Materials effects on the electrode-sensitive bipolar resistive switches of polymer:gold nanoparticle memory devices

Electronic devices with an polystyrene (PS) layer blended with Au nanoparticles capped with conjugated 2-naphthalenethiol (Au–2NT NPs) sandwiched between Au and Al electrodes exhibit bipolar resistive switches sensitive to the electrodes. This paper reports the effects of materials, including electrode materials, capping ligands of Au nanoparticles and matrix polymers, on the electrical behavior of the polymer:nanoparticle memory devices. Although the devices using Cu to replace Au as the top electrode exhibit resistive switches similar to those with Au, the threshold voltage for the resistive switch is higher, and the current density for the devices in the low conductivity state is lower. However, the threshold voltage and the current density are almost the same as those with Au as the top electrode, when a semiconductor, MoO₃, is used to replace Au as the top electrode of the devices. The effects of these electrodes are attributed to the charge transfer at the contacts between Au–2NT NPs and the electrodes. The resistive switches are also sensitive to the capping organic ligand of the Au nanoparticles. The threshold voltage decreases and the current density increases, when conjugated benzenethiol is used to replace 2-naphthalenethiol. However, the current density dramatically decreases and the threshold voltage increases, when 2-benzeneethanethiol, a partially conjugated molecule, is adopted as the capping ligand of the Au nanoparticles. The effect of the capping ligands is ascribed to their effect on the charge tunneling across the Au–2NT NPs in the active layer and the contacts between Au–2NT NPs and electrodes. The devices with poly(methyl methacrylate) (PMMA) replacing PS as the polymer matrix exhibit resistive switches almost the same as those with PS, which indicates that the Au–2NT NPs rather than the polymer is the active material responsible for the resistive switches.     

Temperature-sensitive asymmetrical bipolar resistive switches of polymer:nanoparticle memory devices

The interface between two bulk electronic materials can significantly affect the electrical behavior of electronic devices. But the interface between a bulk metal and metal nanoparticles has been rarely explored. This paper reports significant temperature effect on the asymmetrical resistive switches of polymer:nanoparticle memory devices. The devices have architecture of a polystyrene layer admixed with gold nanoparticles capped with conjugated 2-naphthalenethiol sandwiched between Au and Al electrodes. The devices exhibit significant resistive switches at room temperature. However, the resistive switches become less significant at temperature below 200 K, and they are not noticeable at 103 K. The temperature effect suggests that the resistive switches are assisted by the thermal energy. The charge transport through the devices has different mechanisms at high and low temperatures. At temperature above 220 K, the Poole–Frenkel emission is an important mechanism for the charge transport. At temperature below 220 K, the temperature-independent Fowler–Nordheim tunneling becomes an important process.     

Geometry-based finite-element modeling of the electrical contact between a cultured neuron and a microelectrode

The electrical contact between a substrate embedded microelectrode and a cultured neuron depends on the geometry of the neuron-electrode interface. Interpretation and improvement of these contacts requires proper modeling of all coupling mechanisms. In literature, it is common practice to model the neuron-electrode contact using lumped circuits in which large simplifications are made in the representation of the interface geometry. In this paper, the finite-element method is used to model the neuron-electrode interface, which permits numerical solutions for a variety of interface geometries. The simulation results offer detailed spatial and temporal information about the combined electrical behavior of extracellular volume, electrode-electrolyte interface and neuronal membrane.     

Nonvolatile memory and opto-electrical characteristics of organic memory devices with zinc oxide nanoparticles embedded in the tris(8-hydroxyquinolinato)aluminum light-emitting layer

The nonvolatile organic memory devices based on the tris(8-hydroxyquinolinato)aluminum (Alq₃) emitting layer embedded with zinc oxide nanoparticles (ZnO-NPs) are reported. The devices have a typical tri-layer structure consisting of the Alq₃/ZnO-NPs/Alq₃ layers interposed between indium tin oxide (ITO) and aluminum (Al) electrodes. An external bias is used to program the ON and OFF states of the device that are separated by a four-orders-of-magnitude difference in conductivity. No significant degradation of the device is observed in either the ON or OFF state after continuous stress (∼10⁵ s) and multicycle (∼10³ cycles) testings. These nanoparticles behave as the charge trapping units, which enable the nonvolatile electrical bistability when biased to a sufficiently high voltage. Impedance spectroscopy, capacitance–voltage (C–V) and current–voltage (I–V) analysis are used to verify the possible physical mechanism of the switching operation. Moreover, it is found that the location of the ZnO-NPs could affect the memory and opto-electrical characteristics of the devices, such as the ON/OFF ratio, threshold voltage and turn-on voltage, which can be attributed to the influence of the ZnO-NPs and diffused Al atoms in the bulk of the Alq₃ layer.     

