S. I. Shah

Laser ablation and the production of polymer films

The formation of high-quality thin films of polytetrafluoroethylene (PTFE) is important in many applications ranging from material reinforcement to molecular electronics. Laser ablation, a technique widely used to deposit a variety of inorganic materials, can also be used as a simple and highly versatile method for forming thin polymer films. The data presented show that PTFE films can be produced on various supports by the evaporation of a solid PTFE target with a pulsed ultraviolet laser. The composition of the ablation plume suggests that PTFE ablation and subsequent film formation occur by way of a laser-induced pyrolitic decomposition with subsequent repolymerization. The polymer films produced by this method are composed of amorphous and highly crystalline regions, the latter being predominantly in a chain-folded configuration with the molecular axis aligned parallel to the substrate surface.

Effect of energetic bombardment on the magnetic coercivity of sputtered Pt/Co thin-film multilayers

Pt/Co multilayers are an attractive candidate for a magneto‐optical recording medium. However, films sputter deposited in Ar have coercivities too small (100–350 Oe) to be practical in recording. By sputter depositing multilayers in Kr or Xe instead of Ar, we achieved coercivities ∼1000 Oe, suitable for recording. We attribute the lower coercivity of Ar‐sputtered films to interfacial mixing of Pt and Co layers by energetic bombardment from Ar gas atoms that recoil from the Pt target.

Effect of oxide layer on the hysteresis behavior of fine Fe particles

The effects of surface oxidation on the structural and magnetic properties of fine Fe particles prepared by the evaporation technique have been studied using transmission electron microscopy, x‐ray photoelectron spectroscopy, superconducting quantum interference device magnetometry, and Mossbauer spectroscopy. By varying the argon pressure, particles were obtained with sizes in the range of 60–350 A. The hysteresis behavior was found to be strongly dependent on the variation in the amount of surface oxidation. The differences in the magnetic behavior due to variation in size and oxidation have been explained by considering a shell/core model for the particle morphology with the shell consisting of Fe oxides surrounding the α‐Fe core.

X‐ray photoelectron spectroscopy of initial stages of nucleation and growth of diamond thin films during plasma assisted chemical vapor deposition

We have carried out x‐ray photoelectron spectroscopy on diamond thin films deposited by microwave assisted chemical vapor deposition technique using a H2‐CH4 plasma. Films grown for different lengths of time, from 1 min to several hours, were analyzed for the surface composition in the C 1s and Si 2p regions. The results indicate the presence of SiC in the initial stage of nucleation due to the carbon interaction with the Si substrate. Graphite starts to form in early stages of nucleation as the substrate becomes supersaturated with carbon. A diamond peak starts to appear after the incubation period and a simultaneous decrease in the carbide and graphite peak intensities was observed. Graphite is preferentially etched during the growth but SiC remains as an impurity even after several hours of deposition.

Superconductivity and resputtering effects in rf sputtered YBa2Cu3O7−x thin films

Superconducting stoichiometric YBa2Cu3O7−x films have been grown by rf sputtering of a single bulk material target in an Ar+10% O2 sputtering atmosphere on a variety of substrates. Films on SrTiO3 substrates exhibited the best post‐annealing superconducting properties with the critical onset temperature of 90 K and zero resistance at 67 K. The maximum current density for these samples was 1×105 A/cm2 at 4.2 K. An extensive loss of film underneath the target was observed during the growth of these films which can be attributed to resputtering due to oxygen anion or energetic neutral particle bombardment of the substrate.

Magnetic and structural properties of Fe‐FeO bilayers

The structural and magnetic properties of sputtered Fe/Fe‐O films were studied by x‐ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy, transmission electron microscopy, and superconducting quantum interference device. XPS studies showed the presence of FeO and Fe2O3 on the surface of as‐made samples. Microstructure studies showed a uniform nanostructure with the grain size in the range of 50–150 A with smaller grains corresponding to thinner films. The coercivity at 10 K was found to increase substantially with decreasing film thickness below 60 A. A high Hc (2.7 kOe) was observed in samples with a thickness about 20 A. Magnetization curves showed a planar anisotropy with a shifted hysteresis loop characteristic of an exchange anisotropy between the Fe and Fe‐O coating. The coercivity was found to drop steeply with increasing temperature. This may be attributed to the superparamagnetic behavior of the Fe‐O surface layer.

Growth and microstructure of Bi‐Sr‐Ca‐Cu‐O thin films

Thin superconducting Bi‐Sr‐Ca‐Cu‐O films have been grown by reactive magnetron sputtering with an average cation ratio of 2:2:1:2 on single‐crystal (100)MgO. Films show a superconducting transition onset at 117 K, but do not go to zero resistance until 83–84 K. Secondary electron microscopy and electron beam microprobe showed two major phases with different compositions and morphologies. The dominant phase was a lamellar phase with composition very close to the 2:2:1:2 cation ratio. The second phase had a needle‐like morphology, which was deficient in Bi, and had excess Cu. The nonsuperconducting Sr14−xCaxCu24O41 phase had been known to exist in bulk; therefore, the needle phase could be an intermediate step in the formation of Sr14−xCaxCu24O41 phase as Bi deficiency in the needles increased with increasing annealing temperature and/or time.

Effect of oxygen on the nucleation and growth of diamond thin films

We have used x‐ray photoelectron spectroscopy to study the effects of oxygen on the nucleation and growth of diamond films during microwave plasma assisted chemical vapor deposition. The high film growth rate in the presence of oxygen in the feed gas mixture was found to be related to the accelerated saturation of carbon onto the Si substrate surface. This reduces the incubation period and promotes a much faster diamond nucleation and growth than with oxygen‐less plasmas. In the presence of oxygen, diamond has been detected as early as 1 min into the deposition, whereas without oxygen it was not observed until after 40–50 min.

Melting and freezing behavior of ultrafine granular metal films

Abstract The melting and freezing behavior of small connected and isolated Bi particles embedded in a SiO 2 matrix has been studied as a function of the mean particle size. Melting temperatures have been found to be strongly reduced with decreasing particle size, consistent with a r −1 type behavior as predicted by a number of theoretical approaches to the solid-liquid melting transition. When the freezing behavior of these particles was studied large undercoolings were found. In addition, strong fluctuations in the freezing time were observed in isothermal measurements at large undercoolings below the bulk Bi freezing temperature.

Magnetic and structural properties of Co/CoO bilayers

The magnetic and structural properties of Co/CoO bilayers prepared by sputtering have been studied in films with Co thickness in the range of 65 to 250 A. A two‐phase structure consisting of hcp Co and fcc CoO was observed in the films. The coercivity was found to change inversely proportional to the Co thickness and to the sputtering rate. The highest Hc (9 kOe) was obtained in films exposed to ambient conditions for long periods of time. A shift in the hysteresis loop was found in field‐cooled samples, indicating a strong exchange coupling between the Co and CoO phases.