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MATERIALS RESEARCH

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Crystal growth of layered perovskite Sr2 Nb2 O7 and Sr2 Ta2 O7 film by the sol-gel technique Kumi Okuwada Microelectronics Engineering Laboratory, Toshiba Corporation, Kawasaki 210-8582, Japan

Shin-ichi Nakamura Research and Development Center, Toshiba Corporation, Kawasaki 210-8582, Japan

Hiroshi Nozawa Department of Energy Science and Technology, Kyoto University, Kyoto, 606-8501, Japan (Received 11 March 1998; accepted 25 June 1998)

High dielectric and low loss capacitor thin films of Sr2 Nb2 O7 (SN), Sr2 Ta2 O7 (ST), and their solid solution Sr2 (Nbx , Ta12x )2 O7 (SNT) were investigated using the sol-gel technique. The SN film grows with the (0n0) orientation in the case of heating at over 700 ±C. Heat treatment at a lower temperature results in the polycrystal ST-type structure. The SNT at x , 50% also resulted in the ST type. The dielectric constant for the SN film was 45, within 10% variation at 60.5 MVycm. Dielectric loss (tan d) was 0.3–0.5%. The small variation in dielectric constant and the paraelectricity with low loss are suitable for capacitor applications.

I. INTRODUCTION

Layered perovskites attracted attention as hightemperature superconductors and as new ferroelectric materials due to their attractive features. However, they have been kept at a respectful distance in semiconductor device manufacturing, because they are complex oxides which consist of many elements, and it is often difficult to control their compositions. Another reason is that they include elements which are troublesome from the viewpoint of semiconductor manufacturing, for example, lead or bismuth. Sr2 Ta2 O7 and Sr2 Nb2 O7 are layered-type oxides constructed with only two metal elements.1 They have the same crystal structures, but their Curie temperatures (TC ) are very different. Sr2 Ta2 O7 is paraelectric at room temperature and its TC is 2107 ±C,2 whereas Sr2 Nb2 O7 shows ferroelectric characteristic at room temperature and its TC is 1342 ±C.2 The properties of their solid solution, Sr2 (Nbx , Ta12x )2 O7 , are changeable in accordance with its composition. They are also attractive as highdielectric capacitor materials at high frequencies because of their small dielectric loss. Figure 1 shows their crystal structures.2 Understanding them is facilitated by comparing them with wellknown bismuth layered perovskites which can be generally represented as (Bi2 O2 )21 (Mm21 Rm O3m11 ).2,3 Following this formula, SrBi2 Ta2 O9 and Bi4 Ti3 O12 can be represented as (Bi2 O2 )21 (SrTa2 O7 )22 and (Bi2 O2 )21 (Bi2 Ti3 O10 )22 , respectively. The perovskite units constructed by oxygen octahedrals are sandwiched between (Bi2 O2 )21 layers. (Bi2 O2 )21 layer can be replaced with other 21 ions. When it is replaced as Sr21 , it is Sr21 (SrTa2 O7 )22 . Because Ta51 and Nb51 have almost J. Mater. Res., Vol. 14, No. 3, Mar 1999

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the same ion sizes, Sr2 Nb2 O7 has the same structure, that is, Sr21 (SrNb2 O7 )22 . The ferroelectricity is derive