High Concentration Of Point Defects Research Articles

Microstructures in water‐weakened single crystals of quartz

Models of hydrolytic weakening based on the influence of chemical environment on the concentrations of point defects stimulated a series of experiments by Ord and Hobbs [1986] in which single crystals of natural quartz were deformed in a solid medium apparatus under a range of buffered oxygen, water, and hydrogen fugacities. Microstructures in these specimens have been examined using visible light and transmission electron microscopy in an attempt to identify processes responsible for the observed relationships between chemical environment and strength. Preheating treatments, conducted for ∼20 hours at the pressure, temperature, and chemical conditions of each ensuing deformation experiment, modified each specimen microstructurally such that the quartz was not in the form of a low‐dislocation‐density single crystal when deformation testing began. Specimens fractured axially during pretreatment, particularly those in chemical environments for which water fugacity remained high. Fractures later healed giving rise to arrays of fluid inclusions in regions with moderate densities of dislocations. Some deformation and recrystallization, concentrated at healed fractures and specimen end zones, also accompanied pretreatment. “Mn‐buffered” specimens, for which f O2 and f H2O.were high, were densely fractured; in less strained regions, subsequent deformation was restricted to the vicinity of healed fractures, while regions of higher plastic strain and recrystallization show evidence from inclusions for abundant early fractures, now obliterated. “Ta‐buffered” specimens, of low f O2 and f H2O, are also deformed close to axial fractures, but here fractures are comparatively rare. The marked variation in the degree of fracturing seems to be an example of stress‐corrosion cracking in response to chemical environment. Dislocations are developed in material away from healed fractures in one deformed Ta‐buffered specimen. If point defects introduced by diffusion are responsible for water weakening of quartz crystals, the complex microstructures seen in the deformed specimens of Ord and Hobbs [1986] require that point defect distributions were neither uniform nor controlled by bulk diffusion from the sample edges. The clear association between plastic strain and healed fracture does point to important local effects either of “water” upon dislocation activity or of healed‐fracture structure having been involved in the nucleation of dislocations. Regardless of the details, we infer from the dependence of the degree of fracturing upon chemical environment, together with the high correlation between locations of plastic strain and regions of earlier fracture, that the relationship between chemical environment and macroscopic strength of natural quartz crystal is not simply a matter of water weakening as a direct result of high point defect concentrations. Instead, fracture, inferred to be important in natural fluid‐rock interactions, also seems to be an experimentally critical process which might nucleate the dislocations necessary to initiate plastic deformation in the laboratory.

Enhanced strain relaxation in Si/GexSi1−x/Si heterostructures via point-defect concentrations introduced by ion implantation

It is shown that strain relaxation during annealing of Si/GexSi1−x/Si heterostructures is significantly enhanced if the strained GexSi1−x layers are implanted with p (B) or n (As) type dopants below the amorphization dose. Comparison of strain relaxation during in situ annealing studies in a transmission electron microscope between unimplanted and implanted structures reveals that the latter show residual strains substantialy below those for unimplanted structures. We propose that this enhanced relaxation is caused by increased dislocation nucleation probabilities due to the high point-defect concentrations arising from implantation.

A Comparison of Minority‐Carrier Lifetime in As‐Grown and Oxidized Float‐Zone, Magnetic Czochralski, and Czochralski Silicon

Dislocation‐free p‐type float‐zone (FZ), magnetic Czochralski (MCZ), and Czochralski (CZ) silicon wafers with resistivity of ∼50 Ω‐cm were investigated before and after oxidation. Prior to any heat‐treatment, FZ material showed highest minority‐carrier lifetime with lifetimes approaching 5 ms. Lifetime in as‐grown FZ material appears to be limited primarily by point defects generated due to thermal gradient and temperature fluctuation. As‐grown CZ material showed lowest lifetimes of the order of 550 μs because of high concentration of point defects and residual impurities. This paper demonstrates for the first time that, unlike CZ, crucible‐grown MCZ material of resistivity as low as 30–60 Ω‐cm can have a lifetime as high as 2.5 ms. MCZ lifetime is limited by the residual impurities from the crucible and poly, even though the melt convection is reduced considerably. With the starting materials of comparable quality, the MCZ lifetime could be better than FZ because it has fewer defects due to stable liquid/solid interface. After oxidation, both FZ and MCZ samples gave a lifetime of ∼550 μs which could not be explained on the basis of oxidation‐induced point defects, because lifetime was limited by impurities diffused‐in during high‐temperature processing. Unlike FZ and MCZ, CZ material gave a postoxidation lifetime of only 100 μs due to high density of decorated defects, even though the impurity diffused‐in concentration was the same.

