Charge Storage Characteristics Research Articles

Investigation of electrical and compositional properties of SiO2/Au/SiO2 for nonvolatile memory application

In this work, the effects of annealing temperature on the compositional, morphological and electrical properties of SiO2/Au/SiO2 trilayer structure are investigated. SiO2 and Au layers were deposited using RF magnetron sputtering and e-beam evaporation techniques, respectively. Transmission electron microscopic images reveal the formation of Au nanocrystals. Atomic force micrographs show the variation in surface roughness with annealing temperature. Rutherford backscattering spectroscopy was employed to estimate the thickness of the trilayer structure. X-ray photoelectron spectroscopy results indicate the formation of Au nanocrystal and SiO2 tunneling and blocking layers. The hysteresis curve indicates the role of Au nanocrystals in charge storage characteristics of the device structure.

Visible-light enhanced charge storage characteristics of amorphous Ni-doped HfO2 films

Photo-induced changes of capacitance–voltage curves for amorphous Ni-doped HfO2 films are probed under different visible light illumination conditions. The illumination-induced minority carriers injection effect enhances the negative shift of flat band voltage, and results in a significant enlargement of memory window. This enlargement exhibits negligible dependence on light wavelength but strong dependence on light intensity in the visible light region. A large memory window width of 6.12 V is obtained under illumination using 650 nm red light with an intensity of 5 mW cm−2. Acceptable endurance and retention properties show potential applications on new-type photosensitive nano-floating-gate nonvolatile memory devices.

Role of annealing temperature on charge storage characteristics of Au nanocrystals with HfO2 tunneling and blocking layers

In this paper, we demonstrate the fabrication and charge storage characteristics of Au nanocrystals located between two hafnium oxide films acting as the tunneling and blocking layers serving as nonvolatile memory structure. The structure, composition and electrical properties of the device are drastically affected by annealing temperature. The inter-diffusion and stoichiometric changes of various layers are investigated using X-ray Photoelectron Spectroscopy and Rutherford Backscattering Spectroscopy. Electrical measurements show an improvement in leakage characteristics with annealing. The C-V hysteresis loop with and without Au nanocrystals are compared to understand charge storage behavior of the device structures. The occurrence of well-defined hysteresis loop with Au nanocrystals indicates significant role of Au nanocrystals in charge storage characteristics of the device.

Promising carbon nanosheet-based supercapacitor electrode materials from low-grade coals

The abundantly available low-grade coals were efficiently used to produce supercapacitor electrode materials by using wet-chemical methods. The low-grade coals were first chemically converted to graphene-like carbon nanosheets (GCNSs), and then their electrochemical parameters were extensively investigated to know their basic supercapacitor properties. The formation of the GCNSs was established by using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopic (XPS) techniques. The carbon flakes of the size range of 2–50 μm were found to present in the coal-derived carbon nanomaterials. HRTEM analysis revealed the formation of layered GCNSs along with micropores/mesopores during the oxidative processes. The thermally stabled coal-derived GCNSs were found as promising electrode materials for supercapacitor demonstrating novel charge storage characteristics. The specific capacitance value of ∼125, 84 and 95 F g−1 with high rate capability over 1000 cycles were observed in an acidic electrolyte (H2SO4 1M) with considerably high power density and energy density.

Dynamic modeling of a microbial fuel cell considering anodic electron flow and electrical charge storage

To describe the anodic electron flow and electric charge storage behavior of an MFC system from an electrical perspective, a dynamic model based on a novel electrical equivalent circuit is developed. Conventional equivalent circuits typically have series impedances to model the system from the standpoint of terminal quantities: output voltage and current. However, the conventional approaches do not properly explain internal anodic electron flow and double-layer charge storage characteristics of MFCs. The proposed model uses an equivalent capacitance in parallel and series resistances to accurately model and characterize the anodic electron flow, electrical charge storage, and the dynamic characteristics of both output voltage and current. Two straightforward test methods are proposed to determine the equivalent circuit parameters. Experimental results showed the validity of proposed MFC model.

