Grain Boundary Potential Barrier Research Articles

Pressure-Engineered Ti3C2Tx MXene with Enhanced Conductivity and Accelerated Reaction Kinetics of Lithium Storage.

We studied the structure-function relationship of compressed Ti3C2Tx MXene using high-pressure in situ synchrotron radiation, impedance spectroscopy, Hall effect measurements, and first-principles calculations. With increasing pressure, the conductivity of Ti3C2Tx MXene increases along with its continued lattice shrinkage. A pressure range of 0.4-2.2 GPa exhibits a sharp decrease in resistance, which decreases by more than one order of magnitude from 3.3 × 104 to 1.4 × 103 Ω. A pressure range of 2.2-6.6 GPa exhibits a steady resistance with a slight decrease of 0.2%. As the pressure drops to atmospheric conditions, the resistance increases slightly to 4.2 × 103 Ω. This is accompanied by a transformation of the semiconductor into metal. An irreversible increase in conductivity is observed owing to an increase in the electron concentration and a decrease in the grain-boundary potential barrier. Furthermore, abundant Ti3C2Tx undergoing prepressure treatments (0.4, 2.0, and 4.0 GPa) was first prepared using a double-anvil hydraulic press. The recycled samples retain an accordion-like layered structure with slight lattice shrinkage while the voids between the sheets contract considerably, increasing the density. Correspondingly, electrochemical results show a pressure threshold of 2.0 GPa based on the rapid quenching from the hydraulic press. This weakens the electric polarization in redox reactions and increases the ionic transport rate for the formation of a Ti3C2Tx anode owing to pressure improving the conductivity and interlaminar densification. Our study shows a new, simple, and universal way to regulate various MXenes and also promotes the application of MXene-based materials in energy storage and related fields.

The Hydrogenation Impact on Electronic Properties of p-Diamond/n-Si Heterojunctions.

The undoped polycrystalline diamond films (PDFs) have been deposited on n-type silicon (Si) by Hot Filament Chemical Vapor Deposition (HF CVD) technique. The reaction gases are a mixture of methane and hydrogen. The obtained PDFs were characterized by scanning electron microscopy (SEM) and Raman spectroscopy which, in addition to the diamond phase, also confirms the presence of sp hybridized carbon bonds. As-grown CVD diamond layers are hydrogen terminated and show p-type conductivity. The effect of the level of hydrogenation on the electrical properties of p-diamond/n-Si heterojunctions has been investigated by temperature dependent current–voltage (J-V/T) characteristics. The obtained results suggest that the energy distribution of interface states at the grain boundary (GB) subjected to hydrogenation becomes shallower, and the hole capture cross-section can be reduced. Hydrogenation can lead to a significant reduction of the GB potential barrier. These results can be interesting from the point of view of hydrogen passivation of GBs in microelectronics.

Fabrication of in-situ grown and Pt-decorated ZnO nanoclusters on new-type FTO electrode for room-temperature detection of low-concentration H2S

In this work, an in-situ grown and Pt-decorated ZnO nanoclusters (NCs) gas sensor was fabricated by employing etched FTO glass as the growth substrate and gas-sensing electrode via facile dip-coating assisted hydrothermal route and physical mixing method, which exhibited superior gas-sensing performance for low-concentration H2S at room temperature. Benefiting by the in-situ growth, good ohmic contacts between ZnO NCs and FTO electrode are formed, which maximize the gas-sensing performance and enhancing the stability of the sensor. On this basis, ZnO NCs with top cross-linked structure can provide adequate grain-boundary potential barriers and self-assembled electron pathways, as well as larger effective surface area and gas diffusion space. What's more, Pt nanoparticles (NPs) decoration on the surface of ZnO NCs can introduce more catalytic sites and proper Schottky barriers for gas-sensing tests. Due to their synergistic effects, the in-situ grown and Pt-decorated ZnO NCs sensor exhibit excellent room-temperature gas-sensing behaviors for H2S. Among samples, 1.5 at% Pt-decorated ZnO NCs sensor possesses the highest response value, excellent selectivity, repeatability and stability, which achieves the room-temperature detection of H2S at ppb level. In addition, the effect of Pt decoration on the gas-sensing behaviors under high temperature condition and the mechanism of enhancing gas-sensing performance were also systematically investigated.

