Condensed Matter & Materials Physics


Instability, intermixing and electronic structure at the epitaxial LaAlO3/SrTiO3(001) heterojunction

December 2010 

S. A. Chambers, M. H. Engelhard, V. Shutthanandan, Z. Zhu, T. C. Droubay, L. Qiao, P. V. Sushko, T. Feng, H. D. Lee, T. Gustafsson, E. Garfunkel, A. B. Shah, J.-M. Zuo, Q. M. Ramasse, Surface Science Reports 65, 317-352 (2010).

The LaAlO3/SrTiO3(001) (LAO/STO) interface has attracted widespread interest over the past several years because of the common, but not universal, observation of conductivity near the interface under certain deposition conditions, despite the fact that both constituent materials are band insulators (see seminal work by Ohtomo and Hwang, Nature, 2004).

The presumed cause of LAO/STO interface conductivity is alleviation of the so-called "polar catastrophe", which results from forming a junction between a polar material (LAO) and a nonpolar material (STO). Elementary electrostatic considerations suggest that layer-by-layer growth of LAO on STO and abrupt interface formation will lead to a diverging electric potential as a result of the accumulation of dipoles within the LAO film.


Using several independent analytical methods, we have shown that there is a strong tendency for the LAO/STO interface, as prepared by on-axis PLD, to intermix rather than form an atomically abrupt configuration. Moreover, classical and ab initio calculations show that interfacial intermixing is energetically preferable to abruptness, and that forming an intermixed interface represents a thermodynamically favorable process.

Above: individual Ti L2,3 (b) and O K (c) loss spectra taken every 1 Angstrom along the line shown in the HAADF image in (a) for a thick - 25 unit cells (uc) - LAO film.

Below: layer-projected density of states calculated for several of the lowest energy intermixed configurations (a,b,c) of LAO(3 uc) / STO(6 uc) shows small or negligible band bending in the vicinity of the interface, in agreement with the XPS-based measurements.


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