Topological insulators based on V-VI compounds: epitaxial films and nanowires
In this project, we will study and harbour the interplay of strong spin-orbit coupling manifesting in topo- logical surface states, magnetic order and structural properties of V2VI3 compound heterostructures. The compounds (Bi,Sb)2(Te,Se)3 are topological insulators (TI) with bandgaps of about 0.2 eV. The TI layers are deposited by molecular beam epitaxy and are analyzed in structural and magnetotransport properties. Thin epitaxial (Bi0.2Sb0.8)2Te3 layers doped by V (or Cr) are ferromagnetic (MTI). We have already observed the zero-field Quantum Anomalous Hall Effect and indications of axion physics in such layers at mK temperatures. In this project we will continue our studies on such homogeously doped layers, as well as expand our studies to heterostuctures of TI, MTI, insulator and metal layers on semiconductor substrates. While the assembly of such heterostructures is no trivial task, our vast MBE experience and significant preliminary work make this a very attainable goal.
On the materials side, while we now know how to controllably work with the materials, very funda- mental questions remain to be understood, in particular with regards to the exact cause and nature of the ferromagnetic state. Moreover, a clear understanding of why the magnetically doped layers offer perfect bulk insulation whereas their non-magnetic counterparts do not is still missing.
From the device point of view on homogeneous layers, a key focus will be exploring an already ob- served transition as a function of sample thickness between 2D and 3D topological insulator physics, in which the 2D transport appears to be well described by Maxwell transport physics, whereas the thicker 3D layers require axion terms to be added into the electromagnetic action. For heterostruc- tures, a MTI/TI/MTI trilayer structure, for example, is proposed to show a ν = 0 QAH plateau in lateral transport. TI layers with superconducting (SC) contacts may induce SC and host Majorana-like states.
In addition to expanding to more sophisticated layer structures, we will also enhance our measure- ment toolkits by complementing our well established lateral magnetotransport with C-V and admit- tance measurements in vertical heterostructures. Vertical transport in capacitor and tunneling diode structures promises deeper insights in the density of states, spin polarization and energy gaps of the top or bottom topological surface states (TSS) and promote a deeper understanding of magnetism, axion physics, surface state properties and quantum transport in (M)TIs.
The MBE growth of individual layers in heterostructures will be optimized with respect to composi- tion, layer thickness, doping, interface quality and low defect density. Homogeneous 2D and (quasi-) 3D TI (hetero-)structures with insulating bulk are beneficial for lateral and vertical transport devices. Insulating epitaxial buffer layers such as ZnTe/n-InAs(111) and metal top gates with dielectric insu- lator layer deposited by MBE or atomic layer deposition allow to separately tune the top and bottom TSS carrier densities. The lithographic processing of such structures for vertical or lateral transport with in part sub-µm dimensions will be developed. Complementary to the structural and transport studies, the properties of surface states, possible edge states and magnetic dopants in such layer structures will be studied by STM, STS, ARPES and ResPES in cooperating SFB 1170 projects.