Ultra-low Switching Research Articles

SMART: A Secure Magnetoelectric AntifeRromagnet-Based Tamper-Proof Non-Volatile Memory

The storage industry is moving toward emerging non-volatile memories (NVMs), including the spin-transfer torque magnetoresistive random-access memory (STT-MRAM) and the phase-change memory (PCM), owing to their high density and low-power operation. In this paper, we demonstrate, for the first time, circuit models and performance benchmarking for the domain wall (DW) reversal-based magnetoelectric-antiferromagnetic random access memory (ME-AFMRAM) at cell-level and at array-level. We also provide perspectives for coherent rotation-based memory switching with topological insulator-driven anomalous Hall read-out. In the coherent rotation regime, the ultra-low power magnetoelectric switching coupled with the terahertz-range antiferromagnetic dynamics result in substantially lower energy-per-bit and latency metrics for the ME-AFMRAM compared to other NVMs including STTMRAM and PCM. After characterizing the novel ME-AFMRAM, we leverage its unique properties to build a dense, on-chip, secure NVM platform, called SMART: A Secure Magnetoelectric Antiferromagnet- Based Tamper-Proof Non-Volatile Memory. New NVM technologies open up challenges and opportunities from a data-security perspective. For example, their sensitivity to magnetic fields and temperature fluctuations, and their data remanence after power-down make NVMs vulnerable to data theft and tampering attacks. The proposed SMART memory is not only resilient against data confidentiality attacks seeking to leak sensitive information but also ensures data integrity and prevents Denial-of-Service (DoS) attacks on the memory. It is impervious to particular power side-channel (PSC) attacks which exploit asymmetric read/write signatures for 0 and 1 logic levels, and photonic side-channel attacks which monitor photo-emission signatures from the chip backside.

Room-temperature developed flexible biomemristor with ultralow switching voltage for array learning.

As one of the emerging neuromorphic computing devices, memristors may break through the limitation of traditional computers with a von Neumann architecture. However, the development of flexible memristors is limited by the high-temperature fabrication process, large operating voltage and non-uniform distribution of resistance. The room-temperature process has attracted great attention due to its advantages of low thermal dissipation, low cost and excellent compatibility with flexible electronics. Here, we proposed a fully physical vapour deposition (PVD) process for fabricating a memristor without additional heat treatment. The device showed excellent resistive switching characteristics with ultralow set/reset voltages (0.48 V/-0.39 V), uniform distribution (10%/15%), stable retention characteristic, multilevel storage behavior and reliable flexibility (radius of 10 mm). With continuously modulated conductance, typical synaptic plasticities were simulated by our flexible biomemristor, including excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), long-term potentiation/depression (LTP/LTD) and learning-forgetting curve. Furthermore, the array learning behavior like that of the human brain was simulated with these trainable biomemristors. This study paves a new way for developing low-cost, wearable, neuromorphic computing electronics at room temperature and expands the applications of artificial synapse arrays.

Low-Current-Density Magnetic Tunnel Junctions for STT-RAM Application Using MgO$_{{x}}$ N$_{\text{1}-{x}}\,\,({x}=\text{0.57}$ ) Tunnel Barrier

