Papers by Keyword: Vacancy

Abstract: Precursor phenomena of melting in pure metals (In, Pb, Bi and Sn) and alloys of the systems Pb-Bi and In-Sn with different compositions have been investigated by means of Mechanical Spectroscopy (MS), i.e. dynamic modulus and damping measurements. MS tests evidenced that a sharp drop of dynamic modulus E takes place in a temperature range ΔT before the formation of the first liquid: the modulus variation ΔE and the corresponding temperature range ΔT depend on the specific metal or alloy. The modulus drop is consistent with a relevant increase of interstitial concentration (self-interstitials assuming the dumbbell configuration), as predicted by the Granato’s theory of melting. The increase of damping in the same temperature range of modulus drop supports this explanation. Owing to their dumbbell configuration self-interstitials interact with the flexural vibration of samples and the periodic re-orientation under the external applied stress leads to energy loss and damping increase. The increase of self-interstitials has the effect to weaken interatomic bonds (modulus drop) and favours the collapse of crystal lattice (melting).

Abstract: The paper presents the results of both ab initio and thermodynamic analysis of vacancy and divacancy formation and hydrogen interaction with them in alpha (bcc) iron. Ab initio calculations were performed by DFT method using LAPW in WIEN2k package. Monovacancy formation energy was found to be 2.15 eV and divacancy binding energy 0.22 ± 0.01 eV. Equlibrium fraction of vacancies bound into divacancies is of the order of 10^–5 even at the highest temperatures close to bcc → fcc transformation point. Hydrogen has a strong interaction with monovacancies (vacancy-hydrogen binding energy decreasing from 0.60 to 0.31 eV for the first–fifth H atom inside a single vacancy) but has only a small effect on divacancy formation energy that is equal to 0.28, 0.19 and 0.17 for the case of joining of VH + V, VH + VH and VH₂ + VH₂, respectively. This means that the presence of hydrogen cannot significantly increase the equilibrium concentration of divacancies.

Abstract: In binary systems Kirkendall shift is a well-known phenomenon. We investigated nanoscale diffusion in the framework of a recently published continuum model [Erdélyi and Schmitz, Acta. Mater. 60 (2012) 1807]. In thin films the usual vacancy creation and annihilation mechanisms, leading to the Kirkendall shift on larger scales, cannot operate in the same way. On this length-scale the characteristic distances between vacancy sinks and sources can be comparable to the dimension of the sample, causing differences in the development of the Kirkendall effect. Our group recently reported results in simulating nanoscale Kirkendall shift. In present work we show how using conventional method for velocity reconstruction used in multifoil experiments can be misleading if the distribution of vacancy sinks and sources is not uniform.

Abstract: The article presents the results of first-principle modeling of a defectless (7,7) carbon nanotube and (7,7) nanotubes containing single and double vacancy defects, as well as Stone–Wales defects. These types of defects are often found in real nanotubes and affect their properties. We have established that reliable results can be obtained by using models of more than 1.5 nm in length. It turned out that a single vacancy defect has the least influence on Young modulus, and double n type vacancy defect in the most influential. The elongation at break also depends on the defect type and is 30-60% less than for perfect tubes.

Abstract: The vibrational behavior of defected graphene sheets was investigated via finite element analysis. The simulations were carried out for perfect and imperfect nanosheets. This study was conducted to examine the influence of vacant sites on these nanostructures. In the current study, a graphene sheet is considered as a space frame. The natural frequency and corresponding mode shapes of the perfect and defective nanosheets were obtained and compared. Results are presented as diagrams stating the natural frequency of graphene sheets with respect to the amount of vacancy defects. The results indicate that the natural frequency of nanosheets reduced by introducing atomic defects to the configuration of these nanomaterials. Such impurities lower the vibrational stability of graphene sheets.

