Argon annealing induced morphological transformation from nanowire to nanoflake and high photocurrent gain of ZnO:Al films

• ZnO and ZnO:Al nanowires were synthesized using thermal oxidation method.
• Argon annealing triggered change in morphology from nanowire to nanoflake for ZnO:Al.
• Depth profile reveals Ar annealing triggered Al. migration from bulk to the surface.
• Substantial increment of photocurrent gain for ZnO:Al owing to Ar annealing.

Pristine ZnO nanowire grown via thermal oxidation technique has been one of the facile synthetic routes. However, nucleation of Zn thin film over ultra thin metal layer and subsequent heat treatment would open up the possibilities of metal incorporation into ZnO matrix. The present work demonstrates the formation of ZnO:Al nanowires by oxidizing bi-layered Zn/Al film and subsequent investigation of photoresponse phenomenon of as grown and argon annealed samples in comparison to pristine ZnO nanowire. For as grown ZnO:Al, surface localized native defects effectively form complex with aluminium related defects (AlZn), (generated in small numbers); thereby restricting the donor capacity of aluminium. However, argon annealing provides additional defect centres (both native and AlZn) and at the same time help dissociating the complexes. These phenomena account additional surface charge density and surface reconstruction, causing morphological transformations from nanowire to nanoflake. SIMS depth profile studies reflect that post synthesis argon annealing triggers aluminium surface doping, In this study we have demonstrated high photocurrent gain of ZnO:Al nanowires after argon annealing, attributed to oxygen chemisorption-desorption phenomenon. Photoluminescence spectra revealed that argon annealing has resulted in improvement of near band edge emission and decrease of green luminescence intensity of ZnO:Al sample. In this study violet-blue emission was observed at 3.11 eV after argon annealing of both ZnO and ZnO:Al samples, attributed to emission from the localized defect related complexes of oxygen vacancy and characteristic peak at 3.0 eV may be attributed to interstitial zinc (Zni) related complex.