Negative Differential Capacitance Research Articles

Capacitance spectroscopy of InAs quantum dots inserted in an AlGaAs/GaAs HEMT for photodetector applications

We have investigated the electrical characteristics of an AlGaAs/GaAs high electron mobility transistor, in which a layer of InAs self-assembled Quantum Dots (QDs) was inserted below the 2DEG channel. A Negative Differential Capacitance (NDC) appeared in the capacitance–voltage characteristics at a bias of 1 V and at low temperatures (even at room temperature) under different illumination powers using white light bulbs. This results in an increase in negative differential conductance with the increase in frequency and optical power. This also applies to the NDC except that it decreases with increasing frequency. The numerical simulation of the energy band structure of the device confirmed that the conduction band lowers to its minimum at a special bias value of 1 V. The numerical analysis of the evolution of the energy levels in the QD-HEMT follows the appearance of multiple capacitance peaks and their behavior with the gate voltage.

Validating negative differential capacitance for advanced low-power devices

Validating negative differential capacitance for advanced low-power devices

Negative differential capacitance in ultrathin ferroelectric hafnia

Negative differential capacitance in ultrathin ferroelectric hafnia

Inductive Type Impedance of Mo/<i>n</i>-Si Barrier Structures Irradiated with Alpha Particles

In silicon microelectronics, flat metal spirals are formed to create an integrated inductance. However, the maximum specific inductance of such spirals at low frequencies is limited to a value of the order of tens of microhenries per square centimeter. Gyrators, devices based on operational amplifiers with approximately the same specific inductance as spirals, are also used. Despite the fact that such solutions have been introduced into the production of integrated circuits, the task of searching for new elements with high values of specific inductance is relevant. An alternative to coils and gyrators can be the effect of negative differential capacitance (i.e., inductive type impedance), which is observed in barrier structures based on silicon.The purpose of the work is to study the low-frequency impedance of Schottky diodes (Mo/n-Si) containing defects induced by α-particles irradiation and determination of the parameters of these defects by methods of low-frequency impedance spectroscopy and DLTS (Deep Level Transient Spectroscopy).Unpackaged Schottky diodes Mo/n-Si (epitaxial layer of 5.5 μm thickness and resistivity of 1 Ohm∙cm) produced by JSC “Integral” are studied. Inductance measurements were carried out on the as manufactured diodes and on the diodes irradiated with alpha particles (the maximum kinetic energy of an αparticle is 5.147 MeV). The impedance of inductive type of the Schottky diodes at the corresponding DC forward current of 10 µA were measured in the AC frequency range from 20 Hz to 2 MHz. DLTS spectra were used to determine the parameters of radiation-induced defects. It is shown that irradiation of diodes with alpha particles produces three types of radiation-induced defects: A-centers with thermal activation energy of E1 ≈ 190 meV, divacancies with activation energies of E2 ≈ 230 meV and E3 ≈ 410 meV, and Ecenters with activation energy of E4 ≈ 440 meV measured relative to the bottom of c-band of silicon.

Simulations of phase transitions and capacitance, of simple ionic fluids in porous electrodes

In this work, we simulate the capacitance of a simple charged fluid model consisting of equal-sized, spherical monovalent ions, immersed in an implicit solvent. Apart from Coulomb forces, the particles interact via a truncated and shifted Lennard-Jones potential. This is a possible crude model for hydrophobic ions (assuming an implicit aqueous solvent) and allows for a possible demixing transition in the solution. Such a phase separation may be driven by charging a narrow slit-like pore, immersed in a dilute (“gas-like”) bulk phase, akin to a capillary condensation. We highlight the dramatic effect that boundary conditions and regulatory control have on electrical properties (e.g., the differential capacitance) at the point of transition. For example, the surface potential displays a vertical (first-order) transition, as a function of surface charge density, when the latter is constrained to be uniform over the pore surfaces. At short separations, the transition is gradual (i.e., not first-order), which leads to a regime with an apparent ”non-physical” negative differential capacitance. Under conditions of constant surface potential, the negative capacitance region becomes inaccessible, as a first-order (horizontal) transition is reasserted. This is true for both conducting and non-conducting surfaces.

