Electronic density of states in highly tetrahedral amorphous carbon

Abstract

Highly tetrahedral amorphous carbon (ta-C) films and its hydrogenated form (ta-C:H) have been deposited using a filtered cathodic arc. In this paper, the optical and electronic band-gaps of amorphous diamond and its hydrogenated form ta-C:H, are shown to be in the range 2 eV. The H content of ta-C:H is found to be about 3 at.% using elastic recoil detection analysis (ERDA) and electron energy loss spectroscopy (EELS) is used to show that ta-C:H does keep its tetrahedral structure. Both materials are found to be stable at temperatures up to 1000 K. The room temperature conductivity of 10 −7−10 −8 Ω −1 cm −1 is shown to be thermally activated, with activation energies in the range 0.2–0.3 eV. A detailed study made from the current-voltage characteristics of ta-C/ p-Si heterojunctions shows that current flow is space-charge limited (SCLC) and influenced by bulk traps. The density of states profile, N( fE), in the region of the quasi-Fermi level is calculated using the differential method and found to be of the order 10 18 cm −13 eV −1. Optical absorption data is coupled with the electrical results to confirm that the valence band density of states is of the order 10 21 cm −13 eV −1. It is also shown that there is a reduction of nearly one order of magnitude in the N( E) at the quasi-Fermi level upon hydrogenation. SCLC measurements are complemented with surface admittance measurements (SAM) which yield a value of 10 11 cm −2 eV −1 for the interfacial density of states at the ta-C/ p-Si junction. A decrease in N ss is also observed at the ta-C:H/ p-Si interface in agreement with the SCLC results.