Space Charge Limited Conduction Model Research Articles

Multilevel Memristive GeTe Devices

Phase‐change memories have reached an advanced degree of maturity, although, to be able to meet the increasing storage demand, multilevel capability is needed. A GeTe memristor is obtained in an amorphous state and it is subjected to a specific thermal treatment which initiates the transition toward the crystalline state. It is found that this crystalline state initialization process is highly beneficial for subsequently obtaining a large number of intermediate resistive states between the high and low resistive states. Multiple resistance levels are achieved by operating the devices in both DC sweeps and rectangular pulse modes in the low‐voltage subthreshold regime. The conduction is modeled using a space charge limited conduction model, showing three distinct conduction regions in the high resistive state which merge toward a single conduction region as the low resistive state is approached. The obtained memristors can be used as multilevel nonvolatile memories or as synapses in neuromorphic computing.

I-V and impedance characterization of a solution processed perovskite based heterojunction photodetector

Herein, we report the investigation of electrical and photo response properties of hybrid organic-inorganic perovskite based photodetector. Surface morphology of perovskite thin film examined through Field emission scanning electron Microscope (FESEM) revealed smooth surface with complete coverage of the glass substrate. Different elements present in perovskite thin films, as well as their quantitative ratio are identified by Energy-dispersive X-ray spectroscopy (EDX) analysis. The current-voltage (I-V) measurements are carried out in dark and under illumination conditions to investigate the charge transport process and photodetector performance. The important diode electrical parameters (ideality factor, saturation current, and barrier height) are extracted by the analysis of I-V characteristics using Shockley model. The observed rectification and photo response behavior is analyzed using energy level diagram. The photodetector demonstrates good photoresponse and exhibits responsivity and detectivity value of 0.4 A/W and detectivity of 3 × 1011 Jones, respectively at -5 V. The charge transport mechanisms are examined under the framework of space charge limited conduction (SCLC) model, which helps to extract the mobility, trap density and trap energy. Moreover, electrochemical impedance spectroscopy (EIS) analysis is performed at different applied voltage and employed to determine the different device parameters such as recombination and charge transport resistance, chemical capacitance, mobility and life time. The mobility estimated from impedance analysis show good agreement with the mobility calculated from SCLC model.

Plausible carrier transport model in organic-inorganic hybrid perovskite resistive memory devices

We demonstrate thermally assisted hopping (TAH) as an appropriate carrier transport model for CH3NH3PbI3 resistive memories. Organic semiconductors, including organic-inorganic hybrid perovskites, have been previously speculated to follow the space-charge-limited conduction (SCLC) model. However, the SCLC model cannot reproduce the temperature dependence of experimental current-voltage curves. Instead, the TAH model with temperature-dependent trap densities and a constant trap level are demonstrated to well reproduce the experimental results.

Temperature-dependent space-charge-limited conduction in BaTiO3 heterojunctions

We have investigated the space-charge-limited conduction (SCLC) in two different metal–insulator–metal junctions of the form: Au/BaTiO3 (BTO)/Nb:SrTiO3 (Nb:STO) and Au/BTO/La0.67Ca0.33MnO3 (LCMO) at various temperatures. The SCLC model has been employed to determine various parameters relevant to the charge conduction in these systems. While the trap density increases with decreasing temperature, the ratio of free to trapped carriers (θ) reduces for both the junctions, which can be understood as the thermally activated process. The extracted activation energies of 0.071 eV for Au/BTO/Nb:STO and 0.154 eV for Au/BTO/LCMO indicate the presence of shallow trap level. Moreover, the Fermi level at thermal equilibrium approaches the intrinsic limit with increasing temperature. Comparing both the junctions, we observe lower θ and deeper trap level in BTO/LCMO junction.

