Showing papers on "Biasing published in 2023"

Chirality-Induced Spin Selectivity (CISS) Effect: Magnetocurrent–Voltage Characteristics with Coulomb Interactions I

Abstract: One of the manifestations of chirality-induced spin selectivity (CISS) is the appearance of a magnetocurrent. Magnetocurrent is the observation that the charge currents at finite bias in a two terminal device for opposite magnetizations of one of the leads differ. Magnetocurrents can only occur in the presence of interactions of the electrons either with vibrational modes or among themselves through the Coulomb interaction. In experiments on chiral molecules assembled in monolayers, the magnetocurrent seems to be dominantly cubic (odd) in bias voltage while theory finds a dominantly even bias voltage dependence. Thus far, theoretical work has predicted a magnetocurrent which is even bias. Here we analyze the bias voltage dependence of the magnetocurrent numerically and analytically involving the spin–orbit and Coulomb interactions (through the Hartree–Fock and Hubbard One approximations). For both approximations it is found that for strong Coulomb interactions the magnetocurrent is dominantly odd in bias voltage, confirming the symmetry observed in experiment.

A Self‐Powered CNT–Si Photodetector with Tuneable Photocurrent

Abstract: A photodetector with bias‐tuneable current is realized by adding a film of single‐walled carbon nanotubes (CNT), forming a CNT/Si3N4/Si capacitor, to a prefabricated Pt–Ti/Si3N4/Si metal–insulator–semiconductor (MIS) diode. Electrical characterization of the entire device is performed to extract the temperature‐dependent ideality factor and Schottky barrier height in the framework of the thermionic emission theory. The CNT/Si3N4/Si capacitor increases the reverse current of the parallel Pt–Ti/Si3N4/Si MIS diode by adding a Fowler–Nordheim tunneling current at high reverse voltage bias. This feature endows the photodetector with two different photocurrent levels, photoresponsivity up to 370 mA W−1 and external quantum efficiency up to 50% at 950 nm wavelength. The device also shows a different photoresponse when light is focused on the CNT/Si3N4/Si region or around the Pt–Ti/Si3N4/Si structure. The photodetector can also be used as an optoelectronic Boolean logic device, in which the applied voltage bias and the incident light are the two input signals, and the photocurrent is the output. Furthermore, light generates a photocurrent at zero voltage and a photovoltage at zero current, making the device a self‐powered photodetector.

Exchange bias coupling and bipolar resistive switching at room temperature on GaSb/Mn multilayers for resistive memories applications

Abstract: This work present structural, morphological, magnetic, and electrical properties of GaSb/Mn multilayer deposited via DC magnetron sputtering at room temperature and at 423 K. The samples are characterized by forming layers of 3, 6 and 12 periods of the GaSb/Mn structure. Through XRD patterns, it was possible to stablish the formation of GaSb, Mn3Ga, and Mn2Sb2 phases. FTIR measurements present an optical interference associated with periodicity and the homogenous thickness of the layers. HR-SEM shows the multilayer architecture with columnar microstructure in the formation of layers with grain nucleation on the surface. A ferromagnetic-like behavior was observed in the multilayers at room temperature related to the domains and interlayers interaction. Additionally, the hysteresis curves present shifts attributed to the effect of exchange bias coupling. I-V curves show RESET-SET states of the multilayer system with bipolar resistive behavior, which can be modified by external magnetic fields. The resistive switching evidenced corresponds to the conductive mechanism based on the capacitive conductance and the formation of conductive filaments in multilayer structure.

