Mechanism Of Field Electron Emission Research Articles

Application of Murphy – Good Plot Parameters Extraction Method on Electron Emission from Carbon Fibers

The significance of the Fowler-Nordheim-type plots lies in the possibility of extracting useful and reliable physical parameters of the field electron emitters. This is achieved by the parameter's extraction methods. We report on the application of two parameters extraction methods on field emission data from bundles of carbon fibers (CFs) grouped in nickel tubes and operated inside a typical field emission microscope setup. These methods are the Murphy-Good plot and the conventional Fowler-Nordheim plot iterative method. The physical parameters include the area extraction parameter, the notional (actual) emission area, the formal area efficiency factor and the voltage conversion length. The results obtained from the two methods are discussed and compared to shed light on the controversial nature of these methods. The mechanism of field electron emission is analyzed based on the parameters extracted.

Mechanism of Field Electron Emission by a Field Cathode Coated with a Tunnel-Thin Diamond Film

The field-emission characteristics of cathode arrays based on nanocomposite diamond–graphite film structures with a reinforcing tunnel-thin diamond coating are investigated. These field emission characteristics are not related to the morphology of the emitting surface and cannot be explained in terms of the Fowler–Nordheim model. The effect of the thickness of the diamond coating on the field-emission characteristics and dielectric strength of the cathode heterostructure is determined. The obtained regularities are interpreted based on increasing transparency of potential barriers for electron tunneling due to a decrease in the mirror image forces.

Charge injection in thin dielectric layers by atomic force microscopy: influence of geometry and material work function of the AFM tip on the injection process

Charge injection and retention in thin dielectric layers remain critical issues for the reliability of many electronic devices because of their association with a large number of failure mechanisms. To overcome this drawback, a deep understanding of the mechanisms leading to charge injection close to the injection area is needed. Even though the charge injection is extensively studied and reported in the literature to characterize the charge storage capability of dielectric materials, questions about charge injection mechanisms when using atomic force microscopy (AFM) remain open. In this paper, a thorough study of charge injection by using AFM in thin plasma-processed amorphous silicon oxynitride layers with properties close to that of thermal silica layers is presented. The study considers the impact of applied voltage polarity, work function of the AFM tip coating and tip curvature radius. A simple theoretical model was developed and used to analyze the obtained experimental results. The electric field distribution is computed as a function of tip geometry. The obtained experimental results highlight that after injection in the dielectric layer the charge lateral spreading is mainly controlled by the radial electric field component independently of the carrier polarity. The injected charge density is influenced by the nature of electrode metal coating (work function) and its geometry (tip curvature radius). The electron injection is mainly ruled by the Schottky injection barrier through the field electron emission mechanism enhanced by thermionic electron emission. The hole injection mechanism seems to differ from the electron one depending on the work function of the metal coating. Based on the performed analysis, it is suggested that for hole injection by AFM, pinning of the metal Fermi level with the metal-induced gap states in the studied silicon oxynitride layers starts playing a role in the injection mechanisms.

Field emission properties of a-C and a-C:H films deposited on silicon surfaces modified with nickel nanoparticles**Project supported by the National Natural Science Foundation of China (Grant No. 51105186).

The a-C and a-C:H films are deposited on silicon surfaces modified with and without nickel nanoparticles by using mid-frequency magnetron sputtering. The microstructures and morphologies of the films are analyzed by Raman spectroscopy and atomic force microscopy. Field emission behaviors of the deposited films with and without nickel nanoparticles modification are comparatively investigated. It is found that the hydrogen-free carbon film exhibits a high field emission current density and low turn-on electric field compared with the hydrogenated carbon film. Nickel modifying could increase the current density, whereas it has no significant effect on the turn-on electric field. The mechanism of field electron emission of a sample is discussed from the surface morphologies of the films and nickel nanoparticle roles in the interface between film and substrate.

