Investigation of Growth Evolution in c -Axis SrBi 2 Nb 2 O 9 Epitaxial Thin Films
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TABLE I. Substrate and film lattice parameters and lattice mismatch.
Substrate
Lattice Constant(s) at
Lattice Mismatch t to
25 °C
SrBi2 Nb 20 9 (001)
SrBi 2Nb 20 9 (001)
a = 5.51 b = 5.51/ c = 25.I10 A
SrTiO3 (00l)
a = 3.905 A'
+0.2%
LaA10 3-Sr
a = 7.737
A
- 0.7%
2A1TaO 6 (LSAT) (001)
t Lattice mismatch (at 25 C) - a su - a im afilm speculative at best since the film and both substrates are nearly matched at room temperature. However, although the substrates are both perovskites with similar lattice constants, the surface termination and resulting effects of surface chemistry on nucleation and growth may be quite different. With a buffered-HF etch,6 a combination of etch and anneal , and to some extent high temperature annealing alone, SrTiO 3 has been previously shown to facet with a charge neutral TiO 2 termination layer. The surface structure of (001) LSAT may be more complex since any stoichiometric termination of (001) LSAT (i.e., the AO layer LaO-SrO or the B0 2 layer A10 2-Alo1.Tao. 50 2) is not charge neutral. The surface termination of LSAT has not been reported. We believe this termination condition may play a role in the resulting SrBi2Nb 2O9 film topology. Film Growth SrBi 2Nb 2 0 9 films were grown by PLD with a KrF excimer laser (248 nm, Lambda Physik EMG103MSC) in an on-axis geometry in a radiatively-heated furnace 8 with a targetto-substrate distance of 6.5 cm. Film growth was carried out in a 110 mTorr 03 / 02 mix (-8% 03) at a temperature of 877 'C. A laser pulse energy of 150 mJ, fluence of 2-3 J/cm 2, and a 4 Hz pulse rate were used to grow films of varying thickness. Single, one-inch diameter targets with bismuth-rich composition SrBi 2.3Nb 2 0 9 were used9 as the source material for this study. Details on target fabrication are described elsewhere. After growth, films were quenched in one atm. of 02 by dropping the films to the water-cooled bottom of the PLD chamber immediately after growth. Extensive details on the optimized growth conditions for growth of c-axis SrBi 2Nb 2O 9 and SrBi 2Ta 2O9 films are 0 described elsewhere.°' "' Characterization Determination of phase purity and epitaxial orientation was accomplished with a Picker 4-circle x-ray diffractometer with Cu Ks radiation and a graphite monochromator. Highresolution transmission electron microscopy (HRTEM) images were obtained with a JEOL 4000 EX microscope operating at 400 keV. Topographic information was collected by AFM in "tapping mode" (non-contact method) with silicon tips on silicon cantilevers. 32
RESULTS AND DISCUSSION Crystallinity In any study of anisotropic properties, growing highly crystalline films with the desired orientation (c-axis in this case) is requisite. As expected from the near in-plane lattice match between SrBi 2Nb 2O 9 and the substrate materials explored in this study, films grow readily with a c-axis orientation. 4-circle x-ray diffraction characterization of growths on (001) SrTiO 3 and (001) LSAT are shown in Figs. 1 and 2, respectively. Complementary analysis by Rutherford backscat
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