A Novel Method to Analyze the Relationship between Thermoelectric Coefficient and Energy Disorder of Any Density of States in an Organic Semiconductor.

In this work, a unified method is proposed for analyzing the relationship between the Seebeck coefficient and the energy disorder of organic semiconductors at any multi-parameter density of states (DOS) to study carrier transport in disordered thermoelectric organic semiconductors and the physical meaning of improved DOS parameters. By introducing the Gibbs entropy, a new multi-parameter DOS and traditional Gaussian DOS are used to verify this method, and the simulated result of this method can well fit the experiment data obtained on three organic devices. In particular, the impact of DOS parameters on the Gibbs entropy can also influence the impact of the energy disorder on the Seebeck coefficient.

A Novel Density of States (DOS) for Disordered Organic Semiconductors.

In this work, we proposed a novel theory of DOS for disordered organic semiconductors based on the frontier orbital theory and probability statistics. The proposed DOS has been verified by comparing with other DOS alternatives and experimental data, and the mobility calculated by the proposed DOS is closer to experimental data than traditional DOS. Moreover, we also provide a detailed method to choose the DOS parameter for better use of the proposed DOS. This paper also contains a prediction for the DOS parameters, and it has been verified by the experimental data. More importantly, the physical meaning of the proposed DOS parameter has been explained by equilibrium energy theory and transport energy theory to make this proposed model more rational. Compared with the improved DOS based on Gaussian and exponential DOS, this work is a new attempt to combine probabilistic theory with physical theory related to DOS in disordered organic semiconductors, showing great significance for the further investigation of the properties of DOS.

Collective Transport for Nonlinear Current-Voltage Characteristics of Doped Conducting Polymers.

Origin of nonlinear transport phenomena in conducting polymers has long been a topic of intense controversies. Most previous knowledge has attributed the macroscopic nonlinear I-V characteristics to individual behaviors of elementary resistors in the network. In this Letter, we show via a systematic dimensionality-dependent transport investigation, that understanding the nonlinear transport in conducting polymers must include the collective transport effect in a percolation network. The possible mediation of percolation threshold p_{c} by controlling the samples' dimensionality unveiled the collective effect in growth of percolation paths driven by electric field, enabling us to draw a smooth connection between two typically observed nonlinear phenomena, dissipative tunnelinglike and threshold-limited transport, which have been controversial for years. The possible microscopic origins of the collective transport are discussed within the Coulomb blockade theory.

Charge Trapping and Emission Properties in CAAC-IGZO Transistor: A First-Principles Calculations.

The c-axis aligned crystalline indium-gallium-zinc-oxide field-effect transistor (CAAC-IGZO FET), exhibiting an extremely low off-state leakage current (~10-22 A/µm), has promised to be an ideal candidate for Dynamic Random Access Memory (DRAM) applications. However, the instabilities leaded by the drift of the threshold voltage in various stress seriously affect the device application. To better develop high performance CAAC-IGZO FET for DRAM applications, it's essential to uncover the deep physical process of charge transport mechanism in CAAC-IGZO FET. In this work, by combining the first-principles calculations and nonradiative multiphonon theory, the charge trapping and emission properties in CAAC-IGZO FET have been systematically investigated. It is found that under positive bias stress, hydrogen interstitial in Al2O3 gate dielectric is probable effective electron trap center, which has the transition level (ε (+1/-1) = 0.52 eV) above Fermi level. But it has a high capture barrier about 1.4 eV and low capture rate. Under negative bias stress, oxygen vacancy in Al2O3 gate dielectric and CAAC-IGZO active layer are probable effective electron emission centers whose transition level ε (+2/0) distributed at -0.73~-0.98 eV and 0.69 eV below Fermi level. They have a relatively low emission barrier of about 0.5 eV and 0.25 eV and high emission rate. To overcome the instability in CAAC-IGZO FET, some approaches can be taken to control the hydrogen concentration in Al2O3 dielectric layer and the concentration of the oxygen vacancy. This work can help to understand the mechanisms of instability of CAAC-IGZO transistor caused by the charge capture/emission process.

Surface-Doping-Induced Mobility Modulation Effect for Transport Enhancement in Organic Single-Crystal Transistors.

