Work performed in detail within the respective objectives:
Objective 1 (Nanoparticle synthesis) was done together with University of Duisburg-Essen, Prof. Stephan Schulz. The following nanoparticle samples were synthesized: 5g Sb2Te3, annealed at 400°C; 5g Bi2Te3 with new thermally robust precursor, but not annealed; 4g of Te-enriched Bi2Te3 – another gram of this charge of material was directly delivered to a cooperation partner for THz spectroscopy, 5g of Bi2Te3, annealed at 300 °C, 5g of Bi2Te3 annealed at 400 °C, all samples including SEM, EDX, IR, XRD and most of them XPS analysis. Meanwhile, the first ternary samples were successfully synthesized: (SbxBi2-x)Te3 as well as Bi2(SexTe3-x). Additionally, also a larger amount of Bi2Te3 nanoparticles was produced with the intention to use this material as reference for additional studies on the processing parameters.
Objective 2 (Nanoparticle processing and interrupted early stage sintering) was developed by the implementation of a hot press together with a heating jacket into a glove box in order to provide a controlled experimental environment for the interrupted early stage sintering; Different sintering tools were tested, in part home-made by our workshop. The choice of the right sintering tool turned out to be extremely important to prevent pellets from cracking or even breaking when removed from the tool. Also, thin foils of high melting metals (Ta or W) were added between the pellets and the tools and thereby used to prevent the direct contact, the latter in order to avoid diffusion of Fe into the pellets. Sintering of test samples was done from the delivered nanoparticle batches; We optimized sintering temperature, time and pressure in order to control the porosity.
Owing to the fact that the start of the project was at IFW Dresden, one experimental study from this objective was continued there. This study dealt with Bi2Se3 nanoparticles obtained from ball milling and further compacted to pellets by spark plasma sintering. It could be shown that also these nanograined samples showed the signatures of a weak antilocalization connected with the existence of electrons in surface states. Still, the characteristic signatures of the weak antilocalization were weak compared to these signatures in Bi2Te3 nanoporous samples from the synthesis in ionic liquids combined with the mild hot pressing procedure.
In Objective 3 (Transport characterization), we designed and adapted several sample holders and measurement routines for two existing equipments such as a 4T cryogenics, as well as a Quantum Design MPMS measurement system. With that, high quality transport data could be acquired. Further, we analyzed all data by the Hikami–Larkin–Nagaoka model. For that, fitting routines were programmed. Also, the model needed to be developed slightly in order to account for the spin-orbit coupled transport. By doing so, we obtained exciting results, such as a coherence length of the spin-orbit coupled transport that is longer than the size of the nanoparticles.
Further spectroscopic characterization of the samples was done together with a collaboration partner, Prof. Martin Mittendorff (University of Duisburg-Essen). His team characterized samples by THz spectroscopy that were specifically produced for this purpose. By his characterization it could be shown that exciting properties of the surface states are not a low-temperature phenomenon. In contrast, a plasmon resonance closely connected with the high mobility of the electrons in surface states could be evidenced up to temperatures close to room temperature. Further, it could be modelled from his measurements that the electrons in the surface states dominate the transport properties at low frequencies.
For Objective 4 (towards devices), we developed a processing routine for topological insulator materials in thin films quality, as well as established microstructuring, so that the device fabrication can now start. The first devices will be thin film transistors (as a preliminary stage for spin devices), that are currently being fabricated. Our status of device fabrication is the following: we established a sputtering routine for SnTe films. We further established processing routines for two dielectrics, atomic layer deposition of Al2O3 as well as reactive sputtering of Si3N3, as well as for all the required metallization (Ta as adhesion layer and anti-diffusion layer, Au as contact layer). Currently our transistor devices are characterized by a too high leakage current through the gate dielectric. Hence, we now stepwise increase the thickness of the gate dielectric layer. We further already established an etching routine to artificially porosify the SnTe thin film and thereby increase the surface-to-volume ratio. Since atomic layer deposition conformly covers the surface of the underlying layer, these artificially porosified layers could then be used for devices as well. This work is ongoing and not yet at the level of publication.
