Electron Trapping Process Research Articles

Temperature accelerated dielectric breakdown of PECVD low-k carbon doped silicon dioxide dielectric thin films

The dielectric breakdown strength of carbon doped silicon dioxide thin films with thickness d from 32 nm to 153 nm is determined at 25 °C, 50 °C, 100 °C, 150 °C and 200 °C, using I–V measurements with metal-insulator-semiconductor (MIS) structures. It is found that the dielectric breakdown strength, EB, decreases with increasing temperature for a given film thickness. In addition, a film thickness dependence of breakdown is also observed, which is argued to show an inverse relation to thickness d in the form of EB∝(d-dc)-n. The exponential parameter n and critical thickness limit dc also exhibit temperature dependent behavior, suggesting a temperature accelerated electron trapping process. The activation energy for the temperature acceleration was shown to be thickness dependent, indicating a thickness dependent conduction mechanism. It is thereafter demonstrated that for relatively thick films (thickness >50 nm), the conduction mechanism is Schottky emission. For relatively thin films (thickness <50 nm), the Schottky conduction mechanism was obeyed at low field region while FN tunnelling was observed as a prevail one in the high field region.

Bipolar charge transport, injection, and trapping studies in a model green-emitting polyfluorene copolymer

Experimental studies of charge injection and transport of holes and electrons in LUMATION™ Green 1300 Series light-emitting polymer (LEP) by a combination of experimental techniques are reported. It is found that hole mobility is lower than electron mobility and the former exhibits steeper electric-field dependence thus reducing the misbalance between charge mobilities at higher device operating voltages. An approach to quantitatively analyze charge injection and trapping in organic semiconductors is proposed. Based on our analysis, hole current is limited by injection from the anode and trapping in the bulk of the polymer. Further, we found that hole trapping is approximately independent of electric field and injection efficiency increases with increasing electric field. Injection limitation of holes from poly(3,4-ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS) electrode is believed to be the result of nonuniform contact between the PEDOT:PSS and LEP rather than the energy barrier. On the other hand, electron injection is close to ohmic and the steady-state electron current is affected by trapping, mainly due to deep traps prevailing at low electric fields and with an estimated concentration of 1016cm−3. Electron trapping is found to be significantly reduced in dual-carrier devices, which is believed to be the effect of faster exciton formation and recombination rates, compared to electron trapping processes.

Low-frequency magnetic and resistance noise in magnetic tunnel junctions

We have studied the voltage fluctuations of current-biased, micron-scale magnetic tunnel junctions. We find that the spectral power density is $1/f$-like at low frequencies and becomes frequency independent at high frequencies. The frequency-independent background noise is due to Johnson-Nyquist noise and shot noise mechanisms. The nature of the $1/f$ noise has its origin in two different mechanisms. In the magnetic hysteresis loops this noise power is strongly field-dependent and is due to thermal magnetization fluctuations in both the ``free'' and ``fixed'' magnetic layers. We attribute these magnetic fluctuations to thermally excited hopping of magnetic domain walls between pinning sites. At high temperatures, this magnetic noise is found to track the dc resistance susceptibility but it is not in quantitative agreement with the fluctuation dissipation relation, indicating that the magnetic structure is not in equilibrium. A second mechanism for the $1/f$ noise, connected with defects in the tunnel barrier but not related to the overall magnetization fluctuations, was found at fields for which the magnetic structure in the free and fixed layers is well aligned. We attribute this noise to electron trapping processes having thermally activated kinetics and a broad distribution of activation energies. Below $\ensuremath{\sim}25\mathrm{K}$ the noise power is temperature independent suggesting that the kinetics are dominated by tunneling. Our results show that the thermal stability of both the magnetic layers and the quality of the tunnel barrier are important factors in reducing the low-frequency noise in magnetic tunnel junctions.

Mechanism of electron localization at edge-sharing units in amorphousSiO2

We have presented a model of the electron-trapping process at edge-sharing ${\mathrm{SiO}}_{4}$ units that are supposed to exist as structural defects especially at the surface of silica-based materials. It has been shown that the edge-sharing ${\mathrm{SiO}}_{4}$ dimer can trap an electron, giving rise to a paramagnetic threefold-coordinated silicon and a negatively charged nonbridging oxygen. The former Si-related paramagnetic center exhibits a characteristic of the so-called ${E}^{\ensuremath{'}}$ center. The electrical level of the related electron trapping center is $\ensuremath{\sim}2\mathrm{eV}$ below the conduction band.

