@article{FiedlerElKarehEremeevetal.2014,
  author    = {Fiedler, Sebastian and El-Kareh, Lydia and Eremeev, Sergey V. and Tereshchenko, Oleg E. and Seibel, Christoph and Lutz, Peter and Kokh, Konstantin A. and Chulkov, Evgueni V. and Kuznetsova, Tatyana V. and Grebennikov, Vladimir I. and Bentmann, Hendrik and Bode, Matthias and Reinert, Friedrich},
  title     = {Defect and structural imperfection effects on the electronic properties of BiTeI surfaces},
  series = {New Journal of Physics},
  volume    = {16},
  journal   = {New Journal of Physics},
  number    = {075013},
  issn      = {1367-2630},
  doi       = {10.1088/1367-2630/16/7/075013},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-119467},
  year      = {2014},
  abstract  = {The surface electronic structure of the narrow-gap seminconductor BiTeI exhibits a large Rashba-splitting which strongly depends on the surface termination. Here we report on a detailed investigation of the surface morphology and electronic properties of cleaved BiTeI single crystals by scanning tunneling microscopy, photoelectron spectroscopy (ARPES, XPS), electron diffraction (SPA-LEED) and density functional theory calculations. Our measurements confirm a previously reported coexistence of Te- and I-terminated surface areas originating from bulk stacking faults and find a characteristic length scale of ~100 nm for these areas. We show that the two terminations exhibit distinct types of atomic defects in the surface and subsurface layers. For electronic states resided on the I terminations we observe an energy shift depending on the time after cleavage. This aging effect is successfully mimicked by depositon of Cs adatoms found to accumulate on top of the I terminations. As shown theoretically on a microscopic scale, this preferential adsorbing behaviour results from considerably different energetics and surface diffusion lengths at the two terminations. Our investigations provide insight into the importance of structural imperfections as well as intrinsic and extrinsic defects on the electronic properties of BiTeI surfaces and their temporal stability.},
  language  = {en}
}
@article{ElKarehBihlmayerBuchteretal.2014,
  author    = {El-Kareh, Lydia and Bihlmayer, Gustav and Buchter, Arne and Bentmann, Hendrik and Bl{\"u}gel, Stefan and Reinert, Friedrich and Bode, Matthias},
  title     = {A combined experimental and theoretical study of Rashba-split surface states on the ( √3x√3) Pb/Ag (111)R30° surface},
  doi       = {doi:10.1088/1367-2630/16/4/045017},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-112786},
  year      = {2014},
  abstract  = {We report on a combined low-temperature scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) investigation of the ( √3x√3) Pb/Ag (111)R30° surface alloy which provides a giant Rashba-type spin splitting. With STS we observed spectroscopic features that are assigned to two hole-like Rashba-split bands in the unoccupied energy range. By means of STS and quantum interference mapping we determine the band onsets, splitting strengths, and dispersions for both bands. The unambiguous assignment of scattering vectors is achieved by comparison to ARPES measurements. While intra-band scattering is found for both Rashba bands, inter-band scattering is only observed in the occupied energy range. Spin- and orbitally-resolved band structures were obtained by DFT calculations. Considering the scattering between states of different spin- and orbital character, the apparent deviation between experimentally observed scattering events and the theoretically predicted spin polarization could be resolved.},
  language  = {en}
}
@phdthesis{Volkmann2004,
  author    = {Volkmann, Thorsten},
  title     = {Lattice gas models and simulations of epitaxial growth},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-13812},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2004},
