Efficient high-current field emission from arrays of CNT columns

At present carbon nanotubes (CNT) are the most prospective cathode material for triode applications because of their strong field emission (FE) at low electric fields. The FE homogeneity of flat CNT cathodes, however, is still limited by their rather fast and uncontrolled growth which usually leads to strongly varying field enhancement and current carrying capability of the individual emitters. Therefore, patches with multiple emitters might provide a suitable strategy to improve the homogeneity and current stability of CNT cathodes. Accordingly, structured arrays of CNT based columns on flat n-Si substrates were fabricated applying the atmospheric-pressure CVD method with a volatile catalyst source (ferrocene/xylene mixture). Substrates of 6x6 mm² size and 4 quadrants of 2x2 mm² with varying patch diameter and two pitch sizes were structured by means of photolithography and selective chemical etching of the thermal 0.3 mum thick SiO₂ oxide layer. Preferential growth of CNT in the opened windows of the SiO₂ layer was achieved by adjusting the parameters of the synthesis process, i.e. the temperature in the reaction zone, the growth time, the concentration of the catalyst in the feeding solution, the gas-carrier flow rate etc. as described elsewhere. The resulting multiwall CNT for growth times of 30 s and 2 min formed vertically aligned arrays of uniform columns of about 20 mum and 50 mum net height with a pitch to patch ratio of 160/50, 100/50, 100/30 and 100/10 (mum). Obviously, preferential CNT column growth embedded in a floor of shorter CNT is fairly achieved.