Hot Electron Energy Loss in Tunnel Cathode Structures

Abstract

A variety of scattering processes occur within the metal—oxide—metal tunnel cathode, with the result that most of the injected electrons fail to escape into vacuum. By considering the over‐all behavior it is possible to identify the most important loss mechanisms and where they occur. Electron emissionmeasurements suggest that the internal hot electron velocity distribution in the exit metal films is isotropic as a result of numerous phononscattering events. It is also found that the electron attenuation length in the exit metal film is not a strong function of energy over most of the observable range. As a direct consequence one obtains the result that the shape and mean energy of the emitted electron energy distribution are determined principally by the scattering in the oxide layer. Collision ionization is unlikely; hence optical phononscattering is believed to be the principal oxide energy loss mechanism. If the scattering is random and independent of energy, both the mean energy loss (ΔE) and mean square deviation (δ2) should depend linearly on oxide thickness (x). Within the limits of the available data this is shown to be reasonably correct, and a mean rate of energy lossdE/dx≈0.03 eV/Å is obtained. This corresponds to about one optical phonon emission event for every 4‐Å travel measured along the field direction. The consistency of the argument is tested by showing that δ2 also varies linearly with oxide thickness. The minimum ingredients for a model of the hot electronscattering processes are outlined.