Characterization of donor states in ZnO

We performed electrical and optical measurements on as-grown ZnO which exhibits n-type conductivity. So far, neither the origin of the residual conductivity nor the electrical properties of the responsible defects is fully understood. We investigated shallow and deep donors in ZnO materials grown with pulsed laser injection using admittance spectroscopy. We identifed shallow donors with ionization energies as low as 15 meV which may be attributed to native defects. Annealing in nitrogen ambient enhances the conductivity by further lowering the ionization energy of the shallow donors. Using optically excited admittance spectroscopy we also found deep defects. They are strongly metastable and account for a significant part of the persistent photoconductivity in our ZnO materials