Electronic structure of copper in zinc oxide

Copper influences the electrical and optical properties of zinc oxide to a high degree. Previously this has been explained within the frame of a model assuming Cu⁺⁺ ions to be substitutional for divalent zinc. The observation of additional signals in the Electron Spin Resonance spectrum, however, proved this assumption to be wrong. Satisfying quantitative agreement with experiment is obtained assuming neutral copper atoms at an interstitial site having 3d⁹ 4s² electron configuration and a²D_3/2 ground state. The infrared absorption bands as well as the spin lattice relaxation mechanism give further confirmation of Cu_i⁰.