Cold Cathode Applications Research Articles

Metal Coated, fs-Laser Fabricated Silicon Spikes as Electron Emitters for Cold Cathode Applications

AbstractWe have studied the effect of thin metal coatings on the electron emission characteristics of self-assembled silicon microstructures with nearly identical sharp features. We have employed a common template of spikes produced by fs-laser self-driven structuring of Si on which several different metals have been deposited. We find that, in the pristine state and in vacuum conditions achievable in device applications, all metal coatings do not result in marked change of either the minimum electric field necessary for emission or the maximum obtainable current density. In contrast, the durability of the emitters depends strongly on the metal used and is always enhanced with respect to bare Si. Furthermore, no signs of degradation were found within the 3-day time scale of our experiments with gold and chromium. On the contrary, these two metal coatings resulted in emission characteristics improving with time in typical operation conditions.

Novel cold cathode materials and applications

Field emission (FE) is based on the physical phenomenon of quantum tunneling, in which electrons are injected from the surface of materials into vacuum under the influence of an applied electric field. A variety of field emission cold cathode materials have been developed to date. In this review, we shall focus on several kinds of novel cold cathode materials that have been developed in the past decade. These include materials for microfabricated field-emitter arrays, diamond and related films, carbon nanotubes, other quasi one-dimensional nanomaterials and printable composite materials. In addition, cold cathode materials have a wide range of applications such as in flat panel displays, high-power vacuum electronic devices, microwave-generation devices, vacuum microelectronic devices and vacuum nanoelectronic devices. Applications are in consumer goods, military industries and also space technology. A comprehensive overview of the various applications is presented. Recently, recognizing the strong possibility that vacuum nanoelectronic devices using quasi one-dimensional nanomaterials, such as carbon nanotubes may emit electrons with driving voltages comparable to that of a solid-state device, there is a growing interest in novel applications of such devices. With such exciting opportunities, there is now a flurry of activities to explore applications far beyond those considered for the conventional hot cathodes that operate on thermionic emission. We shall discuss the details of a number of fascinating potential applications.

Field emission measurement techniques for the optimisation of carbon nanotube cathodes

We have developed advanced measurement techniques for the investigation of the field emission (FE) properties of high-technology materials. Flat samples up to 30 mm in diameter can be optimised either for strong or weak electron emission, i.e., cold FE cathodes operating at field levels below 10 V/μm or high-voltage vacuum devices preventing FE up to 200 V/μm. The integral measurement system with luminescent screen (IMLS) provides a quick overview of the microscopic distribution and long- and short-term stability of the emission current, and facilitates a controlled conditioning of cold cathode samples at various pressures. The field emission scanning microscope (FESM) is incorporated within a UHV surface analysis system and enables high-resolution emitter distributions and local FE investigations down to the nanoscale. Exemplary results on carbon nanotube samples will be given to demonstrate that the combination of both instruments yields a very effective means for an improved understanding and tailoring of materials for cold cathode applications.

Application of nanocarbon cold cathodes in lighting elements

AbstractWe have manufactured cathodoluminescent lamps equipped with field emitters composed of carbon nanotubes and nanosized graphite crystallites. The nanocarbon film material was obtained by chemical vapour deposition onto Si and Ni substrates and was used as grown in the vacuum light‐emitting devices. Various light‐emitting arrangements were realized, showing wide possibilities for different applications, including flat panel displays, indicators and light signs. A record power efficiency of >30% was achieved for the cylindrical diode lamps with nanocarbon cold cathodes. This shows their suitability as lighting elements. Copyright © 2004 John Wiley & Sons, Ltd.

