Papers by Keyword: Vacancy

Abstract: Utilizing positron annihilation spectroscopy for studying the energetics, kinetics or charge states of open volume point defects in semiconductors is seldom straight forward. Although obtaining usable experimental results with the technique is usually fairly easy, designing a suitable experiment for a specific case and/or interpreting the results in an unambiguous manner can be challenging. The goal of this lecture is to give advice and suggestions on what to consider when planning experiments with Positron Annihilation Spectroscopy (PAS) in semiconductors, through various example cases. This contribution is not meant to be scientific, rather educational.

Abstract: Tetragonal zirconia (t-ZrO₂) has broad applications for structural ceramics, such as solid oxide fuel cells, electrolysis materials, membranes, and biomedical applications. However, its electronics and magnetics properties are rarely well studied. We carried out spin-polarized calculations to understand the vacancy characteristics in t-ZrO₂. We apply neutral single Zr vacancy (V_Zr) and O vacancy (V_O) for supercell calculations up to 108 atoms. The calculated band gap of t-ZrO₂ is 3.81 eV, which becomes a narrowing band gap by V_Zr. We find that both vacancies systems induce outward relaxations of atoms near vacancies, creating lower symmetry from pristine D_4h to D_2d and C_2v for V_Zr and V_O cases, respectively. The calculated magnetic moment V_Zr is 4 mB due to four electrons of the valence band being occupied in the majority state. Four oxygen electrons of p-orbitals dominate the spin densities near V_Zr. In contrast, the magnetic moment in the case of V_O is 0 µB. Thus, since they have a high magnetic moment in the case of V_Zr, t-ZrO₂ is potentially used as material for diluted magnetic semiconductors.

Abstract: In order to increase productivity and reduce the cost of wafers, we have developed a high temperature chemical vapor deposition (HTCVD) method that can realize the high-speed growth of 4H-SiC crystals. Tokuda et al. reported an interesting study in which the carrier lifetime of a substrate grown by HTCVD (HTCVD substrate) was considerably shorter than that of the substrate grown by physical vapor transport (PVT); moreover, bipolar degradation was highly suppressed when the HTCVD substrate was applied to PiN diodes [1]. Herein, we demonstrate that the short carrier lifetime of the HTCVD substrate is mainly attributable to the carbon vacancy (V_C) and that V_C particularly diffuses from the HTCVD substrate to the epitaxial layer.

Abstract: The discovery of Carbon Nanotubes (CNT) has opened the doors for revolutionary applications in the mechanical, aerospace, and electrical sectors. However, to fully utilize the potential of carbon nanotubes, there is a persisting need to identify all sorts of structural modifications that can be observed in any type of manufacturing procedure for CNTs. Thus, the presented study investigates the mechanical properties of CNTs with variable waviness and defect density. Furthermore, the study is performed using classical Molecular Dynamics simulations (MD). The structures are then characterized with single or multiple vacancy defects along the axis of the nanotube structure, which is modeled as wavy structures to replicate their natural structure. After the simulation results were analyzed, it was observed that the increase in the surrounding temperature from 300K to 1500K reduces the overall tensile strength of the CNT sample from 89-47 GPa. However, introducing a single vacancy defect to the same structures was shown to reduce the tensile strength to 41 GPa at 1500K and 62 GPa at 300K.

Abstract: The sensing characteristics of pristine, Ni-doped, and C-vacancy graphene towards CO and NO₂ gas molecules were studied using density functional theory (DFT). The adsorption energies, electronic properties, charge transfer, and stable geometries were calculated to evaluate the gas-surface interaction mechanisms. Both pristine and vacancy graphene have smaller CO and NO₂ adsorption energies and charge transfer than the Ni-doped graphene, whereas the adsorption energy on Ni-doped vacancy graphene is higher than that of Ni-doped graphene. The results indicate that both CO and NO₂ gas molecules only attach to pristine graphene through weak physical adsorption. Stronger chemisorption occurs when the gas molecules adsorb on the surface of vacancy, Ni-doped, and Ni-doped vacancy graphene. Additionally, the results demonstrated that Ni-doped vacancy graphene has higher sensitivity and selectivity towards the NO₂.

Abstract: We report the possible existence of defect recovery and the magnetic behavior in the T’-Pr_2-xCe_xCuO₄ (T’-PCCO) with x = 0.10 nanoparticles through the partially reduction annealing process. The powders of T’-PCCO nanoparticles were synthesized by using the chemically dissolved method followed by partially reduction annealing at 700 °C for 5 h in argon atmosphere. The high-resolution neutron powder diffraction (HRPD) technique has been employed to study the nuclear structure, vacancy, and the magnetic properties of the T’-PCCO nanoparticles. It is found that there is an increase of oxygen occupancy at the in-plane oxygen, O(1), and the apical oxygen, O(3), which signifies the decrease of the number of the vacancy on their sites. Meanwhile, the out plane oxygen, O(2), seems to be unchanged in the partially reduced samples. The Fourier difference profile shows an enhancement of the neutron scattering density at all the critical sites of O(1), O(3), and the Cu site. This may lead to the idea of the defect recovery affecting the whole magnetic moments which is responsible for the absence of weak ferromagnetism in the T’-PCCO nanoparticles.

Abstract: Because the well-known site-competition and step-controlled epitaxy rules cannot reasonably describe all the incorporation processes of the main impurities (Al and N) into 4H-SiC during epitaxy, the concept of replacement incorporation was proposed and applied to explain the experimental results published so far. In this model, the transient formation of C or Si vacancies at the surface or sub-surface of terraces is proposed to play a key role by destabilizing the impurities sitting on them. In addition to the availability of these vacancies at the surface, desorption was proposed to be a very important limiting process for Al incorporation while only occasionally relevant for N incorporation. The main 4H-SiC epitaxial growth parameters are reviewed and discussed according to the proposed replacement model.

Abstract: In this work, the effect of different inhibitors on the thermal stability of the magnetic properties in Fe_73.5Cu₁M₃Si_13.5B₉ nanocrystalline alloys, where M = Nb, W, Mo, was investigated. Nanocrystalline alloy with tungsten has the greatest thermal stability. The change in the magnetic properties in the ageing process was associated with vacancies and vacancy clusters, the formation of which is facilitated by large atoms of inhibitory elements occupying free positions in the substitution solid solution.

Abstract: In order to provide an accurate prediction of the flow stress during the thermal deformation process, a physical model is established. In the model, it is assumed that the flow stress is mainly composed of short-range stress and long-range stress. The short-range stress is a friction stress needed to be overcome when dislocations pass short-range obstacles and through the lattice, and the long-range stress is athermal stress caused by the interaction of dislocation substructure. The model is established mainly based on the evolution of dislocation density which is described as a competitive process of work hardening and recovery. Meanwhile, the interaction between vacancies and dislocations is also taken into account. The effect of solutes and precipitation on stress is quantified. In addition, some experiments have been performed using two steels containing different amounts of Nb under various deformation conditions. The experimental results indicate the prediction accuracy of the model is satisfactory.

Abstract: The evaluation of the necessary duration of a molecular dynamics experiment for the calculation of the diffusion coefficient at migration of different point defects in Ni (vacancy, bivacancy, self-interstitial atom, hydrogen atom) is held in the present work. It is shown that at the temperature higher than 0.6 of melting point is usually enough the simulation during of 100 ps for this. When calculating of the diffusion coefficient of impurity in the metal crystal, for example, of hydrogen, the decrease of error of mean-square displacements of impurity atoms can be achieved by introducing of a large number of the impurities.