Correlation of Raman Spectra and Bonding in DLC Films Deposited by Laser Ablation and Laser-Plasma Ablation Techniques
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Correlation of Raman Spect aind Baonding in DLC Films Deposited by Laser ANbtion and Laser-PI~asm Abl•baon Techniques A. Rengan, J. Narayan., Dept of Materials Science and Engineering, North Carolina State University, Raleigh, N. C. - 27695. J. L. Park 1., Solid State Div., Oak Ridge National Laboratory, Oak Ridge, TN37831, and, M. Li., Chemistry Dept., University of North Carolina, Chapel Hill, NC-27514
ABSTRACT We have deposited diamondlike carbon (DLC) films on a variety of substrates from 250 C and higher. The effects of deposition temperature on the properties of DLC films deposited by a conventional laser ablation technique are compared with that of a unique laser-plasma deposition scheme. The calculated values of neff, the effective number of valence electrons, suggest that, with the increase in the deposition temperature, the diamondlike component (sp3 bonds) remains invariant for the laser deposited samples, and increases for the laser-plasma deposited films. Raman measurements show that the Raman allowed 'G' band upshifts to -1600 cm-1 for both deposition schemes. However, the disorder induced 'D'band remains invariant at -1370 cm"1 for the laser ablated samples, and downshifts to -1350 cm 1 for the laser-plasma deposited samples. These results suggest a correlation between the diamondlike content (sp3 bonds) and the Raman shift of the 'D' band. I. INTRODUCTION The laser deposition process manifests nonequilibrium features during deposition such as atomically and electronically excited (as well as ionized) species and fast quenching, which lead to the formation of films in metastable states with unique properties. Malshe et al [1] and others [2, 3] have shown that the physical evaporation of graphite atoms has resulted in the deposition of hard diamondlike films. Herein, it is shown that further nonequilibrium features can be incorporated into laser ablated plasma by coupling capacitively stored energy to the laser ablated spot in synchronism with the laser pulse. Uniform depositions over a significantly larger area are characteristic of this deposition technique, and the amorphous hard carbon films deposited by this method show improved optical properties as well as increased hardness as compared to films deposited by the conventional laser ablation method [4]. These improvements result from an increase in the sp 3 to sp 2 ratio in the DLC films, as estimated from neff values originally derived by Savvides [5]. Briefly, since most 7 -> n* transitions occur at energies below -7 eV, and a -> a* transitions make no contributions below - 9eV, it is possible to estimate the fraction of sp2 bonds (and hence sp 3 bonds = 1- sp 2 ) using 'neff' data at energies 1
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below 5 eV. The contributions of the 7c-> t* transitions as calculated for graphite and a-> a* transitions as determined for diamond are shown in Fig 1 [5]. The DLC films are characterized by spectroscopic ellipsomet
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