Positive Charge Buildup Research Articles

Hot-carrier charge trapping and trap generation in HfO2 and Al2O3 field-effect transistors

We present a comprehensive experimental study of hot-carrier trap generation and charging effects in high-κ dielectrics using field-effect transistors fabricated with HfO2 and Al2O3 gate insulator stacks and polycrystalline silicon gates. The experiments utilize substrate injection of hot carriers generated either optically or by direct injection in the dark from a forward-biased p–n junction. Comparison of charge-trapping measurements taken using these two techniques on n-channel field-effect transistors (nFETs) and p-channel field-effect transistors (pFETs) finds that enhanced charge trapping occurs when hot holes are present (in the light or in the dark in pFETs but only under illumination in nFETs). A fundamental understanding of the conditions for hot-carrier damage in nFETs is obtained by studying the dependence on light wavelength, temperature, and substrate bias. In particular, the wavelength dependence reveals that the hot-carrier damage depends on a combination of the electron and photon energies. Study of the time dependence of the gate current indicates the buildup of positive charge in the dielectric during stressing. The density of interface traps generated by hot-carrier stressing is estimated using the capacitance–voltage characteristic, and charge transfer experiments to probe the existence of slow states are performed. Finally, the experimental findings are discussed in the context of a speculative picture in which hot holes act as a precursor to damage in the oxide.

Coordination of alkenes and alkynes to a cationic d(0) zirconocene alkoxide complex.

This paper describes the synthesis of base-free (C5R5)2Zr(OtBu)+ cations, the direct observation of nonchelated alkene and alkyne adducts of these cations, and studies of the thermodynamic and dynamic properties of these novel species. Reaction of %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%ZrMe%@sb@%2%@sbx@%%@mx@% (Cp' = C5H4Me) with tert-butyl alcohol followed by [Ph3C][B(C6F5)4] in benzene yields [%@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@mx@% ][B(C6F5)4] (1), which exists as %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%ClR%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% solvent adducts in C6D5Cl and CD2Cl2 solutions. Addition of ligands L (L = ethylene, propylene, propyne, 2-butyne, CO, phenylacetylene, allene, 1-hexene, cis-2-butene) to 1 in CD2Cl2 at -89 degrees C results in reversible formation of %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%L%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% adducts. NMR data for %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%H%@sb@%2%@sbx@%C=%@/bd@%CHMe%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% (4) indicate that the propylene coordinates unsymmetrically and is polarized with positive charge buildup at Cint. Equilibrium constants, defined by Keq = [Zr-L][1]-1[L]-1, vary in the order CO > propyne > 2-butyne > phenylacetylene > ethylene > allene > propylene > 1-hexene > cis-2-butene > vinyl chloride. Loss of L from %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%L%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% to give 1 appears to proceed via associative displacement by CD2Cl2 in most cases.

Effects of gate bias stressing in power vdmosfets

The effects of gate bias stressing on threshold voltage and mobility in power VDMOSFETs and underlying changes in gate oxide-trapped charge and interface trap densities are presented and analyzed in terms of the mechanisms responsible. In the case of positive bias stressing, electron tunneling from neutral oxide traps associated with trivalent silicon ?Sio defects into the oxide conduction band is proposed as the main mechanism responsible for positive oxide-trapped charge buildup, while subsequent hole tunneling from the charged oxide traps ?Sio+ to interface-trap precursors ?Sis-H is shown to be the dominant mechanism responsible for the interface trap buildup. In the case of negative bias stressing, hole tunneling from the silicon valence band to oxygen vacancy defects ?Sio / Sio? is shown to be responsible for positive oxide-trapped charge buildup, while subsequent electro-chemical reactions of interfacial precursors ?Sis?? with the charged oxide traps ?Sio+ Sio? and H+ ions are proposed to be responsible for interface trap buildup.

Novel examples of three-dimensional aromaticity: 1,3-dehydro-silaadamantane dications. A theoretical (DFT, GIAO NMR, NICS) study.

