Driving force in electromigration

The driving force in electromigration is generally assumed to consist of a "direct force" due to the applied field and an "electron-wind" term due to momentum exchange with the current carriers. In the case of the "direct force" it is uncertain how effectively the charge is screened. In the case of the "electron wind" it is unclear whether the moving atom senses the carrier's change in lattice momentum or real momentum. A method first utilized by Bosvieux and Friedel, and expanded by Sorbello, resolves these problems by calculating driving forces on the unscreened ion resulting from the nonuniformities in spatial carrier distribution. These theories depend, however, on an incomplete picture of the spatial variations in the electric field, which inevitably accompany current flow past localized scatterers. Furthermore, "screening" of the charge of a lattice defect is related to inhomogeneities in carrier density and the resulting spatial inhomogeneity in the rate of current production which a uniform field would provide. Terms of this latter sort have never received explicit consideration.