Formation Of Defect Complexes Research Articles

Chemically induced volume change of CeO2 − δ and nonstoichiometric phases

Deeper understanding of thermal and chemical volume change in non-stoichiometric CeO2−δ is very important for the development of reactive ceria ceramics applied for thermochemical cycles. Dilatometric studies at 1538K–1690K under reduced and air atmosphere were performed using polycrystalline ceria rods, which were produced from powder synthesized via Pechini method. Based on chemical expansion and weight loss during reduction, the oxygen-deficiency parameter δ ranging between 0.02 and 0.09 was calculated. Above 1650K, however, δ cannot be calculated from weight loss data due to initiating ceria evaporation. Log δ- vs. log pO2 curves reveal a discontinuous development, which is interpreted in terms of defect complex formation. Cooling down CeO2−δ from high temperatures under vacuum conditions results in unmixing and phase transformations according to the phase diagram going along with faint discontinuities in dilatometric curves. Reduced ceria ceramics (δ=0.02–0.09) consist of α-CeO1.985, coexisting with structurally related CeO1.778 and CeO1.714, while the expected β-phase (CeO1.818) was not observed. The combination of dilatometric studies, XRD and thermal analyses on CeO2−δ reoxidation shows a multi-step mechanism. CeO1.778 and CeO1.714 re-oxidize at temperatures of 373K while the transformation from CeO1.985 to CeO2 requires temperatures >900K.

On Space Charge and Spatial Distribution of Defects in Yttria-Stabilized Zirconia

Space charge in 8YSZ is examined theoretically. The Poisson-Boltzmann equation is solved by free energy minimization. The Helmholtz free energy of a crystal containing various defects is minimized subject to two constraints: The total dopant concentration is fixed and the maximum amount of surface charge is limited by site density. Large dopant concentration leads to defect concentrations which deviate from Boltzmann distribution. When free energy is minimized assuming all defects attain their equilibrium distributions, the corresponding electrochemical potentials are equilibrated through the crystal. These calculations correspond to long annealing times. Calculations are also conducted for a case in which only oxygen vacancies are allowed to equilibrate at lower temperature after a high temperature anneal. At lower temperatures, cations are frozen corresponding to high temperature anneal. In this case, only the electrochemical potential of oxygen vacancies equilibrates. In all cases Y segregates to grain boundaries/free surfaces lowering the corresponding oxygen vacancy concentration, consistent with reported effect of space charge on grain boundary conductivity. The results show a significant concentration of oxygen vacancies form defect complexes with Y dopant. Thus, free oxygen vacancy concentration is considerably lower than the usually assumed electroneutrality condition based on total dopant concentration.

Ionic conductivity of acceptor doped sodium bismuth titanate: influence of dopants, phase transitions and defect associates

The temperature dependent ionic conductivity of NBT results from an interplay of defect complex formation, phase coexistence, and dopant concentration.

Dielectric Properties of Eu-Doped CaCu3Ti4O12 with Different Compensation Mechanisms

To get a better understanding of the influence of rare-earth element doping, CaCu3Ti4O12 (CCTO) samples with a partial substitution of Ca with Eu with different compensation mechanisms were designed and prepared by solid-state reaction. All the ceramics were single phase, while the dielectric constants and thermally activated energy values for dielectric relaxation in Eu-doped ceramics were both lower than those of CCTO. Ca0.875Eu0.1Cu3Ti4O12 (CECT1) exhibited a slight decrease in both the permittivity and electric resistance of grain boundaries compared with CCTO, while Ca0.85Eu0.1Cu3Ti4O12 (CECT2) underwent a sharp decrease in permittivity associated with an abnormally large resistance. The different dielectric behavior indicates that the dielectric properties of CCTO are sensitive to the valence states of cations and defects. The variation of permittivity is related to the localization of carriers, which, according to the XPS results, should be caused by the presence of oxygen vacancies. The formation of defect complexes between cations and oxygen vacancies leads to the increase in resistance and prevents the hopping between Cu+ and Cu2+, which is an important source of the polarization in grain boundaries.