A DFT study of the hitherto elusive 1,3-dehydro-silaadamantane dications 2(2+)-5(2+) has been carried out. Computed nucleus independent chemical shift (NICS) values are strongly indicative of three-dimensional heteroaromaticity in the resulting caged dications (via 2-electron, 4-center homoconjugation). In the optimized structures, silicon is pyramidalized. Although charge calculations (NPA and MKS) indicate significant positive charge build-up at silicon(s), the (29)Si GIAO NMR chemical shifts are unusually shielded. The latter finding agrees with the recent DFT calculations on 7-silanobornadien-7-ylium monocation 10(+), suggesting that silicon shielding is a consequence of unusual bonding and homoconjugation in the dications. Both NICS values and silicon shielding decrease in going from 2(2+) to 5(2+). Natural bond orbital (NBO) analysis was utilized to shed light on the origin of the three-dimensional heteroaromaticity in these dications.

Catalytic asymmetric benzylic C-H activation by means of carbenoid-induced C-H insertions.

Tetrakis[N-[4-dodecylphenyl)sulfonyl]-(S)-prolinate]dirhodium [Rh(2)(S-DOSP)(4)]-catalyzed decomposition of methyl aryldiazoacetates in the presence of substituted ethylbenzenes results in benzylic C-H activation by means of a rhodium-carbenoid-induced C-H insertion. A Hammet study showed that positive charge buildup occurred on the benzylic carbon in the transition state of the C-H activation step. C-H activation of toluene and isopropylbenzene is possible, but a competing double cyclopropanation occurs with these substrates. The C-H activation is highly regioselective and enantioselective, and in certain cases, moderate diastereoselectivity is also possible.

Radiation-induced leakage currents: atomic scale mechanisms

We link atomic-scale MOS gate oxide silicon dangling bond defects called E' centers to radiation-induced leakage current (RILC). We present evidence that strongly suggests that RILC tolerance is processing dependent and that this tolerance appears to be correlated with lower E' center generation. We furthermore note that in oxides subjected to quite high irradiation levels, the density of (generally electrically neutral) E' centers is far greater than would be expected for the hole trap E' centers involved in the radiation-induced positive charge build-up observed in thicker oxides.

Channel-length impact on radiation-induced threshold-voltage shift in N-MOSFET devices at low gamma ray radiation doses

SiO/sub 2/ gate dielectric and Si/SiO/sub 2/ interface are two important components which will shape the future of the MOSFETs and integrated circuits (IC's) technologies for ionizing radiation environment applications. This study discuss their size effects on irradiated NMOS device response. N-channel MOSFET's of different gate size were first irradiated with /sup 60/Co gamma rays source at several total doses (low doses). Then, they were investigated by using both voltage- and frequency-charge pumping (CP) methods. On one hand, all transistors reveal two radiation-induced oxide charge traps formation mechanisms, caused by low radiation total doses. Initially, there is a build-up of positive charge in the oxide layer, followed later by diminution of net positive charge (turn around effect), while the interface traps exhibit a linear increase with radiation doses. On the other hand, the total dose response is shown to depend on gate length of NMOS device.

A generalized exo‐anomeric effect. Substituent and solvent effects on the conformational equilibria of 2‐(arylseleno)cyclohexanones