Bulk and contact electrical properties by the magneto-transmission-line method: Application to GaAs

The application of a magnetic field to the measurement of planar contact resistances and semiconductor sheet resistances by the standard transmission-line method yields additional information about both bulk and contact properties. In particular, the intrinsic semiconductor mobility and sheet carrier concentration are determined, thus eliminating the need for a separate Hall-effect measurement. The technique is applied to GaAs and AlGaAs/InGaAs layers designed for MESFET and MODFET applications, respectively.     

Tuning the energy gap and charge balance property of bipolar host by molecular modification: Efficient blue electrophosphorescence devices based on solution-process

Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet–triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO–LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (E_T = 3.0 eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2 cd A⁻¹ and a power efficiency of 11.5 lm W⁻¹, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving.     

The effect of charge transfer state on the open-circuit voltage of small-molecular organic photovoltaic devices: A comparison between the planar and bulk heterojunctions using electroluminescence characterization

This work studies the open-circuit voltage (V_OC) of planar and bulk heterojunction organic photovoltaic (OPV) devices by means of electroluminescent (EL) technique to resolve the charge transfer (CT) states between donor and acceptor. The OPV devices containing a small part of bulk heterojunction increases the V_OC as a result of the enhanced CT process as compared with a complete planar structure. Red shift of the CT charge transfer was observed by increasing the bulk volume, which indicates the increased degree of interaction between both molecules and excitons. By characterizing the EL spectra of OPV devices and relating them to the CT absorption, the interfacial property between the donor and acceptor is shown to be crucial for determining the V_OC in small-molecule OPV devices. Detailed analysis of the energetic loss was also used to interpret the V_OC under the effect of CT states.     

On the correlation between morphology and Amplified Spontaneous Emission properties of a polymer: Polymer blend

We investigate the Amplified Spontaneous Emission (ASE) properties of a prototypical host-guest polymer polymer blend, namely poly(9,9-dioctylfluorene) (PF8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) blend, with different concentration ratio. We show that the initial F8BT content increase causes an increase of the F8BT ASE threshold, even leading to ASE suppression for F8BT contents between 25% and 75%. ASE is then recovered upon further increase of the F8BT relative content. We demonstrate that the ASE properties of the PF8:F8BT are dominated by morphology effects, like submicrometric phase segregation, determining the net gain of the active waveguides.     

Enhancement of the electroluminescence of organic light emitting devices based on Ir(ppy)3 by doping with metallic and magnetic nanoparticles

Magnetic nanoparticles embedded in the active layer of the Organic Light Emitting Diodes (OLEDs) significantly increases the electroluminescence and the charge transport without influencing the transparency of these devices. A brief comparison was done in order to identify which parameter influences these properties, by comparing the CoFe₂O₄ magnetic nanoparticles with CoFe₂ metallic magnetic nanoparticles, the latter one being obtained by thermal reduction in hydrogen of cobalt ferrite nanoparticles. CoFe₂ have shown a better efficiency of the metallic nanoparticles where probably the main advantage is the higher magnetization property instead of the coercive field. Concerning the charge transport across the OLEDs, these nanoparticles reduce the electron injection, acting as filling traps, which directly increases the electroluminescence and the current at the same voltage.     

The contact resistance at the interface between a disc electrode and an infinite slab: Mixed-boundary-value solutions

We consider the contact-resistance problem that arises when a circular-disc electrode is in imperfect contact with a semiconductor slab, the imperfect contact being modelled by an infinitely thin layer of resistive material at the interface between the disc electrode and the slab. The resulting mixed-boundary-value problem is solved through the use of basis functions that satisfy the boundary conditions outside the source region identically. Calculations of the source current-density and the total slab resistance (including the effect of the contact resistance) are performed for homogeneous slabs of different thicknesses and with different substrate resistivities, for a wide range of values of the contact resistivity of the interface layer. The results obtained show that the presence of a contact resistance tends to make the source current density distribution more uniform. They also confirm the existence of upper and lower bounds for the difference between the total slab resistance and the layer contact-resistance, as predicted by Foxhall and Lewis. Although applied only to slabs of uniform resistivity, the method can be readily extended to slabs of nonuniform resistivity.     