Electrically programmable solid-state metasurfaces via flash localised heating

Abstract: In the last decades, metasurfaces have attracted much attention because of their extraordinary light-scattering properties. However, their inherently static geometry is an obstacle to many applications where dynamic tunability in their optical behaviour is required. Currently, there is a quest to enable dynamic tuning of metasurface properties, particularly with fast tuning rate, large modulation by small electrical signals, solid state and programmable across multiple pixels. Here, we demonstrate electrically tunable metasurfaces driven by thermo-optic effect and flash-heating in silicon. We show a 9-fold change in transmission by <5 V biasing voltage and the modulation rise-time of <625 µs. Our device consists of a silicon hole array metasurface encapsulated by transparent conducting oxide as a localised heater. It allows for video frame rate optical switching over multiple pixels that can be electrically programmed. Some of the advantages of the proposed tuning method compared with other methods are the possibility to apply it for modulation in the visible and near-infrared region, large modulation depth, working at transmission regime, exhibiting low optical loss, low input voltage requirement, and operating with higher than video-rate switching speed. The device is furthermore compatible with modern electronic display technologies and could be ideal for personal electronic devices such as flat displays, virtual reality holography and light detection and ranging, where fast, solid-state and transparent optical switches are required.

Voltage-Modulated van der Waals Interaction in Single-Molecule Junctions.

Abstract: Understanding how molecular geometry affects the electronic properties of single-molecule junctions experimentally has been challenging. Typically, metal-molecule-metal junctions are measured using a break-junction method where electrode separation is mechanically evolving during measurement. Here, to probe the impact of the junction geometry on conductance, we apply a sinusoidal modulation to the molecular junction electrode position. Simultaneously, we probe the nonlinearity of the current-voltage characteristics of each junction through a modulation in the applied bias at a different frequency. In turn, we show that junctions formed with molecules that have different molecule-electrode interfaces exhibit statistically distinguishable Fourier-transformed conductances. In particular, we find a marked bias dependence for the modulation of junctions where transmission is mediated thorough the van der Waals (vdW) interaction. We attribute our findings to voltage-modulated vdW interactions at the single-molecule level.

Anisotropy-assisted bias-free spin Hall nano-oscillator

Abstract: Ferromagnet/nonmagnet (FM/NM) bilayer-based spin Hall nano-oscillators (SHNOs)—a sub-class of spintronic oscillator devices—have promising potential toward realizing low-power physical reservoir computing systems because of their inherent nonlinearity and miniature form factor. However, most of the studies on SHNOs indicate that an external biasing magnetic field is necessary for their operation, creating a bottleneck for their practical implementation in designing small and compact RC hardware. In this report, using micromagnetic simulation, we demonstrate biasing field-free operation of a FM/NM bilayer-based SHNO by exploiting the magnetic anisotropy. Our results reveal that the magnetic anisotropy in the FM layer provides active control over the DC tunability of auto-oscillation frequency and the threshold value of current needed for sustained auto-oscillations. We show that the increase in uniaxial anisotropy substantially modifies the spatial profile of auto-oscillation and eventually leads to the reduction in the threshold current for auto-oscillation, which could be utilized to design low-power computing hardware using SHNO devices.

Bias history impacts the analog resistance change of HfOx-based neuromorphic synapses

Abstract: Resistive random-access memory (RRAM) devices have been widely studied for neuromorphic, in-memory computing. One of the most studied RRAM structures consists of a titanium capping layer and a HfOx adaptive oxide. Although these devices show promise in improving neuromorphic circuits, high variability, non-linearity, and asymmetric resistance changes limit their usefulness. Many studies have improved linearity by changing materials in or around the device, the circuitry, or the analog bias conditions. However, the impact of prior biasing conditions on the observed analog resistance change is not well understood. Experimental results in this study demonstrate that prior higher reset voltages used after forming cause a greater resistance change during subsequent identical analog pulsing. A multiphysics finite element model suggests that this greater analog resistance change is due to a higher concentration of oxygen ions stored in the titanium capping layer with increasing magnitude of the reset voltage. This work suggests that local ion concentration variations in the titanium capping layer of just tens of atoms cause significant resistance variation during analog operation.

