Surface-barrier analysis for niobium and tantalum and tantalum-on-niobium from periodic deviations in the thermionic Schottky effect

Abstract

The Richardson emission constants and periodic deviations in the Schottky effect have been used to investigate the surface barrier for thermionic electron emission from polycrystalline tantalum and niobium and thin films of tantalum-on-niobium. The data have been fit to the Miller and Good theory and a Herring and Nichols complex reflection coefficient, μ, for the surface potential has been determined. The following conclusions can be drawn: The one-dimensional image nature of the barrier in the region of the barrier maximum has been confirmed. The tantalum and niobium experimental amplitudes increased faster than predicted by the theory. This would indicate: either a field-dependent error exists in the theoretical amplitude; or the surface reflection process should be described by an energy-dependent reflection coefficient. The average μ = 0.35 exp(0.63 i) for niobium and μ = 0.36 exp(0.75 i) for tantalum indicate the Sommerfield surface bandwidth model proposed to fit the periodic deviations from molybdenum and rhenium is inadequate for tantalum and niobium. The controlled deposition of tantalum-on-niobium lowered the work function of niobium, and the experimental results confirm work on the molybdenum-on-rhenium surface alloy system that : (a) Richardson emission constants are strongly dependent on the micropatch distribution of the emitter surface while (b) the periodic deviations parameters appear to be independent of it.