Origin of a needle-like granular structure for ultrananocrystalline diamond films grown in a N2/CH4 plasma

Microstructural evolution as a function of substrate temperature (T S) for conducting ultrananocrystalline diamond (UNCD) films is systematically studied. Variation of the sp2 graphitic and sp3 diamond content with T S in the films is analysed from the Raman and near-edge x-ray absorption fine structure spectra. Morphological and microstructural studies confirm that at T S = 700 °C well-defined acicular structures evolve. These nanowire structures comprise sp3 phased diamond, encased in a sheath of sp2 bonded graphitic phase. T S causes a change in morphology and thereby the various properties of the films. For T S = 800 °C the acicular grain growth ceases, while that for T S = 700 °C ceases only upon termination of the deposition process. The grain-growth process for the unique needle-like granular structure is proposed such that the CN species invariably occupy the tip of the nanowire, promoting an anisotropic grain-growth process and the formation of acicular structure of the grains. The electron field emission studies substantiate that the films grown at T S = 700 °C are the most conducting, with conduction mediated through the graphitic phase present in the films.