  abstract  = {In this PhD thesis, we develop models for the numerical simulation of epitaxial crystal growth, as realized, e.g., in molecular beam epitaxy (MBE). The basic idea is to use a discrete lattice gas representation of the crystal structure, and to apply kinetic Monte Carlo (KMC) simulations for the description of the growth dynamics. The main advantage of the KMC approach is the possibility to account for atomistic details and at the same time cover MBE relevant time scales in the simulation. In chapter 1, we describe the principles of MBE, pointing out relevant physical processes and the influence of experimental control parameters. We discuss various methods used in the theoretical description of epitaxial growth. Subsequently, the underlying concepts of the KMC method and the lattice gas approach are presented. Important aspects concerning the design of a lattice gas model are considered, e.g. the solid-on-solid approximation or the choice of an appropriate lattice topology. A key element of any KMC simulation is the selection of allowed events and the evaluation of Arrhenius rates for thermally activated processes. We discuss simplifying schemes that are used to approximate the corresponding energy barriers if detailed knowledge about the barriers is not available. Finally, the efficient implementation of the MC kinetics using a rejection-free algorithm is described. In chapter 2, we present a solid-on-solid lattice gas model which aims at the description of II-VI(001) semiconductor surfaces like CdTe(001). The model accounts for the zincblende structure and the relevant surface reconstructions of Cd- and Te-terminated surfaces. Particles at the surface interact via anisotropic nearest and next nearest neighbor interactions, whereas interactions in the bulk are isotropic. The anisotropic surface interactions reflect known properties of CdTe(001) like the small energy difference between the c(2x2) and (2x1) vacancy structures of Cd-terminated surfaces. A key element of the model is the presence of additional Te atoms in a weakly bound Te* state, which is motivated by experimental observations of Te coverages exceeding one monolayer at low temperatures and high Te fluxes. The true mechanism of binding excess Te to the surface is still unclear. Here, we use a mean-field approach assuming a Te* reservoir with limited occupation. In chapter 3, we perform KMC simulations of atomic layer epitaxy (ALE) of CdTe(001). We study the self-regulation of the ALE growth rate and demonstrate how the interplay of the Te* reservoir occupation with the surface kinetics results in two different regimes: at high temperatures the growth rate is limited to one half layer of CdTe per ALE cycle, whereas at low enough temperatures each cycle adds a complete layer. The temperature where the transition between the two regimes occurs depends mainly on the particle fluxes. The temperature dependence of the growth rate and the flux dependence of the transition temperature are in good qualitative agreement with experimental results. Comparing the macroscopic activation energy for Te* desorption in our model with experimental values we find semiquantitative agreement. In chapter 4, we study the formation of nanostructures with alternating stripes during submonolayer heteroepitaxy of two different adsorbate species on a given substrate. We evaluate the influence of two mechanisms: kinetic segregation due to chemically induced diffusion barriers, and strain relaxation by alternating arrangement of the adsorbate species. KMC simulations of a simple cubic lattice gas with weak inter-species binding energy show that kinetic effects are sufficient to account for stripe formation during growth. The dependence of the stripe width on control parameters is investigated. We find an Arrhenius temperature dependence, in agreement with experimental investigations of phase separation in binary or ternary material systems. Canonical MC simulations show that the observed stripes are not stable under equilibrium conditions: the adsorbate species separate into very large domains. Off-lattice simulations which account for the lattice misfit of the involved particle species show that, under equilibrium conditions, the competition between binding and strain energy results in regular stripe patterns with a well-defined width depending on both misfit and binding energies. In KMC simulations, the stripe-formation and the experimentally reported ramification of adsorbate islands are reproduced. To clarify the origin of the island ramification, we investigate an enhanced lattice gas model whose parameters are fitted to match characteristic off-lattice diffusion barriers. The simulation results show that a satisfactory explanation of experimental observations within the lattice gas framework requires a detailed incorporation of long-range elastic interactions. In the appendix we discuss supplementary topics related to the lattice gas simulations in chapter 4.},
