Investigation of Two-Dimensional Electron Gases with Angular Resolved Photoemission Spectroscopy

Isabella Gierz
EPFL, 2011
EPFL thesis

Abstract

In this thesis we study the electronic structure of different two-dimensional (2D) electron systems with angular resolved photoemission spectroscopy (ARPES). This technique is based on the photoelectric effect and directly probes the electronic structure of a system. By carefully analyzing the measured band structure with respect to peak position and line width we can determine the complex self-energy Σ that describes the renormalization of the electron's energy and the change in lifetime due to many-body interactions. The 2D electron systems investigated in this work are surface alloys on Ag(111), bismuth trimers on Si(111) and epitaxial graphene monolayers grown on SiC(0001). Surface alloys on Ag(111) are formed by depositing 1/3 of a monolayer of bismuth, lead or antimony (alloy atoms) on the clean silver surface. Although (Bi,Pb,Sb) and Ag atoms are immiscible in the bulk they form long-range ordered surface alloys, where every third Ag atom is replaced by an alloy atom. These systems as well as the Bi trimers on Si(111) show a spin splitting of the 2D band structure due to the Rashba-Bychkov (RB) effect. The RB model states that in a symmetry broken environment (such as the surface of a semi-infinite crystal) the spin-orbit interaction will lift the spin-degeneracy of the band structure. Such a spin-split band structure bares great potential for applications in the field of spintronics, e.g. in a Datta-Das spin field effect transistor. In the present work we investigate the origin of the observed giant spin splitting in surface alloys, especially the interplay between structural parameters and the atomic spin-orbit interaction. Furthermore, we will show that it is possible to transfer these concepts to a semiconducting substrate, which is better suited for spintronics applications. The third system under investigation — graphene — is an ideally two-dimensional crystal. It consists of a single layer of carbon atoms arranged in a honeycomb lattice, and its charge carriers are confined within a plane that is just one atom thick. These charge carriers behave like massless Dirac particles and possess extremely high carrier mobilities. This makes graphene a promising material system for high-speed electronic devices. In order to reach this ambitious goal one needs reliable methods for the large-scale production of high quality graphene films. Epitaxial growth on silicon carbide (0001) substrates is the method of choice in this case, as it offers the advantage of a precise thickness control and a semiconducting substrate at the same time. However, the presence of the substrate reduces the carrier mobility of graphene's charge carriers considerably. Therefore, it is necessary to decouple the graphene layer from the substrate after epitaxial growth. A second issue that needs to be addressed, are viable doping methods for graphene. As graphene's peculiar band structure results from a sensible interplay between electrons and crystal lattice it is not an option to replace single atoms of the graphene lattice by dopants as is common practice when doping silicon. In order to preserve its band structure, graphene is usually doped by adsorbing atoms or molecules on its surface. As graphene grown on SiC is n-doped due to charge transfer from the substrate, appropriate means for p-type doping are clearly required. In this thesis, we will present a new growth method for quasi free-standing graphene on SiC(0001) and viable means for p-type doping.

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Isabella Gierz
EPFL, 2011
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/162116?ln=en

About this result

Related concepts (26)

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Related publications (49)

Related publications

In this thesis, angular resolved photoemission spectroscopy (ARPES) is used to study the electronic structure of different two-dimensional electron systems (2DES). This technique is very surface sensitive and the most direct method to probe the surface band structure of a system. In addition to the band dispersion also signatures from interactions can be detected with ARPES. A careful analysis of peak position and linewidth gives access to the complex self-energy Sigma, which describes the energy renormalization and the lifetime of the electrons due to many-body interactions. By using circularly polarized light it is possible to excite electrons selectively and to extract additional information about the electronic states in the surface. 2DESs exist typically either at interfaces or at surfaces. We put our focus on the surface states of topological insulators and investigate Bi2Te2Se and TiTe1.5Se0.5. The first is known to be a topological insulator with the Fermi energy located in the bulk band gap. A time-dependent doping behavior is observed and we show that the Coulomb interaction in this material is strongly dependent on the charge carrier density and not negligible for very low concentrations. TiTe1.5Se0.5 was predicted to be a topological insulator. We are able to present the surface band structure, but due to the fact that ARPES can only probe the occupied states it is impossible to see the predicted topological surface states. The third system under investigation is a monolayer of epitaxial graphene on SiC(0001) decorated by small amounts of Tl adatoms. ARPES measurements at very low temperature and very low impurity concentrations show that Tl adatoms not only give rise to electron doping but have also a large effect on the quasiparticle scattering rate. The adatoms introduce disorder and act on the graphene electronic structure both as Coulomb long-range scatterers as well as short-range scatterers with a delta-like potential. We show that also for charged impurities short-range scattering can play an important role and is not always negligible. By modeling the self-energy for long- and short-range scattering, we are able to extract a strong short-range scattering potential delta=-3.2 +/- 1 eV. A detailed understanding of the underlying scattering mechanisms in graphene is very important for the development of novel impurity-graphene-based electronics. ARPES experiments are also performed on the surface alloys on Ag(111). For these measurements, we use circularly polarized light and take advantage of the fact that the interaction between light and matter changes for different polarizations. Surface alloys on Ag(111) are formed by depositing 1/3 of a monolayer of bismuth or antimony on the clean Ag(111) surface, where every third Ag atom is replaced by such an alloy atom. These systems are known for their Rashba type spin splitting and the resulting chiral spin structure in the two-dimensional (2D) band structure. We show that circular dichroism (CD) in the angular distribution of the photoelectrons depends strongly on the experimental setup and the energy of the photons. Only under special circumstances it is possible to use CD to access the orbital angular momentum and - in systems with high spin-orbit coupling - maybe even the spin texture of the surface. Because of the final state effects, interpretation of CD data is quite complicated and has to be done very cautiously.

