Space Charge Limited Conduction Research Articles

Dielectrophoretic alignment and electrical characterization of CuO nanowire-based systems

Dielectrophoresis is used to assemble nanowires between metallic electrodes to form scalable functional interconnects. The dielectrophoresis parameters are investigated for semiconductor copper oxide (CuO) nanowires that are desirable for energy conversion and storage, gas sensors and nanoelectromechanical systems. Experimental yields of multiple- and single-nanowire interconnects are explored at dielectrophoresis frequencies from 500 Hz to 500 kHz. The electrical properties of nanowire-electrode physical contact interfaces formed by dielectrophoresis, metal deposition, and dry mechanical transfer are investigated. The electrical transport mechanism in these interconnects is determined to be ohmic conductivity at small electric field, which is overtaken by an order of magnitude higher space charge limited conductivity for electric fields above 105 V/m. The developed method is demonstrated as a promising route to produce nanowire interconnects towards scalable on-chip nanoelectromechanical systems.

Spray pyrolysis deposited iron tungstate memristive device for artificial synapse application

Proliferation in research investigations on Neuromorphic computing applications is evidenced recently, with memristor as a promising and potential candidate. For such an application, an electronic synaptic device is an essential building block. We utilize the spray deposition technique to fabricate a tungstate (FeWO4, FWO) based RS device (Ag/FWO/FTO), which can mimic the basic and advanced synaptic properties. Further, we use X-ray diffraction, field emission scanning electron microscopy, and energy-dispersive X-ray techniques to characterize the so formed device. Moreover, we report the structural refinement of the X-ray diffractograms through the Rietveld refinement method. The electrical measurement reveals that the RS device exhibits analog-type bipolar RS characteristics amenable to modulation using external bias. In addition, we investigate different memristive properties and exemplify that the fabricated device is a non-ideal memristor device using the double valued charge-flux feature. Additionally, the device emulates bio-synaptic characteristics such as potentiation-depression and four complex spike-time-dependent plasticity learning rules. We also report the conduction mechanism of the device dominated by trap-controlled space-charge limited conduction and Ohmic conduction mechanisms. A possible filamentary RS mechanism was formulated based on experimental electrical results and charge transport fitting results. The successful mimicking of all four complex STDP learning rules by a single device reflects the uniqueness of the present work. The results of the present investigation portrayed that the iron tungstate is a promising RS material for neuromorphic computing applications.

High Curie temperature BiFeO3-BaTiO3 lead-free piezoelectric ceramics: Ga3+ doping and enhanced insulation properties

BiFeO3-BaTiO3 is a promising high-temperature lead-free piezo-ceramics due to its high Curie temperature (TC > 500 °C) and excellent piezoelectric properties. However, a high leakage current was often detected in this system, which severely affects its applications. In this work, the 0.7BiFe(1−x)GaxO3-0.3BaTiO3 (BFGax-BT, 0.00 ≤ x ≤ 0.10) system was designed to reveal the reason of leakage-current density decreased by their leakage mechanism. Because of the suppression of generation of Fe2+ and oxygen vacancy (VO∙∙), the leakage mechanism from space charge limited conduction mechanism found in Ga-free BF-BT ceramics turns into Ohmic conduction and Schottky emission mechanism as Fe3+ was replaced with Ga3+. A good combination of piezoelectric properties (d33 = 174 pC N−1 and kp = 29%) and high Curie temperature (TC = 497 °C) was achieved in the BFGa0.06-BT ceramic owning the lowest leakage-current density. This work will provide the clues for preparing the high insulation and high piezoelectric performance BF-BT-based ceramics by suppressing the valence change of Fe3+ and VO∙∙ appearance.

