The digital information processing system has benefited tremendously from the invention and development of complementary metal-oxide-semiconductor (CMOS) integrated circuits. The relentless scaling of the physical dimensions of transistors has been consistently delivering improved overall circuit density and performance every technology generation. However, the continuation of this trend is in question for silicon-based transistors when quantum mechanical tunneling becomes more relevant; further scaling in feature sizes can lead to increased leakage current and power dissipation. Numerous research efforts have been implemented to address these scaling challenges, either by aiming to increase the performance at the transistor level or to introduce new functionalities at the circuit level. In the first approach, novel materials and device structures are explored to improve the performance of CMOS transistors, including the use of high-mobility materials (e.g. III-V compounds and germanium) as the channel, and multi-gate structures. On the other hand, the overall circuit capability could be increased if other state variables are exploited in the electronic devices, such as the electron spin degree of freedom (e.g. spintronics). Here we explore the potential of germanium nanowires in both CMOS and beyond-CMOS applications, studying the electronic and spintronic transport in this material system. Germanium is an attractive replacement to silicon as the channel material in CMOS technology, thanks to its lighter effective electron and hole mass. The nanowire structures, directly synthesized using chemical vapor deposition, provide a natural platform for multi-gate structures in which the electrostatic control of the gate is enhanced. We present the realization and scaling properties of germanium-silicon-germanium core-shell nanowire n-type, [omega]-gate field-effect transistors (FETs). By studying the channel length dependence of NW FET characteristics, we conclude that the intrinsic channel resistance is the main limiting factor of the drive current of Ge NW n-FETs. Utilizing the electron spins in semiconductor devices can in principle enhance overall circuit performance and functionalities. Electrical injection of spin-polarized electrons into a semiconductor, large spin diffusion length, and an integration friendly platform are desirable ingredients for spin based-devices. Here we demonstrate lateral spin injection and detection in Ge NWs, by using ferromagnetic metal contacts and tunnel barriers for contact resistance engineering. We map out the contact resistance window for which spin transport is observed, manifestly showing the conductivity matching required for spin injection. / text

I present a detailed study of the magnetic propertie of electron-beam evaporated Pdo.4Nio.6 alloy thin films by means of ferromagnetic resonance measurements on extended films of varying thickness and anisotropic magnetoresistance measurements lithographically patterned high aspect-ratio ferromagnetic electrodes, respectively. The results reveal that the direction of the magnetization with respect to the film plane strongly depends on the electrode lateral dimensions, transitioning from in-plane magnetization for extended films to out of the plane magnetization for electrode width below 2-3 microns, reaching ~58 degrees for electrode widths of about 100nm (nanowires). This behavior arises from a competition between the film demagnetizing vector, which leads to in-plane magnetization for extended films , and an intrinsic uniaxial anisotropy, which overcomes the magnetostatic energy for laterally constrained films, pulling the magnetization off plane.

The spintronic materials graphene and FeRh are of great scientific and technological interest due to their unique properties. Graphene's remarkable electronic transport and low spin interaction suggest it could be a near-perfect spin-transport material, while the equiatomic alloy FeRh undergoes a first-order antiferromagnetic (AF) to ferromagnetic (FM) phase transition when heated through a critical temperature ~370 K. Combining these materials could lead to a single multifunctional spin injection, transport and detection device in which a range of stimuli - heat, magnetic field, strain etc. - could be used to manipulate the device state. However, realisation of such a multifunctional device is extremely challenging. This thesis describes the progress made in developing a novel method of spin injection into graphene, and details a study of the metamagnetic phase transition in FeRh nanowires suitable for use as spin injection and detection electrodes. The measured values of spin lifetime and spin diffusion length in graphene are an order of magnitude lower than those predicted theoretically. In this project, a novel 1D contact geometry was investigated to determine whether the dwelling of spins underneath tunnel barrier contacts was the cause of the discrepancy. Although these devices exhibited very high charge carrier mobility - indicating successful device fabrication, defect-free graphene flakes and low levels of contamination - no spin signals were observed. Through a thorough investigation of this unexpected result it was determined that the quality of the graphene/- ferromagnetic interface was limiting the polarisation of injected spin current. The use of FeRh as a novel spin injection and detection material was investigated through magnetic force microscopy imaging of the AF and FM phases during heating and cooling sweeps. The results from FeRh full-films showed a strong dependence on surface morphology, as certain surface types were observed to favour the FM phase. These behaviours were confirmed in patterned nanowire devices, which indicated that the dependence on surface topology dominated over spatial confinement effects. In order to perform these studies a magneto-transport measurement system capable of performing measurements over a wide temperature range 2 K - 500 K in a rotatable magnetic field of up to 750 mT was developed. The noise base of the completed system was measured at just 10% above the theoretical minimum level.

