Growth, Morphology, and Conductivity in Semimetallic/Metallic Films on Si(001)

This dissertation deals with the study of epitaxial growth of semimetallic (Bi) and metallic (Ag) films on Si(001) as well as in situ electrical transport study of those films via surface manipulation. The focus of the transport measurements is to study the influence of the surface morphology or structure on the resistance of the film. The most important challenge is the preparation of high quality films with well-defined morphology under ultra high vacuum conditions. In spite of the large lattice mismatch and different lattice geometry, it is possible to grow epitaxial Bi(111) films on Si(001) substrates, which are surprisingly smooth, relaxed and almost free of defects. Due to the two-fold symmetry of the substrates, the Bi(111) film is composed of crystallites rotated by 90 with respect to each other. Annealing of 6 nm film from 150 K to 450 K enables the formation of a periodic interfacial misfit dislocations, which accommodates a remaining lattice mismatch of 2.3 %. The surface/interface roughness and the bulk defect density of the film found to be extremely low, indicating the high crystalline quality of the film with atomically smooth surface and abrupt interface. Similar to the Bi films, Ag grows in a (111) orientation on Si(001) with two 90 rotated domains. The remaining strain of 2.2 % (tensile) is accommodated by the formation of an ordered network of dislocations. The Ag film exhibits atomically smooth surface. Those Bi films and Ag films were used as model systems to study the influence of the surface morphology on the electrical resistance. Surprisingly, all the Bi films (3 - 170 nm thicknesses) have shown an anomalous behavior of conductance with temperature and thickness. As in the case of doped semiconductor, the conductance increases exponentially from 150 K to 300 K and saturates at 350 K before finally decreasing with temperature. This behavior hints to the long predicted semimetal-to-semiconductor transition in the Bi films. However, the thickness dependent conductance behavior agrees with a previously observed metallic surface state, because the conductance does not change with thickness at 80 K. In situ measurements of the resistance during additional Bi deposition on the smooth Bi(111) films exhibit a square root dependent with coverage after a linear increase at very low coverage (1 % of a BL). Due to the extreme electronic properties of Bi, such as a large Fermi wavelength and a large electron mean free path, this behavior is not supported by the classical electron scattering model of Fuchs-Sondheimer. Since the Bi(111) surface state possesses two orders of magnitude higher number of carriers than in the bulk, the surface acts as a dominant channel of electron transport. During additional deposition of Bi, carriers are scattered at the adatoms and small islands, resulting in dramatic increase of surface resistance. Experimental results of nucleation and growth behavior at initial stages in Bi(111) homoepitaxy and the concept of 2D metallic surface states allow to explain the square root dependent of the resistance with coverage. Additionally, from the initial rise of resistance, a 2D surface state conductivity was determined, assuming that the surface states are completely destroyed after additional 0.5 BL Bi deposition. Applying the Boltzmann equation, the scattering mean free path at the 2D surface states was roughly estimated to be 15 nm. The situation becomes much simpler in the case of the resistance behavior during deposition of Ag on a smooth Ag(111) film. The Fuchs-Sondheimer model works quite well and qualitatively demonstrate the increase of film resistance due to the diffuse scattering caused by surface roughness. Furthermore, in a complex situation such as surface alloying via Au deposition on Ag films, the resistance increases dramatically even at room temperature, suggesting that the scattering efficiency in this case is even higher than the case of normal surface roughness.In dieser Dissertation