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Interplay Between Interfacial Properties and Dielectric and Ferroelectric Behaviors of Barium Strontium Titanate Thin Films Y. Liang, S. A. Chambers, S. Gan,(a) and D. R. Baer Supported by DOE LTR Program. (a) Postdoctoral Research Fellow. This project is designed to gain a fundamental understanding of how interfaces affect the dielectric and ferroelectric properties of barium strontium titanate (Ba1-xSrxTiO3 or BST) thin films, and to use such knowledge to improve the design and processing of BST thin-film-based devices. The project aims to address two specific issues of significant concern in BST thin-film technology: 1) the effect of interfacial chemistry and stress on the dielectric and ferroelectric properties of BST thin films, and 2) ferroelectric behavior at the nano-scale level. One of the most important steps towards understanding the interplay between interfacial properties and dielectric and ferroelectric behaviors of BST is the growth of high quality BST films on Si substrates. Successful epitaxial growth of crystalline BST on Si(001) is thought to require the formation of a two-dimensional interfacial silicide layer involving either Ba or Sr as the initial step (McKee et al. 1998). Bulk thermodynamics suggests that this thin silicide layer is required to stabilize the interface (Barin and Knacke 1973-1977). It has been proposed that deposition of 1/4 monolayer (ML) of Ba (Sr) to form an ordered c(4 x 2) interface layer in which alkaline earth metal atoms replace Si atoms in the top layer is the essential first step for successful molecular beam epitaxial (MBE) growth of BaTiO3 (SrTiO3) on Si(001). In the initial phase of this project, the PNNL team is working to prepare, isolate, and characterize this ultrathin silicide layer using Sr as the alkaline earth metal. Si(001)-(2 x 1) surfaces were prepared in ultra high vacuum (UHV) by rapid desorption of the native oxide layer grown after the surface polish. These surfaces were exposed to Sr from an effusion cell in an oxide MBE chamber as a function of evaporation rate, substrate temperature, and total dose. The resulting interfaces were characterized during growth with reflection high-energy electron diffraction (RHEED), and after growth with low-energy electron diffraction (LEED), x-ray photoemission (XPS), and x-ray photoelectron diffraction (XPD) in an analytical chamber appended to the MBE system. Contrary to what has been previously published (McKee et al. 1998), we find that only a negligible amount of Sr (~0.1 ML by XPS) is adsorbed at substrate temperatures of 800° to 900°C. This result is independent of both the evaporation rate over the range of 0.02 to 0.5 Å/sec, and the total dose from 10 to 200 Å. There is no change in the RHEED pattern [it remains (2 x 1), characteristic of the substrate], and no change in the (00) beam intensity during deposition. These results suggest that the small amount of adsorbed Sr deposited at those temperatures is likely bound to point defects or steps, and that the incident Sr that does not diffuse to a defect trap site within a small residence time on the surface reevaporates. In addition to the interaction of Sr with the Si surface, we have also studied the domain structure of single-crystal BaTiO3 using atomic force microscopy (AFM). Figure 4.8 is an AFM image of a BaTiO3 surface with different domains on the surface. Domains of 90° and 180° are clearly evident. Our current effort is to investigate the thermal stability of these domain structures at the elevated temperature condition. Figure 4.8. An AFM image showing domain structures of 90° and 180° on a BaTiO3 surface. References Barin, I., and O. Knacke, Thermochemical Properties of Inorganic Substances (Springer, Berlin, 1973-1977). McKee, R. A., F. J. Walker, and M. F. Chisholm, Phys., Rev. Lett. 81, 3014 (1998). William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147