Space Charge Limited Conduction Mechanism Research Articles

Resistive switching behavior in ZnO:Ca thin films deposited by a pulsed laser deposition technique

Calcium-doped ZnO (CZO) nanopowders were synthesized using the sol–gel method. The structural characteristics were investigated by X-ray diffraction (XRD) and the analysis reveals that our samples are crystalized in a wurtzite hexagonal structure. The morphological properties and the chemical composition of the nanoparticles were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The obtained powders are stoichiometric with crystallites in a nanometric scale aggregated in micrometric particles. Then, pulsed laser deposition (PLD) technique was used to grow Ca-doped ZnO thin films with different doping concentrations (1, 3, and 5%), on p-type Si substrates. The Ca doping effect on the electrical properties of the CZO films was investigated by current–voltage characteristics. A resistive switching (RS) effect was observed in the ITO/ZnO:Ca/Au structures. The RS behavior is dependent on the Ca doping concentration. The charge transport mechanisms of the devices were studied. In the positive bias voltage region, the transport is dominated by Ohmic and space-charge limited conduction mechanisms under low and high electric fields, respectively.

Investigation of resistive switching properties in acceptor-induced Sr(Fe,Ti)O3 thin film memristor

In this work, the amorphous SrFe0.1Ti0.9O3-δ (a-SFT10) thin film was prepared on FTO/Glass substrate by chemical solution deposition process and then annealed at 500 ℃. Step Profiler measurements and SEM images revealed that the thickness of the a-SFT10 film was around 300 nm. The a-SFT10 thin film was found to have a smooth surface but was not crystalline. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical states of the film. Fe2+ and Fe3+ coexist in a-SFT10, according to XPS results, with Fe3+ being the dominating state. The mixed valence state of iron ions may be related to oxygen vacancies. Current-voltage characteristics were conducted to study the electrical properties. The uniform switching voltages, high switch ratio and good endurance were obtained. The ratio of high and low resistance (RH/RL) is up to 103 under positive bias voltage. The physical mechanisms underlying resistance switching behaviors were also investigated. The results show that the Ohmic, space charge limited current and Schottky conduction mechanisms dominated in different voltage ranges of A-SFT10 films, respectively. The oxygen vacancies associated with the acceptor doping play a significant role in the current characteristic in Au/a-SFT10/FTO/Glass device. Our findings suggest that the device has potential applications in future resistive random access memory.

Coexistence of Write Once Read Many Resistive Switching with Negative Differential Resistance in Bi‐Rich Bi12FeO20 Ideal Sillenite

The materials showing write once read many (WORM) types of resistive switching characteristics are potential candidates for permanent storage. The negative differential resistance‐based memory devices have the potential to perform like nonlinear systems together with resistive switching simultaneously. The successful synthesis of Bi‐rich Bi12FeO20 sillenite material as phase pure thin film on fluorine tin oxide (FTO) using a solution process together with thermally deposited copper (Cu) as the top contact is demonstrated. Cu/Bi12FeO20/FTO/Glass resistive random‐access memory (RRAM) devices show the WORM characteristics with Ion/Ioff ratio103 and negative differential resistance simultaneously. Further, Bi12FeO20 active material also shows nonvolatile characteristics with analog switching for 200 cycles or more with large retention time, ≈5 × 103 s, and endurance for 103 cycles, substantiating the stability of the device. The space charge limited conduction and Schottky mechanisms explain the formation of a conductive filament in a high resistance state (HRS) and rupturing of a conductive filament in a low resistance state (LRS). The observed negative differential resistance is attributed to trapping, detrapping, and Schottky barrier interface between the top electrode and active layer together with ambient humidity.

Coexistence of Bipolar and Unipolar Resistive Switching Behavior in Amorphous Ga₂O₃ Based Resistive Random Access Memory Device

Herein, the memory device with a structure of Ag/Ga2O3/Pt has been successfully achieved, and the coexistence of bipolar resistive switching (BRS) and unipolar resistive switching (URS) behaviors are observed. The results show that the set process of bipolar and unipolar modes can be performed only when a positive voltage is applied with the same compliance current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {cc}}$ </tex-math></inline-formula> ) of 1 mA. For the reset process, the conversion process between bipolar and unipolar modes occurs by changing the polarity of sweeping voltages without <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {cc}}$ </tex-math></inline-formula> . The conduction mechanisms in two modes are conducting filaments (CFs) for the low resistance state (LRS). But for the high resistance states (HRS), it is schottky emission in BRS and space charge limited conduction mechanism in URS, respectively.