Multilayer light emitting devices with organometal halide perovskite: Polymer composite emission layer: The relationship of device performance with the compositions of emission layer and device configurations

The formation of composite layers comprising polymers such as poly(ethylene oxide) (PEO) and organometal halide perovskites (Peros) is an effective way to improve the morphology integrity of Pero films and the performance of Pero light emitting devices. Herein, we report the influences of the ratio of the organic content to inorganic content in the precursors as well as the compositions of PEO: Pero films on their morphology, crystal structure and electroluminescent property. Multilayer Pero light emitting devices show the external quantum efficiency of ca. 4.0% and power efficiency of 7.9 lm W⁻¹.     

Chemical reaction concerns of gate metal with gate dielectric in Tagate MOS devices: an effect of self-sealing barrier configurationinterposed between Ta and SiO2

Chemical reaction of gate metal with gate dielectric for Ta gate MOS devices has been experimentally investigated both by electrical and physical measurements: capacitance-voltage (C-V), current-voltage (I-V), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), electron diffraction measurements. In spite of the chemical reaction of Ta with SiO₂ consuming ~1-nm-thick in gate oxide, the interface trap densities of ~2×10¹⁰ cm^-2 eV ^-1 at midgap and ideal channel mobility characteristics have been observed in the Ta gate MOS devices with 5.5-nm-thick thermal oxide gate dielectric. Considering the experimental data with theoretical calculation based on thermodynamics together, a barrier layer model has been developed for the Ta gate MOS systems. The physical mechanism involved is probably self-sealing barrier layer formation resulting from the chemical reaction kinetics in the free-energy change of Ta-Si-O system     

低温退火对具有HfO2/TiN栅结构的MOS电容电学性能的影响

The effects of low temperature annealing,such as post high-k dielectric deposition annealing(PDA),post metal annealing(PMA)and forming gas annealing(FGA)on the electrical characteristics of a metal–oxide–semiconductor(MOS)capacitor with a TiN metal gate and a HfO2dielectric are systematically investigated.It can be found that the low temperature annealing can improve the capacitance–voltage hysteresis performance significantly at the cost of increasing gate leakage current.Moreover,FGA could effectively decrease the interfacial state density and oxygen vacancy density,and PDA could make the flat band positively shift which is suitable for P-type MOSs.     

Influence of the polysilicon doping on the electrical quality of thin oxides: a confrontation between vertical and horizontal furnaces

In this work, the influence of polysilicon doping on thin oxides (thickness equal or below 10 nm) quality and reliability (thickness equal or below 10 nm) in MOS capacitors with polysilicon gate is evaluated. By observing the polysilicon deposed in vertical and horizontal furnaces, a higher degradation in the oxide–silicon interface at high doping concentration has been found. In the case of vertical furnaces, a more evident charge trapping in the constant current stress (CCS) V(t) curves and Qbd (ERCS) degradation have also been noticed. Resistivity measurements at different concentrations show a saturation effect just in correspondence of the oxide degradation. From a morphological point of view, the poly deposited in vertical furnaces consists of grains which are larger than the ones found in horizontal furnace polysilicon and contains lower microdefectivity. Starting from these observations a model explaining the polysilicon morphology role in the oxide reliability can be proposed. According to it, the degradation of the interface is caused by the phosphorus coming from the “in situ” doped polysilicon. The hypothesis is that, at high concentrations and in presence of very large polysilicon grains, phosphorous cannot segregate at the interfaces among the polysilicon grains and, moving through the thin oxide, damages the silicon interface. This model has been confirmed by electrical, AFM and TEM analysis and all the collected data have been related to the finished devices performances (yield and reliability of CMOS flash memories, 0.25 μm technology and below).     

Methods of separation of semiconductor devices by reliability with the use of a low-frequency noise and X-ray irradiation

Methods of the separation of semiconductor devices (SDs) by reliability with the use of parameters of low-frequency (LF) noise under the effect of X-ray irradiation are considered. Different methods of the separation of SDs by reliability depending on their constructive features are suggested.     

