Dissertations / Theses on the topic 'Tunnel junctions'

Spintronics discusses about fundamental physics and material science in mostly nanometer size structures. Spintronics also delivers many promising technologies for now and the future. One of the interesting spintronic structures is called “Magnetic Tunnel junction” (MTJ). A typical MTJ consists of a thin (1-3nm) insulator layer sandwiched between two ferromagnetic layers. In this work, I present MTJ with perpendicular magnetic anisotropy (PMA) using an MgO tunnel barrier. The effect of different heavy metals (HMs) adjacent to the ferromagnets (FMs) on tunneling magnetoresistance (TMR) and PMA of the junctions are discussed. Namely, Ta, Mo, Ta/Mo, W, Ir, and Hf have been utilized in HM/FM/MgO structures, and magneto-transport properties are explored. It is shown that when Ta/Mo is employed, TMR values as high as 208%, and highly thermally stable PMA can be obtained. Some physical explanation based on electronic band structure and thermochemical effects are discussed. In the last part of this work, the newly discovered tunneling anisotropic magnetoresistance (TAMR) effect in antiferromagnets is studied, and clear TAMR is demonstrated for NiFe/IrMn/MgO/Ta structures.

It has been predicted that gold, aluminum, and copper do not fundamentally change the graphene band structure when they are in close proximity to graphene, but merely increase the doping. My data confirms this prediction, as well as explores other consequences of the metal/graphene interface. First, I present a technique to fabricate thin oxide barriers between graphene and aluminum and copper to create tunnel junctions and directly probe graphene in close proximity to a metal. I map the differential conductance of the junctions versus tunnel probe and back gate voltage, and observe mesoscopic fluctuations in the conductance that are directly related to the graphene density of states. I develop a simple theory of tunneling into graphene to extract experimental numbers, such as the doping level of the graphene, and take into account the electrostatic gating of graphene by the tunneling probe. Next, results of measurements in magnetic fields will also be discussed, including evidence for incompressible states in the Quantum Hall regime wherein an electron is forced to tunnel between a localized state and an extended state that is connected to the lead. The physics of this system is similar to that encountered in Single Electron Transistors, and some work in this area will be reviewed. Finally, another possible method of understanding the interface between a metal and graphene through transport is presented. By depositing disconnected gold islands on graphene, I am able to measure resonances in the bias dependent differential resistance, that I connect to interactions between the graphene and gold islands.

Tunnel junctions for use in solar cells and monolithic multi junction solar cells are studied experimentally. The current density-voltage characteristic of an AlGaAs/AlGaAs tunnel junction having a mesa resistance of 0.11 mO·cm2 is determined using time-averaged measurements. A tunneling peak higher than the operating point of a solar cell is recorded by this method, with a value of ∼950 A/cm2. Due to the unstable nature of the negative differential resistance region of the current density-voltage curve, measurements of the tunneling peak and valley current densities are obscured. A time-dependent analysis is performed on this sample, from which a tunneling peak of a value larger than 1100 A/cm 2 is determined. An A1GaAs/InGaP tunnel junction having a tunneling peak of 80 A/cm2 is presented. Multi junction solar cells fabricated using indium tin-oxide as transparent top electrodes are measured. These cells have a maximal efficiency of 25.1% at 3 suns illumination and 26.1% at 20 suns, ∼40% lower efficiency than the standard multi junction solar cell.

Within this work an investigation into the tunnelling magnetoresistance (TMR) will be presented. A base numerical model is developed to describe the tunnelling through a magnetic tunnel junction (MTJ) so that a simple analytic model can be compared. These models have been extended to the crystalline barrier MTJs. This numerical model was based upon an enhanced Wentzel-Kramers-Brillouin (EWKB) method to describe the tunnelling current density. By correctly considering realistic MTJ parameters, the key result was found to be the correct handling of the effective masses in of the three MTJ layers. The extracted barrier-heights of 3.5-4eV is much higher than found previously and closer to the half band-gap result expected. It is then clear that the correct treatment of the parameters produces a far more realistic result. The key parameter which can be extracted from the I-V characteristics is the product b m*d V , where m* is the effective mass of the barrier, d is the effective barrier thickness and Vb is the effective barrier height. The analytic solution is a transparent model in which the key material parameters are visible and simple enough to be applied by experimental researchers to MTJs. The accurate modelling of both the prefactor and exponent are crucial to estimating the TMR. A simplified analytic result was produced that is in good agreement with numerical and experimental results. The numerical and analytic model are then extended to describe the TMR through a crystalline Fe(001)/MgO(001)/Fe(001) trilayer system. The calculation is based on the free-electron-like numerical solution providing a functional dependence of the TMR. The results were found to be in excellent agreement with the ab initio models and experiment. Furthermore a simplified analytic expression shows the TMR is dependent on the band-widths of the tunnelling electron states, the coupling and the thickness of the barrier. These models will be of great benefit to both experimental and theoretical researchers.

The demands from electronic devices have always been to be portable, fast, non-volatile, more intelligent and to consume low energy. One way towards this goal is to introduce multifunctionality of materials in devices. Ferromagnetism and ferroelectricity are two order parameters that can be coupled in a limited number of multiferroics and their coexistence implies the control over magnetisation and polarisation with both electric and magnetic fields. Similar properties were observed at ferromagnetic/ferroelectric thin film interfaces and attracted attention, since high quality thin film devices can be easily obtained nowadays through monitoring in real time of their structural and physical properties. This effect was observed also in tunnel junction configurations, devices which are formed from metallic electrodes separated by a very thin insulating barrier. By combining a barrier with various ferroelectric order parameters (ferroelectric, antiferroelectric, ferrielectric) and ferromagnetic electrodes, multi-field controlled multi-state non-volatile memory devices can be obtained. Tunnelling processes, especially in junctions containing d orbital elements are not completely understood and need deeper investigation. In this thesis, multiferroic tunnel junctions with La0:7Sr0:3MnO3/PbTiO3/Co structure are shown to be functional down to 3 unit cells. Moreover, the domain structure is shown to change with thickness, going through complex patterns including toroidal flux closure structures. The fabrication and working principle of the novel antiferroelectric tunnel junctions are reported for the first time using La0:7Sr0:3MnO3/PbZrO3/Co structures. Both investigated systems exhibit a multiferroic interface characterised by a magnetoelectric coupling which can be tailored by switching the ferroelectric polarisation.