Elucidation of inter/intra molecular H-bonding mediated charge transportation mechanism in water swelled, hydrophilic organic PVA-Co3O4 nanocomposite films

In this article, we have explored the water molecules mediated charge transportation kinetics in Polyvinyl alcohol (PVA)-Cobalt Oxide (Co3O4) flexible nanocomposite films that rely heavily on the comprehension of surface and interface chemistry. Understanding the chemical interactions at the aqueous hydrophilic PVA-Co3O4 interface is essential in recognising the H-bonding revolution for complex nanocomposite films. The amount of water intake with time evolution has been monitored by the contact angle and wettability measurements. The chemical interactions and the revolution of the weak dangling bonds have been studied by Fourier transform infrared spectroscopy (FTIR), Raman and photoluminance (PL) spectra of the water-swollen hydrophilic PVA-Co3O4 nanocomposite films. The dissociation of water molecules following the fragmentation of inter/intra-molecular dangling bonds is correlated with the water diffusion kinetics and saturation in the sorption capacity of water-swollen PVA-Co3O4 nanocomposite films. The trap-states development due to the interfacial charge accumulation, after absorption of water molecules, are favourable for the trap-assisted space charge limited current conduction (TA-SCLC) mechanism and increase current density ∼ 102 times as compared to the dehydrated nanocomposite films. Meticulous tuning of the strong interfacial interactions and the manifestations of fragmented H+ ions have been identified as the primary craftsman for enhancing the dielectric constant up to eight times in the presence of 30 μL deionized water. Due to the disintegration of the free water molecules at the measurement voltage, the ac conductivity (charge transport) changes from the Jonscher's power law type variation to the jump relaxation model (JRM). The origin and accumulation of H+ ions at the grain boundary interfaces have been analyzed with impedance spectroscopic (IS) studies. An electrical equivalent circuit model is proposed for the water-mediated charge transport mechanism in swelled, hydrophilic, organic nanocomposite films.

Remotely tuned multistate resistive switching in MoS2/NiMnIn thin film heterostructure for highly flexible ReRAM application

In this article, the Cu/MoS2/NiMnIn memory structure was fabricated over stainless steel substrate for flexible electronics applications. The bipolar resistive switching characteristics were observed in two different fabricated devices (i) without sulfur vacancies (D1) and (ii) with sulfur vacancies (D2) in MoS2 thin film. Three different resistance states such as the high resistance state (HRS), intermediate resistance state (IRS) and low resistance state (LRS) have been detected for the D2 memory device. It could be ascribed to the two filamentary models based on the movement of sulfur vacancies and the formation of Cu metallic filament with applied bias voltage. Moreover, the Ohmic and modified space charge limited conduction mechanisms clearly explain the current conduction in different resistance states of device D2. The fabricated MoS2 thin film-based memory structure exhibits stable resistive switching behavior with a high OFF/ON ratio of ∼3.6 × 103, good consistency of ∼3600 endurance cycles and excellent data retention capability up to 3000 s. Moreover, the remote tuning of device D2 was thoroughly investigated and an appreciable change in SET voltage was detected with external temperature and magnetic field. Additionally, the external magnetic field altered the switching states and enhanced the multi-bit data storage capability of the memory device. This can be ascribed to the effect of the Lorentz force on ionic movement in the presence of the external magnetic field. The mechanical flexibility of the memory structure was tested for 1000 bending cycles at various bending angles in both the tensile and compressive bending modes. Hence, the present study opens up new ways for the futuristic flexible device for high data storage and neuromorphic computing applications.

