Carrier Transport Behavior Research Articles

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

We propose a research topic to comprehensively understand the intrinsic unipolar SiOx-based resistive-switching random-access memory (ReRAM) characteristics and mechanisms that can provide deep insights of device design and performance improvement for next-generation nonvolatile memory applications, as required for low-power portable electronics, highly-integrated of compact memory design, and other applications. Several main research targets will be pursued. Firstly, we will 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 in both metal-insulator-semiconductor (MIS) and metal-insulator-metal (MIM), and one diode - one resistive switching element (1D-1R) architectures. These ideas will be combined with the use of horizontal and vertical device structure designs (active device size and thickness), composition optimization (SiOx, x < 2), electrical controlling (compliance current limitation, stopped voltage sweeping, and polarity effect) and external factors (series-resistance and ambient effects) for understanding and characterizations of resistive switching mechanism. Secondly, modeling of resistive switching mechanism, including temperature effect, pulse response and carrier transport behaviors will be performed, to develop a compact model in energy diagram, trap-level information, dipole polarization in SiOx resistive switching layer, even for computer-aided design (CAD) in very-large-scale integration (VLSI) design. Finally, for the first time, an intriguing non-Von Neumann computing architecture, named synapse-based neuromorphic system, will be demonstrated in SiOx-based ReRAM, combining with bio-inspiration and biomimetics process illustrations. The novel applications in the physiology and pathology discussions in reactive oxygen species (ROS) production and regulation processes will be clarified. This work will present the comprehensively investigation of SiOx-based resistive switching characteristics, mechanisms, applications for future post-CMOS devices era.

Fabrication and characterization of alternating-current-driven ZnO-based ultraviolet light-emitting diodes

Alternating-current-driven ZnO-based light-emitting diodes (LEDs) with an Au/ZnMgO/ZnO/ZnMgO/GaN/In structure have been fabricated. Both polarity-controlled electroluminescence (EL) and ultraviolet emission were achieved when driven by an alternating current (AC) under a much lower root-mean-square voltage than that of conventional inorganic AC thin-film electroluminescent devices. This ZnO-based LED can be regarded as a series incorporating a metal-insulator-semiconductor diode and a p-i-n diode. The EL mechanisms were discussed in terms of the band diagrams and carrier transport behavior with reference to the semiconductor heterojunction theory.

Scaling and carrier transport behavior of buried-channel In0.7Ga0.3As MOSFETs with Al2O3 insulator

In this paper, we investigate the scaling and carrier transport behavior of sub-100nm In0.7Ga0.3As buried-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) with Al2O3 as gate dielectric. The device combines a 3-nm Al2O3 layer grown by atomic-layer-deposition (ALD) and a 13-nm In0.52Al0.48As insulator grown by molecular-beam-epitaxy (MBE). Our long channel device with Lg=200nm exhibits excellent subthreshold characteristics, such as subthreshold-swing (S) of 68mV/decade at VDS=0.5V, indicating a very good interface quality between Al2O3 and In0.52Al0.48As. In addition, a short-channel device with Lg=60nm maintains electrostatic integrity of the device, such as subthreshold-swing (S)=90mV/decade and drain-induced-barrier-lowering (DIBL)=100mV/V at VDS=0.5V. We show well-behaved electrostatic scaling behavior that follows a modified FD-SOI MOSFET model. Our experimental and theoretical research suggest that further device optimization in the form of a self-aligned contact structure and aggressive EOT scaling would lead to high-performance III–V MOSFETs for multiple types of applications.

Band-like transport observed in TIPS-pentacene thin film by time-resolved microscopic optical second-harmonic generation imaging

Carrier transport in single-crystalline grains of 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene was studied using time-resolved microscopic optical second-harmonic generation (TRM-SHG). TRM-SHG is used to directly visualize carrier injection and transport behavior in TIPS pentacene. A strong SHG signal observed at the edge of a FET source electrode at low temperature suggests the presence of a potential drop associated with the carrier injection process. An accurate carrier mobility calculation, taking into account the potential drop, shows a negative slope with temperature that indicates band-like transport.

