Doctor of Philosophy

dissertationThis dissertation explored and developed technologies for silicon based spin lattice devices. Spin lattices are artificial electron spin systems with a periodic structure having one to a few electrons at each site. They are expected to have various magnetic and even superconducting properties when structured at an optimal scale with a specific number ѵ of electrons. Silicon turns out to be a very good material choice in realizing spin lattices. A metal-oxide-semi conductor field-effect nanostructure (MOSFENS) device, which is closely related to a MOS transistor but with a nanostructured oxide-semi conductor interface, can define the spin lattices potential at the interface and alter the occupation ѵ with the gate electrode potential to change the magnetic phase. The MOSFENS spin lattices engineering challenge addressed in this work has come from the practical difficulty of process integration in modifying a transistor fabrication process to accommodate the interface patterning requirements. Two distinct design choices for the fabrication sequences that create the nanostructure have been examined. Patterning the silicon surface before the MOS gate stack layers gives a "nanostructure first" process, and patterning the interface after forming the gate stack gives a "nanostructure last process." Both processes take advantage of a nano-LOCOS (nano-local oxidation of silicon) invention developed in this work. The nano-LOCOS process plays a central role in defining a clean, sharp confining potential for the spin lattice electrons

Quantum shot noise in mesoscopic superconductor-semiconductor heterostructures

Shot noise in a mesoscopic electrical conductor have become one of the most attentiondrawing subject over the last decade. This is because the shot-noise measurements provide a powerful tool to study charge transport in mesoscopic systems [1]. While conventional resistance measurements yield information on the average probability for the transmission of electrons from source to drain, shot-noise provides additional information on the electron transfer process, which can not be obtained from resistance measurements. For example, one can determine the charge ‘q’ of the current carrying quasi-particles in different systems from the Poisson shot noise SI = 2q�I� [2] where �I� is the mean current of the system. For instance, the quasi-particle charge is a fraction of the electron charge ‘e’ in the fractional quantum Hall regime [3, 4, 5]. The multiple charge quanta were observed in an atomic point contact between two superconducting electrodes [6]. Shot-noise also provides information on the statistics of the electron transfer. Shot noise in general is suppressed from its classical value SI = 2e�I�, due to the correlations. In mesoscopic conductors, due to the Pauli principle in fermion statistics, electrons are highly correlated. As a results, the noise is fully suppressed in the limit of a perfect open channel T = 1. For the opposite limit of low transmission T � 1, transmission of electron follows a Poisson process and recovers the Schottky result SI = 2e�I� [2]. For many channel systems, shot-noise is suppressed to 1/2 × 2e�I� for a symmetric double barrier junction [7, 8], to 1/3 in a disordered wire [9, 10, 11, 12, 13, 14] and to 1/4 in an open chaotic cavity [15, 16, 17]. When a superconductor is involved, the shot-noise can be enhanced by virtue of the Andreev reflection process taking place at the interface between a normal metal and a superconductor. In some limiting cases, e.g. in the tunneling and disordered limit, the shot-noise can be doubled with respect to its normal state value [18, 19, 20, 21]. One of the main results of this thesis is an extensive comparison of our experimental data on conductance and shot noise measurements in a S-N junction with various theoretical models. In addition to measure shot-noise in a two-terminal geometry, one can also perform the fluctuation measurements on multi-terminal conductors. Whereas shotnoise corresponds to the autocorrelation of fluctuations from the same leads, crosscorrelation measurements of fluctuations between different leads provide a wealth of new experiments. For example, the exchange-correlations can be measured directly from these geometry [22]. Experimental attempt in mesoscopic electronic device was the correlation measurements [14, 23] on electron beam-splitter geometry [24] which is the analogue to the Hanbury-Brown Twiss (HBT) experiment in optics. In their experiment, Hanbury-Brown and Twiss demonstrated the intensity-intensity correlations of the light of a star in order to determine its diameter [25]. They measured a positive correlations between two different output photon beams as predicted to the particles obeying Bose-Einstein statistics. This behavior is often called ‘bunching’. On the other hand, a stream of the particles obeying Fermi-Dirac statistics is expected to show a anti-bunching behavior, resulting in a negative correlation of the intensity fluctuations. Latter one was confirmed by a Fermionic version of HBT experiments in single-mode, high-mobility semiconductor 2DEG systems [14, 23]. Whereas in a single electron picture, correlations between Fermions are always negative1 (anti-bunching), the correlation signal is expected to become positive if two electrons are injected simultaneously to two arms and leave the device through different leads for the coincident detection in both outputs2. One simple example is the splitting of the cooper pair in a Y-junction geometry in front of the superconductor. Fig.1.1 shows the possible experimental scheme of the correlation measurement as described here and the sample realized in an high-mobility semiconductor heterostructures. Since all three experiments were done3, only one left unfolded, ‘The positive correlations from the Fermionic system’. The main motivation of this thesis work was to find a positive correlations in the device shown in Fig.1.1. In a well defined single channel collision experiment on an electron beam splitter, it has theoretically been shown that the measured correlations are sensitive to the spin entanglement [29, 30]. This is another even more exciting issue and we would like to mention that the experimental quest for positive correlations is important for the new field of quantum computation and communication in the solid state, [31, 32] in which entangled electrons play a crucial role. A natural source of entanglement is found in superconductors in which electrons are paired in a spin-singlet state. A source of entangled electrons may therefore be based on a superconducting injector.[33, 34, 27, 35, 36, 37, 38, 38, 39, 40, 41] Even more so, an electronic beamsplitter is capable of distinguishing entangled electrons from single electrons.[29, 42] However, the positive correlations have not been observed in solid-state mesoscopic devices until today. This thesis is organized as follows. Chapter 2 is devoted to the theoretical background of the electrical transport and the current fluctuations. We introduce the basic concept of electrical transport and the shot noise in normal state and superconductor-normal metal (S-N) junction. We also briefly review the theoretical proposals and arguments about the current-current cross-correlations in threeterminal systems. In Chapter 3, we describe the sample fabrication techniques which have been done in our laboratory such as e-beam lithography, metallization and etching. We present also the characterization of our particular system, niobium (Nb) / InAs-based 2DEG junction. Chapter 4 describes the reliable low-temperature measurement technique for detecting the noise. We characterize our measurement setup using a simple RC-circuit model. In Chapter 5, our main results about the shot noise of S-N junction are presented in detail

