Changes In Surface Resistance Research Articles

Nondestructive characterization of surface resistance for copper(II) phthalocyanine thin films using a near-field scanning microwave microscope

We observed the surface resistance of copper(II) phthalocyanine (Cu-Pc) thin films dependence on substrate heat-treatment conditions by a nondestructive characterization method using a near-field scanning microwave microscope (NSMM) at an operating frequency f=5.2–5.3 GHz. The changes in surface resistance were investigated by NSMM by measuring the reflection coefficient S 11. The surface resistance of Cu-Pc thin films depends on the grain alignment. When the Cu-Pc thin films were annealed at 150 °C for 3 h, the films are aligned with the b-axis normal to the substrate and have a lower surface resistance.

Preparation of Cr–Si multilayer structures for thin film heater applications

Multilayered thin film structures of Cr–Si have been prepared on top of Al2O3 substrates by RF magnetron sputtering technology, and the electrical and thermodynamic properties of the thin film structures were assessed up to the temperature of 500 °C. The thickness of the Cr–Si film was ranged to about 1 μm, and a post-annealing was carried out to achieve more reliable film structures. In measurements of temperature coefficient of resistance (TCR), chrome-rich films show the metallic properties, whereas silicon-rich films do the semiconductor properties. Optimal composition between chrome and silicon was obtained as 1:2 in the Auger electron spectroscopy analysis, and there is approximately 15% change or less of surface resistance from room temperature to 500 °C. Compared to the conventional heating elements, the thin film heater has additional advantages of extremely low outgassing and high ramping speed in addition to the temperature uniformity.

Studies of surface resistance of copper(II) phthalocyanine thin films by using a near-field scanning microwave microscope

We observed the surface resistance of copper(II) phthalocyanine (Cu–Pc) thin films dependence on substrate heating temperature by using a near-field scanning microwave microscope (NSMM). The NSMM system was coupled to a dielectric resonator with a distance regulation system at an operating frequency f=4.5–5.5 GHz. The crystal structures and surface resistance of Cu–Pc thin films due to different heating temperatures were observed by using X-ray diffraction including the 2 θ and φ-scan and the near-field scanning microwave microscope. As the temperatures increased from room temperature to 200 °C, the crystal structure of Cu–Pc thin films was transformed from monoclinic α-Phase to thermally stable monoclinic β-Phase. The changes in surface resistance of Cu–Pc thin films due to different substrate heating temperatures were investigated by NSMM by measuring the reflection coefficient S 11. The surface resistance of Cu–Pc thin films depends on the morphology and crystal alignment of these films. When the substrate temperature for deposition was 150 °C, the minimum surface resistance of Cu–Pc thin films was obtained.

Preparation and Characterization of Conductive Polymer Nano-Films

We investigate the electrically conductive polypyrrole(PPy)and poly(3,4-ethylenedioxythiophene, PEDOT)nano-films prepared in a continuous roll-to-roll process by vapor-phase polymerization method. Thin films of ferric chloride doped conductive PPy and PEDOT on plastic substrates, in which neither matrix polymers nor binders are used for the film forming process, are obtained at nano-level thickness. PPy and PEDOT film thickness was varying depend on reaction time and reaction temperature. The surface resistance change with time of exposure to monomer vapors, and the value was in the range of 103 ∼ 105 Ω/sq. at 20–60 nm thick films and showed up to 600Ω/sq. for the > 600 nm thick films. These thin films had a very highly ordered surface morphology. Especially, in the case of PEDOT film, the growth of highly ordered conductive PEDOT crystalline microstructure, which high anisotropy, parallel to the substrate film was fabricated, was observed by AFM.

A four-point probe correlation of oxygen sensitivity to changes in surface resistivity of TiO 2(0 0 1) and Pd-modified TiO 2(0 0 1)

The objective of the current study was to investigate changes in the surface electrical resistivity of single crystal TiO 2(0 0 1) under ultra-high vacuum conditions for gas sensor studies. Using a linear four-point probe (FPP), we investigated the effects of surface stoichiometry and of exposure to oxygen on the surface resistivity. We also investigated the effect of palladium (Pd) particles of varying sizes on the surface resistivity using both FPP and scanning tunneling microscopy. Our results showed that there was a strong dependence of the resistivity on the average Ti oxidation state of the clean TiO 2(0 0 1) surface. A decrease in the surface resistivity of TiO 2(0 0 1) was observed after the surface was reduced by Ar + ion sputtering, whereas an increase in the resistivity of the sputtered-TiO 2 surface was detected after reacting with gas-phase oxygen. In addition, we found that the presence of Pd particles enhanced the sensing performance to oxygen, and that the degree of enhancement depended strongly on the size of the Pd nanoparticles.

