Double Tunnel Junctions Research Articles

Fabrication of room-temperature operational single-electron devices using Au nanoparticles.

Au nanoparticles (5-10 nm in diameter) single electron devices (SEDs) were fabricated utilizing both electron beam lithography and granular film deposition. Both multiple-tunnel junction (MTJ) and double-tunnel junction (DTJ) were fabricated by adjusting the number of Au nanoparticles between the electrodes. Coulomb blockade effects were clearly observed at room temperature from the MTJs. The threshold voltage of the MTJ-SED with a large gap was about 1.5 V, whereas that with a small gap was about 0.8 V, respectively. When the gap was below 20 nm, a periodic Coulomb staircase was observed from the DTJ-SED at room temperature, where the charging energy was about 0.114 eV.

Sub-picowatt volatile memory cell based on double-barrier tunnel junction

A bistable resistor as a volatile memory is proposed. A silicon island sandwiched with a double barrier oxide tunnel junction is used as storage node. The write operations are conducted by direct tunneling. The charges are localized on one side of the two silicon-oxide interfaces, and the position of the charge is interchanged upon the polarity of the applied voltages. The bistable data states are clearly identified and all operations are performed under a voltage below 1 V and a current below 1 pA. The present device can be a promising candidate for ultra low-power capacitorless dynamic random access memory.

Size Effect of Inserted Iron Nanoparticles in the MgO-Based Double Barrier Magnetic Tunnel Junction

The transport properties of stacked CoFeB (1.86)/MgO (0.2)/Fe-nanoparticles ( tFe)/MgO (0.5)/CoFeB(0.6) double barrier magnetic tunnel junctions have been characterized. In this investigation, stacked film is prepared by magnetron sputter system, annealed at 250 °, and patterned into a micron-scale device using standard electron beam lithography in conjunction with two-angle ion beam etching. First, the structure, magnetic and electrical properties are examined by using transmission electron microscopy, alternating gradient magnetometers, and lock-in techniques. Stacked film having 1.5 nm lateral size of iron nanoparticles (sample S1) exhibits an in-plane magnetic anisotropy, nonetheless the stacked film having 2 nm lateral size of iron nanoparticles (sample S5) possesses out-of-plane magnetic anisotropy. In addition, the micron scaled S1 device displays not only a normal in-plane magnetoresistance (MR) curve but also an anomalous out-of-plane MR behavior, whereas the S5 device only barely shows magnetic response in the in-plane MR. A hysteresis-free linear region in out-of-plane MR curve has shown field sensitivity of ~ 36 Ω/Oe that is promising for magnetic field sensor applications.

Tunneling processes in asymmetric double barrier magnetic tunnel junctions with a thin top MgO layer

Dynamic conductance dI/dV and inelastic electron tunneling spectroscopy (IETS) d2I/dV2 have been measured at different temperatures for double barrier magnetic tunnel junctions with a thin top MgO layer. The resistance in the antiparallel state exhibits a normal tunnel-like behavior, while the resistance in the parallel state shows metallic-like transport, indicating the presence of pinholes in the thin top MgO layer. Three IETS peaks are the zero-bias anomaly, interface magnons, and barrier phonons in both the parallel and antiparallel states. The zero-bias anomaly is the strongest peak in the parallel state and its intensity decreases with temperature. The magnon has the largest intensity in the antiparallel state and its intensity also decreases with temperature. The origins of the dips and peaks in the dI/dV-V curve are also discussed.

Single-electron spin interplay for characterization of magnetic double tunnel junctions

Building on spin-modified Coulomb blockade theory, we derive an approximate analytical solution which matches closely to the existing numerical models in the limit of a large difference in the resistance of the two tunnel junctions. Using this, we are able to explore the properties of an $I(V)$ measurement for a double tunnel junction incorporating a nanoparticle in various magnetic configurations. It is shown that this information allows, under certain conditions, seven of the device's physical parameters to be determined in a single $I(V)$ sweep, including individual junction capacitances and spin-dependent resistances. The tunnel magnetoresistance and spin polarizations are also examined in the light of this solution and are shown to be similar in nature and dominated by the magnetic properties of the most resistive barrier. It is shown that spin injection and detection are theoretically possible in a single device containing a nonmagnetic nanoparticle, suggesting a novel means of measuring the tunnel spin polarization of ferromagnetic materials.

