Formation Of Defect Complexes Research Articles

Enhanced electrical properties in Bi(Fe0·95Mn0·05)O3 modified Na0·5Bi0·5TiO3 thin film

0·7Na0·5Bi0·5TiO3–0·3Bi(Fe0·95Mn0·05)O3 (NBT–BFOMn) and (Na0·5Bi0·5)TiO3 (NBT) thin films were fabricated on LaNiO3(l 00)/Si substrate by metal organic decomposition. Both films crystallise into the single perovskite structures with strong (l 00) diffraction peaks. The leakage current of NBT–BFOMn can be suppressed due to the decrease in the content for oxygen vacancies and the formation of defect complexes. Compared with the round shaped polarisation–electric field loop for pure NBT, much improved ferroelectric property with a remnent polarisation Pr of 28 μC cm−2 and coercive field Ec of 152 kV cm−1 at 630 kV cm−1 has been obtained for NBT–BFOMn thin film. In addition, the higher change of relative dielectric content ϵr variation versus voltage indicates the enhancement in the ferroelectric nature of NBT–BFOMn thin film. The ϵr and dissipation factor tanδ on frequency for NBT–BFOMn show small dispersion tendency with ϵr of 370 and tanδ of 0·167 at 100 kHz.

First-principles study of native defects in LiTi2O4

In this study, we report on a fundamental first-principles investigation of native point defects in spinel LiTi2O4, an alternative electrode material for lithium ion batteries (LIBs). Under O-rich conditions, Ti vacancy (VTi) and Li antisite (LiTi) are energetically favorable, but Ti interstitial (Tii) has highest formation energy and is hence unstable. While Li interstitial (Lii) is the most stable configuration in O-poor conditions, and O vacancy (VO), Ti interstitial (Tii), Li antisite (LiTi) and Ti antisite (TiLi) have almost the same formation energies, suggesting spontaneous formation of defect complexes. According to the calculated electronic structures of defects, no localized states are found in the gap between the O 2p and Ti t2g states, which is considerable related to the charge compensation process between O and Ti ions. Besides, Li vacancy (VLi) and Li interstitial (Lii) are the most stable defect types with negligible structural changes, while other defects will induce large octahedral disorders and suppress Li ions mobility. These results not only reveal the original defect information of LiTi2O4, but also provide practical guidance to the development of this material.

Defect structure and chemical bonding of p-type ZnO:Sb thin films prepared by pulsed laser deposition

X-ray photoelectron spectroscopy and x-ray diffraction were used to understand the local environment of Sb in pulsed laser deposited ZnO:Sb films, which switched to p-type electrical conductivity from as-deposited n-type behavior after being annealed at 600 °C under an O2 flow. Tensile in-plane stress was observed in the as-deposited n-type films, and its magnitude increased after the annealing induced p-type switch. This stress can be explained by larger Sb sitting on smaller Zn sites in both the as-deposited and annealed films. Annealing resulted in oxygen incorporation and a more electronegative environment for the cations, as indicated by shifts to higher binding energy of both the Zn2p and Sb3d XPS peaks. The XPS results also show the formation of Sb2O5 in the p-type samples, and indicate a reorganization of the local coordination of Sb. Based on these results and density functional theory computations of the formation and ionization energies of defect complexes, we propose that acceptor complexes are responsible for the p-type conductivity.

Role of oxygen vacancies in room-temperature ferromagnetism in cobalt-substitutedSrTiO3

Ferromagnetic insulating behavior has recently been demonstrated in cobalt-substituted ${\mathrm{SrTiO}}_{3}$ at room temperature [Posadas et al., Phys. Rev. B 87, 144422 (2013)]. Experimentally it was found that a well-defined hysteresis loop only occurs at high Co concentrations of 30%--40%. X-ray photoelectron spectroscopy also indicated that Co substitutes for Ti with Co being in high-spin +2 oxidation state. In this work we employ density-functional theory to explain the experimentally observed properties of cobalt-substituted ${\mathrm{SrTiO}}_{3}$. We examine in detail the role of oxygen vacancy (OV) defects and their formation of defect complexes with the Co ions as the origin of the ferromagnetic insulating behavior. Our first-principles thermodynamic calculations indicate that OV defects are much more likely to occur next to Co atoms where their formation energies could be reduced by as much as 1.28 eV compared to that in bulk ${\mathrm{SrTiO}}_{3}$. We also find that Co in these Co-OV complexes occurs in the high-spin state in agreement with core level spectroscopy, and identify a linear arrangement of the Co-OV defect complexes to be the most energetically favorable structure. These defect complexes are also shown to interact ferromagnetically and that their magnetic interaction is found to be short ranged, consistent with the relatively high Co concentrations needed experimentally for ferromagnetism to be observed in cobalt-substituted ${\mathrm{SrTiO}}_{3}$.

