Van Der Pauw Technique Research Articles

Layered GaTe Crystals for Radiation Detectors

In this work we investigated a new method of growing detector grade large GaTe layered chalcogenide single crystals. GaTe ingots (2" diameter) were grown by a novel method using graphite crucible by slow crystallization from a melt of high purity (7 N) Ga and zone refined (ZR) Te precursors in an argon atmosphere. GaTe samples from the monocrystalline area of the ingot have been cleaved mechanically and characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis by x-rays (EDX), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), transfer length method (TLM), resistivity measurements using van der Pauw technique, Hall effect, and capacitance-voltage (C-V) measurements. Our investigations reveal a novel method of growing superior quality GaTe crystals for large volume inexpensive nuclear radiation detectors.

Finite element analysis of the effect of electrodes placement on accurate resistivity measurement in a diamond anvil cell with van der Pauw technique

The van der Pauw technique is widely used to determine resistivity of materials. In diamond anvil cell the compressed sample will make the contact placement change under high pressure. Using finite element analysis, we study the effect of contact placement error induced by pressure on the resistivity measurement accuracy of van der Pauw method. The results show the contact placement has a significant effect on determination accuracy. This method can provide accurate determination of sample resistivity when the spacing b between the contact center and sample periphery is less than D/9 (sample diameter). And the effect of contact placement error on accuracy rapidly increases as the contact location is closing to the sample center. For the same contact placement, the contact size error has a more obvious effect on the semiconductor sample.

Enhanced conductivity in iodine doped polyaniline thin film formed by thermal evaporation

Thermal evaporation technique was employed to deposit pristine and iodine doped polyaniline (PANI) thin films on glass substrates. PANI was synthesized by the chemical oxidation method. Iodine doping was carried out by evaporation. The polymer synthesized was characterized by Thermo Gravimetric Analysis (TGA), Fourier Transform Infra Red (FTIR) and Ultraviolet–Visible (UV–VIS) spectroscopy. The evaporation temperature was optimized from TGA measurements. The thin film was deposited in vacuum at 1.33 × 10 − 4 Pa by thermal evaporation of PANI. The polymer film was characterized by FTIR and UV–VIS spectroscopy. The surface morphology of the films was studied by field emission scanning electron microscopy. The resistivity was measured by van der Pauw technique. The conductivity of the doped films was seen to increase with the iodine concentration and many fold increase in conductivity was observed in comparison to the pristine films. The increase in conductivity is due to the generation of polaron band in the band gap upon iodine doping.

Charge carrier removal rates in n-type silicon and silicon carbide subjected to electron and proton irradiation

Abstract In contrast to irradiated Si, the information on removal rates of charge carriers in irradiated SiC is meager and often contradictory. This is due to serious difficulties in crystal growth which, in turn, may have a profound effect on properties of initial materials. In the present work we compare the influence of electron (≈1 MeV) and proton (8, 15 MeV) irradiation on float-zone Si (FZ-Si) and 4H-SiC (CVD). Electrical measurements were carried out with the help of the Van der Pauw technique and capacitance−voltage technique at 1 kHz for FZ-Si and SiC, correspondingly.

Van der Pauw method for measuring resistivity of a plane sample with distant boundaries

In the paper, we derive an algorithm which follows from the original van der Pauw's technique for measuring resistivity with the added advantage of allowing contacts to be positioned a distance away from the boundary. For a large sample area, we show that the resistivity calculated by our algorithm is equivalent to the resistivity calculated by the original van der Pauw's method. In practice, this configuration is easier to achieve and can eliminate errors associated with contacts that are not placed exactly at the edge.

Electrical behavior of free-standing porous silicon layers

Abstract The electrical behavior of porous silicon (PS) layers has been investigated on one side of p-type silicon with various anodization currents and electrolytes. The two contact I–V characteristic is assigned by the metal/porous silicon rectifying interface, whereas, by using the van der Pauw technique, a nonlinear dependence of the current vs voltage was found. By using Dimethylformamide (DMF) in electrolyte, regular structures and columns were formed and porosity increased. Our results showed that by using DMF, surface resistivity of PS samples increased and became double for free-standing porous silicon (FPS). The reason could be due to increasing surface area and adsorbing some more gas molecules. Activation energy of PS samples was also increased from 0.31 to 0.34 eV and became 0.35 eV for FPS. The changes induced by storage are attributed to the oxidation process of the internal surface of free-standing porous silicon layers.

