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Epoxy nano–micro composite specimen prepared with micro silica and ion trapping nanoparticle, by shear mixing process, was exposed to gamma radiation and its performance for space charge and charge trap characteristics were analysed. The threshold for space charge accumulation of epoxy nanocomposites reduces and rate of space charge accumulation increases with an increase in dosage of gamma irradiation. The average growth of space charge density during poling and charge decay rate during depoling are relatively higher for gamma-irradiated specimens than the virgin specimen. The initial surface potential has a marginal reduction with increase in the dosage of gamma radiation, but the surface potential decay rate has increased significantly. Trap distribution characteristics indicate more number of shallow traps and increase in charge mobility after irradiation. The relative permittivity and loss tangent of the specimens have high impact due to gamma irradiation. The activation energy calculated from DC conductivity by Arrhenius law reduces with increment in radiation dose. Laser-induced breakdown spectroscopy reflected no change in elemental composition with gamma-irradiated specimen. The variation in plasma temperature and ion line to atomic line intensity ratio with dosage of gamma radiation have direct correlation to the Vickers hardness number of the specimens.

Density functional theory (DFT) calculations of polyethylene (PE) HVDC cable insulation have been performed for systems containing four different chemical impurities: acetophenone, cumene, $\alpha$-methyl styrene and $\alpha$-cumyl alcohol. Systems were generated by molecular dynamics (MD) equilibration at four different temperatures relevant for cable insulation applications: 277 K, 293 K, 343 K and 363 K. With the goal of gaining better measure of variations in hole and electron traps energies, four initial configurations were also stochastically generated at each temperature, which yielded four different final configurations after equilibration. The counterpoise correction scheme was implemented for DFT calculations, by distributing ghost atoms thought any empty pockets of space in between the PE chains. The PBE functional was selected for DFT simulations. The resulting band gaps were in agreement with those of earlier GGA-based studies, and thus lower by 3 eV than empirical band gaps. For all impurities, the first HOMO state and the first two LUMO states were generally located on the impurity molecule, forming one hole trap and two electron traps, but certain configurations generated increased electron trap numbers, or eliminated hole traps. No dependence could be derived between temperature and trap depth for either electron or hole traps. Mean electron trap energies were largely in agreement with results from earlier studies, they were deepest for acetophenone, and they varied by as much as 0.6 eV between different configurations. Hole traps are universally shallow and vary by up to 0.7 eV between configurations, and are similar in depth for all impurities. Results suggest that electron trap depths correlate with the presence of molecular features such as oxygen atoms and conjugated double bonds. The dependence of trap depth on the spatial configuration of the impurity molecule suggests that results could be improved by more precise quantum mechanical treatment of the dynamics of the impurity. Täthetsfunktionalteori (DFT) har använts för beräkningar av isolering till HVDC kablar som består av polyeten innehållande fyra olika kemiska orenheter: acetofenon, kumen, alfa-metylstyren och alfa-kumylalkohol. System att studera genererades genom molekylärdynamisk ekvilibrering vid fyra olika temperaturer relevanta för tillämning till kabelisolering: 277 K, 293 K, 343 K och 363 K. För att få ett mått på de variationer som existerar i energierna på hål- och elektronfällor genererades stokastiskt fyra initialkonfigurationer vid varje temperatur, vilket fyra olika konfigurationer efter relaxering. Motviktskorrigering implementerades för DFT-beräkningar, genom att fördela "spökatomer" i de tomrum som bildas mellan PE-kedjorna i den amorfa fasen. PBE-funktionalen användes för DFT-simuleringar. De resulterande bandgapen stämde överens med tidigare GGA-baserade studier, och var därmed runt 3 eV smalare än empiriskt uppmätta bandgap. För alla orenheter var det första HOMO-tillståndet och de två första LUMO-tillstånden i allmänhet placerade på orenheten, vilket resulterade i en hålfälla och två elektronfällor, men vissa konfigurationer gav upphov till fler elektronfällor, eller eliminerade hålfällorna. Inget samband kunde härledas mellan temperaturen och djupet på fällorna för vare sig elektron- eller hålfällor. Medelvärdet på elektronfällornas energier överensstämde till stor del med resultat från tidigare studier, energierna var högst för acetofenon, och de varierade med så mycket som 0,6 eV mellan olika konfigurationer. Hålfällorna var genomgående grunda, varierade med upp till 0,7 eV mellan olika konfigurationer, och hade likartat djup för alla orenheter. Resultaten indikerar att variationerna elektronfällornas medeldjup uppstår på grund av orenheternas olika molekylära uppbyggnad: förekomst av syreatomer och konjugerade dubbelbindningar i orenheterna leder till djupare elektronfällor. Det faktum att djupet på elektron- och hålfällor varierar mellan olika rumsliga konfigurationer av av orenheten och polyetenstrukturen ger en antydan om att resultaten kan komma att förbättras om dynamiken hos orenheten simuleras med mer exakta kvantmekanisk metoder.

In this paper, the literature on vacuum flashover and mechanisms are reviewed, and the development of the Electron triggered polarization relaxation (ETPR) theory is the main content. New evidence is provided to prove the validity of the ETPR theory. Polymer composites show sine-variations in vacuum flashover with the filler content increase is a substantial supporting for ETPR theory. The trap depth and polarization variations corresponding to the transitions of the topological relationship of fillers in the polymer matrix are considered to be the leading causes of the regular variations of flashover. An experiment of grain ejection is designed to verify the application of ETPR. The findings can provide a better understanding of the essential role of dielectric on the flashover, which also supplies a base for surface insulation on the gas atmosphere.

