Academic literature on the topic 'FIELD EMISSION OF ELECTRONS'

The electron emission characteristics of aluminum, molybdenum and carbon nanotubes were studied. The experiments were setup to study the emission behavior as a function of temperature and exposure to oxygen. Changes in the surface work function as a result of thermal annealing were monitored with low energy ultra-violet photoelectron spectroscopy for flat samples while field emission energy distributions were used on tip samples. The change in the field emission from fabricated single tips exposed to oxygen while in operation was measured using simultaneous Fowler-Nordheim plots and electron energy distributions. From the results a mechanism for the degradation in the emission was concluded. Thermal experiments on molybdenum and aluminum showed that these two materials can be reduced at elevated temperatures, while carbon nanotubes on the other hand show effects of oxidation. To purely reduce molybdenum a temperature in excess of 750 ºC is required. This temperature exceeds that allowed by current display device technology. Aluminum on the other hand shows reduction at a much lower temperature of at least 125 ºC; however, its extreme reactivity towards oxygen containing species produces re-oxidation. It is believed that this reduction is due to the outward diffusion of aluminum atoms through the oxide. Carbon nanotubes on the other hand show signs of oxidation as they are heated above 700 ºC. In this case the elevated temperatures cause the opening of the end caps allowing the uptake of water. Oxygen exposure experiments indicate that degradation in field emission is two-fold and is ultimately dependent on the emission current at which the tip is operated. At low emission currents the degradation is exclusively due to oxidation. At high emission currents ion bombardment results in the degradation of the emitter. In between the two extremes, molybdenum tips are capable of stable emission.

This thesis is concerned with the research of the electron field emission properties of carbon based materials. Low emission threshold fields have been observed from both amorphous carbon thin films and carbon nanotubes. The emission mechanism can be subdivided into two groups depending on the type of electric field enhancement. These are the amorphous carbon flat films with non-geometric field enhancement and carbon nanotubes with high surface geometric field enhancement. Amorphous carbon thin films are deposited using an rf-plasma enhanced chemical vapour deposition technique. Changing the deposition conditions such as the addition of Argon or Nitrogen modifies the electronic properties. This induces variations in the sp2 concentration and its distribution within the films. The electron field emission properties from amorphous carbon thin films show a close relationship to its sp2 configuration. A model based on non-geometric field enhancement is proposed to explain the variation in the field emission characteristics. Nano-structured amorphous carbon films custom "designed" using ion beam assisted deposition with sp2 cluster sizes of around 60 nm have also been investigated. The field emission threshold field was shown to be controlled by the film's intrinsic stress and the local carbon density. With increasing stress, there is a concomitant increase in the local density, which is postulated to decrease the distance between the carbon graphitic "planes". This results in enhancement of the electron emission at lower fields. Stress within the films also induces changes to the band structure of the nano-structured carbon which are beneficial to the field emission process. Field emission from carbon nanotubes that are embedded in a polymer matrix has been investigated. The emission threshold fields are observed to be dependent on the nanotube density. The effect of electric field screening is used to explain the reduction of field enhancement observed in these films with increasing nanotube density. The field emission properties are compared with those films which have vertically aligned and in e-beam fabricated nanotube arrays. Results indicate that field emission properties from non-aligned nanotube films are comparable in performance to the best designed arrays in the literature. Although this study shows carbon based materials to have superior field emission properties, integrating the cathodes to fabricate commercial devices could prove to be very challenging.

In the quest for reliable, repeatable and stable field electron emission that has commercial potential, whilst many attempts have been made, none yet has been truly distinguishable as being successful. Whilst I do not claim within this thesis to have uncovered the secret to success, fundamental issues have been addressed that concern the future directions towards achieving its full potential. An exhaustive comparison is made across the diverse range of materials that have, over the past 40-50 years, been postulated and indeed tested as field emitters. This has not previously been attempted. The materials are assessed according to the important metrics of turn on voltage, Eon, and maximum current density, Jmax, where low Eon and high Jmax are seen as desirable. The nano-carbons, carbon nanotubes (CNTs), in particular, perform well in both these metrics. No dependency was seen between the material work function and its performance as an emitter, which might have been suggested by the Fowler Nordheim equations. To address the issues underlying the definition of the local enhancement factor, β, a number of variations of surface geometry using CNTs were fabricated. The field emission of these emitters was measured using two different approaches. The first is a Scanning Electrode Field Emission Microscope, SAFEM, which maps the emission at individual locations across the surface of the emitter, and the parallel plate that is more commonly encountered in field emission measurements. Finally, an observed hysteretic behaviour in CNT field emission was explored. The field emitters were subjected to a number of tests. These included; in-situ residual gas analysis of the gas species in the emitter environment, a stability study in which the emitters were exposed to a continuing voltage loop for 50 cycles, differing applied voltage times to analyse the effects on the emitted current, and varying maximums of applied field in a search for hysteresis onset information. These studies revealed the candidate in causing the hysteresis is likely to be water vapour that adsorbs on the CNT surface. A six step model if the emission process was made that details how and when the hysteresis is caused.

