Papers

AVS2004 Session EM-ThA: Multifunctional Materials

Thursday, November 18, 2004 2:00 PM in Room 304B

Thursday Afternoon

Start	Invited?	Item
2:00 PM		EM-ThA-1 Computational Design of Multifunctional Electronic Materials N.A. Spaldin (University of California, Santa Barbara) In this talk we show how modern theoretical and computational methods can be used to design technologically relevant electronic materials with dual- or multi-functionality. We choose multiferroic magnetoelectrics as an example; These are materials that are both ferromagnetic and ferroelectric in the same phase, and therefore have a spontaneous magnetization which can be switched by an applied magnetic field, a spontaneous polarization which can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature, or have been synthesized in the laboratory, so we begin by determining the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. Then we identify the chemistry behind the additional electronic or structural driving forces that must be present for ferromagnetism and ferroelectricity to occur simultaneously. Finally we describe the successful prediction and subsequent synthesis of a new multiferroic material.
2:40 PM		EM-ThA-3 Tailoring of the Microwave Dielectric Properties of Ba_{1-x.√sub x}TiO₃ Based Thin Films by Acceptor Doping M.W. Cole (U.S. Army Research Laboratory); R.G. Geyer (National Institute of Standards and Technology); W.D. Nothwang (U.S. Army Research Laboratory) In this work, material compositional design and optimized film processing methods, were employed to simultaneously lower the dielectric loss and enhance the dielectric tunability of Ba_{1-x.√sub x}TiO₃ (BST) based thin films without compromising the device impedance matching (permittivity less than 500) and device control voltage (less than 10 Volts) requirements. The films compositional design was achieved by Mg doping BST from 3 to 10 mol-percent via the metalorganic solution deposition (MOSD) technique and post-deposition annealing in an oxygen ambience. The films dielectric loss at these doping levels was identical, tan delta of 0.007, and the permittivity values ranged from 339 to 220. Device quality values of tunability, 40 and 32 percent, for the 3 and 7 mol-percent doped BST films, respectively, were achieved by elevating the applied bias to 474 kV/cm. This device quality tuning is compatible with voltage requirements of current semiconductor based systems. The results suggest that the low level acceptor doping from 3 to 7 mol-percent, optimized precursor solution concentration (0.43 M), and oxygenated post-deposition thermal processing were found to work in concert to lower dielectric loss, limit defect density concentration, optimize film microstructure, and eliminate undesirable film/electrode interfacial phases. The enhanced dielectric and insulating properties of the 3 to 7 mol-percent Mg doped BST thin films make them excellent candidates for integration into tunable devices.
3:00 PM		EM-ThA-4 Strain-effected Phase Transition Temperature of BST Thin Films J.A. Bellotti, W. Chang, S.B. Qadri, S.W. Kirchoefer, J.M. Pond (Naval Research Laboratory) Ba_{0.5√sub 0.5}TiO₃ (BST) thin films were grown on (100) MgO and (100) LaAlO₃ substrates by rf/magnetron sputtering in the thickness range of 50 to 300 nm. Recent studies have demonstrated the importance of film strain on the microwave frequency electronic properties of ferroelectric thin films. We have observed that stress can result in Curie temperature shifts of nearly 300 K in epitaxial thin films of SrTiO₃¹. These highly strained films are also shown to have large directional dependences on the applied electric field. We report on efforts to relate lattice spacing and the resultant substrate-induced stress, as measured by x-ray diffraction, with the measured Curie temperature and microwave tunability and loss. XRD phase analysis as a function temperature will be correlated with the observed directionally-dependent and temperature-dependent microwave properties. Additional characterization of non-epitaxial films and bulk substrate crystals will be presented as a baseline for understanding the strain effect in thin ferroelectric layers. ¹"Using Strain to Control Microwave Tunability and Loss in Barium Strontium Titanate Thin Films", AVS 50th International Symposium, Session EM-WeM5, Invited Talk, 2003. .
3:20 PM		EM-ThA-5 Remote Plasma Enhanced Chemical Vapor Deposition of La2O3 P.R. McCurdy, T. Vazquez, S. Kohli, E.R. Fisher (Colorado State University) Rare earth oxides, such as La₂O₃, show potential for high-k gate insulator applications. La₂O₃ has a large conduction band offset with Si(100) of 2.3 eV¹ and a dielectric constant of ~30 eV.² La₂O₃ films on Si(100) were deposited by remote plasma enhanced chemical vapor deposition (RPECVD), using lanthanum acetyl-acetonate (La(acac)), argon and oxygen. Post deposition annealing was performed to reduce carbon and hydrogen content. Film composition and characteristics were examined using x-ray photoelectron spectroscopy, spectroscopic ellipsometry and XRD. Various parameters have been studied, including gas flow rate, substrate temperature (T_s), temperature of the La(acac), and plasma power. Under optimal conditions we were able to produce La₂O₃ films with less than 5% carbon content. This is the first report of La₂O₃ being deposited by RPECVD. ¹ J. Robertson, J. Non-Cryst. Solids 303 (1) (2002) 94. ² A. I. Kingon, J.-P. Maria, S. K. Streiffer, Nature 406(2000) 1032. .