Optimization of spin-orbit magnetic-state readout in metallic nanodevices

183 p.This thesis presents the first steps of the optimization of the magnetic-state readout component for the envisioned magneto-electric spin-orbit (MESO) logic device. We established that (i) reducing the device dimension of ferromagnetic materials/ strong spin-orbit coupling non-magnetic materials nanostructured devices leads to an enhancement of the output signals; (ii) spurious effects in the device due to the local configuration can be avoided by proper design of the ferromagnetic and spin-orbit coupling material electrodes; (iii) interface properties and interfacial spin-charge interconversion have to be carefully considered when studying spin transport in metallic devices and such interface might be applicable for the MESO-logic devices. Even tough, we did not achieve the required values for the realization of cascaded gates with MESO devices, we did find a guideline for further improvement of the output signals. Besides the independent scaling laws for voltage and charge output signals, the use of other materials systems with large spin-charge interconversion efficiency and high resistivities seems to be promising for enhanced output signal. Further experiments are required to demonstrate the use of our device as a curren

Enhanced upconversion photoluminescence by novel plasmonic structures

The emerging field of plasmon-enhanced upconversion photoluminescence has a significant impact on a variety of technologies, including high-efficiency solar energy systems and biotechnology. To date, the upconversion efficiency of best reported rare-earth doped upconversion nanoparticles cannot meet the requirements of practical utilizations in these fields. Therefore, it is of great significance to find new approaches for the enhancement of upconversion efficiency. This thesis mainly aims to explore the enhanced upconversion photoluminescence by several novel plasmonic nanostructures. In this PhD work, I first studied the properties of rare-earth doped upconversion nanomaterials, which are capable of the spectral conversion of the otherwise lost sub-band-gap photons from the solar spectrum. The extra Gd3+ ion doping strategy was introduced in the hydrothermal synthesis process, which can provide an approach to tune the geometry and upconversion efficiency of upconversion nanoparticles (UCNPs). To achieve higher upconversion efficiency, advances in the experimental improvements in plasmon-enhanced upconversion photoluminescence (UCPL) efficiency are explored, by using Au mesoporous film, Au nanotriangle array or nanohole array substrates for the enhancement of upconversion photoluminescence. It is demonstrated that the best plasmonic nanostructures can achieve about 360 times UCPL enhancement. These experimental results demonstrated the great potential of the plasmonic effect for UCPL enhancement. Furthermore, a triplet-triplet annihilation based upconversion nanoparticles (TTA-UCNPs) were synthesized, which have much higher intrinsic upconversion efficiency than the rare-earth based upconversion nanoparticles. A plasmon-enhanced upconversion photoluminescence substrate was designed for high performance photocatalysis applications under solar simulator (AM 1.5 G) irradiation. Five times faster photocatalytic activity rate was achieved by this plasmonic/TTA-UCNPs/Au@TiO2 system, which demonstrates great value of plasmonic and upconversion mechanisms. The combination of excellent plasmonic substrate and high efficiency TTA-UCNPs makes it possible for the realization of industrial level applications of the plasmonic and upconversion in the photocatalytic field.Open Acces

