Dissertations / Theses on the topic 'Heterostructures Heterostructures'
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1
Mand, R. S. "Characterisation and applications of heterostructures : Characterisation of GaAs/GaAlAs heterostructures and GaAs/GaAlAs double heterostructure electronic and photonic switches." Thesis, University of Bradford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371479.
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Leung, Chi Wah. "Metallic magnetic heterostructures." Thesis, University of Cambridge, 2002. https://www.repository.cam.ac.uk/handle/1810/34608.
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This work studied sputter deposited conventional spin valves (SV) and related structures. In SV layered structures, two ferromagnetic layers are separated by a non-magnetic spacer. Under an external magnetic field, the relative orientation of the magnetization changes in the ferromagnets, exhibiting the giant magnetoresistive effect. The controlled switching of ferromagnets in convention SV is facilitated by the exchange bias (EB) effect, which is achieved by depositing an antiferromagnetic layer next to one of the ferromagnetic layers in a magnetic field. Two highly related investigations were performed in this work. In the first part the exchange bias effect in the Ni80Fe20/Fe50Mn50/Co trilayer structure was studied. Samples were deposited in a low field condition that permitted EB to be established in NiFe/FeMn but not in FeMn/Co bilayer structures. Temperature-dependent magnetic measurements were performed on the trilayer sample, as well as the corresponding NiFe/FeMn and FeMn/Co bilayer samples. Recent literature on similar system showed that an AF spiral could be formed in the trilayer, which was probed by relative EB directions of the NiFe and Co layers. In this work, no exchange bias was found to propagate from the NiFe/FeMn system into the FeMn/Co system, showing that the AF spiral was induced by the specific magnetic treatment and was not the cause of EB effect. Besides, exchange bias field and coercivity of the samples indicated the influence of the EB system in the presence of an adjacent EB system. Explanations of the effect were made with some existing EB models. In the second part of the work, conventional SV of target structure Nb/NiFe/Cu/Co/FeMn/Nb was studied in a 'built-up samples' strategy. A batch of these built-up samples, which corresponded to the different stages of the deposition of the target top conventional SV structure, were prepared by terminating the sputtering process after a certain number of layers were deposited. These samples were thoroughly characterized by structural, magnetic and electrical measurements. In terms of structural characterization by x-ray techniques, more reliable information concerning the morphology and microstructure of the layers was obtained by probing the built-up samples, instead of relying solely on the information of the full SV structure. For the electric and magnetic measurements, a number of unexpected observations were made in the built-up samples, although the final performance of the full SV structure was of comparable quality to the literature. These results showed the ability of the 'built-up samples' strategy in critical characterization and optimization of magnetic multilayered structures.
3
Zhao, F. "Graphene-diamond heterostructures." Thesis, University College London (University of London), 2015. http://discovery.ucl.ac.uk/1462910/.
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Graphene obtained by mechanical exfoliation of graphite displays unique electronic properties with high mobility and saturation velocity. However, this is not a scalable technique, the film being limited to small area. Large area synthesis of good quality graphene has been achieved by CVD. The choice of substrate apparently influences the electronic properties of graphene. Most of reports have used SiO2-Si due to the widespread availability, but it is a poor choice of the material to degrade the graphene performace. In this thesis, more ideal platforms are introduced, including single crystal diamond (SCD), nanodiamond (ND), and diamond-like-carbon (DLC). It was found that different terminations for substrates caused strong effects for graphene properties. For H-terminated diamond, it was found that a p-type layer with good mobility and a small bang gap, whilst when N/F-terminations are introduced it was found that a layer with more metallic-like characters arises. Furthermore, different orientations of H-terminated SCD(100)/(111) were found to induce different band-gap of graphene. Simulation analysis proves the difference. However, the mobility results of graphene-H-terminated ND herostructure are better than graphene supported by SCD, which offers the prospect of low cost sp2 on sp3 technology. Raman and XPS results reveal the influence from the C-H band of ND surface. Impedance measurements show two conductive paths in the graphene-HND heterostructure. Graphene FET was built on this heterostructure, which exhibited n-type and high mobility. The family of amorphous carbon films, DLC, appeal to a preferable choice of graphene supporting substrate since IBM built the high-frequency graphene FET on DLC. For N-termination it was found that the optical band gap of DLC shrinked, whilst for F-terminated DLC it was found that fluorine groups reduce the DLC’s surface energy. Owing to different phonon energies and surface trap densities, graphene-DLC heterostructures give different electronic properties and offer the prospect for 2D lateral control applications.
4
Slobodskyy, Taras. "Semimagnetic heterostructures for spintronics." Doctoral thesis, [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=983425892.
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Fromhold, Timothy Mark. "Magnetotunnelling in semiconductor heterostructures." Thesis, University of Nottingham, 1990. http://eprints.nottingham.ac.uk/14162/.
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Experimental studies of magnetotunnelling in heterostructures have revealed series of resonances due to electrons tunnelling from a 2DEG in a lightly-doped emitter into magnetoquantised states in the collector contact of a single-barrier structure (Hickmott, 1987 and Snell et al. 1987) or in the quantum well of a double-barrier structure (Eaves et a1., 1988 and Leadbeater et a1., 1989). These experiments are very suitable for theoretical analysis since a transverse magnetic field (parallel to the barrier interfaces) has little effect on the electronic states of the 2DEG, provided the diamagnetic energy is much less than the binding energy of the bound state of the accumulation layer potential. The tunnelling electrons then have a small range of transverse momenta between +PF and -PF, where PF = l'lkF is the Fermi momentum in the 2DEG. This range determines the positions of the orbit centres of the magnetoquantised states into which the electrons are injected after emergence from the tunnel barrier. For the single-barrier heterostructures described in this thesis, these are interfacial Landau states corresponding to classical orbits in which the electron skips along the barrier interface. For double-barrier structures there are interfacial states at high magnetic fields and traversing states at low magnetic fields. Owing to the high electric field in the quantum well, the corresponding classical orbits are cycloidal trajectories which intersect both barrier interfaces (traversing states) or just one barrier interface (skipping states). The variation of the tunnel current I with magnetic field B and voltage V is calculated using the Bardeen transfer-Hamiltonian approach within a WKB approximation. The accumulation layer potential is modelled according to a simple variational solution. This enables a physical interpretation of the experimental results to be given in terms of the effect of the magnetic field on the effect ive barri er hei ght and the ampli tudes of the magnetoquantised wave functions at the barrier interfaces. Both of these effects are required to account for the observed dependence of current on magnetic field I(B) and the amplitudes of the oscillatory structure revealed in the derivative plots of dI/dB and d2I/dB2 accounts for: The model (a) the observation of two series of resonances corresponding to +PF and -PF electrons in experiments on (InGa)As/InP single-barrier structures. (b) the absence of the +PF series of resonances in GaAs/(A1Ga)As single-barrier structures. (c) the changeover from traversing to skipping states in GaAs/(A1Ga)As double-barrier structures and the characteristic decrease in oscillatory amplitudes in the changeover region.
6
Ko, D. Y. K. "Quantum tunnelling in heterostructures." Thesis, University of Exeter, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384673.
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Chung, Sung-Yong. "Si/SiGe heterostructures materials, physics, quantum functional devices and their integration with heterostructure bipolar transistors /." Columbus, Ohio : Ohio State University, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1132244278.
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Colson, Harry George. "Strain relaxation in semiconductor heterostructures." Thesis, University of Surrey, 1997. http://epubs.surrey.ac.uk/843607/.
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The strain relaxation behaviour of and oriented InxGa1-xAs layers grown on GaAs substrates has been investigated using surface profilometry and high resolution X-ray diffraction. The strain relaxation behaviour of single InxGa1-xAs layers (grown by various methods) has been studied using double crystal X-ray diffraction. The layer strain has been shown to be predictable, following the empirical relaxation law given by epsilonr = k/d where epsilonr is the residual strain, d is the layer thickness and k is a constant (= 0.84 nm +/- 0.18 nm). In addition, it is shown, using previously published data, that this law is applicable to other semiconductor alloy systems. Results show that the relaxation behaviour of oriented material is very similar to that of oriented material. However, the critical thickness is 1.23 times that for material in agreement with theory. It is shown that plastic relaxation of good quality epitaxially strained layers is accompanied by roughening of the surface in the form of striations and that the maximum striation height is always less than 20 nm regardless of layer thickness. Measurements of strain relaxation in multi-quantum well type samples show good agreement with a simple geometric theory in which the minimum barrier thickness needed to decouple strained layers of thickness hw is given as 0.62hw.
