Redistribution Of Vacancies Research Articles

Direct spectroscopic monitoring of conductance switching in polythiophene memory devices

A large number of nonvolatile memory devices have been reported with both inorganic and organic components, and many of these involve changes in device resistance between a high conductivity “ON” state and a low conductivity “OFF” state. The mechanism of memory action in many of these devices is uncertain, and may be based on many phenomena, including redox reactions, metal filament formation, charge storage in “floating gates”, and redistribution of oxide vacancies. We report here a Raman spectroscopic probe of organic polymer memory devices which permits direct monitoring of the doping state and conductivity of polythiophene in a 3-terminal device. The polymer conductance is controlled by voltage pulses between the source and gate electrodes in FET geometry, while the conductance state is read out by a separate circuit between source and drain. The conductance was directly correlated with the Raman determination of the density of polarons in the polymer film, which was shown to control both the “electroforming” process and the conductance switching in working memory devices. The polymer conductance change requires a redox counter-reaction at the gate electrode, and atmospheric effects on performance indicate that water and oxygen reduction are involved. The observations are consistent with a redox process between the gate and source electrodes which modulates the polaron concentration and source–drain conductivity. This mechanism provides a framework for optimization of the device by changing its composition and geometry, particularly the identity of the redox counter-reaction and control of ion mobility.

Analysis of Erbium and Vanadium Diffusion in Porous Silicon Carbide

Experimental data on diffusion of erbium and vanadium in porous and nonporous silicon carbide at 1700 and 2200°C have been used for modelling diffusion in porous SiC. It is shown that the consideration of pore structure modification under annealing via vacancy redistribution allows for satisfactory description of dopant diffusion. As expected, important contribution to the diffusion in the porous medium is found to be made by the walls of the pores: in SiC, the vacancy surface diffusion coefficient on the walls appears to exceed that in the bulk of the material by an order of magnitude. When thermal treatment transforms pore channels into closed voids, pathways for accelerated diffusion cease to exist and diffusion rates in porous and nonporous SiC become similar.

High-temperature redistribution of cation vacancies and irreversible magnetic transitions in the Fe1−x S nanodisks observed by the Mössbauer spectroscopy and magnetic measurements

The hexagonal pyrrhotite Fe1−x S nanodisks with the NiAs-type structure were synthesized by thermal decomposition of ferrous chloride and thiourea in oleylamine. The Mossbauer spectroscopy and magnetic measurements data indicate that a mixture of antiferromagnetic (AFM) and ferrimagnetic (FRM) phases with the NC (N ≥ 3) and 2C-type superstructures is present in the Fe1−x S compound at temperatures between 80 K and Neel temperature T N. At T 370 K, a redistribution of iron vacancies takes place, and the vacancy ordering transforms from the NC (N ≥ 3) to 2C-type which essentially increases the magnetization with maximum value at 470 K. Heating the sample above the Neel temperature 565 K leads to a random distribution of vacancies, and this state is quenched upon subsequent cooling of the sample to 300 K. This gives rise to a pure AFM structure with a zero magnetic moment due to a total compensation of the moments in neighboring iron layers. Thus, the high-temperature redistribution of cation vacancies leads to irreversible magnetic transformations in the Fe1−x S nanoparticles.

Diffusion in porous silicon carbide

By the example of vanadium and erbium diffusion in porous silicon carbide, the semiconductor porous structure modification during thermal annealing has been simulated and the effect of this modification on impurity diffusion has been considered. A comparison of calculated and experimental profiles of the erbium and vanadium distributions in porous silicon carbide shows that the consideration of porous structure modification due to vacancy redistribution makes it possible to adequately describe diffusion in the porous semiconductor.

