Showing papers on "Van der Pauw method published in 2019"

Mapping the conductivity of graphene with Electrical Resistance Tomography.

Abstract: Electronic applications of large-area graphene films require rapid and accurate methods to map their electrical properties. Here we present the first electrical resistance tomography (ERT) measurements on large-area graphene samples, obtained with a dedicated measurement setup and reconstruction software. The outcome of an ERT measurement is a map of the graphene electrical conductivity. The same setup allows to perform van der Pauw (vdP) measurements of the average conductivity. We characterised the electrical conductivity of chemical-vapour deposited graphene samples by performing ERT, vdP and scanning terahertz time-domain spectroscopy (TDS), the last one by means of a commercial instrument. The measurement results are compared and discussed, showing the potential of ERT as an accurate and reliable technique for the electrical characterization of graphene samples.

Towards phase pure CZTS thin films by SILAR method with augmented Zn adsorption for photovoltaic applications

Abstract: Cu2ZnSnS4 (CZTS) thin films were deposited from a single cationic bath by Successive Ionic Layer Adsorption and Reaction (SILAR) method. Regular SILAR route for CZTS had the drawback of preferential adsorption of copper and tin cations in comparison with zinc. This resulted in Cu3SnS4 (CTS) and Cu2S phases rather than phase pure CZTS films. A modified SILAR route, with a separate bath for Zn2+ ions, circumvented the difficulty and hence led to phase pure CZTS thin films. UV–visible absorption spectra of the CZTS thin films showed absorption coefficients of ~ 104 cm−1 and a band gap of 1.5 eV. Combined van der Pauw and Hall measurements of CZTS thin films showed a resistivity of approximately 1.51 × 102 Ωcm, carrier density of ~ 1.28 × 1017 cm−3, and mobility ~ 0.32 cm2 V−1s−1. A completely solution processed P–N junction was fabricated and characterized by forming glass/FTO/TiO2/CdS/CZTS multilayer.

Characterization of electro-conductive textile materials by its biaxial anisotropy coefficient and resistivity

Abstract: Woven and knitted fabrics were chosen to determine their electro-conductive properties. Van der Pauw equation modified by Wasscher for anisotropic materials was used. The electrical conductivity of textile materials was anisotropic as indicated by the biaxial anisotropy coefficient being in the range from 1.1 to 7.2. Resistance was higher when current flows between electrodes arranged on the line parallel to the warp and course direction in the case of woven or knitted fabrics respectively. Results of analysis confirmed the effect of sample surface roughness on its electro-conductive properties. Sample with a smooth surface will conduct electrical current better than sample with a rough one. Electrical resistivity of the fabrics indicated that they can be used as paths for conveying electrical signals.

Co3O4 thin films prepared by hollow cathode discharge

Abstract: Semiconducting crystalline Co3O4 thin films were deposited on glass, stainless steel and Si substrates using three different PVD methods: (i) RF magnetron sputtering, (ii) high-power impulse magnetron sputtering (HiPIMS), and (iii) hollow cathode discharge (HCD). All layers were sputtered from pure cobalt target (or nozzle) in reactive atmosphere and post-annealed on air. Properties of deposited layers have been studied and discussed with respect to their potential applications. The surface morphology of the films was analyzed by SEM, their crystalline structure by XRD and Raman spectroscopy, chemical composition by EDS, electrical properties by Van der Pauw method and the specific surface area was measured by standard BET analysis. The layers prepared by the hollow cathode discharge compared to the magnetron-prepared films exhibited higher porosity, higher resistivity, higher activation energy, and higher deposition rate during the sputtering process.

