Minimum Resistivity Research Articles

Investigation of low temperature bonding process using indium bumps

Hybrid pixel detector modules are state of the art components of X-ray cameras in synchrotron radiation experiments. Temperature-sensitive, high-Z sensor materials like Germanium, Cadmium-Telluride, Cadmium-Zinc-Telluride and MCT-sensors require an especially low temperature bonding process. The investigation presented in this paper focuses on a low temperature compression bonding process using a ductile bump material, like indium, and a dedicated UBM surface that is able to form intermetallic compounds with this bump metal. The test samples are based on a test chip design with the size of the MEDIPIX3 readout chip and an identical number of bumps, arranged in a 256 × 256 bump matrix with 55 μm pitch in X- and Y-directions. In addition to the indium bumped test chip, two typical UBM-pad configurations were investigated on the corresponding interconnect side: a pad-defined electroplated Ni-Au-UBM and a passivation opening-defined, PVD-deposited NiV-Au pad. Daisy chain structures, 4-Point-probe resistance measurement structures and isolation test structures were implemented to characterize the influence of bonding pressure, bonding temperature and dwell time of bonding pressure on the electrical interconnection resistance. Good bonding results and a corresponding minimum electrical resistance of daisy chains and individual interconnects were achieved at temperatures of 80̂C and 100̂C. A minimum bonding pressure of 3.2 MPa was necessary to achieve reliable bonding results. Daisy chain failures occurred especially in the corner of the chips. After electrical characterization, pull tests were performed to investigate the corresponding bonding area. Pull test results showed a larger bonding area for the pad-defined, electroplated UBM pad configuration in comparison to the passivation opening defined bonding pad.

Unconventional transport properties of the itinerant ferromagnet EuTi1−xNbxO3(x=0.10–0.20)

We report the temperature and magnetic field dependence of resistivity ($\rho$) for single-crystalline EuTi$_{1-x}$Nb$_{x}$O$_3$ ($x$=0.10$-$0.20), an itinerant ferromagnetic system with very low Curie temperature ($T_C$). The detailed analysis reveals that the charge conduction in EuTi$_{1-x}$Nb$_{x}$O$_3$ is extremely sensitive to Nb concentration and dominated by several scattering mechanisms. Well below the $T_C$, where the spontaneous magnetization follows the Bloch's $T^{3/2}$ law, $\rho$ exhibits $T^2$ dependence with a large coefficient $\sim$10$^{-8}$ $\Omega$ cm K$^{-2}$ due to the electron-magnon scattering. Remarkably, all the studied samples exhibit a unique resistivity minimum at $T$$=$$T_{\rm min}$ below which $\rho$ shows logarithmic increment with $T$ (for $T_{\rm C}$$<$$T$$<$$T_{\rm min}$) due to the Kondo scattering of Nb 4$d^1$ itinerant electrons by the localized 4$f$ moments of Eu$^{2+}$ ions which suppresses strongly with applied magnetic field. In the paramagnetic state, $T^{2}$ and $T^{3/2}$ dependence of the resistivity have been observed, suggesting an unusual crossover from a Fermi-liquid to a non-Fermi-liquid behavior with increasing $T$. The observed temperature and magnetic field dependence of resistivity has been analysed using different theoretical models.

Electrical and thermal properties of a (Ba1-x-ySrxCay)TiO3-based PTC thermistor for preheating light oil

