Junction Stacks Research Articles

Modeling the Effects of Niobium Surface Roughness on Electrical Conductivity of Nb/Al-AlO$_{x}$/Nb Josephson Junctions

Technology Computer Aided Design (TCAD) is being developed for superconductor electronics (SCE). The AlO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{x}$</tex-math></inline-formula> tunnel barriers used in SCE have thicknesses on the order of the surface roughness of sputtered niobium. Surface roughness is not included in a standard CMOS TCAD process simulator and is among the unique modules needed for SCE. An empirical model for surface roughness is developed for TCAD process simulators. This model is merged with the existing sputtering module and coupled with chemical mechanical polishing and aluminum oxidation to generate process-simulated structures of the Nb/Al-AlO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{x}$</tex-math></inline-formula> /Nb junction stack. Electrical simulations of these junctions are used to extract room temperature conductance. The inherent statistical variation of the proposed surface roughness model lends itself to large scale parallel simulation of thousands of junctions. Statistical simulations are performed and statistical analysis is given.

Modular Protein-DNA Cocrystals as Precise, Programmable Assembly Scaffolds.

High-precision nanomaterials to entrap DNA-binding molecules are sought after for applications such as controlled drug delivery and scaffold-assisted structural biology. Here, we engineered protein-DNA cocrystals to serve as scaffolds for DNA-binding molecules. The designed cocrystals, isoreticular cocrystals, contain DNA-binding protein and cognate DNA blocks where the DNA-DNA junctions stack end-to-end. Furthermore, the crystal symmetry allows topology preserving (isoreticular) expansion of the DNA stack without breaking protein-protein contacts, hence providing larger solvent channels for guest diffusion. Experimentally, the resulting designed isoreticular cocrystal adopted an interpenetrating I222 lattice, a phenomenon previously observed in metal-organic frameworks (MOFs). The interpenetrating lattice crystallized dependably in the same space group despite myriad modifications at the DNA-DNA junctions. Assembly was modular with respect to the DNA inserted for expansion, providing an interchangeable DNA sequence for guest-specified scaffolding. Also, the DNA-DNA junctions were tunable, accommodating varied sticky base overhang lengths and terminal phosphorylation. As a proof of concept, we used the interpenetrating scaffold crystals to separately entrap three distinct guest molecules during crystallization. Isoreticular cocrystal design offers a route to a programmable scaffold for DNA-binding molecules, and the design principles may be applied to existing cocrystals to develop scaffolding materials.

Ultrafast laser-induced magneto-optical response of CoFeB/MgO/CoFeB magnetic tunneling junction

Understanding of ultrafast spin dynamics is crucial for future ultrafast and energy efficient magnetic memory and storage applications. We study the ultrafast laser-induced magneto-optical response of a CoFeB/MgO/CoFeB magnetic tunneling junction (MTJ), when exciting with a short laser pulse, as a function of magnetic configuration and pump fluence. The ultrafast magnetization of the MTJ drops rapidly in the timescale of 0.33–0.37 ps, which is driven by both electron-spin scattering and spin transport mutually. Subsequently, the energy from the electron and spin reservoirs transfers into the lattice with the timescale of 1.5–2.0 and 5.0–15.0 ps through the electron–phonon and spin–phonon interactions, respectively. Our results suggest that the interfacial spin-orientation of CoFeB/MgO/CoFeB could modulate the interaction constant between spins and phonons. These findings provide insight into the role of MTJ interface in spin dynamics, which will be helpful for opto-spintronic tunnel junction stack designs and applications.