Molecular doping has conventionally been an effective way to improve the electrical-transport performances in organic field-effect transistors (OFETs), while corresponding mechanisms associated with specific doping techniques have been less investigated and discussed in detail. Here, based on ultrathin dinaphtho[2,3-b:2',3'-f]-thieno[3,2-b]thiophene (DNTT) single crystals, robust transconductance enhancements are realized in OFETs upon surface molecular doping realized via van der Waals epitaxially growing crystalline 1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) onto the single crystal's surface. It is proposed that it is the mobility modulation effect (MME) from the interactions between charge-transfer interface and gate electric field, that contributes to more weighted bulk carriers, and finally improves charge-transport performances. The evaluations are further supported by scanning Kelvin probe microscopy (SKPM) surface potential characterizations, which manifest the gate-induced more delocalized holes near the charge-transfer interfaces. Space-charge-limited current (SCLC) investigations, numerical calculations, and theoretical mobility modeling are also performed to corroborate the analysis. This study can deepen the understanding of charge transport in doped semiconductors and provide effective ways for optimizing the electrical performance of organic devices.

High-Stability Memristive Devices Based on Pd Conductive Filaments and Its Applications in Neuromorphic Computing.

Memristive devices with high-density and high-speed performance have considerable potential for neuromorphic computing applications in data storage and artificial synapses. However, current memristive devices that are based on conductive filaments, such as silver, are unstable owing to the high mobility and low thermodynamic stability of the filaments. A high-quality SnSe film was deposited using the pulsed laser deposition technology, and high-performance Pd/SnSe/NSTO devices were fabricated. High-stability memristive devices can not only implement simple arithmetic function but also exhibit the centralized distribution of SET/RESET voltage and cell-cell uniformity. The SET/RESET power can achieve approximately 4.1 and 61 µW power. The possibility of Pd filament formation and Pd2+ diffusion in SnSe thin films is first confirmed by combining high-resolution transmission electron microscopy, energy-dispersive spectrometer mapping, and first principle calculation. The formation and destruction process of Pd filaments can simulate the influx and extrusion kinetics of K+, Ca2+, or Na+ in biological synapses and implements considerable synaptic functions. This study thus provides a new idea for improving device performance using different filament materials, which can greatly facilitate the development of neuromorphic computing.

A tied Fermi liquid to Luttinger liquid model for nonlinear transport in conducting polymers.

Organic conjugated polymers demonstrate great potential in transistors, solar cells and light-emitting diodes, whose performances are fundamentally governed by charge transport. However, the morphology-property relationships and the underpinning charge transport mechanisms remain unclear. Particularly, whether the nonlinear charge transport in conducting polymers is appropriately formulated within non-Fermi liquids is not clear. In this work, via varying crystalline degrees of samples, we carry out systematic investigations on the charge transport nonlinearity in conducting polymers. Possible charge carriers' dimensionality is discussed when varying the molecular chain's crystalline orders. A heterogeneous-resistive-network (HRN) model is proposed based on the tied-link between Fermi liquids (FL) and Luttinger liquids (LL), related to the high-ordered crystalline zones and weak-coupled amorphous regions, respectively. The HRN model is supported by precise electrical and microstructural characterizations, together with theoretic evaluations, which well describes the nonlinear transport behaviors and provides new insights into the microstructure-correlated charge transport in organic solids.

Spike Encoding with Optic Sensory Neurons Enable a Pulse Coupled Neural Network for Ultraviolet Image Segmentation.

Drawing inspiration from biology, neuromorphic systems are of great interest in direct interaction and efficient processing of analogue signals in the real world and could be promising for the development of smart sensors. Here, we demonstrate an artificial sensory neuron consisting of an InGaZnO4 (IGZO4)-based optical sensor and NbOx-based oscillation neuron in series, which can simultaneously sense the optical information even beyond the visible light region and encode them into electrical impulses. Such artificial vision sensory neurons can convey visual information in a parallel manner analogous to biological vision systems, and the output spikes can be effectively processed by a pulse coupled neural network, demonstrating the capability of image segmentation out of a complex background. This study could facilitate the construction of artificial visual systems and pave the way for the development of light-driven neurorobotics, bioinspired optoelectronics, and neuromorphic computing.

Possible Luttinger liquid behavior of edge transport in monolayer transition metal dichalcogenide crystals.