Enhanced field emission of self‐organized laser‐produced Si and Ni micro‐tip arrays with incorporated carbon nanotubes

AbstractResults are reported on low‐field electron emission from periodic arrays of micro‐tips produced by laser‐assisted evaporation of single‐crystal Si wafers in a vacuum and Ni wafers in liquid. To improve the emission from the micro‐tips, single‐wall carbon nanotubes were incorporated into surface layers of the tips during or after micro‐relief formation. The best samples showed emission at threshold fields as low as 0.3–1 V µm−1 for Si substrate and 1.5–2 V µm−1 for Ni substrate, as measured with the flat screen technique. The emission was stable upon cyclic variation of the electric field. Sufficient improvement of the emission surface uniformity (a number of emission sites per square unit) and lower threshold fields were achieved using carbon nanotube incorporation compared to ‘undoped’ Si and Ni micro‐tips. The experimental data explain the field emission from the tip arrays by a complex geometric field enhancement, including nanotubes and narrow conducting channels in the tip body. The arrays of micro‐tips are promising for a wide range of cold cathode applications. Copyright © 2004 John Wiley & Sons, Ltd.

Low-field electron emission of self-organized laser-produced micro-tip arrays with incorporated carbon nanotubes

Low-field electron emission is reported from quasi-periodical arrays of micro-tips produced by laser-assisted evaporation of single crystal Si wafers in vacuum and Ni wafers in liquids. To improve the emission from the micro-tips, single-wall carbon nanotubes were incorporated into surface layers of the tips during or after micro relief formation. The best samples showed the emission at threshold fields of as low as 0.3–1 V/μm for Si substrates and of 1.5–2 V/μm for Ni substrates as measured with the flat screen technique. Improvement of the emission surface uniformity and lower threshold fields were achieved by incorporating of carbon nanotubes when compared with ‘undoped’ Si and Ni micro-tips. The experimental data and microstructural analysis enables us to explain the emission from the tip arrays by a complex geometric field enhancement including nanotubes and narrow conducting channels in the tip body. The arrays of micro-tips are promising for a wide range of cold cathode applications.

Nano-carbon materials for cold cathode applications

Nano-carbon thin film materials were obtained by chemical vapor deposition in dc discharge activated hydrogen–methane gas mixture. Film structure, surface morphology and phase composition was studied by Raman, electron microscopy and electron spectroscopy methods. A highly efficient cold electron emission was found for the films composed from well graphitized nano-structures including carbon nanotubes and graphite nano-crystallites. Electron emission tests and analysis exhibit non-classical behavior of the nano-carbon cold cathodes. The statistical analysis of the cold emission show normal distribution of parameters of separate emission centers with narrow standard deviation. The possible mechanism of the cold emission is discussed.

Carbon-Based Nanocomposite Materials For Cold Cathode Sources

ABSTRACTTo find an alternative for CVD diamond films and carbon nanotubes for low-field electron emission sources and cold cathode applications, we have designed a novel class of nanomaterials with excellent field emission properties – dielectric/carbon nanocomposites, in which different insulating nanoparticles such as silicon oxide, boron nitride in cubic and hexagonal forms, and nanodiamond were used inside a pyrocarbon matrix. The thickness of pyrocarbon shells covering the dielectric particles can be controlled in a wide range during the CVD process. The best samples of the nanocomposites with all four kinds of dielectric material show excellent field emission properties with threshold fields of as low as 0.5–1 V/μm, good surface uniformity and long-term stability. An effect of post-growth treatment with oxygen and hydrogen plasma on the emission was also studied. A mechanism of the emission is discussed basing on quantum properties of nanostructured thin (two-dimensional) carbon covering the dielectric particles. The mechanism could be suitable for wide range of nanostructured sp2-bonded carbon objects.

Field-emission characteristics of carbon buckypaper

Carbon buckypaper was investigated with respect to field-emission properties and long-term stability, especially under well-defined stress conditions. The carbon buckypaper field emitters showed a threshold field strength of about 1 V μm−1 for an emission current of 1 nA. Field-emission current densities of more than 10 mA cm−2 are measured for higher applied field strengths. The main focus of our investigations is the interaction of field emission and vacuum conditions. In this contribution, reproducible stress tests with microdischarges are presented. Higher emission current densities of about 100 mA cm−2 increased ionization, and luminescence out of the emission zone even appears before the microdischarges start. After arcing with microdischarges, the carbon buckypaper field emission has lower emission current density and lower variation range. However, the contribution demonstrates the advantages of carbon buckypaper as a large-array electron field emitter for cold-cathode applications.

