Grain Boundary Barrier Height Research Articles

Synergy effect of microwave annealing and high-pressure hydrogen annealing on Poly-Si thin-film transistor

Grain boundary (GB) is a significant factor that deteriorates the transfer characteristics of poly-Si thin-film transistors (TFTs). In this study, we utilized the synergistic effect of microwave annealing (MWA) and high-pressure hydrogen annealing (HPHA) to effectively reduce grain boundary trap (GBT) density, resulting in improved field-effect mobility (μ) and subthreshold swing (SS). To investigate the synergistic effect of MWA and HPHA, the transfer characteristics of rapid thermal annealing and forming gas annealing devices were compared and analyzed as control devices. Furthermore, the mechanism of SS and mobility enhancement can be quantitatively understood by lowering the GB barrier height. In addition, Raman spectroscopy proved that poly-Si crystallinity was improved during MWA. Our results showed that MWA and HPHA play a vital role in reducing GBT density and improving poly-Si TFT characteristics.

Synergistic effect of co-doping of nano-sized ZnO and Nb2O5 on the enhanced nonlinear coefficient of TiO2 varistor with low breakdown voltage

In this study, the enhanced nonlinear coefficient (8.6) of TiO2 varistor with a low breakdown voltage (3.58 V/mm) was achieved by co-doping of nano-sized ZnO and Nb2O5. Meanwhile, the TiO2 varistor exhibited a low leakage current (0.096 mA) and high relative density (98.73%). Further analysis indicated that such improvement could be attributed to the changes in the microstructure including the grain size, pores, defects distribution and grain boundary characteristics in particular. Separately, SEM results demonstrated that the dopant of nano-sized ZnO could promote the sintering process of TiO2 varistor and increase the density of the sample. The XPS and EPR analysis showed that Nb2O5 could help to produce free electrons and Ti3+ ions through donor doping, while ZnO would be beneficial to generate oxygen vacancies and other defect ions through acceptor doping. On the one hand, the defects generated in the grain could result in the grain semiconducting; on the other hand, some defects diffusing to the grain boundary would participate in the formation of grain boundary potential barrier, thus the height of grain boundary barrier was improved. Consequently, the breakdown voltage descended while the nonlinear coefficient ascended.

Effects of post-deposition CdCl2 annealing on electronic properties of CdTe solar cells

The effects of post-deposition CdCl2 annealing temperature on the electronic properties of CdTe solar cells were investigated. CdTe devices incorporate Mg doped ZnO as a buffer layer and selenization using a CdSe interlayer for reducing the buffer/absorber interface recombination and increasing solar energy absorption respectively. The post-deposition CdCl2 annealing treatments were done under separate, inert atmospheres of nitrogen and helium across the temperature range 380–430 °C. Electrical characterization of devices is carried out including temperature dependent current-voltage characteristics, admittance spectroscopy, and Shockley-Read-Hall recombination analysis. The best improvements in device efficiency are obtained upon annealing at temperature 410 °C. This anneal correlated with reduced back contact barrier in CdTe and reduced grain-boundary barrier height which is beneficial for enhanced charge transport.

Combining high energy efficiency and fast charge-discharge capability in calcium strontium titanate-based linear dielectric ceramic for energy-storage

The high energy density and power density dielectric ceramics with wide frequency and temperature stability are competitive materials for pulse power capacitors. This research proposes an effective strategy for enhancing energy storage performance of Ca0.5Sr0.5TiO3 ceramics through the introduction of Sn4+ ion. This ion restrains grain growth, increases high insulating grain boundaries, and thus enhances the barrier height of grain boundaries, which are demonstrated in SEM micrographs and complex impedance. The Ca0.5Sr0.5Ti0.97Sn0.03O3 (SnCST3) ceramics possess high energy storage density of 2.06 J/cm3 and efficiency of 95% under breakdown strength of 330 kV/cm, which get benefit from wide band gap of 5.3 eV. While the exceptional stability with minimal fluctuation (<15%) in the wide frequency range of 1–1000 Hz and temperature range of 20–120 °C are also achieved in this system. Furthermore, the pulse discharge performance are performed to appraise practical applications of pulse power capacitors. The prominent power density of 32.2 MW/cm3 and current density of 537.4 A/cm2 are obtained. Meanwhile, stored energy are released in very short duration of 13 ns as well as outstanding temperature stability is realized in the temperature range of 20–120 °C for the SnCST3 ceramic, confirming it as one competitor for lead-free high power capacitors.

