Transport phenomena in topological insulators
Electrical transport in topological insulators (TIs) is very sensitive to surface treatments and damages. During the first funding period we developed a wet-chemical etching process which allows us to fabricate HgTe microstructures using electron beam lithography that show a quantized conductance in the quantum spin Hall (QSH) state with a very high yield. Additionally, the gating technique was improved by employing hafnium oxide insulators by atomic layer deposition (ALD) at low temper- atures (35◦C). These techniques are now used to fabricate transport devices for the investigation of unperturbed electronic transport in topologically protected Dirac systems based on HgTe. Especially, the fabrication of quantum point contacts (QPCs) has become an important tool for this investigation. A QPC provides a perfect element for controlling the interactions between helical edge channels on either side and it is possible to implement QPCs as a local source for electron injection and detection. Owing to the new fabrication techniques the exploration of different transport regimes becomes fea- sible. The characterization and the transition between ballistic, diffusive and hydrodynamic transport regime is a main subject for the new funding period which additionally provides a strong overlap with theory projects of the SFB 1170. The progress in growth allows us to extend theses investigations on Weyl and Kane semimetals. With the implementation of QPCs into the side walls of a transport channel a local measurement of the electron temperature becomes possible. This will allow us to start investigating thermoelectric transport properties of systems with linear band dispersion.
The strategy for the new funding period is to further minimize both thermal and radiative process loads during fabrication and develop minimal invasive processes. This will be achieved for example by exploring alternative etching techniques, such as inductively coupled plasma (ICP) etching at low temperatures (0◦C), by improving our existing wet etching process and by novel electron beam lithography techniques reducing the electrons penetrating the surfaces of the TI films. Additionally an optical maskless lithography tool will be used for structures in the μm range with the advantage of avoiding strong forces to the surface of the sample as inherit by mask based optical lithography. Given the importance of ohmic contacts for the device performance we will continue to optimize the existing technology for example by investigating local annealing techniques. This will be an important issue not only for contacting double quantum well structures separately but also for all other SFB 1170 projects relying on clean transport data of HgTe, especially at applied magnetic field.