Efficient collection of photogenerated carriers by inserting double tunnel junctions in III-nitride p-i-n solar cells

The strain-induced piezoelectric polarization significantly affects the performances of III-nitride p-i-n solar cells. It tilts the energy-band of intrinsic InGaN layers towards a detrimental direction for drifting carriers, and induces a discontinuity at GaN/InGaN hetero-interfaces that hinders the collection of photocurrent. In this study, we have numerically demonstrated a general strategy to overcome the issues by inserting n+/p+/n+ and p+/n+/p+ GaN-based double tunnel junctions into the n- and p-sides of the device, respectively. The energy-band tilting in the intrinsic InGaN layer is hence absent, mainly attributed to high doping concentration of double tunnel junctions, screening piezoelectric polarization sheet charges, boosting the carrier collection efficiency. The impact of energy-barrier discontinuity is also alleviated due to the strong tunneling of photogenerated carriers, efficiently contributing to the photocurrent of the device. As a result, the incorporation of double tunnel junctions into devices offers the potential to realize efficient high indium III-nitride solar cells.

MgO-Based Double Barrier Magnetic Tunnel Junctions With Synthetic Antiferromagnetic Free Layer

CoFeB/Ru/CoFeB has been used as the middle free layer in MgO-based double barrier magnetic tunnel junctions (DBMTJs). The tunneling magnetoresistance (TMR) ratio, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> (bias voltage at half maximum TMR ratio) and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> (output voltage, defined as V multiplied by TMR ratio) have been investigated as a function of annealing temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) and Ru thickness (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> ) in the free layer. Magnetization data reveal that the two CoFeB layers in CoFeB/Ru/CoFeB are antiferromagnetically coupled. Compared with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> of only 0.31 V for single barrier MTJs (SBMTJs) annealed at 375°C, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> for DBMTJs is up to 0.66 V. By increasing Ta, TMR ratio first increases, reaching the highest critical value, and then decreases. The highest TMR ratio is 181% with t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> = 0.6 nm, which is much higher than that obtained in the DBMTJs with the pure CoFeB as the free layer. The improved TMR ratio is mainly due to the relatively thorough crystallization of the CoFeB layers in the free layer. The thermal annealing has been proven to be an effective method to remove the dissimilarity of the top and bottom CoFeB/MgO interfaces. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> in the positive and negative voltage branches follows the same trend as that of TMR ratio with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> .

Spin-Torque Oscillators Using Perpendicular Anisotropy in CoFeB—MgO Magnetic Tunnel Junctions

A new class of magnetic tunnel junctions (MTJ) with perpendicular anisotropy has been recently used for fast switching applications. A theoretical investigation for these materials to be applied as microwave nano-oscillators is here reported. We demonstrate micro-magnetically the possibility to have both high frequency and high power microwave emission at zero field in a double magnetic tunnel junction. Our predictions give rise to the design of a more compact and easily embedded spin-torque oscillators for all-on-chip applications. To increase the oscillator power, we also demonstrate the possibility to obtain frequency locking at zero field by applying a low current at microwaves.

Superlattice and Quantum Dot Unipolar Barrier Infrared Detectors

We report device performance and provide theoretical analysis of a modified long-wavelength complementary barrier infrared detector structure that incorporates a double tunnel junction contact designed for simpler material growth while retaining the robustness for processing. We also provide analysis of a mid-wavelength quantum dot barrier infrared detector and explain its spectral shape and turn-on characteristics.