Grain-boundary-limited carrier mobility in polycrystalline silicon with negative temperature dependence: modeling carrier conduction through grain-boundary traps based on trap-assisted tunneling

Seto’s model based on thermionic emission (TE) theory explains that the grain-boundary (GB)-limited carrier mobility in polycrystalline silicon (poly-Si) has a positive temperature dependence. Although this model is widely accepted as a standard model for GB-limited carrier mobility in poly-Si, many experimental measurements under steady-state conditions support extremely weak or negative temperature dependence. In this report, we formulate carrier conduction through GB traps by utilizing the trap-assisted tunneling (TAT) model based on non-radiative multi-phonon transition theory. Self-consistent calculation of poly-Si under steady-state conditions reveals that TAT current exceeds TE current in regions with high GB potential barriers. In contrast to Seto’s model, our novel model reproduces the negative dependence of GB-limited carrier mobility on temperature. From the viewpoint of consistency with experimental measurements, our model seems to be suitable for describing carrier conduction in poly-Si under steady-state conditions.

HUMIDITY SENSORS BASED ON SnO2 -Co3 O4 -Nb2 O5 -Cr2 O3 SEMICONDUCTOR VARISTOR CERAMICS

The varistor effect in SnO2 -Co3 O4 -Nb2 O5 -Cr2 O3 ceramics with different additions is accompanied by a strong humidity-sensitivity effect in the low-field electrical conductivity. The low-field electrical conductivity and dielectric permittivity of ceramics are increased when air relative humidity rises from 10 to 93%. Such effects are observed both in the samples prepared by solid-phase sintering without or with CaO addition and in the samples prepared by liquid-phase sintering with CuO or V2 O5 additions. The lowest humidity-sensitivity effect was found in the liquid-phase sintering samples due to the more solid structures and the secondary phases which are located between the SnO2 grains. For the studied samples the values of humidity sensitivity coefficient 1.8·103 -3.2·105 at low electric fields and the nonlinearity coefficient 30-57 at high electric fields (E1 = 5.3-7 kV·cm-1) were calculated. The properties of oxide ceramics are explained by electrical conductivity which is controlled by the grain-boundary potential barriers. The barrier height decreases with the increase both of air relative humidity at low electric fields and of voltage at high electric fields. Strong humidity sensitivity is a key factor for sensor application of SnO2 - based ceramics.

Electron Mobility across Grain Boundaries in Graphene Synthesized using Chemical Vapor Deposition Process

Chemical vapor deposition (CVD) is currently the only method for large-scale synthesis of graphene. However, the CVD process introduces grain boundaries (GBs) when individual grains coalesce with various mismatch angles. These GBs contain atomic dislocations and defects, which are believed to alter graphene's mechanical, electrical, and thermal properties. Specifically, the GBs can act as “potential barriers” when charges move from one grain to neighboring grains. This barrier effect will not only change the electrical conductivity but also the thermal conductivity of graphene. Besides high-resolution, 3-dimensional topography images, Atomic force microscopy (AFM) can also obtain the electrical properties at the nanoscale. In this report, the potential barrier effect of graphene GBs is studied with AFM. During the experiment, the probe is brought into contact with the graphene while positively (or negatively) biased. This process injects net charges into the graphene. The electrostatic potential across the GBs can be measured by AFM as an indication of the potential barrier effect. GBs with lower potential difference correspond to lower potential barrier, and vice versa. The dependency of the barrier effect on the mismatch angles was also measured. Considering the 6 folds’ symmetry of graphene atomic lattice, the mismatch angle is in the range of 0° ∼ 30°, with 30° the maximum mismatch angle. Our results can be well fitted with a sinusoidal function with π/3 period, which supports our hypothesis that higher mismatch angle contains higher density of dislocations and defects that increase the potential barrier of GBs.

Temperature Dependence According to Grain Boundary Potential Barrier Variation in Vertical NAND Flash Cell with Polycrystalline-Silicon Channel.

