Structural analysis of coexisting tetragonal and rhombohedral phases in polycrystalline Pb(Zr 0.35 Ti 0.65 )O 3 thin fil
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Bryan C. Hendrix, Steven M. Bilodeau, and Jeffrey F. Roeder ATMI, Inc., 7 Commerce Drive, Danbury, Connecticut 06810
Sean Brennan Stanford Synchrotron Radiation Laboratory, Stanford, California 94305 (Received 12 July 2002; accepted 21 October 2002)
Structural properties of polycrystalline Pb(Zr0.35Ti0.65)O3 (PZT) thin films grown by metalorganic chemical vapor deposition on Ir bottom electrodes were investigated. Symmetric x-ray diffraction measurements showed that as-deposited 1500 Å-thick PZT films are partially tetragonal and partially rhombohedral. Cross-section scanning electron microscopy showed that these films have a polycrystalline columnar microstructure with grains extending through the thickness of the film. X-ray depth profiling using the grazing-incidence asymmetric Bragg scattering geometry suggests that each grain has a bilayer structure consisting of a near-surface region in the tetragonal phase and the region at the bottom electrode interface in the rhombohedral phase. The required compatibility between the tetragonal and rhombohedral phases in the proposed layered structure of the 1500 Å PZT can explain the peak shifts observed in the symmetric x-ray diffraction results of thicker PZT films.
I. INTRODUCTION
Thin films of ferroelectric materials such as Pb(ZrxTi1−x)O3 (PZT) can be used in a variety of applications in the microelectronics industry. PZT thin films can be used in nonvolatile memories,1,2 high-frequency capacitor applications,3 pyroelectric detectors,4 and microelectromechanical systems.5,6 These applications require PZT thin films of various thicknesses. The structural and electrical properties of PZT films can change significantly with film thickness.7 Among other effects, previous studies have shown that changing the thickness of the PZT films can affect the coercive field,8 the domain structure, and the out-of plane lattice parameter9 of PZT. Bulk PZT materials have been investigated for many years; however, recent results by Noheda et al.10 and Guo et al.11 suggest that there are features of the bulk PZT phase diagram12 that were not recognized previously. Thin films of PZT also behave differently from the bulk due to the differences in the boundary conditions
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e-mail: [email protected] e-mail: [email protected] J. Mater. Res., Vol. 18, No. 1, Jan 2003
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imposed on the material. Specifically, the nonequilibrium conditions present during thin film deposition and the effects of substrate mechanical constraint may cause thin films to be under significant stress. A number of theoretical works based on the Landau–Ginzburg– Devonshire free energy expansion13,14 have shown that applied stresses can affect the stability regions for the various phases in the bulk PZT phase diagram.15–18 Similar results were obtained by Ramer et al. using local density-functional theory.19 Previous work in our group has focused on the observed phase evolution of PZT thin films with the Ti/Zr ratio of 65/35 as the film thickness is varied
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