Direct Observation of the Barrier Structure of a Praseodymium-Doped Grain Boundary in a Zinc Oxide Bicrystal by Charge Collection Microscopy

A zinc oxide bicrystal, in which the grain boundary was doped with praseodymium oxide, was prepared by hot pressing. The grain boundary showed characteristic varistor breakdown. Scanning electron microscope (SEM) observation revealed that the amount of visible praseodymium oxide along the interface was variable. Charge collection microscopy, using the remote electron beam induced current (REBIC) configuration, was carried out and the grain boundary was found to be electrically active in the praseodymium oxide rich regions, showing contrast consistent with the presence of a double Schottky barrier (DSB). In Pr poor regions the DSB contrast was lost. The observations are consistent with the idea of barrier enhancement with doping in varistors.     

Relationship between Electrical Activity and Grain Boundary Structural Configuration in Polycrystalline Silicon

Temperature dependent electron beam induced current (EBIC) technique has been applied to investigate the electrical activities of grain boundaries (GBs) in polycrystalline silicon. The GB character, misorientation and orientation of GB plane, were analyzed using a FE-SEM/EBSP/OIM system prior to the EBIC measurements. The EBIC contrasts were found to depend on GB character; low GBs showed weak contrasts compared with general GBs at any temperatures, and also demonstrated to vary at GB irregularities such as boundary steps. These results indicate that electrical properties depend on the orientation of the GB plane as well as the misorientation. On the other hand, there existed less differences in temperature dependence of EBIC contrast irrespective of GB characters. The EBIC contrast decreased with increasing temperature, showed a minimum around 250 K, then increased again with further increasing temperature. The resulting temperature dependence of EBIC contrast probably comes from the combination of two types of recombination processes of carriers. One is related to a shallow level associated with an inherent GB structure, though the exact energy levels also would probably depend on GB structures, and the other to a deep level associated with impurities segregated at GBs, which acts as recombination center.     

Mechanisms of reversible conformational transitions in a single molecule

The reversible interconversion between two nonplanar conformations of single Zn(II) Etioporphyrin I molecules adsorbed on a NiAl(110) surface at 13 K was induced by a scanning tunneling microscope (STM). The threshold voltage for the conformational change at negative sample bias depends linearly on the tip-sample distance, suggesting an electrostatic force mechanism. The reverse conversion involves inelastic electron tunneling via a molecular electronic resonance at 1.25 eV. In contrast with the photon-induced conformational changes, an electrically induced mechanism is realized with the STM.     

Comparison between the EBIC and XBIC contrasts of dislocations and grain boundaries

The X-ray-beam-induced current (XBIC) method is used to calculate the contrasts of dislocations and grain boundaries perpendicular to a surface as a function of the diffusion length of minority charge carriers and the X-ray probe width. The results are compared with the contrasts of the same defects determined via the electron-beam-induced current (EBIC) techniques. It is demonstrated that the XBIC contrasts of grain boundaries and dislocations can be several times greater than those obtained in the EBIC mode in the case of a rather narrow X-ray beam. The XBIC contrast always exceeds that of EBIC in semiconductors with a large diffusion length even if the X-ray beam is rather wide.     

Effect of boundary treatments on quantum transport current in the Green’s function and Wigner distribution methods for a nano-scale DG-MOSFET

In this paper, we conduct a study of quantum transport models for a two-dimensional nano-size double gate (DG) MOSFET using two approaches: non-equilibrium Green’s function (NEGF) and Wigner distribution. Both methods are implemented in the framework of the mode space methodology where the electron confinements below the gates are pre-calculated to produce subbands along the vertical direction of the device while the transport along the horizontal channel direction is described by either approach. Each approach handles the open quantum system along the transport direction in a different manner. The NEGF treats the open boundaries with boundary self-energy defined by a Dirichlet to Neumann mapping, which ensures non-reflection at the device boundaries for electron waves leaving the quantum device active region. On the other hand, the Wigner equation method imposes an inflow boundary treatment for the Wigner distribution, which in contrast ensures non-reflection at the boundaries for free electron waves entering the device active region. In both cases the space-charge effect is accounted for by a self-consistent coupling with a Poisson equation. Our goals are to study how the device boundaries are treated in both transport models affects the current calculations, and to investigate the performance of both approaches in modeling the DG-MOSFET. Numerical results show mostly consistent quantum transport characteristics of the DG-MOSFET using both methods, though with higher transport current for the Wigner equation method, and also provide the current–voltage (I–V) curve dependence on various physical parameters such as the gate voltage and the oxide thickness.     

