Positive Charge Buildup Research Articles

Impact of hole trap-detrap mechanism on X-ray irradiation induced threshold voltage shift of radiation-hardened GAA TFET device

We demonstrate the X-ray irradiation effect on a radiation-hardened gate-all-around tunnel FET (GAA TFET) device. The radiation-hardened device has a high and fixed oxide electric field, which is responsible to attain equilibrium state of the device. Once the device reaches equilibrium, the threshold voltage starts to saturate for any variation in the gate bias. Therefore, the fixed electric field in the oxide region is reduced with the application of negative bias to the gate, which varies the equilibrium trapped charge concentration in the oxide. The negative gate bias causes electrons of oxide and holes from the bias terminal to recombine, which ultimately reduces the electric field in the oxide region. This variation in charge concentration results in a new threshold voltage when the device has been re-irradiated. Further, the unique carrier injection mechanism of the TFET device plays a significant role in trapping and detrapping mechanisms along the channel. The report consists of positive charge trapping and detrapping of charge carriers along the channel of the GAA-TFET device. In addition, the report also deals with the impact of applying excessive gate bias to the device and its impact on electric fields. For the X-ray irradiation analysis, several figures of merit are taken into consideration before coming to the conclusion of the report, such as geminate yield, trapped charges in the oxide region, and time-varying positive charge build-up in the oxide. The device under consideration has also been irradiated with several radiation doses and time durations to verify its robustness.

Positive and negative charge trapping GaN HEMTs: Interplay between thermal emission and transport-limited processes

This paper investigates the kinetics of buffer trapping in GaN-based normally-off high-voltage transistors. The analysis was carried out on transfer-length method (TLM) structures.By means of a custom setup, (i) we investigated the trapping and de-trapping processes induced by a large vertical bias and identified different mechanisms, responsible for the storage of negative and positive charge in the buffer. (ii) temperature-dependent analysis was carried out to evaluate the time constants associated to negative and positive charge build-up. Remarkably, the results indicate that the activation energy for negative charge trapping is ~0.3 eV, which is much lower than the ionization energy of carbon acceptors (0.8–0.9 eV). This result is explained by considering that trapping and de-trapping are not dominated by thermal processes (thermal emission from acceptors), but by transport mechanisms, that limit the transfer of charge to trap states. (iii) in the recovery experiments, after low stress bias negative charge trapping dominates. After high stress bias, also the effect of positive charge generation is detected, and the related activation energy is evaluated.The results presented within this paper clearly indicate that the trapping and de-trapping kinetics of normally-off GaN HEMTs are the results of the interplay of transport-limited conduction processes, that result in a low thermal activation (Ea ~ 0.3 eV), compared to that of CN acceptors.

On the Origin of Regioselectivity in Palladium‐Catalyzed Oxidation of Glucosides

AbstractThe palladium‐catalyzed oxidation of glucopyranosides has been investigated using relativistic density functional theory (DFT) at ZORA‐BLYP−D3(BJ)/TZ2P. The complete Gibbs free energy profiles for the oxidation of secondary hydroxy groups at C2, C3, and C4 were computed for methyl β‐glucoside and methyl carba‐β‐glucoside. Both computations and oxidation experiments on carba‐glucosides demonstrate the crucial role of the ring oxygen in the C3 regioselectivity observed during the oxidation of glucosides. Analysis of the model systems for oxidized methyl β‐glucoside shows that the C3 oxidation product is intrinsically favored in the presence of the ring oxygen. Subsequent energy decomposition analysis (EDA) and Hirschfeld charge analysis reveal the role of the ring oxygen: it positively polarizes C1/C5 by inductive effects and disfavors any subsequent buildup of positive charge at neighboring carbon atoms, rendering C3 the most favored site for the β‐hydride elimination.

Electronically Modified Cobalt Aminopyridine Complexes Reveal an Orthogonal Axis for Catalytic Optimization for CO2 Reduction.

