Growth of Carbon Nitride Thin Films by Low-energy Ion Beam Assisted Evaporation: on the Production of Fullerene-like Microstructure

Low-energy (<100eV) ion bombardment has been shown to be a key parameter in the production of hard and elastic carbon nitride (CNx) thin films with fullerene-like (FL) structure [1]. The production of FL-CNx is mostly reported for reactive magnetron sputtering but still the growth mechanisms are not clear [2]. In this way, the growth by ion beam assisted deposition (IBAD) would provide a better understanding since the source of neutrals and ions are independent. In this work we study the growth of CNx films produced by e-beam evaporation of graphite and simultaneous low-energy N2 ion assistance at different substrate temperatures. The low-energy ions were provided by an End-Hall ion source. The samples were characterized by spectroscopic ellipsometry, elastic recoil detection analysis (ERDA), Raman and high resolution transmission electron microscopy (HRTEM). Despite the low-energy ions, a high thermal-activated re-sputtering of the deposited films has been observed, indicating its chemical origin. In addition, the maximum nitrogen content in the films is limited to ~25 at.%, which should be related to the re-sputtering process. The Raman spectra and the optical properties indicate the dominance of sp2 hybrids. However, the results differ from those of FL-CNx, indicating that the sp2 hybrids are not arranged in a FL microstructure. This is also corroborated by HRTEM, where the samples were found to be mainly amorphous. Therefore, our results suggest that low-energy ion bombardment is not the main driving force for the promotion of FL arrangements, although this condition may be necessary to avoid damage on the growing microstructure. In this sense, the incorporation of neutrals, such as CxNy species formed during magnetron sputtering, may play a dominant role for the growth of FL-CNx [3].
1. Sjöstrom et al. Phys. Rev. Lett. 75 (1995) 1336. 2. Hultman et al. MRS Bulletin 28/3 (2003) 194. 3. Neidhardt et al. J. Appl. Phys. (in press).