Carrier Transport Behavior Research Articles

Annulated Thienyl-Vinylene-Thienyl Building Blocks for π-Conjugated Copolymers: Ring Dimensions and Isomeric Structure Effects on π-Conjugation Length and Charge Transport

A series of annulated thienyl-vinylene-thienyl (ATVT) building blocks having varied ring sizes, isomeric structures, and substituents was synthesized and characterized by spectroscopic, electrochemical, quantum chemical, and crystallographic methods. It is found that ATVT ring size and isomeric structure critically affect the planarity, structural rigidity, optical absorption, and redox properties of these new π-units. Various solubilizing substituents can be introduced on the annulated hydrocarbon fragments, preserving the ATVT planarity and redox properties. The corresponding π-conjugated copolymers comprising ATVT units and electron-deficient units were also synthesized and characterized. The solubility, redox properties, and carrier transport behavior of these copolymers also depend remarkably on the annulated ring size and the ATVT unit isomeric structure. One of the copolymers composed of an ATVT with five-membered rings (1), (E)-4,4′,5,5′-tetrahydro-6,6′-bi(cyclopenta[b]thiophenylidene), and a naph...

Rashba Effect and Carrier Mobility in Hybrid Organic-Inorganic Perovskites.

The outstanding photovoltaic performance in hybrid organic-inorganic perovskites (HOIPs) relies on their desirable carrier transport properties. In the HOIPs, strong spin-orbit coupling (SOC) and structural inversion asymmetry give rise to a giant spin splitting in the conduction and valence bands, that is, the Rashba effect (RE), a subject intensively studied in spintronics. Here we show that this giant RE can manifest itself in charge transport and is the key to understanding carrier mobility and its temperature dependence in the HOIPs. The RE greatly enhances acoustic-phonon scattering (APS) and alters the temperature dependence of carrier mobility from T(-3/2) to T(-1). Meanwhile, it reduces polar-optical phonon scattering (POPS). In CH3NH3PbI3, the carrier mobility is limited by the APS for temperatures up to 100 K, above which the POPS becomes dominant. The effective polar coupling is moderate, α = 1.1, indicating that band conduction is still a valid description of charge transport. Our results account for the observed carrier transport behaviors over the entire temperature range and highlight the importance of SOC in charge transport in the HOIPs.

Transition of Carrier Transport Behaviors with Temperature in Phosphorus-Doped Si Nanocrystals/SiO2 Multilayers.

High-conductive phosphorus-doped Si nanocrystals/SiO2(nc-Si/SiO2) multilayers are obtained, and the formation of Si nanocrystals with the average crystal size of 6 nm is confirmed by high-resolution transmission electron microscopy and Raman spectra. The temperature-dependent carrier transport behaviors of the nc-Si/SiO2 films are systematically studied by which we find the shift of Fermi level on account of the changing P doping concentration. By controlling the P doping concentration in the films, the room temperature conductivity can be enhanced by seven orders of magnitude than the un-doped sample, reaching values up to 110 S/cm for heavily doped sample. The changes from Mott variable-range hopping process to thermally activation conduction process with the temperature are identified and discussed.

Tunable electronic properties of graphene through controlling bonding configurations of doped nitrogen atoms

Single–layer and mono–component doped graphene is a crucial platform for a better understanding of the relationship between its intrinsic electronic properties and atomic bonding configurations. Large–scale doped graphene films dominated with graphitic nitrogen (GG) or pyrrolic nitrogen (PG) were synthesized on Cu foils via a free radical reaction at growth temperatures of 230–300 °C and 400–600 °C, respectively. The bonding configurations of N atoms in the graphene lattices were controlled through reaction temperature, and characterized using Raman spectroscopy, X–ray photoelectron spectroscopy and scanning tunneling microscope. The GG exhibited a strong n–type doping behavior, whereas the PG showed a weak n–type doping behavior. Electron mobilities of the GG and PG were in the range of 80.1–340 cm2 V−1·s−1 and 59.3–160.6 cm2 V−1·s−1, respectively. The enhanced doping effect caused by graphitic nitrogen in the GG produced an asymmetry electron–hole transport characteristic, indicating that the long–range scattering (ionized impurities) plays an important role in determining the carrier transport behavior. Analysis of temperature dependent conductance showed that the carrier transport mechanism in the GG was thermal excitation, whereas that in the PG, was a combination of thermal excitation and variable range hopping.

