Scanning Anode Field Emission Microscopy Research Articles

Scanning anode field emission microscopy of a single Si emitter

Emitter tip radius nonuniformity results in exponential variations in emission current and a relatively low array utilization. Here, we provide a method of mapping the current and field-factor from a single emitter over a small area using a scanning anode field emission microscope. A dull W probe is used as the anode, and an array of emitters is fabricated on silicon (Si) wafers. We use a relatively wide spaced (100 μm pitch) emitter array with each emitter having an integrated Si pillar. Current-voltage characteristics are used to extract the field-factor and to experimentally demonstrate the mapping of the currents and field-factor of a single emitter. From emission spot sizes, the emission half-angles are measured to be <14° at anode voltages 2.5 kV and a minimum resolvable feature of 2–3 μm at 1.8 kV. We also determine the field-factor dependence with the distance between the anode and the emitter, where limiting the current becomes essential to prevent early burn-out of the emitter that could reduce the current. We also simulated the maximum currents tolerated by the pillar to assess the thermal effects on the emitter. Finite element modeling confirms the experimental trend in the field-factor with the distance between the anode and the emitter tip, resulting in a value of approximately 105 cm−1 for an emitter tip radius of 5 nm and an emitter-anode distance of 50 μm.

Quantitative characterization of field emission parameters: Application to statistical analysis of individual carbon nanotubes/nanofibers

In this study, the authors present a detailed procedure for the quantitative measurement of the field emission properties of a large number of vertically aligned carbon nanotubes/nanofibers (CNs) using a scanning anode field emission microscope (SAFEM). This method provides the statistical distribution of all the relevant emitter parameters such as field enhancement factor, emitter height, and maximal current before failure. In order to extract the correct absolute enhancement factor of each CN, an analytical electrostatics model has been developed taking into account for the “tip-to-tip” geometry of anode and cathode in the SAFEM set-up. This analytical model has been validated by finite elements electrostatic simulations. Experimental measurements of enhancement factor distributions determined at several anode–cathode distances show the importance of this procedure to obtain quantitative correct values. A good correlation between the enhancement factor and the CN length has been observed. Additionally, the correlation between the maximum current before failure and the enhancement factor has also been investigated. Unlike in previously reported experiments, no clear dependence between these two parameters has been obtained. This result can be explained in our case by a large dispersion of CN crystalline quality or CN–substrate electrical contact resistance in the array used in this study.

Microscopic analysis of performance variations in carbon nanotube field emission cathodes: Implications for device optimization

AbstractCarbon nanotubes (CNTs) are considered very promising for the realization of low‐cost field emission electron sources. However, despite intensive research and development efforts, the fabrication of reliable CNT cathodes for high current density (>100 mA/cm2) applications remains a formidable challenge. In this study we use scanning anode field emission microscopy (SAFEM) to investigate the microscopic origins of macroscopic emission performance variations in chemical vapor deposition (CVD) grown CNT planar field emission cathodes. The field enhancement distributions are determined and the field emission properties of individual emission sites on the cathodes are probed. Contact I(V) measurements are carried out to estimate the resistance of individual emitters. The degradation behavior of individual sites is also studied and can be related with the macroscopic cathode performances. Scanning (SEM) and transmission electron microscopy (TEM) provide additional information on the contact and structural properties of the cathodes. Our results indicate that the sample macroscopic performances depend strongly on the individual emitter field emission properties in terms of maximum current before degradation and contact resistance.

Review Article: Rare-earth monosulfides as durable and efficient cold cathodes

In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, the authors first describe the successful deposition of lanthanum monosulfide via pulsed laser deposition on Si and MgO substrates. These thin films have been characterized by x-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy, and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the scanning anode field emission microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of low work function materials having a 〈100〉 orientation perpendicular to the surface and outcropping it are surrounded by a matrix of amorphous materials with a higher work function. To date, LaS thin films have been used successfully as cold cathode emitters with measured emitted current densities as high as 50 A/cm2. Finally, we describe the successful growth of LaS thin films on InP substrates and, more recently, the production of LaS nanoballs and nanoclusters using pulsed laser ablation.

Field Emission from Self‐Assembled Arrays of Lanthanum Monosulfide Nanoprotrusions

The field emission properties of LaS nanoprotrusions called nanodomes, formed by pulsed laser deposition on porous anodic alumina films, have been analyzed with scanning anode field emission microscopy. The voltage necessary to produce a given field emission current is ~3.5 times less for nanodomes than for thin films. Assuming the same work function for LaS thin films and nanoprotrusions, that is, ~1?eV, a field enhancement factor of ~5.8 is extracted for the nanodome emitters from Fowler‐Nordheim plots of the field emission data. This correlates well with the aspect ratio of the tallest nanodomes observed in atomic force micrograph measurements.