Deep-Trap States of GaN-Based Light Emitting Diodes Analyzed by Space Charge Limited Conduction Model

Deep-Trap States of GaN-Based Light Emitting Diodes Analyzed by Space Charge Limited Conduction Model

A Novel Varying-Bias Read Scheme for MLC and Wide Temperature Range TMO ReRAM

Resistance of transition metal oxide (TMO) resistive random access memory (ReRAM) depends sharply on temperature, resulting in drastic memory window loss at high temperature. Thus, it is difficult to design the ReRAM that can serve a wide range of operating conditions. It is especially challenging to achieve multi-level-cell (MLC) ReRAM because of the large temperature dependency. This letter investigates both the temperature and read bias dependencies of WO x ReRAM, and found both can be well understood by a modified space-charge limited conduction model. Using this model, we have designed a novel read scheme that varies the read bias according to the device temperature and compensates for the temperature effect on cell resistance. Since TMO ReRAM devices depend on defect states, cell-to-cell and cycle-to-cycle variations are naturally large. An algorithm is designed to address the variability. A 1-Mb WO x ReRAM array is fabricated to both characterize the bias and temperature dependencies and verify the new read scheme. A large and constant memory window is preserved for MLC across a wide temperature range (−40 °C–125 °C), suitable for high-reliability applications.

Examination of the temperature dependent electronic behavior of GeTe for switching applications

The DC and RF electronic behaviors of GeTe-based phase change material switches as a function of temperature, from 25 K to 375 K, have been examined. In its polycrystalline (ON) state, GeTe behaved as a degenerate p-type semiconductor, exhibiting metal-like temperature dependence in the DC regime. This was consistent with the polycrystalline (ON) state RF performance of the switch, which exhibited low resistance S-parameter characteristics. In its amorphous (OFF) state, the GeTe presented significantly greater DC resistance that varied considerably with bias and temperature. At low biases (<1 V) and temperatures (<200 K), the amorphous GeTe low-field resistance dramatically increased, resulting in exceptionally high amorphous-polycrystalline (OFF-ON) resistance ratios, exceeding 109 at cryogenic temperatures. At higher biases and temperatures, the amorphous GeTe exhibited nonlinear current-voltage characteristics that were best fit by a space-charge limited conduction model that incorporates the effect of a defect band. The observed conduction behavior suggests the presence of two regions of localized traps within the bandgap of the amorphous GeTe, located at approximately 0.26–0.27 eV and 0.56–0.57 eV from the valence band. Unlike the polycrystalline state, the high resistance DC behavior of amorphous GeTe does not translate to the RF switch performance; instead, a parasitic capacitance associated with the RF switch geometry dominates OFF state RF transmission.

Investigation of resistive switching behavior of Ag/SnOx/ITO device

SnOx thin film was deposited by reactive magnetron sputtering and the resistance switching behavior of Ag/SnOx/ITO was investigated. The endurance testing indicates that HRS resistance decreases with an increase in the number of cycles. After annealing, the memory performance is enhanced, and the ratio of the device resistance of HRS and LRS increases greatly. The abnormal transformation sequence from HRS to LRS was observed for the annealed device and can be explained by electron trapping and detrapping based on the analysis of x-ray diffraction and the Raman spectrum. The temperature-dependent I−V measurement indicates that the thermal activation process is responsible for the temperature range of 300 to 200 K; however, the carrier transport can be ascribed to the nearest-neighbor hopping conduction mechanism for the temperature range of 200 to 100 K. The general conduction mechanism of Ag/SnOx/ITO device can be elucidated by the trap-controlled space charge limited conduction model, and the conductive schematic in the SET and RESET processes has been given.

Effect of the Luminescent Layer Thickness on the Conduction Mechanism in Light-Emitting Devices

Light-emitting structures were prepared with new polymer material based on ethylhexyloxy-phenyles used as emissive layer. Ohmic and Schottky contacts, made from golden and indium-tin oxide, were deposited as thin film anodes for investigation of the conduction mechanism at the electrode interfaces and in the bulk of devices. The influence of the polymer film thickness over the electrical characteristics was investigated. For golden anode, space charge formation and bulk limited conduction mechanism was found. For indium-tin oxide anode, complex behavior of the structures was found, which suggests mixed mode of injection limited conduction and electric field dependence of the charges mobility according to the space charge limited conduction model.