MoS2/WSe2 vdW Heterostructures Decorated with PbS Quantum Dots for the Development of High-Performance Photovoltaic and Broadband Photodiodes.

van der Waals heterostructures (vdWHs) overcoming the lattice and processing limitations of conventional heterostructures provide an opportunity to develop high-performance 2D vdWH solar cells and photodiodes. However, it is challenging to improve the sensitivity and response speed of 2D vdWH photovoltaic devices due to the low light absorption efficiency and electron/hole traps in heterointerfaces. Here, we design a PbS/MoS2/WSe2 heterostructure photodiode in which a light-sensitive PbS quantum dot (QD) layer combined with a MoS2/WSe2 heterostructure significantly enhances the photovoltaic response. The electron current in the heterostructure is increased by the effective collection of photogenerated electrons induced by PbS QDs. The device exhibits a broadband photovoltaic response from 405 to 1064 nm with a maximum responsivity of 0.76 A/W and a specific detectivity of 5.15 × 1011 Jones. In particular, the response speed is not limited by multiple electron traps in the PbS QDs/2D material heterointerface, and a fast rising/decaying time of 43/48 μs and a -3 dB cutoff frequency of over 10 kHz are achieved. The negative differential capacitance and frequency dependence of capacitance demonstrate the presence of interface states in the MoS2/WSe2 heterointerface that hamper the improvement of the response speed. The scheme to enhance photovoltaic performance without sacrificing response speed provides opportunities for the development of high-performance 2D vdWH optoelectronic devices.

Experimental investigation of the gate voltage range of negative differential capacitance in ferroelectric transistors

Experimental investigation of the gate voltage range of negative differential capacitance in ferroelectric transistors

Effects of Interface Trap on Transient Negative Capacitance Effect: Phase Field Model

Ferroelectric materials have received significant attention as next-generation materials for gates in transistors because of their negative differential capacitance. Emerging transistors, such as the negative capacitance field effect transistor (NCFET) and ferroelectric field-effect transistor (FeFET), are based on the use of ferroelectric materials. In this work, using a multidomain 3D phase field model (based on the time-dependent Ginzburg–Landau equation), we investigate the impact of the interface-trapped charge (Qit) on the transient negative capacitance in a ferroelectric capacitor (i.e., metal/Zr-HfO2/heavily doped Si) in series with a resistor. The simulation results show that the interface trap reinforces the effect of transient negative capacitance.

IMPROVEMENT OF PARAMETERS OF ACTIVE FILTERS FOR PROCESSING OF MEASURING ELECTRICAL SIGNALS FROM PRIMARY SENSORS OF SENSORS

An important area of metrology is measuring time and frequency. In the equipment of radio measuring devices for measuring time and frequency parameters of radio signals and/or electrical circuits, the main functional element is electric filters. The efficiency of functioning of devices for measuring time and frequency parameters of radio signals depends on the quality of electric filters, stability of their parameters and characteristics. At the same time, there are requirements for frequency distortion and inertial properties of other stages of processing the measuring signals and elements and circuits of interstage connections. Therefore, the efficiency of frequency selection of measuring signals of primary sensors significantly depends on the quality of electric filters. The principle of the electric filter operation is based on functional dependence of inductive and capacitive elements on frequency, as well as on the phase ratios of voltage and current on them. Nowadays, not only positive but also negative differential capacitance is used in the synthesis and design of electronic circuits. The concept of negative differential capacitance is especially useful in practical implementation of active filters, noise compensators, capacitive sensors, capacitive transducers, etc. The aim of the paper is to increase the frequency overlap ratio for the active filters in the sensors of physical quantities using active elements with a negative value of the differential capacitance.

Light-induced capacitance enhancement and successive carrier escape in InGaN/GaN multiple quantum wells

We observed large enhancement of capacitance with strong voltage sensitivity in InGaN/GaN multiple quantum wells with additional laser illuminations. We have found that the observed negative differential capacitance and its related capacitance peaks in the capacitance-voltage profile are due to the photogenerated charge separation and accumulation at the well/barrier interfaces and its subsequent carrier escape by the applied forward bias. By analyzing temperature dependent photocurrent spectra simultaneously, it is shown that photocarrier separation and strong carrier escape simultaneously occur in an individual quantum well. We can analyze the contribution of a single individual quantum well to the total capacitance of the device, resulting from the nanometer scale carrier separation and accumulation, and clarify the detailed process of accumulation and escape of carriers in the respective quantum wells.