We investigate the electrical characteristics according to changing temperature on trap distribution in the energy gap of grain boundary (GB) and interface trap density (D(it)) between polycrystalline-silicon (poly-Si) channel and tunnel oxide in Vertical NAND (VNAND) flash cell with poly-Si channel. We confirmed that there are two factors changing GB potential barrier height such as trap distribution in GB and D(it) using technology computer-aided design (TCAD) simulation. Also, we found that the electrical characteristics according to changing temperature are significantly dependent on height and position of GB potential barrier in VNAND flash cell with poly-Si channel. We expect that it is required to develop more accurate extraction method for trap distribution in each GB and D(it) for better understanding temperature dependence of electrical characteristics in VNAND Flash cell.

The effect of quenching on semiconductive properties and magnetic phase transition temperature of multiferroic Pb(Fe1/2Nb1/2)O3 ceramics

ABSTRACTNeel temperature of Pb(Fe1/2Nb1/2)O3 ceramics sintered at 1120°C and quenched to room temperature was revealed to be 25 K higher than that of the same ceramics slowly cooled at a rate of 50 K/h. A small singlet component, typical for compositionally ordered perovskites, is present in the room-temperature Mossbauer spectrum of the slowly cooled sample. It proves that the quenched sample is more compositionally disordered than the slowly-cooled one. Substantially higher room-temperature resistivity values and positive temperature coefficient of resistivity of the slowly cooled sample are attributed to the formation of the grain-boundary potential barriers due to oxidation during slow cooling.

Probing Grain-Boundary Chemistry and Electronic Structure in Proton-Conducting Oxides by Atom Probe Tomography.

A laser-assisted atom-probe-tomographic (LAAPT) method has been developed and applied to measure and characterize the three-dimensional atomic and electronic nanostructure at an yttrium-doped barium zirconate (BaZr0.9Y0.1O3-δ, BZY10) grain boundary. Proton-conducting perovskites, such as BZY10, are attracting intense interest for a variety of energy conversion applications. However, their implementation has been hindered, in part, because of high grain-boundary (GB) resistance that is attributed to a positive GB space-charge layer (SCL). In this study, LAAPT is used to analyze BZY10 GB chemistry in three dimensions with subnanometer resolution. From this analysis, maps of the charge density and electrostatic potential arising at the GBs are derived, revealing for the first time direct chemical evidence that a positive SCL indeed exists at these GBs. These maps reveal new insights on the inhomogeneity of the SCL region and produce an average GB potential barrier of approximately 580 mV, agreeing with previous indirect electrochemical measurements.

Origin of Oxygen-Induced Abnormal Hump in Bottom-Gated Polycrystalline Zinc Oxide Thin Film Transistors

In this paper, we investigate the origin of abnormal hump after O2 annealing in bottom-gated polycrystalline ZnO thin film transistors (TFTs). The channel width dependency and X-ray diffraction (XRD) analysis show that the hump is not related with the parasitic edge or highly conductive upper layer by the superior crystal quality to channel/gate oxide interface. It is generally known that there is the grain boundary potential barrier (GBPB) caused by the intrinsic point defects such as Znix, VOx and VZnx localized in the grain boundary of ZnO film. X-ray photoelectron spectroscopy (XPS) and The room temperature photoluminescence (RTPL) analysis reveals that the thermal annealing in O2 gas reduces GBPB near ZnO surface forming the additional neutral zinc oxide lattice by the chemical interaction of defects. That is, the oxygen-induced hump is well explained by the reduced GBPB near ZnO surface, which is also supported by the clean transfer curve of ZnO TFTs with floating Schottky contact on the back side of the channel.

Small Rb+ doping in CaCu3Ti4O12-A possible approach to reduce dielectric loss

Ca1−xRbxCu3Ti4O12 (x=0, 0.01, 0.02 and 0.03) ceramics were synthesized by the sol-gel method. Doping Rb+ reduces dielectric loss, which reaches minimum when x=0.02. By measuring properties of electrical conduction, larger leakage current density and height of grain-boundary Schottky potential barrier (ϕB) were found in the doped samples, and ϕB became maximum when x=0.02. These results are attributed to the increase in the amount of oxygen vacancies and the formation of Cu-rich/Ti-poor grain-boundary layers, and it can be concluded that the dielectric loss in CCTO ceramic can be reduced by manipulating the composition and electrical properties of grain boundary.