Despite an extensive study and deep analysis of the tunnelling assisted transport the detailed processes that occur inside dielectrics are still unknown. The knowledge of these processes is crucial for the design and studies on future spintronic nanostructures. We propose a system of dusted MgO-based magnetic tunnel junctions to probe the processes inside the insulating barrier in order to gain more information about the physics of electron tunnelling.

El requisit de sistemes informàtics i d’emmagatzematge de dades d’alt rendiment en l’era de l’Internet de les coses (IOT) aconsegueix els límits de la tecnologia actual. Les memòries flaix DRAM i NAND presenten importants desavantatges com la volatilitat de les dades i limitacions de velocitat. D’altra banda, el coll d’ampolla de von Neumann limita el rendiment informàtic a l’imposar una limitació física per a la comunicació entre les parts de l’ordinador. S’han proposat noves tecnologies de memòria, com ReRAM, en dispositius alternatius que combinen alt rendiment, baix preu i alta densitat. Un tipus prometedor d’ReRAM és una unió de túnel ferroelèctric (FTJ), que és composta per una capa ultrafina de material ferroelèctric intercalat entre dos elèctrodes metàl·lics. La direcció de la polarització (P) modula les propietats de barrera a la interfície amb els electrodes, canviant la conductivitat dels electrons. Els estats d’alta i baixa resistència (HRS i LRS, respectivament) es poden estabilitzar en el dispositiu invertint P. Per tant, la informació es pot escriure aplicant un pols extern de voltatge per polaritzar la mostra en una direcció particular i emmagatzemar-la en la seva resistència. No obstant això, altres efectes impulsats pel camp elèctric també poden causar commutació resistiva en FTJ. Aquesta tesi té com a objectiu explorar els fenòmens de variació resistiva en unions de túnels ferroeléctricos de Hf0.5Zr0.5O2 que es poden utilitzar en l’emmagatzematge de dades d’alta eficiència. Òxids basats en HfO2 han estat utilitzats durant dècades com a elements en ReRAM causa de la seva canvi de resistència causat per reaccions redox. No obstant això, el descobriment de la ferroelectricidad al HfO2 dopat obre les portes per utilitzar la inversió de polarització com a fenomen per controlar la resistència i, per tant, per escriure informació. Aquí, s’utilitzen capes epitaxials d’HZO amb un gruix inferior a 5 nm. Anàlisi elèctrics i estructurals han permès identificar la coexistència de la inversió ferroelèctrica genuïna i moviment iònic com a mecanismes per induir canvis de resistència en el mateix elèctrode. Es va trobar que la variació de resistència causada per moviment iònic s’aprofita de les fronteres de gra incoherents entre les fases de gra ortorrómbica (ferroelèctrica) i monoclínica (paraeléctrica). A l’dissenyar la microestructura de la capa, utilitzant un substrat amb un paràmetre de xarxa diferent, s’optimitza la variació de resistència per inversió ferroelèctrica i se suprimeix el moviment iònic. A més, a l’segellar les fronteres de gra amb una capa dielèctrica addicional augmentar la finestra de voltatge d’escriptura per a la variació resistiva purament ferroelèctrica i va afectar significativament el rendiment de l’aparell. Capes ultrafines (2 nm) de HZO van ser crescudes en substrats scandate per demostrar robustes propietats memristivas. Es demostren cicles de potenciació / depressió reproduïbles i mesuraments de spike-timing-dependent plasticity (STDP). Aquests resultats, combinats amb la compatibilitat de les capes de HZO amb la tecnologia CMOS actual, la producció amigable amb el medi ambient i el bon rendiment, mostren que les unions de túnel ferroelèctriques de HZO són alternatives viables per a la seva aplicació en memòries no volàtils. A més, les propietats memristivas fonamentals de la modulació de la conducció mostren que poden usar-se en dispositius amb inspiració neuromòrfica.
El requisito de sistemas informáticos y de almacenamiento de datos de alto rendimiento en la era del Internet de las cosas (IoT) alcanza los límites de la tecnología actual. Las memorias flash DRAM y NAND presentan importantes desventajas como la volatilidad de los datos y limitaciones de velocidad. Por otro lado, el cuello de botella de von Neumann limita el rendimiento informático al imponer una limitación física para la comunicación entre las partes del ordenador. Se han propuesto nuevas tecnologías de memoria, como ReRAM, en dispositivos alternativos que combinan alto rendimiento, bajo precio y alta densidad. Un tipo prometedor de ReRAM es una unión de túnel ferroeléctrico (FTJ), que és compuesta por una capa ultrafina de material ferroeléctrico intercalado entre dos electrodos metálicos. La dirección de la polarización (P) modula las propiedades de barrera en la interfaz con los electrodes, cambiando la conductividad de los electrones. Los estados de alta y baja resistencia (HRS y LRS, respectivamente) se pueden estabilizar en el dispositivo invirtiendo P. Por lo tanto, la información se puede escribir aplicando un pulso externo de voltaje para polarizar la muestra en una dirección particular y almacenarla en su resistencia. Sin embargo, otros efectos impulsados por el campo eléctrico también pueden causar conmutación resistiva en FTJ. Esta tesis tiene como objetivo explorar los fenómenos de variación resistiva en uniones de túneles ferroeléctricos de Hf0.5Zr0.5O2 que se pueden utilizar en el almacenamiento de datos de alta eficiencia. Óxidos basados en HfO2 han sido utilizados durante décadas como elementos en ReRAM debido a su cambio de resistencia causado por reacciones redox. Sin embargo, el descubrimiento de la ferroelectricidad en el HfO2 dopado abre las puertas para utilizar la inversión de polarización como fenómeno para controlar la resistencia y, por lo tanto, para escribir información. Aquí, se utilizan capas epitaxiales de HZO con un grosor inferior a 5 nm. Análisis eléctricos y estructurales han permitido identificar la coexistencia de la inversión ferroeléctrica genuina y movimiento iónico como mecanismos para inducir cambios de resistencia en el mismo electrodo. Se encontró que la variación de resistencia causada por movimiento iónico se aprovecha de las fronteras de grano incoherentes entre las fases de grano ortorrómbica (ferroeléctrica) y monoclínica (paraeléctrica). Al diseñar la microestructura de la capa, utilizando un sustrato con un parámetro de red diferente, se optimiza la variación de resistencia por inversión ferroeléctrica y se suprime el movimiento iónico. Además, al sellar las fronteras de grano con una capa dieléctrica adicional aumentó la ventana de voltaje de escritura para la variación resistiva puramente ferroeléctrica y afectó significativamente el rendimiento del dispositivo. Capas ultrafinas (2 nm) de HZO fueron crecidas en sustratos scandate para demostrar robustas propiedades memristivas. Se demuestran ciclos de potenciación/depresión reproducibles y mediciones de spike-timing-dependent plasticity (STDP). Estos resultados, combinados con la compatibilidad de las capas de HZO con la tecnología CMOS actual, la producción amigable con el medio ambiente y el buen rendimiento, muestran que las uniones de túnel ferroeléctricas de HZO son alternativas viables para su aplicación en memorias no volátiles. Además, las propiedades memristivas fundamentales de la modulación de la conducción muestran que pueden usarse en dispositivos con inspiración neuromórfica.
The requirement for high-performance data storage and computing systems in the Internet of Things (IoT) era reaches the limits of the current technology. DRAM and NAND flash memories show significant drawbacks as data volatility and limitations of speed. On the other hand, Von Neumann bottleneck limits computing performance by imposing a physical limitation for communication between parts of the computer. New memory technologies, like resistive random-access memory (ReRAM), have been proposed as alternative devices that combine high performance, low price, and high density. One promising type of ReRAM is a ferroelectric tunnel junction (FTJ), which is composed of an ultrathin layer of ferroelectric material sandwiched between two metallic electrodes. The ferroelectric polarization (P) direction modulates the barrier properties at the interface with the electrodes, changing electrons' conductivity. A high and low resistance states (HRS and LRS, respectively) can be stabilized in a device by reversing P. Therefore, information can be written by applying an external voltage pulse to polarize the sample in one particular direction and store in its resistance. Nevertheless, other electric field-driven effects can also cause resistive switching in FTJ. This thesis aims to explore the resistive switching phenomena in Hf0.5Zr0.5O2 ferroelectric tunnel junctions that could be used in highly efficient data storage. HfO2-based oxides have been explored as ReRAM elements due to their resistance change caused by redox reactions. However, the discovery of ferroelectricity in doped-HfO2 opens doors to use polarization reversal as a phenomenon to control the resistance and, therefore, to writing information. Here, epitaxial HZO films with a thickness smaller than 5 nm are used. Electrical and structural analyses have allowed identifying the coexistence of genuine ferroelectric switching and ionic-like motion as mechanisms to induce resistance change in the same junction. It was found that ionic-motion-driven resistive switching takes advantage of incoherent grain boundaries between orthorhombic (ferroelectric) phase and monoclinic (paraelectric) phase grains. By engineering the film's microstructure, using a substrate with a different in-plane lattice parameter, the ferroelectric switching was optimized, and the ionic motion was suppressed. Also, sealing the grain boundaries with an extra dielectric layer increased the writing voltage window for purely ferroelectric switching and significantly impacted device performance. Ultrathin (2 nm) HZO films grown on scandate substrates were used to demonstrate robust memristive properties associated with polarization reversal. Reproducible potentiation/depression cycles and spike-timing-dependent plasticity (STDP) measurements were demonstrated. These results, combined with HZO films' compatibility with current CMOS technology, environmentally friendly production, and good performance, show HZO tunnel junctions are feasible alternatives for application in non-volatile memories. Also, memristive properties of modulation of conduction show they can be used in neuromorphic inspired devices.
Universitat Autònoma de Barcelona. Programa de Doctorat en Física

