Voltage-controlled Negative Differential Resistance Research Articles

Electron emission and ultraviolet electroluminescence from valence-band states and defect conduction bands of electroformed Al-Al2O3-Ag diodes

Electroforming of metal-insulator-metal (MIM) diodes is a soft dielectric breakdown which results in the formation of a conducting filament through the diode. It is a critical step in the development of conducting states between which switching can occur in resistive switching memories. Conduction, electron emission into vacuum (EM), and electroluminescence (EL) have been studied in two groups of electroformed Al-Al2O3-Ag diodes with amorphous anodic Al2O3 thicknesses between 20 nm and 49 nm. EM and EL appear simultaneously with the abrupt current increase that characterizes electroforming of Al-Al2O3-metal diodes. There is voltage-controlled differential negative resistance in the current-voltage (I-V) characteristics after electroforming. There is a temperature-independent voltage threshold for EM, VEM ≅ 2.2 V. Three EM regions occur. In region I, VEM < VS ≲ 4 V, where VS is the applied voltage, and there is an exponential increase of EM. There is a second exponential increase of EM in region III when VS exceeds a second temperature-independent voltage threshold, UEM. UEM is ∼6.6 V for one group of Al-Al2O3-Ag diodes; it is ∼7.9 V for the second group. EM is nearly constant in region II for 4 V ≲ VS ≲ UEM. Two band-pass filters have been used to characterize EL from electroformed Al-Al2O3-Ag diodes. The long-pass (LP) filter plus photomultiplier responds to photons with energies between ∼1.8 eV and ∼3.0 eV. The short-pass (SP) filter measures ultra-violet (UV) radiation between ∼3.0 eV and ∼4.2 eV. Corresponding to region I of EM, there are exponential increases of EL for VS greater than temperature-independent voltage thresholds: VLP ≅ 1.5 V and VSP ≅ 2.0 V. There is a second exponential increase of UV with the SP filter in region III for VS greater than a temperature-independent voltage threshold, USP. USP ≅ 7.9 V for one group of electroformed Al-Al2O3-Ag diodes and USP ≅ 8.8 V for the second group; USP > UEM. Both groups exhibit EM from valence band states of amorphous Al2O3. The difference in UEM and USP of the two groups of electroformed Al-Al2O3-Ag diodes is attributed to the presence or absence of a defect conduction band formed from the ground state of F0- or F+-centers, oxygen vacancies in amorphous Al2O3. The observation of exponentially increasing EM or EL in the low conductivity state of electroformed Al-Al2O3-metal diodes is not consistent with switching mechanisms of MIM diodes that involve rupture of the conducting filament since rupture that affects diode current, if it occurs, should also cut off EM and EL.

Voltage-controlled negative differential resistance in metal-sputtered alumina-Si structures

We report on the observation of voltage-controlled stable and repeatable negative differential resistance (NDR) in Al/Al2O3/n-type Si structures. The NDR is shown to be stable and repeatable both during multiple scans on the same device and between different devices. NDR peak-to-valley ratios up to 100 were measured, with a valley at 2 × 10−13 A. The maximum peak-to-valley ratio was observed for a voltage step of 0.05 V with dV/dt = 0.1 V s−1 and a step delay time of 0.1 s. The effect is transient, meaning that it tends to disappear for dc conditions. The observed NDR is markedly different from other previously reported NDRs in insulators since it only depends on interface trapping and is not influenced by the leakage current through the device which is below the noise floor of our measurement system (2 × 10−14 A). The origin of the observed NDR is attributed to charging and discharging of traps at the metal-insulator interface.