Field distributions in the rat tibia with and without a porousimplant during electrical stimulation: a parameteric modeling

Expeditious post-operative ingrowth of bone is necessary for clinically successful fixation of porous joint prostheses. Electrical or electromagnetic fields to stimulate bone growth into porous implants have been used; however, they produced nonconvincing data. This was partially attributable to the lack of quantification of the localized electric fields produced in the pores of the implants. Therefore, this study set out: i) to quantify the local electric field values induced into the surface pores of nonconducting implants by "capacitive" coupling and to determine the magnitude of the macroscopically applied capacitively coupled electrical currents to induce specific electric field amplitudes in the pores, ii) to identify the important dielectric properties of the implant-tissue interface, and iii) to create the basis for successfully applying electrical fields in an animal model to stimulate bone ingrowth. A finite element method was used to calculate the electric field gradients and current densities present in a rat tibia modeled with a porous intramedullary implant when capacitively stimulated. Results indicated that while the current density in the pores are reduced in comparison to the region just outside the pore by about one order of magnitude, a significant current density still exists in the pore region. Furthermore, the presence of the implant increases the current densities in the trabecular bone while decreasing these values in the cortical bone. Replacing the trabecular bone in the pore by saline increases the current density in the pore by three-fold, but decreases the voltage gradient by a similar factor.     

Light emitting devices based on Si nanoclusters： the integration with a photonic-crystal and electroluminescence properties

We present the properties and potentialities of light emitting devices based on amorphous Si nanoclusters. Amorphousnanostructures may constitute an interesting alternative to Si nanocrystals for the monolithic integration of optical andelectrical functions in Si technology. In fact, they exhibit an intense room temperature electroluminescence (EL). The ELproperties of these devices have been studied as a function of current and of temperature. Moreover, to improve theextraction efficiency of the light, we have integrated the emitting system with a 2D photonic crystal structure opportunelyfabricated by using conventional optical lithography to reduce the total internal reflection of the emitted light. The extractionefficiency in such devices increases by a factor of 4 at a resonance wavelength.     

Dye-sensitized solid-state photovoltaic cells: Suppression of electron-hole recombination by deposition of the dye on a thin insulating film in contact with a semiconductor

The heterojunction n-SnO₂/Ru-dye/p-CuI prepared by deposition of the ruthernium bipyridyl dye on a meso-porous film of SnO₂ followed by deposition of p-CuI was found to be inactive with respect to visible light photoresponse and dark current rectification. However, n-SnO₂/Al₂O₃/Ru-dye/p-CuI where the dye is coated on a thin film on Al₂O₃ first deposited on SnO₂, delivered a short-circuit current density of 1.7 mAcm⁻² and an open-circuit voltage of 350 mV, behaving as a dye-sensitized solid-state photovoltaic cell. This result is explained as a transfer of energetic electrons released by excitation of the dye molecules to the conduction band of SnO₂ via tunneling across the thin layer of Al₂O₃. The implications of the result on suppression of recombination in dye-sensitized photovoltaic cells are discussed.     

The correlation of the electrical properties with electron irradiation and constant voltage stress for MIS devices based on high-k double layer (HfTiSiO:N and HfTiO:N) dielectrics

This paper describes the influence of e-beam irradiation and constant voltage stress on the electrical characteristics of metal-insulator-semiconductor structures, with double layer high-k dielectric stacks containing HfTiSiO:N and HfTiO:N ultra-thin (1 and 2 nm) films. The changes in the electrical properties were caused by charge trapping phenomena which is similar for e-beam irradiation and voltage stress cases. The current flow mechanism was analyzed on the basis of pre-breakdown, soft-breakdown and post-breakdown current-voltage (J-V) experiments. Based on α-V analysis (α=d[ln(J)]/d[ln(V)]) of the J-V characteristics, a non-ideal Schottky diode-like current mechanism with different parameters in various ranges of J-V characteristics is established, which limits the current flow in these structures independent of irradiation dose or magnitude of applied voltage during stress.     

Organic light-emitting devices with a 2-(4-biphenyl)-5-(4-butylphenyl)-1,3,4-oxadiazole layer between the α-naphtylphenyliphenyl diamine and 8-hydroxyquinoline aluminum

Organic light-emitting devices (OLEDs) with a 2-(4-biphenyl)-5-(4-butylphenyl)-1,3,4-oxadiazole layer between the α-naphtylphenyliphenyl diamine and 8-hydroxyquinoline aluminum were fabricated using a vacuum evaporation method. Compared to the different thickness of the buffer layer, the OLEDs with the 1.0 nm buffer layer showed the maximum power efficiency. The enhancements in power efficiency result from an improved balance of hole and electron injections. After comparing among different density buffer layer, PBD are good candidates for hole-injecting buffer layer, and 1.0 nm PBD buffer layer shows better operational durability and life.