Nanowall networks of ZnO grown on SiO2/Si substrate were found to exhibit persistent photoconductivity (PPC). The relaxation rate of the persistent photocurrent was enhanced by a higher oxygen level in the ambient suggesting that PPC was closely related to the ZnO surface. Surface modification with hydrogen peroxide can significantly reduce the PPC relaxation time, implying that surface oxygen deficiency was responsible for the effect. The transition between the neutral and the metastable singly ionized states of the surface oxygen vacancy was suggested to account for the phenomenon and it was supported by the temperature and wavelength dependence of the PPC.

Persistent Photoconductivity in ZnO Nanostructures Induced by Surface Oxygen Vacancy. Z.G.Yin, X.W.Zhang, Z.Fu, X.L.Yang, J.L.Wu, G.S.Wu, L.Gong, P.K.Chu: Physica Status Solidi - Rapid Research Letters, 2012, 6[3], 117–9