High switching speed, endurance, and low-current-based perpendicular magnetic tunnel junction (p-MTJ) memory is attracting wide interest as a key promising candidate for next-generation spintronic memory technology. p-MTJ-based spin-transfer torque RAM (STT-RAM) has been extensively investigated, and despite the promise, there is concern about the high switching current density and low stability with regard to scaling. In this work, the current controllability of p-MTJ in iron (Fe)-enriched Co20Fe60B20 with a newly designed MgO x N1– x tunnel layer is systematically investigated, with the expectation that the introduction of N minimizes the oxidation of Fe to improve the performance of the device. A facile, plasma-based oxynitridation (MgO x = 0.57N1– x =0.43) of MgO through RF-sputter deposition serves as a reliable procedure to establish a tunnel barrier for an MTJ structure fabricated with ~300-nm diameter and pinned with synthetic antiferromagnetic (SAF) [Co/Pt] ${_{n}}$ multilayer stack. Current-controlled tunneling magnetoresistance (TMR) up to ~65% was observed at room temperature (RT) with ultralow switching current density ( ${J}_{c}$ ) of 136 ± 17 kA/cm2. TMR along with tunnel conductance ( ${g}$ ( ${V}$ )) was measured to be highly stable in the read-bias regime (−200 to +200 mV) for MgO x N1– x as compared to the reported MgO barrier. The analogous MgO x N1– x -based MTJ structures were modeled using the nonequilibrium Green’s function (NEGF) with appropriate tunnel barrier parameters and incorporating modulated barrier height as compared with the MgO barrier. The current–voltage characteristics of the modeled device showed close agreement with experimental data indicating high spin current. Based on the field-induced magnetization analysis, the macro-magnetic reversal analysis suggests the free-layer switching duration of ~3 ns. These observations show the strong candidature of MgO x N1– x ( ${x} = {0.57}$ ) MTJs for STT-RAM device application.

Ultrafast low-energy all-optical switching

The realization of ultrafast integrated opto-optical switches with ultra-low switching energies remains an ongoing challenge. Broadband, silicon-compatible devices relying on gap plasmons and saturable absorption in graphene could pave the way forward.

Facile Synthesis of Ti3C2Tx-Poly(vinylpyrrolidone) Nanocomposites for Nonvolatile Memory Devices with Low Switching Voltage.

MXenes, an emerging class of two-dimensional (2D) transition-metal carbide materials, have received increasing attention for their interesting physiochemical properties. For not only MXenes but also other 2D materials, delamination is a requisite step for the exploitation of their unique properties. In this work, a facile method for exfoliating Ti3C2Tx MXene to nanosheets of small size with the aid of poly(vinylpyrrolidone) (PVP) is designed, which has never been reported to our knowledge. Since both hydrophobic methylene groups and hydrophilic amide groups are provided with PVP, this method is applicable in a wide range of solvents, such as ethanol, water, and chloroform. Considering the charge detrapping and trapping behavior of 2D transition-metal materials in PVP dielectric, a memory device with the configuration of reduced graphene oxide (rGO)/Ti3C2Tx-PVP/Au is directly fabricated with these well-dispersed Ti3C2Tx-PVP composites by the solution process technique. Interestingly, the resultant device exhibits a typical bistable electrical switching, ultralow switching voltage (∼0.9 V), and a nonvolatile rewritable memory effect with the function of flash. This work might pave the way of using MXenes for future data storage, which is an indispensable field nowadays.

Switching Transients in Memristors

Transient performance of nanowire memristors realized using different material systems is discussed. The approach is validated by comparing simulated results with experimental data obtained for a ZnO nanowire memristor. The ZnO nanowire memristors demonstrate bipolar resistive switching with an ROFF/RON ratio of 684 that is 3 times higher than the previous best report. The transient switching model, derived from the physical mechanisms for memristor switching, show a material dependent switching delay greatly influenced by the mobility of the oxygen vacancies. Measured switching delay of 372 µs for ZnO memristor show excellent agreement with the simulated data. The switching delays for other material systems, namely – TiOx, TaOx, HfOx, and ZrOx are calculated to be 2.5 s, 5.5 ns, 11.8 ps, and 7.15 ps, respectively, for identical device geometry (length 2 µm and diameter 300 nm). Upon scaling devices down to 50 nm, the delay is observed to decrease by 3-4 orders of magnitude. ZrOx based memristors showed the shortest switching delay owing to a mobility as high as 370 cm2/V-s. Experimental data for ZnO memristors suggest rise and fall times shorter than 7 µs and 10 ns, respectively. Ultralow switching power of 261 µW and 155 µW are achieved for SET and RESET switching, respectively. Measured switching energy less than 83 nJ and slew rates greater than 0.02 V/µs are attained.