Fully printed ZnO-based valency-change memories for flexible and transparent applications

Printable and flexible memory devices have attracted a great deal of attention in several emerging technological applications for the development of flexible electronics, such as interconnections/wearables/smart devices for the internet of things (IoT). In this work, we report on the fabrication of flexible, transparent, and fully inkjet-printed resistive random access memory (ReRAM) cells using poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/ZnO/PEDOT:PSS structures. The electrical characteristics were studied, including the determination of space charge limited conduction (SCLC) as the dominant charge transport mechanism. In addition, the bending performance and the transparency of the devices was tested in order to confirm the reliable operation and reproducibility of the cells. The switching for the printed structures of PEDOT:PSS/ZnO/PEDOT:PSS was led through the formation and dissolution of a stable oxygen vacancy filament, as confirmed by conductive atomic force microscopy. While the conduction mechanism for the high resistance state (HRS) was attributed to the SCLC mechanism. The switching of the memory cells, their endurance and retention properties were analysed and indicated the stability of the HRS and low resistance state for more than 104 cycles and 105 s comparable to ZnO-based ReRAM produced by clean-room techniques. The study of the mechanical flexibility of the cells shows that up to 700 bending cycles can be reached without significantly changing the switching characteristics.

A review on electrical transport properties of thin film Schottky diode

The article outlines an inclusive list of thin film Schottky diodes (TFSD) references. The review audits the fabrication and characterization of the TF metal-semiconductor (MS) diode, a TFSD. The work functions of metal (ϕ m) and semiconducting material (ϕs ) determines whether the established MS contact is ohmic or rectifying. Current-voltage (I – V) and capacitance-voltage (C – V) characterizations are essential electrical transport measures of TFSDs. The I – V and C – V outcomes are conferred, and archetypal results are parroted. The TFSD device is emblematic of a heterojunction diode (HJD). The rectification ratio (RR), saturation current (I0), ideality factor (n), Schottky barrier height (ϕB), and carrier concentration may all be calculated using I – V and C – V data. Thermionic emission (TE) and/or space charge limited conduction mechanisms (SCLC) may conduct electricity in TFSDs. Anderson’s model can theoretically be used to construct a TFSD energy band diagram.

Enhanced resistive switching properties of HfAlOx/ZrO2- based RRAM devices

Resistive Random-Access Memory (RRAM) devices are recognized as potential candidates for next-generation memory devices, due to their smallest cell size, high write/erase speed, and endurance. Particularly, the resistive switching (RS) characteristics in oxide materials have offered new opportunities for developing CMOS-compatible high-density RRAM devices. In this study, the RS behavior of HfAlOx/ZrO2 thin films sandwiched structure between TiN bottom electrode and Au top electrodes were investigated. It was found that Au/HfAlOx/ZrO2/TiN stacks were superior in terms of RS performance when compare to Au/HfAlOx/TiN memory stacks. The devices demonstrated a good resistance ratio of high resistance state and low resistance state ∼103 for Au/HfAlOx/TiN and ∼105 for Au/HfAlOx/ZrO2/TiN stacks, respectively. Both stacks showed good retention characteristics (>104 ​s) and endurance (>103 cycles). The experimental current-voltage characteristics fitted with different conducting mechanisms, the linear lower bias region is dominated by ohmic conductivity, whereas the non-linear higher bias region was dominated by space-charge limited current conduction mechanism.

Photo-electrical Characterization of New CuAlNi/n-Si/Al Schottky Photodiode Fabricated by Coating Thin-Film Smart Material

Micro/nano scale thin-film shape memory alloys (SMAs) have been used in many different miniaturized systems. Using them as thin-film metal components in fabrication of Schottky photodiodes has started a few years ago. In this work, a new SMA-photodiode device with CuAlNi/n-Si/Al structure was produced by coating nano-thick CuAlNi SMA film onto n-Si wafer substrate via thermal evaporation. The photoelectrical I-V, C-V and I-t photodiode signalization tests were performed under dark and varied artifical light power intensities in room conditions. It was observed that the new device exhibited photoconductive, photovoltaic and capacitive behaviors. By using conventional I-V method, the diode parameters such as electrical ideality factor (n), Schottky barrier height (ϕb) and rectification ratio (RR) of the produced photodevice for the condition of dark environment were computed as 12.5, 0.599 eV and 1266, respectively. As good figure of merits, the photodiode’s performance parameters of responsivity (Rph), photosensivity (%PS) and spesific detectivity (D*) maxima values determined for at -5 V reverse voltage bias and under 100 mW/cm2 of light power intensity condition are as 0.030 A/W (or 30 mA/W), 18693 and 1.33×1010 Jones, respectively. The current conduction mechanism analysis revealed that the space charge limited conduction (SCLC) mechanism is the dominant current conduction mechanism. By the drawn reverse squared C-2-V plots, the values of diffusion potential (Vd), donor concentration (ND), Fermi level (EF) and also barrier height (ϕb) were determined for the SMA-photodiode. The results indicated that the new SMA-photodiode device can be useful in optoelectronic communication systems and photosensing applications.