Abstract: We investigate the recombination activity of a bulk silicon defect limiting the lifetime of high quality n-type float-zone (FZ) silicon wafers. By isochronal annealing between 200 and 1100 °C, a defect was found to become activated upon annealing at 450–700 °C, causing an order of magnitude reduction in the bulk lifetime. From photoluminescence imaging, it was evident that recombination active circular patterns were present in these low lifetime samples, suggesting the defect (s) originates from the growth conditions of the ingot. When the samples were passivated by SiN_x:H films, a substantial improvement in the bulk lifetime resulted, which we postulate occurred due to hydrogenation of the bulk defects. In contrast, when the samples were annealed at high temperatures (800–1100 °C), the circular recombination active patterns were removed, and the bulk lifetime improved, with the highest lifetime achieved at an annealing temperature of 1100 °C. The experimental results suggest that the defect limiting the lifetime in this FZ material is related to a lattice-impurity defect, which can be permanently annihilated upon annealing at >1100 °C.

Abstract: We observed for the first time the thermally stable point positron-sensitive center of a vacancy type in n–FZ–Si (P) material irradiated at RT by ~ 0.9-MeV electrons. The center that emerges after isochronal annealing at T_anneal.≈ 260 – 280 oC is found to be similar to the vacancy-group-V-atom complex revealed in the same Si material irradiated by 15-MeV protons; the detecting of the centers by the positron trapping is finalized at T_anneal.≥ 520 oC. The annihilation gamma-quanta to be emitted from the positron trap gives rise to a characteristic positron lifetime τ₂ (I₂ ~ 38–19 %) ≤ 276 – 294 ps which is somewhat longer than the one predicted for unrelaxed single vacancy τ_V.≈ 254 – 261 ps. Our data suggested a configuration of the complex V_opPV_op, wherein the atom of phosphorus is tied to a split open vacancy volume 2V_op. It is argued that V_op volume detected by the positron trapping may be formed by extended semi-vacancy, V_s-ext , or by the relaxed inwards vacancy, V_inw , thus resulting in a distorted V_s-extPV_s-ext or V_inwPV_inw configurations.

Abstract: The spectrum of defects produced by 5 MeV electron irradiation in oxygen-lean p-type silicon strongly contaminated with interstitial copper (Cu_i) is studied using the deep-level transient spectroscopy. It is observed that the room-temperature irradiation creates a large amount of Cu_PL centers (complexes including one substitutional and three interstitial Cu atoms). The analysis shows that this process is govern by formation of the substitutional copper atoms due to the direct reaction between irradiation-induced vacancies and mobile Cu_i species. This reaction consumes nearly all irradiation-induced vacancies and affects strongly the standard spectrum of radiation defects.

Abstract: An experimental method is defined that reduces the thermal conductivity in Si films by ~90 % compared to control samples, while keeping the thermoelectric power factor almost unchanged. This is done by creating vacancy-rich films via high-energy self-implantation of Si, followed by rapid-thermal annealing. TCAD simulations suggest that this approach is scalable for application in thin-film thermoelectric generators, as an alternative to more expensive and less Earth-abundant materials such as bismuth telluride. This approach to Si thermoelectrics could be straight-forward for scale-up to thin-film device dimensions, something that is a major challenge for other methods used for Si thermal conductivity reduction.

Abstract: Rapid thermal annealing (RTA) of Czochralski silicon wafers at around 1260°C installs a depth profile of some vacancy species. Subsequent oxygen precipitation in such wafers is vacancy-assisted. The data on RTA-installed vacancy profiles - and the corresponding precipitate density profiles - suggest that there is a slow-diffusing vacancy species (V_s) along with two fast-diffusing species: a Watkins vacancy (V_w) manifested in irradiation experiments and fast vacancy (V_f) responsible for the high-T vacancy contribution into self-diffusion. The V_s species are lost during cooling stage of RTA, and the loss seems to occur by conversion of V_s into V_f followed by a quick out-diffusion of V_f. A model based on this scenario provides a good fit to the reported profiles of oxide precipitate density in RTA wafers for different values of T_RTA and different cooling rates.