Grain Boundary Potential Research Articles

Study of Urbach tail, bandgap energy and grain-boundarycharacteristics in CdS by modulated photocurrentspectroscopy

Thin cadmium sulfide films were deposited on glass substrates bychemical bath deposition method. The planar photoconductivitywas measured as a function of the incident photon energy atdifferent temperatures.From the fit of experimental results to the existing models,various optoelectronic parameters of CdS, including those relatedto the temperature dependence of bandgap and the Urbach tail,were evaluated. The measurement technique also helped toevaluate the donor concentration (3×1016 cm-3) in the sample used.In addition to the existing models, the photoconductivity modelwas extended to account for the effect of incident light inlowering the grain-boundary potential barrier. This allowed themeasurement of both, the grain-boundary potential barrier (0.02-0.04 eV)and the energy of the dominant defect level at grain boundaries(0.11-0.13 eV).

Effect of Electric Field on the Migration of Grain Boundaries in Alumina

The effect of an external electric field on the grain‐boundary migration in Al2O3 ceramics has been investigated. The boundary migration is dependent on the direction and magnitude of the applied bias, and the observed boundary migration behavior is attributed to the presence of an electrostatic potential that inherently forms at the grain boundaries of Al2O3 ceramics. The results give experimental evidence that the boundary phenomena in oxide ceramics are related to the grain‐boundary potential.

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.

Grain Boundary Engineering for Superplasticity

The origin of heterogeneity of deformation and fracture has been discussed in relation to grain boundary sliding and migration in polycrystals. The effects of the grain boundary character distribution (GBCD) and geometrical configuration of grain boundaries on sliding-assisted deformation and fracture are particularly important issues to obtain a large and uniform deformation by avoiding intergranular fracture which restricts superplastic deformation in polycrystals. The contribution of sliding at random boundaries is considered as the major factor affecting superplastic deformation by grain boundary sliding. The optimization of grain boundary microstructure is likely obtained by controlling GBCD and the boundary area of random boundaries. It has been shown that an increase of the frequency of random boundaries can enhance deformation and result in a large deformation, from the most recent observations of grain boundary microstructure in fine-grained Al-Li alloy polycrystals superplastically deformed. The potential of grain boundary engineering for superplasticity is discussed.

Varistor characteristics in PTCR-type (Ba,Sr)TiO3 ceramics prepared by single-step firing in air

This paper deals with donor, acceptor-codoped (Ba0.4Sr.6)TiO3 ceramics with distinct varistor characteristics at room temperature, which were prepared by single-step firing in air. The materials, with the Curie point at around −90 °C, exhibited a large PTCR (positive temperature coefficient of resistivity) effect of more than seven orders of magnitude in the temperature range −90 °C (the resistivity ρ ≈ 103 Ω · cm) to room temperature (ρ > 1010 Ω · cm). An apparent dielectric constant of >20000 and tan δ < 0.05 (at 100 kHz) were observed for the present materials at room temperature, and moreover, the materials exhibited nonlinear current-voltage characteristics with the nonlinear coefficient, α, in the range 7–12 and the varistor field, Ev, in the range 0.3–1.0 kV/cm. The value of α in the present materials increased systematically with increasing ρ in their PTCR temperature range. It has been found that there exists a close correlation between α and the grain-boundary potential barrier height, eφ, obtained from the ρ-T characteristic of the materials. An almost linear relationship was also found to exist between α and log Ev for the present materials.