The electrical properties of a (Ba[Formula: see text]SrxCay)TiO3-based positive temperature coefficient (PTC) thermistor were determined by measuring resistivity with increasing temperature, and its thermal properties were measured with a cartridge heater made of a (Ba[Formula: see text]Sr[Formula: see text]Ca[Formula: see text])TiO3-based PTC thermistor. The (Ba[Formula: see text]SrxCay)TiO3-based PTC thermistor were fabricated by a conventional solid-state reaction method with Sb2O3, BaTiO3, SrTiO3, SiO2, and CaTiO3. Their electrical properties were determined by room temperature resistivity ([Formula: see text]C), minimum resistivity ([Formula: see text]), maximum resistivity ([Formula: see text]), resistivity jump rate (log [Formula: see text]/[Formula: see text]), Curie temperature ([Formula: see text]), and withstanding voltage. The effect of the microstructure on the electrical properties was investigated to improve the withstand voltage. The cartridge heater was fabricated with the (Ba[Formula: see text]Sr[Formula: see text]Ca[Formula: see text])TiO3-based PTC thermistor and its thermal properties were determined by the temperature of light oil in a 100 cc beaker. The temperature of light oil was maintained at 60[Formula: see text]C without fluctuations in temperature after reaching 60[Formula: see text]C at 220[Formula: see text]V, and thus (Ba[Formula: see text]SrxCay)TiO3-based PTC thermistor was able to operate as a self-regulating heater element for preheating light oil.

Factors affecting the properties of highly conductive flexible ultrathin ITO films in confined large area magnetron sputtering in three dimensions

A large area magnetron source with the strongly confined magnetic field from all direction is applied for the deposition of flexible ultrathin ITO (UT-ITO) films of thickness 30 nm at room temperature for their applications as transparent electrodes. The films show a minimum resistivity of ∼5.0 x 10-4 Ωcm and high transmittance &amp;gt;80% at wavelengths of 400-700 nm. Measurements and data reveal that a high plasma density, high energy flux, and a relatively low concentration of negative oxygen ions (NOIs) to the flux of positive ions (PIs) induce lower mechanical stress to the growing films, which enables a lower resistivity and superior crystallinity with the smooth surface. The capability of the magnetron source and the characteristic plasma properties are studied in light of the resulting film properties. The considerably lower resistivity with higher carrier concentration and mobility of the UT-ITO films prepared at a high power density of 3 W/cm2 and a low O2 gas flow can be attributed to the growth of crystallized UT-ITO films, resulting in the combination of the oxygen vacancy and substitution of Sn4+ to In3+ site through the deposition of a high energy flux and a low flux ratio of NOIs to PIs.

Novel rare earth Gd and Al co-doped ZnO thin films prepared by nebulizer spray method for optoelectronic applications

In this study, for the first time, rare earth element gadolinium and aluminum co-doped zinc oxide (Gd:AZO) films were prepared on insulating glass plates using cost-effective nebulizer spray method with various Gd co-doping levels (0, 0.5, 1 and 1.5 at.%). The deposited films were characterized using X-ray diffraction, FT-Raman, AFM, EDAX, UV-VIS spectroscopy, Photoluminescence (PL) spectrum and Hall Effect measurement at room temperature. From XRD study, it is confirmed that the Gd and Al ions are incorporated into ZnO lattice. Film crystallinity is slightly reduced due to Gd content by increasing lattice defects. Topology of AFM images displays a slight increase of Gd:AZO thin film roughness from 20 nm to 36 nm, with an increase of thickness from 232 to 324 nm respectively. Elemental mapping and EDAX studies confirmed the existence of Zn, O, Al and Gd elements in the prepared Gd:AZO thin films. Spray deposited pristine AZO films showed maximum optical transmittance ∼91% in entire wavelength spectrum and energy gap value ∼3.31eV. The observed PL spectra showed as a UV emission at 387 nm for deposited films. The obtained minimum resistivity and maximum figure of merit values are 3.42 × 10−4Ωcm, and 18.68 × 10−3(Ω/sq)−1, respectively for 1.5 at.% Gd co-doped AZO thin film. Both values are decent enough for opto-electronic devices.

