The inner workings of sp2 clusters in nitrogen-incorporated tetrahedral amorphous carbon thin films according to optical analyses

Abstract

The conduction mechanism in nitrogen-incorporated tetrahedral amorphous carbon (ta-C: N) thin films is mainly attributed to the conjugation in sp2 clusters. Here, ta-C:N thin films (about 100 nm) were deposited by laser controlled pulsed cathodic vacuum arc (Laser-Arc) at different nitrogen gas flow rates to investigate the electrical properties of such films. Mechanical and structural properties indicate an increase in sp2 fraction with a rise in nitrogen incorporation into the deposited films. However, after an initial decrease in electrical resistivity, it plateaus at higher flow rates while the sp2 content is still rising. Hence, to gain better understanding of the electronic properties, the dielectric constants ε1 and ε2 were obtained from the optical constants [n] and [k], which were numerically determined from reflectance [R] and transmittance [T] measurements in the wavelength range from 190 to 2500 nm (0.5 to 6.5 eV). The dielectric constant ε1 was used to calculate the dielectric volume of the atom, which is interpreted as a measure of the conjugated π electron system. Furthermore, modeling of the dielectric constants yielded the number of conjugated electrons per atom. Through the dielectric volume and number of conjugated electrons, it was discovered that the electrical conductivity does not only depend on the sp2 content and cluster size, but on how many electrons are conjugated and how much space these electrons occupy. The ta-C:N samples with the highest electrical conductivity, were either characterized by the largest occupied volume or the highest number of conjugated electrons.