Metal Tunnel Junctions Research Articles

Micromagnetic modeling of double spin-torque magnetic tunnel junction devices

Emerging nonvolatile magnetoresistive random access memory exhibits high endurance and long data retention compared to flash memory. Additionally, devices with double spin torque magnetic tunnel junctions (dsMTJ) featuring two magnetic reference layers demonstrate enhanced torques, fast switching, and reduced switching currents. To accurately model these devices, we adopt a coupled spin and charge transport approach allowing to describe spin-transfer torques in metallic spin valves and magnetic tunnel junctions on equal footing. Our findings indicate the critical influence of metallic non-magnetic spacers (NMS) properties separating the free layer from the second reference layer on the switching speed improvement in dsMTJ devices.

Spin-polarisation measurement using NbN-insulator-ferromagnet tunnel junction with oxidized barrier

We report a two-step process for the fabrication of superconductor-insulator-normal metal tunnel junctions using NbN as the superconducting electrode and its surface oxide as the insulating tunnel barrier, and investigate its efficacy in measuring spin-polarisation of ferromagnets using the Meservey-Tedrow technique. We observe that for NbN film thickness below 10 nm, under the application of parallel magnetic field, the superconducting density of states show clear “Zeeman” splitting into spin-up and spin-down sub-bands. Tunnelling measurements on devices where ferromagnetic Co is used as the normal electrode shows that these devices can be used to measure spin polarisation of a ferromagnet at temperatures up to 1.6 K. The simplicity of our fabrication process makes NbN a very attractive candidate for spin polarisation measurements. Some limitations of this process are also discussed.

Tunable directional emission from electrically driven nano-strip metal-insulator-metal tunnel junctions.

Electrically driven nanoantennas for on-chip generation and manipulation of light have attracted significant attention in recent times. Metal–insulator–metal (MIM) tunnel junctions have been extensively used to electrically excite surface plasmons and photons via inelastic electron tunneling. However, the dynamic switching of light from MIM junctions into spatially separate channels has not been shown. Here, we numerically demonstrate switchable, highly directional light emission from electrically driven nano-strip Ag–SiO2–Ag tunnel junctions. The top electrode of our Ag–SiO2–Ag stack is divided into 16 nano-strips, with two of the tunnel junctions at the centre (SL and SR) acting as sources. Using full-wave electromagnetic simulations, we show that when SL is excited, the emission is highly directional with an angle of emission of −30° and an angular spread of ∼11°. When the excitation is switched to SR, the emission is redirected to an angle of 30° with an identical angular spread. A directivity of 29.4 is achieved in the forward direction, with a forward-to-backward ratio of 12. We also demonstrate wavelength-selective directional switching by changing the width, and thereby the resonance wavelength, of the sources. The emission can be tuned by varying the periodicity of the structure, paving the way for electrically driven, reconfigurable light sources.

Coherence Between Different Propagating Surface Plasmon Polariton Modes Excited by Quantum Mechanical Tunnel Junctions

AbstractCoherence between different surface plasmon polariton (SPP) modes excited by inelastically tunneling electrons in biased metal–insulator–metal tunnel junctions (MIM‐TJs) is demonstrated. By employing a dedicated SPP stripe waveguide with MIM‐TJ, an effective double‐slit configuration similar to the Young's experiment is realized for an electrically biased SPP source. The spatial correlation between different SPP modes originates from a single inelastic tunneling event and leads to strong interference in the far‐field, observed as alternate bright and dark fringes in the Fourier plane. The measured fringe‐spacing inversely follows the stripe waveguide length, with upper limit dictated by the SPP propagation length, confirming the SPP mediated spatial correlation. Finite difference time domain simulations support the experimental findings. Also, the experimental and simulation results unambiguously demonstrate the two‐step plasmonic decay process in plasmonic MIM‐TJs. The results presented here provide a simple and robust demonstration of the inherent coherence existing between different decay channels (plasmons and photons) of the inelastically tunneling electrons and can be exploited for plasmonic applications with tailored spatial coherence with implications in plasmon amplification and quantum information processing.

