High-density Field Emitter Arrays Research Articles

Fabrication of bulk and epitaxial germanium field emitter arrays by dry etching techniques

We present the fabrication and characterization of novel high density field emitter arrays (FEAs) on CVD-grown epitaxial germanium on (001) silicon. In particular we propose a heterostructure made up of silicon as substrate and of germanium as active layer, exploiting the infrared transparency of Si and the infrared sensitivity of Ge to realize a semi-transparent photo-assisted electron beam source. We used a completely dry etching process in fluorinated gases (SF6) due to its significant under-etching for both silicon and germanium. High aspect ratio silicon and germanium FEAs, with minimum tip radii of 25nm and 40nm, respectively, and lower aspect ratio Ge/Si FEAs with minimum tip radii of 50nm were fabricated. The realized FEAs show good emission behavior with field emission characteristics straight related to tip geometry: low electric field threshold for silicon and germanium tips (<18V/@mm) and enhancement factor of more than 250 and 130, respectively; conversely for the epitaxial germanium we obtained 32V/@mm for electric field threshold and 70 for enhancement factor. Current emission time stability for silicon, for germanium and for Ge/Si field emitter arrays were demonstrated.

Picosecond photoassisted electron emission from gated p-silicon high density field emitter array

A 1 mm square array of oxidation sharpened p-type Si field emitter tips (2.4 μm spacing and 1 μm gate aperture) produces 40 ps rise time pulsed electron emission into the vacuum when excited by 110 fs duration optical pulses (λ=0.78 μm). The observed emission quantum yield for pulsed excitation of 0.04% is partially accounted for by gate shadowing and overlap of optical absorption depth and an estimated depletion layer thickness. These results suggest that more efficient photoexcited picosecond time scale electron emission is possible from optimized semiconductor field emitter structures.

High current density Si field emission devices with plasma passivation and HfC coating

A novel self-aligned process was developed to fabricate gated Si field emission devices. At a gate voltage of 100 V, the emission current from an array of 100 tips increased from 283 to 460 /spl mu/A and the turn-on voltage decreased from 31 to 21 V after H/sub 2/ plasma passivation using an inductively coupled plasma (ICP) source for 2 min. The improvements correspond to a 1.28-eV reduction in the effective work function of the emitters and the instability of the emission current decreased from /spl plusmn/1,25 to /spl plusmn/0.25% after H/sub 2/ plasma passivation. Emitter tips were also coated with Mo silicide and HfC. The emission current increased from 230 /spl mu/A for uncoated emitters to 268 /spl mu/A for emitters coated with Mo silicide and 389 /spl mu/A for emitters coated with HfC. The turn-on voltage decreased from 50 to 41 and 25 V while the breakdown voltage increased from 126 to 129 and 143 V when Mo silicide and HfC were used for coating, respectively, which correspond to reductions of 0.95 and 2.23 eV, respectively, in the effective work function of the emitters. Single emitter tips have similar emission characteristics as high-density field emitter arrays, indicating excellent emission uniformity from the arrays.

Derivation of the image interaction for non-planar pointed emitter geometries: application to field emission I–V characteristics

Spindt et al. have demonstrated that high density field emitter arrays fabricated using thin-film techniques and electron beam microlithography operate at very low applied voltages, compared with that for conventional etched wire field emission tips. These arrays are capable of operating at effective current densities far higher than can be obtained with etched emitters. The current-voltage curves obtained from Spindt devices generally follow Fowler-Nordheim behavior, but differ dramatically in slope of the F-N curves. It has been suggested that the apparent emitting area per tip is a few atomic sites. By contrast, the data for the etched emitters indicate a much greater emitting area (about 100 atomic sites), from which it is concluded that there are anomalously high fields on Spindt cathodes. This difference was explained by assuming that the emitting atoms form a protuberance on the tip of the cone with a field magnification factor of about 5. Closed form analytical solutions for the image interaction for tips modeled as a cone, paraboloid, hyperboloid, and sphere on cone have been derived. Numerical results show that the classical image interaction for these non-planar emitter geometries is diminished in magnitude relative to the planar image interaction for the same external bias potential. Simple analytic expressions for the image interaction for these non-planar geometries are obtained by fitting of the numerical results. The tunneling barriers have been constructed by adding the image potential for each model of the tip and the corresponding geometrically dependent bias potential. It is found that the bias potentials (fields) for the model emitters significantly modify the shape of the tunneling barriers, and the resulting forms predict a dramatically enhanced current relative to the “classical” Fowler-Nordheim result.

Derivation of the image interaction for non-planar pointed emitter geometries: application to field emission I–V characteristics

Abstract Spindt et al. have demonstrated that high density field emitter arrays fabricated using thin-film techniques and electron beam microlithography operate at very low applied voltages, compared with that for conventional etched wire field emission tips. These arrays are capable of operating at effective current densities far higher than can be obtained with etched emitters. The current-voltage curves obtained from Spindt devices generally follow Fowler-Nordheim behavior, but differ dramatically in slope of the F-N curves. It has been suggested that the apparent emitting area per tip is a few atomic sites. By contrast, the data for the etched emitters indicate a much greater emitting area (about 100 atomic sites), from which it is concluded that there are anomalously high fields on Spindt cathodes. This difference was explained by assuming that the emitting atoms form a protuberance on the tip of the cone with a field magnification factor of about 5. Closed form analytical solutions for the image interaction for tips modeled as a cone, paraboloid, hyperboloid, and sphere on cone have been derived. Numerical results show that the classical image interaction for these non-planar emitter geometries is diminished in magnitude relative to the planar image interaction for the same external bias potential. Simple analytic expressions for the image interaction for these non-planar geometries are obtained by fitting of the numerical results. The tunneling barriers have been constructed by adding the image potential for each model of the tip and the corresponding geometrically dependent bias potential. It is found that the bias potentials (fields) for the model emitters significantly modify the shape of the tunneling barriers, and the resulting forms predict a dramatically enhanced current relative to the “classical” Fowler-Nordheim result.