Size and density controlled Ag nanocluster embedded MOS structure for memory applications

Metal-nanoclusters (NC), deposited by magnetron-based nanocluster source coupled with quadrupole mass filter (QMF) assembly having independent control over their size and density, are used in fabricating NC-based non-volatile memory (NVM) devices. The effect of diameter and density on the NVM charge storage characteristics are presented where Ag is used as the metal NC. The Ag-NC, sandwiched between HfO2 tunnel and control oxides, is deposited by using the combination of the above two instruments. No annealing is performed at any stage of the device fabrication. The largest hysteresis loop area in the capacitance-voltage (C−V) characteristics of metal-oxide-semiconductor (MOS) characteristics is observed for a cluster density of 1×1011cm−2. Further, an NC size dependent hysteresis loop area is observed with the MOS devices where the NC diameter is varied from 3 to 1.5nm keeping the NC density at 1×1011cm−2. The device performance is found to be improved with a reduction of the NC size and shows its best with the NC diameter of 1.5nm. The storage time of the NVM devices also increases with the decrease in the NC diameter and exhibits their best performances for the NCs with a diameter of 1.5nm.

Characterising the Charge Storage Mechanisms in Electrochemical Capacitors Using a Combination of Electrochemical Impedance Spectroscopy (EIS) and Step Potential Electrochemical Spectroscopy (SPECS)

Electrochemical capacitors are energy storage devices which have been demonstrated to exhibit both high specific capacitance and high specific power. Electrochemical capacitors are a promising energy storage technology due to their unique combination of specific energy and power output in addition to being relatively inexpensive and environmentally friendly. The performance of electrochemical capacitors is largely influenced by the electrode material properties. The specific properties of the electrode will determine both the nature and magnitude of the charge storage processes occurring within the electrode, such as double layer capacitance (non-faradaic) and redox reactions (faradaic; pseudo-capacitance). For example, activated carbons store charge almost exclusively via double layer capacitance, whereas metal oxides, such as ruthenium oxide and manganese oxide, will store charge via a combination of double layer and pseudo-capacitance. In metal oxides, pseudo-capacitance arises due to the reduction and oxidation of the material via proton insertion (and de-insertion), with ruthenium oxide and manganese dioxide exhibiting different charge storage characteristics. Understanding how the charge storage mechanism is influenced by material (and electrolyte) properties is vital for designing electrodes with optimised performance characteristics. However, this requires an understanding of how each charge storage mechanism contributes to capacitive performance. In terms of evaluating electrode performance, conventional electrochemical methods, such as cyclic voltammetry and constant current charge-discharge, cannot differentiate the capacitance contributions from charge storage processes involved. Characterising the different charge storage contributions from double-layer charge storage (non-faradaic) and pseudo-capacitive redox processes (faradaic) is a vital step in relating electrode performance to its material properties. In this work, both electrochemical impedance spectroscopy (EIS) and step potential electrochemical spectroscopy (SPECS) have been applied to electrochemical capacitor electrodes as a performance analysis method to determine the charge storage contributions from different processes. EIS has a number of advantages over other techniques, namely, the different time constants of electrochemical processes can be utilized to separate the different mechanisms occurring at the electrode. The SPECS experiment uses the same principle as EIS, i.e. separating the electrochemical mechanisms based on their different time constants. The SPECS procedure involves applying a small (±25 mV) potential step to the working electrode followed by a long equilibration time (300 s). This process is repeated over and entire charge-discharge cycle. By scanning at such a slow rate, the electrode has time to equilibrate at each potential, and the maximum charge storage capabilities of the electrode can be accessed. Each of the different charge storage processes occurring at the electrode has a unique time- dependent current response, and hence each potential step profile can be fitted to a model describing each of these processes. From this, values for series resistance (RS), double layer capacitance (CDL), diffusion limited capacitance (CD) and residual capacitance (CR) can be extracted. When the potential is stepped over an entire capacitor cycling range, contributions from each process can be determined at each point in the cycle. Additionally, by varying the equilibration time over which the current response is analysed, the scan rate can be effectively increased therefore the electrode behaviour can be analysed over a range of scan rates. This allows the development of a Ragone diagram for the different charge storage processes, indicating how specific charge storage mechanisms contribute to the power and energy characteristics of different electrode materials. This technique has been applied to a range of commonly used electrochemical capacitor systems including activated carbon, manganese dioxide and ruthenium oxides.

