Electron Emission Area Depends on Electric Field and Unveils Field Emission Properties in Nanodiamond Films

摘要

In this paper we study the effect of actual, locally resolved, field emission(FE) area on electron emission characteristics of uniform semimetallicnitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) field emitters. Toobtain the actual FE area, imaging experiments were carried out in a vacuumsystem in a parallel-plate configuration with a specialty anode phosphorscreen. Electron emission micrographs were taken concurrently with $I$-$V$characteristics measurements. It was found that in uniform (N)UNCD films thefield emitting site distribution is not uniform across the surface, and thatthe actual FE area depends on the applied electric field. To quantify the actual FE area dependence on the applied electric field, anovel automated image processing algorithm was developed. The algorithmprocesses extensive imaging datasets and calculates emission area per image. Bydoing so, it was determined that the emitting area was always significantlysmaller than the FE cathode surface area of 0.152 cm$^2$ available. Namely, theactual FE area would change from $5\times10^{-3}$ \% to 1.5 \% of the totalcathode area with the applied electric field increased. Finally and most importantly, it was shown that when $I$-$E$ curves asmeasured in the experiment were normalized by the field-dependent emissionarea, the resulting $j$-$E$ curves demonstrated a strong kink and significantdeviation from Fowler-Nordheim (FN) law, and eventually saturated at a currentdensity of $\sim$100 mA/cm$^2$. This value was nearly identical for all studied(N)UNCD films, regardless of the substrate.