Conductive Surface Layer Research Articles

Experimental determination of the near-surface conductivity profiles of metals from electromagnetic induction (eddy current) measurements

We report experimental measurements that test an inverse method for determining the electrical properties of conducting surface layers on metals. We test the method's ability to determine the spatial variation of the near-surface electrical conductivity of flat, layered (one-dimensional) metal plates from experimental measurements of the frequency-dependent impedance of a small right-cylindrical air-core coil placed next to the metal surface and driven by an alternating current. This is an inverse problem for the diffusion equation with complex wavevector. We fit the experimental measurements to a recent closed-form analytic solution for the impedance of a conductivity profile that varies as a constant plus a hyperbolic tangent with depth into the sample. The model profile depends on three parameters, which roughly correspond to: (1) the thickness of the surface layer, (2) the change in conductivity and (3) the sharpness of the transition from surface to bulk conductivity. Data were obtained by measuring the impedance of an air-core coil for flat Cu and Ti plates that have surface layers that extend to various depths. We extended the range of the study by using simulated data, which were obtained by solving the forward problem numerically. Good estimates for the 'average' conductivity profile were obtained when the conductivity was maximum (minimum) at the surface and decreased (increased) monotonically to the bulk conductivity as a function of depth into the sample. The thickness, conductivity change and the sharpness of the profile were successfully inferred from experiment in most cases.

The validity of the Smoluchowski equation in electrophoretic studies of lipid membranes.

The Helmholtz-Smoluchowski theory, widely used for the calculation of zeta potential from the measured electrophoretic mobility (EPM), is known to be invalid in the region where the mobility is affected by the surface conductivity and polarization of the electrical double layer. In this region, the zeta potential found according to the Smoluchowski equation (zeta sm) is not identical to the true electrostatic potential at the hydrodynamic plane of shear (zeta), which is considered in the Gouy-Chapman-Stern theory of the electrical double layer. As a result, zeta sm cannot be used for the subsequent calculation of surface potential and surface charge density of a membrane studied. here we suggest a simple way, allowing one to decide between the validity and nonvalidity of the Smoluchowski equation in various sets of experimental conditions used in electrophoretic measurements on lipid membranes. We calculated the dimensionless criterion Rel, accepted in the Dukhin theory of electrophoresis as a measure of the extent of the influence of surface conductivity and double layer polarization on EPM. The Rel changes, with membrane charge density, ionic strength and vesicle radius, were found to be helpful in finding the combinations of these three parameters, corresponding to the validity of the Smoluchowski equation.

Ion Beam Synthesis by Tungsten-Implantation Into 6H-SIC

AbstractWe studied high-dose implantation of tungsten into 6H-silicon carbide in order to synthesize a highly conductive surface layer. Implantation was performed at 200 keV at room temperature. Subsequently, the samples were annealed in two steps at 500°C and 700°C or 950°C, respectively. The influence of dose and annealing temperature on the reaction of W with SiC was investigated. Rutherford Backscattering Spectrometry (RBS), X-Ray Diffraction (XRD) and Auger Electron Spectroscopy (AES) contributed to study structure and composition of the layer as well as chemical states of the elements. During implantation sputtering became significant at a dose exceeding 1.0×1017 W+cm−2. Formation of tungsten carbide and suicide was observed already in the as-implanted state. An annealing temperature of 950°C was necessary to crystallize tungsten carbide. However, tungsten suicide remained amorphous at this temperature. Therefore, a mixture of polycrystalline tungsten carbide and amorphous tungsten suicide evolved under these conditions. The resistivity of such a layer implanted with 1.0×1017 W+ cm−2 and annealed at 950°C is 565 μΩcm.

Ion beam modification of polymers

The implantation of polymers has received considerable attention in recent years, primarily to examine doping of conducting polymers and to increase the surface conductivity (by many orders of magnitude) of highly insulating polymers. The interest in these studies was partly motivated by possible applications to microelectronic device fabrication. More recently it has been observed that ion implantation can under some conditions lead to the formation of a hard (e.g. as hard as steel, ca. 3 MPa) and conducting surface layer. This paper will review the ion beam modification of polymers resulting from ion implantation with reference to fundamental ion-solid interactions. This leads us to examine whether or not implantation of polymers is a contradiction in terms.