10 Gb/s burst-mode driver circuit with on-chip bias switch for in-Vehicle optical networks

Abstract: The recent progress of in-vehicle communication networks has been highlighted by the intensive investigation of optical packet communication systems using a modulation and detection device. This paper proposes a burst-mode driver circuit with an on-chip bias switch to achieve a stable and quick response in bias switching operation. The proposed circuit comprises a driver core and two types of bias circuits. Moreover, the proposed circuit changes output bias voltages using a MOS transistor as a switch. To verify the operating principle, we design the proposed circuit using a high-voltage tolerant 65-nm CMOS technology and obtain the post-layout simulations and measurement results. As a result, we achieve quick bias switching operation with 90%faster response time than the conventional one.

30 GHz plasmonic slot MoTe2 photodetector integrated with silicon photonic circuits at telecom wavelength

Abstract: An experimental demonstration of a waveguide-integrated plasmonic slot photodetector based on MoTe2 with 30 GHz 3 dB roll-off bandwidth at telecom wavelength. To overcome the intrinsic low carrier mobility and weak light-matter interaction when applying two-dimensional material for optoelectronic devices, here we numerically and experimentally show a novel concept of the plasmonic slot structure to eliminate the transit time constant (τ=L^2/μV) from the device which is related to the material mobility. The nanometer-wide plasmonic slot offers a ‘squeezed’ mode that allows the 2d material can effectively absorb the light via band-to-band transitions with an overlap factor (Г) increasing more than 3 times compared to the bare waveguide structure. The ultra-narrow slot width reduces the carriers’ drift route to tens of nanometers and is only limited by the RC time constant. The MoTe₂ serves as the semiconducting light-absorbing material with its layer-dependent bandgap that encompasses the standard O-band wavelength for communications (1,260 nm -1,360 nm). The device's static performance under 1 V bias voltage also shows a high photoresponsivity of 0.8 A/W at 1310 nm with a low dark current of 90 nA. Furthermore, we study the slot width-dependent frequency response and static response to validate our concept, which shows that both the frequency and static response are inversely proportional to the slot width. The concept is not restricted to materials and the platform. This may pave the way for developing high-performance optoelectronic devices with materials that have unique optical and electric properties but suffer from low mobilities.^1–13 It has been shown that introducing plasmonic structures, cavities, resonators, or nanoparticles into waveguide-loaded systems can improve light-matter interactions.^14–17 Still, one of their biggest challenges today is the frequency response of TMDCbased devices for telecommunications or information processing.^18–30 For photodetectors using TMDCs as the lightabsorbing medium, this is particularly crucial.

A Liquid Crystal Tunable Metamaterial Unit Cell for Dynamic Metasurface Antennas

Abstract: This communication presents a liquid crystal (LC)-based electronically tunable metamaterial unit cell that can serve as a building block for dynamic metasurface antennas (DMAs). The metamaterial unit cell was designed based on a complementary electric-inductive-capacitive (cELC) resonator, whose structure was modified into double metal layers for ease of loading the LC mixture. The equivalent capacitance, and thus the transmission coefficient of the metamaterial unit cell, can be tuned by external biasing voltages. The metamaterial unit cell couples to the transverse magnetic (TM) field, making it easy to be embedded into waveguide-fed structures such as microstrip lines or parallel-plate waveguides. In addition, it is individually addressable, which meets the design requirement of DMAs. Experimental results show that the transmission coefficient of the metamaterial unit cell can be continuously tuned from −2.7 to −6.5 dB at 29 GHz with a biasing voltage swinging from 2.5 to 10 Vrms. The response times are around 150 ms for both switch-ON and swtich-OFF cases.

NAM+: Towards Scalable End-to-End Contextual Biasing for Adaptive ASR

Abstract: Attention-based biasing techniques for end-to-end ASR systems are able to achieve large accuracy gains without requiring the inference algorithm adjustments and parameter tuning common to fusion approaches. However, it is challenging to simultaneously scale up attention-based biasing to realistic numbers of biased phrases; maintain in-domain WER gains, while minimizing out-of-domain losses; and run in real time. We present NAM+, an attention-based biasing approach which achieves a 16X inference speedup per acoustic frame over prior work when run with 3,000 biasing entities, as measured on a typical mobile CPU. NAM+ achieves these run-time gains through a combination of Two-Pass Hierarchical Attention and Dilated Context Update. Compared to the adapted baseline, NAM+ further decreases the in-domain WER by up to 12.6% relative, while incurring an out-of-domain WER regression of 20% relative. Compared to the non-adapted baseline, the out-of-domain WER regression is 7.1 % relative.