Abnormal Raman spectra in Er‐doped BaTiO3 ceramics

Greatly enhanced and abnormal Raman spectra were discovered in the nominal (Ba1 − xErx)Ti1 − x/4O3 (x = 0.01) (BET) ceramic for the first time and investigated in relation to the site occupations of Er3+ ions. BaTiO3 doped with Ti‐site Er3+ mainly exhibited the common Raman phonon modes of the tetragonal BaTiO3. Er3+ ions substituted for Ba sites are responsible for the abnormal Raman spectra, but the formation of defect complexes will decrease spectral intensity. A large increase in intensity showed a hundredfold selectivity for Ba‐site Er3+ ions over Ti‐site Er3+ ions. A strong EPR signal at g = 1.974 associated with ionized Ba vacancy defects appeared in BET, and the defect chemistry study indicated that the real formula of BET is expressed by (Ba1 − xEr3x/4)(Ti1 − x/4Erx/4)O3. These abnormal Raman signals were verified to originate from a fluorescent effect corresponding to 4S3/2→4I15/2 transition of Ba‐site Er3+ ions. The fluorescent signals were so intense that they overwhelmed the traditional Raman spectra of BaTiO3. The significance is that the abnormal Raman spectra may act as a probe for the Ba‐site Er3+ occupation in BaTiO3 co‐doped with Er3+ and other dopants. A new broad EPR signal at g = 2.23 was discovered, which originated from Er3+ Kramers ions. Copyright © 2014 John Wiley & Sons, Ltd.

Material Studies on Si:C Epitaxial Films Grown by CVD

We report the material properties of epitaxial Si:C in terms of layer defectivity and carbon substitutionality in view of its use as Source/Drain stressors for nMOSFET devices. Our discussion includes a comparison of disilane versus trisilane as Si precursor. The expected increase in defectivity with increasing carbon content is followed by an epitaxial breakdown (between 2.3% and 5% total C). The increase in defectivity is attributed to the formation of non-stoichiometric Si-C defect complexes. Raman spectroscopic studies revealed the presence of both random and ordered C distributions in the Si:C lattice. A correlation between these ordered distributions and defect structures is presented. The material properties are affected by both process conditions and the details of the process sequence. Typically, a single step deposition process results in a more ordered C structure while a cyclic deposition and etch (CDE) process results in a lower defectivity and an enhanced C substitutionality.

An EBIC and SRP study on thermal donors in proton implanted p‐type magnetic Czochralski silicon

AbstractThe concentration of defects in silicon can be changed by ion implantations and thermal annealing steps. Some of the defects incorporated during these processes form complexes which can act as donors or acceptors and their concentration can even overcome the initial doping concentration of the material, and hence, cause an inversion of the doping type. One way to introduce defects and also to catalyse the formation of defect complexes is by proton implantation. For our project, a high resistivity p‐type magnetic Czochralski (m:Cz) grown silicon wafer was implanted with 4 MeV protons at an implantation dose of 2 ×1014 cm–2. The wafer was broken into pieces and these pieces were annealed at different temperatures resulting in different concentrations of defect complexes. The defects were investigated in the proton implanted region and in the substrate region beyond the penetration depth of the protons (where only the heat treatment caused a change in the defect concentrations). The local doping type and the relative change of the minority carrier diffusion length were measured using Electron Beam Induced Current (EBIC). Furthermore, resistivity profiles were measured using Spreading Resistance Profiling (SRP). The results show several changes of the doping type, the minority carrier diffusion length and the resistivity profile. In addition to donor complexes already reported in the literature, the formation of additional high temperature donor complexes has been observed. The data shows that the presence of hydrogen in silicon has a far higher influence on the changes of the electrical properties of the material at annealing temperatures between 400 °C and 510 °C than at higher annealing temperatures. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of magnetic and ferroelectric properties of BiFeO3 by Er and transition element (Mn, Co) co-doping