Effects of electron and ion irradiation on the dislocation structure of [001] twist boundaries in FeS alloys

Previous work has demonstrated that the dislocation structures of small and large angle [100] twist boundaries in FeS alloys are changed as S is implanted. It was not clear from that work, however, to what extent the radiation damage accompanying the implantation played a role in the change of boundary structure. In the present work the influence of irradiation on the dislocation structures of small angle and large angle (near Σ = 5) twist boundaries in the FeS system was studied using 80 keV S + ions, 80 keV Ar + ions, 1.5 and 0.8 MeV Ne + ions and 800 keV electrons. Electron irradiation produced no change in structure of the boundary in the irradiated region. However, a neighboring region showed a change of structure which is due possibly to diffusion of S related to the high concentration of point defects produced by the electron irradiation (radiation-induced segregation). Ne + ion irradiation also had no effect on the dislocation structure of the boundaries. With a similar mass as S, Ar + ions create a defect structure very similar to that of implanted S +. Ar + ion implantation caused no change in the structure of the large angle boundary, but did transform small areas of the small angle grain boundary. This is substantially different behavior from that caused by S + ion implantation. These results lead to the conclusion that the observed change of structure as a result of implantation of S in the vicinity of [001] twist boundaries in FeS alloys is due primarily to the presence of the S, and that the point defects which are a product of the irradiation may affect only the kinetics of the structural transformation.

Ion Implantation-Induced Amortization of Ceramic Oxides

ABSTRACTA mechanism of amorphization by ion implantation in ceramic oxides is studied using a rate theory approach. It is proposed that the production of highly localized lattice distortions causes lattice destabilization and the ensuing transition to the amorphous state. These distortions can be caused by a large point defect buildup. It is argued that point defect retention occurs because of the impossibility of producing antisite defects. It is proposed that point defects on each sublattice can shield or trap point defects on the other sublattice. Similarly, metallic impurities may shield oxygen vacancies or trap oxygen interstitial ions, preventing anion Frenkel pairs from recombining. These effects are modeled in a-alumina for low temperature implantations (e.g., around 78 K), where point defects are immobile. It is shown that, at these temperatures, recombination is strongly hindered by the radiation-induced point defects themselves, rather than by the implanted impurities. The high point defect concentration attained by this mechanism is sufficient to raise the free energy of the crystal above the free energy of the amorphous solid.

Dislocations in shock-loaded titanium diboride

The structure of shock-loaded polycrystalline titanium diboride was examined with transmission electron microscopy. The shock wave from ballistic impact produces prismatic and basal slip in grains favorably oriented with respect to the shock wave. It can be deduced from annealing experiments with the formation of stacking fault hexagons that there is a high concentration of point defects in deformed regions from the motion of dislocation jogs. Weak-beam microscopy shows that the dislocations in TiB2 are dissociated into partial dislocations. The stacking fault energy measured from a screw dislocation in the basal plane was found to be 120 mJ/m2. Widely dissociated dislocations in the shocked sample suggest that residual stresses are present in some regions.

Lattice imaging study of in-depth disordering of Si-implanted GaAs

Cross-sectional high resolution transmission electron microscopy has been used to obtain direct information on the in-depth radiation damage distribution of weakly damaged GaAs by Si-ion implantation. A comparison is made between the experimental data and the calculated (using TRIM computer simulations) deposited energy by nuclear stopping for the same conditions. In particular a diffusion zone, with 200–300 A width, of high point defect concentration beyond the damage peak is detected. These point defects are interpreted as As interstitials. By direct observation, information concerning the damage- and radiation-enhanced diffusion in implanted III–V compound semiconductors is obtained.

The intermediate ferroelectric phase in PbZrOa with high concentration of defects

Abstract Data is presented on the influence of a d.c. electric field on the temperature dependence of electric permittivity and remanent polarization in PbZrO3 ceramic samples with high concentration of point defects arising during the technological process and formed additionally by heating in a vacuum.

RBS and optical investigations of defects in weakly damaged GaAs

Weakly damaged GaAs layers were investigated by optical transmission and reflection measurements and the Rutherford backscattering (RBS) channeling technique. The samples show characteristic near edge optical properties which are caused by high concentrations of point defects. The analysis of the temperature dependence of the RBS minimum yield points at the existence of slightly displaced lattice atoms with concentrations ≳ 50 at.%. The results suggest, in agreement with those of the optical measurements, that vacancies, antisite defects and/or complexes are responsible for the high concentration of slightly displaced atoms.

A generalized overlap-damage model of amorphous phase and point defect generation during ion implantation into silicon

Abstract A generalized model is presented for describing point defect and amorphous phase generation kinetics during ion implantation into silicon. The model considers the heterogeneity of the damage clusters by dividing them into three different areas: into an amorphous core (a-Si), an inner periphery of high point defect concentration (undetectable by BPR), and an outer periphery of low point defect concentration (detectable by EPR). Weil-known experimental BPR results may be interpreted by this model.

