Nitrogen containing hydrogenated amorphous carbon for thin-film field emission cathodes

Abstract

Field emission measurements using 0.3 m thick nitrogen containing hydrogenated amorphous carbon films ͑a-C:H:N͒ on n ϩϩ -Si cathodes are reported. Onset emission fields as low as 4 V m Ϫ1 have been obtained using a flat plate anode configuration. Uniform emission is observed over the entire cathode area at current densities below 7ϫ10 Ϫ2 mA cm Ϫ2 . At higher current density preferential emission from spots is observed. The spot emission is imaged using the ITO coated plate anode. A model based on the a-C:H:N acting as a space charge interlayer on the n ϩϩ -Si is proposed to explain the emission at low electric fields.

Influence of sp2 clusters on the field emission properties of amorphous carbon thin films

“…However, in both of these types of films there has been a reduction in the spin density and this was partly attributed to passivation of defects. [4,17] It can be seen from Fig. 3 (a) that E th drops with addition of N similar to that reported elsewhere [4,16] but in contrast to the DLC films of Silva et al, [18] no reduction in N s is observed (Fig 3 (b)).…”

“…In order to demonstrate that it is the size of the cluster that is still important in determining the emission mechanism rather than the absolute concentration of the clusters we have made measurements of the threshold field and spin density as a function of N content in the low defect polymeric films. Addition of N has been shown previously to reduce the threshold field in diamond-like carbon (DLC) films [4] and also in ta-C films [17]. However, in both of these types of films there has been a reduction in the spin density and this was partly attributed to passivation of defects.…”

“…[4,5] The low concentration of the more conductive sp 2 clusters is unable to screen the field at the front surface. In this way emission, from these low bias deposited films would be more correctly described as being determined by "the back contact" rather than the "front surface".…”

“…[3] An alternative mechanism in which the presence of a large internal electric field at the film/substrate interface resulting in hotelectron transport -the "back contact" model -has also been proposed. [4] The reduction of the threshold field with N content [4], the dependence of the threshold field with film thickness [5], and the non-uniformity of emission across a film surface have also been reported. [6,7] The relative proportion of the different hybridised C states influences the electronic structure with the optical properties being largely controlled by the size of sp 2 clusters.…”

Carbon based electronic materials: applications in electron field emission

“…[17] In this model the a-C film acts as an interlayer with the true cathode being the underlying Si or metal substrate, with the crucial factor being the heterojunction that formed at the back contact. [18] In the case of DLC and ta-C films the density [16] of unpaired electron states near to the Fermi level is higher (10 20 cm -3 ). These films have a high sp 2 content as well as a large band gap (>2 eV) and as a consequence the wavefunction overlap of the clusters will result in electron delocalization [19] and/or enhanced hopping between the clusters that will enhance the connectivity, as shown in Fig.…”