Influence of porosity on the properties of nanostructured tin oxide thin film

Nanostructured tin oxide (SnO2) thin films are deposited using reactive r.f. sputtering by varying deposition time and substrate temperature during deposition. A wide range of tin oxide film thicknesses between 100 nm to 550 nm are deposited by varying the deposition time while substrate temperature during deposition is also varied from room temperature to 400 °C. The homogeneity of the deposited film during deposition is ensured using secondary electron microscopy system (SIMS). The crystal structure and microstructure of the deposited films are investigated using x-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively while the sheet resistance of deposited film is determined by four point probe method. The x-ray diffraction study showed that deposited films are orientated along (110) plane at elevated temperature, manifested with increase of grain size with thickness and substrate temperature except at 300 °C where it decreases. Microstructural studies using cross sectional SEM reveals that 470 nm tin oxide film deposited at room temperature shows highest porosity. The porosity of the films is quantified by using spectroscopic ellipsometry results and it varies from 23.2% to 8.99% for the film deposited at room temperature and 400 °C respectively. LPG response for all deposited films is also measured. The LPG sensitivity for the sensor film of various thicknesses shows linear response with porosity. SnO2 thin films deposited from room temperature to 300 °C shows no significant variation in sensitivity for LPG. The sensor response for LPG decreases sharply for the film deposited above 300 °C. Minimum resistivity 0.3536 Ω.cm is obtained for the tin oxide film deposited at 400 °C. The mechanism of LPG sensing property of nanostructured tin oxide thin film is also explained.

Volume of precursor solution effect on the properties of SnO2 thin films prepared by nebulized spray pyrolysis technique

Undoped SnO2 thin films have been deposited on amorphous glass substrates with different precursor solution volume (10, 15, 20 and 25 ml) using simple and cost-effective nebulized spray pyrolysis technique. The influence of precursor solution on structural, optical, photoluminescence and electrical properties had been studied. The X-ray diffraction spectra prove the polycrystalline nature of SnO2 with tetragonal structure. All the films show a preferred growth orientation along (110) diffraction plane. The average transmittance of SnO2 thin films varied between 82 and 75% in the visible as well as IR region. The band gap energy decreases from 3.74 to 3.64 eV corresponding to direct transitions with the precursor solution volume had increased from 10 to 20 ml and then increased as 3.72 eV for 25 ml. SEM pictures demonstrated polyhedrons like grains. EDX confirmed the existence of Sn and O elements in all the prepared SnO2 thin films. Photoluminescence spectra at room temperature revealed that the four emission bands in all the samples such as sharp dominant peak at 361 nm with shoulder peak at 377 nm (UV region), a broad and low intensity peak at 492 nm (blue region) and 519 nm (green region). The electrical parameters were examined by Hall effect measurements, which demonstrated that the film prepared at 20 ml precursor solution volume possess minimum resistivity 2.76 × 10−3 Ω-cm with activation energy 0.10 eV and maximum figure of merit 1.54 × 10−2 (Ω/sq)−1.

Tunable resistivity in ink-jet printed electrical structures on paper by plasma conversion of particle-free, stabilizer-free silver inks

Printable metal inks are typically composed of premade nanoparticles that require postdeposition thermal sintering to produce crystalline, electrically conductive features. In this paper, it is shown that particle-free Ag inks made from simple, water-soluble metal salts such as silver nitrate can be ink-jet printed and converted into electrical features with tunable resistivity at low temperature (&amp;lt;100 °C) by exposure to a pure argon plasma. X-ray diffraction confirms that the converted inks are crystalline, and four-point probe electrical measurements show that the sheet resistances are a function of the pressure and power in the plasma. From cross-sectional scanning electron microscopy analysis, it is found that the morphology of the converted silver layer becomes increasingly dense with increasing plasma treatment time, which explains the measured changes in sheet resistance, and that the thickness of the layer is ∼1.5 μm, which yields a minimum resistivity of ∼6 × 10−8 Ω m, approximately 3.8 times higher than bulk resistivity of silver. Interestingly, the resistivity can be varied over a span of 6 orders of magnitude which allows resistor–capacitor filter devices to be fabricated exhibiting varying cut-off frequencies from a single material and geometry.

