Phase transitions of self-polarized PZT thin films
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Phase transitions of self-polarized PZT thin films Gunnar Suchaneck1, Gerald Gerlach1, Alexander Deyneka2, Lubomir Jastrabik2, Sulkhan T. Davitadze3 and Boris A. Strukov3 1 Dresden University of Technology, Institute for Solid State Electronics, D-01062 Dresden, Germany. 2 Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic. 3 Moscow State University, 11119899, Moscow, Russia ABSTRACT
Optical properties and thermal properties of self-polarized thin PZT films were determined by variable angle spectral ellipsometry and by the AC hot strip method, respectively. Analyzing the temperature dependencies of the optical gap and the specific heat, evidence of two not as yet observed phase transitions in the ferroelectric was provided. The origin of these phase transitions was attributed to film stress caused by substrate/PZT thin film lattice mismatch and to the presence of a negative space charge layer in the PZT film at the bottom electrode/PZT interface.
INTRODUCTION
Second order phase transitions are associated with slight distortions of the crystal lattice which cause a change in crystal symmetry. They do not display a temperature hysteresis and are not accompanied by a discontinuous change in volume and entropy [1]. Evidence of an unknown up to now second order transition at about 200°C in multi-target reactive sputtered PZT thin films was reported in [2]. This phase transition was manifested by a kink in the temperature dependence of the optical gap. In fact, when the distribution of electronic charge within the crystal is changed by the excitation of an electron across the energy gap, this affects the lattice mode frequencies and therefore the entropy of the crystal [3]. Thus, a kink in the temperature dependence of the optical gap corresponds to an entropy change. On the other hand, second order phase transitions are accompanied by pretransitions anomalies of physical properties, for instance the heat capacity. The entropy change associated with ordering can be calculated by the anomalous contribution c(T)-c0(T) to the heat capacity according to [1]: Tc
c( T ) − c 0 ( T ) dT , (1) T 0 with Tc as the transition temperature where the order parameter vanishes and with c0(T) as the regular part of the specific heat. Therefore, it is the goal of this contribution to show the appearance of two not as yet observed phase transitions in self-polarized PZT thin film by investigating both the heat capacity and the optical gap. ∆S =
∫
D8.4.1 Downloaded from https://www.cambridge.org/core. YBP Library Services, on 20 Aug 2018 at 16:56:01, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-718-D8.4
A suitable method to study thermal properties of thin films deposited onto massive substrates is the AC heater-probe method which is also called a 3ω method. The basic idea of the heater-probe method is that a metal strip or wire made of a metal with high thermal resistivity coefficient serves as a
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