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We fabricate X-ray detectors based upon superconducting Ta/Al/Al x O y /Al hetero tunnel junctions. Device characteristics are significantly influenced by the fact that the gap energies on the two sides of the tunnel barrier are not equal. We present experimental data showing the dependence of signal height and polarity on bias voltage in heterojunctions. The best energy resolution obtained in a single Ta/Al/Al x O y /Al heterojunction is 60 eV at 1.487 keV. We also report the observation of correlated signals from two separate tunnel junctions attached to one common Ta absorber strip.

The charging and quantum-size effects of all-metal artificial atoms (a tunnel barrier containing a metal particle) are studied by using a Hubbard tight-binding Hamiltonian in the unrestricted Hartree-Fock approximation, which includes intra-atomic and interatomic Coulomb interactions. The charging energy as a function of the electron number added to the artificial atom can be modulated by the artificial atoms is predicted. Furthermore, different charge distributions for the nonequivalent atoms effects of quantum size and structure in the cluster and thus the conductance resonances in all-metal in the artificial atoms are found, which have obvious influence on the electronic transport through all-metal artificial atoms.

Magnetic tunnel junctions were fabricated with tunnel barriers that had a thin film of Fe(0-8 /spl Aring/ thick) inserted in the middle to study the effect of FeO/sub x/ doping on the magneto-electron transport properties of the junction. Tunneling magnetoresistance (TMR) ratio of 24% was realized with the tunnel junction with a 2 /spl Aring/-thick Fe interlayer, whereas the junction without doping did not exhibit the TMR effect. The FeO/sub x/ doping also considerably improved the thermal stability of the junction, retaining the TMR ratio of 15% at 350/spl deg/C.

Ivar Giaever shared the Nobel Prize in Physics with Leo Esaki and Brian D. Josephson. More precisely: The Nobel Prize in Physics 1973 was divided, one half jointly to Leo Esaki and Ivar Giaever “for their experimental discoveries regarding tunnelling phenomena in semiconductors and superconductors, respectively,” and the other half to Brian David Josephson “for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects.”

We show that magnetic dipolar interactions can stabilize superfluidity in atomic gases but the dipole alignment direction required to achieve this varies, depending on whether the flow is oscillatory or continuous. If a condensate is made to oscillate through a lattice, damping of the oscillations can be reduced by aligning the dipoles perpendicular to the direction of motion. However, if a lattice is driven continuously through the condensate, superfluid behavior is best preserved when the dipoles are aligned parallel to the direction of motion. We explain these results in terms of the formation of topological excitations and tunnel barrier heights between lattice sites., Comment: 6 pages, 7 figures

We numerically study the transport behavior of a graphene cavity delimited by two constrictions and divided into two dots by the tunnel barrier induced by the electrostatic action of a negatively biased gate. Performing an analysis, based on an envelope function approach, of the dependence of the conductance on the position of the tunnel barrier, we observe a maximum when the barrier is at the middle of the cavity, an effect similar, even though less pronounced, to that recently studied in devices based on semiconductor heterostructures.

We demonstrated that a large TMR ratio of 753 % has been observed at 2 K in a MTJ using a Co2MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. At room temperature (RT), we also have observed a large TMR ratio of 217 %, which value at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio was still large. In order to improve the interface, we investigated the TMR effect in Co2MnSi/CoFeB(0–2 nm)/MgO/CoFe MTJs. TMR ratio was enhanced by inserting a thin CoFeB layer at the Co2MnSi/MgO interface. The MTJ with CoFeB thickness of 0.5 nm exhibited the highest TMR ratio. From the conductance–voltage measurements for the fabricated MTJs, we inferred that the highly spin polarized electron created in Co2MnSi can conserve the polarization through the 0.5 nm thick FeB layer.

We investigated perpendicular-anisotropy magnetic tunnel junctions (p-MTJ) with two types of MTJ structures prepared by etching. In MTJs with a base electrode which consisted of a tunnel barrier, a synthetic antiferromagnetic reference layer and a seed layer, the coercive force of a perpendicular-anisotropy free layer decreased with decreasing the MTJs sizes. This is assumed to be due to a partial damage of an upper synthetic antiferromagnetic layer which caused a parallel magnetic field on the free layer. In MTJs with a base electrode which consisted of the seed layer, the coercive force of the perpendicular-anisotropy free layer increased gradually with decreasing the MTJ size.

