Abstract
This thesis presents the studies of the band structure and optical properties of narrow-gap heterostructures with HgTe/HgCdTe and broken-gap Alb/InAs/GaSb/AlSb quantum wells (QWs). A unique property of these solid-state systems is high sensitivity of their band structure to internal parameters, such as thickness and composition of the QWs. It allows changing the band structure from normal to inverted inducing topological phase transition from trivial band insulator to quantum spin Hall insulator (QSHI). One of the most remarkable features of QSHI is its unique transport properties, expressed by the current being transmitted via spin-polarized one-dimensional edge channels with quantized conductance.Transport properties of QSHI are of great scientific interest regarding fundamental effects as well as practical applications. Edge channels can be used for the dissipationless current transfer, resistance standards, in spintronics and quantum computing. Topological insulator states have been observed both in HgTe/HgCdTe and InAs/GaSb quantum wells structures. However, in both cases, further application of the amazing physical properties of these systems is limited by their low bandgap energy.In the first part of this Ph.D. project, we present experimental studies of innovative QW geometry on the basis of trilayer InAs/GaSb/InAs structures. The results of magnetoabsorption and photoluminescence spectroscopy show that new geometry increases the bandgap compared to bilayer InAs/GaSbQWs. We also confirm the theoretical prediction that trilayer design can be combined with strained QW layers to amplify the value of inverted bandgap. Presented results prove that InAs/GaSb/InAs can be a basis for QSHI with a large bulk band-gap and pave the way towards ultimately plausible high-temperature topological conductivity.We also investigate trilayer HgTe/CdHgTe/HgTe heterostructures with double QWs. In this system interaction between the states in two QWs results in rich topological behavior. We show that the band structure of double HgTe QWs experiences multiple topological phase transitions driven by temperature. The first transition is similar to the one observed previously in single HgTe QWs. However, the second transition can lead to a formation of a novel phase, high order topological insulator.Another property of HgTe/HgCdTe and Alb/InAs/GaSb/InAs/AlSb heterostructures which is realized in narrow gap structures is carrier dispersion mimicking that of relativistic particles. Energy-momentum conservation laws within quasirelativistic dispersion prohibit non-radiative Auger recombination, the main fundamental factor limiting the operation of interband radiation sources. Thus, the structures with quazirelativistic dispersion can be used for the development of solid-state sources of infrared radiation. They are especially appealing in the wavelength range from 24 to 60 microns where A3B5–based unipolar quantum cascade lasers cannot operate due to high lattice absorption.The goal of the second part of this Ph.D. project is to study the band structure and radiative properties of HgTe/CdTe based heterostructures with QWs and develop the physical foundation for the far-infrared semiconductor lasers. We present the results of emission from samples with Hg1-yCdyTe/Hg1-xCdxTe QWs and analyze their band structure in order to determine the most efficient designs for the amplification of long-wavelength radiation. We demonstrate that maximum suppression of Auger recombination is achieved in structures with thin HgTe QWs with zero cadmium concentration surrounded by HgCdTe barriers with 70% CdTe. Finally, we investigate the pumping parameters and show that the increase of the wavelength of optical excitation improves the characteristics of stimulated emission. These results demonstrate the potential of HgTe/CdHgTe heterostructures as a basis for far-infrared and terahertz sources.