Multiscale simulation on electromigration of the oxygen vacancies in metal oxides

The energetics and dynamics of electromigration of the oxygen vacancy is investigated with first-principles calculations and kinetic Monte Carlo methods. To simulate the charged oxygen vacancy under external fields within the first-principles approach, we introduce a slab model with electron-accepting dopants in the surface. The analysis of the density of states confirms that the oxygen vacancies are positively charged. When the external field is applied, the total energy of the slab linearly changes with respect to the position of the charged vacancy in the field direction, which allows for probing local permittivity around the vacancy site. The activation energy of vacancy migration is lowered along the field direction in a manner that the charge state of the vacancy is maintained along the migration path. Kinetic Monte Carlo simulations based on the first-principles inputs are also carried out and it is shown that the high-temperature condition is important for the fast redistribution of charged vacancies.

Pulsed laser deposited Y3Fe5O12 films: Nature of magnetic anisotropy II

Recently we found epitaxial Fe-deficient yttrium iron garnet Y3Fe5−tO12−3t/2 (YIG) films pulsed laser deposited onto the (111) and (001) face of Gd3Ga5O12 single crystal experience, respectively, strong rhombohedral and tetragonal distortions and possess unusual magnetic anisotropy. Using the crystal field theory, we explained these anomalies and showed in YIG(111) films ferric ion vacancies preferentially occupy octahedral complexes, order themselves along the [111] growth direction, cause rhombohedral lattice distortions, generate strong uniaxial and significantly reduce cubic crystalline anisotropy, Manuilov et al., [J. Appl. Phys. 106, 123917 (2009)]. Oxygen vacancies govern electrical neutrality and in spite of high concentration (in access of 7 at. %) they are perfectly ordered that is indicated by narrow ferromagnetic resonance linewidth. The present paper aims to compare growth anisotropies induced in (111) and (001) epitaxial Fe-deficient YIG films. Ordering of vacancies appeared to be different in (111) and (001) oriented YIG films. In highly Fe-deficient YIG(001) we found much smaller imbalance in redistribution of vacancies between the octa- and tetrahedral magnetic sublattices. This feature together with a weaker crystal field in tetrahedral Fe3+–O2− complexes result in reduced growth induced magnetic anisotropy in (001) oriented tetragonal distorted YIGs compared to rhombohedral distorted YIG(111) films.

Formation of Schottky-type metal/SrTiO3 junctions and their resistive properties

Motivated by the successful use of strontium titanate with different doping metals for memory cells on the basis of resistive switching and the recent findings on the major importance of oxygen vacancy redistribution in this compound, the present work shows the possibility of a non-volatile resistance change memory based on vacancy-doped SrTiO3. The formation of corresponding metal/SrTiO3−δ junctions (δ>0) in an electric field will be discussed as well as the switching between ohmic and Schottky-type contact behavior. A notable hysteresis in the current–voltage characteristics is used to carry out Write, Read, and Erase operations exemplifying the memory cell properties of such junctions. But whereas the electric field-induced formation of Schottky-type junctions is explainable by oxygen vacancy redistribution, the resistive switching needs to be discussed in terms of vacancies serving as electron trap states at the metal/oxide interface.

Thermodynamics of atomic ordering in nonstoichiometric transition metal monoxides

The XRD studies of structural vacancy redistribution in titanium, vanadium and niobium monoxides undergoing atomic-vacancy ordering or structural vacancy clusterization have shown the necessity of a many-cluster approach to model the free energy function of nonstoichiometric monoxides.

