Negative Differential Conductance Region Research Articles

Structural Inhomogeneities and Nonlinear Phenomena in Charge Transfer under Cold Field Emission in Individual Closed Carbon Nanotubes

The structure and phenomena arising from charge transfer in cold field emission mode in a single closed carbon nanotube (CNT) under cold field emission conditions are studied. Inhomogeneities of the structure of CNT in the form of two types of superlattices are found by studying microphotographs obtained by AFM, SEM, and TEM. The features of charge transfer in a quasi-one-dimensional carbon nanotube emitter with a small gap between the anode and cathode are studied under conditions of low-voltage field emission. It is established that the I-V characteristics reveal voltage thresholds and resonant peaks, which are associated with the opening of conduction channels in the region of van Hove singularities. In the region of peaks in the I-V characteristics, the emission current exceeds the one calculated using the Fowler–Nordheim (F-N) function by one to three orders of magnitude. The I-V characteristic is not that the curve straightens in F-N coordinates. It is found that the peaks in the I-V characteristics have distinct regions of negative differential conductivity.

Optically-triggered deterministic spiking regimes in nanostructure resonant tunnelling diode-photodetectors

This work reports a nanostructure resonant tunnelling diode-photodetector (RTD-PD) device and demonstrates its operation as a controllable, optically-triggered excitable spike generator. The top contact layer of the device is designed with a nanopillar structure (500 nm in diameter) to restrain the injection current, yielding therefore lower energy operation for spike generation. We demonstrate experimentally the deterministic optical triggering of controllable and repeatable neuron-like spike patterns in the nanostructure RTD-PDs. Moreover, we show the device’s ability to deliver spiking responses when biased in either of the two regions adjacent to the negative differential conductance region, the so-called ‘peak’ and ‘valley’ points of the current–voltage (I–V) characteristic. This work also demonstrates experimentally key neuron-like dynamical features in the nanostructure RTD-PD, such as a well-defined threshold (in input optical intensity) for spike firing, as well as the presence of spike firing refractory time. The optoelectronic and chip-scale character of the proposed system together with the deterministic, repeatable and well controllable nature of the optically-elicited spiking responses render this nanostructure RTD-PD element as a highly promising solution for high-speed, energy-efficient optoelectronic artificial spiking neurons for novel light-enabled neuromorphic computing hardware.

Interband cascade infrared photodetectors based on Ga-free InAs/InAsSb superlattice absorbers

We present an interband cascade infrared photodetector based on Ga-free type-II superlattice absorbers. Substituting the more standard InAs/GaSb superlattice for a Ga-free superlattice with InAs/InAsSb requires an inverted carrier extraction path. A hole-ladder in the electron-barrier, instead of an electron-ladder in the hole-barrier, is employed to achieve photovoltaic operation. At elevated temperatures, seven negative-differential-conductance (NDC) regions are observed that arise from electrons tunneling through the electron barriers of the seven cascade stages. The detector operates in the photovoltaic mode at room temperature with a cutoff wavelength of 8.5 μm. At the NDC regions, the device features responsivity peaks under laser illumination reaching 0.45 A/W at room temperature at λ = 5.27 μm. This exceeds its highest measured low-temperature value of 0.22 A/W at this wavelength by a factor of 2.

Single-Photon Counting with Semiconductor Resonant Tunneling Devices.

Optical quantum information science and technologies require the capability to generate, control, and detect single or multiple quanta of light. The need to detect individual photons has motivated the development of a variety of novel and refined single-photon detectors (SPDs) with enhanced detector performance. Superconducting nanowire single-photon detectors (SNSPDs) and single-photon avalanche diodes (SPADs) are the top-performer in this field, but alternative promising and innovative devices are emerging. In this review article, we discuss the current state-of-the-art of one such alternative device capable of single-photon counting: the resonant tunneling diode (RTD) single-photon detector. Due to their peculiar photodetection mechanism and current-voltage characteristic with a region of negative differential conductance, RTD single-photon detectors provide, theoretically, several advantages over conventional SPDs, such as an inherently deadtime-free photon-number resolution at elevated temperatures, while offering low dark counts, a low timing jitter, and multiple photon detection modes. This review article brings together our previous studies and current experimental results. We focus on the current limitations of RTD-SPDs and provide detailed design and parameter variations to be potentially employed in next-generation RTD-SPD to improve the figure of merits of these alternative single-photon counting devices. The single-photon detection capability of RTDs without quantum dots is shown.