Ab-Initio Studies of Acceptor Impurities and Stability of Complexes in Ge

The application of germanium (Ge) as a promising material for complementary metal-oxide semiconductors (CMOS) technology is attracting attention due to its narrow band gap, high carrier mobility and low voltage operations. Recent experimental and theoretical studies revealed that dopants in Ge cluster with vacancies forms with lower energies and contribute to low activation of dopants impurity in Ge. DFT electronic simulation have been used to provide concise information about the formation of defect clusters and complexes that influence diffusion mechanisms in Ge. Our main objective in this study is to use simulations based on ͑DFT to calculate the structural, electronic properties and interactions between acceptors and Ge in different configurations with a view to finding the most energetically stable configuration of various defect complexes in Ge. By means of density functional theory (DFT), we present ab-initio calculation of interactions between A (A: Be,Mg, Ca, Sr, Ba, Ga, In) vacancy complexes (A-VGe) and vacancy-interstitial complexes (A-VGeIA) in two configurations of the hexagonal (H) and tetrahedral (T) in Ge. These calculations employed a projector augmented wave (PAW) pseudopotentials within the generalized gradient approximation (GGA). The geometric structures and formation energies of A-VGe and A-VGeIA in both the T and H configurations for the neutral charge state were obtained. The formation energies of A-VGe were low and energectically favourable with Ga-VGe and Ca-VGe forming with the lowest formation energies at -1.24 and -0.38 eV respectively. For the A-VGeIA, the results of the H configuration were more energetically favourable and forms with lower formation energies than the T configuration. The Ca-VGeICa and Be-VGeIBe forms with the lowest formation energies of -2.59 and -1.49 eV respectively. The stability of the A-VGeIA and A-VGe complexes in Ge were obtained from their binding energies. The binding energies (Eb ) which are defined as the energy required to split up the defects cluster into well separated non-interacting defect is given as Eb = E(formation)(VGe) + E(formation)(AGe) - E(formation){defect-complex}, where E(formation)(VGe), E(formation)(AGe) and E(formation)(defect-complex) are the formation energies of the neutral charge state of the germanium vacancy, interstitials and defect-complexes. The above equation could be interpreted as the energy gain of the bonded structure with respect to the isolated components. Positive binding energies suggest that the defect complexes are stable and cannot easily dissociate. The calculated binding energies of both the A-VGe and the A-VGeIA displayed the ability of these complexes to form without dissociation. For the A-VGeIA, the H configuration forms with more favourable binding energies than the T configuration. In summary the detailed calculated results we have presented are expect to be useful in the process modeling of Ge-based devices.

Low‐Temperature Ionic Conductivity of an Acceptor‐Doped Perovskite: I. Impedance of Single‐Crystal SrTiO3

Low‐temperature conductivity mechanisms were identified in acceptor‐doped SrTiO3 single crystals quenched from high temperatures under reducing conditions. Impedance spectroscopy measurements made on samples of the prototypical perovskite structure doped with iron provided a framework for creating a complete conductivity model for a well‐defined point defect system. The dominant conductivity mechanism in the room‐temperature range was identified as being controlled by oxygen vacancy hopping. The activation energy for oxygen vacancy migration, an often debated value in the perovskite community, is determined to lie within the range of 0.59–0.78 eV for the iron‐doped system with the bottom of this range approaching the intrinsic value for oxygen vacancy hopping in an undoped single crystal. At low temperatures, oxygen vacancies form defect complexes with iron impurities, and the observed range of activation energies is explained and modeled in terms of an oxygen vacancy trapping mechanism.

Extended x-ray absorption fine structure spectroscopy and x-ray absorption near edge spectroscopy study of aliovalent doped ceria to correlate local structural changes with oxygen vacancies clustering

This study provides atomic scale insight to understand the role of aliovalent dopants on oxygen vacancies clustering and dissociation mechanism in ceria system in order to enhance the performance of oxy-ion conductor. Dopants induced microscale changes in ceria are probed by extended X-ray absorption fine structure spectroscopy, X-ray absorption near edge spectra, and Raman spectroscopy. The results are explored to establish a correlation between atomic level structural changes (coordination number, interatomic spacing) → formation of dimer and trimer type cation-oxygen vacancies defect complex (intrinsic and extrinsic) → dissociation of oxygen vacancies from defect cluster → ionic conductivity temperature. It is a strategic approach to understand key physics of ionic conductivity mechanism in order to reduce operating temperature of electrolytes for intermediate temperature (300–450 °C) electrochemical devices for the first time.