Investigation of Mg δ-doping for low resistance N-polar p-GaN films grown at reduced temperatures by MOCVD

In this study we explored p-type δ-doping for the deposition of N-polar p-GaN films at 900 °C, for application in device structures containing high In composition active layers. Various δ-doping process parameters were investigated, including the duration and molar flow of the Mg precursor during each pulse as well as the δ-doping period. The results were compared to those obtained for layers grown using continuous doping. As the δ-doping period was reduced from 25 to 5 nm, the p-GaN bulk resistivity decreased from 6.8 to 2.8 cm. The p-layer resistivity rose with increasing [Mg] independent of the doping scheme. For similar average [Mg], however, continuous doping resulted in a lower resistivity compared to δ-doping. This effect was more pronounced at higher [Mg], indicating that high local concentrations of Mg resulted in degradation of the electrical performance. After optimization of the p-layer structure and contact fabrication process, a minimum contact resistance of 2.77 m cm2 and a minimum resistivity of 1.33 cm were achieved.

Effect of yttrium(Y) on structural, morphological and transport properties of CdO thin films prepared by spray pyrolysis technique

Cadmium oxide (CdO) and yttrium (Y) doped CdO (Y: CdO) thin films have been prepared onto glass substrate at temperature 300 °C by spray pyrolysis technique. The effects of yttrium (Y) doping on the structural, morphology, optical and electrical properties were studied systematically. The X-ray diffraction (XRD) study confirms that CdO films are polycrystalline in nature with cubic structure having lattice parameter of 0.4658 nm. Surface topographic and nano-structural analysis indicates cluster grain size and porosity decreased substantially with increase of yttrium (Y) content in CdO films. The optical transmittance exhibits excellent optical transparency, with an average transmittance of >70% in the visible range for 2 to 4% yttrium (Y) doping. The optical band gap widens in Y: CdO film from 2.24 to 2.62 eV through Burstein- Moss shift. Hall measurement confirms that material is of n type with a minimum resistivity of 4.7 × 10−4 Ω-cm with carrier concentration of 4.2 × 1021 cm−3 were achieved for 2% yttrium (Y) doping.

Effect of growth temperature on the structural and optoelectronic properties of epitaxial indium oxide films

High quality monocrystalline indium oxide (In2O3) films have been epitaxially grown on SiO2 (0001) substrates by the metal-organic chemical vapor deposition (MOCVD) method. The influences of different growth temperatures on the structural, morphological and optoelectronic properties of the films were studied in detail. The film deposited at 650 °C exhibited the narrowest X-ray linewidth with an epitaxial relationship of In2O3 (1 1 1)∥SiO2 (0001). The highest Hall mobility of 27.84 cm2 V−1 s−1 with a minimum carrier concentration of 5.03 × 1019 cm−3 and a minimum resistivity of 4.24 × 10−3 Ω cm was obtained for the film prepared at 650 °C. The average transmittance for all the samples in the visible range exceeded 82% and the optical band gap of the films was calculated about 3.7 eV.

Re‐parameterisations of the Cole–Cole model for improved spectral inversion of induced polarization data

ABSTRACTThe induced polarization phenomenon, both in time domain and frequency domain, is often parameterised using the empirical Cole–Cole model. To improve the resolution of model parameters and to decrease the parameter correlations in the inversion process of induced polarization data, we suggest here three re‐parameterisations of the Cole–Cole model, namely the maximum phase angle Cole–Cole model, the maximum imaginary conductivity Cole–Cole model, and the minimum imaginary resistivity Cole–Cole model. The maximum phase angle Cole–Cole model uses the maximum phase φmax and the inverse of the phase peak frequency, τφ, instead of the intrinsic charge‐ability m0 and the time constant adopted in the classic Cole–Cole model. The maximum imaginary conductivity Cole–Cole model uses the maximum imaginary conductivity instead of m0 and the time constant τσ of the Cole–Cole model in its conductivity form. The minimum imaginary resistivity Cole–Cole model uses the minimum imaginary resistivity instead of m0 and the time constant τρ of the Cole–Cole model in its resistivity form.The effects of the three re‐parameterisations have been tested on synthetic time‐domain and frequency‐domain data using a Markov chain Monte Carlo inversion method, which allows for easy quantification of parameter uncertainty, and on field data using 2D gradient‐based inversion. In comparison with the classic Cole–Cole model, it was found that for all the three re‐parameterisations, the model parameters are less correlated with each other and, consequently, better resolved for both time‐domain and frequency‐domain data. The increase in model resolution is particularly significant for models that are poorly resolved using the classic Cole–Cole parameterisation, for instance, for low values of the frequency exponent or with low signal‐to‐noise ratio. In general, this leads to a significantly deeper depth of investigation for the , , and parameters, when compared with the classic m0 parameter, which is shown with a field example. We believe that the use of re‐parameterisations for inverting field data will contribute to narrow the gap between induced polarization theory, laboratory findings, and field applications.