Strain modulating electronic band gaps and SQ efficiencies of semiconductor 2D PdQ2 (Q\u2009=\u2009S, Se) monolayer

We studied the physical, electronic transport and optical properties of a unique pentagonal PdQ2 (Q = S, Se) monolayers. The dynamic stability of 2Dwrinkle like-PdQ2 is proven by positive phonon frequencies in the phonon dispersion curve. The optimized structural parameters of wrinkled pentagonal PdQ2 are in good agreement with the available experimental results. The ultimate tensile strength (UTHS) was calculated and found that, penta-PdS2 monolayer can withstand up to 16% (18%) strain along x (y) direction with 3.44 GPa (3.43 GPa). While, penta-PdSe2 monolayer can withstand up to 17% (19%) strain along x (y) dirrection with 3.46 GPa (3.40 GPa). It is found that, the penta-PdQ2 monolayers has the semiconducting behavior with indirect band gap of 0.94 and 1.26 eV for 2D-PdS2 and 2D-PdSe2, respectively. More interestingly, at room temperacture, the hole mobilty (electron mobility) obtained for 2D-PdS2 and PdSe2 are 67.43 (258.06) cm2 V−1 s−1 and 1518.81 (442.49) cm2 V−1 s−1, respectively. In addition, I-V characteristics of PdSe2 monolayer show strong negative differential conductance (NDC) region near the 3.57 V. The Shockly-Queisser (SQ) effeciency prameters of PdQ2 monolayers are also explored and the highest SQ efficeinciy obtained for PdS2 is 33.93% at −5% strain and for PdSe2 is 33.94% at −2% strain. The penta-PdQ2 exhibits high optical absorption intensity in the UV region, up to 4.04 × 105 (for PdS2) and 5.28 × 105 (for PdSe2), which is suitable for applications in optoelectronic devices. Thus, the ultrathin PdQ2 monolayers could be potential material for next-generation solar-cell applications and high performance nanodevices.

Characteristics of Series‐Connected Resonant Tunneling Diode‐Pair Configuration for High Output Power Operation

Herein, a series‐connected resonant tunneling diode (RTD)‐pair configuration is characterized using the dynamic capacitance characteristics of the RTD. To achieve enhanced output power characteristics, the RTD‐pair cell is utilized in the RTD oscillator design to increase the voltage span and current span of the negative differential conductance (NDC) region. The results indicate that not only the output power can be significantly improved compared with the single‐RTD oscillator, but also it operates at a small capacitance region, making the RTD‐pair oscillator beneficial for high frequency and high output power operation.

Pulsed terahertz radiation from a double-barrier resonant tunneling diode biased into self-oscillation regime

The study of the bolometer response to terahertz (THz) radiation from a double-barrier resonant tunneling diode (RTD) biased into the negative differential conductivity region of the I–V characteristic revealed that the RTD emits two pulses in a period of intrinsic self-oscillations of current. The bolometer pulse repetition rate is a multiple of the fundamental frequency of the intrinsic self-oscillations of current. The bolometer pulses are detected at two critical points with a distance between them being half or one-third of a period of the current self-oscillations. An analysis of the current self-oscillations and the bolometer response has shown that the THz photon emission is excited when the tunneling electrons are trapped in (the first pulse) and then released from (the second pulse) miniband states.

N-polar GaN/AlN resonant tunneling diodes

N-polar GaN/AlN resonant tunneling diodes are realized on a single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current–voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8 ± 0.8 kA/cm2 at a forward bias of ∼8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ∼−4 V. These resonant and threshold voltages are well explained with the polarization field, which is opposite to that of the metal-polar counterpart, confirming the N-polarity of the resonant tunneling diodes (RTDs). When the device is biased in the NDC-region, electronic oscillations are generated in the external circuit, attesting to the robustness of the resonant tunneling phenomenon. In contrast to metal-polar RTDs, N-polar structures have the emitter on the top of the resonant tunneling cavity. As a consequence, this device architecture opens up the possibility of seamlessly interfacing—via resonant tunneling injection—a wide range of exotic materials with III-nitride semiconductors, providing a route towards unexplored device physics.

