Thesis (Ph. D.)--Oregon Graduate Center, 1989.

This work presents a study of the character and distribution of structural defects in (00l)CdTe buffer layers grown on GaAs substrates by metal organic vapour phase epitaxy (MOVPE). These are of importance as hybrid substrates for the growth of Cd(_x)Hg(1-x)Te (CMT), a prominent infrared detector material. The 14.5% mismatch between CdTe and GaAs leads to a high-density of dislocations at the CdTe/GaAs interface, and threading through the layer. The presence of linear and planar defects is detrimental to the performance of CMT devices and it is desirable to reduce the density of dislocations to below l0(^5)cm(^-2). Results of high resolution X-ray diffraction (HRXRD) studies on a series of MOVPE grown CdTe/GaAs epilayers of different thickness and on a single thick layer which was repeatedly etched and remeasured are reported. Threading dislocation density was estimated from HRXRD full width at half maximum (FWHM) using a relationship proposed by Gay, Hirsch and Kelly and was found to decrease rapidly in the initial stages of layer growth. An optimum buffer layer thickness of 8µm is proposed for the subsequent growth of CMT. Rocking curves were recorded from a single thick CdTe/GaAs epilayer for wavelengths in the range 0.69-1.95A at the Daresbury SRS, and this data is extrapolated to infinite absorption to obtain an estimate of the rocking curve width representative of the surface of a thick layer. A number of models which attempt to describe the reduction in threading dislocation density with increasing thickness are reviewed and a new model based on the coalescence of like dislocations is developed. The models are compared to published data for layers with misfit in the range 0.23-14.6%. The models previously developed by Tachikawa and Yamaguchi and by Durose and Tatsuoka are shown to be the more appropriate for describing the dislocation density distribution in highly mismatched layers. Results of the transmission electron microscopy of CdTe/GaAs epilayers, both in plan view and cross-section, showing the character of dislocations threading through thick CdTe buffer layers are presented. Many dipoles composed of 30º type dislocations with Burgers vectors parallel to the interface were observed and found to adopt an orientation whereby the component dislocations had no effect on misfit strain. In cross-section, 30º type dislocations with Burgers vectors inclined to the interface were found to be numerous, 60º and screw type threading dislocations were also observed, but the cross-section projection axis made analysis of these types difficult. The screw and edge components of 30º , 60º and screw type threading dislocations are compared and it is found that the Burgers vector component causing layer tilt is of magnitude (a(_o)/2) for each type. Annealing of CdTe/GaAs epilayers under di-methyl cadmium flow was found to have no deleterious effect on layer morphology but did not result in a narrower X-ray rocking curve. Based upon HRXRD FWHM, 8µm thick CdTe buffer layers grown by MOVPE are estimated to have a dislocation density of about 3xl0(^8)cm(^-2). Even after many of the threading dislocations have been bent over at the CdTe/CMT interface, their density in MOVPE grown CMT/CdTe/GaAs is still greatly in excess of the l0(^5)cm(^-2) desired. The issues discussed and conclusions drawn in this thesis represent a thorough study of the continuing progress towards high quality MOVPE growth of CMT.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references (leaves 123-128). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references (leaves 123-128). Also available on the Internet.

Topological insulators (TIs) are a new state of quantum matter that has fundamentally challenged our knowledge of insulator and metals. They are insulators in the bulk, but metallic on the edge. A TI is characterized by a so-called topological invariant. This characteristic integer number is associated to every mapping between two topological spaces and can be defined for an electronic system on the lattice. Due to the bulk-edge correspondence a non-trivial value leads to topologically protected edge states. To get insight into the electronic characteristics of these edge/surface states, however, an effective continuum theory is needed. Continuum models are analytical and are also able to model transport. In this thesis we will address the suitability of continuum low-energy theories to describe the topological characteristics of TIs. The models which are topologically well-defined are called topological k.p Hamiltonians. After introducing a necessary background in chapter 1 and 2, we will discuss in the methodological chapter 3 the strategies that have to be taken into account to allow for studying topological surface states. In chapter 4 we will study two different model classes associated to a spherical basis manifold. Both have an integer topological invariant, but one shows a marginal bulk-edge correspondence. In chapter 5 we will study a different continuum theory where the basis manifold corresponds to a hemisphere. We then apply all these ideas to a time-reversal invariant TI -- uniaxially strained Mercury Telluride (HgTe). We determine the spin textures of the topological surface states of strained HgTe using their close relations with the mirror Chern numbers of the system and the orbital composition of the surface states. We show that at the side surfaces with $C_{2v}$ point group symmetry an increase in the strain magnitude triggers a topological phase transition where the winding number of the surface state spin texture is flipped while the four topological invariants characterizing the bulk band structure are unchanged. In the last chapter we will give a summary.

