Study of electron field emission from arrays of multi-walled carbon nanotubes synthesized by hot-wire dc plasma-enhanced chemical vapor deposition

Multi-walled carbon nanotubes have been grown on 7 nm Ni-coated substrates consisting of 300 μm thick highly n-doped (1 0 0) silicon covered with a diffusion barrier layer (10 nm thick) of SiO₂ or TiN, by combining hot-wire chemical vapor deposition and direct current plasma-enhanced chemical vapor deposition at low temperature (around 620 °C). Acetylene gas was used as carbon source and ammonia and hydrogen were used either for dilution or etching. Growth of dense aligned nanotubes could be observed only if the ammonia content was minimized (∼5%). In order to improve the electron field emission properties of the films, different geometrical factors have been taken into account: average length, length/radius ratio and spacing between nanotubes. The nanotube growth rate was controlled by the substrate temperature and the pressure in the reactor, and the nanotube height by the growth time. The nanotube diameter was controlled by the catalyst dot volume, and the nanotube spacing was adjusted during the patterning process of the catalyst dots. Using optical lithography, 1 μm Ni dots were obtained and several multi-walled nanotubes with diameter and length in the range 60-120 nm and ∼2.3 μm were grown on each dot. Thus, based on a two-dimensional square lattice with a lattice translation vector of 4 μm, I-V characteristics yielded an onset electric field of 16 V/μm and a maximum emission current density of 40 mA/cm², due to the large screening effect. Using electron-beam lithography, 100 nm Ni dots were obtained and individual multi-walled nanotubes were grown on each dot. Based on a square lattice with 10 μm translation vector, I-V characteristics gave an onset field of 8 V/μm and a maximum emission current density of 0.4 A/cm².