Low Macroscopic Field Research Articles

Low-macroscopic field emission structure: Using the shape of the conductor to identify a local difference in electric potential

The emission of electrons from an insulating crystal deposited on a conductor occurs at a macroscopic electric field of a few volts per micrometer, 3 orders of magnitude below the field emission from a clean metal. This is due to the local field enhancement induced by the presence of the insulating crystal. The emission profiles depend on the shape of the conductive substrate; analyzing these profiles enables the local difference in electric potential and the opening angle to be traced. Given the thickness of the crystal, the local difference in potential indicates the local field enhancement of a few volts per nanometer applied to the conductor.

Advances in Novel Low-Macroscopic Field Emission Electrode Design Based on Fullerene-Doped Porous Silicon

Perspective low-macroscopic field (LMF) emission prototype cathodes based on fullerene C60—doped porous silicon were realized via a two-stage technique comprising the electrochemical etching process of a monocrystalline silicon wafer and functionalization of the acquired porous silicon (PS) matrix with silver-doped fullerene-based carbon structures. The resulting LMF cathode prototypes were studied with SEM and EDS techniques. The formation of an amorphous silver-doped C60-based layer consisting of nanosized aggregates on the matrix surface was established. The emission characteristics of the prototypes were analyzed, crucial parameters including threshold field strength values, emission current density, and effective potential barrier height for electrons were considered. A novel LMF emission model is suggested. It was established that the emitter prototypes realized during this study are on par with or superior to modern and promising field cathodes.

尖鋭構造を持たない炭素膜からの電界放出

Field emission features from Si and W tips are drastically improved upon coating even with simple carbon films. In this paper, in order to obtain clues to clarify the mechanism, we examine atomistic STM observations including local barrier height (LBH) and field emission imagings of the carbon films. And we also examine the field emission features from the shape-controlled W tips before and after the carbon coatings, enabling us to estimate the local electric field at the tip apex. From the STM observations, we find that the carbon film consists of nm-scale grains with various electronic properties and that the FE current varies grain by grain and is higher near the edge of each grain. The effective work functions evaluated from the slopes of the Fowler-Nordheim (FN) plot are constant regardless of the thickness of the carbon coating, although they are largely scattered for the films thinner than 1 nm. On the basis of the observed results, we discuss the mechanism of low macroscopic field emission from carbon films.

Morphology-dependent low macroscopic field emission properties of titania/titanate nanorods synthesized by alkali-controlled hydrothermal treatment of a metallic Ti surface

One-dimensional (1D) and two-dimensional (2D) titania/titanate nanostructures are fabricated directly on a self-source metallic titanium (Ti) surface via in situ surface re-construction of a Ti substrate using potassium hydroxide (KOH) under a hydrothermal (HT) condition. The effect of temperature and the concentration of KOH on the variations in morphology and titania-to-titanate phase changes are studied and explained in detail. A growth model is proposed for the formation process of the platelet-to-nanorod conversion mechanism. The field emission (FE) properties of titania/titanate nanostructures are studied, and the effects of the morphologies (such as 1D nanorods, 2D nanoplatelets, and a mixture of 1D nanorods and 2D platelets) on the FE properties of the samples are investigated. The samples depict a reasonable low turn-on field and emission stability. The FE mechanism is observed to follow standard Fowler–Nordheim (FN) electron tunneling. The geometrical field enhancement factor (β) is measured to be very high, and is compared with theoretical values calculated from various existing models to explore the feasibility of these models. The surface modification of metallic Ti by a simple non-lithographic bottom-up method and the low-macroscopic FE properties can provide a potential alternative to field emission displays for low-power panel technology.

Field-Induced Crystalline-to-Amorphous Phase Transformation on the Si Nano-Apex and the Achieving of Highly Reliable Si Nano-Cathodes.