Les architectures d’ordinateur classiques sont optimisées pour le traitement déterministe d’informations pré-formatées et ont donc des difficultés avec des données naturelles bruitées (images, sons, etc.). Comme celles-ci deviennent nombreuses, de nouveaux circuits neuromorphiques (inspirés par le cerveau) tels que les réseaux de neurones émergent. Des nano-dispositifs, appelés memristors, pourraient permettre leur implémentation sur puce avec une haute efficacité énergétique et en s’approchant de la haute connectivité synaptique du cerveau.Dans ce travail, nous étudions des memristors basés sur des jonctions tunnel ferroélectriques qui sont composées d’une couche ferroélectrique ultramince entre deux électrodes métalliques. Nous montrons que le renversement de la polarisation de BiFeO3 induit des changements de résistance de quatre ordres de grandeurs et établissons un lien direct entre les états de domaines mixtes et les niveaux de résistance intermédiaires.En alternant les matériaux des électrodes, nous révélons leur influence sur la barrière électrostatique et les propriétés dynamiques des memristors. Des expériences d’impulsion unique de tension montrent un retournement de polarisation ultra-rapide. Nous approfondissons l’étude de cette dynamique par des mesures d’impulsions cumulées. La combinaison de leur analyse avec de l’imagerie par microscopie à force piézoélectrique nous permet d’établir un modèle dynamique du memristor. Suite à la démonstration de la spike-timing-dependent plasticity, une règle d’apprentissage importante, nous pouvons prédire le comportement de notre synapse artificielle. Ceci représente une avance majeure vers la réalisation de réseaux de neurones sur puce dotés d’un auto-apprentissage non-supervisé
Classical computer architectures are optimized to process pre-formatted information in a deterministic way and therefore struggle to treat unorganized natural data (images, sounds, etc.). As these become more and more important, the brain inspires new, neuromorphic computer circuits such as neural networks. Their energy-efficient hardware implementations will greatly benefit from nanodevices, called memristors, whose small size could enable the high synaptic connectivity degree observed in the brain.In this work, we concentrate on memristors based on ferroelectric tunnel junctions that are composed of an ultrathin ferroelectric film between two metallic electrodes. We show that the polarization reversal in BiFeO3 films can induce resistance contrasts as high as 10^4 and how mixed domain states are connected to intermediate resistance levels.Changing the electrode materials provides insights into their influence on the electrostatic barrier and dynamic properties of these memristors. Single-shot switching experiments reveal very fast polarization switching which we further investigate in cumulative measurements. Their analysis in combination with piezoresponse force microscopy finally allows us to establish a model describing the memristor dynamics under arbitrary voltage signals. After the demonstration of an important learning rule for neural networks, called spike-timing-dependent plasticity, we successfully predict new, previously unexplored learning curves. This constitutes an important step towards the realization of unsupervised self-learning hardware neural networks

