Pressure dependency of emission currents of Si, Mo, Au, and SiC field emitters

The emission currents of Si and Mo cone emitters and of Au and SiC volcano emitters have been measured at pressures ranging from 10^-9 to 10^-5 torr. It is observed that, independent of the emitter material, the current stays almost constant to about 10^-9 torr and thereafter decreases significantly. Temporal fluctuations, except for the SiC emitters, increase with increasing pressure. As for the SiC emitters, they stay almost constant throughout the pressure range. This behavior is mainly attributed to the large emitter resistance. From Fowler-Nordheim analysis of the silicon devices at 2.5×10^-9 torr and 8×10^-6 torr, it is concluded that the drop in emission current is caused by an increase of the local work function at the emission sites. This increase is caused by the arrival of gas species at the emission sites, which at a pressure of 10^-5 torr is about 10 monolayers/second and consist mainly of CO (90%). Operating the devices above 2×10^-5 torr can cause irreversible damage to tip morphology due to sputtering effects. From these experiments it is concluded that a safe operating regime for the above materials is at pressures at or below 1×10^-7 torr