A. Navitski

Homogeneous Field Emission Cathodes With Precisely Adjustable Geometry Fabricated by Silicon Technology

Silicon-based cathodes with precisely aligned field emitter arrays of sharp tips applicable for miniaturized electron sources were successfully fabricated and characterized. This was made possible by an improved fabrication process using wet thermal oxidation, wet etching, and reactive-ion etching steps with adjustable anisotropy. As substrate materials, both p-doped silicon and n-doped silicon were used. The cathode chips contain about 3 × 105 Si tips/cm2 in a triangular array with tip heights of 2.5 μm, tip radii of less than 30 nm, and spacing of 20 μm. Well-aligned field emission (FE) and excellent homogeneity from all tips (i.e., 100% efficiency) and maximum stable currents of typically 0.1 μA (0.6 μA) for p (n)-type Si were reproducibly achieved. The current-voltage characteristics of the p-Si tips exhibit the expected saturation at around 10 nA with around ten times better current stability, whereas the n-Si tips show the usual Fowler-Nordheim behavior. Additional coating of the Si tips with 5-nm Cr and 10-nm Au layers resulted in improved stability and at least five times higher average FE current limits (about 3 μA) at about 30% higher operation voltage.

Stable field emission of single B-doped Si tips and linear current scaling of uniform tip arrays for integrated vacuum microelectronic devices

Advanced Si-based semiconductor technology is most suitable to fabricate uniform nanostructures as integrated field emitter arrays for novel vacuum electronic devices. In order to improve the field emission homogeneity and stability of p-type silicon tip arrays for pulsed sensor applications, the authors have systematically studied the influence of the fabrication parameters on the tip shape and on the specific operating conditions. Based on detailed design calculations of the field enhancement, they have fabricated two series of hexagonal arrays of B-doped Si-tips in a triangular arrangement. The first (second) type contains three (four) patches with different number of tips (1, 91, 547 and 1, 19, 1027, 4447 for the first and second type, respectively) of about 1 (2.5) μm height, ∼20 (20) nm apex radius, and 20 (10) μm pitch. The field emission properties of both individual tips and complete arrays were investigated with a field emission scanning microscope at a pressure of 10−9 mbar. The current plateau...

Efficient high-current field emission from arrays of carbon nanotube columns

Arrays of entangled carbon nanotube (CNT) columns on patterned n-Si substrates were fabricated by an atmospheric chemical vapor deposition with a ferrocene/xylene mixture. Preferential CNT growth in the opened Si windows of a 300 nm thick SiO2 layer was achieved by adjusting the parameters of the synthesis process. The resulting multiwalled CNT formed vertically aligned columns up to 50 μm net height in four arrays of 10, 30, or 50 μm patch diameter and 100 or 160 μm pitch. The field emission (FE) properties of such structured cathodes were measured by a FE scanning microscope using tip anodes of adjusted size. Well-aligned FE from nearly 100% of the patches at electric field lower than 10 V/μm was observed, but less pronounced FE occurred between the arrays too. High current capability of most patches up to milliamperes, suggesting multiple CNT emitters per patch, was achieved. Integral FE measurements in the diode configuration with luminescence screen and processing under N2 and O2 exposures of up to 3...

Efficient high-current field emission from arrays of CNT columns

At present carbon nanotubes (CNT) are the most prospective cathode material for triode applications because of their strong field emission (FE) at low electric fields. The FE homogeneity of flat CNT cathodes, however, is still limited by their rather fast and uncontrolled growth which usually leads to strongly varying field enhancement and current carrying capability of the individual emitters. Therefore, patches with multiple emitters might provide a suitable strategy to improve the homogeneity and current stability of CNT cathodes. Accordingly, structured arrays of CNT based columns on flat n-Si substrates were fabricated applying the atmospheric-pressure CVD method with a volatile catalyst source (ferrocene/xylene mixture). Substrates of 6×6 mm2 size and 4 quadrants of 2×2 mm2 with varying patch diameter and two pitch sizes were structured by means of photolithography and selective chemical etching of the thermal 0.3 μm thick SiO2 oxide layer. Preferential growth of CNT in the opened windows of the SiO2 layer was achieved by adjusting the parameters of the synthesis process, i.e. the temperature in the reaction zone, the growth time, the concentration of the catalyst in the feeding solution, the gas-carrier flow rate etc. as described elsewhere [1]. The resulting multiwall CNT for growth times of 30 s and 2 min formed vertically aligned arrays of uniform columns of about 20 μm and 50 μm net height with a pitch to patch ratio of 160/50, 100/50, 100/30 and 100/10 (μm) as ahown in Fig. 1. Obviously, preferential CNT column growth embedded in a floor of shorter CNT is fairly achieved.