This work is dedicated to the study of spin dynamics in systems based on the semiconductor gallium arsenide (GaAs) that are suitable for use in spintronic devices. We explored two types of model structures using experimental methods of ultrafast laser spectroscopy and transport measurements. In the ferromagnetic semiconductor (Ga,Mn)As, we investigated laser-induced magnetization precession. We found out that transfer of both energy and angular momentum from the circularly polarized laser light can trigger magnetization precession, the latter one being identified as a new phenomenon, the "optical spin transfer torque". Furthermore, we demonstrate the possibility to control the energy-transfer-induced magnetization dynamics both optically and electrically using piezo-stressing. When dealing with purely non-magnetic structures for spintronics, we studied the Spin-Injection Hall Effect (SIHE) in GaAs/AlGaAs heterostructures with a special type of spin- orbit (SO) coupling that are lithographically patterned to create nanodevices. We managed to observe precession of the electron spin in the SO field directly in the space domain by extending the original detection method. This finding, together with the direct detection of a pure spin current, helped to propose a working spin Hall effect transistor.

Spintronics, also known as spin electronics, or magnetoelectronics, refers to the study of the role that electron and (less frequently) nuclear spins play in solid state physics, and a group of devices that specifically exploit both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge. As a principal type of spintronic device, a spin-valve is a device that uses ferromagnetic electrodes to polarize and analyze the electronic spins. The electrical resistance of the device depends sensitively on the relative magnetization of its two ferromagnetic electrodes, a phenomenon referred to as Giant Magnetoresistance (GMR). Having been successfully applied in the field of data storage, GMR also shows potential for future logic devices. Organic semiconductors possess many advantages in electronic device applications. Therefore, using organic semiconductors in spintronics is very interesting and promising, in part, because of their exceptionally long spin-decoherence times. This thesis concerns itself with the scientific study of magnetic field and spin effects in organic spin valves (OSV) and organic light emitting diodes (OLED). Three projects were finished, achieving a better understanding of the transportation of charge and spin carriers inside organic films, and paving the way to enhancing the spin diffusion length and the organic magnetoresistance (OMAR) effect. Firstly, C60 films were used as the spin-transport layer of OSV devices, because of its low hyperfine coupling and high mobility, which prior work suggested to be beneficial. Subsequently we studied the spin injection and transport properties by measuring the devices' magnetoresistance (MR) response at various biasing voltages, V, temperatures, T and different C60 film thickness. But we do not observe a significantly increased spin-diffusion length compared to OSV devices based on other organic semiconductors. We propose conductivity mismatch as a likely cause of the loss of spin-valve signal with increasing C60 layer thickness. There exists some disagreement in the scientific literature regarding whether OSV operate in the so-called tunneling regime or the so-called injection regime. To shed light on this question, we fabricated spin-valve devices made of organic semiconductor thin films of rubrene sandwiched between ferromagnetic cobalt and iron electrodes. Current-voltage (I-V) characteristics in Co/AlOx/rubrene/Fe junctions with a rubrene layer thickness, d, ranging from 5-50 nm, were measured, and we found two different modes of conductivity. The first mode, tunneling, occurs in relatively thin junctions, d < 15 nm, and decays exponentially with increasing rubrene thickness. We determined the tunneling decay length to be 1 nm. The tunneling mode is also characterized by a weak temperature dependence and a nearly parabolic differential conductance. The second mode, injection followed by hopping, occurs in relatively thick devices, d ≥ 15 nm, and can be identified by strongly temperature dependent, highly non-linear I-V traces that are similar to those commonly measured in organic injection devices such as OLEDs. We observed MR in devices with a rubrene thickness of 5 nm and 10 nm. Those devices are clearly in the tunneling regime. For the 15 nm device, for which the tunneling current is just barely measurable we could not observe MR. In the third project, we show that the performance of both OMAR and OSV devices very sensitively depends on whether the metallic layers are deposited by thermal evaporation or electron-beam evaporation. A strongly reduced spin diffusion length and an enhanced OMAR response can be achieved in devices fabricated by electron-beam evaporation. Then we showed that the difference must be attributed to the generation of traps resulting from the exposure of the organic layer to X-ray bremsstrahlung that is generated during the e-beam evaporation process. We also used the thermally stimulated current technique (TSC) to characterize these traps.