Gamma Irradiation Effects on n-ZnSe/n-Si Isotype Heterojunctions

To get an insight into the isotype heterojunction (IHJ) properties, the influence of gamma irradiation (GI) on the structural and electrical properties of n-ZnSe/n-Si has been presented. The ZnSe thin films were deposited onto the n-Si substrate by thermal evaporation technique. The X-ray diffraction (XRD) studies revealed the nanocrystalline nature of ZnSe thin films with prominent (111) orientation. The gamma irradiated samples displayed no crystallographic phase transformation up to 10 kGy irradiation doses. But noticeable and inconsistent modifications in the different lattice parameters were observed due to irradiation-induced effects. From the analysis of I-V characteristics, it has been found a similar trend in the variation of lattice mismatch, Schottky barrier height and interface trap parameter at different irradiation doses. Thus demonstrating the poor rectification properties of n-ZnSe/n-Si IHJs due to intrinsic and gamma-induced defects, and their role in the space charge limited conduction (SCLC) mechanism that significantly dominating over the thermionic emission (TE) mechanism across the barrier.

Studies of Structural, Electrical, and Magnetic Characteristics of Double Perovskite Ceramic: La2FeMnO6

Herein, detailed studies of structural, electrical, and magnetic characteristics of a double perovskite lanthanum iron manganate (La2FeMnO6) are reported. From the Rietveld analysis of X‐ray diffraction data and pattern, the orthorhombic structure of the compound is determined. The least‐squares refined lattice parameters of the compound are: a = 5.5370 Å, b = 7.7988 Å, and c = 5.5069 Å. The frequency–temperature dependence of the dielectric parameters is explained by the Maxwell–Wagner model to provide the polarization mechanism. Analysis of temperature dependence of impedance and modulus data of different frequency has shown the contributions of grains and grain boundaries in the electrical characteristics of the material. This analysis also shows the semiconductor characteristics, through a negative temperature coefficient of resistance nature. Based on both impedance and modulus spectroscopy, the dielectric relaxation process is found to be of a non‐Debye type. The study of scaling behavior of impedance and modulus data suggests that relaxation phenomena are nearly independent of temperature. Detailed analysis of the frequency dependence of conductivity data of different temperatures using the Jonscher's power law has shown the existence of different types of conduction mechanisms in the material. The nature of the current–voltage (I–V) curves suggests that the material behaves as a semiconductor diode type to be used for the rectifying purpose. The conduction mechanism follows the space charge‐limited conduction (SCLC) process which is obtained from leakage current analysis. The study of magnetic hysteresis of the material suggests that the material exhibits soft ferromagnetic ordering.

The effect of the top electrode on the switching behavior of bipolar Al2O3/ZnO RRAM

In this letter, the atomic layer deposited (ALD) resistive random access memory(RRAM) device with Au/Al2O3/ZnO/FTO structure (ox-RRAM) exhibiting bipolar characteristics is proposed to compare with Ag/Al2O3/ZnO/FTO (conduction bridge RRAM(CBRAM)) device to demonstrate the effect of electrode material on device behavior. Although both devices have undergone similar processes during the fabrication, the proposed device with Au top electrode shows oxygen vacancy-based valance change mechanism (VCM) of switching with two stable states. The latter follows conductive metallic bridge-based switching (CBRAM) with multilevel storage. However, both devices have shown self-compliance feature, with a maximum current settled to 10–20 mA. Also, they have distinguishable memory window (Roff/Ron) > 102. The current conduction mechanism in both devices follows ohmic conduction and space charge limited conduction (SCLC) at low resistance state (LRS) and high resistance state (HRS), respectively. It reveals that the stacking of oxide layers and deposition method influence the electronic conduction mechanism, whereas the top electrode has a prominent role in the switching mechanism. It is also reported that the endurance and retention of the proposed ox-RRAM are better than its CBRAM counterpart.