Naturally occurring spin-valve-type magnetoresistance (SVMR), recently observed in Sr2FeMoO6 samples, suggests the possibility of decoupling the maximal resistance from the coercivity of the sample. Here we present the evidence that SVMR can be engineered in specifically designed and fabricated core-shell nanoparticle systems, realized here in terms of soft magnetic Fe3O4 as the core and hard magnetic insulator CoFe2O4 as the shell materials. We show that this provides a magnetically switchable tunnel barrier that controls the magnetoresistance of the system, instead of the magnetic properties of the magnetic grain material, Fe3O4, and thus establishing the feasibility of engineered SVMR structures., Comment: Copyright (2013) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

We explore theoretically the effect of incorporating a thin tunnel barrier between the electron and hole transport layers of organic heterostructure photovoltaic devices. The device efficiency can be improved significantly by controlling the rates of microscopic processes associated with exciton dissociation and recombination at the interface between the electron and hole transport layers. The effects of different parameters are examined, and conclusions for organic photovoltaic device design are discussed.

Efforts to achieve efficient injection of spin-polarized electrons into a semiconductor, a key prerequisite for developing electronics that exploit the electron's spin degree of freedom, have so far met with limited success. Here we report experimental studies of lateral spin injection and detection through electrodeposited Fe/GaAs tunnel contacts. We demonstrate spin injection efficiencies two orders of magnitude higher than for state-of-the-art contacts fabricated via ultra-high-vacuum methods, including those with MgO or Al2O3 tunnel barriers. To account for this enhancement, we propose that an iron oxide layer that forms at the Fe/GaAs interface during electrodeposition, being magnetic, acts as a tunnel barrier with a spin-dependent height, presenting quantum spin transport calculations for such systems. This serendipitous discovery of greatly enhanced efficiency of spin injection into GaAs via electrodeposited contacts introduces a promising new direction for the development of practical semiconductor spintronic devices., Comment: pdf file, 28 pages, 6 figures

Spin injection into GaAs and Si (both n and p-type) semiconductors using Fe3O4 is achieved with and without a tunnel barrier (MgO) via three-terminal electrical Hanle measurement. Interestingly, the magnitude of spin accumulation voltage (Delta V) in semiconductor is found to be associated with a drastic increment in Delta V in Fe3O4 based devices for temperature

We have studied how the insertion of sub-monolayer amounts of Mn impurities in the middle of the oxide tunnel barrier of Fe/GeO2 on p-type Ge affects the spin transport, using three-terminal Hanle measurements. Strikingly, the magnitude of the Hanle spin voltage is strongly reduced by increasing the amount of Mn dopants and is even completely absent for devices having an amount of Mn impurities equivalent to a 0.2-nm-thick layer. This demonstrates that magnetic impurities in the tunnel barrier are detrimental to the spin transport in ferromagnet/oxide/semiconductor junctions, and that the localized states associated with such magnetic impurities do not produce three-terminal Hanle spin signals.

We have investigated the electrical contacts between YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) and Au. The dI/dV characteristics of the c-axis-oriented YBCO/Au/Nb junctions reveal the existence of a tunnel barrier at the YBCO/Au interface. The experimental results of the YBCO/LaGaO/sub 3//Au/Nb junction suggest p-type degenerate semiconductor-like behavior of YBCO along the c-axis direction. The anomalously small I/sub c/R/sub n/ of a few /spl mu/V in the c-axis direction are interpreted as the band bending at the YBCO surface. On the other hand, the experimental I/sub c/R/sub n/ value of 120 /spl mu/V in the a-axis-oriented YBCO/Au/Nb junction is understood within the scope of conventional theory. >