This paper analyses the neutron irradiation impact on the electrical performances of unstressed, ON-state, OFF-state, and negative gate bias (NGB) stressed AlInN/GaN HEMTs. These irradiations have resulted in the creation of electron traps that are causing a decrease in the drain current and an increase in the access resistance of the unstressed, ON-state, or OFF-state stressed AlInN/GaN devices. These degradations have been correlated with gamma spectrometry measurements and transmutation reactions occurred during the thermalized neutron irradiation have been highlighted. Despite these phenomena, a rise in drain current and a reduction in access resistance have been observed when NGB stressed AlInN/GaN HEMTs were irradiated with a fluence of $1.2\times 10^{\mathbf {12}}$ neutrons/cm2. The differences between the electrical behaviors of unstressed, ON-state, OFF-state, and NGB stressed devices observed after the neutron bombardment are related to the presence of electron traps in these device structures.

Polymer light-emitting electrochemical cells (PLECs) and light-emitting diodes (PLEDs) receive interest for large-area lighting and signage applications. During operation of a PLEC, a p-i-n junction develops where electrons and holes are injected into the film and are transported along n- and p-doped regions to the intrinsic (i) region, where they recombine under light emission. Conceptually, this resembles the PLED device architecture equipped with Ohmic charge-injection and transport layers. The similarity between the i-region of the PLEC and the emissive layer of the PLED is obvious; however, implication of this has not been examined in detail so far. For example, for PLEDs it is known that electron traps hinder the electron transport, and that hole trap formation dictates the long-term durability. Here, for PLECs the electrical and optical response to electrical driving and breaks are studied, the current response to external light irradiation is probed, and degradation is followed with long-term absorption and capacitance measurements. The electron traps in PLECs are identified and it is found that hole trap formation limits the device lifetime, in the same manner as established for PLEDs. It is concluded that charge traps in semiconducting polymers present important, but so far overlooked intrinsic performance limitations for PLECs.

In this study, we investigate the degradation characteristics of E-mode GaN High Electron Mobility Transistors (HEMTs) with a p-GaN gate by designed pulsed and prolonged negative gate (VGS) bias stress. Device transfer and transconductance, output, and gate-leakage characteristics were studied in detail, before and after each pulsed and prolonged negative VGS bias stress. We found that the gradual degradation of electrical parameters, such as threshold voltage (VTH) shift, on-state resistance (RDS-ON) increase, transconductance max (Gm, max) decrease, and gate leakage current (IGS-Leakage) increase, is caused by negative VGS bias stress time evolution and magnitude of stress voltage. The significance of electron trapping effects was revealed from the VTH shift or instability and other parameter degradation under different stress voltages. The degradation mechanism behind the DC characteristics could be assigned to the formation of hole deficiency at p-GaN region and trapping process at the p-GaN/AlGaN hetero-interface, which induces a change in the electric potential distribution at the gate region. The design and application of E-mode GaN with p-GaN gate power devices still need such a reliability investigation for significant credibility.

In last three decades, great progress has been made in the field of organic electronics. Researchers have put tremendous efforts to make new materials and device architectures, which has resulted in a great commercial success of organic light emitting diodes in mobile phone and television display screens. Despite that, still today it is challenging to make organic electronic devices that are efficient in performance, stable in operation and are economical in production at the same time. The objective of this thesis is to understand fundamental charge transport properties of small molecules based organic semiconductors and to develop novel organic electronic device architectures. One of the prime requirements for efficient organic optoelectronic devices is to have ohmic charge injection contacts. Therefore, first a charge injection strategy for making ohmic hole contacts is developed. Using this strategy, ohmic hole contacts are achieved on organic semiconductors with an ionization energy up to 6 eV. As a result, the hole transport in a wide range of organic small molecules with ionization energy between 5 to 6 eV could be investigated. Despite the difference in their chemical structures, similar bulk hole mobilities in the range of $1\times10^{-4}$ cm$^{2}$V$ ^{-1} $s$ ^{-1} $ were observed for all molecules. The hole transport was also investigated using molecular multiscale simulations, an excellent agreement was obtained with the experimental results. Despite fullerene derivatives being known as electron conductors, It was found that the fullerene derivative ICBA has a very good hole mobility of $1.4\times10^{-3}$ cm$^{2}$V$ ^{-1} $s$ ^{-1} $ which is the same as bulk electron mobility, demonstrating the intrinsic bipolar charge-trasnport character of organic semiconductors. It is found that charge trapping is causing the frequently observed unipolarity in organic semiconductors, causing preferential conduction of either holes or electrons. This limits the efficiencies and stabilities of the organic optoelectronic devices. By investigating charge trapping in a wide range of organic semiconductors, we have identified that when the electron affinity is lower than 3.6 eV, electron transport becomes trap limited and when ionization energy is higher than 6 eV hole transport becomes trap limited. As a result, within this energy window of about 2.4 eV trap-free charge transport is observed. Combining this energy window for trap-free transport with our developed charge injection strategy, an efficient and stable single layer OLED based on a neat thermally activated delayed fluorescence emitter is demonstrated. The OLED has a maximum external quantum efficiency of 19% at a luminance of $500 $ cdm$^{-2}$and a lifetime to 50% of initial luminance of $1000$ cdm$^{-2}$ of 1,880 h. It has an exceptionally low operating voltage of 2.9 V at a luminance of $10,000$ cdm$^{-2}$, which resulted in a maximum power efficiency of $ 87 $ lmW$^{-1} $.

