Showing papers on "Van der Pauw method published in 2003"

Charge and spin diffusion in mesoscopic metal wires and at ferromagnet/nonmagnet interfaces

Abstract: Issues related to ``charge-spin coupling'' and the diffusive transport of nonequilibrium spin polarized electrons in nonmagnetic materials and at the interface between ferromagnetic and nonmagnetic materials are discussed theoretically. Equations that govern charge and spin transport in three dimensions are derived and appropriate boundary conditions are discussed. These results are applied to a numerical calculation of spin accumulation and diffusion in a two-dimensional Van der Pauw cross. A detailed model of spin transport at ferromagnetic-nonmagnetic metal interfaces is reviewed, the limiting cases of high and low interface conductance are treated, and the relevance of ``resistance mismatch'' for a variety of experimental systems is analyzed.

DC conductivity measurements in the Van der Pauw geometry

Abstract: A new methodology for conductivity measurements, where square metallic samples are measured with the Van der Pauw technique, has been successfully implemented. The uncertainty obtained is 0.04% and a comparison between national metrology institutes gives an agreement of the measurement values within 0.035%. Major advantages of the new method are that smaller reference samples are required and only a single dimensional measurement is needed.

Improved electrical characterization of Al–Ti ohmic contacts on p-type ion implanted 6H-SiC

Abstract: This paper deals with the electrical characterization of low resistance Al–Ti 72/28 wt% ohmic contacts to a p-type ion implanted 6H-SiC layer. Transmission line model (TLM) structures were realized on the top of MESA islands defined in this ion implanted layer. A metal scheme composed of Al-1%Si(350 nm)/Ti(80 nm) was deposited by sputtering, photolithography defined and annealed at 1000 °C in Ar for 2 min. TLM structures were measured as a function of the temperature in the range 25–290 °C. The TLM data were mainly analysed by a two-dimensional finite difference simulation tool that takes into account the current crowding effect at the contact periphery. Extracted contact resistivity values fall in the low range of data from the literature. The sheet resistance values computed from the TLM data agreed with those measured using Van der Pauw devices realized next to the TLM structures.

High Resistivity Measurement of SiC Wafers Using Different Techniques

Abstract: To establish fast, nondestructive, and inexpensive methods for resistivity measurements of SiC wafers, different resistivity-measurement techniques were tested for characterization of semi-insulating SiC wafers, namely, the four-point probe method with removable graphite contacts, the van der Pauw method with annealed metal and diffused contacts, the current-voltage (I-V) technique, and the contactless resistivity-measurement method. Comparison of different techniques is presented. The resistivity values of the semi-insulating SiC wafer measured using different techniques agree fairly well. As a result, application of removable graphite contacts is proposed for fast and nondestructive resistivity measurement of SiC wafers using the four-point probe method. High-temperature van der Pauw and room-temperature Hall characterization for the tested semi-insulating SiC wafer was also obtained and reported in this work.

Epitaxial (La,Sr)TiO3 as a conductive buffer for high temperature superconducting coated conductors

Abstract: The transport and structural properties of (La,Sr)TiO3 epitaxial thin films grown by pulsed-laser deposition is presented. In particular, the potential use of (La,Sr)TiO3 as a conductive buffer layer for subsequent growth of high temperature superconducting films for coated conductors is discussed. Van der Pauw measurements of film resistivity as a function oxidation conditions show that, for undoped LaTiO3 films, the resistivity increases rapidly as background oxygen pressure is increased, which is consistent with the formation of the LaTiO3þx phase. Sr doping of LaTiO3 significantly enhances the conductivity of thin film materials when synthesized under oxidizing conditions. The transport behavior for Sr-doped LaTiO3 films correlates with structural data showing no significant shift in lattice spacing as oxygen partial pressure is increased during film growth. In addition, the epitaxial growth of (La,Sr)TiO3 on biaxially textured Ni alloy tapes is demonstrated. These results suggest that (La,Sr)TiO3 is a viable candidate as a conducting buffer for superconducting film growth on biaxially textured metal tapes. � 2003 Elsevier Ltd. All rights reserved.