AbstractThe effects of substitution and solvent on the conformational equilibria of 2‐[(4‐R‐substituted‐phenyl)seleno]cyclohexanones are described. The conformational equilibria were determined by comparison of the linewidths of the H‐2 resonances in the 1H NMR spectra of the conformationally averaged systems with those of the anancomeric (highly biased) 4‐isopropyl‐2‐substituted cyclohexanones. The substituent (R = NMe2, OMe, Me, H, F, Cl, CF3, NO2) and solvent ((CD3)2CO, CD3CN, CD2Cl2, CDCl3) effects are discussed in terms of electrostatic effects and the possible stabilizing orbital interactions. The values of Keq (axial‐equatorial) increase as the substituent becomes more electron withdrawing, in agreement with the dominance of nSe → π*C=O or σC‐Se → π*C=O orbital interactions in the axial conformers. The increase in the proportion of the equatorial isomers in more polar solvents for a given substituent suggests a damping of the dipolar interactions in the equatorial isomers. However, the proportion of the equatorial isomers in a given solvent increases as the substituent becomes more electron withdrawing, indicating that electrostatic interactions do not dominate in controlling the conformational equilibria. Analysis of the equilibrium data by means of a dual substituent parameter approach indicates the best correlation with σI and σ+R substituent constants in CD2Cl2 and with σI and σ°R substituent constants in CD3CN, with similar sensitivities to the resonance and polar effects. The correlations are interpreted in terms of accommodation of effective positive charge on the selenium atom in the axial isomers in CD2Cl2, and a lesser sensitivity to the buildup of positive charge in the more polar solvent CD3CN. Comparison of the IR μCO‐stretching frequencies for the axial and equatorial ArSe‐substituted anancomeric systems (R = NO2, NMe2) indicates a higher stretching frequency for the NO2‐substituted isomers. In the case of the NMe2‐substituted compounds, μCO appears at a higher frequency in the equatorial isomer, whereas in the case of the NO2‐substituted compounds, μCO is less sensitive to the axial or equatorial orientation of the substituent. The results are consistent with the operation of nse → π*c=0 or σC‐Se → π*C=O orbital interactions in the axial isomers. The JC2‐H2 values in the axially‐substituted anancomeric isomers are of greater magnitude than those in the equatorially‐substituted isomers, which is also consistent with the operation of the orbital interactions described above. There is, however, no marked substituent effect on the JC2–H2 values within the series of axial or equatorial isomers. We argue that this does not support the dominance of σC‐Se → π*C=O orbital interactions. Examination of crystal structures reported in the literature for related compounds indicates a particular gauche orientation about the C2–Se bond, which lends further support to the operation of an nSe → π*C=O orbital interaction. We suggest that the latter interaction is a manifestation of a generalized exo‐anomeric effect.

Study of the positive charge buildup into buried oxide of SIMOX SOI structure during bias–temperature stress

The buildup of fixed and mobile charge in the buried oxide (BOX) of silicon implanted by oxygen (SIMOX) silicon-on-insulator (SOI) structures during bias–temperature (BT) cycling has been studied by the thermally stimulated polarization (TSP) current technique and C–V measurements. Two polarization processes have been observed: the first process with activation energy of 0.3 eV is likely related to the positively charged ion transport across the BOX, the second process with activation energy about 1.2 eV is associated with space charge polarization. It was found that the ion transport is created simultaneously with the process of lateral positive charge buildup near the BOX/substrate interface when the bias is applied to the structure at temperatures above 280°C.

The effect of near-interface network strain on proton trapping in SiO/sub 2/

The buildup of positive charge during annealing in forming gas at 600/spl deg/C was compared for various types of Si/SiO/sub 2/ interfaces. Our data suggest a correlation between the presence of stressed bonds in the SiO/sub 2/ network near the Si/SiO/sub 2/ interface, and the ratio of fixed vs. mobile positive charge (protons) detected near the interface after performing a forming-gas annealing. We further propose that the presence of these stressed bonds near the interface is correlated with the oxygen deficiency at the interface and with the confinement of the oxide due to the presence of a Si cover layer. A model based on first-principles quantum mechanical calculations shows a significant decrease in the overall proton binding energy with increasing network strain near the interface. These calculations support our model of mobile proton generation at Si/SiO/sub 2/ interfaces with large densities of stressed bonds.

Electrical characterization of oxide and Si/SiO/sub 2/ interface of irradiated NMOS transistors at low radiation doses

N-channel MOSFET's of different dimensions were first irradiated with /sup 60/Co Gamma rays source at several total doses (low doses) and then their gate oxides, SiO/sub 2/ and interfaces, Si/SiO/sub 2/ were characterized by using the standard charge pumping (CP) and the extended standard CP to low frequencies techniques. Both techniques reveal two radiation-induced oxide charge traps formation mechanisms, caused by low radiation total doses. Initially, there is a build-up of positive charge in the oxide layer, followed later by diminution of net positive charge (turn around effect) due to generation of negative charge or/and transformation of positive oxide traps to interface traps. While the interface traps exhibit a linear increase with radiation doses.