Humidity sensing with weak acid-doped polyaniline and its composites

In this paper, we report the behaviour of humidity sensors with polyaniline-based conducting polymers doped with different weak acidic dopants. Changes in surface resistivity of films were monitored as a function of relative humidity. The acidic dopants used were camphosulphonic acid (CSA), diphenyl phosphate (DPPH), and maleic acid (Mac) blends of these were formed with styrene–butyl acrylate copolymer for improving the mechanical stability. The sensitivity of responding the level of relative humidity was compared for three composites wherein, although low sensitivity the Mac-doped films were found to be repeatable and more stable compared to the others. Films prepared out of styrene–butyl acrylate copolymer with different concentrations of PANI–Mac were used for sensing humidity ranging between 20 and 95% relative humidity. The films exhibited almost linear behaviour within a chosen range of humidity.

Electron paramagnetic resonance and surface resistance measurements in MgB 2 and Mg 1− xLi xB 2

The temperature dependence of the electron paramagnetic resonance and surface resistance of MgB 2 and MgB 2 doped with lithium is studied. A single narrow symmetric line having an isotropic g value of 2.0036 is observed which is attributed to mobile spin 1/2 holes in the hexagonal boron planes. Both the g value and line width are independent of temperature in the normal state but there is a marked decrease in the intensity of the spectrum as the temperature is lowered to T c. The surface resistance is observed to decrease linearly with temperature in the normal state of the undoped material. In the doped material there is some deviation of the surface resistance from the linear temperature dependence starting at 150 K suggesting the possibility of the existence of a spin gap in the less than optimally doped material. Large changes in surface resistance as a function of dc magnetic field are observed in the superconducting state resulting from dissipation due to vortex movement.

Estimating evapotranspiration for a temperate salt marsh, Newcastle, Australia

AbstractEvapotranspiration was studied at a salt marsh site in the Hunter River estuary, NSW, Australia, during 1996–8. Estimates of actual evapotranspiration (Ea) were obtained for three sites using the eddy correlation method. These values were compared with results obtained with the Penman and Penman–Monteith equations, and with pan evaporation. The Penman–Monteith method was found to be most reliable in estimating daily and hourly evapotranspiration. Surface resistance values averaging 12 s m−1 were derived from the eddy correlation estimates. Recent tidal flooding and rainfall were found to decrease surface resistance and increase Ea/Ep ratios. Estimates of evapotranspiration obtained using the Penman–Monteith method were shown to be sensitive to changes in surface resistance, canopy height and the method used to estimate net radiation from incoming solar radiation. These results underline the importance of accurately estimating such parameters based on site‐specific data rather than relying on empirical equations, which are derived primarily for crops and forests. Copyright © 2001 John Wiley & Sons, Ltd.

An anomalous weak link model for microwave surface impedance ofYBa2Cu3O7-δ films in a weak dc magnetic field

By incorporating an anomalous weak link (AWL), the coupled-grainmodel is modified to describe the anomalous decrease of microwave surfaceimpedance (Zs) of YBa2Cu3O7-δ thin films in aweak dc magnetic field (Bdc). The critical current density of the AWL hasa minimum at zero Bdc, a linear dependence on Bdc for low Bdcand a saturation at high Bdc, unlike a conventional weak link. Using thismodel, the field dependences of surface resistance are calculated at differentrf input powers and resonant modes. The dimensionless parameters rH,defined as the ratio of the change of surface resistance and reactance, arealso computed. The simulation results agree with the measurementqualitatively. We believe that the anomalous Zs behaviour may originatefrom the AWL.

The nonlinear surface impedance of YBa2Cu3O7−δ thin films in zero and large applied fields

We present coplanar resonator measurements of the nonlinear microwave surface impedance of laser ablated and electron-beam coevaporated YBa2Cu3O7−δ thin films at 8 GHz in zero and applied magnetic fields, including the effects of irradiation with heavy ions. Correlations between the changes in surface resistance and reactance as a function of microwave current suggest that there are different contributions to the nonlinear behavior at low and high microwave currents in both zero and applied fields. In zero field, we suggest that microwave-induced flux lines are responsible for the observed nonlinear behavior, resulting in thermal runaway at large, sample dependent, microwave currents. The observed nonlinearity completely masks any possible contribution from the intrinsic pair breaking expected for a d-wave superconductor. In a magnetic field at relatively low currents, the nonlinearity can be described by the nonlinear dynamics of flux lines pinned by defects with a continuous range of pinning strengths. At higher currents, microwave field nucleation of flux lines is again considered important. On irradiation with 3.6 MeV/nucleon Xe-129 ions, the microwave losses and nonlinearity are significantly decreased in both zero and applied fields, which could have important implications for microwave device applications.

