Field emission energy distribution (clean surfaces)

Abstract

The emission of electrons from a cold cathode upon application of a strong electric field is called field emission. Since the electrons must tunnel through the classically forbidden barrier outside the solid, field emission was one of the first confirmations of the new quantum theory of the 1920's.¹ A field of tens of million volts per centimeter is required to obtain a reasonable current. In order to achieve such hgh fields at reasonable voltages, the cathode or emitter is usually etched to a very sharp point (∼1,000 Å in radius). Therefore, several thousands of volts applied to the anode will produce the desired field. In 1937, Miiller2 developed what is known as the field emission microscope. The success of Miiller's microscope was a consequence of his realization that, if he produced a small hemispherically shaped tip that was thermally smoothed and cleaned, he would project a greatly magnified image of the spatial distribution of electrons tunneling from the emitter onto a fluorescent screen. Such a field emission pattern for clean tungsten is shown in Figure 1. The image on the screen is a nearly stereographic projection of the hemispherical end of the emitter. Because of its small size, the emitter is usually part of a single crystal and thus exposes all crystallographic orientations, so that individual crystal planes can be located and identified in the field emission pattern. The changes in the field emission pattern with exposure to adsorbed atoms or molecules have been used very successfully to study surface processes, such as diffusion, adsorption and desorption kinetics, or work function changes.³