Aging effects on the energy gap of ZnO films

What is it about?

Control of the band bending properties of ZnO is a key step for the development of a new generation of transparent optoelectronic devices. Unlike ZnO bulk single crystals (on which are based most of the related previous works), high quality polar ZnO thin films offer an opportunity to amplify these effects in a way that they can affect properties like optical absorption or photoconductivity. This work provides an easy way of dramatically change the photoconductivity spectra and the effective energy gap of ZnO thin films by means of simple oxygen or vacuum annealing. More surprising, the film exposure to ambient air conditions after these thermal annealing results in a stable gap in the case of an oxygen annealing while for a vacuum annealing the gap evolves in time during several days until a flat band condition is reached. These behaviors have been monitored by photoconductivity spectroscopy and valence band x-ray photoemission spectroscopy (XPS) which proves to be a novel combination of techniques to follow the evolution of the band bending and the effective energy gap. These striking results can be explained taking into account the Franz-Keldysh effect and the presence of different adsorption kinetics of water and hydrogen on the surface film for both types of thermal annealing, as core level XPS shows. Our work suggests that a key optoelectronic property as the optical absorption edge (or the corresponding photoconductivity edge) can tuned in future integrated ZnO devices by means of simple thermal annealing in a reversible and reproducible way. Besides, the observed ageing effects also provide information about the behavior of these devices at atmospheric conditions.

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