XPS analysis of ZnO:Ga films deposited by magnetron sputtering: Substrate bias effect

This work focuses on X-ray photoelectron spectroscopy (XPS) and combined Raman and photoluminescence experiments performed on ∼500 nm thick ZnO:Ga films deposited by magnetron sputtering. The substrate bias voltage applied during the deposition was varied between 0 V (grounded) and −120 V in order to study the effect of Ga-doping on the ZnO wurtzite structure of the films and its electrical properties, when using a fixed doped composition of Ga2O3 (4.5 wt.%) in the ZnO sputtering target. XPS analysis revealed that ZnO dominates in all samples, while the Ga amount is the highest (∼2.9 at.%) for a substrate bias polarization of −60 V, diminishing substantially for either decreasing or increasing substrate polarizations. This increase in Ga concentration is responsible for the enhancement of electronic transport properties, resulting in a minimum electrical resistivity of ∼300 µΩ·cm. Moreover, the atomic layers closer to the surface are deficient in zinc for higher bias, due to the etching effect of Ar+ ions and subsequent Zn re-evaporation. From the Raman experiments, it was observed that the dynamics of the A1 and E1 phonons correlates with the decrease of the electrical resistivity. Photoluminescence studies revealed two broad bands, being one near the ZnO near-band-edge (3.4 eV) and another at higher energies (∼3.6 eV). The band centered at higher energies is more prominent for the case of the more electrically-conductive films, and is ascribed to electron transitions from the conduction band to single ionized oxygen vacancies. The lifetime of the polar-nature longitudinal optical phonons is in the range of 0.1–0.2 ps, which is quite small due to the Fröhlich interactions with gallium dopant atoms and other defects.

Photocatalytic properties of plasma-synthesized zinc oxide and tin-doped zinc oxide (TZO) nanopowders and their applications as transparent conducting films

Transparent conducting zinc oxide and tin-doped zinc oxide (TZO) nanopowders were synthesized for the first time using a novel plasma-assisted chemical vapor synthesis route. The injected precursors were volatized completely and rapidly followed by chemical reactions and subsequent quenching to yield fine nanopowder. The amount of tin nitrate was varied to obtain 3 and 5 at.% Sn designated as TZO1 and TZO2 respectively. XRD diffraction peaks of TZO1 nanoparticles indicated the presence of wurtzite structure without any tin oxide peaks except in TZO2 sample and SEM micrographs revealed spherical particles. The nanosized powders would make an excellent material for use as photocatalyst due to high surface to volume ratio. Optical examinations indicated that the band gap in TZO1 was redshifted to 3.16 eV from 3.22 eV in undoped ZnO nanoparticles. The photocatalytic properties of ZnO and TZO nanopowders were investigated using the methylene blue dye degradation under UV light irradiation and kinetic analyses indicated that the photodegradation of methylene blue followed pseudo-first order kinetic model using Langmuir–Hinshelwood mechanism. Furthermore, the TZO1 nanoparticles exhibited superior photocatalytic activity compared with ZnO and the improvement was ascribed to increase in specific surface area and enhanced oxygen vacancies as revealed from the XPS O 1s and PL spectra. Deposited films showed a hexagonal wurtzite structure and exhibited a c-axis preferred orientation perpendicular to the substrate. A minimum resistivity of 1.4 × 10− 3 Ωcm was obtained at lower doping amount of 3 at.% Sn as in TZO1 film and all the films exhibited an average transmission of 80% indicating their suitability as a promising material in optoelectronic applications. Optical constants of the films were determined, which varied with doping amount. The photo-current properties of ZnO and TZO films were investigated and only TZO1 film showed photo response property when irradiated with UV lamp.

