Cold Cathode Electron Emitters Research Articles

Understanding temporal evolution of microstructures on metal-assisted chemically etched Ge surface and its applications

We report on metal-assisted chemical etching (MaCE) of single crystalline p-Ge(100) substrates at an elevated temperature, using dilute H2O2 solution, for various etching times in the range of 30–1200 s. We carry out atomic force microscopic and scanning electron microscopic studies to investigate the temporal evolution of microstructures on Ge substrates. It is observed that at lower etching times rough surfaces evolve, whereas higher etching times lead to the formation of pyramidally textured surfaces. In order to understand the present observations numerical simulation studies, based on continuum theory of stress-induced morphological instability, are carried out. Further, in an attempt to demonstrate the tunable multifunctional properties of pyramidally textured-Ge surfaces, we study light trapping and cold cathode electron emission properties of the same as a function of etching time. Interestingly, the optical reflectance of pyramidally textured surfaces reduces systematically with increasing etching time and the one evolved under an etching time of 1200 s can go down to an unprecedented low value of 0.23% over a broad spectral range (600–3000 nm). In addition, the turn-on potential for cold cathode electron emission from the etched-Ge surfaces also reduces steadily with increasing etching time with the lowest one being 1.7 V µm−1 (with the field-enhancement factor of 15,171) for the highest etching time of 1200 s. These are by far the best known results for any chemically etched-Ge surface and offer tremendous potential to fabricate various Ge-based optoelectronic devices.

Mathematical modelling of the electric field of carbon nanotube arrays used in cold cathode electron emission devices

Influence of the ratios of the carbon nanotube arrays geometrical parameters (height to radius – H/R, distance between the individual nanotube to height – S/H) on their properties to enhance the electric field strength at their tips and to accumulate electrostatic charge have been investigated. It was done with the help of their electric field mathematical modelling. Such modelling has shown that dependences of the emission current density on S/H ratio have maximums that are determined both by H/R and the applied electric field strength levels.

Cold cathode electron emission with ultralow turn-on fields from Au-nanoparticle-decorated self-organized Si nanofacets

Fabrication of highly dense conical nanostructures and their subsequent controlled metallization make them ideal candidates for enhancing cold cathode electron emission efficiency.

Experimental and simulation studies on temporal evolution of chemically etched Si surface: Tunable light trapping and cold cathode electron emission properties

Anisotropic alkaline etching of single crystalline p-Si(100) substrates is carried out for different times (in the range of 30–2400 s). This leads to the formation of randomly distributed pyramidal structures on Si surfaces, as observed from atomic force microscopy (AFM) and scanning electron microscopy images. During early stages of etching, rough surfaces evolve, but for longer etching times, pyramidally textured surfaces (having dimensions in the range of 0.2–2 μm) are formed. The formation of pyramidal structures is explained in light of simulation studies based on the continuum theory of stress-induced morphological instability. The power spectral density plots extracted from the experimental AFM images and the simulated images show that while the correlation length increases for lower etching times, it gets saturated for higher etching times. These facts corroborate well with our experimental results that reveal increasing pyramidal size with etching time. In addition, we study the temporal evolution of antireflection and field emission properties of such pyramidally textured-silicon substrates in line with their potential use in solar cells and moderate level cold cathode electron emission, respectively. For instance, it is interesting to note that surface reflectance of these pyramidally textured surfaces (formed at higher etching times) can be brought down to as low as 0.4% over a broad spectral range, viz., 300–3000 nm. Likewise, the field emission data show that turn-on potential decreases with increasing etching time (e.g., 2.62 V μm−1 for an etching time of 1200 s).

Fabrication of Stable Carbon Nanotube Cold Cathode Electron Emitters with Post-Growth Electrical Aging.

We fabricated carbon nanotube (CNT) cold cathode emitters with enhanced and stable electron emission properties and long-time stability with electrical aging as a post-treatment. Our CNT field emitters showed improved electrical properties by electrical aging. We set the applied bias for effective electrical aging, with the bias voltage defined at the voltage where Joule heating appeared. At the initial stage of aging, the electron emission current started to increase and then was saturated within 3 h. We understood that 5 h aging time was enough at proper aging bias. If the aging bias is higher, excessive heating damages CNT emitters. With the electrical aging, we obtained improved electron emission current from 3 mA to 6 mA. The current of 6 mA was steadily driven for 9 h.

Surfing Silicon Nanofacets for Cold Cathode Electron Emission Sites.

