R. D. Forrest

Effect of aspect ratio and anode location on the field emission properties of a single tip based emitter

The effect on the field emission characteristics of the aspect ratio of an isolated emitter, together with the position of the anode electrode are reported. We show by computational simulation that the field enhancement factor β is only dependant on the emitter height h, radius r, when the anode to cathode separation D is greater than three times the height of the emitter away from the tip. In this regime the enhancement factor is independent of the anode location and approaches a value depicted by h and r alone and is described by the expression β0=(1+h∕αr)m where α=2 and m=1. As the anode is brought close to the tip of the emitter, the emitter tip and anode approximate a parallel plate configuration and the enhancement factor tends to unity. Extracted enhancement factor and threshold fields are described by a modified applied electric field taking D−h as the separation. Comparison with previously reported experimental results is also given.

Interpretation of enhancement factor in nonplanar field emitters

A comparison of the field emission properties of exposed nanotubes lying on a tipped carbon nanorope, with the emission properties from a sharpened iron tip of similar dimensions is performed. By varying the electrode separation it is observed that the threshold field for emission for both structures decreases as the electrode separation initially increases; however, for sufficiently large electrode separations, the threshold field is observed to reach an asymptotic value. Our results show that the field enhancement factor is fundamentally associated with the electrode separation, and depending on the experimental conditions in order to obtain a true value for electric field a set of alternative definitions for enhancement factors is required. We further confirm our experimental synopsis by simulation of the local electrostatic field which gives results similar to those obtained experimentally.

Thermionic emission from defective carbon nanotubes

Using a nanomanipulation system contained within a scanning electron microscope we investigate the thermionic electron emission from multiwall carbon nanotubes. Peak emission currents of 65nA are measured. The carbon nanotubes being grown at low temperature by the chemical vapor deposition method are defective with poor thermal conductivity. We believe it is crucial for the thermal conductivity to be poor in order to obtain significant thermionic emission from the carbon nanotubes. This allows for the carbon nanotube during electron emission to be at high temperatures, and thus give higher emission efficiencies. At the highest emission current levels we estimate the temperature of the nanotubes to be approximately 2900K.

Reactive ion etching of quartz and Pyrex for microelectronic applications

The reactive ion etching of quartz and Pyrex substrates was carried out using CF4/Ar and CF4/O2 gas mixtures in a combined radio frequency (rf)/microwave (μw) plasma. It was observed that the etch rate and the surface morphology of the etched regions depended on the gas mixture (CF4/Ar or CF4/O2), the relative concentration of CF4 in the gas mixture, the rf power (and the associated self-induced bias) and microwave power. An etch rate of 95 nm/min for quartz was achieved. For samples covered with a thin metal layer, ex situ high resolution scanning electron microscopy and atomic force microscopy imaging indicated that, during etching, surface roughness is produced on the surface beneath the thin metallic mask. Near vertical sidewalls with a taper angle greater than 80° and smooth etched surfaces at the nanometric scale were fabricated by carefully controlling the etching parameters and the masking technique. A simulation of the electrostatic field distribution was carried out to understand the etching pro...

Branched carbon nanofiber network synthesis at room temperature using radio frequency supported microwave plasmas

Carbon nanofibers have been grown at room temperature using a combination of radio frequency and microwave assisted plasma-enhanced chemical vapor deposition. The nanofibers were grown, using Ni powder catalyst, onto substrates kept at room temperature by using a purposely designed water-cooled sample holder. Branched carbon nanofiber growth was obtained without using a template resulting in interconnected carbon nanofiber network formation on substrates held at room temperature. This method would allow room-temperature direct synthesized nanofiber networks over relatively large areas, for a range of temperature sensitive substrates, such as organic materials, plastics, and other polymers of interest for nanoelectronic two-dimensional networks, nanoelectromechanical devices, nanoactuators, and composite materials.

Study of the current stressing in nanomanipulated three-dimensional carbon nanotube structures

We report the fabrication of free-standing carbon nanotube structures. The welding of individual carbon nanotubes to other nanotubes and metal substrates has been performed, on a selective basis, to produce joints of both good electrical conductivity and mechanical integrity, without the need for a joining material. As a result of this unique process, we study the damage to the microstructure of the nanotube as a function of current. When the current densities are in excess of 2×106A∕cm2, particular care must be taken with regard to the quality of the nanotube and the heat dissipation. This is crucial for the use and application of nanotubes in any future device structure for it gives the upper limits to the “average” current density calculations. This process now allows for the fabrication of bespoke carbon nanotube devices for the prototyping of device performance.

Role of nanostructure on electron field emission from amorphous carbon thin films

The mechanism of electron field emission from different forms of amorphous carbon (a-C) thin films is discussed. We show that it is possible to explain electron emission from a variety of amorphous carbon films by understanding the nature of electron states near the Fermi level. The films can be described as consisting of conductive sp2 C clusters lying within a more insulating sp3 C matrix. We show that the trend in the threshold field for emission can be explained in terms of improvements in the connectivity between these sp2 clusters and that cluster concentration and size can be in turn controlled by the choice of deposition conditions. The presence of the dielectric inhomogeneity between the two regions of sp2 clusters and sp3 C matrix is also shown to be very important in understanding the apparently low barrier heights that appear in the analysis of the current-voltage emission characteristics using the Fowler–Nordheim theory. This article attempts to set the framework for a unified model for elect...

Modification of electron field emission properties from surface treated amorphous carbon thin films

The field emission properties of amorphous hydrogenated carbon (a-C:H) films and nitrogenated a-C:H (a-C:H:N) films subjected to 10 keV Co60 ion implantation are investigated as a function of ion dose. The average threshold electric field for conditioned nitrogenated and non-nitrogenated unimplanted films is found to be 27 and 29 V/μm. Implantation of C60 ions to a dose of 7.5×1013 cm−2 results in an increase in the threshold field of both the nitrogenated and non-nitrogenated films. However, implantation to a dose of 1.25×1014 cm−2 results in a reduction in the threshold fields to values close to those found in the unimplanted samples. At larger doses of 2.65×1014 cm−2, the threshold field for the non-nitrogenated samples remain largely unchanged though there is an increase in the refractive index consistent with an increase in the optical density of the film. In the nitrogenated film, the average threshold field at the highest dose again increases to values comparable to those found for C60 implantation...

On the importance of the electrostatic environment for the transport properties of freestanding multiwall carbon nanotubes

Electrical measurements of freestanding multiwall carbon nanotubes using high resistance tunnelling contacts reveal a power law behaviour, I alpha V alpha + 1, with alpha as high as 5.2, followed by a transition to an offset ohmic behaviour. The freestanding electrode geometry allows for a distinction between the predictions from Luttinger liquid and environmental quantum fluctuation (EQF) theories to be made. The high values of exponents found are explained within the EQF formulism, where reflections resulting from the impedance discontinuity caused by the freestanding geometry are included.

Fabrication of a self-aligned microtip field emission array

In this work we describe the development of a process to fabricate a self-gated cold cathode microtip emitter array from laser crystallized silicon using mature large area techniques and materials familiar to the microelectronics industry. A scanning electron microscopy study demonstrates the evolution of the devices with reactive ion etch time and indicates how they may be tailored to optimize field emission performance. The surface density of the gated emitter structures was estimated to be 108–109 per cm2, similar to that reported for emission arrays fabricated using cutting edge methods. A photoresist masking technique to potentially enhance device performance was investigated. Two terminal field emission operation of the device is demonstrated with a threshold field of 18 V/μm for a current of 10 nA.