Boundary breakdown time of ZnO varistors under nanosecond pulse current

Conductive properties of zinc oxide (ZnO) varistor ceramics depend on the boundary barrier structure. By using the photoconductivity switches (PCSS) technology, the boundary breakdown characteristics of ZnO varistors for nanosecond electric pulse response was measured under bias voltage 700 - 1600 V and the relationship between conductance and material structure was analyzed. Charging time of grain boundary capacitor was ∼ 100 ns and shortened with the increase of the bias electric field. The barrier height of ZnO grain boundary was influenced by bias electric field and the grain boundary capacitance was changed accordingly. The calculating numerical results are in agreement with the measured values.

Investigation on electrical properties of indium tin oxide thin films by effective control of crystallographic orientation

Although indium tin oxide (ITO) film has been widely used in optoelectronic devices, it remains difficult to systemically determine the structural properties' influence on electrical properties for ITO. In this work, ITO films are grown by metal organic chemical vapor deposition (MOCVD). By introducing nucleation layers, the structural properties could be controlled effectively so that the dependence of electrical properties on crystallographic orientation and O2 partial pressure could be investigated separately. It is found that (1) for the samples grown with the same doping level, the decrease in carrier concentration with increasing oxygen flow rate is purely attributed to the increasing O2 partial pressure rather than different crystallographic orientation; (2) electron mobility of the ITO films is strongly affected by crystallographic orientation and it decreases with (100)-grain density. Analysis by using carrier's transport theory reveals that the averaged barrier height of grain boundary scattering increases with the (100)-grain density.

Electrical properties of ZnO varistor ceramics modified by rare earth-yttrium and gallium dopants

Scanning electron microscopy, electric field-current density measurements, capacitance-voltage testing, X-ray diffraction analysis, and energy dispersive X-ray spectroscopy were performed to study the effects of rare earth-yttrium and gallium doping on the microstructure and electrical properties of ZnO varistors. Our results indicated that the introduction of gallium and yttrium strengthened the barrier height of grain boundaries and led to a decrease in the grain resistance. Both the voltage gradient and nonlinear coefficient of the samples increased and then decreased as the gallium doping level was increased at a given rare earth-yttrium. The residual voltage ratio and the leakage current varied with the opposite relationship. The findings of this study could help to improve the protective effects of surge protection devices assembled from ZnO ceramic varistors and enhance the stability of power systems.

Enhanced carrier mobility in Si nano-crystals via nanoscale phosphorus doping

Usually, the conductivity of semiconductor can be increased after phosphorus (P) doping but the electron mobility will be obviously reduced due to the strong impurity scattering. Here, we report a novel phenomenon that the carrier mobility in Si nano-crystals is significantly enhanced after nanoscale P doping. It is found that the electron mobility reaches to 30.3cm2/Vs, which is improved by one order of magnitude compared with that of un-doped one (1.6cm2/Vs), while the room temperature conductivity is as high as 1.5×103S/cm by appropriately P doping. The clear experimental evidences are present to demonstrate that both the reduction of barrier height of grain boundaries and the surface defects states due to P doping improve the corresponding electron mobility in nano-crystalline Si films.

Temperature dependence of zinc oxide grain resistivity in ZnO varistor ceramics

Temperature dependence of the bulk resistivity of zinc oxide grains in ZnO varistor ceramics was studied in the range 77–460 K. The pulse method was used for the measurement of the grain resistivity. In the examined temperature range the resistivity of ZnO grains exhibits the negative temperature coefficient (the resistivity is decreased with increase in temperature). Two activation energies of electrical conduction, 0.014 eV (below 270 K) and 0.048 eV (above 270 K) were observed. They are related to two shallow donor levels contributing to the electrical conduction of zinc oxide grains. It was shown that ZnO varistor ceramics with the higher grain resistivity have the higher low-field grain-boundary barrier height.