Pulsed laser deposited Y3Fe5O12 films: Nature of magnetic anisotropy I

Recently we sintered by pulsed laser deposition (PLD) technique the epitaxial Fe-deficient yttrium iron garnet (YIG) films with ferromagnetic resonance (FMR) linewidth as narrow as 0.9 Oe, the uniaxial anisotropy as high as Hu=−880 Oe, and demonstrated them feasible for magnetostatic waves band pass filter application [Manuilov et al., J. Appl. Phys. 105, 033917 (2009)]. Here we explore the origin of unusually high noncubic magnetic anisotropy. Using the angular resolved FMR spectroscopy we found that in addition to strong uniaxial anisotropy, cubic magnetic anisotropy experienced almost fivefold reduction compared to standard YIG grown by liquid phase epitaxy. Molecular field theory was employed to calculate saturation magnetization 4πMs, cubic magnetocrystalline K1, and uniaxial anisotropy Ku in garnets with Fe vacancies. The modeling utilizes crystal field parameters that we revealed from earlier published experimental data on diamagnetic ion substituted Y3Fe5O12 and Fe-substituted isomorphous diamagnetic garnets. Consistent single ion anisotropy crystal field theory perfectly fits experimentally observed high saturation magnetization, reduction in cubic, and appearance of strong uniaxial anisotropy in PLD-grown Fe-deficient YIG films. The redistribution of Fe vacancies between different magnetic sublattices was quantified and confirmed that in YIG(111) films ferric ions preferentially leave vacant octahedrally coordinated sites. Simulation of growth induced anisotropy proves the ordering of Fe3+ vacancies within octahedral sites. At equal number of available ferric ions and vacancies, the latter populate the octahedrons with distortion axis perpendicular to the film surface with the probability equal to 0.67. Deformation blockage of octahedral complexes with distortion axes directed along the film surface reduces this probability down to 0.14.

Vacancy ordering in γ-Fe 2O 3 nanocrystals observed by 57Fe NMR

The identification by 57Fe internal field nuclear magnetic resonance (NMR) of hyperfine fields at four Fe sites in the (average) tetragonal unit cell of vacancy-ordered γ-Fe 2O 3 (maghemite) is reported. The effects of vacancy redistribution due to annealing the partially vacancy-ordered form has been observed in the 57Fe lineshape. In addition, the reduction of the particle size of the vacancy-ordered form has been observed to gradually eliminate the vacancy ordering and then to cause a transition from ferrimagnetism to superparamagnetism.

Experimental Evidences of the Space Charge Activity in Pb(Ti,Zr)O3 and (Ba,Sr)TiO3 Ferroelectric Thin Film Structures

Highly inhomogeneous electronic structure of Pb(Ti,Zr)O3 sol-gel thin ferroelectric films with metal electrodes including a n-type semiconducting near-electrode layer and a p-n junction in the film's interior is revealed by means of a dielectric spectroscopy and photoelectric study. Charged oxygen vacancies accumulation near the electrodes is believed as a reason for a decrease of coercive field with decreasing a speed of a bias voltage variation during measurement of C-V dependences in Ni/PZT/Pt thin film structures. Slow capacitance relaxation and transient charging/discharging currents in laser ablated (Ba,Sr)TiO3 thin films with conducting oxide electrodes after a step voltage switching on and off are explained by the p-n junction formation due to the oxygen vacancy redistribution and near-electrode Schottky barriers charging/discharging processes.

Oxygen vacancy migration and time-dependent leakage current behavior of Ba0.3Sr0.7TiO3 thin films

The leakage current response of high-permittivity columnar-grown (Ba,Sr)TiO3 thin films has been studied at elevated temperatures under dc load. We observe a thermally activated current prior to the onset of the resistance degradation with an activation energy of EA=1.1eV. A point defect model is applied to calculate the migration of electronic and ionic defects under the dc field as well as the current response of the system. We find that the peak in current is not caused by a space-charge-limited transient of oxygen vacancies, but related to a modulation of the electronic conductivity upon oxygen vacancy redistribution. Furthermore, we show that after the redistribution of electronic and ionic defects, no further increase in conductivity takes place in the simulation.

GISAXS study of hydrogen implanted silicon

The grazing incidence small angle X-ray scattering (GISAXS) technique was used to study monocrystalline silicon samples implanted with H2 ions at an energy of 31keV and to the dose of 1016ions/cm2. Samples were annealed isochronally at different temperatures. Although the H depth distribution was expected to be smooth initially, nanosized features, like agglomerates of defects, have been detected. Annealing destroys this feature due to the relaxation of defects, i.e. redistribution of vacancies and hydrogen. Above 300°C a well defined film with highly correlated borders is formed on the edge of the layer rich in defects, whose thickness is slowly decreasing from 25 to 22nm with increasing annealing temperature. Moreover, defects as well as hydrogen are migrating towards the surface with increasing annealing temperature, as indicated by the increase in surface roughness.

