Microstructural evolution as a function of film thickness of nitrogen incorporated ultrananocrystalline diamond (NUNCD) films, grown using bias-enhanced microwave plasma chemical vapor deposition with gas mixtures of N2/CH4, is systematically investigated. It is observed that by controlling the growth time, the morphology, the microstructure, and the electrical properties of NUNCD films can be manipulated. The growth of NUNCD films starts with the formation of amorphous carbon on Si surface prior to the nucleation of diamond. In the growth time of 10 min, the films retain rod-shaped diamond grains, whereas in the films grown for 30 min, needle-like diamond grains are formed, which comprises a diamond core encased in a sheath of sp2-bonded graphite phase. On increasing the growth time to 60 min, the growth of acicular grains ceases and large proportion of graphite clusters or defective diamond clusters (n-diamond) is formed. The salient features of such materials with unique granular structure are that their electrical properties can be tuned in wide range such that they are especially useful in practical applications. High-electrical conductivity and superior field emission properties of nitrogen incorporated ultrananocrystalline diamond films are achieved by the needle-like diamond grains encased by sp2-bonded graphitic grain boundaries.
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