A study of piezoelectric orthorhombic Ta 2 O 5
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A study of piezoelectric orthorhombic Ta2 O5 B. R. Jooste and H. J. Viljoen Department of Chemical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588–0126 (Received 12 June 1996; accepted 7 April 1997)
In 1985 it was first reported that monoclinic Ta2 O5 has piezoelectric properties comparable to ZnO. In this work we report on the deposition, characterization, and qualitative assessment of the piezoelectric behavior of orthorhombic Ta2 O5 . Reactive magnetron sputtering was used to deposit thin films of Ta2 O5 onto substrates of 316L stainless steel. Without substrate heating the crystallinity was poor. A rapid thermal anneal improved the crystallinity. The orthorhombic phase was dominantly present on all substrates. The piezoelectric property was qualitatively assessed, including a high temperature test at 650 ±C.
I. INTRODUCTION
The polymorphism of Ta2 O5 has been extensively studied and there is general agreement that a low temperature form b –Ta2 O5 and a high temperature form a –Ta2 O5 exist. Jehn and Olzi1 proposed a phase diagram for the Ta–O2 system in the temperature range 1100–2500 ±C. It was observed that the transformation of the b phase into the a phase begins at about 1360 ±C, but the b form exists metastably above this temperature until the melting point of the b phase is reached at 1785 ±C.2 The thermal stability of Ta2 O5 makes it an attractive material for high temperature applications. However, the formation of suboxides at elevated temperatures could be of concern. Boiko et al.3 studied the interaction between Ta and Ta2 O5 . Films were vacuum annealed at 337 ±C or 437 ±C and leakage currents were compared with those of similar films annealed in air (presence of moisture provides proton source); no noticeable differences were measured. Oxygen depletion occurs near the interface, but TEM micrographs revealed that the oxygen diffuses along grain boundaries extending from surface dislocations into the Ta. This process causes the appearance of “thinning” regions in the film with finely distributed islands of a separate metal phase. The coexistence of metal and oxide phases instead of a suboxide phase is consistent with the work reported by Dubrovskaya et al.4 and Raisman and Holzberg.5 Dubrovskaya et al.4 found that the Ta–O system is of a simple eutectic variety and only two phases exist over the entire composition interval: Ta and Ta2 O5 . Recently, Ta2 O5 has been investigated because of its high-dielectric constant for use in high-density dynamic random access memory (DRAM) devices. Several studies were reported on the effect of substrates, thermal annealing, and deposition conditions on the leakage current of Ta2 O5 films. In contrast, very little has been published on the piezoelectric properties of Ta2 O5 . The J. Mater. Res., Vol. 13, No. 2, Feb 1998
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first report on the piezoelectric properties of Ta2 O5 was presented by Nakagawa and Gomi6 who deposited monoclinic b –Ta2
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