Direct evidence of band-bending at grain boundaries of ZnO:SnO2 films: Local probe microscopic studies

Defect states in a semiconductor are known to affect its optoelectronic properties and control its chemical composition. In this paper, based on conductive atomic force microscopy (cAFM) and electrostatic force microscopy (EFM) measurements, we demonstrate charge trapping at grain boundaries of ZnO:SnO2 (ZTO: zinc tin oxide) films, indicating the existence of local band-bending. Electrostatic force microscopy measurements reveal that charge trapping is more at grain boundaries in comparison to grains. On the other hand, cAFM and Kelvin probe force microscopy (KPFM) measurements provide an experimental existence of local band-bending at grain boundaries of ZTO films. In addition, compositional analysis of the films confirms the presence of oxygen vacancies, resulting in the formation of defect states within the band gap which further control the charge transport. The present observations are explained in the basis of defect-dependent potential barrier formation due to oxygen vacancies in the film. The present findings help to understand nanoscale charge transport in ZTO thin films using a combination of local probe techniques which should be useful in fabricating ZTO-based transparent optoelectronic devices.