Electron Trapping Rate Research Articles

Redirecting the electron flow to coordinate oxidation and reduction reactions for efficient photocatalytic H2O2 production

Energy-efficient photocatalytic production of H2O2 from water holds the potential to replace the anthraquinone process. However, the impracticality of oxygen bubbling in such processes necessitates addressing the H2O2 production in oxygen-deficient environments. Here, we demonstrated the generation of singlet oxygen (1O2) as a highly active water oxidation derivative that redirects the electron flow for H2O2 production in oxygen-deficient environments. Ground-state oxygen was in-situ generated on photocatalysts and undergoes transition to 1O2, surpassing other competitive electron-consuming reactions. Self-generated 1O2 undergoes preferential photo-reduction to H2O2, benefiting from a small activation energy (0.2 eV), facilitated electron trapping rate, adjacent oxidation/reduction sites and suppressed competitive energy transfer from excited-states to air 3O2. A remarkable solar-to-chemical conversion (SCC) efficiency of 0.25 % was obtained for H2O2 generation in oxygen-deficient conditions. This work unveils a novel concept of redirecting electron flow towards H2O2 production, taking advantage of highly active in-situ generated 1O2.

Towards the optically stimulated luminescence dating of unheated flint

Flint tools are important archaeological finds, but unheated specimens remain challenging to date. The optical luminescence properties of the microcrystalline flint differ from sedimentary quartz grains, and optically stimulated luminescence (OSL) has, therefore, only occasionally been used to date heated flint specimens. However, if OSL dating could be used on unheated samples, this should expand the number of dateable flint samples from the archaeological record. This study investigates the luminescence sensitivity to blue, green, and infrared stimulation in flints collected from four sites in eastern Denmark. While stimulations at each wavelength yielded some luminescence emissions, blue OSL provided the strongest emission, and subsequent investigations here focus on measurements using this signal. The shape of the decay curves and the photo-ionising cross-sections for the blue OSL signal components are similar between natural and irradiated OSL and are also comparable to decay curves from calibration quartz. Dose–response curves show characteristic doses of ∽400 Gy, and dose recovery tests demonstrate that a SAR protocol can recover laboratory doses. The measured laboratory fading rates are considerable, where flint slices stored for up to six weeks provide average g-values of up to 9 % per decade. Also, the flint is less thermally stable compared to calibration quartz. However, a natural OSL signal remains in Cretaceous flints (albeit at 50 % or less of laboratory saturation), showing that signal loss did not outpace the electron trapping rate. The presence of a dose-dependent, natural OSL signal and acceptable dose recovery indicate that optical dating of flint surfaces could be feasible, at least in the flints from eastern Denmark, and that flint from other regions should be investigated.

Charge Carrier Recombination Dynamics in MAPb(BrxCl1-x)3 Single Crystals.

Understanding carrier recombination processes in MAPb(BrxCl1-x)3 crystals is essential for their photoelectrical applications. In this work, carrier recombination dynamics in MAPb(BrxCl1-x)3 single crystals were studied by steady-state photoluminescence (PL), time-resolved photoluminescence (TRPL), and time-resolved microwave photoconductivity (TRMC). By comparing TRPL and TRMC, we find TRPL of MAPb(BrxCl1-x)3 (x < 0.98) single crystals is dominated by a hole trapping process while the long-lived component of TRMC is dominated by an electron trapping process. We also find both electron and hole trapping rates of MAPb(BrxCl1-x)3 (x < 0.98) crystals decrease with an increase in Br content. A temperature-dependent PL study shows there are shallow trap states besides the deep level trap states in the MAPb(Br0.82Cl0.18)3 crystal. The activation energy for holes in shallow trap states detrapped into the valence band is ∼0.1 eV, while the activation energy for free holes to be trapped into deep trap states is ∼0.4 eV. This work provides insight into carrier recombination processes in MAPb(BrxCl1-x)3 single crystals.

Charge Carrier Recombination Dynamics in MAPb(Br1−yIy)3 Single Crystals

Studying the carrier recombination process in MAPb(Br1−yIy)3 single crystals (SCs) is important for its application in the optoelectronic field. In this work, a series of MAPb(Br1−yIy)3 SCs with varied Br/I compositions have been studied. Steady-state photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra and time-resolved microwave photoconductivity (TRMC) were used to understand the radiative and non-radiative recombination processes of MAPb(Br1−yIy)3 SCs. By comparing the dynamics of TRPL and TRMC, we conclude that the dynamics of TRPL is dominated by the electron trapping process, which is in accordance with the fast decay component of TRMC kinetics, whereas the slower decay component in TRMC is dominated by the hole trapping process. Moreover, we find both the electron and hole trapping rates in mixed-halide perovskite MAPb(Br1−yIy)3 (0 &lt; y &lt; 1) SCs are higher than that of mono-halide perovskite MAPbBr3 SCs and MAPbI3 SCs. This suggests mixed-halide crystals could introduce additional electron and hole trapping densities, which could be related to the fluctuation of Br/I compositions in the crystals. This work is helpful for understanding carrier recombination process in mixed-halide perovskite SCs.

