Electron Field Emission from Undoped and Doped DLC Films
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ABSTRACT Electron field emission and electrical conductivity of undoped and nitrogen doped DLC films have been investigated. The films were grown by the PE CVD method from CH 4:H2 and CH4 :H2 :N2 gas mixtures, respectively. By varying nitrogen content in the gas mixture over the range 0 to 45%, corresponding concentrations of 0 to 8 % (atomic) could be achieved in the films. Three different gas pressures were used in the deposition chamber: 0.2, 0.6 and 0.8 Torr. Emission current measurements were performed at approximately 10-6 Torr using the diode method with emitter-anode spacing set at 20 gm. The current - voltage characteristics of the Si field electron emission arrays covered with DLC films show that threshold voltage (Vth) varies in a complex manner with nitrogen content. As a function of nitrogen content, Vth initially increases rapidly, then decreases and finally increases again for the highest concentration. Corresponding Fowler-Nordheim (F-N) plots follow F-N tunneling over a wide range. The F-N plots were used for determination of the work function, threshold voltage, field enhancement factor and effective emission area. For a qualitative explanation of experimental results, we treat the DLC film as a diamond-like (sp 3 bonded) matrix with graphite-like inclusions. INTRODUCTION DLC films are very attractive for field emitter applications. Previous results demonstrated improved field emission from silicon tips by coating DLC films [1-6]. Also, nitrogen doping is known to strongly influence emission properties of DLC films [7-10]. Nevertheless, the effect of deposition conditions and nitrogen doping on field emission properties of DLC films remains poorly understood. In this work, the electron field emission properties of silicon tip arrays coated with undoped and nitrogen doped DLC films are investigated. The properties of the DLC films strongly depend on the microstructure which can be varied by the deposition conditions. The influence of DLC deposition conditions on electron field emission is studied in detail. The correlation between the film properties and field emission characteristics is investigated. EXPERIMENT Cathode formation The arrays of silicon emitter tips were fabricated by wet chemical etching of (100) Si n-type wafers
(Nd=I015 cm"3) patterned with Si3N4 masking material. Tip sharpening was performed by oxidation of
the as-etched tips at 900 0 C in wet oxygen. The oxide formed is then removed in HF:H 2 0 solution. This sharpening technique allows the production of tips with a radius of curvature of 10-20 nm and 2 5 2 height of 4jim. Our arrays have been fabricated over areas of 8x8 cm with 2.5x10 tips/cm . The radii before and after DLC film deposition were estimated by scanning electron microscopy. DLC films with thickness in the range 60-80nm were grown on flat silicon wafers and silicon tip arrays by plasma enhanced chemical vapor deposition (PE CVD) from a CH 4 :H2 :N2 mixture. Nitrogen content in the gas mixture was varied within the range from 0 to 45%. In-situ-gas-phase doping al
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