468289 Epitaxial Growth and Atomic Characterization of Fe-BTO(111) on SiC (0001) Using MgO Template Layer

Sunday, November 13, 2016: 4:40 PM
Golden Gate 5 (Hilton San Francisco Union Square)
Sue J. Celestin and Katherine S. Ziemer, Chemical Engineering, Northeastern University, Boston, MA

Barium titanate is one of the most important ferroelectric materials for many applications in microelectronic, especially for ceramic capacitor production [1]. Recently, the thermoelectric properties of BaTiO3 (BTO), previously overlooked, gained attention with the recent discovery of a high Seebeck coefficient (100µV/K) and low resistivity (200µΩ cm) in layered cobalt oxide, NaCo2O4 [2]. Because ferroelectric oxides are typically non-toxic, composed of commonly occurring elements, and available at relatively low-costs, much effort has been devoted to understanding the origin of the enhanced Seebeck coefficient in oxides, and to explore thermoelectric applications for perovskites oxides.

Many investigations have provided experimental evidence of spin and orbital degeneracy being the dominant contribution to the enhanced Seebeck coefficient in cobalt oxides [3]. NaCo2O4 is a structurally complex material in which edge-shared distorted octahedral (CoO6) of oxygen ions form a 2D triangular lattice and sodium ions in a prism site between CoO2 layers. It was found that the d-orbital of the cobalt ion located in the center of the octahedra is split in a way that allows the cobalt to always be in a low spin state regardless of its oxidation state. This results in orbital degeneracy and therefore high entropy due to spin state of the metal atom [2]. We are further exploring the microscopic origin of the enhanced Seebeck coefficient in the metal-oxides manipulating the spin entropy of a BTO through controlled doping with Fe. The goal is to enable tuning of the Seebeck coefficient while enhancing the conductivity and thus increasing the thermoelectric properties of the material.

The growth of high quality epitaxial Fe-BTO films with atomically sharp interfaces is first needed before investigating the spin state-thermoelectric relationship. Molecular beam epitaxy (MBE), which provides control of crystal growth at the atomic level, will be used to grow Fe-BTO on 6H-SiC. Through the use of controlled parameters, the interface formation mechanisms will be investigated for effective heteroepitaxy of high-quality Fe-BTO on 6H-SiC by reflection high energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS). Understanding the relationship between processing conditions and chemistry and structure will be used to link key electronic and thermoelectric properties. This understanding will enable the development of processing strategies to enhance the Seebeck coefficient and electrical conductivity of BTO.

In our previous work, RHEED studies supported the importance of a thin, 2.5 nm, crystalline MgO(111) layer for improved epitaxy and crystal structure of tetragonal BTO(111)[4]. Understanding from our previous results, will allow us to explain interface formation mechanisms for Fe-BTO/MgO/SiC heterostructures and influence the film crystal quality. In this study, the use of a magnesium oxide (MgO) template layer and the interface formation mechanisms will be investigated for effective heteroepitaxy of high-quality Fe-BTO films on 6H-SiC.

Author for correspondence: k.ziemer@neu.edu

[1]. E. Beaudrouet, A. Viviet, M. Lejeune, C. Santerne, F. Rossignol, C. Dossou-Yovo, M. Mougenot, R. Noguera, “Stability of aqueous barium titanate sus- pensions for MLCC inkjet printing”, J.Am. Ceram.Soc., 97 [4] (2014) 1248–1255.

[2]. I. Terasaki, Y. Sasago, K. Uchinokura Large thermoelectric power in NaCo2O4 single crystals. Physical Review B, 56, 1997.

[3]. G.D. Tang, X.N. Xu, C.P.Tang, L.Qiu, Y. Lv, Z. H. Wang The spin entropy suppression induced by Fe3+ in NaCo2O4 Journal Of Applied Physics of Physics 2010, 107

[4]. T. L. Goodrich, Z. Cai, M. D. Losego, J.-P. Maria, L. F. Kourkoutis, D. A. Muller, and K. S. Ziemer, "Improved epitaxy of barium titanate by molecular beam epitaxy," Journal of Vacuum Science Technology B, vol. 26, p. 024803, 2008.



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