Vacancy Defect Clusters Research Articles

Nonstoichiometry accommodation in SrTiO3thin films studied by positron annihilation and electron microscopy

Accommodation of nonstoichiometry in SrTiO${}_{3}$ pulsed laser deposited (PLD) films was investigated using positron annihilation lifetime spectroscopy and (scanning) transmission electron microscopy. Increasing PLD laser fluence changed the stoichiometry from Ti to Sr deficient. Cation vacancy defects were detected, and the concentration ratio of Sr to Ti vacancies, [${V}_{\mathrm{Sr}}$]/[${V}_{\mathrm{Ti}}$], was observed to increase systematically in the Sr-deficient region, although no change in the electron microscopy lattice images was detected. Increasing Ti deficiency resulted in the accommodation of SrO layers in planar defects, and in the formation of vacancy cluster defects. A change from ${V}_{\mathrm{Ti}}$ to ${V}_{\mathrm{Sr}}$ defect positron trapping was also detected.

Suppression of vacancy defects in epitaxial La-doped SrTiO3 films

Variable energy positron annihilation lifetime spectroscopy of high-mobility La-doped SrTiO3 grown by molecular beam epitaxy found that the films contained sufficiently low concentrations of Sr vacancies and vacancy cluster defects to allow the observation of positron annihilation events from the perfect lattice. This enabled the concentrations of charged cation vacancies to be estimated, and these were found to be at least an order of magnitude below the La-dopant concentrations.

Defects and Microstructures in the Surface Layer of Single-crystal Silicon In-duced by High-current Pulsed Electron Beam

In order to investigate the microstructures of nonmetallic material induced by high-speed deformation, the high-current pulsed electron beam (HCPEB) technique was used to irradiate the single-crystal silicon. The surface microstructures induced by electron beam were studied by transmission electron microscope (TEM). The experimental results showed that a large number of defect structures were formed by the HCPEB irradiation. Among them, the typical defect structures were the parallel screw dislocations and the extrinsic stacking faults. In the meantime, the HCPEB irradiation induced high density of vacancy cluster defects. The surface stress with very high value and strain rate led to the integral shift of (111) crystal plane, which might be the dominating reason of the formation of the massive vacancy cluster defects. In addition, the mixtures of nanocrystal and amorphous in the surface of single-crystal silicon can be formed by HCPEB technique.

Thermal stretching of defective nanowires: the coupled effects of vacancy cluster defects, operating temperature, and wire cross-sectional area

Extensive atomistic simulations of the thermal stretching of defective nanowires (NWs) were performed using the embedded-atom molecular dynamics modeling approach. The nucleation and propagation of dislocations are described via quantitative dislocation-based analyses. The investigation focuses on the coupled effects of various vacancy cluster (VC) defects, operating temperature, and wire cross-sectional area on the mechanical properties and plastic deformations of defective NWs. With increasing internal stress of a stretched wire, a rapidly moving dislocation loop that transferred atoms to fill up the original vacancy cluster before the wire yielded was found (i.e. it carried the vacancies away from the inside of the wire and formed a notch at the wire edge). The heterogeneous nucleation of dislocations from the notch site propagated along the {111}〈112〉 partial dislocations and formed stacking faults or perfect dislocations on the {111} activated planes. Simulation results show a decreasing yield strength with increasing VC size for a given wire sectional area and temperature. Quasi-linear decreasing Young’s moduli were observed with increasing operation temperature. For a given operation temperature, NW Young’s modulus increased with increasing NW size. Two typical deformation regimes under various operation temperatures were found: (i) a high-temperature-induced pre-melting phenomenon and a thermal softening effect caused low-stress plastic flow and rapid pillar-necking deformation, and (ii) step-wise glides, slip bands, and cross-slips proceeded along the activated glide planes in the low-temperature hard-brittle structure. These two regimes were thoroughly characterized via the evolutions of microscopic dislocations and the changes of true stress. For operation at high temperatures, the ultra-thin 1/5-type pentagonal ring chains exhibit a relatively robust structure, which can potentially be used as building blocks and components for high-temperature nanoelectromechanical systems (NEMS) devices in the future.

