Origin of charge density at LaAlO3-on-SrTiO3 heterointerfacespossibility of intrinsic doping Page: 2 of 14
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explains the high mobility based on carrier redistribution due to an
increased dielectric constant.
Recently, Ohtomo and Hwang [11 2] reported the existence of a conducting electron layer
at the hetero-interface between two nominal insulators, SrTiO3 and LaAlO3. This is a remarkable
result and has intrigued many researchers in the field [3 4]. Equally remarkably, Ohtomo and
Hwang [1 2] found that for the TiO2/LaO interface between SrTiO3 and LaAlO3 has a sheet
carrier density of ~10" electrons/cm2 and a mobility of 104 cm2V s-1, as inferred from
conductivity and Hall-effect measurements; each of these is strikingly large.
Some insight into the possible sheet charge densities at a SrTiO3/LaAlO3 interface can be
seen from the following considerations, which relate to an intrinsic doping mechanism. SrTiO3
consists of charge neutral SrO and TiO2 layers, whereas the AlO2 and LaO+ layers in LaAlO3
have net charge and for an ideal planar interface yield a net interface charge equal to half that of
the last plane . Indeed a neutralizing charge at the interface is required to avoid a polarization
catastrophe that arises due to this net interface charge. If left uncompensated, the energy
associated with this polarization grows indefinitely as the thickness of the LaAlO3 layer
increases. Therefore, electrons have to be promoted to the conduction band of LaAlO3 at some
point. The charge that is necessary to prevent this polarization catastrophe is equal to half an
electron per unit cell, one is led to 1/2 of an electron per unit cell or 3.2 x 1014 cm-2 . Note that
this estimate applies only for perfectly stoichiometric LaAlO3 and in this sense is an approximate
upper limit in the intrinsic case. Any defects will reduce this number. In any event, clearly this
line of reasoning cannot explain the very large charge densities observed. Of course, in the case
of lower carrier densities, an intrinsic doping mechanism may become operative. Hence, an
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Siemons, W. Origin of charge density at LaAlO3-on-SrTiO3 heterointerfacespossibility of intrinsic doping, article, April 29, 2010; United States. (digital.library.unt.edu/ark:/67531/metadc926390/m1/2/: accessed January 21, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.