Initiation of high power microwave dielectric interface breakdown

A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream side of the dielectric interface. The previously observed emission of low energy x-ray radiation from the interface is explained by bremsstrahlung generated by impacting electrons having initially a higher energy than the average emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density above the dielectric surface and eventually leading to a gaseous breakdown.

On the preferential emission of heavy isotopes in sputtering

The isotopic composition of secondary ions of boron were measured at a small emission angle by bombarding boron nitride with xenon ions at energies ranging from 100 eV to 1.5 keV. An ion gun was used to generate the ion beam. The secondary ions were detected by a quadrupole mass spectrometer. A flood electron gun was used to neutralize the positive charge build-up on the target surface. The secondary-ion flux was found to be enriched in heavy isotopes at lower incident ion energies. The heavy isotope enrichment was observed to decrease with increasing primary-ion energy. Beyond 350 eV, light isotopes were sputtered preferentially with the enrichment increasing to an asymptotic value of 1.27 at 1.5 keV. This trend is similar to that of the isotopic enrichment observed earlier when copper was sputtered with xenon ions in the same energy range.

Current oscillations in thin metal–oxide–semiconductor structures observed by ballistic electron emission microscopy

Quantum interference oscillations of electrons in a thin SiO2 layer were observed by ballistic electron emission microscopy (BEEM). With BEEM, electrons are injected across the gate of a metal–oxide–semiconductor (MOS) structure and directly into the conduction band of the SiO2. The MOS capacitor consisted of a 5 nm thick Pd film deposited on a 2.8±0.2 nm oxide thermally grown on Si(100). Oscillations with up to four peaks in an energy range of 0–3 eV above the injection threshold were noted. Their magnitude is of the order of 30% of the underlying BEEM current. The oscillations were most salient and their energy location repeatable at points of the sample that were previously not exposed to the electron beam. Even modest exposures caused a buildup of positive charge. This charge resulted in energy shifts, as well as a weakening of the oscillations, both of which are a consequence of the added scattering and local field inhomogeneities associated with the random distribution of the positive charge. Solutions of the Schrödinger equation that included a built-in oxide potential of 0.20 V and image force effects at both interfaces gave excellent fits to the experimental data for an effective electron mass in the oxide mox=0.63±0.09mo. The uncertainty in mox arises from an uncertainty of ±0.2 nm in the determination of the oxide thickness by ellipsometric methods. Nevertheless, the obtained value is well above the generally accepted value of 0.5mo.

Localized degradation studies of ultrathin gate oxides

We present studies on the limits of oxide reliability on a local, microscopic scale, using scanning tunneling microscope (STM)-based ballistic electron emission microscopy/spectroscopy (BEEM/S). In these studies, electrons are injected from the STM tip into the conduction band of a SiO2 layer that is imbedded in a metal–oxide–semiconductor (MOS) structure. The electron energy is determined both by the tip bias that can be set up to −13 V and by the applied oxide bias. Combining the two biases can heat electrons to energies that are unreachable in thin oxides by conventional Fowler–Nordheim injection methods. Our studies indicate that breakdowns are difficult to achieve for 7.1 nm oxides. A local breakdown was not observed even for an injected charge dosage of 1.8×103 C/cm2 at equivalent Fowler–Nordheim stress fields of ∼25 MV/cm, although defect densities in the oxide were as high as ∼5×1013/cm2. Evidence of anode hole injection is also observed under high oxide biases ∼8 MV/cm. Therefore we conclude that trap creation and hole injection processes are not sufficient to cause breakdowns at arbitrary locations on the 7.1 nm oxides. Whereas electron trapping is dominant during electron injection for 7.1 nm oxides, only a positive charge buildup was observed in the 2.8 nm oxides while stressing with only 1 eV electrons. For 2.8 nm oxides, a local breakdown did not occur for dosages of 3.1×103 C/cm2 at equivalent fields >43 MV/cm. The observed breakdowns were accompanied by gate metal failure and are hence believed to occur at weak spots in MOS capacitors. We conclude that an intrinsic breakdown limit of SiO2 has not yet been reached.