Charge storage characteristics and tunneling mechanism of amorphous Ge-doped HfOx films

Amorphous Ge-doped HfOx films have been deposited on p-Si(100) substrates by means of RF magnetron sputtering. Microstructural investigations reveal the partial oxidation of doped Ge atoms in the amorphous HfOx matrix and the existence of HfSiOx interfacial layer. Capacitance–voltage hysteresis of the Ag-/Ge-doped HfOx/Si/Ag memory capacitor exhibits a memory window of 3.15 V which can maintain for >5 × 104 cycles. Current–voltage characteristics reveal that Poole–Frenkel tunneling is responsible for electron transport in the Ge-doped HfOx film.

Durable Li-S batteries with nano-sulfur/graphite nanoplatelets composites

Sulfur composites were prepared with graphite nanoplatelets (GNPs) for the first time, wherein the GNPs served as a scaffold for tethering the nano-sulfur (nano-S) particles. Three protocols were employed: (i) an in situ route, wherein the GNPs (40wt%) were introduced in the precursor solution of nano-S, (ii) by ex situ mixing of solid GNPs (40wt%) with solid nano-S, and (iii) by using a combination of methods (i) and (ii). The sulfur particles are uniformly and contiguously attached to the ribbon like graphitic nanoplatelets in the nano-S/GNPs (ex situ 20wt%+in situ 20wt%) composite, whereas in the other two composites, the sulfur particles tend to agglomerate over different regions of the platelet fibers. A comparative study of the electrochemical performances of half-cells based on the three composites, nano-S and S revealed that the nano-S/GNPs (ex situ 20wt%+in situ 20wt%) composite retains 73% of its’ original capacity at the end of 100 cycles with a reversible capacity of 610mAhgsulfur−1. The cell also showed an outstanding rate capability for the cell capacity reverted to nearly the same value, when the C-rate was ricocheted to 0.1 C after going through higher C-rates. The role of GNPs and the dependence of the method of embedding sulfur particles on GNPs demonstrate that GNPs have tremendous promise to unlock the potential of sulfur for producing cells with reproducible, stable and durable charge storage characteristics.

All‐Solid‐State Cable‐Type Supercapacitors with Ultrahigh Rate Capability

All‐Solid‐State Cable‐Type Supercapacitors with Ultrahigh Rate Capability

Effects of thermal annealing on the charge localization characteristics of HfO2/Au/HfO2 stack

To be compatible with the mainstream nano CMOS technology and to further increase the density and to reduce power consumption of non-volatile memory, high-k dielectric will become the major technology option for next generation non-volatile memory technology. To ensure the required retention time and to maintain the scalability of floating gate memory transistor, the charge store in future memory transistor should be accompanied with the high-k insolated metal or conductive clusters, islands, or nano-particles. This work proposes a simple method to fabricate high-k isolated metal cluster array. An HfO2/Au/HfO2 stack was first grown by using atomic layer deposition (ALD) and thermal evaporation, respectively, for HfO2 and Au film deposition. After a high-temperature thermal annealing, a number of HfO2-buried Au islands with diameter of about 5 to10nm were obtained. Capacitance–voltage (C–V) measurements show that the charge storage characteristics of the Au-embedded HfO2 structure were affected greatly by the annealing conditions. Depending on the annealing temperature (it should be governed by thickness of Au layer also), the thermal annealing may lead to the formation of Au islands/clusters, the improvement of HfO2 blocking property as a result of defect removal, or the deterioration of the blocking property of HfO2 due to the crystallization of HfO2 film. Process optimization should be conducted for further improving the charge localization characteristics.

Improvement of charge storage characteristics of silicon based polyvinylidene fluoride thin film through heat treatment and multiple layer stack

To utilize polyvinylidene fluoride (PVDF) in novel integrated electrostatic devices, charge storage characteristics of PVDF thin film on wafer are analyzed and improved. Heat treatment method and layer thickness/stack are identified as two influencing factors. Among the four comparing heat treatment methods, the method of room temperature drying for 2 h followed by 90 °C baking for 3 h achieves PVDF films with better charge storage ability due to increased surface roughness and film compactness. Both initial surface potential and retain rate in 30 min increase as PVDF film thickness increases. Multiple layer stack exhibits better charge storage stability due to increased surface roughness and additional charge traps introduced by inter-layer interfaces. In this paper, optimal charge storage characteristics are obtained for three layer stack with 16 μm single layer thickness. Averagely, initial surface potential reaches −548.6 V and retain rate in 30 min reaches 91.0% @−600 V grid voltage & −6 kV needle voltage.

