Ultraviolet and visible Raman spectroscopic investigations of nanocrystalline carbon thin films grown by bias‐assisted hot‐filament chemical vapor deposition

Abstract

AbstractThe structural bonding in a series of nanocrystalline carbon thin films grown under different substrate biasing conditions was analyzed using Raman spectroscopy in both the visible and ultraviolet (uv) regimes of the spectrum. The nanocrystalline carbon thin films studied were deposited by the hot‐filament chemical vapor deposition (HFCVD) technique using a 2% concentration of methane in hydrogen. The films were deposited on molybdenum substrates under various substrate biasing conditions. A positive bias (forward) produced a continuous flow of electrons from the filament on to the substrate, whereas a negative bias (negative) caused the substrate to be bombarded with positive ions. Films were also grown under no bias, for comparison. Differences in the Raman spectra obtained with visible (λL = 514.5 nm) and uv (λL = 244.0 nm) excitation sources were investigated. Apart from the basic features consisting of D and G bands at around 1360 and 1580 cm−1 in the case of visible Raman spectra, an extra feature at around 1060 cm−1, denoted a T band, appeared in the uv Raman spectra only. Hence, uv Raman scattering measurements clearly revealed the presence of sp3‐bonded carbon atoms. The position and its intensity ratio with respect to the G peak [(I(T)/I(G)] were used to provide a reliable means to measure the sp3 C bonding fraction, which is both semi‐quantitative and non‐destructive. The sp3 content estimated within the microstructure of n‐C thin films was found to be around 60–80%. Further, probing the samples with two different photons (visible and uv) allowed us to estimate qualitatively the amount and clustering of sp2 sites. This technique provided a fast and reliable microstructural characterization of disordered carbons. The dispersion in the Raman features is specific to each carbon system and, therefore, can be used as a fingerprint. These findings point at the similarities of bias‐assisted HFCVD nanocrystalline carbon (n‐C) materials and the tetrahedrally bonded amorphous carbon (ta‐C) materials grown by ion‐beam assisted deposition (IBAD). Copyright © 2003 John Wiley & Sons, Ltd.