Correlation between the composition, phase, band structure, ferroelectric and leakage responses of Bi1-Ba Fe1- TiyO3 thin films

• Synthesis optimization to achieve correct phase. • Substitution reduces Bi 3+ volatilization, stabilizes structural phase and leakage. • Resistive and dielectric characteristics identified in Polarization hysteresis. • Mechanisms of leakage current have been identified and discussed. • Schematic band diagram with and without substituent have been constructed. Despite significant attention given to multiferroics, they are still far from being used in practical applications. This is because high electrical leakage current limited their use on a commercial scale. The current work demonstrates the effect of substitution in reducing leakage current and improving the electrical properties of BiFeO 3 films. Chemical solution deposition method is used to prepare substituted and co-substituted thin-films of BiFeO 3 on fluorine doped tin oxide glass substrate. Detailed structural analysis of synthesis optimization has been characterized, including X-ray diffraction and Raman analysis. Polarization hysteresis loops exhibit resistive characteristics, indicating leakage behavior. Specifically, high leakage current is observed in BiFeO 3 thin film. Detailed analysis revealed the presence of ohmic and space charge limited conduction mechanisms. Substitution of Ba 2+ for Bi 3+ reduces Bi 3+ volatilization and suppresses the leakage current. The co-substitution of Ba 2+ and Ti 4+ is observed to be effective in reducing leakage characteristics in films.

Effect of annealing temperature on the physical properties of NiO thin films and ITO/NiO/Al Schottky diodes

Herein, fabrication and characterisation of p-NiO/Al Schottky barrier diodes have been realised by dc reactive magnetron sputtering. In addition, effect of post-annealing temperature was studied. A detailed investigation on the physical properties of the NiO thin films was carried out using structural, morphological, optical, and spectroscopic characterisation tools. The metal–semiconductor interface behaviour has been analysed by current–voltage characterisation by implementing the Thermionic emission and Chueng model. The fabricated diode rectification ratio of 103 has been recorded at pm 3V, and barrier height varied from 0.53 to 1.10 eV as a function of annealed treatment. A space charge-limited conduction mechanism and tunnelling through the interface trap density of p-NiO/Al Schottky diodes were explained by power law characteristics. Furthermore, we have evaluated the Schottky device's electrical characteristics through the ac measurements which include capacitance densities, dielectric constants, tangent loss, AC conductivity, and Nyquist plots. The present work could provide a better understanding and insight into the p-NiO/Al Schottky junction.

Dielectric properties and impedance spectroscopy of NASICON type Na3Zr2Si2PO12

We report the temperature dependent dielectric properties and impedance spectroscopy investigation of Na3Zr2Si2PO12 in the frequency range of 20 Hz–2 MHz. The Rietveld refinement of x-ray diffraction pattern confirms the monoclinic phase with C2/c space group. The d.c. resistivity behavior shows its strong insulating nature at low temperatures, and follows Arrhenius law of thermal conduction with an activation energy of 0.68 eV. Moreover, detailed analysis of temperature dependent I–V data shows a transition from ohmic to space charge limited conduction (SCLC) mechanism at around 5 V applied electric field. The decrease in electric permittivity (εr) with frequency is explained based on the space polarization mechanism and its increment with temperature by thermal activation of charge carriers. The dielectric loss (D = tanδ) peak follows the Arrhenius law of thermal activation with an energy of 0.25 eV. We observe an enhancement in a.c. conductivity with frequency and temperature due to the decrease in the activation energy, which results in enhancing the conduction between defect states. Further, we observe an abrupt increase in the a.c. conductivity at high frequencies, which is explained using the universal Jonschers power law. The analysis of a.c. conductivity shows two types of conduction mechanisms namely correlated barrier hopping and non-overlapping small polaron tunnelling in the measured temperature range. The imaginary part of the electric modulus confirms the non-Debye type relaxation in the sample. The shifting of the relaxation peak towards higher frequency side with an increase in temperature ensures its thermally activated nature. The scaling behavior of the electric modulus shows similar type of relaxation over the measured temperature range. The combined analysis of electric modulus and impedance with frequency shows the short-range mobility of charge carriers.

