Oxygen Point Defects Research Articles

Optimization of carrier mobility in Pr-doped SrTiO3 thin films through controlled Sr-segregation for optoelectronic applications

Functional oxides employed in electronic components hold significant promise for future electronic devices. Strontium titanate based thin films have several emergent features due to unsaturated bonds, dimension restriction, production of oxygen vacancies and point defects that make them attractive for optoelectronic applications. Here we prepared Pr-doped strontium titanate thin films via RF magnetron sputtering in a pure Argon environment at various gas pressures followed by heat treatment. The structural parameters, analyzed via XRD and Raman spectroscopy, were correlated with the electrical transport properties. Intriguing morphological features such as Stranski–Krastanov (SK) growth and cation segregation observed by FESEM and AFM analysis were further examined to elucidate the conduction mechanism in the films. The optical studies unveiled significant transparency in the visible spectrum. Photoluminescence emission studies at specific wavelengths shed light on the involvement of oxygen vacancies. By carefully controlling annealing conditions and optimizing sputtering parameters, thin films devoid of Sr segregation with highest reported carrier mobility of 33.9 cm2/V s were prepared. The high mobility leading to enhanced conductivity render the films suitable for a wide range of optoelectronic applications.

Regulation of oxygen defects in AlGaN-based epilayers grown on high temperature annealed AlN template

The regulation of oxygen (O) point defects in AlGaN-based epilayers grown on high-temperature annealed AlN (HTA-AlN) template by metal–organic chemical vapor deposition (MOCVD) is systematically investigated in this work. The concentration of O impurity in AlN epilayers is restricted to 1 × 1017 cm−3, but decreases by 59 % and 27 % when the AlN layer is replaced by the Al0.5Ga0.5N interlayer (IL) and AlN/Al0.5Ga0.5N superlattice (SL), respectively. By analyzing the growth conditions, the O impurity can be inhibited by raising V/III ratio or decreasing Al component, which increases the formation energy of O point defects according to first-principles calculation. This work provides accurate guidance for regulating O point defects in AlGaN-based materials.

Profiles of oxygen and titanium point defects in ferromagnetic TiO2 films

Experimentally it is shown that without any oxygen manipulation for TiO2, a strong room temperature ferromagnetism could be expected only in ultra-thin films, with the ideal thickness below 100 nm. Both bulks and nano-powders of TiO2 are diamagnetic, indicating that the surface and its nano-sublayers play very important roles in tailoring the magnetic properties in this type of compound. To shed a new light on the defect-related magnetism in the typical case of anatase TiO2 surfaces, we have performed a series of quantum-mechanical calculations for TiO2 slabs containing Ti or O vacancies in different distances from the (001) surface. The lowest formation energies were obtained for the Ti vacancies in the first sub-surface layer and the O vacancies within the surface. The computed magnetic states reflect complicated structural relaxations of atoms influenced by both the surface and vacant atomic positions. O atoms cannot contribute much to magnetic moment when Ti vacancies are isolated and far from the surface. Ti vacancies in TiO2 are only metastable. The formation energy of Ti interstitials is lower than for Ti vacancies since high-temperature annealing, especially with a lot of O2 available that would fill up O-related defects, and as a result, eliminate most of Ti vacancies. Lower temperatures, less O2, and shorter exposure times may enable not only partial elimination of Ti vacancies but also can facilitate their diffusion into different states of aggregations. In the ferromagnetic films (i.e. thin films below 100 nm), it looks like that the O atoms are located closer to the Ti vacancies.

Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO2 Photoanodes for Oxygen Evolution

AbstractAmong the various photoanode materials investigated for photoelectrochemical water splitting cells, TiO2 stands out due to its abundance, stability, and favorable valence band edge for water oxidation. In this study, the importance of introducing and combining oxygen and titanium vacancy point defects in anatase TiO2 photoanodes to improve their performance is unveiled, achieving a photocurrent density of 0.73 (±0.015) mA cm−2 at +1.23 VRHE under 100 mW cm−2 of simulated sunlight or 26.4 mA cm−2 at +1.23 VRHE under 100 mW cm−2 of 365 nm light. The characterization by X‐ray photoelectron spectroscopy, surface photovoltage, and electron paramagnetic resonance demonstrates that these oxygen and titanium vacancies can have both collective and localized positive effects on the material, leading to a narrowing of the bandgap, an increase in donor density, and an increase in hydroxyl groups on the surface of TiO2. These result in enhanced light absorption, conductivity, and photovoltage, as well as a more negative flat‐band potential and increase in hole flux to the semiconductor–electrolyte interface. These findings provide valuable insights into the role of point defects in modulating the properties of TiO2 and have important implications for the development of high‐performance TiO2‐based devices.

A solid-state electrochemical device for studying thermodynamic properties of non-stoichiometric oxides: Application to UO2+x

We report attempts at determining the evolution of the oxygen activity in hyper-stoichiometric uranium dioxide for very low values of deviation from stoichiometry, using a solid-state electrochemical device. To this end, solid-state oxidation cycles coupled with oxygen activity measurements are carried out on a dense polycrystalline uranium oxide sample. We thus follow the evolution of the oxygen activity in the material as it oxidises or reduces, for temperatures ranging from 1093 to 1273 K. The data are subsequently analysed using an oxygen point defect model and equilibrium constants, corresponding to the various supposed point defect equilibria, are determined. We show that the model faithfully reproduces the trends observed in the experimental data. A more detailed analysis of the equilibrium constants obtained enables us to identify more precisely the limitations of the experimental method. In particular, we highlight the fact that the presence of residual oxygen in the device prevents us from determining the desired physical quantities for stoichiometry deviations less than 10−3. Ways of circumventing these difficulties which would further allow us to determine point defect diffusion coefficients are outlined.

Oxygen Non-Stoichiometry and Point Defect Equilibria in (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ

Compositionally complex oxides (CCOs), also referred to as high entropy oxides, have several different cations (typically 5 or more) occupying a single site. CCOs have shown unique dielectric, ionic conductivity, and thermal conductivity properties, in addition to promising electrochemical properties for solid oxide fuel cells (SOFCs) [1,2]. In this work, the A-site of the common SOFC cathode La0.8Sr0.2MnO3-δ (LSM) is modified with additional lanthanide and alkali elements, resulting in the composition (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ. Typically, the average lattice parameter is correlated with bond length and strength, and thus energetics of oxygen vacancy formation energy. In the compositionally complex system, there is a wide range of cation radii on the A-site and so bond strengths are expected to vary significantly, resulting in a distribution of oxygen vacancy formation energies. Initial results have shown that the amount of oxygen released during reduction is significantly greater for the studied CCO in comparison with La2/3Sr1/3MnO3-δ, the latter having the same amount of total acceptor dopant and with a similar lattice parameter as the CCO. The significant enhancement in oxygen loss is an indicator of a lower reduction enthalpy and suggests a significant impact from atomic structure that is not captured in long-range structure parameters (i.e., average lattice parameter). The oxygen non-stoichiometry measured by thermogravimetric analysis in ~10-5 – 1 atm O2 at 1200 oC – 1450 oC will be presented and the point defect equilibria analysis will be discussed. Comparisons with existing high temperature experimental and modeling studies on LSM will also be discussed.SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525References[1] Xu, Y., et al., J. Adv. Ceram., 11 (2022) 794, DOI:10.1007/s40145-022-0573-7[2] J. Dabrowa, et al., J. Mater. Chem. A, 8 (2020) 24455; DOI:10.1007/s40145-022-0573-7

First-principles study on the heterogeneous formation of environmentally persistent free radicals (EPFRs) over α-Fe2O3(0001) surface: Effect of oxygen vacancy