Aligned carbon nanotube based sensors for strain sensing applications

Abstract: This paper presents an aligned carbon nanotube (CNT)-based strain sensor. Vertical aligned carbon nanotubes (VA-CNT), synthesized by chemical vapour deposition (CVD), were knocked down onto polymeric films, in order to obtain a thin 10 × 10 × 0.05 mm CNT patch. Different polymeric substrates, ADEXepoxy, polyethylene terephthalate (PET) and polyimide (PI) were used. The samples’ morphology before and after the knock down process, specifically their alignment, was observed by scanning electron microscopy (SEM). The good quality of the synthesized VA-CNT was assessed by Raman spectroscopy. Furthermore, transmission electron microscopy (TEM) analysis was carried out to determine the average wall number and diameters (inner and outer) of the VA-CNT. A MATLAB software with an adapted Van der Pauw method for anisotropic conductors was developed to determine the electric properties of the obtained samples, which were strained in the transverse (X) and parallel (Y) directions with respect to the CNT alignment. The electric anisotropy, defined as electric resistance ratio between obtained measurements along the X (Rxx) and Y (Ryy) -axes, decreases with deformation increment when the sample was strained in the Y-direction, while it increases when strained in the X-direction. Moreover, the obtained Gauge factor values showed a much sensitive response to deformation, i.e., approximately 47% increase in GF values, when the samples are strained transversely to CNT alignment. These results showed that the piezoresistive CNT/polymeric based sensor produced is suitable for strain sensing applications.

The properties study of transparent conductive oxides (TCO) of tin dioxide (ATO) doped by antimony obtained by spray pyrolysis

Abstract: Transparent conductive coatings based on thin films of metal oxides are widely used in various optoelectronic devices and appliances. The article is devoted to the study of the morphological, structural, electrical and optical properties of transparent conductive oxide. Tin dioxide thin films are obtained by spray pyrolysis technique on glass substrates. Analysis of the structural properties showed that SnO2 has a tetragonal crystal structure. Analysis of the morphological properties showed that the grain size of the films increases at a deposition temperature from 450 to 550 ℃. Dependencies of the transmittance coefficient of samples obtained on the solutions volume and transmittance coefficient of samples obtained on the doping levels have become the result of studying the optical properties of transparent conductive oxide. The transmittance is almost independent on the amount of substance sprayed onto the substrate. However, the transmittance is greatly influenced by the chemical composition of the films. The main electrical parameter affecting the TCO quality is conductivity or surface resistance. Surface resistance is measured by probe methods, the most accurate of which is the Van der Pauw method. Surface resistance consistently decreases with increasing solution volume, precursor concentration and impurity concentration.

The composition, structure and properties of four different glassy carbons

Abstract: Glassy carbon (GC) is a class of disordered carbon materials that is known to be superelastic and non-graphitizing up to 3000 °C. The maximum heat treatment temperature is often used as a proxy to denote structure and physical properties. GC synthesised at low temperatures (~1000 °C) is often classified as Type I GC which has advantages of higher elastic modulus, resistance to oxidation, and lower permeability to gases. Type II GC is synthesised at higher temperatures (>2000 °C), has fewer impurities, is more electrically conductive, and is rated to a higher service temperature. Here Type I and II GC samples sourced from two suppliers are investigated using Rutherford backscattering spectrometry and elastic recoil detection analysis for composition, Raman spectroscopy, transmission electron microscope imaging, X-ray and neutron diffraction for structure determination, nanoindentation for mechanical properties, and Van der Pauw measurements for resistivity. The results show that the broad classifications of Type I or Type II do not correlate with the physical properties of the samples. We conclude that the quoted maximum heat treatment temperature alone is not sufficient to specify the properties of GC and that a careful microstructural examination of the material should be used to inform materials selection.

A new application of CexZr1-xO2 as dense diffusion barrier in limiting current oxygen sensor