Grazing incidence small-angle X-ray scattering study of defects in deuterium implanted monocrystalline silicon

Grazing incidence small-angle X-ray scattering was used to study monocrystalline silicon samples implanted with deuterium ions at an energy of 24 keV and to the dose of 2 × 1016 ions/cm2. Samples were annealed isochronally at different temperatures in the range from 393 to 973 K. Due to the relaxation of the defects structures, i.e. redistribution of vacancies and deuterium, strong particle like contribution is observed in addition to the rough surface scattering, already at 393 K annealing. During the annealing, this particles (agglomerations of vacancies) are gradually dissolved till 623 K annealing temperature. Another agglomeration mechanism takes over at about 773 K when a different type of particle growth is observed, and these are dissolved again at about 973 K. The sizes of detected particles are in 2–3 nm range. Also, the interference type of scattering from a film of about 30 nm thickness (the top layer, mostly unaffected by implantation) is observed. This film is gradually getting thinner with the increasing annealing temperature, due to the redistribution of the defects and the structure relaxation.

Slow Dielectric Relaxation in SrRuO 3 /Ba 0.8 Sr 0.2 TiO 3 /SrRuO 3 Ferroelectric Thin Film Capacitor Structures

Slow dielectric relaxation in barium strontium titanate thin film capacitor structures with strontium ruthenate electrodes has been investigated. It is shown that in the films containing a relatively high vacancy concentration (more than 10 17 cm m 3 ), slow capacitance relaxation during the time interval of several minutes after biasing switching on and off takes place due to the oxygen vacancy accumulation in near-cathode layer and the diffusional spreading of this layer after biasing switching on and off, respectively. At lower vacancy concentration (∼0.5 10 17 cm m 3 ), the considerable capacitance relaxation has not been obtained but transient currents under the step voltage stimulation appear in the time interval of 0.05-2.5 s. These currents are explained by near-electrode Schottky potential barrier capacitances charging and discharging, in combine with the oxygen vacancy redistribution in the film. Relations approximating the relaxation processes have been derived and the corresponding parameter values, close to the published data, have been obtained.

Rb-doped potassium titanyl phosphate for periodic ferroelectric domain inversion

This article reports the dielectric, conductive, and ferroelectric properties of Rb-doped potassium titanyl phosphate (RKTP) crystals, of formula type RbxK1-xTiOPO4, where x=0.0065 or 0.013, both of which are shown to undergo ferroelectric switching at room temperature with coercive fields varying from 2.3 to 2.7 kV/mm. The measured spontaneous polarizations are 0.18(±0.01) and 0.19(±0.01) Cm-2 for RbxK1−xTiOPO4 with x=0.0065 and x=0.013, respectively. The dependence of the conductivity of Rb:KTP on electric-field amplitude at high fields leads to distorted ferroelectric hysteresis loops. As a result of thermal effects and the redistribution of vacancies, the switching times and switching current peaks vary throughout a series of switching processes at a frequency of 20 Hz. The measured switching times range from 2 to 6 ms and are inversely proportional to the peak current densities. The dielectric relaxation of Rb:KTP is found to be further from the Debye response than that of KTP. A correlation effect is seen to play an important role in reducing the ac conductivity of Rb:KTP at low frequencies relative to that of KTP. The results are discussed in the context of obtaining a KTP derivative, which is suitable for periodic patterning by electric field poling at room temperature.