Nano-scale vacuum channel transistors possess merits of higher cutoff frequency and greater gain power as compared with the conventional solid-state transistors. The improvement in cathode reliability is one of the major challenges to obtain high performance vacuum channel transistors. We report the experimental findings and the physical insight into the field induced crystalline-to-amorphous phase transformation on the surface of the Si nano-cathode. The crystalline Si tip apex deformed to amorphous structure at a low macroscopic field (0.6~1.65 V/nm) with an ultra-low emission current (1~10 pA). First-principle calculation suggests that the strong electrostatic force exerting on the electrons in the surface lattices would take the account for the field-induced atomic migration that result in an amorphization. The arsenic-dopant in the Si surface lattice would increase the inner stress as well as the electron density, leading to a lower amorphization field. Highly reliable Si nano-cathodes were obtained by employing diamond like carbon coating to enhance the electron emission and thus decrease the surface charge accumulation. The findings are crucial for developing highly reliable Si-based nano-scale vacuum channel transistors and have the significance for future Si nano-electronic devices with narrow separation.

Zener tunneling in conductive graphite/epoxy composites: Dielectric breakdown aspects

The electrical responses of conductive graphite/epoxy composites subjected to an applied electric field were investigated. The results showed that reversible dielectric breakdown can easily occur inside the composites even under low macroscopic field strengths. This is attributed to the Zener effect induced by an intense internal electric field. The dielectric breakdown can yield new conducting paths in the graphite/epoxy composites, thereby contributing to overall electrical con- duction process.

Low-macroscopic field emission properties of wide bandgap copper aluminium oxidenanoparticles for low-power panel applications

Field emission properties of CuAlO2 nanoparticles are reported for the first time, with a low turn-on field of approximately2 V µm − 1 and field enhancement factor around 230. The field emission process follows the standardFowler–Nordheim tunnelling of cold electron emission. The emission mechanism is found tobe a combination of low electron affinity, internal nanostructure and large fieldenhancement at the low-dimensional emitter tips of the nanoparticles. The field emissionproperties are comparable to the conventional carbon-based field emitters, and thuscan become alternative candidate for field emission devices for low-power panelapplications.

Nanometer-scale distribution of field emission current from the arc-prepared carbon thin film

In order to clarify the mechanism of low-macroscopic-field (LMF) electron emission from carbon related materials, atomistic properties including local tunneling barrier height (LBH) and field emission (FE) current distributions of the arc-prepared carbon thin film have been measured by using scanning tunneling microscopy. From the LBH images, it is found that the carbon thin film surface consists of nanometer-scale graphite grains whose crystallinities and electronic properties are different although the local work function of the film is rather homogeneous. The obtained FE images show that the FE currents are varied more than one order of magnitude, depending on the grains, and that the FE current is higher at the edge of the individual grains. The authors cannot find any specific emission sites that give extremely higher emission currents, indicating that the specific emission sites such as high-aspect-ratio nanoprotrusions or atomistic defects are not responsible to the LMF electron emission from carbon related materials.

Field penetration into amorphous‐carbon films: consequences for field‐induced electron emission

AbstractThe phenomenon of low‐macroscopic‐field (LMF) electron emission is sometimes exhibited by thin films of non‐metallic materials. Both wide‐band‐gap semiconductors and hopping conductors such as amorphous carbon exhibit this phenomenon, and its origin has been under discussion for some years. This paper first uses a simple theory to discuss field penetration into hopping conductors with a high density of localised states near the charge‐neutrality level. This theory is physically similar to widely used existing theories, in that it assumes continuous charge distributions and uses the one‐dimensional Poisson's equation. The theory suggests that little field penetration occurs, and that models of LMF emission based on uniform barrier lowering are not viable. Unexpected self‐consistency difficulties are also created for some types of local‐field‐enhancement model. A re‐assessment of the validity of this simple field‐penetration theory suggests the following. Contrary to a basic assumption of this type of theory, the granularity of the screening charge distribution is a dominant feature of the physics associated with LMF emission from nanoscale‐sized emission sites on thin films of hopping conductors. The authors think that the screening is carried out by discrete charges in localised states, although details are not clear. This conclusion is potentially very far‐reaching. In particular, new forms of field‐penetration theory may be required before definitive decisions can be made about the self‐consistency of some models of local field enhancement. Copyright © 2007 John Wiley & Sons, Ltd.