Epitaxial structure and transport in LaTiO3+x films on (001) SrTiO3

Abstract: The structure and transport properties of LaTiO3+x epitaxial thin films grown on (001) SrTiO3 by pulsedlaser deposition is examined. Four-circle X-ray diffraction indicates that the films possess the defect perovskite LaTiO3 structure when deposited in vacuum, with the higher X compounds forming at moderate oxygen pressures. The crystal structure of the LaTiO3 films is tetragonal in the epitaxial films, in contrast to the orthorhombic structure observed in bulk materials. A domain structure is observed in the films, consisting of LaTiO3 oriented either with the [110] or [001] directions perpendicular to the substrate surface. Z-contrast scanning transmission electron microscopy reveals that this domain structure is not present in the first few unit cells of the film, but emerges approximately 2 – 3 nm from the SrTiO3/LaTiO3 interface. Upon increasing the oxygen pressure during growth, a shift in the lattice d-spacing parallel to the substrate surface is observed, and is consistent with the growth of the La2Ti2O7 phase. However, van der Pauw measurements show that the films with the larger d-spacing remain conductive, albeit with a resistivity that is significantly higher than that for the perovskite LaTiO3 films. The transport behavior suggests that the films grown at higher oxygen pressures are LaTiO3+x with 0.4 < x < 0.5.

Formation of highly conductive polycrystalline GaAs from annealed amorphous (Ga,As)

Abstract: Polycrystalline (Ga,As) grown by molecular-beam epitaxy (MBE) is compared to polycrystalline (Ga,As) formed by annealing amorphous (Ga,As) also grown by MBE. Both amorphous and polycrystalline materials were grown at 100 °C and crystallinity is controlled by the As overpressure. As the amorphous material was annealed at varying temperatures from 300 to 500 °C several properties of the material changed such as levels of excess As in the material, grain structure of annealed material, strength and adhesion of crystallized layer, and conductivity of the material. After annealing at temperatures around 400 °C, the material is specular, polycrystalline, has good adhesion, and is very conductive. Resistivity values of less than 2 mΩ cm and acceptor concentrations near 5×1020 cm−3 were detected according to Hall/van der Pauw measurements. Conduction is believed to be due to the large amount of excess As in the material forming an As conduction path when the material is annealed. Material grown in a polycrystalline form by MBE differs in both grain structure and conductivity from material crystallized from an amorphous form.

InNAsSb Single Crystals with Cutoff Wavelength of 11–13.5 μm Grown by Melt Epitaxy

Abstract: We grew, for the first time, InNAsSb single crystals with a room temperature cutoff wavelength of 11–13.5 µm on (100) InAs substrates by melt epitaxy (ME). The crystals were characterized using X-ray diffraction, electron-probe microanalysis (EPMA), temperature-dependent transmittance and Van der Pauw measurements. An obvious redshift of cutoff wavelength was observed with increasing N concentration from 13 to 19% in the epilayers. The cutoff wavelength varies from 12.5 to 8.9 µm in the temperature range from 300 to 77 K. The thick epilayers show a good macroscopic homogeneity of the composition. An electron mobility of 55,800 cm2/Vs with a carrier density of 1.08×1016 cm-3 was obtained at 77 K, indicating possible applications for infrared photodevices.

Finite-element analysis of the electric field distribution in conductance cell

Abstract: The finite-element method (FEM) for the modeling of the electric field distribution in impedance cells designed according to the assumptions of the van der Pauw method of electrical conductivity (specific conductance) measurement has been used. Particularly, the effect of electrode-electrolyte solution interface (EESI) on the electric field distribution has been studied. It has been found that the EESI of current (driving) electrodes influences the cell electric field distribution. It has been concluded, as well, that the potential difference of the potential electrodes-being a measure of the conductivity of the electrolyte solution filling the cell-does not depend on whether EESI exists, provided the current electrodes are driven from a current source. The results of measurement experiments confirming the FEM studies are presented in the following paper as well.