The formation mechanism and stability of p-type N-doped Zn-rich ZnO films

Nitrogen-doped Zn-rich ZnO films [ZnO:(Zn, N)] were deposited on quartz substrates using a radio-frequency (RF) magnetron sputtering and ion implantation technique. Hall-effect measurements confirmed that a p-type ZnO:(Zn, N) film with a hole concentration of ~1016 cm−3, which exhibits significantly higher stability than p-type ZnO:N film prepared under non-Zn-rich conditions, is obtained by optimized post-annealing condition. With the help of X-ray photoelectron spectroscopy, Auger electron spectroscopy, Raman spectroscopy (Raman), ultraviolet and visible spectrophotometer and first-principles calculations, it is found that a certain concentration of zinc interstitial (Zni) donor defects which easily bond to substitutional nitrogen (NO) to form defect complexes (denoted as Zni@NO) were observed in the p-type ZnO:(Zn, N) film. Further theoretical and experimental investigations indicate that the relatively stable p-type conductivity of ZnO:(Zn, N) film is attributed to the formation of passive complex (Zni–2NO), which can form an impurity band (IBM) above the valence band maximum, resulting in a decrease in the acceptor ionization energy and an improvement in the stability of p-type ZnO:(Zn, N) film. This p-type formation mechanism is consistent with donor–acceptor co-doping method. Nevertheless, the p-type performance of the ZnO:(Zn, N) film would still gradually decline over time. The remaining interstitial nitrogen atoms (Ni) in p-type film is easy trapped by the acceptor NO to form a dual-donor defect (N2)O, which is one of possible important factors for the eventual instability of p-type ZnO:(Zn, N) films.

Implications of Accelerated Recombination-Active Defect Complex Formation for Mitigating Carrier-Induced Degradation in Silicon

A three-state model is used to explore the influence of the accelerated formation of recombination-active defect complexes on the mitigation of carrier-induced degradation in p-type silicon containing boron and oxygen. Defect formation is observed to be a critical factor for the speed at which carrier-induced degradation can be mitigated. Defect formation also plays a critical role in determining the effectiveness of mitigation at elevated temperatures. It is observed that under conventional conditions, at a processing temperature of $\text{200}\;^\circ{\text{C}}$ , approximately $170\;\text{s}$ are required to form and passivate $99\%$ of possible defects. The experimentally demonstrated improved effectiveness of carrier-induced defect passivation with a process time of $10\;\text{s}$ at temperatures over $\text{300}\;^\circ {\text{C}}$ is consistent with a substantial acceleration of defect formation.

Acceptor doping effects in (K0.5Na0.5)NbO3 lead-free piezoelectric ceramics

In this paper, the effects of acceptor dopants on the synthesis, sintering and properties of KNN ceramics were studied for (K0.5Na0.5)(Nb0.994A0.006)O3−δ (A=Ge4+, Ga3+, Zn2+, Mn2+, Ni2+, and Cu2+) ceramics. The results reveal that the eutectic phase(s) is (are) formed between the acceptor oxide and alkali metal carbonate, thereby significantly improving the sinterability of the materials. Compared to the trivalent and tetravalent acceptor dopants, divalent dopant can lead to higher Qm values, most likely due to the formation of defect complexes.

Determination of the Free Gibbs Energy of Plate-Like Precipitates of Hydrogen Molecules and Silicon Vacancies Formed after H<sup>+</sup> Ion Implantation into Silicon and Annealing

Hydrogen implantation at room temperature into monocrystalline silicon leads to the formation of complex defects and also to the appearance of in-plane compressive stress. During annealing hydrogen atoms and vacancies co-precipitate into platelets lying on two types of habit planes. These platelets play a decisive role in the fracture of the material that can occur during further annealing and which is used for the manufacture of SOI wafers. Thus, their stress assisted nucleation mechanism has to be well understood. Here, we develop a formalism based on the Volmer’s model which allows calculating the variation of the free Gibbs energy of the system following the nucleation of a platelet. In an unstressed crystal, this energy only relies on the habit plane of the platelet. When the system is under stress, this energy also depends on a term coupling this stress and the strain field generated by the platelet. Because those energies control the nucleation rate of the platelets variants, we could calibrate our model using the transmission electron microscopy observations of the platelets occurrences as a function of depth and, thus, as a function of the magnitude of the intrinsic stress and the angles between the stress direction and Burgers vectors of the considered platelets. These experimental distributions allowed us adjusting the parameters describing the Gibbs free energy of platelets.

Clarification of difference for transition between photoluminescence and cathode-luminescence based on GaMnN

