Observation of ferroelectricity induced by defect dipoles in the strain-free epitaxial CaTiO3 thin film

CaTiO₃ is a well-known incipient ferroelectric material that does not undergo a ferroelectric phase transition in spite of the intriguing dielectric constant behavior. Especially, unlike a prototypical incipient ferroelectric SrTiO₃, the paraelectric state of CaTiO₃ cannot be easily destroyed by small perturbations, including cation doping and epitaxial strain. We present that a nearly strain-free epitaxial CaTiO₃ film grown at a low oxygen partial pressure exhibits polarization–voltage hysteresis loops and the distinct difference of piezoresponse force microscopy phase signals, implying that a ferroelectric phase is induced. Such results are shown even at room temperature. We suggest that the observed ferroelectric behavior in CaTiO₃ film comes from the defect dipoles composed of vacancies inside the film. Using electron-probe microanalysis and optical absorption spectra measurements, we found that CaTiO₃ film has considerable Ca and O vacancies, forming the localized defect state in electronic structure. This work highlights the importance of vacancies and their clusters, such as defect dipoles, in understanding the electronic properties of perovskite oxide thin films, including ferroelectricity.     

Kinetics of accumulation of vacancy complexes in dislocated silicon under 60Co γ-ray irradiation

Abstract

Radiation defect accumulation in ⁶⁰Co γ-ray-irradiated n-type Si single crystals (ρ=150ωcm) with various densities of dislocations (N_D = 1 × 10⁴ to 1 × 10⁷ cm ^?2) introduced at plastic deformation was studied. The temperature dependences of the Hall coefficient were measured. The probabilities of interaction of vacancies with oxygen, phosphorus atoms, and dislocation line elements were determined. It has been established that with the increase of N_D they can increase at the expense of complication of dislocation structure, decrease during formation of impurity atmosphere near dislocations and compensation of deformation fields, and they do not change if complex formation of vacancies with impurities occurs far from dislocations. Kinetics of A- and E-centre accumulation in the crystals containing dislocations with different impurity atmosphere was described.     

Diffuse scattering of electrons in the process of ordering of the solid solution of boron in palladium PdB<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> (<Emphasis Type="Italic">y</Emphasis> ≈ 1/6)

The cluster model for the transition state is applied to explain the features of diffuse scattering of electrons, which is observed in the process of ordering of boron atoms and structure vacancies in the cubic (with the B1 structure) solid solution PdB _y of boron in palladium. To simulate the diffuse intensity surface in the case of the short-range substitution order, clusters in the form of tetrahedron, octahedron, cube, and fcc unit cell including 4, 6, 9, and 14 sites of nonmetallic sublattice, respectively, are used. It has been shown that the experimental shape of the diffuse scattering contours in PdB_0.163 is best described by the model in the form of a cluster combining an octahedron and a cube.     

Stability and magnetism of vacancy in NiO: A GGA+U study

Native vacancy defects in typical strongly correlated oxides NiO have been investigated using the GGA+U method. The defect formation energies under different conditions are determined and magnetisms induced by vacancies are studied. Our results indicate that the predominant defect is Ni vacancy under oxygen-rich condition and the most stable ionization state varies with different Fermi level. The Ni vacancy forms shallow acceptor, suggesting the native p-type conductivity originates from the cation vacancy. In addition, after introducing a Ni vacancy, a half-metallic antiferromagnet or half-metallic ferromagnet can form according to different ionization state of cation vacancy.     

Probing the out-of-plane distortion of single point defects in atomically thin hexagonal boron nitride at the picometer scale

Crystalline systems often lower their energy by atom displacements from regular high-symmetry lattice sites. We demonstrate that such symmetry lowering distortions can be visualized by ultrahigh resolution transmission electron microscopy even at single point defects. Experimental investigation of structural distortions at the monovacancy defects in suspended bilayers of hexagonal boron nitride (h-BN) accompanied by first-principles calculations reveals a characteristic charge-induced pm symmetry configuration of boron vacancies. This symmetry breaking is caused by interlayer bond reconstruction across the bilayer h-BN at the negatively charged boron vacancy defects and results in local membrane bending at the defect site. This study confirms that boron vacancies are dominantly present in the h-BN membrane.     