Separation of current density and electric field domains caused by nonlinear electronic instabilities

In 1963 Ridley postulated that under certain bias conditions circuit elements exhibiting a current- or voltage-controlled negative differential resistance will separate into coexisting domains with different current densities or electric fields, respectively, in a process similar to spinodal decomposition of a homogeneous liquid or disproportionation of a metastable chemical compound. The ensuing debate, however, failed to agree on the existence or causes of such electronic decomposition. Using thermal and chemical spectro-microscopy, we directly imaged signatures of current-density and electric-field domains in several metal oxides. The concept of local activity successfully predicts initiation and occurrence of spontaneous electronic decomposition, accompanied by a reduction in internal energy, despite unchanged power input and heat output. This reveals a thermodynamic constraint required to properly model nonlinear circuit elements. Our results explain the electroforming process that initiates information storage via resistance switching in metal oxides and has significant implications for improving neuromorphic computing based on nonlinear dynamical devices.

Leveraging nMOS Negative Differential Resistance for Low Power, High Reliability Magnetic Memory

We propose, demonstrate, and assess a nontunneling-based nMOS voltage-controlled negative differential resistance (V-NDR) concept for overcoming the intrinsic efficiency and reliability shortcomings of magnetic random access memory memories (MRAM). Using nMOS V-NDR circuits in series with MRAM tunnel junctions, we experimentally observe 40 times reduction in current during switching, enabling write termination and read margin amplification. Large scale Monte Carlo simulations also show 5X improvement in write energy savings and demonstrate the robustness of the scheme against device variability.

Negative Resistance Behaviour and Molecular Reorientation Properties of Zinc Oxide Nanoparticles Based Liquid Crystals for High Image Quality Liquid Crystal Displays

The octylcyanobiphenyl (8CB) liquid crystal was doped by ZnO nanoparticles to improve the dielectric anisotropy parameters, as the image quality of liquid crystal displays (LCDs) is strongly dependent on the dielectric anisotropy of the liquid crystal. The dielectric anisotropy parameters of the liquid crystals have been determined. The magnitude of dielectric anisotropy of the 8CB liquid crystal was increased with nano-nennatic composite liquid crystal system. The current voltage chracteristics of 8CB and ZnO doped 8CB liquid crystals were determined using current voltage chracteristics. The liquid crystals exhibited a voltage-controlled differential negative resistance behaviour (VCNR). The obtained results indicate that the ZnO nanoparticles increase the dielectric anisotropy parameters of the 8CB liquid crystal

Metal-insulator transition upon heating and negative-differential-resistive-switching induced by self-heating in BaCo0.9Ni0.1S1.8

The layered compound BaCo1−xNixS2−y (0.05 < x < 0.2 and 0.05 < y < 0.2) exhibits an unusual first-order structural and electronic phase transition from a low-T monoclinic paramagnetic metal to a high-T tetragonal antiferromagnetic insulator around 200 K with huge hysteresis (∼40 K) and large volume change (∼0.01). Here, we report on unusual voltage-controlled resistive switching followed by current-controlled resistive switching induced by self-heating in polycrystalline BaCo1−xNixS2−y (nominal x = 0.1 and y = 0.2). These were due to the steep metal to insulator transition upon heating followed by the activated behavior of the resistivity above the transition. The major role of Joule heating in switching is supported by the absence of nonlinearity in the current as function of voltage, I(V), obtained in pulsed measurements, in the range of electric fields relevant to d.c. measurements. The voltage-controlled negative differential resistance around the threshold for switching was explained by a simple model of self-heating. The main difficulty in modeling I(V) from the samples resistance as function of temperature R(T) was the progressive increase of R(T), and to a lesser extend the decrease of the resistance jumps at the transitions, caused by the damage induced by cycling through the transitions by heating or self-heating. This was dealt with by following systematically R(T) over many cycles and by using the data of R(T) in the heating cycle closest to that of the self-heating one.

Dielectric anisotropy and electrical properties of the copper phthalocyanine (CuPc): 4–4′-n-Heptylcyanobiphenyl (7CB) composite liquid crystals

Dielectric anisotropy and electrical properties of the copper phthalocyanine (CuPc): 4–4′-n-heptylcyanobiphenyl (7CB) composite liquid crystals have been investigated. Liquid crystals exhibit dielectrically-controlled positive dielectric anisotropy (p-type Δɛ) and the dielectric anisotropy changes from positive to negative dielectric anisotropy (n-type Δɛ) behavior with frequency of applied electrical field. The critical frequency fc values for the 7CB and CuPc doped 7CB liquid crystals were found to be 602kHz and 518kHz, respectively. This indicates that CuPc doping causes a decrease in the critical frequency. The splay elastic coefficients of the 7CB and CuPc doped 7CB liquid crystals were determined and CuPc doping increases the splay elastic coefficient of 7CB liquid crystal. The 7CB liquid crystal exhibits a voltage-controlled differential negative resistance (VCNR) behavior and CuPc doping affects significantly VCNR behavior of 7CB.