Metal oxides with oxygen vacancies have a significant impact on catalytic activity for the transformation of organic pollutants in waste-to-energy (WtE) incineration processes. This study aims to investigate the influence of hematite surface oxygen point defects on the formation of environmentally persistent free radicals (EPFRs) from phenolic compounds based on the first-principles calculations. Two oxygen-deficient conditions were considered: oxygen vacancies at the top surface and on the subsurface. Our simulations indicate that the adsorption strength of phenol on the α-Fe2O3(0001) surface is enhanced by the presence of oxygen vacancies. However, the presence of oxygen vacancies has a negative impact on the dissociation of the phenol molecule, particularly for the surface with a defective point at the top layer. Thermo-kinetic parameters were established over a temperature range of 300–1000 K, and lower reaction rate constants were observed for the scission of phenolic O-H bonds over the oxygen-deficient surfaces compared to the pristine surface. The negative effects caused by the oxygen-deficient conditions could be attributed to the local reduction of FeIII to FeII, which lower the oxidizing ability of surface reaction sites. The findings of this study provide us a promising approach to regulate the formation of EPFRs.

Oxygen Non-Stoichiometry and Point Defect Equilibria in (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ

In this work, the A-site of the common SOFC cathode La0.8Sr0.2MnO3-δ (LSM) is modified with additional lanthanide and alkali elements, resulting in the composition (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ. We show that the amount of oxygen released during reduction measured by thermo-gravimetric analysis is significantly greater for this “high entropy” perovskite oxide (HEPO) in comparison with La2/3Sr1/3MnO3-δ, the latter having the same amount of total acceptor dopant and with a similar average A-site radius. The origins of enhanced reduction will be discussed.

A Perspective on ferroelectricity in hafnium oxide: Mechanisms and considerations regarding its stability and performance

Ferroelectric hafnium oxides are poised to impact a wide range of microelectronic applications owing to their superior thickness scaling of ferroelectric stability and compatibility with mainstream semiconductors and fabrication processes. For broad-scale impact, long-term performance and reliability of devices using hafnia will require knowledge of the phases present and how they vary with time and use. In this Perspective article, the importance of phases present on device performance is discussed, including the extent to which specific classes of devices can tolerate phase impurities. Following, the factors and mechanisms that are known to influence phase stability, including substituents, crystallite size, oxygen point defects, electrode chemistry, biaxial stress, and electrode capping layers, are highlighted. Discussions will focus on the importance of considering both neutral and charged oxygen vacancies as stabilizing agents, the limited biaxial strain imparted to a hafnia layer by adjacent electrodes, and the strong correlation of biaxial stress with resulting polarization response. Areas needing additional research, such as the necessity for a more quantitative means to distinguish the metastable tetragonal and orthorhombic phases, quantification of oxygen vacancies, and calculation of band structures, including defect energy levels for pure hafnia and stabilized with substituents, are emphasized.

System of excitons bound at oxygen centers and stimulated emission of CdS crystals

The paper presents the results of a study of the optical properties of CdS on the basis of the band anticrossing theory (BAC) with the involvement of the wider source data to the analysis results. Depending on the growth conditions, the presence and change in the concentration of oxygen and intrinsic point defects, which determine the composition of the crystals, have been taken into account. Data from studies of the structural features of crystals have also been used. The concept of nonuniform distribution of oxygen isoelectronic centers in the CdS volume due to their predominant segregation in layers of stacking faults (SFs) and also on dislocations has been introduced. To analyze the optical data, the capabilities of the method for constructing band models, which collect extensive and multilateral information about specific samples have been used. A refined model multizone of [Formula: see text] with SFs, which determines the spectrum of edge emission, has been presented. The nature of the edge emission A2B6 crystals with a small deviation from stoichiometry (CdS type) was explained as a spectrum of bound excitons at oxygen centers in SF layers. The article gives recommendations for the diagnostics of CdS crystals, suitable for creating stable luminescent systems or lasers.

