Unusual Sequential Annealing Effect in Achieving High Thermal Stability of Conductive Al-Doped ZnO Nanofilms

Emerging interactive sensor electronics requires metal oxide electrodes that possess long-term atmospheric stability and electrical conductivity to function under harsh conditions (e.g., high temperatures) in air. In this study, we report a rational method to accomplish the long-term thermal stability of conductive Al-doped ZnO (AZO) nanofilms, which have been thermally unstable due to inevitable crystal defects. Our method utilizes a sequential thermal annealing in air and Zn vapor atmosphere. An initial annealing was performed in air, followed by a second annealing in a Zn vapor atmosphere. Air tolerance tests on the resulting AZO nanofilms revealed the stable electrical resistivity (∼10-4 ω·cm) in air, even at temperatures up to 500 °C. Conversely, when annealing was performed in the reverse sequence, the electrical resistivity of the AZO nanofilms significantly increased by 5 orders of magnitude during tolerance tests. Photoluminescence data further supported the results of the air tolerance tests. The unusual effect of the annealing-atmosphere sequence is discussed in terms of the presence of dual anion/cation vacancies and the sequential benefits when these vacancies are compensated during annealing. The applicability of these thermally stable AZO electrodes for use in nanochannel sensor devices is demonstrated. Furthermore, we show that the proposed sequential annealing method is applicable for Ga-doped ZnO films, supporting its use as a platform fabrication method. Thus, the proposed fundamental concept for tailoring thermally stable conductive metal oxide electrodes provides a foundation for designing interactive electronic devices that are stable for a long period.