Carbon Buckypaper field emission investigations

We investigated Carbon Buckypaper with respect to field emission properties, especially long-term field emission stability and under well-defined stress conditions. The Carbon Buckypaper field emitters showed a threshold field strength of about 1 V μm −1 for an emission current of 1 nA. Field emission current densities of more than 10 mA cm −2 are measured for higher applied field strengths. The main focus of our current investigations is the interaction of field emission and vacuum conditions. In this contribution reproducible stress tests with controlled micro-discharges are presented. At higher emission current densities of about 100 mA cm −2 increased ionization and even luminescence out of the emission zone appears before the micro-discharges start. After arcing with micro-discharges the Carbon Buckypaper field emission has lower emission current density and lower variation range. However, the contribution demonstrates the advantages of Carbon Buckypaper as a large array electron field emitter for cold cathode applications.

Defect induced lowering of work function in graphite-like materials

Different carbon materials have shown unusually high efficiency of electron field emission. The device application of the carbon cold cathodes requires a fundamental understanding of the emission mechanism. We describe the emission properties of carbon thin films grown by chemical vapor deposition. The structure, phase composition and electronic properties peculiar to the film material were investigated. New model of electron emission sites and mechanism of field electron emission are proposed. In accordance with the proposed model the electron emission in carbon materials occurs from sp3-like defects in an sp2 network of graphite-like material. The corresponding mechanism of field emission consists of electrons escaping into vacuum by tunneling from the Fermi level of the graphite material through the atomically thin sp3-like layer and the energy barrier on its surface.

Growth and characterization of rare-earth monosulfides for cold cathode applications

We report the successful growth of bulk cubic lanthanum sulfide (LaS) and neodymium sulfide (NdS). Powder x-ray diffraction scans of the samples show the rocksalt phase with a lattice constant a0=5.857(2) Å and 5.694(2) Å for LaS and NdS, respectively. In Raman scattering, one observes the vibrational density of states (cubic symmetry) and identifies the longitudinal optical and transverse acoustic phonons at 261(284) and 100(92) cm−1 with LaS(NdS), respectively. These rare-earth monosulfides offer attractive alternatives to the commonly used cesiated surfaces to reach negative electron affinity at various III–V semiconductor surfaces.

Aligned carbon nanotube films for cold cathode applications

Thin film material of oriented multiwall carbon nanotubes was obtained by noncatalytical chemical vapor deposition in a glow-discharge plasma. The film phase composition, surface morphology, and structural features were studied by Raman and electron microscopy techniques. Low-voltage electron field emission of thin film nanotube material was obtained and examined in diode configuration. The I–V curves in Fowler–Nordheim coordinates were linear and the corresponding threshold average field was about 1.5 V/μm. The emission current density was up to 50 mA/cm2 at the field of 5 V/μm. The emission site density reached 107 cm−2 at the same value of electric field.

Anomalous Field Enhancement in Planar Semiconducting Cold Cathodes from Spontaneous Ordering in the Accumulation Region

ABSTRACTWide band gap semiconductors exhibit a low electron affinity and may prove suitable for cold cathode applications. We introduce a simple closed-form analytic approximation for the stability of electrons in the electron accumulation layer of planar Low Electron Affinity (LEA) semiconducting cathodes. This analysis extends our previous results, which used Runge-Kutta numerical integration of the linearized equations of motion for the electric potential and quasi Fermi level. The model shows conditions in which the electrons in the accumulation layer form a two dimensional array of regions of higher and lower electron density. This instability could lead to field enhancement without surface roughness and could account for observed electron emission at low applied fields.