We report on observations of thickness dependent Josephson coupling and multiple Andreev reflections (MAR) in vertically stacked molybdenum disulfide (MoS2) - molybdenum rhenium (MoRe) Josephson junctions. MoRe, a chemically inert superconductor, allows for oxide free fabrication of high transparency vertical MoS2 devices. Single and bilayer MoS2 junctions display relatively large critical currents (up to 2.5 uA) and the appearance of sub-gap structure given by MAR. In three and four layer thick devices we observe orders of magnitude lower critical currents (sub-nA) and reduced quasiparticle gaps due to proximitized MoS2 layers in contact with MoRe. We anticipate that this device architecture could be easily extended to other 2D materials., Comment: 18 pages, 6 figures including Supporting Information

As dimensions of devices shrink new phenomena are revealed. We present a technique which allows the dimension of a point contact to ‘‘shrink’’ down to a single atom. Furthermore, this technique enables us to adjust a tunnel barrier between two inherently stable electrodes. Utilizing this technique we show that it is possible to apply a pressure or a tensile force on a single atom contact and study the resulting effects on the transport properties of this microscopic structure.

We investigated a magnetic tunnel junction (MTJ) with a tunnel barrier formed by N2O plasma. Compared with a MTJ with a tunnel barrier formed by conventional O2 plasma, the MTJ fabricated with N2O plasma shows much lower specific junction resistance and a comparably high tunneling magnetoresistance ratio. In particular, it was found that N2O plasma oxidation is quite important in the junction with a thin tunnel barrier, for which O2 plasma cannot be used. From x-ray photoelectron spectroscopy, we observed that N2O plasma oxidation leads to the slight nitridation of the Al2O3 layer and significantly reduces the oxidation of the bottom electrode, especially for a thin tunnel barrier. Thus, we conclude that the use of N2O plasma in forming the tunnel barrier is effective for achieving a low junction resistance and for minimizing the oxidation of the bottom electrode during plasma oxidation in MTJs.

We have developed a magnetic tunnel junction (MTJ) pattern technique that involves transforming the magnetic layer above the tunnel barrier in unwanted areas into an insulator, thus providing insulation between different MTJ devices without suffering common tunnel barrier shorting problems. With this technique, 90%–100% yielding MTJ devices have been observed. MTJ results using this process are superior to an etching based process. Switching distribution of patterned magnetic bits is also narrower using this novel technique. Process control and the ability to stop on the tunnel barrier have been demonstrated.

A quantum-statistical theory of injection of spin-polarized current into a semiconductor in ferromagnet/tunnel barrier/semiconductor system is presented. The presence of Schottky barrier in the semiconductor is taken into account. The case of degenerated and non-degenerated semiconductors are considered. Both the diffusive and ballistic transport regime are investigated. The dependence of current polarization on barrier thickness and temperature is calculated.