3C-silicon carbide (3C-SiC) Schottky barrier diodes (SBDs) on silicon (Si) substrates (3C-SiC-on-Si) have been found to suffer from excessive subthreshold current, despite the superior electrical properties of 3C-SiC. In turn, that is one of the factors deterring the commercialization of this technology. The forward current–voltage ( $I$ – $V$ ) characteristics in these devices carry considerable information about the material quality. In this context, an advanced technology computer-aided design (TCAD) model is proposed and validated with measurements obtained from a fabricated and characterized platinum/3C-SiC-on-Si SBD with scope to shed light on the physical carrier transport mechanisms, the impact of traps, and their characteristics on the actual device performance. The model includes defects originating from both the Schottky contact and the heterointerface of 3C-SiC with Si, which allows the investigation of their impact on the magnification of the subthreshold current. Furthermore, the simulation results and measured data allowed for the identification of additional distributions of interfacial states, the effect of which is linked to the observed nonuniformities of the Barrier height value. A comprehensive characterization of the defects affecting the carrier transport mechanisms of the investigated 3C-SiC-on-Si power diode is thus achieved, and the proposed TCAD model is able to accurately predict the device current both during forward and reverse bias conditions.

Thanks to their superior transport properties, indium gallium arsenide (InGaAs) metal-oxide-semiconductor field-effect transistors (MOSFETs) constitute an alternative to conventional silicon MOSFETs for digital applications at ultrascaled nodes. The successful integration of this technology is challenged mainly by the high defect density in the gate oxide and at the interface with the semiconductor channel, which degrades the electrostatics and could limit the potential benefits over Si. In this work, we: 1) establish a systematic modeling approach to evaluate the performance degradation due to interface traps in terms of electrostatics and transport of InGaAs dual-gate ultrathin body (DG-UTB) FETs and 2) investigate the effects of random interface-trap concentration as another roadblock to the scaling of the technology, due to statistical variability of the threshold voltage. Variability is assessed with a Technology CAD (TCAD) simulator calibrated against multi-subband Monte Carlo (MSMC) simulations. The modeling approach overcomes the TCAD limitations when dealing with ultrathin channels (i.e., below 5 nm) without altering crucial geometrical parameters that would compromise the dependability of the variability analysis. Our results indicate that interface-trap fluctuation becomes comparable with the other variability sources dominating the total variability when shrinking the device dimensions, thus contrasting the trend of reduced variability with scaling. This, in turn, implies that interface and border traps may strongly limit the benefits of InGaAs over Silicon if not effectively reduced by gate process optimization.

We present an investigation of vertical leakage in GaN-on-Si epitaxial stack through electrical characterization and device simulations. Different structures of increasing complexity have been fabricated and analyzed in order to achieve a complete understanding of the main transport mechanisms. We have clarified the role of the Si substrate through comparison of identical structures built on p-type and n-type Si substrates. We show that in the case of p-Si substrates the leakage current is sustained by carrier generation in the Si depletion region. We also find that experiments on structures grown on n-doped silicon are consistent with considering electron injection from the substrate through the AlN/Si barrier as the main current limiting mechanism. Our insights are supported by device simulations that consistently reproduce the experimental capacitance–voltage and current–voltage characteristics as a function of temperature for all the considered structures.

Electron trapping in ferroelectric and non-ferroelectric HfO2-based layers was studied at room temperature by charge injection and photodepopulation techniques. The comparison of inferred energy distribution and density of trapped electrons in differently processed samples shows insignificant impact of Al- and Si-doping on trapping properties suggesting intrinsic nature of the observed traps. A comparison to the HfZrO4 layers is provided. The volume concentration of deep traps, most of which are energetically distributed between 2 and 3.5 eV below the HfO2 conduction band, is found to be in the range of 1019 cm−3.

Owing to the formation of interface and new feature of which, the properties of nanodielectrics can be improved. ‘Hard/soft interface’ and its trap distribution can be tailored by functionalised groups. Molecular simulation results show that the interaction energy and electrostatic potential are larger for the soft interface, which indicates the greater bonding strength with the polymer matrix and electrostatic force on charge carriers. Charge transport simulation indicates that the accumulation of homo-charges would form a reverse electric field and distort electric field distribution. The injection depth would be restricted at the vicinity of sample/electrodes due to the greater trapping effect of deep traps, thus weakening the distortion in the sample bulk, thereby decreasing carrier energy and delaying the formation of impact ionisation. Based on the accumulation of carrier energy Φ = Eeλ, the idea of suppressing electron free path and carrier energy to enhance the insulation breakdown is confirmed. The classified effects of nanofillers during dc breakdown and corona-resistant are further understood from carrier energy. The introduced interfacial trap is effective in trapping carriers due to the low carrier energy under dc voltage, while ineffective in blocking the energetic charges during corona-discharge, but nanoparticles exert blocking and scattering effect against the energetic charges.

In this article, an analytical predictive model of interface charge traps in symmetric, long-channel double-gate, junctionless transistors (JLTs) is proposed based on a charge-based model. Interface charge traps arising from exposure to chemicals, high-energy ionizing radiation, or aging mechanism could degrade the charge-voltage characteristics. The model is predictive in a range of temperatures from 77 to 400 K. The validity of the approach is confirmed by extensive comparisons with numerical technology computer-aided design (TCAD) simulations in all regions of operation from deep depletion to accumulation and from linear to saturation.

This paper reports physics based TCAD simulations of multi-frequency C-V curves of In0.53Ga0.47As MOSCAPs including the AC response of the border traps. The calculations reproduce the experimental inversion and accumulation capacitance versus frequency, and provide a means to profile the space and energy density of states of border traps. A sensitivity analysis of the results to border traps' distribution is carried out changing the trap volume and the oxide capacitance.

