Frenkel Pair Formation Research Articles

Radiation defect distribution in silicon irradiated with 600 keV electrons

Low-doped n-type float zone silicon was irradiated with 600 keV electrons to fluences from 2×10 13 to 1×10 15 cm −2. Radiation defects, their introduction rates and full-depth profiles were measured by two complementary methods – the capacitance deep level spectroscopy and the high-voltage current transient spectroscopy. Results show that, in the vicinity of the anode junction, the profile of vacancy-related defect centers is strongly influenced by electric field and an excessive generation of vacancies. In the bulk, the slope of the profile can be derived from the distribution of absorbed dose taking into the account the threshold energy necessary for Frenkel pair formation and the dependency of the defect introduction rate on electron energy.

Investigation of the Effect of Shock-wave Propagation in Metals on Point Defect Production using Thomas-Fermi-Dirac Microscopical Model

The process of high-speed deformation in metals is considered as a propagation of shock-wave with some velocity. The description of this phenomenon is based on microscopic Thomas-Fermi-Dirac and so called "jelly" model of metal with degenerated electron gas and cold ionic subsystem. This model allows to investigate the stationary high-speed deformation structure in metals on microscopic level as a propagation of shock-wave in cold plasma. It is shown that during the shock wave propagation in metals in the shock wave front an effective electrical field is formed which tends to come back the displaced ions to initial equilibrium positions in crystal lattice. This electrostatic energy plays the role of potential energy of ions. The effective electrical field profile and kinetic energy profile for moved ions formed during the short time of shock wave propagation have been obtained. The developed theoretical model allows to suggest a physical mechanism for the explanation of formation of displaced atoms from nodal points during high speed deformation. When the kinetic energy of ions in shock wave front overcomes the threshold displacement energy (the sum of potential energy of ions in electrical field and energy for Frenkel pair formation) the point defects are produced. The obtained results are used to analyze the point defect formation in shock wave front moved with the different velocities.

Radiation-stimulated erosion of field electron emitters

The features of erosion of the surface of point field electron emitters bombarded by helium ions were studied. An analysis of the ion-bombardment-induced atomic-scale surface roughness showed evidence of a nondynamic character of the surface atom displacement for the primary ion energies below a threshold for the stable Frenkel pair formation and cathode sputtering. A quasi-static mechanism of the surface erosion is considered, according to which the process is related to the displacement of metal atoms into low-coordinated positions upon evolution of the energy of formation of interstitial helium atoms.

Modeling and Simulation for Microelectronic Materials Research

Recent development in modeling and simulation for microelectronic materials research in the areas of Cu alloy interconnect is presented. A model for defect generation and diffusion through a Frenkel pair formation process at grain boundaries under electromigration conditions was developed. The model has consistently predicted the activation energies well for all the interconnect materials, namely Al, Al-Cu, Cu, and Cu alloys. A virtual simulation procedure was developed to screen a beneficial dopant to Cu based on the assumption that grain boundary (GB) diffusion dominantly controls the electromigration performance. The procedure covers dopant segregation to GB, dopant bulk diffusion, dopant and Cu diffusion along GB, and the effect of presence of a dopant on Cu diffusion at GB by dragging and blocking (stuffing) mechanisms. The success of the procedure has convincingly shown that modeling and simulation is the first choice to design advanced materials systems for next generation of interconnect.

The influence of an external electric field and irradiation energy on the efficiency of Frenkel pair formation in silicon crystals

indent 0 pt Local irradiation of p-Si and n-Si followed by measurement of the bulk photovoltage along the sample is used to show that the Coulomb interaction energy between unlike charged components of Frenkel pairs is negligible compared to energies imparted to a silicon atom when n-Si is irradiated with electrons having energies of 6–8 MeV. It is asserted that when n-Si is irradiated by 8-MeV electrons, the cascade mechanism for defect formation prevails over the diffusion mechanism.