The design of effective electrocatalysts for carbon dioxide reduction requires understanding the mechanistic underpinnings governing the binding, reduction, and protonation of CO2. A critical aspect to understanding and tuning these factors for optimal catalysis revolves around controlling the electronic environments of the primary and secondary coordination sphere. Herein we report a series of para-substituted cobalt aminopyridine macrocyclic catalysts 2-4 capable of carrying out the electrochemical reduction of CO2 to CO. Under catalytic conditions, complexes 2-4, as well as the unsubstituted cobalt aminopyridine complex 1, exhibit icat/ip values ranging from 144 to 781. Complexes 2 and 4 exhibit a pronounced precatalytic wave suggestive of an ECEC mechanism. A Hammett analysis reveals that ligand modifications with electron-donating groups enhance catalysis (ρ < 0), indicative of positive charge buildup in the transition state. This trend also extends to the CoI/0 potential, where complexes possessing more negative E(CoI/0) reductions exhibit greater icat/ip values. The reported modifications offer a synthetic lever to tune catalytic activity, orthogonal to our previous study of the role of pendant hydrogen bond donors.

Storage and release of buffer charge in GaN-on-Si HEMTs investigated by transient measurements

This letter investigates the kinetics of the non-monotonic trapping mechanisms responsible for dynamic on-resistance (RDS,ON) in GaN-on-Si enhancement-mode HEMTs. We describe the time-dependences of electron trapping at carbon on the nitrogen site (CN) acceptors and for the first time we investigate the kinetics of the build-up of positive charge at the buffer/strain-relief layer interface. Part of the analysis is carried out on two-terminal ohmic devices, by a novel setup capable of measuring current transients (from 10 μs to 10 s) after stressing with a negative substrate bias.

Ionizing-Radiation Response and Low-Frequency Noise of 28-nm MOSFETs at Ultrahigh Doses

Total ionizing dose (TID) mechanisms are investigated in 28-nm MOSFETs via dc static and low-frequency noise measurements. nMOSFETs and pMOSFETs are irradiated up to 1 Grad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) and annealed at high temperatures. TID sensitivity depends on the channel length, the channel width, and the bias condition. Halo implantations improve the radiation tolerance of shorter-channel transistors. Worst case bias for TID-induced degradation is found with high electric field applied to the gate during irradiation, due to increased charge trapping in the upper corner of the shallow trench isolation (STI) and in the gate oxide. DC and low-frequency noise measurements show that, at doses up to ~100 Mrad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), radiation-induced degradation is primarily due to the positive charge buildup in the STI oxides. At ultrahigh doses approaching 1 Grad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), TID degradation is influenced by charge buildup in the gate oxide and traps at or near the gate/dielectric interface and/or along the STI sidewalls. Worst case degradation is found in narrower and longer-channel devices, due to the enhanced charge buildup in the STI oxide and along its interfaces.

Oligothiophene Synthesis by a General C–H Activation Mechanism: Electrophilic Concerted Metalation–Deprotonation (eCMD)

Oxidative C-H/C-H coupling is a promising synthetic route for the streamlined construction of conjugated organic materials for optoelectronic applications. Broader adoption of these methods is nevertheless hindered by the need for catalysts that excel in forging core semiconductor motifs, such as ubiquitous oligothiophenes, with high efficiency in the absence of metal reagents. We report a (thioether)Pd-catalyzed oxidative coupling method for the rapid assembly of both privileged oligothiophenes and challenging hindered cases, even at low catalyst loading under Ag- and Cu-free conditions. A combined experimental and computational mechanistic study was undertaken to understand how a simple thioether ligand, MeS(CH2)3SO3Na, leads to such potent reactivity toward electron-rich substrates. The consensus from these data is that a concerted, base-assisted C-H cleavage transition state is operative, but thioether coordination to Pd is associated with decreased synchronicity (bond formation exceeding bond breaking) versus the "standard" concerted metalation-deprotonation (CMD) model that was formalized by Fagnou in direct arylation reaction. Enhanced positive charge build-up on the substrate results from this perturbation, which rationalizes experimental trends strongly favoring π-basic sites. The term electrophilic CMD (eCMD) is introduced to distinguish this mechanism from the standard model, even though both mechanisms locate in a broad concerted continuum. More O'Ferrall-Jencks analysis further suggests eCMD should be a general mechanism manifested by many metal complexes. A preliminary classification of complexes into those favoring eCMD or standard CMD is proposed, which should be informative for studies toward tunable catalyst-controlled reactivity.

Gold(III)-Catalyzed Decarboxylative C3-Benzylation of Indole-3-carboxylic Acids with Benzylic Alcohols in Water.

