Uniformity of Misfit Strain in Heteroepitaxial (Ba,Sr)Tio 3 Films on SrRuO 3 /SrTiO 3

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(Ba,Sr)Ti03 (•

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Substrate Figure 1.Lattice deformation in heteroepitaxial (Ba,Sr)TiO 3 films due to lattice misfit. Mat. Res. Soc. Symp. Proc. Vol. 574 ©1999 Materials Research Society

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Poor squareness

Figure 2. Typical drawbacks in ferroelectric properties observed in heteroepitaxial (Ba,Sr)TiO 3 films. through the heteroepitaxial growth. The lattice misfit strain was accompanied by enhancement of ferroelectricity in the heteroepitaxial (Ba,Sr)TiO 3 films. Ferroelectric hysteresis was observed at 200'C in a (BaO. 6Sr0 .4)TiO 3 film, for which the inherent Curie temperature is known to be about 0°C. This indicates that heteroepitaxial (Ba,Sr)TiO 3 films can be used in ferroelectric nonvolatile memories instead of conventional ferroelectric materials such as Pb(Zr,Ti)0 3 or SrBi 2Ta 2O9 . However, the authors observed various types of hysteresis loops in heteroepitaxial (Ba,Sr)TiO 3 films in a series of earlier studies. As illustrated in Fig. 2, they present several drawbacks from the viewpoint of nonvolatile memory applications: one is asymmetry in the hysteresis loop what is called "imprint" and the other is poor squareness. The purpose of this study was to clarify the reasons that these drawbacks arise in the ferroelectric hysteresis loop of heteroepitaxial (Ba,Sr)TiO 3 films. We therefore carefully examined the crystallinity, because we assumed that the ferroelectric properties are strongly influenced by the crystal structure. Since the ferroelectricity is induced by misfit strain, the uniformity of strain was quantitatively evaluated. EXPERIMENT Ferroelectricity and crystal structure were compared for two specimen of heteroepitaxial (Ba,Sr)TiO 3 film. The specimens were prepared on substrates of single crystalline SrTiO 3 with a (100) orientation. Heteroepitaxial SrRuO 3 films with a thickness of 50 nm were deposited as bottom electrodes using a SrRuO 3 ceramic target [7, 8]. The (Ba,Sr)TiO 3 films were deposited under different sputtering conditions from a polycrystalline Bao.75 Sr0 .25TiO 3 target with a diameter of 4 inches. Subsequently, other SrRuO 3 films were also grown as top electrodes without breaking vacuum. The Ba 0 .75Sr0.25TiO 3 films and both SrRuO 3 films were deposited at 500-700°C by radio-frequency (13.56 MHz) magnetron sputtering. A mixture of Ar:0 2 gas with a 4:1 ratio was introduced, which held the total pressure at 0.3-0.7 Pa during deposition. Radiofrequency power of 100-300 W was applied to the targets. The substrates were positioned at a distance of 100-140 mm from the targets. The structure of the specimen is schematically illustrated in Fig. 3. Table I shows the composition and thickness of two specimen of heteroepitaxial (Ba,Sr)TiO 3 film. The composition was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) employing an internal standard. The thickness was measured with an Alpha-step 200 (Tencor Instruments). The Ba content x=Ba/(Ba+Sr) in both (Ba,Sr)TiO 3 films was almost the same as that of the target (x=0.75), whereas the atomic ratio