|Ph.D Student||Koenigsfeld Noam|
|Subject||Field Emission from Diamond, Carbon Layers and Wide-Band-Gap|
|Department||Department of Physics||Supervisor||Professor Emeritus Rafael Kalish|
This work presents a study of key elements in the electron field emission process from diamond, carbon films, and other wide band gap semiconductors.
Field emission (FE) is the extraction of electrons from the materials surface by applying an intense electric field. The theory of FE from a metal surface presented by Fowler and Nordheim in 1928 describes the process of electrons escaping into vacuum by tunneling through the surface potential barrier. The intense electric field narrows the barrier to the conditions where the tunneling probability becomes substantial. In the description of FE from semiconductors (SC), the three stages of the emission process need to be considered. These stages are: injection of carriers into the SC, transport through the SC to the surface, and escape through the surface barrier. The theory of FE from SC was developed by Stratton in 1955 and others who followed. The effects of space charge, field penetration, and surface states related to SC were considered in these theories.
The FE measurements in this study were performed in a new experimental setup built in the framework of this research. Measurements were performed in ultra high vacuum; samples were introduced through an introduction chamber, heated in vacuum, and tested at room temperature. The experiment was fully automated, connected to a PC with a program for control and data collection. An image of the emission sites was recorded or alternately, a configuration where the anode distance can be varied was used. CVD diamond films were prepared with boron doping in a hot filament reactor. Un-doped films and hydrogen plasma treatments were performed in a micro-wave reactor. Ion implantation was used to introduce damage into the sample and study its effect. Ohmic contacts to the samples substrate or surface were prepared by electron beam evaporation and annealing. Different characterization methods were also used which included: optical and electron microscopy, scanning probe techniques, Raman spectroscopy, and electron spectroscopy.
The results presented in this work address all three stages of the emission process. The escape from the surface was studied in regards to the effects of CVD diamond surface condition and surface roughness influence on its FE properties. The hydrogenated diamond surface was found to exhibit reproducible FE results only when non-diamond components were removed from the surface before the hydrogen plasma treatment and the sample was baked in vacuum to remove water vapor from the surface. These results are discussed in the context of upward band bending at the surface proposed as the origin of diamond surface conductivity, which is induced
by a water layer adsorbed on the surface. The surface roughness was reduced by cluster ion-beam irradiation, which has the advantage of preserving the bulk material properties, including the existence of grain boundaries.
The emission TOF was found to increase for surfaces with lower roughness, indicating that geometrical field enhancement is involved in emission rather than emission from grain boundaries.
The conditioning effect, where FE is enhanced after the first emission current is extracted, was carefully investigated. New insights as to the origin of emission enhancement were made. We examined the option that graphite inclusions, introduced by discharge conditioning events, are responsible for the enhanced emission. Furthermore, nano-scale features that resemble carbon nano-tubes were discovered in the discharge affected area using a high resolution scanning electron microscope.
The transport of carriers to the surface was studied in relation to conduction channels. Graphitic single ion tracks were introduced into extremely flat tetrahedral amorphous carbon layers. The graphitic channels improved emission by enhancing the electric field around the channel at the surface. The injection at the back contact interface was studied in wide band gap SC with contacts that have graded potential barriers. The application of such structures was found to depend on the ability of the external field to penetrate into the SC and influence the junction potential barrier shape.