An Explicit Analytical Solution to the Grain Boundary Barrier Height in Undoped Polycrystalline Semiconductor Thin-Film Transistors

Based on discrete grain analysis and U-shaped distribution of density of states (DOS) for grain boundary (GB) traps, a physical-based explicit analytical solution to the GB potential barrier height (ψ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) is developed for undoped polycrystalline semiconductor thin-film transistors (TFTs). The explicit solution is derived using the Lambert W function, without additional approximations introduced. The validity and accuracy of the solution is demonstrated comparing the model with both numerical calculations and experimental ψ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> data of polycrystalline Si TFTs. Furthermore, it is found that a previous widely used Seto's model could be consistent to the proposed model in the above-threshold region, in this case deep states DOS dominates ψ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> , where the monoenergetic trap density of Seto's model roughly corresponds to the deep states DOS multiplying by 3-4 units of the thermal energy kT. Finally, the analytical model is applied in ZnO TFTs.

Effects of potential barrier height and its fluctuations at grain boundaries on thermoelectric properties of polycrystalline semiconductors

We study theoretically the effects of grain-boundary potential barriers and their fluctuations in height on the electrical conductivity and the Seebeck coefficient of polycrystalline semiconductors. On the basis of an energy-filtering model, we first derive analytical expressions of the electrical conductivity σ and the Seebeck coefficient α for polycrystalline semiconductors of any degeneracy, assuming grain-boundary potential barriers of a uniform height. These expressions in closed form enable us to perform numerical calculations of σ and α for polycrystalline semiconductors having grain-boundary potential barriers of fluctuated heights. The fluctuated barrier model is applied to the reported experimental data on the electrical conductivity and the Seebeck coefficient of a boron-doped polysilicon sample. Then, the expected enhancement of the thermoelectric power factor due to the energy-filtering effect by the grain-boundary barriers is presented with the mean value and the standard deviation of the potential barrier height as variable parameters. The calculated results suggest that diminishing the fluctuations of the potential barrier height is the key issue for enhancing the power factor by energy filtering.

Thickness dependence of electrical properties of polycrystalline GaSbAs thin films grown on glass substrates: Analysis on the basis of a two-band conduction model using a differential Hall-effect method

Polycrystalline films of undoped p-type GaSb0.8As0.2 with different thicknesses ranging from 0.1 to 1μm were grown on glass substrates at a substrate temperature of 300°C by molecular-beam deposition. Hall-effect measurements in the temperature range of 10–400K revealed the coexisting valence-band and impurity-band conduction. A differential Hall-effect method was applied for the purpose of obtaining concentration and mobility of holes in the surface-side region of the film eliminating the effects of the underlying substrate-side region. Furthermore, a two-band conduction model was applied for the purpose of obtaining concentration and mobility of holes in the valence band eliminating the effects of the impurity-band conduction. The concentrations of a shallow acceptor level, a deep acceptor level, and a compensating donor level are deduced through fitting the experimental temperature dependence of valence-band hole concentration by the solution of the charge-balance equation. The mean value and the standard deviation of the grain-boundary barrier height as well as the mean free path are deduced through fitting the experimental temperature dependence of mobility by assuming the thermionic emission of holes over grain-boundary potential barriers with fluctuated heights. The estimated results of the above parameters are presented as a function of the distance from the substrate interface. Especially, it is shown that the mean free path of valence-band holes is almost unchanged while the height of grain-boundary potential barriers decreases with the distance from the substrate. The obtained results suggest the necessity of improving the electrical properties of GaSbAs layers near at the beginning of their deposition by some method, e.g., by inserting buffer layers, in the practical application for thin film transistors.

Effects of grain-boundary potential barrier height and its fluctuation on conductivity of polycrystalline semiconductors in the ionized-impurity-scattering dominated case

We propose a new conduction model based on an energy filtering model for thermionic emission over non-uniform grain-boundary barriers in polycrystalline semiconductors of arbitrary degeneracy in which the relaxation time of carriers is dominated by ionized-impurity scattering. It is shown that the non-linear curves of Arrhenius plots of electrical conductivity in several polycrystalline samples of different semiconductors (FeS2, WO3, SnO2, ZnO) in literature are well fitted using the model with the mean value and the standard deviation of the grain-boundary potential barrier height and the concentration of ionized impurities as the three fitting parameters.