Multifunctionality in ferromagnetic shape memory alloy-based resistive switching memory for flexible ReRAM application

Multifunctional flexible electronics is the ongoing demand for fabricating wearable data storage and communication devices. The magnetoelectric (ME) heterostructure consisting of piezoelectric (AlN) and ferromagnetic magnetic shape memory alloy [FSMA (Ni–Mn–In)] was fabricated over stainless steel (SS) substrate for resistive random access memory application. The Cu/AlN/FSMA/SS metal–insulator–metal based memory cell displays bipolar resistive switching (RS) behavior. The formation of Cu metallic filament at a particular SET voltage leads the memory cell in a low resistance state (LRS) from its pristine high resistance state (HRS). The LRS and HRS are explained well by Ohmic and space charge limited conduction mechanisms, respectively. The fabricated memory cell displays excellent endurance and data retention capability with a high OFF/ON ratio of ∼1.2 × 103. Furthermore, the multifunctionality of the ME heterostructure-based RAM was investigated by tuning the SET voltage with ambiance temperature and external magnetic field remotely. A significant change in the SET voltage could be ascribed to the temperature and magnetic field-induced strain transferred to the AlN piezoelectric layer from the magnetostrictive FSMA (Ni–Mn–In) bottom electrode. The residual Lorentz force explains the remotely tuned LRS and HRS in the transverse magnetic field for multi-bit data storage applications. Moreover, the RS characteristics remain stable even after 800 bending cycles as well as with bending angle (0°–180°). Hence, the present ME heterostructure integrated with flexible SS substrate can be a better choice for highly flexible, low-cost, and multifunctional futuristic RAM applications.

Intrinsically ionic conductive nanofibrils for ultra-thin bio-memristor with low operating voltage

Memristors integrated with low operating voltage, good stability, and environmental benignity play an important role in data storage and logical circuit technology, but their fabrication still faces challenges. This study reports an ultra-thin bio-memristor based on pristine environment-friendly silk nanofibrils (SNFs). The intrinsic ionic conductivity, combined with high dielectric performance and nanoscale thickness, lowers the operation voltage down to 0.1–0.2 V, and enables stable switching and retention time over 180 times and 105 s, respectively. Furthermore, the SNF-based memristor device in a crossbar array achieves stable memristive performance, and thus realizes the functions of memorizing image and logic operation. By carrying out variable-temperature electrical experiments and Kelvin probe force microscopy, the space charge-limited conduction mechanism is revealed. Integrating with low operating voltage, good stability, and ultra-thin thickness makes the SNF-based memristors excellent candidates in bioelectronics.

Investigations on interface charge conduction mechanisms for chemically grown manganite – Manganite structure: Hysteretic current – Voltage characteristics

In the present study, high quality LaMnO3–δ/La0·7Ca0·3MnO3/LaAlO3 (LMO/LCMO/LAO) manganite – manganite structure was successfully prepared using low cost chemical solution deposition (CSD) method. Temperature dependent resistive switching (RS) was performed across the manganite – manganite structure and various electronic processes have been employed to understand observed RS behavior. Various theoretical models have been used to understand the charge conduction involved in the hysteretic current – voltage (I–V) characteristics at different temperatures. Charge carrier conduction processes have been investigated for separate I–V cycles of RS characteristics where thermionic emission and space charge limited conduction mechanisms have been identified as appropriate charge carrier conduction mechanisms. Temperature dependent barrier height and other obtained fitting parameters have been discussed in detail.