Low-macroscopic field emission from silicon-incorporated diamond-like carbon film synthesized by dc PECVD

Silicon-incorporated diamond-like carbon (Si–DLC) films were deposited via dc plasma-enhanced chemical vapor deposition (PECVD), on glass and alumina substrates at a substrate temperature 300 °C. The precursor gas used was acetylene and for Si incorporation, tetraethyl orthosilicate dissolved in methanol was used. Si atomic percentage in the films was varied from 0% to 19.3% as measured from energy-dispersive X-ray analysis (EDX). The binding energies of C 1s, Si 2s and Si 2p were determined from X-ray photoelectron spectroscopic studies. We have observed low-macroscopic field electron emission from Si–DLC thin films deposited on glass substrates. The emission properties have been studied for a fixed anode–sample separation of 80 μm for different Si atomic percentages in the films. The turn-on field was also found to vary from 16.19 to 3.61 V/μm for a fixed anode–sample separation of 80 μm with a variation of silicon atomic percentage in the films 0% to 19.3%. The turn-on field and approximate work function are calculated and we have tried to explain the emission mechanism there from. It was found that the turn-on field and effective emission barrier were reduced by Si incorporation than undoped DLC.

Low-macroscopic field emission from nanocrystalline Al doped SnO2 thin films synthesized by sol–gel technique

We have observed low-macroscopic field electron emission from wide bandgap nanocrystalline Al doped SnO2 thin films deposited on glass substrates. The emission properties have been studied for different anode-sample spacings and for different Al concentrations in the films. The turn-on field and approximate work function were calculated and we have tried to explain the emission mechanism from this. The turn-on field was found to vary in the range 5.6–7.5 V/μm for a variation of anode sample spacing from 80–120 μm. The turn-on field was also found to vary from 4.6–5.68 V/μm for a fixed anode-sample separation of 80 μm with a variation of Al concentration in the films 8.16–2.31%. The Al concentrations in the films have been measured by energy dispersive X-ray analysis. Optical transmittance measurement of the films showed a high transparency with a direct bandgap ∼3.98 eV. Due to the wide bandgap, the electron affinity of the film decreased. This, along with the nanocrystalline nature of the films, enhanced the field emission properties.

Electro-optical characteristics and field-emission properties of reactive DC-sputtered p-CuAlO 2+x thin films

P-type transparent-conducting CuAlO 2+ x thin films were deposited on silicon and glass substrates by reactive direct current sputtering of a prefabricated metal powder target having 1:1 atomic ratio of Cu and Al in oxygen-diluted argon atmosphere. XRD spectrum confirmed the proper phase formation of the material. UV-Vis-NIR spectrophotometric measurements showed high transparency of the films in the visible region with direct and indirect band gap values around 3.90 and 1.89 eV, respectively. The room temperature conductivity of the film was of the order of 0.22 S cm −1 and the activation energy was ∼0.25 eV. Seebeck coefficient at room temperature showed a value of +115 μV/K confirming the p-type nature of the film. Room temperature Hall effect measurement also indicated positive value of Hall coefficient with a carrier concentration ∼4.4×10 17 cm −3. We have also observed the low macroscopic field emission, from the wide band gap p-CuAlO 2+ x thin film deposited on glass substrate. The emission properties have been studied for different anode-sample spacing. The threshold field was found to be as low as around 0.5–1.1 V/μm. This low threshold is attributed primarily to the internal nanostructure of the thin film, which causes considerable geometrical field enhancement inside the film as well as at the film/vacuum interface.

