Resistance Change of Cobalt and Niobium Films When Exposed To
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PHYSICAL REVIEW B VOLUME 55, NUMBER 20 15 MAY 1997-II Resistance change of cobalt and niobium films when exposed to hydrogen and carbon monoxide A. L. Cabrera,* and W. Garrido-Molina Facultad de Fı´sica, Pontificia Universidad Cato´lica de Chile Casilla 306, Santiago 22, Chile J. Colino† Physics Department, University of California, San Diego, La Jolla, California 92093-0319 D. Lederman Physics Department, West Virginia University, Morgantown, West Virginia 26506-6315 Ivan K. Schuller Physics Department, University of California, San Diego, La Jolla, California 92093-0319 ~Received 4 June 1996; revised manuscript received 16 January 1997! The resistance of thin cobalt and niobium films was monitored during 1000 s exposures to hydrogen or carbon monoxide at a fixed pressure (1026 torr! and also during their removal. Upon an adsorption-desorption cycle the resistivity changed in a ‘‘sawtooth’’ fashion, similar to the changes previously observed in niobium foils. The resistivity increase by the adsorbed gas seems to be directly related to weakly adsorbed states on the surface. The magnitude of the resistivity changes due to gas adsorption is related to the surface roughness of the film. @S0163-1829~97!02219-4# I. INTRODUCTION quantities:21 two H atoms for every four Pd atoms in the lattice ~0.50 atomic fraction!. Several theoretical models A strong surface adsorption of gaseous molecules is ex- have been developed to explain the large bulk diffusion of pected to modify the surface electrical conductivity. Since hydrogen in Pd and its absence in Nb and Ni.22–24 both the surface and bulk contribute to the total electrical Other studies have claimed that resistance increases due conductivity of a solid, an effect due to surface adsorption is to surface adsorption of gases such as oxygen molecules on more readily detectable in a thin film. In particular, transition silver films.25,26 Moreover, ultraviolet photoemission spec- metals are excellent candidates for such studies. Most tran- troscopy measurements on the O/Ag and O 2/Ag have shown sition metals are good catalysts for the production of hydro- that a p orbital of the adsorbed molecules is located above 1,2 carbons from the reaction of H 2 with CO. An important the Fermi level of the silver film where conduction electrons intermediate step for these reactions is the adsorption and can be additionally scattered.27 dissociation of both molecular H 2 and CO on the surface. In Several explanations have been advanced to explain general, two hydrogen chemisorption states (b1 and b2), and changes in film resistance with adsorbed gas. The Suhrmann molecular and dissociated CO states are observed on the sur- model28 claims that the molecular orbitals of a molecule lo- face of transition metals. cated on the surface overlap with the conduction bands. The Nickel, cobalt, and iron adsorb hydrogen on the surface Sachtler model28 hypothesizes an effective decrease in the with almost no diffusion into the bulk.3–10 The adsorption of thickness of the metal film when the molecules are adsorbed hydrogen and CO on polycrystalline Ni and Co surfaces8–10 on the surface. A model based on the Fuchs-Sondheimer and single crystal Ni with different surface orientations3,11,12 theory28 hypothesizes that the surface adsorbed molecules has been extensively studied. increase the resistance of the film because they increase the Shanabarger studied the adsorption of hydrogen on thin diffuse scattering of electrons reflected from the surface. Ni ~Refs. 13 and 14! and Fe ~Ref. 15! films by monitoring This assumes that the surface morphology of the film is per- their resistance upon the adsorption of hydrogen. The low fectly planar. A refinement of this theory, known as ‘‘the activation energies for desorption observed in these studies scattering hypothesis,’’28 is a more general case. In this implies that this effect depends on the b1 state. The fact that model, surface roughness increases diffuse scattering, effec- the resistance of the film is sensitive to this adsorption state tively making the relative effect of the adsorbed molecules and not to the higher energy one (b2) implies that this effect on the resistivity smaller. is not related to an electron transfer mechanism. In this paper, we study the effect of H 2 or CO adsorption Niobium allows some diffusion of hydrogen atoms16 into on the resistance of Co and Nb films. The resistance change subsurface sites ~0.0028 atomic fraction!. In this case, resis- in a 40 nm Nb film exposed to 1000 L of either hydrogen or tance changes have also been observed upon the absorption CO is twice that fora6nmCofilmexposed to the same of H.16,17 The inhibition or enhancement of hydrogen absorp- dosage of CO. The film resistance increases monotonically tion by metallic overlayers has also been observed in this with hydrogen or CO exposure and the absolute change after type of experiments.18–20 Pd, on the other hand, is the only 1000 s of exposure decreases with increasing film thickness. metal which absorbs hydrogen into the bulk in large These results imply that the difference in uptake is a direct 0163-1829/97/55~20!