EPFL2015

At the surface of a solid, quantum mechanics allows the existence of two-dimensional Bloch waves as solutions of the Schrödinger equation. These "surface" states are interesting both for fundamental and practical reasons, and have been extensively explored, e.g., on pure metals. Conversely, relatively few studies exist of the electronic states of artificially-grown two-dimensional structures. This work is an investigation, mainly by angle-resolved photoemission, of different heterogeneous surfaces of defined stoichiometry, formed by a large-Z p metal on a noble metal substrate, which constitute what is called a "surface alloy". A breakthrough has been obtained on the Bi/Ag(111) (√3 × √3)R30° structure, which presents a band dispersion split into two spin polarized replicas in momentum space by the spin-orbit coupling. The effect is known as Rashba-Bychkov splitting in semiconductors, and it is a genuine effect of the symmetry breaking occurring at the surface, but in Bi/Ag(111) it is much larger than reported in any other compound. This sets this class of materials among the suitable candidates for spintronics applications, which have been object of rising interest in the last years. We have put our attention on other surface alloys, varying substrates (Ag(111) and Cu(111)) and adsorbates (Bi, Pb, Sb) in order to set the basis for a unifying description of the quantities involved in the determination of the size of the splitting, and in particular to discriminate between atomic and structural contributions. An analogous band structure is found in all the systems, consisting namely of two "sets" of bands with negative effective mass, originating from the hybridization between the surface states of the substrate and the p electrons of the adsorbate. We have identified a significant in-plane component of the electric potential gradient, which is not present in pure metal surfaces, as the new element most likely responsible for the large size of the splitting. At the same time, the results show that a necessary condition for it to be effective in enhancing the splitting is a strong atomic spin-orbit parameter. Our findings are confirmed by first-principle calculations, which reproduce quantitatively the band structure, and by a nearly free electron model, which qualitatively supports our interpretation of the role of the in-plane gradient. A parallel project of this thesis has concerned Yb-based Kondo systems, as studied by hard x-ray photoemission and inelastic x-ray scattering. At these high energies, photoemission probes a thicker region below the sample surface, where heavy-fermion compounds stabilize in a different electronic configuration with respect to that of the bulk. We have shown that the temperature dependence of the 4f spectral function is in line with the predictions of the Anderson impurity model, a point which has often been questioned on the basis of previous results of low-energy photoemission. On the other hand, by a comparison with inelastic scattering, we have noticed that the latter leads to a better agreement with the data from thermodynamic and transport measurements. We conclude that, despite the unique capability of photoemission of accessing directly the electron spectral function, photon-in–photon-out spectroscopies are a more trustworthy probe of the bulk electronic structure.

EPFL2009

Organic semiconductor materials have been widely applied in optoelectronic devices to replace their inorganic counterparts and explore new fields of applications. Due to their high extinction coefficients, chemical tunability and solubility, cyanine dyes are an interesting class of molecules to be employed in organic semiconductor devices. Their beneficial properties have been demonstrated in organic solar cells (OSC), photodetectors and recently in organic light-emitting electrochemical cells (OLEC). The focus of this thesis is to analyse different aspects regarding the dynamics of electronic and ionic charges in cyanine based devices. Models and conclusions are not restricted to cyanine based devices but can potentially be applied in various systems where combined ionic and electronic charges are present. In order to focus on electronic processes in OSCs, a combination of steady-state, transient and impedance analysis is used. It is resolved that trapped charges can be introduced at the dye/hole extracting layer interface by using specific solvents for the cyanine layer fabrication. These measurements also reveal the low hole mobility of cyanine films which in turn limits the possible upper layer thickness in OSCs. A frequently used way to enhance the conductivity of organic semiconductors is the addition of extra charges by chemical doping. This concept is employed for a near-infrared absorbing dye which is used as donor in a bilayer solar cell. The dependence of all figures of merit versus doping concentration and layer thickness is studied in detail. It is found that the performance of the optimised cell is not further improved by doping, but that it presents a good option if thicker layers are required, e.g. on rough substrates. Furthermore, the stability and degradation pathway of the p-doped species is analysed. This information is of general relevance as they also occur after charge transfer at the heterojunction interface of an OSC and during operation of OLECs. The dynamics of ionic charge carriers is separated into two contributions. In a first step, the steady-state diffusion profile of the counter ion within a cyanine/fullerene bilayer structure is scrutinised using different profiling techniques. An unexpected constant ion profile is found throughout the C60 layer. Simultaneously, the potential within the acceptor is measured by the Kelvin probe technique. A mechanism of combined electron and ion transfer from the C60 to the dye and vice versa is proposed which is in agreement with the experimental results. A prerequisite for OLEC operation is the mobility of ionic charge carriers. Their dynamics in cyanine salts under the effect of an external field is therefore studied in such devices. Several methods are developed and applied to characterise the dynamic p-i-n structure in detail, i.e. the position and width of the intrinsic region as well as the doping concentrations in p- and n-doped regions. The combination of all these methods helps to understand the transient behaviour of these cyanine devices but can also be adapted to other OLEC systems. A major drawback of cyanine dyes as emissive material is their low fluorescence quantum yield in solid films. To overcome this restriction and improve the OLEC efficiency, host-guest systems with various visibly emitting dyes are studied. The gain in fluorescence quantum yield in the film can directly be transferred to an increase in external quantum efficiency of the device.

EPFL2017

EPFL2009

EPFL2017

EPFL2015