Defect-induced dissociation ofH2in silicon

Ab initio molecular-dynamics simulations of intrinsic defects and hydrogen in crystalline silicon reveal an unexpected process with considerable implications. The vacancy $(V)$ and the self-interstitial $(I),$ both rapid diffusers in $c\ensuremath{-}\mathrm{Si},$ dissociate interstitial ${\mathrm{H}}_{2}$ molecules with a substantial gain in energy: $V+{\mathrm{H}}_{2}\ensuremath{\rightarrow}{V,\mathrm{H},\mathrm{H}}+4.0\mathrm{eV}$ and $I+{\mathrm{H}}_{2}\ensuremath{\rightarrow}{I,\mathrm{H},\mathrm{H}}+1.7\mathrm{eV}.$ The dissociation of ${\mathrm{H}}_{2}$ is caused by the lattice strain associated with the defect, and occurs whenever ${\mathrm{H}}_{2}$ molecules are in the vicinity of strained Si-Si bonds. After the dissociation, the two H's may either bind to the defect that caused the strain or diffuse away from it. The calculated Frenkel pair formation energy is 8.2 eV. The reaction ${\mathrm{H}}_{2}{+\mathrm{H}}_{2}\ensuremath{\rightarrow}{V,{\mathrm{H}}_{4}}+I$ releases less than 0.1 eV, suggesting that ${\mathrm{H}}_{2}$'s in otherwise perfect Si will not generate intrinsic defects.

A mechanism of fast Frenkel-pair formation by electronic excitation in alkali halides

We analyse the results of femtosecond pump-probe spectroscopy which demonstrate that Frenkel defect pairs are formed in KBr prior to the holes relaxation into their most stable V K centre state. The ab initio Hartree Fock calculations which account for lattice polarisation have revealed that the fast process of the F-H pair formation in KBr prior to the hole self-trapping is driven by instability of the one-centre hole state towards formation of the two-centre state and by the interaction of the relaxing holes with electrons.

Effects of native defects on carrier concentrations in heavily Si-doped and adjoining lightly doped GaAs layers

Generation of native defects during growth of heavily Si-doped GaAs and their effects on carrier concentrations in heavily doped and adjoining lightly Si-doped GaAs layers are investigated. The mechanism of their intrusion from the heavily doped layer into the lightly doped layers is discussed. The behavior of the native defects during annealing after growth is also studied. As Si doping concentration increases, the concentration of triply ionized gallium vacancy (VGa3−), generated by a Frenkel-pair defect formation process, increases. The limit of free-carrier concentration in the heavily doped layers is caused by this VGa and not by electron occupation of a highly localized state of the donor-related DX center. VGa also causes carrier compensation in the adjoining underlayers. However, the carrier concentration in the adjoining overlayer grown on the heavily doped layer is not affected. We infer that drift is the predominant process for VGa flow into the lightly doped layers. This drift is caused by the electric field induced by a surface–Fermi-level pinning, mainly in an early growth stage of the heavily doped layer. On the other hand, the diffusion process of VGa from the heavily doped layer during growth is negligible. Therefore, the carrier concentration in the layers grown on the heavily doped layer is not affected. During annealing after growth the VGa, which is supersaturated in the lightly doped underlayer, disappears as the result of a first-order reaction, so the carrier concentration is recovered. These results not only suggest a carrier compensation mechanism, but are also useful in improving the characteristics of devices consisting of structures with heavily Si-doped GaAs layers.

Radiation defect formation and evolution in C0.03Cr20Ni16Mn6 steel under low-temperature neutron irradiation and their effect on physical and mechanical properties of the steel

Radiation defect formation in austenitic steel under low-temperature irradiation was analysed. Certain approximations were made to calculate the neutron interaction with a multicomponent system, such as austenitic steel, and atomic displacements in cascade regions. These approximations were used to calculate a threshold energy for stable Frenkel pair formation. The level of gas transmutant generation by neutron irradiation was estimated. A qualitative interpretation of the microstructural changes in austenite stainless steel under low-temperature neutron irradiation was formulated. The radiation defect types dominating physical and mechanical properties were considered. A model to describe an influence of these defects on steel strength properties, plasticity, and thermal expansion characteristics was suggested. C0.03Cr20Ni16Mn6 steel property variation under neutron irradiation to the fluence of 1 × 10 23n/m 2 ( E ≥ 0.1 MeV) at 77 K was calculated within the mechanisms selected. The results obtained were compared to data on low-dose mechanical properties, Young's modulus and the thermal linear expansion coefficient of C0.03Cr20Ni16Mn6 steel specimens, irradiated to the fluence 1 × 10 22 n/m 2, 5 × 10 22 n/m 2, and 1 × 10 23n/m 2 at 77 K and tested at the same temperature as well as after heating to room temperature.