This work is focused on the development and the characterization of polycrystalline magnetic tunnel junctions (MTJ) with strontium titanium oxide barrier, SrTiO3, identified as a low band gap tunnel barrier by literature. Such barrier could fulfill the critical application requirement: having a lower resistance area product (RA) in MTJ, or its corollary, having a thicker barrier at constant RA, while keeping the tunnel magnetoresistance ratio (TMR) high enough. Former studies have shown that SrTiO3 deposited by ion beam sputtering (IBS) could crystallize at an unusual low temperature (< 400°C) which could make it compatible with the magnetic layers of MTJs. In a first place, MTJs with a tunnel barrier made of a well known material in spintronics, namely MgO, were deposited. This preliminary work allowed us to highlight the specific parameters affecting the transport properties in MTJs deposited by IBS, including the oxidation type. In a second place, CoFeB/SrTiO3/CoFeB MTJs were developed using IBS and a SrTiO3 ceramic target, learning from our experience on MgO based MTJs. Many combinations of different parameters (including deposition, oxidation and annealing parameters) were explored, leading to two opposite tendencies with systems having either a high TMR (up to 18 %) or a low RA (down to 2.6 Ohm.µm²). SrTiO3 based MTJs were then patterned for the first time and submitted to electrical tests. These tests showed that the MTJs having a low RA exhibited an ohmic behaviour while the MTJs having a large TMR showed the expected tunnel characteristics. Furthermore, the latter MTJs showed an intrinsic dielectric breakdown. In parallel, microstructural characterizations have shown that SrTiO3 based MTJs and MgO based MTJ were alike morphologically. Nevertheless, these observations alone were not enough to assess on the crystalline state of SrTiO3. Many possibilities/tracks aiming at determining the crystallisation temperature of SrTiO3, in the range of extremely low thicknesses used in MTJs, are identified., Ce travail de thèse porte sur l'élaboration et la caractérisation de jonctions tunnel magnétiques (JTM) polycristallines à barrière d'oxyde de titane de strontium, SrTiO3, qui se situe parmi les nouvelles barrières tunnel aux bandes interdites les plus étroites, recensées par la littérature. De telles barrières pourraient répondre à un besoin applicatif crucial : avoir un produit résistance x surface, RA, plus faible dans les JTM, ou à son corollaire, avoir une épaisseur de barrière plus forte à RA égal tout en conservant une magnétorésistance tunnel, TMR, élevée. De précédents travaux ont montré que le SrTiO3 présente une température de cristallisation inhabituellement basse (< 400°C) lorsqu'il est déposé par pulvérisation par faisceau d'ions (IBS) ce qui peut le rendre compatible avec les électrodes magnétiques standards constitutives des JTM. Le dépôt par IBS restant une technique pour le moins exotique au regard de l'état de l'art des JTM, nous avons dans un premier temps élaboré des JTM à barrière d'oxyde de magnésium, MgO, matériau phare de la spintronique. Cette étude a permis de mettre en avant les paramètres spécifiques à cette technique de dépôt influant sur les propriétés de transport des JTM, notamment le type d'oxydation. Dans un second temps, nous avons réalisé des JTM CoFeB/SrTiO3/CoFeB par IBS à partir d'une cible céramique de SrTiO3, en nous inspirant du travail effectué sur le MgO. Les influences de plusieurs paramètres de dépôt, d'oxydation et de recuit ont été analysées, conduisant à deux tendances opposées avec des systèmes présentant soit à une TMR élevée (18 %), soit un RA faible (2.6 Ohm.µm²). Des JTM SrTiO3 ont ensuite été nanostructurées pour la première fois et les tests électriques ont montré que les JTM ayant un bas RA présentaient un comportement ohmique alors que celles ayant une TMR élevée présentaient le comportement tunnel attendu. De plus, ces dernières présentent un claquage diélectrique intrinsèque à l'oxyde. En parallèle, des études microstructurales ont montré une qualité morphologique des JTM SrTiO3 semblable à celle des JTM MgO à l'état de l'art. Toutefois, ces observations n'ont pas permis de statuer sur le caractère cristallisé ou non des barrières en SrTiO3. Plusieurs pistes visant à déterminer la température de cristallisation du SrTiO3 dans la gamme des épaisseurs extraordinairement faibles des barrières tunnel ont été proposées.

We investigated the electronic structure as well as the magnetic properties of a Co2MnSi film on MgO(100) element-specifically at the interface to a MgO tunnel barrier by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The electronic structure of the Co atoms as a function of the capping layer thickness remained unchanged, whereas the XA spectra of Mn indicate an increase of the unoccupied d states. The experimental findings are consistent with the interfacial structure proposed in the work by B. Hulsen et al. [Phys. Rev. Lett. 103, 046802 (2009)], where a MnSi layer is present at the interface to the MgO with oxygen atoms at top positions in the first MgO layer. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769180]

It is shown that at sufficiently low temperatures, one-dimensional arrays of ultrasmall tunnel junctions (for example, semiconductor superlattices) may exhibit an unusual type of electron transfer. This process can be considered as fast ``Bloch'' oscillations with frequency ${\mathit{f}}_{\mathit{B}}$=\ensuremath{\varepsilon}/h (where \ensuremath{\varepsilon} is the electron energy change due to tunneling through one tunnel barrier), modulated with lower single-electron-tunneling frequency ${\mathit{f}}_{\mathit{S}}$=I/e (where I is the dc electric current through the array).

