Vacancy States Research Articles

Topological superconductivity and anti-Shiba states in disordered chains of magnetic adatoms

Regular arrays of magnetic atoms on a superconductor provide a promising platform for topological superconductivity. In this work, we study the effects of disorder in these systems, focusing on vacancies realized by missing magnetic atoms. We develop approaches that allow treatment of ferromagnetic dense chains as well as long-range hopping ferromagnetic and helical Shiba chains at arbitrary subgap energies. Vacancies in magnetic chains play an analogous role to magnetic impurities in a clean $s$-wave superconductor. A single vacancy in a topological chain gives rise to a low-lying ``anti-Shiba'' state below the band edge of a regular magnetic chain. Proliferation of the anti-Shiba band formed by a finite density of hybridized vacancy states leads to deterioration of the topological phase, which exhibits unusual fragility in a particular parameter region in dilute chains. We also consider local fluctuation in the Shiba coupling and discuss how vacancy states could contribute to experimental verification of topological superconductivity.

Band gap states in nanocrystalline WO3 thin films studied by soft x-ray spectroscopy and optical spectrophotometry

Nanocrystalline tungsten trioxide (WO3) thin films prepared by DC magnetron sputtering have been studied using soft x-ray spectroscopy and optical spectrophotometry. Resonant inelastic x-ray scattering (RIXS) measurements reveal band gap states in sub-stoichiometric γ-WO3−x with x = 0.001–0.005. The energy positions of these states are in good agreement with recently reported density functional calculations. The results were compared with optical absorption measurements in the near infrared spectral region. An optical absorption peak at 0.74 eV is assigned to intervalence transfer of polarons between W sites. A less prominent peak at energies between 0.96 and 1.16 eV is assigned to electron excitation of oxygen vacancies. The latter results are supported by RIXS measurements, where an energy loss in this energy range was observed, and this suggests that electron transfer processes involving transitions from oxygen vacancy states can be observed in RIXS. Our results have implications for the interpretation of optical properties of WO3, and the optical transitions close to the band gap, which are important in photocatalytic and photoelectrochemical applications.

First-principles DFT+Uinvestigation of charged states of defects and fission gas atoms inCeO2

Cerium dioxide $({\mathrm{CeO}}_{2})$ is considered as a model material for the experimental study of radiation damage in the standard nuclear fuel uranium dioxide $({\mathrm{UO}}_{2})$. In this paper, we present a first-principles study in the framework of the DFT+$U$ approach to investigate the charged point defects and the incorporation of the fission gases Xe and Kr in ${\mathrm{CeO}}_{2}$ and compare it with published data in ${\mathrm{UO}}_{2}$. All intrinsic charge states are considered for point defects in contrast to previous published studies. Our calculations prove that ${\mathrm{CeO}}_{2}$ shows similar behavior to ${\mathrm{UO}}_{2}$ in the formation of point defects with the same charge states under stoichiometric and nonstoichiometric conditions. The charge states of vacancies have an important effect on the incorporation of fission gas atoms in ${\mathrm{CeO}}_{2}$. The bound Schottky defect with the two oxygen vacancies along the (100) direction is found to be energetically preferable to trap Xe and Kr atoms both in ${\mathrm{CeO}}_{2}$ and ${\mathrm{UO}}_{2}$. Xe and Kr atoms in the cation vacancy sites under nonformal charge states (different from $4\ensuremath{-}$) in ${\mathrm{CeO}}_{2}$, unlike in ${\mathrm{UO}}_{2}$, lose electrons to their neighboring atoms, which is traced back to the absence of the +5 valence state for Ce in contrast to its existence for U.

Magnetic interactions between vacancy-induced intrinsic magnetic impurities in single-wall carbon nanotubes

Spin-half paramagnetism induced by point detects was found in graphene recently, micromechanism of this magnetic response can be explained well by the intrinsic magnetic impurity theory. In this paper, we apply this theory to two types of single-walled carbon nanotubes (SWCNs) and calculate the properties of various magnetic interactions for comparison. Interestingly, magnetic interactions have different behaviors in these systems. Following our calculation, within a short length, the interactions can be suppressed by ether size effect or a tiny band gap, and then exhibit exponentially decaying. However, in the absence of a band gap, the RKKY interaction could leave a tiny tail at long range, which determines long range magnetic order. Further more, when a tiny band gap exist in the systems, the Heisenberg coupling is the dominate one due to the expanded wavefunction. According to these result, vacancy states in different types of SWCNs could form different magnetic order, bringing abundant candidates for application.

