Effect of gases on the field emission properties of ultrananocrystalline diamond-coated silicon field emitter arrays

We performed studies of electron emission from ultrananocrystalline diamond (UNCD)-coated, ungated silicon field emitters as a function of in situ exposure to various gases during current versus voltage and current versus time measurements. The emitter arrays were fabricated by a subtractive tip fabrication process and coated with UNCD films using microwave plasma chemical vapor deposition with a ${\mathrm{CH}}_4/\mathrm{Ar}$ plasma chemistry. The emission characteristics of the coated tip arrays were studied in the diode configuration; using a 2 mm diameter anode with rounded edges to suppress arcing. Significant enhancement of the electron emission was observed, increasing from 35% to 100%, after the emitting surface was exposed to ${\mathrm{H}}_2$ at pressures in the ${10}^{\text{−5}}$ and ${10}^{\text{−4}}\hspace{0.17em}\mathrm{Torr}$ range. Upon termination of the ${\mathrm{H}}_2$ exposure, the current decreased to the initial value of 2 μA. The emission current subsequently remained stable at 2 μA upon continued evacuation down to the base pressure below ${10}^{\text{−9}}\hspace{0.17em}\mathrm{Torr}.$ The emission current variation is repeatable with ensuing hydrogen exposure, indicating that the enhancement is due to the hydrogen exposure. Negligible emission current variations are observed at pressures less than ${10}^{\text{−5}}\hspace{0.17em}\mathrm{Torr}.$ Exposure to either Ar or ${\mathrm{N}}_2$ resulted in a reduction of the emission current for ambients of up to ${10}^{\text{−5}}\hspace{0.17em}\mathrm{Torr}.$ This effect is reversible. The effect of the investigated gases on the emission characteristics of UNCD-coated Si tip arrays is attributed to a modification of the effective work function at the localized sites from where electrons are being emitted.