Ion-Assisted Pulsed Laser Deposition of Amorphous Tetrahedral-Coordinated Carbon Films
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Tetrahedral-
Coordinated Carbon Films
T. A. Friedmann, D. R. Tallant, J. P. Sullivan, M. P. Siegal, and R. L. Simpson,
Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT A parametric study has been performed of amorphous tetrahedral carbon (a-tC) films produced by ion- assisted pulsed laser deposition (IAPLD). The ion voltage, current density, and feed gas composition (nitrogen in argon) have been varied. The resultant films were characterized by thickness, residual stress, Raman spectroscopy, and electrical resistivity. The Raman spectra have been fit to two gaussian peaks, the so called graphitic ("G") peak and the disorder ("D") peak. It has been found that the magnitude of the D peak and the residual compressive stress are inversely correlated. At low beam voltages and currents, the magnitude of the D peak is low, increasing as the ion beam voltage and current are raised. The ion beam voltage has the most dramatic effect on the magnitude of the D peak. At low voltages (200-500 V) the magnitude of the D peak is greater for ion beams with high percentages of nitrogen possibly indicative of C-N bonding in the films. At higher voltages (500 - 1500V) the D peak intensity is less sensitive to the nitrogen content of the beam.
INTRODUCTION Recently, the growth of amorphous carbon films with a high fraction of sp3 bonds and low hydrogen contents (amorphous tetrahedral carbon or a-tC) has received much attention. Many techniques have been used to grow these films including pulsed laser deposition[1-6], dual ion beam deposition[7, 8], energy filtered ion beam deposition[9-1 1], and the filtered cathodic arc[ 1214]. Typically, carbon atoms and ions ranging in energy from 10 - 1000 eV are condensed upon room temperature substrates. Each of these techniques offers its unique advantages and disadvantages. For example, PLD offers a quick way to deposit films and vary deposition parameters, but particulates generated in the ablation process can cause problems in the deposited films. Ion beam deposition provides control over the carbon ion species and energies, however, the growth rates are usually too slow for commercial applications. The filtered cathodic arc provides a high current means of obtaining technologically useful growth rates, but problems remain in terms of commercialization of this technology (e.g. limited cathode lifetimes). Previously, we have reported a study on a-tC films grown by unassisted PLD in both vacuum and varying background ambients of nitrogen, argon and hydrogen[5, 6]. In general, we found that as the laser generated ablation plume became more energetic, it condensed into a film with increased diamond-like character. Changes in the deposition parameters were manifested in the Raman spectra and residual compressive film stresses. Symmetric Raman spectra (centered near 1570 cm-') and high residual stress (> 5 GPa) correlated with high laser energy densities (up to 45 J/cm 2). Background gasses slowed the ablation plume resulting in less symmetric Raman spectra and lower residual compres
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