Influence of external β-irradiation in oxide glasses

Radiation effects in oxide glass as external β-irradiation has been investigated by Raman spectroscopy, Cr 3+ luminescence and EPR spectroscopy to analyze microstructure evolution. Comparison between simplified and complex glasses (high-level nuclear waste glass analogue) was undertaken. Migration and segregation of sodium have been confirmed on simplified irradiated glasses by a polymerization increase and dissolved oxygen presence. Densification under irradiation has also been demonstrated with the increase of three-membered silicate rings in silicate glass and the decrease of the average Si–O–Si angle in the borosilicate glass. In both glasses, Cr 3+ crystal field was increased, suggesting a Cr–O length decrease. The complex borosilicate glass responds differently: densification effect and Na segregation were not detected. Under β-irradiation, electron-trapping processes on Cr 6+ occurred, forming Cr 4+ and Cr 5+ species.

Field Effect of Photoluminescence from Excitons Bound to Nitrogen Atom Pairs in GaAs

Field effect of photoluminescence due to excitons bound to nitrogen atom pairs in GaAs has been investigated for uniformly doped and atomic-layer-doped samples grown on (001) GaAs substrates. The intensities of excitonic photoluminescence lines due to distant nitrogen atom pairs decrease much more rapidly than those from closer pairs when the electric field is increased. In addition, photoluminescence due to the nearest neighbor pairs in atomic-layer-doped samples exhibits much more stable characteristics than that of uniformly doped samples against an applied electric field. This stability is observed only when the electric field is applied in either the [110] or [110] direction. This anomalous field effect can be explained by considering the electron trapping process to the isoelectric N traps modulated by the electric field.

Electrical stressing effects in solid-phase crystallized polysilicon thin film transistors

The evolution of the electrical parameters of solid-phase crystallized polysilicon thin film transistors under various stressing-gate and drain-bias values was studied. Under VGS stressing, the threshold voltage Vth exhibits an initial negative shift attributed to hole trapping and a subsequent turnaround, occurring sooner for larger stressing VGS, towards positive shifts with a logarithmic time dependence, indicating an electron trapping process occurring in the oxide and at the polysilicon/SiO2 interface. The subthreshold swing, the midgap trap state density and the transconductance exhibit a concomitant logarithmic degradation, attributed to a stress-induced acceptor trap creation at the interface and in the polysilicon active layer, corroborating the electron trapping process inferred from the Vth data. The simultaneous application of a small VDS stressing in combination with the VGS stressing suppresses this Vth turnaround effect and enhances the degradation of all parameters. The reduction of the turnaround stressing time as well as the increase in the subthreshold swing and the midgap trap state density correlated with the increase of the stressing current IDS, indicating a concomitant increase of the stress-induced trap creation and the electron trapping.

Photochemistry of Diflunisal in Nonionic (Brij-35) Micelles: Influence of the Microenvironment on Photoionization, Electron Trapping, and Persistent Radical Effect

The photochemistry of diflunisal (DF) in nonionic micelles of polyoxyethylene lauryl ether (Brij-35) has been investigated by combining steady-state and time-resolved spectroscopic techniques. Photophysical and photochemical measurements account for an efficient aggregation of the drug with the micellar system. Shortening of the singlet and lengthening of the triplet lifetimes of DF is observed upon incorporation into micelles, respectively. The former effect is responsible for changes in the relative weight of the mono- and biphotonic pathways of photoionization. The quenching constant related to the trapping by the ground state of DF of the hydrated electrons photoejected is markedly reduced in the presence of Brij-35. The exit dynamic of DF from the micellar cage is the almost exclusive rate-controlling step for the electron-trapping process. Both nature of the photoionization and efficiency of the electron trapping provide basic information about the localization of DF in the micellar cage. The “persi...

The theory of galvanoluminescence in the anodic oxide films obtained by aluminum anodization in ammonium tartrate

The experimental as well as the theoretical aspects of galvanoluminescence (GL) during the anodization of aluminum in the barrier-layer forming electrolyte ammonium tartrate are considered in this paper. The experimental results are explained by the avalanche theory, where the electron trapping processes and the influence of interference effects on the measured GL intensity have been taken into account as well. It is assumed that the luminescence is produced by impact excitation of luminescent centers (LCs). The electric field strength F in the parts of the oxide film occupied by avalanches is calculated in addition to its influence on the ionization coefficient α, mean free path for electron capture by trap x 0 and the number of the avalanche electrons. The formula for spectral intensity of GL is also derived.

Statistical Thermodynamics and Photographic Sensitivity

By the methods of statistical thermodynamics, it has been proved that a conduction electron cannot be trapped above 45 K in an AgBr microcrystal by a shallow positive potential well with a depth of about 0.05 eV. A trap with a depth of 0.2 eV cannot be effective above 155 K. The direct electron trapping process of the Hamilton nucleation and growth theory, in which an electron is trapped at room temperature by the shallow positive potential well of a silver ion on a surface kink site, without the participation of an interstitial silver ion in the actual trapping process at the trapping site, violates the second law of thermodynamics. This process would result in a decrease in the entropy and an increase in the Helmholtz free energy of the system.