Aquesta tesi estudia les propietats de magnetotransport en unions túnel on un dels elèctrodes és l’òxid ferromagnètic La0.7Sr0.3MnO3 (LSMO). En concret, ens interessem per dos fenòmens diferents: (i) magnetoresistència (MR) en unions túnel amb un sol elèctrode magnètic i (ii) filtratge d’espí en unions túnel magnètiques. L’efecte túnel és extremadament dependent de les interfícies i una bona qualitat de les heteroestructures resulta crucial per a obtenir un bon rendiment dels dispositius. És per aquest motiu que bona part d’aquesta tesi es dedica al creixement (per polvorització catòdica) i caracterització de capes primes, a l’estudi de les interfícies de les heterostructures i a la fabricació de les unions. Pel que fa a les unions amb un únic elèctrode magnètic, ens centrem en l’estudi del transport túnel en funció de la temperatura i del camp magnètic aplicat en unions de Pt/LaAlO3/LSMO. En el nostre treball hem identificat diferents mecanismes físics que juguen un paper important en la MR d’aquest sistema: la magnetoresistència túnel anisòtropa (TAMR), de l’ordre de 4 % a baixa temperatura, i una altra contribució a la magnetoresistència, de l’ordre de 17%. A més, TAMR a baix camp magnètic s’atribueix a la rotació de dominis magnètics. En el cas d’unions amb dos elèctrodes magnètics, també cal tenir en compte l’orientació relativa entre les magnetitzacions d’aquests. El sistema que estudiem és Fe/MgO/LSMO, en el qual s’espera un valor de magnetoresistència túnel (TMR) gran degut a la combinació del filtratge d’espín per part del Fe/MgO i la semimetal·licitat del LSMO. Com a conseqüència de la formació de FeOX en la interfície Fe/MgO, obtenim diferent signe de TMR per a diferents unions: una TMR negativa de 4% a baixes temperatures s’atribueix a un FeOX magnèticament desordenat i una TMR positiva de 25% a 70 K s’atribueix a la ordenació magnètica del FeOX a la intercara amb el MgO, que dóna lloc a filtratge d’espín. Quan el gruix de la barrera d’MgO es redueix a 1.2 nm, aquesta capa ordenada de FeOX s’acobla antiferromagnèticament amb la de LSMO donant lloc a un comportament molt interessant de la MR especialment quan es mesura aplicant el camp magnètic fora del pla de la capa. La formació de FeOX en aquesta estructura no s’ha aconseguit eliminar ni amb creixement in-situ ni amb recuits, i se suggereix que la barrera d’MgO és permeable a l’oxigen de la manganita, fet que duria a l’oxidació del Fe. Per altra banda, amb l’objectiu de fabricar unions amb una barrera túnel magnètica que actuï com a filtre d’espín, hem estudiat la possibilitat d’utilitzar capes primes de La2CoMnO6 (LCMO) com a barrera. Aquest material és ferromagnètic, aïllant i amb estructura de doble perovskita, però hi ha pocs treballs sobre la seva preparació en capes primes. A més a més, en aquests les capes estan crescudes per depòsit de làser pulsat i els gruixos són superiors a 100 nm, i per tant no aptes per actuar com barreres aïllants en filtres d’espí. L’objectiu principal s’ha orientat cap a un estudi detallat del creixement, optimització i caracterització de les propietats de capes primes de LCMO. En aquest sentit, s’han aconseguit capes (de 20 a 5 nm) epitaxials, aïllants i ferromagnètiques amb temperatures de Curie prop del 230 K i anisotropia magnètica perpendicular. S’han crescut heterostructures epitaxials de LCMO/LSMO, les propietats magnetoresistives de les quals es preveuen aprofundir en futurs treballs.
This thesis studies the magnetotransport properties of tunnel junctions in which one of the electrodes is the ferromagnetic oxide La0.7Sr0.3MnO3 (LSMO). In particular, we focus on two different phenomena: (i) magnetoresistance (MR) in tunnel junctions with a single magnetic electrode and (ii) spin filtering in magnetic tunnel junctions. The tunneling effect is extremely sensitive to the interfaces and good quality of the heterostructures is crucial toward the optimal performance of the devices. For this reason, much of the thesis is dedicated to the growth (by sputtering) and characterization of thin films, to the study of interfaces in heterostructures and to the fabrication of junctions. With respect to the junctions with a single magnetic electrode, we concentrate on the tunnelling transport as a function of temperature and magnetic field applied in Pt/LaAlO3/LSMO junctions. In our work, we have identified the different physical mechanisms which play a relevant role on the MR of this system: tunnelling anisotropic magnetoresistance (TAMR), of the order of 4 % at low temperature, and another contribution to the MR, of the order of 17 %. Furthermore, TAMR at low magnetic field is attributed to rotation of magnetic domains. In the case of junctions with two magnetic electrodes, we must also take into account the relative orientation between their magnetizations. The studied system is Fe/MgO/LSMO, in which a large tunnel magnetoresistance (TMR) is expected due to the combination of spin filtering from the Fe/MgO and the half-metallicity of LSMO. As a consequence of the formation of FeOX at the Fe/MgO interface, we obtain different sign of the TMR for different junctions: a negative TMR of 4 % at low temperatures is ascribed to a magnetically disordered FeOX and a positive TMR of 25 % at 70 K is attributed to magnetic ordering of the FeOX at the interface with MgO, which results in spin filtering. When the MgO barrier thickness is reduced to 1.2 nm, this ordered FeOX coupled antiferromagnetically to the LSMO layer gives rise to an interesting magnetoresistive behaviour, especially when measured with the magnetic field applied out-of-plane. We have not been able to avoid the formation of FeOX in this heterostructure, even for in-situ growth or annealings, and we suggest that the MgO barrier is permeable to the oxygen from the manganites, which would be responsible for the oxidation of the Fe. On the other hand, aiming at the fabrication of junctions with magnetic tunnel barrier which acts as spin filter, we have studied the possibility of using La2CoMnO6 (LCMO) thin films as barrier. This material is ferromagnetic, insulating and possesses perovskite structure, but there are only a few works on its thin film growth. What is more, such works are performed with pulsed laser deposition and thicknesses are above 100 nm, thus not suitable as insulating barriers in spin filters. We have performed a detailed study of the growth, optimization and characterization of LCMO thin films. In this regard, we have achieved epitaxial, insulating, ferromagnetic thin films (from 20 to 5 nm), with Curie temperatures around 230 K and perpendicular magnetic anisotropy. LCMO/LSMO heterostructures, whose magnetoresistive properties remain to be studied in future work, have also been grown.