Resonance energy shift for mössbauer impurity atoms trapped by vacancies

AbstractAn estimation of the second order Doppler shift is made for a simple lattice model containing impurities and vacancies by employing the double time Green's function technique at the high temperature limit. The changes in the force constants due to the presence of substitutional impurities and the vacancies near to the impurity are evaluated from the Debye relation and are used to estimate the contributions. It is found that the two contributions (one due to the impurity and the other due to the vacancy near to the impurity) arising from the last two terms of the lattice Hamiltonian yield a negative shift of the absorption line, which is expected to be significant for high concentrations of point defects.

Defect structure and color centers in synthetic fluorochlorapatite

The conditions for the synthesis of the powdery calcium halophosphates fluorapatite, chlorapatite, and fluorochlorapatite, which takes place at T = 1000~ in the air, suggest that there is a high concentration of point defects in the anionic and cationic sublattices of the crystal. Diffusion of the halogen vacancies along the 63 axis results in the formation of complicated defects when they combine with the oxygen ions replacing the halogens. In [i, 2] it was shown that the formation of such complexes in unactivated fluorapatite and chlorapatite takes place according to a mechanism involving charge compensation of vacancies of different sign, which is highly probable from the point of view of the maintenance of the electrical neutrality of the crystal.

Mean field theories of sublattice melting in ionic crystals

A number of materials with essentially ionic binding exhibit a transition in the solid state to a high conductivity state which has been identified with positional disorder of one of the ionic species. In this paper the contrasting roles of the disorder dependence of energy and entropy in determining the nature of the transition are examined within the mean field approximation at high point defect concentration. A generalization of the Mott-Littleton defect theory is used to illustrate the role of various physical characteristics such as polarizability, elastic properties, etc., and two types of phenomenological description of the transition are discussed.

The deformation and twinning of graphite crystals under a point load

Abstract The deformation produced by point loading on the cleaved basal surfaces of graphite crystals has been studied in the scanning electron microscope. The experiments were of two types, viz. sliding and static contact. In the first a small tungsten stylus (tip radius ∼ 1 μm) was slid on the graphite surface inside the scanning electron microscope and the resulting damage observed as it occurred. The marked anisotropy and the limited number of slip systems of the graphite structure result in a characteristic deformation mode which involves the cleavage of thin flakes followed by folding of the partially separated layers. In the second type of experiment the deformation produced by a diamond indenter was studied. These experiments showed that the hardness value of c. 4.8 × 107 N/m2 for naturally-occurring graphite and 22 × 107 N/m2 for neutron-irradiated graphite are determined by a cleavage and folding process and are not related to the basal plane shear strength. It is suggested that the mechanism of folding in both types of experiment is one of mechanical twinning. An important feature of the deformation process is that the dislocation walls forming the twin boundaries are mobile under the applied stress and are free to run in the newly formed chips and flakes in a direction parallel to the basal plane. The greatly increased hardness of neutron-irradiated graphite is explained by the reduction in mobility of the twin boundaries due to the high point defect concentration.

Elastic Constants of Dilute Mo-Re Alloys: A Second-Neighbor Model

The effects of a relatively high concentration of point defects on the elastic properties of a bcc crystal in the Elliott-Taylor approximation have been studied by using a defect perturbation model extending up to second neighbors. Numerical calculations have been performed for two experimentally studied Mo-Re (7 and 7.4%) alloys. It is observed that the changes in the second-neighbor force constants are very small and the perturbation may well be described by a nearest-neighbor defect model. The results obtained in the Elliott-Taylor approximation are seen to be very near to those obtained in the lowest-order concentration theory.

A note on the transformation in non-stoichiometric rutile

Abstract Intersecting, nearly orthogonal faults on {131} planes are formed when rutile is annealed in a reducing atmosphere. These faults have been observed to become aligned at high temperatures and this phenomenon appears to be associated with a mechanism of rutile transformation. It is tentatively coricludetl that the transformation is related to an instability of the rutile lattice containing a high point defect concentration.

The surface structure of silver crystals after argon-ion bombardment

Three single crystals of silver in the form of thin sheets with surfaces parallel to (111), (110) and (100), respectively, have been bombarded with argon ions at several energies in the range 12–4000 eV. Electron-diffraction patterns, taken by transmission and reflection, show short arcs and some streaks not present in the diagrams taken before bombardment. It has been found that after bombardment the surface contains disoriented crystallites. In general, the crystallite orientations are related to the original orientation by tilts about 〈112〉 axes lying in or near the surface of the crystal. It is suggested that the bombarding ions can easily penetrate the lattice if directed parallel to close-packed directions and are arrested near dislocation lines giving rise to a high concentration of point defects. These in turn give rise to a net current of interstitial atoms diffusing away from the primary dislocation. This net transfer of material is thought to be the cause of the tilting of lattice blocks.