Department of Physics

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Author: search.filters.author.Gangopadhyay, SubhashisSubject: search.filters.subject.Thin films

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    Thin Cu film resistivity using four probe techniques: Effect of film thickness and geometrical shapes
    (AIP, 2018-05) Gangopadhyay, Subhashis
    Precise measurement of electrical sheet resistance and resistivity of metallic thin Cu films may play a significant role in temperature sensing by means of resistivity changes which can further act as a safety measure of various electronic devices during their operation. Four point probes resistivity measurement is a useful approach as it successfully excludes the contact resistance between the probes and film surface of the sample. Although, the resistivity of bulk samples at a particular temperature mostly depends on its materialistic property, however, it may significantly differ in the case of thin films, where the shape and thickness of the sample can significantly influence on it. Depending on the ratio of the film thickness to probe spacing, samples are usually classified in two segments such as (i) thick films or (ii) thin films. Accordingly, the geometric correction factors G can be related to the sample resistivity r, which has been calculated here for thin Cu films of thickness up to few 100 nm. In this study, various rectangular shapes of thin Cu films have been used to determine the shape induced geometric correction factors G. An expressions for G have been obtained as a function of film thickness t versus the probe spacing s. Using these expressions, the correction factors have been plotted separately for each cases as a function of (a) film thickness for fixed linear probe spacing and (b) probe distance from the edge of the film surface for particular thickness. Finally, we compare the experimental results of thin Cu films of various rectangular geometries with the theoretical reported results.
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    Formation of Monolayer Graphene by Annealing Sacrificial Nickel Thin Films
    (ACS, 2009-09) Gangopadhyay, Subhashis
    Graphene films have been formed by annealing Ni thin films at 800 °C under vacuum conditions. The Ni thin films are deposited on Si/SiO2 and, following annealing, have a polycrystalline morphology with grain sizes on the order of 1 μm. Following growth, the Ni is removed by etching, and the graphene is transferred as a single continuous layer onto a separate surface. The fraction of monolayer graphene is investigated using optical and electron microscopy and Raman spectroscopy and is shown to be >75%.
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    Oxidation mechanism of thin Cu films: A gateway towards the formation of single oxide phase
    (AIP, 2018) Gangopadhyay, Subhashis; Pande, Surojit
    Controlled thermal oxidations of thin copper films at relatively lower temperatures (up to 500°C) leading towards the formation of a single phase of copper oxide are investigated where the oxidation temperature, duration, oxygen partial pressure, film thickness and the crystallographic orientations play very crucial roles to significantly control the final phase of the copper oxide. Thin Cu films of thicknesses 100-1000 nm were deposited on glass and silicon substrates using the vacuum assisted thermal evaporation technique. Oxidations of those Cu films were performed at different temperatures for variable durations in air ambient as well as oxygen ambient conditions. Four probe resistivity measurement, x-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible (UV-Vis) spectroscopy, scanning electron microscopy (SEM) and x-ray photoemission spectroscopy (XPS) techniques have been used to characterize the oxide films. At a thermodynamic equilibrium, it has been observed that the oxide phase is solely determined by the oxidation temperature, however, the oxygen partial pressure can significantly alter this temperature range. In case of thermal oxidation in air, the initial oxidation of the copper films starts at about 150 °C, but a well ordered crystalline phase of the cuprous oxide (Cu2O) is observed only above 200 °C. However, the cupric oxide (CuO) phase starts to appear only above 320 °C. The details of the oxidation mechanism of the Cu film are explained with a probable schematic model in terms of thermal diffusion as well as the chemical reactivity.