UV laser processing of diamond films: effects of irradiation conditions on the properties of laser-treated diamond film surfaces

KrF excimer laser etching and smoothening of polycrystalline diamond films grown on Mo and W substrates by arc discharge plasma chemical vapour deposition (CVD) were investigated. The effects of angle of laser beam incidence and pulse duration on the surface roughness of the laser-treated diamond films were determined. A scanning laser beam was used to polish the diamond films over a large area. Laser-controlled diamond growth by means of pulse irradiation during CVD was performed. The electrical conductivity of the diamond films as well as the electrical resistance of the laser-produced conductive pathways were studied in the temperature range 300 ⩽ T ⩽ 800 K. On the basis of the electrical resistance measurements, the laser-induced conductive surface layer was found to be unstable under oxidation in air at 450°C and in an atomic hydrogen atmosphere in the course of the diamond CVD process.

Investigation of enhanced conductivity in dye-coated tin oxide thin films using Hall voltage measurements

The effects of dye coatings upon the transport properties of tin oxide thin films are studied. Conductivity enhancement factors ranging in magnitude from 2 to 20 were observed in fully characterized films painted with a crystal violet dye derivative. The conductivity enhancement is shown to be due to a combination of mobility and carrier density increase. Both of these effects are attributed to a reduction in surface scattering within a highly conductive surface layer rather than to more exotic models such as exciton coupling of conduction electrons. Preliminary studies using Langmuir-Blodgett films of the crystal violet dye did not, however, produce an increase in the tin oxide film conductivity.

Inversion of the COPROD2 Data by a Method of Modelling.

Separate inversions of the North American Central Plains (NACP) and Thompson Belt (TOBE) anomalies are undertaken using a method in which a 'least-blocked' model is sought. The algorithm first constructs a 3-column starting model by the juxtaposition of 1D 'least-layered' inversions of the TE data at 3 sites covering the region under investigation and then minimizes the least squares fit of the TE and TM apparent resistivity and phase responses by adjusting the positions of the horizontal and vertical boundaries and the resistivities of the resulting blocks in the composite starting model. The procedure is repeated with successively 4, 5, ... columns until the addition of further columns is no longer justified by a significant improvement in the fit. Inversion of the COPROD2 data by this method reveals a conductive surface layer of sediments slightly less than 3 km thick across the whole region, underlain by an NACP anomaly roughly 120 km wide and composed of two adjoining conductive blocks extending to the basement 60 km deep and whose top surfaces are at depths of roughly 12 km and 19 km respectively, and a TOBE anomaly consisting of a single narrow block only about 7 km wide rising from the basement to the base of the surface sedimentary layer. The TOBE anomaly is found to be much more conductive than the NACP anomaly.

Polypyrrole grown at the surface of sulfonated polyethylene films. Electrical conductivity and stability

New data on the electrical conductivity of polypyrrole (PPy) grown at the surface of sulfonated polyethylene (SPE) thin films and its stability in air are presented. The influence of the sulfonation degree of PPy-SPE samples and that of pyrrole polymerization time on conductivity levels have been investigated. It is shown that the conductivity derived using the thickness of the conductive surface layer from infrared transmission spectra of PPy-SPE films allows a convenient definition of the percolation transition observed for the electrical conductivity in these materials. The level of electrical stability of PPy involving sulfonic groups is discussed in the light of conductivity data for PPy containing other different counterions.

Smoothening of diamond films with an ArF laser

Polycrystalline diamond films of various microstructures were modified by large-area ArF laser irradiation. The modified film surfaces were characterized employing electron microscopy, mechanical profilometry and optical reflectometry. Laser irradiation was found to smooth the surface finish over a wide range of spatial frequencies covering microroughness imposed by the individual crystallites as well as waviness with periods exceeding the crystal sizes. No smoothening was achieved, however, for chemically non-uniform diamond films. The results can be rationalized in terms of laser-induced surface evaporation. Resistivity measurements revealed the simultaneous formation of a highly stable conductive surface layer upon laser irradiation.

