Reheating induced atomic migration in Al-doped ZnO (AZO)films: Effect on the growth of AZO/ZnO bilayer

In this work, reheating-induced transition in Al, Zn and O atomic positions in Al-doped ZnO (AZO)thin films is reported, and its application is shown for the growth of AZO/ZnO bilayers. Here thickening ZnO films were deposited on the AZO-coated soda lime glass (SLG)substrates by adopting two different fabrication routes. In the first process, ZnO film was deposited on the AZO-coated SLG substrate for appropriate duration immediately after the deposition of the AZO layer, whereas in the second process, the bottom AZO-coated SLG was cooled to room temperature after its deposition and then reheated again to 623 K for depositing the ZnO layer. To compare and interpret the microstructure and optoelectronic properties of these bilayers with respect to ZnO layer thickness and/or process type, the bottom AZO layer was also reheated for the same duration (as that of the second process). Atomic migration, followed by chemical reaction between these species is observed for all these reheated samples, which affected their optoelectronic properties. More precisely, some of the Al³⁺ cations are found to be un-substituted from the Zn²⁺ lattice sites, which then reacted with the excess oxygen present in these films to form Al<inf>x</inf>O<inf>y</inf>. These unsubstituted lattice sites are then filled by the Zn²⁺ cations, which were originally positioned in the interstitial sites. This atomic rearrangement is then found to alter the optoelectronic properties of these single and bilayers.