Josephson effect in twisted cuprates

&lt;sec&gt;To tunnel, or not to tunnel, that is the question for a Josephson junction constructed by superconductors with unidentified pairing symmetry. Theoretically, Josephson tunneling is forbidden between two &lt;i&gt;d&lt;/i&gt;-wave superconductors twisted by 45°. This is in sharp contrast to persistent tunneling between two &lt;i&gt;s&lt;/i&gt;-wave superconductors. Experimentally, however, Josephson tunneling is observed in twisted bicrystalline cuprates at around 45°, against the expectation that cuprate superconductors possess &lt;i&gt;d&lt;/i&gt;-wave pairing. Due to technical uncertainties, the early studies on twisted bulk cuprates were not widely recognized. The recent advent of van der Waals stacking has allowed a fresh look at this problem. Indeed, twisted thin flakes of cuprates have been realized and the corresponding pairing symmetry has been revisited both experimentally and theoretically. In this work, we overview the recent development on twisted cuprates. After summarizing the theoretical treatment and recent proposals, we introduce the technical progress of making the twisted cuprate junctions in van der Waal stacking, and discuss the recent experimental results of &lt;i&gt;s&lt;/i&gt;-, &lt;i&gt;d&lt;/i&gt;-, or &lt;i&gt;d&lt;/i&gt; + i&lt;i&gt;d&lt;/i&gt;-wave pairing. In the end, we propose possible directions for future exploration in this field.&lt;/sec&gt;&lt;sec&gt; This paper has three major sections: theories on twisted cuprates in Section 1, techniques of realizing twisted cuprates in Section 2, and experimental results on twisted cuprates in Section 3. Specifically, in Section 1, both the early theory and the latest theoretical proposals are introduced. After discussing the calculated angular dependence of Josephson tunneling between two &lt;i&gt;d&lt;/i&gt;-wave or &lt;i&gt;s-&lt;/i&gt;wave superconductors, we summarize the predicted features from the emergent &lt;i&gt;d&lt;/i&gt;+i&lt;i&gt;d-&lt;/i&gt;wave pairing. They include unconventional temperature dependence of the critical Josephson current, doubling in frequency of the Fraunhofer pattern or Shapiro steps, and spontaneous Kerr rotation or emergence of Josephson diode effect. In Section 2, the technological progress of van der Waals stacking of cuprate superconductors is presented. Ultrathin twisted Josephson junctions of cuprates can be realized by either dry stacking together with oxygen post-annealing or cryogenic stacking at tens of degrees below 0 °C. In Section 3, the recent experimental results on van der Waals stacked twisted cuprates are reviewed. Tunneling in twisted underdoped cuprates realized by post-annealing indicates the existence of &lt;i&gt;s&lt;/i&gt;-wave pairing and strong deviation from pure &lt;i&gt;d&lt;/i&gt;-wave pairing. This result is contrasted with another study on cryogenically stacked junctions. There, signatures of &lt;i&gt;d&lt;/i&gt;+i&lt;i&gt;d&lt;/i&gt;-wave pairing, such as fractional Shapiro steps, are reported. Still, our recent experiments on 45°-twisted junctions with ultraclean interfaces, which are also realized by cryogenic stacking, show standard Fraunhofer patterns and AC Josephson effect with only integer steps, indicating the absence of &lt;i&gt;d &lt;/i&gt;+ i&lt;i&gt;d&lt;/i&gt;-wave pairing. These results have far-reaching influence on understanding the pairing symmetry of twisted cuprates. Future efforts to study the twisted cuprates may include: extending to a wider pool of materials, pushing the thickness to the atomic limit, and adopting other characterization tools. The twisted cuprates may also find applications in high temperature superconducting quantum bit as well as Josephson diodes.&lt;/sec&gt;

Circuit models of simultaneously biased intrinsic Josephson junction stacks for terahertz radiations in high-bias regime

Experiment-based circuit models for Josephson plasma emitters (JPEs) made of cuprate superconductor Bi2Sr2CaCu2O8+δ single crystals are proposed. Introducing an LCR resonator shunting an RCSJ parallel circuit allows us to reproduce experimental radiation intensity as a function of device voltage. Mismatch in the resonance frequencies of the total device and the LCR part is related to the wide-range tuning of the radiation frequency of JPE. It is found that coupling capacitances for simultaneously biased operations provide significant increases in radiation intensities, which have been reported as synchronized radiation from multiple mesas in the literature.

