Bonding structure in nitrogen doped ultrananocrystalline diamond

Abstract

The transport properties of diamond thin films are well known to be sensitive to the sp2/sp3-bonded carbon ratio, the presence of the grain boundaries and other defects, and to the presence of various impurities. In order to clarify the roles these factors play in the conduction mechanisms of nitrogen-doped ultrananocrystalline diamond (UNCD), Raman scattering, near edge x-ray absorption fine structure (NEXAFS), soft x-ray fluorescence (SXF), and secondary ion mass spectroscopy (SIMS) measurements were performed. Transmission electron microscopy analysis of nitrogen doped UNCD has previously indicated that the films are composed of crystalline diamond nano-grains with boundaries of amorphous carbon, and NEXAFS measurements reveal that the global amount of sp2-bonded carbon in these films increases slightly with nitrogen doping. The nitrogen content is quantified with high-resolution SIMS analysis, while NEXAFS and SXF indicates that the nitrogen exists primarily in tetrahedrally coordinated sites. These measurements indicate that the overall grain boundary volume of nitrogen-doped ultrananocrystalline diamond is increasing, while the grains themselves remain pure diamond. This supports our previously reported hypothesis that grain boundary conduction is the mechanism for the observed increase in conductivity in ultrananocrystalline diamond with nitrogen doping.