Plasma sprayed ceramics for low macroscopic field emitters

Plasma sprayed ceramics for low macroscopic field emitters

Low-macroscopic field emission from fibrous ZnO thin film prepared by catalyst-free solution route

Fiber-structured zinc oxide thin films were synthesized by catalyst-free chemical process from zinc acetate dihydrate at an elevated temperature of 1073K on Si substrate. X-ray diffraction spectra confirmed the proper phase formation of the films. Scanning electron micrograph depicted the formation of ZnO fibers with an average diameter of 500nm with an average aspect ratio ∼150. Field emission properties of the film showed considerably low turn-on field around 1.4V/μm. This low-threshold field-emission of the films are attributed to the conducting nanostructure inside the film as well as on the tip of the fibers at the film–vacuum interface, which causes the geometrical field enhancement at the emitter-tip and provides the required barrier field for field-emission tunneling. The emission current was as high as ∼70μA at the field of 3.6V/μm. The high emission current, high stability and low turn-on field make the ZnO fibers a strong candidate for field-emission displays.

Low macroscopic field electron emission from surface of plasma sprayed and laser engraved TiO 2, Al 2O 3+13TiO 2 and Al 2O 3+40TiO 2 coatings

The paper deals with investigation of electron field emission from plasma sprayed and laser engraved TiO 2 and Al 2O 3–TiO 2 coatings of two compositions (13 and 40 wt.% of TiO 2). The coatings were plasma sprayed onto aluminum rolls coated initially with Ni20Cr alloy. Their technology included also grinding and polishing prior to CO 2 laser treatment. The treatment was similar to that applied for anilox rolls manufacturing and allowed a production of three different patterns with the lines densities of 60, 100 and 200 cm −1 and depths of approximately 80, 50 and 10 μm correspondingly. The patterns were engraved under angle of 60° that results in a honeycomb geometry of cells. Measurements of emission current under macroscopic electric field up to 150 V/μm for all the samples were carried out. The SEM observations of engraved coating morphology were made in order to find the electron emission spots and XRD analysis allowed to identify the crystal phases present in the coatings. A tentative explanation of a field emission mechanism basing onto Fowler–Nordheim (FN) model is also presented.

Low-threshold field emission from transparent p-type conducting CuAlO 2 thin film prepared by dc sputtering

We have observed the low-macroscopic field (LMF) emission, at a relatively lower threshold, from a wide bandgap CuAlO 2 thin film having p-type conductivity deposited on glass substrate. The emission properties have been studied for different anode-sample spacing. The range of the threshold field is calculated and we have tried to explain the emission mechanism therefrom. The threshold field has been found to be as low as 0.9 V/μm. This unusually low threshold is attributed primarily to the internal nanostructure of the thin film, which causes geometrical field enhancement inside as well as at the film/vacuum interface. Also, secondary effect of the presence of surface states, causing further field enhancement may not be ruled out.

Low-macroscopic-field electron emission from piezoelectric thin films and crystals

Low-macroscopic-field (LMF) electron emission is an interesting and still somewhat enigmatic phenomenon. It occurs under conditions when the electric field estimated from the macroscopic device geometry is by orders of magnitude lower than the local field value ∼10 7 V cm −1 necessary to provide a sufficient transparency of the potential barrier for tunneling electrons. The mechanisms that convert the LMF into a high local field can range from trivial field enhancement caused by surface roughness to more intricate mechanisms, which are partly not well understood. We have developed and prepared field emitters based on piezoquartz single crystals as well as on textured thin films of piezoactive compounds ZnO, ZnS, CdS, Zn x Cd 1 −x S, ZnSe, CdSe, SiO 2 and tested them in flat diode cells and in Mueller's field emission microscopes. All the cathodes generate electron emission in electric fields ∼10 5 V cm −1 as determined from the macrogeometry of the devices. A remarkable feature is the high emission current stability at j⩾1 A cm −2 even under vacuum ∼10 −5 Torr. The cathodes have been utilised to make prototypes of flat matrix displays with cathodoluminescent screen brightness up to 300 cd m −2. The characteristics of elementary emitters show also a rather high uniformity which, in the case of multitip arrays, can only be attained by a complicated technology that provides each tip with an individual damping resistor. To explain the unique properties of the piezoelectric field emitters, a model has been suggested which predicts the appearance of local electric fields up to 10 7 V cm −1, due to combined enhancing action of the direct and inverse piezoeffects in samples with a large ratio of dimensions along different directions. The electric field in the near-surface region of such piezoelectric emitters can achieve values sufficient to cause Zener's effect, and conditions of effective negative electron affinity (NEA) can then be realised at the emitter surface. This model accounts for both the high emission stability in poor vacuum and the high spatial uniformity of the emission over the matrix emitter surface. Indeed, the state of the emitter surface plays only a minor role because of the effective NEA regime, and the piezoelectric film itself performs the function of a damping resistor. The model is corroborated by the existence of a correlation between the slope of Fowler-Nordheim plots and the band-gap width of the piezoelectric film. Another argument in its favour is that, contrary to the above results, no measurable emission has been found for such non-piezoactive materials as Al 2O 3 and fused quartz, even at macroscopic fields up to 10 6 V cm −1. The piezoelectric LMF emitters can be exploited in various micro- and nanoelectronic vacuum devices.