Topological insulators (TIs) are a new state of quantum matter that has fundamentally challenged our knowledge of insulator and metals. They are insulators in the bulk, but metallic on the edge. A TI is characterized by a so-called topological invariant. This characteristic integer number is associated to every mapping between two topological spaces and can be defined for an electronic system on the lattice. Due to the bulk-edge correspondence a non-trivial value leads to topologically protected edge states. To get insight into the electronic characteristics of these edge/surface states, however, an effective continuum theory is needed. Continuum models are analytical and are also able to model transport. In this thesis we will address the suitability of continuum low-energy theories to describe the topological characteristics of TIs. The models which are topologically well-defined are called topological k.p Hamiltonians. After introducing a necessary background in chapter 1 and 2, we will discuss in the methodological chapter 3 the strategies that have to be taken into account to allow for studying topological surface states. In chapter 4 we will study two different model classes associated to a spherical basis manifold. Both have an integer topological invariant, but one shows a marginal bulk-edge correspondence. In chapter 5 we will study a different continuum theory where the basis manifold corresponds to a hemisphere. We then apply all these ideas to a time-reversal invariant TI -- uniaxially strained Mercury Telluride (HgTe). We determine the spin textures of the topological surface states of strained HgTe using their close relations with the mirror Chern numbers of the system and the orbital composition of the surface states. We show that at the side surfaces with $C_{2v}$ point group symmetry an increase in the strain magnitude triggers a topological phase transition where the winding number of the surface state spin texture is flipped while the four topological invariants characterizing the bulk band structure are unchanged. In the last chapter we will give a summary.

Dans cette thèse, j’étudie la compressibilité électronique de deux isolants topologiques tridimensionnels : Le tellurure de mercure (HgTe) sous contrainte et le séléniure de bismuth (Bi2Se3).Je présente des mesures d’admittance électronique à basse température résolues en phase sur une large gamme de fréquence. Cela permet d’extraire la capacité quantique associé à la densité d’états et la résistivité des matériaux étudiés.Nous montrons qu’un isolant topologique intrinsèque présente une réponse dominée par les états de surface topologiques sur une large gamme d’énergie qui s’étend au-delà du gap de transport du matériau massif. Ce régime, appelé « écrantage de Dirac », est caractérisé par une compressibilité électronique proportionnelle à l’énergie de surface et une haute mobilité.Dans la suite, nous nous intéressons à la limite de ce régime. Nous observons qu’à haute énergie et sous l’influence de forts champs électriques perpendiculaires, des états excités massifs de surface sont peuplés ce qui se manifeste expérimentalement de différentes façons : Une chute dans la constante de diffusion électronique, un pic de conductivité ainsi que l’apparition d’un deuxième type de porteurs en magnéto-transport et de métastabilité dans la relation charge-tension.Un modèle théorique basé sur un traitement quasi-relativiste du Hamiltonien de surface est présenté. Il permet d’identifier la dépendance en énergie et champ électrique des états massifs de surface.Cette thèse est complémenté par des résultats expérimentaux sur Bi2Se3 obtenu par croissance sur nitrure de bore mettent en évidence l’importance de la pureté des interfaces d’isolants topologiques. / This thesis discusses the electronic compressibility of two representative three dimensional topological insulators: Strained mercury telluride (HgTe) and bismuth selenide (Bi2Se3).I present low temperature phase-sensitive electron admittance data over a broad frequency range. This allows to extract the quantum capacitance related to the density of states and the resistivity of the investigated materials.We show that the response of an intrinsic topological insulator is dominated by topological surface states over a large energy range exceeding the bulk material’s transport gap. This regime, named “Dirac screening” is characterized by an electron compressibility proportional to the surface Fermi level and a high mobility.Subsequently, we investigate the limits of this regime. At high energy and large perpendicular electric fields we observe the population of excited massive surface states. Experimentally, these manifest themselves in multiple signatures: A drop in the electronic diffusion constant, a peak in the conductivity, appearance of a second carrier type in magneto-transport and meta-stability in the charge-voltage relation.A theoretical model based on a quasi-relativistic treatment of the surface Hamiltonian is presented. It allows to identify the electric field and energy dependence of the massive surface states.This thesis is complemented by experimental results on Bi2Se3 grown on boron nitride, where we demonstrate the importance of clean surfaces for the study of electronic properties in topological insulators.