Fourier transform infrared spectroscopic study of nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films, which possess n-type conduction with enhanced electrical conductivities, were prepared by pulsed laser deposition and they were structurally studied by Fourier transform infrared (FTIR) spectroscopy. The film with a nitrogen content of 7.9 at.% possessed n-type condition with an electrical conductivity of 18 S/cm at 300 K. The FTIR spectra revealed peaks due to nitrogen impurities, C = N, C-N, and CH_n (n = 1, 2, 3) bands. The sp²-CH_n/(sp²-CH_n + sp³-CH_n), estimated from the area-integration of decomposed peaks, were 24.5 and 19.4% for undoped and 7.9 at.% doped films, respectively. The nitrogen-doping not only form the chemical bonds between carbon and nitrogen atoms such as C = N and C-N bonds but also facilitate the formation of both sp² and sp³ bonds, in particular, the sp³-CH_n bond is preferentially formed. From the analysis of the FTIR spectra, it was found that the hydrogen content in the film is increased with an increase in the nitrogen content. The increased hydrogen content might be owing to the enhanced volume of grain boundaries (GBs) between UNCD grains, and those between UNCD grains and an a-C:H matrix, which is caused by a reduction in the UNCD grain size. The CH_n peaks predominantly come from an a-C:H matrix and GBs. Since the nitrogen-doping for a-C:H has been known to be hardly effective, the n-type conduction with the enhanced electrical conductivities might be attributed to the sp²-CH_n formation at the GBs.