Non-Ohmic conduction in In2O3–Bi2O3 ceramics

The semiconductor In2O3–Bi2O3 ceramics exhibit unusual behaviour: The current quasi-saturation (current limiting) in dc current–voltage characteristic is accompanied by low-frequency (∼1Hz) current oscillations. In this paper some electrical properties of In2O3–Bi2O3 ceramics are studied and a mechanism of non-Ohmic conduction in this material is suggested. The electrical conduction is controlled by the grain-boundary potential barriers. Assuming that the barrier heights at different grain boundaries in a sample are not identical, the current quasi-saturation is explained qualitatively by capture of electrons at the interface states and respective increase in the height of key barriers. A mechanism of current oscillations is related to the current quasi-saturation. An increase in the height of key barriers leads to a raise of the voltage drop at these barriers and to Joule heating of barrier regions followed by decrease in the barrier height. The suggested mechanism of non-Ohmic conduction is confirmed by the obtained experimental data.

Potential barrier at grain boundaries in polycrystalline silicon: influence of grain-boundary character and copper/iron contamination

We used Kelvin probe force microscopy to measure the potential barrier associated with grain boundaries in B-doped p-type polycrystalline silicon. The potential barrier at non-contaminated grain boundaries varied approximately from 5 to 90 meV according to grain-boundary character. The random boundaries had a higher potential barrier than the coincidence boundaries. This barrier increased with increasing twist component of the grain boundary when the misorientation angle of two adjoining grains was almost the same. Copper and iron contamination caused a significant increase in the grain-boundary potential barrier, making the potential barrier depend more strongly on misorientation, probably because of anisotropy of grain-boundary segregation.

Inline Bulk-lifetime Prediction on as-cut Multicrystalline Silicon Wafers

Solar cell production using multicrystalline silicon is rife with uncertainties about material quality and its impact on subsequent processing steps. However, inline electrical metrology can provide predictive information about bulk silicon quality before processing resources are applied, allowing later metrology to focus on process control. In this work, we show an industrial algorithm which enables bulk lifetime prediction in as-cut multicrystalline wafers, agnostic of wafer origin, using an inline Quasi-Steady-State Photoconductance (QSSPC) measurement setup from Sinton Instruments. In addition, we demonstrate a robust method to extract emitter saturation current density from post-diffusion wafer measurements. Finally, we extend the use of inline QSSPC with a corrected doping measurement that utilizes the theory of grain-boundary potential barriers to interpret excess conductance.

Analysis of temperature dependence of electrical conductivity in degenerate n-type polycrystalline InAsP films in an energy-filtering model with potential fluctuations at grain boundaries

Hall-effect measurements were performed in the temperature range of 10-400 K on degenerate n-type polycrystalline InAsxP1−x films deposited on glass substrates by molecular-beam deposition at 240 or 320 °C. For the analysis of the temperature dependence of electrical conductivity of these films, we use a model developed in our preceding paper, which is based on an energy-filtering model with potential fluctuations at grain boundaries, but with refining it so as to be applicable for a non-parabolic conduction band. Using the refined version of our energy-filtering model, the experimental curves of temperature-dependent electrical conductivity are fitted to deduce the mean free path as well as the mean value and the standard deviation of the barrier height. The analysis results show that, in almost of the InAsxP1−x films investigated, the grain-boundary potential barriers fluctuate in height across the Fermi level. Owing to the substantially low potential barrier height, the InAsxP1−x film with x = 0.65 exhibited a high electron mobility of about 250 cm2/(Vs) in spite of the low substrate temperature of 320 °C.

Conduction model covering non-degenerate through degenerate polycrystalline semiconductors with non-uniform grain-boundary potential heights based on an energy filtering model

We propose a new conduction model associated with thermionic emission of carriers over potential barriers at grain boundaries, which is applicable to non-degenerate through degenerate polycrystalline semiconductors. We first assume the grain-boundary potential barriers of a uniform height and derive an analytical expression of the electrical conductivity on the basis of an energy filtering model with fully incorporating the Fermi-Dirac distribution of carriers. We then introduce fluctuations of the potential barrier height as a Gaussian distribution into the model. It is shown that the non-linear curves of Arrhenius plots of electrical conductivity in various polycrystalline semiconductors (Si, CdS, CdSe, ZnO, WO3, In2Se3) in literature are well fitted using the model with the effect of fluctuations taken into account, being regardless of whether carriers are degenerate or non-degenerate, without assuming other conduction mechanisms such as tunneling and variable range hopping.