Optical spin injection in InAs/GaAs quantum dots (QDs) structures under cryogenic temperature has been investigated in this work using continuous-wave optical orientation spectroscopy. Circularly polarized luminescence from trions in the QDs was used as a measure for the degree of spin polarization of the carriers in the QD ground states. The efficiency of spin conservation of the carriers during the injection process into the QDs and also the influence of the nuclear spins in the QDs were studied both under zero and external magnetic field. It was shown in zero magnetic field that the spin states were less conserved during the injection process for correlated excitons and hot free carriers. While under the external magnetic field, measurements were done in Faraday configuration. Confined electron motion yielding the quantized Landau levels in the InGaAs wetting layer (WL) and lifting of the Landau level spin degeneracy was observed. Also possible spin thermalization in the InGaAs WL during spin injection process was found. Finally, the quench of hyperfine induced spin relaxation by dynamic nuclear polarization (DNP) in the QDs was discovered and believed to be a stronger effect under weak/zero magnetic field.

Spintronics, an abbreviation of spin based electronics and also known as magneto electronics, has attracted a lot of interest in recent years. It aims to explore the role of electrons’ spins in building next generation electric devices. Using electrons’ spins rather than electrons’ charges may allow faster, lower energy cost devices. Spin Hall Effect is an important subfield of spintronics. It studies spin current, spin transport, and spin accumulation in paramagnetic systems. It can further understanding of quantum physics, device physics, and may also provide insights for spin injection, spin detection and spin manipulation in the design of the next generation spintronics devices. In this experimental work, two sets of experiments were prepared to detect the Spin Hall Effect in metallic systems. The first set of experiments aims to extract Spin Hall Effect from Double Hall Effect in micrometer size metal thin film patterns. Our experiments proved that the Spin Hall Effect signal was much smaller than the theoretically calculated value due to higher electrical resistivity in evaporated thin films. The second set of experiments employs a multi-step process. It combines micro fabrication and electrochemical method to fabricate a perpendicular ferromagnet rod as a spin injector. Process description and various techniques to improve the measurement sensitivity are presented. Measurement results in aluminum, gold and copper are presented in Chapters III, IV and V. Some new experiments are suggested in Chapters V and VI.

No description available.

Le développement de nouveaux dispositifs spintroniques à base de semi-conducteurs (SC) nécessite la création d'une population électronique polarisée en spin dans ces matériaux. De ce point de vue, le germanium est un matériau prometteur pour les applications en spintronique à cause de la forte mobilité des porteurs de charge ainsi que de la symétrie d'inversion du cristal diamant à l'origine de temps de vie de spin très longs. Dans ce manuscrit, nous discutons deux approches pour l'injection et la détection électrique de spins dans le germanium. La première approche consiste à utiliser une barrière tunnel et un métal ferromagnétique (FM) comme injecteur de spin. L'insertion d'une barrière tunnel à l'interface FM/SC permet de résoudre le problème fondamental du désaccord de conductivité. Nous avons utilisé deux injecteurs différents : Py/Al2O3 et CoFeB/MgO. Les mesures sont réalisées en géométrie à trois contacts et l'accumulation de spins dans le germanium est démontrée par la mesure de l'effet Hanle. Dans le cas d'une barrière d'Al2O3, les spins injectés s'accumulent sur des états localisés à l'interface oxyde/Ge et cette accumulation est observée jusqu'à 220 K. Dans le cas d'une barrière de MgO, les spins sont réellement injectés dans le canal de Ge et un signal de 20-30 µV est encore observé à température ambiante. Nous discutons dans la deuxième approche l'utilisation du semi-conducteur magnétique (Ge,Mn) comme injecteur de spins dans le Ge. Nous avons tout d'abord étudié les propriétés structurales et magnétiques de films minces de (Ge,Mn) fabriqués par épitaxie par jets moléculaires à basse température. En faisant varier