Influence of temperature and interface charge on the grain-boundary conductivity in acceptor-doped SrTiO3 ceramics

The temperature dependence of both the grain-boundary potential barrier height and the conductivity across the grain-boundary space-charge depletion layer in acceptor-doped SrTiO3 ceramics has been investigated by a numerical simulation technique. The underlying model is that of a back-to-back double Schottky barrier at the grain boundary. The influence of the amount of positively charged donorlike grain-boundary interface states on the charge transport behavior of the grain-boundary region is also discussed. An interpretation in terms of a defect chemistry model of the bulk and the space-charge depletion layer, which on both sides surrounds the grain-boundary core is presented. The temperature behavior of the potential barrier at the grain boundary can be divided into three different regimes: a linear regime, and subsequently, saturation and decreasing regimes. The theoretical explanation for this behavior is given. Two different spatial conductivity profiles at the grain boundaries have to be considered, which clearly can be identified by two different characteristic thermal activation energies of the effective grain-boundary conductivity. The barrier height itself is not equal to the the thermal activation energy of the effective grain-boundary conductivity. The electrical characteristics of the grain boundaries can be influenced deliberately by decorating the grain boundaries with suitable dopants.

Electrical properties of ZnSe-CdS alloy films

Thin films of ZnSexCdS1−x (t ∼ 0.6 μm) over the entire range of x, were deposited on glass substrates at two temperatures, Ts (350 and 470 K) by vacuum evaporation. X-ray diffraction studies showed that all the films were polycrystalline in nature. Films prepared at 470 K were nearly stoichiometric. Grain size increased with substrate temperature, Ts. The electrical conductivity and Hall measurements were carried out by d.c. van der Pauw technique. Hall effect studies/hot probe test showed that all the films were of n-type conductivity. Hall mobility increased with Ts. In addition, mobilities increased with temperature in films of all compositions, indicating the dominance of grain-boundary scattering. Grain-boundary potentials were in range 0.03–0.06 eV.

An analytical model for the above-threshold characteristics of polysilicon thin-film transistors

An analytical model for the above-threshold characteristics of long-channel, small-grain and thin channel polysilicon thin film transistors (TFT's) is presented. This model is constructed by considering the barrier potential and the carrier trapping effect at grain boundaries of the channel. A band tail state located at E/sub c/-0.15 eV is taken into account to simulate the I-V characteristics. Based on the model, the theoretically simulated results show good agreement with experimental data of plasma-passivated and unpassivated TFT devices in a wide range of gate and drain biases and temperature. The correlation of transconductance to gate bias is also investigated. It is found that the decrease of grain-boundary barrier potential with gate voltage enhances the transconductance, while this enhancement effect becomes insignificant and causes a decrease of transconductance at high gate bias.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Metallurgical factors influencing the corrosion of aluminum, Al-Cu, and Al-Si alloy thin films in dilute hydrofluoric solution

The corrosion behavior of sputter-deposited Al, Al-Cu, and Al-Si alloy thin films in dilute hydrofluoric (HF) acid solution was investigated. These materials maintain a thin aluminum oxide film in dilute HF solutions and, consequently, are susceptible to localized corrosion. Pit densities increase for the alloys with Cu and, to a lesser extent, Si additions. Open circuit potentials (OCP) are more positive for such alloys relative to the OCP of pure Al. Metastable pits in Al-Cu alloys are formed in Cu-depleted zones at grain boundaries which are galvanically coupled to adjacent θ-Al2Cu precipitates. Metastable pits in Al-Si alloys are formed in the Al matrix which is galvanically coupled to adjacent elemental Si nodules. θ-Al2Cu has different electrochemical characteristics than Al, even though both maintain a thin Al oxide in dilute HF solutions. θ-Al2Cu has a more positive OCP than pure Al and facilitates cathodic reactions at enhanced rates relative to pure Al. Hence, its presence raises the potential of the adjacent pure Al grain boundary to potentials which increase the probability of metastable pitting. Evidence is also presented which suggests that metastable pit growth may be cathode limited. A new hypothesis describing one mechanism by which θ-Al2Cu supports cathodic electron transfer re-actions is discussed.

