Recent progress in thermal field electron source performance

Abstract

The electron emission properties of a thermal-field electron (TFE) source consisting of a 〈100〉 oriented tungsten emitter with an overlayer of zirconium will be reported. In addition, the electronic and geometric structure of the unsually low work function (2.6 eV) and thermally stable Zr/W(100) surface will be discussed. It has been shown through Auger spectroscopy and field emission microscopy investigations that the low work function surface is confined to the (100) plane of tungsten and consists of a Zr/O/W composite structure which extends several tens of atomic layers into the bulk. Heating in oxygen causes the Zr to disappear from the surface along with an increase in work function from 2.6 to 5.5 eV. However, subsequent heating in vacuum at T ∼ 2000 K restores both the Zr/O/W surface composition and low work function. The low work function surface is unaffected by small amounts of carbon impurities, but nitrogen, presumably in the form of ZrN, increases the work function to 3.9 eV which remains unchanged up to 1800 K. Both thermionic and retarding work function measurements of the Zr/O/W surface yield a work function in the range of 2.5 to 2.7 eV. Further, this low work function surface is stable with respect to repeated temperature cycling to ∼2100 K. Field emission studies using 〈100〉 oriented emitters with a ZrO overlayer have shown the low work function (100) plane to be the dominant region of emission. Operation of such TF cathodes at 1800 K and residual pressure of 1 to 3 × 10 -8 Torr appears feasible with life in excess of 5000 h. Current fluctuations in the frequency interval 1 to 5000 Hz were ∼0.23% for a highly apertured beam current of 30 nA. The angular intensity of electron current from such TF sources can exceed 1 × 10 -3 A/sr without limiting emitter life.