Optical and electronic properties of nitrogen-implanted diamond-like carbon films
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Optical and electrical measurements on nitrogen ion-implanted diamond-like carbon films are presented. Raman scattering measurements, which probe the crystallinity of the film surface, indicate that nitrogen implantation reduces the finite crystallographic order in the pristine carbon films. The absence of molecular vibrations in the infrared absorption spectra of the films argues against a polymeric structure of the ion-implanted films. Spectroscopic ellipsometry experiments determine the change in the optical constants of the carbon film due to nitrogen implantation. Electrical dc conductivity measurements are interpreted within the framework of a schematic density of states picture of graphitic 7r-electrons in an amorphous carbon system. Taken collectively, the optical and electrical measurements suggest that nitrogen implantation increases the density of localized states within the 1.5 eV bandgap of the quasi-amorphous carbon film, thereby reducing the bandgap and increasing the conductivity of the nitrogen-implanted films.
I. INTRODUCTION Crystalline and amorphous carbon nitride films have attracted considerable technological and scientific interest recently. For example, Liu and Cohen1 have suggested that if carbon could replace the silicon atoms in the y8-Si 3 N 4 structure (space group Clh), that C 3 N 4 could have a bulk modulus comparable to or greater than that of diamond. On the other hand, amorphous carbon nitride has been suggested as a substitute for amorphous silicon nitride in electronic applications such as gate insulators in thin film transistors.2 Jones and Stewart3 first reported the growth of amorphous carbon films doped with nitrogen from the plasma decomposition of N 2 and various hydrocarbons. They found that their films demonstrated a decrease in both the optical bandgap and structural disorder with increasing nitrogen concentration. Han and Feldman4 grew amorphous carbon nitride films in a capacitively coupled rf plasma reactor using CH4 and N 2 gases. They also observed that nitrogen incorporation decreased the bandgap and increased the structural order of the films. Ricci et al.5 performed the same growth experiment but added chlorine gas to the process to reduce the hydrogen content of the films. They observed that their films underwent a progressive crystallization with post growth heat treatments which they ascribed to a kind of Mott-Anderson transition. Implanted nitrogen has been used as an n-type dopant in diamond,6 and to increase the wear properties of amorphous carbon coatings.7 Amir and Kalish8 have implanted nitrogen into amorphous hydrogenated carbon films, but found that the changes observed in the film J. Mater. Res., Vol. 9, No. 1, Jan 1994
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properties were the result of damage related effects instead of doping. In this study, we implanted nitrogen ions into quasiamorphous, diamond-like carbon films. The properties of the films have been measured optically and electrically, and the results have been examined within
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