An Ultra-Low Power Consumption High-Linearity Switching Scheme for SAR ADC

An ultra-low power consumption high-linearity switching scheme for successive approximation register (SAR) analog-to-digital converter (ADC) is presented with a mixed switching method. Based on the combination of C-2C dummy capacitors, the charge sharing technique and monotonic switching method, the proposed switching method achieves high-energy saving and high linearity. Compared with the conventional SAR ADC, the proposed method consumes no reset energy and achieves 98.9% less switching energy and 87.2% reduction in capacitor area. Moreover, the proposed scheme obtains good performance in linearity. Furthermore, the common-mode voltage variation of the proposed scheme is smaller than other published schemes, which is important for decreasing input-dependent offset of the comparator.

A 99.8% Energy-Reduced Two-Stage Mixed Switching Scheme for SAR ADC Without Reset Energy

An ultra-low power consumption two-stage mixed switching scheme for successive approximation register (SAR) analog-to-digital converter (ADC) is presented. Using two simple switches, the novel switching scheme divides the capacitor arrays into two sub-arrays: stage-one and stage-two arrays. The two sub-arrays convert high and low bits cycles, respectively. Once the high bits conversion cycles are completed, the corresponding sub-arrays are split off from the capacitor arrays. This decreases the number of capacitors used in the rest conversion procedure. Thus, the proposed method improves the energy efficiency of SAR ADC. Thanks to C–2C dummy capacitors and two-stage capacitor arrays, the novel architecture achieves 86% reduction in capacitor area than conventional SAR ADC. Furthermore, based on the charge sharing technique and monotonic switching method, the proposed switching scheme does not consume reset energy and achieves 99.8% less switching energy than the conventional switching method. In addition, the proposed scheme is less sensitive to capacitor mismatch because of its great performance in linearity.

Single-Plasmon Thermo-Optical Switching in Graphene

While plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.

Polymer Memory Devices: Control of Resistive Switching Voltage by Nanoparticle‐Decorated Wrinkle Interface (Adv. Electron. Mater. 5/2019)

In article number 1800503, Juqing Liu, Wei Huang, and coworkers propose an effective strategy combining wrinkle surface and nanoparticle decoration to control the resistive switching voltage in polymer memory devices. By manipulating the nanoparticle density on the wrinkled surface of a reduced graphene oxide electrode, the optimized device exhibits high performance nonvolatile memory with an ultra-low switching voltage, high ON/OFF ratio, long retention time, and excellent reproducibility.

Temperature sensitivity and short-term memory in electroforming-free low power carbon memristors

We report temperature dependent electrical characteristics of two-terminal Ag/a-COx/ta-C/Pt memristors. In these asymmetric devices, defects at the Ag/a-COx interface are passivated by oxygen. This alleviates Fermi level pinning and hence increases the height of the Schottky barrier formed at the interface. Electric-field-induced detrapping of electrons from sp2-related defects in the ta-C causes the observed resistive switching. This occurs entirely in the insulating regime, i.e., with conductance ≪ 2e2/h, enabling ultralow power resistive switching (∼6 nW). Nonlinear temperature dependent ON/OFF ratios and short-term memory characteristics (governed by thermal detrapping kinetics) suggest suitability for temporal neuromorphic computing and sensing applications.

Performance‐Enhancing Selector via Symmetrical Multilayer Design

AbstractTwo‐terminal selectors with high nonlinearity, based on bidirectional threshold switching (TS) behaviors, are considered as a crucial element of crossbar integration for emerging nonvolatile memory and neuromorphic network. Although great efforts have been made to obtain various selectors, existing selectors cannot fully satisfy the rigorous standard of assorted memristive elements and it is in great demand to enhance the performance. Here, a new type of Ag/TaOx/TaOy/TaOx/Ag (x < y) selector based on homogeneous trilayered oxides is developed to attain the required parameters including bidirectional TS operation, a large selectivity of ≈1010, a high compliance current up to 1 mA, and ultralow switching voltages under 0.2 V. Tunable operation voltages can be realized by modulating the thickness of inserted TaOy. All‐TaOx‐based integrated 1S1R (one selector and one memristor) cells, prepared completely by magnetron sputtering and no need of a middle electrode, exhibit a nonlinear feature, which is quite characteristic for the crossbar devices, avoiding undesired crosstalk current issues. The tantalum‐oxide‐based homojunctions offer high insulation, low ion mobility, and rich interfaces, which is responsible for the modulation of Ag conductive filaments and corresponding high‐performance cation‐based selector. These findings might advance practical implementation of two‐terminal selectors in emerging memories, especially resistive random access memories.

