Boundary Barrier Research Articles

Dielectric properties of high permittivity ceramics based on Dy2/3CuTa4O12

Abstract The paper reports on the dielectric properties of Dy 2/3 CuTa 4 O 12 ceramic—a new material exhibiting very high permittivity exceeding 10 4 at low frequencies and broad steps in the permittivity versus temperature plots. Impedance spectroscopy studies performed in a frequency range 10 Hz to 2 MHz and within a temperature range from −160 to 650 °C revealed two distinct contributions in impedance and electric modulus formalisms, which were attributed to semiconducting grains and more resistive grain boundaries. Two plateaus of about 40,000 and 50 were found in the real part of the permittivity versus frequency plots. Resistances and capacitances of grains, estimated on the basis of the impedance data, are 1.5–2 orders and about 400 times, respectively, lower than those of grain boundaries. The influence of sintering temperature within a range 1180–1250 °C on the dielectric properties and the microstructure of the ceramics was also investigated. The permittivity values were found to increase significantly with increasing sintering temperature. The observed high dielectric constant behavior was explained in terms of Maxwell–Wagner polarization related to internal (grain boundary) barrier layer capacitance effects.

Analysis of Photon-Induced Drain Current in Polycrystalline-Silicon Thin-Film Transistors

The electrical properties of illuminated polycrystalline-silicon thin-film transistors (poly-Si TFTs) were investigated by measurement and device simulation. Several features were revealed in n-channel poly-Si TFTs: the flat-band voltage (Vfb) shifts to a positive direction on the order of 0.1 V. The trap density increases around the midgap. In the on-state, the grain boundary barrier height slightly decreases owing to the increase in donor-type and the decrease in acceptor-type trap densities. In the off-state, the reverse current of pn-junction at the drain edge becomes much larger than the generation current or gate-induced drain leakage current, showing almost linear dependence on light intensity. A hump appears in off-current below Vfb, which is derived from the transition of carrier transport from the reverse diffusion at pn-junction in the off-state to the drift-diffusion in the on-state. Considering all these features, a surface-potential-based drain current model was proposed. Calculations using the model were compared with device simulation and measurement by varying the light intensity or drain voltage. The good agreements between them confirm the validity of the model.

Grain Boundary Effect on the Dielectric Properties of Nanocrystalline β-CuSCN

The physics of grain boundaries in semiconductors is of interest both from a fundamental as well as technical point of view because they play crucial role in controlling properties. The grain boundary effect in the nanocrystalline materials are more because the volume fraction of atoms lying at the grain boundaries of the nanocrystalline materials are more as compared with conventional coarse-grained polycrystalline materials. The most attractive hole transporting electrolyte material of dye sensitized solar cell is CuSCN because of its band gap, valence band edge position and p-type semiconducting nature. We have synthesised nanocrystalline beta phase of CuSCN using co-precipitation technique. The grain boundary effect on the dielectric behaviour has been investigated using impedance spectroscopy in a wide frequency range 1 Hz to 1 MHz under dc bias voltages 0 V to -4.2 V. The results are analyzed with double (or back-to-back) Schottky grain boundary barrier model and discussed in detail.

Optical emission and electron capture of rare-earth trivalent ions located at distinct sites in [formula omitted] thin films

We present photoluminescence and decay of photo excited conductivity data for sol-gel SnO2 thin films doped with rare earth ions Eu3+ and Er3+, a material with nanoscopic crystallites. Photoluminescence spectra are obtained under excitation with several monochromatic light sources, such as Kr+ and Ar+ lasers, Xe lamp plus a selective monochromator with UV grating, and the fourth harmonic of a Nd:YAG laser (4.65 eV), which assures band-to-band transition and energy transfer to the ion located at matrix sites, substitutional to Sn4+. The luminescence structure is rather different depending on the location of the rare-earth doping, at lattice symmetric sites or segregated at grain boundary layer, where it is placed in asymmetric sites. The decay of photo-excited conductivity also shows different trapping rate depending on the rare-earth concentration. For Er-doped films, above the saturation limit, the evaluated capture energy is higher than for films with concentration below the limit, in good agreement with the different behaviour obtained from luminescence data. For Eu-doped films, the difference between capture energy and grain boundary barrier is not so evident, even though the luminescence spectra are rather distinct.

