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Zinc oxide (ZnO) nanocrystal films are of interest for new applications in thin film transistors and as transparent conductive oxides. Previous work has concentrated on achieving highly conductive, metallic films. This work focusses on the less explored insulating to semi-insulating regime, which enables obtaining deeper insights into the roles of surface states and defect states trapped at the nanocrystal interfaces. We examine the effects of various post-deposition treatments including controlled dosing with ultraviolet light, filling the voids between nanocrystals with a matrix material deposited by atomic layer deposition, and thermal annealing of the nanocrystal films. Both Mott and Efros-Shklovskii variable range hopping are observed depending on the carrier concentration in the nanocrystals. Using the above post-treatments to transition the films between the two conduction mechanisms enables determining the Fermi level density of states and the electron localization length. To interpret our results, we propose a model based on the assumption of nanocrystals consisting of quasi-neutral cores surrounded by shells depleted by surface OH trap states. The model suggests that the primary source of the increased conductivity in ZnO nanocrystal films based on post-treatments is an increase in the ability to tunnel between nanocrystals due to a reduction of the distance between the quasi-neutral nanocrystal cores.
Bibliographical noteFunding Information:
This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013. Part of this work was carried out in the College of Science and Engineering Minnesota Nano Center which receives partial support from the National Science Foundation through the NNCI program. The authors also acknowledge very helpful discussions with nano transport theorists Boris Shklovskii and Kostya Reich.
© 2018 IOP Publishing Ltd.
- charge transport
- ES-Mott crossover
- variable range hopping
- zinc oxide
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PubMed: MeSH publication types
- Journal Article
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