Planar Tunnel Research Articles

Spin-transfer torque in double tunnel junctions with ferromagnetic barriers and nonmagnetic electrodes

Spin and charge transport in a double planar tunnel junction with two ferromagnetic barriers and nonmagnetic electrodes is analysed theoretically. Tunnelling current and both in-plane and normal (out-of-plane) components of the spin-transfer torque exerted on ferromagnetic barriers are determined in the free-electron-like model in the coherent tunnelling limit. It is shown that the bias voltage dependence of the current and both components of the torque are nonmonotonic. The current and in-plane torque can be enhanced due to the resonant tunnelling through resonance states located below the Fermi level. These states also lead to oscillations of the current and torque with the thickness of the central layer. The normal torque can change sign with increase of the bias or the thickness of the central layer, which is due to the position change of the resonance states and the spin-splitting of these states. When electrons tunnel via the resonant states, the angular dependence of the torque can deviate strongly from the sinusoidal dependence observed in single junction with ferromagnetic electrodes and nonmagnetic barrier. The maximum of spin torque in the double junction with ferromagnetic barriers is often shifted from the configuration with perpendicular orientation of magnetic moments toward the configurations close to the antiparallel configuration.

Experimental Study of Floating-Gate-Type Metal–Oxide–Semiconductor Capacitors with Nanosize Triangular Cross-Sectional Tunnel Areas for Low Operating Voltage Flash Memory Application

The floating-gate (FG)-type metal–oxide–semiconductor (MOS) capacitors with planar (planar-MOS) and three-dimensional (3D) nanosize triangular cross-sectional tunnel areas (3D-MOS) have successfully been fabricated by introducing rapid thermal oxidation (RTO) and postdeposition annealing (PDA), and their electrical characteristics between the control gate (CG) and FG have been systematically compared. It was experimentally found in both planar- and 3D-MOS capacitors that the uniform and higher breakdown voltages are obtained by introducing RTO owing to the high-quality thermal oxide formation on the surface and etched edge regions of the n+ polycrystalline silicon (poly-Si) FG, and the leakage current is highly suppressed after PDA owing to the improved quality of the tetraethylorthosilicate (TEOS) silicon dioxide (SiO2) between CG and FG. Moreover, a lower breakdown voltage between CG and FG was obtained in the fabricated 3D-MOS capacitors as compared with that of planar-MOS capacitors thanks to the enhanced local electric field at the tips of triangular tunnel areas. The developed nanosize triangular cross-sectional tunnel area is useful for the fabrication of low operating voltage flash memories.

Thermoelectric Effects in Planar Tunnel Junctions

Thermoelectric Effects in Planar Tunnel Junctions

Temperature dependance of the tunneling density of states in sub-micron planar metal/oxide/graphene junctions

We present the tunneling measurements of sub-micron metal/insulator/graphene planar tunnel junctions up to room temperature. We observe a gate independent gap, as previously observed only by low temperature STM [Y. Zhang et al., Nat. Phys. 4, 627 (2008)]. No gap appears at temperatures above 150 K, which is four times smaller than the theoretically expected Tc, from the accepted mean field model [T. O. Wehling et al., Phys. Rev. Lett. 101, 216803 (2008)]. We show that taking into account an additional vibrational effect of out-of-plane phonon soft modes the gap may disappear from the measurements at temperatures much lower than the calculated Tc.

Thermopower, figure of merit and spin-transfer torque induced by the temperature gradient in planar tunnel junctions

The thermopower, charge and thermal conductance, and figure of merit as well as the spin-transfer torque generated by the temperature gradient in the planar tunnel junction consisting of ferromagnetic layers and the nonmagnetic tunnel barrier are investigated in the free-electron-like spin-polarized one-band model. In particular, the influence of the parameters of the junction as well as the influence of the relative orientation of magnetic moments on the studied phenomena are investigated. The thermopower can be related to the voltage drop generated by the temperature difference between electrodes under the condition that the charge current vanishes. It depends on the magnetic configuration of the junction. In junctions with high barriers the thermopower is maximal in the antiparallel configuration and it can be enhanced in junctions with strong spin-splitting of the electron bands. The component of the torque studied in the present paper is oriented in the plane formed by magnetic moments and it appears in the absence of the bias voltage. Its magnitude is insensitive to the sign of the temperature difference in contrast to the bias-induced in-plane torque which strongly depends on the polarization of the bias. The studied torque is usually smaller than the torque generated by the bias: however, it can be significant in junctions with low barriers.