Final report on EUROMET Project 427, EUROMET.EM-S7: Comparison of AC and DC conductivity standards

The electrical conductivity of non-ferrous metals with conductivities in the range 2.2 MS/m to 35.8 MS/m, provided by NPL in the form of bars (600 mm by 80 mm by 10 mm) and square blocks (80 mm side by 10 mm), was measured using an AC method at NPL and a DC method at PTB and VSL. The purpose of the comparison was to establish if there is a difference in the electrical conductivity measured for AC and DC conditions. Conductivity standards are used by industries involved with non-destructive testing (NDT) and evaluation (NDE) to calibrate the instruments used. These instruments operate at frequencies of at least 60 kHz and it was therefore necessary to establish agreement between AC and DC methods.For all cases the uncertainty for AC conductivity measurements is considerably larger than that for DC measurements due to the increased complexity of the unique NPL bridge method. For DC measurements on blocks the PTB and VSL values agreed within the measurement uncertainties provided. A small difference between the values for a bar made from Nordic gold was removed when the CMC uncertainty for PTB was used. A better agreement between the DC measurement for blocks is believed to be due to a reduced sensitivity to material homogeneity as well as fewer dimensional measurements for the Van der Pauw technique used. A significant difference between the measured AC and DC conductivities for the titanium bar was found. This is believed to be due to a variation in the properties of the material through the thickness. The AC method is surface sensitive and so such a change in properties will influence the value compared to DC methods that are a volume average.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by EUROMET, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Theory of Breakdown Position Determination by Voltage- and Current-Ratio Methods

A theory of the current-ratio (CR) technique in uniform semiconductors, which is widely used to locate gate oxide breakdown (BD) spots in one dimension (i.e., distance from source or drain), is proposed and verified. The theory shows that the CR method is a special case of generalized van der Pauw technique and, as such, can easily be generalized to locate oxide BD spots in two dimensions. We develop the theoretical framework of this new class of BD-spot characterization techniques and then validate the theory by experiments. We conclude by discussing the implications of locating BD spots in two dimensions for reliability projections of ultrathin gate oxides.

Defects and transport in Gd-doped BaPrO3

The electrical conductivity of BaPr1−x GdxO3−δ has been characterized by means of the four-point van der Pauw technique at 200–1100 °C as a function of pO2 and pH2O. The contributions from ionic charge carriers were investigated by the EMF of concentration cells and the H+/D+ isotope effect on the total conductivity. BaPr1−x Gd x O3−δ is predominately a p-type electronic conductor under oxidizing conditions, while ionic conduction is barely measurable. Gd(III) substituted for Pr(IV) is charge compensated mainly by electron holes, with protons and oxygen vacancies contributing significantly but as minority defects only at low temperatures (wet conditions) and at high temperatures, respectively. The conductivity behaviour has been modelled under these assumptions to extract thermodynamic parameters for the defect reactions at play. The practical use of this material is limited by its poor chemical stability.

Offset reduction in Hall effect measurements using a nonswitching van der Pauw technique

A nonswitching van der Pauw technique, which uses two electrically isolated alternating current sources operating at two different frequencies and two lock-in amplifiers, is suggested for Hall effect measurements. Parasitic offset voltage, typical for this type of measurements, is reduced by averaging two sets of data accumulated simultaneously. Application of the technique is particularly useful when the offset changes on a time scale comparable to the measurement cycle.

Synthesis of Nano-Dimensional ZnO and Ga Doped ZnO Thin Films by Vapor Phase Transport and Study as Transparent Conducting Oxide

We report synthesis of polycrystalline ZnO and Ga doped ZnO (ZnO:Ga) thin films (approximately 80 nm) on Si and quartz substrates in a non-vacuum muffle furnace, a simple and cost-effective route, without any catalyst/reactive carrier gases, at relatively low processing temperature of 550 degrees C. The crystalline phases of the films are identified by grazing angle X-ray diffraction (GAXRD). The growth of ZnO films is examined with scanning electron microscope (SEM) as a function of deposition time. An optical transmission of approximately 90% is observed for pure ZnO film having a resistivity of approximately 2.1 Omega-cm as measured by van der Pauw technique. Doping with Ga results in single phase ZnO:Ga films, retaining an optical transmission of about 80% and three orders of magnitude decrease in resistivity as compared to pure ZnO film.

Hall Effect measurements on p-n-p InP structures

The electrical properties of p-type layers of indium phosphide (InP), formed by the diffusion of zinc into n-type material, are studied by Hall Effect measurements. A wide range of diffusion conditions are used and both homogeneously doped specimens and those containing a zinc atom concentration gradient are produced. A non-correspondence of atom and carrier concentrations is indicated, confirming previous four point resistivity studies. Carrier profiles are achieved by both serial sectioning and multiple specimen techniques. Contacting procedures are developed from which plots of carrier mobility versus carrier concentration, in the range 5 x 10(17) - 5 x 10(19) cm- 3, are produced for p-n-p InP. In the main, present results showed good reproducibility and conformed to the "rules" of the Van der Pauw technique.