Defect microstructures in polycrystalline pure copper induced by high-current pulsed electron beam——the vacancy defect clusters and surface micropores

In this paper, high-current pulsed electron beam (HCPEB) was used to irradiate the polycrystalline pure copper. The vacancy defect clusters of the irradiated surface layer have been investigated by using transmission electron microscopy. Very dense vacancy defect clusters involving square cells, vacancy dislocation loops and stacking fault tetrahedra were formed after HCPEB irradiation. It suggests that the very high stress and high strain rate induced by rapid heating and cooling due to HCPEB irradiation could cause the shifting of whole atomic planes synchronously, which is the probable mechanism of the formation of the vacancy defect clusters. Additionally, it was established by scanning electron microscopy investigations that dense, fine and dispersed micropores on the irradiated surface of pure copper can be successfully fabricated by using HCPEB irradiation. The dominating formation mechanism of surface micropores should be attributed to the formation of supersaturation vacancies within the near-surface introduced during HCPEB irradiation and vacancy migration along grain boundaries and (or) dislocations towards the irradiated surface. The present results indicate that HCPEB technique may become a new method for rapid synthesis of surface porous materials.

The vacancy defect clusters in polycrystalline pure nickle induced by high-current pulsed electron beam

High-current pulsed electron beam (HCPEB) technique is employed to irradiate the samples of polycrystalline pure nickel. The microstructures of the irradiated surface layers are investigated by using transmission electron microscopy (TEM). After HCPEB irradiation, very high value of residual stress is induced in the irradiated surface layer, which leads to the formation of dense dislocation walls and twins. Furthermore, a larger number of vacancy defect clusters including dislocation loops, stacking fault tetrahedra (SFT) and voids are also formed. Among three vacancy defect clusters, the number of SFT is much more than that of two other vacancy defect clusters. The lower dislocation density near the regions with dense SFT is observed and voids are likely to be present in these regions. It suggests that the stress with very high value and strain rate induced by rapid heating and cooling due to HCPEB irradiation could cause the shifting of the whole atomic plane synchronously.

Effect of vacancy defect clusters on the optical property of the aluminium filter used for the space solar telescope

We investigate the microstructures of the pure aluminium foil and filter used on the space solar telescope, irradiated by photons with different doses. The vacancy defect clusters induced by proton irradiation in both samples are characterized by transmission electron microscopy, and the density and the size distribution of vacancy defect clusters are determined. Their transmittances are measured before and after irradiating the samples by protons with energy E = 100 keV and dose φ = 6 × 1011/mm2. Our experimental results show that the density and the size of vacancy defect clusters increase with the increase of irradiation doses in the irradiated pure aluminium foils. As irradiation dose increases, vacancies incline to form larger defect clusters. In the irradiated filter, a large number of banded void defects are observed at the agglomerate boundary, which results in the degradation of the optical and mechanical performances of the filter after proton irradiation.

Nature of vacancy defect clusters in chalcogenic semiconductors of AIIBVI group irradiated by neutrons

AbstractAn experimentally proven model is proposed stating that vacancies of cadmium atoms are dominating in kernels of the defect clusters (DCs) formed in single CdS crystals by the irradiation with fast 1 MeV reactor neutrons. The so‐called subthreshold effects play the essential role in cluster formation. The subthreshold effects consequently lead to the excitation and ionization of the K‐shell of Cd atoms followed by an increase of charge of Cd ions. This finally result in an ejection of these ions from the kernels to periphery of DCs due to the electrostatic force (electrostatic explosion). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The vacancy defect clusters in polycrystalline pure aluminum induced by high-current pulsed electron beam

The specimens of polycrystalline pure aluminum were irradiated with high-current pulsed electron beam （HCPEB）. The microstructure of vacancy defect clusters has been investigated in detail by using transmission electron microscopy （TEM）. The results reveal that large numbers of vacancy cells including dislocation loop and even stacking fault tetrahedra （SFT） can be formed in the specimens of polycrystalline pure aluminum irradiated with HCPEB. For the specimen irradiated with one pulse, vacancy dislocation loops were formed in the vacancy cells. SFTs became the dominating structures after five pulses. For the specimen irradiated with ten pulses, dislocation loops were frequently present and SFTs were only formed in some local zones of vacancy cells. In the vicinity of SFT formation, dislocation-free or very low dislocation densities were observed. It is suggested that high stress and strain rate induced by rapid heating and cooling due to HCPEB irradiation could cause the shifting of whole atomic planes synchronously. This is the more probable mechanism of the formation of SFTs.

