Charge Retention Characteristics Research Articles

Analysis on charge-retention characteristics of sub-threshold synaptic IGZO thin-film transistors with defective gate oxides

We provide a quantitative analysis on the charge-retention characteristics of sub-threshold operating In–Ga–Zn–O (IGZO) thin-film transistors (TFTs) with a defective gate-oxide for low-power synaptic applications. Here, a defective SiO2 is incorporated as the synaptic gate-oxide in the fabricated IGZO TFTs, where a defect is physically playing the role as an electron trap. With this synaptic TFT, positive programming pulses for the electron trapping are applied to the gate electrode, followed by monitoring the retention characteristics as a function of time. And this set of the programming and retention-monitoring experiments is repeated in several times for accumulating effects of pre-synaptic stimulations. Due to these accumulated stimulations, electrons are expected to be getting occupied within a deeper trap-state with a higher activation energy, which can lead to a longer retention. To verify these phenomena, a stretched exponential function and respective inverse Laplace transform are employed to precisely estimate a retention time and trap activation-energy for transient experimental results.

Fabrication and characteristics of electret ultrasonic‐sensor using laser patterning

AbstractIn this study, in an electret capacitor sensor (ECS), which is an electromechanical exchange device consisting of an electret sandwiched between two electrodes, ridges as spacers and grooves as gaps were formed in the PTFE (polytetrafluoroethylene) layer of the insulating electrode by laser ablation. The ECSs were then fabricated using PTFE electrets, and the charge retention and airborne ultrasonic wave transmitting and receiving characteristics were investigated. The charge retention characteristics of the ECS with laser‐patterned insulating electrodes were slightly lower than those of the silica‐aggregate electret. As the thickness of the insulating layer increased or the groove width decreased, the transmitting and receiving sensitivity increased significantly and then peaked and degraded at the measurements of transmitting and receiving. The ECS with the highest peak sensitivity obtained by laser patterning had a bandwidth comparable to that of the conventional ECS with silica aggregate electret, but the peak sensitivity was improved by up to 8 dB. Therefore, laser patterning could contribute to the improvement of ECS performance.

レーザパターニングを用いたエレクトレット超音波センサの製作と特性評価

In this study, in an electret capacitor sensor (ECS), which is an electromechanical exchange device consisting of an electret sandwiched between two electrodes, ridges as spacers and grooves as gaps were formed in the PTFE layer of the insulating electrode by laser ablation. The ECSs were then fabricated using PTFE electrets, and the charge retention and airborne ultrasonic wave transmitting and receiving characteristics were investigated. The charge retention characteristics of the ECS with laser-patterned insulating electrodes were slightly lower than those of the silica-aggregate electret. as the thickness of the insulating layer increased or the groove width decreased, the transmitting and receiving sensitivity increased significantly and then peaked and degraded at the measurements of transmitting and receiving. The ECS with the highest peak sensitivity obtained by laser patterning had a bandwidth comparable to that of the conventional ECS with silica aggregate electret, but the peak sensitivity was improved by up to 8 dB. Therefore, laser patterning could contribute to the improvement of ECS performance.

Sustainable charged composites with amphiphobic surfaces for harsh environment–tolerant non-contact mode triboelectric nanogenerators

In this work, the sustainable charged composites with amphiphobic surfaces are reported regarding harsh environment tolerance and robust contactless mode triboelectric nanogenerators (TENGs). SiO2 and polytetrafluoroethylene (PTFE) particles are well dispersed in thermoplastic polyurethane (TPU) and polydimethylsiloxane (PDMS), respectively. Both surfaces become very rough after a chemical etching method and the surface of PTFE-PDMS is fluorinated using a fluorinated alkylsilane, resulted in the formation of amphiphobic surfaces. This significantly enhances the environment tolerance of the contactless mode TENGs as well as the charge retention characteristics of the composites. As a practical non-contact mode application, a speed sensor fabricated using two composites can stably detect a low vehicle speed less than 3.75 km/h when exposed to high humidity (∼ 99%) and various chemical oils. Over 50% of initial output voltage are still generated after 2 weeks with no more decrease after 5 days, showing a 1.4 times slower charge decay rate, compared with the reference Ni-PTFE based TENG.

