V. Semet

Carbon nanotubes as field emission sources

Micro and nano-structurally rich carbon materials are alternatives to conventional metal/silicon tips for field emission sources. In particular, carbon nanotubes exhibit extraordinary field emission properties because of their high electrical conductivity, their high aspect ratio “whisker-like” shape for optimum geometrical field enhancement, and remarkable thermal stability. This paper will review the PECVD growth process, and the microfabrication techniques needed to produce well defined carbon nanotube based micro-electron sources for use in novel parallel e-beam lithography and high frequency microwave amplifier systems.

Plasma enhanced chemical vapour deposition carbon nanotubes/nanofibres - How uniform do they grow?

The ability to grow carbon nanotubes/nanofibres (CNs) with a high degree of uniformity is desirable in many applications. In this paper, the structural uniformity of CNs produced by plasma enhanced chemical vapour deposition is evaluated for field emission applications. When single isolated CNs were deposited using this technology, the structures exhibited remarkable uniformity in terms of diameter and height (standard deviations were 4.1 and 6.3% respectively of the average diameter and height). The lithographic conditions to achieve a high yield of single CNs are also discussed. Using the height and diameter uniformity statistics, we show that it is indeed possible to accurately predict the average field enhancement factor and the distribution of enhancement factors of the structures, which was confirmed by electrical emission measurements on individual CNs in an array.

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.

Fabrication and electrical characteristics of carbon nanotube-based microcathodes for use in a parallel electron-beam lithography system

This article presents an overview of the “Nanolith” parallel electron-beam (e-beam) lithography approach. The e-beam writing head consists of an array of microguns independently driven by an active matrix complementary metal–oxide–semiconductor circuit. At the heart of each microgun is a field-emission microcathode comprised of an extraction gate and vertical carbon nanotube emitter, whose mutual alignment is critical in order to achieve highly focused electron beams. Thus, in this work, a single-mask, self-aligned technique is developed to pattern the extraction gate, insulator, and nanotubes in the microcathode. The microcathode examined here (150×150 gates, 2 μm gate diameter, with multiple nanotubes per gate) exhibited a peak current of 10.5 μA at 48 V when operated with a duty cycle of 0.5%. The self-aligned process was extended to demonstrate the fabrication of single nanotube-based microcathodes with submicron gates.

Electrical and field emission investigation of individual carbon nanotubes from plasma enhanced chemical vapour deposition

Abstract Plasma enhanced chemical vapour deposition (PECVD) is a controlled technique for the production of vertically aligned multiwall carbon nanotubes for field emission applications. In this paper, we investigate the electrical properties of individual carbon nanotubes which is important for designing field emission devices. PECVD nanotubes exhibit a room temperature resistance of 1–10 kΩ/μm length (resistivity 10 −6 to 10 −5 Ωxa0m) and have a maximum current carrying capability of 0.2–2 mA (current density 10 7 –10 8 A/cm 2 ). The field emission characteristics show that the field enhancement of the structures is strongly related to the geometry (height/radius) of the structures and maximum emission currents of ∼10 μA were obtained. The failure of nanotubes under field emission is also discussed.

Electron emission through a multilayer planar nanostructured solid-state field-controlled emitter

We have measured the field electron emission (FE) from a surface covered with two ultrathin layers of semiconductor, 4 nm GaN on 2 nm Al0.5Ga0.5N. The threshold field was 50 V/μm, with stable FE current densities up to 3×10−2u2009A/cm2. We have also measured the FE dependence with field and temperature and determine then an effective surface tunneling barrier ⩽0.5u2002eV, coexisting with an effective thermal activation energy of ∼0.85u2002eV. To interpret these experimental results, we propose a dual-barrier model, related to the nanostructured layers, with a serial two-step mechanism for the electron emission, taking into account the space charge formation in the quantum well structure at the surface.

Field emission behavior of vertically aligned ZnO nanowire planar cathodes

A field emission (FE) study by scanning anode field emission microscopy was performed to evaluate the FE properties of vertically aligned zinc oxide (ZnO) nanowire arrays electrodeposited on a plane conductive surface. The specific FE behaviors of the cathode observed experimentally are (1) a turn-on macroscopic field of about 6 V/μm for a FE current density JFEu2009=u20095u2009×u200910−4 A/cm2, (2) a stable FE characteristics for 5u2009×u200910−4u2009<u2009JFEu2009<u20095u2009×u200910−2 A/cm2, and (3) a brutal shut down of FE when JFE crossed a limiting value of ∼0.05 A/cm2 due to a rapid evolution of the nanowires toward a bulbous tip geometry or a complete melting. A physical process of FE from ZnO nanostructures is proposed from the experimental data analyses. An effective surface barrier of about 1 eV was determined from the experimental Fowler–Nordheim plot and the presence of a Zn enriched surface was assumed in considering the possibility of important modifications of the crystallography and charge transfers at the surface of ZnO nanowires duri...

Electron emission from arrays of carbon nanotubes/fibres ☆

Abstract The overall aim of this work is to produce arrays of field emitting microguns, based on carbon nanotubes, which can be utilised in the manufacture of large area field emitting displays, parallel e-beam lithography systems and electron sources for high frequency amplifiers. This paper will describe the work carried out to produce patterned arrays of aligned multiwall carbon nanotubes (MWCNTs) using a dc plasma technique and a Ni catalyst. We will discuss how the density of the carbon nanotube/fibres can be varied by reducing the deposition yield through nickel interaction with a diffusion layer or by direct lithographic patterning of the Ni catalyst to precisely define the position of each nanotube/fibre. Details of the field emission behaviour of the different arrays of MWCNTS will also be presented.

Recent progress in the characterization of electron emission from solid-state field-controlled emitters

The solid-state field-controlled emitter (SSE) structure is an ultrathin large gap semiconductor (UTSC) layer on a metallic surface. Quantum calculations show that the electron emissions from the SSE cold cathodes result from a serial two-step mechanism: first is the injection into the UTSC of a large concentration of electrons through the Schottky junction with the consequence of a significant lowering of the emission barrier, followed by the emission of the electrons from the UTSC surface, a current modulated by the applied voltage. The determination of the current densities versus the applied field from the experimental total current versus applied voltage data, measured for the SSE planar cathodes, used results obtained with electron optics numerical simulations of the field distributions across the planar area in front of a hemispherical probe. Discussions about this methodology are presented. This allows a comparison between the experimental results and the theoretical predictions. The specific beha...

Microguns with 100-V electron beams

The microgun is a combination of a nanotip and a microlens which is composed of two planar micron-size bore electrodes and a coplanar four-pole deflector microfabricated on the same Si chip. The focusing and deflection characteristics of the microgun, working as an immersion lens at 100 V, have been studied both experimentally and by numerical simulations. Results show unique electron optics properties due mainly to the coherence of the electron beam emitted from the nanotip and to the noninteraction of the incident electrons with the different microelectrodes. The focus spot can reach nanometric dimensions with minimum aberrations and a deflection amplitude of ∼2.5u2009mrad/V.