Coulomb blockade in vertical, bandgap engineered silicon nanopillars

Vertically oriented, bandgap engineered silicon double tunnel junction nanopillars were fabricated and electrically addressed. The devices were tested at liquid nitrogen and room temperatures. Distinctive staircase steps in current were observed at cryogenic temperatures indicative of the Coulomb blockade effect present in asymmetric double tunnel junction structures. These features disappeared when the device was measured at room temperature.

Gold Nanoparticles on Oxide-Free Silicon–Molecule Interface for Single Electron Transport

Two different organic monolayers were prepared on silicon Si(111) and modified for attaching gold nanoparticles. The molecules are covalently bound to silicon and form very ordered monolayers sometimes improperly called self-assembled monolayers (SAM). They are designed to be electrically insulating and to have very few electrical interface states. By positioning the tip of an STM above a nanoparticle, a double barrier tunnel junction (DBTJ) is created, and Coulomb blockade is demonstrated at 40 K. This is the first time Coulomb blockade is observed with an organic monolayer on oxide-free silicon. This work focuses on the fabrication and initial electrical characterization of this double barrier tunnel junction. The organic layers were prepared by thermal hydrosilylation of two different alkene molecules with either a long carbon chain (C11) or a shorter one (C7), and both were modified to be amine-terminated. FTIR and XPS measurements confirm that the Si(111) substrate remains unoxidized during the whole chemical process. Colloidal gold nanoparticles were prepared using two methods: either with citrate molecules (Turkevich method) or with ascorbic acid as the surfactant. In both cases AFM and STM images show a well-controlled deposition on the grafted organic monolayer. I-V curves obtained by scanning tunneling spectroscopy (STS) are presented on 8 nm diameter nanoparticles and exhibit the well-known Coulomb staircases at low temperature. The curves are discussed as a function of the organic layer thickness and silicon substrate doping.

CoFe alloy as middle layer for strong spin dependent quantum well resonant tunneling in MgO double barrier magnetic tunnel junctions

We report on spin dependent quantum well (QW) resonances in the CoFe alloy middle layer of CoFe/MgO/CoFe/MgO/CoFeB double barrier magnetic tunnel junctions. The $dI/dV$ spectra reveal clear resonant peaks for the parallel magnetization configurations, which can be related to the existence of QW resonances obtained from first-principles calculations. We observe that the differential tunneling magnetoresistance (TMR) exhibits an oscillatory behavior as a function of voltage with a sign change as well as a pronounced TMR enhancement at resonant voltages at room temperature. The observation of strong QW resonances indicates that the CoFe film possesses a long majority spin mean-free path, and the substitutional disorder does not cause a significant increase of scattering. Both points are confirmed by first-principles electronic structure calculation.

Complementary barrier infrared detector (CBIRD) with double tunnel junction contact and quantum dot barrier infrared detector (QD-BIRD)

The InAs/GaSb type-II superlattice based complementary barrier infrared detector (CBIRD) has already demonstrated very good performance in long-wavelength infrared (LWIR) detection. In this work, we describe results on a modified CBIRD device that incorporates a double tunnel junction contact designed for robust device and focal plane array processing. The new device also exhibited reduced turn-on voltage. We also report results on the quantum dot barrier infrared detector (QD-BIRD). By incorporating self-assembled InSb quantum dots into the InAsSb absorber of the standard nBn detector structure, the QD-BIRD extend the detector cutoff wavelength from ∼4.2μm to 6μm, allowing the coverage of the mid-wavelength infrared (MWIR) transmission window. The device has been observed to show infrared response at 225K.

Nonequilibrium Superconductivity in Double Tunnel Junction with Spatially Inhomogeneous Superconducting Electrode

To study nonequilibrium superconductivity in the electrodes with a spatially inhomogeneous order parameter (SIOP), Pb1/Sn1-I-Pb2/Sn2-I-Pb3 double tunnel junctions have been produced. Their current-voltage characteristics at the temperature T=4.2 K and dependencies representing order parameter change in the electrode with SIOP under quasiparticle injection have been studied. It is shown that for a Pb/Sn sandwich the “dirty limit” conditions hold. Estimations of the γm proximity effect parameter, \(\tau_{R}^{\mathrm{eff}}\) effective recombination time for the Pb/Sn sandwich and τT tunneling time in an injector junction have been made.