Теория возмущений для четырёхполюсника с сегнетоэлектрическим конденсатором с отрицательной дифференциальной ёмкостью

Теория возмущений для четырёхполюсника с сегнетоэлектрическим конденсатором с отрицательной дифференциальной ёмкостью

Simulation of Structure Parameters' Influence on the Threshold Voltage of Normally‐Off p‐GaN/AlGaN/GaN Transistors

The influence of different parameters of p‐GaN/AlGaN/GaN structure on a threshold voltage of transistors is studied. The parameters that are varied are the thickness of the p‐type GaN layer, its doping concentration, doping concentration of the GaN channel layer, and Schottky barrier height. Increasing the thickness of the p‐GaN layer is found to increase the threshold voltage. Also, increasing doping concentration of the p‐GaN layer increases the threshold voltage. The doping concentration of the GaN channel layer has the decisive effect on the possibility to use capacitance measurement for the threshold voltage assessment. Simulations show an interesting result of the Schottky barrier height threshold voltage dependence. For higher Schottky barrier height, lower threshold voltage is received, which is connected with the contribution of the negative differential capacitance of the p‐GaN layer. This starts to increase for higher positive voltage if the Schottky barrier height is lower.

Structural Correlation of Ferroelectric Behavior in Mixed Hafnia-Zirconia High-k Dielectrics for FeRAM and NCFET Applications

ABSTRACTThe recent discovery of ferroelectric behavior in doped hafnia-based dielectrics, attributed to a non-centrosymmetric orthorhombic phase, has potential for use in attractive applications such as negative differential capacitance field-effect-transistors (NCFET) and ferroelectric random access memory devices (FeRAM). Alloying with similar oxides like ZrO2, doping with specific elements such as Si, novel processing methods, encapsulation and annealing schemes are also some of the techniques that are being explored to target structural modifications and stabilization of the non-centrosymmetric phase. In this study, we utilized synchrotron-based x-ray diffraction in the grazing incidence in plane geometry (GIIXRD) to determine the crystalline phases in hafnia-zirconia (HZO) compositional alloys deposited by atomic layer deposition (ALD). Here we compare and contrast the structural phases and ferroelectric properties of mechanically confined HZO films in metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) structures. Both MIM and MIS structures reveals a host of reflections due to non-monoclinic phases in the d-spacing region between 1.75Å to 4Å. The non-monoclinic phases are believed to consist of tetragonal and orthorhombic phases. Compared to the MIS structures a suppression of the monoclinic phase in MIM structures with 50% zirconia or less was observed. The correlation of the electrical properties with the structural analysis obtained by GIIXRD highlights the importance of understanding the effects of the underlying substrate (metal vs. Si) for different target applications.

Physics of ferroelectric differential capacitance based upon free energy, and implications for use in electronic devices

In this paper, it is shown that for stable, steady state operation of devices typical of microwave and millimeter wave electronics, no negative differential capacitance is possible with conventional thinking. However, it may be possible, with strain engineering of materials, to obtain some if not all elements of the differential capacitance tensor which are negative. Rigorous derivations are provided based upon analyzing the physics using thermodynamic phenomenological free energy. It should be emphasized that, even with strain engineering, and possible discovery of some negative capacitive elements, stable operation will not be obtained because the thermodynamics precludes it.

Ferroelectric differential capacitance affected by stresses and strains and implications for use in electronic devices

AbstractThere has been a continuing interest in finding ways to shrink the dimensions of electronic devices, especially active devices like transistors, down to become compatible with the ever decreasing size scales, using fabrication processes such as at Intel, with the trend toward 10 nm. Eventual gate length in MOSFETs is roughly half this dimension. This had been one of the (not the only one) of interests in using ferroelectric (FE) materials in the gate of FETs, for example, when the FE material displays a negative polarization effect. Such ideas as escaping the Boltzmann tyranny have loomed large, regarding the subthreshold voltage, and have motivated this interest. It is generally understood that for stable, steady state operation of devices typical of microwave and millimeter wave electronics, no negative differential capacitance is possible with conventional thinking. However, it may be possible, perhaps in transient regions of the hysteretic curve, with strain engineering of materials, to obtain some if not all elements of the differential capacitance tensor which are negative. The analysis is based upon analyzing the physics using thermodynamic phenomenological free energy.

Negative capacitance phenomena depending on the wake-up effect in the ferroelectric Si:HfO2 film

Negative capacitance (NC) phenomena is recently suggested as a breakthrough that can lead to the improved transistor by decreasing the subthreshold swing. To understand the NC phenomena, we investigated the effects of the ferroelectric properties such as the remnant polarization depending on the switching cycles in Si-doped HfO2 thin film capacitor. The ferroelectric properties were controlled by using the wake-up effect, in which the remnant polarization enhances as the number of switching cycles increase. The relation between the wake-up effect and the voltage drop region with the negative differential capacitance were elucidated. The dynamic hysteresis loops were fitted based on Landau-Khalatnikov equation, and the free energy as a function of polarization was obtained. The Landau coefficients showed that the double-well feature of the free energy becomes more apparent due to the wake-up. Based on the wake-up effect on NC phenomena, we show that the NC phenomena is well described by Landau-Ginzburg theory of ferroelectrics.