Can magnetotelluric data resolve lower-crustal conductors?

Abstract It is commonly accepted that the lower crust is electrically conductive. The most commonly used measurement technique is magnetotellurics (MT) using natural plane waves, but also a few controlled-source experiments have been carried out to study this problem. Previous work using MT and controlled-source data has shown that in areas devoid of a sedimentary layer a hypothetical homogeneous lower crust can be resolved accurately. Adding a conductive surface layer degrades the resolving power of MT and controlled-source data. We have studied this problem quantitatively by varying the conductances (product of the conductivity and thickness) of the sedimentary and lower-crustal layers. Both MT data and the data calculated with a controlled-source (grounded dipole) technique are used for investigation. We show that for sedimentary conductances of 200–2000 S, MT data combined with controlled source data may identify a conducting lower crust provided its resistivity is less than a few hundred Ω m, but its thickness and resistivity cannot be resolved.

A study of undoped and molybdenum doped, polycrystalline, tin oxide thin films produced by a simple reactive evaporation technique

Uniformly thick, conductive, tin oxide thin films ranging in thickness from 3 to 85 nm were deposited by reactive evaporation of tin, in the presence of oxygen, onto heated glass substrates. The conductivity of the films could be increased by doping with molybdenum using a simple, novel technique which is described. Film resistivity was found to increase with the time of exposure to laboratory ambients subsequent to deposition. Mobility and carrier density data for the more conductive films are presented. An approximately constant sheet resistance was found for the undoped films from which it is inferred that conduction is probably dominated by a highly conductive surface layer. The sheet resistance of the doped films increased with decreasing thickness. The films produced consisted entirely of tin oxide phases as indicated by X-ray diffraction results and were highly transparent (T>85%). There was no significant difference between the optical absorption edge (3.6-3.9 eV) for doped and undoped films. A direct optical transition was found to be the dominant absorption process. The direct bandgap was found to lie in the region 4.1+or-0.1 eV.

Resistance and 1/f noise of Au, Al, and Ge contacts to (Hg,Cd)Te

The resistance and 1/f noise of Au and Al contacts to ion-sputtered p-type Hg0.79Cd0.21Te and of Ge contacts to Hg0.7Cd0.3Te have been measured at temperatures of 12, 40, 77, and 295 K. The Au and Al contacts were ohmic at all temperatures whereas the Ge contacts were partially rectifying. The specific contact resistance for Au and Al varied by about a factor of 10 up to 9×10−4 Ω cm2 and 3×10−3 Ω cm2, respectively, at 295 K, and there was little variation in resistance down to 12 K. The 1/f noise of the Au and Al contacts could be described as a power spectral density of resistance fluctuations SR which varied with the contact diameter d as SR∝d−m, where 5≤m≤6 for Au and 2≤m≤3 for Al. The values of m suggest that whereas the 1/f noise of the Au contacts originated at the Au/(Hg,Cd)Te interface or in the underlying (Hg,Cd)Te, the 1/f noise of the Al contacts originated in a surface conduction layer next to the contact. The magnitude of the 1/f noise was significantly larger than expected for any fundamental 1/f noise source.

Magnetic field of a direct current in a cylindrical-shaped conductor imbedded in a resistive half-space beneath a conductive surface layer