In atomically-thin two-dimensional (2D) semiconductors, the nonuniformity in current flow due to its edge states may alter and even dictate the charge transport properties of the entire device. However, the influence of the edge states on electrical transport in 2D materials has not been sufficiently explored to date. Here, we systematically quantify the edge state contribution to electrical transport in monolayer MoS2/WSe2 field-effect transistors, revealing that the charge transport at low temperature is dominated by the edge conduction with the nonlinear behavior. The metallic edge states are revealed by scanning probe microscopy, scanning Kelvin probe force microscopy and first-principle calculations. Further analyses demonstrate that the edge-state dominated nonlinear transport shows a universal power-law scaling relationship with both temperature and bias voltage, which can be well explained by the 1D Luttinger liquid theory. These findings demonstrate the Luttinger liquid behavior in 2D materials and offer important insights into designing 2D electronics.

A Review for Compact Model of Thin-Film Transistors (TFTs).

Thin-film transistors (TFTs) have grown into a huge industry due to their broad applications in display, radio-frequency identification tags (RFID), logical calculation, etc. In order to bridge the gap between the fabrication process and the circuit design, compact model plays an indispensable role in the development and application of TFTs. The purpose of this review is to provide a theoretical description of compact models of TFTs with different active layers, such as polysilicon, amorphous silicon, organic and In-Ga-Zn-O (IGZO) semiconductors. Special attention is paid to the surface-potential-based compact models of silicon-based TFTs. With the understanding of both the charge transport characteristics and the requirement of TFTs in organic and IGZO TFTs, we have proposed the surface-potential-based compact models and the parameter extraction techniques. The proposed models can provide accurate circuit-level performance prediction and RFID circuit design, and pass the Gummel symmetry test (GST). Finally; the outlook on the compact models of TFTs is briefly discussed.

Correction: A review of carrier thermoelectric-transport theory in organic semiconductors.

Correction for 'A review of carrier thermoelectric-transport theory in organic semiconductors' by Nianduan Lu et al., Phys. Chem. Chem. Phys., 2016, 18, 19503-19525.

Eliminating Negative-SET Behavior by Suppressing Nanofilament Overgrowth in Cation-Based Memory.

Negative-SET behavior is observed in various cation-based memories, which degrades the device reliability. Transmission electron microscopy results demonstrate the behavior is caused by the overgrowth of the conductive filament (CF) into the Pt electrode. The CF overgrowth phenomenon is suppressed and the negative-SET behavior is eliminated by inserting an impermeable graphene layer. The graphene-based devices show high reliability and satisfying performance.

Super non-linear RRAM with ultra-low power for 3D vertical nano-crossbar arrays.

Vertical crossbar arrays provide a cost-effective approach for high density three-dimensional (3D) integration of resistive random access memory. However, an individual selector device is not allowed to be integrated with the memory cell separately. The development of V-RRAM has impeded the lack of satisfactory self-selective cells. In this study, we have developed a high performance bilayer self-selective device using HfO2 as the memory switching layer and a mixed ionic and electron conductor as the selective layer. The device exhibits high non-linearity (>10(3)) and ultra-low half-select leakage (<0.1 pA). A four layer vertical crossbar array was successfully demonstrated based on the developed self-selective device. High uniformity, ultra-low leakage, sub-nA operation, self-compliance, and excellent read/write disturbance immunity were achieved. The robust array level performance shows attractive potential for low power and high density 3D data storage applications.

A review of carrier thermoelectric-transport theory in organic semiconductors.

Carrier thermoelectric-transport theory has recently become of growing interest and numerous thermoelectric-transport models have been proposed for organic semiconductors, due to pressing current issues involving energy production and the environment. The purpose of this review is to provide a theoretical description of the thermoelectric Seebeck effect in organic semiconductors. Special attention is devoted to the carrier concentration, temperature, polaron effect and dipole effect dependence of the Seebeck effect and its relationship to hopping transport theory. Furthermore, various theoretical methods are used to discuss carrier thermoelectric transport. Finally, an outlook of the remaining challenges ahead for future theoretical research is provided.

Thermal crosstalk in 3-dimensional RRAM crossbar array.

