Hopping and optical absorption of electrons in nano-porous crystal 12CaO·7Al2O3

Recently it has been discovered that a nano-porous main group oxide 12CaO·7Al₂O₃ (C12A7) can be converted from a wide-gap insulator to a good transparent conductor. Using ab initio modelling we explain good conductivity of this material by very small barriers for hopping of localised electrons between neighbouring positive cages. We show that optical absorption of C12A7 in infrared region and at energies higher than 2.7 eV is due to inter-cage and intra-cage electron transitions, respectively. The proposed mechanisms can be useful in further search for conducting transparent media.     

SearchFill: A stochastic optimization code for detecting atomic vacancies in crystalline and non-crystalline systems

We present an implementation of a stochastic optimization algorithm applied to location of atomic vacancies. Our method labels an empty point in space as a vacancy site, if the total spatial overlap of a “virtual sphere”, centered around the point, with the surrounding atoms (and other vacancies) falls below a tolerance parameter. A Metropolis-like algorithm displaces the vacancies randomly, using an “overlap temperature” parameter to allow for acceptance of moves into regions with higher overlap, thus avoiding local minima. Once the algorithm has targeted a point with low overlap, the overlap temperature is decreased, and the method works as a steepest descent optimization.Our method, with only two free parameters, is able to detect the correct number and coordinates of vacancies in a wide spectrum of condensed-matter systems, from crystals to amorphous solids, in fact in any given set of atomic coordinates, without any need of comparison with a reference initial structure.     

B-site vacancy induced Raman scattering in BaTiO3-based ferroelectric ceramics

Defects, in particular vacancies, play a crucial role in substituted perovskite systems, influencing the structural features that underpin ferroelectricity. B-site vacancies introduce cation disorder in the perovskite lattice and are in fact one of the main driving forces for relaxor behaviour in barium titanate (BaTiO₃, BT) based ferroelectrics. In this work, material systems are carefully selected to qualitatively study the change in B-site vacancy concentration for heterovalent substituted BT-based ferroelectric polycrystals. Raman spectroscopy was used to investigate those systems, and B-site vacancy specific Raman modes were identified unambiguously by comparison with charge-compensated BT, where B-site vacancies are absent. This study validates the hypothesis that vacancies induce Raman scattering because of symmetry breaking in the BT lattice, establishing this method as a vital tool to study substitutional defects in ceramic materials.     

Improvement of on/off ratio in ZnO thin-film transistor by using growth interruptions during metalorganic chemical vapor deposition

We developed a new method to realize enhancement-mode zinc oxide (ZnO) thin-film transistors (TFT) by metalorganic chemical vapor deposition (MOCVD). We used growth interruptions during MOCVD to encourage complete oxidation of deposited ZnO film, where diethylzinc and oxygen were used as sources. With this method, turn-off characteristics were significantly improved, and threshold voltage was shifted to positive voltages. ZnO TFTs grown at 450 °C showed 10⁷ on/off ratio with 18 cm²/V s mobility, and + 5 V threshold voltage. Our data support that the surface layer is also important in determining ZnO TFT characteristics.     

Electric transport properties of rare earth doped YbxCa1-xMnO3 ceramics (part I: Optimization of ceramic processing)

A comprehensive study was performed in order to find the effect of different calcination and sintering conditions on the physical properties of calcium manganite ceramics in dependence of temperature T and partial pressure of oxygen p(O₂). The eventual formation of oxygen vacancies during sintering was investigated and the results were confirmed by monitoring the release of oxygen using a ZrO₂ oxygen sensor. The phase transition behavior was studied using thermogravimetric analysis (TGA) in a wide range of p(O₂) ≈ 10^?1 MPa down to 10^-19 MPa at high temperatures accompanied by dilatometry- and XRD-measurements. Furthermore, the present study reveals for the first time a way for reducing and preventing crack formation that may occur during sintering. The present systematic research provides essential fundamental information before performing electrical measurements necessary in order to understand important factors about charge carriers and electrical transport mechanism.     