A strategy for the gold(III)-catalyzed decarboxylative coupling reaction of indole-3-carboxylic acids with benzylic alcohols has been developed. This cascade reaction is devised as a straightforward and efficient synthetic route for 3-benzylindoles in moderate to excellent yields (50-93%). A Hammett study of the protodecarboxylation gives a negative ρ value, suggesting that there is a buildup of positive charge on the indole ring in the transition state. Furthermore, comparison of initial rates in H2O and in D2O reveals an observed kinetic solvent isotope effect (KSIE = 2.7). This simple protocol, which affords the desired products with CO2 and water as the coproducts, can be achieved under mild conditions without the need for base or other additives in water.

Graphene-based positron charge sensor

We utilize a graphene field-effect transistor to measure back-gate charging by positrons. The device consists of an exfoliated graphene flake transferred onto hexagonal Boron Nitride, placed on a 1 cm2 substrate of 500 μm thick conducting p-Si capped by 285 nm-thick SiO2. It is placed at close proximity to a 25 μCi 22Na positron source emitting a constant flux of positrons, which during the measurement annihilate within the back-gate. We demonstrate that when the back-gate is allowed to float, the charging current of ≈20 fA causes the buildup of positive charge which capacitively couples to the graphene device and is detected as a variation in the two-terminal conductance. Furthermore, a prolonged exposure to positrons causes a shift in the graphene transport characteristics, associated with local charges at the immediate environment of the graphene flake. Our results demonstrate the utility of two-dimensional layered materials as probes for charging dynamics of positrons in solids.

Mechanistic Insights into the B(C6F5)3-Initiated Aldehyde–Aniline–Alkyne Reaction To Form Substituted Quinolines

A substoichiometric quantity of the Lewis acid B(C6F5)3 is sufficient to initiate the aldehyde–amine–alkyne reaction, in a one-pot methodology that enables the synthesis of a range of functionalized quinolines. Optimization studies revealed that key requirements for the high-yielding tricomponent reaction initiated by B(C6F5)3 at raised temperatures include an excess of the in situ generated imine (which acts as a hydrogen acceptor) and an alkyne substituent able to stabilize positive charge buildup during the cyclization. Mechanistic experiments revealed that under these conditions B(C6F5)3 is acting as a Lewis acid-assisted Brønsted acid, with H2O–B(C6F5)3 being the key species enabling catalytic quinoline formation. This was indicated by deuterium labeling studies and the observation that the cyclization of N-(3-phenylpropargyl)aniline using B(C6F5)3 under anhydrous conditions afforded the zwitterion [(N-H-3-B(C6F5)3-4-Ph-quinolinium], which does not undergo protodeboronation to release B(C6F5)3 and the quinoline product under a range of conditions. Finally, a brief substrate scope exploration demonstrated that this is an operationally simple and effective methodology for the production of functionalized quinolines.

Fabrication of radiation hardened SOI with embedded Si nanocrystal by ion-cut technique

The ion-cut technique has been proposed to improve the top Si crystalline quality of the radiation hardened silicon-on-insulator (SOI). Si ion implantation prior to wafer bonding and splitting is performed to reduce the lattice damage induced by direct Si implantation through top Si film. Atomic-resolution transmission electron microscopy studies reveal that the top Si film possesses nearly perfect crystalline quality. Photoluminescence spectroscopy, x-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy have corroborated the existence of the embedded Si nanocrystals. The pseudo-MOS transistors are fabricated on the hardened and unhardened SOI wafers for a quick and effective evaluation on the electrical properties of SOI wafers. The results indicate that the improvement in the total ionizing dose tolerance of the hardened SOI wafer can be attributed to the generation of deep electron and proton traps which reduce the positive charge build-up defects in the buried oxides.

Coulomb explosion of “hot spot”

The study presented in this paper has shown that the generation of hard x rays and high-energy ions, which are detected in pinch implosion experiments, may be associated with the Coulomb explosion of the hot spot that is formed due to the outflow of the material from the pinch cross point. During the process of material outflow, the temperature of the hot spot plasma increases, and conditions arise for the plasma electrons to become continuously accelerated. The runaway of electrons from the hot spot region results in the buildup of positive space charge in this region followed by a Coulomb explosion. The conditions for the hot spot plasma electrons to become continuously accelerated have been revealed, and the estimates have been obtained for the kinetic energy of the ions generated by the Coulomb explosion.