This work is mainly divided into three parts. Firstly, with the aim of integrating electron field emitter with other circuit elements on a single chip, silver-silicon dioxide (Ag-SiO2) nanocomposites are fabricated and studied. The Ag-SiO2 nanocomposites are synthesised by Ag implantation into thermally oxidised SiO2 layers on Si substrates and their fabrication processes are fully compatible with existing integrated circuit technology and their threshold fields are less than 20 V/mum. The local field enhancement mechanisms were studied and the fabrication processes of these layers optimised. Secondly, the electron field emission (FE) properties of two-dimensional quantum confinement structure were studied. Band gap modulated amorphous carbon (alpha-C) nanolayers were synthesised by pulsed laser deposition. In these structures, electrons are confined in a few nm thick low band gap sp2 rich alpha-C layer, which is bound by the vacuum barrier and a 3 nm thick high band gap sp3 rich alpha-C base layer. Anomalous FE properties, including negative differential conductance and repeatable switching effects, are observed when compared to control samples. These properties will be discussed in terms of resonant tunnelling and are of great interest in the high-speed vacuum microelectronic devices. Finally, due to the interesting electrical transport properties and rare FE characteristics of metal quantum dots (QDs), cobalt QDs were synthesized in a SiO2 matrix by ion implantation. Staircase-like current-field characteristics were observed for the first time from these samples and give an experimental insight into existing Coulomb Blockade effects in the metal QDs during the FE process. Moreover, these samples also achieve excellent FE properties with threshold fields less than 5 V/mum and are comparable with other popular FE materials.

Flat panel displays based on electron field emission can provide the benefits of the high resolution of a cathode ray tube display while possessing the portability of a liquid crystal display. To date, the problem with a field emission flat panel display based on silicon is that it usually involves complex photolithography processing, making it too complex and expensive to be commercially viable. In this thesis, the emphasis of the research is to fabricate a three terminal silicon device for flat panel display based on field emission technology without using photolithography processes. Laser crystallised amorphous silicon is chosen for our material which creates a rough silicon surface whose roughness gives rise to field enhancement. Furthermore, this process is widely used in the display industry to fabricate silicon based display driver thin film transistors, which can be readily incorporated. It is important to understand the electron field emission mechanism from the laser crystallised amorphous silicon and to find optimum conditions for emission. In the course of our research, we established a regime for super sequential lateral growth or a hybrid sequential lateral solidification and super lateral growth in Nd:YAG crystallisation of amorphous silicon. Excimer laser crystallised amorphous silicon under optimum conditions gives emission currents of the order of 10-5A (current densities ~ 0.04 A/cm2) at threshold fields less than 15 V/mum in a diode configuration, without the need for a forming process. Through experiments, we concluded that the field emission mechanism from these samples is not controlled purely by surface phenomena, contrary to what was suggested by the Fowler Nordheim theory. Instead, it is the diffusion of the underlay metals into the silicon that create clusters of silicide that allow the electrons to become "hot" while travelling between the clusters. Lastly, a novel process illustrating that a three-terminal device based on laser crystallised amorphous silicon can be fabricated without the need for photolithography. However, the field emission data showed that some fine-tuning of the process is still required.

Large area displays such as laptop computers and flat screen televisions have enormous market potential. There are numerous technologies in existence today, including LCD (liquid crystal displays) and plasma, however they each suffer from unique limitations, mainly the size of the display and operating conditions. There is still a need for a method of depositing electronic materials over large areas at suitable temperatures. This is important since the construction of panels using sub-modules such as crystalline silicon wafers is very difficult and expensive. The displays based on liquid crystals that use thin film transistor (TFT) driven active matrix addressing is the most common flat screen technology at present. However, there are great problems associated with yield in fabrication, especially for screen dimensions over 14 inches. Hence the screens are very expensive. Field emission displays utilising the phenomenon of field emission in which electrons escape their work function and "jump" from the surface of the semiconductor into a vacuum has been proposed as a competing technology. Each pixel is controlled by many tip based emitters, therefore improving the yield. This thesis explores the field emission properties and mechanisms of tip based emitters, and also explore the possibility of utilising carbon nanotubes as electron sources for field emission displays.