Current-voltage characteristic of the contact of a plasma with an electrode with a thin dielectric film on the surface

A model of the current flowing in the contact of a plasma with an electrode with a thin dielectric film on the surface has been developed to describe the observed features of the current-voltage characteristics of such a contact: a segment with small current near the floating potential corresponding to the film recharging and a segment with large current, where the mechanism of field electron emission through the film into the plasma occurs, leading to an N-shaped current-voltage characteristic. Such current-voltage characteristics are observed for W, Al, and Ta electrodes with a self-oxide film and for a stainless steel electrode with a silicon oxide film in a beam-plasma discharge in hydrogen. The model is based on the calculation of the equilibrium potential of the film surface faced to the plasma for the region of the negative bias of the electrode with respect to the plasma potential. Balance involves currents of ions and electrons from the plasma, secondary emission currents, and field electron emission current from the electrode into the plasma through the insulator. The film recharging voltage calculated within the model is in exact agreement with experimental data. This allows the determination of the thickness of the dielectric layer on the electrode surface from its current-voltage characteristic.

The Fowler−Nordheim Plot Behavior and Mechanism of Field Electron Emission from ZnO Tetrapod Structures

Field emission measurements of current-voltage characteristics are reported for tetrapod structures of ZnO. The nonlinear Fowler-Nordheim (FN) plot is analyzed according to a model of calculation based on saturation of conduction band current and predominance of valence band current at high-field values. The simulated FN plot exhibits similar features to those observed experimentally. The model of calculation suggests that the slope variation of the FN plot, in the high-field and low-field regions, does not depend on the magnitude of saturation. Instead, it is a characteristic of the energy band structure and voltage-to-barrier-field conversion factor of the emitting material.

Field electron emission from HfN xO y thin films deposited by direct current sputtering

HfN x O y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm 2, and at the electric field of 24 V/μm, the current density is up to 1 mA/cm 2. The field electron emission mechanism of the HfN x O y thin film is also discussed.

Low pressure chemical vapor deposition synthesis of InN nanowires and their field electron emission

InN nanowires were prepared on Si substrates by low pressure chemical vapor deposition using Au as the catalyst. Scanning electron microscopy showed that the diameter of these nanowires is 60—100 nm, and the length is larger than 1 μm. High resolution transmission electron microscopy showed that the synthesized nanowires are a mixture of hexagonal and cubic phase. The field electron emission characteristics of these InN nanowires are good and the field emission current is stable. The turn-on electric field was 10.02 V/μm(the current density being 10 μA/cm2)，and at the high electric field of 24 V/μm，the current density was as high as to 5.5 mA/cm2. The field electron emission mechanism of these nanowires is discussed.

Mechanism of field electron emission from carbon nanotubes

Field electron emission (FE) is a quantum tunneling process in which electrons are injected from materials (usually metals) into a vacuum under the influence of an applied electric field. In order to obtain usable electron current, the conventional way is to increase the local field at the surface of an emitter. For a plane metal emitter with a typical work function of 5 eV, an applied field of over 1 000 V/μm is needed to obtain a significant current. The high working field (and/or the voltage between the electrodes) has been the bottleneck for many applications of the FE technique. Since the 1960s, enormous effort has been devoted to reduce the working macroscopic field (voltage). A widely adopted idea is to sharpen the emitters to get a large surface field enhancement. The materials of emitters should have good electronic conductivity, high melting points, good chemical inertness, and high mechanical stiffness. Carbon nanotubes (CNTs) are built with such needed properties. As a quasi-one-dimensional material, the CNT is expected to have a large surface field enhancement factor. The experiments have proved the excellent FE performance of CNTs. The turn-on field (the macroscopic field for obtaining a density of 10 μA/cm2) of CNT based emitters can be as low as 1 V/μm. However, this turn-on field is too good to be explained by conventional theory. There are other observations, such as the non-linear Fowler-Nordheim plot and multi-peaks field emission energy distribution spectra, indicating that the field enhancement is not the only story in the FE of CNTs. Since the discovery of CNTs, people have employed more serious quantum mechanical methods, including the electronic band theory, tight-binding theory, scattering theory and density function theory, to investigate FE of CNTs. A few theoretical models have been developed at the same time. The multi-walled carbon nanotubes (MWCNTs) should be assembled with a sharp metal needle of nano-scale radius, for which the FE mechanism is more or less clear. Although MWCNTs are more common in present FE applications, the single-walled carbon nanotubes (SWCNTs) are more interesting in the theoretical point of view since the SWCNTs have unique atomic structures and electronic properties. It would be very interesting if people can predict the behavior of the well-defined SWCNTs quantitatively (for MWCNTs, this is currently impossible). The FE as a tunneling process is sensitive to the apex-vacuum potential barrier of CNTs. On the other hand, the barrier could be significantly altered by the redistribution of excessive charges in the micrometer long SWCNTs, which have only one layer of carbon atoms. Therefore, the conventional theories based upon the hypothesis of fixed potential (work function) would not be valid in this quasi-one-dimensional system. In this review, we shall focus on the mechanism that would be responsible for the superior field emission characteristics of CNTs. We shall introduce a multi-scale simulation algorithm that deals with the entire carbon nanotube as well as the substrate as a whole. The simulation for (5, 5) capped SWCNTs with lengths in the order of micrometers is given as an example. The results show that the field dependence of the apex-vacuum electron potential barrier of a long carbon nanotube is a more pronounced effect, besides the local field enhancement phenomenon.