Graphical abstractDisplay Omitted Charge traps in (100)Ge/Sc2O3/HfO2 stacks are studied by ESR and electrical probing.The utility of Sc2O3 as passivation layer on Ge is studied by conventional ESR.Trapping centers in (100)Ge/Sc2O3 entities are tracked by ESR as g=2.0017 defects.Defect generation in (100)Ge/Sc2O3 entities may be suppressed by sufficient supply of O2.Oxygen deficiency causes thermal degradation of (100)Ge/Sc2O3/HfO2 stacks. The usefulness of employing the rare-earth Sc2O3 high-? (~16) dielectric in passivating Ge surfaces for metal-oxide-semiconductor device application is studied in terms of occurring inherent paramagnetic point defects, operating as detrimental charge traps. To assess their atomic nature, low temperature K-band electron spin resonance (ESR) experiments have been carried out in combination with electrical probing, including capacitance-voltage, internal photoemission and photoconductance observations, on (100)Ge/(GeO2)/Sc2O3/HfO2 stacks grown by atomic layer deposition subjected to various processing steps. In stacks with Ge directly contacted with Sc2O3, without an additional O-supply processing step, ESR reveals the presence of a prominent signal of powder pattern shape, centered at g=2.0017(2). It is ascribed to O-deficiency centers located in the Sc2O3-based dielectric, which from combined electrical analysis are concluded to be at the origin of substantial electron trapping (~6i?1012cm-2) in near-interfacial layers. Generally, the data suggest that O-deficiency caused by oxidation of Ge (O-scavenging) emerges as a crucial parameter in defect generation, leading to the conclusion that the suppression of defect generation will require a sufficient supply of oxygen during critical manufacturing steps.

Electron and hole transport properties of the organic small molecule N,N0 -Di(1-naphthyl)-N,N0 - diphenyl-(1,10 -biphenyl)-4,40 -diamine are investigated by space-charge-limited current measurements. The hole transport shows trap-free behavior with a mobility of 2.3 108m2 /Vs at vanishing carrier density and electric field. The electron transport, on the other hand, shows heavily trap-limited behavior, which leads to highly unbalanced transport. A trap concentration of 1.3 1024m3 was found by modeling the electron currents, similar to the universal trap concentration found in conjugated polymers. This indicates that electron trapping is a generic property of organic semiconductors, ranging from vacuum-deposited small-molecules to solution-processed conjugated polymers.

A ns Nd:Yag laser, at intensity of 1010 W/cm2 is employed to generate carbon and aluminum non-equilibrium plasmas at a temperature of about 33 eV accelerating ions at energies of the order of 130 eV per charge state. The ion emission occurs manly along the normal to the target surface and can be detected using ion collectors employed in time-of-flight configuration. The application of magnetic field along the axe of the ion emission permits to focalize the ion emission enhancing the detected ion current. The formation of electron traps, due to the magnetic force lines, drives the ion acceleration improving their kinetic energy. Different applications can make use of these results to increase the flow of charged particles and their energy employing appropriate static magnetic fields, as it will be presented and discussed.

Luminescence (thermoluminescence (TL) and optically stimulated luminescence (OSL)) methods are important for dosimetric studies. Understanding the TL and OSL traps of dosimetric materials is critical to explain their different and similar properties. In this study, TL and OSL signals of natural aragonite mineral were recorded using lexsyg smart TL/OSL reader after different beta irradiations. TL and OSL trap properties were compared with each other using the obtained TL glow curves and OSL signals. In sequential measurements, while the TL signals were observed after the OSL measurements, the OSL signals were not observed after the TL measurements. In conclusion, TL and OSL traps are located close to each other within the band gap of aragonite mineral and the OSL traps affect the TL signals if the TL measurements are done firstly. Cukurova University, FED-2017-8045

Spectral analysis of optically excited currents in single-crystal (100)InAs/amorphous (a-)Al2O3/ metal structures allows one to separate contributions stemming from the internal photoemission (IPE) of electrons into alumina and from the trapping-related displacement currents. IPE spectra suggest that the out-diffusion of In and, possibly, its incorporation in a-Al2O3 lead to the development of 0.4 eV wide conduction band (CB) tail states. The top of the InAs valence band is found at 3.4560.10 eV below the alumina CB bottom, i.e., at the same energy as at the GaAs/a-Al2O3 interface. This corresponds to the CB and the valence band offsets at the InAs/a-Al2O3 interface of 3.160.1 eV and 2.560.1 eV, respectively. However, atomic-layer deposition of alumina on InAs results in additional low-energy electron transitions with spectral thresholds in the range of 2.0–2.2 eV, which is close to the bandgap of AlAs. The latter suggests the interaction of As with Al, leading to an interlayer containing Al-As bonds providing a lower barrier for electron injection. received: 2016-09-05 accepted: 2016-11-17 published: 2016-12-15 ispartof: Journal of Applied Physics vol:120 issue:23 pages:235701-1 status: published

Using state-of-the-art simulation tools ranging from semi-classical Monte-Carlo to full-quantum atomistic approaches, the competitiveness of III-V compounds for next-generation high-performance logic switches is confirmed. A planar double-gate ultra-thin-body (DG-UTB), a triple-gate FinFET, and a gate-all-around nanowire (NW) transistor have been designed according to the ITRS specifications for two technology nodes with physical gate lengths of Lg=15 nm and 10.4 nm. A thorough performance comparison of digital and analog figures of merit at these nodes reveals that for Lg=15 nm, the performance of planar and 3-D architectures is comparable. At LG=10.4 nm, the III-V NW promises the highest performance, especially when lowering the supply voltage from 0.59 V to 0.50 V. It also significantly outperforms its strained silicon counterpart. Finally, the effects of series resistance combined with interface traps, surface roughness, alloy scattering, and electron-phonon interactions have been found to deteriorate the III-V ballistic ON-current by 50–60%.

