Gerwin Hassink

Ph.D. thesis

Summary

Two-dimensional electron systems have always attracted attention for their remarkable properties. In this thesis, two-dimensional electron layers in two correlated-electron systems were studied: LaTiO3/LaAlO3 for a possible Mott insulating state and LaAlO3/SrTiO3 for its conducting interface. Single monolayers of LaTiO3 embedded in LaAlO3 become a two-dimensional Mott insulator as shown by transport and ellipsometry measurements. Such layers can be hole-doped by a second, nearby interface, where the doping can be modeled by a balance between surface oxidation and electron transfer. For the (conducting) LaAlO3//SrTiO3 interface careful evaluation of literature data confirms a critical deposition pressure below which the electron conduction is primarily bulk-like and controlled by oxygen vacancies. A phase diagram is proposed to show how both the deposition pressure and the cool-down pressure control the conducting state of the interface. The electron mobility evolves gradually with the deposition pressure, as it is only indirectly influenced by the changes in the dielectric constant. It influences the electron distribution perpendicular to the interface: closer to the interface the electron scattering is stronger due to the interface defects, so the spatial distribution influences the average mobility even for samples with similar electron sheet densities. This strong dependence on the local polarization gives rise to an alternative model for the interface conduction in LaAlO3//SrTiO3 based on the discontinuity in polarization across the interface. Comparison between LaAlO3//SrTiO3 and SrTiO3//LaTiO3//LaAlO3 interfaces show that the donor sites are the same for both interfaces. However, the different strain/polarization at the second interface does not result in a potential well through which electron transport can occur. Combining both interfaces creates double-interface structures that have a more robust (with respect to oxygen treatments) interface doping. The doping is a function of the interface separation and can be modeled similarly to the LaTiO3/LaAlO3 system. Here the balance is between the electron binding energy at the donor interface and the electron transfer dipole.