Showing papers on "Van der Pauw method published in 2016"

Heteroepitaxy of N-type β-Ga2O3 thin films on sapphire substrate by low pressure chemical vapor deposition

Abstract: This paper presents the heteroepitaxial growth of ultrawide bandgap β-Ga2O3 thin films on c-plane sapphire substrates by low pressure chemical vapor deposition. N-type conductivity in silicon (Si)-doped β-Ga2O3 films grown on sapphire substrate is demonstrated. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen (O2) as precursors. The morphology, crystal quality, and properties of the as-grown thin films were characterized and analyzed by field emission scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, photoluminescence and optical, photoluminescence excitation spectroscopy, and temperature dependent van der Pauw/Hall measurement. The optical bandgap is ∼4.76 eV, and room temperature electron mobility of 42.35 cm2/V s was measured for a Si-doped heteroepitaxial β-Ga2O3 film with a doping concentration of 1.32 × 1018 cm−3.

Twin-Induced InSb Nanosails: A Convenient High Mobility Quantum System

Abstract: Ultra narrow bandgap III–V semiconductor nanomaterials provide a unique platform for realizing advanced nanoelectronics, thermoelectrics, infrared photodetection, and quantum transport physics. In this work we employ molecular beam epitaxy to synthesize novel nanosheet-like InSb nanostructures exhibiting superior electronic performance. Through careful morphological and crystallographic characterization we show how this unique geometry is the result of a single twinning event in an otherwise pure zinc blende structure. Four-terminal electrical measurements performed in both the Hall and van der Pauw configurations reveal a room temperature electron mobility greater than 12 000 cm2·V–1·s–1. Quantized conductance in a quantum point contact processed with a split-gate configuration is also demonstrated. We thus introduce InSb “nanosails” as a versatile and convenient platform for realizing new device and physics experiments with a strong interplay between electronic and spin degrees of freedom.

Modification of electrical and optical properties of CuO thin films by Ni doping

Abstract: Undoped and Ni-doped CuO thin films were deposited onto glass substrates using a spin-coating technique at different doping concentrations (undoped, 2, 4, 6, and 10 %). X-ray diffraction patterns for undoped and Ni-doped CuO thin films indicated that the films were polycrystalline, with preferential growth in the (002), (111), and (−311) directions. Atomic force microscopy images revealed that the surface morphologies of the films were not uniform. Scanning electron microscopy images confirmed the presence of agglomerated particles on the surfaces; the coverage increased with the doping level. A Hall effect system with a van der Pauw configuration was used to investigate the electrical properties of the CuO films. The free charge carrier concentration decreased and hole mobility increased with increasing Ni concentration, with the exception of the 10 % Ni-doped CuO sample. Ultraviolet–visible spectroscopy measurements of the film samples indicated an average transmittance of 30–40 % in the visible range. The optical band gap decreased slightly for low-level doping and increased from 2.03 to 2.22 eV for 10 % Ni incorporation. The electrical and optical properties of the CuO films were modified by Ni doping, i.e. the band gap decreased and the mobility increased almost linearly, with the exception of the 10 % Ni-doped sample. SEM images of a undoped b 2 % c 4 % d 6 %, and e 10 % Ni-doped CuO thin films.

Hydrothermal derived nanostructure rare earth (Er, Yb)-doped ZnO: structural, optical and electrical properties

Abstract: The structural, optical and electrical properties of undoped and rare-earth (Er, Yb) doped zinc oxide (ZnO) nanopowder samples synthesized by hydrothermal method were investigated. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy. The optical properties of undoped and rare-earth (Er, Yb) doped ZnO were carried out with UV–visible diffuse reflectance spectroscopy techniques. XRD results reveal that Yb and Er doped ZnO nanopowders have single phase hexagonal (Wurtzite) structure without any impurities. SEM analysis indicate that dopants with different radii affected the surface morphology of ZnO nanostructures. The optical band gap of all samples were calculated from UV–Vis diffuse reflectance spectroscopy data. We have obtained band gap values of undoped, Er and Yb doped ZnO as 3.24, 3.23, 3.22 eV, respectively. Electrical characterization of the samples were made in the 280–350 K temperature range using Van der Pauw method based on Hall effect measurement. The carrier concentrations decreased for both Er and Yb doping while the Hall mobility and electrical resistivity increased with Yb, Er doping compared to undoped ZnO nanopowder at room temperature. The temperature dependent resistivity measurements of Er doped ZnO showed a metal–semiconductor transition at about 295 K, while Yb doped ZnO showed characteristic semiconductor behavior.