(Invited) Environmental Resistance of Resistive Random Access Memory

Environmental resistance of memory devices must be clarified for putting them into practical use. This should be emphasized in particular for resistance random access memory (ReRAM), which switching mechanism is still unclear, because responses to environmental stresses are closely related to the mechanism. As for the influence of radiation exposure, with increasing density of memory devices, the issue of the generation of soft errors by the radiation, ex. cosmic rays, is becoming more and more serious. So far, soft errors are concerned about only in some semiconductor memories such as SRAM and DRAM, in which stored data are easily inverted by weak current that is generated by incident secondary cosmic rays. On the other hand, there is very little report on reliabilities of ReRAM against exposure to radiation, which might be due to a common belief that ReRAM, which resistive switching is believed to be caused by ion migration, is free from this kind of error. However, if resistive switching of ReRAM is caused by change in the charge state of oxygen vacancies (VO) as proposed by Kamiya et al. [1], soft errors of data inversion might be occurred by charge excitation such as due to ultraviolet irradiation (UV) irradiation. In this paper, we fabricated Pt/NiO/indium tin oxide (ITO) structure, which has transparent ITO electrode, and therefore the irradiation of light to the NiO memory layer became possible. By using this structure, we investigated data retention characteristics against UV irradiation to ReRAM [2]. As a result, soft errors were confirmed to be caused by UV irradiation in both low- and high-resistance states. An analysis of the wavelength dependence of light irradiation on data retention characteristics suggested that electronic excitation from the valence to the conduction band and to the energy level generated due to the introduction of VO caused the errors. Based on a statistically estimated soft error rates, the errors were suggested to be caused by the cohesion and dispersion of VO owing to the generation of electron-hole pairs and valence changes by the ultraviolet irradiation. Therefore, it was suggested that establishing a method to improve resistance to UV irradiation in consideration of the band gap, in-gap states, and the charge state of VO in metal oxides may be a key factor for practical use of ReRAM. The effect of moisture and atmosphere gas on the reliability of ReRAM will also be discussed. [1] K. Kamiya et al., APL 100, 073502 (2012). [2] K. Kimura et al., APL 108, 123501 (2016).

Deep-level defect distribution as a function of oxygen partial pressure in sputtered ZnO thin-film transistors

Deep-level defect states in sputtered ZnO thin-film transistors were investigated as a function of oxygen partial pressure during sputtering growth. Photo-induced threshold voltage-shift measurements under monochromatic illumination were used to characterize the deep-level defect distribution. Intrinsically, the defect states of oxygen vacancies were ionized to Vo+ and Vo2+ while the photon energy was absorbed within the bandgap, resulting in the shift of threshold voltage. Extracted deep-level defect distribution from this shift of threshold voltage was clearly confirmed in the range of 1.8–2.1 eV below the conduction band minimum and this region was suppressed with increasing oxygen partial pressure. These deep-level defect states can have a detrimental effect on device performance, such as threshold voltage shift and photo-induced leakage current. The photo instability of the devices occurred under visible light due to the photo-ionization of deep-level trapped charges associated with oxygen vacancies.

Impact of nitrogen and carbon on defect equilibrium in ZrO2

We investigate the electronic properties of nitrogen and carbon impurities in ZrO2 using density functional theory with a hybrid functional. It is commonly accepted that N substitutes on the O site and is stable in the −1 charge state (NO−). The NO− acceptors are then compensated by donor defects such as O vacancies in the +2 charge state. We test the validity of this assumption by determining the formation energies of all relevant charge states of nitrogen and oxygen vacancies as a function of Fermi level. We also examine the effects of carbon, a common unintentional impurity in ZrO2. We find that carbon impurities have relatively low formation energies and would indeed incorporate easily during growth of ZrO2, but they do not affect the equilibrium between nitrogen and oxygen vacancies. Our results show that the Kröger-Vink picture is valid for common growth conditions and the dominant defects are NO− and VO+2.