FABRICATION AND CHARACTERIZATION OF THIN FILM RESISTORS FOR GaAs-BASED POWER AMPLIFIERS

Abstract: This paper presents a comprehensive study of sputtered TaN thin film resistor with a low resistivity of only 150 µΩ-cm, its comparison with thin film resistors fabricated by evaporated NiCr and sputtered NiV. Sheet resistance (Rs), temperature coefficient of resistance (TCR), and voltage coefficient of resistance (VCR) were measured using a standard Van Der Pauw (VDP) structure. Thickness was measured by a profilometer as well as cross-section SEM. Biased-drift tests and thermal tests were performed to check the reliability of the thin film resistors.

A resistivity peak close to Tc in Nd2−xCexCuO4−y single crystals

Abstract: We present results of the electrical resistivity of a Nd1.84Ce0.16CuO4−y superconducting (Tc=22.4 K) single crystal in magnetic fields up to 13 T. The measurements were performed by the van der Pauw method in both possible configurations of the electrodes. In one of the configurations a resistivity peak has been observed close to the critical temperature, whereas the peak was not visible for the other configuration. We propose a simple model assuming an inhomogeneous distribution of cerium, which may explain this effect. Results of AC-susceptibility measurements on the crystal and its fragments support this model.

Annealing Behavior of Donorlike Defects Induced by High-Fluence Irradiation of High-Energy Particles in p-Type Silicon

Abstract: decreases with fluence. 5) These reports suggest that hole concentration should decrease with an increase in fluence. However, these causes cannot lead to the conversion of the p layer to the n layer at all. In order to convert the p layer to the n layer, the high-fluence irradiation must create donorlike defects, which supply electrons to the conduction band. In this article, from the annealing behavior of the majority-carrier concentration, the possible origin of the donorlike defects is discussed. The B-doped single-crystalline Si wafers used here were fabricated by the Czochralski method. The resistivity of the wafers was � 10 � cm, that is, the B acceptor density (NA) was � 2 � 10 15 cm � 3 . From radioactivation analyses, the concentrations of O and C in the wafers were determined to be 8 � 10 17 cm � 3 and 1 � 10 15 cm � 3 , respectively. The thickness and area of the samples for Hall effect measurements were 220 mm and 5 � 5 mm 2 , respectively. After the formation of good ohmic contacts at four corners of the sample, the samples were irradiated by 1 MeV electrons with 1 � 10 17 cm � 2 fluence or by 10 MeV protons with 3 � 10 12 {5 � 10 14 cm � 2 fluence. Hall effect measurements were conducted by the van der Pauw method in a magnetic field of 1.4 T from low to high temperature (T), every 5 K, and it took about 15 min to obtain the data at each temperature.

Effect of substrate bias voltage on the texture and microstructure of Cu thin films deposited by ion beam deposition

Abstract: Cu thin films deposited by non-mass separated ion beam deposition under various substrate bias voltages were investigated. The film textures and microstructure were analyzed by X-ray diffraction and field emission scanning electron microscopy, and the resistivity of the film was measured with the Van der Pauw method. It was found that the optimum negative substrate bias voltage for Cu films was −50 V. The Cu films deposited without substrate bias voltage showed a columnar grain structure with small grains and random orientation. However, when a substrate bias voltage of −50 V was applied, the Cu films had a non-columnar structure with a strong (111) texture and large grains. The electrical resistivity of the Cu films decreased remarkably with increasing negative substrate bias voltage, and reaching a minimum value of 1.8±0.13 μΩ cm at the substrate bias voltage of −50V.

Charge and Spin Diffusion in Mesoscopic Metal Wires and at Ferromagnet/Nonmagnet Interfaces

Abstract: Issues related to ``charge-spin coupling'' and the diffusive transport of nonequilibrium spin polarized electrons in nonmagnetic materials and at the interface between ferromagnetic and nonmagnetic materials are discussed theoretically. Equations that govern charge and spin transport in three dimensions are derived and appropriate boundary conditions are discussed. These results are applied to a numerical calculation of spin accumulation and diffusion in a two-dimensional Van der Pauw cross. A detailed model of spin transport at ferromagnetic-nonmagnetic metal interfaces is reviewed, the limiting cases of high and low interface conductance are treated, and the relevance of ``resistance mismatch'' for a variety of experimental systems is analyzed.