wird das epitaktische Wachstum dünner halbmetallischer Wismut- bzw. metallischer Silberfilme auf Si(001) sowie der elektrische Transport durch diese Filme untersucht. Im Fokus der Transportmessungen steht die Untersuchung des Einflusses von Oberflächenmorphologie bzw. -struktur auf die Leitfähigkeit. Hierfür sind Filme mit hoher Kristallqualität unter Ultrahochvakuum-Bedingungen eine entscheidende Voraussetzung. Trotz des großen Unterschieds der beiden Gitterkonstanten und der verschiedenen Gittergeometrie ist es möglich, Bi(111) Filme auf Si(001) Substrate aufzuwachsen, die überraschend glatt, entspannt und praktisch frei von Defekten sind. Bedingt durch die 2-zählige Symmetrie des Substrates besteht der Bi(111) Film aus Mikrometer großen (111) Kristalliten, die jeweils um 90° gegeneinander verdreht sind. Eine verbleibende Gitterfehlanpassung von 2.3% wird durch die Ausbildung eines periodischen Netzwerks von Versetzungen an der Grenzfläche angepasst. Die Oberflächen- bzw. Grenzflächenrauhigkeit und die Volumendefektdichte des Films sind extrem gering, was die hohe Kristallgüte des Films mit einer atomar glatten Oberfläche und einer abrupten Grenzfläche widerspiegelt. Ähnlich wie für Wismut wachsen auch Silber-Filme mit einer (111) Orientierung und um jeweils 90° gedrehten Domänen auf Si(001). Auch hier passt ein Netzwerk von Versetzungen eine verbleibende Gitterfehlanpassung von 2.2% an. Auch hier ist der Silberfilm atomar glatt. Diese Silber- und Wismut-Filme wurden dann als Modellsystem für die Untersuchung des Einflusses der Oberflächenmorphologie auf den elektrischen Widerstand verwendet. Überraschenderweise zeigten alle Wismutfilme (3 - 170 nm Dicke) ein anomales Verhalten als Funktion der Schichtdicke und Temperatur. Wie im Fall eines dotierten Halbleiters stieg die Leitfähigkeit zwischen 150 und 300 K exponentiell an, zeigte ein Plateau bei 350 K bevor sie mit steigender Temperatur abfiel. Dieses Verhalten ließe sich mit dem lang vorhergesagten Halbleiter/Halbmetallübergang für dünne Bi-Filme erklären. Die zusätzlich beobachtete schichtdickenunabhängige Leitfähigkeit bei 80 K ist mit einem in der Literatur beschriebenen metallischen Oberflächenzustand zu erklären. Bei weiterem Bedampfen mit Wismut werden die Ladungsträger an den dabei entstehenden Wismutinseln gestreut und eine Erhöhung des Widerstands beobachtet, die nach einem linearen Anstieg eine wurzelförmige Abhängigkeit von der zusätzlichen Bedeckung aufweist. Auf Grund der besonderen elektronischen Eigenschaften von Wismut - wie eine große Fermiwellenlänge und eine große freie Weglänge der Elektronen - kann dieses Verhalten nicht durch ein klassisches Streumodell nach Fuchs-Sondheimer erklärt werden. Da jedoch der metallische Oberflächenzustand von Bi(111) bis zu zwei Größenordnungen mehr Ladungsträger aufweist wie das Filmvolumen stellt dieser den dominanten elektronischen Transportkanal dar. Bei weiterem Aufdampfen von Wismut bei 80 K werden die Elektronen an den isolierten Adatomen bzw. 2dim. Inseln gestreut, was einen deutlichen Anstieg des Widerstands bewirkt. Mit einer STM Analyse der Inseldichte und -größe in diesem Wachstumsbereich konnte der beobachtete wurzelförmige Anstieg des Oberflächenwiderstands erklärt werden. Unter der Annahme, dass der Oberflächenzustand nach Aufdampfen einer halben Atomlage Wismut vollständig zerstört ist, konnte die Leitfähigkeit im Oberflächenzustand auch quantitativ bestimmt werden. Die mittlere freie Streulänge für Transport im Oberflächenzustand konnte zu 15 nm bestimmt werden. Die Erklärung des Widerstandsanstiegs bei der Abscheidung weiteren Silbers auf ultradünne Silberfilme bei tiefen Temperaturen ist wesentlich einfacher. Die Beschreibung über Fuchs-Sondheimer erklärt den durch das Aufrauhen der Oberfläche bewirkten Widerstandsanstieg qualitativ sehr gut. Im Falle der Abscheidung von Gold auf die Silberfilme bewirkt die Oberflächenlegierung einen auch bei Zimmertemperatur deutlichen Anstieg des Widerstands. Offensichtlich ist die Streueffizienz in diesem Fall deutlich höher als für rauhe Oberflächen ist