We present a quantitative analysis of the loss of electroluminescence in light-emitting diodes (LEDs) based on poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) due to the combination of non-radiative trap-assisted recombination and exciton quenching at the metallic cathode. It is demonstrated that for an MEH-PPV LED the biggest efficiency loss, up to 45%, arises from extrinsic non-radiative recombination via electron traps. The loss caused by exciton quenching at the cathode proves only to be significant for devices thinner than 100 nm. Removal of electron traps by purification is expected to enhance the efficiency of polymer LEDs by more than a factor of two. (C) 2012 Elsevier B.V. All rights reserved.

The electron trapping in MOS capacitors with amorphous Al2O3 as an insulating layer was studied through pulsed capacitance-voltage technique. A positive shift of the voltage value corresponding to a constant capacitance (VC) was observed. The dependences of the voltage instability with the applied bias and the charging time were investigated. Two different contributions could be distinguished: a hysteresis phenomenon observed on each measurement cycle, and a permanent accumulated VC-shift to which each measurement cycle contributes. A physical model based on tunneling transitions between the substrate and defects within the oxide was implemented. From the fitting procedure within the energy range covered in our measurements (1.7-2.7 eV below the conduction band edge), the trap density was found to decrease exponentially with trap energy depth from 3.0 × 1020 cm-3eV-1 to 9.6 × 1018 cm-3eV-1, with a uniform spatial distribution within the first 2 nm from the semiconductor interface for the hysteresis traps. Fil: Sambuco Salomone, Lucas Ignacio. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Laboratorio de Física de Dispositivos Microelectrónica; Argentina Fil: Campabadal, F.. Instituto de Microelectronica de Barcelona; España Fil: Faigon, Adrián Néstor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Laboratorio de Física de Dispositivos Microelectrónica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentina

N -channel operation of thin-film transistors based on 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) with a 9-nm-thick poly(methyl methacrylate) (PMMA) gate buffer layer was examined. The uniform coverage of the ultrathin PMMA layer on an SiO 2 gate insulator, verified by X-ray reflectivity measurement, caused the increase of electron field-effect mobility because of the suppression of electron traps existing on the SiO 2 surface. In addition, air stability for n -channel operation of the NTCDA transistor was also improved by the PMMA layer which possibly prevented the adsorption of ambient water molecules onto the SiO 2 surface.

On the basis of experimental evidence and DFT calculations, we propose a simple yet viable way to stabilize and chemically activate gold nanoclusters on MgO. First the MgO surface is functionalized by creation of trapped electrons, (H+)(e-) centers (exposure to atomic H or to H2 under UV light, deposition of low amounts of alkali metals on partially hydroxylated surfaces, etc.); the second step consists in the self-aggregation of gold clusters deposited from the gas phase. The calculations show that the (H+)(e-) centers act both as nucleation and activation sites. The process can lead to thermally stable gold cluster anions whose catalytic activity is enhanced by the presence of an excess electron. © 2008 American Chemical Society.

The adsorption of small amounts of alkali metal atoms (Li, Na, K, Rb, and Cs) on the surface of MgO powders and thin films has been studied by means of EPR spectroscopy and DFT calculations. From a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), a tentative assignment of the preferred adsorption sites is proposed. All atoms bind preferentially to surface oxide anions, but the location of these anions differs as a function of the deposition temperature and alkali metal. Lithium forms relatively strong bonds with MgO and can be stabilized at low temperatures on terrace sites. Potassium interacts very weakly with MgO and is stabilized only at specific sites, such as at reverse corners where it can interact simultaneously with three surface oxygen atoms (rubidium and cesium presumably behave in the same way). Sodium forms bonds of intermediate strength and could, in principle, populate more than a single site when deposited at room temperature. In all cases, large deviations of the hfccs from the gas-phase values are observed. These reductions in the hfccs are due to polarization effects and are not connected to ionization of the alkali metal, which would lead to the formation of an adsorbed cation and a trapped electron. In this respect, hydrogen atoms behave completely differently. Under similar conditions, they form (H(+))(e(-)) pairs. The reasons for this different behavior are discussed.

We elucidate five considerations for accurate estimation of electron tunneling from the gate edges of high-electron mobility transistors (HEMTs). These considerations are listed as follows: 1) edge roughness; 2) net charge at the AlGaN/passivation interface; 3) dielectric constant of the medium above the HEMT surface; 4) nontriangular potential barrier; and 5) negligible angular tunneling from the gate edge. Using these considerations, we calculate the reverse gate current IG of AlGaN/GaN HEMTs based on thermionic trap-assisted tunneling (TTT) and direct tunneling (DT) mechanisms. These calculations establish that the observed rise in IG for a gate voltage beyond the threshold is due to tunneling from the gate edges. The calculations also show that the TTT mechanism can predict the measured G of AlGaN/GaN HEMTs over a wide range of gate voltages and temperatures and point to the possibility of a rapid rise in IG at high gate voltages due to the DT mechanism. � 2007 IEEE.

Mixed phase rutile/anatase catalysts show increased reactivity compared with the pure phases alone. However, the mechanism causing this effect is not fully understood. The electronic properties of the interface and the relative energy of the electron in each phase play a key role in lowering the rate of recombination of electron hole pairs. Using density functional theory and the +U correction, we calculated the bands offsets between the phases taking into account the effect of the interface. Our model included several thousands atoms, and thus is a good representation of an interface between actual nanoparticles. We found rutile to have both higher conduction and valence band offsets than rutile, leading to an accumulation of electrons in the anatase phase accompanied by hole accumulation in the rutile phase. We also probed the electronic structure of our heterostructure and found a gap state caused by electrons localized in undercoordinated Ti atoms which were present within the interfacial region. Interfaces between bulk materials and between exposed surfaces both showed electron trapping at undercoordinated sites. These undercoordinated (typically four) atoms present localized electrons that could enable reduction reactions in the interfacial region, and could explain the increased reactivity of mixed-phase TiO2 photocatalyst materials.

