Travis L. WADE, Marcin KONCZYKOWSKI

En collaboration avec Haiming DENG et Lia KRUSIN-ELBAUM, City College of New York, NY, USA

Topological Insulators (TIs) are a new class of materials that have a band gap in the bulk that is spin-orbit locked i.e. Dirac states on the surface. Accessing surface conduction channels; however, is a major challenge because the Fermi level does not lie between the band gap due to crystal imperfections. Therefore, high aspect ratio surface-to-volume of nanowires and thin films provides an alternate solution. Electrodeposition is an economical and flexible method to grow TIs with tuneable electronic properties as well as complex structures such as p-n junctions and TI-superconductor interfaces.

Goals:

• Exploring the growth conditions of Bi2Te3 and Sb2Te3 thin films and characterizing their microstructures, composition and electronic properties with SEM and low temperature transport measurements.
• Fabrication and low temperature transport measurement of nanowires embedded in anodized aluminum oxide (AAO) matrix of different diameter sizes and with different growth conditions.
• Fabrication and low temperature transport measurement of TI hetero-structures of p-n junctions and superconductor (Pb)-TI interfaces on films or nanowires.

Figure1. Resistance vs temperature plot; there are two super-conducting (SC) transitions. The first transition is at 7.6K (corresponding to lead) while the other transition occurs at 4.9K which corresponds to the SC proximity effect of the TI film.
Figure 2. Resistance vs magnetic field at 4.2K, it shows that the super conductivity disappears at around 1000 Oe.
Figures 3 and 4 are. Rxx and Rxy vs magnetic field (insert) plots at 4.2K for TI thin films. For both Sb2Te3 and Bi2Te3 thin films, at small magnetic fields, the magneto resistance is Linear and positive suggesting weak anti-localization (WAL), which is one of the features of TIs due to their strong spin-orbit surface state. The inserts show that the Sb2Te3 film is p-type and the Bi2Te3 is n-type.

From calibrating the deposition voltage on films, we found that between 125mV to 150mV deposition voltage, the stoichiometry of Sb2Te3 is almost constant. However, from transport measurements, we found that they are very different in carrier density and disorder. These are the data of Sb2Te3 nanowires of different diameter or deposition voltage.

Figure 5 is the data of 55nm diameter nanowire that shows WAL while figure 6 does not because of its larger diameter. (the aspect ratio of surface VS bulk is much higher in 55nm than 100nm).
Figure 7 and 8 are resistance vs temperature plots that shows semi-conducting behaviour which suggests strong disorder in our nanowire, confirmed by TEM diffraction, figure 9 (photo by Pierre-Eugene Coulon, LSI).