Resistive switching characteristics and mechanisms in silicon oxide memory devices

Abstract Intrinsic unipolar SiOx-based resistance random access memories (ReRAM) characterization, switching mechanisms, and applications have been investigated. Device structures, material compositions, and electrical characteristics are identified that enable ReRAM cells with high ON/OFF ratio, low static power consumption, low switching power, and high readout-margin using complementary metal-oxide semiconductor transistor (CMOS)–compatible SiOx-based materials. These ideas are combined with the use of horizontal and vertical device structure designs, composition optimization, electrical control, and external factors to help understand resistive switching (RS) mechanisms. Measured temperature effects, pulse response, and carrier transport behaviors lead to compact models of RS mechanisms and energy band diagrams in order to aid the development of computer-aided design for ultralarge-v scale integration. This chapter presents a comprehensive investigation of SiOx-based RS characteristics and mechanisms for the post-CMOS device era.

Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging.

In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p-n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2-MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.

Analysis of the Electrical Properties of an Electron Injection Layer in Alq3-Based Organic Light Emitting Diodes.

We investigated the carrier transfer and luminescence characteristics of organic light emitting diodes (OLEDs) with structure ITO/HAT-CN/NPB/Alq3/Al, ITO/HAT-CN/NPB/Alq3/Liq/Al, and ITO/HAT-CN/NPB/Alq3/LiF/A. The performance of the OLED device is improved by inserting an electron injection layer (EIL), which induces lowering of the electron injection barrier. We also investigated the electrical transport behaviors of p-Si/Alq3/Al, p-Si/Alq3/Liq/Al, and p-Si/Alq3/LiF/Al Schottky diodes, by using current-voltage (L-V) and capacitance-voltage (C-V) characterization methods. The parameters of diode quality factor n and barrier height φ(b) were dependent on the interlayer materials between Alq3 and Al. The barrier heights φ(b) were 0.59, 0.49, and 0.45 eV, respectively, and the diode quality factors n were 1.34, 1.31, and 1.30, respectively, obtained from the I-V characteristics. The built in potentials V(bi) were 0.41, 0.42, and 0.42 eV, respectively, obtained from the C-V characteristics. In this experiment, Liq and LiF thin film layers improved the carrier transport behaviors by increasing electron injection from Al to Alq3, and the LiF schottky diode showed better I-V performance than the Liq schottky diode. We confirmed that a Liq or LiF thin film inter-layer governs electron and hole transport at the Al/Alq3 interface, and has an important role in determining the electrical properties of OLED devices.

Theoretical Study of Triboelectric-Potential Gated/Driven Metal-Oxide-Semiconductor Field-Effect Transistor.

Triboelectric nanogenerator has drawn considerable attentions as a potential candidate for harvesting mechanical energies in our daily life. By utilizing the triboelectric potential generated through the coupling of contact electrification and electrostatic induction, the "tribotronics" has been introduced to tune/control the charge carrier transport behavior of silicon-based metal-oxide-semiconductor field-effect transistor (MOSFET). Here, we perform a theoretical study of the performances of tribotronic MOSFET gated by triboelectric potential in two working modes through finite element analysis. The drain-source current dependence on contact-electrification generated triboelectric charges, gap separation distance, and externally applied bias are investigated. The in-depth physical mechanism of the tribotronic MOSFET operations is thoroughly illustrated by calculating and analyzing the charge transfer process, voltage relationship to gap separation distance, and electric potential distribution. Moreover, a tribotronic MOSFET working concept is proposed, simulated and studied for performing self-powered FET and logic operations. This work provides a deep understanding of working mechanisms and design guidance of tribotronic MOSFET for potential applications in micro/nanoelectromechanical systems (MEMS/NEMS), human-machine interface, flexible electronics, and self-powered active sensors.