Perovskite Solar Cells in the Shadow: Understanding the Mechanism of Reverse‐Bias Behavior toward Suppressed Reverse‐Bias Breakdown and Reverse‐Bias Induced Degradation

Abstract: In recent years, the power conversion efficiencies of halide perovskite solar cells (PSCs) have reached 25.7%. Further commercialization puts forward higher requirements for the stability of PSCs under different stresses. Current research on the stability of PSCs predominantly focuses on investigating the effects of temperature, humidity, oxygen, ultraviolet light, and electrical bias stress, while stability under reverse bias has been poorly studied among the many factors. When large‐area application of PSCs panels occurs, the shading effect and local hot spots of PSCs modules in real scenarios all indicate that the research on reverse bias stability of PSCs is urgently needed. Within this review, a bird's‐eye view of the recent advances on the reverse bias stability issue and reverse bias behavior of PSCs is obtained, and the related mechanisms behind that have been proposed so far are reviewed and discussed. Furthermore, future directions are provided for further improving the reverse bias stability of PSCs via increasing the reverse breakdown voltage (VRB). This review aims to promote the establishment of well‐established reverse bias degradation and reverse breakdown mechanisms of PSCs as well as to establish standardized test procedures for reverse bias stability issues on the road to commercialization of PSCs.

60-GHz Single Flux Quantum Pulse Transfer Circuit for Serial Biasing

Abstract: Serial Biasing (SB) is a technique to reduce the total DC bias current of Rapid Single Flux Quantum (RSFQ) circuits by partitioning a design into several islands with isolated grounds and sequential biasing. In this paper we focus on the design of a driver-receiver pair (DRP) to transfer pulses between islands that are galvanically isolated for pulse streams. We discuss both DRP itself and the structure for its testing, that comprises several DRPs connected in series, on-chip pseudo random binary sequence (PRBS) generator for circuit stimulation, and HF output interface. We use the layout of DRPs’ chain as an example to illustrate the advantage of the grapevine (GV) approach, introduced earlier, to manage the bias current flowing into and out of an island. The GV current management technique is analyzed by both electromagnetic simulations and measurement, compared, and contrasted with the so-called ‘straightforward’ (SF) approach. The maximum operational frequency for SF test structure was 10 GHz with zero margins for the SB current. Measurements of the GV structure at 10 GHz demonstrated BER of 10 ⁻¹² with ±5.8% margins for SB current. We observed the correct operation of the 5-island DRP chain up to 60 GHz using the grapevine approach for SB current management. All chips were fabricated at MIT Lincoln Laboratory using SFQ5ee fab node.

Investigating spatial macroscopic metastability of perovskite solar cells with voltage dependent photoluminescence imaging

Abstract: Metastability is a characteristic feature of perovskite solar cell (PSC) devices that affects power rating measurements and general electrical behaviour. In this work the metastability of different types of PSC devices is investigated through current–voltage (I–V) testing and voltage dependent photoluminescence (PL-V) imaging. We show that advanced I–V parameter acquisition methods need to be applied for accurate PSC performance evaluation, and that misleading results can be obtained when using simple fast I–V curves, which can lead to incorrect estimation of cell efficiency. The method, as applied in this work, can also distinguish between metastability and degradation, which is a crucial step towards reporting stabilised efficiencies of PSC devices. PL-V is then used to investigate temporal and spatial PL response at different voltage steps. In addition to the impact on current response, metastability effects are clearly observed in the spatial PL response of different types of PSCs. The results imply that a high density of local defects and non-uniformities leads to increased lateral metastability visible in PL-V measurements, which is directly linked to electrical metastability. This work indicates that existing quantitative PL imaging methods and point-based PL measurements of PSC devices may need to be revisited, as assumptions such as the absence of lateral currents or uniform voltage bias across a cell area may not be valid.

