Optical constants and associated functions of metastable diamondlike amorphous carbon films in the energy range 0.5–7.3 eV

Abstract

The complex refractive index N(ω)=n+ik and the complex dielectric constant ε(ω)=ε1+iε2 are presented for diamondlike amorphous carbon (a-C) films in the photon energy range 0.5–7.3 eV. The effective number of valence electrons neff per carbon atom, the static dielectric constant ε0,eff, and the energy loss function Im[−1/ε(ω)] are deduced via the use of sum rules and are used to interpret the optical data. The a-C films were deposited using an unbalanced magnetron gun to sputter a graphite target (effective sputtering area of 20 cm2) in ultrapure argon gas. The magnetron is characterized by a high deposition flux of condensing atoms (1.5×1014–1.2×1016 cm−2 s−1) and a concomitant high ion flux (6×1014–2.5×1016 cm−2 s−1). A series of films were prepared by sputtering at different power levels in the range 5–500 W. Insulating substrates were used which allowed the films to self-bias negatively with respect to the plasma, so that the films were bombarded during their growth by Ar+ ions of energy 16–13 eV at an Ar+/C arrival rate ratio varying from about 4 to 2. A transition in the optical properties, physical properties (density, conductivity, microhardness), and microstructure is observed with the most rapid changes occurring at low sputtering power. The optical data are discussed in terms of interband transitions appropriate to amorphous semiconductors, and by comparison with the optical constants and the band structure of crystalline graphite and diamond. We find that films possess a metastable bonding configuration of a mixture of sp3 (tetrahedral) and sp2 (trigonal) bonds with the average coordination of the carbon atoms varying from 3.76 to 3.44. This fourfold-to-threefold transition in bonding is attributed to ion-induced structural modification of the amorphous carbon matrix. Weak plasma peaks at about 5 eV and the trends in neff and ε0,eff indicate that the π electron is localized leading to a hopping conductivity and a large optical gap, E0=0.40–0.74 eV.