Investigation of the effect of different dopants on the trap states of ZnO-based and SnO2-based varistors

The electrical properties of ZnO varistors and novel SnO2 varistors have been attributed to their grain boundary barriers. In this report, the techniques of capacitance-voltage (C-V) measurement and deep level transient spectroscopy (DLTS) were employed to investigate the effect of different dopants on the electronic states of ZnO-based and SnO2-based varistors. Two trap levels were determined in typical ZnO varistors: trap L1 at Ec-0.15±0.01eV and trap L2 at Ec-0.25±0.01eV. The different dopants affected the parameters of those traps significantly. Two deeper trap levels, trap S1 at Ec-0.30±0.03eV and trap S2 at Ec-0.69±0.03eV, were identified in both SnO2-CoO-Nb2O5 and SnO2-CoO-Nb2O5-Cr2O3 varistors. The variations in the donor density and trap density appear to be associated with the addition of trivalent Cr3+. The features of these trap levels and the defect theory related to the SnO2 varistors need to be further studied.

Piezoresistive Sensitivity, Linearity and Resistance Time Drift of Polysilicon Nanofilms with Different Deposition Temperatures

Our previous research work indicated that highly boron doped polysilicon nanofilms (≤100 nm in thickness) have higher gauge factor (the maximum is ∼34 for 80 nm-thick films) and better temperature stability than common polysilicon films (≥ 200nm in thickness) at the same doping levels. Therefore, in order to further analyze the influence of deposition temperature on the film structure and piezoresistance performance, the piezoresistive sensitivity, piezoresistive linearity (PRL) and resistance time drift (RTD) of 80 nm-thick highly boron doped polysilicon nanofilms (PSNFs) with different deposition temperatures were studied here. The tunneling piezoresistive model was established to explain the relationship between the measured gauge factors (GFs) and deposition temperature. It was seen that the piezoresistance coefficient (PRC) of composite grain boundaries is higher than that of grains and the magnitude of GF is dependent on the resistivity of grain boundary (GB) barriers and the weight of the resistivity of composite GBs in the film resistivity. In the investigations on PRL and RTD, the interstitial-vacancy (IV) model was established to model GBs as the accumulation of IV pairs. And the recrystallization of metastable IV pairs caused by material deformation or current excitation is considered as the prime reason for piezoresistive nonlinearity (PRNL) and RTD. Finally, the optimal deposition temperature for the improvement of film performance and reliability is about 620 °C and the high temperature annealing is not very effective in improving the piezoresistive performance of PSNFs deposited at lower temperatures.

Self-organization of In nanostructures on Si surfaces

Methods of forming various In nanostructures on Si surfaces are demonstrated. Using a high-index Si(311) surface, isolated nanotriangles and wires were grown by optimizing the deposition rate and substrate temperature. In contrast, nanodots were formed by the deposition of In on a Si(111)–In-31×31 surface at room temperature (RT) deposition. On a Si(111)–In-4×1/31×31 coexisting surface, nanowires were selectively grown in the Si(111)–In 4×1 area by RT deposition through the nucleation promoted by the boundary barrier produced by the surrounding 31×31 area. Details were studied using scanning tunneling microscopy.

Nucleation and Grain Boundaries in Thin Film CdTe/CdS Solar Cells

AbstractThe early stage formation mechanisms operating during the sublimation growth of CdTe films on CdS has been evaluated using a growth interrupt methodology for deposition under 100 Torr of N2. Key stages of the growth were identified and are discussed in terms of the processes of island nucleation, island growth/coalescence, channel formation and secondary nucleation that have been reported for other materials systems. It was demonstrated that the grain size could be manipulated by means of controlling the gas pressure in the range 2 – 200 Torr, with the grain diameter increasing with pressure linearly as D (μm) = 0.027(± 0.011) × P (Torr) + 0.90(± 0.31). For a series of solar cells made using such material, the performance parameters were seen to increase with grain size up to a plateau corresponding to grains of ∼4 μm in this case. Equivalent circuit parameters for resistive components arising from grain boundaries, and the contact to the CdTe, were measured. It is considered that grain boundary barriers in CdTe are harmful to PV performance, and that the plateau in performance occurs when the grain size is increased to the level where the contact resistance is greater than that due to grain boundaries.