Formation and Annihilation of Hydrogen Related Donor States in Proton Implanted and Subsequently Plasma Hydrogenated N-Type Float Zone Silicon

The formation of hydrogen-related donor states was studied in proton-implanted and hydrogen plasma-treated Float-Zone silicon. H-implantations were executed at energies of 1 or 3 MeV, and fluences of 1x10E14 1/cm2. 13.56-MHz RF-plasma hydrogenations were done at 150 W for 15 min or 1 hour at substrate temperatures between 350 C and 500 C. Analysis was done by two-point-probe spreading-resistance measurements. H-plasma exposure for 15 min caused an enhanced donor concentration close to the projected ion range Rp. The donor states can be related to hydrogenated vacancy defects. Near the wafer surface n-type doping is also enhanced. Applying higher substrate temperatures (> 400 C) during plasma exposure, acceptor-like states overcompensate for the enhanced n-type doping near the surface. After 1-hour H-plasma exposure excessive donor states in the wafers subsurface region are completely annealed. At the surface n-type doping is still overcompensated for by acceptor-like defects attributed to hydrogenated vacancy defect clusters.

STM/STS investigation of the interaction of Si with atomic scale vacancies on cleaved GaAs

Cross-sectional scanning tunnelling microscopy (STM) images on a freshly cleaved III–V semiconductor laser facet regularly indicate atomic scale defects, such as steps and vacancies on the non-polar (1 1 0) surface. In this work, studies were made when sub-monolayer quantities of Si were deposited at 280 °C on clean cleaved GaAs(1 1 0). Selective adsorption of Si on atomic defects has been observed with STM imaging of the surface. The simultaneous use of STM and scanning tunnelling spectroscopy (STS) allows the quantification of the Fermi shifts present at atomic steps and vacancies before and after Si adsorption. Localised current–voltage measurements, performed using STS, on the atomic steps and vacancy defect clusters of p-type GaAs(1 1 0) surfaces showed Fermi-level shifts of ∼0.8 and 0.7 eV towards mid-gap position, respectively. However, measurements taken from silicon-coated atomic steps and defect clusters showed that the Fermi-level reverted back towards the ‘ideal’ flat band position by ∼0.4 eV. This behaviour was distinctively different to silicon adsorbed at defect-free surface of GaAs(1 1 0). These results indicated the passivating potential of silicon on the defect sites of GaAs(1 1 0) surface when deposited under these conditions. The ability to measure and control surface properties on the atomic scale is crucial to the success of nano-technology and the STM/STS studies demonstrate the effectiveness of the technique for this role.

Quantitative Depth Profiles of Vacancy Cluster Defects Produced by MeV Ion Implantation in Si: Species and dose Dependence

AbstractMonte Carlo simulation codes such as TRIM or MARLOWE show a net displacement of interstitials with respect to vacancies from the Frenkel pairs produced in implant cascades. As a result, defect profiles after recombination have an excess of vacancies (Vex) in the shallow region of these implants, with excess interstitials near the projected range Rp. Typically, accurate estimates of the Vex distribution are computer intensive as the complete history of each damage cascade must be recorded. In this work we introduce a method for fast estimation of the Vex profile based on the derivative of the recoil distribution obtained from binary collision simulations using the Kinchen-Pease approximation for damage production. Also, Au labeling has been used to quantitatively analyze and compare the experimental depth profiles of Vex produced by implants of B, Si, and Ge into Si for a range of doses. Specific conditions like matching of the projected range of the implanted ions and matching of the number of Frenkel pairs created have been compared. This detailed study and comparison with simulation provides the first systematic, quantitative measurements of the Vex concentrations over such a range of species and doses. In analogy to the “+ number” for excess interstitials from ion implantation, we determine the range of “minus (-) numbers”, that is the number of excess vacancies per implanted ion for the different species. For Si it is constant in the dose range studied at -0.05, while for B it is -0.001 to -0.004 and for Ge, -0.1 to -0.2. The general shapes of the Vex profiles are similar to those obtained from simulation. Evidence is presented that one significant difference, namely an experimentally observed vacancy depletion near the surface, may be due to the diffusion and annihilation of vacancies at the surface. In addition, an increased nucleation probability of vacancy defects close to Rp may account for the growth of excess vacancies from close to Rp towards the surface with increasing implant dose.