Carrier Recombination Processes in GaAs Wafers Passivated by Wet Nitridation.

As one of the successful approaches to GaAs surface passivation, wet-chemical nitridation is applied here to relate the effect of surface passivation to carrier recombination processes in bulk GaAs. By combining time-resolved photoluminescence and optical pump—THz probe measurements, we found that surface hole trapping dominates the decay of photoluminescence, while photoconductivity dynamics is limited by surface electron trapping. Compared to untreated sample dynamics, the optimized nitridation reduces hole- and electron-trapping rate by at least 2.6 and 3 times, respectively. Our results indicate that under ambient conditions, recovery of the fast hole trapping due to the oxide regrowth at the deoxidized GaAs surface takes tens of hours, while it is effectively inhibited by surface nitridation. Our study demonstrates that surface nitridation stabilizes the GaAs surface via reduction of both electron- and hole-trapping rates, which results in chemical and electronical passivation of the bulk GaAs surface.

Time-dependent dielectric breakdown characterizations of interlayer dielectric damage induced during plasma processing

We proposed an electrical evaluation method on the basis of leakage current–stress time (I–t) measurement for plasma-induced damage (PID) in interlayer dielectric films. The I–t measurement, which is widely used for the time-dependent dielectric breakdown (TDDB) test is applied to the evaluation of PID in thick SiO2 films. We focus on the electron trapping process during the TDDB test. The initial stage of time evolution of leakage current is monitored, and compared among the samples damaged under various plasma conditions. It was found that the initial electron trapping rate depends on the amount of damage, i.e., the rate increases with increasing in process time (tpr) and dc self-bias voltage (Vdc). The proposed technique can be used to assess the reliability degradation of interlayer dielectric films caused by PID.

The structural and electrical comparison of Y2O3 and Ti-doped Y2O3 dielectrics

A Ti-doped Y2O3(Y2Ti2O5) dielectric on polycrystalline silicon followed by rapid thermal annealing results in improved characteristics including a higher effective dielectric constant, higher breakdown electric field, lower electron trapping rate, and larger charge-to-breakdown when compared with Y2O3. The performance of high-k Y2O3 and Y2Ti2O5 dielectrics were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), capacitance-voltage, and current density-voltage. Incorporating Ti into the Y2O3 dielectric imparts improvements in the structural and electrical performance of the material. The Y2Ti2O5 dielectric with 800°C annealing treatment has the best performance among all the samples tested.

Photoexcited carrier trapping and recombination at Fe centers in GaN

Fe doped GaN was studied by time-resolved photoluminescence (PL) spectroscopy. The shape of PL transients at different temperatures and excitation powers allowed discrimination between electron and hole capture to Fe3+ and Fe2+ centers, respectively. Analysis of the internal structure of Fe ions and intra-ion relaxation rates suggests that for high repetition rates of photoexciting laser pulses the electron and hole trapping takes place in the excited state rather than the ground state of Fe ions. Hence, the estimated electron and hole capture coefficients of 5.5 × 10−8 cm3/s and 1.8 × 10−8 cm3/s should be attributed to excited Fe3+ and Fe2+ states. The difference in electron capture rates determined for high (MHz) and low (Hz) (Fang et al., Appl. Phys. Lett. 107, 051901 (2015)) pulse repetition rates may be assigned to the different Fe states participating in the carrier capture. A weak temperature dependence of the electron trapping rate shows that the potential barrier for the multiphonon electron capture is small. A spectral feature observed at ∼420 nm is assigned to the radiative recombination of an electron in the ground Fe2+ state and a bound hole.

Electron thermalization and trapping rates in pure and doped alkali and alkaline-earth iodide crystals studied by picosecond optical absorption

Although light continues to be emitted from insulating crystals used as scintillators over a period of nanoseconds to microseconds after stopping of an energetic particle, much of what determines the nonlinearity of response goes on in the first picoseconds. On this time scale, free carriers and excitons are at high density near the track core and thus are subject to nonlinear quenching. The hot (free) electrons eventually cool to low enough energy that trapping on holes, dopants, or defects can commence. In the track environment, spatial distributions of trapped carriers determined on the picosecond time scale can influence the proportionality between light yield and the initial particle energy throughout the whole light pulse. Picosecond spectroscopy of optical absorption induced by a short pulse of above-gap excitation provides a useful window on what occurs during the crucial early evolution of excited populations. The laser excitation can be tuned to excite carriers that are initially very hot ($\ensuremath{\sim}$3 eV) relative to the band edges, or that are almost thermalized ($\ensuremath{\sim}$0.1 eV excess energy) at the outset. Undoped and doped samples of NaI:Tl(0%, 0.1%), CsI:Tl(0%, 0.01%, 0.04%, 0.3%), and $\mathrm{Sr}{\mathrm{I}}_{2}$:Eu(0%, 0.2%, 0.5%, 3%) are studied in this work.