Restoring the surface layer of Bi2Sr2CaCu2O8+δ single crystals by current injection

In 2010, we reported a new method to electronically tailor the hole concentration of the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212). We discovered that current injection along the c-axis of Bi-2212 provides a convenient tool to reversibly increase the carrier concentration. In this study, we have investigated the current injection effect on the surface junction of Bi-2212 by a series of current injection experiments carried out in samples of slightly overdoped and of extremely underdoped Bi-2212 crystals. The c-axis transport characteristics of bulk and surface intrinsic Josephson junctions are examined in a wide range of doping by current injection. Before injection treatment, typically, our samples demonstrate non-superconducting characteristics across their surface junctions, because standard preparation procedures cause the topmost CuO2 layers to be heavily degraded by oxygen loss. The electronic doping enables to substantially restore the hole concentration of the topmost CuO2 layers. The contact resistance is significantly reduced, and the intrinsic junction at the surface gets back its superconducting tunneling characteristic. We examine systematically the doping results of surface and bulk junction stacks and discuss the enhancement of their superconducting properties.

Josephson junctions based on amorphous MoGe: prospects for use in superconducting electronics

We have fabricated and characterized all-MoGe Josephson junctions with a very thin Al/AlO x /(Al) barrier, where the amorphous MoGe films exhibit superconducting transition temperatures up to 7 K. Due to the uniformity of the surface morphology of the MoGe films, the junctions demonstrate high uniformity of their tunneling properties. The experimental data on the temperature dependence of the subgap current agree well with theoretical calculations. The results obtained imply that Josephson tunnel junctions based on amorphous superconductors are promising candidates for use in superconducting electronics, especially in applications requiring multiple stacked junctions or the creation of a nonequilibrium quasiparticle distribution.

High-performance voltage controlled multilevel MRAM cell

In the recent past, spin-transfer torque (STT) and spin-orbit torque (SOT) based magnetic random access memories (MRAMs) have been studied for future energy efficient and non-volatile memory applications. Multilevel cell (MLC) design has emerged as one of the promising solutions to enhance the storage density of these MRAMs. However, the conventional MLC design adds a larger magnetic tunnel junction (MTJ) stack that makes it difficult to maintain low switching current and high speed. Moreover, it becomes very difficult to reduce the driver transistor size. This paper describes the application of voltage controlled magnetic anisotropy effect to design energy efficient and fast MLC MRAM cell. So far, this approach has been reported only in single-bit MTJ devices. In the proposed MLC the voltage control is able to reduce both SOT and STT switching currents. The results show that the voltage control in MLC enhances energy efficiency and switching speed by more than 80 times and 3 times, respectively, in comparison to conventional SOT based MLCs. The reduction in switching currents also achieves smaller transistor size and enhances area efficiency by 3.5% as compared to conventional SOT-MLC. The effect of different channel materials on SOT switching current has also been explored. Furthermore, the system level evaluation shows that voltage controlled MLC outperforms STT-MRAM and SOT-MRAM for designing cache memory.

(Invited) Vertically Stacked Junction Devices Fabricated Using Single-Crystal Graphene on SiC Substrate

Two types of vertically stacked graphene junction diodes were fabricated in this study. Samples of single-crystal graphene measuring 100 mm2 were epitaxially grown on SiC substrate using the thermal decomposition method and were bonded using the direct bonding technique. The direct-bonded stacked junction diode exhibited nonlinear current-voltage characteristics and acted as a far-infrared emitter. Fowler-Nordheim tunneling phenomena with a strong nonlinear behavior was observed in the tunneling diode with a thin insulative layer (air gap or structured water). By using simple device-assembly processes, vertically stacked graphene diodes with new functions were successfully fabricated.