Negative Resistance Behavior of Ferroelectric Bismuth Titanate Ceramics at Low Field

Ferroelectric Bi4Ti3O12ceramics are fabricated by conventional solid-state reaction process. The current-voltage characteristic of Bi4Ti3O12sample exhibits a voltage-controlled negative differential resistance behavior at low field (E≤100V/mm), and an obvious PTC effect appears at around 100°C on the resistance-temperature curve. Based on conducting filament model about electrical transport, instead of Heywang-Jonker model, the experimental results of Bi4Ti3O12ceramics are suitably explained.

Voltage-controlled multiple-valued logic design using negative differential resistance devices

This paper demonstrates a concise and novel voltage-controlled multiple-valued logic (MVL) design using the standard BiCMOS technique. This MVL circuit is constructed by a voltage-controlled negative differential resistance (NDR) circuit, which is integrated by the standard Si-based metal–oxide-semiconductor field-effect-transistor (MOS) and SiGe-based heterojunction bipolar transistor (HBT). There exists a two-peak current–voltage curve by connecting two integrated MOS–HBT–NDR elements in parallel as we suitably determine the width/length ( W/ L) of the MOS devices. In particular, each peak current can be effectively modulated by the corresponding controlled voltage. Using this special characteristic, we can obtain the three logic states with arbitrary sequence at the output.

Investigation on Resistive Memory Switching Mechanism of NiO

ABSTRACT Experimental investigations on the resistive memory switching in sub-micron sized NiO memory cell are presented to elucidate the resistive memory switching mechanism. The voltage or current-biased I-V measurements show that the resistive switching transitions can be regarded as the combination of a voltage-controlled negative differential resistance phenomenon and a current-controlled negative differential resistance phenomenon. Along with experimental observations of multiple resistance states, these indicate that the memory switching in NiO would come from the percolative formation and rupture of filamentary conducting paths. Pulse experiments further suggest that the memory switching would come from local domains inside filaments.

Investigation of refractive index dispersion and electrical properties in carbon nano-balls’ doped nematic liquid crystals

Electrical and refractive index dispersion properties of a carbon nano-balls’ (Fullerene C 60) doped Liquid crystal (LC) composite, are investigated by concerning the frequency-dependent dielectrical properties, laser-induced effects on current–voltage ( I – V ) characteristics and optical characteristics suggest that the samples exhibit voltage-controlled differential negative resistance behavior. Dielectric spectra reflect the presence of different relaxation modes at higher frequencies. The refractive index dispersion parameters (oscillator energy and strength, optical moments) were also determined by optical characterization. As a result, C 60 doping changes photoconductivity, dielectric and refractive index properties of LC in a favorable manner.

Electroforming and switching effects in yttrium oxide

Electroforming and switching in sandwich structures based on anodic films of yttrium oxide have been studied. After being electroformed, the structures exhibit voltage-controlled negative differential resistance with N-shaped I–V characteristics. Typical values of the threshold current and voltage are Ith ∼ 10–30 μA and Vth ∼ 0.5–1.5 V for a film thickness d ∼ 100 nm. The switching channels, consisting of non-stoichiometric Y2O3−x, are formed in the initial oxide films during the preliminary electroforming. A possible switching mechanism associated with the non-equilibrium metal–insulator transition is discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Monte Carlo simulation of current–voltage characteristics in metal–insulator–metal thin film structures