Photogeneration of Donor Pairs in ZnSe{Ag} Crystals

Experimental evidence is presented that photoinduced defect reactions in ZnSe{Ag} crystals give rise to the formation of (Ag0i)2 interstitial donor pairs. As a result, the induced impurity photoconductivity spectrum shows, along with the band at 0.29 eV due to isolated Ag0i atoms, a series of narrow bands in the range 0.3–0.6 eV (quasi-line spectrum). The energy positions and relative intensities of these bands are accounted for in terms of different pair separations in (Ag0i)2 . The charge instability of Agi under band-gap excitation, associated with multiple electron trapping processes, leads to atomic instability, which gives rise to ionization-enhanced diffusion and the formation of (Ag0i)2 donor pairs.

Electron trapping centres and cross sections in LiNbO3 studied by 57Co Mössbauer emission spectroscopy

Fast electron trapping processes and aliovalent charge states following the 57Co(EC)57Fe decay are studied in undoped, 5.4 mol% Mg-doped and 0.1 mol% Fe-doped LiNbO3 in various thermochemical reduction (TCR) states. Static 57Co Mössbauer emission spectra of congruent Mg:LiNbO3 recorded at T = 4.2 K in external magnetic field of 4.6 T are presented. Trapping cross section ratios are derived for Fe3+Li, Nb5+Li and Mg2+Li. A method to determine trap concentrations for TCR states of LiNbO3 is outlined. The electron-capture distance of the traps is found to be 2.7±1.4 nm. As this is much smaller than the 6 keV Auger-electron penetration depth, it is concluded that the distribution of the aliovalent charge states at 4.2 K is determined mainly by the 600 eV Auger electrons.

Stress polarity dependence of the activation energy in time-dependent dielectric breakdown of thin gate oxides

Stress polarity dependence of the activation energies in the two time dependent dielectric breakdown measurements, the constant-current injection (Q/sub bd/ testing) and the constant-voltage stressing (t/sub bd/ testing) are investigated for gate oxides with the thickness ranging from 10 to 4 nm. A remarkable polarity dependence of the activation energies appears in the t/sub bd/ testing when the oxide thickness decreases. This phenomenon is found to be due to a characteristic temperature dependence of the gate current density during the whole t/sub bd/ testing period for thinner oxides, which is considered as a result from the temperature dependence of the electron trapping process during the stressing.

Electron trapping process in ferroelectric lead–zirconate–titanate thin-film capacitors

The effects of constant voltage stress on ferroelectric lead–zirconate–titanate (PZT) thin-film capacitors have been studied. Two stress effects are observed in the PZT capacitance–voltage (C–V) characteristics. The first is that the capacitance is reduced, and the second is a voltage shift of the C–V curve. These effects are found to depend on the stress electric field and the injected charge fluence. A correlation between the stress effects and the electron trapping inside the films is established. The injected charge fluence can be calculated from the leakage current. The trapping efficiency, electron capture cross section, and the trap density are obtained from the measured injected charge influence and the C–V voltage shift. The calculated value of the electron capture cross-section σT is 1.89×10−19 cm2 and the neutral electron trap density NT is 1.20×1013 cm−2.

Radiation induced paramagnetic centres in nuclear glasses by EPR spectroscopy

An electron paramagenetic resonance (EPR) investigation of a series of glasses irradiated at room temperature with β or X radiation sources has been made in order to predict the long term behavior of glasses used in the nuclear waste disposal. EPR spectra of the irradiated glasses have been measured and treated using computer simulations. Simulation of the g factor and hyperfine interaction distributions have led to the identification of different paramagnetic centers in these irradiated glasses. The following paramagnetic defects were identified: boron oxygen hole center (BOHC), HC 1 center, two different hole centers, peroxy radical (Oxy defect), E′ centers, and electron trapping processes for iron and zirconium ions: Zr 4++ e −→ Zr 3+ and Fe 3+ + e −→ Fe 2+. In particular, no paramagnetic defects associated with aluminum ions are detected in these irradiated glasses.

