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

Structural defects within Ta 2 O 5 monoclinic structures were obtained by crystallisation at 850 °C in atmosphere O 2 –H 2 O 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 Ta 5+ vacancies along a crystallographic direction [001]. H 2 O gas of different partial pressures was obtained by bubbling O 2 gas through deionised water or Na 2 CO 3 –K 2 CO 3 aqueous solution maintained at different temperatures in a range of 25–45 °C. The amount of structural defects increases dramatically with H 2 O 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 Na 2 CO 3 –K 2 CO 3 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 0.8 K 0.2 ) 2 Ta 4 O 11 of natrotantite type structure. Assuming that Ta 5+ 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 Ta 2 O 5 concordant with H enrichment at defects was identified by time-of-flight secondary ion mass spectrometry (ToF-SIMS). [ABSTRACT FROM AUTHOR]