Multilayer graphene-based flash memory

For the memory element based on a Si/HfO2/multilayer graphene/SiO2/Si structure, the write/erase and charge storage characteristics are evaluated. A sufficiently large work function of electrons in multilayer graphene (MLG) makes it possible to increase the time of storing the charge injected into it. Using MLG in flash memory devices allows one to increase the speed and/or reduce the voltage of reprogramming such devices.

Temperature Effects on Charge Storage and Transfer of Nanocrystalline CdSe Embedded Zr-Doped HfO2 MOS Memory Device

The influence of temperature on the charge storage and transfer characteristics of the nanocrystalline CdSe embedded Zr-doped HfO2 high-k dielectric on the p-type Si wafer has been studied. With the increase of temperature, the hole-trapping capacity is increased but the electron-trapping follows the opposite trend, which results in a small reduction of the memory window. From the frequency dispersion measurement result, at high temperatures, holes are loosely trapped at the nanocrystal/high-k interface and electrons are mainly trapped in the bulk nanocrystal. The high temperature induces a larger leakage current because it enhances the charge supply and transfer in the high-k stack. At the low applied voltage, the charge transfer follows the Schottky emission mechanism. In the high, positive gate voltage region, charges are transferred following the Poole-Frenkel mechanism at the low temperature and the Fowler-Nordheim mechanism at the high temperature. The temperature has little effect on the sample's leakage current-gate voltage hysteresis due to the large charge trapping capability. In summary, temperature is an important factor in the operation of the nanocrystals embedded high-k memory device because it affects the charge storage and transfer mechanisms.

High performance pseudocapacitor based on 2D layered metal chalcogenide nanocrystals.

Single-layer and few-layer transition metal dichalcogenides have been extensively studied for their electronic properties, but their energy-storage potential has not been well explored. This paper describes the structural and electrochemical properties of few-layer TiS2 nanocrystals. The two-dimensional morphology leads to very different behavior, compared to corresponding bulk materials. Only small structural changes occur during lithiation/delithiation and charge storage characteristics are consistent with intercalation pseudocapacitance, leading to materials that exhibit both high energy and power density.

Charge storage characteristics of nonvolatile memories with chemically-synthesized and vacuum-deposited gold nanoparticles

Carrier injection and charge loss characteristics of nonvolatile memories with chemically-synthesized (CS) and vacuum-deposited (VD) gold nanoparticles (Au-NPs) have been investigated. Compared to CS counterparts, the memories with VD Au-NPs exhibit a higher dot density of 3.77 × 1011 cm−2, leading to a larger memory window. Further, the energy from valence-band edge to vacuum level (EVB_vac) of tunneling oxide for the samples with CS and VD Au-NPs is found to be 9.04 and 9.85 eV respectively. The small EVB_vac value of the memories with CS Au-NPs is resulted from the formation of a thin chemical oxide (SiOx) on thermally-grown SiO2 tunneling layer during the chemically synthesized process, contributing to a slow erasing behavior. Besides, the programming of the memories with VD Au-NPs is saturated at high gate bias, which has been well-explained by the electrons induced potential coupling between Au-NPs. Superior data retention property and high temperature dependence of charge loss are observed for the memories with CS Au-NPs, which can be ascribed to the thick tunneling oxide layer by the additional SiOx film.