Effects of oxidation and CdCl2 treatment on the electronic properties of CdTe polycrystalline films

Electronic and structural properties of polycrystalline CdTe films which were grown at the atmosphere of vacuum, Ar, Ar/O2 mixture, by close space sublimation were studied. The effects of O2 and CdCl2 on the CdTe films were investigated from the energy band and electronic properties perspective. Based on the electrical transport measurement from 300 K to 125 K, the height of grain boundary barrier was calculated from the mobility and was found to be increased with the increasing partial pressure of O2 in CdTe deposition process. XPS spectra showed that the O–Te interaction causes the band bending downward, resulting in the increased barrier at grain boundary. XPS spectra also showed that post annealing with CdCl2 enhanced the p-doping in CdTe intensively and improved the quantum efficiency of CdS/CdTe solar cells.

Characteristics of Porous Sb-Doped Barium Titanate Ceramics Fabricated by Adding Graphite

Porous Sb-doped barium titanate (Sb-BaTiO3) ceramics were fabricated by adding various amounts of graphite powders. The density, structure, microstructure, porosity and electrical resistivity of the porous Sb-BaTiO3 ceramics produced without and with graphite were investigated. All the sintered ceramics showed a tetragonal perovskite structure, irrespective of the amount of graphite added. The porosity of the ceramics increased and the grain size decreased with increasing graphite addition which mainly due to the exothermic reactions of the graphite and oxygen molecules in the ceramics. The prepared porous Sb-BaTiO3 ceramics exhibit PTCR behavior where the PTCR jump of the ceramics with graphite was about 103 which is higher than that of the ceramics without graphite. The increasing in the PTCR jump with increasing graphite addition was attributed mainly due to the increase in the electrical barrier height of grain boundaries and the porosity. It was found that the graphite is an effective pore forming agent for fabricating porous BaTiO3-based ceramics.

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.

Effect of Nitrogen Doping on the Electrical Properties of 3C-SiC Thin Films for High-Temperature Sensors Applications

3C-SiC is a promising structural material for piezoresistive sensors used in high-temperature applications. For sensor development, the preparation of sensor materials and study of its electrical properties, such as resistivity, barrier height of grain boundaries, and temperature coefficient of resistivity, are important in addition to structural properties and these have to be optimized. In the present work, 3C-SiC thin film with in situ doping of nitrogen is prepared through low-pressure chemical vapor deposition by using methyl trichloro silane, ammonia, and hydrogen as precursors. Electrical properties of deposited 3C-SiC thin films with varying nitrogen doping concentration through four probe technique are studied. Atomic force microscopy investigations are carried out to study the grain size on and average root-mean-squared roughness 3C-SiC thin films. A decrease in the degree of crystallinity is observed in nitrogen-doped 3C-SiC thin films. The sheet resistivity of nitrogen-doped 3C-SiC thin film is found to decrease with increase in temperature in the range from 303 to 823 K. The sheet resistivity, average temperature coefficient of resistance, and barrier height of the grain boundaries of film doped with 17 at.% of nitrogen are 0.14 Ω cm, −1.0 × 10−4/K, and 0.01 eV, respectively. Comparing all the nitrogen-doped 3C-SiC thin films, the film doped with 17 at.% of nitrogen exhibits an improved structural and electrical properties and it can be used as sensing material for high-temperature applications.

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.

Explicit calculation for grain boundary barrier height in polysilicon TFTs based on quasi-two-dimensional approach

A physical-based explicit calculation to the height of grain boundary barrier has been derived based on the quasi-two-dimensional approach at discrete grain boundaries. The analytical solution is obtained by using the Lambert W function, combining both the uniform distributed deep states and the exponential tail states. The proposed scheme is demonstrated as an accurate and computationally efficient solution in a closed form, which can serve as a basis for the discrete-grain-based models of mobility and drain current in polysilicon thin film transistors. It is verified successfully by comparisons with both numerical simulation and experimental data.