Large-Area Arrays of Polymer-Tethered Gold Nanorods with Controllable Orientation and Their Application in Nano-Floating-Gate Memory Devices.

In this work, it is reported that large-area (centimeter-scale) arrays of non-close-packed polystyrene-tethered gold nanorod (AuNR@PS) can be prepared through a liquid-liquid interfacial assembly method. Most importantly, the orientation of AuNRs in the arrays can be controlled by changing the intensity and direction of electric field applied in the solvent annealing process. The interparticle distance of AuNR can be tuned by varying the length of polymer ligands. Moreover, the AuNR@PS with short PS ligand are favorited to form orientated arrays with the assistance of electric field, while long PS ligands make the orientation of AuNRs difficult. The orientated AuNR@PS arrays are employed as the nano-floating gate of field-effect transistor memory device. Tunable charge trapping and retention characteristics in the device can be realized by electrical pulse with visible light illumination. The memory device with orientated AuNR@PS array required less illumination time (1 s) at the same onset voltage in programming operation, compared to the control device with disordered AuNR@PS array (illumination time: 3 s). Moreover, the orientated AuNR@PS array-based memory device can maintain the stored data for more than 9000 s, and exhibits stable endurance characteristic without significant degradation in 50 programming/reading/erasing/reading cycles.

ZIF-8/ZIF-67 derived ZnS@Co-N-C hollow core-shell composite and its application in lithium‑sulfur battery

Lithium‑sulfur batteries are energy conversion devices that possess much higher energy densities than traditional lithium-ion batteries. However, the shuttle effect of the chemical reaction in the positive electrode of lithium‑sulfur batteries significantly reduces their specific capacity and charge retention characteristics. Therefore, we fabricated a novel hollow core-shell structure to serve as a separator modification layer and inhibit the shuttle effect. A ZIF-8/ZIF-67 core-shell precursor was first prepared by a room temperature solution reaction. This precursor was then treated by a vulcanization and calcination process to form the end product, in which ZnS was the core active site and CoN doped carbon nanocages were the outer shell. The discharge capacity and the cycling durability of a lithium‑sulfur battery were significantly improved by coating this novel structure on a polyethylene (PE) separator. Electrochemical measurements and microstructural analysis demonstrated that the unique hollow structure served as a polysulfide reservoir. Moreover, the Co-N-C shell and ZnS core of this structure also chemically interacted with and had a catalytic effect on the polysulfides, which enhanced the cycle and rate performance of the battery. Therefore, the lithium‑sulfur battery prepared using a ZnS@Co-N-C core-shell composite-coated PE separator showed excellent electrochemical performance.

Charge transport characteristic and charge stability enhancement mechanism of polyimide aerogel as an ultralight weight electret

In this study, polyimide aerogel (PIA) is applied as an ultralight-weight and thermally stable electret material, meanwhile its charge transport characteristic and charge stability enhancement mechanism are investigated. It is found that PIA can exhibit excellent electret properties at 120 °C after corona charging at room temperature, and its charge stability acquires further promotion after charging at elevated temperatures or heat treatment, which far exceeds conventional polyimide-based electrets. Further research indicates that the outstanding charge retention characteristics of PIA originate from the enhanced local defect density, fast retrapping effect, weakened internal electric field by dipole polarization, and prolonged conduction path, which are inextricably linked to its porous structure. In addition, the high gas breakdown strength in the pores of PIA effectively limits the breakdown induced conductivity, thereby ensuring high space charge density and large piezoelectric response of PIA electrets. Considering the extremely low density and inherently excellent properties of polyimide substances, PIA may have promising prospects as an electret in the lightweight sensing and energy conversion devices at elevated temperatures.

