Strained Hgte 3D Topological Insulator

First proposed as a new class of matter depicted as a bulk insulator with conducting surfaces, topological insulators have recently stimulated an intensive research activity due to their unique electronic and spin properties. In particular, surface conduction is expected to occur from mass-less Dirac fermions with spin orientation locked to momentum.

A general requirement for turning a conventional insulator into its topological counterpart is the realization of an inverted band structure. Such inversion can be achieved experimentally using confinement or band-offset effects in hetero-structures, but may also be intrinsically present in the band structure of low band-gap materials with very large spin-orbit interaction (SOI).

Since the SOI scales with Z⁴, the family of intrinsic topological insulator material thus shrinks to very heavy elements. In particular, the most widely investigated Bi_1-xSb_x and Bi₂Se₃/Bi₂Te₃ compounds have shown, from angle resolved photo-emission spectroscopy, to exhibit such surfaces states in the form of Dirac fermions, but usually suffer from parasitic bulk conduction which has so far prevented the demonstration of convincing surface transport experiments.

In this picture, HgTe appears to be a good candidate for topological insulator structures, and we will show that the epitaxial growth of intentionally strained HgTe can provide “insulator-like” band structure material of very high quality. In particular, we will present extensive material characterization of the bulk crystal quality of HgTe and the surfaces and interfaces of HgTe/HgCdTe/CdTe structures designed for electronic transport experiments.

Samples processed into top-gated Hall-bar structures are investigated using variable magnetic field, low-temperature, electronic transport set-up. Strong experimental signatures of surface transport of Dirac particles will be given for different epitaxial configurations. Finally, perspective work as well as potential applicative developments will be discussed.