AC properties of Pr0.7Ca0.3MnO3 ceramics

Ceramic Pr1-xCaxMnO3(x=0.3) samples are prepared by solid-state reaction and measured using direct current (DC) and alternating current (AC) methods in different magnetic and electrical fields. A Curie temperature of 150 K is determined by I-V measurements in field for Pr0.7Ca0.3MnO3, which is consistent with that from vibrating sample magnetometer (VSM). The AC measurement shows that the grain boundary resistance reduces with magnetic field increasing, while the grain almost keeps unchanged, and it indicates that the colossal magnetoresistance (CMR) effect in low field mainly comes from the grain boundary for the Pr0.7Ca0.3MnO3 ceramic. The barrier height of grain boundary is 117 meV for the Pr0.7Ca0.3MnO3 ceramic, obtained by fitting the temperature spectrum of impedance, and it is well coincident with that from fitting the R-T data. A trap state model is proposed to explain all the measured data.

Temperature dependent conductivity of polycrystalline Cu2ZnSnS4 thin films

The temperature-dependent conductivity of Cu2ZnSnS4 (CZTS) thin films prepared by sulfurization of different sputtered ZnS/Cu/Sn stacks and also of the same stack annealed for different times was investigated from 30-300 K. Fitting of the through-thickness conductivity requires a model including Mott variable-range hopping (M-VRH), nearest-neighbor hopping (NNH), and thermionic emission over grain boundary (GB) barriers. The GB barrier height varies sensitively from 50–150 (±5) meV with annealing and especially with [Cu]/([Zn] + [Sn]) ratio but is independent of [Zn]/[Sn] ratio. These results are critical for understanding the behavior of solar cells based on polycrystalline CZTS absorber layers.

Preparation and electrical properties of (1 − x)(Ba,Y)TiO3 · xPbTiO3 materials containing low-melting B2O3-PbO-SiO2 glass additions

We have studied the electrical properties of (1 − x)(Ba,Y)TiO3 · xPbTiO3-based solid solutions containing different percentages of low-melting glass. The materials were prepared by solid-state reactions, sintered in air at temperatures from 1100–1300°C, and characterized by complex impedance measurements in wide frequency and temperature ranges. The results demonstrate that the positive temperature coefficient of resistance effect in these materials, containing different percentages of low-melting glass, is contributed mainly by both the grain boundaries and surface layer. With increasing glass content, the ratio of the maximum resistivity to the minimum resistivity increases and the influence of the surface layer on the positive temperature coefficient of resistance effect becomes stronger. With increasing glass content, the grainboundary potential barrier height in the (1 − x)(Ba,Y)TiO3 · xPbTiO3 materials increases.

Influence of the grain boundary barrier height on the electrical properties of Gallium doped ZnO thin films

The pulsed laser deposition (PLD) technique is used to deposit Gallium doped zinc oxide (GZO) thin films on glass substrates at 250 with different Gallium (Ga) doping concentration of 0, 1.0, 3.0 and 5.0%. The influence of Ga doping concentration on structure, chemical atomic compositions, electrical and optical properties was investigated by XRD, XPS, Hall measurement and UV spectrophotometer, respectively. The relationship between electrical properties and Ga doping concentration was clarified by analyzing the chemical element compositions and the chemical states on the GZO films. It is found that the carrier concentrations and oxygen vacancies in the GZO films increase with increasing Ga doping concentration. The lowest resistivity (3.63 × 10 −4 Ω cm) and barrier height of grain boundaries (14 mV) were obtained with 3% Ga doping. In particular, we suppose the band gap of 5% Ga doping sample larger than that of 3% Ga doping sample is due to the quantum size effect from the amorphous structure rather than Moss–Burstein shift.