Chaotic behavior of a stack of intrinsic Josephson junctions at the transition to branching for overcritical currents

Phase dynamics of a stack of coupled intrinsic Josephson junctions was investigated in the framework of capacitively coupled Josephson junctions with diffusion current model. We study the transition from the current-voltage characteristic specific to Josephson junctions arrays with small dissipation and weak coupling between the junctions to the arrays with strong coupling between the junctions and high dissipation. Low dissipative arrays of Josephson junctions are characterized by the absence of branching for overcritical currents which appears for highly dissipative arrays. Described branching appears due to charging on the superconducting layers and charge traveling waves generation. Arrays of Josephson junctions with intermediate values of coupling and dissipation parameters are characterized by the chaotic behavior, confirmed by positive Lyapunov exponent, and branching on the current voltage characteristic for both sub- and overcritical currents.

Stacked Josephson Junction Arrays for the Pulse-Driven AC Josephson Voltage Standard

We developed a fabrication process to establish large arrays of up to 5-stacked Josephson junctions for the Josephson Arbitrary Waveform Synthesizer (JAWS). SNS-type Josephson junctions with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> barriers are used for this application. By modifying our standard window process, e.g. to add a CMP (chemical mechanical polishing) and an ALD (atomic layer deposition) step, the yield of this process was increased. An output voltage of 1 V RMS could be achieved by using 4 JAWS arrays in series with a total number of 60,000 5-stacked junctions. The investigation of the current-voltage characteristics (IVC) showed an interesting and potentially useful feature. The required pulse amplitude per junction for getting the maximum Shapiro step width reduces for higher junction stacks. We found that the pulse power linearly reduces with the square of the junction number in the array. The origin of this effect might be a strong neighbour-neighbour interaction of the junctions, which are closely arranged within the stacked configuration. This might generate a strong self-synchronization of the junctions.

Improvement of Non-Uniform Temperature Distributions in Intrinsic Josephson Junction Stacks

Terahertz (THz) electromagnetic (EM) wave emitters have been expected for applications because these waves are suitable for non-destructive inspection, telecommunication technologies, and other uses. Strong and coherent THz EM waves are known to radiate from large-sized intrinsic Josephson junction (IJJ) stacks in which the self-heating effect is considerably larger and the temperatures in the mesa are non-uniformly distributed. In this study, we numerically investigate and discuss the temperature and current distributions in large-sized IJJ stacks because these parameters are difficult to analyze through experimental investigation. The temperature and current distributions can be obtained by self-consistently solving the non-linear diffusion equation in an equivalent circuit of the mesa by considering the temperature dependence of several parameters. As shown in the numerical results, self-heating caused the local temperature of the center in the mesa to exceed the critical temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ). To improve the non-uniform temperature and current distributions, we propose an improvement in which an external heat generation system is placed on the edges of the mesa. By applying this measure, the temperature distribution is improved because the edges of the IJJ mesa are heated externally. Furthermore, the local temperatures throughout the mesa were held below Tc, and the current - voltage characteristics were improved using the proposed improvement.

Controllable preparation and photoelectric applications of two-dimensional in-plane and van der Waals heterostructures

&lt;sec&gt;Since the discovery of graphene, two-dimensional (2D) materials have received continuous attention and carried out in-depth exploration and development due to their excellent properties. With the exploration of the preparation of new 2D materials, one began to consider the synergistic effects produced by the in-plane junction and interlayer stacking to compensate for the defects of a single material and obtain some new properties. Matching the lattice structure to achieve specific functionalization, or using van der Waals force to achieve stacking, helps to introduce a new degree of freedom by combining different 2D materials, and open a new window for the research and practical application of 2D materials.&lt;/sec&gt;&lt;sec&gt;From the perspective of atomic manufacturing, in this article we introduce the controllable preparation and optoelectronic applications of 2D planar and van der Waals heterojunction materials. First, we briefly introduce the common 2D materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides and black phosphorus used in the preparation of heterojunctions and related concepts of heterojunctions. Second, we review, in principle, the commonly used characterization methods including scanning probe-based techniques, spectrum-based, electron-based imaging techniques and others. Third, we summarize the preparation methods of planar and vertical heterojunctions. Basically, mechanical transfer method such as wet or dry method can be used to produce various vertical heterostructures of 2D materials, but usually lack the scalability. On the other hand, chemical vapor deposition method provides a scalable route to producing the planar heterostructure and vertical structure of 2D materials. Several strategies have been developed to produce various heterostructures. In addition, the recent development of twist-angle and quasi-crystalline bi-layer graphene is briefly reviewed. Fourth, the properties and applications of 2D van der Waals heterostructures such as field-effect transistor, light emitting diode, solar cell, flexible optoelectronic devices and plasmonic applications are introduced. Finally, the problems in the field are discussed, and the outlook is provided. &lt;/sec&gt;