les paramètres de croissance, nous avons pu observer des nanocolonnes de GeMn cristallines ou amorphes, ainsi que des films et des nanoparticules de Ge3Mn5. Nous nous sommes concentrés sur l'anisotropie magnétique de ces nanostructures. Finalement, la croissance de (Ge,Mn) sur GOI a été optimisée en vue de son utilisation comme injecteur de spins dans le germanium et différentes méthodes d'intégration de ce matériau dans les dispositifs de spintronique « tout semi-conducteur » sont discutées. / Creation of spin polarization in non-magnetic semiconductors is one of the prerequisite for creation of spintronics based semiconductor devices. Germanium is interesting for spintronics applications due to its high carrier mobilities and its inversion symmetry that gives long spin lifetimes. In this manuscript, we discuss two approaches for electrical spin injection and detection in Germanium. The first approach is to use a tunnel barrier and a ferromagnetic metal as a spin injector. The tunnel barrier at the interface circumvents the conductivity mismatch problem. Two different spin injectors are used: Py/Al2O3 and CoFeB/MgO. The measurements are performed in three-terminal geometry and the proof of spin accumulation is given by Hanle measurements. In case of Al2O3, the spin accumulation is predicted to be in localized states at the oxide/Ge interface and the spin signal is observed up to 220 K. However in MgO based devices, true injection in Ge channel is predicted and spin signal of 20-30 µV is observed at room temperature. The second approach of using ferromagnetic semiconductor (Ge,Mn) as spin injector is also discussed. The structural and magnetic properties of (Ge,Mn) thin-films grown by low-temperature molecular beam epitaxy (LT-MBE) are studied. Depending on the growth parameters, crystalline/amorphous GeMn nanocolumns and Ge3Mn5 thin films or nanoclusters have been observed. Magnetic anisotropy in these nanostructures is also studied. Finally, the growth of (Ge,Mn) films on GOI substrates is shown and different ways to use (Ge,Mn) as a spin injector in Ge are discussed to achieve all-semiconductor based spintronics devices.

This thesis work aims at a better understanding of magneto-optical properties of dilute nitrides and II-VI diluted magnetic semiconductor quantum structures. The thesis is divided into two parts. The first part gives an introduction of the research fields, together with a brief summary of the scientific results included in the thesis. The second part consists of seven scientific articles that present the main findings of the thesis work. Below is a short summary of the thesis. Dilute nitrides have been of great scientific interest since their development in the early 1990s, because of their unusual fundamental physical properties as well as their potential for device applications. Incorporation of a small amount of N in conventional Ga(In)As or Ga(In)P semiconductors leads to dramatic modifications in both electronic and optical properties of the materials. This makes the dilute nitrides ideally suited for novel optoelectronic devices such as light emitting devices for fiber-optic communications, highly efficient visible light emitting devices, multi-junction solar cells, etc. In addition, diluted nitrides open a window for combining Si-based electronics with III-V compounds-based optoelectronics on Si wafers, promising for novel optoelectronic integrated circuits. Full exploration and optimization of this new material system in device applications requires a detailed understanding of their physical properties. Papers I and II report detailed studies of effects of post-growth rapid thermal annealing (RTA) and growth conditions (i.e. presence of N ions, N2 flow, growth temperature and In alloying) on the formation of grown-in defects in Ga(In)NP. High N2 flow and bombardment of impinging N ions on grown sample surface is found to facilitate formation of defects, such as Ga interstitial (Gai) related defects, revealed by optically detected magnetic resonance (ODMR). These defects act as competing carrier recombination centers, which efficiently decrease photoluminescence (PL) intensity. Incorporation of a small amount of In (e.g. 