Space Charge Limited Conduction Research Articles

Carrier transport mechanisms of iodine-doped plasma polymerised N, N, 3, 5 tetramethylaniline thin films

Plasma polymerised N, N, 3, 5 tetramethylaniline (PPTMA) thin films were produced through plasma polymerisation process using aniline derived -N(CH3)2 as a precursor where N, N, 3, 5 tetramethylaniline (TMA) was used at ambient temperature. As-deposited and iodine incorporated PPTMA layers with smooth and pinholes-free surfaces of several thicknesses (d) with structural and electrical (direct current, DC) features were evaluated via various characterisation techniques. The X-ray photoelectron spectroscopic analyses showed that carbon, nitrogen, and oxygen were present on the thin films’ surface with different possible groups CO, C-H, C-N, and CC. Both sets of PPTMA films for DC electrical assessment followed Ohmic and non-ohmic conduction in the low (VLower) and high (VHigher) voltage areas, respectively. At the VHigher region for both films, space-charge-limited conduction (SCLC) tackle is found to be operative, and the current density (J) increases with T and d. The total trap concentration, the free carrier concentration, and the carrier mobility of PPTMA thin films were changed from 4 ± 0.05 × 1030 m−3, 2 ± 0.04 × 1021 m−3 and 1.3 ± 0.02 × 10−13 m2 V−1s−1 to 1.2 ± 0.06 × 1027 m−3, 3.7 ± 0.03 × 1022 m−3, and 1.5 ± 0.05 × 10−12 m2 V−1s−1 respectively, due to the gradual iodine addition. The activation energy (ΔE) values of iodine-doped and as-deposited PPTMA films at an applied voltage (VA = 2 V) from lower (TL) to higher (TH) temperatures were ~ 0.11 to ~ 0.73 eV (at VA =10 V, 0.10–0.75 eV) and 0.19–0.83 eV (at VA =10 V, 0.18–0.87 eV), respectively. The conductivity of iodine-doped PPTMA films was higher compared to the as-deposited ones.

Forming-free bipolar resistive switching characteristics in Al/Mn3O4/FTO RRAM device

The bipolar resistive switching (BRS) phenomenon was demonstrated in Mn3O4 using an aluminium/Mn3O4/fluorine-doped tin oxide resistive random access memory (RRAM) device. The fabricated RRAM device showed good retention, non-volatile behaviour, and forming-free BRS. The current–voltage characteristics and the temperature dependence of the resistance measurements were used to explore conduction mechanisms, thermal activation energy, and temperature coefficient of resistance. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) was ∼102. The device showed different conduction mechanisms in LRS and HRS modes, such as ohmic conduction and space charge limited conduction. The formation and rupture of conducting filaments of oxygen vacancies took place by changing the polarity of external voltage, which may be responsible for resistive switching characteristics in the device. This device is suitable for application in future high-density non-volatile memory RRAM devices.

Vertical organic phototransistors realized by femtosecond laser ablation patterned metal/insulator bilayer source electrodes: Performance and numerical modeling

Compared with their conventional lateral counter parts, vertical organic phototransistors (VOPTs) have the advantage of fast response speed due to short channel lengths, and may find wide applications in the field of high speed photo detection. By operation of VOPTs, the source electrode plays the vital role. We report on fabrication, characterization and analysis of VOPTs with pulsed laser ablation patterned Al/LiF bilayer source electrode. A maximal photoresponsivity of 760 mA/W, external quantum efficiency ( EQE ) of 145%, specific detectivity of 1.6 × 10 11 Jones were obtained for un-optimized devices. For describing the output characteristics of the devices, a rough numerical model was developed by combining the theory of thermionic emission at metal/organic interface and space charge limited current conduction. • We innovatively achieved the manufacture of vertical channel field-effect transistors by using a porous metal/insulator bilayer source electrode. • We patterned the porous electrode by femtosecond laser ablation. • We proposed a phenomenological device model to getting quantitative insight into the working mechanism of VOPTs. • The un-optimized device in this work showed a maximal photoresponsivity of 760 mA/W, EQE of 145%, specific detectivity of 1.6 × 10 11 Jones.

