This work is devoted to the study of mercury cadmium telluride based heterostructures employing techniques such as terahertz spectroscopy and magnetophotoconductivity. An important goal of this work is to obtain a better understanding of the topological boundary states in these materials and their influence on electro-optical effects. The investigations focus on two-dimensional HgTe quantum wells as well as three-dimensional CdₓHg₁₋ₓTe films in both topologically trivial and non-trivial regimes. Using continuous and pulsed terahertz excitation, fascinating optoelectronic phenomena were discovered in the course of this study.
A detailed investigation of the terahertz photoconductivity in HgTe QWs with inverted and non-inverted parabolic as well as linear band dispersion revealed that these structures manifest a distinct sign-alternating photoresponse in magnetic field. Upon increase of the external, out-of-plane magnetic field, the photoconductivity systematically changes its sign. This remarkable effect was observed in QWs corresponding to both topologically trivial and non-trivial regimes. Additionally, it was studied in samples with different geometries including conventional Hall bar and Corbino disk design. Notably, the analysis of the photoconductivity in Corbino disk samples brought up an elegant optoelectronic method to probe the carrier mobility in such systems.
During the research on the sign-alternating photoconductivity, a qualitatively different behavior of the photoresponse was observed in a high-mobility 20 nm QW. Instead of a single or double sign inversion in magnetic field, pronounced ω/ωc -periodic oscillations were observed in photoresistivity for the Fermi energy lying in the conduction band. Further analysis demonstrated that these oscillations have the same origin as microwave-induced resistance oscillations (MIRO), which have previously been observed only in systems with
ultra-high mobility, such as ultra-clean high-mobility GaAs quantum wells, under illumination with microwave radiation. Similar to Shubnikov-de Haas oscillations, which result from an interplay of Fermi energy and cyclotron energy, MIRO express the commensurability between the photon energy and the cyclotron energy. Recently, their terahertz analogue, the terahertz-induced resistance oscillations (TIRO), were observed in GaAs quantum wells as well as in three-dimensional HgTe topological insulators. The results presented in this work represent the first observation of such MIRO-like oscillations in HgTe quantum wells.
In addition to two-dimensional systems, the optoelectronic properties of three-dimensional CdₓHg₁₋ₓTe films were studied in the last part of this thesis. The investigation of topologically non-trivial CdₓHg₁₋ₓTe films with an inverted band ordering revealed cyclotron resonance involving the two-dimensional surface states. The surface state resonance was detected in terahertz radiation transmission as well as photogalvanic experiments. However, it was absent in the investigated CdₓHg₁₋ₓTe films with higher cadmium contents corresponding to the topologically trivial regime, demonstrating the origin of the resonance in the surface state carriers. Moreover, experiments with different film designs showed the importance of sharp interfaces between topologically trivial and non-trivial layers for the formation of fully two-dimensional topological surface states