Re-amorphization of GeSbTe alloys not through a melt-quenching process

Ge–Sb–Te (GST) alloys are one of the most successful chalcogenides used in rewritable optical discs and electric non-volatile memories. In these materials the structural phase transition between amorphous and crystalline states is used for recording. Here, we report that a melt-quenching method is not necessarily the only way to form a high-resistance phase. We found that amorphous-like grains with high resistance are formed in the GST crystal film by holding it at above the crystallization temperature under an external magnetic field, followed by cooling accompanied with pulse current injection. The treatment may open new route for ultra-low current switching phase change memory.

Voltage control of unidirectional anisotropy in ferromagnet-multiferroic system.

Demonstration of ultralow energy switching mechanisms is imperative for continued improvements in computing devices. Ferroelectric (FE) and multiferroic (MF) order and their manipulation promise an ideal combination of state variables to reach attojoule range for logic and memory (i.e., ~30× lower switching energy than nanoelectronics). In BiFeO3 (BFO), the coupling between the antiferromagnetic (AFM) and FE order is robust at room temperature, scalable in voltage, stabilized by the FE order, and can be integrated into a fabrication process for a beyond-CMOS (complementary metal-oxide semiconductor) era. The presence of the AFM order and a canted magnetic moment in this system causes exchange interaction with a ferromagnet such as Co0.9Fe0.1 or La0.7Sr0.3MnO3. Previous research has shown that exchange coupling (uniaxial anisotropy) can be controlled with an electric field. However, voltage modulation of unidirectional anisotropy, which is preferred for logic and memory technologies, has not yet been demonstrated. Here, we present evidence for electric field control of exchange bias of laterally scaled spin valves that is exchange coupled to BFO at room temperature. We show that the exchange bias in this bilayer is robust, electrically controlled, and reversible. We anticipate that magnetoelectricity at these scaled dimensions provides a powerful pathway for computing beyond modern nanoelectronics by enabling a new class of nonvolatile, ultralow energy computing elements.

Configurable ultra-low operating voltage resistive switching between bipolar and threshold behaviors for Ag/TaOx/Pt structures

An ultra-low operating voltage bipolar resistive switching is observed in Ag/TaOx/Pt devices. They show a typical bipolar resistive switching with both low operating voltages and high cycling endurance when the compliance current (ICC) is 0.3 mA. Moreover, the operating voltage is considerably influenced by the grain size of the film. The VForming increases dramatically when the grain size exceeds a critical value. Meanwhile, the bipolar resistive switching and threshold switching in Ag/TaOx/Pt devices can be converted to each other by changing the magnitude of the ICC. Finally, a model based on the migration of Ag+ is proposed to explain the ultra-low operating voltage and the critical effect of grain size. The model is proved by simulation. These findings may lead to ultra-low power memories and contribute to a further understanding of the resistive switching effect.

Shielded Gate SiC Trench Power MOSFET with Ultra-Low Switching Loss

A shielded gate trench silicon carbide (SiC) metal oxide semiconductor field effect transistor (SG-TMOS) is proposed and investigated by simulation in this paper. The impact of shielded gate design in SG-TMOS on Miller charge (Qgd) as well as conduction resistance (Ron) are comprehensively discussed, showing a tradeoff between Qgd and Ron. Furthermore, the Huang’s Figure of Merit (HFOM) of the SG-TMOS with reasonable design of SG is reduced more than 20%, compared with the conventional trench MOSFET (C-TMOS). Therefore, the proposed SG-TMOS is a competitive next generation device structure for ultra-high switching speed SiC MOSFET.

