• PDF / 664,410 Bytes
• 10 Pages / 612 x 792 pts (letter) Page_size
• 6 Downloads / 218 Views

DOWNLOAD

REPORT

---

T. Tsuchiyab) Department of Materials Science and Technology, Science University of Tokyo, Yamazaki 2641, Noda, Chiba 278-0022, Japan (Received 25 August 1999; accepted 14 December 1999)

Orientation-controlled epitaxial thin films of bismuth layer-structured ferroelectrics, SrBi2Ta2O9 (SBT) and SrBi2Nb2O9 (SBN), were prepared on single-crystal SrTiO3 (STO) substrates by the coating-pyrolysis process. Most of the SBT (SBN) films showed the (106) and (001) orientations on STO(110) and (001), respectively. The degree of orientation, in terms of the ratio of peak intensity to the background level in the x-ray diffraction ␾-scan profile for the film, greatly increased with a decrease in the oxygen partial pressure, p(O2), of annealing atmosphere at 800 °C. Interestingly, coexistence of the (110)-oriented grains with the (106)-oriented ones on STO(110) [and the (100)-oriented grains with the (001)-oriented ones on STO(001)] was observed exclusively in the SBT films annealed at 700–750 °C under p(O2) of 10 Pa. Atomic force microscopy observations showed that the surface morphology of the SBT films remained almost unchanged, i.e., comprising round-shaped grains of submicrometer size, whereas that of the SBN films drastically changed, according to the variation in orientation of substrate surfaces or in annealing conditions, i.e., temperature, p(O2), and time.

I. INTRODUCTION

Strontium bismuth tantalate, SrBi2Ta2O9 (SBT), has been intensively studied comparable to Pb(Zr,Ti)O3 for realizing the so-called ultimate memory, ferroelectric random-access memory (FeRAM).1–3 Advantages of FeRAM using the SBT films over the conventional memories include nonvolatility, high-speed read–write operation, high endurance up to 1012 operating cycles, low-voltage operation, and so on.4 Since SBT has a highly anisotropic (layered-perovskite) crystal structure, its ferroelectric properties are also thought to be anisotropic theoretically.5 In spite of such an expectation, most of the studies have focused on the fabrication of polycrystalline thin films on platinized silicon substrates,1–3,6,7 and very few experimental results are available concerning the anisotropy in ferroelectric

a)

Permanent address: Department of Materials Science and Technology, Science University of Tokyo, Yamazaki 2641, Noda, Chiba 278-0022, Japan. b) Cooperation program between Agency of Industrial Science and Technology and Science University of Tokyo. J. Mater. Res., Vol. 15, No. 3, Mar 2000

http://journals.cambridge.org

Downloaded: 16 Apr 2015

properties.8–10 Since the growth of SBT single crystals is very difficult, fabrication of epitaxial SBT films and, especially, precise control of their orientation are extremely important to elucidate the anisotropic ferroelectricity of SBT. Recently, Lettieri et al. prepared the (001)ortho- and (110)ortho-oriented SBT thin films using the latticematched LaAlO3–Sr2AlTaO6 (LAST) (001) and LaSrAlO4 (100) substrates, respectively.11 We also reported the successful growth of the (001)tetra- [(001)ortho-] and (106) tetra -