Thermal stability of amorphous carbon films grown by pulsed laser deposition

The thermal stability in vacuum of amorphous tetrahedrally coordinated carbon (a-tC) films grown on Si has been assessed by in situ Raman spectroscopy. Films were grown in vacuum on room-temperature substrates using laser fluences of 12, 22, and 45 J/cm(2) and in a background gas of either hydrogen or nitrogen using a laser fluence of 45 J/cm(2). The films grown in vacuum at high fluence (>20J/cm(2)) show little change in the a-tC Raman spectra with temperature up to 800 degrees C. Above this temperature the films convert to glassy carbon (nanocrystalline graphite). Samples grown in vacuum at lower fluence or in a background gas (H-2 or N-2) at high fluence are not nearly as stable. For all samples, the Raman signal from the Si substrate (observed through the a-tC film) decreases in intensity with annealing temperature indicating that the transparency of the a-tC films is decreasing with temperature. These changes in transparency begin at much lower temperatures (similar to 200 degrees C) than the changes in the a-tC Raman band shape and indicate that subtle changes are occurring in the a-tC films at lower temperatures. (C) 1996 American Institute of Physics.