When a metallic surface is subjected to a sufficiently strong electric field, electron emission can occur without the need for thermal excitation. This phenomenon, known as cold field emission, arises from the narrowing of the potential barrier and the formation of the Schottky–Nordheim barrier. Once the barrier is sufficiently lowered and thinned, electrons with energies near the Fermi level can tunnel through, giving rise to a current of free electrons. Such electron sources play a key role in applications including electron microscopy, electron beam lithography, and X-ray generation.
Over the past decades, the field-emission properties of various refractory metals, semiconductors, and carbon nanotubes have been investigated. Nevertheless, for many high-resolution applications—most notably in electron microscopy—the tungsten tip with an apex radius of 100–200 nm remains the material of choice. Tungsten offers outstanding durability and a low sputtering yield under ion bombardment.
The deposition of thin surface coatings on tungsten tips can significantly modify their emission behavior. Properly designed coatings are capable of reducing the relatively high work function of tungsten, lowering the turn-on voltage, extending emitter lifetime, enhancing beam brightness, and improving electron-beam focusing. In addition, protective layers suppress tungsten atom desorption caused by energetic ion impacts, thereby reducing reactivity. In some cases, the coating may act as a tunneling filter, or conversely enable resonantly enhanced tunneling, both of which influence the emission probability. The key challenge, however, lies in choosing the appropriate thickness and material of the coating so as to ensure stability while achieving the desired electronic properties.
This dissertation focuses on the fabrication and subsequent deposition of thin oxide layers on tungsten cathodes, followed by an in-depth analysis of their field-induced electron emission. Two oxidation techniques were employed: thermal oxidation and anodization. The Murphy–Good formalism was applied as the primary framework for emission analysis. The main experimental tool is an field electron microscope operating in the ultra-high vacuum regime, which allows for detailed characterization of the field emission. Complementary methods, such as scanning electron microscopy, were also employed to provide valuable insights into the morphology and geometry of the fabricated electrodes.