Persistent photoinduced changes in charge states of transition-metal donors in hydrothermally grown ZnO crystals

Electron paramagnetic resonance (EPR) has been used to monitor photoinduced changes in the charge states of deep transition-metal donors (Mn, Fe, Co, and Ni), shallow donors (Al and Ga), and lithium acceptors in a hydrothermally grown ZnO crystal. All of these impurities except the lithium were unintentionally present. Changes in charge states were photoinduced at low temperature using 325nm light. Before illumination, the transition-metal donors were present in divalent and trivalent charge states, and all the lithium acceptors were in the singly ionized charge state. During illumination, neutral lithium acceptors were formed, the Fe3+, Ni3+, and Mn3+ spectra disappeared (i.e., they converted to divalent states), and the Mn2+ and Co2+ spectra increased. When the light was removed, these changes in charge states were persistent and stable at low temperature. The thermal stabilities of the changes were monitored during a pulsed anneal (the sample was warmed in steps to sequentially higher temperatures and returned each time to a lower monitoring temperature). Electrons were released from Fe2+ donors in the 90–120K range, from Ni2+ and Co2+ donors in the 110–140K range, and from Mn2+ donors in the 220–300K range. Thermal activation energies for Fe2+ and Ni2+ are estimated to be 240 and 280meV, respectively.