Double and triple resonance behaviour in large systems of LC-shunted intrinsic Josephson junctions

We study the current-voltage characteristics and phase dynamics of intrinsic Josephson junctions shunted by inductive and capacitive circuit elements. Here we investigate double and triple resonances that leads to charging of the superconducting layers in shunted stacks of junctions. We study numerically the dynamical response of the system to sweeping dc-bias currents, such as would typically be used in experiments. Our simulations reveal a series of features in the current-voltage characteristic of the system. In particular, we show that for larger numbers of junctions (N⪆15) the shunting causes charging to occur even along the outermost branch of the current-voltage characteristic. This charging on the outermost branch is associated with travelling wave solutions that become more stable than fully synchronous solutions in the shunted stacks with larger numbers of junctions. We observe a transformation of the travelling wave solution into a standing wave solution associated with a longitudinal plasma wave.

A novel BIST for monitoring aging/temperature by self-triggered scheme to improve the reliability of STT-MRAM

This paper proposes a novel methodology to design high reliable STT-MRAM, with self-activated built-in-self-test (BIST) against aging/temperature-induced degradation. During sensing operation, tunneling magnetoresistance (TMR) is monitored, and real-time BIST is activated prior to permanent damage in Magnetic tunnel junction (MTJ) stack. To evaluate the feasibility of the test scheme, the proposed technique was involved in MRAM array implementation using 28-nm CMOS and 40-nm MTJ. HSPICE MOS Reliability Analysis (MOSRA) is used to evaluate the amount of electrical stress to the actual device aging degradation. Compared with previous periodical BIST method, the proposed self-triggered BIST saves ~31.1% cumulative power consumption over 12 years. And the proposed technique can improve reliability in the wear-out failure period.

Asymmetric magnetization reversal of the Heusler alloy Co2FeSi as free layer in an CoFeB/MgO/Co2FeSi magnetic tunnel junction

We report the in-plane magnetization reversal behavior of the Co2FeSi Heusler alloy free layer in a bottom-pinned magnetic tunnel junction film with 149% TMR. Using magneto-optic indicator film imaging, we visualize the magnetic domain dynamics of this buried layer integrated within a full magnetic tunnel junction stack. The magnetic domain dynamics reveal anisotropic magnetization reversal within Co2FeSi under applied in-plane magnetic fields. While the reversal behavior under in-plane fields applied perpendicular to the in-plane exchange bias direction indicates smooth, coherent rotation away from the easy axis, asymmetric magnetic reversal behavior is observed along the easy axis. Reversed domains propagate from the interior of the film laterally outward toward the edges as the free layer magnetization switches into parallel alignment with the pinned synthetic antiferromagnetic reference layer (IrMn/CoFe/Ru/CoFeB). On the other hand, domains propagate from the film edges inward for the free layer transition into antiparallel alignment with the reference layer. These results have important implications for Heusler magnetic tunnel junction device performance.

Nonvolatile and Neuromorphic Memory Devices Using Interfacial Traps in Two-Dimensional WSe2/MoTe2 Stack Channel.