9
Warburton, Paul Anthony. "Quasiparticle trapping in superconducting heterostructures." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318419.
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10
Leadbeater, Mark Levence. "Resonant tunnelling in semiconductor heterostructures." Thesis, University of Nottingham, 1990. http://eprints.nottingham.ac.uk/28733/.
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This thesis examines the electrical transport properties of a series of n-type GaAs/(AIGa)As double barrier resonant tunnelling devices with well widths between 50 angstrem and 2400 angstrem . The current-voltage characteristics show peak-to-valley ratios as high as 25:1 and as many as seventy resonances, with clear evidence of quantum interference effects at room temperature. The application of a high magnetic field parallel to the current flow produces magnetooscillations in the transport properties which allow the sheet charge density in the accumulation layer to be determined. The energy level in the well over a wide range of bias is obtained from analysis of thermal activation of resonant tunnelling. The contributions of elastic scattering and LO phonon emission to the valley current are investigated spectroscopically with a magnetic field and two phonon modes of the (AIGa)As barrier are observed. The buildup of space charge in the quantum well at resonance leads to intrinsic bistability in the current and differential capacitance of an asymmetric structure. Magnetoquantum oscillations due to a degenerate electron gas in the well are used to measure this charge buildup and demonstrate that the tunnelling process is truly sequential. The bistability is dramatically enhanced at high magnetic fields when the lowest energy Landau level of the well can accommodate a high electron density. In a strongly asymmetric sample, a new kind of bistability is observed where the off-resonant current exceeds the resonant current due to enhancement of charge buildup by intersubband scattering. The modulation of the scattering rate by a magnetic field produces periodic oscillations in the width of the bistability. In a magnetic field applied perpendicular to the current, the resonances are broadened as a consequence of the conservation of canonical momentum. The transition from electric to magnetic quantisation in wide wells is investigated and tunnelling into interfacial Landau levels is observed. The angular dependence of the resonances is used to probe conduction band anisotropy. In a tilted field, a completely new type of magneto-oscillations is observed.
11
Chrastina, Daniel. "Transport in silicon-germanium heterostructures." Thesis, University of Warwick, 2001. http://wrap.warwick.ac.uk/2574/.
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The work presented here describes the electrical characterization of n- and p-type strained silicon- germanium systems. Theories of quantum transport in low magnetic fields at low temperature are discussed in terms of weak-localization: the traditional theory is shown not to account for the dephasing in a 2-dimensional hole gas behaving in a metallic manner and emergent alternative theories, while promising, require refinement. The mobility as a function of sheet density is measured in a p-type pseudomorphic Si0.5Ge0.5 across the temperature range 350 mK–282 K; it is shown that calculations of the mobility based on semi-classical scattering mechanisms fail below 10 K where quantum transport effects become relevant. A room temperature Hall scattering factor has been extracted. A new functional form has been presented to fit the resistivity as a function of temperature, below 20 K: traditional theories of screening and weak localization appear not to be applicable. It is also demonstrated that simple protection circuitry is essential if commercial-scale devices are to be meaningfully investigated. Mobility spectrum analysis is performed on an n-type strained-silicon device. Established analysis methods are discussed and a new method is presented based on the Bryan’s Algorithm approach to maximum entropy. The breakdown of the QHE is also investigated: the critical current density compares well to that predicted by an existing theory. Finally, devices in which both electron and hole gases can be induced are investigated. However, it is shown that the two carrier species never co-exist. Design rules are presented which may allow more successful structures to be created. Results are presented which demonstrate the success and the utility of implanted contacts which selectively reach different regions of the structure.
12
White, James Antony. "Many-body effects in heterostructures." Thesis, University of Exeter, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235959.
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Sarkozy, Stephen Joseph. "Mesoscopic transport in undoped heterostructures." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612534.
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English, David. "Spin dynamics in semiconductor heterostructures." Thesis, University of Southampton, 2012. https://eprints.soton.ac.uk/340899/.
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This thesis presents experimental investigations of the electron spin dynamics (i.e. the spin relaxation rate, This thesis presents experimental investigations of the electron spin dynamics(i.e. the spin relaxation rate, Γs, and the g-factor, g ∗ ) inGaAs/ AlGaAs based semiconductor heterostructures and the effect of symmetry breaking perturbations such as; an applied external electric field, shear strain or graded alloy composition in the barriers. Spin-polarised electron populations are generated and detected using optical methods. Quantum wells grown on a (001) zinc-blende substrate display isotropic inplane spin dynamics. The above perturbations act to lower the symmetry and therefore the spin dynamics are allowed to show in plane anisotropy. However, the microscopic origin of the anisotropy of Γs is different to that of g∗ . This thesis contains a full study of the anisotropy of both Γs and g∗ for all of the above perturbations. This reveals the microscopic effects on the band edges of the perturbations. It has previously been shown theoretically that strain applied in the plane of (110) or (111) oriented quantum wells can act to cancel the spin-orbit field from bulk inversion asymmetry. In this thesis, we study the effect on the spin dynamics of of strain applied to a (110) quantum well and also investigate a (111) sample for its suitability in such experiments.
15
Wee, Siew Fong. "Interdiffusion of semiconductor alloy heterostructures." Thesis, University of Surrey, 1998. http://epubs.surrey.ac.uk/844156/.
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This thesis is concerned with a quantitative study of intermixing in GaAs/AlGaAs and ZnSe/ZnCdSe single quantum well semiconductor structures. In this study, a method of iterative isothermal anneals and photoluminescence used to characterize this phenomenon has enabled the evolution of the diffusion coefficients for the interdiffusion process with anneal time to be followed. The blue-shift emissions arising from this method are predicted by a model based on Fick's law of diffusion. This model is developed in an attempt to relate the energy shift that is observed experimentally to the diffusion length. The mixing is modelled using an error function expression to solve the diffusion equation so as to describe the variation in well shape which is attributed to compositional disordering induced during thermal processing. Using this approach, where evidence of intermixing was monitored, the emission would be expected to shift measurably. Data has been taken to cover a wide temperature range to establish values for the activation energy EA. From this data, it has been found that the diffusion coefficients at various temperatures are thermally activated with an energy of 3.6 +/- 0.2 eV in GaAs/AlGaAs. The data is compared with the available literature data taken under a wide range of experimental conditions. We show that despite the range of activation energies quoted in the literature all the data appears to be consistent with a single activation energy. Departures from the 'mean' value are ascribed to experimental uncertainties in determining the diffusion coefficients for example, to fluctuations in the composition of the material, to techniques used, or to a wide range of perturbations. Photoluminescence observations on ZnSe/ZnCdSe show that an improvement in the optical quality of these quantum well structures was found for anneals at temperatures (~500°C). A value of EA = 2.9 +/- 0.3 eV was derived from the experiments for the interdiffusion process over a 250 K temperature range and four decades of interdiffusion coefficient. The interdiffusion process of both these systems was inferred to be Fickian with no dependence on alloy composition or strain.
16
Yu, Sisheng. "Spin Dynamics in Antiferromagnetic Heterostructures." The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1586599000240225.
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Sarı, Hüseyin. "DX centers in InAlAs heterostructures /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2001. http://wwwlib.umi.com/cr/ucsd/fullcit?p9997570.
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Boddison-Chouinard, Justin. "Fabricating van der Waals Heterostructures." Thesis, Université d'Ottawa / University of Ottawa, 2018. http://hdl.handle.net/10393/38511.
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The isolation of single layer graphene in 2004 by Geim and Novoselov introduced a
method that researchers could extend to other van der Waals materials. Interesting and new properties arise when we reduce a crystal to two dimensions where they are often different from their bulk counterpart. Due to the van der Waals bonding between layers, these single sheets of crystal can be combined and stacked with diferent sheets to create novel materials.
With the goal to study the interesting physics associated to these stacks, the focus of this work is on the fabrication and characterization of van der Waals heterostructures.
In this work, we first present a brief history of 2D materials, the fabrication of heterostructures, and the various tools used to characterize these materials. We then give a description of the custom-built instrument that was used to assemble various 2D heterostructures followed by the findings associated with the optimization of the cleanliness of the stack's interface and surface. Finally, we discuss the results related to the twisting of adjacent layers of stacked MoS2 and its relation to the interlayer coupling between said layers.