Boosted Output Voltage of BiSbTe‐Based Thermoelectric Generators via Coupled Effect between Thermoelectric Carriers and Triboelectric Charges

AbstractThe present work demonstrates the very first approach to enhancing the output voltage of a thermoelectric generator by introducing highly charged polyimide‐based dielectrics. High charge density over 320 µC m−2 in C60‐containing block polyimide (PI‐b‐C60) having excellent charge retention characteristics is obtained through an ion injection process, attached at the cooling part of a thermoelectric generator consisting of Cu/p‐type BiSbTe/Au/Cr/SiO2/Si. This significantly increases the electric potential difference across the thermoelectric generator (TEG) from 4.2 to 8.2 mV at the temperature difference of 20 K, and this increase can be maintained over 1 day. Density functional theory studies support that the enhancement depends on the dielectric constant and the insulating properties of the dielectric. Finally, a TEG consisting of 81 p‐type BiSbTe legs with PI‐b‐C60 generates an output voltage of 0.632 V, and the charged energy of the 10 mF‐capacitor is boosted by 30 s. These results demonstrate the possibility of large‐area TEG fabrication without the need for modification of the TE materials by introducing a new mechanism which is based on the coupling effect between thermoelectric carriers and triboelectric charges.

Memory Properties of Zr-Doped ZrO2 MOS-like Capacitor

The high-k-based MOS-like capacitors are a promising approach for the domain of non-volatile memory devices, which currently is limited by SiO2 technology and cannot face the rapid downsizing of the electronic device trend. In this paper, we prepare MOS-like trilayer memory structures based on high-k ZrO2 by magnetron sputtering, with a 5% and a 10% concentrations of Zr in the Zr–ZrO2 floating gate layer. For crystallization of the memory structure, rapid thermal annealing at different temperatures between 500 °C and 700 °C was performed. Additionally, Al electrodes were deposited in a top-down configuration. High-resolution transmission electron microscopy reveals that ZrO2 has a polycrystalline–columnar crystallization and a tetragonal crystalline structure, which was confirmed by X-ray diffraction measurements. It is shown that the tetragonal phase is stabilized during the crystallization by the fast diffusion of oxygen atoms. The capacitance–voltage characteristics show that the widest memory window (ΔV = 2.23 V) was obtained for samples with 10% Zr annealed at 700 °C for 4 min. The charge retention characteristics show a capacitance decrease of 36% after 10 years.

Fabrication and characteristics of electret using fine silica powder and its application of ultrasonic sensor

Electrets were prepared by spin-coating the dispersion of fine silica powders, and their charge retention characteristics were investigated. Finer silica aggregates were formed on the PTFE surface layer by spin-coating the dispersed solution of fine silica powder than those formed by spray-coating colloidal silica. Electrets using fine silica powder also showed better charge retention characteristics than those using colloidal silica. Such an improvement in the charge retention might be due to the suppression of discharge in the excessive gap caused by coarse silica aggregates. Then, ECSs were fabricated using the prepared electrets, and their transmitting and receiving characteristics of airborne ultrasound were also investigated. The peak sensitivity of the ECS using a spin coating of fine powdered silica dispersion was slightly improved from those using spray coating of colloidal silica. On the other hand, the peak frequency and frequency band were greatly improved using spin coating of fine powdered silica dispersion. Therefore, ECS using fine silica powder is expected to be applied as a broadband airborne ultrasonic sensor.

Optimization of MIS type Non-Volatile Memory Device with Al-Doped HfO2 as Charge Trapping Layer

The reduction in physical size of the Non-volatile memory (NVM) demands the use of high-k dielectrics due to loss in charge trapping behavior. The large bandgap of Al2O3 (∼7.0 eV) proves it suitable for blocking and tunneling layer, while the high dielectric constant of HfO2 proves it suitable for charge trapping layer (CTL). In this paper, we propose the application of Al doped HfO2 used as CTL. The doping concentration has been varied by varying the number of sub-cycles (sequential cycles of HfO2, Al2O3) by ALD system. The optical energy bandgap of Al doped HfO2 observed to be increases from 4.91−5.20 eV as the Al incorporation content increases with 1:9−1:4 ALD sub-cycles. Oxygen vacancy defect in the thin film decreased to 17.87% for 1:4-Al doped HfO2 sample as verified from the XPS. The increase in Al incorporation leads to charge loss towards silicon substrate due to high bandgap energy. In addition, charge traps are not well performed due to reduction of internal defects in the film. After 10 years, the NVM device with Al doped HfO2 (1:9) CTL has improved charge retention characteristics by 14% as compared to a normal HfO2 charge trap layer NVM device.