PolySOI MOSFETs have been fabricated on undoped and doped polycrystalline silicon films and characterized to study the effect of doping on grain boundary passivation. The grain boundary trap density (NST) and threshold voltages have been extracted experimentally to evaluate the extent of grain boundary passivation by the dopants. Charge sheet model based on the effective doping concentration has been employed to analytically estimate the threshold voltages using the experimentally determined grain boundary trap density and grain size (Lg) as model parameters. The variation of threshold voltages with increasing doping concentration for the range of NA ? (NST/Lg) has been studied both by simulation and experiments and the results are presented. Analytically estimated threshold voltages and experimental results show that the threshold voltage falls with increase in the dopant concentration and that this effect is indeed due to the reduction in NST as a result of the grain boundary passivation by the dopants. � 2005 Elsevier B.V. All rights reserved.

We present some typical applications of our device simulation model "MOLED" which has proven to be a very useful tool for obtaining insight into the functioning of multi-layer organic light-emitting devices (OLED's). Our general approach consists of combining experimental measurement and numerical device modelling in a deliberate and coordinated way. Taking a step-by-step, evolutive approach, starting with simple single layer devices and gradually moving up to more complex multi-layer architectures with a partially doped emission layer, we could progressively determine unknown parameters in the model and at the same time shed light on specific aspects of OLED functioning. For example, electrode characterization was done by measuring and modeling single layer devices with different electrode materials, while the simulation of time of flight signals gave us insight into the transient state and the effects of correlated disorder on the macroscopic mobility. The internal electric field in multi-layer OLED's could also be investigated by varying the thicknesses of the hole transport and electron transport layers. Applying this method to the standard OLED device structure that has received broad attention in the literature, we have found a number of surprising results. From our experiments, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. The device simulations fully clarify the situation, giving insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we found that there is a leakage of unrecombined holes towards the cathode at low voltages. We also found a strong variation of the electric field in the Alq3 layer due to space charge effects. By using the laser dye derivatives DCM-TPA with electron trapping capabilities and DCM-II with both electron and hole trapping capabilities as dopants in a standard OLED architecture, we could study the effect on transport and emission characteristics. In the case of the exclusively electron trapping dopant, a blue-shift of the emission color with increasing bias is observed which we find is due to a splitting of the recombination zone

By using pyran-containing donor-acceptor dyes as doping molecules in organic light-emitting devices (OLEDs), we scrutinize the effects of charge trapping and polarization induced by the guest molecules in the electro-active host material. Laser dyes 4-(dicyanomethylene)-2-methyl-6-[2-[(julolidin-9-yl)phenyl]ethenyl]-4H-pyran (DCM2) and the novel 4-(dicyanomethylene)-2-methyl-6-[2-[(4-diphenylamino)phenyl]ethenyl]-4H-pyran (DCM-TPA) are used as model compounds. The emission color of these polar dyes depends strongly on doping concentration, which we have attributed to polarization effects induced by the doping molecules themselves. Their frontier orbital energy levels are situated within the bandgap of the tris(8-hydroxyquinoline)aluminum (Alq3) host matrix and allow the investigation of either electron trapping or both electron and hole trapping. In the case of DCM-TPA doping, we were able to show that electron trapping leads to a partial shift of the recombination zone out of the doped Alq3 region. To impede charge-recombination processes taking place in the undoped host matrix, a charge-blocking layer efficiently confines the recombination zone inside the doped zone and gives rise to increased luminous efficiency. For a doping concentration of 1 wt.-% we obtain a maximum luminous efficiency of 10.4 cd A-1. At this doping concentration, the yellow emission spectrum shows excellent color saturation with CIE chromaticity coordinates x, y of 0.49 and 0.50, respectively. In the case of DCM2 the recombination zone is much less affected for the same doping concentrations, which is ascribed to the fact that both electrons and holes are being trapped. The experimental findings are corroborated with a numerical simulation of the doped multilayer devices

Carrier free paths in Chemical Vapour Deposition (CVD) diamond films depend on the presence of traps, which therefore strongly affect the performance of those CVD diamond based devices which rely on the electronic properties of the material, like radiation detectors. For the same reason, these devices can in turn be used as tools to study carrier dynamics. It is well known that some traps may be saturated by pre-irradiation with ionizing radiation (e.g. β-particles), a process called pumping or priming. Not all traps behave in the same way. Due to the large bandgap of diamond, both shallow (not affected by pumping) and deep traps for electrons and holes may exist. We measured, using 5.5 MeV 241 Am α-particles, the response of high quality CVD diamond based detectors after successive annealing steps performed at selected temperatures. The analisys of the decay of the detector efficency with annealing time at several temperatures allows a quantitative evaluation of the activation energy of these defects. Two main trapping centres connected to the pumping process were found, both related to holes, having activation energies of about 1.6 eV and 1.3 eV respectively.

Nominally undoped p-GaS layered single crystals were grown using the Bridgman technique. Thermally stimulated current measurements in the temperature range 10-300 K were performed at a heating rate of 0.10 K/s. The analysis of the data revealed six trap levels at 0.05, 0.06, 0.12, 0.63, 0.71, and 0.75 eV. The calculations for these traps yielded 1.2 × 10-21, 2.9 × 10-23, 2.4 × 10-21, 8.0 × 10-9, 1.9 × 10-9 and 4.3 × 10-10 cm2 for the capture cross sections and 1.6 × 1013, 5.0 × 1012, 7.3 × 1012, 1.2 × 1014, 8.9 × 1013 and 2.6 × 1013 cm-3 for the concentrations, respectively.