Rapid thermal annealing of LPCVD-SOI substrates for sensor applications

Abstract Rapid thermal anneals (RTA) have been performed on boron and arsenic-implanted LPCVD polysilicon films. We have investigated the electrical properties of highly implanted films (4 × 10 14 , 8 × 10 14 and 5 × 10 15 cm −2 ) using Hall techniques. The experimental results are compared with those obtained on conventionally annealed (CTA) films. The electrical dopant efficiency and Hall mobility are measured at room temperature in the two series. We find that they are higher in RTA films. Thus the RTA-processed polysilicon substrates have the lowest resistivity. This result is of the highest importance in using implanted LPCVD amorphous- or polycrystalline silicon layers for low-cost, thin, film sensors. The density, N t , of trapping centers at grain boundaries (GB) is derived from electrical data (i.e., from the Hall carrier concentration and the GB potential barrier height, E B) . We find that it does not depend significantly on the implant species and heat-treatment. In contrast, it depends strongly on the implantation dose. The temperature coefficient of the resistivity is found to be positive or negative depending on the implantation dose. This striking result is clearly interpreted in terms of GB barrier potential scattering.

Metastable Pitting Corrosion of Aluminum, Al-Cu, and Al-Si Thin Films in Dilute HF Solutions and its Relevancy to the Processing of Integrated Circuit Interconnections

ABSTRACTWe have investigated the pitting corrosion of Al, Al-Cu, and Al-Si thin films in dilute hydrofluoric acid solution. Additions of Cu and, to a lesser extent, Si cause an increase in open circuit potentials (OCP), pit densities and pit sizes as compared to pure Al. Pits in Al-Cu alloys are associated with Cu depleted grain boundaries which are galvanically coupled to adjacent θ-phase precipitates. Pits formed in Al-Si alloys are associated with the Al matrix which is galvanically coupled to Si nodules. θ-Al2Cu has different electrochemical characteristics than Al even though both maintain a thin Al surface oxide. θ-Al2Cu has a more positive OCP, above the average pit repassivation potential for pure Al. Since θ-Al2Cu supports cathodic reactions at enhanced rates compared to pure Al, its presence raises the potential of the adjacent pure Al grain boundary to values near its average breakdown potential and above its average repassivation potential. Metastable pitting is thus promoted.

Effect of NO2 in air on the electrical conductance of In2O3 films with and without added ZnO prepared by R.F. Sputtering

The conductance of indium oxide films with and without added zinc oxide consisting of ultrafine particles with a mean particle size of ⩽30 nm, decreased by a factor of about 500 upon changing from air to 10 p.p.m. NO2 in air at 150° C. The dramatic decrease in the conduction caused by NO2 adsorption is mainly attributed to the increase in the activation energy,E, in the Arrhenius relationshipG=Goexp (-E/kT), while the pre-exponential factor,Go, is almost constant. The conductance measured in air and NO2 /air for the films with and without added zinc oxide can be fitted to this relationship in the temperature range below 250° C. The decrease in the carrier concentration is considered as the origin of the decrease in conductance caused by NO2 adsorption. The alternative explanation in terms of an increase in the grain-boundary potential, i.e. the mobility gap, can be ruled out because the depletion layer width is larger than the mean particle size.

The influence of Mn on the grain-boundary potential barrier characteristics of donor-doped BaTiO3 ceramics

Two compositions of BaTiO3 positive temperature coefficient of resistance ceramics, prepared identically except for the fact that a small addition of Mn (0.04 at. %) was made to one of them, were studied. The samples were sintered simultaneously in air at 1400 °C for 1 h and then annealed at 1200 ° for 5 h, using a muffle furnace. Room-temperature dielectric measurements in the audio- and radio-frequency ranges confirmed that Mn has a negligible effect on the bulk resistance. Arrhenius plots of resistivity vs 1/[Tε′m(T)] were found to give straight lines for Tc&amp;lt;T&amp;lt;Tmax (where ε′m is the relative permittivity of the specimen measured at a constant frequency of 30 kHz, Tc is the ferroelectric transition temperature, Tmax is the temperature corresponding to the maximum in resistivity, and T is the absolute temperature), in accordance with the well-known Heywang model. The height of the potential barriers at different temperatures, as calculated from the slopes of these plots, were found to increase by about 40% (from ∼0.34 to ∼0.50 eV) by the addition of Mn. A small increase in the acceptor-state density at the grain surfaces, which was again obtained from these plots, was observed in Mn-doped specimens (3.9×1013 cm−2 as compared to 2.7×1013 for Mn-free specimens). It was also found that the inclusion of Mn had a negligible effect on ε′m above Tc.