Abstract: CexZr1-xO2 (x = 0.25, 0.5 and 0.75) was synthesized by solid phase chemical reaction synthesis and characterized by X-ray diffraction (XRD), thermal expansion, Hebb-Wagner method and DC van der Pauw method. ZrO2 has a high melting point and CeO2 has a variable valence state, which can make the CexZr1-xO2 material have mixed ionic-electronic conductivity and stability. The synthesis method has the advantages of having a lower synthesis temperature and no waste liquid discharge, which is beneficial to energy conservation and environmental protection. A limiting current oxygen sensor was prepared with Ce0.75Zr0.25O2 dense diffusion barrier and ZrO2 stabilized by 9 mol% Y2O3 (9YSZ) solid electrolyte by Pt sintered-paste method. Limiting current plateau of the oxygen sensor was obtained and the effects of operate temperature (T), oxygen concentration (x(O2)) and water vapor pressure (p(H2O)) on the limiting current was studied, respectively. The results show that the Ce0.75Zr0.25O2 material has maximum electronic and total conductivity at 800 oC and is the most suitable ceramic material to be a dense diffusion barrier of limiting current oxygen sensor. The oxygen sensor exhibits good sensing characteristics under different research conditions, including different T, x(O2) and p(H2O). The limiting current is related to various research factors, for example, log(IL·T) depends linearly on 1000/T, IL depends linearly on x(O2) and IL is not influenced obviously by p(H2O). The experiment supplements the application of mixed conductor material CexZr1-xO2 (x = 0.25, 0.5 and 0.75) as a dense diffusion barrier in limiting current oxygen sensor.

Study of Implantation Defects in CVD Graphene by Optical and Electrical Methods

Abstract: A Chemical Vapor Deposition graphene monolayer grown on 6H–SiC (0001) substrates was used for implantation experiments. The graphene samples were irradiated by He+ and N+ ions. The Raman spectra and electrical transport parameters were measured as a function of increasing implantation fluence. The defect concentration was determined from intensity ratio of the Raman D and G peaks, while the carrier’s concentration was determined from the relations between G and 2D Raman modes energies. It was found that the number of defects generated by one ion is 0.0025 and 0.045 and the mean defect radius about 1.5 and 1.34 nm for He+ and N+, respectively. Hole concentration and mobility were determined from van der Pauw measurements. It was found that mobility decreases nearly by three orders of magnitude with increase of defect concentration. The inverse of mobility versus defect concentration is a linear function, which indicates that the main scattering mechanism is related to defects generated by ion implantation. The slope of inverse mobility versus defect concentration provides the value of defect radius responsible for scattering carriers at about 0.75 nm. This estimated defect radius indicates that the scattering centres most likely consist of reconstructed divacancies or larger vacancy complexes.

Superior positive relative humidity sensing properties of porous nanostructured Al:ZnO thin films deposited by jet-atomizer spray pyrolysis technique

Abstract: A highly responsive resistive type humidity sensor is fabricated from porous aluminium-doped ZnO (Al:ZnO, AZO) thin films deposited by using jet-atomizer spray pyrolysis technique. The deposited thin films are calcined at various temperatures, ranging from 450 to 650 °C, to improve the crystallinity, electrical and optical properties. Electrical resistivity of the films is measured by Van der Pauw technique. Lowest resistivity of 13 Ω·cm is obtained for 3 at.% AZO samples calcined at 500 °C. Humidity sensing properties like responsivity, sensitivity, repeatability, response time and recovery time of the AZO samples are investigated at room temperature in the range of 10–90% of relative humidity (RH). Among all the samples, 5 at.% AZO thin films show the highest responsivity of 733% at 90% RH. In contrast to other ZnO based sensors, it is found that the resistance of the AZO sensors, prepared in our method, increases with the increase of relative humidity. A sensing mechanism of the AZO and ZnO thin films is proposed to explain the water adsorption technique and the cause of increase in resistance of the sensors during water adsorption.

Application of an extended van der Pauw method to anisotropic magnetoresistance measurements of ferromagnetic films