Formation process of interfaces and microdefects in nanostructured Ag studied by positron lifetime spectroscopy

Nanostructured Ag (polycrystalline Ag with nanometre-sized grains), synthesized by inert-gas condensation plus in situ vacuum compaction, has been investigated by positron lifetime spectroscopy (PLS). The results indicate that there is a common character, i.e. only three lifetime components (, and ) are resolvable from the lifetime spectrum on each of the specimens synthesized under the whole compacting pressure range investigated (from 0.15 to 1.50 GPa). Corresponding to the three lifetime components, there are three types of defect (traps) in nanostructured Ag: (a) vacancy-like (VL) defects, (b) vacancy-cluster (VC) defects and (c) larger voids. Compacting pressure and annealing treatment has great influences on the positron annihilating behaviour. The lifetimes and corresponding intensities decreased irreversibly with compacting pressure and annealing temperature, indicating that the VL defects and VC defects are both mechanically and thermally unstable, and so it is inappropriate to consider them as structural elements in nanostructured Ag. Moreover, the interfaces in n-Ag can be considered as superpositions of VL defects on the normal ordered boundaries, and since the number and size of the VL defects change with external conditions, the interfaces in n-Ag can stay in various metastable states, which implies that its interfacial structures may range from total random states (gaslike) to complete ordered structures, depending on the number and size of the VL defects contained. The forming process of bulk nanostructured Ag revealed by PLS can be roughly divided into three stages: (1) formation stage of interfaces (compacting pressure GPa); (2) rapid elimination of the three types of defect (0.6 GPa < p < 1.1 GPa); (3) gradual elimination of those defects (p > 1.1 GPa). Based on these results obtained on nanostructured Ag, a density criterion that of the polycrystalline counterpart is proposed for the formation of bulk nanostructured materials.

Story of stacking fault tetrahedra

Stacking fault tetrahedra, although they have a peculiar structure, are the most general type of vacancy clustered defects in f.c.c. metals and alloys. Placing these stacking fault tetrahedra at the center, the story of point defect reaction is told. The structure of the defect and the energy relation are first described. Various experimental treatments which lead to the formation of stacking fault tetrahedra are described, featuring characteristic point defect processes in each. Quenching from high temperature leads to an understanding of the nucleation processes. Electron irradiation focuses on the relation with the local accumulation of vacancies, and the stochasticity of point defect reaction is detected in terms of the behavior of the defects during irradiation. Microstructural evolution by irradiation with neutrons and ions is well illustrated by the formation of stacking fault tetrahedra directly from collision cascades. Subcascade formation, estimation of deposited energy density, impact effect from cascade collision to point defect reaction, and roles of freely migrating point defects are included in the story related to the formation of stacking fault tetrahedra. Finally, plastic deformation, especially recent results on high-speed heavy deformation, presents the striking result of the formation of high density stacking fault tetrahedra even in pure aluminum, which leads to a proposal of plastic deformation without dislocation.

Vacancy Related Defects in La0.5Sr0.5CoO3-δ Thin Films

ABSTRACTLaser ablated La0.5Sr0.5CoO3-δ thin films have been studied by Doppler-broademng-detected positron annihilation using a variable-energy positron beam. The oxygen partial pressure during cooling from the growth temperature was altered through the range 760 torr to 10−5 torr to change the oxygen non-stoichiometry of the films.The measured Doppler broadened lineshape parameter S was found to increase with increasing oxygen nonstoichiometry. For films cooled with an oxygen partial pressure of ≤ 10−4 Torr positron trapping to monovacancy type defects is infered. It is proposed that Sr ion oxygen vacancy complexes are likely trapping sites.For the film cooled in 10−5 torr oxygen the magnitude of the increase in S, with respect to that measured from the film cooled in 760 Torr oxygen, showed positron trapping to vacancy cluster defects is occuring.