BiFeO3(BFO), Bi0.8Er0.2FeO3 (BEFO), Bi0.8Er0.2Fe0.9Mn0.1O3 (BEFMO) and Bi0.8Er0.2Fe0.9Co0.1O3(BEFCO) nanoparticles were prepared by sol–gel method having an average size of 200 nm for BFO, under100 nm for BEFO and under 60 nm for BEFMO and BEFCO. Phase transition from a rhombohedral symmetry (R3c) for BFO to an orthorhombic symmetry (Ibmm) for BEFO, BEFMO and BEFCO has been observed. The phase transformation combined with size reduction has significantly improved both ferroelectric and ferromagnetic behaviors of the doped samples in a similar way. The formation of dipolar defect complexes (DDCs) in the doped samples also contributes to the improved ferroelectric property with saturated polarization (Ps) of 0.375 μC/cm2 and remanent polarization (Pr) of 0.244 μC/cm2 for BEFMO. Size effect may also impact the simultaneously developed Pr for BEFMO and BEFCO. Owning to the interactions between the ferromagnetic and antiferromagnetic microdomains, improved saturated magnetization (Ms) and remanent magnetization (Mr) are also observed in BEFMO.

Ion-beam-induced thin film stress in lithium niobate

The dominating modification of crystalline solids by energetic ions is the formation of lattice defects, which accumulate with ongoing irradiation. Many materials exhibit a phase transition from crystalline to the amorphous state at higher ion fluence. However, this ion-beam-induced structural modification involves the formation of mechanical stress, which is generally disadvantageous for the successful application of ion irradiation in the micro-device technology. Hence, a fundamental understanding of the ion-beam-induced stress evolution is crucial for the effective use of ion beam technology. Lithium niobate (LiNbO3) is a promising candidate for the application of integrated photonic structures. However, for the fabrication of such structures in LiNbO3 ion irradiation is indispensable. In order to get a fundamental and comprehensive understanding of the ion-beam-induced defect and stress evolution in LiNbO3, irradiations with varying parameters (ion energy and irradiation temperature) over a wide range of ion fluence for different crystallographic orientations were performed. The ion-beam-induced defect and stress evolution were studied by means of in situ Rutherford backscattering spectrometry and laser reflection technique, respectively.The investigations demonstrate that ion-beam-induced defect and stress evolution in LiNbO3 is highly anisotropic. Moreover, a complex stress evolution is observed, i.e. with increasing ion fluence different stress formation and stress relaxation processes occur. It will be demonstrated that effects such as radiation-induced viscosity or anisotropic deformation that were suggested by previous models cannot explain the stress evolution in LiNbO3.This work presents a new approach that describes the anisotropic stress and defect evolution in LiNbO3 by a complex defect formation mechanism, i.e. the presence of different defect types and their transformation into each other. Each defect type strains the surrounding crystal matrix and leads to a macroscopic deformation. The total stress is the superposition of the individual stress caused by different defect types.

Dopant compensation in HfO2 and other high K oxides

The theory of doping limits in semiconductors and insulators is applied to the case of wide gap oxides, crystalline, or amorphous, and used to explain that impurities do not in general give rise to gap states or a doping response. Instead, the system tends to form defect complexes or undergo symmetry-lowering reconstructions to expel gap states out of the band gap. The model is applied to impurities, such as trivalent metals, carbon, N, P, and B, in HfO2, the main gate dielectric used in field effect transistors.

Ab initio study of the formation of vacancy and hydrogen-vacancy complexes in palladium and its hydride

We report on the results of ab initio calculations of vacancy and hydrogen-vacancy complexes in palladium and palladium hydride. Comparative analysis of the energies of the formation of defect complexes in palladium and its hydride has revealed that the formation of vacancy clusters is easier in the palladium hydride structure. Investigation of hydrogen-vacancy complexes in bulk crystalline palladium has shown that a hydrogen atom and a vacancy interact to form a stable hydrogen-vacancy (H-Vac) defect complex with a binding energy of Eb = −0.21 eV. To investigate the initial stage in the formation of hydrogen-vacancy complexes (Hn-Vacm), we consider the clusterization of defects into clusters containing H-Vac and H2-Vac complexes as a structural unit. It is found that hydrogen-vacancy complexes form 2D defect structures in palladium in the (100)-type planes.

Composition dependent intrinsic defect structures in SrTiO₃.