/13999~6!/$10.0055 13 999 © 1997 The American Physical Society 14 000 A. L. CABRERA et al. 55 result of the surface morphology since the adsorption kinet- ics in Nb is not much different from those in Co. II. EXPERIMENT Thermal desorption and resistivity were measured for Co and Nb films exposed to H 2 and CO in a modified AMETEK ~Thermox Instruments Division! system designed for gas analysis. This consists of a six-way stainless steel cross ~base pressure 1029 torr! mounted on a vacuum system equipped with a sample manipulator, an Ar ion sputtering gun, a quad- rupole mass spectrometer, a variable leak valve, and a glass viewport. Gas evolution and resistivity measurements were per- FIG. 1. Resistance changes of Nb films when exposed to H 2 formed on 40, 200, and 400 nm films of Nb (1.130.3 cm 2 ~500–1500 s! and after removal of the gas. Curve a, 40 nm; curve b, 400 nm. Solid line for curve b corresponds to fits to Eq. ~3.1!. area! sputtered onto sapphire substrates. The same type of experiments were performed on 6, 15, or 30 nm films of Co (0.931.2 cm 2 area! evaporated in ultrahigh vacuum ~UHV! a) and a 400 nm ~curve b) Nb film upon exposure to hydro- on mica. The Co and Nb sources used for the films were gen. Between 0 s and 500 s the samples are in an UHV 99.999% pure. Elemental surface composition was moni- environment, and the data for these times represents the tored with Auger electron spectroscopy or x-ray photoelec- background measurement of the instrument. The films are tron spectroscopy using a hemispherical analyzer ~Physical exposed to hydrogen from 500 s up to 1500 s and then the Electronics, System 5100!. Spectra in the 15 to 1000 eV gas is pumped out of the chamber. When the gas is pumped energy range show typical surface C and O without any other out, the resistance decreases ~between 1500 and 2500 s!. impurity. Figure 2~a! shows the resistance changes of the same 40 The crystal structure of the films was characterized with nm ~curve a) and 400 nm ~curve c) Nb films and the resis- high and low angle x-ray diffraction ~XRD! using a rotating anode x-ray diffractometer. Low angle XRD, performed in a 29 medium-resolution configuration using Cu Ka radiation (l50.15418 nm!, was also used to measure the thickness of some of the films. The thicknesses of the nominally 15 nm thick Co and 40 nm thick Nb films were 15.4 nm and 41.4 nm, respectively, thus confirming that the film thicknesses estimated from the quartz thickness monitor used during deposition were correct to within 5%. Electrical contacts were attached to the films with a con- ductive emulsion of small metal particles in ethyl acetate ~Loctite ‘‘Quick Grid’’!. Two insulated gold wires were spot welded to the film, at the location of the conductive paint on one side and connected to pins of a UHV feedthrough on the other side. The electrical resistance of this configuration was measured with a Keithley MicroOhmmeter ~model 580!. In order to clean the surface, the samples were subjected to two cycles of heating to 450 K followed by Ar ion sput- tering. Ar ion sputtering was carried out on a 131cm2area with an ion current of 1 mA for two minutes which removes less than 1 nm of metal. Assuming the absence of contami- nants after this process, the samples were exposed to 1000 L ~1L51026 torr s! of hydrogen or CO at 320 K while the resistivity was measured. For each exposure, the cleaning cycle was repeated prior to the gas exposure. III. RESULTS AND DISCUSSION The resistivity of the Co and Nb films increased during FIG. 2. ~a! Resistance changes of Nb and Co films exposed to exposure to hydrogen of CO ~at 320 K! and decreased to CO ~500–1500 s! and after the removal of the gas. Curve a,40nm approximately the initial value when the gas was removed Nb film; curve b, 10 nm Co film; curve c, 400 nm Nb film. Solid from the chamber. These changes were quite reproducible lines for all the curves correspond to curve fitting using Eq. ~3.1!. and a ‘‘sawtooth’’ pattern was obtained for cycles of ~b! Normalized resistance change multiplied by thickness of film adsorption/desorption on films of different thicknesses. (Nl) in a number of layers.