Field-Emission Properties of Vertically Aligned Carbon Nanotube Cathodes of Varying Geometries

This work characterizes the bulk emission properties of carbon nanotube (CNT) forest cathodes fabricated with various geometries. Geometries explored include dense nanotube forests of varying height grown on $5\times 5$ mm silicon (Si) substrates and discrete, patterned CNT pillars fabricated using UV photolithography. Dense forests heights ranged from $526~\mu \text { m}$ to 1.41 mm, with packing fraction for dense forest calculated to be 4e10 nanotubes/cm2 based on an average nanotube separation distance of 100 nm for fixed growth dense forests. Patterned sample micro pillar heights ranged from 47 to $393~\mu \text { m}$ with pillar widths on tested samples ranging from 250 to $270~\mu \text { m}$ . Properties explored include emission current, turn-on field, and emission current performance over time. A parallel plate electron beam diode with a 100- $\mu \text { m}$ A-K gap and an automated test apparatus were developed to provide a configurable experiment that provides accurate and repeatable measurements for dc, dc sweep, and timed performance testing. Operating voltages for the voltage sweeps spanned from 0 to −350 V. Testing has shown evidence of a hysteresis effect on the emission current tied to the applied field history as well as shifting of the turn-on field magnitude throughout the testing period, suggesting a conditioning effect during use. Three separate emission regions in the I–V curves during sweep testing have also been observed. In the geometric study, dense forests and patterned samples were sweep tested up to a peak applied voltage of −250 V, with the taller samples generally performing better. Currents produced in the geometric study from the dense forest emitters ranged from 36.8 to $572.34~\mu \text { A}$ , with current densities ranging from 15 to 2.29 mA/cm2. Currents produced from the patterned micropillar emitters ranged from 39.4 to $317.51~\mu \text { A}$ , with current densities ranging from 67 to 3 mA/cm2. DC time testing showed a relatively stable output current over a 4.5-h testing period. The findings of this work aim to explore CNT emitters as a viable alternative to thermionic cathodes used in large RF systems and to characterize connection between CNT emitter geometry and the resulting emission performance. The insights gained from this work will be used for informed design and optimization of future emitters.