Dynamics of ordering in highly degenerate models with anisotropic grain-boundary potential: Effects of temperature and vortex formation.

Monte Carlo computer-simulation techniques are used to elucidate the equilibrium phase behavior as well as the late-stage ordering dynamics of some two-dimensional models with ground-state ordering of a high degeneracy, $Q$. The models are $Q$-state Potts models with anisotropic grain-boundary potential on triangular lattices---essentially clock models, except that the potential is not a cosine, but a sine function of the angle between neighboring grain orientations. For not too small $Q$, these models display two thermally driven phase transitions, one which takes the system from a low-temperature Potts-ordered phase to an intermediate phase which lacks conventional long-range order, and another transition which takes the system to the high-temperature disordered phase. The linear nature of the sine potential used makes it a marginal case in the sense that it favors neither hard domain boundaries, like the standard Potts models do, nor a wetting of the boundaries, as the standard clock models do. Thermal fluctuations nevertheless cause wetting to occur for not too small temperatures. Specifically, we have studied models with $Q=12 \mathrm{and} 48$. The models are quenched from infinite to zero as well as finite temperatures within the two low-temperature phases. The order parameter is a nonconserved quantity during these quenches. The nonequilibrium ordering process subsequent to the quench is studied as a function of time by calculating the interfacial energy, $\ensuremath{\Delta}E$, associated with the entire grain-boundary network. The time evolution of this quantity is shown to obey the growth law, $\ensuremath{\Delta}E(t)\ensuremath{\sim}{t}^{\ensuremath{-}n}$, over an extended time range at late times. It is found that the zero-temperature dynamics is characterized by a special exponent value which for the $Q=48$ model is $n\ensuremath{\simeq}0.25$ in accordance with earlier work. However, for quenches to finite temperatures in the Potts-ordered phase there is a distinct crossover to the classical Lifshitz-Allen-Cahn exponent value, $n=\frac{1}{2}$, for both values of $Q$. This supports the conjecture that the zero-temperature dynamics for models with soft domain boundaries belong to a special universality class, and that all models with nonconserved order parameter, independent of ordering degeneracy and softness and origin of domain boundaries, obey the classical growth law at finite temperatures. In quenches to the Potts-ordered phase vortices and antivortices occur and annihilate mutually without pinning the ordering process. The ordering dynamics for quenches into the intermediate phase is also found to be described by an effectively algebraic growth law.

Voltage dependence of dc resistance in polycrystalline ferrimagnetic materials

The voltage dependence of dc resistance of polycrystalline GdtY3−tFe5O12 (t=1.0, 1.1, 1.6, and 2.2) is experimentally studied at different temperatures. The data is explained on the basis of Krausse’s theory. It is found that gadolinium substitution in YIG does not affect the grain-boundary potential barrier height (φ0). The temperature, however, affects φ0. The parameter z, in Krausse’s model, which is found to be temperature independent, increases with the increase in the gadolinium substitution in YIG.