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Rodriguez-Vega, Martin Alexander. "Disorder Effects in Dirac Heterostructures." W&M ScholarWorks, 2016. https://scholarworks.wm.edu/etd/1477068246.
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In this dissertation, we study theoretically heterostructures based on Dirac mate- rials, i.e. materials, such as graphene in which the electrons behave as massless Dirac fermions at low energies. We first examine how the presence of long-range disorder affects the electronic ground state of a double layer graphene heterostruc- ture formed by two graphene layers separated by a thin dielectric film. We then identify the necessary conditions for the formation of an interlayer exciton conden- sate in such a system. We also comment on the effect of long-range disorder on the broken symmetry ground state induced by electron-electron interactions in bilayer graphene. Then, we study the transport properties of heterostructures obtained by stacking a graphene layer on the surface of a strong three-dimensional topological insulator (TI). In particular, we determine the non-equilibrium current-induced spin density accumulation for these systems using linear response theory and taking into account the effects of long- and short-range disorder both in the limit of strong and weak tunneling between the graphene layer and the TI. Finally, using some of the theoretical approaches developed to characterize the effect of long-range disorder in Dirac materials, we study the effect of long-range inhomogeneities in first-order phase transitions. In particular, we present a theoretical model to describe the ef- fect of inhomogeneities on the relaxation dynamics of vanadium dioxide films after a photo-induced metal-insulator transition.
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Salomon, Damien. "Croissance, propriétés optiques et intégration d'hétérostructures radiales InGaN/GaN autour de fils auto-assemblés de GaN crûs sur saphir et silicium." Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENY052.
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Ce travail est consacré à la réalisation de diodes électroluminescentes visibles à base de fils de GaN crûs sur Si(111) par épitaxie en phase vapeur de précurseurs organo-métalliques. Nous cherchons en particulier à comprendre les mécanismes de croissance des fils de GaN et les propriétés structurales et optiques de puits quantiques InGaN/GaN cœur/coquille déposés autour de ceux-ci. La croissance de fils orientés le long de l'axe -c sur saphir est dans un premier temps détaillée et expliquée. Nous montrons que l'injection de silane pendant la croissance des fils permet de former une couche de passivation de SiNx autour de ceux-ci. L'arrêt de l'injection de silane après quelques dizaines de secondes ne modifie pas la géométrie fil et ce procédé peut donc être utilisé pour contrôler le positionnement le long du fil de la zone de dépôts des puits quantiques InGaN/GaN. Ce procédé est ensuite transféré sur substrat Si(111) grâce au dépôt préalable d'une fine couche tampon d'AlN sur le substrat. Le dépôt de puits quantiques InGaN/GaN sur les facettes non-polaires m de ces fils et l'influence de différents paramètres de croissance sur leur émission de lumière sont étudiés. Nous montrons notamment l'existence de plusieurs familles des puits quantiques dans les fils dont les longueurs d'onde d'émission ont pu être indexées à l'aide de cartographies de cathodoluminescence. La concentration en indium des puits quantiques déposés a été estimée en comparant les énergies d'émissions des puits à des simulations utilisant la théorie k.p dans l'approximations 8 bandes pour les électrons et les trous et est comprise entre 8 et 24%. Enfin, des structures LED complètes ont été déposées sur les fils de GaN par MOVPE et une électroluminescence bleue à 450 nm à température ambiante est mesurée sur des fils uniques et sur des assemblées de fils sur siliciumThis work reports on the realization by metal organic vapor phase epitaxy of visible light emitting diodes based on GaN wires grown on Si(111) with a focus on understanding the wires growth mechanisms and the properties of InGaN/GaN core/shell heterostructures grown around them. First we report the MOVPE growth of –c oriented GaN wires on sapphire. We demonstrate that the injection of silane during the growth induces the formation of a SiNx passivation layer around the GaN wires, preventing the lateral expansion. The silane flow can be stopped after a certain time without modifying the wire geometry. This phenomenon is used to control the position of the InGaN/GaN multiple quantum well shells along the wires. The wire growth on sapphire has then been transferred to silicon substrate thanks to the deposition of a thin AlN buffer layer prior to the wire growth. The deposition of InGaN/GaN core/shell heterostructures on the non-polar m-plane side facets of the wires and the influence of different growth parameters on the light emission properties of the quantum wells are then studied. Several types of quantum wells grown on different facets of the wire surface are observed. These different families emit light at different wavelengths that have been indexed thanks to cathodoluminescence mapping. The indium concentration in the quantum wells deposited is estimated between 8 and 24 %, depending on the growth conditions. This estimation has been made by comparing the emission wavelength of the quantum well to the recombination energy of electrons and wells simulated using the 8x8 band k.p theory for electron and hole masses. Finally, complete LED structures have been deposited on GaN wires by MOVPE and blue electroluminescence at 450 nm has been measured on single wires and assemblies of wires on Si(111)
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González, Cuxart Marc. "Magnetic metal-organic / topological insulator heterostructures." Doctoral thesis, Universitat Autònoma de Barcelona, 2019. http://hdl.handle.net/10803/667359.
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Topological Insulators (TIs) have become one of the wonder materials of condensed
matter physics over the last decade due to their novel properties, possessing
an insulating bulk in coexistence with metallic boundaries. They present
an inverted band gap consequence of strong spin orbit coupling, which gives rise
to the conductive boundary states with linear dispersion, characteristic of Dirac
fermions, and helical spin polarization. Numerous materials have been predicted
and observed to have TI signatures, holding great perspective for the realization
of novel applications in spintronics, quantum computing and metrology.
The experimental realization of three-dimensional TIs with the the Fermi
Level located well in the bulk band gap is a challenging task because of their
relatively small gap of hundreds of meV, and their high sensibility to crystal
defects and impurities. These can induce electron doping that activates bulk
conduction channels, thus burying the contribution of the surface states to the
transport. Molecular Beam Epitaxy (MBE) has been reported to be the most
suitable growth method to overcome this hindrance, due to its capability to grow
single crystals with fine control over the crystal defects and impurity level. The
first part of this thesis deals with the growth of high-quality TIs that maintain
their pristine insulating bulk behaviour. By using MBE, we studied the impact of
different substrates and growth parameters to the synthesis of Bismuth Telluride
(Bi2Te3) thin films, and the growth of the ternary compound Bismuth-Antimony
Telluride.
We were able to grow insulating Bi2Te3 thin films with complete suppression
of the \twin" domains, mirror-symmetric domains that contribute to the
self-doping of the crystal. By a combination of the initial interaction with the
lattice-matched Barium Fluoride substrate and the high working temperatures,
the growth of Bi2Te3 single-crystalline films is achieved already from the first
layer. More importantly, the films present low-doping level with the the Fermi
Level kept in the bulk band gap. The correlation between the lack of \twin" domains
(measured by Re
ection High-Energy Electron Diffraction, X-ray Diffraction
and Atomic Force Microscopy) and the low-doping level measured by Angle-
Resolved Photoemission Spectroscopy (ARPES), indicates the relation between
the crystal quality and the capability to preserve the bulk insulating character.
This result contrasts to other TIs grown on more conventional substrates,
typically presenting large lattice misfits that lead to the formation of an initial
polycrystalilne or amorphous seed layer. In parallel, we explored a complemeniii
tary approach to the growth of insulating Bi2Te3, based on the addition of Sb at
the expense of Te atoms. A sequence of Bismuth-Antimony Telluride films with
different x content were measured by X-ray Photoemission Spectroscopy (XPS)
and ARPES, showing that the the Fermi Level can be gradually brought to the
bulk valence band. The realization of such TIs, with a controlled level of the the
Fermi Level position is of special interest for counteracting the n-doping effects
typically induced by the addition of magnetic materials.
The second and more extended part of this thesis is devoted to the study
of interfaces formed by magnetic Metal-organic molecules deposited on the TI
thin films. Interfacing TI surfaces with magnetic materials can give rise to novel
magnetoelectronic phenomena, involving the manipulation of spin-torques (Inverse
Edelstein Effect), or the realization of spin polarized edge states (Quantum
Anomalous Hall Effect). The realization of such spin-related effects rely on the
capability to control the interfacial magnetic and electronic interactions. The
use of organic molecules to cage magnetic ions has been proved to be a versatile
approach to engineer inter-ions and ions-surface interactions, due to the
exible
design that molecules offer and to their ability to form structurally perfect selfassembled
structures. Moreover, they can also act as building blocks for covalent
or coordination structures via on-surface reactions.
