The Cu was diffused from a constant source into oxide single crystals. Layer-wise etching and atomic absorption analysis were used to determine the concentration profile. At 1100 to 1250C, the data could be described by:

D (cm2/s) = 2 x 107exp[-4.8(eV)/kT]

It was concluded that the Cu was present mainly as Cu2+ on Zn lattice-sites. A decrease in the electrical conductivity, with increasing Cu concentration, was attributed to the action of Cu2+Cu2+] associates.

Diffusion of Cu in ZnO Single Crystals. Mueller, G., Helbig, R.: Journal of Physics and Chemistry of Solids, 1971, 32[8], 1971-7