Defect chemistry of grain boundaries in proton-conducting solid oxides

The defect chemistry of charged grain boundaries in an acceptor-doped oxide in equilibrium with water vapour is examined theoretically. The basis of the theoretical approach is that the formation of charged grain boundaries and attendant space-charge zones is governed by differences in the standard chemical potentials of oxygen vacancies and hydroxide ions between bulk and grain-boundary core, that is, by the thermodynamic driving energies for defect redistribution. A one-dimensional continuum treatment is used to predict the space-charge potential and defect concentrations in the grain-boundary core as a function of water partial pressure, temperature and acceptor dopant concentration for various values of the two thermodynamic driving energies. The results are discussed with respect to experimental data in the literature for acceptor-doped perovskite oxides (e.g. BaZrO₃) and fluorite oxides (e.g. CeO₂).     

Static and Dynamic Electron Holography of Electrically Active Grain Boundaries in SrTiO3

The dynamics of electrostatic potential barriers at grain boundaries (GBs) in Nb-doped SrTiO₃ bicrystals is investigated using a unique combination of bulk and in-situ TEM electrical measurements across isolated GBs, coupled with electron holography under in-situ applied bias. The Nb bulk-doped bicrystals exhibit a positive GB potential that suppresses reversibly under applied bias greater than the nonlinearity threshold in the current-voltage curve. This suppression is interpreted as break-down of the potential barrier to current transport.The results on Nb bulk-doped bicrystals have been compared to those in which Mn has been added as a grain boundary specific dopant. This acceptor doping of the grain boundary causes an appreciable increase in the grain boundary resistance and extension of the nonlinear regime. A preliminary account of static electron holography shows a relatively flat potential profile across the GB, indicating probable compensation of donor states at the GB core with Mn-acceptors. Interestingly, the phase profile under applied bias in this case exhibits a reversible dip at the GB which is interpreted as an activation of GB trap states due to Mn-acceptor dopants trapping extra electrons (the majority charge carriers) at the GB core, inducing a negative GB potential, and diminishing current transport until the threshold bias is exceeded.The synergistic combination of nanoscale TEM measurements coupled with traditional macroscopic electrical measurements is emphasized.     

Analysis of electro-active regions and conductivity of BaMnO3 ceramic by impedance spectroscopy

Polycrystalline BaMnO₃ ceramic powders were prepared using the conventional mixed oxide route accompanied with several milling processes. Single phase formation was verified by recording the X-ray diffraction pattern of the powder as well as sintered pellet at room temperature. Scanning electron micrograph and energy dispersive X-ray spectrum of cross-sectional view have shown that sintered pellet is highly porous and contains only Ba, Mn and O elements, respectively. Analysis of impedance spectroscopy was carried out via the complex impedance and complex modulus formalisms. These results have shown that BaMnO₃ behave as semiconducting material. Furthermore, as a consequence of electrically inhomogeneous nature of the sample, it was observed that the electroactive regions (such as grain, grain boundary and sample-electrode interface) are overlapped in the applied frequency domain with dominant grain boundary effect. An equivalent circuit model (R _g C _g)(R _gb Q _gb)(R _e Q _e) was employed to fit the temperature dependent impedance spectroscopy data. Study of grain and grain boundary conductivities suggest that grains are more conductive than grain boundaries and conduction mechanism followed correlated barrier hopping (CBH) model.     

Effect of contamination with iron on the electron-beam-induced current contrast of extended defects in multicrystalline silicon

The effect of contamination with iron on the recombination activity of extended defects in multicrystalline silicon has been studied by the electron-beam-induced current (EBIC) technique. It has been shown that this process does not lead to the appearance of EBIC contrast of the ??3 and ??9 grain boundaries. It has been revealed that iron diffusion results in a significant increase in the contrast of dislocations introduced by plastic deformation and of traces behind the dislocations in single-crystal silicon, while the dislocation contrast in multicrystalline silicon remains practically unchanged.     