Characterization and field emission properties of lanthanum monosulfide nanoprotrusion arrays obtained by pulsed laser deposition on self-assembled nanoporous alumina templates

Three distinct types of nanostructures—nanodomes, nanodots, and nanowires—have been simultaneously self-assembled by pulsed laser deposition of lanthanum monosulfide on anodic alumina films containing hexagonal arrays of pores about 50nm wide and 500nm deep. The nanostructures have been characterized by x-ray diffraction, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM). Nanodomes preferentially grow on the boundary separating regions (grains) of the alumina template that have near perfect pore ordering, and their density is ∼109∕cm2. The diameter of a nanodome at the base is about 100nm and their aspect ratio (height/diameter at the base) is between 1 and 3. Additionally, nanodots nucleate on top of the alumina walls that separate adjacent pores. They have a diameter of ∼50nm, a density equal to the pore density (1010∕cm2), and an aspect ratio less than 1. Finally, cross sectional FE-SEM images of the templates indicate that LaS nanowires grow inside the pores with a density of 1010∕cm2. They have a diameter of 50nm and a maximum length equal to the length of the pores (∼500nm). The field emission properties of the LaS nanodomes and nanodots have been analyzed via the scanning anode field emission microscopy technique (SAFEM). For a fixed SAFEM probe to cathode distance, the applied voltage necessary to extract the same FE current is found to be ∼3.5 times less for a LaS thin film deposited on alumina templates compared to the value recorded for LaS thin films deposited on Silicon substrates. Assuming a LaS work function of ∼1eV (as recorded for LaS thin films grown on silicon substrates), a field enhancement factor of ∼5.8 is extracted for the nanoscale emitters from Fowler-Nordheim plots of the FE data. The field enhancement effect accrues from the concentration of electric field lines at the tip of the nanodome and nanodot emitters. The value of 5.8 can be correlated to an aspect ratio of 2.7 for the dominant emitter, which is in good agreement with the aspect ratio of the tallest nanodomes observed in AFM measurements.

Patchwork field emission properties of lanthanum monosulfide thin films

The field emission properties of lanthanum monosulfide (LaS) films, deposited on Si substrates by pulsed laser deposition, have been thoroughly analyzed via the scanning anode field emission microscopy technique. Using the conventional Fowler-Nordheim relation, the work function of LaS thin films has been extracted from the slope of the plot ln(J∕F2) vs 1∕F, where J is the field emission current density and F is the local applied electric field. The threshold for an emission current density of 1mA∕cm2 occurs around a 230V∕μm electric field applied across the vacuum gap. This leads to an outstanding, reproducible effective work function value of ∼1eV across a 1cm2 sample area.

Scanning anode field emission microscopy analysis for studies of planar cathodes

In a scanning anode field emission microscope the field emission current is extracted by a small spherical anode brought, at micrometric proximity, in front of a planar cathode surface. Therefore, the field over the emission area is not uniform and direct quantitative interpretation from the total field emission current versus applied voltage is misleading. The potential distribution of the system composed of a sphere in front of a plane have been calculated to determine the field distribution over the surface in order to define the active field emitting zone and to extract the current density versus the local applied field from the measured total field emission current versus applied voltage characteristics.

53.4: A New Coated CNT Cathode Using an ITO Ink

We developed a new coated cathode using an ink composed of double-wall carbon nanotubes (DWNT) dispersed into an organic In2O3-SnO2 solution. A thin film cathode was fabricated by spraying the ink onto a substrate. The turn-on voltage of a diode using the cathode was 2.2V/um and the brightness uniformity was within 3%. A scanning anode field emission microscope (SAFEM) measurement showed that the average field enhancement factor (β) of this cathode was 900 and its full width at half maximum was 200. The β was three times higher and the distribution was one-half of the reported values [7].

Prospects and Limitations of Carbon Nanotube Field Emission Electron Sources

Today the most mature technology to produce gated micro field electron emitter arrays is the so-called Spindt-type metal micro-tip process. The drawbacks of the Spindt-type process are the expensive production, the critical lifetime in technical vacuum and the high operating voltages. Carbon nanotubes (CNT) can be regarded as the potential second-generation technology to Spindt-type metal micro-tips. The use of CNT as field enhancing structures in field emission electron sources can bring several advantages such as longer lifetime and operation in poor vacuum due to the high chemical inertness as well as low operation voltages and perhaps most important very low cost production techniques. In the present contribution we show that the field electron emission (FE) of CNT thin films can be accurately described by Fowler-Nordheim tunneling and that the field enhancement factor ? influences the emission properties most prominently. We have used scanning anode field emission microscopy to investigate the local field emission properties of randomly oriented carbon nanotube deposits. In the technically interesting applied electric field range of 30 V/?m an emission site density larger than 5×106 cm?2 could be measured. We will discuss however that emitter degradation at high emission currents limits the full exploitation of this high emission site density. The emission degradation becomes apparent for emission currents in the ?A range for a single emitter and the field emission I-V characteristics suggests that power dissipation due high contact or intra CNT resistance is the cause of the emitter degradation. Therefore, although the fundamental properties of CNTs are very favorable for the use as field emission tips, these properties alone will not guarantee their success in this area. Our investigations clearly show that a perfect control of the catalytic CNT growth process is needed for successful CNT field emitter technology, at least for high current applications.