Time resolved spectroscopy for radiation damage processes induced by electronic excitation in insulators

We review current studies by means of femtosecond time resolved spectroscopy on defect formation induced by valence-electron excitation in halide crystals. We first summarize the characteristics of several transient relaxed configurations of electron-hole pairs or excitons which play important roles in the defect formation process. Then, the results on the self-trapping process of holes in this material are described to show the specific feature revealed by the femtosecond spectroscopy. Finally, the dynamics of the Frenkel pair formation in alkali halide crystals is discussed based on the results obtained for several crystals at wide temperature range from 5 to 300 K. Emphasis is placed on the new feature of damage mechanism which takes place during the lattice relaxation of the localized electron-hole pairs.

Spatial distribution of the wave function of the self-trapped exciton in AgCl.

The results are presented of a high-frequency electron nuclear double resonance (ENDOR) study of the lowest triplet state of the self-trapped exciton in AgCl. Direct information about the spatial distribution of the wave function is obtained, which confirms that the exciton in its triplet state is built up from a strongly localized self-trapped hole and a very diffuse electron. The electron is mainly contained in a hydrogenlike 1s orbital with a Bohr radius of 15.1\ifmmode\pm\else\textpm\fi{}0.6 \AA{}, but near its center the electron density deviates from spherical symmetry and reflects the ${\mathit{D}}_{4\mathit{h}}$ charge distribution of the hole. From a comparison with the results of an ENDOR study of the self-trapped hole, it is concluded that the microscopic and electronic structure of a self-trapped hole is not significantly altered by the attraction of a shallow electron. No evidence is obtained for the formation of Frenkel pairs in AgCl upon ultraviolet irradiation at low temperatures. \textcopyright{} 1996 The American Physical Society.

Decay of the self-trapped exciton and Frenkel-pair formation in NaF: An ab initio study.

The adiabatic potential-energy surface connecting the triplet self-trapped exciton state to up to the third-nearest-neighbor Frenkel pairs is calculated for ${\mathrm{Na}}_{18}$${\mathrm{F}}_{4}$ clusters using an all electron ab initio Hartree-Fock approach. The correlation effect brings out more details, such as the positions of the nearest-neighbor Frenkel pairs, but does not alter substantially the result of unrestricted Hartree-Fock calculation. No potential barrier larger than 0.2 eV is observed up to the third-nearest-neighbor separation, and it is concluded that the self-trapped excitons can evolve dynamically into stable Frenkel pairs. \textcopyright{} 1996 The American Physical Society.

Femtosecond time-resolved spectroscopy of the photo-induced structural changes in alkali halide crystals

Dynamics of the formation of self-trapped excitons (STE's) and Frenkel pairs upon band-gap excitation in alkali chlorides has been studied at temperatures from 6 to 300 K It is found that the relaxation of electronhole pairs can be divided into two distinct stages with different nature Frenkel pairs are formed at both stages, whereas the off-center STE's are generated only at the second stage. Characteristic features of relaxation pathways in chlorides have been discussed on comparing with those in bromides

Dynamics of lithium ions in lithium oxide

Abstract It is known that high ionic conductivity in fluorite-structured materials is associated with thermally created Frenkel-type disorder at temperatures well-below the melting point. Using crystalline Li2O as a model system, we studied the mechanism of formation of Frenkel pairs at temperatures close to the transition to the fast-ionic phase, and ionic motion at temperatures above the transition point. The dynamical behaviour of lithium oxide has been studied using both lattice dynamics within the quasi-harmonic (QH) approximation and molecular dynamics (MD).

Displacement-threshold energies in Si calculated by molecular dynamics.

Molecular-dynamics calculations of simple defect production in bulk Si are reported. The Tersoff three-body potential-energy function is used to provide a realistic lattice-atom interaction potential. The displacement-threshold energy ${\mathit{E}}_{\mathit{d}}$ is calculated as a function of the initial primary knock-on momentum direction, and the formation energies for resulting defect structures are obtained. In addition, we determine the effect of the vacancy-interstitial separation on the anisotropy in ${\mathit{E}}_{\mathit{d}}$, where the criterion for stable Frenkel-pair defect formation is assumed to require a certain minimum separation. The resultant angular dependent ${\mathit{E}}_{\mathit{d}}$ versus (\ensuremath{\theta},\ensuremath{\varphi}) is compared with that proposed by Hemment and Stevens (HS) to account for the magnitude and anisotropy of the observed energetic electron-induced radiation damage in Si. The anisotropy of our ${\mathit{E}}_{\mathit{d}}$ surface is consistent with that of HS provided a lower limit of 4 \AA{} is placed on the initial Frenkel-pair separation for the production of long-lived damage. The relatively large formation energy of 8.4 eV which we obtain for such widely separated vacancy-interstitial pairs indicates that the individual point defects are essentially isolated (i.e., noninteracting). More closely spaced vacancy-interstitial complexes are also identified, which have lower formation energies. While it is argued that these complexes are relatively short lived, they may be relevant to dynamical processes such as beam-enhanced molecular-beam epitaxy, atomic mixing at interfaces, and ``subthreshold'' damage production.

