Intrinsic Structural Defects Research Articles

Flux pinning and magnetic relaxation in Ga-doped LiFeAs single crystals

The effect of nonmagnetic Ga3+ impurities on structural and superconducting properties of LiFeAs single crystals has been studied. The out-of-plane magnetization of the doped material exhibits a remarkable double-peak structure. The presence of a low-field peak observed both in doped and undoped LiFeAs is suggestive of intrinsic structural defects, while a secondary high-field fishtail maximum, which shifts progressively with temperature, is associated with the extrinsic pinning centers created by Ga. The superconducting transition temperature of Ga doped LiFeAs is suppressed by about 4.8 K/at. %. However, a set of superconducting parameters including the superconducting transition temperature, the coherence length, the upper critical field, and the irreversibility field are not significantly reduced in a sample doped with 0.5 at. % Ga. At this Ga concentration, the critical current density of doped LiFeAs is about four times that of our undoped material in intermediate magnetic fields at 5 K. The scaling of the normalized pinning force density Fp = JcB vs the applied field normalized by Bmax (Bmax denotes the peak position of Fpmax) in the temperature range 5–13 K indicates a single type of predominant flux-pinning mechanism provided by Ga additions. Analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning model. The magnetic relaxation measurements combined with the peak position of the critical current density in the B-T phase diagram suggests an elastic–plastic transition of the vortex lattice at higher temperatures and fields. The observed vortex behavior of Ga doped LiFeAs strongly resembles that of YBa2Cu3O7 doped with nonmagnetic impurities.

Cu-Doped ZnO Hemispherical Shell Structures: Synthesis and Room-Temperature Ferromagnetism Properties

We report a simple chemical vapor deposition method to fabricate Cu-doped ZnO hemispherical shell structures with room-temperature ferromagnetism (RTFM). Through a series of controlled experiments by changing the growth temperature and reaction time, we observe the evolution of product morphology from whole spherical structures into partially broken shells and hemispherical shells at different temperatures together with the reinforced hemispherical shells with the reaction time extended. The growth mechanism of the ZnO:Cu hemispherical shell structures has been proposed to involve the synthesis of Cu-doped Zn sphere, surface oxidation, and sublimation of the inner Zn from the broken shell. The structural and optical properties of the ZnO:Cu system with different Cu contents (below 4%) were investigated by X-ray diffraction, Raman spectroscopy, and photoluminescence measurements indicating that the Cu ions were successfully substituted into the Zn2+ lattice sites, and more intrinsic structural defects were...

Intrinsic defects and dopability of zinc phosphide

Zinc phosphide (Zn${}_{3}$P${}_{2}$) could be the basis for cheap and highly efficient solar cells. Its use in this regard is limited by the difficulty in $n$-type doping of the material. In an effort to understand the mechanism behind this, the energetics and electronic structure of intrinsic point defects in zinc phosphide are studied using generalized Kohn-Sham theory and utilizing the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional for exchange and correlation. Novel ``perturbation extrapolation'' is utilized to extend the use of the computationally expensive HSE functional to this large-scale defect system. According to calculations, the formation energy of charged phosphorus interstitial defects are very low in $n$-type Zn${}_{3}$P${}_{2}$ and act as ``electron sinks,'' nullifying the desired doping and lowering the Fermi-level back toward the $p$-type regime. This is consistent with experimental observations of both the tendency of conductivity to rise with phosphorus partial pressure, and with current partial successes in $n$-type doping in very zinc-rich growth conditions.

Influence of oxygen vacancy on the electronic structure of CaCu3Ti4O12 and its deep-level vacancy trap states by first-principle calculation

The electronic structure, formation energy, and transition energy levels of intrinsic defects have been studied using the density-functional method within the generalized gradient approximation for neutral and charged oxygen vacancy in CaCu3Ti4O12 (CCTO). It is found that oxygen vacancies with different charge states can be formed in CCTO under both oxygen-rich and poor conditions for nonequilibrium and higher-energy sintering processes; especially, a lower formation energy is obtained for poor oxygen environment. The charge transition level (0/1+) of the oxygen vacancy in CCTO is located at 0.53 eV below the conduction-band edge. The (1+/2+) transition occurs at 1.06 eV below the conduction-band edge. Oxygen vacancies of Vo1+ and Vo2+ are positive stable charge states in most gap regions and can act as a moderately deep donor for Vo1+ and a borderline deep for Vo2+, respectively. The polarization and dielectric constant are considerably enhanced by oxygen vacancy dipoles, due to the off-center Ti and Cu ions in CCTO.

