V2VI3 compounds are a topological insulator (TI) material system which has a large inverted band gap up to 0.3 eV, promising room temperature spintronic applications. Doped V2VI3 materials also show superconductivity (SC) or ferromagnetism (FM). The metallic surface states of pure V2VI3 TI structures and hybrid structures combining epitaxial TI with SC or FM systems will enable many fundamental transport and spectroscopy studies of topologically protected and correlated electron systems.
In this materials science project we will optimize molecular beam epitaxy (MBE) of intrinsic as well as doped, thin V2VI3 layers and self-assembled nanowires on semiconductor substrates in order to perform magnetotransport and spectroscopy studies. The materials crystallize in quintuple layers which are just weakly bound to each other and to the substrate by Van-der-Waals forces. This structural feature may cause rather inert layer surfaces embodying topologically protected Dirac-like surface states, but it also represents a challenge in epitaxial growth and for achieving insulating bulk behavior, so far. At present, BiSbSe3 or BiSbTeSe2 bulk crystals reveal the lowest carrier densities. Nevertheless, electron transport in TI surface states of epitaxial layers is obscured by dominating bulk- or defect-related conductivity, until now. The various conductive channels will be identified by extensive studies of crystal structure and carrier densities assigned to surfaces, interfaces and defects. Adequately passivated substrates and cap layers may prevent extrinsic 2D excess carriers at interfaces. Extended defects like domain boundaries and dislocations in Bi2Se3 can be widely prevented by lattice-matched substrates with proper surface, like Bi2Se3 on InP(111). Charged point defects have to be reduced by optimization of MBE growth and layer parameters like growth temperature and layer composition. As these parameters also influence interface and defect properties, our aim of fabricating V2VI3 TI layers with insulating bulk requires systematic MBE growth studies accompanied by detailed structural and electrical characterization.
Homogeneous ferromagnetic TI layers like (CrBiSb)2Te3 will be fabricated by the substitutional incorporation of magnetic materials at adequate growth conditions. Superconductivity in TI layers can be reached by dopants intercalated between the quintuple layers. Besides the fabrication and characterization of such TI, FM and SC V2VI3 layers, also epitaxial hybrid structures, multilayers and TI layers just covered by a dopant will be studied.
MBE techniques will be developed also for gold-catalyzed and homo-catalyzed growth of freestanding V2VI3 nanowires, which are expected to minimize crystal defects and maximize the surface to bulk ratio. The high degree of control over the growth conditions, most notably the tunable Selenium to Bismuth ratio, offers opportunities for different growth regimes where the composition and crystal structure of the nanowires may be altered. Controllable fabrication of topological insulator nanostructures can enable the explicit proof of Majorana bound states. Tunneling spectroscopy on proximity induced topological superconducting nanowires are expected to show a quantized zero-bias peak and the superconducting gap that should exhibit a closing at the phase transition between topological and non-topological phase.
Thus, our main objectives are the MBE growth of high quality TI layers and nanowires, their detailed structural, transport and spectroscopic characterization, the realization of TI, FM and SC hybrid structures, and their study with respect to TI properties and electronic correlations.
[B01.1] S. Schreyeck, N. V. Tarakina, G. Karczewski, C. Schumacher, T. Borzenko, C. Brüne, H. Buhmann, C. Gould, K. Brunner, and L. W. Molenkamp, Molecular beam epitaxy of high structural quality Bi2Se3 on lattice matched InP(111) substrates, Appl. Phys. Lett. 102, 041914 (2013).
[B01.3] N. V. Tarakina, S. Schreyeck, M. Luysberg, C. Schumacher, G. Karczewski, K. Brunner, C. Gould, H. Buhmann, R. Dunin-Borkowski, and L. W. Molenkamp, Suppressing twin formation in Bi2Se3 thin films, Adv. Mat. Interf. 2014, 1400134 (2014).