As a first approach to tune the interfacial properties with Metal-organic molecules,
we showed how the ligand chemistry allows a progressive control over the
magnetic interactions between a hosted Co ion and a prototypical Au surface.
The spin states and magnetic moments are comprehensively studied thanks to the
complementary use of local spectroscopic Scanning Tunneling Spectroscopy and
non-local magnetic sensitive X-ray Magnetic Circular Dichroism (XMCD) techniques,
which are supported theoretically by Density Functional Theory (DFT).
We were able to continuously cover the range of magnetic Co ion-substrate interactions,
from a strong interacting scenario where the magnetic moment is
quenched, to a gradual decrease of the interaction revealed by a lower Kondo
screening of the spin. In addition, by changing the Au surface for a TI surface,
the interfacial interactions reach the weakest limit in which the molecular
magnetic structure is completely decoupled from the substrate electrons.
Thereafter, we explored the electronic and magnetic interactions between the
Topological Surface State of the Bi2Te3 thin film and Co ions caged in two different
planar molecules such as Cobalt - Tetrakis (4-Promophenyl) Porphyrin
(CoTBrPP) and Cobalt - Phthalocyanine (CoPc). We found a Metal-organic /
TI interface with unperturbed electronic and magnetic properties. This is assessed
by a coverage dependent ARPES study in which the Topological Surface
State persists upon the deposition of one (CoTBrPP or CoPc) molecular layer.
On the other hand, XMCD and Scanning Tunneling Spectroscopy measurements
reveal the preservation of the pristine CoTBrPP magnetic moment and electronic
structure respectively. Furthermore, a comprehensive Scanning Tunneling Microscopy
(STM) and DFT study of the CoTBrPP adsorption geometry describes
weak molecule-surface interactions, and corroborates the electronic decoupling of
the Metal-organic layer from the TI surface. In an analogue study with CoPc we
find slightly stronger interactions yet within the non-perturbative regime, that
suggesting ligand chemistry can be used to tune magnetic interactions without
affecting the overall properties of each component of the heterostructure.
Subsequently, the Br-functionalized CoTBrPP on Bi2Te3 system was used to
induce on-surface synthesis of Metal-organic coordination networks on TI. These
more entangled structures are of great interest as a framework in which magnetic
ions could arrange in ordered and mechanically stable arrays. Two different
coordination phases are selectively created after CoTBrPP dehalogenation upon
thermal activation. We track the chemical reaction by XPS, and investigate the
morphological and electronic properties of the final products by combining Scanning
Tunneling Spectroscopy (STS) and DFT calculations. We conclude that the
resulting structures consists of CoTPP coordinated with Te atoms incorporated
from the substrate, and thanks to the supporting DFT calculations, we are able to
explain the presence of linear chains and irregular coordinated networks. In parallel,
the presence of unperturbed Topological Surface State upon the formation
of the Metal-organic structures is confirmed by a coverage-dependent ARPES
study.
Overall, the first part of the thesis constitutes an extensive study of MBE
grown of Bi2Te3 thin films, in which different substrates and growth conditions
are discussed. Furthermore, the results provide a route for the enhancement of
the crystal quality of simple diatomic TIs, crucial for the preservation of their
bulk insulating behaviour. The results presented in the second part conceive the
capabilities of organic molecules to tune magnetic interactions between Co atoms
and Bi2Te3 films, and pave the way for the on-TI surface synthesis of magnetic
supramolecular structures.
22
Thurmer, Dominic J. "Nanomembrane-based hybrid semiconductor-superconductor heterostructures." Doctoral thesis, Universitätsbibliothek Chemnitz, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-71217.
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The combination of modern self-assembly techniques with well-established top-down processing methods pioneered in the electronics industry is paving the way for increasingly sophisticated devices in the future[1]. Nanomembranes, made from a variety of materials, can provide the necessary framework for a diverse range of device structures incorporating wrinkling, buckling, folding, and rolling of thin films[2, 3]. Over the past decade, an elegant symbiosis of bottom-up and top-down methods has been developed, allowing the fabrica- tion of hybrid layer systems via the controlled release and rearrangement of inherently strained layers [4]. Self-assembled rolled-up structures[4, 5] have become increasingly at- tractive in a number of fields including micro/nano uidics[6], optics[7](including metama- terial optical fibers[8]), Lab on a Chip applications[9], and micro- and nanoelectronics[10]. The use of such structures for microelectronic applications has been driven by the versatility in contacting geometries and the abundance of material combinations that these devices offer. By allowing devices to expand in the third dimension, certain obstacles that inhibit 2D structuring can be overcome in elegant ways. Similarly, recent progress in nanostructured superconducting electronic structures has been receiving increased attention[11]. The advancement of such devices has been mo- tivated by their use in quantum computation[12], high sensitivity radiation sensors[13], precision voltage standards[14] and superconducting spintronics[15] to name a few. Combining semiconductor with superconductor materials to create new hybrid geometries is advantageous because it adds the functionalities of the semiconductor, including high charge carrier mobilities, gating possibilities, and refined processing technologies.
The main focus of the work presented in this thesis is the development of new methods for controlling strain behavior and its applications toward novel semiconduc- tor/superconductor heterostructures based on nanomembranes. More specifically, the goal is to integrate inherently strained semiconductor layer structures with superconducting materials to create innovative electronic devices by the controlled releasing and rearrangement of thin films. By rolling up pre-patterned semiconductor/superconductor layers, device geometries have been realized that are not feasible using any other technique. In this way, superconducting hybrid junctions, or Josephson junctions, have been created and their basic properties investigated.
The Josephson effect, and junctions displaying this quantum coherent behavior, have found many essential uses in diverse areas of science and technology. Many research groups around the world are involved in finding new materials and fabrication methods to tune the properties and structure of such Josephson devices further[11]. The inclusion of semi- conductors, for example, allows for a greater control of the charge carrier density within the junction area, thus allowing for "transistor-like" behavior in these superconducting devices.
By rolling up the superconductor contacts using a strained semiconductor as scaffolding, the fabrication of hybrid nano-junctions is simplified drastically, removing the need for complicated processing steps such as electron-beam or nano-imprint lithography. Furthermore, the technique allows many nanometer-sized devices to be created in parallel on a single chip which has the advantage that it can be scaled up to full-wafer processing.
First, post-growth processing techniques of epitaxial layers are developed in order to extend the control of hybrid device fabrication. Here, three unique concepts for controlling the rolling behavior of strained semiconductor nanomembranes are presented. First an optical method for inhibiting the rolling of the strained layers is described. Next, a selective etching method for destroying the inherent strain within the semiconductor layer is introduced. Finally, a method by which the strain gradient across a trilayer stack is altered in situ during rolling is presented. Next, the fabrication of a hybrid nanomembrane-based superconducting device is presented. Various experimental details of the fabrication process are analyzed, and the electronic properties of the completed device are investigated. The devices created here highlight the fabrication process in which nanometer-sized structures are created using self-assembly techniques and standard microelectronics fabrication methods, presenting a new method to circumvent more complicated processing techniques.
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[2] Y. G. Sun, W. M. Choi, H. Q. Jiang, Y. G. Y. Huang and J. A. Rogers. Controlled buckling of semiconductor
nanoribbons for stretchable electronics. Nature Nanotechnology 1, 201{207 (2006).
[3] O. G. Schmidt and K. Eberl. Nanotechnology - Thin solid films roll up into nanotubes. Nature 410, 168 (2001).
[4] O. G. Schmidt, C. Deneke, Y. Nakamura, R. Zapf-Gottwick, C. Mller and N. Y. Jin-Phillipp. Nanotechnology
{ Bottom-up meets top-down. Advanced Solid State Physics 42, 231 (2002).
[5] V. Ya. Prinz, V. A. Seleznev, A. K. Gutakovsky, A. V. Chehovskiy, V. V. Preobrazhenskii, M. A. Putyato
and T. A. Gavrilova. Free-standing and overgrown InGaAs/GaAs nanotubes, nanohelices and their arrays. Physica
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[6] D. J. Thurmer, C. Deneke, Y. F. Mei and O. G. Schmidt. Process integration of microtubes for
uidic applications.
Applied Physics Letters 89, 223507 (2006).
[7] R. Songmuang, A. Rastelli, S. Mendach and O. G. Schmidt. SiOx/Si radial superlattices and microtube optical
ring resonators. Applied Physics Letters 90, 091905 (2007).