A comment on a rigid-ion potential for UO2

We report a revision of the rigid-ion potential for UO2 introduced by Sindzingre and Gillan (see ibid., vol. 21, p. 4017, 1988). In its original form, the potential is unstable at the reported lattice parameter. We have re-fitted the potential to remove this problem, whilst keeping as close as possible to the original parametrization. The calculated static quantities are little changed, but there is a small increase in the O Frenkel pair formation energy. In dynamics both the superionic transition temperature and the rate of O diffusion in the superionic state are altered, but there is no effect on the calculated thermal conductivity. The conclusions of previous work with the original potential are not affected by the changes we propose.

Individual formation parameters of charged point defects in silver chloride.

The analysis of the original measurements of the subsurface electrical potential in oriented single crystals of AgCl [Hudson, Farlow, and Slifkin, Phys. Rev. B 36, 4651 (1987)] has been corrected for two previously neglected effects. The resulting values for the surface potential ${\mathrm{\ensuremath{\Phi}}}_{\mathit{S}}$ have been reanalyzed to resolve the Frenkel-pair formation enthalpy and entropy into the contributions from the two distinct defects comprising the pair. It is shown that the effects of divalent impurities and of impurity-vacancy association are important even for temperatures that appear intrinsic in Arrhenius ionic-conductivity plots. The new values of the formation free energies are somewhat different from those of the earlier results, and the decomposition of these into the formation enthalpy and entropy for both the silver interstitial and the silver vacancy yields surprisingly different results. For example, for formation at the (100) surface, ${\mathit{H}}_{\mathit{V}}$, ${\mathit{H}}_{\mathit{i}}$, ${\mathit{S}}_{\mathit{V}}$, and ${\mathit{S}}_{\mathit{i}}$ are 0.12 eV, 1.35 eV, -13.2k, and 23.0k, respectively, apparently involving very large effects on the entropy of phonon frequency relaxations.

Electrical measurements on CuBr

In an attempt to clarify the relative contributions of bulk and interfacial effects to the overall electrical conductivity of CuBr, four-probe van der Pauw-type measurements were performed over an extensive range of temperature, 165–488°C. While our conductivity results agree reasonably well with previously reported data in the low temperature range, T<385°C, characteristic of the γ phase, they are almost an order of magnitude higher for the β phase (385°C< T<469°C). The source of the discrepancy was traced to the irreversibility of the Cu-electrode in contact with the β-phase. In contrast, the Cu-electrode was found to be reversible under low frequency ac excitation for the γ phase. Under dc current conditions, however, the electrodes deteriorated even in the γ phase after removal of approximately 1 μm of Cu from the electrode. The enthalpy of migration obtained from the new conductivity data is 0.35 eV for the β phase and 1.1 eV for the γ phase. The Frenkel pair formation enthalpy is neglegible in the β phase, and 1.50 eV in the γ phase.

Effect of 58Ni MeV ion irradiation on the resistivity of Bi 1.7Pb 0.35Sr 2Ca 2Cu 3O 10+x at low ion fluence

Heavy ion 58Ni MeV ion irradiation at room temperature of high T c Bi 1.7Pb 0.35Sr 2Ca 2Cu 3O 10+ x is reported. We show through in-situ resistivity measurements that at low fluence, formation of stable Frenkel pairs takes place together with possible defect annealing. The changes in T c at low fluence is not appreciable. This is possibly a result of chemical substitution of Cu by Ni, a technique suitable to engineer new materials for potential applications.

Formation and direct recombination of Frenkel pairs in silver bromide

The frequency dependent conductivity of solid silver bromide has been measured in the 100 MHz to 60 GHz frequency range, at different temperatures. The microwave conductivity is found to be dominated by a thermally activated Debye-type relaxation process. The relaxation is consistently explained in terms of the frequent formation and direct recombination of Frenkel pairs.