Structural and Electrical Characteristics of FeTiVO6 Double Perovskite

In this paper, studies of structural as well as electrical characteristics of the double perovskite material FeTiVO6 (iron titanium vanadate), synthesized by a high-temperature mixed oxide reaction method have been discussed. The room temperature X-ray diffraction analysis confirms the formation of a single-phase orthorhombic structure without any secondary phase. All the electrical characteristics (i.e., dielectric, impedance, conductivity and modulus) of the sample, studied at various temperatures (25–300°C) and frequencies (1[Formula: see text]kHz–1[Formula: see text]MHz), provide many remarkable characteristics of the material. The dielectric parameters as a function of frequency explain the presence of different polarization mechanisms based on the Maxwell–Wagner double-layer model. Impedance analysis describes the grain (bulk) and grain boundary (bulk interior) effect on the material using the equivalent RQC-RC circuits. The presence of non-Debye type of relaxation behavior in the material is confirmed by the depressed semicircles of Nyquist plots. The conductivity study provides information about the CBH and OLPT type of conduction phenomenon. The temperature dependence of leakage current behavior follows the Ohmic (semiconductor) and space charge limited conduction mechanisms at a different range of applied fields. The occurrence of the room temperature hysteresis loop obtained from the PE loop tracer confirms the ferroelectric behavior of the studied compound.

Inkjet-printed bipolar resistive switching device based on Ag/ZnO/Au structure

In this Letter, we report an inkjet-printed resistive switching device based on an Ag/ZnO/Au structure. The device exhibits bipolar resistive switching behavior, a low operation voltage of about 0.7 V, a high on/off ratio of 107, a long retention time exceeding 104 s, and good endurance. The conduction mechanism of the device in low and high resistive states was studied and showed good consistency with the theory of Ohmic and space charge limited conduction mechanisms, respectively.

Electroforming-Free Bipolar Resistive Switching Memory Based on Magnesium Fluoride.

Electroforming-free resistive switching random access memory (RRAM) devices employing magnesium fluoride (MgFx) as the resistive switching layer are reported. The electroforming-free MgFx based RRAM devices exhibit bipolar SET/RESET operational characteristics with an on/off ratio higher than 102 and good data retention of >104 s. The resistive switching mechanism in the Ti/MgFx/Pt devices combines two processes as well as trap-controlled space charge limited conduction (SCLC), which is governed by pre-existing defects of fluoride vacancies in the bulk MgFx layer. In addition, filamentary switching mode at the interface between the MgFx and Ti layers is assisted by O–H group-related defects on the surface of the active layer.

Low-temperature electrical characteristics of nanocrystalline BaTiO3 thin films for cryogenic applications

Ferroelectric thin films are vital in a large number of applications due to their promising properties. Although ferroelectric films' electrical response is studied at high temperatures, their low-temperature behavior is not well understood. In the present study, the εr-V and I-V characteristics of BaTiO3 (BTO) thin films are explored at cryogenic temperatures. BTO thin films have systematically deposited different Ar/O2 ratios by RF sputtering and investigated their structural, microstructural, optical, and electrical properties. The microstructures of these films are well-packed with uniform grain growth and revealed lower rms roughness. The thickness of the films (~170 nm) is obtained by cross-sectional scanning electron microscope images and transmittance spectra. There is not much variation in the optical bandgap, which shows the full stoichiometry and similar thickness of the films. The εr-V response of the film measured at 387 K shows the asymmetry in dielectric constant maxima value. It signifies that there are movable ions and charge accumulation at the interface of film and electrodes. The dielectric response obtained as a temperature function exhibited a Tc of 387 K, indicating the phase transition from ferroelectric tetragonal phase to paraelectric cubic phase. The I-V characteristics of the film measured at 300 K revealed the leakage current of 10−6 Amp, whereas, at 43 K, it is 8.83 × 10−10 Amp. The slope of the I-V curve of the film measured between 1 and 5 V displayed the Ohmic nature. The slope of the film obtained at 6–10 V indicated the space charge limited conduction mechanism, which corresponds to discrete trap carriers. Therefore, trap carriers assisted discrete conduction mechanism is one of the leading factors controlling the conduction process in BTO oxide thin films. The acquired electrical response of the BTO films deposited at 50 Ar is promising for cryogenic devices.