Study of ZnO and Ni-doped ZnO synthesized by atom beam sputtering technique

Zinc oxide (ZnO) and Ni-doped zinc oxide (ZnO:Ni) films are prepared by atom beam sputtering with an intent of growing transparent conducting oxide (TCO) material and understanding its physical properties. The crystalline phases of the films are identified by the grazing angle X-ray diffraction (GAXRD) technique. Thicknesses of the films are measured by ellipsometry. Chemical states of the elements present in the films are investigated by X-ray photoelectron spectroscopy (XPS), which indicates the presence of Ni in the ZnO environment in a divalent state. Average transmission across the ZnO:Ni film was determined to be ∼83% in the visible region, which is less than that (∼90%) of undoped ZnO films. The resistivity measured by van der Pauw technique of the ZnO:Ni film (∼9×10-3 Ω cm) is two orders of magnitude smaller as compared to its undoped counterpart (1 Ω cm). For ZnO:Ni film an average carrier concentration of ∼1.4×1019 cm-3 was observed by Hall measurements. Two important mechanisms reported in the literature viz. influence of d–d transition bands and electron scattering from crystallites/grains are discussed as the possible causes for the increase in conductivity on Ni doping in ZnO.

Thermal sensors based on Sb2Te3 and (Sb2Te3)70(Bi2Te3)30 thin films

Thin films of Sb2Te3 and (Sb2Te3)70(Bi2Te3)30 alloy and have been deposited on precleaned glass substrate by thermal evaporation technique in a vacuum of 2 × 10−6 Torr. The structural study was carried out by X-ray diffractometer, which shows that the films are polycrystalline in nature. The grain size, microstrain and dislocation density were determined. The Seebeck coefficient was determined as the ratio of the potential difference across the films to the temperature difference. The power factor for the (Sb2Te3)70 (Bi2Te3)30 and (Sb2Te3) is found to be 19.602 and 1.066 of the film of thickness 1,500 A, respectively. The Van der-Pauw technique was used to measure the Hall coefficient at room temperature. The carrier concentration was calculated and the results were discussed.

Thickness dependence of the conductivity of thin films (La,Sr)FeO 3 deposited on MgO single crystal

Thin films of La 0.6Sr 0.4FeO 3− δ of different thicknesses have been deposited on single crystal MgO substrate by pulsed laser deposition (PLD). The deposited films are characterized by XRD before and after annealing, by scanning electron microscopy (SEM) for morphological characterization and by the Van der Pauw (VDP) technique for determination of the conductivity. The temperature dependence of the conductivity in air for samples of different thickness has been investigated. The electrical conductivity of the films increases with increasing film thickness but the conductivity of all films is less than the value of the bulk material. The apparent conductivity versus temperature shows a maximum at a certain temperature ( T max). This characteristic temperature ( T max) decreases as the film thickness increases and reaches the value for bulk for thicker films. All of the samples show the same activation energy of the conductivity in the low temperature limit.

Properties of highly oriented spray-deposited fluorine-doped tin oxide thin films on glass substrates of different thickness

Transparent conducting thin films of fluorine-doped tin oxide (FTO) have been deposited onto the preheated glass substrates of different thickness by spray pyrolysis process using SnCl 4·5H 2O and NH 4F precursors. Substrate thickness is varied from 1 to 6 mm. The films are grown using mixed solvent with propane-2-ol as organic solvent and distilled water at optimized substrate temperature of 475 °C. Films of thickness up to 1525 nm are grown by a fine spray of the source solution using compressed air as a carrier gas. The films have been characterized by the techniques such as X-ray diffraction, optical absorption, van der Pauw technique, and Hall effect. The as-deposited films are preferentially oriented along the (2 0 0) plane and are of polycrystalline SnO 2 with a tetragonal crystal structure having the texture coefficient of 6.19 for the films deposited on 4 mm thick substrate. The lattice parameter values remain unchanged with the substrate thickness. The grain size varies between 38 and 48 nm. The films exhibit moderate optical transmission up to 70% at 550 nm. The figure of merit ( φ) varies from 1.36×10 −4 to 1.93×10 −3 Ω −1. The films are heavily doped, therefore degenerate and exhibit n-type electrical conductivity. The lowest sheet resistance ( R s) of 7.5 Ω is obtained for a typical sample deposited on 4 mm thick substrate. The resistivity ( ρ) and carrier concentration ( n D) vary over 8.38×10 −4 to 2.95×10 −3 Ω cm and 4.03×10 20 to 2.69×10 21 cm −3, respectively.

