Particular structural defects in Ta₂O₅ from crystallisation of amorphous thin films in O₂–H₂O atmosphere

Graphical abstract: (a,b) HRTEM images of structural defects in mL-Ta₂O₅ monoclinic structures resulting of a crystallisation of amorphous thin films in O₂–H₂O atmosphere compared in (c) to simulated images of the 11L-Ta₂O₅ structure with [001] columns of Ta⁵⁺–O²⁻ vacancies or only Ta⁵⁺ vacancies; (d) ToF-SIMS depth profiles of H⁻, ¹⁸O⁻, TaO⁻ and ³⁰Si⁻ through a Ta2O5 thin film on Si substrate. - Abstract: Structural defects within Ta₂O₅ monoclinic structures were obtained by crystallisation at 850 °C in atmosphere O₂–H₂O of amorphous thin films deposited by injection metal-organic chemical vapour deposition (i-MOCVD) on Si substrates. These defects, observed by high resolution transmission electron microscopy (HRTEM), contain alignments of Ta⁵⁺ vacancies along a crystallographic direction [001]. H₂O gas of different partial pressures was obtained by bubbling O₂ gas through deionised water or Na₂CO₃–K₂CO₃ aqueous solution maintained at different temperatures in a range of 25–45 °C. The amount of structural defects increases dramatically with H₂O partial pressure and annealing time from HRTEM observations and X-ray diffraction results. Such defects are thermally stable, at least up to 850 °C. Using Na₂CO₃–K₂CO₃ aqueous solution, similar structural defects including mainly K⁺ species were formed. Na⁺ species contributes mostly to the formation of another phase of composition close to (Na_0.8K_0.2)₂Ta₄O₁₁ of natrotantite type structure. Assuming that Ta⁵⁺ vacancies are occupied by H⁺ in a form of bridging hydroxyl groups Ta–OH–Ta in order to satisfy electrical neutrality, the presence of significant levels of hydrogen atoms within Ta₂O₅ concordant with H enrichment at defects was identified by time-of-flight secondary ion mass spectrometry (ToF-SIMS)