High-resolution core-level photoemission measurements on the pentacene single crystal surface assisted by photoconduction

Upon charge carrier transport behaviors of high-mobility organic field effect transistors of pentacene single crystal, effects of ambient gases and resultant probable ‘impurities’ at the crystal surface have been controversial. Definite knowledge on the surface stoichiometry and chemical composites is indispensable to solve this question. In the present study, high-resolution x-ray photoelectron spectroscopy (XPS) measurements on the pentacene single crystal samples successfully demonstrated a presence of a few atomic-percent of (photo-)oxidized species at the first molecular layer of the crystal surface through accurate analyses of the excitation energy (i.e. probing depth) dependence of the C1s peak profiles. Particular methodologies to conduct XPS on organic single crystal samples, without any charging nor damage of the sample in spite of its electric insulating character and fragility against x-ray irradiation, is also described in detail.

Photogenerated carriers transport behaviors in L-cysteine capped ZnSe core-shell quantum dots

The photoexcited carrier transport behavior of zinc selenide (ZnSe) quantum dots (QDs) with core–shell structure is studied because of their unique photoelectronic characteristics. The surface photovoltaic (SPV) properties of self-assembled ZnSe/ZnS/L-Cys core–shell QDs were probed via electric field induced surface photovoltage and transient photovoltage (TPV) measurements supplemented by Fourier transform infrared, laser Raman, absorption, and photoluminescence spectroscopies. The ZnSe QDs displayed p-type SPV characteristics with a broader stronger SPV response over the whole ultraviolet-to-near-infrared range compared with those of other core–shell QDs in the same group. The relationship between the SPV phase value of the QDs and external bias was revealed in their SPV phase spectrum. The wide transient photovoltage response region from 3.3 × 10−8 to 2 × 10−3 s was closely related to the long diffusion distance of photoexcited free charge carriers in the interfacial space–charge region of the QDs. The strong SPV response corresponding to the ZnSe core mainly originated from an obvious quantum tunneling effect in the QDs.

Extrinsic and intrinsic performance effects on the electrical property in few-layer graphene

The effects of extrinsic and intrinsic performances on the electrical property of few-layer graphene (FLG) are investigated. This study uses the ultraviolet irradiation technique to tune the electrical parameters of FLG for analyzing the extrinsic/intrinsic contribution to the electrical conductivity. A correlation between the temperature-dependent electrical properties, phonon and impurity scatterings, and thermal activation of charge carriers is identified. The observed temperature evolution of resistivity is understood from the competition among the effects of phonon and impurity scatterings and thermal activation of charge carriers. It is important to identify the carrier transport behavior for enhancing the FLG-based device performance.

Effect of Plasma Induced Surface Damages on the Organic Thin Films by Using Metal Mesh

In this paper we have investigated the effect of plasma induced damages on the organic small molecular electron transport layers in organic light emitting devices with such electroluminescent materials as tris(8-hydroxyquinolinato) aluminum (Alq3) layers was used for studying plasma-induced degradation. We presently use metal mesh that could reduce the influence of plasma exposure of the organic layer films. The photoluminescence, atomic force microscopy, scanning electron microscopy and current-voltage characteristics of electron transport material was measured after exposing to argon plasma. The high-energy species in plasma induces the sputtering of organic layers, quenching of emission sites, and the creation of defect sites in organic layers. In the current-voltage characteristics, negative differential resistance phenomenon was observed corresponding to electron-only device structure of Indium Tin Oxide (ITO)/Alq3/LiF/Al. The carrier transport behavior in the electron-only device was described.

Carrier transport behaviors depending on the two orthogonally directional energy bands in the ZnO nanofilm affected by oxygen plasma

An oxygen plasma treatment of ZnO nanostructures has frequently been used for obtaining a desired optoelectrical property. Nevertheless, a detailed study regarding carrier transport behaviors affected by the plasma has scarcely been managed, especially in the thin film structure, owing to its more complex physics than those of a one-dimensional nanostructure. Herein, we demonstrate an analysis of carrier transport behaviors on an oxygen plasma-treated ZnO nanofilm (50 nm thick) on a SiO2/Si substrate. By comparison with the as-grown sample, we observed drastic changes in carrier transport behavior according to the short exposure times of 30 s and 60 s. The plasma effect leading to the distinction was confirmed to originate from the bombardment of energetic ions near the surface and the diffusion of various oxygen ions and radicals into the host. The mechanism of the resulting carrier transport was comprehended through the revelation of two orthogonally directional energy band structures (surface band bending in the surface layer and localized energy bending at the grain boundary). Furthermore, we experimentally observed that the increased electrical barrier of the grain boundary, due to negatively absorbed oxygen ions, could be helpful in impeding persistent photoconductivity and in reducing dark current.