SOI TFET $I_{\rm ON}/I_{\rm OFF}$ Enhancement via Back Biasing

The effect of back biasing on the performance of a planar tunnel field-effect transistor (TFET) implemented on a silicon-on-insulator substrate is investigated. It is found that reverse back biasing reduces the subthreshold swing SS and increases the range of drain current over which SS is less than ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> ) <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ln</i> (10); hence, it is effective for improving the TFET on/off current ratio for low operating voltages (≤ 0.5 V).

Resonant tunnel magnetoresistance in double-barrier planar magnetic tunnel junctions

We present a theoretical approach to calculate the spin-dependent current and tunnel magnetoresistance (TMR) in a double-barrier magnetic tunnel junction (DMTJ), in which the magnetization of the middle ferromagnetic metal layer can be aligned parallel or antiparallel in relation to the fixed magnetizations of the left and right ferromagnetic electrodes. The electron transport through the DMTJ is considered as a three-dimensional problem, taking into account all transmitting electron trajectories as well as the spin-dependent momentum conservation law. The dependence of the transmission coefficient and spin-polarized currents on the applied voltage is derived as an exact solution to the quantum-mechanical problem for the spin-polarized transport. In the range of the developed physical model, the resonant tunneling, nonresonant tunneling, and enhanced spin filtering can be explained; the simulation results are in good agreement with experimental data.

Electric transport across Sr1−xLaxCuO2/Au/Nb planar tunnel junctions and x-ray photoelectron and Auger-electron spectroscopy on Sr1−xLaxCuO2thin films

Thin-film planar tunnel junctions with the electron-doped infinite-layer superconductor Sr${}_{1\ensuremath{-}x}$La${}_{x}$CuO${}_{2}$ (SLCO) with $x\ensuremath{\sim}0.15$ as bottom electrode, a thin Au interlayer, and Nb as top electrode were fabricated and characterized. Measurements of electric transport across these junctions provide information on the interface and surface properties of the SLCO thin films. No Cooper pair tunneling is observed; however, nonlinear current-voltage characteristics give evidence for quasiparticle (QP) tunneling across a thin insulating SLCO barrier at the SLCO/Au interface, with a single gap value $\ensuremath{\sim}$$1.4\phantom{\rule{0.28em}{0ex}}$meV, originating from superconducting Nb. The absence of a superconducting SLCO gap in the QP conductance curves indicates a thin normal-conducting SLCO layer below the insulating SLCO barrier. To examine its origin, x-ray photoelectron spectroscopy (XPS) and x-ray Auger-electron spectroscopy (XAES) on SLCO thin films were performed. We observe a Cu valence of +1 in the SLCO surface layer (within $\ensuremath{\sim}$$3\phantom{\rule{0.28em}{0ex}}$nm thickness) and of +2 in deeper regions, as expected for fully oxidized CuO${}_{2}$ planes in the bulk. Hence, the XPS and XAES results for the SLCO films are consistent with the QP tunneling spectra observed for our planar SLCO/Au/Nb junctions.

Electron Tunneling Spectroscopy of SmB&lt;SUB&gt;6&lt;/SUB&gt; Studied by &lt;I&gt;In Situ&lt;/I&gt; Nano-Break-Junction Method

Tunneling spectra of intermediate-valence semiconductor SmB6 are reported for in-situ break junctions, being able to make nano-scale planar tunnel junctions. The electron tunneling using break junction method is a powerful probe of the intrinsic energy gap. The investigated tunneling conductance dI/dV curves are mostly reproducible and symmetric with respect to the applied voltage. Two kinds of characteristic energy gaps are observed at 2E(d) = 20 mV and 2E(a) = 9 mV, which coincides well with those previously studied by point-contact spectroscopy and the activation energy fitted by our electrical resistivity data. The positions of the gap structures are independent of the zero-bias conductance, implying no additional voltage drop induced by the break junctions. The small anomaly at the activation energy 2E(a) indicates a relatively low density of in-gap states. Furthermore, the results of magnetic properties reveal the ratio of Sm2+:Sm3+ = 3.7:6.3 and the antiferromagnetic nature at high temperature.

Josephson tunnel junctions in a magnetic field gradient

We measured the magnetic field dependence of the critical current of high-quality Nb-based planar Josephson tunnel junctions in the presence of a controllable nonuniform field distribution. We found skewed and slowly changing magnetic diffraction patterns quite dissimilar from the Fraunhofer-like ones typical of a homogeneous field. Our findings can be well interpreted in terms of recent theoretical predictions [R. Monaco, J. Appl. Phys. 108, 033906 (2010)] for a uniform magnetic field gradient, leading to Fresnel-like magnetic diffraction patterns. We also show that Fiske resonances can be suppressed by an asymmetric magnetic field profile.