Investigation of electrical and magnetic properties of rare-earth based double perovskite: Dy2CoMnO6

In this report, the detailed studies of structural, dielectric, impedance, conductivity, electrical modulus, and leakage-current characteristics of the oxide method synthesized -double perovskite Dy2CoMnO6 ceramic have been provided. From the Rietveld analysis, the structure of the compound is found to be monoclinic. The frequency-temperature dependence of the dielectric properties is discussed by using the Maxwell-Wagner model. The analysis of the Nyquist plot suggests the occurrence of grain and grain boundary effect in the compound. The frequency response of conductivity analysis was explained by Jonscher’s power law. The conduction mechanism follows the space charge limited conduction (SCLC) process which is obtained from leakage current analysis.

Structural, dielectric, ferroelectric and electrical behavior of (Bi0.5Ba0.25Sr0.25)(Ti0.25Mn0.25Fe0.5)O3

The Rietveld refined X-ray diffraction pattern of (Bi0.5Ba0.25Sr0.25)(Ti0.25Mn0.25Fe0.5)O3 sample synthesized through the high temperature mixed oxide solid state route confirms the tetragonal (space group: P4mm) symmetry. The scanning electron micrographs clearly explain the morphology of the sample. Temperature and frequency dependent dielectric parameters as well as the PE-hysteresis loop explain the validity of Curie Weiss law and ferroelectricity present in the material below the TC. The non-Debye type of relaxation, effect of grain and grain boundary on impedance is illuminated by the impedance analysis. The Field-leakage current (JE) characteristics revealed the space charge limited and Ohmic-type conduction process occurring in the material.

Composition and Surface Morphology Invariant High On–Off Ratio from an Organic Memristor

Material composition plays a crucial role in the device performance; thus, nonvolatile memory devices from a small molecule named 5-mercapto-1-methyl tetrazole (MMT) in an insulating polymer matrix of poly(4-vinyl pyridine) (PVP) were fabricated. The composition of the active material in the device was varied to observe its influence on the device’s electronic properties. The device with a more or less weight ratio of MMT has a much smoother surface morphology, whereas when the contributions of MMT and PVP were equal, the average surface roughness increased. However, the maximum on–off current ratio for all the devices is 105, suggesting that the MMT molecule does not show any change in its characteristic properties when surrounded by an insulating material. When the device was fabricated without the polymer matrix, the surface morphology of the device completely changed as it was filled with large holes. These holes provide short-circuited pathways for the current by forming the direct metal contact between the top and bottom electrodes. The carrier transport through these devices follows various conduction mechanisms. Some of the dominating conduction mechanisms are direct tunneling and trap-free and trap-assisted space–charge-limited conduction.

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.

Interfacial reaction induced digital-to-analog resistive switching in TiO2-based memory devices

In the study, TiO2 based resistive random access memory (RRAM) was fabricated by sol-gel technique. By changing the thickness of the TiO2 layer, the devices showed a transition of bipolar digital-type to analog-type memristive behavior under cyclic voltage sweeping process, along with well-defined high and low resistance states. The creation and rupture of nanoscale conductive filaments by drift of the oxygen vacancies was responsible for the digital behavior. The digital-type conduction mechanisms were explained as trap-controlled space charge-limited current conduction mechanism at high resistance state and Ohmic conduction mechanism at low resistance state, respectively. Schottky barrier change induced by the formation of the interfacial layer between Al and TiO2 was responsible for the analog behavior. The analog-type switching mechanisms were explained as trapping and de-trapping of carriers. The interfacial microstructure between TiO2 and electrode would affect the resistive switching behavior of the devices remarkably.