Low Temperature Sol-Gel Processed Zirconium Oxide Based Transparent Resistive Random Access Memory Devices

In this work, solution-processed ZrO2-based resistive random-access memory (RRAM) device with ITO (Indium Tin Oxide) as a bottom electrode (BE) was fabricated at a low thermal budget by sol-gel spin coating technique. The grown devices (ZrO2/ITO) were found to be amorphous by XRD measurements and exhibit 80% transmittance in the visible region. The resistive switching characteristics of solution-processed ZrO2 thin films were investigated. The W/ZrO2/ITO device exhibits good cyclic endurance with an on-off ratio (~10) and reliable switching and good retention at room temperature. The conduction mechanism for the low resistance state is ohmic, whereas the conduction mechanism of the high resistance state is ohmic at low applied voltages, while a space charge limited conduction (SCLC) at the high applied voltages. The combined ohmic and space charge limited conduction mechanisms together indicate that the RS behavior in the ZrO2 layer is due to the formation and rupture of conducting filaments. Results suggest that solution-processed ZrO2 has great potential to develop transparent and flexible resistive random access memory devices.

Vacuum and Low-Temperature Characteristics of Silicon Oxynitride-Based Bipolar RRAM.

This study investigates the switching characteristics of the silicon oxynitride (SiOxNy)-based bipolar resistive random-access memory (RRAM) devices at different operating ambiances at temperatures ranging from 300 K to 77 K. The operating ambiances (open air or vacuum) and temperature affect the device’s performance. The electroforming-free multilevel bipolar Au/Ni/SiOxNy/p+-Si RRAM device (in open-air) becomes bilevel in a vacuum with an on/off ratio >104 and promising data retention properties. The device becomes more resistive with cryogenic temperatures. The experimental results indicate that the presence and absence of moisture (hydrogen and hydroxyl groups) in open air and vacuum, respectively, alter the elemental composition of the amorphous SiOxNy active layer and Ni/SiOxNy interface region. Consequently, this affects the overall device performance. Filament-type resistive switching and trap-controlled space charge limited conduction (SCLC) mechanisms in the bulk SiOxNy layer are confirmed.

Cu x S nanosheets with controllable morphology and alignment for memristor devices

In electrochemical metallization memristor, the performance of resistive switching (RS) is influenced by the forming and fusing of conductive filaments within the dielectric layer. However, the growth of filaments, mostly, is unpredictable and uncontrollable. For this reason, to optimize ions migration paths in the dielectric layer itself in the Al/Cu x S/Cu structure, uniform Cu x S nanosheets films have been synthesized using anodization for various time spans. And the Al/Cu x S/Cu devices show a low operating voltage of less than 0.3 V and stable RS performance. At the same time, a reversible negative differential resistance (NDR) behavior is also demonstrated. And then, the mechanism of repeatable coexistence of RS effect and NDR phenomenon is investigated exhaustively. Analyses suggest that the combined physical model of space-charge limited conduction mechanism and conductive filaments bias-induced migration of Cu ions within the Cu x S dielectric layer is responsible for the RS operation, meanwhile, a Schottky barrier caused by copper vacancy at the Cu x S/Cu interface is demonstrated to explain the NDR phenomenon. This work will develop a new way to optimize the performance of non-volatile memory with multiple physical attributes in the future.

Effects of the Operating Ambiance and Active Layer Treatments on the Performance of Magnesium Fluoride Based Bipolar RRAM.

This study investigates switching characteristics of the magnesium fluoride (MgFx)-based bipolar resistive random-access memory (RRAM) devices at different operating ambiances (open-air and vacuum). Operating ambiances alter the elemental composition of the amorphous MgFx active layer and Ti/MgFx interface region, which affects the overall device performance. The experimental results indicate that filament type resistive switching takes place at the interface of Ti/MgFx and trap-controlled space charge limited conduction (SCLC) mechanisms is dominant in both the low and high resistance states in the bulk MgFx layer. RRAM device performances at different operating ambiances are also altered by MgFx active layer treatments (air exposure and annealing). Devices show the better uniformity, stability, and a higher on/off current ratio in vacuum compared to an open-air environment. The Ti/MgFx/Pt memory devices have great potential for future vacuum electronic applications.