2009.02.19

Audiotorium, Eigineering 9th Bld, Asano Campus

Professor A. L. Shluger 

Department of Physics and Astronomy and London Centre for Nanotechnology,

University College, London

Title：Electron trapping by grain boundaries in positive and negative electron affinity materials

Schedule：15:00  ~ 17:00   19th Feb 2009

Place：Audiotorium, Engineering 9th Bld, Asano Campus

Abstract：

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)