Charge carrier transport properties of Mg-doped Al0.6Ga0.4N grown by molecular beam epitaxy

We have studied the charge carrier transport properties of Mg-doped AlGaN epilayers with Al composition ∼60% grown by plasma-assisted molecular beam epitaxy. At room temperature, free hole concentration up to 8.7 × 1017 cm−3 was measured, and hole mobility values in the range of 10–17 cm2 V−1 s−1 can be reliably achieved. Significantly, a minimum resistivity of 0.7 Ω cm was measured, and its dependence on the Mg dopant incorporation was identified. Detailed temperature-dependent charge carrier transport studies further revealed that the hole concentration exhibited a negligible dependence on temperature near room temperature, but increases drastically for temperatures above 300 °C, suggesting the importance of impurity band conduction at room temperature.

Effect of ambient air flow on resistivity uniformity of transparent Ga-doped ZnO film deposited by atmospheric pressure plasma jet

Transparent gallium-doped zinc oxide films were deposited on large area (117 mm × 185 mm) glass substrates by atmospheric pressure plasma jet method, using two different enclosure conditions—with and without side opening boundary. The opening allows gas exchange between the growth ambient and outside atmosphere. The spatial resistivity distribution was apparently influenced by the air flow field. The case with side opening has higher overall non-uniformity, and the film near the opening has the highest resistivity, caused by considerable reduction in carrier concentration (−32%) and mobility (−15%). Computer fluid dynamics simulations show that air was drawn in, through the side opening, from outside atmosphere, resulting in a much higher oxygen mass fraction (+50%) near the opening. As such, the film near the opening was deposited in O-rich conditions. The degradation of electrical conductivity in O-rich conditions cannot be explained by oxygen vacancies. We suggest that it is due to the formation of dopant-defect complex GaZn−VZn (the lowest formation energy) and reduction of hydrogen impurity (reduced hydrogen partial pressure) in O-rich conditions. Therefore, the tool enclosure, gas flow field, and scanning trajectory must be carefully designed to achieve minimum resistivity non-uniformity on large-area substrates.

Geographic variation and plasticity in climate stress resistance among southern African populations of Ceratitis capitata (Wiedemann) (Diptera: Tephritidae)

Traits of thermal sensitivity or performance are typically the focus of species distribution modelling. Among-population trait variation, trait plasticity, population connectedness and the possible climatic covariation thereof are seldom accounted for. Here, we examine multiple climate stress resistance traits, and the plasticity thereof, for a globally invasive agricultural pest insect, the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae). We also accounted for body size and population genetic connectivity among distinct populations from diverse bioclimatic regions across southern Africa. Desiccation resistance, starvation resistance, and critical thermal minimum (CTmin) and maximum (CTmax) of C. capitata varied between populations. For thermal tolerance traits, patterns of flexibility in response to thermal acclimation were suggestive of beneficial acclimation, but this was not the case for desiccation or starvation resistance. Population differences in measured traits were larger than those associated with acclimation, even though gene flow was high. Desiccation resistance was weakly but positively affected by growing degree-days. There was also a weak positive relationship between CTmin and temperature seasonality, but CTmax was weakly but negatively affected by the same bioclimatic variable. Our results suggest that the invasive potential of C. capitata may be supported by adaptation of tolerance traits to local bioclimatic conditions.