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In, on the other hand, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai related defects formed during the growth. In Paper III, the first identification of an interfacial defect at a heterojunction between two semiconductors (i.e. GaP/GaNP) is presented. The interface nature of the defect is clearly manifested by the observation of ODMR lines originating from only two out of four equivalent <111> orientations. Based on its resolved hyperfine interaction between an unpaired electronic spin (S=1/2) and a nuclear spin (I=1/2), the defect is concluded to involve a P atom at its core with a defect/impurity partner along a <111> direction. Defect formation is shown to be facilitated by N ion bombardment. In Paper IV, the effects of post-growth hydrogenation on the efficiency of the nonradiative (NR) recombination centers in GaNP are studied. Based on the ODMR results, incorporation of H is found to increase the efficiency of the NR recombination via defects such as Ga interstitials. In Paper V, we report on our results from a systematic study of layered structures containing an InGaNAs/GaAs quantum well, by the optically detected cyclotron resonance (ODCR) technique. By monitoring PL emissions from various layers, the predominant ODCR peak is shown to be related to electrons in GaAs/AlAs superlattices. This demonstrates the role of the SL as an escape route for the carriers confined within the InGaNAs/GaAs single quantum well. The last two papers are within a relatively new field of spintronics which utilizes not only the charge (as in conventional electronics) but also the quantum mechanical property of spin of the electron. Spintronics offers a pathway towards integration of information storage, processing and communications into a single technology. Spintronics also promises advantages over conventional charge-based electronics since spin can be manipulated on a much shorter time scale and at lower cost of energy. Success of semiconductor-based spintronics relies on our ability to inject spin polarized electrons or holes into semiconductors, spin transport with minimum loss and reliable spin detection. In Papers VI and VII, we study the efficiency and mechanism for carrier/exciton and spin injection from a diluted magnetic semiconductor (DMS) ZnMnSe quantum well into nonmagnetic CdSe quantum dots (QD’s) by means of spin-polarized magneto PL combined with tunable laser spectroscopy. By means of a detailed rate equation analysis presented in Paper VI, the injected spin polarization is deduced to be about 32%, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin injection process. In Paper VII, we present evidence that energy transfer is the dominant mechanism for carrier/exciton injection from the DMS to the QD’s. This is based on the fact that carrier/exciton injection efficiency is independent of the width of the ZnSe tunneling barrier inserted between the DMS and QD’s. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with wide barriers, pointing towards increasing spin loss.

Cette thèse de doctorat propose d'explorer un nouveau paradigme de propagation de l'information de spin sur de très longues distances après encodage sur la polarisation de lumière cohérente. L'objectif principal de ce manuscrit est de fournir une étude détaillée de l'injection de spin dans des composants optoélectroniques III-V à géométrie verticale. Pour atteindre cet objectif, nous nous concentrons sur l'étude de l'injection optique et électrique de spin dans des structures « Light Emitting Diodes » (LEDs) et des structures « Vertical External Cavity Surface Emitting Lasers » (VECSELs) à base de semiconducteurs III-V. Nos investigations et résultats sont présentés suivant trois axes majeurs.La première partie regroupe un état de l'art sur l'injection de spin dans les composants optoélectroniques III-V et se concentre sur les phénomènes physiques engagés dans la conversion d'une accumulation de spin en information de polarisation lumineuse. Une discussion sur l'injection et le transport de spin dans des structures semi-conductrices est suivie par une analyse orientée-composant sur l'injection de spin dans les LEDs et les VCSELs.La deuxième axe s'articule autour de notre travail expérimental sur le développement et l'optimisation sur LEDs III-V d'un injecteur de spin MgO/CoFeB/Ta ultra-fin présentant une aimantation perpendiculaire à la rémanence magnétique. Nous nous focalisons en premier lieu sur l'optimisation de la barrière tunnel MgO pour maximiser l'injection de porteurs polarisés en spin et détaillons par la suite le développement et la caractérisation d’un injecteur