Effect of doping- and field-induced charge carrier density on the electron transport innanocrystalline ZnO

The charge transport properties of thin films of sol–gel processed undoped and Al-dopedzinc oxide nanoparticles with variable doping level between 0.8 and 10 at.% wereinvestigated. The x-ray diffraction studies revealed a decrease of the average crystallitesizes in highly doped samples. We provide estimates of the conductivity and the resultingcharge carrier densities with respect to the doping level. The increase of chargecarrier density due to extrinsic doping was compared to the accumulation of chargecarriers in field effect transistor structures. This allowed us to assess the scatteringeffects due to extrinsic doping on the electron mobility. The latter decreases from4.6 × 10−3 to4.5 × 10−4 cm2 V−1 s−1 with increasing doping density. In contrast, the accumulation leads to an increasing mobility up to1.5 × 10−2 cm2 V−1 s−1. The potential barrier heights related to grain boundaries between the crystallites werederived from temperature dependent mobility measurements. The extrinsic doping initiallyleads to a grain boundary barrier height lowering, followed by an increase due todoping-induced structural defects. We conclude that the conductivity of sol–gel processednanocrystalline ZnO:Al is governed by an interplay of the enhanced charge carrier densityand the doping-induced charge carrier scattering effects, achieving a maximum at 0.8 at.%in our case.

The contribution of grain boundary barriers to the electrical conductivity of titanium oxide thin films

Titanium oxide thin films were prepared by reactive magnetron sputtering. The reactive gas pulsing process was implemented to control the oxygen injection in the deposition process and, consequently, to tune the oxygen concentration in the films from pure titanium to stoichiometric TiO2, maintaining a homogeneous in-depth concentration. The electrical conductivity of the films was investigated as a function of the oxygen injection time, the metalloid concentration and temperature, in the range 90–600K. The curved Arrhenius plots of the conductivity were examined taking into account the grain boundary limited transport model of Werner (J. H. Werner [Solid State Phenom. 37–38, 213 (1994)]). The grain barrier heights were found to depend significantly on the oxygen supplied into the deposition process and thus, on the oxygen-to-titanium atomic ratio in the films. The analysis as a function of temperature showed that the conduction mechanism in the coatings was not solely limited by the oxygen-to-titanium atomic ratio, but also by the grain boundary scattering.

ZnO-Bi2O3-Sb2O3세라믹스의 전기적 특성

In this study, it has been investigated on the changing behavior of electrical properties in <TEX>$ZnO-Bi_2O_3-Sb_2O_3$</TEX> (Sb/Bi=2.0, 1.0 and 0.5) ceramics. The samples were prepared by conventional ceramic process, and then characterized by I-V, C-V curve plots, impedance and modulus spectroscopy (IS & MS) measurement. The electrical properties of ZBS systems were strongly dependent on Sb/Bi. In ZBS systems, the varistor characteristics were deteriorated noticeably with increasing Sb/Bi and the donor density and interface state density were increased with increasing Sb/Bi. On the other hand, we observed that the grain boundary reacted actively with the ambient oxygen according to Sb/Bi ratio. Especially the grain boundaries of Sb/Bi=0.5 systems were divided into two types, i.e. sensitive to oxygen and thus electrically active one and electrically inactive intergranular one with temperature. Besides, the increased pyrochlore and <TEX>$\beta$</TEX>-spinel phase with Sb/Bi ratio caused the distributional inhomogeneity in the grain boundary barrier height and the temperature instability. To the contrary, the grain boundary layer was relatively homogeneous and more stable to temperature change and kept the system highly nonlinear at high Bi-rich phase contents.

