Electromigration in Zinc Single Crystals

The electromigration of high-purity Zn single crystals resistively heated in an argon atmosphere has been measured by the marker-motion technique in the temperature range of 360 to 395°C with current densities of ∼5×${10}^3$ A/${\mathrm{cm}}^2$. The cylindrical specimens were selected with $c$ axes oriented within 25° perpendicular or parallel to the direction of current flow. Accurate measurements of both longitudinal and transverse strain rates were used to calculate the anode-directed atom-drift velocity $v_a$, and hence the effective charge on the moving ion, $Z^{*}$. The effective charge is found to be anisotropic: $\frac{{Z_{\mathrm{II}}}^{*}}{f}=-2.55\mathrm{}\pm 0.15$ and $\frac{{Z_{\perp }}^{*}}{f}=-5.6\mathrm{}\pm 0.4$, where $f$ is the correlation factor, which has been shown to be approximately 0.78. The anisotropy of the ion velocities is in the opposite sense and is greater than the anisotropy of the diffusivities. This results from the large anisotropy of the effective charge due to the band structure. It appears that this is the first demonstration of the crystalline anisotropy of the electromigration driving force.