Conductivity and tunneling density of states in granular Cr films

We have measured the tunneling differential conductances, $G(V)$, of four $\text{Al}/{\text{AlO}}_{x}/\text{Cr}$ planar tunnel junctions as well as the conductivities, $\ensuremath{\sigma}(T)$, of the Cr electrodes at liquid-helium temperatures. The Cr electrodes were made to be granular with dimensionless intergrain tunneling conductance spanning from $g\ensuremath{\simeq}1$ to $g⪢1$, and the dimensionality of the granular array $d=3$. For the samples with $g⪢1$, we found that the measured $G(V)$ curves display large zero-bias singularities which obey a $\text{ln}\text{ }V$ law at low bias voltages ($\ensuremath{\lesssim}$ a few millielectron volt) while crossing over to a $\sqrt{V}$ law at high bias voltages. Simultaneously, the conductivities of the Cr electrodes reveal $\text{ln}\text{ }T$ dependence below a characteristic temperature. These results are explained in terms of the recent theory of granular metals. In a sample with $g\ensuremath{\simeq}1$, in addition to the conductivity dependence $\ensuremath{\sigma}\ensuremath{\propto}\text{ln}\text{ }T$, we observed a universal scaling behavior of the normalized differential conductance $[G(V,T)\ensuremath{-}G(0,T)]/\sqrt{T}$ with the combined parameter $\sqrt{e|V|/{k}_{B}T}$ in a wide temperature interval of 2.5--32 K. This result is not yet understood.

Electrical Control of Magnetic States

Charge and spin transport in artificial magnetic structures, like nanopillar spin valves, planar tunnel junctions, and mesoscopic double-barrier junctions (single-electron transistors) are briefly surveyed, with particular emphasis on magnetoresistance effects and current-induced magnetic switching and spin dynamics. The methods of spin current generation are also discussed, particularly the method based on spin Hall effect.

Probing semiconductor band structures and heterojunction interface properties with ballistic carrier emission:GaAs/AlxGa1−xAsas a model system

Utilizing three-terminal tunnel emission of ballistic electrons and holes in a planar tunnel transistor with a Mott-barrier collector, we have developed a method to self-consistently determine the energy gap of a semiconductor and band discontinuities at a semiconductor heterojunction without using a priori material parameters. Measurements are performed on lattice-matched $\text{GaAs}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ (100) single-barrier double heterostructures with ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ as the model ternary III--V compounds. Electronic band gaps of the AlGaAs alloys and band offsets at the GaAs/AlGaAs (100) interfaces are measured with a resolution of several meV at 4.2 K. The direct-gap $\ensuremath{\Gamma}$ band offset ratio for the GaAs/AlGaAs (100) interface is found to be 59:41 $(\ifmmode\pm\else\textpm\fi{}3\mathrm{%})$. Reexamination of our previous experiment [W. Yi et al., Appl. Phys. Lett. 95, 112102 (2009)] revealed that, in the indirect-gap regime, ballistic electrons from direct tunnel emissions probe the X valley in the conduction band, while those from Auger-like scattering processes in the metal base film probe the higher-lying L valley. Such selective electron collection may be explained by their different momentum distributions and parallel momentum conservation at the quasiepitaxial Al/GaAs (100) interface. We argue that the present method is in principle applicable to arbitrary type-I semiconductor heterostructures.

Transport measurements on microcrystals of oriented CeIn 3 and CeCoIn 5 thin films

We present results of a systematic growth study on the substrate temperature-driven de-wetting of CeIn 3 thin films as a reference compound for the heavy-fermion superconductor CeCoIn 5. This study is motivated by the last results on the improved surface morphology and in-plane orientation preference of c-axis oriented CeCoIn 5 thin films. In apparently phase pure CeIn 3 thin films, as far as x-ray diffraction can tell, we find evidence for minor In segregation by means of percolating resistance contributions of In in the films. We argue that further efforts into improving the films' morphology towards optimized surfaces for the preparation of macroscopic planar tunnel junctions may be futile. As an alternative approach, we present results of electronic transport measurements on CeIn 3 and CeCoIn 5 microcrystals isolated from the thin films by means of focused particle beam induced processing, such as focused ion beam milling and deposition.

