Electron field emission from polycrystalline diamond films
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Electron field emission from polycrystalline diamond films Qing Zhang, S.F. Yoon, J. Ahn, Bo Gan, and Rusli Microelectronics Centre, Nanyang Technological University, Singapore (Received 19 February 1999; accepted 28 September 1999)
The influence of the carbon network structure of polycrystalline diamond films that were prepared from a mixture of H2, CH4, and N2 using microwave-enhanced plasma chemical vapor deposition on electron field emission has been systematically investigated. With increasing nitrogen gas flow ratio of [N2]/[H2 + CH4 + N2], the film hardness and surface roughness of the as-grown films decreased, and the concentration ratio of amorphous sp2-bonded carbon clusters and mixed sp2–sp3 carbon structures to tetrahedrally bonded amorphous carbon phases increased. Correspondingly, the turn-on voltage for electron emission decreased. After the surface post-treatment by pure hydrogen plasma exposure, the concentration ratio was clearly found to have increased dramatically and the turn-on voltage decreased significantly for the films produced at small nitrogen flow ratio. Our results suggest that the influence of the concentration ratio on electron field emission is much more significant than that of the surface roughness of the polycrystalline diamond films studied in this paper. I. INTRODUCTION
Chemical-vapor-deposited (CVD) diamond films are known for their chemically inert properties and low, or even negative, electron affinity.1–4 These sole characteristics make CVD diamond-based electron emitters more favorable for emitting electrons at a lower electric field and for a longer time under a higher emission current density, compared to silicon and other metallic emitters.5 However, it has been found that negative electron affinity only plays a limited role in the electron field emission. The electron emission from CVD diamond films is strongly affected by several other factors. A high nitrogen incorporation in polycrystalline diamond films has been reported to reduce the emission turn-on voltage significantly.6 Incorporated non-diamond phases (especially graphite clusters) can greatly enhance the emissivity.7–10 Besides these, the presence of microprotrusions on the emitting surface11,12 and hydrogen plasma surface treatment20 have also been observed to have significant effects on the emissivity. In this paper, we focus on the electron field emission from polycrystalline diamond films prepared using a microwave plasma CVD system from a mixture of H2, CH4, and N2. The carbon network structure and film surface morphology are presented to show the correlation with the field emission characteristics. The influence of pure hydrogen plasma surface treatment on the field emission characteristics is also examined. II. EXPERIMENTAL CONDITIONS
Polycrystalline diamond films used in this study were prepared using the AsTex microwave plasma deposition system. The substrate used was a highly phosphorus 212
J. Mater. Res., Vol. 15, No. 1, Jan 2000
doped (100) silicon
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