High density 3-dimensional (3D) crossbar resistive random access memory (RRAM) is one of the major focus of the new age technologies. To compete with the ultra-high density NAND and NOR memories, understanding of reliability mechanisms and scaling potential of 3D RRAM crossbar array is needed. Thermal crosstalk is one of the most critical effects that should be considered in 3D crossbar array application. The Joule heat generated inside the RRAM device will determine the switching behavior itself, and for dense memory arrays, the temperature surrounding may lead to a consequent resistance degradation of neighboring devices. In this work, thermal crosstalk effect and scaling potential under thermal effect in 3D RRAM crossbar array are systematically investigated. It is revealed that the reset process is dominated by transient thermal effect in 3D RRAM array. More importantly, thermal crosstalk phenomena could deteriorate device retention performance and even lead to data storage state failure from LRS (low resistance state) to HRS (high resistance state) of the disturbed RRAM cell. In addition, the resistance state degradation will be more serious with continuously scaling down the feature size. Possible methods for alleviating thermal crosstalk effect while further advancing the scaling potential are also provided and verified by numerical simulation.

High uniformity and improved nonlinearity by embedding nanocrystals in selector-less resistive random access memory.

The sneak path problem is one of the major hindrances for the application of high density 3D crossbar resistive random access memory (RRAM). For the selector-less RRAM devices, nonlinear (NL) current-voltage (I-V) characteristics are an alternative approach to minimize the sneak paths. In this work we have demonstrated metallic IrOx nanocrystal (IrOx-NC) based selector-less crossbar RRAM devices in an IrOx/AlOx/IrOx-NC/AlOx/W structure with very reliable hysteresis resistive switching of >10 000 cycles, stable multiple levels, and high temperature (HT) data retention. Moreover, an improvement in the NL behavior has been reported as compared to a pure high-κ AlOx RRAM. The origin of the NL nature has been discussed using the hopping model and Luittenger's 1D metal theory. The nonlinearity can be further improved by structure engineering and will improve the sensing margin of the devices, which is rewarding for crossbar array integration.

Thermoelectric Seebeck effect in oxide-based resistive switching memory.

Reversible resistive switching induced by an electric field in oxide-based resistive switching memory shows a promising application in future information storage and processing. It is believed that there are some local conductive filaments formed and ruptured in the resistive switching process. However, as a fundamental question, how electron transports in the formed conductive filament is still under debate due to the difficulty to directly characterize its physical and electrical properties. Here we investigate the intrinsic electronic transport mechanism in such conductive filament by measuring thermoelectric Seebeck effects. We show that the small-polaron hopping model can well describe the electronic transport process for all resistance states, although the corresponding temperature-dependent resistance behaviours are contrary. Moreover, at low resistance states, we observe a clear semiconductor-metal transition around 150 K. These results provide insight in understanding resistive switching process and establish a basic framework for modelling resistive switching behaviour.

Crystal structure and magnetic properties of ultrafine nanocrystalline SmCo3 compound.

The single-phase ultrafine nanocrystalline SmCo(3) compound with a high coercivity of 33 kOe and a Curie temperature of 925 K was prepared using a simple and efficient method, which took advantages of the concurrent processes of nanocrystallization and densification during spark plasma sintering. The crystal structure of the nanocrystalline SmCo(3) compound was constructed. As compared with the conventional microcrystalline SmCo(3) compound, a large axial ratio c/a = 4.920 and an expansion of the unit cell volume of 2.97% were obtained in the lattice structure of the nanocrystalline SmCo(3). The relationship between the magnetic properties and the nanocrystalline structure was analyzed. A specific magnetic transition from the weak ferromagnetic to the strong ferromagnetic state was discovered in the nanocrystalline SmCo(3) compound, which was considered to be related to the large anisotropic strain in the crystal lattice.

Preparation and properties characterization of the single-phased Sm2Co17, nanocrystalline alloy.

A novel route for the preparation of the single-phased Sm2Co17 nanocrystalline bulk with ultrafine grain sizes was proposed. It was found that the nanocrystalline Sm20Co17 has a hexagonal crystal structure at the room temperature, which shows a different thermal stability from the conventional polycrystalline alloy. The intrinsic coercivity of the nanocrystalline Sm2Co17 with a hexagonal crystal structure was greatly increased as compared with the single-phased polycrystalline alloy with a rhombohedral structure. The microhardness and the elastic modulus of the nanocrystalline Sm2Co17 bulk were increased as high as 1.8 and 2.6 times, respectively, when compared with the polycrystalline parent alloy.