Manipulation of the Two-Dimensional States at Titanates Surfaces studied by ARPES

The transition metal oxides are known for their versatile physical properties and are therefore an active field of research towards possible new functionalities and applications. This thesis focuses on the subgroup of titanates with perovskite structure that shows a wide range of interesting bulk properties, ranging from para-, ferro-, and antiferroelectric orders to superconductivity. Since the finding of a two-dimensional electron gas (2DEG) at the interface and surface of titanates, the interaction of the bulk properties with the 2DEG is of particular interest. In this thesis, ways to manipulate the 2DEG formation at the surface of titanates by ferro- and paraelectric bulk properties and the surface crystal structure are investigated by the help of angle-resolved photoelectron spectroscopy (ARPES). Measured are films of CaTiO3, BaTiO3, SrTiO3, and BaxSr(1-x)TiO3 grown on different substrates by pulsed laser deposition (PLD) as well as vicinal SrTiO3 substrates. The investigated quantum paraelectric CaTiO3 hosts a 2DEG at its surface with dxy orbital character that is the only metallic state of the system. The 2DEG shows indications of a Rashba-type spin splitting and has a band bottom of approximately at 400 meV. Therefore the TiO6 octahedrons at the film surface are likely strong tetragonal distorted. The symmetry of the grown film is given by the pseudo-cubic unit cell of CaTiO3 with a c(4x2) reconstruction. Under irradiation with ultra-violet light, the reconstruction is changed to a p(2x2) symmetry with folded Fermi surfaces according to the reconstruction. The affinity of the 2DEG at the CaTiO3 surface to arrange with the surface reconstruction makes it feasible to further manipulate the system with different surface terminations and formations, with the goal to tailor its two-dimensional transport properties. BaTiO3 is a well known ferroelectric material. In this thesis, 20 unit cell thick films of BaTiO3 grown on SrTiO3 and KTaO3 substrates are studied. The films exhibit ferroelectric properties, measured by piezo-response force microscopy (PFM) and show indications of domains that are locked in in-plane directions. The strong in-plane electric field is responsible for a Wannier-Stark localization (WSL) of the conduction electron, resulting in a smearing of the observed two-dimensional states in reciprocal space. These effects are also observable in small overlayers of SrTiO3 grown on top, and fade with increasing SrTiO3 film thickness. The 2DEG observed at the vacuum surface of SrTiO3 can be directly affected by the surface properties. On vicinal SrTiO3 surfaces, the surface relaxation is altered by the terrace width. This affects the tetragonal distortion of the TiO6 octahedron and results in changes of the energy splitting of the surface localized bands. Apart from this, the surface band bending is altered due to the electron affinity of the vicinal substrates. The surface band bending can also be altered by the dielectric properties of the surface region. This is likely responsible for the presence of only the dxy bands at the surface of SrTiO3 thin films grown on STO. At the surface of Ba doped SrTiO3 films the 2DEG forms similar as on the SrTiO3 thin films. For the different dopings investigated, the samples turn insulating at different temperatures, likely due to the changed behavior of the dielectric properties with temperature.


  • Thesis submitted - Forthcoming publication

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