Anisotropic Diffusion of Charges on Au Nanoclusters Embedded in Al2O3 Dielectrics

Recent transistors are one of state‐of‐the‐art products of semiconductor industry. As it has been required to be minimized to the nanoscale regime, the classical model from the microelectronics is often challenged by its discrepancy. Therefore, understanding the behavior of electrons in such regime is essential for transistor applications in nanoscale. Herein, the charge trapping and anisotropic diffusion behaviors of electrons in multistacked nanostructures composed of a blocking oxide, a charge storage layer of Au nanoclusters (NCs), a tunneling oxide, and silicon substrate are reported. Electrons are injected from the substrate to Au NCs via quantum tunneling effect. The charge retention and diffusion characteristics of the NCs are investigated by measuring surface potential. To explore the spatial dependence, a line‐ and dot‐shaped injection of charges is examined. Both of the accumulated electrons decay following an exponential function of time. However, the injected electrons with a line‐shape in Au NCs are found to have a larger time constant than those with a dot‐shape. This result implies that the initial spatial distribution of electrons plays a role in determining the equilibrium distribution of the charges in the nanostructure.

Graphene Oxide as a Dielectric and Charge Trap Element in Pentacene-Based Organic Thin-Film Transistors for Nonvolatile Memory.

In this report, the dielectric nature of graphene oxide (GO) was exploited for the successful implementation of low-power pentacene thin-film transistors suitable for nonvolatile memory applications. Two different types of devices were fabricated on indium tin oxide-coated glass substrates with two different metals, viz., gold and aluminum, as the source and drain contacts. The performance of the devices was analyzed from their field-effect characteristics. Both the devices showed dominant p-type charge transport behavior. The breakdown electric field was determined to be 1.02 × 108 V/m. The current transport mechanism was explained from the output characteristics using the Fowler–Nordheim tunneling theory. Capacitance–voltage (C–V) measurements have been employed to determine the value of the oxide capacitance and to examine the memory effect. The hysteresis behavior observed from the C–V characteristics show the suitability of the device for memory applications with a low operating voltage of 3 V. The charge trapping behavior of GO was explained by the energy band diagram. Frequency-dependent C–V measurements in the range 100 kHz to 1 MHz were also performed to account for the memory window obtained in the devices. The charge retention and endurance characteristics were evaluated under a constant voltage stress to check the reliability of device operation.

Optical memory characteristics of solution-processed organic transistors with self-organized organic floating gates for printable multi-level storage devices

Abstract Exploring optical memory functions in nonvolatile organic field-effect transistor (OFET) memories with top-gate/bottom-contact (TG/BC) configurations can offer effective routes for developing printable, high-density organic memory circuits capable of multi-level data storage. Here, we use a solution process to fabricate TG/BC OFET devices with organic floating-gate structures and investigate their memory characteristics under light illumination. A solution-processable organic composite of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and poly(methylmethacrylate) is employed to self-organize organic floating-gate structures on a solution-processed semiconductor layer composed of poly(3-hexylthiophene) (P3HT). The floating-gate OFET devices programmed with blue, green, and red light exhibit large threshold voltage (Vth) shifts of approximately 30 V and stable charge retention characteristics even under light illumination. The devices also exhibit high sensitivity to incident light during programming, and the degree of Vth shift and the on-state current can be tuned using light and programming voltage to facilitate distinct storage and readout of multi-level data.

High Density Formation and Magnetoelectronic Transport Properties of Fe3Si Nanodots

