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

摘要

Complex oxides exhibit a wide range of physical properties making them very attractivefor future electronic and device applications. Although more and more studied, additionalscientific investigations are required, especially in oxide interfaces, where new andamazing phenomena can arise. A prototypical example is the LaAlO3-SrTiO3 interfacethat appear to be conducting, magnetic or even supra-conducting while these propertiesare not present in the LaAlO3 and SrTiO3 bulk insulator compounds. The conductivityarises from the formation of a highly localized electron gas at the interface which exhibitsa different behavior than the one at semiconductor interfaces. Even nowadays, its exactorigin, intrinsic versus extrinsic, is still intensively debated. The existence of an electricfield in LaAlO3 used as a key feature of models based on an intrinsic origin is highlycontroversial. In these models, the closing of the band gap with increasing LaAlO3 filmthicknesses 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 anelectric 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 existenceof an electric field in LaAlO3. Second, the metal/insulator transition was tuned withregards to the intensity of the electric field in the film, which was controlled by the compositionof a solid solution between LaAlO3 and SrTiO3. The theoretical results matchthe experimental one where, nevertheless, extrinsic origin mechanisms are allowed anddefects are present. These two works are in favors of an intrinsic origin of the electronicgas observed in LaAlO3-SrTiO3 heterostructures. In addition, a relationship between thesheet carrier density and spatial extension of the gas was established and thus setting anintrinsic threshold to the sheet carrier concentration. At lower density the electrons arestrictly localized close to the interface while above this value the carriers start to spillinto the SrTiO3 substrate.