Morphology and emission characteristics of an HfC〈110〉 thermal-field electron source

Abstract

The emission characteristics and morphology of three ⟨110⟩ oriented HfC field emitters operated at elevated temperatures and electric field were investigated. During operation, a ridge is formed at the emitter apex due to the growth and intersection of either two {111} or two {100} planes. The latter planes were the primary facets that formed during operation. Modeling analysis showed that for a surface energy ratio of γ111/γ100=1.15, a vertex forms at the apex and for γ111/γ100≠1.15, the ridge would rotate from a ⟨110⟩ direction (for γ111/γ100<1.15) to a ⟨100⟩ crystal direction (for γ111/γ100>1.15). For a well aligned HfC⟨110⟩ emitter, it appeared from the preferred ridge alignment that γ111/γ100>1.15. For a large radius emitter with a ridge length of >40nm, the emission distribution was a twofold symmetric “bow tie” shape with the maximum angular intensity (I′) at the corners of the ridge structure. For a smaller radius emitter with a ridge length of <40nm, the emission distribution was a single oblong shaped pattern with the maximum I′ at the center. Axial work function values at 1600 K ranged from 3.0 eV for the ridge formed along the ⟨100⟩ direction to 3.8 eV for the ridged formed along the ⟨110⟩ direction. Axial values of the full width at half maximum of the energy distribution increased from 1.0 to 4.0 eV as I′ increased from 0.08 to 7.0 mA/sr. Low frequency axial current fluctuations for emitters with a large emitting area at T=1600K were 3%–7% and increased to 15% for emitters with a smaller emitting area.