SUB- AND SUPER-POISSONIAN NOISE IN SI QUANTUM DOTS USING FULLY SELF-CONSISTENT 3D SIMULATION

The 3D Monte Carlo simulation of an Si dot-based double-tunnel junction shows not only the possibility of shot noise suppression down to the Fano factor of 0.5, but also of super-Poissonian noise. The counting statistics of the tunneling events provides a clear interpretation of the different noise regimes according to the balance between the different tunneling rates involved.

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.

Diffusive model of current-in-plane-tunneling in double magnetic tunnel junctions

We propose a model that describes current-in-plane tunneling transport in double barrier magnetic tunnel junctions in diffusive regime. Our study shows that specific features appear in double junctions that are described by introducing two typical length scales. The model may be used to measure the magnetoresistance and the resistance area product of both barriers in unpatterned stacks of double barrier magnetic tunnel junctions.

Competition between cotunneling, Kondo effect, and direct tunneling in discontinuous high-anisotropy magnetic tunnel junctions

The transition between Kondo and Coulomb blockade effects in discontinuous double magnetic tunnel junctions is explored as a function of the size of the CoPt magnetic clusters embedded between AlO${}_{x}$ tunnel barriers. A gradual competition between cotunneling enhancement of the tunneling magnetoresistance (TMR) and the TMR suppression due to the Kondo effect has been found in these junctions, with both effects having been found to coexist even in the same sample. It is possible to tune between these two states with temperature (at a temperature far below the cluster blocking temperature). In addition, when further decreasing the size of the CoPt clusters, another gradual transition between the Kondo effect and direct tunneling between the electrodes takes place. This second transition shows that the spin-flip processes found in junctions with impurities in the barrier are in fact due to the Kondo effect. A simple theoretical model able to account for these experimental results is proposed.

Field sensing in MgO double barrier magnetic tunnel junctions with a superparamagnetic Co50Fe50 free layer

Linear response and low frequency noise have been investigated in MgO double barrier magnetic tunnel junctions with a superparamagnetic Co50Fe50 free layer. Linear and hysteresis-free switching was observed for the Co50Fe50 thickness t ≤ 1 nm. A tunneling magnetoresistance ratio of up to 108% and large magnetic field sensitivity value of 61%/mT were obtained at room temperature when t = 1.0 nm. The angular dependence of magnetoresistance suggests that weak coupling between superparamagnetic islands in a 1.0 nm free layer permits continuous rotation of magnetization, whereas the islands in a 0.8 nm layer switch rather independently. The frequency dependence of noise power spectrum density and field dependence of Hooge parameter (α) also behave differently for junctions with 0.8 and 1.0 nm free layers. The noise sensitivity of 1.0 nm free layer junctions is independent of bias, and it is estimated to reach 400 pT/Hz0.5 at 500 kHz.

Asymmetric Voltage Behavior of the Tunnel Magnetoresistance in Double Barrier Magnetic Tunnel Junctions

In this paper, we study the value of the tunnel magnetoresistance (TMR) as a function of the applied voltage in double barrier magnetic tunnel junctions (DMTJs) with the left and right ferromagnetic (FM) layers being pinned and numerically estimate the possible difference of the TMR curves for negative and positive voltages in the homojunctions (equal barriers and electrodes). DMTJs are modeled as two single barrier junctions connected in series with consecutive tunneling (CST). We investigated the asymmetric voltage behavior of the TMR for the CST in the range of a general theoretical model. Significant asymmetries of the experimental curves, which arise due to different annealing regimes, are mostly explained by different heights of the tunnel barriers and asymmetries of spin polarizations in magnetic layers.