[8] E. J. Smith, Z. W. Liu, Y. F. Mei and O. G. Schmidt. Combined surface plasmon and classical waveguiding through
metamaterial fiber design. Nano Letters 10, 1{5 (2010).
[9] G. S. Huang, Y. F. Mei, D. J. Thurmer, E. Coric and O. G. Schmidt. Rolled-up transparent microtubes as
two-dimensionally confined culture scaffolds of individual yeast cells. Lab on a Chip 9, 263{268 (2009).
[10] C. C. B. Bufon, J. D. C. Gonzalez, D. J. Thurmer, D. Grimm, M. Bauer and O. G. Schmidt. Self-assembled
ultra-compact energy storage elements based on hybrid nanomembranes. Nano Letters 10, 2506{2510 (2010).
[11] G. Katsaros, P. Spathis, M. Stoffel, F. Fournel, M. Mongillo, V. Bouchiat, F. Lefloch, A. Rastelli,
O. G. Schmidt and S. De Franceschi. Hybrid superconductor-semiconductor devices made from self-assembled
SiGe nanocrystals on silicon. Nature Nanotechnology 5, 458{464 (2010).
[12] Y. J. Doh, J. A. van Dam, A. L. Roest, E. P. A. M. Bakkers, L. P. Kouwenhoven and S. De Franceschi.
Tunable supercurrent through semiconductor nanowires. Science 309, 272{275 (2005).
[13] F. Giazotto, T. T. Heikkila, G. P. Pepe, P. Helisto, A. Luukanen and J. P. Pekola. Ultrasensitive proximity
Josephson sensor with kinetic inductance readout. Applied Physics Letters 92, 162507 (2008).
[14] S. P. Benz. Superconductor-normal-superconductor junctions for programmable voltage standards. Applied Physics
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[15] Y. C. Tao and J. G. Hu. Superconducting spintronics: Spin-polarized transport in superconducting junctions with
ferromagnetic semiconducting contact. Journal of Applied Physics 107, 041101 (2010).
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Yildirim, Hasan. "Nonlinear Optical Properties Of Semiconductor Heterostructures." Phd thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/12607438/index.pdf.
Full textAbstract:
The nonlinear optical properties of semiconductor heterostructures, such as GaAsAl/GaAs alloys, are studied with analytic and numerical methods on the basis of quantum mechanics. Particularly, second and third-order nonlinear optical properties of quantum wells described by the various types of confining potentials are considered within the density matrix formalism. We consider a Pöschl-Teller type potential which has been rarely considered in this area. It has a tunable asymmetry parameter, making it a good candidate to investigate the effect of the asymmetry on the nonlinear optical properties. The calculated nonlinear quantities include nonlinear absorption coefficient, second-harmonic generation, optical rectification, third-harmonic generation and the intensity-dependent refractive index. The effects of the DC electric field on the corresponding nonlinearities are also studied. The results are in good agreement with the results obtained in other types of quantum wells, such as square and parabolic quantum wells. The effects of the Coulomb interaction among the electrons on the nonlinear intersubband absorption are considered within the rotating wave approximation. The result is applied to a Si-delta-doped, square quantum well in which the Coulomb interaction among the electrons are relatively important, since there has been no work on the nonlinear absorption spectrum of the Si-delta-doped quantum well. The results are found to be new and interesting, especially when a DC electric field is included in the calculations.
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Sharma, Adesh. "Electronic transport in GaAs-AlGaAs heterostructures." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239264.
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Wong, Siu Ling. "Magneto-optical studies of semiconductor heterostructures." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335820.
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Martin, Robert W. "Quantum magnetotransport in strained layer heterostructures." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315751.
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Kinder, David. "Magneto-optical studies of semiconductor heterostructures." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389017.
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Wiggins, Graham C. "Fourier transform spectroscopy of semiconductor heterostructures." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.276832.
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Chang, Chin-chi. "Magneto-optical studies of semiconductor heterostructures." Thesis, University of Oxford, 1998. http://ora.ox.ac.uk/objects/uuid:f25fb6fb-18d9-4750-8ca2-191201c27fba.
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This thesis is primarily concerned with far infrared effects in semiconductor heterostructures. These properties have been studied as a function of magnetic fields at low temperature using various optical detection techniques. Cyclotron resonance has been studied in CdTe quantum wells. The results are compared with calculations using the memory function approach, which demonstrate that there is a large reduction in the resonant coupling due to level occupancy effects. Semimetallic GaSb/InAs superlattices have been studied by cyclotron resonance experiments. In samples with low InAs/GaSb ratios (~1), a pinning between the heavy hole subbands is predicted by theory which results in a suppression of heavy hole resonance at high magnetic fields. Photoluminescence measurements on a series of ultra-high mobility GaAs/AlGaAs heterojunctions have been performed. It has been found that the modulation caused by far infrared beam is entirely dependent on the filling factors, which is understood as the Landau level coupling effects between the subbands of 2DES. A GaAs/AlGaAs coupled quantum well photodiode has been studied by photoluminescence and photocurrent under the influence of far infrared beam. An enhancement of the photocurrent in the device is observed when the infrared photons are resonant with the intersubband transition between the anti-crossing electronic subbands of the coupled quantum wells, which makes the structure a potential tuneable far infrared detector. Most of the experimental works have been modelled with a k.p theory using momentum matrix approach. The self-consistency incorporated in this model proves to be useful while dealing with semimetallic or nonintrinsic systems. These calculations offer invaluable clues to the semiconductor heterostructures investigated.
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Gelsthorpe, Andrew James. "A.E.S. characterisation of small dimensional heterostructures." Thesis, University of York, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.369332.
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Liu, Xian Wei. "Dislocations in strained-layer semiconductor heterostructures." Thesis, Open University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299912.
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Pettersen, Eirik. "Electron transport in gated undoped heterostructures." Thesis, University of Cambridge, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627583.
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Vaughan, Thomas Alexander. "Magneto-optics of InAs/GaSb heterostructures." Thesis, University of Oxford, 1995. http://ora.ox.ac.uk/objects/uuid:52b3d4c8-04f2-4ee8-b5a5-382934807722.
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The optical properties of InAs/GaSb heterostructures under applied magnetic fields are studied in experimental and theoretical detail. The InAs/GaSb system is a type-II "crossed-gap" system, where the valence band edge of GaSb lies higher in energy than the conduction band edge of InAs. This leads to a region of energy above the InAs conduction band where conduction and hole states mix. Thin-layer superlattices remain semiconducting due to confinement effects, but thick-layer superlattices experience charge transfer which leads to intrinsic carrier densities approaching 1012 cm-2 per layer. Existing multi-band modeling techniques based on the k·p formalism are discussed, and a method of solving superlattice band structure (the "momentum-matrix" technique) is presented. The quantizing effects of the superlattice layers and applied magnetic fields are investigated, and the selection rules for optical transitions are derived. Standard cyclotron resonance (CR) is used to study effective masses in InAs/GaSb structures. The heavy hole mass is found to be strongly orientation-dependent, with a mass in the [111] orientation reduced 25% from the [001] mass. The electron mass is found to be roughly isotropic with respect to growth orientation, but shows variation with the InAs width due to quantum confinement effects. CR of InAs/GaSb heterojunctions display hitherto unexplained oscillations in linewidth, intensity, and effective mass. A model is proposed which explains the oscillations, based on the intrinsic nature of the InAs/GaSb system. CR is performed on an InAs/GaSb heterojunction using a free-electron laser, where due to the high intensities (on the order of MW/cm2) the absorption process saturates. This saturation allows for a determination of non-radiative relaxation lifetimes, and through the energy dependence of these lifetimes the magnetophonon effect is observed, allowing a direct measurement of LO-phonon-assisted energy relaxation rates. Coupling is introduced into the standard CR experiment, either by tilting the sample with respect to the magnetic field, or by applying a metal grating to the surface. These coupled CR experiments have striking qualitative results which allow for determination of subband separation energies and coupling matrix elements. Photoconductivity experiments are performed on thin-layer (semiconducting) superlattices, showing optical response at far-infrared wavelengths (5-20 μm). The results are compared with k·p calculations. One sample is processed for vertical transport, in which conduction occurs perpendicular to the superlattice layers. Strong optical response from this sample indicates the viability of InAs/GaSb-based far-infrared detectors. The momentum-matrix technique is used to predict optimum parameters for semiconducting superlattices with band gaps in the far-infrared. Semimetallic structures are studied via a multi-band self-consistent model, with results corroborating with and extending previous work. Intrinsic structures under applied magnetic field are modeled theoretically for the first time.
