Hard amorphous carbon (ta-C) films were implanted with 20 keV N+ ions into saturation at two different temperatures. Monitoring the composition ratio [N]/[C] using in situ elastic recoil detection analysis, saturation levels [N]/[C] of 0.35 and 0.16 have been found for implantations at room temperature and 400 °C, respectively. Raman and x-ray photoelectron spectroscopy analyses of the implanted films indicate the presence of N2 molecules inside room-temperature implanted samples, but not for the case of implantation at 400 °C. Cross-section transmission electron microscopy images show the presence of near-surface voids, interpreted as remnants of N2-filled bubbles, in the former case. Annealing experiments show that about 50% of the total N inventory consists of nitrogen only weakly trapped inside the carbon matrix in room-temperature implanted films, this fraction being significantly reduced in samples held at 400 °C during implantation. © 1997 American Institute of Physics.