison of the formation and dissolution of this extended defect signature with TEM analyses indicates that this level is a signature of the rodlike {311} defects that are known to store the interstitials responsible for transient enhanced diffusion. These results suggest that the small interstitial clusters are either the precursors of the {311} defects or that they compete with {311} defects as sinks for self-interstitials.