Intrinsic point defect complexes in SrTiO3 under different chemical conditions are studied using density functional theory. The Schottky defect complex consisting of nominally charged Sr, Ti and O vacancies is predicted to be the most stable defect structure in stoichiometric SrTiO3, with a relatively low formation energy of 1.64 eV per defect. In addition, the mechanisms of defect complex formation in nonstoichiometric SrTiO3 are investigated. Excess SrO leads to the formation of oxygen vacancies and a strontium-titanium antisite defect, while a strontium vacancy together with an oxygen vacancy and a titanium-strontium antisite defect are produced in an excess TiO2 environment. Since point defects, such as oxygen vacancies and cation antisite defects, are intimately related to the functionality of SrTiO3, these results provide guidelines for controlling the formation of intrinsic point defects and optimizing the functionality of SrTiO3 by controlling nonstoichiometric chemical compositions of SrO and TiO2 in experiments.

Defect ordering and defect–domain-wall interactions in PbTiO3: A first-principles study

The properties of ferroelectric materials, such as lead zirconate titanate (PZT), are heavily influenced by the interaction of defects with domain walls. These defects are either intrinsic or are induced by the addition of dopants. We study here PbTiO3 (the end member of a key family of solid solutions) in the presence of acceptor (Fe) and donor (Nb) dopants, and the interactions of the different defects and defect associates with the domain walls. For the case of iron acceptors, the calculations point to the formation of defect associates involving an iron substitutional defect and a charged oxygen vacancy (Fe-Ti'-V-O ''). This associate exhibits a strong tendency to align in the direction of the bulk polarization; in fact, ordering of defects is also observed in pure PbTiO3 in the form of lead-oxygen divacancies. Conversely, calculations on donor-doped PbTiO3 do not indicate the formation of polar defect complexes involving donor substitutions. Last, it is observed that both isolated defects in donor-doped materials and defect associates in acceptor-doped materials are more stable at 180 degrees. domain walls. However, polar defect complexes lead to asymmetric potentials at domain walls due to the interaction of the defect polarization with the bulk polarization. The relative pinning characteristics of different defects are then compared, to develop an understanding of defect-domain-wall interactions in both doped and pure PbTiO3. These results may also help in understanding hardening and softening mechanisms in PZT.

Structural, ferroelectric and electrical properties of Fe-doped K0.5Bi4.5Ti4O15 thin films prepared by using a chemical solution deposition

The effects of Fe3+ ion doping on the structural, ferroelectric and electrical properties of K0.5Bi4.5Ti4O15 (KBTi) thin films were investigated. Pure KBTi and Fe-doped KBTi (KBTFe) thin films were prepared by using a chemical solution deposition method. All thin films were annealed at 750 °C for 3 min by rapid thermal annealing under an oxygen atmosphere for crystallization. The KBTFe thin films exhibited well-defined hysteresis loops, large remanent polarization (48.5 µC/cm2 at an applied electric field of 264 kV/cm) and low leakage current density (8.27 × 10−8 A/cm2 at an applied electric field of 100 kV/cm). The improved ferroelectric and electrical properties of the KBTFe thin films could be explained by the micro-structural change and the formation of defect complex, which minimized the interactions between the chemical defects and the domain walls.

Novel X7R BaTiO3 ceramics co-doped with La3+ and Ca2+ ions

• Structural and dielectric properties of (Ba 1−x La x )(Ti 1−x/2 Ca x/2 )O 3 were studied. • A pseudo-cubic ceramic with x = 0.03 is a novel dielectric for X7R applications. • A symmetric (2 0 0) XRD peak and sharp Raman band are characteristics of pseudo-cubic. • The unit cell volume ( V 0 ) of BLTC increases linearly with x , satisfying Vegard’s law. • Formation of defect complexes with much larger Ti-site Ca 2+ is responsible for X7R. (Ba 1− x La x )(Ti 1− x /2 Ca x /2 )O 3 (0.03 ⩽ x ⩽ 0.06) (BLTC) ceramics were prepared using a mixed oxides method. The structure, microstructure, and dielectric properties of BLTC were investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscope, and dielectric measurements. Both symmetric (2 0 0) XRD peak indicative of cubicity and sharp 308 cm −1 Raman band indicative of tetragonality are characteristics of pseudo-cubic symmetry of BLTC. The unit cell volume of BLTC increased linearly with x , satisfying Vegard’s law. A strong “Raman charge effect” appeared at 836 cm −1 because of the aliovalent double substitution in BLTC. The x = 0.03 sample because of its higher permittivity ( ε ′ = ∼2000) and lower loss (tan δ < 0.04) is a novel dielectric for X7R applications. The formation of La Ba - Ca Ti ″ - La Ba defect complexes was responsible for the dielectric-temperature stability of BLTC.