Defect detection in solar cells via electroluminescence,LBIC, and EBIC methods

Structural defects affecting the efficiency of multicrystalline silicon solar cells are investigated by the electron-beam-induced current (EBIC) mode of a scanning electron microscope and the laser-beam-induced current (LBIC) method. It is experimentally demonstrated that the LBIC technique is more sensitive to electrically active 2D defects than the EBIC approach at large values of the diffusion length and excitation-beam penetration depth. The comparison of LBIC (or EBIC) and electroluminescence images enables us to reveal the correlation between solar-cell short circuits and electrically active structural defects.     

Microstructural evolution of AZ31 magnesium alloy subjected to sliding friction treatment

Microstructural evolution and grain refinement mechanism in AZ31 magnesium alloy subjected to sliding friction treatment were investigated by means of transmission electron microscopy. The process of grain refinement was found to involve the following stages: (I) coarse grains were divided into fine twin plates through mechanical twinning; then the twin plates were transformed to lamellae with the accumulation of residual dislocations at the twin boundaries; (II) the lamellae were separated into subgrains with increasing grain boundary misorientation and evolution of high angle boundaries into random boundaries by continuous dynamic recrystallisation (cDRX); (III) the formation of nanograins. The mechanisms for the final stage, the formation of nanograins, can be classified into three types: (i) cDRX; (ii) discontinuous dynamic recrystallisation (dDRX); (iii) a combined mechanism of prior shear-band and subsequent dDRX. Stored strain energy plays an important role in determining deformation mechanisms during plastic deformation.     

Microstructures of grain boundaries in (001)-oriented strontium bismuth tantalate thin films grown by pulsed laser deposition

(001)-oriented strontium bismuth tantalate thin films have been grown on Pt/TiO₂/SiO₂/Si (100) substrates by pulsed laser deposition. The room-temperature current–electric field dependence of the films has been investigated, which revealed a space-charge-limited conduction mechanism. The microstructures of grain boundaries and structural defects in these films were also examined by transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The grains of the films deposited at 550 °C exhibited polyhedral morphologies, and the average grain size was about 50 nm in length and 35 nm in width. At a small misorientation angle (8.2°) tilt boundary, a regular array of edge dislocations with about 3-nm periodic distance was observed, and localized strain contrast near the dislocation cores was also observed. The Burgers vector b of the edge dislocation was determined to be [110]. At a high misorientation angle (39.0°) tilt grain boundary lattice strain contrast associated with the distortion of lattice planes was observed, and the mismatching lattice images occurred at about 2 nm along the boundary. The relationship between microstructural defects at grain boundaries and leakage currents of these films is also discussed. Received: 8 September 2000 / Accepted: 18 December 2000 / Published online: 28 February 2001     

Study of the electrical and optical properties of silicon containing oxygen precipitates

The properties of n-type silicon with oxygen precipitates introduced by three-stage annealing were studied by the electron beam induced current (EBIC) method, deep-level transient spectroscopy (DLTS), and photoluminescence (PL). The presence of extended defects with concentration of ≤10⁹ cm^?3 is revealed by the EBIC method. The concentration of electrically active defects formed in silicon due to oxygen precipitation is estimated from the EBIC contrast and is compared to that obtained from the DLTS data. Comparing the spectra of samples with oxygen precipitates with those of plastically deformed crystals, we can assume that the DLTS and PL spectra of silicon with oxygen precipitates are mainly determined by dislocations.     

Electrical activities of stacking faults and partial dislocations in 4H-SiC homoepitaxial films

The electrical activities of stacking faults (SFs) and partial dislocations in 4H-SiC homoepitaxial films were investigated by using the electron-beam-induced current (EBIC) technique. The basal plane dislocation was dissociated into Si_(g) 30^° and C_(g) 30^° partials under electron-beam irradiation, with a SF formed in between. The SF shows bright contrast at RT and dark contrast at 50 K in EBIC images. The reasons were discussed according to the quantum-well state of SF. C_(g) 30^° partial is always more electrically active than Si_(g) 30^° partial at each specific accelerating voltage. The EBIC contrasts of those two partials were discussed with the number of recombination centers.     