Change in surface resistance of an Ag(100) electrode by adsorbed bromide

The change Δ R of the DC resistance of epitaxial Ag(100) films in contact with 0.04 M KClO 4+0.01 M KBr aqueous electrolyte by adsorption of bromide scales approximately linearly with the Br coverage Θ x of the fourfold hollow surface sites, as determined by surface X-ray scattering, but not with the coverage Θ e as obtained by chronocoulometry. The difference in the potential dependence of Δ R and Θ e is explained by assuming that the bromide ions adsorbed in the double layer off the hollow sites have no electron exchange with the metal. The order–disorder transition in the adsorbate between lattice gas at low coverages and a commensurate c(2×2) structure does not influence the development of the DC-resistance. Most probably only a negligible fraction of the electrons at the Fermi surface is scattered coherently by the superstructure.

Surface roughness effects on magnetoresistive and magnetic properties of NiFe thin films

The effect of surface roughness on magnetoresistance (MR) and magnetic properties of Ta/NiFe films is investigated as a function of surface roughness and NiFe thickness. The surface roughness effects on the MR ratio, Δρ/ρ, of Ta/NiFe films can be separated into two different regimes. In regime I, Δρ/ρ of NiFe films linearly decreases with increasing roughness mostly due to the increase in resistivity of NiFe, while Δρ remains constant. In regime II at increased roughness, domain wall pinning is induced by surface roughness causing a degradation of the uniaxial anisotropy of NiFe and leading to the decrease in Δρ, resulting in the steeper MR ratio decrease. The change in MR properties of NiFe films with varying thicknesses can be correlated with the change in surface resistivity, ρs. Strongly textured NiFe(111) grown onto smooth substrate yields higher anisotropy and lower coercivity than those deposited onto rough substrates over a wide range of NiFe thicknesses.

Charge-reducing Effect of Chemically Amplified Resist in Electron-Beam Lithography

The charging effect of chemically amplified resists in electron-beam (EB) lithography is described. The results of pattern placement error caused by the charging effect using SiO2 substrates with various thicknesses revealed that the resist with polyhydroxystyrene (PHS) has the ability to reduce the pattern placement error due to the charging effect as compared to the resist, SAL601. From the measurement of the surface resistance of the resist, it was found that the surface resistance of the resist with PHS decreases from 1017Ω to 1012Ω after UV exposure, even though no change in surface resistance was observed for SAL601. These results imply the following: (1) the charge-reducing effect of the resist with PHS is related to EB bombardment-induced conductivity of resist and (2) the deposited charge in the resist with PHS dissipates at a rate sufficient to avoid charging problems as a result of a leakage current through the surface of SiO2 film or electron-beam-induced current (EBIC). In this paper, we report the experimental results and the possible mechanism to explain the charge-reducing effect of the resist with PHS.

Feed-Induced Changes in Transport Across the Rumen Epithelium

The effect of feeding strategy on rumen epithelial growth and transport capacity were studied in sixteen dairy cows. There was a significant effect of feeding strategy on transport of butyrate, sodium and chloride ions, which could not be explained by changes in epithelial surface area, structure or resistance.

Sensitive measurement of the surface impedance of superconducting single crystals using a sapphire dielectric resonator

A highly sensitive system based on a sapphire dielectric resonator has been developed to investigate the microwave properties of superconducting single crystals. The resonator is fabricated from a machined piece of single crystal sapphire, with a hole bored along its axis to accommodate samples. Samples are mounted on a independently heated sapphire rod, which is inserted into the region of high microwave field at the centre of the resonator. Crystals can be mounted with either their c-axis or ab-planes parallel to the microwave field. In the former configuration the predicted smallest detectable change in surface resistance is /spl sim/2.6/spl times/10/sup -4/ /spl Omega/ and is /spl sim/3.7/spl times/10/sup -4/ /spl Omega/ for the latter. Microwave measurements at 10 GHz on high quality YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// single crystals demonstrate the potential of the system for measuring the surface impedance as a function of field (up to 1.75 T parallel and 0.18 T perpendicular to microwave field) and temperature.