No description available.

In the present thesis, the properties at ferromagnet/semiconductor interfaces, relevant for semiconductor spintronics applications, are addressed. Semiconductor spintronics refers to the possibility of storing information using the electron spin, additional to the electron charge, for enhanced flexibility in nanoscale semiconductor devices. The system under focus is the Fe/ZnSe(001) heterostructure, where ZnSe is a wide gap semiconductor ideally compatible with GaAs. The heterostructures are grown on GaAs(001) substrates by molecular beam epitaxy. From various electron-beam based diffraction, spectroscopy and microscopy techniques, it is shown that Fe grows epitaxially and predominantly in a layer-by-layer mode on ZnSe(001) with no presence of chemically reacted phases or interdiffusion. An in-plane uniaxial magnetic anisotropy (UMA) is detected for thin Fe films on ZnSe(001) by magnetometry, thus opposing the cubic symmetry of bcc Fe. From first principles calculations, the unidirectional sp3-bonds from ZnSe are shown to induce this uniaxiality. Moreover, an in-plane anisotropic lattice relaxation of Fe is found experimentally, seemingly as a consequence of the sp3-bonds, giving an additional UMA contribution via magneto-elastic coupling. It is proposed that these two effects are responsible for the much-debated UMA observed in Fe/semiconductor structures in general. The interface magnetism is probed by x-ray magnetic circular dichroism and Mössbauer spectroscopy. It is found that the magnetic moment at the interface is comparable or even enhanced with respect to the bulk Fe. These two experiments are believed to provide the first unambiguous proof of a persistent bulk magnetic moment at a transition metal/semiconductor interface. Spin-polarised transport measurements are performed on Fe/ZnSe/FeCo magnetic tunnel junctions. A magnetoresistance of 16% is found at low temperature, which evidences both the existence of interface spin polarisation, as inferred from the bulk magnetic moment above, and that the spin polarisation can be transmitted across the semiconductor barrier layer.

Recognising that the characterisation of actual interfaces in magnetic multilayer systems will provide valuable insight for the integration of spintronics in practical devices, a study of interface effects in various structures is presented. Magnetometry measurements are performed for a range of Fe thicknesses (0.4 - 23 nm) grown by molecular beam epitaxy on GaAs and InAs substrates in order to determine the factors governing the evolution of the magnetic moment of epitaxial Fe grown on a zinc-blende semiconductor. A greater reduction of the Fe magnetic moment is observed for films grown on InAs as compared to GaAs, as the Fe films reach a bulk-like moment (within 10% deviation) at a thickness of ~5.2 nm and ~2.2 nm, respectively. From this direct comparative study it is concluded that interface and interdiffusion effects are the dominant mechanisms influencing the value of the magnetic moment for ultra-thin Fe films on GaAs and InAs. Spin injection at this interface is performed, by detecting optical polarisation in the oblique Hanle geometry from a Fe/AlGaAs/GaAs spin-light emitting diode structure. The electrical and magnetic properties of the system are presented, and a ~1% injection polarisation at room temperature, rising to ~4% at 77 K is reported. A study of the deposition and growth of MgO thin (3 - 39 nm) films in conjunction with magnetic layers is also performed. Crystallinity of MgO grown on GaAs is obtained, and epitaxial growth of Fe and Co on MgO is demonstrated. Polarised neutron reflectivity results again indicate a slight decrease in Fe and Co magnetic moments due to interfacial oxide layers. MgO is also incorporated in a pseudo-spin-valve structure which demonstrates epitaxy-induced magneto-crystalline anisotropy. It is concluded that the interface quality is a critical parameter for spintronic devices. Atomic-scale defects and intermixing in real samples mean that current theoretical estimates of ~100% injection efficiency in perfect systems remain unattainable. However by increasing atomic-level structural control of interfaces, a substantial increase in efficiency might be achieved, similarly to the recent breakthrough in tunnelling magneto-resistance ratios which have reached 1000%.

