A surface science based window to transport properties: The electronic structure of Te-based chalcogenides close to the Fermi level

Being insulating in the bulk, topological insulators (Tis) possess topologically protected surface states with a helical spin texture within the bulk band gap. Unfortunately, conventional binary TIs exhibit high intrinsic defect densities, pushing the Fermi level in the bulk conduction or valence band, making them metallic. Therefore, the intrinsic disorder makes the utilization of the pure topologically protected surface states difficult. New methods will be presented in order to counteract the disorder and tune the TI, especially the Dirac point.Two different approaches are used. The first employs the intrinsic electrical field of p- and n-type TIs in a heterostructure resulting in the creation of a topological p-n junction. Thus the electrical field of the buried p-n junction tunes the band structure at the surface, depending on the thickness of the top TI layer in the heterostructure. The second approach uses the alloying of n- and p-type doped TIs, leading to a TI with a Dirac point coinciding with the Fermi energy within 5 meV and being well apart from each bulk band. For both methods, the band structure is mapped by angle resolved photoemission spectroscopy (ARPES).In PCMs disorder plays also an important role. The highly disordered phase, i.e. the amorphous phase, can be switched within nano-seconds into the more ordered crystalline phase, accompanied by a change of the optical and electrical contrast. The second part of this thesis describes the electrical properties and the atomic structure of p-type PCMs.Firstly the PCM GeTe is investigated, exhibiting bulk bands with a giant Rashba-type spin splitting. The fact that this splitting results from ferroelectricity, leads to the possibility of changing the helicity of the Rashba-split bands by electric fields in a non-volatile fashion. Piezo force microscopy experiments on GeTe validate the ferroelectric behavior and determine the as-grown polarization. The Rashba-split bands are resolved in (spin polarized) ARPES measurements, confirming the existence of giant Rashba-split surface and bulk bands. The second investigated PCM is the technologically relevant PCM, Ge2Sb2Te5 (GST225) in its meta-stable rock-salt phase. The 3D structure of the upper valence bands of GST225 is mapped by ARPES with variable photon energy. An M-shaped top valence band revealing little dispersion in (111) direction and a hexagonal band shape in the k plane perpendicular to (111) is found being clearly distinct from the usually assumed isotropic, parabolic valence band centered at Γ. The valence band maximum is about 100 meV below the Fermi level, but the disorder broadened tails of the valence band cut the Fermi energy. These results are compared with magneto-transport experiments of the same type of epitaxial samples. It is shown that a simple band model can describe the main effects with an accuracy of about 50%. Indications of a topological surface state are found by using spin analysis and two-photon excitation. Finally, STM and STS experiments are conducted on the surface of meta-stable GST225, which are compared with DFT calculations of the relaxed surface structure. DFT and STM reveal an intact Te layer and detect the disordered subsurface layer made of Ge, Sb and vacancies. In addition, STS experiments find fluctuations of the electrostatic potential within the Te layer, which agrees with simulations of the fluctuation at the surface, using independently measured acceptor density of the material. DFT calculations observe tetrahedral and octahedral Ge-Te bonding configurations at the surface layers which is not yet observed in experiments