Study of point defect clusters in high purity single crystals of silicon grown by Czochralski and float-zone methods by diffuse X-ray scattering technique

Results of study of point defect clusters in high purity ( ∼ 10 kΩ · cm ) dislocation-free Czochralski (Cz) and float-zone (FZ) grown crystals (∼ 50 mm diameter, 11 mm thick, p-type) are reported. A multicrystal X-ray diffractometer set in (+, −, −, +) geometry and employing a well-collimated Mo K α 1 beam was used. High resolution X-ray diffractometry, absolute integrated intensity measurements and diffuse X-ray scattering (DXS) measurements were carried out. The specimens gave sharp diffraction curves and the integrated intensities were found to be close to the theoretical value for an ideally perfect silicon crystal. DXS measurements were made around 111 reciprocal lattice point (RLP) along four directions of K ∗ : [111], [1̄1̄1̄], [01̄1] and [011̄]. K ∗ is the vector which connects the elemental reciprocal volume under investigation with the nearest RLP. From the analysis of the DXS intensity versus K ∗ plots, it was found that the source of DXS is interstitial defect clusters in Cz-grown crystals and vacancy defect clusters in FZ-grown crystals. By using a phenomenological model, the values of cluster size R cl, cluster volume A cl and number of defects per cluster N cl, respectively, were found to be: 0.8 μm, 2.02 × 10 −16 cm 3, 1.26 × 10 6 for Cz specimen and 0.6 μm, 1.32 × 10 −16 cm 3, 8.3 × 10 5 for the FZ specimen.

Formation of vacancy clustered defects from cascade collisions during heavy-ion irradiation and their annihilation by freely-migrating interstitial atoms

Heavy-ion irradiation of several fcc metals and their ordered alloys was performed. The behavior of ion-irradiation induced point defect clusters during subsequent electron irradiation indicates that about 90% of the clusters are of vacancy type and 10% of interstitials. For any given ion and energy, a wide variation of the collision cascade zone size was found. In general, the larger zones were associated with a larger number of point-defect clusters. A collision zone of a given size was associated with the same average number of clusters even when they were produced by ions of different energy. The dependence of cluster formation on ion energy, target material, and angle of incidence of the ion beam can be correlated with a unique parameter, the depth in the foil of defect cluster formation. The defect yield increases linearly with irradiation dose for shallow depth distributions but then increases with a square-root relationship for deeper distributions. These results are analyzed in terms of a variation in the role of freely-migrating interstitials for the two cases.

An Examination of the Factors Which Determine the Thermal Spike Lifetime of a Displacement Cascade

AbstractThe thermal spike phase of a displacement cascade is thought to be of fundamental importance to a number of issues in radiation effects. For example, the formation of stable vacancy clusters and the efficiency of ion mixing are thought to be directly controlled by the duration of the liquid-like state during the thermal spike. A number of factors have been proposed to be of significance to the lifetime of the thermal spike. For example, materials which have a high melting temperature, or which exhibit strong electron-phonon coupling, which provides an efficient method of energy transport to the surrounding material, are expected to have short lifetimes. We have examined these factors in detail and demonstrated that, in both elemental metals and alloys, electron-phonon coupling provides the most consistent explanation for variations in the thermal spike lifetime and, hence, the efficiency of ion mixing and the number of stable vacancy defect clusters formed.

Characters of NTD FZ(H2) Si kept at room temperature for three years

It has been found that the hydrogen-defect shallow donors do not appear, and some strong neutron radiation-induced Si-H IR bands, such as 1832 and 2054 cm-1 disappear; but the strength of some others, e.g. 1979 and 2065 cm-1 increases greatly for NTD FZ(H2) Si kept at room temperature for 3 years. These changes indicate that at room temperature simple point defects diffuse and interact, and hydrogen atoms trap the vacancy and vacancy cluster defects. This gives important information in the discussion of the micro-structures of the above Si-H IR bands.

Recoil energy effects and defect processes in neutron irradiated metals

Subdivision of primary recoil energy into sub-cascades in 14 MeV neutron irradiated fcc metals is analyzed in reference to the primary recoil energy spectrum, 10–20 keV per sub-cascade depending on the kind of metals. Shape and size of sub-cascade groups are also related to the recoil energy, including the anisotropy with reference to the incident neutron direction. The extent of the overlapping of sub-cascades to produce vacancy clustered defects are classified from the spacing between sub-cascades, significant overlapping for narrow spacing less than 5 nm in Ag and Au, and no overlapping for wide separation more than 10 nm in Ni and Cu. The amount of point defects annihilated within the cascade zone is estimated and the roles of interstitial atoms released from the zone are categorized to propose a simplified kinetics modeling of the microstructure development. A discussion is made on the irradiation rate dependence from the defect cluster formation mechanism and from the degree of cascade-overlap. Analysis and diagnosis of the neutron-temperature history in fission reactor irradiation leads to a proposal of an essential improvement of the reactor irradiation.