Fast and slow lifetime degradation in boron‐doped Czochralski silicon described by a single defect

A demonstration that boron–oxygen related degradation in boron‐doped Czochralski silicon could be caused by a single defect with two trap energy levels is presented. In this work, the same two‐level defect can describe the fast and slow lifetime decay with a capture cross‐section ratio of electrons and holes for the donor level of σ_n/σ_p = 19 ± 4. A model is proposed for the multi‐stage degradation involving a single defect, in which the product of the slow reaction is a reactant in the fast reaction. After thermal processing, a population of interstitial oxygen (O_i) exists in a certain state (the precursor state) that can rapidly form defects (fast degradation) and another population of O_i exists in a state that is required to undergo a slow transformation into the precursor state before defect formation can proceed (slow degradation). Kinetic modelling is able to adequately reproduce the multi‐stage degradation for experimental data. Dark annealing is also shown to impact the extent of ‘fast’ degradation. By decreasing the dark annealing time on pre‐degraded wafers, a more severe ‘fast’ degradation of the samples can be enabled during subsequent illumination, consistent with this theory. The paper then discusses possible candidates for the chemical species involved. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The effects of O deficiency on the electronic structure of rutile TiO2

Ab initio density functional calculations (plane wave GGA, CASTEP) were performed to determine the effect of O deficiency on the electronic structure of rutile, TiO₂. O deficiency was introduced through either the removal of O or the insertion of interstitial Ti atoms. At physically realistic concentrations of O vacancies in the rutile lattice (i.e. 25% and less) O deficiency results in the population of the bottom of the conduction band, the location of the Ti 3d orbitals in the pure structure, increasingly with increasing vacancy concentration. We propose that this could be confused with the formation and population of gap states especially where O vacancies occur in isolated positions in the lattice. In contrast, Ti interstitials introduce a defect state into the energy gap, without an overall reduction in the size of the energy gap. O vacancies result in a spin polarized solution, whereas Ti interstitials do not.     

Electronic states induced by surface defects on GaSb(110)

Electronic state calculations for point defects on the GaSb(110) surface are presented using a cluster, in order to indicate theoretically the usefulness of the defect model as a mechanism of the Fermi level pinning in Schottky barriers. The results demonstrate that the presence of atomic Ga at surface Sb vacancy sites in addition to surface Ga vacancies gives electronic states localized near the top of the valence band which can be responsible for the pinning observed experimentally.     

Band structure of the superconducting MgB2 compound and modeling of related ternary systems

Band structure of a novel superconductor—magnesium diboride—is studied by the self-consistent FP-LMTO method. Density of states near the Fermi level of MgB₂ and its electronic properties are governed by the metal-like boron 2p orbitals in the planar network of boron atoms. The modification of the band structure of MgB₂ upon doping the boron (with Be, C, N, and O substitutional impurities) and the magnesium (with Be, Ca, Li, and Na substitutional impurities) sublattices or upon the introduction of structural vacancies (boron nonstoichiomety) is analyzed. The electronic structures of MgB₂ and hypothetical CaB₂ are also studied as functions of pressure.     

First principles study of the magnetism driven by cation defects in CeO2: the important role of O2p states

The magnetism driven by cation defects in undoped CeO2 bulk and thin films is studied by the density functional theory corrected for on-site Coulomb interactions （DFT＋U） with U = 5 eV for the Ce4f states and U = 7 eV for the O2p states. It is found that the Ce vacancies can induce a magnetic moment of the -4 gB/supercell, which arises mainly from the 2p hole state of the nearest neighbouring O atom （-1μB on per oxygen） to the Ce vacancy. The effect of the methodology is investigated, indicating that U = 7 eV for the O2p state is necessary to obtain the localized O2p hole state in defective ceria with cation vacancies.     

Effect of the degree of ordering of structural vacancies on the fine structure of the valence-band top in cubic boron nitride

The electronic energy structure of the defect system of c-BN_x with ZnS-type structure is calculated in the multiple-scattering theory by the local coherent potential method. The cluster version of the MT approximation is used to calculate the crystal potential. The effect of the relaxation of the crystal lattice on the electronic structure of nonstoichiometric boron nitride c-BN_0.75 is studied and a comparison is made with the electronic energy structure of c-BN in the same approximation. Fiz. Tverd. Tela (St. Petersburg) 39, 1064–1065 (June 1997)     

Influence of nitrogen doping on the defect formation and surface properties of TiO2 rutile and anatase

Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N 2p states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti 3d band gap states at elevated temperatures. The increased O vacancy formation triggers the 1 x 2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.     