Conduction behavior conversion for Cu-doped ZnS/n-type Si devices with different Cu contents

Currents through Cu-doped ZnS (ZnCuS)/n-type Si structures were studied. The electrical conduction investigations suggest that the carrier transport behavior is governed by the Poole–Frenkel emission for ZnCuS/n-type Si devices having the low Cu concentration. However, the carrier transport behavior is governed by the thermionic emission for ZnCuS/n-type Si devices having the high Cu concentration. The photoluminescence result revealed that sulfur vacancy (VS) is the origin of conduction behavior conversion. It is shown that the increased Cu concentration leads to the reduced formation probability of VS. The dependence of VS on the film composition was identified for providing a guide to control the current transport behavior of ZnCuS/n-type Si devices.

Electrical and Optical Analysis on Green Organic Light Emitting Device Using Finite Element Method.

PSS); Bis[(1-naphthyl)-N-phenyl]benzidine (α-NPD); 4,4'-N,N'-dicarbazole-biphenyl (CBP) doped with Ir(ppy)3; a 2-(4-Biphenylyl)-5-(4-tert-butylphenyl-1,3,4-oxadiazole) (Bu-PBD). We demonstrate that the refractive indexes of each material affect the emission property and barrier height of the interface influences the behavior of charges and the generation of exciton.

Surface group modification and carrier transport properties of layered transition metal carbides (Ti2CT(x), T: -OH, -F and -O).

In spite of recent significant research into various two-dimensional (2D) materials after the emergence of graphene, the development of a new 2D material that provides both high mobility and an appropriate energy band gap (which are crucial for various device applications) remains elusive. In this report, we demonstrate that the carrier transport behaviour of 2D Ti2CTx, which belongs to the family of 2D transition metal carbides and nitrides, can be tuned by modifying the surface group Tx (-OH, -F, and -O). Our results show that 2D Ti2C(OH)xFy and Ti2COx films can be obtained via simple chemical treatment, thermal annealing, and mechanical exfoliation processes. For the first time, we study the carrier transport properties of 2D Ti2CTx field effect transistors (FETs), obtaining the high field effect carrier mobilities of 10(4) cm(2) V(-1) s(-1) at room temperature. The temperature dependent resistivity of the Ti2COx film exhibits semiconductor like Arrhenius behaviour at zero gate voltage, from which we estimate the energy gap of 80 meV. One interesting feature of the FETs based on transition metal carbides is that the field effect mobility at room temperature is less sensitive to the measured transport gaps, which may arise from the dominant charge transport of activated carriers over the narrow energy gaps of the transition metal carbides. Our results open up the possibility that new 2D materials with high mobilities and appropriate band gaps can be achieved, and broaden the range of electronic device applications of Ti2CTx films.

Inhomogeneous carrier transport at contact/GaN interfaces and thermal instability of vertical light-emitting diodes.

The carrier-transport behavior at the interface of a contact and n-type GaN was investigated for group III nitride vertical light-emitting diodes (LEDs). Three types of samples were investigated including dry-etched flat Ga-polar n-GaN (GA), dry-etched flat N-polar n-GaN (NF), and wet-etched roughened N-polar n-GaN (NR). Schottky diodes fabricated using a palladium contact revealed that carrier transport at the contact/GaN interface could be understood in terms of the barrier inhomogeneity model, in which the local shallow barriers dominated the overall carrier transport. Kelvin probe force microscopy showed that the peculiar crystallographic structures and native defects (acting as the local shallow barriers) led to the largest tunneling probability of the NR samples. The study was followed up by forming a TiN/Al contact on the NR surfaces to study the thermal instability of the vertical LEDs, revealing that the vertical LEDs were degraded after a thermal annealing process. This could be related to the n-contact size, indicating that the degradation of vertical LEDs was likely due to the failure of Ohmic patches (contacts formed on the defects and/or the tip/edge of hexagonal cones) at elevated temperature.

Structural, magnetic and electrical characterization of the La0.7Ca0.3Co1–x MnxO3 (x=0, 0.7 and 1) compounds prepared by a simple method