Magnetic tunnel junctions (MTJs) have come to the technological fore in recent times due to their high applicability to recording media, and the wealth of information that can be obtained in understanding spin-polarised tunnelling phenomena. In 1997 it was first reported that performing a high-vacuum annealling step on MTJs significantly increased tunnelling magnetoresistance (TMR), the figure-of-merit in these devices. Since then research has continued apace into both understanding this increase and determining how to maximise TlvlR via annealling. Recently the annealling step has taken an even larger role in MgO based MTJs, as it is integral in creating the interfaces needed to obtain giant TMR. This thesis contains work based on annealling MTJs using two techniques that have previously not been used to study annealling, despite being highly applicable. The first technique is in situ transport measurements during annealling, which can be used to determine the barrier profile throughout the anneal process. The second technique is soft X-ray resonant magnetic scattering (SXRMS) which has two advantages; firstly, using the incoming X-rays tuned to an absorption edge, buried .interfaces can be probed and, secondly, by switching the photon helicity, magnetic properties of these interfaces can be monitored. The first study comprises the development of MgO plasma oxidised MTJs and uses in situ transport measurements to show that during annealling the Mn moves to the barrier interfaces, which affects the sample performance. The second and third studies use in situ transport in conjunction with SXRMS. One study investIgates the barrier evolution and interface sharpness during the anneal using AIO based MTJs. This shows that the improvement of the barrier quality appears to playa larger role than improving the magnetic interfaces. Lastly; CoFeBjMgOjCoFeB MTJs are measured by both techniques as they are annealed at various temperatures around the CoFeB crystallisation temperature. Upon annealling at 200°C the barrier quality is improved and TMR increases. Annealling at temperatures above this does not im;prove the barrier but causes part-crystallisation of the CoFeB causing a large increase in TMR. SXRMS results showed conflicting results depending upon the cumulative anneal process. These results were modelled qualitatively to understand the crystallisation/ diffusion behaviour seen in the MTJ.

Inelastic Electron Tunnelling Spectroscopy (IETS) [1-5] provides a means to characterise the phonon spectrum of a molecule by measuring the phonon-assisted tunnelling current through a potential barrier impregnated with target molecules. Traditionally, this technique has used Metal - Insulator - Metal (MIM) junctions, and the molecules of interest are adsorbed on to the insulator during junction fabrication. At low applied voltage V, tunnelling through the barrier is elastic. However, inelastic tunnelling caused by electron interaction with vibrational states in the adsorbed molecules can create additional conduction channels, occurring when V reaches a value of hω/e, where ω is a molecular vibrational mode. These lead to peaks in the d^2 I / dV^2 vs. V characteristics for each additional channel, giving a spectrum of the molecular vibration modes. As energy separations in the vibrational spectrum are relatively small compared to the electronic spectrum, the full vibrational spectrum is measured only at T < 30K. However, it may be possible to measure part of the spectrum even at room temperature, raising the possibility of a molecular detector. This project is concerned with fabricating nanoscale tunnel junctions based on Si nanowires (NWs) made by electron-beam lithography (EBL), for the purpose of IETS measurements, at 300K. A Si/SiO2 tunnel barrier/Al structure is used, where the Al NW crosses an oxidised Si NW. This allows the fabrication of tunnel junctions down to 50nm x 120nm in area and tunnelling occurs across a 10nm thick SiO2 layer. The reduction in device dimensions to the nanoscale may increase the sensitivity of the device to molecules adsorbed on the tunnel junction. Furthermore, the use of Silicon on insulator (SOI) material allows modulation of the tunnel junction using the back gate formed by the SOI substrate, control the Fermi energy and electron concentration in the NW, and hence the IETS characteristics of the device. In principle, an IETS sensor may be possible using such a configuration. In principle, a switchable IETS measurements are performed at 300K for ammonium hydroxide (NH4OH), acetic acid (CH3COOH), and propionic acid (C3H6O2) molecules. The I-V , dI/dV - V , and d2^I/dV^2-V characteristics of the tunnel junction are measured before and after the adsorption of molecules on the junction using vapour treatment or immersion. Peaks can be observed in the d^2I/dV^2-V characteristics in all the cases following molecules adsorption. These peaks may be attributed to vibrational modes of N-H and C-H bonds. Simulation of IETS characteristics modelled based on a combination of elastic, inelastic tunnelling and Schottky barriers at the Si / SiO2 / Al interface in the device. A comparison has been made between the simulation results and experimental measurements which showed very good agreement. This device modelling can be used to predict experimental characteristics and allow thermal broadening of the IETS peaks to be investigated.

