Hydrogen donor formation in a solution-processed ZnO film by UV light irradiation

Abstract

A transparent ZnO film was deposited by the spin-spray method at a low temperature of 90 °C. Ultraviolet (UV) light irradiation to the film for 60 min decreased the film resistivity drastically by three orders of magnitude from 55.3 to 4.43 × 10−2 Ω cm. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile and 1H solid-state nuclear magnetic resonance (NMR) spectroscopy analysis revealed that the as-deposited ZnO film before UV treatment contained a large amount of water in the ZnO crystal lattice, and hydrogen donors, including interstitial hydrogen in bond-centered sites (Hi), substitutional hydrogen on the oxygen lattice site (HO), and three O–H bonds in a zinc vacancy (VZn−H3), were generated in the ZnO film after UV treatment. From these results, the mechanism of the hydrogen donor formation was proposed in which UV irradiation to photocatalytic ZnO decomposed the water in the ZnO crystal lattice to form H+ and OH− ions, which associated with the oxygen and zinc vacancies, resulting in the formation of Hi, HO, and VZn−H3.