Elastic and anelastic properties of densified vitreous B2O3: Relaxations and anharmonicity

The elastic and anelastic properties of densified B${}_{2}$O${}_{3}$ glasses, melt quenched under pressures of 2 and 4 GPa, were investigated by measuring the sound velocity and the acoustic attenuation of longitudinal and shear ultrasonic waves in the megahertz range over the temperature interval between 8 and 300 K. Densification from 1826 to 2373 kg/m${}^{3}$ leads to an extraordinarily large growth of both bulk and shear moduli but leaves the Poisson's ratio nearly constant. In the glass compacted at 4 GPa, the elastic moduli become larger by a factor of five than those characterizing normal vitreous B${}_{2}$O${}_{3}$ (v-B${}_{2}$O${}_{3}$) as a consequence of modifications of the chemical bonding in the network. The thermally activated relaxations of intrinsic structural defects, which dominate the acoustic behaviors of normal glass below 150 K, giving rise to an intense attenuation peak and a corresponding steep decrease in sound velocity, are increasingly depressed by growing densification. Above 150 K, the ultrasonic velocity is mainly regulated by the vibrational anharmonicity and shows a nearly linear decrease as the temperature is increased, with a substantially smaller slope with increasing densification. Modeling the relaxation losses and the related velocity variations by an asymmetric double-well potential model that has a distribution of both the barrier potential and the asymmetry, it has been possible to separate the relaxation and the anharmonic contributions to the sound velocity. The former has been ascribed to local motions of boroxol rings formed by connected BO${}_{3}$ planar triangles, the basic units building up the network of v-B${}_{2}$O${}_{3}$, while the latter has been interpreted in terms of the Akhiezer mechanism concerning the ``thermal vibration viscosity.''

Identification of ZnO Defect Structure by PL Spectroscopy

Defect structure of ZnO determines the optoelectronic characteristics of ZnO crystal and film. The identification and modulation of the defect structure is the foundation of the manufacture of optoelectronic devices. Although a good deal of research has been carried out about intrinsic defects and doping defects in ZnO, it is difficult to obtain a conclusive result accepted by all. In the paper, ZnO film is prepared by sol-gel method and the defect structure is expressed by photoluminescent (PL) spectroscopy. Based on some basic rules the intrinsic defect structure of ZnO is confirmed and the interaction of impurity Li and intrinsic defect is discussed. At the same time, some new electronic levels are proposed. It is surprising that although there are fourteen peaks in PL spectra of ZnO film, almost all the peaks can be identified by these basic rules.

Effect of magnetic field on the reconstruction of the defect-impurity state and сathodoluminescence in Si/SiO2 structure

Impurity states in Si/SiO2 structure have been studied using cathodoluminescence (CL). It has been found that intrinsic structure defects in Si/SiO2 are sensitive to the action of magnetic field, which can be revealed due to changes in Si/SiO2 optical properties. The most sensitive to magnetic field (about 35 per cent) is the intensity of the 1.9 eV CL band attributed to non-bridge oxygen atoms.

Control of room-temperature defect-mediated ferromagnetism in VO2 films

We report interesting ferromagnetic properties and their control in a vanadium-based oxide system driven by stoichiometric defects. Vanadium oxide (VO2) thin films were grown on c-plane sapphire substrates by a pulsed laser deposition technique under different ambient conditions. The ferromagnetism of the epitaxial VO2 films can be switched on and off by altering the cooling ambient parameters. In addition, the saturated magnetic moments and coercivity of the VO2 films were found to be a function of the oxygen partial pressure during the growth process. The room-temperature ferromagnetic properties of VO2 films were correlated with the nature of the microstructure and the growth parameters. The origin of the induced magnetic properties are qualitatively understood to stem from intrinsic structural and stoichiometric defects.

A Comparative Study of Structural and Optical Properties of ZnO Thin Film Prepared on Sapphire Substrate by PLD and Annealed

ZnO thin films are grown on sapphire substrate by employing a Nd:YAG (1064nm) laser ablation in 1.3 Pa oxygen ambient. X-ray diffraction (XRD) and atomic force microscopy (AFM) observation show that as-grown thin film possess (0002) textured feature and form columnar inclined grains with a small size. The as-grown films show a little oxygen vacancies and Zn interstitial by the analysis of X-ray photoelectron spectroscopy (XPS) and Raman. The annealing treatment in oxygen ambient causes obvious improvement on the structure, surface morphology, compositions and optical properties by the analysis of XRD, AFM, XPS, Raman and photoluminescence (PL) spectra. It can be concluded that the increase of UV emission is oriented to the improvement of structure and decrease of intrinsic defects. The blue emission (430 nm) is attributed to the electronic transition from shallow donor level of Zn interstitial to top level of valence band.

