Low energy post-growth irradiation of amorphous hydrogenated carbon (a-C:H) films

Abstract

a-C:H films were deposited on Si by applying the ion beam deposition (IBD) method using C6H14 vapor mixture with H2 (tsubstrate=15–200 °C). Then a-C:H films were irradiated by electron or Ar+ (20–700 eV) beams. Current densities were 4–6 mA/cm2 for electrons and 0.5–3 mA/cm2 for ions. Properties of the films were investigated by Raman spectroscopy (RS), ellipsometry, X-ray photoelectron spectroscopy (XPS), scanning electron (SEM), transmission electron (TEM), high-resolution electron (HREM) microscopies, and mechanical measurements. sp3 bonds were dominated when the substrate temperature was 15 °C and the ion energy was lower than 1000 eV. We observed a graphite peak in C1s spectra when the temperature was higher than 150 °C. The C1s peak was extended and shifted to a higher energy range using additional H2. The concentration of sp3 bonded carbon atoms was decreased in the surface layer after ion bombardment of DLC films and sp2 bonds were predominant with exceeding some critical power densities. The optical measurements showed that irradiation with P≤1.4 W/cm2 made the properties of DLC closer to diamond. The opposite trend was at power density P>1.4 W/cm2. This critical power density of Ar+ beams depended on properties of unirradiated carbon films. Direct irradiation of a-C:H had little or no changes at all on crystal structure. Great changes were observed when a-C:H films were coated by SiO2. Irradiation caused the electrical breakdown phenomena and in breakdown areas we saw crystal (∼6 nm) islands identified as graphite or lonsdaleite.