Oxygen vacancies as a link between the grain growth and grain boundary conductivity anomalies in titanium-rich strontium titanate

Titanium-rich (Sr/Ti?=?0.995) strontium titanate (ST) ceramics, air-sintered in a temperature range of 1400–1625?°C, were reported to possess anomalies in the grain growth and analogous anomalies in the grain boundary (GB) conductivity activation energy. However, these two interface-related phenomena, occurring at GBs, could not be associated with each other using a simple “brick-layer” model. In this work we revise the topic and advocate that the deviation from the model comes from the oxygen vacancies localized at GBs of the rapidly-cooled ST ceramics. To verify this, we annealed the ceramics in oxygen and performed their systematic and comparative analysis using impedance spectroscopy. A levelling-off in the GB conductivity activation energy, which increases for ≤1.24?eV, and a four-fold decrease in the GB permittivity are observed after annealing. Thus, we confirm a key role of oxygen vacancies in relation between the grain growth and GB conductivity anomalies of as-sintered Ti-rich ST ceramics.     

Blue photoluminescence from continuous freestanding β-SiC/SiOxCy/Cfree nanocomposite films with polycarbosilane (PCS) precursor

Novel continuous freestanding β-SiC/SiO_xC_y/C_free nanocomposite films, namely, β-SiC nano-crystals in amorphous SiO_xC_y and free C cluster matrix material, were fabricated by melt spinning the polycarbosilane (PCS) precursor. Effects of oxidation curing time and sintering temperatures on the photoluminescence (PL) properties of nanocomposite films were investigated. The PL spectra show two strong blue emissions at 416 nm and 435 nm, which are unchanged neither with oxygen content nor with β-SiC crystallite size. The PL intensity of the films is enhanced by increasing curing time when sintered at 1200 °C. However, a reversed trend is identified after the films were sintered at 1300 °C. Spectroscopy and microscopy studies indicate that the radiative recombination of carriers is ascribed to the oxygen mono- and di-vacancy from SiO_xC_y at the surfaces of β-SiC nano-crystals, whereas the photogeneration of carriers occurs in the β-SiC nano-crystals cores. The obtained results are expected to have important applications in advanced optoelectronic devices.     

Charge trapping during constant current stress in Hf-doped Ta2O5 films sputtered on nitrided Si

Metal-insulator-silicon structures containing Hf-doped Ta₂O₅ dielectric films sputtered on rapid thermally nitrided Si are shown to have very good reliability properties. Stress-induced leakage currents are low, both at low and at high-fields. It is found that charge trapping during the stress is the dominant wear-out mode for very long stress times of 500 s even for injected current densities J_s as high as 100 mA/cm². Stress curves approach saturation at long stress time, indicating that the trap generation rate is very low, even compared to the observed reduced trapping at pre-existing traps.Applying a trapping kinetics model, two trapping sites with characteristic trapping times τ₁ = 3.2 s and τ₂ = 49 s were determined and attributed to pre-existing defects in the bulk Hf:Ta₂O₅ layer and not in the interfacial SiO_xN_y layer. It was found that both τ₁ and τ₂ do not depend on J_s, which may be explained by the presence of a mechanism of charging the active sites through field activated emission of charge from them.     

Investigation of ordering and disordering in B-doped Ni76Al24 by residual resistometry

Homogeneous long-range ordering (LRO) kinetics in Ni₇₆Al₂₄ + 0.19 at.% B was studied by residual resistometry. The LRO relaxations were composed of two simultaneous processes showing dramatically different rates strongly dependent on temperature. The resulting very high activation energy of the processes was in agreement with previous results indicating its correlation with the value of the “order-disorder” transition temperature. The results were compared with the literature data and discussed in terms of the microscopic image of vacancy migration in the L1₂ superstructure.     

高氧空位氧化锌晶体的制备及光催化性能

利用气相沉积法,在低氧气氛下制备高缺陷的ZnO晶体。分别将样品在800℃、900℃、1000℃下通氧退火1h,对ZnO晶体做表面修饰。PL光谱实验和光催化降解亚甲基蓝实验表明ZnO晶体的氧空位、表面态和光催化活性间存在内在联系。1000℃下退火的样品表面缺陷程度多于800℃和900℃下退火样品,光催化活性也优于后两者。