Many metal–insulator–metal (MIM) thin film sandwich structures exhibit an electroforming process, following which the resulting direct current (DC) current–voltage I–V characteristic exhibits ohmic conduction and voltage-controlled differential negative resistance (VCNR) at low and high voltages, respectively. One of the more successful models of these phenomena is the filamentary model, in which it is assumed that the electroforming process establishes a population of ohmic filaments within the insulating matrix, which span the metal contacts. The VCNR behaviour results from the progressive cessation of conduction in individual filaments owing to Joule heating effects. Early work showed that by postulating plausible probability distributions of filament resistances, I–V characteristics typical of those obtained experimentally could be derived. More recently more realistic normal distributions of filament resistances, radii and cross-sectional areas have been considered, and these too have yielded characteristics in accordance with experiment. In the present work normal distributions of filament resistances and radii are considered, but in contrast to previous work a novel method using a Monte Carlo simulation was employed. Furthermore, the effects of using a combination of two different filament resistivity values were explored. Variations in conductance and shifts in the current peak were explicable using existing theory. The approach adopted in this work lends itself to the exploration of more complex filament distributions, and therefore to a better representation of the underlying filament population, including those having a multivariate distribution.

The VCNR properties of nano-structured ceria thin films

Nano-structured ceria thin films were prepared by using the sol–gel dip-coating method. The films were heated at various temperatures. A constant voltage source is used to measure current–voltage (I–V) properties. We found voltage-controlled differential negative resistance (VCNR) properties in these ceria thin films. The VCNR properties exhibit two distinct characteristics. One is that the VCNR behavior is very obvious for the negative I–V characteristics and is not symmetric about the voltage. The other is that in the negative I–V characteristics region, the current increases sharply at low voltage until it reaches a maximum, and subsequently the current decreases to a minimum but not zero, and then the current continues to rise linearly with increasing of voltage. The conduction mechanism of the above phenomena is discussed and the influence of operating temperature, dopant concentration and heating temperature on the VCNR properties of ceria thin films is analyzed.

Poole-Frenkel conductivity prior to electroforming in evaporated Au-SiO x-Au sandwich structures

It is well known that evaporated metal-silicon oxide-metal sandwich structures exhibit a field-assisted lowering of the potential barrier between donor-like centres and the conduction band edge, known as the Poole-Frenkel effect. This behaviour is indicated by a linear dependence of log J on V1/2, where J is the current density and V is the applied voltage. Less well known is that when a suitable electrode metal is used, and after the application of a few volts under moderate vacuum conditions, the structure undergoes a process known as ‘electroforming’, after which characteristic features such as voltage-controlled differential negative resistance (VCNR) and electron emission are exhibited. The work described in the present investigation is concerned with the d.c. properties during voltage cycling prior to electroforming. In agreement with previous work Poole-Frenkel conductivity was observed during the first voltage cycle, but the value of the Poole-Frenkel field-lowering coefficient β generally increased with the number of voltage cycles before the onset of electroforming, frequently attaining a value of up to twice the theoretical value. No significant variation in the value of β was observed in samples which did not subsequently electroform. Enhanced β values have previously been identified with the existence of a non-uniform electric field region within the insulator, and it was therefore concluded that a high-field region is established during the voltage cycling, which is necessary before electroforming can take place.

Electroforming and memory effects in thick-film sandwich devices based on Bi2O3/In2O3/Ru2

Abstract The electrical properties of thick-film sandwich devices based on composites of bismuth, ruthenium and indium oxides are presented. The devices are shown to exhibit Poole-Frenkel bulk limited conduction in the temperature range 290 K to 393 K. The activation energies for the compositions were calculated at between 7-5 to 13meV, leading to the conclusion that hopping is the dominant conduction mechanism. After electroforming, the devices exhibited voltage-controlled differential negative resistance and an increase in the conductivity under atmospheric pressure conditions at between 2 to 2-5 orders of magnitude, indicating the possibility of producing memory-switching devices based on the thick-film metal-resistor-metal configuration.

