Poisoning and reactivation processes in oxide-type cathodes: Part II. Single crystal BaO films

Abstract

The work described here is a continuation of the studies reported in part I of shelf-life poisoning and reactivation observed with mixed Ba/Sr oxide-type cathodes after exposure in a vacuum tube ambient. Emphasis in part II is placed on the identification and determination of the basic mechanisms involved in the individual processes that make up the overall effects observed in part I. In order to achieve this, single crystal BaO films deposited on an inert Ir substrate were studied with respect to poisoning and reactivation by the various individual gas components found in typical vacuum ambients. The electronic work function changes were separated into their components of surface dipole and bulk-type Fermi level changes using the analysis of shifts in the low energy electron reflection (LEER) patterns obtained with an incident electron beam. Corresponding compositional changes were correlated with the electronic changes using multiple Auger energy analysis and also by noting changes in the characteristic shapes of the LEER patterns. The results showed that poisoning during shelf-life takes place chiefly by O 2, CO 2 and H 2O. O 2 poisoning occurs mainly by neutralizing O-vacancy donors and is more of a bulk-type effect altering the Fermi level. CO 2 and H 2O, on the other hand, form partial monolayers of surface carbonates and hydroxides which alter the surface dipole, i.e. electron affinity. These electronic changes in band structure are verified by the compositional measurements indicating the presence of an adsorbed surface layer with CO 2 and H 2O but not with O 2. The surface layers have also been identified as the carbonates and hydroxides from characteristic LEER shapes as well as from comparison to dissociation temperatures and gas desorption results of part I. The poisoning rate for a 10 Langmuir exposure is about 1 eV change in Fermi level for O 2, about 1 eV change in electron affinity for CO 2 and about 1 3 eV change in electron affinity for H 2O. The actually observed vacuum ambient poisoning rates are in very good agreement with that expected from the above mentioned gas poisoning rates when combined in the ratios of the partial pressures of these gases found in typical vacuum ambients. Reactivation is largely a desorption of the gas adsorbed during shelf-life and this is usually completed for oxide-type cathodes when the reactivation temperature reaches ∼1200 K, a typical operating temperature. Reactivation can also be enhanced by choosing substrate materials which will aid in the generation of O-vacancy donors or in the formation of a Ba surface layer. Care must be taken though to avoid interaction effects such as the formation of an oxide residue at the interface which may impede the donor generation process.