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

摘要

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 325 nm 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 Fe~(3+), Ni~(3+), and Mn~(3+) spectra disappeared (i.e., they converted to divalent states), and the Mn~(2+) and Co~(2+) 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 Fe~(2+) donors in the 90-120 K range, from Ni~(2+) and Co~(2+) donors in the 110-140 K range, and from Mn~(2+) donors in the 220-300 K range. Thermal activation energies for Fe~(2+) and Ni~(2+) are estimated to be 240 and 280 meV, respectively.