Electrode effects in dielectric spectroscopy measurements on (Nb+In) co-doped TiO2

Abstract: Recently, several papers reported the discovery of giant permittivity and low dielectric loss in (Nb+In) co-doped TiO2. A series of tests was performed which included the measurement of the frequency dependence of the dielectric permittivity and alternating current (ac) conductivity of co-doped (Nb+In)TiO2 as a function of electrode type, sample thickness, and temperature. The data suggest that the measurements are strongly affected by the electrodes. The consistency between four-contact van der Pauw direct current conductivity measurements and bulk conductivity values extracted from two-contact ac conductivity measurements suggest that the values of colossal permittivity are, at least in part, a result of Schottky barrier depletion widths that depend on electrode type and temperature.

Closed form expressions for sheet resistance and mobility from Van-der-Pauw measurement on 90° symmetric devices with four arbitrary contacts

Abstract: Sheet resistance and Hall mobility are commonly measured by Van der Pauw’s method. Closed form expressions are known for four point-sized contacts. Recently, for devices with fourfold rotational symmetry a closed form expression for the sheet resistance was given for contacts of arbitrary size. In this paper we discuss its accuracy, link it to the equivalent circuit diagram of the device, and add another expression that determines the Hall mobility with 0.02% accuracy.

High-Throughput Synthesis, Screening, and Scale-Up of Optimized Conducting Indium Tin Oxides

Abstract: A high-throughput optimization and subsequent scale-up methodology has been used for the synthesis of conductive tin-doped indium oxide (known as ITO) nanoparticles. ITO nanoparticles with up to 12 at % Sn were synthesized using a laboratory scale (15 g/hour by dry mass) continuous hydrothermal synthesis process, and the as-synthesized powders were characterized by powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy. Under standard synthetic conditions, either the cubic In2O3 phase, or a mixture of InO(OH) and In2O3 phases were observed in the as-synthesized materials. These materials were pressed into compacts and heat-treated in an inert atmosphere, and their electrical resistivities were then measured using the Van der Pauw method. Sn doping yielded resistivities of ∼10–2 Ω cm for most samples with the lowest resistivity of 6.0 × 10–3 Ω cm (exceptionally conductive for such pressed nanopowders) at a Sn concentration of 10 at...

The growth of zinc phthalocyanine thin films by pulsed laser deposition

Abstract: Zinc phthalocyanine (ZnPc) thin films were prepared by pulsed laser deposition (PLD) using KrF laser (λ = 248 nm, τ = 5 ns). The effect of laser fluence (in the region from 10 to 100 mJ/cm2) and repetition rate of 50 and 200 Hz to the film growth and its properties was investigated. The growth of ZnPc thin film was in situ monitored using transmission measurement in ultraviolet-visible spectral range. The optical properties in conjunction with density functional theory/time-dependent density functional theory calculations suggested the growth of the film in β-phase. X-ray diffraction also revealed crystalline character of the film. The electrical properties analyzed by van der Pauw method exhibited resistivity ρ ≈ 108–1010 Ω cm. Fourier transform infrared spectroscopy analyses revealed low deterioration of PLD deposited ZnPc films. We demonstrate that, by finely tuning the deposition conditions, PLD is a successful technique for fabrication of ZnPc thin films.

Electrical properties of n-type AlGaN with high Si concentration

Abstract: The electrical properties of Si-doped AlGaN layers (AlN molar fractions: 0.03–0.06) with the donor concentrations (N D) from 8.8 × 1017 to 4.5 × 1020 cm−3 were investigated by variable-temperature Hall effect measurement using the van der Pauw method. A minimum resistivity of 3.6 × 10−4 Ω cm was obtained for Si-doped AlGaN with a smooth surface at room temperature. We found that the activation energy of the Si donor is affected by the Coulomb interaction in the AlGaN layer with N D values from 8.8 × 1017 to 2.5 × 1020 cm−3. In several AlGaN layers, the free-electron concentration did not vary with sample temperature, as expected in the case of degeneracy. The localization of GaN in the AlGaN layer was speculated as a cause of degeneracy of samples.

High temperature Hall measurement setup for thin film characterization.

