Electron Trapping Polycrystalline Materials with Negative Electron Affinity

Letter to be published in Nature Materials (November issue)

Electron Trapping Polycrystalline Materials with Negative Electron Affinity

Keith P. McKenna* & Alexander L. Shluger

Department of Physics and Astronomy and The London Centre for Nanotechnology, UniversityCollegeLondon, Gower Street, London, WC1E 6BT, UK.

*Corresponding author: email: k.mckenna@ucl.ac.uk   tel: +44 (0)20 7679 9932

Article available online (from October 12): [Link]

Insulating materials, such as MgO, are widely used as substrates for thin films and metallic clusters and employed as insulating barriers in spintronic devices. In most cases they are polycrystalline yet the ability of boundaries between crystallites to trap electrons is not well understood. In this letter we investigate the electron trapping properties of grain boundaries in MgO and alkali halides by first principles calculations. Our results show that conduction band electrons, which may be introduced by an applied electrical voltage or irradiation, can be trapped at grain boundaries in MgO, NaCl and LiF. We find that the nature of the electron trapping in these negative electron affinity (NEA) materials is unusual in that the electron is confined in the empty space inside the dislocation cores rather than associated with interfacial ions (see figure).

We demonstrate that this surprising effect can be explained using a simple model which should be applicable to other NEA materials, such as boron nitride and alumina. These grain boundaries represent novel examples of systems that are capable of confining electrons and may be probed experimentally by transport measurements or using scanning probes. These effects can also lead to electrons escaping from thin films grown on MgO substrates and reduced tunnel barrier heights in magnetic tunnel junctions affecting the performance of electronic devices.


Figure 1: The (310)[001](36.8°) tilt grain boundary in MgO together with an electron density iso-surface illustrating how electrons can be confined inside the dislocation core (darker colour higher electron density).

Notes for Editors:

1. About the London Centre for Nanotechnology
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