de spin possédant une aimantation perpendiculaire à la rémanence magnétique.La troisième partie contient le travail principal de cette thèse de doctorat. Elle est entièrement consacrée à notre recherche expérimentale sur l'injection de spin dans les structures « Vertical External Cavity Surface Emitting Lasers ». Nous commençons par introduire un model vectoriel permettant la compréhension théorique de la sélection de polarisation dans les structures VECSELs injectées en spin. Nous rapportons ensuite la mesure de biréfringence d'une structure VECSEL designée pour le pompage optique en utilisant une technique expérimentale originale basée sur la mesure du décalage en fréquence entre les deux modes de polarisation orthogonaux TE et TM. Ultérieurement, nos observations et résultats sur l'injection optique de spin dans les VECSELs sont détaillés, analysés et commentés. L'étude est étendue à l'estimation des temps de vie caractéristiques du système par mesures de Photoluminescence résolues en temps afin d'évaluer l'efficacité de conversion de l'information de spin. Pour finir, les résultats préliminaires sur l'injection électrique de spin dans les VECSELs sont présentés. / This Ph.D Thesis proposes to explore a new paradigm of spin-information propagation over very long distances after encoding on coherent light polarization. The main objective of this manuscript is to provide a detailed study of spin-injection into III-V semiconductor based opto-electronic devices with vertical geometries. To achieve this goal, we focus on the study of optical and electrical spin-injection in III-V semiconductor based Light Emitting Diodes (LEDs) and Vertical External Cavity Surface Emitting Lasers (VECSELs). Our investigations and results are presented on three axes.The first part regroups a state-of-the-art of spin-injection into semiconductors optoelectronic devices and focuses on the physical phenomena engaged in the conversion of a spin accumulation into light polarization information. A discussion on spin-injection and spin-transport into III-V semiconductor structures is followed by a more device-oriented review on spin-injection in LEDs and VCSELs.The second axis is articulated around our experimental work on the development and the optimization on III-V semiconductors LEDs of an ultra-thin MgO/CoFeB/Ta spin-injector with perpendicular magnetization at magnetic remanence. We focus on the MgO tunnel barrier optimization for maximizing the spin-injection efficiency and further detailed the development and the characterization of the spin-injector with perpendicular magnetization at remanence.Finally, the third part contains the main work of this Ph.D thesis. It is fully dedicated to our experimental research on spin-injection in Vertical External Cavity Surface Emitting Laser structures. A vectorial model allowing the theoretical understanding of polarization selection in spin-injected VECSELs is first introduced. Next, we report the birefringence measurement of a VECSEL designed for optical pumping using an original frequency detuning measurement between the two orthogonal TE- and TM-modes. Afterward, our observations and results on optical spin-injection in VECSELs are displayed, analyzed and commented. The study is farther extended to the measurement of the system's characteristic lifetimes using Time Resolved Photo-Luminescence in order to evaluate the spin-information conversion efficiency. Finally the preliminary results on electrical spin-injection experiment are presented.

No description available.

Die Erzeugung spinpolarisierter Ladungsträger in einem Halbleiter gilt als Grundvoraussetzung zur Realisierung spintronischer Bauelemente. Einen möglichen Ansatz zu deren Realisierung stellen Ferromagnet/Halbleiter(FM/HL)-Hybridstrukturen dar, deren Herstellung jedoch mit einigen Schwierigkeiten verbunden ist. Durch die Vermischung des ferromagnetischen Materials mit dem Halbleiter werden die elektronischen Eigenschaften der Hybridstruktur verändert und die Spininjektionseffizienz stark verringert. Durch das gezielte Einfügen einer dünnen Oxidschicht in den FM/HL-Grenzübergang kann die Diffusion unterdrückt, die Kristallqualität verbessert und die Effizienz der Struktur erhöht werden. Diese Arbeit beschäftigt sich mit dem Wachstum und der Charakterisierung dünner Oxidschichten, hergestellt mittels Molekularstrahlepitaxie. Zwei Seltenerdoxide, La2O3 und Lu2O3, werden auf GaAs-Substraten