Riassunto analitico
Two dimensional topological insulators (2DTIs) are a new intriguing class of materials characterized by an insulating bulk and gap-less one dimensional helical edge states.
The InAs/GaSb coupled quantum well in particular, shows a peculiar band alignment with coexisting spatially separated electrons and
holes.
By varying the thickness of the InAs and GaSb layers the band alignment can be tuned from a normal insulator band structure, into an
inverted one, where the lower electron sub-band lies below the highest hole sub-band. At the crossing point of the two bands, as a result of the coupling between electrons and holes, which are at the same energy with opposite momentum, a bulk hybridization gap opens up.
Recently, motivated by the hypothesis of the presence of one dimensional helical edge state within the bulk gap, InAs/GaSb in its inverted phase has been recently proposed as a quantum spin Hall insulator, since these states are topologically protected by time reversal symmetry, and states with opposite spin counter propagate on each edge dissipationless..
Even if this prediction has already been verified by few experimental-research group, a consistent identification of an InAs/GaSb
sample as a two dimensional topological insulator needs to be based on a careful determination of the contribution of the different subband to the electrical transport properties, as the Fermi energy is tuned continuously by applying a gate voltage. In this view, the analysis of the Shubnikov de-Haas oscillations, detected with magneto-transport measurements, is an optimal strategy to estimate the carrier density, the effective mass and the quantum scattering time of the charge carriers. Moreover, even if the transport properties of this system are sufficiently understood in the deep trivial and inverted regime, its behavior at the critical point represented by the transition between these two phases is still unexplored.
One theory, predicts that at this critical condition, the two bands may overlap in one point showing a zero gap band structure with a linear dispersion at the Gamma point of the Brillouin zone, exactly like it happens for the HgTe/CdTe quantum well, the first material known to be a 2DTIs. In this favorable case, InAs/GaSb systems may be the second candidates for the observation of single valley Dirac fermions.
The final goal of this thesis work, is the characterization of an InAs/GaSb quantum well, epitaxially grown with critical parameters, in order to study its transport properties in this particular condition, and to give a clear description of the conduction contributions in the different charge carriers regimes, as obtained with electrostatic gating.
|
