Pulsed-laser deposition of carbon: from DLC to cluster-assembled films

Carbon films have been synthesized on (100) Si substrates at room temperature by KrF Excimer Pulsed Laser Deposition (PLD) of highly oriented pyrolitic graphite. Changing laser power density (from 0.5 to 19 MW mm-2) and ambient conditions (vacuum; molecular nitrogen at 1 Pa; helium, from 0.6 Pa to 2 kPa), a range of structures and microstructures, from DLC to nanosized cluster assembled (CA) films were obtained. The microstructure and morphology of the films were studied by Scanning Electron Microscopy. The surface of all films deposited in vacuum and in nitrogen appears uniform, flat, featureless. In CA films, the nature-pressure of ambient gas and the energy deposited by each laser pulse determine threshold fluences of increasing value. These correspond to the sequence of dense columns, followed by nodule-like morphology and by an open dendritic structure. Atomic Force Microscopy on several CA films allowed to estimate the size distribution and relative abundancies of aggregates of carbon clusters. Film morphology is correlated with the ballistic mechanisms that occur when the plasma plume constituents interact with ambient gas atoms. For all films, local carbon coordination was studied by visible Raman spectroscopy. UV Raman spectra were collected for vacuum- and nitrogen-deposited films. The films are hydrogen-free, as shown by Fourier Transform Infra-Red spectroscopy, and result invariably structurally disordered. Vacuum-deposited films undergo a transition from mainly disordered graphitic to up to 80% tetrahedral amorphous carbon (ta-C) above a threshold power density. Transmission Electron Energy Loss spectra on selected samples confirmed ta-C formation. The same transition is absent in films deposited in N2 atmosphere. All CA films are structurally disordered, sp2 coordinated, and belong to the family of carbon nanoglasses. The mechanisms underlying the graphite to ta-C transition are discussed with reference to the different atmospheres