Wide-Angle Scanning Graphene-Biased Terahertz Coding Meta-Surface

Abstract: We demonstrate a reconfigurable beam steerable meta-surface through a graphene-biased slot-array over a grounded quartz substrate. More specifically, the graphene meta-elements can be dynamically tuned to program the radiations by applying adequate DC bias voltages to different gating pads, capable of turning on or off the releasing slots of the guided fields as adjustable switches. In particular, such a graphene-biased terahertz meta-surface will achieve a wide-angle steerable beam at a fixed frequency and the scanning directions can further be modulated when varying the frequency at a certain state of the graphene, thus should pave the way for building up more advanced reconfigurable transceivers and sensors in terahertz wireless electronics.

Drain-bias dependence of low-frequency Y22 signals for Fe-related GaN traps in GaN HEMTs with different Fe doping concentrations

Abstract: The bias dependence of trap properties for GaN HEMTs with Fe-doped GaN layers was investigated using low-frequency Y-parameters measurement in the two-port network. This measurement technique can be estimated quasi-equilibrium trap properties and location by changing the combination between gate/drain current response and gate/drain input voltage with small AC signal overlapped fixed DC bias. We focused on Y22 which can be detected only in GaN traps. Activation energies for the GaN traps were estimated bias-by-bias for wide drain voltage (VDS) ranging from 2 V below the knee voltage to 15 V for the saturation region with high current at the gate voltage of 0 V (on-state condition). Fe-related GaN traps with low-frequency Y22 characteristics were identified experimentally using GaN HEMTs including GaN layers with different Fe doping concentrations. The Fe-related GaN trap signals in the imaginary part of Y22 (Im(Y22)) appeared at a peak near 200 Hz at a low VDS of 3 V under on-state conditions and moved to a higher frequency with an increase of VDS. The amplitudes of the trap peaks enhanced around VDS of 3 V. The maximum value in the amplitude of the trap peaks was large for the high-concentration Fe doping. On the other hand, the peak frequency depending on VDS had a similar trend for both the low- and high-concentration Fe doping. The activation energy for the Fe-related trap decreased for VDS up to 7 V and saturated for both Fe doping concentrations. Device simulation was performed to analyze the location in GaN HEMTs for GaN trap response and bias dependence of the activation energy. To simulate the characteristics of Im(Y22), drain current (ID) responses against AC (sine wave) input signal of VDS were calculated at the frequency corresponding to the trap peak. Because ID led VDS in phase, the trap effect is considered to change the trapped charge. From two-dimensional plots for the difference in ionized trap density, trap response regions were located under the gate edge at the drain side. Because the imaginary part of the admittance vector between ID and VDS corresponded with Im(Y22), the Fe-related traps under the gate of the drain side changed the charge condition according to the AC signal and generated the peak in Im(Y22) depending on the frequency. Furthermore, the saturation of the activation energy in the bias-dependence characteristics occurred due to decreasing the thermal conductivity at high temperatures. The temperature in GaN HEMTs at high VDS became high because of the self-heating effect in addition to the high ambient temperature. The internal temperature increase induced the trap peak frequency increase, and the activation energies were saturated.

Large-Area Niobium Titanium Nitride Superconducting Microstrip Single-Photon Detector Fabricated Using a Photolithography Process

Abstract: In this paper, we present a large-area niobium titanium nitride (NbTiN) superconducting microstrip single-photon detector (SMSPD) fabricated using a photolithography process. We fabricated a 1-μm-wide NbTiN-SMSPD covering 420 × 420 μm ² active area with a filling factor of 20% using an i-line stepper. The detector is cooled down to approximately 2.2 K, and its performance is evaluated at the wavelengths from visible to near-infrared. The detector operates stably without a shunt resistor and shows single-photon sensitivity for photons with a wavelength of 1550 nm. The bias current dependence of the detection count rate is measured at the wavelengths from 406 to 1550 nm. The saturation of the detection count rate with increasing the bias current was successfully observed at wavelengths below 850 nm, suggesting an internal detection efficiency approaching 100%. Furthermore, we evaluated the timing jitter performance using the femtosecond pulse laser with a wavelength of 1550 nm. Consequently, we achieved a system timing jitter of 85 ps for a light spot diameter of 100 µm.

