Prospects and Limitations of Carbon Nanotube Field Emission Electron Sources

Today the most mature technology to produce gated micro field electron emitter arrays is the so-called Spindt-type metal micro-tip process. The drawbacks of the Spindt-type process are the expensive production, the critical lifetime in technical vacuum and the high operating voltages. Carbon nanotubes (CNT) can be regarded as the potential second-generation technology to Spindt-type metal micro-tips. The use of CNT as field enhancing structures in field emission electron sources can bring several advantages such as longer lifetime and operation in poor vacuum due to the high chemical inertness as well as low operation voltages and perhaps most important very low cost production techniques. In the present contribution we show that the field electron emission (FE) of CNT thin films can be accurately described by Fowler-Nordheim tunneling and that the field enhancement factor ? influences the emission properties most prominently. We have used scanning anode field emission microscopy to investigate the local field emission properties of randomly oriented carbon nanotube deposits. In the technically interesting applied electric field range of 30 V/?m an emission site density larger than 5×106 cm?2 could be measured. We will discuss however that emitter degradation at high emission currents limits the full exploitation of this high emission site density. The emission degradation becomes apparent for emission currents in the ?A range for a single emitter and the field emission I-V characteristics suggests that power dissipation due high contact or intra CNT resistance is the cause of the emitter degradation. Therefore, although the fundamental properties of CNTs are very favorable for the use as field emission tips, these properties alone will not guarantee their success in this area. Our investigations clearly show that a perfect control of the catalytic CNT growth process is needed for successful CNT field emitter technology, at least for high current applications