Field electron emission from individual carbon nanotubes of a vertically aligned array

Field electron emission behavior of individual multiwalled carbon nanotubes (MWNTs), that are elements of a vertically aligned array grown on a Si wafer, were analyzed with a scanning anode field emission microscope. The electron emission of each MWNT followed the conventional Fowler–Nordheim field emission mechanism after their apexes were freed from the erratic adsorption species using a conditioning process at room temperature. The conditioning process led to stable emission currents and reduced their variations ΔI/I to less than 30% between different MWNTs of the array. This opens the possibility for using MWNTs in an array as independent electron sources for massively parallel microguns.

Microscopic characterization of electron field emission

We report on the functional capabilities of a scanning anode field emission microscope (SAFEM) which combined with a phosphor screen is used to investigate and correlate individual electron emission site characteristics of low threshold thin film electron emitters in the micrometer regime. Spatially recorded extraction voltage V(x,y) maps under constant emission current or emission current I(x,y) maps under constant anode voltage reveal spatially divergent emission properties on thin film emitters. The V(x,y) maps are used to derive the field enhancement β(x,y) maps which give a better description of the thin film emission properties as compared to electric threshold fields which depends on anode-cathode geometry. Individual emission site current stability of thin film emitters can be investigated with the SAFEM, and a high-resolution field emission microscope to investigate the environmental stability of single carbon nanotubes mounted on filaments as a function of partial gas pressures and temperature.

Collective emission degradation behavior of carbon nanotube thin-film electron emitters

The current-induced emission degradation of a carbon nanotube (CNT) thin-film electron emitter is studied under constant emission current for different current levels, using a scanning anode field emission microscope. A permanent emission degradation is observed for emission currents higher than 300 nA per CNT and is associated with resistive heating at the CNT–substrate interface for the sample under investigation. A second field-induced emission degradation mechanism, associated with the removal of CNTs from the substrate, is also reported.

Characterization of thin film electron emitters by scanning anode field emission microscopy

Scanning anode field emission microscopy is used to map the electron emission current I(x,y) under constant anode voltage and the electron extraction voltage V(x,y) under constant emission current as a function of tip position on carbon based thin film emitters. The spatially resolved field enhancement factor β(x,y) is derived from V(x,y) maps. It is shown that large variations in the emission site density (ESD) and current density can be explained in terms of the spatial variation of the field enhancement β(x,y). Comparison of β(x,y) and I(x,y) shows that electron emission currents are correlated to the presence of high aspect ratio field enhancing structures. We introduce the concept of field enhancement distribution f(β), which is derived from β(x,y) maps to characterize the field emission properties of thin films. In this context f(β)dβ gives the number of emitters on a unit surface with field enhancement factors in the interval (β,β+dβ). It is shown experimentally for the carbon thin film emitters investigated that f(β) has an exponential dependence with regard to the field enhancement factor β. The field enhancement distribution function f(β) can be said to give a complete characterization of the thin film field emission properties. As a consequence, the emitted current density and ESD can be optimized by tuning f(β) of the emitting thin film.

Printing Gel-like Catalysts for the Directed Growth of Multiwall Carbon Nanotubes

Microcontact printing is used to transfer an Fe(III)-containing gel-like catalyst precursor from a hydrophilized elastomeric stamp to a substrate. The catalytic pattern activates the growth of multiwall carbon nanotubes using chemical vapor deposition of acetylene. Our results show that the choice of the catalyst is of extreme importance. Most of the aqueous and ethanolic Fe(III) inks used give rise to drying effects on the stamp surface, which lead to the formation of islands of the catalyst within the pattern. To avoid these shortcomings, we developed a catalyst precursor, which has better performance on the stamp and as a catalyst on the substrate. Simple aging of the ethanolic Fe(III) ink results in a polymerized gel-like catalyst, which can be printed homogeneously on the substrate with excellent contrast. Changing the concentration of the catalyst in the ink allows the density of the carbon nanotubes in the film to be tuned. A scanning anode field emission microscope was used to investigate the microscopic field emission properties of the samples. The emission images reproduce the topographical contrast nicely and prove the high quality of the patterning process.