摘要
Photoinduced atomic structural transitions of negative-U defects: neutral oxygen vacancies (VO0), accompanied by lattice relaxation, can form ionized 1+ and 2+ vacancy defects in ZnO materials, giving rise to an optoelectronic phenomenon named "persistent photoconductivity," thereby limiting the applications of ZnO materials in optoelectronic fields. Nevertheless, very little is known about the kinetics of the separation-recombination interactions between an electron and an ionized oxygen vacancy, constituting a photoexcited charge pair, in nanoscale ZnO material systems, especially when considering the effect of electric fields. In this report, we describe the charge-separation kinetics of photoexcited VO0 defects in ZnO nanowire (NW) field-effect transistor (FET) systems, examined through modulation of the surface electric field of the ZnO NW. We apply oxygen plasma treatment to tailor the doping concentration within the ZnO NWs with the goal of modulating the electric field within their surface space-charge layers. X-ray photoelectron spectroscopy and low-frequency current-noise spectroscopy are applied to identify the change in the density of oxygen-vacancy defects near the NW surface after oxygen plasma treatment. A model describing the initial stage of the photoconductance responses associated with the formation of the photoinduced ionized 1+ state of the oxygen-vacancy defects (VO+) in the fully depleted ZnO NW FETs in the low-photoconductance regime upon UV excitation is proposed to extract the charge-separation probabilities of the photoexcited electron/VO+ pair. Accordingly, the charge-separation probability increases from approximately 0.0012 to 0.042 upon increasing the electric field at the NW surface from approximately 7.5×106 to 5.0×107 V m-1. Moreover, we employ modified Braun empirical theory to model the effect of the electric field on the charge-separation behavior of photoexcited electron/VO+ pairs in ZnO NWs, obtaining a reactivity parameter of 4×104 m s-1. In addition, the mechanism of thermally activated charge capture associated with the recovery of photoexcited oxygen vacancies VO2+ in ZnO NW FETs is also investigated, obtaining a thermally activated barrier and a capture cross-section prefactor of approximately 100 meV and approximately 5×10-27 cm2, respectively, for the transition of ionized oxygen vacancies from charged 2+ to 1+ oxygen-vacancy states. This report provides insight into the fundamental physics behind the effect of the electric field on the charge separation and recombination kinetics of photoexcited electron/VO+ pairs and physical information regarding the mechanism of the persistent-photoconductivity recovery in ZnO NW FET systems, potentially benefiting the future applications of nanoscale ZnO materials in optoelectronic devices, phototransistors, and photodetectors.