Unusual Sequential Annealing Effect in Achieving HighThermal Stability of Conductive Al-Doped ZnO Nanofilms
2020
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.
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