Geometric Control Over the Edge Diffraction of Electrically Excited Surface Plasmon Polaritons by Tunnel Junctions

Plasmonic metal–insulator–metal tunnel junctions (MIM-TJs) readily excite surface plasmon polaritons (SPPs) by inelastic electron tunneling, well below the diffraction limit, eliminating the need for bulky optical elements. The highly confined MIM cavity mode (MIM-SPP) excited by the tunneling electrons dominantly outcouples to photons and single-interface SPPs which further outcouple as photons and give characteristic features in the Fourier plane of observation. Here, we employ SPP waveguides, directly integrated with MIM-TJs, to explore the diffraction of single-interface SPPs at the edges of the metal stripe waveguides. In addition to the leakage of the SPP modes through the metal electrodes, SPP edge diffraction presents as an additional channel for SPP outcoupling, not limited by the thickness of the waveguide. Edge diffraction manifests as a straight-line feature in the Fourier plane of the far-field and by systematically varying the width of the waveguides, we show that the in-plane momentum of the SPPs can be controlled, which in turn leads to control over the edge diffraction. We fabricated MIM-TJs with integrated plasmonic stripe waveguides of different widths, and the propagation and confinement of the SPP modes along the edges of the SPP waveguides are directly identified by the experimental back focal plane imaging, which are further corroborated by numerical simulations. Our findings can be exploited for applications ranging from sensing to nonlinear plasmonics, where focusing and localization of SPP fields are necessary.

Thermally Stable All‐Perovskite Tandem Solar Cells Fully Using Metal Oxide Charge Transport Layers and Tunnel Junction

All‐perovskite tandem solar cells offer a promising avenue to go beyond the efficiency limit of single‐junction devices. Their efficiencies have been increasing rapidly in the past few years; however, their commercial viability is hindered by the instability under thermal stressing. Herein, comprehensive device design strategies are proposed to achieve thermally stable all‐perovskite tandem solar cells while retaining the advantages of solution processing. Metal oxides, i.e., NiOx and SnO2, are used for the hole and electron transport layers in both wide bandgap and narrow subcells. The metal‐based recombination layer is replaced with a stable and conductive indium tin oxide nanocrystals film to fabricate an all metal‐oxide‐based tunnel junction. Based on those design strategies, the encapsulated all‐perovskite tandem solar cells retained 85% of their initial efficiency after stressing at 85 °C for 2500 h and maintained >80% of their initial performance after 900 h operation at the maximum power point and operating temperature of ≈65 °C. Achieving such thermal stability represents a crucial step toward commercial viability of all‐perovskite tandem solar cells.

Uniform light emission from electrically driven plasmonic grating using multilayer tunneling barriers

Light emission by inelastic tunneling (LEIT) from a metal–insulator–metal tunnel junction is an ultrafast emission process. It is a promising platform for ultrafast transduction from electrical signal to optical signal on integrated circuits. However, existing procedures of fabricating LEIT devices usually involve both top-down and bottom-up techniques, which reduces its compatibility with the modern microfabrication streamline and limits its potential applications in industrial scale-up. Here in this work, we lift these restrictions by using a multilayer insulator grown by atomic layer deposition as the tunnel barrier. For the first time, we fabricate an LEIT device fully by microfabrication techniques and show a stable performance under ambient conditions. Uniform electroluminescence is observed over the entire active region, with the emission spectrum shaped by metallic grating plasmons. The introduction of a multilayer insulator into the LEIT can provide an additional degree of freedom for engineering the energy band landscape of the tunnel barrier. The presented scheme of preparing a stable ultrathin tunnel barrier may also find some applications in a wide range of integrated optoelectronic devices.