Thermodynamic lattice instability driving bulk amorphization in Eu 3+-doped Al 2O 3 mesoporous composites

X-ray diffraction, thermogravimetric analysis (TG), and differential thermal analysis (DTA) were performed on mesoporous Al 2O 3 doped with different amounts (up to 1.5 wt.%) of Eu 3+ cations into pores through an aqueous Eu 3+ solution. As such, the Eu 3+ cations occupy the pores in groups in crystalline Eu 2O 3 with an average 30-nm crystallite size. During the processing, the Eu 2O 3 nanoparticles in pores chemisorb oxygen (or moisture and other gases from air) in a huge amount in 45–55 wt.% on their high-energy surfaces through pores. On heating, it desorbs in steps leaving behind their surfaces, mostly at 450–700 K, leading to an instability of their crystalline structure. A redistribution of excess vacancies leads their phase transformation to an amorphous structure as an alternative to maintain an optimal Gibb's free energy. It occurs by an anomalously fast local diffusion of Eu 3+/Al 3+ cations through mobile vacancies in pores. It results in an endothermic peak at 520–650 K in DTA as per the Eu 3+ contents. The results present a new method for processing bulk amorphous solids in doped Al 2O 3 ceramics.

The effect of cyclic deformation in a 13% Cr ferritic steel on structure and Barkhausen noise level

It has been demonstrated that there is a relation between changes in the microstructure of the chromium ferritic steel and the Barkhausen noise (BN) intensity resulting from cyclic deformation. The non-linear dependence of the BN on the number of cycles has been shown. The dislocations formed in the course of cyclic deformations have been identified as prefect screw ones of Burgers vector b= a/2 1 1 ̄ 1 . It was found that both the cell size and dislocation structure and redistribution of vacancies considerably affect the BN intensity. Dislocation structures and redistribution of vacancies have been correlated with the course of the BN intensity, which enables non-destructive assessment of the condition of the chromium ferritic steel microstructure subjected to cyclic deformation. Persistent slip bands were identified after a large number of cycles, close to the crack initiation. They are of the highest density near the crack tip. Their structure is similar to that of deformation bands in a heavily deformed material.

Slow capacitance relaxation in (BaSr)TiO3 thin films due to the oxygen vacancy redistribution

Capacitance relaxation in Ba0.8Sr0.2TiO3 thin film structures has been investigated. Slow decrease of the capacitance after a bias voltage switching on is explained by suppressing the film dielectric permittivity by the field of a p-n junction originated as a result of the oxygen vacancy migration in the bias field. After the bias switching off, the p-n junction gradually disappears due to the vacancy diffusion, and the capacitance increases. The relation determining capacitance increase after the bias switching off has been obtained in agreement with experimental data. The proposed relaxation mechanism is considered as a certain type of the size effect when the p-n junction depleted region spreads over the film thickness.

Modeling electromigration and the void nucleation in thin-film interconnects of integrated circuits

The modern tendency for increasing the productivity of microelectronic devices at the expense of the size shrinkage and the development of densely packed multilevel microelectronic structures stipulates the rising concern for the reliability of integrated circuits. The damage of integrated circuits is mainly caused by electromigration in thin-film interconnects. The current-induced redistribution of vacancies and the action of vacancy sinks/sources lead to heterogeneous volume deformations, which, in turn, cause the rise of mechanical stresses. The interconnect failure is initiated by the nucleation of voids taking place on the crystalline structure heterogeneities like triple points, inclusions, etc. or in the plug region of multilevel metallizations. In the latter case the interconnect damage is also caused by the edge depletion. Mechanical stresses induced by electromigration strongly influence the nucleation process. In the present work we propose a general 3D model for electromigration and the rise of mechanical stresses in a passivated aluminum interconnect. A system of differential equations describing electromigration and induced deformation of an interconnect is derived. We also propose a kinetic model for the void nucleation, elaborated on the basis of the classical theory of the new phase nucleation. Integral equations for the time to the void nucleation are deduced. Based on these models numerical calculations for the void formation in a triple point of the interconnect crystalline structure and for both failure mechanisms in the plug region have been carried out. The times to nucleation and characteristic sizes of voids are calculated as functions of temperature and electric current density. The results obtained agree well with experimental data.