Comparison of high-κ Nd2O3 and NdTiO3 dielectrics deposited on polycrystalline silicon substrates

High-κ Nd2O3 and NdTiO3 films deposited on polycrystalline silicon treated at various post-rapid thermal annealing temperatures were fabricated as high-κ gate dielectrics. The NdTiO3 samples demonstrated a higher effective dielectric constant, lower leakage current, higher breakdown electric field, smaller gate voltage shift, and lower electron trapping rate than the high-κ Nd2O3 polyoxide dielectric samples. Annealing effects were analyzed using electrical measurements and multiple material analysis techniques included X-ray diffraction and atomic force microscopy. Incorporating Ti content into Nd2O3 dielectrics resulted in noticeable improvements in electrical performance and material quality. Results indicate that NdTiO3 dielectrics show promise for future high-κ dielectric electronic device applications.

Ultrafast Electron Trapping at the Surface of Semiconductor Nanocrystals: Excitonic and Biexcitonic Processes

Aging of semiconductor nanocrystals (NCs) is well-known to attenuate the spontaneous photoluminescence from the band edge excitonic state by introduction of nonradiative trap states formed at the NC surface. In order to explore charge carrier dynamics dictated by the surface of the NC, femtosecond pump/probe spectroscopic experiments are performed on freshly synthesized and aged CdTe NCs. These experiments reveal fast electron trapping for aged CdTe NCs from the single excitonic state (X). Pump fluence dependence with excitonic state-resolved optical pumping enables directly populating the biexcitonic state (XX), which produces further accelerated electron trapping rates. This increase in electron trapping rate triggers coherent acoustic phonons by virtue of the ultrafast impulsive time scale of the surface trapping process. The observed trapping rates are discussed in terms of electron transfer theory.

Ab initio approach to the rate of radiative electron trapping and electron–hole recombination in B‐, C‐, and N‐doped anatase

AbstractWe propose a first‐principle method for evaluating the rate of radiative electron–hole recombination and electron trapping on impurities in semiconductors. The method is based on the Einstein–Planck theory of photon modes and the perturbation theory with dipole approximation for the electron–field interaction. The calculations employing the LMTO‐TB band structure method have been done for anatase doped with boron, carbon, and nitrogen. We find that the doping cannot induce radiative processes of electron trapping or electron–hole recombination.

Electron Self-Injection and Trapping into an Evolving Plasma Bubble

The blowout (or bubble) regime of laser wakefield acceleration is promising for generating monochromatic high-energy electron beams out of low-density plasmas. It is shown analytically and by particle-in-cell simulations that self-injection of the background plasma electrons into the quasistatic plasma bubble can be caused by slow temporal expansion of the bubble. Sufficient criteria for the electron trapping and bubble's expansion rate are derived using a semianalytic nonstationary Hamiltonian theory. It is further shown that the combination of bubble's expansion and contraction results in monoenergetic electron beams.

Electrical and Reliability Improvement in Polyoxide by Fluorine Implantation

We show that the incorporation of fluorine into the oxide grown on polysilicon (polyoxide) not only improves the electrical characteristics (i.e., lower leakage current, higher electrical breakdown field), but also improves the reliability (lower electron trapping rate, larger ). This improvement is believed to be due to the stress relaxation of the polyoxide and smoother polysilicon/polyoxide interface by the fluorine incorporation. The optimum fluorine dose shows the best characteristics such as over and . However, excessive fluorination seems to result in performance degradation due to the generation of nonbridging oxygen centers.

Characteristics of the Inter-Poly Al2O3 Dielectrics on NH3-Nitrided Bottom Poly-Si for Next-Generation Flash Memories

The effects of surface NH3 nitridation of the bottom poly-Si film, and post-deposition annealing (PDA) temperature, on the electrical properties and reliability characteristics of aluminum oxide (Al2O3) inter-poly capacitors, were evaluated in this study. The polarity-dependent dielectric properties of Al2O3 inter-poly dielectrics (IPDs) were strongly affected by both surface nitridation and the annealing temperature. For positive gate bias, IPDs with NH3 surface nitridation significantly suppressed the formation of an additional layer with a lower dielectric constant, during the post-annealing process, and obtained a smoother interface, compared to those without nitridation treatment. Furthermore, the presence of a thin Si–N layer made the PDA more effective in eliminating the traps existing in the as-deposited films, and improved dielectric characteristics, under negative polarity. As a result, the smoother interface and smaller electron trapping rate contributed to the drastically reduced leakage current, enhanced breakdown field, and charge to breakdown (Qbd) of the Al2O3 inter-poly capacitors with surface NH3 nitridation. Moreover, the electrical properties of Al2O3 IPD were heavily dependent upon the PDA temperature. The sample exhibited optimal quality in terms of leakage current, electron trapping rate and Qbd when annealed at 900°C. X-ray photoelectron spectroscopy and Auger electron spectroscopy analyses have shown that this occurrence arises from various compositions, under different annealing conditions and excess oxygen, which can act as an electron trapping center, playing an important role in determining IPD electrical properties.