Highly uniform and stable electron field emission from B-doped Si-tip arrays for applications in integrated vacuum microelectronic devices

In order to improve the uniformity and field emission stability of p-type silicon tip arrays for pulsed sensor applications, we have systematically studied the influence of the fabrication parameters on the tip shape and the specific operating conditions. Based on detailed design calculations of the field enhancement, we have fabricated a series of hexagonal arrays of B-doped Si-tips in a triangular arrangement, each containing a different number of tips (91, 575 and 1300) of 1 μm height, 20 nm apex radius, and 20 μm pitch. The field emission properties of both individual tips and complete arrays were investigated with by field emission scanning microscopy. The current plateaus of these tips typically occur at about 10 nA and 60 V/μm field level. In this carrier depletion range, single tips provide the highest current stability (<; 4%) and optical current switching ratios of ~2.5. Rather homogeneous emission of the tip arrays leads to an almost linear scaling of the saturation current (2 nA/tip) and to a much improved current stability (<; 1%) measured over 1 hour.

4.1: Field emission spectroscopy of carbon nanotube cathodes

We have constructed a measurement system for field emission spectroscopy (FES) of cold cathodes. Its commissioning with a tungsten needle yielded an electron energy resolution of less than 50 meV. Combined FES and current-field measurements on column arrays of entangled multiwall carbon nanotubes resulted in a work function of Φ = 4.86 ± 0.4 eV which confirms the literature data.

Optimization of ion-track etching and electrochemical Cu nanocone deposition for field emission cathodes

We have fabricated patch-structured nanocone (NC) cathodes by using asymmetrically etched ion-track membranes which were metallized and fixed on flat holders. The conical channels of these templates were filled by potentiostatic copper deposition resulting in freestanding Cu-NC of ~ 28 μm length, ~ 3 μm base, and 160-240 nm tip diameter. Fairly homogeneous and well-aligned field emission from all patches was obtained due to the high NC (105 cm-2) and emitter densities (~ 3/patch). Local measurements of selected spots and patches show distinct current jumps at μA levels. SEM images of the processed areas reveal morphological changes proving a successive destruction of single Cu-NCs. Nevertheless, improved current values of ~ 300 μA at reduced fields of 30-50 V/μm have been achieved for the best Cu-NC patches.

Scaling of the field emission current from B-doped Si-tip arrays

We have fabricated a test chip with various hexagonal arrays of B-doped Si tips (height ~ 3 μm, apex radius <; 30 nm, number 1-4447, resistivity 4 Ωcm, 100 orientation) in triangular arrangement (pitch 10 μm, density 1.16×106 cm-2) in order to systematically investigate the field emission current scaling with the number N of tips. Regulated voltage scans for 1 nA revealed rather efficient emission from nearly all tips of the arrays at an average field of 15 V/μm. The expected current plateau was always obtained at fields around 20 V/μm, but its width strongly increased with N. In this carrier depletion range, the single tip provided a much higher stability (<; 5%) of the current (2-3 nA) than at lower (>; 50 %) and higher currents (>; 30%). Integral current measurements of the hexagonal arrays resulted in a statistically improved current stability (<; 1%) but only a weak increase of the total current with N0.28 yet. These results will be discussed with respect to the remaining inhomogeneity of the tips.

P2–23: Field emission properties of aligned pure and TiO 2 -coated carbon nanotube block arrays

We report about field emission properties of various arrays of vertically aligned CNT blocks. Multiple emitters per block resulted in nearly aligned and efficient cathode performances. Stable currents up to 300 (100) µA have been achieved for the pure (TiO2 coated) CNT blocks.

Efficient field emission of gold and platinum nanowire patch arrays

Metallic nanostructures with controllable dimension and high aspect ratio provide an interesting alternative to carbon nanotubes for triode applications which require highly efficient and homogeneous field emission cathodes. Noble metal nanowires (NW) were electrochemically synthesized into the randomly distributed cylindrical pores of polycarbonate templates fabricated by GeV-ion irradiation, selective pore etching and subsequent metallization with a stable backing layer. After dissolution of the polycarbonate, freestanding solitary or clustered gold NW were found depending on the irradiation fluence and the NW length [1]. Systematic studies of Au-NW cathodes revealed promising field emission (FE) properties [2]. Therefore, regular patch arrays of gold or platinum NW were fabricated by using shadow masks during irradiation. A test mask with four quadratic arrays of patches of 50 μm diameter and different pitch size (100 and 150 μm) was used to optimize the fluence (106 − 108/cm) and NW length (7–28 μm) with respect to emitter efficiency and alignment [3]. A second mask with a hexagonal array of 150 μm diameter patches and 320 μm pitch was then employed to develop cathodes suitable for triode devices.