J.K. Lee and A.A. Demkov, Phys. Rev. B 78 193104 (2008)
The internal electric field as discussed in the context of "polar catastrophe" in the
case of polar semiconductors by Harrison is clearly seen.
We show the results for two structures that differ in the thickness of the LAO layer,
the intrinsic electric field is on the order of 0.24 V/Å. The experimental band
gap of LAO is 5.6 eV, and in a free standing
film this electric field would result in the dielectric breakdown if the film
thickness exceeds 23 Å. The situation is changed dramatically, however, by the
presence of an STO layer since the band gap of SrTiO3 is only 3.2
eV. Taking into consideration the LAO/STO valence band offset (calculated value
0.1 eV) , to gain this energy it takes only three and a half unit cells or
about 13 Å of LAO. Obviously this distance is sufficiently small for electron
tunneling to occur, thus the critical thickness for charge transfer is about
four LAO layers.
