Localization phenomena in INAS/GASB composite quantum wells with disorder

Official URL: http://risc01.sabanciuniv.edu/record=b2325806 (Table of contents)

A bilayer structure of indium arsenide (InAs) and gallium antimonide (GaSb) has been proposed as a two-dimensional (2D) electronic system tunable to different states of matter namely the trivial and the topological insulator (TI) phases. 2D TI which is also called quantum spin Hall (QSH) insulator is characterized by an insulating bulk and non-dissipative counterpropagating edge states with opposite spin polarizations. These features identify the quantum spin Hall effect where the helical edge states are topologically immune against backscattering guaranteed by time-reversal symmetry. Lowtemperature electronic transport measurements may serve to examine the transport properties of InAs/GaSb quantum wells and therefore to inquire into the essence of its bulk and edge behavior. This work is aimed to study the constituents necessary to obtain a robust quantum spin Hall insulator based on InAs/GaSb bilayer heterostructure. We focus on the localization phenomena in InAs/GaSb bilayer quantum wells which are intentionally disordered by silicon atoms. We observed that Si doping near the InAs/GaSb interface significantly alters the transport behavior of these heterostructures. First, we investigated the localization of trivial edge states driven by silicon impurities. As confirmed by recent experimental studies, conductance quantization due to nontrivial edge states, which is the most significant highlight of the QSH effect is obscured by spurious conductivity arising from trivial edge states. In this thesis, we present an experimental observation of the strong localization of trivial edge modes in an InAs/GaSb heterostructure which is weakly disordered by silicon delta-like dopants within the InAs layer. The edge conduction which is characterized by a temperature-independent behavior at low temperatures and a power law at high temperatures is observed to be exponentially scaled with the length of the edge. Results of comprehensive analyses on measurements done with a range of devices are in agreement with the localization theories in quasi-onedimensional electronic systems. Spin-orbit interaction is one of the main ingredients of the TIs and in particular in disordered and thus low mobility electronic systems it leads to a weak anti-localization characteristic in low field magnetoconductance measurements. As the charge carriers are depleted using a top gate electrode, we observed a crossover from weak anti-localization (WAL) to weak localization (WL). This occurs when the dephasing length decreases below the spin-orbit characteristic length as a result of enhanced electron-electron interactions at lower carrier concentrations. The same crossover is observed with increasing temperature. The linear temperature behavior of inelastic scattering rate indicates that the dominant phase breaking mechanism in our 2D system is due to electron-electron interactions. Finally, we compare three heterostructures (Wafers A, B, and C) all delta-doped with silicon atoms at different spatial positions in the growth direction near the InAs/GaSb interface. We found the transport behavior of these heterostructures to be very different from each other which can be attributed to the vertical position of disorder within the layered structures. Since the silicon atoms are donors to InAs and acceptors to GaSb, it matters where they are located with respect to the InAs/GaSb interface considering the interfacial effects and potential fluctuations induced by impurities