Structural, magnetic and electrical properties of the La0.7Ca0.3Co1–xMnxO3 (x=0, 0.7 and 1) samples prepared by a simple method were systematically studied and it was found that the crystal structure was transformed from rhombohedral for La0.7Ca0.3CoO3 (LCCO) and La0.7Ca0.3Co0.3Mn0.7O3 (LCCMO) samples to orthorhombic for La0.7Ca0.3MnO3 (LCMO) sample. The AC magnetic susceptibility measurements showed that LCCO sample underwent a transition from paramagnetic (PM) to ferromagnetic (FM) phase at Curie temperature, TC∼155 K and below Curie temperature, the glassy ferromagnetism nature was observed. In LCCMO sample, clear evidence of spin glass (SG) state was observed at low temperature. PM-FM phase transition at about TC∼260 K and long range FM order at low temperatures were observed in LCMO sample. Both the LCCO and LCCMO samples exhibited insulating behavior in the whole range of measuring temperature whereas the LCMO sample underwent a clear metal-insulator (MI) transition at about TMI∼263 K, corresponding to Curie temperature. Metallic region of ρ(T) curve of the LCMO sample was fitted to the model of electron-electron and electron-magnon scattering. The charge carrier transport behavior in all the samples was compared based on polaronic models.

Heavily Doped poly(3,4‐ethylenedioxythiophene) Thin Films with High Carrier Mobility Deposited Using Oxidative CVD: Conductivity Stability and Carrier Transport

The transparent conductingpoly(3,4‐ethylenedioxythiophene) (PEDOT) is of interest for various optoelectronic device applications. Here, the conductivity stability of PEDOT processed using oxidative chemical‐vapor‐deposition (oCVD) with FeCl3 as an oxidant is primarily dominated by the change in carrier density when aged in air. To establish the mechanism for the conductivity decrease, the changes in carrier density and carrier mobility of PEDOT films are separately monitored using an AC Hall Effect measurement system. The measured electrical properties reveal that a decrease in carrier density dominates the conductivity decrease during annealing. X‐ray diffraction analysis made on the HBr‐ and MeOH‐rinsed PEDOT samples identifies the Fe‐related dedoping phase of Fe(OH)2 and provides the dedoping mechanism. The carrier transport study demonstrates heavily doped oCVD PEDOT with the carrier density higher than ~1020 cm–3, and in this regime, an increase in carrier density yields lower carrier mobility which shows that the carrier transport is governed by the ionized impurity scattering mechanism due to increased dopant counter‐anions. These findings of the mechanisms for PEDOT conductivity decrease and carrier transport behavior may be important to organic optoelectronic device applications that show a strong effect of air‐exposure and low‐temperature annealing on the device stability and performance.

Effect of ligand self-assembly on nanostructure and carrier transport behaviour in CdSe quantum dots

Adjustment of the nanostructure and carrier behaviour of CdSe quantum dots (QDs) by varying the ligands used during QD synthesis enables the design of specific quantum devices via a self-assembly process of the QD core–shell structure without additional technologies. Surface photovoltaic (SPV) technology supplemented by X-ray diffractometry and infrared absorption spectroscopy were used to probe the characteristics of these QDs. Our study reveals that while CdSe QDs synthesized in the presence of and capped by thioglycolic acid, 3-mercaptopropionic acid, mercaptoethanol or α-thioglycerol ligands display zinc blende nanocrystalline structures, CdSe QDs modified by l-cysteine possess wurtzite nanocrystalline structures, because different end groups in these ligands induce distinctive nucleation and growth mechanisms. Carboxyl end groups in the ligand served to increase the SPV response of the QDs, when illuminated by hν ≥ Eg,nano-CdSe. Increased length of the alkyl chains and side-chain radicals in the ligands partially inhibit photo-generated free charge carrier (FCC) transfer transitions of CdSe QDs illuminated by photon energy of 4.13 to 2.14 eV. The terminal hydroxyl group might better accommodate energy released in the non-radiative de-excitation process of photo-generated FCCs in the ligand's lowest unoccupied molecular orbital in the 300–580 nm wavelength region, when compared with other ligand end groups.

Study on the resistance distribution at the contact between molybdenum disulfide and metals.

Contact resistance hinders the high performance of electrical devices, especially devices based on two-dimensional (2D) materials, such as graphene and transition metal dichalcogenide. To engineer contact resistance, understanding the resistance distribution and carrier transport behavior at the contact area is essential. Here, we developed a method that can be used to obtain some key parameters of contact, such as transfer length (Lt), sheet resistance of the 2D materials beneath the contacting metal (Rsh), and contact resistivity between the 2D materials and the metal electrode (ρc). Using our method, we studied the contacts between molybdenum disulfide (MoS2) and metals, such as titanium and gold, in bilayer and few-layered MoS2 devices. Especially, we found that Rsh is obviously larger than the sheet resistance of the same 2D materials in the channel (Rch) in all the devices we studied. With the increasing of the back-gate voltage, Lt increases and Rsh, ρc, Rch, and the contact resistance Rc decrease in all the devices we studied. Our results are helpful for understanding the metal–MoS2 contact and improving the performances of MoS2 devices.

