Dynamic graphitization of ultra-nano-crystalline diamond and its effects on material resistivity

Detailed structural and electrical properties of ultra-nano-crystalline diamond (UNCD) films grown in H2/CH4/N2 plasma were systematically studied as a function of deposition temperature (Td) and nitrogen content (% N2) to thoroughly evaluate their effects on resistivity. It was found that even the films grown with no nitrogen in the synthetic gas mixture could be made as conductive as 10−2 O cm. The overall resistivity of all the films was tunable over 4 orders of magnitude through varying growth parameters. On a set of 27 samples, Raman spectroscopy and scanning electron microscopy show a progressive and highly reproducible material phase transformation from ultra-nano-crystalline diamond to nano-crystalline graphite as deposition temperature increases. The rate of this transformation is heavily dependent on the N2 content estimated by secondary ion mass spectroscopy. The addition of nitrogen greatly increases the amount of 𝑠𝑝2 bonded carbon in the films, thus enhancing the physical connectivity in the grain boundary (GB) network that has high electronic density of states. However, the addition of nitrogen greatly slows down crystallization of the 𝑠𝑝2 phase in the GBs compromising electron transport. Therefore, the proper balance between GB connectivity and crystallinity is the key in resistivity engineering of UNCD.