Quasi-noble-metal graphene quantum dots deposited stannic oxide with oxygen vacancies: Synthesis and enhanced photocatalytic properties

Quasi-noble-metal graphene quantum dots (GQDs) deposited stannic oxide (SnO2) with oxygen vacancies (VOs) were prepared by simply sintering SnO2 and citric acid (CA) together. The redox process between SnO2 and GQDs shows the formation of oxygen vacancy states below the conduction band of stannic oxide. The produced VOs obviously extend the optical absorption region of SnO2 to the visible-light region. Meanwhile, GQDs can effectively improve the charge-separation efficiency via a quasi function like noble metal and promote the visible-light response to some degree. In addition, the samples calcinated at 450°C reveals the best performance because of its relatively high concentrations of VOs. What is more, the possible degradation mechanism has been inferred as extended visible-light response as well as raised charge-separation efficiency has also been put forward. Our work may offer a simple strategy to combine the defect modulation and noble metal deposition simultaneously for efficient photocatalysis.

Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs2SnI6 and Cs2TeI6.

Vacancy-ordered double perovskites of the general formula A2BX6 are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated [BX6] units with A-site cations occupying the cuboctahedral voids. Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs2SnI6 has recently been explored for applications in photovoltaic devices. To elucidate the structure-property relationships of these materials, we have synthesized solid-solution Cs2Sn1-xTexI6. However, even though tellurium substitution increases electronic dispersion via closer I-I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility. Density functional calculations of native defects in Cs2SnI6 reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material tolerant to these defect states. The increased covalency of Te-I bonding renders the formation of iodine vacancy states unfavorable and is responsible for the reduction in conductivity upon Te substitution. Additionally, Cs2TeI6 is intolerant to the formation of these defects, because the defect level occurs deep within the band gap and thus localizes potential mobile charge carriers. In these vacancy-ordered double perovskites, the close-packed lattice of iodine provides significant electronic dispersion, while the interaction of the B- and X-site ions dictates the properties as they pertain to electronic structure and defect tolerance. This simplified perspective based on extensive experimental and theoretical analysis provides a platform from which to understand structure-property relationships in functional perovskite halides.

Low energy ion irradiation of TiO2(110) – understanding evolution of surface morphology and scaling studies

ABSTRACTScaling properties of TiO(110) surfaces, after irradiation with 3 keV Ar ions, have been investigated here. Ion irradiation leads to the spontaneous formation of self-assembled Ti-rich nanostructures (NS) as well as Ti vacancy states. The NS thus formed are elliptical in shape and exhibit a shape transformation to less elliptical nature at high fluences. Scaling studies also reflect this transition. Scaling studies further reveal that the surfaces are self-affine in nature and with increasing fluence the regions of correlated structures, or fluctuations, spread and become bigger. This is connected to the formation of bigger NS, causing long-ranged correlations on the surface. On the other hand, at short length scales, the surfaces become less jagged as reflected by the roughness exponent of irradiated surfaces.

Handshake electron transfer from hydrogen Rydberg atoms incident at a series of metallic thin films.

Thin metallic films have a 1D quantum well along the surface normal direction, which yields particle-in-a-box style electronic quantum states. However the quantum well is not infinitely deep and the wavefunctions of these states penetrate outside the surface where the electron is bound by its own image-charge attraction. Therefore a series of discrete, vacant states reach out from the thin film into the vacuum increasing the probability of electron transfer from an external atom or molecule to the thin film, especially for the resonant case where the quantum well energy matches that of the atom. We show that "handshake" electron transfer from a highly excited Rydberg atom to these thin-film states is experimentally measurable. Thicker films have a wider 1D box, changing the energetic distribution and image-state contribution to the thin film wavefunctions, resulting in more resonances. Calculations successfully predict the number of resonances and the nature of the thin-film wavefunctions for a given film thickness.

Optical properties of group-3 metal hexaboride nanoparticles by first-principles calculations.

LaB6 nanoparticles are widely used as solar control materials for strong near-infrared absorption and high visible transparency. In order to elucidate the origin of this unique optical property, first-principles calculations have been made for the energy-band structure and dielectric functions of R(III)B6 (R(III) = Sc, Y, La, Ac). On account of the precise assessment of the energy eigenvalues of vacant states in conduction band by employing the screened exchange method, as well as to the incorporation of the Drude term, dielectric functions and various physical properties of LaB6 have been reproduced in excellent agreement with experimental values. Systematic examinations of dielectric functions and electronic structures of the trivalent metal hexaborides have clarified the origin of the visible transparency and the near-infrared plasmon absorption of R(III)B6 nanoparticles.

