Total-Energy Distribution of Field-Emitted Electrons and Single-Plane Work Functions for Tungsten

Total-energy-distribution measurements in the temperature range 77 to 900°K along the 〈310〉, 〈211〉, 〈111〉, and 〈611〉 directions of W are in good agreement with the classical Fowler-Nordheim model for field emission. Work functions obtained from the I(V) characteristics and energy-distribution results from these directions are reasonable. In contrast, energy-distribution results along the 〈100〉 and 〈110〉 directions do not agree with the free-electron model of Fowler and Nordheim nor do they yield sensible work-function values. Coating the emitter with a low-work-function Zr-O coadsorbate caused the 〈100〉 energy-distribution results to appear normal and yield sensible values of work function. Temperature coefficients of the work function for the 〈110〉, 〈112〉, 〈100〉, 〈111〉, 〈116〉, and 〈310〉 directions of clean W are -17.0, -14.3, -10.9, 3.5, 5.0, -3.2 (×10^-5)eV/deg, respectively. Upon coadsorbing Zr-O, the temperature coefficient of the 〈100〉 work function becomes negligible. The amplitude of low-frequency thermal-induced flicker noise was found to be least on the close-packed crystal faces. These results, when discussed in the light of the bulk band structure of W, indicate the plausibility of utilizing field-emission energy-distribution measurements to illustrate certain types of anisotropies in the energy surfaces of metals or semiconductors at or near the Fermi level.