Abstract: Hall measurement using the van der Pauw technique is a common characterization approach that does not require patterning of contacts. Measurements of the Hall voltage and electrical resistivity lead to the product of carrier mobility and carrier concentration (Hall coefficient) which can be decoupled through transport models. Based on the van der Paw method, we have developed an automated setup for Hall measurements from room temperature to ∼500 °C of semiconducting thin films of a wide resistivity range. The resistivity of the film and Hall coefficient is obtained from multiple current-voltage (I-V) measurements performed using a semiconductor parameter analyzer under applied constant "up," zero, and "down" magnetic field generated with two neodymium permanent magnets. The use of slopes obtained from multiple I-Vs for the three magnetic field conditions offer improved accuracy. Samples are preferred in square shape geometry and can range from 2 mm to 25 mm side length. Example measurements of single-crystal silicon with known doping concentration show the accuracy and reliability of the measurement.

Electrical Resistance Tomography of Conductive Thin Films

Abstract: The electrical resistance tomography (ERT) technique is applied to the measurement of sheet conductance maps of both uniform and patterned conductive thin films. Images of the sheet conductance spatial distribution and local conductivity values are obtained. Test samples are tin-oxide films on glass substrates, with electrical contacts on the sample boundary. Some samples are deliberately patterned in order to induce null conductivity zones of known geometry, while others contain higher conductivity inclusions. Four-terminal resistance measurements among the contacts are performed with a scanning setup. The ERT reconstruction is performed by a numerical algorithm based on the total variation regularization and the $L$ -curve method. ERT correctly images the sheet conductance spatial distribution of the samples. The reconstructed conductance values are in good quantitative agreement with independent measurements performed with the van der Pauw and the four-point probe methods.

EMI Shielding Effectiveness of Polyacrylonitrile Fabric With Polyaniline Deposition by Reactive Ink-Jet Printing and Model Approach

Abstract: This study reports the simple, inexpensive, and in situ deposition of polyaniline (PANI) on polyacrylonitrile (PAN) woven fabric with the reactive ink-jet printing technique aimed at the manufacturing of multilayered electromagnetic interference (EMI) shields. The electrical characteristics of fabric samples are measured by the van der Pauw method. The surface resistance value of the treated fabrics steadily decreases when the number of the PANI-printed layers increases and reaches the lowest surface resistance value of 20 Ω/sq. The PANI-printed PAN structures are characterized and evaluated in terms of their electromagnetic shielding effectiveness (SE), absorption (insertion loss), and reflection characteristics. These characteristics are determined by means of a network analyzer with a frequency range from 2.6 to 18 GHz. One layer of PAN fabric ink-jet printed with conducting PANI, which gives a surface weight equal to ca 20 g/m2, shows a SE of ca 5 dB. However, tripling the surface weight of PANI on PAN fabric shows a SE of ca. 12 dB, whereas a fivefold increase in the PANI surface weight shows a SE of ca. 22 dB. The equivalent analog circuit model with an approximation approach for predicting the SE, reflection coefficient, and insertion loss for the cases, which are not experimentally tested is proposed.

Electrical Resistance Tomography of Conductive Thin Films

Abstract: The Electrical Resistance Tomography (ERT) technique is applied to the measurement of sheet conductance maps of both uniform and patterned conductive thin films. Images of the sheet conductance spatial distribution, and local conductivity values are obtained. Test samples are tin oxide films on glass substrates, with electrical contacts on the sample boundary, some samples are deliberately patterned in order to induce null conductivity zones of known geometry while others contain higher conductivity inclusions. Four-terminal resistance measurements among the contacts are performed with a scanning setup. The ERT reconstruction is performed by a numerical algorithm based on the total variation regularization and the L-curve method. ERT correctly images the sheet conductance spatial distribution of the samples. The reconstructed conductance values are in good quantitative agreement with independent measurements performed with the van der Pauw and the four-point probe methods.

Considerations on electrical impedance measurements of electrolyte solutions in a four-electrode cell

Abstract: A tetrapolar probe to measure the electrical properties of electrolyte solutions was implemented with gold electrodes according to the van der Pauw method. Electrical impedance spectroscopy (EIS) measurements of different concentrations of phosphate buffer saline (PBS) solution and an oral mucosal tissue sample dispersed in PBS were performed in the galvanostatic mode using a four-electrode cell (tetrapolar probe). Taking advantage of using a potentiostat/galvanostat for carrying out the electrical measurements, a simple and rapid method using a three-electrode electrochemical cell is described for: a) cleaning of electrodes, b) verification of surface chemical state of electrode material and c) choice of current supplied to electrodes for EIS measurements. Results of this research shown a depolarization effect due to the addition of oral mucosa tissue cells into the PBS solution.