Point sources exhibit low threshold electron emission due to local field enhancement at the tip. In the case of silicon, however, the realization of tip emitters has been hampered by unwanted oxidation, limiting the number of emission sites and the overall current. In contrast to this, here, we report the fascinating low threshold (∼0.67 V μm-1) cold cathode electron emission from silicon nanofacets (Si-NFs). The ensembles of nanofacets fabricated at different time scales, under low energy ion impacts, yield tunable field emission with a Fowler-Nordheim tunneling field in the range of 0.67-4.75 V μm-1. The local probe surface microscopy-based tunneling current mapping in conjunction with Kelvin probe force microscopy measurements revealed that the valleys and a part of the sidewalls of the nanofacets contribute more to the field emission process. The observed lowest turn-on field is attributed to the absence of native oxide on the sidewalls of the smallest facets as well as their lowest work function. In addition, first-principle density functional theory-based simulation revealed a crystal orientation-dependent work function of Si, which corroborates well with our experimental observations. The present study demonstrates a novel way to address the origin of the cold cathode electron emission sites from Si-NFs fabricated at room temperature. In principle, the present methodology can be extended to probe the cold cathode electron emission sites from any nanostructured material.

Radial AlN nanotips on carbon fibers as flexible electron emitters

A facile catalyst-free approach using a simple thermal transport method has been developed to fabricate high-density AlN nanotips on flexible carbon cloth at large scales for use as field emission (FE) emitters. The AlN nanotips exhibit good performance as flexible cold-cathode electron emitters, with a very low turn-on electric field of 1.1–2.3Vμm−1, a low threshold electric field of 1.5–2.5Vμm−1, and a high emission current density. The excellent field emission properties of the AlN nanotips are attributed to the large field enhancement factor of 6895 as well as the combined effect of the tip profile of the AlN nanostructures and the excellent electron transport path of the conductive carbon cloth substrate.

Facile fabrication of aligned SnO2 nanotube arrays and their field-emission property

The aligned SnO2 nanotube arrays on Si substrate have been prepared via liquid-phase deposition with the solution system of SnF2 utilizing ZnO nanorod arrays as sacrificial template. The SnO2 nanotubes are distributed uniformly and grown perpendicularly to the substrate. The SnO2 nanotubes are rutile phase with the outside diameter of ~190–310nm and the wall thickness of ~45 nm. The template-related growth mechanism for the formation of SnO2 nanotube is investigated. Experimentally, the SnO2 nanotube array shows an efficient field-emission property, which is favorable for realizing high performance cold cathode electron emitters.

Position-controlled ZnO nanoflower arrays grown on glass substrates for electron emitterapplication

The electron emission of position-controlled grown ZnO nanoflowers was investigated forapplication in cold cathode electron emission devices. ZnO nanoflower arrays, composed ofseveral nanoneedles with sharp tips, were grown selectively on a conducting glass substrateusing a chemical solution deposition method. The morphology and position of the ZnOnanoflowers were controlled by preparing polymethylmethacrylate submicron patterns usingelectron-beam lithography. Without the patterns, in contrast, vertical ZnO nanoneedleswere randomly grown on the substrates with high density. Several samples preparedat the same conditions exhibited almost the same nanoflower morphology andfield emission characteristics. Comparison of the field emission characteristics ofthe ZnO nanoflower arrays and ZnO nanoneedles showed that the arrays hadexcellent electron emission characteristics, with a low turn-on electric field of0.13 V µm−1 at0.1 µA cm−2 and a high emissioncurrent density of 0.8 mA cm−2 in anapplied electric field of 9.0 V µm−1. Furthermore, light-emitting devices made using ZnO nanoflower arrays demonstratedstrong light emission, and micropixels for display application were clearly displayed.

A New Carbon Nanotube-Based Field Enhanced Thermionic Cathode

ABSTRACTThe electron emission properties and field enhancement effects of carbon nanotubes (CNTs) have been extensively studied. However, all of these studies focus only on the field emission aspect of the materials and its application in cold cathode electron emitters. So far, we have not seen any studies in the literature that link CNTs with thermionic cathodes, which are an equally important cathode type because of their many applications. We present a study of field enhanced electron emissions from a new type of cathode: the CNT-based field enhanced thermionic cathode. This new cathode consists of a metal substrate with CNTs grown on top of its surface. The CNTs are coated with thermionic emission materials (BaO, SrO, and CaO). This unique cathode structure takes advantage of both the field enhancement effect from CNTs and the high electron emission capability of thermionic materials. The electron emission properties of this new cathode, particularly the field enhancement factor and effective work function, are compared with the conventional thermionic cathodes that are made of same oxide coating.