Effect of ionizing radiation in ultrasmall doses on dielectric properties of CdZnTe crystals with anomalously high polarizability

The effect of ultrasmall exposure doses (10–45 R) of gamma-radiation on both real and imaginary parts of the complex dielectric permittivity of CdZnTe crystals in the low-frequency range has been discovered. It is shown that the observed effect is related to (i) changes in the system of intrinsic structural defects, which cause a deviation of the crystal composition from stoichiometry and (ii) growth-related large-scale fluctuations of the electric potential.

Optical properties of ferromagnetic ytterbium‐doped III‐nitride epilayers

AbstractWe have studied the optical and magnetic properties of ytterbium (Yb) implanted GaN epilayers grown on (0001) sapphire by different epitaxial growth methods. Samples were implanted at room temperature with Yb ions and subsequently thermally annealed at 1000 °C in N2 at atmospheric pressure. The characteristic Yb3+ ion photoluminescence (PL) spectra were observed in the spectral range between 970 nm and 1050 nm. Analysis of the PL temperature quenching and PL kinetics suggest that Yb3+ ions are involved in at least two major luminescence centers. The magnetizations versus magnetic field curves show an enhancement of magnetic order for Yb‐implanted GaN epilayer containing intrinsic impurities and structural defects. The hysteretic behavior with a weak coercivity is enhanced when the magnetic field is out‐of‐plane indicating magnetic anisotropy of the Yb‐implanted GaN epilayers (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

(Bi2Te3)0:25(Sb2Te3)0:75 crystal structure improvements with excess Te as studied by AFM, SEM, EBSD and XRD

(Bi2Te3) x (Sb2Te3)1−x solid solutions with x = 0:2, 0.25 and 0.3 − p type thermoelectric compounds with an excess of Tellurium dopant up to 4 wt% were crystallized. By increasing the Bi2Te3 content in the Bi-Sb-Te system, the hole concentration decreased and in consequence caused an optimum Seebeck coefficient and a decrease in electrical conductivity and thermal conductivity, thus resulting in an increase in the figure of merit at x = 0:25. The results showed that optimum thermoelectric properties can be achieved for a mixture of x = 0:25 with 3 wt% Te added. Structural characterizations of this compound in the absence and presence of the dopant were carried out by means of X-Ray diffraction measurement, scanning electron microscopy, electron backscattering diffraction and atomic force microscopy. Detailed analyses confirm that the improvements in thermoelectric parameters due to the intrinsic structure and minimum defects during crystallization of the compounds result from the excess of Te.

Atomic layer deposited beryllium oxide: Effective passivation layer for III-V metal/oxide/semiconductor devices

Electrical and physical characteristics of the atomic layer deposited beryllium oxide (BeO) grown on the Si and GaAs substrates were evaluated as a barrier/passivation layer in the III-V devices. Compared to Al2O3, BeO exhibits lower interface defect density and hysteresis, and smaller frequency dispersion and leakage current density at the same effective oxide thickness, as well as an excellent self-cleaning effect. These dielectric characteristics combined with its advantageous intrinsic properties, such as high thermal stability, large energy band-gap(10.6 eV), effective diffusion barrier, and low intrinsic structural defects, make BeO an excellent candidate for the interfacial passivation layer applications in the channel III-V devices.

Interplay between collective pinning and artificial defects on domain wall propagation in Co/Pt multilayers

The interplay between collective pinning on intrinsic structural defects and artificial pinning at a patterned hole is studied in magnetic multilayers with perpendicular anisotropy. The pinning strength of a patterned hole is measured through its efficiency to stop domain wall (DW) propagation into a consecutive unpatterned nanowire section (using antisymmetric magnetoresistance to detect the direction of DW propagation) whereas collective pinning is characterized by the field dependence of DW velocity. Close to room temperature, collective pinning becomes weaker than artificial pinning so that pinning at the hole compensates nucleation-pad geometry, blocking DW propagation across the nanowire.

Laser-induced damage threshold at different wavelengths of Ta 2O 5 films annealed over a wide temperature range

The laser-induced damage of Ta 2O 5 films annealed at a wide range of temperature (473–1273 K) at the laser wavelengths of 1064 and 355 nm was investigated. The relations between microstructure, optical properties, chemical composition, absorption and laser-induced damage threshold (LIDT) were studied. The dependence of a damage mechanism on laser wavelengths was discussed. It was found that the LIDT either at 1064 or 355 nm first increased and then decreased with increase of annealing temperature. The LIDT at 1064 nm was influenced by the substoichiometric defects, structural defects and thermal diffusion, whereas at 355 nm it was affected mainly by the intrinsic absorption and structural defects. Both the maximum LIDT at the two wavelengths were obtained at the annealing temperature of 873 K, which could be attributed to the lowest defect density in films.

