Insulating oxide films are an essential ingredient in a wide variety of technologically important electronic devices. For example, MgO is often used as a tunnel barrier in magnetic tunnel junctions[1], and a thin dielectric layer is used below the gate electrode in CMOS transistors. In most applications the oxide film is polycrystalline and the atomic structure at interfaces between grains can be very different to that of the bulk crystal. Therefore, it is important to calculate how the electronic structure of the oxide is modified at grain boundaries and to investigate the properties of defects that may be found there. A (310) tilt grain boundary in MgO is studied as a general and simple model interface, and a much more complex HfO2 grain boundary is also investigated because of its application as a gate dielectric.
The calculations are performed using both an embedded cluster methodology[2] and periodic Density Functional Theory. The embedded cluster method combines quantum-mechanical and classical levels of approximation self-consistently enabling large systems to be studied. This is particularly important for oxide materials where polarisation effects are quite long ranged. For the MgO tilt grain boundary we find that the bottom of the conduction band is associated with electrons confined in the gaps at the interface. The (310) grain boundary does not have an electron affinity. However, protons or H- centres located near the interface can modify the electrostatic potential such that electron trapping in the gaps is favoured. One-dimensional transport of electrons along the channels formed at the grain boundary is possible by thermal or optical excitation into conduction band and subsequent re-trapping.
[1] A. Gokcea, E. R. Nowak, S. H. Yang, S. S. P. Parkin, J. Appl. Phys 99, 08A906 (2006)
[2] P. V. Sushko, A. L. Shluger, C. R. A. Catlow, Surf. Sci. 450, 153 (2000) |