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

Abstract

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. KEYWORDS: conductive (Al, Ga)-doped ZnO nanofilms, crystal defects, long-term thermally stable electrode, sequential effect of annealing−atmospheres

Enhancement of pH Tolerance in Conductive Al-Doped ZnO Nanofilms via Sequential Annealing

“…Interestingly, the chemical stability of the sequence-II-annealed samples is consistent with our previously reported thermal stability tests of a series of sequentially annealed AZO nanofilms. Similar to Figure , only the sequence-II-annealed AZO nanofilms showed thermal stability and sufficiently low resistivity during thermal tolerance tests up to 500 °C in ambient air Figure shows the electrical resistance of the as-grown, sequence-I-annealed, and sequence-II-annealed AZO nanofilm devices during the chemical stability tests.…”

“…First, we examined the chemical stability of the as-grown AZO nanofilms. AZO nanofilms were deposited on Al 2 O 3 (110) substrates through pulsed laser deposition (PLD) with deposition parameters optimized to obtain low electrical resistivity . The details of film deposition are provided in the Methods section.…”

“…Note that changes in the contact resistance between Pt electrodes and AZO nanofilms do not affect the measured resistance because the resistance was measured by the four-probe technique. The deposition parameters of the as-grown AZO nanofilms were optimized to obtain low resistivity, resulting in inevitable crystal defects within AZO . Thus, the as-grown AZO nanofilms, which are deposited to be electrically conductive, are not stable in acidic aqueous solutions.…”

“…The sequential annealing method consists of a set of two thermal annealing steps: annealing in air and in Zn vapor atmosphere. These air and Zn vapor annealing steps were employed to compensate for oxygen and Zn defects in AZO nanofilms, respectively. ,, Details of air and Zn vapor annealing steps are provided in the Methods section. In this study, the effects of sequence I annealing (annealing in Zn vapor atmosphere followed by that in air) and sequence II annealing (annealing in air followed by that in Zn vapor atmosphere) on the chemical stability of AZO nanofilms were investigated (Figure a).…”

“…Figure shows the electrical resistance of the as-grown, sequence-I-annealed, and sequence-II-annealed AZO nanofilm devices during the chemical stability tests. Note that the high initial resistance of the sequence-I-annealed AZO nanofilms originates from the remaining Zn defects in the AZO nanofilms, which trap mobile charges . Each sample and data point in Figure corresponds to that of the chemical etching evaluation shown in Figure .…”