Ohmic contact formation to n-type GaN often involves high temperature steps, for example sintering at about 800 °C in the case of Ti-based contacts. Such processing steps might cause changes in the distribution, concentration, and properties of the defects. The present work aims at contributing to the knowledge about defect evolution in GaN upon processing at different temperatures. The processing temperatures are selected according to fabrication procedures for commonly used ohmic contacts to n-GaN: 300 °C (In-based), 550 °C (Ta-based), and 800 °C (Ti-based). Properties and concentration of the defects are studied by the means of deep level transient spectroscopy (DLTS). Changes in carrier capture kinetics are monitored with varying filling pulse duration.

A careful study of the phosphorescence afterglow and the thermoluminescence (TL) of sol-gel-prepared m-ZrO2 nanocrystalline powders in an extended temperature range −100 to 300 °C was carried out. Wavelength-resolved TL proved the existence of a single active luminescence centre in this temperature range. A TL method based on various heating rates was used to derive more reliable trap depths of 0.75, 0.95, 1.25, 1.46 and 1.66 eV whereas deconvolution methods provided somewhat lower values. The most intense room-temperature afterglows (that were easily observable beyond 1000 s) were obtained from samples annealed at 1250 and 1500 °C, and were attributed mainly to depopulation of the 1.25 eV traps.

The aim of this study is to determine the trap parameters (trap depth E, frequency factors) of quartz using various heating rates method and also to investigate the effect of annealing temperature on determining trap depths. The method is based on the positions of the thermoluminescence peaks, obtained from the change in temperature of the peak at maximum caused by changing the heating rate at which the sample is measured. In the present work, powder quartz samples were annealed first at different temperatures before irradiation. Then samples irradiated to different doses were measured with a TL reader at different heating rates and the glow curves were recorded. In order to calculate the trap depth E and the frequency factor s, the glow parameter T-m was determined experimentally from the glow curve by measuring the shift of the maximum peak temperature depending on heating rate beta. The calculation of trap parameters was repeated for each annealing temperature. Then the effect of annealing temperature on trap depths calculated by the various heating rates method was evaluated. Publisher's Version Q2 WOS:000171859000008

Nanocrystalline carbonaceous cluster evolution and electron transport in the N+ beam induced spin coated poly(2,6-dimethyl-1,4-phenylene oxide) thin films as a function of ion fluence has been investigated. Following Robertson’s model and electron diffraction, the narrow optical band gaps were explained in terms of polyaromatic, single crystalline graphitelike clusters. With a threshold fluence of 1×1015 ions/cm2 for cluster growth, the size of the clusters ranged from 2 to 50 nm with the number of aromatic rings varying between 20 and 170 over the entire fluence range upto 8×1016 ions/cm2. A molecular reconstruction/self organization has been envisaged as a possible clue to the above structure evolution upon a critical energy density transferred to the 53 nm implanted layer. Transmission electron microscopy study of fractal scaling in the nanoparticle aggregates revealed a fractal dimension of 1.37±0.02 with the growth process to follow a diffusion limited aggregation model. Electrical conductivity data ...

We investigate the role of grains and grain boundaries (GBs) in the electron transport through poly-crystalline HfO2 by means of conductive atomic force microscopy (CAFM) measurements and trap-assisted tunneling simulations. CAFM experiments demonstrate that the leakage current through a thin dielectric film preferentially flows via the GBs. The current I-V characteristics measured on both types of sites, grains, and GBs are successfully simulated by utilizing the multiphonon trap-assisted tunneling model, which accounts for the inelastic charge transport via the electron traps. The extracted density of electrically active traps, whose energy parameters match those of the positively charged oxygen vacancies in hafnia, is ∼3 × 1019 cm−3 at the grains, whereas a much higher value of (0.9÷2.1) × 1021 cm−3 is required to reproduce the leakage current through the GBs.

We report an observation of ultra-low-frequency self-oscillation of photocurrent in InxGa1–xAs/Al0.15Ga0.85As multiple-quantum-well p–i–n diodes. The photocurrent intensity shows self-oscillations with a characteristic frequency of ~0.1 Hz at low temperatures under reverse bias voltages. The photocurrent self-oscillation depends on applied bias voltage, temperature, illumination power, and indium content of quantum-well layers. These dependences indicate that the photocurrent self-oscillation is attributed to photogenerated carriers trapped in localized centers within InxGa1–xAs quantum-well regions.

We present a quantitative analysis of the loss of electroluminescence in light-emitting diodes (LEDs) based on poly[2-methoxy-5-(2′-ethylhexyloxy)- p-phenylenevinylene] (MEH-PPV) due to the combination of non-radiative trap-assisted recombination and exciton quenching at the metallic cathode. It is demonstrated that for an MEH-PPV LED the biggest efficiency loss, up to 45%, arises from extrinsic non-radiative recombination via electron traps. The loss caused by exciton quenching at the cathode proves only to be significant for devices thinner than 100 nm. Removal of electron traps by purification is expected to enhance the efficiency of polymer LEDs by more than a factor of two. © 2012 Elsevier B.V. All rights reserved.

