Persistent photoconductivity and thermally stimulated current related to electron-irradiation induced defects in single crystal ZnO bulk

Persistent photoconductivity (PPC) in 30 MeV-electron-irradiated ZnO single-crystals is studied by excitation using light emitting diodes (LEDs) with various wavelengths above and below the band gap. The PPC with extremely long time constants of a few tens of days is observed for the illumination with ultraviolet and blue LEDs at 90 K. An electron paramagnetic resonance signal with a gg-value of 1.996 appears after illumination at 77 K, suggesting the presence of an oxygen vacancy related defect. These results support a mechanism of PPC proposed by Van de Walle, suggesting a transfer from the electron-irradiation induced 2+ charge state of the oxygen vacancy to the metastable + charge state due to the illumination. Two thermally stimulated current peaks with ionization energies of 545 and 664 meV appear after electron irradiation. These traps are tentatively assigned to the zinc vacancy and interstitial oxygen.