Impact of acid etching on surfaces and near-surface region in nitrogen plasma terminated polycrystalline diamond

Abstract

In this study, the influence of tri-acid: HClO₄:H₂SO₄:HNO₃ (1:3:4) etching on the surface and near-surface chemical properties of radio frequency (RF) nitrogen plasma terminated polycrystalline diamond (PCD) surfaces is reported. From the C(1s) and N(1s) high resolution X-ray photoelectron spectroscopy (HR-XPS) measured as a function of photon energy, it is clearly shown that acid etching reduced the thickness of the disordered C(sp²) layer created by the nitrogen plasma treatment exposing an upper surface of a larger C(sp³) character alongside the formation of C-O_x bonds. This is accompanied by a reduction in the nitrogen surface concentration and the N(1s) XP peak full width at half maximum (FWHM), suggesting a more homogeneous nitrogen bonding to the diamond surface. High resolution electron energy loss spectroscopy (HREELS) shows that NH(ads) species produced by the plasma processes are preferentially etched alongside the formation of C=O(ads), C-O(ads), and COOC(ads) species, whereas hydrogen is bonded in various CH_y(ads) configurations. The C K-edge NEXAFS spectra show characteristic peaks in the pre-edge structure clearly associated with nitrogen bonding. Following acid etching, the intensities of peaks related to defect states are reduced, whereas the diamond second band-gap at 302.4 eV increases in intensity. N K-edge NEXAFS of the nitrogen plasma terminated surfaces show peaks associated to C-N(ads) and C=N(ads) bonding, which acid etching results in the preferred depopulation of C=N(ads) bonds.