Carrier transport mechanisms of hybrid ZnO nanorod-polymer LEDs

A hybrid polymer-nanorod (NR) light-emitting diode (LED), consisting of a hole-conducting polymer poly (9-vinyl carbazole) (PVK) and ZnO nanorod (NR) composite, with the device structure of glass/indium-tin-oxide (ITO)/PEDOT:PSS/(PVK + ZnO nanorods)/Al is fabricated through a simple spin coating technique. TEM images shows inhomogeneous deposition and the agglomeration of ZnO NRs, which is explained through their low probability of adsorption on PVK due to two-dimensional structural property. In the current-voltage characteristics, negative differential resistance (NDR) phenomenon is observed corresponding to device structure without ZnO NRs. The carrier transport behavior in the LED device is well described by both ohmic and space-chargelimited-current (SCLC) mechanisms. Broad blue electroluminescence (EL) consisting of two sub peaks, are centered at 441 nm and the other at 495 nm, is observed, which indicates that the ZnO nanorod play a role as a recombination center for excitons. The red shift in the position of the EL compared to that photoluminescence is well explained through band offsets at the heterojunction between the PVK and ZnO NRs.

Variable-Range Hopping and Thermal Activation Conduction of Y-Doped ZnO Nanocrystalline Films

ZnO and Y-doped ZnO nanocrystalline films were separately fabricated on the glass substrates by sol-gel spin-coating method. X-ray diffraction patterns of the films show the same wurtzite hexagonal structure and (0 0 2) preferential orientation. Scanning electron microscope images show that grain size and thickness of the nanocrystalline films decrease with increasing doping concentration. The decrease of optical bandgap with the increase of Y doping is deduced from the transmittance spectra. Temperature-dependent resistivity reveals a semiconductor transport behavior for all ZnO and Y-doped ZnO nanocrystalline films. The resulting conductivity originates from the combination of thermal activation conduction and Mott variable-range hopping (VRH) conduction. In the high-temperature range, the temperature-dependent resistivity can be described by the Arrhenius equation, σ (T) = σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> exp[ -(E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> /kT)], which shows the thermal activation conduction. The activation energy Ea increases from 0.47 meV for ZnO film to 0.83 meV for Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.98</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.02</sub> O film. On the contrary, in the low-temperature range, the temperature-dependent resistivity can be fitted well by the relationship, σ(T) = σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h 0</sub> exp[-(T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /T) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/4</sup> ], which indicates the behavior of Mott VRH. The results demonstrate that the crystallization and the corresponding carrier transport behavior of the ZnO and Y-doped ZnO nanocrystalline films are affected by Y doping.

Numerical Analysis Using Finite Element Method On Phosphorescent Organic Light Emitting Diodes

In this paper, we report our numerical simulation on the electronic-optical properties of the phosphorescent organic light emitting diodes (PHOLEDs) devices. In order to calculate the electrical and optical characteristics such as the transport behavior of carriers, recombination kinetics, and emission property, we undertake the finite element method (FEM). Our model includes Poisson’s equation, continuity equation to account for behavior of electrons and holes and the exciton continuity/transfer equation. We demonstrate that the refractive indexes of each material affect the emission property and the barrier height of the interface influences the behavior of charges and the generation of exciton.

Non-parabolicity and band gap re-normalisation in Si doped ZnO

A combinatorial methodology, developed for the rapid optimisation of sputtered transparent conducting oxides, was applied to Si doped ZnO. A wide range of compositions have been explored over a single sample to determine an optimum composition, with respect to the minimisation of resistivity, of x = 0.65% wt. SiO2. A fundamental investigation of the conduction band non-parabolicity yields values of me0=0.35m0 and C = 0.3 eV−1 for the conduction band minimum effective mass and the non-parabolicity factor, respectively. The variation of extracted band gap values with respect to dopant concentration provided an estimate of the magnitude of re-normalization effects. A model is proposed to describe the carrier transport behaviour for a degenerate polycrystalline semiconductor by accounting for the tunnelling of carriers through grain boundaries.