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.

Field electron emission in graphite-like films

Results of investigation of carbon films deposited with the use of gas-phase chemical reactions in the plasma of a dc discharge are presented. Films obtained at different parameters of the deposition process varied widely in their structure and phase composition, from polycrystalline diamond to graphite-like material. Comparative study of the structure and phase composition of the films using Raman spectroscopy, cathodoluminescence, electron microscopy, and diffractometry, as well as the obtained field electron emission characteristics, have shown that the threshold value of the electric field strength for electron emission decreases with a decrease in the size of diamond crystallites and growth of the fraction of non-diamond carbon. The lowest threshold fields (less than 1.5 V/µm) are obtained for films consisting mainly of graphite-like material. A model based on the experimental data is proposed, which explains the mechanism of field electron emission in carbon materials.

Similarity in field electron emission from nanocrystalline diamond and related materials

Results on a complex study of macroscopic and microscopic electronic properties of nanocrystalline diamond and related material (BN, GaN, CN) films are presented. It was found that all the samples studied showed similar dependencies of macroscopic emission parameters (the Fowler–Nordheim work function, the effective emitting area) on the film emissivity. Also, some microscopic properties were found to be common for all the films tested. It was generally observed that field electron emission occurred at nanosized regions on the boundary of high and low resistivity areas, and peaks of the emission intensity were associated with a lowered surface electron potential. Based on the experimental data, a mechanism of field electron emission from the surface of nanocrystalline films is proposed. We suppose that electrons are transported from narrow conducting channels into vacuum through specific low-dimensional regions, the electronic properties of which, due to the quantum size effect, are different from the bulk material. In this case the base material grains play a role of the conductive channel-containing matrix as well as a heat sink. Details of the new low-field emission mechanism including the filling of discrete energy states, and the decrease in the work function under the action of electric field are discussed.

Thermostimulated electron emission from polar cleavage of a triglycine sulfate crystal

This paper reports the results of experimental investigations into thermostimulated electron emission from the positive and negative cleavage surfaces of a ferroelectric triglycine sulfate crystal. It is shown that the current density j of emission from the negative surface is universally higher than that from the positive surface over the entire range from room temperature to the Curie point. The shape of the j(T) curves substantially depends on the degree of natural unipolarity of the studied samples. The results obtained are discussed in the framework of the mechanism of field electron emission from the surface electronic states.

Analysis of physical mechanism of field electron emission from ferroelectrics

Pulsed electron emission from ceramic composition PLZT 2/95/5 and x/65/35 (x = 7;8;9;13) has been observed. Three levels of emission are shown and peculiarities of each of them are interpreted.