GaN:Mn epilayers were grown on Al2O3 substrate uisng molecular beam epitaxy (MBE) and were subsequently implanted with Mn+ ions (1% and 10%). Photoluminescence (PL) with 1% of Mn showed that optical transitions related to Mn revealed the donor-Mn pair (D, Mn) at 2.5eV and the electron-Mn pair (e, Mn) around 3.1eV, and yellow luminescence (YL) around 2.20–2.25eV. Photoluminescence (PL) with 10% of Mn showed the same but enhanced optical transitions as above. However, the new transitions around 1.65eV for the sample with 10% which did not appeared with Mn of 1% were very weakly produced. The results of cathode-luminescence (CL) with 10% of Mn showed transitions related to Mn in PL together with new transitions around 1.72eV. However, the new transitions around 1.72eV for the sample with 10% according to high accelerating voltage were very remarkably activated in contrast with PL transitions which appeared were very weakly produced in samples with Mn of 10%. Transitions around 1.72eV in CL correspond to though around 1.65eV in PL. This result means that deep donor (probably, VN) is detected with increasing accelerating voltage and Mn–VN complex is formed. This is supported by strong electron beam sensitivity of the IR emission bands. It is well known that heavy Mn doping (>∼1019Cm−3) leads to a downshift of the Fermi level and promotes the formation of defect complexes of Mn–VN. In our case, Mn doping concentration is >∼1021Cm−3. Therefore, it is conjectured that the CL transition around 1.72eV corresponds to Mn–VN complex.

The study of optical property of sapphire irradiated with 73 MeV Ca ions

Single crystals of sapphire were irradiated with 73MeV Ca ions at room temperature to the fluences of 0.1, 0.5 and 1.0×1014ions/cm2. Optical properties of these samples were characterized by ultraviolet–visible spectrometry (UV–VIS) and fluorescence spectrometer (PL). In UV–VIS spectra, it is observed the absorbance bands from oxygen single vacancy (F and F+ color centers) and vacancy pair (F2+ and F22+ color centers). The oxygen single vacancy initially increases rapidly and then does not increase in the fluence range from 0.1 to 0.5×1014ions/cm2. When the fluence is higher than 0.5×1014ions/cm2, oxygen single vacancy starts to increase again. Oxygen vacancy pair increases monotonically with fluence for all irradiated samples. The variation of oxygen single vacancy with fluence is probably associated with the recombination of oxygen vacancies with Al interstitials and complex defect formation (such as vacancy clusters). From PL spectra, two emission bands around 3.1 and 2.34eV are observed. The PL intensity of the emission band around 3.1eV decreases for all the irradiated samples. For the emission band around 2.34eV, the PL intensity initially decreases, and then increases with fluence. Meanwhile, the peak position of the emission band around 2.34eV gradually shifts to high energy direction with increase of fluence. The decrease of the intensity of the emission bands around 3.1 and 2.34eV could be induced by stress from the damage layer in the irradiated samples. The shift of peak position for the emission band around 2.34eV is induced by the appearance of emission band from Al interstitials.

Modelling of point defect complex formation and its application to H+ ion implanted silicon

The diffusion and interaction of impurity atoms in semiconductors play an important role in modelling of the technological processes for device fabrication. Being mobile, impurity atoms, vacancies and interstitials can recombine and/or precipitate in the form of stable complexes which leads to the modification of target material properties. Here, we present an analytic model that predicts the concentrations of such complexes as a function of point defect concentrations using the probabilities for point defects to encounter and the probabilities for the formation of specific complexes dependent on their formation energies. This approach is general and can be used in different systems. We applied this model to the formation of different complexes after H+ implantation in silicon at room temperature. The formation energies of the complexes were calculated using the density functional theory. We linked the macroscopic strain measured in the implanted crystal to the individual deformation fields generated by the different complexes and to their concentrations. Such model calibration allowed determining the diffusion coefficients of silicon vacancies and interstitials at room temperature, the time required for the formation of all the complexes, the concentrations of complexes as a function of H concentration and the specific role of some complexes in generating strain. The model can be extended to the case of the systems co-implanted with different ions and can be applied for developing the SmartCut® technology used for creating innovative substrates.

Kinetics of CO2 Reduction over Nonstoichiometric Ceria.

The kinetics of CO2 reduction over nonstoichimetric ceria, CeO2−δ, a material of high potential for thermochemical conversion of sunlight to fuel, has been investigated for a wide range of nonstoichiometries (0.02 ≤ δ ≤ 0.25), temperatures (693 ≤ T ≤ 1273 K), and CO2 concentrations (0.005 ≤ pCO2 ≤ 0.4 atm). Samples were reduced thermally at 1773 K to probe low nonstoichiometries (δ < 0.05) and chemically at lower temperatures in a H2 atmosphere to prevent particle sintering and probe the effect of higher nonstoichiometries (δ < 0.25). For extents greater than δ = 0.2, oxidation rates at a given nonstoichiometry are hindered for the duration of the reaction, presumably because of near-order changes, such as lattice compression, as confirmed via Raman Spectroscopy. Importantly, this behavior is reversible and oxidation rates are not affected at lower δ. Following thermal reduction at very low δ, however, oxidation rates are an order of magnitude slower than those of chemically reduced samples, and rates monotonically increase with the initial nonstoichiometry (up to δ = 0.05). This dependence may be attributed to the formation of stable defect complexes formed between oxygen vacancies and polarons. When the same experiments are performed with 10 mol % Gd3+ doped ceria, in which defect complexes are less prevalent than in pure ceria, this dependence is not observed.