Thermionic emission diffusion model of current conduction in polycrystalline silicon and temperature dependence of mobility

In this paper a comprehensive model of current conduction in polycrystalline silicon (polysilicon) based on the thermionic-emission-diffusion (TED) theory is developed, and on the basis of this model an expression for the effective majority carrier mobility μeff is derived. This expression is quite general in nature and some thermionic emission (TE) theory based expressions for μeff can be obtained from it straightaway under certain simplifying assumptions. In addition, it helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results. Also, the experimental data on Hall mobility, which we obtained under an ohmic conduction regime in the 300–440 K temperature range for dark and illuminated conditions in lightly doped n-type polysilicon samples of different grain sizes, are presented and are interpreted on the basis of the TED model. Under strong illumination, the Hall mobility μHL was observed to vary with temperature T according to the relation μHL=aT−b, where a depended on grain size and was found to be smaller for the smaller grain size. The dark mobility data fitted well into the TED-based expression for μeff considering the interface states associated with grain boundaries to be localized at Ev+0.63 eV in the band gap. The analysis reveals that, generally, the scaling factor is needed if the effect of diffusion is neglected in comparison with the thermionic emission while in essence it is appreciable to be considered. However, in the TED model, as diffusion contribution in controlling the current transport across the grain-boundary potential barrier is well taken care of, the scaling factor is not required.

Electronic transport at grain boundaries in silicon

An experimental investigation is reported on carrier transport across isolated grain boundaries in large-grain cast silicon material. Continuous interface-state densities in the (${10}^{15}$-${10}^{16}$)-${\mathrm{m}}^{\ensuremath{-}2}$${\mathrm{eV}}^{\ensuremath{-}1}$ range are measured for the lower part of the silicon energy gap. The grain-boundary diffusion potentials for this material are shown in some cases to vary appreciably over a single grain-boundary plane; this is thought to be due to a nonuniform spatial distribution of interface charge at the grain boundaries. Numerical calculations by finite-element methods, of the quasi-Fermi-potentials in the vicinity of the grain boundary, suggest that the supply of majority carriers by diffusion to this interface may be the limiting factor controlling their transport. The experiments have consequently been interpreted according to the diffusion model rather than the customary thermionic-emission model of majority-carrier transport across grain boundaries.

On the transport theory of Schottky barriers to polycrystalline silicon thin films

A theoretical investigation of electronic transport in metal contacts to polycrystalline silicon thin films is presented. Calculations based upon the reported values of grain-boundary potentials indicate that the thermionic emission theory may be applied to the majority-carrier transport only for low bias voltages. At larger bias voltages, one needs to take account of the voltage lost to the space-charge regions adjacent to the grain boundaries, and a transition from electrode-limited to bulk-limited majority-carrier transport results. We further demonstrate that the injection of minority carriers can dominate the dark current for a range of grain size and interface state densities at the grain boundaries. Under these conditions, the current obeys an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\exp (qV/2kT)</tex> dependence reminiscent of space-charge recombination, although the origin of the current in this case is minority-carrier diffusion current, with recombination only at grain boundaries in the neutral region. This is a special case of the "high-injection" regime observed in single crystals, but in the present situation it is found even for low bias voltages as a consequence of the band bending at the grain boundaries, which makes the material nearly intrinsic at these points. Finally, we show that the effective minority-carrier diffusion length for the injected carriers under dark conditions itself increases with bias voltage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</tex> approximately as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\exp (qV/2kT)</tex> , in striking contrast to previous treatments.

Role of defects in determining the electrical properties of CdS thin films. I. Grain boundaries and surfaces

The relationship between the electronic properties of mobility and resistivity, and electron scattering from film defects in slowly deposited cadmium sulphide polycrystalline thin films is investigated. The correlation of these properties is accomplished in a direction parallel to the substrate (``in-plane'') where the defects of interest are grain boundaries and surfaces. The Petritz (grain boundary) theory is incorporated with surface-scattering theory to formulate a simple model which predicts the effect of these defects on the electron mobility. Typical values for the grain-boundary potential and surface-scattering length are 0.07 eV and 1100 Å at a substrate temperature of 180°C for these glass-deposited films. The mobility is found to depend strongly on grain size, which was a function of substrate temperature. Hall data are presented as a function of evaporation rate, substrate temperature, film thickness, and film temperature. Finally, the grain-boundary potential is found to depend on the fabrication parameters, especially the deposition rate.