Field-induced electron emission from electrically nanostructured heterogeneous (ENH) materials

This paper discusses the origins of field-induced electron emission from thin films of electrically nanostructured heterogeneous (ENH) materials. Such materials exhibit low macroscopic field (LMF) electron emission: as thin films on a conducting substrate, they emit electrons into vacuum when LMFs, typically about 1–10 V/μm, are applied. An ENH material comprises: a dielectric matrix, which may contain nanoscale inclusions of higher electrical conductivity; conducting channels that open in the dielectric between the inclusions (if present) and between them and the substrate; and an electron emitting channel that opens into vacuum. Electrically nanostructured heterogeneous materials can have a variety of different detailed structures and theories, but all can exhibit LMF emission under suitable circumstances. This paper provides an updated summary of an integrating overview recently presented to explain LMF emission. A central feature is that electrical nanostructure within the film can create internal field enhancement, thereby producing a high local field at the vacuum interface: this enables thermalised electrons to escape rapidly into vacuum by tunnelling. The question of what aspect of the system controls the emission current is a separate issue. Various features and implications of the theory are set out.

Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism

Thin flat dielectric films can be low-macroscopic-field (LMF) electron emitters, able to generate electrons when subject to a macroscopic electric field in the range 1–50 V μm −1. This phenomenon is a known cause of pre-breakdown currents in high-voltage vacuum breakdown, and is now the basis of a broad-area electron-source technology, using carbon-based thin films and other materials. The phenomenon occurs because the dielectric film is, or becomes, an electrically nanostructured heterogeneous (ENH) material, with quasi-filamentary conducting channels between its surfaces. These channels connect to emitting features near or on the film/vacuum surface, or act as electron emitters themselves. The film may contain conducting or semiconducting particles that assist with conductivity and/or act as emitting features. Several forms of thin-film LMF emitter exist: in each case the situation geometry ensures that sufficient field enhancement occurs at the ‘tip’ of the emitting feature for the emission process to be some form of tunnelling field electron emission (probably ‘cold’ in some cases, ‘hot’ in others). This paper explores aspects of the theory of thin-film LMF emission, starting from the need to understand the behaviour of emitters based on amorphous carbon films. A summary review, with extensive references, is given of relevant past work outside the immediate ‘carbon field emission’ context. Relevant aspects of semiconductor field emission theory are noted. Comment is made on the original experiments on diamond field emission, and on theoretical misconceptions in the carbon field emission literature. Analysis of carbon-film emitter behaviour suggests that emission must primarily be due to geometrical field enhancement, that in at least some cases arises from conducting nanostructure inside the film. In one case, published film characteristics can be used to show that sufficient field enhancement should be available. Some problems with an ‘internal field enhancement’ hypothesis are considered and disposed of. Difficulties with Latham’s theory of field-induced emission from ENH materials are pointed out: a new theory, largely qualitative at this stage, can explain longstanding problems: this assumes that dielectric films must be treated as ‘hopping conductors’ not semiconductors. Electron emission takes place via localised surface states: transition to a channel-limited current regime takes place when the surface states no longer have high enough occupation probability to screen the external field, and is accompanied by anomalous band bending at the channel tip. Mathematical theories of band bending and field emission for hopping conductors are required. Some consequences for the design of LMF emitters are noted.