Minimizing biases associated with tracking analysis of submicron particles in heterogeneous biological fluids

Tracking the dynamic motion of individual nanoparticles or viruses offers quantitative insights into their real-time behavior and fate in different biological environments. Indeed, particle tracking is a powerful tool that has facilitated the development of drug carriers with enhanced penetration of mucus, brain tissues and other extracellular matrices. Nevertheless, heterogeneity is a hallmark of nanoparticle diffusion in such complex environments: identical particles can exhibit strongly hindered or unobstructed diffusion within microns of each other. The common practice in 2D particle tracking, namely analyzing all trackable particle traces with equal weighting, naturally biases towards rapidly diffusing sub-populations at shorter time scales. This in turn results in misrepresentation of particle behavior and a systematic underestimate of the time necessary for a population of nanoparticles to diffuse specific distances. We show here via both computational simulation and experimental data that this bias can be rigorously corrected by weighing the contribution by each particle trace on a ‘frame-by-frame’ basis. We believe this methodology presents an important step towards objective and accurate assessment of the heterogeneous transport behavior of submicron drug carriers and pathogens in biological environments.

Microstructure evolution and energy band alignment at the interface of a Si-rich amorphous silicon carbide/c-Si heterostructure

The microstructure evolution of Si-rich amorphous a-SiC:H films obtained under different annealing conditions was investigated by x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The influence of its microstructure on the energy band alignment at a Si-rich a-SiC:H/n-type c-Si hetero-interface was analyzed by ultraviolet visible transmission spectroscopy and ultraviolet photoelectron spectroscopy. The results revealed that the as-deposited Si-rich a-SiC:H film was mainly in an amorphous state. After annealing, Si and SiC quantum dots (QDs) formed, and the crystallinity of the QDs and the proportion of SiC QDs increased with increasing the annealing time at the same annealing temperature. It is found that the energy band alignment at the hetero-interface was influenced by the crystallinity of the films, the sizes of the QDs, and the relative proportion of Si to SiC QDs in a-SiC:H films. Moreover, the contact potential at the hetero-interface decreased with the improved crystallinity of the QDs in a-SiC:H film. The determination of energy band alignment at the Si-rich a-SiC:H/c-Si hetero-interface is beneficial to understanding the carrier transport behavior and designing hetero-structure devices.

(Invited) Intrinsic Unipolar SiOx-Based Resistive Switching Memory: Characterization, Mechanism and Applications

Intrinsic unipolar SiOx-based Resistive-RAM (ReRAM) characterization, mechanism and applications have been investigated. We investigate device structures, material compositions and electrical characteristics to realize ReRAM cells with high ON/OFF ratio, low static power consumption, low switching power, and high readout-margin using CMOS compatible SiOx-based materials. These ideas are combined with the use of horizontal and vertical device structure designs, composition optimization, electrical controlling and external factors for understanding resistive switching (RS) mechanism. Modeling of RS mechanism, including temperature effect, pulse response and carrier transport behaviors are performed, to develop a compact model in energy diagram, trap-level information in SiOx RS layer, even for computer-aided design (CAD) in very-large-scale integration (VLSI) design. Finally, synapse-based neuromorphic system is demonstrated in SiOx-based ReRAM, combining with bio-inspiration and biomimetics process illustrations. This work presents the comprehensively investigation of SiOx-based resistive switching characteristics, mechanisms, applications for future post-CMOS devices era.