Intra-die temperature non uniformity related to front side emissivity dependence during rapid thermal annealing

Abstract: Emissivity variations on the front side of a wafer are due to the different materials present on patterned wafers. When using high ramp rates rapid thermal processing dopant activation at high temperature can cause temperature non-uniformity within a die. We show this effect with patterned test wafers annealed in lamp-based systems and in a conductive heating RTP system (ASM Levitor) [1]. We confirm our previous work obtained with sheet resistance measurements. SIMS profiles measured at different locations of a die show slight variation for a deep 5 keV boron implantation. Oxidation growth on the lamp based system results in a 10 % thickness variation between the centre and the edge of a die. Van der Pauw structures implanted either with n-type or p-type dopant shows fluctuations in the sheet resistance along a die in the lamp-based system. However, this effect is negligible on the tool using conductive heating.

Elimination of burn-in effect in carbon-doped InGaP/GaAs HBTs by hydrogen lateral diffusion

Abstract: Hydrogen lateral diffusion by annealing at low temperature was proposed to eliminate the burn-in effect in carbon-doped InGaP/GaAs heterojunction bipolar transistors (HBTs). After a thermal annealing at 480 °C for 30 min, the current gain variations caused by the electrical stress decreased from 42.7% to 2.6% as the emitter width was reduced from 100 to 5 μm. After the annealing process, the sheet resistance was decreased from 194.4 to 162.7 ohm/sq. as the van der Pauw line widths was reduced from 65 to 5 μm. Effective doping concentration in base layer was increased by removal of incorporated hydrogen atoms. Degradation of device characteristics was not obvious after annealing by comparing the ratio difference of current gain to base sheet resistance.

Heteroepitaxial Growth and Heterojunction Characteristics of Voids-Free n-3C-SiC on p-Si(100)

Abstract: Highly oriented voids-free 3C-SiC heteroepitaxial layers are grown on φ50mm Si (100) substrates by low pressure chemical vapor deposition (LPCVD). The initial stage of carbonization and the surface morphology of carbonization layers of Si(100) are studied using reflection high energy electron diffraction (RHEED) and scanning electron microscopy (SEM). It is shown that the optimized carbonization temperature for the growth of voids-free 3S-SiC on Si (100) substrates is 1100 ℃. The electrical properties of SiC layers are characterized using Van der Pauw method. The I-V, C-V, and the temperature dependence of I-V characteristics in n-3C-SiC-p-Si heterojunctions with AuGeNi and Al electrical pads are investigated. It is shown that the maximum reverse breakdown voltage of the n-3C-SiC-p-Si heterojunction diodes reaches to 220V at room temperature. These results indicate that the SiC/Si heterojunction diode can be used to fabricate the wide bandgap emitter SiC/Si heterojunction bipolar transistors (HBT's).

The Study on Means of Measuring Sheet Resistance of Microareas

Abstract: The means of measuring sheet resistance of microareas are summarized for both our country and aboard.The measuring theory and measuring progress and measuring results of the modified Van der Pauw are analyzed.A new operable method of measuring theory by image manipulation is put forward at last.A new type of testing equipment of four-point-method was made.

Electron Transport Mechanism in GaN/AlGaN HEMT Structures

Abstract: The electron transport mechanism in GaN/AlGaN HEMT (High Electron Mobility Transistors) structures grown with MBE on sapphire substrate was investigated by using the temperature dependence of the Hall coefficient, resistivity, carrier density and Hall mobility. Hall measurements were carried out using Van der Pauw geometry. From the LO-phonon-scattering-limited component of the mobility, we obtain LO phonon energy \hbar w \approx 90 meV and the momentum relaxation time of tm \approx 4 fs. Also, from the temperature dependence of the 2D carrier density, we obtain the donor activation energy Ea \approx 29 meV.