Nanocrystal floating gate memory employing an amorphous carbon (a-C)/SiO2 double-layered tunnel barrier was fabricated. The band gap of a-C and conduction band discontinuity between a-C and Si was estimated to be 1.95 and 0.4 eV, respectively. In addition, interface states density of the a-C/SiO2/channel Si was estimated from the capacitance–voltage measurement. The nanocrystal memory using this tunnel barrier exhibited enhanced charge retention than that employing a single SiO2 tunnel barrier whereas the injection efficiency is comparable between them, which is due to the asymmetrical band profile of the tunnel barrier.

We demonstrate experimentally the electrical ballistic electron spin injection from a ferromagnetic metal / tunnel barrier contact into a semiconductor III-V heterostructure. We introduce the Oblique Hanle Effect technique for reliable optical measurement of the degree of injected spin polarization. In a CoFe / Al2O3 / GaAs / (Al,Ga)As heterostructure we observed injected spin polarization in excess of 8 % at 80K., 5 pages, 4 figures

We report electrical spin injection from a ferromagnetic metal contact into a semiconductor light emitting diode structure with an injection efficiency of 30% which persists to room temperature. The Schottky barrier formed at the Fe/AlGaAs interface provides a natural tunnel barrier for injection of spin polarized electrons under reverse bias. These carriers radiatively recombine, emitting circularly polarized light, and the quantum selection rules relating the optical and carrier spin polarizations provide a quantitative, model-independent measure of injection efficiency. This demonstrates that spin injecting contacts can be formed using a widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing technology., Comment: 14 pages including 3 figures, version accepted by Applied Physics Letters - A. Hanbicki, et al. Appl. Phys. Lett. 80 (7), p.TBD (2002)

A magnetic "spin filter" tunnel barrier, sandwiched between a non-magnetic metal and a magnetic metal, is used to create a new magnetoresistive tunnel device, somewhat analogous to an optical polarizer-analyzer configuration. The resistance of these trilayer structures depends on the relative magnetization orientation of the spin filter and the ferromagnetic electrode. The spin filtering in this configuration yields a previously unobserved magnetoresistance effect, exceeding 100%., Comment: 3.5 pages, 3 figures, submitted to Appl. Phys. Lett

The barrier quality and conduction mechanisms in ferromagnet–insulator–ferromagnet (FIF) junctions are studied by fabricating adjacent ferromagnet–insulator–superconductor (FIS) junctions, using the same barrier and top electrode in both junctions. Transport measurements on the FIS junctions provide information about the transport processes in the FIF junctions. The FIS junctions have a large zero-bias conductance (ZBC) that varies with barrier thickness, in part due to ballistic channels in parallel with the tunnel barrier. As the ZBC increases, the magnetoresistance (MR) in the FIF junctions decreases, implying that these ballistic channels are not magnetoresistive and that their presence reduces the total MR in the FIF junctions.

We report local and non-local measurements in pin-hole dominated mesoscopic multigraphene spin-valves. Local spin-valve measurements show spurious switching behavior in resistance during magnetic field sweeping similar to the signal observed due to spin-injection into multigraphene. The switching behavior has been explained in terms of local Hall effect due to thickness irregularity of the tunnel barrier. Local Hall effect appears due to large local magnetostatic field produced at the roughness in the AlO$_x$ tunnel barrier. The effect of this local Hall effect is found to reduce as temperature is increased above 75 K. The strong local Hall effect hinders spin-injection into multigraphene resulting in no spin signal in non-local measurements., 5 pages, 7 figures

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

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic "pinholes" in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current., 4 pages, 4 figures

We have observed very large tunneling magneto-resistance (TMR) in GaMnAs/AlAs/GaMnAs single-barrier ferromagnetic semiconductor tunnel junctions grown on a (0 0 1) GaAs substrate by low-temperature molecular beam epitaxy. The TMR ratio of 72 % was obtained in a junction with a thin (1.6 nm) AlAs tunnel barrier when the magnetic field was applied along the [1 0 0] axis in the film plane. The TMR ratio was larger when the applied magnetic field direction was along [1 0 0], compared with other directions of [1 1 0] and [−1 1 0]. Also, the TMR ratio was found to decrease with increasing the AlAs barrier thickness.