Electron emission of carbon nitride films and mechanism for the nitrogen-lowered threshold in cold cathode

Carbon nitride films have been deposited by rf reactive magnetron sputtered graphite carbon in an N2 discharge. The process parameters, viz., nitrogen partial pressure (PN2), substrate temperature (Ts), and substrate bias (Vb) were varied in order to investigate their influence on the field emission properties. The effective work function for carbon nitride films determined using the Fowler–Nordheim equation is in the range of 0.01–0.1 eV. Insight is presented into the nitrogen-lowered threshold of cold cathode electron emission of carbon from the perspective of nitride tetrahedron bond formation. The involvement of nonbonding (lone pair) and lone-pair-induced antibonding (dipole) states is suggested to be responsible for lowering the work function and hence the electron emission threshold. It is found that the substrate temperature of 200 °C, floating potential at the substrate, and nitrogen partial pressure of 0.3 Pa are favorable to promote the reaction that lowers the work function.

Nanomaterials for field electron emission: preparation, characterization and application

We review the current development in synthesis and deposition techniques of quasi-one-dimensional nanomaterials and recent achievements in the study of the field electron emission properties of these materials. Major findings are given from our recent experimental study of the field electron emission characteristics of carbon nanotubes, copper sulfide and silicon carbide nanorods. The study reveals that the above nanomaterials are promising candidates as cold cathode electron emitters. Furthermore, we review the recent development of techniques for growing and depositing the above nanomaterials in device structures. Typical device structures and their performance are also shown.

Advances in cold cathode physics and technology

We review recent progress in the physics and technology of cold cathode electron emitters. The characteristics of emission from field emitter arrays, photocathodes, and ferroelectrics are presented, together with a summary of the understanding of the physics involved. The paper concludes with a description of L-band micropulse gun, based on secondary emission in an RF cavity. Emphasis is placed on cathode development for electron guns to drive microwave tubes and RF accelerators.

Electron emission induced modifications in amorphous tetrahedral diamondlike carbon

The cold-cathode electron emission properties of amorphous tetrahedral diamondlike carbon are promising for flat-panel display and vacuum microelectronics technologies. The onset of electron emission is, typically, preceded by “conditioning” where the material is stressed by an applied electric field. To simulate conditioning and assess its effect, we combined the spatially localized field and current of a scanning tunneling microscope tip with high-spatial-resolution characterization. Scanning force microscopy shows that conditioning alters surface morphology and electronic structure. Spatially resolved electron-energy-loss spectroscopy indicates that the predominant bonding configuration changes from predominantly fourfold to threefold coordination.

Low Field-Induced Electron Emission Phenomenon Observed from the Carbon Containing Thin Films

Stable cold-cathode electron emission at fields as low as 3 MV/m has been observed from two types of carbon-containing thin film of thickness in submicron scale, i.e., non-doped fullerene C60 and amorphous diamond films. A transparent anode imaging technique was used to record the spatial distribution of individual sites and the total emission current-voltage characteristic of the films. It is found that supplemental measurements of optical spectra and electrical conductivity of the films can provide important evidence useful for understanding the differences in emission behaviour between two types of film.

Photoconductivity of CVD diamond under bandgap and subbandgap irradiations

We systematically studied the photoconductivity of nominally intrinsic diamond films grown by two CVD methods. In the 200–275 nm wavelength range covering the bandgap energy (5.5 eV or 225 nm), the measured photocurrent showed characteristic behavior that can be quantitatively related to a fast recombination of electrons and effective trapping of holes, mainly by trap states near the valence band edge. In addition to photoconductivity, thermoelectric emission spectroscopy and thermally stimulated current measurements were made. From the results, the density and location of two distinctive trap levels within the energy bandgap was estimated. The results are not only self-consistent, but also agree with other authors' findings, such as a hole-dominated current, without or under bandgap illumination, a Fermi level close to the valence band edge, an electron recombination center in the middle of the bandgap, and a shallow hole-trap state having an extremely high density of ∼ 1019 cm−3. Based on these data, we suggest a bandgap and trap-state model for intrinsic CVD diamond. The details of this model and the characteristic properties of the defects/traps are consistent with experimental results and theoretical findings made by other authors covering a large diversity of areas, such as photoluminescence and cathodoluminescence, photoabsorption, carrier lifetimes and mobilities, electron paramagnetic resonance, cold-cathode electron emission, and photoconductive current switching.