New applications of Fourier series and integral equation methods have yielded results of significance for electrical prospecting. This paper presents the first model calculation in three dimensions of the magnetic field due to a direct current flow in a conductor imbedded in a half-space beneath a conductive surface layer. The conductor is in the shape of a cylinder of semi-infinite length, and the current flow is from a point source in the surface layer. The vertical component of the magnetic field is given here by a Fourier series integral over a function which satisfies a Cauchy singular integral equation. For the two special cases, namely, an outcropping cylinder with no surface layer and a nearly invisible (in the sense of negligible conductivity contrast with its surrounding) cylinder, this integral equation can be solved exactly to yield closed-form expressions for the vertical component of the magnetic field. The exact solution in the first special case is based on a new generalization of the Hankel integral transform (published separately), and it extends the class of exactly solvable singular integral equations. This exact solution has also yielded as useful byproducts two new standard integrals involving Bessel functions. The integral equation approach used here, in contrast to the well-established surface integral equation approach, is suitable for numerical calculations, and typical magnetic field profiles are presented. The magnetic field results given here would be valuable for interpretation in electrical prospecting (based on the measurement of magnetic fields of direct currents), and the mathematical techniques used here would be useful for future calculations in electromagnetism.

Soil hydrophobic effects on infiltration and catchment runoff

AbstractAfter dry summers or drought, eucalypt forest soils at two sites in southeastern Australia developed hydrophobic or non‐wetting surface characteristics that reduced infiltration, measured using a sprinkling infiltrometer. At one site the development of hydrophobic conditions caused the rainfall to runoff conversion efficiency of a forested catchment to increase from 5 per cent to 15 per cent. Under non‐hydrophobic conditions at this site, grassland always generated more runoff than forest. However, one major rainfall‐runoff was recorded at a time of highly hydrophobic forest soil conditions and this storm generated greater runoff on the forested catchment than the grassland catchment.At the second site forest soils have naturally highly conductive surface layers because of a dense network of macropores and pathways for preferential flow. Hydrophobic conditions produced by drought caused soil water movement to be confined to only a few of the larger macropores exposed to surface ponded water. Even so, infiltration rates remained relatively high so that the impacts of hydrophobic soils were not translated into increased catchment runoff as at the first site.

The Formation of an Ionically Conducting Surface Layer on a 50V<sub>2</sub>O<sub>5</sub>-50TeO<sub>2</sub> Glass by the Electrochemical Lithiation

The Formation of an Ionically Conducting Surface Layer on a 50V<sub>2</sub>O<sub>5</sub>-50TeO<sub>2</sub> Glass by the Electrochemical Lithiation

Magnetic-field-induced localization in narrow-gap semiconductors Hg1-xCdxTe and InSb.

Magnetotransport measurements on n-type ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te (x\ensuremath{\simeq}0.2) and InSb and far-infrared magnetooptical spectroscopy in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te are reported. The transport data at magnetic fields below and near the magnetic-field-induced metal-insulator (M-I) transition indicate clear similarity of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te and InSb. At fields well above the M-I transition and at low temperatures, the magnetotransport coefficients for ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te show anomalously weak dependences on field and temperature. This is attributed to shorting of the bulk by a conducting surface layer. Below the M-I transition field, an anomalous Hall effect is observed in both ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te and InSb. We interpret this effect within a model in which the M-I transition takes place in the donor impurity band. The impurity cyclotron resonance observed in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te provides conclusive evidence for donor-bound electrons in this semiconductor and further confirms its similarity to InSb. The cyclotron-resonance data are in agreement with theoretical predictions for hydrogenic donors in a strong magnetic field. These observations provide strong evidence against the Wigner crystallization of electrons in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te.

A downhole magnetometric resistivity technique for electrical sounding beneath a conductive surface layer

The magnetometric resistivity (MMR) method has not been used systematically for vertical electrical sounding because surficial measurements of the magnetic field caused by static current flow from a point source of current at the surface are independent of the form of the variation of earth resistivity with depth. However, data obtained from an adaptation of the MMR method in which the ratio of the magnetic fields below and above a known conductive surface layer is measured as a function of the horizontal range from the current source can be inverted to obtain the unknown resistivity of deeper material. The practical limitation is that the integrated conductivity of the surface layer must not be much smaller than the integrated conductivity of the unknown zone in order to observe diagnostic ratios significantly different from unity. The expressions derived for the magnetic‐field ratio and for the sensitivity of the ratio to changes in the resistivity of a plane‐layered unknown zone are closed‐form inverse Hankel transforms. The transforms are evaluated analytically for a model of constant, uniform resistivity. One possible geotechnical application of the method is the detection of offshore, resistive permafrost beneath the shallow Beaufort Sea. A theoretical design study in which ratios of the magnetic fields at the bottom and top of the sea are synthesized and the sensitivity of those ratios to changes in sea‐floor resistivity is determined as a function of range and depth, respectively, reveals that the resistivity of a permafrost zone 50 m thick buried 20 m below the sea floor can be identified. The maximum range needed is of the order of 200 m, which appears to be smaller than the maximum spacing required for a Schlumberger sounding to yield similar information.