Oxygen point defect stabilized metastable M3‐phase VO2 films

Point defects can stabilize various metastable polymorph phases of complex oxides showing unique physical properties, even though their role in the stabilization of the metastable phase is unclear. In this study, we examined the role of oxygen point defects in stabilizing a polymorph M3-VO2 phase and its transition characteristics. Raman spectroscopy revealed the M3-VO2 phase at the ambient environment for the VO2 film grown on c-Al2O3 substrate at a higher oxygen flow rate. High-resolution XRD and DFT calculation exhibited that the M3-VO2 phase was stabilized by oxygen point defect incorporated into the tetrahedral interstitial site, the most stable defect type in the monoclinic VO2 system. The M3-phase had a higher IMT temperature and a sharper IMT than the M1-phase due to the decrease in interdimer hopping energy and the small energy barrier for the IMT by the slightly zigzag V-V chain. Furthermore, the temperature-dependent Raman spectra showed the unprecedented structural phase transition from the M3-VO2 phase to the R-phase without an intermediate state. Overall, these findings provide fundamental information on the role of oxygen point defects in realizing metastable polymorph phases in promising new technological materials.

Carbon driven oxygen retention in uranium oxycarbide (UCO) fluorite phase

The behavior of oxygen in uranium oxycarbide (UCO) has been investigated using density functional theory. To assess the role of carbon on the stability of oxygen point defects, we first determined the formation energies of oxygen vacancy and interstitial for different carbon configurations. Subsequently, we evaluated the barriers for migration of various oxygen defects in the UCO matrix. Our results show that carbon atoms create strong attraction centers for oxygen atoms, as a consequence the spontaneous formation of oxygen Frenkel pairs in the fluorite structure is promoted. Moreover, the strong affinity of oxygen atoms for sites in the first coordination shell of a carbon center leads to the formation of CO, and thus reducing the mobility of oxygen atoms in the UCO fuel. Upon formation, the CO remains fixed at the carbon site due to significant hybridization with uranium atoms, therefore behaving as trap sites for oxygen atoms. Our findings indicate that the formation of CO molecules increases the retention of oxygen atoms inside the UCO matrix. Consequently, carbon atoms in the UCO matrix contribute to mitigating the deleterious effects that oxygen release from the fuel kernel has on the structural integrity of the silicon carbide layer in tristructural isotropic (TRISO) particles.

Formation of oxygen vacancies in Li-rich Mn-based cathode material Li<sub>1.167</sub>Ni<sub>0.167</sub>Co<sub>0.167</sub>Mn<sub>0.5</sub>O<sub>2</sub>

Using the first-principles method based on the density functional theory, the oxygen vacancy formations in the lithium-rich manganese-based ternary cathode material Li<sub>1.167</sub>Ni<sub>0.167</sub>Co<sub>0.167</sub>Mn<sub>0.5</sub>O<sub>2</sub> are calculated. The changes of oxygen vacancy formation energy with temperature, oxygen partial pressure and point defects in the material are discussed, meanwhile, the effect of oxygen vacancies on the capacity is also discussed. The calculation results show that the increase of temperature and the decrease of oxygen partial pressure can lead the formation energy of an oxygen vacancy to decline. For the charged oxygen vacancies (<inline-formula><tex-math id="M4">\begin{document}$ {\mathrm{V}}_{\mathrm{O}}^{+1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220274_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220274_M4.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M5">\begin{document}$ {\mathrm{V}}_{\mathrm{O}}^{+2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220274_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220274_M5.png"/></alternatives></inline-formula>), the formation energy of an O-vacancy increases with Fermi level increasing. It is also found that the presence of an oxygen vacancy will trigger off a very local charge density redistributions, mainly around the neighboring Mn ions next to the O-vacancy. Furthermore, the effects of point defects, including cation vacancies and substitutional defects in the vicinity of the O-vacancy, on the formation energy of O-vacancy are also calculated. The results show that the presence of Mn vacancy near the O-vacancy is beneficial to the formation of the O-vacancy. In addition, the formation of oxygen vacancy is suppressed when the Mn atoms near the O-vacancy are substituted by the Mo or Fe atoms.