Abstract: We demonstrate anisotropic resistivity measurements using the extended van der Pauw (vdP) method in ferromagnetic Ni80Fe20 (Py) films. We apply it to measure anisotropic magnetoresistance (AMR) and compare the results of the vdP method with the more conventional Hall-bar method along the hard and easy axis of the film and show that the vdP method gives more reliable AMR result. For instance the AMR result along the hard and easy axis of the film are in close agreement. Further, we applied the vdP method to study AMR in a series of Py films with thicknesses ranging between 10–250 nm. The films were grown by sputtering deposition at an angle with respect to the substrate normal and with an in situ magnetic field, both conditions assisting in the definition of in-plane uniaxial anisotropy. The microstructure of Py films was characterized using x-ray reflectivity, diffraction and polar mapping of (1 1 1) planes. We detected no off-normal texture and negligible surface roughness, which indicates that self-shadowing is not dominating in our growth. Yet the films have well defined uniaxial anisotropy. Abrupt changes in the average resistivity versus film thickness were observed, which cannot be explained by the models accounting for the thickness and grain size but strongly correlate with the changes in (1 1 1) texture in the films. We compared our results with the literature and show that independent of growth method, substrate and deposition temperature, the AMR value presents a saturation behavior with thickness at about 100 nm.

Measurement of the Variable Surface Charge Concentration in Gallium Nitride and Implications on Device Modeling and Physics

Abstract: We have evaluated the density of interface trap states ( ${D}_{\textsf {it}}$ ) at the surface of a GaN/AlGaN/GaN heterojunction by the previously described gated van der Pauw experiments, as well as by a UV-assisted gated van der Pauw method, described in this paper. The obtained ${D}_{\textsf {it}}$ values are about two orders of magnitude lower than the assumed by the surface-donor theory and three orders of magnitude lower than the required to compensate the polarization surface charge in GaN. Previous experimental studies using a variety of other techniques reported similarly low ${D}_{\textsf {it}}$ values. We, hence, conclude that the variable midgap surface charge is not responsible for the formation of the 2-D electron gas, and cannot compensate for the large surface polarization charge in GaN. A yet unexplained polarization self-compensating (PSC) surface charge must be invoked to account for experiments. A few comments about the physical nature of the proposed PSC charge are provided.

Band gap determination in multi-band-gap CuFeO 2 delafossite epitaxial thin film by photoconductivity

Abstract: The photoconductivity within a wavelength range of 450–1100 nm was determined for a sample of epitaxial delafossite CuFeO2 film grown by pulsed laser deposition. The film thickness was estimated to be 75 nm. The resistance of the films was determined with four-contact van der Pauw’s method and using monochromatic illumination of the film. The most significant change in resistance resulted in three rapid lineal conductivity increases at photon energies of ~ 1.5 eV (gap-1), ~ 2.1 eV (gap-2) and ~ 2.5 eV (gap-3). The conductivity properties are well correlated with prior optical absorption results obtained in the NIR-VIS region using transmittance spectroscopy.

Poly(3,4-Ethylenedioxythiophene) nanoparticles as building blocks for hybrid thermoelectric flexible films

Abstract: Hybrid thermoelectric flexible films based on poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles and carbon nanotubes were prepared by using layer-by-layer (LbL) assembly. The employed PEDOT nanoparticles were synthesized by oxidative miniemulsion polymerization by using iron(III) p-toluenesulfonate hexahydrate (FeTos) as an oxidant and poly(diallyldimethylammonium chloride) (PDADMAC) as stabilizer. Sodium deoxycholate (DOC) was used as a stabilizer to prepare the aqueous dispersions of the carbon nanotubes. Hybrid thermoelectric films were finally prepared with different monomer/oxidant molar ratios and different types of carbon nanotubes, aiming to maximize the power factor (PF). The use of single-wall (SWCNT), double-wall (DWCNT), and multiwall (MWCNT) carbon nanotubes was compared. The Seebeck coefficient was measured by applying a temperature difference between the ends of the film and the electrical conductivity was measured by the Van der Pauw method. The best hybrid film in this study exhibited a PF of 72 µW m−1K−2. These films are prepared from aqueous dispersions with relatively low-cost materials and, due to lightweight and flexible properties, they are potentially good candidates to recover waste heat in wearable electronic applications.