Monovacancy and substitutional defects in hexagonal silicon nanotubes

We present a first-principles study of the geometrical and electronic structures of a hexagonal single-walled silicon nanotube with a monovacancy or a substitutional defect. The B, C, N, Al and P atoms are chosen as substitutional impurities. It is found that the defect such as a monovacancy or a substitutional impurity results in deformation of the hexagonal single-walled silicon nanotube. In both cases, a relatively localized unoccupied state near the Fermi level occurs due to this local deformation. The differences in geometrical and electronic properties of different substitutional impurities are discussed.     

Topological defect motifs in two-dimensional Coulomb clusters

We study the distribution of topological defects in two-dimensional Coulomb clusters with parabolic lateral confinement. The minima hopping algorithm based on molecular dynamics is used to efficiently locate the ground-?and low-energy metastable states, and their structure is analysed by means of the Delaunay triangulation. The size, structure and distribution of geometry-induced lattice imperfections strongly depends on the system size and the energetic state. Besides isolated disclinations and dislocations, classification of defect motifs includes defect compounds-grain boundaries, rosette defects, vacancies and interstitial particles. Proliferation of defects in metastable configurations destroys the orientational order of the Wigner lattice.     

Influencing range of vacancy defects in zigzag single-walled carbon nanotubes

The influencing range of a vacancy defect in a zigzag single-walled nanotube is characterized with both structural deformation and variation in bandstructure. This paper proposes a microscopic explanation to relate the structural deformation to the bandstructure variation. With an increasing defect density, the nanotubes become oblate and the energy gap between the deep localized gap state and the conducting band minimum state decreases. Theoretical results shed some light on the local energy gap engineering via vacancy density for future potential applications.     

Electronic Structures of Wurtzite GaN with Ga and N Vacancies

The electronic band structures of wurtzite GaN with Ga and N vacancy defects are investigated by means of the first-principles total energy calculations in the neutral charge state. Our results show that the band structures can be significantly modified by the Ga and N vacancies in the GaN samples. Generally, the width of the valence band is reduced and the band gap is enlarged. The defect-induced bands can be introduced in the band gap of GMV due to the Ga and N vacancies. Moreover, the GaN with high density of N vacancies becomes an indirect gap semiconductor. Three defect bands due to Ga vacancy defects are created within the band gap and near the top of the valence band. In contrast, the N vacancies introduce four defect bands within the band gap. One is in the vicinity of the top of the valence band, and the others are near the bottom of the conduction band. The physical origin of the defect bands and modification of the band structures due to the Ga and N vacancies are analysed in depth.     

Vacancies and the energy spectrum of refractory metal compounds: TiC and TiO

The influence of structural vacancies in metal and non-metal sublattices on the electronic structure of titanium carbide and oxide is studied by the nonempirical Hartree-Fock-Slater method in the cluster approximation. The main valence band changes for non-stoichiometric compounds are connected with the narrowing of bands due to subtraction of a number of electronic states and an increase of density of states near the Fermi level. Vacancy states appear to be localized in the unoccupied region of the energy spectrum; their admixture in the valence band is very small. The valency of metal atoms is shown to vary continuously in accordance with the stoichiometry ratio. The vacancy charge is very much smaller than the formal ionicity of the atom removed. It is shown that in defect compounds there is essentially no bonding charge connected with the vacancy center.The results obtained are compared with the data of previous calculations and physico-chemical properties.     

Theoretical explorations on the armchair BN nanotube with defects

A systematic study of armchair boron nitride nanotubes (BNNTs) with defects has been carried out within density functional theory. The effect brought by the defects is localized. The defect sites have major contribution to the frontier molecular orbital and change the conductivity of the BNNTs. The defect sites are reactive centers. The substitution of boron with carbon enhances the field emission of the tubes. Doping or vacancy defect creates active center on nanotubes, thus broadening the applications of nanotubes in chemistry and material sciences through functionalization.     

Localized states of acoustic waves in three-dimensional periodic composites with point defects

Acoustic point defect states in the three-dimensional simple-cubic arrays of water spheres embedded in a mercury host are studied. Two kinds of defects are introduced, one is a sphere defect created by changing its radius, and another is a cubic defect obtained by replacing one of the spheres with a cube. The results show that a defect band appears in the band gap of the perfect crystals. The calculations show that the defect modes are localized around the defect. The influence of the filling fraction and the geometry of the defect on the defect modes are investigated in detail.Received: 5 May 2003, Published online: 11 August 2003PACS: 43.20.+g General linear acoustics - 62.60.+v Acoustical properties of liquids - 61.72.Ji Point defects (vacancies, interstitials, color centers, etc.) and defect clusters