Current increase in thin gate-oxide (3.5–7.0 nm) metal–oxide–silicon structures with the boron-doped polycrystalline-silicon gates biased negatively

We have investigated gate-current instability in boron-doped polycrystalline-silicon gate metal–oxide–silicon structures. Gate-current increase is observed only for oxide thicknesses of 3.5–7.0 nm and for negative gate voltage. The current-increase characteristics are reversible for positive and negative bias cycles. When the gate is negatively or positively biased, the flatband voltage remains constant, suggesting that there is no positive charge buildup at the Si–SiO2 interface. The experimental results suggest that a positive charge buildup or the formation of a conductive region at the gate–SiO2 interface is involved in the current increase. A model for this current instability is presented.

Generalized trapping kinetic model for the oxide degradation after Fowler–Nordheim uniform gate stress

The practicality of modeling the power law degradation observed in thin dielectrics after Fowler–Nordheim stress has been demonstrated on the basis of a generalized trapping approach with appropriate trap cross-section and density profiles. A detailed mathematical analysis of the negative bulk oxide charge kinetics has been established using incomplete Gamma and generalized hypergeometric functions, after assuming exponentially varying trap cross-section and density profiles throughout the oxide. These spatial distributions could be due to the structural nature of the oxide after growth. Moreover, the asymmetry of the charge distribution centroid for negative and positive gate bias stress has been satisfactorily interpreted by neglecting the trapping in the tunneling region near the cathode. Overall this generalized kinetic trapping model provides very good fitting of the variation of the trapped oxide charge with the injection dose for oxide thicknesses between 5.5 and 10 nm. The evolution of the charge centroid is also well predicted but with less accuracy, due to the presence of other concurrent charge generation processes associated with positive and/or negative charge buildup.

High field and high temperature stress of n-SiC MOS capacitors

The development of reliable oxides built on SiC has become a very important issue with respect to either passivation processes or metal-oxide-semiconductor (MOS) applications. The aim of this paper is to present a detailed investigation of Fowler-Nordheim electron injections in n-type silicon carbide (SiC) MOS capacitors. A systematic variation in the temperature and in the average oxide field E OX applied during the stress allowed us to evidence both a positive charge and electron trapping. The positive charge build-up is shown to emerge above a threshold field close to 7 MV cm −1 . It decreases with temperature, and can be dissociated into two different exponential time processes. The overall results agree with mechanisms based on impact ionization in the oxide, leading to pair creation and subsequent hole trapping.

Stress induced leakage current in ultra-thin gate oxides after constant current stress

Constant current stress induced leakage currents are studied in very thin oxide devices, for both stress polarities. This current has been investigated for both positive and negative gate voltage measurements. Stress induced leakage current (SILC) physical nature has been studied and an interpretation has been proposed, considering local oxide weak spots with barrier height close to 1eV. The increasing rate of the SILC versus the injection dose has been studied and compared with the degradation positive charge build-up rate, observed in the same oxide, indicating that hole trapping and stress induced leakage current could have the same physical origin.

High-field Fowler - Nordheim stress of n-type silicon carbide metal-oxide-semiconductor capacitors

Fowler - Nordheim electron injections have been carried out in n-type silicon carbide (SiC) metal-oxide-semiconductor (MOS) capacitors. A systematic variation in the temperature and in the average oxide field applied during the stress allowed us to show both a positive charge and electron trapping. The positive charge build-up is shown to emerge above a critical oxide field close to . It decreases with temperature, and can be dissociated into two different exponential time processes. The overall results agree with mechanisms based on impact ionization in the oxide, leading to pair creation and subsequent hole trapping.