Charge trapping of Ge-nanocrystals embedded in TaZrOx dielectric films

Ge-nanocrystals (NCs) were synthesized in amorphous TaZrOx by thermal annealing of co-sputtered Ge-TaZrOx layers. Formation of spherical shaped Ge-NCs with small variation of size, areal density, and depth distribution was confirmed by high-resolution transmission electron microscopy. The charge storage characteristics of the Ge-NCs were investigated by capacitance-voltage and constant-capacity measurements using metal-insulator-semiconductor structures. Samples with Ge-NCs exhibit a maximum memory window of 5 V by sweeping the bias voltage from −7 V to 7 V and back. Below this maximum, the width of the memory window can be controlled by the bias voltage. The fitted slope of the memory window versus bias voltage characteristics is very close to 1 for samples with one layer Ge-NCs. A second layer Ge-NCs does not result in a second flat stair in the memory window characteristics. Constant-capacity measurements indicate charge storage in trapping centers at the interfaces between the Ge-NCs and the surrounding materials (amorphous matrix/tunneling oxide). Charge loss occurs by thermal detrapping and subsequent band-to-band tunneling. Reference samples without Ge-NCs do not show any memory window.

Study on charge storage characteristics of PP film electret charged by interface polarization method

Electret attracts increasing attention nowadays because of its lasting and stable electrostatic field. To achieve the widespread use of electret material, higher density and better stability of the electret charge storage as well as well-distributed electrostatic field must be ensured at the same time. Based on the mechanism of interface polarization on double-layer media, a novel charging technology for electret is reported in this paper; and PP film is successfully charged through an auxiliary layer to form electret by this proposed method. Effect of charging temperature and charging voltage on the charge storage performance of the as-prepared PP film electret is investigated by means of surface potential measurement. Also its charge storage performance at high temperatures is explored by thermally stimulated discharge technique. Furthermore, its electrostatic field distribution in the directions of X and Y is measured. Results show that the interface polarization charging is more excellent than the corona charging. At a certain temperature, the surface potential of PP film electret increases with increasing charging voltage and both are in a good linear relationship. This is in good agreement with the theoretical analysis in terms of the equation of electret charge accumulation during the charging process. It is shown that in the case of constant charging voltage within the range of 0.53.0 kV, the effect of charging temperature is not obvious when the temperature is below 75 ℃; however, when the temperature is higher than 75 ℃, the surface potential of PP film electret increases with increasing temperature. In addition, its surface potential may change a little with time so it has an excellent charge storage stability. The distribution of its surface potential shows that it exhibits an homogeneous electrostatic field due to interface polarization charging.

Estimation of Equilibrium Capacitance of Polyaniline Films Using Step Voltammetry

The equilibrium charge storage characteristic of polyaniline films has been determined using step voltammetry technique. The step coulogram (plot of charge stored versus potential) consists of five distinct regions, which correspond to the different energy bands exhibited by the film, in keeping with Raman and ESR spectroscopy data. The coulogram is piecewise linear, with constant capacitance in each band. Effect of the type and concentration of the acid on the equilibrium capacitances of the bands has been studied. The polaron band has the highest capacitance, followed by the bipolaron band and then the polaron lattice band. The charge stored at a given potential is found to be proportional to film mass. The film needs to be polarized to a threshold charge before faradaic charging begins. This faradaic threshold is independent of the type and concentration of the acids. The step coulogram is compared with the sweep coulogram obtained from linear sweep voltammetry. At low sweep rates, the sweep coulogram also exhibits five distinct bands. However, with increase in the sweep rate, the bands progressively reduce in number. The capacitance of the polaron band decreases linearly with square root of sweep rate and attains the equilibrium capacitance at zero sweep rate.

Unique Charge Storage Characteristics of FEP/THV/FEP Sandwich Electret Membrane Polarized by Thermally Charging Technology

Utilizing the synergy of three processes (space charge injection, dipole orientation and interfacial polarization) which determine the electret properties, a sandwich electret membrane FEP/THV/FEP (FEP: fluorinated ethylene propylene, THV: tetrafluoroethylene-hexafluoropropylene-vinylidene) is prepared by the laminating method and the thermal charging technology. The surface potential measurement indicates that the sandwich electret membrane exhibits excellent charge storage stability. When washing the sample surface with alcohol, its surface potential first undergoes decay to zero, and then quickly restores to a high value. The surface potential value is associated with the charging electric field and temperature. The best charging condition is 18.75 MV·m−1 and 130°C. A charge storage profile is proposed, and the experimental results are in good agreement with this profile.