LATTICE DIFFUSION, GRAIN-BOUNDARY DIFFUSION AND DEFECT STRUCTURE OF ZNO

The rates of Zn-65(2+) self-diffusion and Ni2+ diffusion have been mea sured along the a axis and the c axis of single-crystal ZnO over tempe rature ranges of 758-1267 degrees C and 900-1550 degrees C, respective ly. Samples were encapsulated in polycrystalline disks during annealin g to inhibit loss of solute or host material through vaporization. Cat ion self-diffusion in ZnO is isotropic within experimental error and m ay be described by an activation energy of 1.80+/-0.09 eV and a pre-ex ponential term, D-o, of 7.26 x 10(-6)cm(2)s(-1). Results for Ni2+ diff usion provide an activation energy of 2.06 eV and a small dependence o n crystallographic direction, D-o, being 9.89 x 10(-5) and 3.16 x 10(- 5) cm(2) s(-1) along the a and c axes, respectively. Diffusion of Zn2 and Ni2+ in polycrystalline ZnO is strongly enhanced along grain boun daries. At temperatures up to 1300 degrees C the temperature dependenc e of delta D-B for Ni2+ provides the same activation energy as found f or volume diffusion. The enhanced transport is attributed to higher de fect concentrations near the boundary. At temperatures >1300 degrees C , delta seems to change with temperature due to incomplete equilibrati on of the sample, an interpretation supported by an observed increase of delta D-B with the time of equilibration with a reducing atmosphere . The increase of delta D-B with decrease in oxygen partial pressure s upports the assignment of doubly-ionized interstitial zinc ions as the predominant point defect but leaves problematic the isotropy observed for both D-o and the activation energy for migration.