Critical behavior of the dimerized Si(001) surface: A continuous order-disorder phase transition in the 2D Ising universality class

The critical behavior of the order-disorder phase transition in the buckled dimer structure of the Si(001) surface is investigated both theoretically by means of first-principles calculations and experimentally by spot profile analysis low-energy electron diffraction (SPA-LEED). We use density functional theory (DFT) with three different functionals commonly used for Si to determine the coupling constants of an effective lattice Hamiltonian describing the dimer interactions. Experimentally, the phase transition from the low-temperature $c(4 {\times} 2)$- to the high-temperature $p(2 {\times} 1)$-reconstructed surface is followed through the intensity and width of the superstructure spots within the temperature range of 78-400 K. Near the critical temperature $T_\mathrm{c} = 190.6$ K, we observe universal critical behavior of spot intensities and correlation lengths which falls into the universality class of the two-dimensional (2D) Ising model. From the ratio of correlation lengths along and across the dimer rows we determine effective nearest-neighbor couplings of an anisotropic 2D Ising model, $J_\parallel = (-24.9 \pm 0.9_\mathrm{stat} \pm 1.3_\mathrm{sys})$ meV and $J_\perp = (-0.8 \pm 0.1_\mathrm{stat})$ meV. We find that the experimentally determined coupling constants of the Ising model can be reconciled with those of the more complex lattice Hamiltonian from DFT when the critical behavior is of primary interest. The anisotropy of the interactions derived from the experimental data via the 2D Ising model is best matched by DFT calculations using the PBEsol functional. The trends in the calculated anisotropy are consistent with the surface stress anisotropy predicted by the DFT functionals, pointing towards the role of surface stress reduction as a driving force for establishing the $c(4 {\times} 2)$-reconstructed ground state

Graphene-Complex-oxide Nanoscale Device Concepts

The integration of graphene with complex-oxide heterostructures such as LaAlO$_3$/SrTiO$_3$ offers the opportunity to combine the multifunctional properties of an oxide interface with the electronic properties of graphene. The ability to control interface conduction through graphene and understanding how it affects the intrinsic properties of an oxide interface are critical to the technological development of novel multifunctional devices. Here we demonstrate several device archetypes in which electron transport at an oxide interface is modulated using a patterned graphene top gate. Nanoscale devices are fabricated at the oxide interface by conductive atomic force microscope (c-AFM) lithography, and transport measurements are performed as a function of the graphene gate voltage. Experiments are performed with devices written adjacent to or directly underneath the graphene gate. Unique capabilities of this approach include the ability to create highly flexible device configurations, the ability to modulate carrier density at the oxide interface, and the ability to control electron transport up to the single-electron-tunneling regime, while maintaining intrinsic transport properties of the oxide interface. Our results facilitate the design of a variety of nanoscale devices that combine unique transport properties of these two intimately coupled two-dimensional electron systems.Comment: 27 pages, 10 figure

Ultrafast Photoinduced Band Splitting and Carrier Dynamics in Chiral Tellurium Nanosheets

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.Comment: 42 pages, 13 figure

Interplay between Forward and Backward Scattering of Spin–Orbit Split Surface States of Bi(111)

The electronic structure at the surface of Bi(111) enables us to study the effect of defects scattering into multiple channels. By performing scanning tunneling spectroscopy near step edges, we analyze the resulting oscillations in the local density of electronic states (LDOS) as function of position. At a given energy, forward and backward scattering not only occur simultaneously but may contribute to the same scattering vector Δ<b>k</b>. If the scattering phase of both processes differs by π and the amplitudes are almost equal, the oscillations cancel out. A sharp dip in the magnitude of the Fourier transform of the LDOS marks the crossover between forward and backward scattering channels

Observation of Ground- and Excited-State Charge Transfer at the C₆₀/Graphene Interface

We examine charge transfer interactions in the hybrid system of a film of C₆₀ molecules deposited on single-layer graphene using Raman spectroscopy and Terahertz (THz) time-domain spectroscopy. In the absence of photoexcitation, we find that the C₆₀ molecules in the deposited film act as electron acceptors for graphene, yielding increased hole doping in the graphene layer. Hole doping of the graphene film by a uniform C₆₀ film at a level of 5.6 × 10¹²/cm² or 0.04 holes per interfacial C₆₀ molecule was determined by the use of both Raman and THz spectroscopy. We also investigate transient charge transfer occurring upon photoexcitation by femtosecond laser pulses with a photon energy of 3.1 eV. The C₆₀/graphene hybrid exhibits a short-lived (ps) decrease in THz conductivity, followed by a long-lived increase in conductivity. The initial negative photoconductivity transient, which decays within 2 ps, reflects the intrinsic photoresponse of graphene. The longer-lived positive conductivity transient, with a lifetime on the order of 100 ps, is attributed to photoinduced hole doping of graphene by interfacial charge transfer. We discuss possible microscopic pathways for hot carrier processes in the hybrid system