Hydrogen-induced degradation in SrTiO 3-based grain boundary barrier layer ceramic capacitors

Hydrogen-induced degradation in SrTiO 3-based grain boundary barrier layer ceramic capacitors was studied through electrochemical hydrogen charging, in which the capacitors were placed in 0.01 M NaOH solution with hydrogen deposited on their electrodes from the electrolysis of water. The properties of the capacitors were greatly degraded after 0.5 h of treatment: The capacitance was dramatically decreased and the dielectric loss was dramatically increased over the frequency range of 10 2–10 5 Hz, the leakage current was increased by orders of magnitude. It was proposed that atomic hydrogen diffused relatively easily along the grain boundaries and induced a reduction reaction to the grain boundary layer, which resulted in the degradation observed. Hydrogen-induced degradation is more serious in SrTiO 3-based grain boundary barrier layer ceramic capacitors than in other ceramic capacitors and great efforts should be made to prevent hydrogen-induced degradation in them.

Bulrush-Like Double Perovskite: Synthesis, Tunneling Magnetoresistance, and Magnetocaloric Effects

The recent observation of room temperature tunneling magnetoresistance (TMR) in half-metallic A2FeMoO6 (A = Ca, Sr, Ba) double perovskites, and their importance to the emerging field of spintronics has led to considerable effort being dedicated to detailed investigations of the physical and chemical properties of these materials. This article will present an review of our recent investigations covering the synthesis, structures, magnetic and transport properties of "bulrush-like" A2FeMoO6 (A = Sr, Ba). Utilizing the high shape anisotropy as well as the reactivity of A2FeMoO6 to water and a sonochemical technique, we managed to manipulate the properties of grain boundary barriers, and thus put forward a new approach for the enhancement of room temperature TMR. The magnetocaloric effects of A2FeMoO6 double perovskites will also be discussed.

Structural and electrical properties of In6Se7 thin films

In6Se7 thin films of different thicknesses were prepared by the thermal deposition technique. Structural studies show that In6Se7 thin films have a polycrystalline nature of the monoclinic phase: its lattice parameters a, b, c and β were calculated for the first time. The thermoelectric power (TEP) measurements indicate that the investigated films are n-type semiconductors. Both the resistivity and the TEP measurements indicate the existence of one conduction mechanism. The temperature coefficient of activation energy, the free charge carrier concentration, the carrier mobility and the grain boundary barrier potential were evaluated. The dark I–V and C–V characteristics of n-In6Se7/p-Si heterojunction at different temperatures were performed. The internal junction parameters were calculated and the existence of the multi-step tunnelling mechanism was confirmed.

A comprehensive analytical on-current model for polycrystalline silicon thin film transistors based on effective channel mobility

A comprehensive analytical ON-state drain current model for poly-Si thin film transistors (TFTs) is developed to accurately fit both transfer and output characteristics of either low or high temperature processed poly-Si TFTs in both n- and p-types, with the same unified analytical form. The model is physical explicitly based and mathematically accurate, with no artificial factors introduced. To accomplish this, (1) the most appropriate model precisely describing the gate voltage dependent mobility degradation effect in poly-Si TFTs is determined from three previously employed models; (2) grain boundary barrier controlled carrier conduction model is adequately incorporated; and (3) a mathematically accurate analytical form for drain current is derived based on gradual channel approximation by introducing a new fitting parameter to describe the effective average channel mobility. Channel length modulation effect is introduced in modeling output characteristic. Applicability and accuracy of the model are demonstrated by fitting both n- and p-types poly-Si TFTs fabricated from very different processes.

Re‐Making of Europe's borders through the European neighbourhood policy

This article examines the implication of the European Neighbourhood Policy (ENP) framework for the development of political community around the problem of re‐bordering processes in Europe. It suggests that the ENP can be viewed as an attempt to reconcile two potentially contradictory bordering processes. The first—'border confirming'—is about confirming border areas of demarcation and division in which borders are conceived as boundary lines, frontier zones, or barriers that protect the Union and its citizens. The second— ‘border transcending ‘—consists in a challenge to open European borders and involves the transformation of the EU's external boundaries into zones of interactions, opportunities and exchanges, where the emphasis is on transcendence of boundaries. In order to sort out some of the contradictions surrounding highly contested phenomena of mobility in the neighborhood, the article critically examines three bordering strategies: state borders, the imperial analogy, and borders as networks. Each corresponds to different forms of territoriality and each implies a different form of control over the population.