Formation and Movement of Cationic Defects During Forming and Resistive Switching in SrTiO3 Thin Film Devices

The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen‐deficient, highly electron‐doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal–insulator–metal structures fabricated from a thin Fe‐doped SrTiO3 (STO) film on n‐conducting Nb‐doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X‐ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect‐rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor‐doped substrate. In particular, the apparent dichotomy between the observation of acceptor‐type defects and increased electronic conductivity in STO is addressed.

Dopants and dopant–vacancy complexes in tetragonal lead titanate: A systematic first principles study

A systematic investigation of dopants in tetragonal lead titanate is presented by screening elements from the third period including K, Ca and all 3d transition metals. Formation energies and equilibrium transition states are determined by means of density functional theory calculations for both cation sites in the perovskite lattice, which allows us to discriminate between donor and acceptor type behavior. The stability of defect dipoles is determined by calculating the binding energy of transition metal-vacancy complexes. The results reveal that the tendency to substitute the Pb-site rather than the Ti-site monotonically increases going from Ti to Zn. The transition from Ti to Pb substitution depends both on the chemical equilibrium conditions and the position of the Fermi energy. This is most evident for Sc and Zn dopants that in principle can occupy both Pb- and Ti-sites depending on preparation conditions. Except for V all acceptor dopants form defect complexes with oxygen vacancies and thus can form defect dipoles causing hardening as well as aging effects. Defect dipoles involving Pb substitution and oxygen vacancies are found to be unfavorable for all dopants considered here.

Structural and electrical properties of Li-doped TiO2 rutile ceramics

Li doped TiO2 rutile-type (TL) polycrystalline ceramics with Li concentration in the range 0≤x≤0.02have been synthesized by a solid state reaction method. The structural properties were investigated using an x-ray diffraction (XRD) technique. The dc conductivity (σdc), obtained from impedance spectroscopy, has been investigated over the temperature range of 400–750°C. The activation energy (Ea) of the samples was calculated from the absolute temperature dependence of the dc conductivity. The physical parameters, such as the lattice parameters, dc conductivity and activation energy, show changes in their behavior with increasing Li concentration. Such observed behaviors can be interpreted in terms of two different defect reaction mechanisms. For instance, for x≤0.01 the Li incorporates into interstitial sites, whereas for higher concentrations (0.01<x≤0.02) the Li+ substitutes the Ti4+ ion in the tetragonal structure, favoring the formation of complex defects.

Modelling of degradation/recovery phenomena in CdS/CdTe ultrathin film solar cells

The degradation/recovery phenomena in ultrathin film solar cells based on CdS/CdTe are theoretically analysed using Sah–Noyce–Shockley theory for generation and recombination in the depletion region. This theory can explain the overlap of the depletion regions at both front and back contacts where the carrier generation and collection are as important as recombination mechanism. The value of physical parameters such as uncompensated defect density, carrier recombination lifetime and band bending at interface are critically important when reducing the thickness of CdTe layer down to sub-micron. The rollover, materials inter-/out-diffusion, complex defect formation and the role of mobile ions are taken into consideration to obtain an insight into the physics of degradation/recovery phenomena in ultrathin CdTe film solar cells. Both mechanisms are precisely analysed drawing the schematics of the energy band diagrams and mobile ions transport paths which in this case is the grain interior. This means that we neglect the metal diffusion through the grain boundaries which are assumed to be completely passivated. This assumption enabled us to study the role of the defects on the carrier transport in the interiors rather than through the boundaries.

Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film

Na0.5Bi0.5Ti1−xFexO3−δ (abbreviated as NBTFex, x=0.005, 0.01, 0.02 and 0.04) thin films were deposited on indium tin oxide/glass substrates via a chemical solution deposition method. The influence of Fe2+ doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 was investigated. All the films crystallize into the single perovskite structure without any secondary phases. The insulating measurement reveals that 2at% Fe2+ doping can effectively reduce the leakage current of Na0.5Bi0.5TiO3 thin film by the formation of defect complexes. The enhanced ferroelectricity is obtained in NBTFe0.02 with a large remanent polarization (Pr) of 20μC/cm2 due to the lowest leakage current and the distortion of TiO6. The normalized capacitance–voltage curves of all films agree with the polarization-electric field hysteresis loops. Also, the relative dielectric constant and dissipation factor of NBTFe0.02 thin film are 450 and 0.094 at 100kHz, respectively.