Die vorliegende Arbeit befasst sich mit magnetischen Tunnelkontakten (magnetic tunnel junctions, MTJs) auf Basis des Oxids Zinkferrit (ZnxFe3-xO4). Dabei soll das Potential dieses Materials durch die Demonstration des Tunnelmagnetowiderstandes (tunnel magnetoresistance, TMR) in zinkferritbasierten Tunnelkontakten gezeigt werden. Dazu wurde ein Probendesign für MTJs auf Basis der „pseudo spin valve“-Geometrie entwickelt. Die Basis für dieseStrukturen ist ein Dünnfilmstapel aus MgO (Substrat) / TiN / ZnxFe3-xO4 / MgO / Co. Dieser ist mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Im Rahmen dieser Arbeit wurden die strukturellen, elektrischen und magnetischen Eigenschaften der Dünnfilme untersucht. Des weiteren wurden die fertig prozessierten MTJ-Bauelemente an einem im Rahmen dieser Arbeit entwickeltem und aufgebautem TMR-Messplatz vermessen. Dabei ist es gelungen einen TMR-Effekt von 0.5% in ZnxFe3-xO4-basierten MTJs nachzuweisen. Das erste Kapitel der Arbeit gibt eine Einführung in die spintronischen Effekte Riesenmagnetowiderstand (giant magnetoresistance, GMR) und Tunnelmagnetowiderstand (TMR). Deren technologische Anwendungen sowie die grundlegenden physikalischen Effekte und Modelle werden diskutiert. Das zweite Kapitel gibt eine Übersicht über die Materialklasse der spinellartigen Ferrite. Der Fokus liegt auf den Materialien Magnetit (Fe3O4) sowie Zinkferrit (ZnxFe3-xO4). Die physikalischen Modelle zur Beschreibung der strukturellen, magnetischen und elektrischen Eigenschaften dieser Materialien werden dargelegt sowie ein Literaturüberblick über experimentelle und theoretische Arbeiten gegeben. Im dritten Kapitel werden die im Rahmen dieser Arbeit verwendeten Probenpräparations- und Charakterisierungsmethoden vorgestellt und technische Details sowie physikalische Grundlagen erläutert. Die Entwicklung eines neuen Probendesigns zum Nachweis des TMR-Effekts in ZnxFe3-xO4-basierten MTJs ist Gegenstand des vierten Kapitels. Die Entwicklung des Probenaufbaus sowie die daraus resultierende Probenprozessierung werden beschrieben. Die beiden letzten Kapitel befassen sich mit der strukturellen, elektrischen und magnetischen Charakterisierung der mittels PLD abgeschiedenen Dünnfilme sowie der Tunnelkontaktstrukturen.

La détection des très faibles valeurs de champ magnétique est un enjeu important pour l’émergence à plus grande échelle de techniques pour le domaine du médical telles que la magnéto cardiographie, ou la magnétoencéphalographie. Les solutions existantes industrialisées reposent sur l’utilisation de jonctions tunnels supraconductrices qui permettent de fabriques des SQUIDS (Superconducting Quantum Intereference Device) qui sont les briques de base des magnétomètres avec des sensibilités de l’ordre de la dizaine de femtotesla. Cependant cette approche impose de travailler à des températures très basses qui ne sont accessibles qu’avec de l’hélium liquide. Un approche récente, développée par le Spec-CEA permet de travailler à l’azote liquide (77K) ce qui lève un certain nombre de contraintes. Le dispositif est un capteur mixte composé d’une boucle supraconductrice de grande taille qui contient une constriction de taille micrométrique sur laquelle est rapportée une magnétorésistance tunnel qui sert de sonde locale du champ magnétique. L’objectif du travail dans ce travail de thèse est de poursuivre le développement de ce type de capteur en utilisant visant des structures tout oxyde. En effet l’intégration complète de ce type de capteur permettrait de gagner encore en termes de performances et d’atteindre une résolution de l’ordre du femtotesla. Pour ce faire le travail vise à intégrer une jonction tunnel tout oxyde directement par épitaxie sur la constriction. La jonction tunnel sera réalisée à partie d’oxydes magnétiques tels que les composés LaSrMnO3 ou SrRuO3 qui sont deux matériaux ferromagnétiques à la température de l’azote liquide
Sensing of extremely weak magnetic signals, such as produced by electrical activity of the human heart and brain, still remains a challenge. A very promising alternative to established field-sensing techniques is a novel, spin electronic based, ultrasensitive device called an all-oxide mixed sensor. It is formed by a superconducting loop, acting as a flux-to-field transformer and field amplifier, combined with a magnetic tunnel junction sensing the field.Our research activities have the goal to improve the performance of the mixed sensor, focusing on its core component – the magnetic tunnel junction (MTJ). The capability of an MTJ is predominantly determined by the quality of the tunnel barrier and by the stability of magnetization states. In this context, oxide materials, known for their remarkable physical properties, have already shown their advantages. Thus, studies on La0.7Sr0.3MnO3/SrTi0.8Nb0.2O3 functional oxide interfaces, exploration of SrRuO3/ La0.7Sr0.3MnO3 exchange bias system, and the final integration of these two components into a magnetic tunnel junction form the main part of our work.In the presented thesis, oxide thin films and heterostructures used for studies were grown by pulsed laser deposition (PLD). We fabricated electronic devices for investigations using clean room microfabrication techniques , e.g. optical lithography, chemically assisted ion beam etching (CAIBE) and sputtering. Temperature dependent magnetic and (magneto-) transport measurements were performed.Metal-semiconductor interfaces formed by the half-metallic ferromagnet La0.7Sr0.3MnO3 (LSMO) and heavily doped semiconductor SrTi0.8Nb0.2O3 (Nb:STO) were studied. Antiferromagnetic coupling at the interface of the LaSrMnO3 and itinerant ferromagnet SrRuO3 was explored. Magnetic tunnel junctions with Schottky barrier were investigated (MTJs with Nb:STO and LSMRO)

In this research, Ni–NiOx–Cr and Ni–NiOx–ZnO–Cr metal-insulator-metal (MIM) junction based tunnel diodes have been investigated for the purpose of a wide-band detector. An MIM diode has a multitude of applications such as harmonic mixers, rectifiers, millimeter wave and infrared detectors. Femtosecond-fast electron transport in MIM tunnel diodes also makes them attractive for energy-harvesting devices. These applications require the tunnel diodes to have high current-asymmetry and non-linear current-voltage behavior at low applied voltages and high frequencies. Asymmetric and non-linear characteristics of Ni–NiOx-Cr MIM tunnel diodes were enhanced in this research by the addition of ZnO as a second insulator layer in the MIM junction to form metal-insulator-insulator-metal (MIIM) structure. Electrical characteristics were studied in a voltage range of for the single-insulator Ni–NiOx–Cr and double-insulator Ni–NiOx–ZnO–Cr tunnel diodes. Since the electrical characteristics of the diode are sensitive to material selection, material arrangement, thickness, deposition techniques and conditions, understanding the diode behavior with respect to these factors is crucial to developing a robust diode structure. Thus, ZnO insulator layer in MIIM junction was deposited by two different techniques: sputtering and atomic layer deposition (ALD). Also, the optical properties were characterized for the sputter deposited NiOx insulator layers by ellipsometry and the impact of annealing was explored for the NiOx optical properties. The Ni–NiOx–Cr MIM tunnel diodes provide low resistance but exhibit a low (~1) current-asymmetry. Asymmetry increased by an order of magnitude in case of Ni–NiOx–ZnO–Cr MIIM tunnel diode. The sensitivity of the MIM and MIIM diodes was 11 V-1 and 16 V-1, respectively. The results suggest that the MIIM diode can provide improved asymmetry at low voltages. The tunneling behavior of the device was also demonstrated in the 4-298K temperature range. It is hypothesized that the improved performance of the bilayer insulator diode is due to resonant tunneling enabled by the second insulator. Finally, the MIM and MIIM devices were investigated for wide-band detection up to 50GHz (RF) and 0.3THz (optical).