gewachsen und die Kristallqualität der Schichten miteinander verglichen. Mit der Heusler-Legierung Co2FeSi als Injektorschicht wird eine FM/Oxid/HL-Hybridstruktur auf Basis einer La2O3/GaAs(111)B-Struktur realisiert und magnetisch und elektrisch charakterisiert. Ein häufig verwendetes Barrierenmaterial in FM/HL-Hybridstrukturen ist Magnesiumoxid (MgO). In dieser Arbeit werden dünne MgO-Schichten auf GaAs(001) an der PHARAO-Wachstumsanlage am BESSY II erzeugt. Dies geschieht durch getrenntes Verdampfen von metallischem Mg bzw. Einleiten von molekularem Sauerstoff in die Wachstumskammer. Um die Oxidation des Halbleitersubstrats zu verhindern, wird vor dem MgO-Wachstum eine dünne Mg-Schicht abgeschieden. Abhängig von der Dicke dieser Schicht sind zwei in-plane-Orientierungen des MgO relativ zum GaAs kontrolliert einstellbar. Darüber hinaus werden Hybridstrukturen mit Eisen Fe als Injektorschicht und schrittweise erhöhter MgO-Schichtdicke gewachsen. Die Eindiffusion von Fe in das GaAs-Substrat nimmt mit zunehmender MgO-Schichtdicke um mehrere Größenordnungen ab. / The generation of spin-polarized charge carriers in a semiconductor is a basic building block for the implemention of spintronic devices. A feasible approach to their implementation are ferromagnet/semiconductur(FM/SC) hybrid structures, whose fabrication is associated with some issues. The intermixing of the ferromagnetic material with the semiconductor leads to distortion of the electrical properties of the hybrid structure and the spin injection efficiency is reduced. By intentionally inserting a thin oxide layer into the FM/SC interface diffusion can be suppressed while the crystal quality and the spin injection efficiency of the structure are both increased. In this thesis the growth and characterization of thin oxide films fabricated by molecular beam epitaxy are discussed. Two rare earth oxides, La2O3 and Lu2O3, are grown on GaAs substrates and their crystal qualities are compared. Based on La2O3/GaAs(111)B full FM/SC hybrid structures are grown with the Heusler alloy Co2FeSi as injection layer and characterized by magnetic and electrical means. Another material used as a barrier in FM/SC hybrid structures is magnesium oxide (MgO). Here, thin MgO layers are grown on GaAs(001) at the PHARAO system at BESSY II. The growth is conducted by the separated evaporation of metallic Mg and introducing molecular oxygen into the growth chamber. To avoid oxidation of the semiconducting substrate a thin Mg layer is deposited prior to the MgO growth. Depending on the Mg layer thickness two different MgO in-plane orientations can be achieved with respect to the GaAs substrate. Furthermore, FM/SC hybrid structures with iron Fe as injection layer are grown while the MgO layer thickness is increased gradually. The indiffusion of Fe into the GaAs substrate is suppressed by several orders of magnitude with increasing MgO layer thickness.

Das Co2FeSi/GaAs-Hybridsystem wurde hinsichtlich seiner Eignung für Anwendungen in der Spintronik untersucht. Die Heusler-Legierung Co2FeSi ist ein aussichtsreicher Kandidat für derartige Anwendungen, weil der vollständig geordneten Kristallphase Halbmetallizität, d. h. eine Spinpolarisation von 100% an der Fermi-Energie, vorhergesagt wird. Zunächst wurde im Rahmen dieser Arbeit die elektrische Spininjektion und Spindetektion in lateralen Transportstrukturen in der sogenannten nicht-lokalen Konfiguration sowohl für die vollständig geordnete, als auch für eine teilweise ungeordnete Kristallphase mittels Spinventil- und Hanle-Messungen nachgewiesen. Die Abhängigkeiten der Spinsignale vom Strom und von der Temperatur konnten erklärt werden und eine Spininjektionsefizienz von 16 bzw. 9% wurde ermittelt. Für den praktischen Einsatz werden allerdings lokale Spinventile benötigt, deren Funktionsfähigkeit für beide kristallinen Ordnungen demonstriert wurde. Der Magnetowiderstand, der ein Maß für die Güte der lokalen Spinventile darstellt, beträgt 0.03% und liegt im Bereich des theoretisch zu erwartenden Wertes. Anhand des sogenannten Fert-Kriteriums konnten die Gründe für diesen niedrigen Wert aufgezeigt werden. Des Weiteren ließ ein Vergleich der lokalen und nicht-lokalen Spinsignale auf eine hohe Spinpolarisation des Co2FeSi schließen. Die Spinextraktion bietet neben der Spininjektion eine weitere Möglichkeit zur Erzeugung einer Spinakkumulation in einem Halbleiter. Die Stromabhängigkeiten von Spininjektion