Nanoscale Tunnel Junctions and Metallic Single-Electron Transistors via Shadow Evaporation and In Situ Atomic Layer Deposition of Tunnel Barriers

Nanoscale metallic tunnel junctions are important elements of state-of-the-art technologies including superconducting qubits, single electronics, nanospintronics, and caloritronics. The most widely...

Voltage- and temperature-dependent rare-earth dopant contribution to the interfacial magnetic anisotropy

The control of magnetic materials and devices by voltages without electric currents holds the promise of power-saving nano-scale devices. Here we study the temperature-dependent voltage control of the magnetic anisotropy caused by rare-earth (RE) local moments at an interface between a magnetic metal and a non-magnetic insulator, such as Co|(RE)|MgO. Based on a Stevens operator representation of crystal and applied field effects, we find large dominantly quadrupolar intrinsic and field-induced interface anisotropies at room temperature. We suggest improved functionalities of transition metal tunnel junctions by dusting their interfaces with rare earths.

Bright upconverted emission from light-induced inelastic tunneling.

Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are very weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, 5 orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions that we believe is due to light-induced inelastic tunneling emission. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes.

(Invited) Isolating an Individual Ion Emitter and Light-Induced Tunneling Upconversion

This talk will review two recent discoveries made in my group. Single Photon Emitters: Intrigued by the promise of single-photon quantum technologies, many have studied the properties of single ion defect emitters, particularly lanthanides in a crystal matrix (including praseodymium, cerium, and erbium). I will report on a way of reliably isolating individual erbium ion emitters in nanocrystals with the potential for 100% individual emitter yield in post-selection. We used a nanoplasmonic (metal nanostructure) aperture optical tweezer to reliably isolate individual erbium ion emitters in individually trapped NaYF4 nanocrystals. The nanoplasmonic aperture solved two major problems: (1) it enhanced the emission rate so that single ions could be detected reliably in a short time, and (2) it isolated the single Er containing nanocrystal from the other nanocrystals in solution. This new demonstration follows our recently published work showing 400× increased emission using the nanoapertures. Light-Induced Upconversion Emission: Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are very weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, 5 orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions due to light-induced inelastic tunneling. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes.

Large and Robust Charge-to-Spin Conversion in Sputtered Disordered WTe <sub>x</sub>

Topological insulators have recently shown great promise for ultralow-power spin-orbit torque (SOT) devices thanks to their large charge-to-spin conversion efficiency originating from the spin-momentum-locked surface states. Recently, Weyl semimetals emerge as an alternative SOT source with spin-polarized surface as well as bulk states, robustness against magnetic and structural disorder, and higher electrical conductivity for integration in metallic magnetic tunnel junctions. Here, we report that long-range disordered sputtered WTex thin films exhibit local chemical and structural order as those of Weyl semimetal WTe2 and conduction behavior that is consistent with semi-metallic Weyl fermion. We find large charge-to-spin conversion properties in thermally annealed sputtered WTex films that are comparable with those in crystalline WTe2 flakes. Besides, the strength of unidirectional spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is 5 to 20 times larger than typical SOT/ferromagnet bilayers reported at room temperature. We further demonstrate room temperature field-free magnetization switching at a current density of 1 – 2 MA/cm2. These large charge-to-spin conversion properties that are robust in the presence of long-range disorder and thermal annealing pave the way for industrial integration of such topological materials. Our results open a new class of sputtered semimetals for memory and computing based on magnetic tunnel junctions as well as broader planar heterostructures consisting of SOT layer/ferromagnet interfaces.

Complementary Resistive Switching Using Metal-Ferroelectric-Metal Tunnel Junctions.