This thesis discusses spin-transfer torques in MgO-based magnetic tunnel junctions. The voltage-field switching phase diagrams have been experimentally determined for in-plane CoFeB/MgO/CoFeB magnetic tunnel junctions. In order to limit the effect of thermal activation, experiments have been carried out using nanosecond voltage pulses, as well as at low-temperature (4.2 K). The bias-dependence of the two spin-torque terms (Slonczewski-like and field-like) has been determined from thermally-excited ferromagnetic resonance measurements, yielding values which are in good agreement with previous reports. Additionally, material parameters such as the effective magnetisation and the damping factor have also been extracted. Using these values as input, the switching voltages as function of the applied magnetic field have been calculated numerically and analytically by solving the modified Landau-Lifshitz-Gilbert equation. Unlike previous studies, the field-like spin-torque has also been included. Moreover, different configurations have been considered for the magnetic anisotropy directions of the reference and free layer, respectively
Diese Arbeit befasst sich mit Spin-Transfer-Torque-Effekten in MgO-basierten magnetischen Tunnelstrukturen. Die Phasendiagramme als Funktion von Spannung und Magnetfeld von CoFeB/MgO/CoFeB-Tunnelstrukturen mit Magnetisierung in der Ebene wurden experimentell bestimmt. Um thermische Anregungseffekte zu limitieren, wurden die Experimente einerseits mit nanosekundenlangen Spannungspulsen und andererseits bei niedrigen Temperaturen (4.2 K) durchgeführt. Die Spannungsabhängigkeit der beiden Spin-Torque-Parameter (in-plane und senkrechter Spin-Transfer-Torque) wurde aus Messungen der thermisch angeregten ferromagnetischen Resonanz bestimmt, wobei sich Werte ergaben, die gut mit vorangegangenen Untersuchungen übereinstimmen. Zusätzlich wurden Werte für Materialparameter wie die effektive Magnetisierung und den Dämpfungsparameter gewonnen. Unter Verwendung der erhaltenen Werte wurden die Schaltspannungen als Funktion des angelegten Magnetfeldes analytisch und numerisch berechnet, indem die erweiterte Landau-Lifshitz-Gilbert-Gleichung gelöst wurde. Im Gegensatz zu vorangegangenen Untersuchungen wurde der senkrechte Spin-Transfer-Torque dabei mit einbezogen. Darüber hinaus wurden verschiedene Konfigurationen für die Richtung der magnetischen Anisotropie der freien und fixierten Schicht berücksichtigt

The discovery of large, room temperature magnetoresistance (MR) in magnetic tunnel junctions in 1995 sparked great interest in these devices. Their potential applications include hard disk read head sensors and magnetic random access memory (MRAM). However, the fabrication of repeatable, high quality magnetic tunnel junctions is still problematic. This thesis investigates methods to improve and quantify the quality of tunnel junction fabrication. Superconductor-insulator-superconductor (SIS) and superconductor-insulator ferromagnet(SIF) tunnel junctions were used to develop the fabrication route, due to the ease of identifying their faults. The effect on SIF device quality of interchanging the top and bottom electrodes was monitored. The relationship between the superconducting and normal state characteristics of SIS junctions was investigated. Criteria were formulated to identify devices in which tunneling is not the principal conduction mechanism innormal metal-insulator-normal metal junctions. Magnetic tunnel junctions (MTJs) were produced on the basis of the fabrication route developed with SIS and SIF devices. MTJs in which tunneling is the principal conduction mechanism do not necessarily demonstrate high MR, due to effects such as magnetic coupling between the electrodes and spin scattering. Transmission electron microscope images were used to study magnetic tunnel junction structure, revealing an amorphous barrier and crystalline electrodes. The decoration of pinholes and weak-links by copper electrodeposition was investigated. A new technique is presented to identify the number of copper deposits present in a thin insulating film. The effect of roughness, aluminium thickness and voltage on the number of pinholes and weak-links per unit area was studied. High frequency testing of read heads at wafer level was performed with a network analyser. Design implications for read head geometry were investigated, independent of magnetic performance. This technique has great potential to aid the rapid development of read and write heads whilst improving understanding of the system.

Since 1995, magnetic tunnel junction structures have been of great commercial interest because of their use in magnetic recording and data storage applications. In addition, the complex way in which the properties of the junction are influenced by the microstructure makes these structures of great scientific interest. The development of the capabilities of devices based on these structures has also lead to new applications for various materials. In particular, amorphous ferromagnets have become popular choices for use as ferromagnetic layers in these structures. Since the properties of magnetic tunnel junctions are determined by structural features less than 1 nm in size, a technique capable of studying these junctions with very high resolution is needed. In this thesis tunnel junction structures with two different types of barrier material are studied using a variety of transmission electron microscopy techniques. The effects of processing conditions were investigated for both structures. Electron microscopy was also used to investigate the origins ofuniaxial magnetic anisotropy in an important amorphous ferromagnet. The TiOx barrier of IrMnlCoFeffiOx/CoFe tunnel junctions formed by radical oxidation was found to be amorphous and its thickness highly dependent on oxidation time. Increased oxidation times led to the formation of oxides of Co and Fe from the lower ferromagnetic layer. Annealing was shown to have little effect on barrier thickness but does lead to diffusion of Mn to the barrier. Mn was observed to reduce oxides of Co and Fe and form MnOx in the barrier. In a study of PtMnlCoFe/AIOxfNiFe junctions, in which the barrier was formed by natural oxidation, the amorphous AIOx barrier width was found to be independent of oxidation time. The increase in the magnetoresistance of the higher oxidation time junctions was attributed to an increase in the barrier height of the oxide. In addition, evidence of oxidation of the ferromagnetic layers was found in a sample which had not been annealed. Magnetic anisotropy, induced by in-field annealing, was measured in amorphous CoFeB thin films and these films were then studied by electron diffraction to examine the short range order in this material. G(r) was obtained for directions parallel and perpendicular to the easy axis of magnetisation. No bond-length anisotropy was observed and an upper limit for the magnitude of the pair-ordering effect on the coordination of the transition metal and metalloid atoms was established.