Calculation of Electron Swarm Parameters in Tetrafluoromethane

The electron swarm parameters and electron energy distribution function (EEDF) are necessary, especially onunderstanding quantitatively plasma phenomena and ionized gases. The EEDF and electron swarm parameters including the reduce effective ionization coefficient (α-η)/N (α and η are the ionization and attachment coefficient, respectively), electron drift velocity, electron mean energy, characteristic energy, density normalized longitudinal diffusion coefficient, and density normalized electron mobility in tetrafluoromethane (CF4) which was analyzed and calculated using the two-term approximation of the Boltzmann equation method at room temperature, over a range of the reduced electric field strength (E/N) between 0.1 and 1000 Td(1Td=10-17 V.cm2), where E is the electric field and N is the gas density of the gas. The calculations required cross-sections of the electron beam, thus published momentum transfer, vibration, electronic excitation, ionization, and attachment cross-sections for CF4 were used, the results of the Boltzmann equation in a good agreement with experimental and theoretical values over the entire range of E/N. In all cases, negative differential conductivity regions were found. It is found that the calculated EEDF closes to Maxwellian distribution and decreases sharply at low E/N. The low energy part of EEDF flats and the high-energy tail of EEDF increases with increase E/N. The EEDF found to be non-Maxwellian when the E/N> 10Td, havingenergy variations which reflect electron/molecule energy exchange processes. In addition, limiting field strength (E/N)limit has been calculated from the plots of (α-η)/N, for which the ionization exactlybalances the electron attachment, which is valid for the analysis of insulation characteristics and application to power equipment.

Sensitive terahertz-wave detector responses originated by negative differential conductance of resonant-tunneling-diode oscillator

We present an experimental characterization of frequency- and bias-dependent detector responses in a resonant-tunneling-diode (RTD) terahertz (THz)-wave oscillator. By tuning the incident THz-wave frequency and the bias voltage applied to the RTD device, the origins of detection signals are identified to be two distinct detection modes. One is based on square-law detection near the peak and valley points of the negative differential conductance (NDC) region, with the detection bandwidth determined by an integrated slot antenna. The other is based on detectable current changes induced by injection locking within the NDC region when the frequency of the incident THz-wave radiation to be detected is coincident with that of the bias-dependent RTD self-oscillation between 0.74 and 0.81 THz, resulting in a minimum noise equivalent power (NEP) of 7.7 pW/√Hz at 0.78 THz at room temperature. Our conclusions demonstrate that an RTD oscillator can be used as a sensitive THz-wave detector within and around the NDC region.

Validation of the model for predicting VAC RTD photonics

This paper discusses the processes of current transfer in resonant tunneling diodes during the transition from the region of positive differential conductivity to the region of negative differential conductivity. The behavior features of the self-consistent potential and tunnel transparency coefficient in the region of peak stresses are demonstrated.

Negative Differential Conductivity of Lanthanum-Oxide-Based Structures

Transparent surface-hydrogenated lanthanum-oxide films (OH–La2O3) 40, 140, and 545 nm in thickness are fabricated using electron-beam evaporation. The electrical and optical characteristics of Al/OH–La2O3/p-Si structures are investigated. Aluminum and silicon substrates of p-type conductivity are used as the upper and lower electrodes of the structure, respectively. The region of negative differential conductivity is found in the voltage dependences of conductivity under forward bias; the possible mechanism of the negative differential conductivity is explained by proton transfer along chains of water molecules bound by hydrogen bonds on the surface of the OH–La2O3 film.

Resonant tunnelling into the two-dimensional subbands of InSe layers

Two-dimensional (2D) van der Waals (vdW) crystals have attracted considerable interest for digital electronics beyond Si-based complementary metal oxide semiconductor technologies. Despite the transformative success of Si-based devices, there are limits to their miniaturization and functionalities. Here we realize a resonant tunnelling transistor (RTT) based on a 2D InSe layer sandwiched between two multilayered graphene (MLG) electrodes. In the RTT the energy of the quantum-confined 2D subbands of InSe can be tuned by the thickness of the InSe layer. By applying a voltage across the two MLG electrodes, which serve as the source and drain electrodes to the InSe, the chemical potential in the source can be tuned in and out of resonance with a given 2D subband, leading to multiple regions of negative differential conductance that can be additionally tuned by electrostatic gating. This work demonstrates the potential of InSe and InSe-based RTTs for applications in quantum electronics.

Отрицательная дифференциальная проводимость структур на основе оксида лантана

Using e-beam evaporation technique, transparent surface-hydrated lanthanum oxide (OH-La2O3) films with a thickness of 40, 140, and 545 nm were obtained. The electrical and optical characteristics of Al/OH-La2O3/p-Si structures were studied, where aluminum and a silicon substrate with p-type conductivity were used as the upper and lower electrodes, respectively. Negative differential conductivity region in the conductivity–voltage characteristics for the forward bias voltage was found; a possible mechanism of negative differential conductivity is explained by proton transport in hydrogen bonded chains of water molecules at the OH-La2O3 film surface.