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KOEBEL, ARNAUD. "Heterostructures gase et inse sur si." Paris 7, 1997. http://www.theses.fr/1997PA077123.
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L'observation d'epitaxie entre semiconducteurs lamellaires (2d), malgre les contraintes de desaccord parametrique, alors que les semiconducteurs tridimensionnels (3d) classiques ne le permettent pas a permis de penser que l'intercalation de composes lamellaires entre semiconducteurs classiques pourrait etre une solution pour realiser des epitaxies a fort desaccord entre semiconducteurs 3d. Pour que ce systeme soit possible, l'accrochage entre les materiaux 3d et 2d doit etre de type van der waals sans liaisons dirigees. L'objet de cette these est l'etude de la structure de l'interface entre gase et si(111), en utilisant deux techniques : microscopie electronique en transmission a haute resolution (hrtem) et ondes stationnaires de rayons x. Les observations hrtem ont mis en evidence la grande sensibilite des couches produites a la preparation initiale de la surface de silicium. Les surfaces h-si(111) ont permis la croissance de couches de gase de la meilleure qualite. Les observations en vue de plan ont montre une microstructure riche constituee de domaines de taille 1000a. Le mode de croissance observe est probablement de type croissance ilots bidimensionnels. Les experiences xsw ont demontre l'existence d'un demi-feuillet de gase a l'interface. L'epitaxie n'est donc pas de type van der waals mais pseudomorphique. La premiere couche complete est deja relaxee par rapport au substrat. La croissance se deroule donc en deux stades : une epitaxie pseudomorphique suivie par une homoepitaxie van der waals de gase. La position du selenium dans le demi-feuillet est principalement de symetrie silicium, mais une petite quantite est dans une position de symetrie gase. D'autres heterostructures 2d/3d ont ete etudiees : inse/si(111), multicouches gase/inse et reprise de croissance de znse sur une heterostructure 2d/3d. Le systeme inse/si est similaire au cas gase/si, mais avec une taille de domaines inferieure, a cause des parametres de croissance differents. Les multicouches gase/inse obtenues sont de qualite moyenne, a cause des temperatures de fusion trop differentes entre gase et inse. La reprise de croissance de znse a donne des couches de qualite diverses, mais le systeme znse/gase/si(111) presente des caracteristiques encouragentes.
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Wang, Zhiguang. "Magnetoelectric Effect in Ferroelectric-Ferromagnetic Heterostructures." Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/48168.
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The magnetoelectric (ME) effect, a coupling effect between magnetic and electric orders, has been widely investigated, both from a fundamental science perspective and an applications point of view. Magnetoelectric composites with one piezoelectric phase and one magnetostrictive phase can be magneto-electrically coupled via elastic strain mediation. Bulk magnetoelectric composites have been intensively studied as magnetic sensors due their significant magnetic-to-electric signal transforming efficiency, which promises high magnetic field sensitivity. In contrast, electric field-controlled magnetization in magnetoelectric thin films is more attractive for information recording and novel electrically-tunable microwave magnetic devices.
For the present work, we prepared a series of magnetoelectric structures capable of modulating the magnetization with an electric field -- all of which display unprecedented magnetic coercive field tunability. These structures show promise for a number of applications, including magnetic memory and spintronics. First, we generated self-assembled BiFeO3-CoFe2O4 (BFO-CFO) nanostructures of varying architectural structures on differently-oriented perovskite substrates. We were able to control aspect ratio through both thickness control and by manipulating growth thermodynamics. The relationship between magnetic shape and strain anisotropy was systematically analyzed using both in-plane and out-of-plane magnetic easy axis data. The BFO-CFO self-assembled structures may be useful for applications, including longitudinal and perpendicular magnetic memory; additionally they can serve as a prototype for analyzing the magnetoelectric effect-based magnetoresistive random-access memory. BFO-CFO grown on piezoelectric Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) shows a large magnetoelectric coupling coeffcient.
Second, we sought to clarify the relationship between ferroelectric/ferroelastic phase transformation and the magnetoelectric effect in CFO films on PMN-PT heterostructures. Elastic strain is an essential component of electro-mechanical-magnetic coupling. Most prior studies that used piezoelectric materials as a strain source assumed that these materials shared a linear relationship (d31 or d33) with the electric field, which is true only with small electric field signals. In contrast, the largest strain is produced during phase transformation in piezoelectric single crystals. In this work, we systematically investigated electric field induced phase transformation in PMN-PT single crystals with different compositions. A signficant finding that emerged from this study is that a large in-plane uniaxial strain can be controlled by an electric field, and this strain can be used to control the magnetic easy axis distribution in the in-plane. The electric field is along the out-of-plane direction, which is perpendicular to the uniaxial strain and the surface of the sample, and thus can be easily incorporated into real device design.
Finally, we identified very large magnetic coercive field tunability in the CFO/PMN-PT monolithic structures -- in fact, more than ten times larger than previously reported magnetoelectric heterostructures. We used a <011> oriented PMN-PT substrate, where a large uniaxial strain can be induced by an electric field. Importantly, since the two in-plane directions have the same dimensions, the uniaxial strain can induce a significant magnetic anisotropy distribution change in the two in-plane directions. A unprecedented magnetic coercive field change of up to 580 Oe has been observed, which shows great potential for applications in both magnetic memory and microwave magnetic devices.Ph. D.
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Kaiser, Franz. "Current and noise in driven heterostructures." kostenfrei, 2009. http://d-nb.info/994443641/34.
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Uddin, Md Tamez. "Metal oxide heterostructures for efficient photocatalysts." Phd thesis, Université Sciences et Technologies - Bordeaux I, 2013. http://tel.archives-ouvertes.fr/tel-00879226.
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Photocatalytic processes over semiconducting oxide surfaces have attracted worldwide attention aspotentially efficient, environmentally friendly and low cost methods for water/air purification as well as forrenewable hydrogen production. However, some limitations to achieve high photocatalytic efficiencies havebeen found due to the fast recombination of the charge carriers. Development of heterostucture photocatalystsby depositing metals on the surface of semiconductors or by coupling two semiconductors with suitable bandedge position can reduce recombination phenomena by vectorial transfer of charge carriers. To draw newprospects in this domain, three different kinds of heterostructures such as n-type/n-type semiconductor(SnO2/ZnO), metal/n-type semiconductor (RuO2/TiO2 and RuO2/ZnO) and p-type/n-type semiconductor(NiO/TiO2) heterojunction nanomaterials were successfully prepared by solution process. Their composition,texture, structure and morphology were thoroughly characterized by FTIR, X-ray diffraction (XRD), Ramanspectroscopy, transmission electron microscopy (TEM) and N2 sorption measurements. On the other hand, asuitable combination of UV-visible diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy(XPS) and ultraviolet photoemission spectroscopy (UPS) data provided the energy band diagram for eachsystem. The as-prepared heterojunction photocatalysts showed higher photocatalytic efficiency than P25 TiO2for the degradation of organic dyes (i.e. methylene blue and methyl orange) and the production of hydrogen.Particularly, heterostructure RuO2/TiO2 and NiO/TiO2 nanocomposites with optimum loading of RuO2 (5 wt %)and NiO (1 wt %), respectively, yielded the highest photocatalytic activities for the production of hydrogen.These enhanced performances were rationalized in terms of suitable band alignment as evidenced by XPS/UPSmeasurements along with their good textural and structural properties. This concept of semiconductingheterojunction nanocatalysts with high photocatlytic activity should find industrial application in the future toremove undesirable organics from the environment and to produce renewable hydrogen.
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Westover, Richard. "Synthesis of Multiple Constituent Ferecrystal Heterostructures." Thesis, University of Oregon, 2016. http://hdl.handle.net/1794/19712.
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The ability to form multiple component heterostructures of two-dimensional materials promises to provide access to hybrid materials with tunable properties different from those of the bulk materials or two-dimensional constituents. By taking advantage of the unique properties of different constituents, numerous applications are possible for which none of the individual components are viable. The synthesis of multiple component heterostructures, however, is nontrivial, relying on either the cleaving and stacking of bulk materials in a “scotch tape” type technique or finding coincidentally favorable growth conditions which allow layers to be grown epitaxially on each other in any order. In addition, alloying of miscible materials occurs when the modulation wavelength is small. These synthetic challenges have limited the ability of scientists to fully utilize the potential of multiple component heterostructures. An alternative synthetic route to multiple component heterostructures may be found through expansion of the modulated elemental reactant technique which allows access to metastable products, known as ferecrystals, which are otherwise inaccessible.