Doping and temperature dependence of the superconducting energy gap in the electron-doped cuprate Pr2−xCexCuO4−δ

In hole-doped cuprate superconductors at low carrier concentrations two energy scales are identified: the superconducting energy gap and the pseudogap. The relation between these energy scales is still a puzzle. In these compounds a measurement of the energy gap is not necessarily a probe of the order parameter. In the electron-doped cuprates the pseudogap does not obscure the superconducting state. Consequently, the superconducting gap can be studied directly in a tunneling experiment. Here we show that by studying superconductor/insulator/superconductor planar tunnel junctions we are able to map the behavior of the gap amplitude for the entire (doping-temperature) phase diagram of the electron-doped cuprate superconductor Pr2-xCexCuO4-δ. The superconducting gap, Δ, shows a BCS-like temperature dependence even for extremely low carrier concentrations. Moreover, Δ follows the doping dependence of Tc. We can therefore conclude that there is a single superconducting energy scale in the electron-doped cuprates.

Competing effects of surface phonon softening and quantum size effects on thesuperconducting properties of nanostructured Pb

The superconducting transition temperature(TC) in nanostructured Pb decreases from 7.24 to 6.4 K as the particle size isreduced from 65 to 7 nm, below which superconductivity is lost ratherabruptly. In contrast, there is a large enhancement in the upper critical field(HC2) in the same size regime. We explore the origin of the unusual robustness ofTC over such a large particle size range in nanostructured Pb by measuring the temperaturedependence of the superconducting energy gap in planar tunnel junctions ofAl/Al2O3/nano-Pb. We show that below 22 nm, the electron–phonon coupling strengthincreases monotonically with decreasing particle size, and almost exactlycompensates for the quantum size effect, which is expected to suppressTC.

Static properties of small Josephson tunnel junctions in an oblique magnetic field

We have carried out a detailed experimental investigation of the static properties of planar Josephson tunnel junctions in presence of a uniform external magnetic field applied in an arbitrary orientation with respect to the barrier plane. We considered annular junctions, as well as rectangular junctions (having both overlap and cross-type geometries) with different barrier aspect ratios. It is shown how most of the experimental findings in an oblique field can be reproduced invoking the superposition principle to combine the classical behavior of electrically small junctions in an in-plane field together with the small junction behavior in a transverse field that we recently published [R. Monaco et al., J. Appl. Phys. vol 104, 023906 (2008)]. We explore the implications of these results in supposing systematic errors in previous experiments and in proposing new possible applications. We show that the presence of a transverse field may have important consequences, which could be either voluntarily exploited in applications or present an unwanted perturbation.

Two-channel Kondo effects inAl/AlOx/Scplanar tunnel junctions

We have measured the differential conductances $G(V,T)$ in several $\text{Al}/{\text{AlO}}_{x}/\text{Sc}$ planar tunnel junctions between 2 and 35 K. As the temperature decreases to $\ensuremath{\sim}16\text{ }\text{K}$, the zero-bias conductance $G(0,T)$ crosses over from a standard $\ensuremath{-}\text{ln}\text{ }T$ dependence to a novel $\ensuremath{-}\sqrt{T}$ dependence. Correspondingly, the finite bias conductance $G(V,T)$ reveals a two-channel Kondo scaling behavior between $\ensuremath{\sim}4$ and 16 K. The observed two-channel Kondo physics is ascribed to originating from a few localized spin-$\frac{1}{2}$ Sc atoms situated slightly inside the ${\text{AlO}}_{x}/\text{Sc}$ interface.

Spin Torque in Double Planar Tunnel Junctions

Transport in a double planar tunnel junction with ferromagnetic electrodes is analyzed theoretically in the zero-temperature limit. The in-plane and out-of-plane components of the spin torque exerted on magnetic moment of the central layer are determined as a function of the angle θ between magnetic moments of one of the external electrodes and of the central layer. Two configurations of the magnetic moments of external electrodes are considered, i.e. parallel and antiparallel ones. It is found that both torque components depend strongly on the thickness of the central layer, especially in junctions with a relatively thin central layer, where enhancement of the torque or a change of its sign can be observed for specific layer thicknesses. In junctions with thick central layer and in the limit of small bias voltage applied to the junction, the in-plane torque is generally smaller in the parallel configuration than in the antiparallel one. The opposite relation is observed for the normal torque.

Static properties of small Josephson tunnel junctions in a transverse magnetic field

The magnetic field distribution in the barrier of small planar Josephson tunnel junctions is numerically simulated in the case when an external magnetic field is applied perpendicular to the barrier plane. The simulations allow for heuristic analytical solutions for the Josephson static phase profile from which the dependence of the maximum Josephson current on the applied field amplitude is derived. The most common geometrical configurations are considered and, when possible, the theoretical findings are compared with the experimental data.