Subpicosecond studies of primary photochemical events of CdS particles with surface modified by various capping agents

Q-CdS particles with surface modified by thiophenol (PhS) or 2-mercaptobenzoxazole (MBO) were prepared and their time-resolved transient absorption spectra were measured in acetonitrile in subpicosecond to picosecond time domain in order to investigate the influences of capping agents on primary photochemical events such as electron and hole trapping processes and decay kinetics of the trapped charge carriers on capped semiconductor Q-particles. In addition, time-resolved transient absorption spectra of aqueous CdS colloidal solutions containing sodium hexametaphosphate (HMP) as a stabilizer against flocculation were also measured in the same time domain and compared to obtain knowledge about the trap sites for electrons photo-produced on capped CdS particles in acetonitrile. The shape and the decay profile of transient absorption spectra obtained by subpicosecond laser flash photolysis of PhS-CdS and MBO-CdS in acetonitrile are markedly different from those of non-capped CdS particles in acetonitrile obtained previously using picosecond laser flash photolysis by Karnat et al. The results suggest that capping agents such as PhS and MBO influence significantly hole trapping process and decay dynamics of the trapped charge carriers on the capped CdS. Furthermore, it seems that the trap site of the electrons generated by laser excitation of PhS-CdS or MBO-CdS in acetonitrile is mainly at the interface of particle-solution and the possibility for the production of solvated electrons is low because the shapes and decay kinetics of the transient absorption spectra of PhS-CdS or MBO-CdS in acetonitrile are clearly different from those HMP-CdS in water in which the photo-production of the hydrated electrons is known.

パレオドースの新評価法 （ＩＩ）

A new method of evaluating the pareodose, the auther presented in 1988, extended the limits of the TL dating method to about 106 years. In the former article, consideration about the electron-trapping process produced the combination of a competition model and an ordinary saturation model. The competition model accounts for the supralinearity in the low dose range, and the ordinary saturation model explains the sublinearity in the high dose range. In the new method, the experimental data decide the coefficients of the equation directly, without the regressive error. Therefore, in many types of samples, the new method brought good results for evaluation of the pareodose. In some cases, however, a few errors have appeared in the low dose range. In the former article, the electron-trapping probability of the competition model was doubly approximated, in order to make a solution easy. The double approximation caused a few errors in the calculation of TL responce, in the low dose range, Consequently, the former equation led a smaller value as a pareodose. For the purpose of a solution, the approximate term of (1-e7-CR) was exchanged for the correct term of (1+e-CR+D)-1. The improved equation has offered better-fitting results with experiments, especially in the low dose range. By this improvement, the new method has enhanced the reliability and has come to be of wide application.

Metal-oxide-semiconductor characteristics of rapid thermal nitrided thin oxides

The electrical properties of thin nitrided oxide (∼100 Å) formed by rapid thermal nitridation (RTN) in pure NH3 have been studied. It is found that the current-voltage characteristic of RTN oxides follows a Fowler–Nordheim tunneling behavior with modifications caused by electron trapping processes at the oxide surface and interface. The trapping density is dependent on the RTN conditions. At the interface, both fixed charge (Nf) and interface state (Dit) densities exhibit turnaround phenomena when the RTN process proceeds. The maximum values of Nf and Dit at the turnaround points are lower for the higher temperature RTN, suggesting a viscous flow related strain relieving mechanism associated with RTN of thin oxides. Films with superior endurance behavior (QBD=20.4 C/cm2 compared with QBD=5.1 C/cm2 of thermal oxide under 10 mA/cm2 constant current stress) have been obtained by RTN at 1000 °C, 10 s.

Identification of electron and hole picosecond trapping processes in doped a-Si:H

Using the picosecond pump and probe technique we identify the electron and hole contributions to the photomodulation response in doped a-Si:H. We have found that electron trapping in doping related defects is about 50 times faster than hole trapping regardless of doping type and concentration, temperature, light exposure or the energy level of the traps. This is an intrinsic property of a-Si:H which originates from a larger electron hopping rate among localized states in the conduction band-tail due to a larger extent of the electron wave function.

パレオドースの新評価法 熱ルミネッセンス法の適用年代拡張の試み

On evaluating the paleodose in TL dating, there are two problems to be solved. The first is the supralinearity of TL response in the low dose range. The second is the sublinearity of TL response in the high dose (over 100 Gy) range. We surmised that these phenomena are concerned with the electron-trapping process. A competition model was used for the supralinearity phenomenon, and an ordinary saturation model was used for the sublinearity phenomenon. We proposed a new equation combining these two models. Furthermore, using this equation, the coefficients were decided directly from the experimental data without regressive error. Also, the change in TL sensitivity after annealing could be represented by the change of only one coefficient. This equation concerning these two phenomena offers well-fitting results with experiments from the low dose range to the high dose (2000 Gy) range. The annual dose rate is distributed in the range of 2.5 mGy/a. The age to which TL dating method is applicable, has been extended to about 106 years by using the equation. On three samples from the archeological sites in Miyagi Prefecture, by this method, the evaluated paleodoses range from 301.3 Gy to 626.5 Gy.