The application of group-IV based metallic-nanodots (NDs) such as silicide- and germanide-NDs to a floating gate in MOS memories has been attracting much attention as charge retention characteristics can be improved and they can be compatible with Si-based semiconductor devices [1]. By replacing nonmagnetic NDs with ferromagnetic NDs, floating gate MOS memories can have the new functionality to control charging and discharging properties in NDs floating gate and to improve retention characteristics by the tunnel magnetoresistance effect between NDs and ferromagnetic gate electorodes [2]. Recently, Fe3Si-NDs have received much attention as the promising materials for application to the Si-based spintronics and/or magnetic memory devices such as magnetoresistive random access memory [3]. Previously, we demonstrated the self-assembling formation of metal (Ni, Pt, Pd, Co and Fe) NDs on thermally grown SiO2 with an areal density as high as ~1012 cm-2 by simply exposing ultrathin metal films to remote H2 plasma (H2-RP) without external heating [4]. In this work, we extended our research to form high density Fe3Si-NDs and to study their magnetization and local I-V characteristics measured with a magnetic probe under external magnetic field application. After conventional wet-chemical cleaning steps for p-type Si(100) wafers, a ~3.6nm-thick SiO2 layer was grown at 850 ˚C. After that, a ~2.0nm-thick Fe layer was first deposited uniformly on the SiO2 layer by electron beam evaporation, and then uniformly with a ~2.0nm-thick amorphous Si (a-Si) layer was covered without air exposure. Subsequently, a ~1.5nm-thick Fe layer was deposited, where the film thicknesses were designed with stoichiometric ratio of Fe:Si=3:1. The Fe/a-Si/Fe trilayer stack was exposed to a H2-RP without external heating. By exposing the trilayer stack to the H2-RP, the formation of NDs with an areal density as high as ~5×1011 cm-2 and with an average height of ~10 nm was observed. Considering the fact that the temperature was raised up to ~480 ˚C in the case of a Fe foil during H2-RP exposure, this result suggests that Fe-Si alloying was promoted with agglomeration by cohesive action of Fe and Si atoms on the SiO2 surface during the H2-RP exposure. In addition, by X-ray diffraction and magnetization measurements, the formation of DO3-orderd NDs was confirmed. To evaluate the local electron transport properties of the DO3-orderd Fe3Si-NDs, a CoPtCr-coated Si cantilever was contacted with the sample surface and kept at the same position, and the I-V characteristics were measured at room temperature, where electrons are injected from the tip. Without any external magnetic field application, the current level was slightly increased with tip bias over ~-3.5 V. When applying a magnetic field of 0.5 kOe in the same direction as the tip magnetization, a distinct increase in the current level was detected, namely, the threshold voltage which is defined as a bias voltage at 0.1 nA was decreased from -4.0 to -0.3 V. With an increase in the magnetic field over 0.6 kOe, no further changes in the I-V curve were observable, which means that the resistance was kept at a significantly decreased level under magnetic fields of 0.5k Oe and over. When the CoPtCr-coated tip was replaced by a Rh-coated tip, almost no change was detected in the I-V curve by the application of a magnetic field of 4.5 kOe, irrespective of the field direction. We also confirmed a remarkable decrease in the current level and an almost complete recovery to the initial current level by applying a magnetic field of 0.5 kOe in the opposite direction to the first magnetization at 0.5kOe. Noted that, when the magnetic field of 0.5 kOe was applied in the same direction again, remarkable increase in the current level was observed. Here, the magnetization direction of the CoPtCr-coated tip is not affected by the magnetic field of 0.5 kOe. Therefore, the observed increase and decrease in the current level, namely, the change in the magnetoresistance can be interpreted in terms that the magnetization of the Fe3Si-NDs was aligned in parallel or antiparallel to that of the CoPtCr-coated tip by the external magnetic field.

Graphene quantum dots as charge trap elements for nonvolatile flash memory

In this study, we investigated the charge storage properties of graphene quantum dots (GQDs) by embedding them between poly(methylmethacrylate) (PMMA) sheets as control and tunneling dielectric layers in a silicon metal-oxide-semiconductor-based nonvolatile memory device (Ag-Pd/PMMA/GQDs/PMMA/Si). The conduction mechanism through the gate dielectric stack embedded with GQDs could be explained by Fowler Nordheim (F–N) tunneling. The GQDs trapped electrons to yield a maximum memory window of ∼5.45 V (at ± 7 V) with a low leakage current density. A control experiment without GQDs obtained very low hysteresis (∼7 mV) even when the gate voltage was swept from +8.5 V to –8.5 V. Thus, the memory window obtained in the present study originated from the GQDs and not from the trapped states in the PMMA/Si interface. The maximum charge density was determined as 12.1 × 1011 cm−2, which is comparable to or even better than that found in conventional semiconductor devices. The endurance and charge retention characteristics were examined to assess the reliability of device operation when the devices were put under a constant stress of ±12 V. The flat-band voltage shifts were determined as 0.16 V and 0.23 V during the programming and erasing operations, respectively, and the charge loss was only ∼14.9% even after 105 s, thereby demonstrating the excellent charge confinement property of the GQDs.