Functional bipolar resistive switching in AlN/Ni–Mn–In based magnetoelectric heterostructure

This report explores the influence of temperature on resistive switching characteristics in the AlN/Ni–Mn–In magnetoelectric (ME) heterostructure-based resistive random access memory (ReRAM) device. The fabricated Cu/AlN/Ni–Mn–In/Si device exhibits a sharp transition from a high resistance state (HRS) to low resistance state (LRS) at a SET voltage. The rupture of the filament from its weakest point at a RESET voltage turn the device back to its HRS. The stable bipolar resistive switching behavior is described by the current–voltage (I–V) characteristic. The HRS and LRS are explained by the trap-controlled space charge limited conduction mechanism and a well-known Ohmic conduction mechanism, respectively. The temperature-dependent resistance has been observed to further confirm the conduction mechanism in HRS and LRS. The current conduction in LRS is explained by an analytical model based on copper metallic filament formation via Cu+ migration from the top to the bottom electrode. A significant change in the SET voltage has been observed with the decrease in temperature. This variation in the SET voltage is explained via strain-mediated coupling in interfacially connected AlN/Ni–Mn–In ME heterostructure. The fabricated device displays an appreciable OFF/ON ratio of the order ∼3 × 103 with good endurance and retention of ∼1000 cycles and ∼900 s, respectively. A slight variation (<40%) in SET and RESET voltages has been observed for total endurance cycles. This study demonstrates the importance of ME heterostructure for futuristic tuneable ReRAM applications.

Temperature dependent performance of ITO Schottky contacts on β-Ga2O3

Sputtered indium tin oxide (ITO) was used as a rectifying contact on lightly n-type (n ∼ 1016 cm−3) β-Ga2O3 and found to exhibit excellent Schottky characteristics up to 500 K, with no thermally driven degradation to this temperature. The barrier height extracted from current–voltage characteristics was 1.15 ± 0.04 eV at 300 K and 0.78 ± 0.03 eV at 500 K, with thermionic behavior of charge carriers over the image force lowered Schottky barriers dominating the carrier transport at low temperatures. The breakdown voltages were 246, 185, and 144 V at 300, 400 and 500 K, respectively. At 600 K, the diodes suffered irreversible thermal damage. The diode on/off ratio was &amp;gt;105 for reverse biases up to 100 V. At higher reverse voltage, the current shows an I ∝ Vn relationship with voltage, indicating a trap-assisted space-charge-limited conduction (SCLC) mechanism. We observed this SCLC relation when the reverse voltage was larger than 100 V for 300 and 400 K and at &amp;lt;100 V at 500 K. The ITO can also be used to make Ohmic contacts on heavily doped Ga2O3 suggesting the possibility of completely optically transparent power devices.

Space-charge limited ionic conductivity enhancement in gel polymer electrolyte capacitors by embedding nanoparticles

In this work, a novel mixture of ionic liquid gel polymer electrolyte (ILGPE), containing 1-butyl-3-methylimidazolium bromide, ammonium acetate and poly(vinyl alcohol), with Si@C and SiO2 embedded nanoparticles is fabricated. The resulting CILGPE is deposited onto interdigitated electrodes. The fabricated electrolytes have been characterised by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and the resulting capacitors have been characterised by impedance spectroscopy (IS) and linear sweep voltammetry (LSV) and modelled. The specific capacitance for capacitor with ILGPE is 0.042Fcm−3, while for the capacitor with SiO2 and Si@C embedded in ILGPE are 0.072 Fcm−3 and 0.06 Fcm−3, respectively. Therefore, the addition of SiO2 and Si@C nanoparticles improves the capacitance by 71% and 42%, respectively. The results of fitting I-V curves exhibit reductions in both the activation voltage (Vtr) and in the voltage that indicates the traps are filled (VTFL) with the addition of nanoparticles, revealing an increase in charge contribution. We propose that the capacitance enhancement is due to the contribution of nanoparticles to: (i) the creation new effective ion transport pathways, (ii) ion dissociation in the electrolyte and (iii) the crystallinity reduction of ILGPE. This work contributes with new findings about the effect of nanoparticles on the Space-charge-limited-conduction (SCLC) mechanism and how these changes in transport mechanism are related to the series and leakage resistances, capacitance, and physicochemical properties.