To test whether the interface between an Fe-alloy electrode and a ${\text{SrTiO}}_{3}$ (STO) tunnel barrier constitutes or not a good spin injector, we have studied the transport and magnetic properties of ${\text{CoFe}}_{2}$ $(7.5\text{ }\text{nm})/{\text{SrTiO}}_{3}$ $(3\text{ }\text{nm})/{\text{CoFe}}_{2}$ (1 nm)/NiFe (8 nm) stacks prepared by pulsed-laser deposition on STO(001) substrates and varied the STO barrier deposition temperature. While this parameter does not strongly influence the magnetic properties of the two electrodes, the resulting barrier height at the bottom ${\text{CoFe}}_{2}/\text{STO}$ interface is lowered if the STO barrier is deposited at $300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ rather than $80\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and an already low-tunnel-magnetoresistance ratio is suppressed. We discuss our findings in terms of the oxide stability of the ${\text{CoFe}}_{2}/\text{STO}$ interface.

Tunneling spectroscopy measurements have been performed on single crystals Bi—Sr—Ca—Cu—O, using the natural surface oxides as the tunnel barrier. The energy gap 2Δ was 58meV at 4·2K and crystals give a sharp single transition at Tc = 90K. Large values of 2Δ/kBTc in 2212 phase are obtained.

Oscillatory structure in the low temperature current-voltage characteristics of Be-doped p-type GaAs/(AlGa)As/GaAs single barrier tunnelling devices is observed. The oscillations have a period Delta V=39 mV, close to h(cross) omega L/e, where h(cross) omega L is the LO phonon energy in GaAs. They result from energy relaxation of hot holes injected through the tunnel barrier. The sheet density in the hole accumulation layer adjacent to the tunnel barrier is obtained from the magneto-oscillations in I(V) for magnetic field B//J. This allows one to calculate the hole effective mass for tunnelling.

We report on ferromagnetic spin-dependent tunneling (SDT) junctions with NiFe/AlN/NiFe and NiFe/AlON/NiFe structures. Good barriers were formed by plasma nitridation and oxy-nitridation of Al films. Tunneling magnetoresistance ratios (TMR) up to 18% were observed at room temperature. The devices exhibit lower resistance-area products than those seen in reference junctions with Al2O3 barriers. The degradation in TMR at higher bias voltages is found to be less than that found in standard alumina junctions. AlN and AlON could thus be alternate materials for the tunnel barrier in SDT junctions.

We show that the barrier profile of in situ grown AlOx tunnel barriers strongly depends on the material choices of the oxide-metal interface. By doing transport measurements on Al and Nb-based metal-oxide-metal tunnel junctions in a wide temperature range and using the phenomenological Simmons' model, we obtain barrier parameters that are qualitatively consistent with the values obtained from the first-principles calculations. The latter suggest that the formation of metal-induced gap states originating from the hybridization between the metallic bands and Al2O3 conduction band is responsible for the tunnel barrier modification. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer.

