Shallow and deep donors in n-type ZnO characterized by admittance spectroscopy

Shallow and deep energy levels in n-type ZnO materials grown by a pulsed laser injection method are investigated. We report thermal ionization energies for residual shallow donors of about Ed = 15 meV. Annealing in nitrogen ambient further lowers these ionization energies. We attribute these residual donors to native defects such as Zn interstitials and oxygen vacancies. Using optical admittance spectroscopy we also identify deep defects with optical ionization energies Eo1 = 2.5 eV and Eo2 = 2.1 eV. One of these centers is bistable with large capture barrier energy and therefore accounts for a significant part of the persistent photoconductivity in the material.