Explicit resistance matrix for a Hall disk with multiple peripheral contacts: Application to a van der Pauw type method for extended contacts

Abstract: A direct analytical method for determining the Resistance Matrix of a Hall disk with an arbitrary number of extended peripheral contacts has been developed. The method does not require the use of any conformal mappings. It works also in the case of large magnetic fields. The resulting explicit formulas involve the angular coordinates of the asymmetrical contacts ends, the sheet resistance, and the Hall angle θH as inputs. The formulas are obtained through the calculation of some definite integrals of analytical functions with integrable singularities at the end of the peripheral contacts. The method can be used for determining the sheet resistance and the Hall mobility of a circular plate with extended contacts on its boundary by utilizing two measurements similar to those used by van der Pauw's method for pointlike contacts.

Effect of atomic ordering on the magnetic anisotropy of single crystal Ni80Fe20

Abstract: We investigate the effect of atomic ordering on the magnetic anisotropy of Ni80Fe20 at.% (Py). To this end, Py films were grown epitaxially on MgO(001) using dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS). Aside from twin boundaries observed in the latter case, both methods present high quality single crystals with cube-on-cube epitaxial relationship as verified by the polar mapping of important crystal planes. However, X-ray diffraction results indicate higher order for the dcMS deposited film towards L12 Ni3Fe superlattice. This difference can be understood by the very high deposition rate of HiPIMS during each pulse which suppresses adatom mobility and ordering. We show that the dcMS deposited film presents biaxial anisotropy while HiPIMS deposition gives well defined uniaxial anisotropy. Thus, higher order achieved in the dcMS deposition behaves as predicted by magnetocrystalline anisotropy i.e. easy axis along the [111] direction that forced in the plane along the [110] direction due to shape anisotropy. The uniaxial behaviour in HiPIMS deposited film then can be explained by pair ordering or more recent localized composition non-uniformity theories. Further, we studied magnetoresistance of the films along the [100] directions using an extended van der Pauw method. We find that the electrical resistivities of the dcMS deposited film are lower than in their HiPIMS counterparts verifying the higher order in the dcMS case.

Thermal oxidation of sputtered nickel nano-film as hole transport layer for high performance perovskite solar cells

Abstract: The effect of rapid oxidation temperature on the sputtered nickel (Ni) films to act as a hole transport layer (HTL) for perovskite solar cell (PSCs) was investigated. A nano-sputtered Ni film with a thickness about 100 nm was oxidized at a range of different oxidation temperatures between 350 and 650 °C to work as HTL in an inverted p–i–n configuration. DC Hall measurement in van der Pauw configuration and photoluminescence spectroscopy were used to measure the charge’s mobility and extraction of nickel oxide (NiO) films. The behaviour of the carrier concentration measurements of NiO layers at different oxidation temperatures showed that the Ni layer oxidized at 450 °C had the highest carrier concentration among the other samples. The performance measurements of the fabricated PSCs showed that the nickel oxide hole-transporting layer which has been oxidized at the optimum oxidation temperature of 450 °C has the highest power conversion efficiency (PCE) of 12.05%. Moreover, the characteristic parameters of the optimum cell such as the open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF) were 0.92 V, 19.80 mA/cm2 and 0.331, respectively.

Effects of phosphorus doping via Mn3P2 on diamond growth along the (100) surfaces

Abstract: In this study, diamond crystals were synthesized via the temperature gradient method at 5.6 GPa and 1230–1245 °C by adding a Mn3P2 dopant and FeNi catalyst. Experimental results showed that Mn3P2 shifted the V-shaped growth region to the upper right by influencing the catalytic properties. The scanning electron microscopy (SEM) photographs of the diamonds synthesized at 1245 °C revealed that there were many dendritic structures in the (100) surfaces. The diamond crystal quality could be improved when the added Mn3P2 was less than 6 wt% at 1245 °C, but it would be seriously damaged when the diamond was heavily doped with Mn3P2. Pits and uneven layered structures appeared on the diamond surfaces when the additive was included at 12 wt% at 1245 °C. The Fourier transform infrared spectroscopy (FTIR) results revealed that the Mn3P2 additive increases the N content of the crystal and that N is present in the diamond crystal in the form of a “C” center. It is worth noting that phosphorus could be doped into diamond crystals by using the Mn3P2 additive and that the other impurities in diamonds were mostly C–N and C–O forms, as confirmed by X-ray photoelectron spectroscopy (XPS). The phosphorus in the sample crystals mainly formed C–P bonds with carbon, while a smaller amount of phosphorus formed P–O bonds with oxygen. The test results of the electrical properties as assessed by the van der Pauw method for the diamond crystals with Mn3P2 doped at 1245 °C revealed a resistivity of 0.516 × 106–9.729 × 106 Ω cm and a negative Hall coefficient, indicative of an n-type semiconductor.