A.c. electrical properties of vacuum deposited Au-SiOx-Au sandwich structures prior to electroforming

Vacuum-evaporated silicon oxide (SiOx films of thickness 150 nm were fabricated in the form of sandwich structures using Au electrodes. It is well known that such samples undergo an electroforming process subsequent to the application of a forming voltage after which voltage-controlled differential negative resistance (VCNR) is observed. The a.c. electrical properties of such structures were studied prior to electroforming in the frequency range 100 Hz–20 kHz and for temperatures in the range 78–523 K. The a.c. conductivity σ(ω) showed a power-law dependence on angular frequency ω, with exponent s in the range 0.5 ⩽ s ⩽ 1.0 below 1 kHz and s ⩾ 1.0 above 10 kHz. The low frequency results obtained at temperatures below approximately 323 K. showed a good agreement with the model of Elliott for amorphous structures, yielding a typical low-temperature activation energy of 1.4 meV, strongly indicating a hopping mechanism. Sample capacitance showed a moderate decrease with frequency and an increase with temperature, eventually becoming constant at temperatures below 300 K for all frequencies. The loss tangent tan δ showed a minimum in its frequency dependence at low temperatures, which shifted to higher frequency with increasing temperatures. For temperatures above 423 K, the minimum in tan δ was not observed below 20 kHz. The results are discussed in terms of existing models, a good correlation being obtained with the model of Goswami and Goswami which assumes a thermal activation process and the existence of ohmic contacts to the SiOx.

N-shaped negative differential resistance in a transistor structure with a resistive gate

Voltage-controlled negative differential resistance (NDR) characteristics in a N-AlGaAs/p/sup +/-GaAs/n-GaAs transistor structure are proposed and demonstrated. The gate, made using self-aligned p-type diffusion, is placed in the n-GaAs collector layer instead of the p/sup +/-GaAs base layer, resulting in a so-called resistive gate. For a fixed gate voltage, the device current is modulated by the applied anode voltage. Under appropriate gate voltage with respect to the anode, the device shows good voltage-controllable NDR characteristics, including large peak-to-valley current ratios (PTV's) and a voltage extension in the N-shaped curve which is equivalent to the common-emitter breakdown voltage in a transistor. A numerical model based on the transistor model for the carrier transport in this device, taking account of the influence of the applied anode voltage on the gate, is proposed. The experimental results show large room temperature PTV's (e.g., 140 at a gate bias of 1.5 V) and large voltage extension in N-shaped curves (about 9 V). Reasonable agreement between theoretical and experimental results is observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Calculations of the current-voltage characteristics of electroformed MIM structures, assuming a normal distribution of filamentary radii

Abstract Voltage-controlled differential negative resistance (VCNR) is known to occur in many electroformed metal-insulator-metal (MIM) structures. Current-voltage characteristics obtained from electroformed MIM devices can be derived using a model in which conduction is assumed to take place through an array of filaments spanning the two electrodes. The model assumes that conduction through the filaments is ohmic but deviates from this as filaments rupture due to Joule heating. Previous results based on filamentary models have been obtained by assuming that the filament resistances are distributed normally or according to some alternative distribution. Here we consider a filamentary model in which the radii of filaments are taken to be normally distributed. The corresponding current-voltage characteristics are derived and found to be typical of those obtained experimentally.

Electroformed MIM structures: dependence of the current-voltage characteristics on the standard deviation of a normal distribution of filamentary radii

In electroformed metal-insulator-metal (MIM) structures, voltage controlled differential negative resistance (VCNR) is frequently observed. Such electrical characteristics are frequently modelled in terms of a matrix of conducting filaments spanning the insulating layer between the electrodes. The filaments are assumed to switch off at varying voltages Vp, depending on their individual resistances ρ, ultimately leading to the VCNR behaviour. The detailed current-voltage characteristics depend both on the nature of the relationship between Vp and ρ, and the probability density function P(ρ) of filament resistances. Often it is assumed that filaments switch off when their local temperatures reach some predetermined value; if the heat produced is exclusively by Joule heating, then Vp = βρ* where β is a constant depending on the geometry of the filaments and the thermal conductivity of the insulator. Previously, triangular and parabolic probability density functions have been investigated while more recently ...