Negative differential conductance in the electron-transport through copper-rich cuprous oxide thin films

Copper deposition onto Cu2O thin films grown on Au(111) results in the formation of monolayer islands with hexagonal and rhombic shapes, as observed with scanning tunnelling microscopy. The differential conductance through the Cu islands is governed by distinct quantum well states (QWS), accompanied by pronounced electron standing wave patterns. Below the onset of the QWS, an extended region of negative differential conductance opens up, in which also the tunnelling current declines markedly with increasing bias voltage. The effect is assigned to the quantised electronic structure of the Cu islands in combination with the p-type conductance behaviour of the oxide film underneath. The latter promotes electron transport across the islands around the Fermi level, but leads to a closure of this transport channel at negative bias.

Computer simulation of an electronic device for generation of electric oscillations by supercooled in electric field superconductors

We present computer simulations of the work of an electronic device for generation of electric oscillation by negative differential conductivity of a superconductor supercooled below the critical temperature. The superconductor is cooled below its critical temperature in the normal state at a small constant electric voltage. To simulate the device, we use formerly derived explicit analytical expressions for the conductivity of nanostructured superconductors supercooled below the critical temperature in an electric field. Computer simulations reveal that the negative differential conductivity region of the current–voltage characteristic leads to excitation of electric oscillations. We simulate a circuit with distributed elements as a first step to obtain gigahertz frequency. This gives a hint that a hybrid device of nanostructured superconductors will work in terahertz frequencies. If the projected setup is successful, we consider the possibility for it to be put in a nanosatellite (such as a CubeSat). The study of layered high-Tc superconductors and EM waves emitted from them in space vacuum would be an important task for future space technology.

Observation of Regions of Negative Differential Conductivity and Current Generation during Tunneling through Zero-Dimensional Defect Levels of the h-BN Barrier in Graphene/h-BN/Graphene Heterostructures

Tunneling and magnetic tunneling are investigated in graphene/h-BN/graphene van der Waals heterosystems. Two new types of systems are found, in which negative differential conductivity regions are implemented due to resonant tunneling through defect levels in the h-BN barrier, and current caused by their presence is generated.

Nonlinear transport, dynamic ordering, and clustering for driven skyrmions on random pinning

Using numerical simulations, we examine the nonlinear dynamics of skyrmions driven over random pinning. For weak pinning, the skyrmions depin elastically, retaining sixfold ordering; however, at the onset of motion there is a dip in the magnitude of the structure factor peaks due to a decrease in positional ordering, indicating that the depinning transition can be detected using the structure factor even within the elastic depinning regime. At higher drives the moving skyrmion lattice regains full ordering. For increasing pinning strength, we find a transition from elastic to plastic depinning that is accompanied by a sharp increase in the depinning threshold due to the proliferation of topological defects, similar to the peak effect found at the elastic to plastic depinning transition in superconducting vortex systems. For strong pinning and strong Magnus force, the skyrmions in the moving phase can form a strongly clustered or phase separated state with highly modulated skyrmion density, similar to that recently observed in continuum-based simulations for strong disorder. As the Magnus force is decreased, the density phase separated state crosses over to a dynamically phase separated state with uniform density but with flow localized in bands of motion, while in the strongly damped limit, both types of phase separated states are lost. In the strong pinning limit, we find highly nonlinear velocity-force curves in the transverse and longitudinal directions, along with distinct regions of negative differential conductivity in the plastic flow regime. The negative differential conductivity is absent in the overdamped limit. The Magnus force is responsible for both the negative differential conductivity and the clustering effects, since it causes faster moving skyrmions to partially rotate around slower moving or pinned skyrmions.

Polariton-like stimulated excitations in rectifying terahertz GaAs/AlAs double barrier nanostructures driven by nonequilibrity of the resonant-tunneling proces

We have studied the periodical-in-voltage oscillations in the current-voltage characteristics of GaAs/AlAs double barrier resonant tunneling diode terahertz detectors. The found oscillations of the negative differential conductance are attributed to LO-phonon-branch polariton excitations stimulated by electrons tunneling through the quantum active region in the resonance nanostructure. It is argued that, due to the conduction-band profiles disposition in a manner of the inversion-like nonequilibrium cascade, the tunneling current supplies the inevitable excessive energy. The RTD serves as an internal amplifier of weak electromagnetic signals, provided by the region of negative differential conductance.

Наблюдение областей отрицательной дифференциальной проводимости и генерации тока при туннелировании через нульмерные уровни дефектов барьера h-BN в гетероструктурах графен/h-BN/графен

AbstractTunneling and magnetic tunneling are investigated in graphene/ h -BN/graphene van der Waals heterosystems. Two new types of systems are found, in which negative differential conductivity regions are implemented due to resonant tunneling through defect levels in the h -BN barrier, and current caused by their presence is generated.