Effects of (Dy, Zn) co-doping on structural and electrical properties of BiFeO3 thin films

Abstract Pure BiFeO3 (BFO) and (Dy, Zn) co-doped (Bi0.9Dy0.1)(Fe0.975Zn0.025)O3–δ (BDFZO) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by a chemical solution deposition method. Effects of (Dy, Zn) co-doping on the structural and electrical properties of BFO were studied. Both the thin films were crystallized as randomly oriented polycrystalline distorted rhombohedral structures, with no detectable impurity and secondary phases. Large remnant polarization (2Pr) of 47 μC/cm2, low coercive electric field (2Ec) of 925 kV/cm at 1143 kV/cm and low leakage current density of 1.30×10−6 A/cm2 at 100 kV were measured for the BDFZO thin film. The improved electrical properties of the BDFZO thin film are ascribed to the stabilization of perovskite structure, defect complex formation between acceptors Zn2+ and oxygen vacancies of [ ( Zn Fe 3 + 2 + ) ′ − ( V O 2 − • • ) ] , pronounced off-center displacement in the perovskite and structural distortion by the Dy and Zn ions doping.

In situ transmission electron microscopy observation of keV-ion irradiation of single-walled carbon and boron nitride nanotubes

Single-walled carbon nanotubes (SWCNNT) and single-walled boron nitride nanotubes (SWBNNT) were irradiated with 30keV Kr+ ions at room temperature. The structural changes during irradiation were monitored in situ using transmission electron microscopy (TEM) and ex situ at certain dose points using Raman spectroscopy. Our measurements demonstrate that metallic/semiconducting SWCNNT and insulating SWBNNT undergo similar structural modifications due to keV-ion irradiation. Point defects due to primary knock-on events accumulate and lead to the formation of defect complexes and eventual amorphization. The loss of atoms due to sputtering reduces the nanotube diameter and leads to shrinkage. Time resolved footage of the ion irradiation of a particular area demonstrated that NT scission and subsequent shortening can take place with some probability, possibly due to collision cascade events.

Effect of Indium Segregation on the Surface versus Bulk Chemistry for Indium-Doped TiO2

This work reports the effect of indium segregation on the surface versus bulk composition of indium (In)-doped TiO(2). The studies are performed using proton-induced X-ray emission (PIXE), secondary-ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS). The results of XPS analysis indicate that annealing of In-doped TiO(2) containing 0.3 atom % In at 1273 K in the gas phase of controlled oxygen activity [p(O(2)) = 75 kPa and 10 Pa] results in a surface enrichment of 2.95 and 2.61 atom % In, respectively. The obtained segregation data are considered in terms of the transport of indium ions from its titanium sites in the bulk phase to the surface where these ions are incorporated into interstitial sites. The effect of oxygen activity on the segregation-induced surface enrichment is considered in terms of the formation of a low-dimensional surface structure and a sublayer, which are charged negatively. The latter is formed as a result of strong interactions between titanium vacancies and interstitial indium ions, leading to the formation of defect complexes. The data obtained in this work may be used for engineering of TiO(2)-based semiconductors with enhanced performance in solar energy conversion.

Mechanism for radiative recombination and defect properties of GaP/GaNP core/shell nanowires

Recombination processes in GaP/GaNP core/shell nanowires (NWs) grown on a Si substrate by molecular beam epitaxy are examined using a variety of optical characterization techniques, including cw- and time-resolved photoluminescence and optically detected magnetic resonance (ODMR). Superior optical quality of the structures is demonstrated based on the observation of intense emission from a single NW at room temperature. This emission is shown to originate from radiative transitions within N-related localized states. From ODMR, growth of GaP/GaNP NWs is also found to facilitate formation of complex defects containing a P atom at its core that act as centers of competing non-radiative recombination.

Role of donor-acceptor complexes and impurity band in stabilizing ferromagnetic order in Cu-doped SnO2 thin films

Our complementary magnetic and photoluminescence measurements reveal the correlation between the donor-acceptor complex and the ferromagnetic order in Cu-doped SnO2 thin films. Oxygen vacancies (VO) and Cu dopants form defect complexes of donor-acceptor pairs, and the associated spin-polarized impurity band leads to the narrowing of bandgap. Electronic structure calculations based on the first-principles method demonstrate that the Cu-VO complex has low formation energy and can stabilize the ferromagnetic coupling. Our results suggest that intrinsic defects and their complexes with dopants play a key role for establishing the ferromagnetic order in doped wide-bandgap oxides.