Understanding Nickel Oxidation and Reduction Processes in SOFC Systems

In Solid Oxide Fuel Cells, Ni-YSZ cermets have many advantages, but degradation can be severe if the anode is exposed to an oxidizing environment. It has been found, in this study and others, that the primary cause of degradation is due to a >60% volume change from Ni to Ni oxide. It is shown here that the Ni is reduced and oxidized in a shorter time period for each redox cycle after the first full cycle. For reduction, this is likely due to the development of a larger surface area, as the cellular NiO surface structure is observed. Also, controlled oxidation of the anode leads to reduced series and reaction resistance values and the Van der Pauw technique indicates that the conductivity of the Ni-YSZ electrode improves after the first redox cycle. Thus, as long as there is enough space for the Ni to expand as it is oxidized, there are actually benefits to cell performance from redox cycling.

Effect of concentration of SnCl 4 on sprayed fluorine doped tin oxide thin films

Transparent conducting thin films of F:SnO 2 have been deposited onto preheated glass substrates by a spray pyrolysis technique by varying the concentration of SnCl 4. A fine spray of the source solution using air as a carrier gas has grown films of thickness up to 1108 nm. Optical absorption, X-ray diffraction, Van der Pauw technique for measurement of a sheet resistance and Hall effect measurements at room temperature for determination of carrier density and conductivity has been used. The as deposited films are of polycrystalline SnO 2 with a tetragonal crystal structure having preferentially orientation along the (2 0 0) direction with texture coefficient as high as 6.92 with average grain size of 48 nm. The maximum value of figure of merit (61.8 × 10 −3 Ω −1) with 87% transmittance at 550 nm has been obtained. The surface morphology of the films has been studied with SEM and AFM techniques. SEM shows continuous periodic arrangement of the smaller grains in between two bigger grains, with pinhole free surface. The films are heavily doped, degenerate and exhibit n-type electrical conductivity. For the optimized sample the values of lowest sheet resistance ( R s) resistivity ( ρ) and mobility ( μ) are 3.71 Ω, 3.7 × 10 −4 Ω cm and 5.81 cm 2 V −1 s −1, respectively.

Effect of solvent ratio on the properties of highly oriented sprayed fluorine-doped tin oxide thin films

Transparent conducting thin films of F:SnO 2 have been deposited onto preheated glass substrates by a spray pyrolysis technique using pentahydrate stannic chloride (SnCl 4·5H 2O) and ammonium fluoride (NH 4F) as precursors and mixture of water and propane-2-ol as solvent. The concentration of SnCl 4·5H 2O and NH 4F is kept fixed and the ratio of water and propane-2-ol solvent in the spraying solution is varied. A fine spray of the source solution using air as a carrier gas has grown films of thickness up to 995 nm. Optical absorption, X-ray diffraction, Van der Pauw technique for measurement of a sheet resistance and Hall effect measurements at room temperature for determination of carrier density and conductivity have been used. The as-deposited films are of polycrystalline SnO 2 with a tetragonal crystal structure and are preferentially having orientation along the (200) direction with texture coefficient as high as 6.16. The average grain size for the as-deposited sample is found to be of the order of 44 nm. The films have moderate optical transmission (up to 70–85% at 550 nm). The figure of merit ( ϕ) values vary from 1.95 · 10 − 3 to 35.68 · 10 − 3 Ω − 1 . The films are heavily doped, degenerate and exhibit n-type electrical conductivity. The lowest sheet resistance ( R s) for the optimized sample is 5.1 Ω. The films have a resistivity of 5.43 · 10 − 4 Ω cm and mobility around 7.38 cm 2 V − 1 s − 1 .

Room-temperature B off-lattice displacement and electrical deactivation induced by H and He implantation

Substitutional boron atoms in silicon experience an off-lattice displacement during ion-irradiation with energetic light ions at room temperature. The off-lattice displacement rate has been measured in a B-doped Si by channelling analyses using nuclear reaction (650keV proton beam, 11B(p,α)8Be). The normalized channeling yield χ of B increases with the ion fluence until it saturates at a value smaller than 1. This indicates that B is not totally displaced in a random site. The carrier concentration in layer, measured by Van Der Pauw and Hall effect techniques, decreases during irradiation until complete de-activation of B occurs. The comparison of electrical and structural analyses is consistent with the formation of small, not electrically active B complexes stable at room temperature in presence of an excess of point defects.