Band structure engineering in 3D topological insulators investigated by angle-resolved photoemission spectroscopy

Three-dimensional topological insulators (3D TIs) are a new state of quantum matter and open up fascinating opportunities for novel spintronic devices due to their unique electronic properties: the simultaneous presence of an insulating energy gap in the bulk and conductive, spin-polarized electronic states at their surface. Unlike the metallic surface states of ordinary (topologically trivial) materials, these (topologically non-trivial) surface states are induced and protected by time reversal symmetry and by a new bulk property, called topology. However, for the usability of TI materials in spintronic devices one needs to find means to engineer their electronic band structure such that the Fermi level falls into the band gap, since most of the 3D TI materials suffer from significant bulk conductivity also at their surfaces. In this thesis different approaches are presented to manipulate the Fermi level and thereby engineer the electronic properties of thin films of typical 3D TI materials, such as Bi2Se3, Bi2Te3, and Sb2Te3, which were grown by molecular beam epitaxy. Their surface electronic structure is investigated using angle-resolved photoelectron spectroscopy. Besides conventional approaches like surface or bulk doping, the successful realization of a vertical topological p-n junction in epitaxial Sb2Te3/Bi2Te3/Si(111) heterostructures is demonstrated for the first time. Besides the verification of the crystalline quality of the bilayers and integrity of the interface, it is shown that it is possible to drive the surface of Sb2Te3 from being p-type into n-type by varying the influence from the lower Bi2Te3 layer, i. e. the built-in electrostatic potential caused by the depletion layer. The experimental findings are supported by solving the Schrödinger and Poisson equations self-consistently and thus simulating the band diagram throughout the heterostructure. Further, a thorough investigation of the crystal structure as well as the rich electronic (spin-) structure of the natural superlattice phase Bi1Te1 = (Bi2)1(Bi2Te3)2 is presented. It is shown by density functional theory that Bi1Te1, contrary to the closely related, prototypical strong 3D TI Bi2Te3, is a weak topological insulator with n0; (n1n2n3) = 0; (001). According to this, surfaces, which are perpendicular to the stacking direction, i. e. parallel to the natural cleavage planes, are expected to be free of topological surface states. Indeed, such surface is shown to exhibit surface states which in fact can easily be confused with topological Dirac cones but do not possess a measurable helical spin polarization which thus confirms the weak topological nature of Bi1Te1