Comparative Study on the Direct Current Electrical Conduction in Monolayer and Bilayer Plasma Polymerized Thin Films

A comparative study on the nature of direct current electrical conduction in the monolayer and bilayer plasma polymerized thin films has been discussed in this article. The plasma polymerized pyrrole (PPPy) monolayer, plasma polymerized N,N,3,5-tetramethylaniline (PPTMA) monolayer and plasma polymerized pyrrole-N,N,3,5-tetramethylaniline (PPPy-PPTMA) bilayer thin films were deposited at room temperature onto glass substrates by using a parallel plate capacitively coupled glow discharge reactor. The current density-voltage (J-V) characteristics and the conductivity-voltage (σ-V) characteristics have been studied to analyze the direct current conduction mechanism in PPPy, PPTMA monolayer and PPPy-PPTMA bilayer thin films. The observed results have been presented in this paper. The J-V and σ-V characteristics of PPPy and PPTMA monolayer thin films of different thicknesses indicated an increase in electrical conduction and also an increase in conductivity in the films of lower thicknesses. It is also observed that PPTMA thin films are more conductive than that of PPPy thin films. As a result the PPPy-PPTMA bilayer thin films of different thicknesses and different deposition time ratios indicated an increase in conductivity as the proportion of PPTMA is increased in the films. In addition to that, from the J-V and σ-V characteristics of PPPy, PPTMA and PPPy-PPTMA thin films of same thickness, it is observed that the current conduction and the conductivity in the bilayer thin films is less compared to the monolayer thin films. This unusual result has been explained by studying ideal and real bilayer thin films. It is also seen that in the low voltage region, the current conduction obeys Ohm’s law while the charge transport phenomenon appears to be the space charge limited conduction (SCLC) in the higher voltage regions.

Space charge limited conduction in anatase and mixed-phase (anatase/rutile) single TiO2 nanotubes

TiO2 nanotube arrays are used for several applications; however, in-depth analysis of charge conduction through an individual nanotube is necessary to predict the behavior of devices and to achieve higher efficiency. Here we report successful isolation and charge transport study through individual nanotubes annealed at different conditions. Anatase or anatase/rutile mixed-phase nanotubes are obtained by annealing in air or nitrogen at temperatures 450 and 650 °C, respectively. Current-voltage measurements in the range of 213 K–413 K indicate that the charge transport through single nanotubes follows Ohmic conduction in lower voltage and trap-assisted space charge limited conduction (SCLC) at higher voltage. Charge transport is controlled by two thermal activation processes resulting in two different values of activation energy, which changes with applied voltage. The activation energy in mixed-phase sample is higher than in the anatase nanotube. Calculated values of trap density vary from 9.28 × 1013/cm3 for anatase nanotubes to 6.60 × 1015/cm3 for mixed-phase (∼52% rutile) and 3.34 × 1016/cm3 in mixed-phase nanotubes (∼76% rutile). The variation of trap density with an increase in rutile phase is attributed to electron trapping by rutile regions arising from a corresponding band alignment. This is also reflected in room temperature conductivity which decreases with increase in rutile content.

Self-rectifying and forming-free resistive switching behaviors in Pt/La2Ti2O7/Pt structure

In this work, we report the resistive switching behavior of an amorphous La2Ti2O7 (LTO) film as sandwiched between two Pt electrodes. The resistive switching is forming-free and highly uniform. Furthermore, it exhibits self-rectifying resistive switching behaviors owing to the Schottky contact and quasi-ohmic contact formed at the top and bottom interfaces, respectively. The mechanism of switching behavior in the device is attributed to the trapping/detrapping-mediated electronic bipolar resistance switching. By fitting the current-voltage characteristics, it indicates the coexistent conduction mechanisms of Schottky emission and space-charge-limited-conduction (SCLC), while the Schottky barrier modified by electron trapping/detrapping plays a dominating role in the resistive switching process.

Optimizing endurance performance of Ga2O3 random resistive access memories by altering oxygen vacancy content

In this work, we propose an effective way to improve the endurance performance of Ga 2 O 3 resistive random access memories by doping ZnO. Excellent bipolar resistive switching behaviors have been observed in atomic-layer-deposited ZnO-doped Ga 2 O 3 RRAMs, which exhibit large memory windows over 10 3 and retention time up to 10 4 . Bipolar resistive switching behaviors can be explained by the formation or rupture of conductive filaments composed of Ag atoms and oxygen vacancies. Current – voltage curves suggest Ohmic conductance of the low resistance states, while the conductive mechanism of the high resistance states corresponds to the trap-assisted space charge limited conduction. Furthermore, the reason why the endurance performance of ZnO-doped Ga 2 O 3 RRAMs is better than pure Ga 2 O 3 RRAMs is analyzed.