Resistive switching functional quantum-dot light-emitting diodes

This study demonstrates quantum-dot light-emitting diodes (QD-LEDs) with a function of resistive switching memory, capable of on/off operation at the same driving current depending on reset/set state. The QD-LEDs were fabricated by spin-coating process and experienced two different annealing conditions, which yielded defective or less-defective V2O5–x layer. One of the annealing conditions produced QD-LEDs with the unusual electrical behaviors of negative differential resistance (NDR), capacitance oscillation, and voltage–current hysteresis curves, signifying so-called resistive switching characteristics. X-ray and ultraviolet photoelectron spectroscopies were used to examine the chemical state of the differently annealed V2O5–x layers. The less stoichiometric V2O5–x layer was found to be responsible for the resistive switching behaviors of the NDR and the low and high resistance states (LRS and HRS, respectively). We discuss the LRS/HRS of V2O5–x for resistive switching in terms of a conductive filament effect, induced by microstructural changes caused by oxygen drift and vacancy annihilation processes in the high defect density V2O5–x layer.

Comparative Study of Negative Differential Capacitance in Ferroelectric Capacitors: P(VDF0.75-TrFE0.25) and P(VDF0.50-TrFE0.50)

The transient response of the negative capacitance (NC) in a P(VDFSUB0.75/SUB-TrFESUB0.25/SUB) ferroelectric capacitor is experimentally compared against that of a P(VDFSUB0.50/SUB-TrFESUB0.50/SUB) ferroelectric capacitor. To measure first-hand the negative capacitance of the capacitor, a basic resistor-capacitor (RC) series circuit consisting of an external resistor and the ferroelectric capacitor was constructed. It was verified that the time duration of the NC could be adjusted based on the value of the external resistor. It was also confirmed that the transient response of the NC of the P(VDFSUB0.75/SUB-TrFESUB0.25/SUB) ferroelectric capacitor versus that of the P(VDFSUB0.50/SUB-TrFESUB0.50/SUB) ferroelectric capacitor was more sensitive to variation in the external resistor values. This is attributed to the reason that the polarization switching of P(VDFSUB0.75/SUB-TrFESUB0.25/SUB) ferroelectric capacitor is faster than that of the P(VDFSUB0.50/SUB-TrFESUB0.50/SUB) ferroelectric capacitor.

Characterization of the Al/Ge/In2Se3/Ga2S3/Al hybrid tunneling barriers designed for Gigahertz/Terahertz applications

In the current work, we report the design and performance of a tunneling amorphous thin film hybrid device made of Ge/In2Se3/Ga2S3 sandwiched between two aluminum thin films. Each of the stacked semiconducting layers are of 200 nm thicknesses. The hybrid device which is composed of a p-n junction between two Schottky shoulders is designed to have two valence band offsets of 0.59 and 0.84 eV at the Ge/In2Se3 and at In2Se3/Ga2S3 interfaces, respectively. The offsets which caused two quantum confinements forces the device to exhibit field effect assisted thermionic and thermionic transport mechanisms under the reverse and forward biasing conditions, respectively. When an alternating current signal is imposed between the terminals of the device, the device conducted by the quantum mechanical tunneling of charge carriers and by the correlated barrier hopping above and below 0.180 GHz, respectively. The hybrid structure are also observed to exhibit series and parallel resonance at the switching frequency between the two conduction mechanisms. It also exhibited negative differential capacitance effect in the frequency domain of 0.18–1.80 GHz. For the Al/Ge/In2Se3/Ga2S3/Al hybrid structure the microwave cutoff frequency reached 100 GHz. The latter value is promising as it indicates the ability of using the device as fast switches and microwave/Tera wave traps.

Origin of Negative Capacitance in Bipolar Organic Diodes

Negative differential capacitance (NC) occurring at low frequencies in organic light-emitting diodes (OLEDs) is a poorly understood phenomenon. We study the origin of the NC effect by systematically varying the number of electron traps in OLEDs based on the polymeric semiconductor poly(p-phenylene vinylene). Increasing the electron trap density enhances the NC effect. The magnitude and observed decrease of the relaxation time is consistent with the (inverse) rate of trap-assisted recombination. The absence of NC in a nearly trap-free light-emitting diode unambiguously shows that trap-assisted recombination is the responsible mechanism for the negative contribution to the capacitance in bipolar organic diodes. Our results reveal that the NC effect can be exploited to quantitatively determine the number of traps in organic semiconductors in a nondestructive fashion.