The main issue in molecular electronics is connecting a molecule to a macroscopic device. The goal of this thesis is to study electronic transport in metallic nanogaps, host structures for the molecules and then to insert a molecule to realize a molecular transistor. The knowledge of transport properties of the host structure is a key point for the molecular transistor realization and the understanding of its electronic properties. Nanogaps are obtained by electromigration of a gold nanowire. When a large current density flows through the nanowire, gold atoms start to move, and eventually breaking the nanowire. The controlled electromigration process developed during this thesis is performed at room temperature, and by limiting atomic displacements, one can obtain a nanometric size gap. The sample is then cooled at 4K to limit any diffusion process which could damage the nanogap, and it is electrically characterized. Fitting the I-V curves by a tunneling model allows us to determinate the electrodes work functions and the inter-electrodes distance, distance to be compared to the molecule size. This characterization may also be used to detect the presence of metallic clusters trapped between the electrodes during the electromigration. The last stage of the molecular transistor realization is the insertion of a single molecule in the gap. This is made in-situ at 4K, under vacuum, by sublimation of a C60 powder by Joule effect. The first tests show that it is possible to obtain a layer of molecules without damaging the nanogap., L'enjeu de l'électronique moléculaire est la connexion de la molécule à un dispositif macroscopique. Le but de cette thèse est d'étudier le transport électronique dans des nanocassures métalliques, structures d'accueil de molécules, puis d'y insérer une molécule pour réaliser un transistor moléculaire. Connaître les propriétés de transport de la structure d'accueil est un point clé pour la fabrication du transistor moléculaire et la compréhension de ses propriétés électroniques. Les nanocassures sont obtenues par électromigration d'un nanofil d'or. Une forte densité de courant entraine le déplacement des atomes d'or et provoque la rupture du nanofil. Le processus d'électromigration contrôlée développé lors de cette thèse est effectué à température ambiante, et permet de limiter les déplacements atomiques afin d'obtenir des coupures de taille nanométrique. L'échantillon est immédiatement refroidi à 4K pour limiter tous processus diffusifs dégradant la nanocassure formée, et il est caractérisé électriquement. L'ajustement des courbes I-V par un modèle tunnel donne les travaux de sortie des électrodes et la distance inter-électrodes, distance à comparer avec la taille de la molécule. La courbe I-V permet aussi de détecter la présence d'agrégats métalliques piégés entre les électrodes lors de l'électromigration. La dernière étape de la réalisation d'un transistor moléculaire est le dépôt de la molécule. Ce dépôt est effectué in-situ à 4K, sous vide, par sublimation d'une poudre de C60 par effet Joule. Les premiers tests montrent qu'il est possible d'obtenir un tapis de molécules sans dégrader les nanocassures.

Results are presented of extensive numerical simulations of electron wave packets transmitted by horns. A detailed quantitative analysis is given of the collimation of the electron wave by horn-like devices. It is demonstrated that the electron wave collimation effect cannot be described in terms of adiabatic wave expansion and semi-classical considerations. It is argued that, in the context of atonic-size electron beam sources, the collimation effect due to a possible horn-like structure in the metal-vacuum potential is much less than the effect stemming from the tunnel barrier itself.

The mechanism for the dissociation of ${\mathrm{CH}}_{4}$ at metal surfaces has been and is still controversial. In large part, this is because many of the experimental data suggest a direct tunneling mechanism, which is not obviously compatible with an observed strong dependence on the surface temperature. We propose here that a new phenomenon, thermally assisted tunneling, caused by a coupling of the tunnel barrier to the lattice, resolves this controversy. Explicit calculations using a simplified model for this process agree qualitatively with a wide range of experimental data for ${\mathrm{CH}}_{4}$ dissociation on Pt(111).

In this work the authors have studied changes in aluminium-oxide-aluminium tunnel barrier parameters that result from annealing at different temperatures with and without an applied potential difference. They identify two processes: irreversible increase in mean barrier height which takes place under all conditions and reversible changes in the internal field produced by applied bias. They explain the observations in terms of a detailed microscopic model involving excess Al3+ in the barrier region and field-induced movement of ions. The model implies that the oxide grows by the classic Cabrera-Mott mechanism with aluminium as the mobile species, and it sheds light on the nature of the oxide and of the interfacial regions in this technologically important system.

Simultaneous monitoring of both capacitance and resistance during the exposure of tunnel junctions to water vapour is shown to be a powerful technique for studying the infusion doping process. Results depend critically on the metal used for the top electrode and show capacitance increases on exposure to water vapour. Observed changes in both capacitance and resistance can be explained in terms of current ideas about tunnel junction structure.

A large exchange splitting of the conduction band in ultrathin films of the ferromagnetic semiconductor EuO was determined quantitatively, by using EuO as a tunnel barrier and fitting the current-voltage characteristics and temperature dependence to tunneling theory. This exchange splitting leads to different tunnel barrier heights for spin-up and spin-down electrons and is large enough to produce a near-fully spin-polarized current. Moreover, the magnetic properties of these ultrathin films (6 nm) show a reduction in Curie temperature with decreasing thickness, in agreement with theoretical calculation [R. Schiller, Phys. Rev. Lett. 86, 3847 (2001)10.1103/Phys. Rev. Lett.86.3847].

