April 2016

Abstracts of the QSIT Lunch Seminar, Thursday, April 14, 2016

Disorder effects in InAs/GaSb topological insulator candidates

Camille Ndebeka-Bandou, Optoelectronics Group,ETH Zurich

InAs/GaSb quantum wells exhibit unique band-gap alignments where, if the layer thicknesses are sufficiently large, the electron level lays below the heavy-hole level. This configuration leads to a band-inversion and a significant hybridization of the conduction and valence states [1]. The system presents a strong ambipolar behavior with the possibility of tuning its conductivity from electron-like to hole-like by the application of an external electric field [2]. Due to this tunable bipolarity, InAs/GaSb quantum wells are promising topological insulator candidates in which non-local transport through edge-states has already been demonstrated [2-4]. However, a significant contribution of the bulk transport still obscures the visibility of the dissipationless edge channel transport. To date, few solutions have been proposed such as the adjustment of the disorder in the sample to lower the carrier mobility in the bulk and suppress the parasitic scattering channels between the edge-states [3,5].

In this work, we present a theoretical analysis of the effect of the static disorder on electron and hole states in such broken-gap structures. As static scatterers, we have considered interface defects and coulombic dopants both located at the InAs/GaSb interface of a 10/15/8/10 nm AlSb/InAs/GaSb/AlSb structure. We have used an eight-band k.p calculation and developed an exact numerical diagonalization of the disordered Hamiltonian. Our numerical approach enables to get a full insight of the disordered eigenenergies and eigenfunctions as well as the disorder-induced intra- and inter-band couplings. Depending on the spin-polarization, the structure exhibits either an insulating or a semi-metallic bandalignment. The presence of static scatterers combined with this broken-gap alignment leads to interesting features. In particular, we show that, as soon as the band-gap is larger than the disorder potential fluctuations, it subsists despite the existence of bound states below the electronic band edge. Moreover, whereas the in-plane carrier wavefunctions are slightly localized in the layer plane, the motion along the growth direction is more delocalized and a stronger admixture between electrons and holes develops. The static disorder reinforces the intrinsic hole-electron hybridization of the InAs/GaSb system.

References:

[1] A. Zakharova, S. T. Yen, and K. A. Chao, Phys. Rev. B 64, 235332 (2001).
[2] I. Knez, R.-R. Du, and G. Sullivan, Phys. Rev. Lett. 107, 136603 (2011).
[3] K. Suzuki, Y. Harada, K. Onomitsu, and K. Muraki, Phys. Rev. B 87, 235311 (2013).
[4] F. Nichele, et al., Phys. Rev. Lett. 112, 036802 (2014).
[5] C. Charpentier, et al., Appl. Phys. Lett. 103, 112102 (2013).

Charge Transport in Semiconductor Nanocrystal Solids

Nuri Yazdani, Laboratory for Nanoelectronics, ETH Zurich

Films of semiconductor nanocrystals (NC-solids) hold great promise as low-cost, solution processable semiconductors with electronic and optical properties that can be tuned by varying the size and composition of the constituent NCs, as well as their surface terminating ligands. NC-solids are highly disordered materials, with a distribution of NC sizes leading to a distribution in on site energies and disordered lattice positions.
Charge transport in NC-solids is governed by a complex interplay of a large number of material parameters. The beauty of these materials is that we can control, or at least precisely measure, all of these parameters. In this talk I will give an introduction to semiconducting NCs and discuss our work in trying to understand charge transport in these complex materials.