Reports study of metal-oxide-semiconductor (MOS) capacitors with 2.2 nm dry and N2O grown gate dielectrics. Interface trap generation during constant voltage stressing at different operating temperatures (from 22°C to 90°C) has been investigated. The effect of nitrogen annealing (20 min) at 400°C on high temperature stress-induced interface traps was also studied., IEEE

Charge carrier transport in disordered organic semiconductors, performed in electronic devices such as optoelectronic and photovoltaic ones, is usually affected by an exponential distribution of localized states in the band-gap (traps) under space-charge limited current. In this paper, we provide a full analysis for the trap-controlled transport of the single-carrier device in the frequency domain. Trap-limited mobility is interpreted in terms of the classical multiple-trapping picture with one transport state and the trapping-detrapping dynamics of the exponential density of traps. This allows us to provide a suitable explanation of the usual experimental features of the mobility dependence on voltage as along with the capacitance spectra. Ministerio de Economía y Competitividad HOPE CSD2007-00007 Generalitat Valenciana PROMETEO / 2009/058

Electron irradiation has been applied to the formation of a semi-insulating 4H-SiC(0001) layer. The resistivity of the semi-insulating layer, which was irradiated with a fluence of 1.9 × 10^[18] cm^[−2] at 400 keV, exceeded 10^[10] Ω cm at room temperature. From capacitance-voltage characteristics of Schottky structure, the depth of the semi-insulating layer was estimated to be 10 μm, indicating that the whole region of lightly-doped n-type epilayer was converted to the semi-insulating layer by electron irradiation. The semi-insulating property can be ascribed to electron trapping at the Z_[1/2] and EH_[6/7] centers generated by electron irradiation. The threshold energy for the generation of Z_[1/2] center was about 100 keV.

The trap-limited electron currents in poly(p-phenylene vinylene) (PPV) derivatives can be modeled using a Gaussian trap distribution that is positioned approximately 0.75 eV below the lowest unoccupied molecular orbital (LUMO) of PPV. Photothermal deflection spectroscopy measurements and internal photo-emission spectroscopy measurements confirm the claim of a Gaussian shaped trap distribution centered at 0.75 eV below the LUMO of PPV. Additionally, two PPV derivatives that differ in the number of conformational defects incorporated during synthesis exhibit identical electron trapping behavior, showing that the traps do not originate from extrinsic impurities of the synthesis or defects in the polymer chains. © 2011 American Institute of Physics.

The trap-limited electron currents in poly(p-phenylene vinylene) (PPV) derivatives can be modeled using a Gaussian trap distribution that is positioned approximately 0.75 eV below the lowest unoccupied molecular orbital (LUMO) of PPV. Photothermal deflection spectroscopy measurements and internal photo-emission spectroscopy measurements confirm the claim of a Gaussian shaped trap distribution centered at 0.75 eV below the LUMO of PPV. Additionally, two PPV derivatives that differ in the number of conformational defects incorporated during synthesis exhibit identical electron trapping behavior, showing that the traps do not originate from extrinsic impurities of the synthesis or defects in the polymer chains. © 2011 American Institute of Physics.

Major deep levels observed in as-grown and irradiated n-type 4H–SiC and 6H–SiC epilayers have been investigated. After low-energy electron irradiation, by which only carbon atoms are displaced, five traps, EH1 (Ec−0.36 eV), Z_1/Z_2 (Ec−0.65 eV), EH3 (Ec−0.79 eV), EH5 (Ec−1.0 eV), and EH6/7 (Ec−1.48 eV), were detected in 4H–SiC and four traps, E_1/E_2 (Ec−0.45 eV), RD_5 (Ec−0.57 eV), ES (Ec−0.80 eV), and R (Ec−1.25 eV), were detected in 6H–SiC. The Z_1/Z_2, EH6/7 centers in 4H–SiC and the E_1/E_2, R centers in 6H–SiC exhibit common features as follows: their generation rates by the e^−-irradiation were almost the same each other, their concentrations were not changed by heat treatments up to 1500℃, and they showed very similar annealing behaviors at elevated temperatures. Furthermore, these defect centers were almost eliminated by thermal oxidation. Taking account of the observed results and the energy positions, the authors suggest that the Z_1/Z_2 center in 4H–SiC corresponds to the E_1/E_2 center in 6H–SiC, and the EH6/7 center in 4H–SiC to the R center in 6H–SiC, respectively. Since the concentrations of these four centers are almost the same for as-grown, electron-irradiated, annealed, and oxidized samples, these centers will contain a common intrinsic defect, most likely carbon vacancy. The authors also observed similar correspondence for other thermally unstable traps in 4H–SiC and 6H–SiC.

The combination of piezoelectric materials and single crystal diamond offers the opportunity for the development of multifunctional micromachined devices under extreme conditions. In this work, the authors report the structural, electrical, optical, and dielectric properties of Pb(Zr0.52, Ti0.48)O3 (PZT) thin films integrated on single crystal diamond (100) substrates. The corresponding field effect transistor based on the metal-piezoelectric-insulator-semiconductor (MPIS) structure was fabricated on a homoepitaxial p-type diamond layer grown on a type-Ib diamond substrate. Different intermediate layers were deposited on single diamond substrates prior to the PZT films growth in order to achieve the best polarization properties. It was observed that the utilization of an Al2O3 buffer layer followed by a SrTiO3 seed layer favored the formation of a single perovskite PZT phase. Transmission electron diffraction patterns revealed that the PZT films included an initial layer at the SrTiO3/PZT interface followed by a well crystallized layer. The PZT film grown on SrTiO3/Al2O3/diamond exhibited much better in-plane polarization than that of the PZT film on Al2O3/diamond. The photoresponse behavior revealed that carriers trapping effect was trivial in the PZT film. The channel electrical conductivity of the MPIS field effect transistor was successfully modulated by the gate bias.