und Spinextraktion unterscheiden sich für beide kristallinen Phasen des Co2FeSi. Das stark unterschiedliche Verhalten konnte anhand des Einflusses der jeweiligen Bandstruktur auf die Spinerzeugung erklärt werden. Des Weiteren konnte aus dem Vergleich zwischen der Messungen und der theoretisch vorhergesagten Bandstruktur der halbmetallische Charakter der vollständig geordneten Kristallphase nachgewiesen werden. / The Co2FeSi/GaAs hybrid system was investigated regarding its suitability for spintronic applications. The Heusler-compound Co2FeSi is a promissing canditate for these kind of applications due to the predicted half-metallicity, i. e. a 100% spin polarisation at the Fermi energy, for its fully ordered crystall phase. The electrical spin injection and detection was demonstrated in lateral tranpost structures in the so called non-local geometry for the fully ordered and for a partly disordered crystall phase by observing spin valve signatures and Hanle characteristics. The current and temperature dependence of the spin signals was explained and a respective spin injection efficiencies of 16 and 9% determined. For practical use one needs local spin valves, which where demonstrated for both crystalline phases. The magnetoresistance, a measure of the goodness of a local spin valve, was 0.03%, i. e. in the theoretically expected range. Making use of the so-called Fert criterion the reasons for this low value could be pointed out. Further the quotient of the local and non-local spin signals implied a high spin polarisation of the Co2FeSi. Spin extraction is another method to create a spin accumulation in a semiconductor. The current dependece of the spin injection and the spin extraction signals strongly depends on the degree of ordering in the Co2FeSi lattice. The different behavior is explanied by the crucial influence of the respective electronic band structure on the spin generation processes. Further, the comparison between the measured signals and the theoretically calculated electronic band structure hints towards the half-metalicity of the fully ordered crystall phase of Co2FeSi.

Carrier, spin, charge current, and spin current injection by one- and two-color optical schemes are investigated within 30-band k·p theory. Parameters of the band model are optimized to give full-Brillouin zone band structures for GaAs and Ge that give accurate Γ-point effective masses and gyromagnetic factors and give access to the L valley, and to the E₁ and E₁+Δ₁ critical points in the linear optical absorption. Calculations of one- and two-photon carrier and spin injection and two-color current injection are performed for excitation energies in the range of 0—4 eV in GaAs and 0—3.5 eV in Ge. Significant spin and spin current injection occurs with 30% spin polarization in GaAs and Ge at photon energy matching the E₁ critical point. Further, the anisotropy and disparity of the current injection between parallel and perpendicular linearly-polarized beam configurations are calculated. For light propagating along a <111> crystal axis, anisotropic contributions in coherent current control and two-photon spin injection give rise to normal current components and in-plane spin components. In Ge, contributions from the holes to spin, electrical current, and spin current injection are investigated. Optical orientation results in 83% spin-polarized holes at the band edge. The effects of carrier dynamics in Ge are treated within a rate-equation model. The detection of spin dynamics in a pump-probe setup is considered, and the Fermi-factor approach is justified for electrons but not for holes. Carrier and current injection are further investigated in single-layer and bilayer graphene within the tight-binding model. In single-layer graphene, the linear-circular dichroism in two-photon absorption yields an absorption coefficient that is twice as large for circularly polarized light compared to linearly polarized light. Coherent current injection is largest for co-circularly polarized beams and zero for cross-circularly polarized beams. For linearly polarized beams, the magnitude of the injected current is independent of beam polarizations. In contrast, the injected current in bilayer graphene shows disparity between parallel and perpendicular configurations of the beams. The resulting angular dependence of the current is a macroscopic, measurable consequence of interlayer coupling in the bilayer.