Spatially resolved photoresponse on individual ZnO nanorods: correlating morphology, defects and conductivity.

Electrically active native point defects have a significant impact on the optical and electrical properties of ZnO nanostructures. Control of defect distribution and a detailed understanding of their physical properties are central to designing ZnO in novel functional forms and architecture, which ultimately decides device performance. Defect control is primarily achieved by either engineering nanostructure morphology by tailoring growth techniques or doping. Here, we report conducting atomic force microscopy studies of spatially resolved photoresponse properties on ZnO nanorod surfaces. The photoresponse for super-band gap, ultraviolet excitations show a direct correlation between surface morphology and photoactivity localization. Additionally, the system exhibits significant photoresponse with sub-bandgap, green illumination; the signature energy associated with the deep level oxygen vacancy states. While the local current-voltage characteristics provide evidence of multiple transport processes and quantifies the photoresponse, the local time-resolved photoresponse data evidences large variations in response times (90 ms–50 s), across the surface of a nanorod. The spatially varied photoconductance and the range in temporal response display a complex interplay of morphology, defects and connectivity that brings about the true colour of these ZnO nanostructures.

AlN and Al oxy-nitride gate dielectrics for reliable gate stacks on Ge and InGaAs channels

AlN and Al oxy-nitride dielectric layers are proposed instead of Al2O3 as a component of the gate dielectric stacks on higher mobility channels in metal oxide field effect transistors to improve their positive bias stress instability reliability. It is calculated that the gap states of nitrogen vacancies in AlN lie further away in energy from the semiconductor band gap than those of oxygen vacancies in Al2O3, and thus AlN might be less susceptible to charge trapping and have a better reliability performance. The unfavourable defect energy level distribution in amorphous Al2O3 is attributed to its larger coordination disorder compared to the more symmetrically bonded AlN. Al oxy-nitride is also predicted to have less tendency for charge trapping.

M‐X‐ray satellite lines of heavy elements due to multiple ionization

The relative intensities of M‐X‐ray satellites for heavy elements have been calculated. These satellite lines arise from X‐ray emission with multiple vacancy states in M and N shells. The origin of multiple vacancies is considered as the Coster–Kronig transition as well as the shake process. The calculated intensities are compared with the experimental results and other theoretical calculations. Copyright © 2016 John Wiley & Sons, Ltd.

Photostability and visible-light-driven photoactivity enhancement of hierarchical ZnS nanoparticles: The role of embedment of stable defect sites on the catalyst surface with the assistant of ultrasonic waves

Zinc sulfide is a UV-active photocatalyst and it undergoes photocorrosion under light irradiation. In this work, the defect sites on ZnS nanoparticles (NPs) surfaces were induced with the help of powerful ultrasonic waves. The defect sites caused (1) suppression of photocorrosion in a large extent under UV light irradiation and (2) enhancement of visible light photo activity. The photocorrosion inhibition was induced by raising valence band (VB) position through the formation of interstitial zinc and sulfur vacancy states in the ZnS band structure and weakening of oxidative capacity of hole. The enhancement of visible light photocatalytic activity may be related to the generation of more defect energy states in the ZnS band gap. Under visible light irradiation, the electron was excited from the ZnS VB to the interstitial sulfur and zinc vacancy states before injecting into the conduction band of ZnS. Therefore, we modified the band gap of ZnS so that it acts as a visible light active photocatalyst. ZnS NPs were prepared using two different classical and ultrasound methods. The prepared ZnS using ultrasound method, exhibited more outstanding photocatalytic activity for degrading reactive black 5 (RB5) under UV and sunlight irradiation in comparison with the classical method. Details of the degradation mechanism under UV light were investigated. This work provides new insights to understanding the photocorrosion stability and visible light activity of bare ZnS photocatalyst.