Effect of Si Layer in The ZnO Thin Films by Pulsed Laser Deposition

Abstract: ZnO thin films and ZnO-Si-ZnO multi-layer thin films have been deposited by pulsed laser deposition (PLD). And then, the films have been annealed at 300°C in oxygen ambient pressure. The optical and structural properties changed by Si layer in ZnO thin film. UV and visible peak position was shifted by Si layer. Electrical properties of the films were improved slightly than ZnO thin film without Si layer. The optical and structural properties of ZnO thin films and ZnOSi-ZnO multi-layer thin films were characterized by PL (Photoluminescence) and XRD(X-ray diffraction method), respectively. Electrical properties were measured by van der Pauw Hall measurements.

Influence of multiple interfaces on oxygen ionic conductivity in gadolinia-doped single crystal oxide electrolyte multi-layer nano films

Abstract: Single-crystal multi-layered thin films of pure and gadolinia-doped ceria and zirconia were heteroepitaxially grown on sapphire (0001) substrates using molecular beam epitaxy. The growth of these films was monitored using in-situ reflection high-energy electron diffraction (RHEED). In addition, the ex-situ characterization techniques including X-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) along with ion channeling were used to further characterize these films. The electrical conductivity in these films was measured using a four-probe van der Pauw technique. Oxygen ionic conductivity at low temperatures appeared to be higher in the four-layer film compared to two-layer film.

Seebeck coefficient and resistivity measurement of polycrystalline Bi in a magnetic field

Abstract: Seebeck coefficient and resistivity of a polycrystalline Bi sample were measured from 15 to 300 K in a magnetic field. The measured Seebeck coefficient at 15 K was -10 /spl mu/V/K at 0 Tesla; however, the Seebeck coefficient enormously varied as a function of magnetic field and was over -1 mV/K at 1.5 Tesla. The Hall coefficient was measured by Van der Pauw method in order to evaluate the carrier density for estimating the mechanism of varying Seebeck coefficient at 15 K, and the carrier density decreased by a factor of 1/30 at 1.5 Tesla. It was shown that one reason for the variation of the Seebeck coefficient was the influence of phonon drag effect, even for polycrystalline Bi. A large Seebeck coefficient was observed below the Debye temperature of Bi in the magnetic field, The resistivity at 15 K also enormously increased in the transverse magnetic field due to the decreased carrier density, and the ratio of the increase of the resistivity compared to the case for no magnetic field was approximately 1000 times at 1.5 Tesla and decreased with increasing temperature. The increase in the resistivity influenced the phonon drag effect and increased the Seebeck coefficient.

Properties of Photoluminescence and Growth of CdIn 2 Te 4 Single Crystal by Bridgeman method

Abstract: A stoichiometric mixture for single crystal was prepared from horizontal electric furnace. The single crystal was grown in the three-stage vertical electric furnace by using Bridgeman method. The quality of the grown crystal has been investigated by the x-ray diffraction and the photoluminescence measurements. The (001) growth plane of oriented single crystal was confirmed from back-reflection Laue patterns. The carrier density and mobility of single crystal measured with Hall effect by van der Pauw method are and at 293 K, respectively. The temperature dependence of the energy band gap of the single crystal obtained from the absorption spectra was well described by the Varshni`s relation, $E_g(T)

Current distribution in spinvalve multilayer films determined from in-situ conductance measurements

Abstract: In this paper, the conductance of top pinned spinvalves was measured during deposition using a four point Van Der Pauw geometry. This measurement has submonolayer resolution. The layers in the spinvalve were Ta (5 nm)/NiFe(5 nm)/CoFe(1 nm)/Ru(0.6 nm)/CoFe(1.5 nm)/IrMn(8 nm)/Ta(5 nm). The thickness of the Cu layers were 2, 3, 4, 5, and 6 nm thick. The in-situ conductance data is given. The bulk conductivity of each layer in the spin valve was measured by growing a film where the layer was 30 nm thick and measuring the asymptotic conductivity.