Study of the transverse conductivity of polycrystalline silicon films fabricated by magnetron sputtering

The transverse conductivity of undoped polycrystalline silicon (PCS) films 0.2–0.6 µm thick, deposited by magnetron sputtering on heavily doped single-crystal substrates is studied. Molybdenum films 0.4 µm thick were used as top electrodes. The dependences of the film resistivity on the electric field strength are obtained, which are described by the space-charge limited conduction model for the quasi-uniform distribution of traps in the PCS band gap.

Realization of polymer charge pump circuits using polymer semiconductors

Vertical diodes made of chromium/gold/poly(3-hexylthiophene) (P3HT)/aluminum are used to implement AC–DC charge pump circuits. A single stage polymer charge pump with 470 nF capacitors is able to supply 7.2 V DC to a load of 1 MΩ || 20 pF when an AC signal with a peak value of 10 V is applied. The circuit can be used up to 10 kHz. High turn-on voltages and internal resistances and capacitances of the diodes limit the DC output voltages. The ramp-up time of the circuit built with 10 nF capacitors is 9.4 ms for a 1 kHz, 10 V peak AC signal. Diodes are modeled and simulated in SPICE using space charge limited conduction model. The simulated charge pump circuit produces consistent results with measurements. Performances of air exposed vertical diodes deteriorate under vacuum, prolonged operation at atmospheric conditions and electrical bias stress resulting in blistering, emphasizing the importance of packaging the devices under inert atmosphere. The DC output voltage and the input impedance of the circuit can be adjusted by using multiple stages.

Effect of annealing temperature on the conduction mechanism for a sol–gel driven ZnO Schottky diode

The conduction mechanism of a sol–gel driven ZnO Schottky diode has been investigated with various annealing temperatures. A dense and pore-free ZnO film has been fabricated for a metal–semiconductor–metal structure via the sol–gel method with Zn acetate as a precursor, ethanol as a solvent and ethanol amine as a chelating agent. ZnO films were analysed by means of field emission scanning electron microscopy, ultraviolet visible absorption analysis and Fourier transform infrared (FTIR) analysis. The absorption shoulder appeared and shifted to a longer wavelength as the heating temperature and duration increased. The intensities of FTIR absorption peaks decreased as the heating temperature and duration increased. In particular, the strong and broad absorption peak of –OH and –NH at approximately 3400 cm−1 almost disappeared after the heating process. The intensity of the x-ray diffraction peak at 2θ = 34° (2 0 0) was strongly dependent on the ZnO film thickness and annealing temperature. Schottky diodes comprising Al/ZnO/Au were fabricated with three different annealing temperatures, i.e. 100 (AZA100), 200 (AZA200) and 300 °C (AZA300). The forward current of AZA300 has been increased more than twice with respect to that of AZA100. The plots of Ln(I) versus Ln(V), Ln(I) versus V1/2 and Ln(I/V) versus V1/2 based on the equations of the space charge limited conduction model, the Schottky conduction model and the Poole–Frenkel conduction model were employed to identify the conduction mechanism. All three plots showed a linear relationship. However, the slope and beta values do not match compared with the theoretical values. This result indicates that the conduction process is not a single conduction mechanism. Therefore, the forward current enhancement may be due to the reduction of –OH and –NH groups (acting as deep traps) instead of a fundamental conduction mechanism change.