Tunable Negative Differential Resistance and Resistive Switching Properties of Amorphous WOₓ Devices

The development of multifunctional electric device is of great significance for improving the integration dense of integrated circuits in the future. Herein, amorphous WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -based memristor was fabricated and the mechanism of resistance switching was studied in detail. The device exhibited a stable coexistence of bipolar resistive switching (RS) and negative differential resistance (NDR) behaviors at room temperature, which could modulated by changing the annealing time at 300 °C. The optimal high-resistance state (HRS)/low-resistance state (LRS) resistance ratio of ~30 was obtained by postannealing in 100 min. However, both RS and NDR behaviors disappeared with the crystallization of amorphous films when the annealing time was 300 min. The Ohmic conduction mechanism should be responsible for the charge transport of LRS, while the HRS transmission was corporately dominated by the trap-controlled space charge limited conduction (SCLC) and Poole-Frenkel (P-F) emission. The change of trap levels modulated by postannealing was calculated and considered to be the reason for the change of RS and NDR behaviors. Raman spectroscopy provided evidence for the formation of strong W-O bonds, which contributed to the disappearance of RS and NDR behaviors.

Tunable multiferroic and forming-free bipolar resistive switching properties in multifunctional BiFeO3 film by doping engineering

The advent of multiferroic-based materials has opened the plethora for high tunable multifunctional materials and ultra-fast operation for future non-volatile memory technology. Multifunctional rhombohedral Bi0.94Y0.06Fe0.95Mn0.05O3 (BYFMO) film is grown on fluorine-doped tin oxide to investigate the electromechanical and resistive switching properties in Ag/BYFMO/FTO RRAM configuration. UV–visible absorbance spectra reveal the semiconducting behavior of BYFMO, and the band-gap is found to be 2.37 eV. The magnetic hysteresis curve manifests the soft ferromagnetic nature by suppressing the spiral spin modulated structure, supported by MFM imaging. The Y-Mn co-doped BFO possesses highly tunable piezoelectric and ferroelectric features with maximum domains preferred along 710 and 1090. Lateral domain growth is observed with the increase in tip bias voltage. The Ag/BYFMO/FTO RRAM shows distinct bipolar resistive switching behavior at the SET (ON), and RESET (OFF) processes are obtained at voltage VSET = +1.7V and VRESET = −2.8V, respectively. The memory window (ON/OFF) between high resistance state and low resistance state is about ~ 100, which can be sustained up to 100 testing cycles and 103s without any degradation, indicating that the BYFMO based device exhibits better endurance and retention properties. Moreover, the resistive switching mechanism of the device can be well explained by space charge limited current conduction, which is well supported by conducting a filamentary model. With excellent piezoelectric and resistive switching performance, the multifunctional BYFMO has enough potential for future non-volatile memory technology.

Temperature-dependent resistive switching behaviour of an oxide memristor

In this work, we report the temperature-dependent transport and resistive switching behaviour of a promising hybrid structure made of La0.7Ca0.3MnO3 (LCMO) and reduced graphene oxide (rGO). The current-voltage (IV) characteristics are non-linear across the studied temperature range of 100 K–300 K, which is also temperature-dependent. The memristive effect is most prominent at 200 K, while the reduction of hysteresis in the IV-curve with decrease in temperature is ascribed to the low thermal energy of the charge carriers. The charge transport in the SET and RESET process at different temperatures can be explained using trap-controlled space charge limited conduction mechanism for temperature >200 K and Poole-Frenkel emission at temperatures below that.

Low-Power and Tunable-Performance Biomemristor Based on Silk Fibroin.

