Thick Epitaxial Pb(Zr_0.35,Ti_0.65)O₃Films Grown on (100)CaF₂Substrates with Polar-Axis-Orientation

Abstract

(100)/(001)-oriented tetragonal Pb(Zr,Ti)O3 (PZT) films, which were thicker than 2 µm, were epitaxially grown on SrRuO3-covered (100)Si, (100)KTaO3, (100)SrTiO3, and (100)CaF2 substrates by metal organic chemical vapor deposition. The volume fraction of the (001)-orientation almost linearly increased as the thermal strain increased during the cooling process from the deposition temperature to the Curie temperature. Consequently, perfectly (001)-oriented, i.e., polar-axis-oriented, thick films were obtained on CaF2 substrates with a remanent polarization of 71 µC/cm2. This approach to grow polar-axis-oriented PZT thick films enabled the intrinsic piezoelectricity of PZT itself to be clarified, and has potential in novel applications.

Direct observation of intrinsic piezoelectricity of Pb(Zr,Ti)O3 by time-resolved x-ray diffraction measurement using single-crystalline films

“…This thickness is about 20 times thicker than that of the previously reported polar-axis oriented films but its remaining strain is very small. 8 In addition, by using time-resolved XRD synchronized with the electrical response measurement, it is able to directly measure intrinsic piezoelectricity originated from the lattice response by an applied pulsed electric field with mega-hertz order while keeping the leakage current negligibly small and excluding extrinsic contribution of domain switching.…”

“…The details of the growth condition was already described elsewhere. 8,9 Film thickness was controlled by the deposition time. We used CaF 2 single crystals covered with SrRuO 3 top layers as substrates because the thermal expansion coefficient of CaF 2 (19 Â 10 À6 K À1 ) 10 was large enough to realize the perfectly polar axis oriented tetragonal films for which the c-axis (polar-axis) is normal to the substrate and the a-axis is in the in-plane.…”

“…We used CaF 2 single crystals covered with SrRuO 3 top layers as substrates because the thermal expansion coefficient of CaF 2 (19 Â 10 À6 K À1 ) 10 was large enough to realize the perfectly polar axis oriented tetragonal films for which the c-axis (polar-axis) is normal to the substrate and the a-axis is in the in-plane. 8 The thermal strain applied at the Curie temperature during the cooling process after the deposition is larger than the misfit strain for the case of thick films because the misfit strain was almost fully released when the film thickness reached to 2000 nm. 8 In addition, CaF 2 has the similar thermal expansion coefficient with that of a-axis of PZT (21 Â 10 À6 K À1 ) 11 below Curie temperature, which results in the small residual strain in the films, as detailed in supplementary material.…”

“…8 The thermal strain applied at the Curie temperature during the cooling process after the deposition is larger than the misfit strain for the case of thick films because the misfit strain was almost fully released when the film thickness reached to 2000 nm. 8 In addition, CaF 2 has the similar thermal expansion coefficient with that of a-axis of PZT (21 Â 10 À6 K À1 ) 11 below Curie temperature, which results in the small residual strain in the films, as detailed in supplementary material. 12 SrRuO 3 layer is used as a bottom electrode due to its high thermal stability and good conductivity, while LaNiO 3 layer was used as a buffer layer to grow SrRuO 3 on CaF 2 substrate.…”

Growth of polar axis oriented tetragonal Pb(Zr,Ti)O3 films on CaF2 substrates with transparent (La0.07Sr0.93)SnO3

“…Therefore, film growth must exhibit a monodomain structure or single axis oriented multidomain. Our previous studies have taken the first step toward eliminating multidomains in tetragonal Pb(Zr,Ti)O 3 films up to 3 mm thick with a small residual strain using CaF 2 substrates and epitaxial growth of Pb(Zr 0.35 Ti 0.65 )O 3 films with perfect (0 0 1) out-of-plane orientation, and have realized polar axis orientation [2]. The key for growing perfectly polar axis orientated films with a small residual strain is using a CaF 2 substrate with the appropriate thermal expansion coefficient [3].…”