Complementary resistive switching (CRS) devices are receiving attention because they can potentially solve the current-sneak and current-leakage problems of memory arrays based on resistive switching (RS) elements. It is shown here that a simple anti-serial connection of two ferroelectric tunnel junctions, based on BaTiO3 , with symmetric top metallic electrodes and a common, floating bottom nanometric film electrode, constitute a CRS memory element. It allows nonvolatile storage of binary states ("1" = "HRS+LRS" and "0" = "LRS+HRS"), where HRS (LRS) indicate the high (low) resistance state of each ferroelectric tunnel junction. Remarkably, these states have an identical and large resistance in the remanent state, characteristic of CRS. Here, protocols for writing information are reported and it is shown that non-destructive or destructive reading schemes can be chosen by selecting the appropriate reading voltage amplitude. Moreover, this dual-tunnel device has a significantly lower power consumption than a single ferroelectric tunnel junction to perform writing/reading functions, as is experimentally demonstrated. These findings illustrate that the recent impressive development of ferroelectric tunnel junctions can be further exploited to contribute to solving critical bottlenecks in data storage and logic functions implemented using RS elements.

Tunneling time probed by quantum shot noise

In typical metallic tunnel junctions, the tunneling events occur on a femtosecond timescale. An estimation of this time requires current measurements at optical frequencies and remains challenging. However, it has been known for more than 40 years that as soon as the bias voltage exceeds one volt, the junction emits infrared radiation as an electrically driven optical antenna. We demonstrate here that the photon emission results from the fluctuations of the current inside the tunneling barrier. Photon detection is then equivalent to a measurement of the current fluctuations at optical frequencies, allowing to probe the tunneling time. Based on this idea, we perform optical spectroscopy and electronic current fluctuation measurements in the far from equilibrium regime. Our experimental data are in very good agreement with theoretical predictions based on the Landauer Büttiker scattering formalism. By combining the optics and the electronics, we directly estimate the so-called traversal time.

The shot noise in quasi one-dimensional magnetic tunnel junctions

Based on the free electron approximation, the shot noise of spin-polarized electron tunneling ferromagnetic metal /insulator/ferromagnetic metal (FM/I/FM) tunnel junctions is investigated. The calculated results show that the Fano factors for electrons with different spin orientations increase with the insulator thickness. When the thickness of insulator is small, the Fano factors for up and down spin electrons decrease slowly with the increase of incident electron energy. For large insulator thickness, the electrons Fano factors for different spin directions are the same when the incident electron energy is located in the low energy region, but decrease sharply and appear resonance when the incident electron energy is located in the high energy region. In addition, with the changes of the angle of magnetic moment in two ferromagnetic layers, the electrons Fano factors for different spin directions show obvious separation characteristics. When the magnetic moments in two ferromagnetic layers are antiparallel, the Fano factors tend to same. At the same time, with the increase of the thickness of the insulator and the incident electron energy, the phase variations of between the tunneling conductances and the Fano factors are always opposite.

Strong Electron Self-Cooling in the Cold-Electron Bolometers Designed for CMB Measurements

We have realized cold-electron bolometers (CEB) with direct electron self-cooling of the nanoabsorber by SIN (Superconductor-Insulator-Normal metal) tunnel junctions. This electron self-cooling acts as a strong negative electrothermal feedback, improving noise and dynamic properties. Due to this cooling the photon-noise-limited operation of CEBs was realized in array of bolometers developed for the 345 GHz channel of the OLIMPO Balloon Telescope in the power range from 10 pW to 20 pW at phonon temperature Tph=310 mK. The negative electrothermal feedback in CEB is analogous to TES but instead of artificial heating we use cooling of the absorber. The high efficiency of the electron self-cooling to Te=100 mK without power load and to Te=160 mK under power load is achieved by:- a very small volume of the nanoabsorber (0.02 μm3) and a large area of the SIN tunnel junctions,- effective removal of hot quasiparticles by arranging double stock at both sides of the junctions and close position of the normal metal traps,- self-protection of the 2D array of CEBs against interferences by dividing them between N series CEBs (for voltage interferences) and M parallel CEBs (for current interferences),- suppression of Andreev reflection by a thin layer of Fe in the AlFe absorber.As a result even under high power load the CEBs are working at electron temperature Te less than Tph. To our knowledge, there is no analogue in the bolometers technology in the world for bolometers working at electron temperature colder than phonon temperature.

