Study of the MgO-coated W emitters by field emission microscopy

The field electron emission cathode was fabricated by vacuum evaporating a magnesium oxide layer, ranging from ∼0.1 to a few tenths of microns thick, onto a tungsten tip. The coated cathodes were heated in air at temperatures between 400°C and 500°C. The electron emission characteristics were studied using the typical method of applying positive voltage to the anode without using any triggering techniques that were suggested elsewhere to be necessary for operating the cathode. The maximum emission current obtained after extensive tip thermal treatment was 4.6 μA for an applied anode voltage of 8 kV, the tip exploding at a higher voltage. This current in most cases was recorded as a series of pulses, without reaching stability. Spatial distributions were obtained for various conditions. The emission images demonstrated a large variation in intensity and brightness. Thermal treatment at elevated temperatures (∼550°C) resulted in short-term stability and a higher beam current. Cooling the sample to near liquid-nitrogen temperatures resulted in a lower emission of current stability. The current–voltage characteristics of this atypical behaviour and the work function changes obtained from Fowler–Nordheim (F–N) plots are presented here.