Raman measurements on plasmon-phonon coupled systems

In this thesis, Raman spectroscopy is used to characterize the interaction between a plasmon and the lattice vibration of a solid state material. Two systems have been analyzed: the first is composed of a metallic nanostrucure and a carbon material (carbon nanotubes and graphene), the second consists of beryllium-doped gallium arsenide nanowires. In the first system, additionally to the electromagnetic enhancement, a cooperative process (dynamical back-action) between the localized surface plasmon-polariton and the lattice vibration can occur. This process leads to a non-linear response of the Raman signal in dependence on the laser power. In this work the occurrence of this non-linearity is experimentally observed and compared with the theoretical prediction. In the second system, the charge-carriers provided by the dopant act as a plasma, interacting with the electric field related to the longitudinal phonon mode of the crystal lattice. This interaction causes a change in the position and width of the Raman peak, which can consequently been used to extrapolate the change carriers concentration and mobility. The appearance of surface phonons, typical of nanostructures, is also observed and discussed

Auger-mediated processes and photoluminescence in group iv semiconductor nanostructures

Group IV semiconductors (Si, Ge) are inefficient light emitting materials due to their indirect bandgap structure. Nanostructures of Si, Ge, and SiGe however, have shown relatively high photoluminescence (PL) quantum efficiency (QE) at low carrier concentrations. At higher carrier concentrations, the PL QE of these nanostructures is drastically reduced due to the onset of a fast non-radiative process attributed to Auger recombination. Moreover, this onset occurs earlier in structures with reduced physical dimensions, than in bulk material. The study of Auger-mediated processes in group IV nanostructures is therefore critical to understanding the physics of carrier recombination and photonic device limitations. This work investigates recombination mechanisms in two such systems: the silicon/silicon germanium three-dimensional (3D) nanostructure system, and the silicon-on-insulator (SOI) system. Recombination mechanisms are studied by several experimental techniques. One approach explores the steady state PL spectroscopy and PL dynamics under pulsed excitations with varying concentrations of photo-generated charge carriers in the investigated systems. Another important technique uses selective, wavelength dependent photoexcitation to generate carriers up to varying depths in the nanostructures, enabling the understanding of local differences in PL properties through the thickness of structures. Several interesting observations are reported and underlying recombination mechanisms are discussed. For the Si/SiGe 3D nanostructure system, these include a reversible degradation of the PL after a few minutes of relative stability, an Auger Fountain mechanism that redistributes charge carriers within the nanostructure, and a severe reduction of the exciton diffusion length. For the SOI system, an apparently successful competition of the radiative recombination of carriers in a condensed excitonic phase with Auger processes is observed. The influence of the Si/SiO2 interface on the recombination mechanism in this system is emphasized. Results of the experiments show that the coexistence of a type II energy band alignment at Si/SiGe interfaces, the electron-hole-droplets in Si, and Auger-mediated processes results in several unusual photoluminescence properties in SiGe and Si nanostructures