The effect of LaNiO 3 buffer layer thickness on the electric properties of Pb(Zr 0.53Ti 0.47)O 3 thin films deposited on titanium foils

Sol-gel derived Pb(Zr 0.53Ti 0.47)O 3 (PZT) thin films were prepared on LaNiO 3 (LNO) buffered titanium foils. The effect of LNO buffer layer thickness on the electric properties of PZT thin films was investigated. The room temperature dielectric constant of PZT thin films increased with increasing LNO thickness. The remnant polarization of PZT thin films on 150 and 250 nm LNO was about 20 uC/cm 2. Curie temperatures of PZT thin films were 310, 330 and 340 °C for LNO of 250, 150 and 50 nm respectively. The current-voltage characteristics of PZT thin films were examined for different LNO buffer layer thicknesses, and the space charge limited conduction model was followed in PZT thin films on 50 nm LNO.

Thermally stimulated discharge current and DC resistivity of ethylene propylene rubberwith various Pb concentrations

DC resistivity of extruded ethylene propylene rubber (EPR) samples with various Pb concentration have been measured under wet conditions as a function of electrical field at selected temperatures in a range from 20 to 100°C. The temperature and electrical field coefficients of resistivity have been calculated. Thermally stimulated discharge current (TSDC) has also been measured and a broad positive peak has been observed for three EPR samples. It has been found that the resistivity of EPR is not sensitive to the Pb concentration within the range of 0 to 5 parts per hundred base resin (phr). The results show that the resistivity of EPR varies non-linearly with both temperature and electrical field. The temperature coefficient of resistivity α of EPR has been measured to be ∼0.1 K−1 for all the samples with various Pb concentration. The electrical field coefficient of resistivity β of EPR at room temperature is small and increases with temperature. Increasing Pb content increases slightly the electrical field coefficient β of resistivity. Based on a space charge limited conduction model, the trap depth of EPR has been estimated. TSDC measurements indicate that doping with Pb increases both the density of charge carriers and the number of deep traps simultaneously. The broad TSDC peak reveals that there must be a distribution rather than just a single value of the trap depth.

Experimental and theoretical investigation of MEH-ppv based Schottky diodes

MEH-PPV (poly[2-methoxy-5-(2 ′-ethyl-hexyloxy)-1,4-phenylene vinylene]) based Schottky diodes have been fabricated and investigated through the analyses of current density–voltage and capacitance–voltage characteristics. Temperature-dependent hole mobility of MEH-PPV is extracted by the space-charge limited conduction (SCLC) model from 300 to 400 K, and the use of the SCLC model is examined in high electric field. The highest measured hole mobility is 0.013 cm 2/Vs at 353 K. The thickness of MEH-PPV in the structures consisting of ITO/MEH-PPV/Al, largely affects the performance of the diodes, and the thinner film displays better device performance. According to capacitance–voltage relations, the effective carrier density and Schottky barrier height of MEH-PPV have been determined to be 2.24 × 10 17 cm −3 and 0.64 eV, respectively.

Current‐voltage characteristics of thin poly(biphenyl‐4‐ylphthalide) films

AbstractThe paper considers the features of the charge transport near to the threshold of the transition of polymer films to the high conductive state, induced by a small uniaxial pressure. The problem has not been solved so far, how the energy structure of a wide‐band‐gap organic dielectric varies near this threshold. The current‐voltage characteristics of poly(biphenyl‐4‐ylphthalide) films at different uniaxial pressures were measured and analyzed. The interpretation of the obtained results is carried out within the framework of the space charge limited conduction model. The estimation of the injection model of transport parameters such as the charge carrier mobility and concentration, trapping state concentration and others are carried out. The analysis of the obtained results allows to make the following preliminary conclusion. Pressure increase promotes formation of a narrow trap band near the quasi‐Fermi level resulting from the increase in the injection. This can give rise to a sharp magnification of the charge carrier mobility and even transition to the metallic state.