Spin-torque quantization and microwave sensitivity of a nano-sized spin diode

Rectification of microwave signal by the spin-torque diode is very promising for its practical applications in microwave imaging. This is due to a very high sensitivity of magnetic tunnel junction under the bias current, which was previously demonstrated in a number of works [1-3]. The decreasing of cross-sectional area of the spin-torque diode up to the nano-sized dimensions below 10 nm allows one to reach high sensitivity without any bias current. Transverse quantization of electron states in the magnetic nanowire based on nano-sized metallic spin valves and magnetic tunnel junctions can create an additional impact not only on the magnetoresistance, but also on the spin-transfer torque in such structures. In this work we present an analysis of the quantization effect of conductance and spin-transfer torques on the microwave sensitivity of nano-sized spin-torque diodes during the reduction of its cross-sectional area. It was found that the magnetoresistance values up to 130 % can be achieved in a magnetic nanowire containing spin-valve diode with the nonmagnetic metal spacer. As a result, the maximum microwave sensitivity of spin-torque diodes based on these structures can be increased several times that opens the way for the further development of highly sensitive microwave detectors.

Single electron transistors with hydrogen treatment of ALD SiO2 in nanoscale metal–insulator–metal tunnel junctions

Over the past five years, fabrication of metal–insulator–metal (MIM) single electron transistors (SET) featuring atomic layer deposition (ALD) of ultrathin tunnel barrier dielectrics (SiO2, Al2O3) has been reported. However, the performance of fabricated devices was significantly compromised by the presence of native metal oxide and problems associated with the nucleation of ALD dielectrics on metal substrates. To overcome the difficulty of dielectric ALD nucleation on metal substrates, we recently developed a fabrication technique in which the native metal oxide naturally forming in the presence of the ALD oxidant precursor is first used to promote the nucleation of ALD dielectrics, and then is chemically reduced by forming gas anneal (FGA) at temperatures near 400 °C. However, despite the elimination of native oxide, low temperature characterization of the devices fabricated using FGA reveals excess ‘switching’ noise of a very large magnitude resulting from charged defects within the junctions. It has been previously reported that remote hydrogen plasma (RHP) treatment of SiO2 thin films effectively eradicates fabrication defects. This work reports a comparative study of Ni-based MIM SET treated with FGA and/or RHP. We show that, using a combination of FGA and RHP treatments, it is possible to obtain MIM junctions free of switching noise and without a detectable contribution of native oxide.

Fingerprint of topological Andreev bound states in phase-dependent heat transport

We demonstrate that phase-dependent heat currents through superconductor-topological insulator Josephson junctions provide a useful tool to probe the existence of topological Andreev bound states, even for multi-channel surface states. We predict that in the tunneling regime topological Andreev bound states lead to a minimum of the thermal conductance for a phase difference $\phi=\pi$, in clear contrast to a maximum of the thermal conductance at $\phi=\pi$ that occurs for trivial Andreev bound states in superconductor-normal metal tunnel junctions. This opens up the possibility that phase-dependent heat transport can distinguish between topologically trivial and nontrivial 4$\pi$ modes. Furthermore, we propose a superconducting quantum interference device geometry where phase-dependent heat currents can be measured using available experimental technology.

Superconducting terahertz receivers for space-borne and balloon-borne radio telescopes

A superconducting heterodyne receiver based on superconductor–insulator–superconductor (SIS) junctions and a bolometric receiver based on superconductor–insulator–normal metal (SIN) tunnel junctions developed at the Kotelnikov Institute is described. The noise temperature and noise equivalent power of such a receiver fit the requirements for the TELIS, BOOMERANG, OLIMPO, and LSPE aerostatborne radio telescopes, and the space-based SPICA and MILLIMETRON projects, among others. Applications of such receivers for medical diagnostics and remote control are described.