The influence of the Ti/Ba ratio on the formation of pyroelectric and piezoelectric quasi-amorphous films of BaTiO 3
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The influence of the Ti/Ba ratio on the formation of pyroelectric and piezoelectric quasi-amorphous BaTiO3 films was investigated. Three types of films, Ti-rich, Ba-rich, and stoichiometric, were pulled through a temperature gradient or subjected to isothermal heating. The quasi-amorphous polar phase only formed in films pulled through the temperature gradient with Ti/Ba ratio within the broad range of 0.95–1.1. This implies that quasi-amorphous pyroelectric and piezoelectric thin films are significantly more tolerant of a deviation from stoichiometry than their crystalline counterparts.
I. INTRODUCTION AND MOTIVATION
The report on the existence of a polar but noncrystalline phase of BaTiO3, called quasi-amorphous,1 was followed by a series of investigations directed at understanding the formation and the origin of polarity of the quasi-amorphous phase. These investigations led to the development of a model that can be summarized as follows. The preparation of quasi-amorphous films of BaTiO3 includes two stages: sputtering of amorphous films and subsequent pulling through a temperature gradient.1 In the amorphous phases of BaTiO3, TiO6 octahedra form a network,2 within which they are connected in three different ways: apex-to-apex, as in perovskite, edge-to-edge, and face-to-face.3 The network is stabilized by Ba ions that compensate for the local charge imbalance. Before nucleation of the perovskite phase may occur, edge-to-edge and face-to-face connections must be severed. Severing non-perovskite bonds while preserving the TiO6 octahedra is accompanied by a large (>10%) volume increase.4 In a thin substrate-supported film, the volume increase is obstructed by substrate clamping. Therefore, in-plane mechanical stress builds up and, if not relaxed, may arrest crystallization completely. In the presence of a temperature gradient, the increasing stress is strongly inhomogeneous and causes partial alignment of the TiO6 octahedra, giving rise to an amorphous structure with permanent macroscopic polarization. In regions where the stress does not accumulate or is relieved, nucleation and subsequent crystallization pro-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0342 2742 J. Mater. Res., Vol. 22, No. 10, Oct 2007 http://journals.cambridge.org Downloaded: 01 Feb 2015
ceed unobstructed. In a real film, voids, cracks, and scratches are the centers of stress relaxation. Therefore, during heating two competing processes take place: closure of the voids due to prenucleation expansion and nucleation in the areas where stress relaxation occurs.5 If the first process prevails, the crystallization is eventually arrested. If the second process dominates, then the crystallization proceeds. The generality of this model has been recently confirmed by the discovery of the pyroelectric and piezoelectric quasi-amorphous phase of SrTiO3 and BaZrO3.6 This discovery has also emphasized the necessity for better understanding the factors suppressing the nucleation, especially
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