Ultrafast Energy Flow and Structural Changes in Nanoscale Heterostructures

A central goal of nanotechnology is the precise construction of nanoscale heterostructures with optimized chemical, physical or biological functionalities. It is known that function stems from structure but, in addition, function always involves nonequilibrium conditions and energy flow. The central topic of this thesis is the ultrafast energy flow in nanoscale heterostructures and how this energy flow drives ultrafast structural changes. The main experimental technique of this work is femtosecond electron diffraction, which probes the lattice response to electronic excitations. The nanoscale heterostructures contain metallic (Au) nanostructures of well-defined 0D or 2D morphology, supported on 2D substrates. In photoexcited heterostructures, thermal equilibrium is restored by electron-lattice interactions, within each component, and electronic and vibrational coupling across their interface. A newly developed model of ultrafast energy flow is used to measure the microscopic couplings, like electron-phonon coupling and interfacial vibrational coupling in nanoscale heterostructures using the observed Debye-Waller dynamics. Ultrafast energy flow in supported metallic nanostructures can initiate a rich variety of real-space motions like anharmonic lattice expansion and surface premelting, which manifest as distinct and quantifiable observables in reciprocal-space. These phenomena have been studied for Au nanoclusters on amorphous thin-film substrates. Au nanoclusters are found to exhibit ultrafast surface premelting at atypically low lattice temperatures and pronounced electron-lattice nonequilibrium conditions. Femtosecond electron diffraction is mostly used to study ultrafast motions related with phonons but in ultrasmall nanocrystals a new observable arises: the motion of the phonons’ frame of reference, meaning the crystal itself. This has been demonstrated for Au nanoclusters attached on graphene using femtosecond electron diffraction experiments, molecular dynamics and electron diffraction simulations. The substrate has a significant effect on the energy flow and the structural motions of ultrasmall, adsorbed nanostructures and, inversely, metallic nanostructures can alter fundamental properties of semiconducting substrates. Surface decoration with plasmonic, quasi-2D nanoislands of Au sensitizes WSe2 to sub-band-gap photons, causes nonlinear lattice heating and accelerates electron-phonon equilibration times. Conclusively, nanoscale heterostructures have a rich variety of nonequilibrium phenomena that affect their structure at ultrafast timescales. Ultrafast diffractive probes, like femtosecond electron diffraction, can provide a detailed, quantitative understanding of this relationship.In dieser Doktorarbeit wird der ultraschnelle Energietransfer in nanoskaligen Heterostrukturen sowie die dadurch verursachten ultraschnellen Strukturänderungen untersucht. Die wichtigste Methode dieser Arbeit ist Femtosekunden-Elektronenbeugung. Diese Methode untersucht die Reaktion des Kristallgitters auf elektronische Anregung. Die Heterostrukturen bestehen aus Gold-Nanostrukturen mit wohldefinierten 0D oder 2D Strukturen, die auf 2D Substraten aufgebracht sind. In mit Licht angeregten Heterostrukturen wird das thermische Gleichgewicht durch Elektron-Phonon-Kopplung in den einzelnen Materialien sowie durch elektronische und phononische Kopplung zwischen den Materialien wiederhergestellt. Ein neu eingeführtes Modell für ultraschnellen Energietransfer wird verwendet, um die ultraschnellen Veränderungen der Gittertemperatur zu beschreiben. Das Modell ermöglicht es, aus der gemessenen Debye-Waller-Dynamik mikroskopische Größen wie Elektron-Phonon-Kopplung und Phonon-Phonon-Kopplung an der Grenzfläche der nanoskaligen Heterostrukturen zu extrahieren. Ultraschneller Energietransfer in metallischen Nanostrukturen können eine Vielzahl an Veränderungen im Kristallgitter hervorrufen, z.B. Gitterausdehnung und Schmelzen der Kristalloberfläche. Diese Veränderungen gemessen werden, für 0D Gold Nanostrukturen die auf 2D Substraten aufgebracht sind. Au-Nanocluster zeigen ultraschnelles Schmelzen der Kristalloberfläche bei außergewöhnlich niedrigen Gittertemperaturen und ausgeprägtem Nichtgleichgewichtszustand zwischen Elektronen und Gitter. Femtosekunden Elektronen Beugung ist eine Methode, die am häufigsten bei der Untersuchung durch Phonen induzierter ultraschneller Bewegungen von Atomen Anwendung findet. In ultrakleinen Nanokristallen stellt sich aber ein neue Herausforderung dar: der Referenzrahmen der Bewegung der Phononen, was der Kristall selber ist. Demonstriert wurde das für Gold 0D Nanostructuren, die auf Graphen. Das Substrat hat einen signifikanten Einfluss auf den Energiefluss und die strukturelle Bewegung von ultrakleinen, adsorbierten Nanostrukturen und in inverser Weise können metallische Nanostrukturen due fundamentalen Eigenschafter halbleitender Proben verändern. Wenn WSe2 mit plasmonische quasi-2D Gold-Nanoinseln bedeckt wird, ändern sich dessen Eigenschaften so, dass Photonen unterhalb der Bandlücke absorbiert werden können. Die resultierende Erwärmung des Gitters folgt einem nichtlinearen Zusammenhang mit der Fluenz des einkoppelnden Lasers und die Elektron-Gitter Relaxationszeit ist reduziert