The effect of nitrogen partial pressure on the bonding in sputtered CNx films: implications for formation of β-C3N4

An investigation of the bonding structure in carbon nitride films deposited by DC magnetron sputtering using a Penning-type opposed target system has been carried out. The changes in the valence band structure have been measured by XPS, UPS and EELS. The density of unpaired electrons has been measured by ESR. As the nitrogen partial pressure and the nitrogen content of the films is initially increased, the structure shows a greater degree of sp2 bonding compared to the pure carbon samples. However, with further increases in the nitrogen partial pressure up to 100% of the sputtering gas pressure, which has very little effect on the nitrogen content of the films, the bonding becomes more sp3-like in character and there is evidence for a reduction in the network terminating CN bonding. Under these circumstances the sp3-like nature becomes comparable to or greater than that for un-nitrogenated films. These results point to the reasons why crystalline β-C3N4 material has been found in these films only with high nitrogen partial pressure even though the nitrogen content is not significantly enhanced in this situation.