Very recently, stacked two-dimensional materials have been studied, focusing on the van der Waals interaction at their stack junction interface. Here, we report field effect transistors (FETs) with stacked transition metal dichalcogenide (TMD) channels, where the heterojunction interface between two TMDs appears useful for nonvolatile or neuromorphic memory FETs. A few nanometer-thin WSe2 and MoTe2 flakes are vertically stacked on the gate dielectric, and bottom p-MoTe2 performs as a channel for hole transport. Interestingly, the WSe2/MoTe2 stack interface functions as a hole trapping site where traps behave in a nonvolatile manner, although trapping/detrapping can be controlled by gate voltage (VGS). Memory retention after high VGS pulse appears longer than 10000 s, and the Program/Erase ratio in a drain current is higher than 200. Moreover, the traps are delicately controllable even with small VGS, which indicates that a neuromorphic memory is also possible with our heterojunction stack FETs. Our stack channel FET demonstrates neuromorphic memory behavior of ∼94% recognition accuracy.

Ar+ ion irradiation of magnetic tunnel junction multilayers: impact on the magnetic and electrical properties

The impact of 400 keV Ar+ irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The ion fluences ranged from to . Below , the anisotropy of the Ta-capped FeCoB free layer was weakly modulated, following a decrease in the saturation magnetization. The tunnel magnetoresistance (TMR), along with the exchange-bias and the interlayer exchange coupling providing a stable magnetic configuration to the reference layer, decreased continuously. Above , a strong decrease in the saturation magnetization was accompanied by a loss of the magnetic coupling and of the TMR. We show there is an ion-fluence window where the modulation of magnetic anisotropy can occur while preserving a large TMR and stable magnetic configuration of the MTJ, and demonstrate that the layers surrounding the free layer play a decisive role in determining the trend of the magnetic anisotropy modulation resulting from the irradiation. Our results provide guidance for the tailoring of MTJ parameters via ion irradiation, which we propose as a potentially suitable technique for setting the magnetic easy-cone state in MTJ for attaining field-free, fast, and non-stochastic magnetization switching.

Etch Characteristics of Nanoscale Patterned Magnetic Tunnel Junction Stacks Using Pulse-Modulated Radio Frequency Source Plasma.

Magnetic tunnel junctions (MTJs) patterned with 70 × 70 nm² square arrays were etched in a CH₄/O₂/Ar gas mixture by pulse-modulated inductively coupled plasma reactive ion etching (ICPRIE). A good etch profile of MTJs with etch slope of approximately 82° was achieved by adjusting the on-off duty ratio of the plasma and pulse frequency. Langmuir probe analysis and optical emission spectroscopy confirmed that the balance between the formation of the passivation layer as an etch byproduct and sputtering effect is responsible for the etch selectivity and etch profile with a high degree of anisotropy. It is concluded that the application of pulse-modulated plasma on ICPRIE can be an effective method to obtain the anisotropic etch profile of nanometer-scale MTJs.

Sub-Band-Gap Photoresponse Caused by Hot Electron Injections in Au Nanorod Decorated van der Waals Semiconductor Monolayers

Hot electrons in metal nanostructures can be exploited in a wide range of optical functions, including photocatalysis, surface-enhanced Raman scattering, photodetectors and photovoltaics. Here, we report the sub-band-gap (Eg) photoresponse in Au-nanorod decorated van der Waals (vdW) semiconductor, MoS2 and WSe2, monolayers (MLs). We found that hot electrons, optically excited in Au nanorod (NR) arrays at sub-Eg radiations, can be injected into vdW ML semiconductors over Schottky barriers, producing substantial photocurrents in n-type MoS2 and p-type WSe2 ML photodetectors, as well as photovoltages in n-MoS2/p-WSe2 ML stack junctions. Moreover, by using spectrally and light-polarization resolved measurements, we showed that these sub-Eg excitations of hot electrons can be modulated by tuning the plasmon resonance to the shape-controlled AuNRs.