Memory Functions of Cadmium Sulfide Embedded Zr-Doped HfO2 High-k Dielectrics

Nonvolatile memory characteristics of CdS embedded Zr-doped HfO2 high-k MOS capacitors have been studied. The n-type nature of the CdS caused the preference of hole-trapping in the bulk high-k layer. The lack of frequency dispersion of the capacitance-voltage curve was due to the passivation of defects at the CdS/high-k interface. The hole trapping/detrapping mechanism was verified with the current-voltage measurement. The loosely trapped holes were quickly released upon the release of the stress voltage. The embedded CdS layer maintained the hole trapping capability after the dielectric breakdown. About half of the originally trapped charges were retained after releasing the stress voltage for 10 years. The CdS embedded samples showed good memory and charge retention characteristics. Therefore, the CdS embedded Zr-doped HfO2 gate dielectric structure is suitable for low voltage nonvolatile memory applications.

A metal/Ba0.6Sr0.4TiO3/SiO2/Si single film device for charge trapping memory towards a large memory window

In this study, we present a metal/Ba0.6Sr0.4TiO3/SiO2/Si (MBOS) structure for charge trapping memory, where the single Ba0.6Sr0.4TiO3 film acts as the blocking layer and charge trapping layer. This MBOS device structure demonstrates excellent charge trapping characteristics, a large memory window up to 8.4 V under an applied voltage of ±12 V, robust charge retention of only 4% charge loss after 1.08 × 104 s, fast switching rate, and great program/erase endurance. These attractive features are attributed to the high density of defect states in the Ba0.6Sr0.4TiO3 film and its inter-diffusion interface with SiO2. The properties of defect states in the Ba0.6Sr0.4TiO3 film are investigated through measurements of photoluminescence and photoluminescence excitation spectroscopy. The energy levels of these defect states are found to be distributed between 2.66 eV and 4.05 eV above the valence band. The inter-diffusion at the Ba0.6Sr0.4TiO3/SiO2 interface is observed by high-resolution transmission electron microscopy. More defect sites were created to obtain a better charge trapping capability and retention characteristics.

Unipolar resistive switching in cobalt titanate thin films

We report giant resistive switching of the order of 104, long-time charge retention characteristics up to 104 s, non-overlapping SET and RESET voltages, Ohmic in low-resistance state (LRS) and space charge limited current (SCLC) mechanism in high-resistance state (HRS) properties in polycrystalline perovskite cobalt titanate thin films. Impedance spectroscopy study was carried out for both LRS and HRS states which illustrates that only bulk resistance changes after resistance switching, however, there is a small change (<10% which is in the pF range) in the bulk capacitance value in both states. These results suggest that in the LRS state current filaments break the capacitor in many small capacitors in a parallel configuration which, in turn, provides the same capacitance in both states even if there was a 90-degree change in phase angle and an order of change in the tangent loss.

Fabrication of InGaZnO Nonvolatile Memory Devices at Low Temperature of 150 degrees C for Applications in Flexible Memory Displays and Transparency Coating on Plastic Substrates.

We directly deposited amorphous InGaZnO (a-IGZO) nonvolatile memory (NVM) devices with oxynitride-oxide-dioxide (OOO) stack structures on plastic substrate by a DC pulsed magnetron sputtering and inductively coupled plasma chemical vapor deposition (ICPCVD) system, using a low-temperature of 150 degrees C. The fabricated bottom gate a-IGZO NVM devices have a wide memory window with a low operating voltage during programming and erasing, due to an effective control of the gate dielectrics. In addition, after ten years, the memory device retains a memory window of over 73%, with a programming duration of only 1 ms. Moreover, the a-IGZO films show high optical transmittance of over 85%, and good uniformity with a root mean square (RMS) roughness of 0.26 nm. This film is a promising candidate to achieve flexible displays and transparency on plastic substrates because of the possibility of low-temperature deposition, and the high transparent properties of a-IGZO films. These results demonstrate that the a-IGZO NVM devices obtained at low-temperature have a suitable programming and erasing efficiency for data storage under low-voltage conditions, in combination with excellent charge retention characteristics, and thus show great potential application in flexible memory displays.