Enhanced thermoelectric properties of lightly Nb doped SrTiO3 thin films

Abstract: Novel thermoelectric materials developed for operation at room temperature must have similar or better performance along with being as ecofriendly as those commercially used, e.g., Bi2Te3, in terms of their toxicity and cost. In this work, we present an in-depth study of the thermoelectric properties of epitaxial Nb-doped strontium titanate (SrTi1−xNbxO3) thin films as a function of (i) doping concentration, (ii) film thickness and (iii) substrate type. The excellent crystal quality was confirmed by high resolution transmission electron microscopy and X-ray diffraction analysis. The thermoelectric properties were measured by the three-omega method (thermal conductivity) and van der Pauw method (electrical resistivity), complemented by Seebeck coefficient measurements. A maximum power factor of 8.9 × 10−3 W m−1 K−2 and a thermoelectric figure of merit of 0.49 were measured at room temperature in 50 nm-thick films grown on lanthanum strontium aluminate. The mechanisms behind this high figure of merit are discussed in terms of a possible two-dimensional electron gas, increase of the effective mass of the electrons, electron filtering and change in strain due to different substrates. The overall enhancement of the thermoelectric properties suggests that SrTi1−xNbxO3 is a very promising n-type candidate for room- to high-temperature applications.

Impact of Silicon Doping on the Properties of Transparent Conducting CdO Thin Films

Abstract: CdO thin films doped with different amounts of silicon ions have been deposited on glass substrates by the vacuum evaporation method in order to improve their properties for application as transparent conducting oxide (TCO) films. The structural, electrical, and optical properties of the host CdO films were systematically studied. The realization of doping with silicon ions was confirmed and studied by the X-ray diffraction (XRD) method. It was concluded that Si ions occupied locations in interstitial positions and structural vacancies of the CdO lattice. The bandgap of Si-doped CdO was blue shifted following the Moss-Burstein (B-M) effect. The dc-electrical behaviors were studied by the Van der Pauw method showing that the prepared Si-doped CdO films were degenerate semiconductors of controllable opto-electrical properties by doping level. The utmost improvements in mobility (μ) and conductivity (σ) took place with the host CdO film doped with ∼2% Si, so that the mobility increased by ∼10 times and the conductivity by ∼100 times compared to the pristine CdO film. The results show that silicon is effective for CdO utilization in applications in the TCO field.

Van der Pauw Method for Measuring the Electrical Conductivity of Smart Textiles

Abstract: The electrical properties of conductive textile fabrics are investigated using the Van der Pauw method. The dependences of the change in the electrical resistance of conductive knitted fabrics under tension perpendicular to the warp direction, as well as at a particular angle to the warp direction of the fabric, are obtained. The data obtained are intended for use in the design of textile electronics, as well as “smart” and highly functional clothing made based on it.

Sheet Resistance Measurement of Inkjet Printed Layers

Abstract: The Van der Pauw method is a method commonly used to measure the resistance and Hall coefficient of a sample. Its advantage lies in its ability to accurately measure the properties of a random sample, ensuring that the sample is approximately two-dimensional (i.e., it is much thinner than it is wide), rigid (without holes), and the electrodes are located on the perimeter. The Van der Pauw method uses a 4-point probe around the sample circumference, unlike the linear 4-point probe: this allows the Van der Pauw method to provide a mean resistance of the sample while the linear array provides resistance in the reading direction. [1] This difference becomes important for anisotropic materials, which can be correctly measured by the various modifications of the Van der Pauw method.

