Homogeneity, activation effects and current limits of structured CNT column cathodes on Si substrates

Regular arrays of entangled carbon nanotube (CNT) columns were grown on p-Si substrates with bimetallic (aluminum and patterned iron) catalyst by water assisted chemical vapor deposition based on a pyrolysis of ethylene. Rounded CNT columns of about 20 μm diameter and 5 μm height in an asymmetric triangular array of 256 and 270 μm pitch were obtained. The field emission (FE) maps yielded high efficiency and good alignment of the CNT emitters. Local FE investigations of single columns revealed initial onset field of ~ 11 V/μm for 1 nA. The FE current was rather stable (± 10 %), first FN-like (<; 0.1 μA) but then saturated above 20 V/μm to 1-2 μA. Above 40-60 V/μm, however, irreversible current jumps up to 10-40 μA occurred reproducibly for all emitters. This strong activation effect was often combined with a visible light emission from hot spots at the top of the columns. Finally stable FN-like current-voltage curves with onset field of ~ 30 V/μm and maximum current of ~40 μA were obtained. This strange FE behavior of CNT cathodes is considered to be caused by the creation of conducting channels into the SiO₂ layer and will be explained by the band structure.