Investigations of the electron field emission properties and microstructure correlation in sulfur-incorporated nanocrystalline carbon thin films

Abstract

Role of sp 2 C cluster size on the field emission properties of sulfur-incorporated nanocomposite carbon thin films Appl.Results are reported on the electron field emission properties of sulfur ͑S͒-incorporated nanocrystalline carbon (n-C:S) thin films grown on molybdenum ͑Mo͒ substrates by hot-filament chemical vapor deposition ͑HFCVD͒ technique. In addition to the conventionally used methane (CH 4 ) as carbon precursor with high hydrogen (H 2 ) dilution, hydrogen sulfide-hydrogen (H 2 S)/H 2 premix gas was used for sulfur incorporation. The field emission properties for the S-incorporated films were investigated systematically as a function of substrate temperature (T S ) and sulfur concentration. Lowest turn-on field achieved was observed at around 4.0 V/m for the n-C:S sample grown at T S of 900°C with 500 ppm of H 2 S. These results are compared with those films grown without sulfur (n-C) at a particular T S . The turn-on field was found to be almost half for the S-assisted film thus demonstrating the effect of sulfur addition to the chemical vapor deposition process. An inverse relation between turn-on field (E C ), growth temperature and sulfur concentration was found. The S incorporation also causes significant microstructural changes, as characterized with non-destructive complementary ex situ techniques: scanning electron microscopy ͑SEM͒, atomic force microscopy ͑AFM͒, and Raman spectroscopy ͑RS͒. S-assisted films show relatively smoother and finer-grained surfaces than those grown without it. These findings are discussed in terms of the dual role of sulfur in enhancing the field emission properties by controlling the sp 2 C cluster size and introducing substantial structural defects through its incorporation. The in-plane correlation length (L a ) of sp 2 C cluster was determined from the intensity ratio of the Dand G-bands ͓I(D)/I(G)͔ in the visible RS as a function of deposition temperature and sulfur concentration using a phenomenological model. The turn-on field was found to decrease with increasing sp 2 C cluster size in general ranging from 0.8 to 1.4 nm. The films having sp 2 C clusters of around 1.4 nm had the lowest turn-on field and steep rising emission currents, providing an estimate of optimum size for L a for the material grown hereby. These findings are assessed in terms of a reduced field emission barrier brought about by the sulfur addition and the need for relatively longer conductive paths capable of withstanding the relatively large emission currents. It is because the sp 2 C cluster size predominate the chemical environment, chemical order, sp 3 content or local conductivity. Besides, although most of the S is expected to be electrically inactive, under the high doping conditions ͑larger S/C͒ hereby employed, there may be some amount of S in donor states, an indication of the availability of conduction electrons. These results also suggest that the behaviors of sulfur-incorporated nanocrystalline carbon thin films are closer to that grown with phosphorus ͑P͒ and Nitrogen ͑N͒ eleme...

Enhancement of electron field emission property with silver incorporation into diamondlike carbon matrix

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