MBE growth and characterization of Zn1-xCrxTe diluted magnetic semiconductor

Master'sMASTER OF ENGINEERIN

Report / Institute für Physik

The 2014 Report of the Physics Institutes of the Universität Leipzig presents a hopefully interesting overview of our research activities in the past year. It is also testimony of our scientific interaction with colleagues and partners worldwide. We are grateful to our guests for enriching our academic year with their contributions in the colloquium and within the work groups. The open full professorship in the Institute for Experimental Physics I has been filled with an outstanding candidate. We could attract Prof. Ralf Seidel from the University of Münster. He is an expert in molecular biophysics that complements the existing strength in cellular biophysics. Prof. Hollands could fill all positions of his ERC Starting Grant, so that the work on the project \"Quantum Fields and Curvature – Novel Constructive Approach via Operator Product Expansion\" is now running at full pace. Within the Horizon 2020 project LOMID \"Large Cost-effective OLED Microdisplays and their Applications\" (2015-2017) with eight European partners including industry the semiconductor physics group contributes with transparent oxide devices. A joint laboratory for single ion implantation was established between the Leibniz-Institute for Surface Modification (IOM) and the university under the guidance of Profs. Rauschenbach and Meijer. The EU IRSES Network DIONICOS \"Dynamics of and in Complex Systems\", a consortium of 6 European and 12 non-European partners, including sites in England, France and Germany as well as in Russia, Ukraine, India, the United States and Venezuela, started in February 2014. In the next four years the Leipzig node headed by Prof. Janke will profit from the numerous international contacts this network provides. With a joint project, Prof. Kroy and Prof. Cichos participate in the newly established priority research programme SPP 1726 \"Microswimmers\", which started with a kick-off workshop in October 2014. In 2014 the International Graduate College \"Statistical Physics of Complex Systems\" run by the computational physics group has commenced its third 3-years granting period funded by Deutsch-Französische Hochschule (DFH-UFA). Besides the main partner Université de Lorraine in Nancy, France, now also Coventry University, UK, and the Institute for Condensed Matter Physis of the National Academy of Sciences of Ukraine in Lviv, Ukraine, participate as associated partners. During the last week of September the TCO2014 conference \"Transparent Conductive Oxides – Fundamentals and Applications\" took place in honor of the 100th anniversary of the death of Prof. Dr. KarlW. Bädeker. In 1907 Karl Bädeker had discovered transparent conductive materials and oxides in Leipzig. About a hundred participants joined for many invited talks from international experts, intense discussion and new cooperations. At the end of November the by now traditional 15th nternational Workshop on Recent Developments in Computational Physics \"CompPhys14\" organized by Prof. Janke took place in Leipzig. Around 60 scientists from over 10 different countries exchanged ideas and discussed recent progress in several fields of computational physics. Work has successfully continued in the Centers of Excellence (Sonderforschungsbereiche) SFB 762 \"Functionality ofOxide Interfaces\" and SFB TRR 102 \"Polymers under Multiple Constraints: Restricted and Controlled Molecular Order and Mobility\" (just renewed for 2015-2019). Our activities and success are only possible with the generous support fromvarious funding agencies for which we are very grateful and which is individually acknowledged in the brief reports

Ab Initio Insights Into Substrate Effects, Structural Identification, and Excitonic States in 2D Materials

The 21st century has seen enormous growth in the study of two-dimensional (2D) materials, beginning with the isolation of graphene but rapidly expanding to include a wide variety of other compounds. Due to their size, 2D materials have immediate appeal for applications in nanoscale electronics. At the same time, uniquely low-dimensional phenomena such as the quantum spin Hall effect, quantum confinement, and 2D superconductivity are of interest to basic physics researchers. This dissertation presents ab initio investigations of three 2D materials. First, we discuss the binding of stanene on various substrates. Stanene, the buckled monolayer form of tin, is predicted to be a 2D topological insulator with symmetry-protected helical edge states. We investigate the effects of strain, chemical functionalization, and substrate–overlayer interactions on the topological band structure of stanene, showing that Al2O3 is an ideal substrate for synthesizing a potential quantum spin Hall insulator. Next, we examine the polymorphic structure of borophene sheets, the monolayer form of boron. We report on research that revealed the complex atomic structure of borophene on the Cu(111) and Cu(100) surfaces, including the crucial role played by simulated scanning tunneling microscopy (STM) data. We discuss the effect of modulation by the substrate on the occurrence of Dirac cones in the borophene band structure. Finally, we discuss the potential for Mg2TiO4 films to host long-lived, strongly bound interlayer excitons. At the DFT level, we obtain the band structure of Mg2TiO4 films grown on MgO and show how the polar films have a band offset favorable for interlayer exciton formation. Motivated by this work, we present ?? and ??-BSE calculations of quasiparticle energies, exciton binding energies, and optical absorption spectra. These calculations more clearly characterize the suite of excitons that exist in Mg2TiO4 and shed light on the importance of film thickness in controlling their relative binding energies. The materials studied in this dissertation are diverse in chemical identity and properties, but are unified by their 2D structure and the crucial role played by their growth substrates, which are discussed throughout

