Origin and properties of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface: a first-principles hybrid functional study.

Complex oxides exhibit a wide range of physical properties making them very attractive
for future electronic and device applications. Although more and more studied, additional
scientific investigations are required, especially in oxide interfaces, where new and
amazing phenomena can arise. A prototypical example is the LaAlO3-SrTiO3 interface
that appear to be conducting, magnetic or even supra-conducting while these properties
are not present in the LaAlO3 and SrTiO3 bulk insulator compounds. The conductivity
arises from the formation of a highly localized electron gas at the interface which exhibits
a different behavior than the one at semiconductor interfaces. Even nowadays, its exact
origin, intrinsic versus extrinsic, is still intensively debated. The existence of an electric
field in LaAlO3 used as a key feature of models based on an intrinsic origin is highly
controversial. In these models, the closing of the band gap with increasing LaAlO3 film
thicknesses finally results to a Zener breakdown and to the metal/insulator transition.
In this Ph.D. thesis we aim to investigate the various consequences of the presence of an
electric field in LaAlO3 through first-principles calculations in pristine LaAlO3-SrTiO3 interfaces.
First, using both experimental and theoretical structural distortions in LaAlO3,
we predicted a lattice expansion via an electrostrictive effect, supporting the existence
of an electric field in LaAlO3. Second, the metal/insulator transition was tuned with
regards to the intensity of the electric field in the film, which was controlled by the composition
of a solid solution between LaAlO3 and SrTiO3. The theoretical results match
the experimental one where, nevertheless, extrinsic origin mechanisms are allowed and
defects are present. These two works are in favors of an intrinsic origin of the electronic
gas observed in LaAlO3-SrTiO3 heterostructures. In addition, a relationship between the
sheet carrier density and spatial extension of the gas was established and thus setting an
intrinsic threshold to the sheet carrier concentration. At lower density the electrons are
strictly localized close to the interface while above this value the carriers start to spill
into the SrTiO3 substrate.