Investigation of carrier transport behavior in amorphous indium–gallium–zinc oxide thin film transistors

Behaviors of carrier transport in amorphous indium–gallium–zinc oxide (a-IGZO) thin film transistors are investigated. It is found that the electron mobility is higher at elevated temperatures, which is contrary to that in crystalline Si devices. Drain current enhancement with regard to temperature at corresponding gate voltage follows the Arrhenius equation. This implies that carrier transport is limited by the potential barrier heights induced by trap states within IGZO, and therefore current conduction is heat-activated to overcome those barriers. In addition, the extracted activation energy decreases with increasing gate voltage, indicating the effective potential barrier height is lowered when abundant electrons are injected into the channel. Furthermore, the relationship between carrier mobility and carrier concentration is also investigated, with the carrier mobility monotonically increasing with carrier concentration. Such behavior can be ascribed to a lowered effective barrier above the conduction band when the Fermi-level rises.

Bifacial sodium-incorporated treatments: Tailoring deep traps and enhancing carrier transport properties in Cu2ZnSnS4 solar cells

Manipulating the nature of defects and carrier dynamics in kesterite Cu2ZnSnS4 (CZTS) solar cells is challenging because of the complex behavior of defects. Among the various strategies used to reduce the defect levels, sodium-incorporated CZTS absorbers are effective and beneficial for defect passivation. In this study, we proposed a bifacial sodium-incorporated treatment (BSIT) in CZTS for the control of defect levels. The defect energy levels of the absorber measured by admittance spectroscopy decrease from 263 to 112meV with increasing Na contents. In addition, impedance measurements of the sample after the BSIT showed an improved carrier dynamics with a prolonged minority carrier lifetime from 0.9 to 1.8μs. This suggests that the post treatment of NaF diffused from the top and bottom surfaces of the CZTS absorbers is beneficial for carrier transport behaviors. Our results demonstrated that by manipulating the sodium content in the BSIT, defect passivation and effective reduction of carrier recombination can be attained, resulting in enhanced CZTS device performance from 4.1% to 5.6%.

Defect-dependent carrier transport behavior of polymer:ZnO composites/electrodeposited CdS/indium tin oxide devices

Currents through the poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) and ZnO nanoparticles (PEDOT:PSS:ZnO)/CdS/indium tin oxide (ITO) hetero-structures are studied. The authors introduced the electrodeposition technique with sulfide treatment to improve the film quality of CdS. It is shown that sulfide treatment leads to a reduction in the number of donor-like defects (that is, sulfur vacancies and cadmium interstitials) in the CdS films, which leads to the conversion of carrier transport behavior from Poole-Frenkel emission to thermionic emission-diffusion for PEDOT:PSS:ZnO/CdS/ITO devices. A correlation is identified for providing a guide to control the current transport behavior of PEDOT:PSS:ZnO/CdS/ITO devices.

Metal Semiconductor Field-Effect Transistor with MoS2/Conducting NiO(x) van der Waals Schottky Interface for Intrinsic High Mobility and Photoswitching Speed.

Molybdenum disulfide (MoS2) nanosheet, one of two-dimensional (2D) semiconductors, has recently been regarded as a promising material to break through the limit of present semiconductors. With an apparent energy band gap, it certainly provides a high carrier mobility, superior subthreshold swing, and ON/OFF ratio in field-effect transistors (FETs). However, its potential in carrier mobility has still been depreciated since the field-effect mobilities have only been measured from metal-insulator-semiconductor (MIS) FETs, where the transport behavior of conducting carriers located at the insulator/MoS2 interface is unavoidably interfered by the interface traps and gate voltage. Moreover, thin MoS2 MISFETs have always shown large hysteresis with unpredictable negative threshold voltages. Here, we for the first time report MoS2-based metal semiconductor field-effect transistors (MESFETs) using NiOx Schottky electrode which makes van der Waals interface with MoS2. We thus expect that the maximum mobilities or carrier transport behavior of the Schottky devices may hardly be interfered by interface traps or an on-state gate field. Our MESFETs with a few and ∼10 layer MoS2 demonstrate intrinsic-like high mobilities of 500-1200 cm(2)/(V s) at a certain low threshold voltage between -1 and -2 V without much hysteresis. Moreover, they work as a high speed and highly sensitive phototransistor with 2 ms switching and ∼5000 A/W, respectively, supporting their high intrinsic mobility results.