Novel dilute nitride semiconductor materials for mid-infrared applications

A new approach to room temperature mid-infrared applications in the 3 -5 µm spectral range has been proposed through the development of novel dilute nitride materials. InAsN shows a large bandgap reduction with a small lattice mismatch when a small amount of nitrogen is introduced into the host InAs. Because the InAsN reported so far had generally poor crystalline quality and exhibited weak luminescence that quenched far below room temperature due to poorly understood localised states, the present thesis investigated the optimisation of the MBE growth of InAsN. The nitrogen incorporation was shown to be inversely dependent on the growth temperature, the growth rate and the arsenic flux, as was interpreted by a kinetic model. Luminescence from InAsN material showed two peaks, originating in bandgap-and localised state-related transitions. The emission related to the bandgap undergoes hardly any energy shift with increasing laser excitation power while it redshifts with increasing temperature. The emission from the localised states blueshifts with increasing laser excitation power and temperature. Rapid thermal annealing on InAsN layers improved the photoluminescence intensity and blueshifted the transition energy. InAsN was successfully grown with high crystalline quality. It showed strong photoluminescence which persisted up to room temperature with a reduced influence of the localised states and wavelength up to 4.5 µm was achieved when 2.5 % N were incorporated. An explanation of the temperature and nitrogen incorporation dependences of the InAsN bandgap was successfully proposed using Varshni's equation and an adapted band anticrossing model, where the nitrogen level energy is dependent on temperature and nitrogen composition. The crystalline quality and luminescence were both further improved by incorporating antimony into InAsN and strain balance in material for 4.2 µm applications was achieved with the novel InAsNSb alloy. A method to determine the composition of the quaternary was derived from bandgap models and lattice constant equation and the addition of antimony in InAsN was shown to enhance the nitrogen incorporation in InAsNSb. Finally, attempts at prototype InAsN and InAsNSb devices were reported, InAsNSb LEDs were demonstrated and electroluminescence from InAsNSb diodes at 3.88 µm at 4K was obtained

Report / Institute für Physik

The 2014 Report of the Physics Institutes of the Universität Leipzig presents a hopefully interesting overview of our research activities in the past year. It is also testimony of our scientific interaction with colleagues and partners worldwide. We are grateful to our guests for enriching our academic year with their contributions in the colloquium and within the work groups. The open full professorship in the Institute for Experimental Physics I has been filled with an outstanding candidate. We could attract Prof. Ralf Seidel from the University of Münster. He is an expert in molecular biophysics that complements the existing strength in cellular biophysics. Prof. Hollands could fill all positions of his ERC Starting Grant, so that the work on the project \"Quantum Fields and Curvature – Novel Constructive Approach via Operator Product Expansion\" is now running at full pace. Within the Horizon 2020 project LOMID \"Large Cost-effective OLED Microdisplays and their Applications\" (2015-2017) with eight European partners including industry the semiconductor physics group contributes with transparent oxide devices. A joint laboratory for single ion implantation was established between the Leibniz-Institute for Surface Modification (IOM) and the university under the guidance of Profs. Rauschenbach and Meijer. The EU IRSES Network DIONICOS \"Dynamics of and in Complex Systems\", a consortium of 6 European and 12 non-European partners, including sites in England, France and Germany as well as in Russia, Ukraine, India, the United States and Venezuela, started in February 2014. In the next four years the Leipzig node headed by Prof. Janke will profit from the numerous international contacts this network provides. With a joint project, Prof. Kroy and Prof. Cichos participate in the newly established priority research programme SPP 1726 \"Microswimmers\", which started with a kick-off workshop in October 2014. In 2014 the International Graduate College \"Statistical Physics of Complex Systems\" run by the computational physics group has commenced its third 3-years granting period funded by Deutsch-Französische Hochschule (DFH-UFA). Besides the main partner Université de Lorraine in Nancy, France, now also Coventry University, UK, and the Institute for Condensed Matter Physis of the National Academy of Sciences of Ukraine in Lviv, Ukraine, participate as associated partners. During the last week of September the TCO2014 conference \"Transparent Conductive Oxides – Fundamentals and Applications\" took place in honor of the 100th anniversary of the death of Prof. Dr. KarlW. Bädeker. In 1907 Karl Bädeker had discovered transparent conductive materials and oxides in Leipzig. About a hundred participants joined for many invited talks from international experts, intense discussion and new cooperations. At the end of November the by now traditional 15th nternational Workshop on Recent Developments in Computational Physics \"CompPhys14\" organized by Prof. Janke took place in Leipzig. Around 60 scientists from over 10 different countries exchanged ideas and discussed recent progress in several fields of computational physics. Work has successfully continued in the Centers of Excellence (Sonderforschungsbereiche) SFB 762 \"Functionality ofOxide Interfaces\" and SFB TRR 102 \"Polymers under Multiple Constraints: Restricted and Controlled Molecular Order and Mobility\" (just renewed for 2015-2019). Our activities and success are only possible with the generous support fromvarious funding agencies for which we are very grateful and which is individually acknowledged in the brief reports