Scalability assessment of Group-IV mono-chalcogenide based tunnel FET

Selection of appropriate channel material is the key to design high performance tunnel field effect transistor (TFET), which promises to outperform the conventional metal oxide semiconductor field effect transistor (MOSFET) in ultra-low energy switching applications. Recently discovered atomically thin GeSe, a group IV mono-chalcogenide, can be a potential candidate owing to its direct electronic band gap and low carrier effective mass. In this work we employ ballistic quantum transport model to assess the intrinsic performance limit of monolayer GeSe-TFET. We first study the electronic band structure by regular and hybrid density functional theory and develop two band k · p hamiltonian for the material. We find that the complex band wraps itself within the conduction band and valence band edges and thus signifies efficient band to band tunneling mechanism. We then use the k · p hamiltonian to calculate self-consistent solution of the transport equations within the non-equilibrium Green’s function formalism and the Poisson’s equation based electrostatic potential. Keeping the OFF-current fixed at 10 pA/μm we investigate different static and dynamic performance metrics (ON current, energy and delay) under three different constant-field scaling rules: 40, 30 and 20 nm/V. Our study shows that monolayer GeSe-TFET is scalable till 8 nm while preserving ON/OFF current ratio higher than 104.

Steep Sub-Boltzmann Switching in AlGaN/GaN Phase-FETs With ALD VO2

We report the first demonstration of ultralow leakage AlGaN/GaN MOS-HEMTs on silicon substrates loaded at the source with atomic layer deposition (ALD)-grown VO2 metal–insulator transition (MIT) material resistors. The resulting GaN phase-transition FET (phase-FET) shows steep sub-Boltzmann switching in its transfer characteristics in both sweep directions with slopes of ~9.9 and ~28.2 mV/decade at 60 °C. The control MOS-HEMTs and the phase-FETs show more than 12-order on/off ratio with ultralow off-state leakage. Because of the MIT, the drain current and the transconductance also show unconventional behavior. This first demonstration of ultralow leakage steep switching in GaN phase-FETs using integration-friendly ALD VO2 opens the door to introducing new functionalities in nitride low-power digital devices, microwave circuits, photonic devices, and power electronics in the GaN-on-silicon platform.

Control of magnetic vortex polarity by the phase difference between voltage signals

Using micromagnetic simulations, we investigate the voltage control of magnetic vortex polarity based on a designed multiferroic heterostructure that contains two separate piezoelectric films beneath a magnetostrictive nanodisk. The results show that controllable switching of vortex polarity can be achieved by proper modulation of the phase difference between two sinusoidal voltage pulses V1 and V2, which are applied to the two separate piezoelectric films, respectively. The frequencies of V1 and V2 are set at the gyrotropic eigenfrequency fG of the nanodisk, and the vortex polarity switching is completed via the nucleation-annihilation process of the vortex-antivortex pair. Our findings provide an additional effective means for ultralow power switching of the magnetic vortex, which lays the foundation for voltage-controlled vortex random access memory.

Tutorial: Integrated-photonic switching structures

Recent developments in waveguided 2 × 2 and N × M photonic switches are reviewed, including both broadband and narrowband resonant devices for the Si, InP, and AlN platforms. Practical actuation of switches by electro-optical and thermo-optical techniques is discussed. Present datacom-and-computing applications are reviewed, and potential applications are proposed for chip-scale photonic and optoelectronic integrated switching networks. Potential is found in the reconfigurable, programmable “mesh” switches that enable a promising group of applications in new areas beyond those in data centers and cloud servers. Many important matrix switches use gated semiconductor optical amplifiers. The family of broadband, directional-coupler 2 × 2 switches featuring two or three side-coupled waveguides deserves future experimentation, including devices that employ phase-change materials. The newer 2 × 2 resonant switches include standing-wave resonators, different from the micro-ring traveling-wave resonators. The resonant devices comprise nanobeam interferometers, complex-Bragg interferometers, and asymmetric contra-directional couplers. Although the fast, resonant devices offer ultralow switching energy, ∼1 fJ/bit, they have limitations. They require several trade-offs when deployed, but they do have practical application.