Systematic oxygen impurity reduction in smooth N-polar GaN by chemical potential control

Process chemical potential control and dislocation reduction were implemented to control oxygen concentration in N-polar GaN layers grown on sapphire substrates via metal organic chemical vapor deposition (MOCVD). As process supersaturation was changed from ∼30 to 3400, the formation energy of the oxygen point defect increased, which resulted in a 25-fold decrease in oxygen incorporation. Reducing dislocations by approximately a factor of 4 (to ∼109 cm−3) allowed for further reduction of oxygen incorporation to the low-1017 cm−3 range. Smooth N-polar GaN layers with low oxygen content were achieved by a two-step process, whereas first a 1 µm thick smooth N-polar layer with high oxygen concentration was grown, followed by low oxygen concentration layer grown at high supersaturation.

Computational Design of Pd Nanoclusters and Pd Single-Atom Catalysts Supported on O-Functionalized Graphene

A crucial step in the preparation of supported metal catalysts is related to the choice of support, since it regulates the anchoring of the metal species in an environment of strong interactions. In this sense, the binding on the support, which may present defects, as well as induced modifications of the electronic structure, helps to determine efficient metal/support combinations for catalysis applications. In this work, first-principles studies have been carried out to model and describe the geometric and electronic properties of O-functionalized graphene as a model carbon support, in addition to a single Pd atom and Pd13 nanocluster (experimentally observed on various carbon supports) as supported metal catalysts. The carbon-based nanomaterial includes experimentally probed abundant oxygen functional groups and point defects, demonstrating high thermal stability at room temperature. The characterization confirms the structural motifs presented in the model, distinguishing the organic functionalities and various defects using spectroscopy techniques. Interestingly, the oxygenated carbon-based support presents a metallic character when the formal charges in the embedded Pd atom and nanocluster have been determined to be [Pd]+ and [Pd13]3+. The hydrogenation process of the adsorbed cluster is also presented, with the determination of a ratio between the adsorbed hydrides and the surface metal atoms close to unity at room temperature, thanks to ab initio molecular dynamics.

Effects of different cusp magnetic ratios and crucible rotation conditions on oxygen transport and point defect formation during Cz silicon crystal growth

The effects of different crucible counter- and iso-rotation rates and cusp magnetic ratios (MRs) on the melt flow, temperature, oxygen content, and point defect formation during the growth of a 5-inch diameter silicon crystal are discussed numerically. In cases without a cusp magnetic field (CMF), the oxygen content gets smaller at a low crucible rotation speed than at a high one, for both counter- and iso-rotation conditions. This trend is reversed under a CMF. The flow field is controlled by the crucible rotation rate, the rotation direction between the crucible and the crystal, and the MR. Since the melt flow motion is retarded by the Lorentz force at a high MR, diffusion has a greater effect on oxygen transport in the bulk melt. It promotes the transport of oxygen atoms from the crucible wall towards the melt surface and the growth interface. The point defect density is quite dependent on the thermal field arising from changes in the growth interface shape. Microdefects in the silicon crystal can be reduced in density and enhanced in the radial uniformity using a high crucible iso-rotation rate and high unbalanced MR. A flatter shape of growth interface can be obtained under these growth conditions.

Clustering of oxygen point defects in transition metal nitrides

Point defects create exotic properties in materials such as defect-induced luminescence in wide-bandgap semiconductors, magnetism in nonmagnetic materials, single-photon emission from semiconductors, etc. In this article, oxygen defect formation in metallic TiN and semiconducting rock salt-(Al,Sc)N is investigated with a combination of first-principles density functional theory, synchrotron-based x-ray absorption spectroscopy (XAS) analysis, and scanning transmission electron microscopy–energy-dispersive x-ray spectroscopy mapping. Modeling results show that oxygen in TiN and rock salt-(Al,Sc)N prefers to be in the defect complex of substitutional and interstitial oxygen (nON + Oi) types. While in TiN, the preferential interstitial sites of oxygen in ON + Oi are at the tetrahedral site, in rock salt-(Al,Sc)N, a split interstitial site along the [111] direction was found to be energetically preferable. Simulations performed as a function of the oxygen partial pressure show that under experimental growth conditions, four oxygen atoms at the substitutional sites of nitrogen (4ON), along with four Ti atoms, decorate around an interstitial oxygen atom at the tetrahedral site (Oi) in the energetically favored configuration. However, in rock salt-(Al,Sc)N, n in nON + Oi was found to vary from two to four depending on the oxygen partial pressure. Theoretical predictions agree well with the experimentally obtained XAS results. These results are not only important for a fundamental understanding of oxygen impurity defect behavior in rock salt nitride materials but will also help in the development of epitaxial metal/semiconductor superlattices with efficient thermionic properties.

