A very promising piezoelectric property of Ta2O5 thin films. I: Monoclinic-trigonal phase transition

Ceramic thin films of tantalum oxide of a new trigonal structure (a=12.713(7) A, α=28.201(0)o, space-group R3) were produced by thermal treatments of amorphous deposits on (001)Si wafers, either by electrostatic spray deposition or by injection metal-organic chemical vapor decomposition. This trigonal phase comes from the transformation of a monoclinic phase 11L- or 25L-Ta2O5. The transformation is reversible under oxygen atmosphere and, from results of TEM investigations, occurs mainly via atomic motions along the z unique axis of the monoclinic structure parallel to the polar three-fold axis of the trigonal structure. The non-centrosymmetry and direction of polar axis of the trigonal phase, identified by high resolution TEM imaging, indicate a possibility of very high electric dipole moments linked to a strong piezoelectricity. From results of XPS analyses of both monoclinic and trigonal structures, the binding energies remain similar to those of Ta2O5. As the formation of the trigonal structure gives rise to an important volume expansion, stresses induced in ceramic thin films are likely influencing both properties of birefringence and piezoelectricity which are presented in a separated article (part II). It is mentioned that the formation of trigonal phase does not occur in bulk Ta2O5 samples, for which an incommensurate phase transition has been observed in a previous work. - Graphical abstract: (a) θ/2θ X-ray pattern of the tantalum oxide layer prepared by i-MOCVD, cooled down at 5 deg. C min-1 after a thermal treatment at 850 deg. C under N2 for 1 h. (b) Rhombohedral cell of the Ta2O5 trigonal structure of R3 space-group decorated with its 10 Ta sites. Highlights: → We examine crystallographic properties of a new Ta2O5 trigonal phase. → Its formation occurs only in thin films. → It comes from a reversible transition with a monoclinic phase. → A possibility of strong piezoelectricity is deduced from the present study.