A Study of Photophysics, Photoelectrical Properties, and Photoconductivity Relaxation Dynamics in the Case of Nanocrystalline Tin(II) Selenide Thin Films

Charge-carrier transport properties in chemically deposited thin films composed of close-packed tin(II) selenide nanocrystals were studied both in the dark and under conditions when internal photoelectric effect is manifested in the nanostructured material. Stationary and time-resolved experiments were performed to characterize the photophysics of quantum-confined nanocrystals synthesized in thin film form. The charge-carrier transport through the films was found to occur predominantly by the thermionic emission mechanism in temperature range from 300 to 585 K. The corresponding crystal boundary barrier height in the case of as-deposited films is ≈160 meV, and the trap states which cause a partial charge-carrier depletion of the nanocrystals are situated below the Fermi level. Thermal band gap energy was found to be 0.90 eV, which is somewhat lower than the value calculated from optical spectroscopic data (1.10 eV). A physical basis for these differences is proposed. Both the internal photoelectric effect and the crystal boundary barrier height modulation upon illumination were found to contribute to the photoresponse of nanocrystalline films, the surface and bulk recombination velocities being comparable. Indirect and direct band gap energy values of 1.23 and 1.54 eV were calculated for the nanocrystalline films on the basis of photoresponse data. The nonequilibrium conductivity was found to relax exponentially with a time constant of 1.78 ms, which corresponds to the average lifetime of minority charge carriers (electrons). Lux-ampere characteristics of the films further support the linear photoconductivity relaxation mechanism, contrary to what is usually found in the case of amorphous materials.

Effects of LaNiO3 bottom electrode on structural and dielectric properties of CaCu3Ti4O12 films fabricated by sol-gel method

Ca Cu 3 Ti 4 O 12 (CCTO) thin films are prepared by a sol-gel method on LaNiO3-coated silicon and Pt∕TiO2∕SiO2∕Si substrate. Compared with the films on Pt, the CCTO on LaNiO3 exhibits a (400) orientation. Dielectric loss of CCTO on LaNiO3 is lower than 0.25 within 100Hz–10kHz, lower than the reported value of CCTO grown on Pt∕TiO2∕SiO2∕Si by pulse laser deposition. Possible reason is that LaNiO3 acts as seed layer for the growth of CCTO. The crystallinity of CCTO is improved and the dielectric properties are enhanced. Complex impedance spectrum of CCTO on LaNiO3 is discussed according to grain boundary barrier layer capacitance model.

Sintering and dielectric properties of Cu 2Ta 4O 12 ceramics

In this work, Cu 2Ta 4O 12 ceramic was investigated as a promising, lead-free, nonferroelectric material with high dielectric permittivity. The results of impedance spectroscopy studies carried out at frequencies 10 Hz to 2 MHz over a wide temperature range from −55 to 700 °C were analyzed in the impedance, dielectric permittivity and electric modulus formalisms. In complex impedance plots two distinct arcs were distinguished, ascribed to the semiconducting grains and to the insulating grain boundaries. Cu 2Ta 4O 12 ceramic was found to exhibit a high dielectric permittivity exceeding 10,000 at low frequencies in the temperature range 150–740 °C. High permittivity of this material was attributed to the formation of internal (grain boundary) barrier layer capacitors. The influence of sintering conditions on microstructure, composition and dielectric properties of Cu 2Ta 4O 12 ceramics was also studied.

Structure, Properties, and Impedance Spectroscopy of CaCu3Ti4O12 Ceramics Prepared by Sol–Gel Process

CaCu3Ti4O12 (CCTO) ceramics with high dielectric constant (2–4 × 104) and low loss (0.04) were prepared by the sol–gel process and sintered at 1050°C for different times. The sintering time has a sensitive influence on the values of the dielectric constant and nonlinear coefficient. Tailored dielectric constant and nonlinear coefficient can be obtained by selecting a suitable sintering time according to different desired device application. The result of current–voltage characteristics and Cole–Cole plots in a broad temperature range (60–400 K) provide more effective evidence of the high dielectric constant supported by the grain boundary barrier layer (GBBL) capacitors model. Below 150 K, the GBBL capacitors effect weakens and gradually disappears with further decrease of temperature, thus leading the dielectric constant to decrease rapidly. Two values of grain activation energy acting at different temperature for each sample were obtained.