Characterisation of Diamond-like Carbon by Raman Spectroscopy and Optical Constants
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Spectroscopy and
C. M6l1ner**, P. Grant, H. Tran, G. Clarke, D. J. Lockwood, H. J. Labbe, B. Mason, R. Berriche* Institute for Microstructural Sciences, * Institute for Aerospace Research, National Research Council of Canada, Ottawa, Ont., K1A 0R6, Canada • *present address: Swiss Federal Institute of Technology, Lausanne, Switzerland, [email protected] ABSTRACT Crystalline diamond coatings and, increasingly, diamond like amorphous carbon (DLC) films are used for tribological and protective layers for their hardness and chemical inertness. They are also under investigation for their electron emitting properties, with possible applications in field emission displays. DLC films were deposited by laser ablation using a KrF excimer laser and fluences between 0.5 and 2 J/cm2. FTIR measurements did not show the presence of hydrogen in the films. Raman spectra allowed for the determination of the nature of the graphitic and diamond bonds (sp2 and sp3 ) as well as information about the disorder and short range order in the films. For a better determination of the sp 3 -content, which is often hidden in the Raman spectra, a correlation with optical properties in the near IR to near UV region was established. These values depended strongly on the substrate temperature and the laser fluence. DLC formation could be demonstrated even at substrate temperatures close to room temperature. Vickers hardness values and first measurements on the electron emissivity of the films can be correlated to the diamond character and the preparation method of the films. INTRODUCTION Diamond like amorphous carbon (DLC) values for hardness, chemical inertness, electrical and optical properties are intermediate to those of graphite and diamond, and, therefore cover a wide range [1-3]. In addition to various sputtering and vapor deposition methods, pulsed laser deposition (PLD) has been shown to be an effective method to generate DLC thin films [4-7]. One major advantage of using PLD instead of other deposition methods is the lack of hydrogen, thus enhancing the density of the films and the index of refraction, which is preferable in some applications. DLC thin films produced by PLD are generally categorized diamond-like, but the degree of diamond-like character varies considerably, and is strongly dependent on the deposition conditions. In addition to the continued lasting interest in diamond as a protective coating, recent interest has also focused on possible applications as a field emitter for displays and other vacuum microelectronic devices [8-1 1]. Crystalline diamond exhibits a negative electron affinity (NEA) for certain crystal surface planes via restructuring [12-14], which can under appropriate conditions, lead to a low work function, and thereby supposedly allow high current densities in field emission setups. A high electron emission, which is due to high micro roughness or low work function, is also observed for amorphous films [8], whereas the exact mechanism for high field emission current is still under discussion. Interest in
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