Magnetic CoFe nanoparticles have been produced by gas-aggregation and incorporated into sputtered MgO tunnel junction structures. Scanning tunnelling microscopy (STM) has been developed as a technique for examining spin accumulation and transport in these nanoscale junctions. The particles were initially characterised for their magnetic properties; x-ray magnetic circular dichroism on 11-14~nm diameter clusters was performed. The orbital-to-spin moment ratio was found to be enhanced over the bulk value and to decrease with increasing average diameter, which complements previous studies on smaller particles. The size dependence of the combined data is found not to follow predicted trends based on reduced orbital moment quenching in the outer shell. In particular for these large particles, the quenching is far more rapid than expected. Magnetometry studies on random arrays of nanoparticles at percolation show interesting effects attributed to complex magnetic dipolar interactions. This includes very broad range anisotropy and large blocking temperatures. For transport measurements, cryogenic STM is used to address individual islands and forms the top electrode of a double magnetic tunnel junction. Single electron charging effects are observed in these confined structures and the charging energy correlates to the size of the particle. New theory was developed to simulate these structures, giving an analytical solution to the current numerical orthodox theory. These solutions showed that TMR measurements, a current major barrier to studying nanospintronics using STM, were unnecessary. We are able characterise the tunnel junction parameters, including spin polarisation and accumulation, in a single I-V sweep of high information density. The spin polarisations of the opposing electrodes are found to be aligned anti-parallel despite a parallel magnetisation axis. Finally the spin lifetime on the island was calculated and found to exceed 1 us, longer than measured in previous studies.

This thesis gives an account of experiments which investigate the detection of light by, and the emission of light from, planar metal-oxide-metal (M-O-M) tunnel junctions. The particular focus of attention is the mediation of these processes by surface plasmons, or surface electromagnetic waves bound to metal-dielectric interfaces, in the two processes. It describes how the coupling of incident bulk radiation to a surface plasmon supported by the junction structure may enhance the response of the device when used as a photodetector. This idea is then extended to cover other electromagnetic resonances supported by the junction system in different operating configurations. There is a brief departure from M-O-M devices to consider how a metal-semiconductor Schottky barrier diode may also have its photoresponse enhanced in a similar manner by coupling to a surface plasma wave localised at the metal-vacuum interface before returning to M-O-M devices to show that, in addition to their use as discrete detectors, they may also be used as integrated detectors of guided radiation. Attention is then turned onto the reverse process of light emission from M-O-M tunnel junctions. When these devices are 'rough' or are corrugated in some manner and pass a current they emit broadband light with an upper frequency cut-off determined by the applied bias, hv= e^V_bias. This light emission process is mediated by the surface plasmons of the structure, of which there are three in the energy range considered. Experimental results on the light emission from residually rough and deliberately roughened junctions are reported. In particular, the results of an experiment are presented which show that the majority of the radiation outcoupled from statistically rough devices is derived from the 'fast' surface plasmon localised at the outer metal surface.

Thermoelectricity can directly convert a temperature difference into a voltage or charge current. Recently, the development of spin caloritronics has introduced spin as another degree of freedom in traditional thermoelectrics. This discovery bodes a new generation of magnetic random access memories (MRAMs), where thermal spin-transfer torque (TSTT) rather than voltage driven spin-transfer torque (STT) is used to switch the magnetization in magnetic tunnel junctions (MTJs). To advance the rising trend of spin caloritronics, the coupling of charge, spin, and heat flow during electron transport in MTJs was systematically studied in this thesis. To begin with, the static transport properties of MTJs were studied by observing current dependent tunnel magnetic resistance (TMR). The observed decrease of TMR with a biased current is attributed to the change in spin polarization of the free ferromagnetic layer. A phenomenological model has been built based on the current dependent polarization, which agrees with our experimental results. Next, the Seebeck rectification effect in MTJs was studied. By applying microwave currents to MTJs, an intrinsic thermoelectric coupling effect in the linear response regime of MTJs was discovered. This intrinsic thermoelectric coupling contributes a nonlinear correction to Ohm's law. In addition, this effect can be controlled magnetically since the Seebeck coefficient is related to magnetization configuration. Finally, TSTT in MTJs was systematically studied. A laser heating technique was employed to apply a temperature difference across the tunnel barrier and ferromagnetic resonance (FMR) spectra were measured electrically through spin rectification. By analyzing the FMR spectra, TSTT in MTJs was observed and the angular dependence of TSTT was found to be different from dc-biased STT. By solving the Landau-Lifshitz-Gilbert equation including STT, the experimental observations were well explained. The discovery of Seebeck rectification refines the previous understanding of magneto-transport and microwave rectification in MTJs and provides a new possibility for utilizing spin caloritronics in high-frequency applications. The study of TSTT in MTJs shows clear experimental evidence of TSTT in MTJs. Further optimization of the design of MTJs may succeed in decreasing the necessary switching fields strength or even achieve a switching by only TSTT in MTJs.
February 2017

Spin-dependent tunneling in magnetic tunnel junctions (MTJs) has recently aroused great research interest and has developed into a separate branch of materials science. The large tunneling magnetoresistance (TMR) observed in MTJs got much attention due to possible applications in non-volatile random access memories and next generation sensors of magnetic field. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35037

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999.
Vita.
Includes bibliographical references (p. 129-134).
by Clifford Takashi Tanaka.
Ph.D.

Spintronics or electronics that utilizes the spin degree of freedom of a single charge carrier (or an ensemble of charge carriers) to store, process, sense or communicate data and information is a rapidly burgeoning field in electronics. In spintronic devices, information is encoded in the spin polarization of a single carrier (or multiple carriers) and the spin(s) of these carrier(s) are manipulated for device operation. This strategy could lead to devices with low power consumption. This dissertation investigates spin transport in one dimensional and two dimensional semiconductors, with a view to applications in spintronic devices.