Point defects in lead sulfide: A first-principles study

The energetic and electronic properties of point defects in lead sulfide (PbS) were studied using first-principles methods. In particular, intrinsic defects including single-site and double-site defects (e.g. Schottky dimers and Frenkel pairs) were considered as well as extrinsic oxygen containing defects. A novel, stable and energetically preferred interstitial site was identified. Convergence of the calculations with supercell size was examined and found to be well-converged for most, but not all, defects in 250 atom supercells. For intrinsic defects, it was found that, after accounting for the chemical potentials of Pb and S in the environment, the lowest formation energies are associated with lead vacancies in S-rich conditions and sulfur vacancies in Pb-rich conditions and not with Schottky defects, as previously reported. Interstitials, Frenkel pairs, and antisite defects were all found to have much larger defect formation energies and are therefore unlikely to be found. The electronic band structure was affected by the presence of intrinsic defects. In particular, for lead vacancies, the Fermi level was shifted below valence band maximum, indicating p-type conductivity; and for lead interstitials, it was shifted above the conduction band minimum, indicating n-type conductivity. In contrast, sulfur vacancies were found to introduce levels deep in the band gap, which may affect the electronic properties significantly. The charged states of the point defects were examined and found to be preferred over the neutral states. The formation energies of oxygen defects were found to be highly competitive energetically with those of intrinsic defects, and therefore, oxygen point defects are expected to play a significant role in determining material properties. In particular, in Pb-rich conditions, oxygen is expected to be drawn into the material to occupy S vacancies.

Solid solution mechanism and thermophysical properties of HfO2-SmTaO4 ceramics

The HfO2-SmTaO4 ceramics were prepared by solid state reaction at 1700 ℃. The XRD shows that HfO2-SmTaO4 ceramics belong to the monoclinic phase (the space group: I12/ (5)). The HfO2-SmTaO4 ceramics doped with 2 % and 4 % mol have a second phase, which is inferred to as Sm0.33TaO7. The solid solution mechanism of HfO2-SmTaO4 ceramics appears to be the replacement of Sm3+ ions with Hf4+ ions. The HfO2-SmTaO4 ceramics have a lower thermal conductivity (1.50 W m−1 K−1 at 800 ℃) than 7–8-YSZ, with a minimum nearly 30 % lower than YSZ. The low thermal conductivity is attributed to the substitution of Sm3+ ions with an equal number of Hf4+ ions; the resulting oxygen vacancy and point defect enhances phonon scattering, reducing the thermal conductivity of HfO2-SmTaO4 ceramics. The thermal conductivity decreases as the doping of the HfO2 is increased, and the experimental and calculated values tend to be similar. The outstanding thermophysical properties indicate that HfO2-SmTaO4 ceramics are potential TBCs.

Dynamic behavior of reversible oxygen migration in irradiated-annealed high temperature superconducting wires

We use atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy to determine the atomic-scale structural, chemical and electronic properties of artificial engineered defects in irradiated-annealed high temperature superconducting wires based on epitaxial Y(Dy)BCO film. We directly probe the oxygen vacancy defects in both plane and chain sites after irradiation with 18-meV Au ions. The plane site vacancies are reoccupied during post-annealing treatment. Our results demonstrate the dynamic reversible behavior of oxygen point defects, which explains the depression and recovery of self-field critical current and critical temperature in irradiation-annealing process. These findings reveal the strong effect of oxygen vacancies in different sites on the superconductivity properties of irradiated Y(Dy)BCO film, and provide important insights into defects engineering of 2G HTS coil wires.