In this work, various carbon, metallic and semiconductor nanostructures were fabricated within collaborations and systematically investigated for potential field emission (FE) cathode applications.
Various carbon-based cathodes were obtained by different chemical vapour deposition methods. Single-walled carbon nanotube (CNT) networks grown on n-Si substrates at ~150°C showed well-homogeneous FE with ~10⁴ emitting sites/cm² at low onset field Eon (1nA) ~2.5 V/μm. During the local measurements ∅ 150 μm sites yielded stable currents up to 0.2 mA. Integral measurements of the whole cathodes revealed fairly homogeneous FE resulting in at least 10 mA/cm². Arrays of entangled CNT bundles of ~2 μm height, 2-3 μm patch and 100, 10 and 5 μm pitch were grown on n-Si wafers with trimetallic Mo/Al/Ni layers. Highly efficient and well-aligned FE at Eon (1nA) ~15 V/μm was obtained from CNT arrays with a pitch of 100 μm, however, the highest current up to 0.5 mA at 400 V was achieved from a spot of 150 μm for cathodes with a pitch of 5 μm. Integral measurements of the whole cathodes showed fairly homogeneous FE at currents up to 4.4 A/cm². CNT columns of ∅=250 μm and different height (h = 70 or 350 μm) forming quadratic arrays with a pitch of 650 μm showed fairly aligned and efficient FE at comparatively low Eon (1nA) ~2 V/μm. Maximum current values up to 600 μA at 15 V/μm were achieved independently of the column height. The FE triode tests of single CNT columns yielded anode-cathode current ratio up to 97 % at a gate (anode) voltage of 247 (2500) V. Structuring of carbon nanowall (CNW) films was successfully performed with a laser for the optimization of their FE properties. Such cathodes exhibited fairly aligned and efficient FE at Eon (1nA) = 10-20 V/μm. Local FE measurements of selected CNW patches revealed maximum current values up to ~100 μA.
Mechanically stable and randomly distributed copper nanocones (Cu-NCs) were fabricated by ion-track template method. Depending on the process parameters, Cu-NCs of ~28 μm length, ~3 μm base, with different ~60-300 nm tip radius and number density were fabricated. The cathode with high number density of Cu-NCs (10⁷ cm-2) yielded stable currents up to 280 μA at 100 V/μm from an emission spot of 30 μm. In contrast, the cathodes with a triangular patch array (∅ 150 μm, 320 μm pitch) of less Cu-NCs (10⁵ cm-2) provided fairly homogeneous and well-aligned FE of all Cu-NC patches at much reduced Eon (1nA) <10 V/μm and demonstrated an average current of 30 μA/patch at 32 V/μm. The FE performance of the Cu-NC cathode was improved by a thin Au coating resulting in an average current of 151 μA at 50 V/μm. Integral FE measurements on the whole Cu-NC cathode showed fairly homogeneous FE at 8 A/cm².
Silicon technology is the most suitable for fabrication of highly-uniform arrays of bare p- and n-type Si tips. Rather homogeneous and well-aligned FE from all tips and stable currents up to ~0.1 (0.6) μA for p-(n-) type tips were achieved. In comparison, Au-coated n-type Si tips showed improved FE uniformity and at least 5 times higher current values (i.e. ~3 μA/tip), at ~30% higher extraction field though. P-Si tips showed a current saturation region of about 10 nA. In this region, emitters provide the highest current stability (<5%) and an optical current switching ratio of ~2.5.
Potential applications of the described above materials are discussed.