This chapter contains sections titled: Introduction SiO 2 as Tunnel Dielectric��-��Historical Perspective Early Work on Silicon Nitride as a Tunnel Dielectric Jet-Vapor Deposition Silicon Nitride Deposition Properties of Gate-Quality JVD Silicon Nitride Films Deposited Silicon Nitride as Tunnel Dielectric N-Channel Floating-Gate Device with Deposited Silicon Nitride Tunnel Dielectric P-Channel Floating-Gate Device with Deposited Silicon Nitride Tunnel Dielectric Reliability Concerns Associated with Hot-Hole Injection Tunnel Dielectric for SONOS Cell Prospects for High-K Dielectrics Tunnel Barrier Engineering with Multiple Barriers Summary References ]]>

The authors use two insulator layers in thermotunnel refrigerator to modify the shape of the tunneling barrier so that electrons with high kinetic energy pass it with increased probability. Theoretical analysis show that the overall tunneling current between the electrodes contains an increased number of high kinetic energy electrons and a reduced number of low energy ones, leading to high efficiency. The particular case of vacuum gap and solid insulator layer is calculated using digital methods. Efficiency remains high in the wide range of the emitter electric field. The cooling coefficient is found to be as high as 40%-50% in the wide range of the emitter electric field., Comment: 9 pages, 3 figures

In this paper, we study the effect of the tunnel barrier thickness non-uniformity in Nb/Al-AlOx/Nb tunnel junctions using the measurement results of the junctioncapacitance (C) and the normal resistance (Rn). The local thickness distribution of the AlOx tunnel barrier in Nb/Al-AlOx/Nb trilayer (RnA ∼ 30 Ω μm2) was studied by high resolution transmission electron microscopy. The specific resistance (RnA) values of the measured junctions range from 8.8 to 68 Ω μm2. We observed scatter in both the junction specific resistance and capacitance data, which is considerably higher than the measurement uncertainty. We also observed noticeable scatter in the RnC product, which does not stem from junction area estimation uncertainties. We discuss the possible reasons that contribute to this scatter. We suggest that the local thickness non-uniformity of the tunnel barrier significantly contributes to the scatter in the RnC product. We confirm this conclusion through an illustrative model based on the barrier imaging data, which results in the variation of the RnC data consistent with the measurements in this paper.

Local barrier height (Φ) images on a Ni (110) sample in ultrahigh vacuum (UHV) conditions show the coexistence of small regions (50 A) with low and high Φ values. Thus, we measured I–s and s–V plots at a fixed point and several tip–sample voltages in order to understand the behavior of the barrier height with different gap distances. A good fitting of these plots with theoretical expressions is obtained for high Φ values, and we evaluate the contribution of the image potential to the tunnel barrier. Anomalously low Φ values in the tunnel regime correspond to quite high values in field emission. The data are consistent with the suggestion that apparently low barrier heights in tunneling are due to forces between tip and sample, resulting in changes of the real gap distance.

This chapter contains sections titled: Introduction What is a Tunnel Junction? Reading the Memory Cell: Control of the Tunnel Barrier Writing to the Memory Cell: Ferromagnetic Layer Magnetization Reversal Actual Performance and Comparison with Calculations Are There Solutions? Conclusions References ]]>

In quasi-magnetic tunnel junctions (QMTJs) with a EuS spin filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions (MTJs). This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory shows excellent agreement with experiment., 4pages, 4Postscript figures, RevTeX4

We examine the conductance behavior of a cavity containing a relatively opaque transverse barrier. We show that the transmission of this structure is strongly dependent on the longitudinal position of the barrier and, in particular, has a maximum value when the barrier is located exactly in the middle of the cavity. In this specific condition the overall conductance is much greater than that associated with the barrier alone. Finally we propose a physical interpretation for the obtained numerical results.