Understanding conductivity in SrCu2O2: stability, geometry and electronic structure of intrinsic defects from first principles

Density functional theory calculations have been performed on stoichiometric and intrinsically defective p-type transparent conducting oxide SrCu2O2, using GGA corrected for on-site Coulombic interactions (GGA + U). Analysis of the absorption spectrum of SrCu2O2 indicates that the fundamental direct band gap could be as much as ∼0.5 eV smaller than the optical band gap. Our results indicate that the defects that cause p-type conductivity are favoured under all conditions, with defects that cause n-type conductivity having significantly higher formation energies. We show conclusively that the most stable defects are copper and strontium vacancies. Copper vacancies introduce a distinct acceptor single particle level above the valence band maximum, consistent with the experimentally known activated hopping mechanism.

Grainboundary electronic structure of ZnO-Bi2O3 based varistor ceramics

Dielectric frequency spectra of ZnO-Bi2O3 based varistor ceramics were measured in the frequency range of 10-2Hz—106Hz from -100℃ to 20℃ by Novocontrol broadband dielectric spectrometer. The intrinsic defect structure of ZnO was calculated according to the theory which assumes that characteristics dielectric loss peaks of ZnO varistor ceramics origin from electronic relaxation process of intrinsic defects in depletion layer. At the same time the micro-electric parameters of grainboundary and the single grainboundary breakdown voltage Vb were obtained. The accordance of theory with experiment in Vb value indicates that the method to calculate grainboundary electronic structure by dielectric spectroscopy proposed in this paper is resonable.

Blue luminescence of SrTiO3 under intense optical excitation

The blue-green photoluminescence emitted by pure and electron-doped strontium titanate under intense pulsed near-ultraviolet excitation is studied experimentally as a function of excitation intensity and temperature. Both emission spectra and time-resolved decays of the emission are measured and analyzed in the framework of simple phenomenological models. We find an interesting blue-to-green transition occurring for increasing temperatures in pure samples, which is absent in doped materials. The luminescence yield and decay rate measured as a function of temperature can be modeled well as standard activated behaviors. The leading electron-hole recombination process taking place in the initial decay is established to be second order, or bimolecular, in contrast to recent reports favoring a third-order interpretation as an Auger process. The temporal decay of the luminescence can be described well by a model based on two interacting populations of excitations, respectively identified with interacting defect-trapped (possibly forming excitons) and mobile charges. Finally, from the measured doping and sample dependence of the luminescence yield, we conclude that the radiative centers responsible for the luminescence are probably intrinsic structural defects other than bulk oxygen vacancies.

Structure and diffusion of intrinsic defects, adsorbed hydrogen, and water molecules at the surface of alkali-earth fluorides calculated using density functional theory

Using periodic density functional theory, we calculate the structure and migration energies of fluorine vacancies and interstitials in the bulk and at the stoichiometric bulk-truncated surface of three alkali-earth fluorides: ${\text{CaF}}_{2}$, ${\text{SrF}}_{2}$, and ${\text{BaF}}_{2}$. We then study the adsorption of water and hydrogen, in both molecular and dissociated form, at the ideal surface, and at neutral and charged vacancies in the surface and subsurface layers. The results demonstrate that in nearly all cases molecular adsorption is strongly favored. For the most probable configurations on the surfaces, we also studied the migration paths and barriers, and found that water is highly mobile on the surface, even when adsorbed at defects. In general, ${\text{CaF}}_{2}$ and ${\text{SrF}}_{2}$ show similar behavior with respect to water, while adsorption energies and migration barriers for ${\text{BaF}}_{2}$ are smaller. Finally, we discuss our results in the context of recent experimental Atomic Force Microscopy studies on ${\text{CaF}}_{2}$ and compare to calculations on other insulating surfaces.

Acoustic behaviour of normal and densified vitreous GeO2

The acoustic properties of GeO2 glass were investigated by measuring acoustic attenuation and sound velocity between 1.5 and 300 K over the frequency range 10–50 MHz. A low-temperature plateau of attenuation followed by a large loss peak, paralleled by corresponding dispersive behaviour in sound velocity, were the main experimental observations. The temperature behaviour of ultrasonic attenuation is explained in terms of thermally activated relaxations of intrinsic structural defects at high temperatures (T > 10 K) and of tunnelling motions at low temperatures (T < 10 K). A direct comparison between the values of tunnelling strength, obtained over the MHz- and kHz-intervals, indicates that the internal-friction plateau arising from the phonon-assisted tunnelling of two-level systems (TLS) is slightly frequency-dependent. Analysis of the temperature dependence of sound velocity showed that anomalous hardening, typical of glasses having tetrahedrally coordinated structures, starts above ∼100 K. Finally, a comparison of sound velocity behaviour in normal and permanently densified GeO2 glasses demonstrates that densification leads to a significant reduction in the number of relaxing particles.