2009.02.19

工学部9号館（総合研究機構）大会議室

Prof. A. L. Shluger

Department of Physics and Astronomy and London Centre for Nanotechnology, University College, London

題目：Electron trapping by grain boundaried in positive and negative electron affinity materials

日時：2009年2月 19日（木）15:00 ~ 17:00

場所：東京大学工学部9号館（総合研究機構）大会議室

概要： There is growing evidence that boundaries between grains in polycrystalline oxide films present favorable paths through which electrons can conduct or tunnel. This has important implications for MOSFETs employing polycrystalline gate dielectrics and also for other nanoscale electronic devices such as magnetic tunnel junctions. In this presentation we will show, by first principles calculations, how the preferential trapping of electrons at defects which are known to segregate to boundaries, such as oxygen vacancies, can open up conducting channels through the oxide. We compare two important materials, MgO and HfO₂, and show that, although they have similar band gaps, their electron trapping properties are quite different. One of the reasons for this is that MgO has a negative affinity while HfO₂ has a positive affinity to electrons^1,2. To investigate the structure and properties of grain boundaries we developed a multi-scale approach linking atomistic calculations, periodic DFT and an embedded cluster method. We have studied several models of MgO and HfO₂ grain boundary structures, calculated their electronic properties and investigated the segregation and diffusion of vacancies and relevant impurities. Dislocations forming as a part of the interface between MgO and HfO₂ grains lead to trapping of electrons and holes in one-dimensional states. Electrons and holes may then subsequently trap at defects and impurities that segregate to grain boundaries. We demonstrate that the flexibility of the HfO₂ lattice and its higher dielectric constant encourages the formation of polarons in the bulk³ and inside the dislocation cores.

 ¹ K. P. McKenna and A. L. Shluger, Nature Mat. 7, 859-862 (2008)

² K. McKenna et al., J. Am. Chem. Soc. 129, 8600 (2007)

³ D. Muñoz Ramo, et al. Phys. Rev. Lett. 99, 155504 (2007)