Low temperature synthesis of α- and β-phase Bi2O3 thin film via B doping: tailoring optical band gap and n- to p-type Bi2O3

Abstract: In this article, we fabricated p-type bismuth oxide (Bi2O3) thin films with tailoring optical band gap by boron (B) doping, for the first time. In addition, an effort is made to see the influence of B doping on the surface morphological, structural, optical and electrical transport properties of Bi2O3 thin films. Field Emission Scanning Electron Microscope (FESEM) images demonstrated that the film surface is covered by well-defined multigonal shaped particles and glassy surface. α-Bi2O3 (monoclinic) and β-Bi2O3 (tetragonal) phase structures are confirmed by X-ray diffraction (XRD) analysis. The average crystallite size is decreased from 46.62 to 23.57 nm with B doping concentration. Moreover, the average strain, stress and dislocation density values are calculated using XRD data. The optical band gaps have changed from 3.70 to 3.99 eV with the texture coefficient values of $$ \left( {11\bar{2}} \right) $$ orientation plane. A minimum refractive index and optical conductivity value are found to be 2.58 and 2.23 × 106 Ω−1 m−1 for 3 at.% B content. Electrical parameters, viz. resistivity, sheet resistance, charge carrier concentration, mobility and conductivity types are investigated using a van der Pauw Hall measurement system. Electrical measurements demonstrated that the resistivity values are found to vary in the range of 1.23–1.82, × 103 Ω-m with increasing B doping concentrations. A high-quality factor is obtained 5.52 × 10−6 Ω−1 for higher doping content at 550 nm wavelength. This work promotes a new vision into the fabrication of p-type Bi2O3 thin films and facilitates their application in the field of optoelectronic devices, viz. window layer coating, p–n junction and photovoltaic applications.

Electron transport in N-polar GaN-based heterostructures

Abstract: Electron transport in N-polar GaN-based high-electron-mobility transistor (HEMT) structures with a combination of In0.18Al0.82N-AlN as the barrier was studied via temperature-dependent van der Pauw Hall and Shubnikov de Haas measurements. In contrast to Ga-polar HEMT structures, no persistent photoconductivity could be detected. In a sample with 10 nm thick InAlN, only one oscillation frequency was observed, demonstrating that a single sublevel is present. From the oscillations, a two-dimensional electron gas carrier density of 8.54 × 1012 cm−2 and a mobility of 4970 cm2/V s were extracted at 1.7 K. This sample was further investigated using ionic liquid gating. The charge density was varied from 7.5 × 1012 cm−2 to 9.6 × 1012 cm−2. The electron mobility significantly declined with decreasing charge density. This is in contrast to Ga-polar HEMT structures, where the electron mobility typically increases slightly as the charge density decreases.

Thermal and Electrical Conductivity of Liquid Al–Si Alloys

Abstract: Aluminum silicon alloys belong to the family of metallic phase change materials that utilizes latent heat to store and release energy. In designing such heat storage systems, knowledge of the thermal conductivity in both the liquid and solid phase is required to determine heat exchange speed. Here, we attempted to evaluate the thermal conductivity of liquid Al–Si alloys both from electrical resistivity and from thermal diffusivity. Electrical resistivity was measured by adapting the van der Pauw method for liquids and is related to thermal conductivity through the Wiedemann–Franz law, but assumptions about the Lorenz number of liquid Al–Si alloys is necessary. We also measured the thermal diffusivity of liquid Al–Si alloys by laser flash to calculate the thermal conductivity. Comparing the thermal conductivity obtained through these two methods, we realized that calculations with the ideal metal’s Lorenz number overestimates the thermal conductivity of liquid Al–Si alloys.