This work focuses on the expansion of the modulated elemental reactants technique for the formation of ferecrystals containing multiple constituents. As a starting point, the synthesis of the first alloy ferecrystals (SnSe)1.16-1.09([NbxMo1-x]Se2) will be discussed. The structural and electrical characterization of these compounds will then be used to determine the intermixing of the first three component ferecrystal heterojunction ([SnSe]1+δ)([{MoxNb1-x}Se2]1+γ)([SnSe]1+δ)({NbyMo1-y}Se2). Then, by synthesizing ([SnSe]1+δ)m([{MoxNb1-x}Se2]1+γ)1([SnSe]1+δ)m({NbxMo1-x}Se2)1 (m = 0 - 4) compounds with increasing thicknesses of SnSe, the interdiffusion of miscible constituents in ferecrystals will be studied. In addition, by comparison of the ([SnSe]1+δ)m ([{MoxNb1-x}Se2]1+γ)1([SnSe]1+δ)m({NbxMo1-x}Se2)1 (m = 0 - 4) compounds to the ([SnSe]1+δ)m(NbSe2)1 (m = 1 - 8) compounds the electronic interactions of the MoSe2 and NbSe2 layers will be determined. Finally, the effects of different alloying strategies and the interdiffusion of miscible constituents will be further examined by the synthesis of ordered ([SnSe]1.15)1([TaxV1-x]Se2)1([SnSe]1.15)1([VyTa1-y]Se2)1 and ([SnSe]1+δ) ([TaxV1-x]Se2) compounds with the effect of isoelectric doping on the charge density wave transition in (SnSe)1.15(VSe2) also being explored.
This work contains previously published and unpublished co-authored material.
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Vuong, Phuong. "Optical spectroscopy of boron nitride heterostructures." Thesis, Montpellier, 2018. http://www.theses.fr/2018MONTS075/document.
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Le nitrure de bore hexagonal (h-BN) est un semi-conducteur à large bande interdite (~ 6 eV) avec une stabilité thermique et chimique très élevées lui offrant la possibilité d'être utilisé dans des dispositifs fonctionnant dans des conditions de fonctionnements extrêmes. La nature indirecte de la bande interdite dans h-BN a été étudiée à la fois par des calculs théoriques et par des expériences. Un exciton indirect et des recombinaisons assistées par phonons dans h-BN ont été observées par photoluminescence.Durant cette thèse, nous avons étudié les propriétés optiques de cristaux massifs et de couches hétéro-épitaxiales de nitrure de bore hexagonal. Nous avons étudié des échantillons provenant de différentes sources et des cristaux qui ont été fabriqués en utilisant différentes méthodes de croissance pour nous permettre de mesurer les propriétés optiques intrinsèques de h-BN. Nous rapportons l'impact des symétries des phonons sur la réponse optique du h-BN en effectuant des mesures photoluminescence résolues par polarisation. L’analyse des données en polarisation, nous permet de mesurer la contribution du phonon manquant, celui qui n'a pas été détectée avant cette thèse. En suite, nous démontrons que l'origine de la structure fine du spectre de PL provient pour chaque réplique phonon d’une diffusion complémentaire de type Raman faisant intervenir le mode de phonon E2g à basse énergie (mode de cisaillement inter-feuillets). Les spectroscopies de photoluminescence et de diffusion inélastique Raman ont été combinées pour quantifier l'influence des effets isotopiques sur les propriétés optiques de h-BN ainsi pour révéler que les modifications des interactions de van de Waals liées à l'utilisation de 10B et 11B ou du bore naturel pour la croissance de cristaux h-BN massifs.Enfin, nous étudions des epitaxis de h-BN crues par Épitaxie sous Jets Moléculaires. L'utilisation conjointe de l’imagerie par microscopie à force atomique (AFM) et de la spectroscopie de photoluminescence permet de comprendre la première observation de recombinaison assistée par phonons dans des épitaxies de h-BN sur le saphir et le graphite. Ce résultat indique que la croissance de h-BN à large échelle par méthode épitaxiales est en voie d'acquérir la maturité nécessaire au développement technologique de h-BNHexagonal boron nitride (h-BN) is a wide bandgap (~ 6 eV) semiconductor with a very high thermal and chemical stability often used in devices operating under extreme conditions. The indirect nature of the bandgap in h-BN is investigated by both theoretical calculations and experiments. An indirect excion and phonon-assisted reombinations in h-BN are observed in photoluminescene spectroscopy.This thesis focus on the optical properties of bulk and epilayers of h-BN. We investigated samples from different sources grown different methods in order to confirm the intrinsic optical properties of h-BN. We report the impact of the phonon symmetry on the optical response of h-BN by performing polarization-resolved PL measurements. From them, we will measure the contribution of all the phonon-assisted recombination which was not detected before this thesis. We follow by addressing the origin of the fine structure of the phonon-assisted recombinations in h-BN. It arises from overtones involving up to six low-energy interlayer shear phonon modes, with a characteristic energy of about 6.8 meV.Raman and photoluminescence measurements are recorded to quantify the influence of isotope effects on optical properties of h-BN as well as the modifications of van de Waals interactions linked to utilization of 10B and 11B or natural Boron for the growth of bulk h-BN crystals.Finally, we study h-BN thin epilayers grown by Molecular Beam Epitaxy at Nottingham University, atomic force microscopy (AFM) images and photoluminescence features are combined to confirm the first observation of phonon-assisted recombination in high quality thin h-BN epilayers grown on c-plane sapphire and Highly Ordered Pyrolitic Graphite. This demontrates that large scale growth of h-BN by epitaxy is getting a technologically required maturity
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Preisler, Edward J. McGill T. C. "Investigation of novel semiconductor heterostructure systems : I. Cerium oxide/silicon heterostructures ; II. 6.1 Å semiconductor-based avalanche photodiodes /." Diss., Pasadena, Calif. : California Institute of Technology, 2003. http://resolver.caltech.edu/CaltechETD:etd-06022003-071834.
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Bristowe, Nicholas Charles. "Theoretical study of polar complex oxide heterostructures." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/265542.
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Plastic strains during investment casting of single crystal Ni-based superalloys arise from differences in thermal contraction between the metal and ceramic mould-and-core. If deformation is above a critical limit, subsequent solution heat treatment of the alloy causes recrystallisation. Crack nucleation and propagation is preferred at the recrystallisation grain boundaries, and this significantly reduces the creep and fatigue properties of the alloy. The as-cast microstructure (Chapter 3) is characteristic of high temperature deformation (~1050�(), where dislocations primarily form loops and networks at the y/y' interface. This validates the process modelling performed by our collaborators. The dislocation density was found to be higher in the interdendritic areas and the dendrite cores were virtually dislocation-free, indicating that deformation occurs at temperatures close to they' solvus, which is 1250-1310�C for CMSX 4, where dislocations accumulate in the interdendritic areas that are the first to precipitate they'. Critical strains for recrystallisation were determined as a function of temperature (Chapter 3). Recrystallisation was found to be sensitive to the surface finish, where alloys with the cast surface were more prone to recrystallisation, tolerating plastic strains of 1-2% at temperatures ~1000�C. On examining the cast surface of CMSX 4, two sources of nucleation for recrystallisation were identified (Chapter 4). Firstly, micro-grains of y', 2-30 ?m deep, forming high angle boundaries with the bulk single crystal were found within the surface eutectic; these grow larger during the heat treatment and maintain high angle misorientations with the matrix. Secondly, in regions where surface eutectic is absent, the metal adheres to the mould and forms intense local deformation, 5- 20 ?m deep, during subsequent detachment. During the heat treatment local surface recrystallisation occurs, where small grains develop in orientations similar to the deformed matrix and subsequently twin to form high angle boundaries. Experimental trials show that in the presence of deformation in the bulk the nuclei in the casting surface can cause recrystallisation. By removing the cast surface with etching, recrystallisation can be completely mitigated . Recrystallisation studies on alloys varying systematically in composition (Chapter 5) show that high . Co alloys (with up to 8 wt%) are more prone to recrystallisation. Co lowers they' solvus temperature and the stacking fault energy of y. Ru, Mo and W appear to have no direct effect on recrystallisation. The nucleating grains for recrystallisation form in orientations similar to the deformed matrix, and subsequently twin to form high angle boundaries and proliferate within the deformed microstructure. They' phase and topologically close packed phases hinder grain boundary migration.
