ANODIC OXIDE-GROWTH ON TANTALUM WITH AC+DC FIELDS .2. MODELING AND THEORY

The small signal ac response of the ionic conductivity process, invest igated in Part I, with its inductive phase relation between ionic curr ent and field, is a manifestation of the dependence of the ionic curre nt density on the previous history of the film. This history dependenc e is probably caused by structural changes in the oxide. Equations pre viously formulated to predict the ionic current density for all situat ions were modified to account for the observed behavior by introducing a distribution of relaxation times. The type of distribution found is consistent with structural changes in a glassy oxide. The relaxation times involved in the adjustment of the structure to a new field are e xpected to be a function of the field, temperature, and the state of t he oxide. For the present experiments, since the state of the oxide co rresponded to steady state (growth at constant current and field for l ong enough), tau(m) could be fitted by a function of field and tempera ture alone. The implications of the fact that tau(m) was close to inve rsely proportional to the ionic current density independent of tempera ture are discussed.