Influence of the fullerene acceptor derivative on the structure, molecular interaction and optoelectronic properties of poly(3-hexylthiophene) blend films: Elucidating key factors

Blend films made of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) with the weight ratio in the range of 1:0 to 1:2 were prepared using the spin coating method. The influence of PC71BM on the structure, morphology and molecular interactions was assessed using the AFM, XRD, and FTIR spectroscopy measurements. The optical absorbance spectrum of the P3HT:PC71BM blend films inspected that the band gap of P3HT decreased from 2.1 eV to around 1.95 eV with increasing the amount of PC71BM in P3HT. The luminescence spectra of the P3HT:PC71BM blend films indicated that the incorporation of PC71BM with the P3HT:PC71BM weight ratio above 1:1.2 leads to the low emission, implying that 1:1.2 is the optimal blend ratio. The time dependent luminescence intensity measurements showed that the incorporation of PC71BM in P3HT delays the recombination process, thus, allowing an enhancement of the separation of the photo-generated charges. The electrical behavior of the P3HT:PC71BM blend films is described by the space charge limited conduction mechanism, in particular, the Poole-Frenkel conduction mechanism. The inclusion of the Cd0.95Hf0.05S QDs into the hole transport layer PEDOT:PSS leads to an improvement of the solar cell performance. At optimum PC71BM concentration, the highest conversion efficiency of 4.7% with VOC = 0.62 V and JSC = 4.2 mA cm−2 was attained.

Voltage-programmable negative differential resistance in memristor of single-crystalline lithium niobate thin film

Negative differential resistance (NDR) in memristor has attracted considerable attention due to its nonlinear dynamic characteristic accompanied by resistive switching behavior. Here, we demonstrated the voltage-programmable NDR effect in an Au/LNO/Pt memristor based on a single-crystalline lithium niobate (SC-LNO) thin film tuned by low-energy Ar+ irradiation. A typical N-Shape NDR behavior occurs in reverse bias and can be programmed by changing the positive sweeping voltage. Moreover, thickness-dependent of the NDR behavior was also investigated. Thanks to the merit of the SC-LNO thin film, our memristor reveals excellent uniformity and reproducibility with low cycle-to-cycle variation (down to 1.82% for Ipeak and 2.94% for Vvalley). The carrier transport mechanism of our device is attributed to the interplay of trap-controlled space-charge-limited conduction (SCLC) and the Fowler–Nordheim (FN) tunneling. The transition from bulk-limited conduction (SCLC) to interfacial one (FN) tuned by Ar+ irradiation is crucial for our NDR devices.

Discrete resistive switching characteristics in metal-free phthalocyanine and Dy-phthalocyanine based devices

The resistive memory devices based on planar metal-free phthalocyanine (H2Pc) and sandwiched Dy-phthalocyanine (DyPc2) have been prepared on an ITO glass substrate with the Al as a top electrode. The current-voltage characteristics of these devices are found to be relevant with the interlayer thickness and the crystalline structure. Both the average values of reset and set voltage increased with the increasing thickness of interlayer. The devices can be cycled more than 150 consecutive cycles when the thickness of H2Pc and DyPc2 are 60 nm and 30 nm, respectively. The resistances of present H2Pc devices are larger than that of DyPc2 devices, which may be related to different orientations of H2Pc and DyPc2 films. Additionally, a high ON/OFF current ratio of about 103 and a long retention ability of over 104 s were achieved for both devices, implying their nonvolatile storage capacity. An analysis of the current-voltage characteristics revealed that the electrical conduction behavior followed the space charge limited conduction mechanism. This study is useful to design advanced resistive memory devices and understand the underlying resistive switching mechanism.