Interface formation between hydrogen terminated Si(111) and amorphous hydrogenated carbon (a-C:H)

In the last few years heterostructures consisting of a-C:H on top of crystalline silicon have revealed a number of interesting properties for electronic applications, for example, as a photodiode (see Ref. 1) or, very recently, as cold cathode electron emitter (see Ref. 2). For those applications the interface properties of the heterostructure play a key role. In this article the electronic and chemical nature of the interface between plasma deposited a-C:H in the form of diamondlike carbon (DLC) and hydrogen terminated silicon (111) was determined by in situ core level and valence band photoemission spectroscopy. The quantitative analysis of core level shifts and intensities indicate that both, an interface layer of mixed composition and a pure a-C:H layer, grow simultaneously with relative growth rates of 2:1 during the initial stages of the interface formation. The interface layer is first carbon rich and then approaches stoichiometric composition which is reached at an interface thickness of 18 Å. Then a reactive SiC interface is completed and acts as a diffusion barrier which prevents further extension of the interface region when the deposition of a-C:H proceeds. Aside from an acceptor passivation of the Si substrate, due to H atoms from the plasma, no overlayer induced band bending is measured. The valence band offset between Si and DLC is determined as 0.7±0.1 eV with the valence band edge of DLC below that of silicon.

Morphology and Electron Emission Properties of Nanocrystalline CVD Diamond Thin Films

ABSTRACTNanocrystalline diamond thin films have been produced by microwave plasma-enhanced chemical vapor deposition (MPECVD) using C60/Ar/H2 or CH4/Ar/H2 plasmas. Films grown with H2 concentration ≤ 20% are nanocrystalline, with atomically abrupt grain boundaries and without observable graphitic or amorphous carbon phases. The growth and morphology of these films are controlled via a high nucleation rate resulting from low hydrogen concentration in the plasma. Initial growth is in the form of diamond, which is the thermodynamic equilibrium phase for grains < 5 nm in diameter. Once formed, the diamond phase persists for grains up to at least 15–20 nm in diameter. The renucleation rate in the near-absence of atomic hydrogen is very high (∼1010 cm2sec−1), limiting the average grain size to a nearly constant value as the film thickness increases, although the average grain size increases as hydrogen is added to the plasma. For hydrogen concentrations less than ∼20%, the growth species is believed to be the carbon dimer, C2, rather than the CH3* growth species associated with diamond film growth at higher hydrogen concentrations. For very thin films grown from the C60 precursor, the threshold field (2 to ∼60 volts/micron) for cold cathode electron emission depends on the electrical conductivity and on the surface topography, which in turn depends on the hydrogen concentration in the plasma. A model of electron emission, based on quantum well effects at the grain boundaries is presented. This model predicts promotion of the electrons at the grain boundary to the conduction band of diamond for a grain boundary width ∼3–4 Å, a value within the range observed by TEM.

UV photoemission study of heteroepitaxial AlGaN films grown on 6H-SiC

This study presents results of UV photoemission measurements of the surface and interface properties of heteroepitaxial AlGaN on 6H-SiC. Previous results have demonstrated a negative electron affinity of AlN on 6H-SiC. In this study Al x Ga 1− x N alloy films were grown by organometallic vapor phase epitaxy (OMVPE) and doped with silicon. The analytical techniques included UPS, Auger electron spectroscopy, and LEED. All analysis took place in an integrated UHV transfer system which included the analysis techniques, a surface processing chamber and a gas source MBE. The OMVPE alloy samples were transported in air to the surface characterization system while the AlN and GaN investigations were prepared in situ. The surface electronic states were characterized by surface normal UV photoemission to determine whether the electron affinity was positive or negative. Two aspects of the photoemission distinguish a surface that exhibits a NEA: (1) the spectrum exhibits a sharp peak in the low kinetic energy region, and (2) the width of the spectrum is hv - E g. The in situ prepared AlN samples exhibited the characteristics of a NEA while the GaN and Al 0.13Ga 0.87N samples did not. The Al 0.55Ga 0.45N sample shows a low positive electron affinity. Annealing of the sample to > 400°C resulted in the disappearance of the sharp emission features, and this effect was related to contaminant effects on the surface. The results suggest the potential of nitride based cold cathode electron emitters.

Monte Carlo study of hot electron and ballistic transport in diamond: Low electric field region

A Monte Carlo simulation of electron transport in the conduction band of diamond as a function of field and film thickness has been performed. It predicts that the energy distribution of field emitted electrons ‘‘heats up’’ when the internal electric field is of the order of 1 V/μm and greater. The energy distribution shifts and becomes broader as the width of the sample increases. With increasing field (≥10 V/μm), there is a transition to quasi-ballistic-like behavior. For thinner films (≤0.1 μm), the transport is more clearly ballistic with the peak energy scaling roughly with the field. It is suggested that if a realistic and viable electron injection mechanism into the conduction band of a diamond–metal or diamond–semiconductor interface could be found for those crystal faces of diamond exhibiting negative electron affinity, then a copious cold cathode electron emitter with field tunable energies is feasible.