Diameter-Independent Hole Mobility in Ge/Si Core/Shell Nanowire Field Effect Transistors

Heterostructure engineering capability, especially in the radial direction, is a unique property of bottom-up nanowires (NWs) that makes them a serious candidate for high-performance field-effect transistors (FETs). In this Letter, we present a comprehensive study on size dependent carrier transport behaviors in vapor-liquid-solid grown Ge/Si core/shell NWFETs. Transconductance, subthreshold swing, and threshold voltage exhibit a linear increase with the NW diameter due to the increase of the transistor body size. Carrier confinement in this core/shell architecture is shown to maintain a diameter-independent hole mobility as opposed to surface-induced mobility degradation in homogeneous Ge NWs. The Si shell thickness also exhibits a slight effect on the hole mobility, while the most abrupt mobility transition is between structures with and without the Si shell. A hole mobility of 200 cm(2)/(V · s) is extracted from transistor performance for core/shell NWs with a diameter range of 15-50 nm and a 3 nm Si shell. The constant mobility enables a complete and unambiguous dependence of FET performance on NW diameter to be established and provides a caliper for performance comparisons between NWFETs and with other FET families.

Two-dimensional Au lattices featuring unique carrier transport preference and wide forbidden gap.

Large-scale 2D Au lattices with honeycomb-like structure are fabricated on Si(111)-7 × 7 surface at room temperature. The growth pattern investigated by reflection high-energy electron diffraction and in situ scanning tunneling microscopy indicates that the 2D Au lattices are composed of two interfacial distinct layers that are completely formed one after another with a close-packed structure. A unique wide forbidden gap of 4.1 eV is measured around the Fermi level of the 2D Au lattices by scanning tunneling spectroscopy. Bias-dependent STM images and theoretical simulations suggest that the in-plane quantum coupling and carrier transport behavior are responsible for the novel electronic properties. In addition to local electronic states, the electronic structures of 2D Au lattices are further modulated by the carrier transport preference that is determined by carrier energy and symmetry of 2D lattices. These findings will provide some references for the controlled fabrication and for routing the carrier transport behavior of low-dimensional metal structures.

Synthesis and Interfacial Electron Transfer of a Composite Film of Graphene and Tungsten Oxide

Composite films of graphene and tungsten oxide were fabricated by dip-coating with ammonium metatungstate as the precursor and polyvinylpyrrolidone as the bridging agent. The as-prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Photocurrent test, electrochemical impedance spectroscopy (EIS), transient photocurrent spectroscopy,and intensity-modulated photocurrent spectroscopy were used to study the transfer process and transport behavior of the charge carriers at the interface of the film electrodes. The results showed that the tungsten oxide nanoparticles were sufficiently composited with graphene. The efficiency of photoelectric conversion improved significantly. The transient constant and the electron-hole lifetime increased after the incorporation of graphene. The electron transit time of the composite film was reduced and was found to be only 47.5% of that of the tungsten oxide film.

Voltage-controlled ferromagnetism and magnetoresistance in LaCoO3/SrTiO3 heterostructures

A LaCoO3/SrTiO3 heterostructure grown on Si (001) is shown to provide electrically switchable ferromagnetism, a large, electrically tunable magnetoresistance, and a vehicle for achieving and probing electrical control over ferromagnetic behavior at submicron dimensions. Fabrication of devices in a field-effect transistor geometry enables application of a gate bias voltage that modulates strain in the heterostructure via the converse piezoelectric effect in SrTiO3, leading to an artificial inverse magnetoelectric effect arising from the dependence of ferromagnetism in the LaCoO3 layer on strain. Below the Curie temperature of the LaCoO3 layer, this effect leads to modulation of resistance in LaCoO3 as large as 100%, and magnetoresistance as high as 80%, both of which arise from carrier scattering at ferromagnetic-nonmagnetic interfaces in LaCoO3. Finite-element numerical modeling of electric field distributions is used to explain the dependence of carrier transport behavior on gate contact geometry, and a Valet-Fert transport model enables determination of spin polarization in the LaCoO3 layer. Piezoresponse force microscopy is used to confirm the existence of piezoelectric response in SrTiO3 grown on Si (001). It is also shown that this structure offers the possibility of achieving exclusive-NOR logic functionality within a single device.