Positional order in lead scandium tantalate (PST) as a "tool" for the investigation of relaxor ferroelectric behavior in thin films

Relaxor ferroelectric thin films exhibit a drastic reduction in the dielectric constant and associated properties in the thin film form, even for thicknesses in the micron range, which are essentially infinity for the size effects typically investigated in conventional ferroelectrics. In order to investigate this phenomenon, the material system Lead Scandium Tantalum (PST) was fabricated by a Sol-Gel technique using mixed acetate and alkoxide precursors. The positional order of the "B" site species was tailored through different annealing procedures to produce ordered (conventional ferroelectric) and disordered (relaxor ferroelectric) PST thin films. With regards to processing, it was found that the dehydration of scandium acetate by acetic acid anhydride, followed by precursor modification via the solvent 2-Methoxyethanol was necessary for a stable "B" site Sc/Ta complex and subsequent stoichiometric thin films. Excess lead was required in amounts greater than 20% to compensate for lead loss during annealing and avoid the formation of secondary phases. Disordered films with measurable properties were only achieved at low annealing temperatures near 700°C. Higher temperatures up to 1000°C resulted in "B" site ordering, and at annealing temperatures above 1000°C significant film degradation was observed. The ordering of PST thin films was limited to low degrees of order (S=0.22) on Pt/Si substrates after annealing at 800°C for 20 minutes. Higher degrees of order were achieved on single crystal substrates by extended time annealing in a controlled atmosphere. An ordering degree of S=0.91 observed for PST films on sapphire substrates annealed at 850°C for 35 hours. Highly ordered films showed the development of a ferroelectric state, and had dielectric properties comparable with ordered ceramic samples. Disordered films fabricated on numerous substrates and under many processing conditions consistently displayed dielectric constant values reduced by an order of magnitude as compared to ceramic or single crystals. A direct comparison of ordered and disordered PST on a common substrate showed that the dielectric constant increased with an increase in "B" site order in thin films, while in ceramics the opposite effect was observed (a decrease in the dielectric constant with an increase in order). In terms of lattice dynamics, ordered PST thin films unexpectedly showed that the transverse optic (TO1) mode softened to the Burns temperature, with the simultaneous appearance of a central mode (CM) attributed to the appearance of polar regions in the paraelectric phase. Higher processing temperatures were seen to increase the dielectric strength of the CM. Ordered and disordered samples displayed drastically different behavior at low frequencies (below 1MHz), while possessing essentially the same lattice dynamics. It was shown by comparing the "In-Plane" and "Out-of-Plane" dielectric response that the commonly observed low values of dielectric constant in relaxor thin films cannot be explained by a passive layer, at least in nominally thick films near 1 micron in thickness. The changes in the dielectric constant introduced as a result of thermal induced stress were small and relatively insignificant as compared to the order of magnitude reduction in the dielectric constant of thin films as compared to ceramics. A study on the effects of grain size from dielectric measurements showed no detectable change in the value of the dielectric constant by reducing the grain size from 325 to 140 nm. In addition, even epitaxial films fabricated on STO and MgO substrates displayed a reduced dielectric response. Self-polarization was observed in PST thin films and the quantitative impact on the dielectric response was estimated to be near a 10% reduction in the dielectric constant at temperatures close to the dielectric maximum. A hypothesis was proposed to explain the reduced dielectric response in relaxor thin films emphasizing that the quantitative contribution of polar regions may be different. Disordered relaxor PST thin films were annealed at 700°C resulting in a Low-Temperature disordered state, while ceramic samples were annealed at temperatures in excess of 1500°C leading to a High-Temperature disordered state. Both states of disorder evolved to the same long range ordered ferroelectric state after annealing at temperatures near 900°C.