Influence of nitrogen on the structure and nanomechanical properties of pulsed laser deposited tetrahedral amorphous carbon

The effect of nitrogen addition on the structure and nanomechanical properties of tetrahedral amorphous carbon, tα-C, has been studied. The tα-C films were grown on Al₂O₃-TiC substrates by reactive pulsed KrF excimer laser ablation of graphite targets at a laser fluence of 10 J cm^-2. Nitrogen contents up to 19 at.% were obtained by increasing the nitrogen partial pressure, P_N2, to 75 mTorr. The sp³ content in the tα-C film as determined by analysis of the XPS C 1s core level spectra had a value of about 76%. Incorporation of a small amount of nitrogen, 2 at.%, reduces the clustering of the sp² phase and improves the nanomechanical properties of the tα-C films, whilst for higher nitrogen concentrations the carbon bonding changes progressively from sp³ to sp². Quantitative analysis of the Raman spectra indicated that incorporation of nitrogen greater than 2 at.% induced a progressive long-range order in the amorphous carbon and an increase in the size of sp² graphitic clusters. Additionally, Raman spectroscopy established the presence of C≡N bonds at high P_N2. To elucidate the influence of the substrate on the measurement of the nanomechanical properties of thin film a continuous measure of hardness and modulus as a function of depth was performed. Both the hardness and Young's modulus were significantly reduced from 56 and 573 GPa for CN_0.02 to 2 and 44 GPa for CN_0.19 at a contact depth of 25 nm. The deterioration of the nanomechanical properties with N incorporation is consistent with the spectroscopic results, which indicate a structural transformation from an amorphous structure consisting predominately of sp³ C bonds to an sp² graphitic-like phase.