Dependence of grain boundary character distribution on the initial grain size of 304 austenitic stainless steel

Abstract

In order to study the dependence of the grain boundary character distributions (GBCD) on the grain size, annealing treatment was carried out on 304 austenitic stainless steel with different initial grain sizes. The evolution of the GBCD was analysed by electron backscatter diffraction. The experimental results showed that abnormal grain growth (AGG) occurred when grain size was small. With a smaller initial grain size, the number density of abnormally large grains and the fraction of low-Σ CSL boundaries increased but the size of abnormally large grains decreased and the random boundaries presented a continuous network. With a larger initial grain size, the fraction of low-Σ CSL boundaries also increased as well as the size of abnormally large grains but the number density of abnormally large grains decreased and the connectivity of random boundary network was disrupted by low-Σ CSL boundaries, especially Σ3ⁿ (n = 1, 2, 3) boundaries. However, with a very large initial grain size, normal grain growth (NGG) occurred, which had no effect on the fraction of low-Σ CSL boundaries and the connectivity of random boundary network.     

Chevron defect at the intersection of grain boundaries with free surfaces in Au

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90 degrees <110> tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.     

Correlation among grain boundary character,carbide precipitation and deformation in Alloy 690

The correlation among grain boundary character, carbide precipitation and deformation in the grain boundary engineering (GBE) treated Alloy 690 samples with and without pre-deformation aged at 715^oC for 15?h was analysed by scanning electron microscopy and electron backscatter diffraction. The fraction of low Σ coincidence site lattice (CSL) grain boundary was enhanced by GBE treatment. The fraction of Σ3 grain boundary decreased, and most of Σ9 and Σ27 grain boundaries disappeared in the deformed GBE samples. After aging treatment, bigger carbide precipitated at coherent Σ3 grain boundary, however, most of plate-like carbide precipitated at incoherent Σ3 grain boundary disappeared in the pre-deformed GBE samples. The larger carbide precipitated on the random grain boundary in the 5% pre-deformed sample, while smaller carbide can be observed in the 15% pre-deformed sample. During the in situ tensile test of the aged GBE samples, grain boundary carbide migrated with the grain boundary migration. The slip bands go across Σ3 grain boundary directly, but cannot go across other grain boundaries. The high density of carbide plate precipitated near incoherent Σ3 and Σ9 grain boundaries can resist the evolution of slip bands. Compared to the Σ3 and Σ9 grain boundaries, Σ27 and random grain boundaries are more easily to form microcrack during deformation. The initiation of grain boundary microcrack not only related to the character of grain boundary but also related to the character of nearby grain boundaries. The phase interface of carbide and matrix is another region to initiate the microcrack.     

Superconductivity as a consequence of an ordering of the electron gas zero-point oscillations

It is shown that the temperature dependence of the value of energy gap in superconductors is characteristic for the order–disorder transition. The obtained relationship between the critical parameters of the Bose–Einstein condensate of electrically charged particles is in accordance with measurement data of superconductor critical parameters. The dependence of the critical temperatures for both – I and II – type superconductors and their Sommerfeld constants (and their Fermi energies) is obtained. It is marked that among the high-temperature superconducting ceramics there are the both – I and II – type superconductors. In total the phenomenon of superconductivity is seen as a consequence of ordering into the zero-point oscillation system of the electron gas in a metal at low temperatures. The obtained estimations of the interaction of zero-point oscillations are in satisfactory agreement with the data measurements of critical parameters of I and II type superconductors.     

In situ and tomographic analysis of dislocation/grain boundary interactions in α-titanium

In situ straining in the transmission electron microscope and diffraction-contrast electron tomography have been applied to the investigation of dislocation/grain boundary and dislocation/twin boundary interactions in α-Ti. It was found that, similar to FCC materials, the transfer of dislocations across grain boundaries is governed primarily by the minimization of the magnitude of the Burgers vector of the residual grain boundary dislocation. That is, grain boundary strain energy density minimization determines the selection of the emitted slip system.     

A method for continuous monitoring of quantum tunneling in a molecular double dot

An application of impedance measurement technique (IMT) for a detection of quantum tunneling in molecular structures is investigated. A charged particle which tunnels in a two-well potential is electrically coupled to a high-quality superconducting LC-circuit (tank) that makes possible a measurement of the electric susceptibility of the molecule at the resonant frequency of the tank. The real part of this susceptibility bears information about the tunneling rate through a measurable parameter—a phase angle between the tank voltage and a bias current applied to the tank. It is shown that the present approach is highly sensitive and allows the monitoring of the tunnel motion of charged nuclei in a single molecule.