Dependence of the conductivity of a porous medium on electrolyte conductivity.

For an arbitrary geometry of insulating, but charged, objects immersed in an electrolyte for which diffusion currents are important, the mathematical problem of the dc electrical conductivity can be mapped onto that of an ordinary conduction problem without diffusion currents but with a conductive surface layer. As a result, using variational arguments we can prove two general theorems which hold irrespective of the geometry of the porous medium: (a) At high salinities, so that the conductivity of the pore fluid, ${\ensuremath{\sigma}}_{f}$, is large, the conductivity of the system as a whole, ${\ensuremath{\sigma}}_{\mathrm{eff}}$, is a linear function of ${\ensuremath{\sigma}}_{f}$, with a slope of 1/F and with an offset proportional to 1/\ensuremath{\Lambda}. (b) For lower values of salinity, ${\ensuremath{\sigma}}_{\mathrm{eff}}$ as a function of ${\ensuremath{\sigma}}_{f}$ is convex-up as long as the conductivity within the double-layer region is independent of the salinity of the pore fluid. The parameters F and \ensuremath{\Lambda} introduced previously [D. L. Johnson, J. Koplik, and L. M. Schwartz, Phys. Rev. Lett. 57, 2564 (1986); D. L. Johnson, J. Koplik, and R. Dashen, J. Fluid Mech. 176, 379 (1987)] are hereby shown to be relevant to the electrolyte problem. An illustration of an ordered suspension is given to show how to implement these ideas.

Ultramicrohardness measurements of coated samples

Imaging of Vickers imprints for in situ hardness test of insulating samples in a scanning electron microscope requires a conductive surface layer to avoid charging. The influence of such coatings on the measured hardness and the elimination of this effect by an extrapolation towards zero thickness of the layer are discussed theoretically. The method is checked by the example of gold-coated silicon

Surface electronic structure and chemisorption on corundum transition-metal oxides: alpha -Fe2O3.

Ultraviolet photoemission spectroscopy has been used to study the electronic structure of both nearly stoichiometric, well-ordered \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ surfaces and surfaces containing point defects. The interaction of ${\mathrm{O}}_{2}$, ${\mathrm{H}}_{2}$O, ${\mathrm{H}}_{2}$, and ${\mathrm{SO}}_{2}$ with both types of surfaces has also been investigated. The energy levels of the five 3d electrons on the ${\mathrm{Fe}}^{3+}$ ions overlap those of the filled O 2p orbitals, leading to a complex valence band about 10 eV wide. Oxygen-vacancy surface defects produced by ${\mathrm{Ar}}^{+}$-ion bombardment result in a conducting surface layer containing both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{0}$ ions. The well-ordered \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$(0001) surface is relatively inert with respect to all four molecules studied, with exposures greater than ${10}^{3}$ L (1 L\ensuremath{\equiv}${10}^{\mathrm{\ensuremath{-}}6}$ Torr sec) necessary before any chemisorption could be seen. For large exposures, the ${\mathrm{O}}_{2}$-surface interaction gives rise to a negative adsorbed species. ${\mathrm{H}}_{2}$O adsorbs dissociatively on both well-ordered and defect surfaces, resulting in adsorbed ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ ions. ${\mathrm{SO}}_{2}$ appears to bond primarily to surface oxygen ions, yielding a complex similar to ${\mathrm{SO}}_{4}$${\mathrm{}}^{2\mathrm{\ensuremath{-}}}$.