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Wang, Jianfeng, and 王建峰. "Field modulation on transport properties in heterostructures composed of perovskite oxides." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2011. http://hub.hku.hk/bib/B47147076.
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Harrison, Paul Anthony. "Resonant tunnelling and luminescence in coupled quantum wells." Thesis, University of Nottingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.363933.
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Straube, Urs Norman. "Fabrication and characterization of p-MOSFETS with a strained SiGe channel." Thesis, University of Southampton, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269943.
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Mauro, Diego. "Electronic properties of Van der Waals heterostructures." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/10565/.
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L’interazione spin-orbita (SOI) nel grafene è attualmente oggetto di intensa ricerca grazie alla recente scoperta di una nuova classe di materiali chiamati isolanti topologici.
Questi materiali, la cui esistenza è strettamente legata alla presenza di una forte SOI, sono caratterizzati dall’interessante proprietà di avere un bulk isolante ed allo stesso tempo superfici conduttrici. La scoperta teorica degli isolanti topologici la si deve ad un lavoro nato con l’intento di studiare l’influenza dell’interazione spin-orbita sulle proprietà del grafene. Poichè questa interazione nel grafene è però intrinsecamente troppo piccola, non è mai stato possibile effettuare verifiche sperimentali. Per questa ragione, vari lavori di ricerca hanno recentemente proposto tecniche volte ad aumentare questa interazione. Sebbene alcuni di questi studi abbiano mostrato un effettivo aumento dell’interazione spin-orbita rispetto al piccolo valore intrinseco, sfortunatamente hanno anche evidenziato una consistente riduzione della qualità del grafene.
L’obbiettivo che ci si pone in questa tesi è di determinare se sia possibile aumentare l’interazione spin-orbita nel grafene preservandone allo stesso tempo le qualità. La soluzione proposta in questo lavoro si basa sull’utilizzo di due materiali semiconduttori, diselenio di tungsteno WSe2 e solfuro di molibdeno MoS2, utilizzati da substrato su cui sopra verrà posizionato il grafene formando così un’eterostruttura -nota anche di “van der Waal” (vdW)-.
Il motivo di questa scelta è dovuto al fatto che questi materiali, appartenenti alla famiglia dei metalli di transizione dicalcogenuri (TMDS), mostrano una struttura reticolare simile a quella del grafene, rendendoli ideali per formare eterostrutture e ancora più importante, presentano una SOI estremamente grande. Sostanzialmente l’idea è quindi di sfruttare questa grande interazione spin-orbita del substrato per indurla nel grafene aumentandone così il suo piccolo valore intrinseco.
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Kneip, Martin K. "Magnetization dynamics in diluted magnetic semiconductor heterostructures." kostenfrei, 2008. http://hdl.handle.net/2003/25822.
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Bach, Peter. "Growth and characterization of NiMnSb-based heterostructures." Doctoral thesis, [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=98030282X.
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Marsden, Alexander J. "Van der Waals epitaxy in graphene heterostructures." Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/77193/.
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Graphene — a two-dimensional sheet of carbon atoms — has surged into recent interest with its host of remarkable properties and its ultimate thinness. However, graphene combined with other materials is starting to attract more attention. These heterostructures can be important for production routes, incorporating graphene into existing technologies, or for modifying its intrinsic properties. This thesis aims to examine the role of van der Waals epitaxy within these heterostructures. First, the graphene-copper interaction during chemical vapour deposition of graphene is investigated. Graphene is found to grow with a mismatch epitaxy of 8 relative to the [001] direction of the Cu(100) surface, despite a mismatch in symmetry and lattice parameter between two. Further, the electronic structure of both graphene and copper is unchanged by the interaction. This highlights the weak interaction between the two, owing to its van der Waals nature. Functionalised graphene is another important heterostructure, and is intensively studied for both graphene production routes and for altering graphene’s properties. Here, it is the change to the homogeneous graphene surface that makes it interesting for van der Waals epitaxy. The effect of functionalisation of graphene with atomic oxygen and nitrogen is presented next. In both cases, only small amounts of functionalisation ( 5 at%) is sufficient to significantly deteriorate the -band structure of the graphene through localisation. For small amounts of nitrogen functionalisation, and greater amounts of oxygen functionalisation, extended topological defects are formed in the graphene lattice. Unlike epoxide oxygen groups, these disruptions to the pristine graphene are found to be irreversible by annealing. Next, the interaction between graphene and the organic semiconducting molecule vanadyl-phthalocyanine (VOPc) is presented. As a result of the van der Waals nature of the graphene surface, VOPc molecules can form crystals microns in size when deposited onto a substrate with an elevated temperature of 155 C; at ambient temperatures, the crystals are only tens of nanometres across. In contrast, the functionalised graphene oxide surface prevents large crystal growth, even at elevated temperatures, because surface functionalities inhibit molecule diffusion. This highlights the importance of graphene as a substrate for molecular crystal growth, even when the growth is not epitaxial. Finally, the supramolecular assembly of trimesic acid (TMA) and terephthalic acid (TPA) is presented. Despite their chemical similarity they display different behaviour as they transition from monolayers to three-dimensional structures: for TMA, the epitaxial chicken wire structure seen at a monolayer templates up through the layers as molecules stack, until a thickness of 20 nm, when random in-plane orientations appear; on the other hand, TPA forms a brickwork structure at the monolayer, which quickly transitions to fibre-like crystals with a bulk structure for the thin films. However, the TPA orientation is still determined by the epitaxy with the graphene substrate, although this is significantly weaker than for TMA.
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Mak, Wing Yee. "Transport experiments in undoped GaAs/A1GaAs heterostructures." Thesis, University of Cambridge, 2013. https://www.repository.cam.ac.uk/handle/1810/252296.
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Pang, Brian SiewHan. "Control of superconductivity in cuprate/manganite heterostructures." Thesis, University of Cambridge, 2004. https://www.repository.cam.ac.uk/handle/1810/34610.
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Research has shown that the spin alignment in an adjacent ferromagnet is capable of suppressing superconductivity. In this project, devices incorporating cuprate/manganite heterostuctres were successfully fabricated to study the effects of spin transport on the high temperature superconductor, YBCO. Deposition of such oxide ferromagnet/superconductor (F/S) multilayers using the 'eclipse' pulsed laser deposition(PLD) technique was also examined. Reproducible multilayers with ultrathin repeats were deposited, which exhibited superconducting and magnetic properties to minimum thicknesses of 3nm for both YBCO and LSMO. Using spin injection, via a ferromagnet, to create a spin imbalance in the superconductor, a suppression of superconducting critical current was observed with increasing injection current. However, the exact cause of this suppression could not be solely attributed to spin-induced nonequilibrium effects, as it proved difficult to eliminate the effects of localized heating, current summation and magnetic field. Interfacial studies of the device junction provided evidence of an alternative current path at the interface. The control of superconductivity was also examined using F/S proximity effects, which improves the understanding of how magnetic and superconducting materials coexist. We observed that oxide F/S samples deposited by high O2 sputtering and 'eclipse' PLD were similar, and that Tc was clearly more suppressed in F/S compared to N(normal metal)/S systems. However, the magnetic moment and exchange coupling, two magnetic properties of significance in ferromagnets, did not, individually, have a major influence on the increased Tc suppression. The Curie temperatures of the multilayers were suppressed with increasing manganite thickness because of structural effects, and also with increasing thickness of the YBCO layer which reduced the coupling between manganite layers. To study the use of the spin-valve effect as a means to control high temperature superconductors, we fabricated an LSMO/YBCO/LC(0.3)MO pseudo spin-valve structure, which is equivalent to a superconductor sandwiched within a spin valve where both parallel and antiparallel configurations of the F layers can be achieved within a single magnetic field sweep. Previous research involving a metallic F/S/F/AF structure, showed that the superconductivity was suppressed when the ferromagnets were in the parallel configuration. From the onset of superconductivity, when the normal metallic behaviour of YBCO switches to superconductivity, a magnetoresistance (MR) peak was observed when the F layers were antiparallel. The MR effect increased with decreasing bias current and temperature, characteristic of a pseudo-spin valve. The result is suggestive of spin transport across the YBCO spacer layer.