Thermoelectrical Characterization of Piezoelectric Diaphragms: Towards a Better Understanding of Ferroelectrics for Future Memory Applications
Authors: Vinuesa Guillermo , Marín Pelayo , Gonzalez Ossorio Oscar , Sahelices Benjamin , Garcia Hector , Castan Helena , Duenas Salvador 
Published: July 2021

In the search of materials with high charge storage densities to be used in the memory field, metal oxide perovskites such as lead zirconate titanate (PZT) result of particular interest due to its low leakage currents. Besides, ferroelectric PZT shows high remanent polarization. These properties make this material promising for high-speed and low-voltage non-volatile memories (J.F.Scott et al., J. Appl. Phys. 70, 382, 1991). Due to its piezoelectric behavior, PZT has been most commonly used in the application of sensors and actuators. More recently, its usefulness in the energy harvesting field (H. Liu, J. Mat. Chem. A
8, 19631, 2020), as well as for controlling electro-chemical processes (W. Quian et al. Nano-micro Letters 2020, https://doi.org/10.1007/s40820-020-00489-z), has been demonstrated. One of the main problems to overcome is the thermal incompatibility between this material and the specific ones of semiconductor devices. However, some efforts have been carried out in this respect (I. Bretos et al. Scientif. Rep.
6:20143, 2016, doi: 10.1038/srep20143). PZT systems exhibit excellent electrical properties such as very high permittivity and thermal stability with low coercive field. Additionally, they present a high electromechanical coupling coefficient, and therefore they can be easily poled and feature a very high Curie Temperature, being fully operational across a wide temperature range. In this work, the variation of the ferroelectric behavior as a function of temperature was investigated. The samples used were PZT commercial structures manufactured by MuRata Manufacturing Co., Ltd.. Element sizes were 9 mm and 14 mm, for 7BB-12-9 and 7BB-20-6 samples, respectively.

In Fig. 1(a), I-V curves in the 100-320 K temperature range show that, at low temperatures, the ferroelectric material requires higher voltage values in order to be polarized, and its polarization current diminishes. Furthermore, the coercive field increases quickly as the temperature drops (Fig. 1(b)). This can be explained by the fact that polarization switching is ruled by domain wall motion, which is a thermally activated process. The experimental results show that domain wall reversal becomes more difficult as temperature decreases.

The impedance analysis was carried out by performing a simultaneous frequency and temperature sweep. Impedance resonances shifted towards higher frequencies as the samples got cooler (Fig. 2(b)) and their peak values grew as temperature decreased (Fig. 2(a)). This can be explained by the fact that the impedance is inversely proportional to the piezoelectric coefficients, which decrease in value with temperature (Silva de Freitas et al. https://doi.org/10.1016/j.sna.2015.11.031).

In Fig. 3(a) the relationship between the coercive field and temperature is depicted for both samples 7BB-12-9 and 7BB-20-6. Here we can see a dependence in size that can only be explained by the fact that the smaller sample’s domain walls have less mobility. This means the crystalline grains of the 7BB-20-6 sample are significantly thinner than those of the 7BB-12-9. Thus, its domain walls have greater mobility, and the sample is easier to polarize.

This is further demonstrated by analyzing Fig. 3(b) which presents the logarithmic plot of the measured relationship between polarization backswitching (Pr – Ps) and the inverse of the temperature for the two different size samples. Pr denotes the remnant polarization, whereas Ps is the saturation polarization. This fits an Arrhenius-like behavior only for temperatures above a certain threshold value ( 150 K for the 7BB-12-9 and 120 K for the 7BB-20-6) which is a new finding when studying previous works on this subject. These measurements verify the fact that the grains of the smaller sample are thicker, thus the movement of its domain walls is more difficult. This causes the saturated state to be less stable. Consequently, when the external electric field is removed, in the smaller sample a larger number of domains return to the situation of non-alignment, increasing the polarization backswitching.

In summary, an exhaustive study of the ferroelectric behavior of PZT as a function of temperature has been carried out in order to deepen the knowledge of the underlying mechanisms for its use in the field of non-volatile memories.

Figure 1