Identification and modeling of microwave loss mechanisms in YBa/sub 2/Cu/sub 3/O/sub 7-X/

It has been proposed that the nonlinear microwave response of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films is due to small- or large-angle grain boundaries which behave as weak links. To identify dominant loss mechanisms in the measured response of stripline resonators, the relative change in surface reactance and resistance, r=/spl Delta/X/sub s///spl Delta/R/sub s/, is used as a characteristic signature for the different mechanisms. We show that Meissner state and flux flow losses can be differentiated from hysteresis loss for a variety of films across a range of microwave powers, frequencies, and temperatures. In addition, a coupled-grain/RSJ model has been augmented to include Josephson currents up to and beyond the critical currents J/sub cj/ of the weak links. The model can account for a distribution of grain and boundary properties including I/sub c/R/sub n/ products, junction capacitances, and grain dimensions. The r-values predicted by the model are discussed and compared to measurements.

Correlation between surface and bulk phenomena in the process of thin palladium hydride and titanium hydride film formation

The measurements of surface potential SP and resistance changes ΔR of thin Pd and Ti films in the process of PdH x( x = 1) and TiH y( y = 2) hydride formation were performed. SP measurements allow differentiation between various forms of hydrogen adspecies. The influence of these forms of the adsorbate on the resistance are determined and discussed.

Angular dependence of microwave dissipation by vortices in YBa2Cu3O7-x thin films.

Using meander line resonant structures, we perform highly sensitive measurements of the changes in surface resistance {Delta}{ital R}{sub {ital s}} of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} thin films in order to probe the vortex dynamics as a function of temperature, applied dc magnetic field, and angle ({theta}) of the applied field relative to the {ital ab} planes. We observe that the component of the magnetic field normal to the planes produces substantially more dissipation than the component parallel to the planes. By using an extension of the London theory to anisotropic superconductors, we can calculate the internal flux densities parallel ({ital B}{sub {ital a}{ital b}}) and perpendicular ({ital B}{sub {ital c}}) to the {ital ab} planes inside a superconductor for an arbitrary field orientation relative to the {ital ab} planes. For low fields, we can define a weight ({delta}) of the contribution to dissipation from {ital B}{sub {ital a}{ital b}} relative to that from {ital B}{sub {ital c}}. This allows us to calculate an ``effective`` internal flux density and hence obtain the total dissipation. Using the results from this theory, we obtain excellent quantitative agreement with our measurements of {Delta}{ital R}{sub {ital s}} vs {theta}.

Microwave dissipation due to vortices in YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films for magnetic fields applied at small angles from the ab-planes

YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films patterned in a meander line resonant structure are used to probe vortex dynamics at microwave frequencies. We perform highly sensitive measurements of the changes in surface resistance /spl Delta/R/sub s/(H,T,/spl theta/) of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films as a function of applied dc field, temperature, and angle of the applied field relative to the ab-planes. We observe that the component of the magnetic field normal to the planes produces substantially more dissipation than the component parallel to the planes. Also, the surface resistance R/sub s/ vs. H data for magnetic fields applied at small angles from the ab-planes shows two regions of markedly different field dependence separated by a crossover field H*. For applied fields H>H* we have linear dependence of R/sub s/ on applied field with a steep slope. For applied fields H<H*, we observe a much smaller slope in the RS versus field data. We believe that the low field region corresponds to the case where we have complete shielding of the field from the ab-planes (transverse Meissner effect), such that the field lines do not cross them and are instead confined to the interplanar region.<<ETX>>

Effects of etching on the morphology and surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7/(-δ) films

The changes in surface morphology and surface resistance of sputtered and laser ablated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films both before and after etching have been examined. Six different etchants were used: citric acid, nitric acid, Br-methanol, EDTA, disodium EDTA, and ion milling. The surface morphologies of the films were examined by reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM), both before and after etching. The surface resistance (R/sub s/) was measured at 94 GHz using a confocal resonator. An amorphous layer was found on the film surfaces after exposure to air. A few of the etches restored some of the surface crystallinity, but most caused increases in the overall surface roughness. Several of the wet etches attacked dislocations. Ion milling caused the largest degradation of surface crystallinity and a corresponding increase in R/sub s/. Some of the chemical etches increased R/sub s/ by less than 15%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>