Integral and local field emission analyses of nanodiamond coatings for power applications

Nanodiamond (ND) powder coatings are very promising for cold cathode applications. The suitability of a (di)electrophoretically deposited ND coating (crystallite size: 1–10 nm) for high current applications was analyzed depending on three types of Si substrates (flat, rough, Si tip array). Nondestructive direct current field emission (FE) investigations were performed by means of a field emission scanning microscope. FE properties like the electrical onset field strength Eon (at 0.5 nA), current I vs E behavior, maximum current carrying ability as well as the local distribution and uniformity of emitters were analyzed by using anodes of different apex diameters D. A decrease of Eon for D=10/100/1000 μm from 203/190/109 V/μm of the flat to 67/44/20 V/μm of the rough, and to ≈7/6.7/4.5 V/μm of the tip array sample is achieved by the geometrical field enhancement of the structures. The reproducible high-current density JREP>15 A/cm2 for D=10 μm of the flat and rough sample exhibits the large potential of the ND coating for power applications. Their nonuniformity due to the μm-roughness results, however, in a low JREP for large areas. In contrast, ND coated tips showed a good uniformity over mm2 size and a moderate JREP≈0.2 A/cm2, basing on about 40% of significantly emitting tips.

Cold cathodes are heating up

One potential application of cold cathodes is flat panel displays. At present, the market for emissive displays is based on cathode ray tubes (CRTs) and liquid crystal displays (LCDs). However, these technologies do not offer the combination of bright, sharp images in a low power, lightweight and flat package. Cold cathodes should offer these advantages in the not-too-distant future. Since the international market for displays by the year 2000 is predicted to reach over $20 billion (US), an increasing number of innovative cold cathode concepts should be expected.

Thin films of aluminum nitride and aluminum gallium nitride for cold cathode applications

Cold cathode structures have been fabricated using AlN and graded AlGaN structures (deposited on n-type 6H-SiC) as the thin film emitting layer. The cathodes consist of an aluminum grid layer separated from the nitride layer by a SiO2 layer and etched to form arrays of either 1, 3, or 5 μm holes through which the emitting nitride surface is exposed. After fabrication, a hydrogen plasma exposure was employed to activate the cathodes. Cathode devices with 5 μm holes displayed emission for up to 30 min before failing. Maximum emission currents ranged from 10–100 nA and required grid voltages ranging from 20–110 V. The grid currents were typically 1 to 104 times the collector currents.

Enhancing low field electron emission from polycrystalline diamond

Abstract The onset of electron emission at modest electric fields can be promoted by treating boron doped thin film CVD diamond with well chosen combinations of gas bombardment and/or gas treatments. The latter are more successful in promoting the highest levels of current density and would appear to be a promising method for using relatively stable crystalline material for cold cathode applications. The exact nature of the modifications that take place are not clear but a combination of increasing the conductivity of electron pathways through the thin film with a lowering of surface work function is implicated.

Low-threshold cold cathodes made of nitrogen-doped chemical-vapour-deposited diamond

BECAUSE diamond surfaces terminated with hydrogen have a negative electron affinity1–4 (the conduction band minimum lies below the vacuum level), they are expected to emit electrons spontaneously. This has led to attempts to develop 'cold cathodes'—miniaturized vacuum diodes that might have applications in microelectronics and flat-panel displays. In previous studies of electron emission from diamond grown by chemical vapour deposition5–9 (CVD), the threshold voltages for emission were more than an order of magnitude too large for use in battery-driven cold cathodes. Although low-threshold emission from caesium-coated, nitrogen-doped high-pressure synthetic diamond was reported recently10, ultimately diamond thin films grown by chemical vapour deposition (CVD) look to be the most promising material for cold-cathode applications. But to obtain low-threshold emission, it is necessary to introduce high concentrations of donor dopants such as nitrogen—something that is difficult for CVD diamond. Here we report that high concentrations of nitrogen can be incorporated into diamond films by using urea as the gaseous nitrogen source, and that such heavily doped films shown very-low-threshold electron emission, which augurs well for cold-cathode technology.

Analysis for an electron source based on microchannel plate principles

An analysis is carried out for the channel output current density of a microchannel plate constructed from a crystalline semiconducting bulk. It is shown that the current density can be increased to the order of 1 A/cm/sup 2/, which is several orders of magnitude higher than that of conventional microchannel plates, allowing its use for cold cathode applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>