Forming‐Free Pt/Ti/AlOx/CeOx/Pt Multilayer Memristors with Multistate and Synaptic Characteristics

Due to their unique electrical performance and simple structure, memristors exhibit excellent application prospects for future information technology. In this work, we fabricated Pt/Ti/AlOx/CeOx/Pt memristors demonstrating electroforming‐free bipolar resistive switching behavior with low operating voltage (−1 to 1 V), stable endurance, and retention. Space‐charge limited conduction (SCLC) as well as the formation and rupture of conductive filaments are responsible for the resistive switching behavior. Increasing the magnitude of the RESET voltage could generate multistate resistive switching. We studied the synaptic characteristics of the device by obtaining multilevel conductance states and investigating the relationship between the device resistance, pulse amplitude, pulse width, and pulse number. By applying programmed pre and postsynaptic spiking pulses, spike‐timing dependent plasticity was observed. This study shows that the device is suitable for multivalue storage and can be used as an electronic synapse device in artificial neural networks.

Facilitation of compliance current for resistive switching and stability of Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub>/ITO

In this work, Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub> multi-layer thin film is deposited on indium tin oxide substrates by using the magnetron sputtering technology. Obvious resistive switching performance can be observed by increasing the compliance current. Ohmic and space charge limited current conduction mechanisms are demonstrated in Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub>. The reproducible and stable resistive switching behaviors in Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub>/ITO device at <i> I</i><sub>cc</sub> = 10<sup>–2</sup> A are reported. The results show that no obvious degradation is found after 365 successive cycles tests.

Effects of interfacial oxide layer formed by annealing process on WORM characteristics of Ag/CuxO/SiOx/n+–Si devices

In this paper, we investigate the effects of a SiOx interfacial layer on ON/OFF current ratios, endurance characteristics and read-disturb immunities of Ag/CuxO/SiOx/n+-Si write-once-read-many-times (WORM) memories. The CuxO active layers were prepared using a sol-gel process. After coating CuxO films on n+-Si substrates, the CuxO/n+-Si samples were annealed in air at 400 ℃ and 600 ℃, respectively, to obtain a SiOx interfacial layer at the CuxO/n + -Si interfaces. The 400 ℃-annealed CuxO device shows an ON/OFF current ratio of 104. However, degradation of OFF state current (IOFF) with increasing read-pulse cycles and stress time is observed. For the 600 ℃-annealed CuxO device, stable ON and OFF state currents can be observed both in an endurance test for over 1.2 × 104 read cycles and in a read-disturb test for 2 × 104 s. Moreover, a higher ON/OFF current ratio of 107 is obtained. A rigorous retention test executed at an elevated temperature of 85 ℃ indicates that the data retention time is expected to last for 10 years. The performance improvement of the 600 ℃-annealed CuxO device is due to an increase in the thickness of the SiOx interfacial oxide layer. The mechanism for the influence of the interfacial layer on memory performance is investigated and illustrated. The OFF state current of the memory is limited by hopping and trap-assisted tunneling transport in the SiOx interfacial layer. In the ON state, conductive paths in the device cause that the carriers can easily migrate by Ohmic and space charge limited conduction.

Nonvolatile resistive switching memory based on monosaccharide fructose film

In this paper, we report resistive random access memory (RRAM) based on a monosaccharide—fructose for nonvolatile memory in biocompatible and “green” electronics. Fructose thin film acts as the resistive switching layer with Al and Ag top electrodes for comparison. Both devices demonstrated highly reproducible nonvolatile bipolar resistive switching behaviors with a large on/off ratio of ∼106 for the Al electrode and ∼105 for the Ag electrode. The forming voltage, set voltage, and memory window are also larger for the Al electrode than the Ag electrode, but the reset voltages are comparable. Dominant conduction mechanisms of fructose films were proposed. At a high resistance state, both electrodes reveal space charge limited conduction, while at a low resistance state, the governing mechanism is Ohm's law, and in addition, the Ag electrode also shows trap-fill limited conduction when approaching the reset voltage. This observation has yet to be reported in RRAM based on natural bio-organic materials.

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.