Biomemristors have attracted significant attention because of their potential applications in logic operations, nonvolatile memory, and synaptic emulators, thus leading to the urgent need to improve memristive performance. In this work, a silk fibroin (SF)-based memristor, integrated with both low power and low operating current simultaneously, has been reported. Doping the SF with Ag and an ethanol-based post-treatment promote microcrystal formation in the bulk of the SF. This induces carrier transport along fixed, short paths and results in a low set voltage, low operating current, and high memristive stability. Such performances can greatly reduce power consumption and heat generation, beneficial for the accuracy and durability of memristor devices. The memristive mechanism of SF-based memristors with different Ag contents is the space-charge-limited conduction (SCLC) mechanism. In addition, the nonlinear transmission property of SF-based memristors suggests useful applications in bioelectronics.

Reverse leakage and breakdown mechanisms of vertical GaN-on-Si Schottky barrier diodes with and without implanted termination

This Letter studies the reverse leakage and breakdown mechanisms of vertical GaN-on-Si Schottky barrier diodes (SBDs) with and without argon-implanted termination (ArIT). The electrical leakage characteristics in the vertical GaN-on-Si SBD without edge termination sequentially go through the thermionic field emission, variable range hopping (VRH), and trap-assisted tunneling conduction mechanisms as the reverse bias increases gradually. Its leakage and breakdown mechanisms are limited by the edge electric field crowding effect. While for the vertical GaN-on-Si SBD with ArIT (ArIT-SBD), the electrons conduction at a low reverse bias, following the space-charge-limited conduction (SCLC) model, is limited by the damage-induced traps in the implanted GaN. As the reverse bias increases up to the occurrence of breakdown, the VRH and SCLC dominate the leakage mechanism of the ArIT-SBD, which stem from intrinsic traps in GaN grown on Si. A rapidly growing leakage under a low reverse bias and enhanced breakdown voltage performance in the ArIT-SBD is attributed to the charging of the damage-induced traps in implanted GaN. This Letter not only gives in-depth insights of vertical GaN-on-Si SBDs but also provides a useful design guidance of implanted termination for high-voltage power devices.

Development of eco-friendly thin film manufacturing process using poeroxo titanium complex solution and potential for resistive random access memory

Here we report on the development of thin film manufacturing processes based on new and sustainable green solutions. We focused on utilizing green based surface solutions. The peroxo titanium complex (PTC) solution-based thin film fabrication method used does not require additional heat treatment and complex processes to remove organic matter. Conditions suitable for thin film fabrication were derived through optimization of precursor concentration and reaction temperature. As a result of I-V test of TiN/TiO2/FTO samples fabricated by applying this technology, it has been proven that it can operate as a resistive random-access memory (RRAM). The measured I-V curve suggests that TiN/TiO2/FTO RRAM exhibits bipolar resistive switching (RS) characteristics through the formation of oxygen vacancy (Vo) filament conduction paths. We also used the double logarithmic plot analysis to confirm that the dominant conduction mechanism of the device is the space charge limited conduction (SCLC) model.

Solution-Processed Memristor Devices Using a Colloidal Quantum Dot-Polymer Composite

In this study, we demonstrate solution-processed memristor devices using a CdSe/ZnS colloidal quantum dot (CQD)/poly(methyl methacrylate) (PMMA) composite and their electrical characteristics were investigated. Particularly, to obtain stable memristive characteristics with a large current switching ratio, the concentration of CdSe/ZnS QDs in the PMMA matrix was optimized. It was found that with the CdSe/ZnS QD concentration of 1 wt%, the memristor device exhibited a high current switching ratio of ~104 and a retention time over 104 s, owing to the efficient charge trapping and de-trapping during the set and reset processes, respectively. In addition, we investigated the operational stability of the device by carrying out the cyclic endurance test and it was found that the memristor device showed stable switching behavior up to 400 cycles. Furthermore, by analyzing the conduction behavior of the memristor device, we have deduced the possible mechanisms for the degradation of the switching characteristics over long switching cycles. Specifically, it was observed that the dominant conduction mechanism changed from trap-free space charge-limited current conduction to trap charge-limited current conduction, indicating the creation of additional trap states during the repeated operation, disturbing the memristive operation.