Absolute determination of radiation bursts and of proportional counters space charge effect through the influence method

When proportional counters are employed in charge integration mode to determine the magnitude of a radiation pulse, so intense that individual detection events take place in a time too short to produce individual output pulses, mostly in pulsed neutron sources, the strong build-up of positive space charge reduces the electric multiplication factor of the proportional detector. Under such conditions the ensuing measurement underestimates the amount of radiation that interacted with the detector. If the geometric characteristics, the filling gas pressure and the voltage applied to that detector are known, it becomes possible to apply an analytical correction method to the measurement.In this article we present a method that allows to determine the absolute value of the detected radiation burst without the need to know the characteristics of the employed detectors. It is necessary to employ more than one detector, taking advantage of the Influence Method.The “Influence Method” is conceived for the absolute determination of a nuclear particle flux in the absence of known detector efficiency and without the need to register coincidences of any kind. This method exploits the influence of the presence of one detector in the count rate of another detector, when they are placed one behind the other and define statistical estimators for the absolute number of incident particles and for the efficiency (Rios and Mayer, 2015 [1,2]). Its practical implementation in the measurement of a moderated neutron flux arising from an isotopic neutron source was exemplified in (Rios and Mayer, 2016 [3]) and the extension for multiple detectors in (Rios and Mayer 2016 [4]).

Synthesis, Cycloaddition, and Cycloreversion Reactions of Mononuclear Titanocene–oxo Complexes

Titanocene–oxo complexes of the type Cpx2Ti═O(L) (Cpx = pentamethylcyclopentadienyl; tetramethylcyclopentadienyl; L = pyridine or derivatives) are synthesized from the corresponding titanocene–ethylene complexes via oxidation with pyridine N-oxides or styrene oxide. These oxo complexes react with alkynes, nitriles, and α,β-unsaturated carbonyls to form titanacycles, which undergo exchange reactions with organic substrates or react with 4-dimethylaminopyridine to regenerate the titanocene–oxo. Mechanistic experiments support a dissociative mechanism in which the first step is rate-determining retrocycloaddition followed by trapping of the reactive [Cpx2Ti═O] species. In the case of the retro-[4+2]-cycloaddition from dioxatitanacyclohexene complexes, a Hammett study gives ρ values of −1.18 and −1.04 for substituents on two different phenyl rings on the metallacycles, suggesting positive charge buildup and a slightly asynchronous cycloreversion in the rate-determining step.

Electric field inhomogeneity in ohmic‐type CdTe detectors measured by time‐of‐flight technique

AbstractThe bias‐induced instability of the internal electric field in Ohmic‐type CdTe gamma‐ray detectors is investigated by using the time‐of‐flight technique. Based on the solution of one dimensional Poisson equation, transient current waveforms at different delays of laser excitation from the bias application and at different temperatures are analysed It is shown that under DC bias application, the internal electric field decreases with depth because of the positive space charge buildup in the detectors. Further, the origin of the space charge is attributed to the injection and successive trapping of holes from the anode side at the early stage of evolution and then later to the thermal release of electrons from a deep donor at 0.64 eV. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

ChemInform Abstract: Direct Substitution of Benzylic Alcohols with Electron‐Deficient Benzenethiols via π‐Benzylpalladium(II) in Water.

An efficient and environmentally benign method for the direct nucleophilic substitution of benzylic alcohols with electron-deficient benzenethiols in water is developed. π-Benzylpalladium(ii) cation complexes show high catalytic activity for S-benzylation, which provides a mechanistic alternative to the borrowing hydrogen methodology. Hammett studies on the rate constants of S-benzylation by various substituted alcohols show good correlation between log(kX/kH) and the σ value of the respective substituents. From the slope, negative ρ values are obtained, suggesting that there is a build-up of positive charge in the transition state. Water plays an important role in the catalytic system for sp(3) C-O bond activation and stabilization of the activated Pd(ii) cation species.

Modeling thermionic emission from laser-heated nanoparticles

An adjusted form of thermionic emission is applied to calculate emitted current from laser-heated nanoparticles and to interpret time-resolved laser-induced incandescence (TR-LII) signals. This adjusted form of thermionic emission predicts significantly lower values of emitted current compared to the commonly used Richardson-Dushman equation, since the buildup of positive charge in a laser-heated nanoparticle increases the energy barrier for further emission of electrons. Thermionic emission influences the particle's energy balance equation, which can influence TR-LII signals. Additionally, reports suggest that thermionic emission can induce disintegration of nanoparticle aggregates when the electrostatic Coulomb repulsion energy between two positively charged primary particles is greater than the van der Waals bond energy. Since the presence and size of aggregates strongly influences the particle's energy balance equation, using an appropriate form of thermionic emission to calculate emitted current may improve interpretation of TR-LII signals.

