Structural changes in diamond and amorphous carbon induced by low-energy ion irradiation

In the present work we describe an investigation of the influence of low-energy ion irradiation (1-keV Ar+) on the surface structure of polycrystalline diamond and amorphous carbon films with various degrees of graphitization. Photoelectron spectroscopy (PES) with excitation energies in the ultraviolet and x-ray regime is employed to monitor the radiation-induced modification of the electronic structure of the surface which is closely linked to the local bonding environment of the carbon atoms.;A comparison of the mean photoelectron escape depth and the thickness of the irradiation affected layer also illustrates the suitability of PES for this investigation. For the chemical vapor deposition (CVD)-diamond film a gradual change from typical diamond features to amorphous carbon is observed for ion doses surpassing 6x10(14) cm(-2). The structural changes in the diamond lattice are expressed in a broadening of the C Is core-level peak, and increasing contributions from p-pi states around 3-4 eV in the valence-band spectra. Likewise the peak located at 13 eV (He II, h nu=40.82 eV), characteristic of diamond, is no longer apparent for ion doses exceeding 3x10(15) cm(-2). The diamond surface clearly shows a tendency to amorphize rather than graphitize under ion irradiation. To complement the results for the diamond film, we irradiated two amorphous carbon films with different microstructures: (A) a predominantly amorphous film, and (B) a film with graphitic inclusions. The destruction of graphitic structures in film (B) is apparent even for the lowest ion dose (1.5x10(14) cm(-2)), and expressed in an increase in the width of the C Is core-level peak and a smearing out of the valence-band spectral features. The resultant valence-band and core-level spectra (total ion dose: 3x10(15) cm(-2)) are very similar to the one obtained for the amorphous film (A), which, on the other hand, does not show any significant changes in its structure upon irradiation. The irradiation of both diamond and graphitic structures with low-energy Arf ions leads to the formation of a predominantly amorphous surface layer.