Electrical characteristics of light-emitting diode based on poly( p-phenylenevinylene) derivatives: CzEH-PPV and OxdEH-PPV

In the light-emitting devices (LEDs) based on the π-conjugated polymers, the relationship between the quantum efficiency and the balance of hole ( μ h) and electron ( μ e) mobility has been investigated. In order to measure the μ h and μ e of the LEDs based on π-conjugated polymers, we fabricated the hole transport device (HTD) and the electron transport device (ETD) by using various metal electrodes with different work functions. For the materials of light emitting layer, we synthesized poly[2-( N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV) and poly[2-{4-[5-(4- tert-butylphenyl)-1,3,4-oxadiazolyl]-phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (OxdEH-PPV) with electron-rich groups. The poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), which is well known material for the polymer-based LED, was synthesized for the reference. We measured the current density vs. applied field ( J– E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J– E curves were analyzed by using the space charge limited conduction (SCLC) model. Based upon the SCLC model, μ h and μ e of MEH-PPV sample was measured to be ∼10 −6 cm 2/V s and ∼10 −8 cm 2/V s, respectively. For CzEH-PPV and OxdEH-PPV samples with electron-rich groups, μ h was similar to μ e with 10 −10–10 −11 cm 2/V s. The μ h and μ e of CzEH-PPV and OxdEH-PPV samples was lower than that of MEH-PPV sample, but more balanced. The quantum efficiency of the LED by using CzEH-PPV or OxdEH-PPV materials was ∼10 times higher than that prepared from MEH-PPV. The balance of the μ h and μ e plays an important role for the quantum efficiency. We analyze the balance of the μ h and μ e and the relatively low mobilities of CzEH-PPV and OxdEH-PPV samples in terms of the heavier effective mass due to the asymmetric dipole distribution in the side chains. The results of photocurrent of the systems qualitatively agreed with the result of the electrical measurements. From AC impedance measurement of the LEDs, we observed that the relaxation time of MEH-PPV was shorter than that of OxdEH-PPV sample because of the higher mobility of MEH-PPV sample.

Electrical and optical characteristics of various PPV derivatives (MEH-PPV, CzEH-PPV, OxdEH-PPV)

To investigate the electrical characteristics of polymer based light emitting diode (LED) devices, we fabricated the hole transport device (HTD) and the electron transport device (ETD). The ITO and Au with high work function were used as electrodes for the HTD, and the Al and Li:Al with low work function were used for the ETD. The active layer materials were poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV), and poly[2-(4-tert-butylphenyl)-5-phenyl-1,3,4-oxadiazole-5(2-ethylhexoxy)-1,4-phenylene vinylene] (OxdEH-PPV). We measured the current density–applied field (J–E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J–E curves were analyzed by using tunneling model, space charge limited conduction (SCLC) model, etc. In the SCLC model, the mobility of the hole and the electron of MEH-PPV is ∼10−6 and ∼10−8cm2/Vs, respectively. For CzEH-PPV and OxdEH-PPV, the hole mobility is similar to the value of the electron mobility with ∼10−10cm2/Vs. The luminescent efficiency of CzEH-PPV or OxdEH-PPV is higher than that of MEH-PPV. The results of photoconductivity of the systems qualitatively agrees with the result of the electrical measurement. We analyze that the balance of the electron and the hole mobility plays an important role for the efficiency of the LEDs.

Leakage current of sol-gel derived Pb(Zr, Ti)O3 thin films having Pt electrodes

The electrical conduction behaviors of sol-gel derived Pb(Zr, Ti)O3 (PZT) thin films on Pt electrodes were analyzed based on a fully depleted film, thermionic field emission, and space charge limited conduction model in the low and high electric field regions, respectively. For films having thicknesses ranging from 150 to 250 nm, no thickness-dependent variation in the dielectric constant was observed due to the relatively large thicknesses. The rather small film-thickness-dependent leakage current characteristics in the low-field region elucidates that the positive space charge density in the film is about 1018 cm−3, which is a smaller value than that of the sputter deposited (Ba, Sr)TiO3 thin films by an order of magnitude. The calculated interfacial potential barrier height and effective mass of electrons were 0.93 eV and 0.09m0, respectively. The slope larger than 2 from the log J vs log V plot in the high-field region implies that the energy level of electron traps are continuously distributed in the energy band gap.