Effect of Nb doping on the structural, morphological, optical and electrical properties of RF magnetron sputtered In2O3 nanostructured films

Abstract: Undoped and niobium (Nb) doped indium oxide (In2O3) thin films are prepared by radio frequency magnetron sputtering technique. The effect of Nb on the structural, morphological, optical and electrical properties of In2O3films are analyzed using techniques such as X-ray diffraction (XRD), micro-Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy, UV–visible spectroscopy, spectroscopic ellipsometry, photoluminescence spectroscopy and Hall effect measurements. XRD analysis reveals that the as-deposited undoped and Nb doped films are polycrystalline in nature with cubic bixbyite structure. Raman analysis supports the presence of cubic bixbyite structure of In2O3in the films. XPS analysis shows a decrease of oxygen deficiency due to Nb and the existence of Nb as Nb4+ in the In2O3lattice. The band gap energy of the films increases with increase in Nb concentration. PL spectra reveal intense UV and visible emissions in all the films. Optical constants of the films are determined using spectroscopic ellipsometry. The thickness of films estimated using FESEM and ellipsometry are in good agreement. The carrier concentration, mobility and nature of carriers are measured using Hall measurement technique in Van der Pauw configuration at room temperature. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new method for simultaneous measurement of Seebeck coefficient and resistivity

Abstract: A new method has been proposed and verified to measure the Seebeck coefficient and electrical resistivity of a sample in the paper. Different from the conventional method for Seebeck coefficient and resistivity measurement, the new method adopts a four-point configuration to measure both the Seebeck coefficient and resistivity. It can well identify the inhomogeneity of the sample by simply comparing the four Seebeck coefficients of different probe combinations, and it is more accurate and appropriate to take the average value of the four Seebeck coefficients as the measured result of the Seebeck coefficient of the sample than that measured by the two-point method. Furthermore, the four-point configuration makes it also very convenient to measure the resistivity by using the Van der Pauw method. The validity of this method has been verified with both the constantan alloy and p-type Bi2Te3 semiconductor samples, and the measurement results are in good agreement with those obtained by commercial available equipment.

Structural, mechanical, and electrical properties of carbon nanoparticles synthesized from diesel

Abstract: We studied the elemental analysis, structural morphology, mechanical, and electrical properties of carbon nanoparticles synthesized from diesel. The spherical carbon particle size in the range of about 10 to 80 nm in diameter was observed in scanning electron microscope (SEM) studies that were identified by Atomic force microscopy (AFM) study as an aggregation of carbon particles of average size 2.5 nm. The surface rms of carbon nanoparticle thin film (CNTF) was measured directly by AFM and found 0.22 nm. The Derjaguin–Muller–Toporov (DMT) elastic modulus of carbon nanoparticles (CNPs) was measured by PeakForce QNM mode of AFM. The minimum and maximum elastic modulus was measured of 0.40 GPa and 43.89 GPa, respectively. The resistivity, conductivity, magneto resistance, mobility, and average Hall co-efficient were measured by “Ecopia Hall-effect measurement system” by four-point Van der Pauw approach at ambient condition. We demonstrated I–V characteristic at the Indium/CNTF thin film interface, w...

Magnetoresistance behaviour in CoFe/Cu multilayers: thin Cu layer effect

Abstract: The magnetoresistance properties of the CoFe/Cu multilayers have been investigated as a function of thin non-magnetic Cu layer thickness (from 2.5 to 0.3 nm). CoFe/Cu multilayers were electrodeposited on Ti substrates from a single electrolyte containing their metal ions under potentiostatic control. The structural analysis of the films was made using X-ray diffraction. The peaks appeared at 2θ ≈ 44°, 51°, 74° and 90° are the main Bragg peaks of the multilayers, arising from the (111), (200), (220) and (311) planes of the face-centered cubic structure, respectively. The magnetic characterization was performed by using vibration sample magnetometer in magnetic fields up to ±1600 kA/m. At 0.6, 1.2 and 2.0 nm Cu layer thicknesses, the high saturation magnetization values were observed due to antiferromagnetic coupling of adjacent magnetic layers. Magnetoresistance measurements were carried out using the Van der Pauw method in magnetic fields up to ±1000 kA/m at room temperature. All multilayers exhibited giant magnetoresistance (GMR), and the similar trend in GMR values and GMR field sensitivity was observed depending on the Cu layer thickness.