Multiferroic nature of intrinsic point defects inBiFeO3: A hybrid Hartree-Fock density functional study

To achieve a fundamental understanding of the multiferroic behavior and electronic properties of intrinsic vacancies in $\mathrm{BiFe}{\mathrm{O}}_{3}$, here we performed first-principles calculations based on hybrid Hartree-Fock density functional theories, which can accurately describe defect electronic structures. Oxygen vacancies, which behave as deep donors with high concentrations under oxygen-poor conditions, reduce the magnetic moments at neighboring Fe ions in the neutral state, while charged oxygen vacancies induce additional ferroelectric polarizations. Cation vacancies, on the other hand, are likely to form under oxygen-rich conditions and result in multiferroic properties distinct from those induced by oxygen vacancies. Bi vacancies act as triple-shallow acceptors and strongly suppress spontaneous polarization regardless of charge states, while Fe vacancies locally interfere with both electric and spin polarization and are thus regarded as multiferroic singular points in $\mathrm{BiFe}{\mathrm{O}}_{3}$. A rich variety of the multiferroic behavior of vacancies can be systematically understood from the localized/delocalized features of defect states, and the different formation conditions for vacancies provide a strategy to tailor the multiferroic properties of $\mathrm{BiFe}{\mathrm{O}}_{3}$ through control of the concentration and charge states of vacancies.

Spectroscopic properties of oxygen vacancies inLaAlO3

Oxygen vacancies in LaAlO3 (LAO) play an important role in the formation of the two-dimensional electron gas observed at the LaAlO3/SrTiO3 interface and affect the performance of MOSFETs using LAO as a gate dielectric. However, their spectroscopic properties are still poorly understood, which hampers their experimental identification. Here we predict the absorption spectra and ESR parameters of oxygen vacancies in LAO using periodic and embedded cluster methods and density functional theory (DFT). The structure, charge distribution, and spectroscopic properties of the neutral and charged (1+ and 2+) oxygen vacancies in cubic and rhombohedral LaAlO3 are investigated. The highest intensity optical transitions [calculated using time-dependent DFT (TDDFT)], from the oxygen vacancy states to the conduction-band states have onsets at 3.5 and 4.2 eV for the neutral vacancy and 3.6 eV for the 1+ charged vacancy in rhombohedral LAO and 3.3 and 4.0 eV for the neutral vacancy and 3.4 eV for the 1+ charged vacancy in cubic LAO, respectively. Also reported are the isotropic g value (2.004026) and hyperfine coupling constants of the 1+ charged oxygen vacancy, which are compared to the experimental data obtained using electron spin resonance (ESR) spectroscopy, and accurately predict both the position and the width (3 mT) of its ESR signature. These results may further facilitate the experimental identification of oxygen vacancies in LAO and help to establish their role at the LAO/STO interfaces and in nanodevices using LAO.

Energetics and electronic structure of tubular Si vacancies filled with carbon nanotubes

We studied the energetics and electronic structure of tubular Si vacancies incorporating a carbon nanotube (CNT), using first-principles total-energy calculations based on the density functional theory. Our calculations show that the incorporated CNT into a Si nanotunnel acts as an atom-thickness liner providing the electrostatically flat nanoscale space inside them by shielding the dangling bond states of tubular Si vacancies. The incorporation of the CNT into the tubular Si vacancies is exothermic with an energy gain up to 7.4 eV/nm depending on the diameters of the vacancy and encapsulated CNT. The electronic states of the vacancy substantially hybridize with those of the CNT, leading to the complex electronic energy band near the Fermi level.

Strain Manipulated Magnetic Properties in ZnO and GaN Induced by Cation Vacancy

The effects of isotropic strains on the magnetic properties in ZnO and GaN induced by cation vacancies are comparatively investigated by density functional theory calculations. The magnetic moments and the couplings between vacancies in different charged states are calculated as a function of strains. The modulation of strain on the magnetic properties relies on the materials and the charge states of cation vacancies in them. As the occurrence of charge transfer in ZnO:V Zn under compression, the coupling between $$V_{\rm{Zn}}^{0} $$ is antiferromagnetic (AFM) and it could be stabilized by strains. Tensions can strengthen the ferromagnetic (FM) coupling between $$V_{\rm{Zn}}^{0} $$ but weaken that of $$V_{\rm{Ga}}^{ - } $$ . The neutral V Ga are always AFM coupling under strains from −6 to +6% and could be stabilized by compressions. The interactions between $$V_{\rm{Ga}}^{ - } $$ are always FM with ignorable variations under strains; however, the FM couplings between $$V_{\rm{Ga}}^{2 - } $$ could be strengthened by compressions. These varying trends of magnetic coupling under strains are interpreted by the band coupling models. Therefore, strain-engineering provides a route to manipulate and design high Curie temperature ferromagnetism derived and mediated by intrinsic defect for spintronic applications.