Sm-doped CdS thin films prepared by spray pyrolysis: a structural, optical, and electrical examination

Virgin and Sm-doped CdS thin films were prepared by spray pyrolysis route on glass substrates. The influences of Sm-doping amount on structural, topographical, optical and electrical properties were examined. It was found from X-ray diffraction data that virgin and 1 and 2 at.% Sm-doped CdS samples grew along (002) plane whereas the preferred orientation changed to (101) starting from Sm doping of 3 at.%. Morphological investigation showed that even though CdS specimen had irregular shaped grains, Sm-doping substantially modified the surface topography of CdS thin films. A maximum transmittance were attained for 4 at.% Sm-doped CdS sample. After Sm-doping, a slight increase was observed in the band gap value of CdS thin films. It was found from the room temperature photoluminescence data that compared to the virgin CdS, the intrinsic defect population of Sm-doped CdS thin films reduced, meaning that more stoichiometric films formed. From the electrical measurements, it was determined that 1 at.% Sm-doped CdS thin films exhibited the best electrical properties, i.e., maximum carrier density and minimum resistivity. Based on all the data, it could be pronounced that 4 at.% Sm-doped CdS displayed the best optical properties, whereas the optimum electrical characteristics were reached for 1 at.% Sm-doped CdS thin films.

Electric field effect on variable-range hopping conductivity in Bi1.9Lu0.1Te3

Electric field effect on low-temperature electrical resistivity of n-type Bi1.9Lu0.1Te3 was examined and analyzed. It was found that with decreasing temperature from 35 down to 2.5 K, the conductivity mechanism changes from “metal” type to “semiconductor” one resulting in appearance of minimum in the specific electrical resistivity, ρ. The temperature of this minimum, Tm, depends on electric field strength, E. At low electric field of 0.15 Vˑm−1 this temperature is equal to ∼11 K and decreases as E increases up to 4.65 Vˑm−1. Variable-range hopping (VRH) conductivity mechanism was applied to explain the ρ(T, E) behavior below Tm. Average hopping distance estimated from the experimental ρ(T, E) dependences decreases as temperature or electric field strength increase, that is the VRH conductivity can be activated both temperature and electric field. It was also found that density of states at the Fermi level rapidly enhances as electric field increases. This enhancement could be attributed to high and sharp density of states within an impurity band formed from the electronic f-levels of Lu. The forming of this impurity band is believed to be responsible for enhancement of the thermoelectric figure-of-merit of Bi2Te3.

Plasma-assisted chemical vapor synthesis of indium tin oxide (ITO) nanopowder and hydrogen-sensing property of ITO thin film

Transparent conducting indium tin oxide (ITO) nanoparticles were synthesized by plasma-assisted chemical vapor synthesis route using indium nitrate and tin nitrate as the precursors. The injected precursors were vaporized in the plasma flame followed by vapor-phase reaction and subsequent quenching of the vaporized precursors produced nanosized ITO. The amount of tin nitrate was varied to obtain 5, 10 and 15 atomic percent Sn designated as ITO1, ITO2 and ITO3, respectively. The grain size of the produced ITO powder increased with increasing plasma torch power. On the other hand, it decreased with increasing plasma gas flow rate. The electrical and optical properties of ITO films prepared by spin-coating a dispersion of synthesized nanoparticles on a glass substrate vary as a function of tin content. Hall effect measurements showed that the minimum resistivity of 6.65 × 10−4 Ωcm was obtained for ITO2 film. ITO1 and ITO2 film exhibited an average transmission of 85% indicating their suitability in optoelectronic applications. The ITO gas sensor was exposed to different concentrations of H2 gas and temperatures to evaluate its gas sensitivity. The optimum operating temperature and gas concentration of H2 showing the highest sensitivity was determined to be 350 °C and 400 ppm, respectively. The linear relation between sensitivity and concentration up to 400 ppm of H2 can benefit the actuator to detect the concentration of H2 and thus making it suitable for high-performance hydrogen gas sensing applications.