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

• Published in: Acta materialia Vol. 94; pp. 181 - 192
• Main Authors: Le Gallic, Marie, Roussel, Hervé, Rapenne, Laetitia, Audier, Marc, Barnes, Jean-Paul
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.08.2015; Elsevier
• Subjects: Amorphous structure; Atmospheres; Chemical Sciences; Crystal defects; Crystallization; Defects; Material chemistry; Partial pressure; Ta2O5; Tantalum oxides; Thin films; ToF-SIMS; Transmission electron microscopy; Vacancies; X-ray diffraction; X-ray diffraction; Transmission electron microscopy; ToF-SIMS; Ta2O5; Defects
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2015.04.041

(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. [Display omitted] 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 (Na0.8K0.2)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).