First Principles Studies of Magnetic Oxides, Spin-Driven Ferroelectricity, and the Effect of Polarization in the Chemistry of Functional Heterointerfaces

Achieving accurate description and understanding of the chemical and physical properties of complex materials enables the further development of their technological applications. Employing density functional theory (DFT) with rotationally invariant Hubbard corrections, we present an extensive study of binary manganese oxides modeling their noncollinear spin patterns and computing their electronic structures in agreement with experimental results. Leveraging on our success in predicting accurately magnetic properties, we explore the noncollinear cycloidal magnetic order in CaMn$_{7}$O$_{12}$, which breaks inversion symmetry generating one of the largest spin-driven ferroelectric polarizations measured to date. Based on a generalized spin-current model with Heisenberg-exchange and Dzyaloshinskii-Moriya interaction energetics we explain the microscopic origin of the polarization, including its direction, coupling to the spin helicity, charge density redistribution, and magnetic exchange interactions. Our experimental collaborators synthesize the proposed material, CaMn$_{7}$O$_{12}$, in films, reporting experimental evidence of its remarkable high temperature charge ordering phase transition and our atomistic insights through DFT calculations elucidate on the structural and electronic coupling of this phase transition. Symmetry breaking and chemical potential mismatch at an interface could lead to novel phenomena and multifunctional properties inaccessible in the bulk; therefore, interfacial engineering of functional heterostructure geometries could guide devices by design. We propose the functional interface between graphene and polydomain ferroelectrics as platform for novel field effect transistors. Here, we present both a theoretical understanding of how ferroelectric polarization direction affects the graphene carrier density and with help from our experimental collaborators we show evidence of our explanations. We predict that the graphene can be \emph{n}- or \emph{p}-type depending on the polarization direction and quantify the changes in carrier density. The functional complex oxide heterostructure, LaAlO$_{3}$/SrTiO$_{3}$ - known for the emergence of two-dimensional electron gas (2DEG) at the interface, is investigated to explain the experimental observation of a surface chemically switchable luminescence process after water treatment. We determine that water dissociates at the surface of LaAlO$_{3}$/SrTiO$_{3}$, which leads to protons on the surface cancelling the ``polar catastrophe and providing accessibility to optical transitions from the 2DEG system. Finally, motivated by the instability of organometal halide perovskites once exposed to water, we study the interaction of water with the (001) surfaces of CH$_{3}$NH$_{3}$PbI$_{3}$ under low and high water concentrations concluding that the orientation of the dipole from the methylammonium molecules heavily influenced the surface interaction with water

Antenna near-field wave studies in magnetized plasmas as part of the optimization of auxiliary heating in fusion devicesAntenna near-field wave studies in magnetized plasmas as part of the optimization of auxiliary heating in fusion devices

C

Report / Institute für Physik

The 2016 Report of the Physics Institutes of the Universität Leipzig presents a hopefully interesting overview of our research activities in the past year. It is also testimony of our scientific interaction with colleagues and partners worldwide. We are grateful to our guests for enriching our academic year with their contributions in the colloquium and within our work groups