Self-doped SnO 2 :F synthesis by aerosol-spray deposition technique and their application in relative humidity sensor devices

Abstract: Fluorine-doped tin-oxide (SnO2:F) thin films were prepared by aerosol-spray deposition technique on soda lime glass substrate by one-step deposition process. Tin (II) fluoride (SnF2) was used as source of tin (Sn) and doping element fluorine (F), to synthesis a self-doped transparent conducting electrode. The doping concentration of the thin films was controlled by varying deposition temperature and the molar concentration of the SnF2 solution. Electrical resistivity of the samples was measured by Van Der Pauw method and the mobility was calculated by Hall measurement technique. SnO2:F thin films grown under optimum condition showed 1.2 × 10−4 Ω-cm resistivity and 6.5 × 1020/cm3 carrier concentration. Surface-free energy of the deposited thin films was calculated from the measurement of contact angle for three different probe liquids using the Wendt theory. Sample grown at optimum condition showed the lowest surface-free energy of 63.55 mJ/m2 with dominating polar component. Reduced graphene oxide was used to produce a resistive humidity-sensing device to study the effectiveness of FTO thin film as back electrode. Maximum of 18.27% responsivity was observed for the relative humidity of 90% along with the sensitivity of 1.16 Ω/% RH.

A limiting Current Oxygen Sensor Constituted of (CeO2)0.95(Y2O3)0.05 as Solid Electrolyte Layer and (CeO2)0.75(ZrO2)0.25 as Dense Diffusion Barrier Layer

Abstract: Using the co-precipitation method to synthesize (CeO2)0.95(Y2O3)0.05 (YDC) and solidreaction method to synthesize (CeO2)0.75(ZrO2)0.25 (ZDC), and the crystal structure, micro-structure,total conductivity and electronic conductivity of the two materials was measured with X-raydiffraction (XRD), scanning electron microscope (SEM), DC van der Pauw and Hebb-Wagnermethods. A limiting current oxygen sensor was prepared with YDC solid electrolyte and a ZDCdense diffusion barrier layer by employing platinum pasting bonding. Sensing characteristics ofthe sensor were obtained at different conditions, including temperature (T), oxygen concentration(x(O2)) and water vapor pressure (p(H2O)), and the influence of various conditions on sensingperformance was studied. The long-term stability of the sensor was measured in an oxygen concentration of 1.2% and at a temperature of 800 °C for 120 h. XRD results show that the phase structure of both YDC and ZDC belongs to the cubic phase. SEM results show that both YDC and ZDC layers are dense layers, which are then qualified to be the composition materials of the sensor. The limiting current (IL) of the sensor is obtained and the sensor exhibits good sensing characteristics to satisfy the Knudsen model. Log(IL·T) depends linearly on 1000/T with a squared correlation coefficient (R2) of 0.9904; IL depends linearly on x(O2) with an R2 of 0.9726; and sensing characteristics are not affected by p(H2O). It was found that the oxygen sensor has good long-term stability.

Effect of atomic ordering on the magnetic anisotropy of single crystal Ni80Fe20

Abstract: We investigate the effect of atomic ordering on the magnetic anisotropy of Ni80Fe20 at.% (Py). To this end, Py films were grown epitaxially on MgO (001) using dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS). Aside from twin boundaries observed in the latter case, both methods present high quality single crystals with cube-on-cube epitaxial relationship as verified by the polar mapping of important crystal planes. However, X-ray diffraction results indicate higher order for the dcMS deposited film towards L12 Ni3Fe superlattice. This difference can be understood by the very high deposition rate of HiPIMS during each pulse which suppresses adatom mobility and ordering. We show that the dcMS deposited film presents biaxial anisotropy while HiPIMS deposition gives well defined uniaxial anisotropy. Thus, higher order achieved in the dcMS deposition behaves as predicted by magnetocrystalline anisotropy i.e. easy axis along the [111] direction that forced in the plane along the [110] direction due to shape anisotropy. The uniaxial behaviour in HiPIMS deposited film then can be explained by pair ordering or more recent localized composition non-uniformity theories. Further, we studied magnetoresistance of the films along the [100] directions using an extended van der Pauw method. We find that the electrical resistivities of the dcMS deposited film are lower than in their HiPIMS counterparts verifying the higher order in the dcMS case.