Nous avons étudié la création, manipulation et détection d’accumulation de spin hors-équilibre dans l’hétérostructure d’oxydes non-magnétique LaAlO3/SrTiO3 (LAO/STO), hôte d’un système d’électron quasi-bidimensionnel (q2DES). Combiné à des expériences d’injection de spin, nous employons l’effet Hanle (à 3 terminaux) pour sonder l’amplitude de l’accumulation de spin dans des jonctions tunnel Co/LAO/STO. Nous observons une large amplification du signal de spin, attribuée à des processus d’effet tunnel séquentiel préservant le spin via des états localisés avec des longs temps de vie de spin. Une importante modulation du signal de spin par effets de champ électrique atteste de la création d’accumulation de spin au sein même du q2DES. Nous avons utilisé la technique de pompage de spin en cavité, induite par résonance ferromagnétique d’une couche de permalloy, pour générer un courant de spin à l’interface LAO/STO, lequel est converti en un large courant de charge au sein du q2DES. Nous l’attribuons à un effet Edelstein inverse, dérivant d’une interaction spin-orbite de type Rashba. Lesquels sont efficacement modulés par effets de champ. Ainsi, nos résultats permettent d’étendre le champ d’intérêt depuis le transport de charge planaire vers l’exploration de phénomènes dépendant du spin dans un canal conducteur prototypique d’oxydes non-magnétique. Nous avons par ailleurs démontré que l’épaisseur critique pour l’observation d’un q2DES à l’interface LAO/STO peut être réduite à une monocouche de LAO en recourant à une variété de couches métalliques. Cela ouvre un nouveau champ d’investigation pour tenter d’identifier les potentiels mécanismes à l’origine de la formation du q2DES. / We investigated the generation, manipulation, and detection of non-equilibrium spin accumulation in the nonmagnetic LaAlO3/SrTiO3 (LAO/STO) oxide heterostructure, which is the host of a quasi-two-dimensional electron system (q2DES). In electrical tunneling spin injection experiments, we made use of the (three-terminal) Hanle effect to probe the magnitude of spin accumulation at Co/LAO/STO interfaces. We report on large amplification effects of the spin signal, ascribed to spin-conserving sequential tunneling processes via localized electronic states of enhanced spin lifetimes. A substantial modulation of the spin signal, by electrostatic field-effect, evidences the successful generation of spin accumulation inside the q2DES. We further resorted to ferromagnetic resonance experiments in a cavity to adiabatically pump a spin current from a permalloy layer toward the LAO/STO interface. We find that the generated spin current is converted into a sizeable planar charge current within the q2DES. This is attributed to an inverse Edelstein effect deriving from a Rashba-like spin-orbit interaction, both of which are efficiently modulated by electrostatic field-effect. Hence, our findings expand the general field of interest from planar charge transport to the exploration of spin-dependent phenomena in a prototypical nonmagnetic conducting oxide channel. Additionally, we have also demonstrated that the critical thickness threshold for the onset of a q2DES at LAO/STO interfaces can be reduced to a single unit cell of LAO when resorting to various metal capping layers. It opens up a new field of investigation to tentatively identify the potential mechanisms driving the formation of the q2DES.

No description available.