Growth of epitaxial tetragonal Pb(Zr,Ti)O 3 thin films with 100% polar-axis-orientation and their electrical properties
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Growth of epitaxial tetragonal Pb(Zr,Ti)O3 thin films with 100% polar-axis-orientation and their electrical properties Hitoshi Morioka, Shintaro Yokoyama, Takahiro Oikawa, Keisuke Saito1 and Hiroshi Funakubo Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama, 226-8502, Japan 1 Application Laboratory, Bruker AXS Yokohama, 221-0022, Japan ABSTRACT Pb(ZrxTi1-x)O3 (PZT) thin films with the various Zr/(Zr+Ti) ratios and the film thickness of 50 and 250 nm were epitaxiallly grown on (100)cSrRuO3//(100)SrTiO3 substrates by metalorganic chemical vapor deposition at 540 ºC. 50 nm-thick tetragonal PZT films consistd of 100% c-axis-orientation for the Zr/(Zr+Ti) ratio ranging from 0.13 to 0.56, while 250 nm-thick tetragonal ones consistd of about 70% c-axis-orientation for the range from 0.19 to 0.45. On the other hand, 50 and 250 nm-thick films composed rhombohedral single phase for the Zr/(Zr+Ti) ratio above 0.56 and 0.60, respectively. Mixture region of tetragonal and rhombohedral phases was observed from 0.45 to 0.60 for 250 nm-thick films, but was not for 50 nm-thick ones. Remanent polarization monotonously decreased with increasing the Zr/(Zr+Ti) ratio for 50 nm-thick films in tetragonal PZT films. These results show that Ti rich tetragonal PZT thin films have large spontaneous polarization with good square-shape hysteresis loops and are applicable for the high-density capacitor-type ferroelectric random access memories.
INTRODUCTION Lead-based perovskites such as Pb(ZrxTi1-x)O3 (PZT) are leading candidate for application in nonvolatile ferroelectric random access memories, piezoelectric actuators, optical wave guides, and pyroelectric sensors because of their superior ferroelectric and piezoelectric properties, especially for large remanent polarization and piezoelectric constant [1,2]. Up to date, the dependences of the ferroelectricity on the film composition and orientation have been investigated using epitaxial and polycrystalline PZT films [3-5]. However, the direct measurement of the spontaneous polarization (Ps), most essential characteristic of the ferroelectric materials, is hardly reported because of the lack of the single crystal and/or the perfectly polar-axis-oriented films. Recently, growth and characterization of tetragonal PZT thin films with perfect polar-axis (c-axis)-orientation were successfully by a few groups including us [6,7]. In the present study, perfectly polar-axis-oriented epitaxial PZT films with the various Zr/(Zr+Ti) ratios were grown on (100)cSrRuO3//(100)SrTiO3 substrates and investigated their crystal structure together with the electrical properties.
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EXPERIMENTAL DITAILS PZT thin films having about 50 and 250 nm in thickness were grown on (100)cSrRuO3//(100)SrTiO3 substrates at 540 ºC by pulsed-metalorganic chemical vapor deposition (MOCVD) from Pb(C11H19O2)2-Zr(O·t-C4H9)4-Ti(O·i-C3H7)4-O2 system [5]. The Zr/(Zr+Ti) ratio and the film thickness of the films were controlled by the input gas concentration of the source gases and
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