Visible Near-Infrared Photodetection Based on Ta2NiSe5/WSe2 van der Waals Heterostructures

Abstract: The increasing interest in two-dimensional materials with unique crystal structures and novel band characteristics has provided numerous new strategies and paradigms in the field of photodetection. However, as the demand for wide-spectrum detection increases, the size of integrated systems and the limitations of mission modules pose significant challenges to existing devices. In this paper, we present a van der Waals heterostructure photodetector based on Ta2NiSe5/WSe2, leveraging the inherent characteristics of heterostructures. Our results demonstrate that this detector exhibits excellent broad-spectrum detection ability from the visible to the infrared bands at room temperature, achieving an extremely high on/off ratio, without the need for an external bias voltage. Furthermore, compared to a pure material detector, it exhibits a fast response and low dark currents (~3.6 pA), with rise and fall times of 278 μs and 283 μs for the response rate, respectively. Our findings provide a promising method for wide-spectrum detection and enrich the diversity of room-temperature photoelectric detection.

Minimising Biasing Word Errors for Contextual ASR With the Tree-Constrained Pointer Generator

Abstract: Contextual knowledge is essential for reducing speech recognition errors on high-valued long-tail words. This paper proposes a novel tree-constrained pointer generator (TCPGen) component that enables end-to-end ASR models to bias towards a list of long-tail words obtained using external contextual information. With only a small overhead in memory use and computation cost, TCPGen can structure thousands of biasing words efficiently into a symbolic prefix-tree, and creates a neural shortcut between the tree and the final ASR output to facilitate the recognition of the biasing words. To enhance TCPGen, we further propose a novel minimum biasing word error (MBWE) loss that directly optimises biasing word errors during training, along with a biasing-word-driven language model discounting (BLMD) method during the test. All contextual ASR systems were evaluated on the public Librispeech audiobook corpus and the data from the dialogue state tracking challenges (DSTC) with the biasing lists extracted from the dialogue-system ontology. Consistent word error rate (WER) reductions were achieved with TCPGen, which were particularly significant on the biasing words with around 40% relative reductions in the recognition error rates. MBWE and BLMD further improved the effectiveness of TCPGen, and achieved more significant WER reductions on the biasing words. TCPGen also achieved zero-shot learning of words not in the audio training set with large WER reductions on the out-of-vocabulary words in the biasing list.

Wear and Corrosion Resistance of CrYN Coating in Artificial Seawater

Abstract: In this study, CrYN coatings were prepared using multi-arc ion plating at various substrate bias voltages (−50 V, −100 V, −150 V, and −200 V). X-ray diffractometry and scanning electron microscopy were used to characterize the composition and microstructure of the coatings. An electrochemical workstation and a ball-on-disk tribometer were used to investigate their corrosion and friction behavior. The results show that grain refinement can be achieved through the addition of yttrium (Y) and that the surfaces of coatings prepared under different bias voltages have varying smoothness and compactness. It was shown that surfaces prepared under −100 V bias voltages were relatively smooth and dense in structure, corresponding to a Y content of 2.83 at.%; CrYN coatings at −100 V were shown to have the highest corrosion potential and a low self-corrosion current, equating to superior corrosion resistance. Additionally, the friction coefficients of deposited CrYN coatings under bias voltages of −100 V were less than 0.2. Therefore, the coatings under bias voltages of −100 V had the minimum wear rate due to its structure, corrosion resistance, and friction.