Dose-rate sensitivity of deep sub-micro complementary metal oxide semiconductor process

Enhancing low dose rate sensitivity (ELDRS) in bipolar device is a major problem of liner circuit radiation hardness prediction for space application. ELDRS is usually attributed to space-charge effect. A key element is the difference in transport rate between holes and protons in SiO2. Interface-trap formation at high dose rate is reduced due to positive charge buildup in the Si/SiO2 interfacial region (due to the trapping of holes and/or protons) which reduces the flow rates of subsequent holes and protons (relative to the low-dose-rate case) from the bulk of the oxide to the Si/SiO2 interface. Generally speaking, the dose rate of metal oxide semiconductor (MOS) device is time dependent when annealing of radiation-induced charge is taken into account. The degradation of MOS device induced by the low dose rate irradiation is the same as that by high dose rate when annealing of radiation-induced charge is taken into account. However, radiation response of new generation MOS device is dominated by charge buildup in shallow trench isolation (STI) rather than gate oxide as older generation device. Unlike gate oxides, which are routinely grown by thermal oxidation, field oxides are produced using a wide variety of deposition techniques. As a result, they are typically thick (100 nm), soft to ionizing radiation, and electric field is far less than that of gate oxide, which is similar to the passivation layer of bipolar device and may lead to ELDRS. Therefore, dose-rate sensitivities of n-type metal oxide semiconductor field effect transistor (NMOSFET) and static random access memory (SRAM) manufactured by 0.18 m complementary metal oxide semiconductor (CMOS) process are explored experimentally and theoretically in this paper. Radiation-induced leakages in NMOSFET and SRAM are examined each as a function of dose rate. Under the worst-case bias, the degradation of NMOSFET is more severe under the low dose rate irradiation than under the high dose rate irradiation and anneal. Moreover, radiation-induced standby current rising in SRAM is more severe under the low dose rate irradiation than under the high dose rate irradiation even when anneal is not considered. The above experimental results reveal that the dose-rate sensitivity of deep sub-micron CMOS process is not related to time-dependent effects of CMOS devices. Mathematical description of the combination between enhanced low dose-rate sensitivity and timedependent effects as applied to radiation-induced leakage in NMOSFET is developed. It has been numerically found that non time-dependent effect of deep sub-micron CMOS device arises due to the competition between enhanced low dose-rate sensitivity in bottom of STI and time-dependent effect at the top of STI. The high dose rate irradiation is overly conservative for devices used in a low dose rate environment. The test method provides an extended room temperature anneal test to allow leakage-related parameters that exceed postirradiation specifications to return to a specified range.

Accelerated Submonomer Solid-Phase Synthesis of Peptoids Incorporating Multiple Substituted N-Aryl Glycine Monomers.

N-Aryl glycines are a chemically diverse class of peptoid monomers that have strong structure-inducing propensities. Yet their use has been limited due to the sluggish reactivity of the weakly nucleophilic aniline submonomers. Here, we report up to a 76-fold rate acceleration of the displacement reaction using aniline submonomers in solid-phase peptoid synthesis. This is achieved by adding halophilic silver salts to the displacement reaction, facilitating bromide abstraction and AgBr precipitation. Mechanistic insight derived from analysis of a series of 15 substituted anilines reveals that the silver-mediated reaction proceeds through a transition state that has considerably less positive charge buildup on the incoming nucleophile and an enhanced leaving group. This straightforward enhancement to the submonomer method enables the rapid room temperature synthesis of a wide variety of N-aryl glycine-rich peptoid oligomers, possessing both electron-withdrawing and -donating substituents, in good yields.

Self-induced electrostatic-boosted radioisotope heat sources

In this paper the possibility of stimulated self-induced electrostatic fields in radioisotope heat sources for power enhancement is discussed. Because electrons have higher mobility than the positively charged fragments from radioactive decay, a build-up of positive charges can be promoted, leading to an internal induced electrostatic field. This, in turn, results in a repulsive force acting on positive charges, endowing them with additional kinetic energy, and heat release after these charged particles are stopped by inelastic collisions with the boundary wall. Utilizing a simplified geometrical model, an analytical expression for the attainable power enhancement is derived. It is shown that the proposed concept could result in power enhancements of 5–10% for beta sources but enhancements are negligible for alpha sources.