Effects of Sulfur Concentration on the Electron Field Emission Properties of Nanocrystalline Carbon Thin Films
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Effects of Sulfur Concentration on the Electron Field Emission Properties of Nanocrystalline Carbon Thin Films S. Guptaa, B. R. Weinerb, B. L. Weissa, G. Morellc a Department of Physics, University of Puerto Rico, San Juan, PO Box 23343, PR00931, USA b Department of Chemistry, University of Puerto Rico, San Juan, PO Box 23346, PR00931, USA c Department of Physical Sciences, University of Puerto Rico, San Juan, PO Box 23323, PR00931, USA ABSTRACT The electron field emission properties of sulfur-assisted nanocrystalline carbon (n-C: S) thin films grown on molybdenum substrates by hot-filament CVD technique using methanehydrogen (CH4/H2) and hydrogen sulfide-hydrogen (H2S/H2) gas mixtures were investigated. The field emission properties of the S-assisted films are reported as a function of sulfur concentration. The incorporation of S caused structural and microstructural changes that were characterized with SEM, AFM and Raman spectroscopy (RS). The S-assisted films show smoother surfaces and smaller grains than those grown without. The lowest turn-on field measured was around 4.5 - 5.0 V/µm films grown with 500 ppm of hydrogen sulfide and at 900 oC. The electron field emission properties of S-assisted films were also compared to those grown without sulfur (i.e., intrinsic). An inverse correlation between the threshold field (Ec) and sulfur concentration was found. These finding are attributed to defect induced states within the electronic band structure. INTRODUCTION Cold cathodes as electron field emitters are potentially useful for field emission displays (FEDs) [1-3]. Recently, electron field emission (EFE) from diamond, disordered carbon and diamond-like carbon (DLC) attracted a great deal of interest for applications that require thin film cathodes and low threshold fields, in contrast to sharp metal tips (Spindt), such as flat panel displays (FPDs) [4-6]. In addition, DLC FED’s are expected to be more cost-effective than the Spindt tips due to their complicated fabrication process [1]. Moreover, the excellent mechanical and chemical stability of carbon-based materials will result in highly reliable and stable emitters, even under extreme conditions. In spite of the fact that the original work on electron field emission from diamond was motivated by the discovery of its negative electron affinity (NEA) [7], the ease of emission from many carbon materials suggests that the NEA is not a prerequisite. So far, there is lack of agreement regarding the mechanism(s) for FE, and more general emission models are desirable [8]. Although, much research has been devoted to the emission from disordered [5] and nanostructured carbon [9], a detailed investigation of the influence of impurity incorporating elements concentration is of interest for application issues as well as for fundamental understanding. The field emission properties for the undoped and doped (N, P, B) materials have been studied by several authors [9-14]. In this study, we report our findings of the EFE properties on n-C: S thin films grown by HFCVD as a func
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