Metal-to-insulator transition near room temperature in graphene oxide and graphene oxide + TiO2 thin films

Abstract: Thin films of graphene oxide (GO) and a composite of graphene oxide with titanium oxide (GO + TiO2) were prepared via an alternative chemical route based on Hummer's method. The morphology and crystalline structure of the GO and GO + TiO2 thin films were investigated by X-ray power diffraction (XRD) and scanning electron microscopy (SEM). Anatase TiO2 nanostructures were incorporated into the bulk and surface of GO. Ultraviolet-visible spectroscopy (UV-vis) and Fourier transform infrared spectroscopy (FTIR) were also performed in order to corroborate with the results obtained by XRD, SEM and TEM. The electrical characterization of GO and GO + TiO2 TFs was performed using the four-probe van der Pauw method from 400 K down to 77 K. The resistivity, carrier density and carrier mobility in the TFs were determined as a function of temperature. It was found that the conductivity and mobility increased with the incorporation of TiO2 on GO; however, the concentration of free-carriers decreased. The analysis of the temperature dependence of the resistivity also showed that both films present a metal-to-insulator transition (MIT) near room temperature, at 280 K and 260 K for GO and GO + TiO2, respectively. Thus, for temperatures near 300 K the carrier density, thermally induced in GO and GO + TiO2 thin films, is so high that both the systems suffer a Mott MIT, which increases the prospects of using these films in a novel class of electronics, spintronics and photonics.

Application of the van der Pauw method for samples with holes

Abstract: Modifications of the original van der Pauw relation were suggested recently. The methods are applicable to samples with a hole, unlike the original van der Pauw relation, but it takes too much time and effort to apply the methods to samples with high randomness. So in this paper we suggest two generalizations of the van der Pauw method which are applicable for two-dimensional homogeneous systems with a finite number of holes. Both methods rely on obtaining a crucial constant of the system, ν. The first method involves setting the probes far from each other while conducting the experiment using a sample with a small hole, approximating a relation that gives ν as a linear function of the area ratio of the hole only. The second method produces an identical sample of known resistivity and thickness to obtain ν, which is believed to be dependent on the geometrical properties only. Unlike the earlier methods, which entailed complex procedures, little in the way of measurements and computation is needed for the new methods. The methods will be very useful in electric experiments or industries which need to measure the resistivity of samples.

Main scattering mechanisms in InAs/GaAs multi-quantum-well: a new approach by the global optimization method

Abstract: In this paper, we present the results of the electrical characterization of semiconductor mutli-quantum-well of InAs/GaAs. The growth by molecular beam epitaxy may induce the formation of other structures such as dislocations and point defects and also quantum dots due to the stress that arise in the lattice caused by mismatches in lattice constants. For the characterization, we used the Hall measurements as a function of temperature using the van der Pauw technique between temperatures of 10 and 310 K. For data analysis, the presence of two conduction channels was observed and it was necessary to use a simple model for splitting the experimental data into two channels. For the mobility analysis process of splitting channels, a set of scattering mechanisms models for carriers in two dimensions was used together with the global optimization method, named cross entropy. With this, the main scattering mechanisms limiting the mobility of each sample were estimated in a robust and statistical fashion. As a result of the fittings, it was possible to infer effects associated to stresses on the InAs/GaAs interfaces and its relaxation across the active layer on the fitting parameters. The fittings can also act as a source of information on impurity and dislocation densities. The cross entropy together with constitutive characteristics changes can be used to improve characteristics of quantum dot samples.

Electrical Behavior of X-Ray Detector Based on PbI 2 Crystal With Coplanar Electrode Structure

Abstract: In this work, we report on the results of the electrical behavior of coplanar electrode structure X-ray detector based on PbI2 crystal. The detector with coplanar electrode similar to the Van der Pauw structure instead of conventional planar structure can partly reduce the influence of trapping effect to charge carrier transport along the $c$ -axis. According to the configuration, four operating modes of the detector were thoroughly studied. Several novel results, such as the negative polarity dark current and photocurrent under zero bias voltage, orientation polarization, short relaxation time of steady-state surface leakage current and fast photocurrent response are obtained by this kind of coplanar structure configuration. The detector has the potential to be developed as a solid state X-ray dosimeter in the medical radiation environment, providing a high sensitivity and good adaptability under positive or negative bias voltage.