  subject      = {Kristallwachstum},
  language  = {en}
}
@phdthesis{Wagner2003,
  author    = {Wagner, Joachim},
  title     = {Optische Charakterisierung von II-VI-Halbleiter-Oberfl{\"a}chen in Kombination mit First-Principles-Rechnungen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-8722},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2003},
  abstract  = {In dieser Arbeit sind Methoden der optischen Spektroskopie, insbesondere die Ramanspektroskopie (RS) und die Reflexions-Anisotropie-Spektroskopie (RAS), angewandt worden, um die Oberfl{\"a}chen von II-VI Halbleitern zu charakterisieren. F{\"u}r die experimentellen Untersuchungen wurde eine eigens f{\"u}r diesen Zweck entwickelte UHV-Optikkammer benutzt. Diese einzigartige M{\"o}glichkeit, II-VI Halbleiterproben aus einer state-of-the-art MBE-Anlage mit einer UHV-Optikanlage zu kombinieren hat gezeigt, dass optische Spektroskopie sehr gut daf{\"u}r geeignet ist, strukturelle Eigenschaften, z.B. Rekonstruktionen, und chemische Bindungen an Oberfl{\"a}chen, sowie die damit verbundene Schwingungsdynamik zu analysieren. Neben den experimentellen Arbeiten wurden u. a. first principles Rechnungen mittels der Dichtefunktionaltheorie im Rahmen der Lokalen-Dichte-Approximation durchgef{\"u}hrt. Damit konnten f{\"u}r die Oberfl{\"a}chen einerseits ihre geometrischen Eigenschaften, d.h die atomare Anordnung der Oberfl{\"a}chenatome, und andererseits auch ihre Dynamik, d.h. die Schwingungsfrequenzen und die Auslenkungsmuster der an der Rekonstruktion beteiligten Atome der Oberfl{\"a}che und der oberfl{\"a}chennahen Schichten, im Rahmen der Frozen-Phonon-N{\"a}herung bestimmt werden. Die Kombination von experimenteller und theoretischer Vibrationsbestimmung von Oberfl{\"a}chen bietet also, neben den klassischen Oberfl{\"a}chen-Analysemethoden wie RHEED, LEED, XPS, Auger und SXRD, ein zus{\"a}tzliches Werkzeug zur Charakterisierung von Oberfl{\"a}chen. Da die Frozen-Phonon-N{\"a}herung nicht elementarer Bestandteil des hier benutzten DFT-Programmcodes fhi96md ist, wurde diese Erweiterung im Rahmen dieser Arbeit durchgef{\"u}hrt. Die theoretische Berechnung von Schwingungsfrequenzen mit dynamischen Matrizen ist in einem Unterkapitel dargestellt. Die so berechneten Schwingungsfrequenzen f{\"u}r verschiedene Oberfl{\"a}chen-Rekonstruktionen konnten erfolgreich am Beispiel der reinen BeTe(100)-Oberfl{\"a}che mit den experimentell mit der UHV-Ramanspektroskopie beobachteten Frequenzen verglichen werden. So gelang erstmalig die optische identifizierung von rekonstruktionsinduzierten Eigenschwingungen einer Oberfl{\"a}che. Nach detaillierter Kenntnis der BeTe(100)-Oberfl{\"a}che wurde die Ramanspektroskopie als Sonde benutzt, um die Entwicklung der BeTe-Oberfl{\"a}che bei unterschiedlichen Behandlungen (Modifikation) zu verfolgen. Dabei dienten die fr{\"u}heren Ergebnisse als Referenzpunkte, um die modifizierten Spektren zu erkl{\"a}ren. Zus{\"a}tzlich wurde ein Konzept zur Passivierung der Te-reichen BeTe(100)-Oberfl{\"a}che entwickelt, um diese Proben ohne einen technisch aufwendigen UHV-Transportbeh{\"a}lter {\"u}ber gr{\"o}ssere Entfernungen transportieren zu k{\"o}nnen (z.B. zu Experimenten an einem Synchrotron). Mit der RAS wurden auch die Oberfl{\"a}chen von weiteren Gruppe II-Telluriden, n{\"a}mlich die Te-reiche (2x1) CdTe(100)-Oberfl{\"a}che, die Te-reiche (2x1) MnTe(100)-Oberfl{\"a}che und die Hg-reiche c(2x2) HgTe(100)-Oberfl{\"a}che untersucht. Schließlich wurde der Wachstumsstart von CdSe auf der BeTe(100)-Oberfl{\"a}che im Bereich weniger Monolagen (1-5 ML) CdSe analysiert, wobei die hohe Empfindlichkeit der Ramanspektroskopie bereits den Nachweis einer Monolage CdSe erlaubte.},
  subject      = {Zwei-Sechs-Halbleiter},
  language  = {de}
}