High-Temperature Stable Amorphous Al _{2– <i>x</i>} (ZrY) _<i>x</i> O ₃ Thin Film Insulators: An Alternative to Crystalline Alumina

In high-temperature settings, amorphous alumina thin film insulators offer distinct advantages over crystalline materials, especially including a reduction in pinholes and leakage currents. Nonetheless, the phase stability of amorphous structures is a significant factor in determining their performance as insulators over a broad temperature range. With increasing temperatures, amorphous alumina undergoes a series of crystallization processes, resulting in irreversible alterations such as cracking and delamination due to volume changes between the different Al₂O₃ polymorphs. In this study, amorphous alumina doped with yttrium and zirconium has been deposited via physical vapor deposition, demonstrating the stability of the amorphous structure up to 1200 °C in ambient air and vacuum. Above 1000 °C, the amorphous films transform into a two-phase system consisting of tetragonal yttria-stabilized zirconia (YSZ) embedded in an amorphous alumina matrix. Compared to undoped alumina, films alloyed with 4.3, 12.2, and 20.7 at. % ZrY exhibited enhanced thermal stability. Samples alloyed with 4-12 at. % ZrY additionally showed stable and reversible resistivity behavior under thermal cycling. Notably, films containing only 4.3 at. % ZrY achieved uncharted resistivity values of (3.33 ± 0.06) × 10⁵ and (2.83 ± 0.07) × 10⁵ Ω·m at 750 and 850 °C, respectively, representing a substantial enhancement relative to undoped alumina, which exhibited a value of (8.33 ± 0.04) × 10⁴ Ω·m at 750 °C.