Quantifying charging damage in gate oxides of antenna structures for WLR monitoring

A new method to quantify the reliability risk for gate oxide with plasma induced charging damage (PID) is established. Based on existing antenna test methodology the quantity of inflicted damage is expressed in a physical meaningful number by means of a simple model applicable for thick oxides (>5 nm). This model takes trap activation, trap filling, detrapping and also traps generation under constant current test condition (“revealing stress”, “diagnostic stress”) into account. For the corresponding development of the measurable external supply voltage with time an equation is derived. Experimental test data from different oxide thicknesses are fitted to this model equation to obtain its main parameters, the cross section values. These cross section values describe the probabilities for the different trap/detrap processes during stress. Cross section values thus found extend published data for lower electric fields to high electric fields necessary for a fast test. The number of plasma induced traps, which was added to the oxide during wafer processing, can now be determined by applying an electron trapping rate (ETR) test method, and combining it with our dynamic trap generation/filling model. The obtained number of PID related traps opens a path to calculate the corresponding reduction of oxide lifetime. Real measurement data are used to illustrate the method and its applicability to fast wafer level reliability (fWLR) monitoring.

Charge collection and absorption-limited x-ray sensitivity of pixellated x-ray detectors

The charge collection and absorption-limited x-ray sensitivity of a direct conversion pixellated x-ray detector operating in the presence of deep trapping of charge carriers is calculated using the Shockley–Ramo theorem and the weighting potential of the individual pixel. The sensitivity of a pixellated x-ray detector is analyzed in terms of normalized parameters; (a) the normalized x-ray absorption depth (absorption depth/photoconductor thickness), (b) normalized pixel width (pixel size/thickness), and (c) normalized carrier schubwegs (schubweg/thickness). The charge collection and absorption-limited sensitivity of pixellated x-ray detectors mainly depends on the transport properties (mobility and lifetime) of the charges that move towards the pixel electrodes and the extent of dependence increases with decreasing normalized pixel width. The x-ray sensitivity of smaller pixels may be higher or lower than that of larger pixels depending on the rate of electron and hole trapping and the bias polarity. The sensitivity of pixellated detectors can be improved by ensuring that the carrier with the higher mobility-lifetime product is drifted towards the pixel electrodes.

High quality Al/sub 2/O/sub 3/ IPD with NH/sub 3/ surface nitridation

In this letter, the effect of surface NH/sub 3/ nitridation on the electrical properties and reliability characteristics of aluminum oxide (Al/sub 2/O/sub 3/) interpoly capacitors is studied. With NH/sub 3/ surface nitridation, the formation of an additional layer with lower dielectric constant during post-annealing process can be significantly suppressed, compared to that without nitridation treatment. Furthermore, the presence of a thin Si-N layer can make post-deposition annealing more effective in eliminating traps existing in the as-deposited films. As a result, a smoother interface and smaller electron trapping rate can contribute to the drastically reduced leakage current, enhanced breakdown field and charge to breakdown (Q/sub bd/) of Al/sub 2/O/sub 3/ interpoly capacitors with surface NH/sub 3/ nitridation.

Thin Oxides Grown on Disilane-Based Polysilicon

This report describes the thin oxide (<10 nm) thermally grown on polysilicon film, which was prepared by disilane (Si2H6) chemical vapor deposition. In comparison with the thin oxide grown on conventional silane (SiH4) polysilicon film, the prepared oxides have lower leakage currents, a lower electron trapping rate and a larger charge to breakdown distribution. These good properties are attributed to the smooth surface of the disilane poly-I film. This film is suitable for use as the interpoly oxide of electrically-erasable programmable read only memory.

High reliability polyoxide fabricated by using TEOS oxide deposited on disilane polysilicon film

This paper reports the TEOS oxide deposited on the polysilicon film which was prepared by using the disilane chemical vapor deposition. Compared to the thermally grown oxide or TEOS (tetra-ethyl-ortho-silicate) oxide deposited on the conventional silane polysilicon film, it had symmetrical J-E characteristics that had lower leakage currents but much higher breakdown field, a lower electron trapping rate, and a much larger charge to breakdown. These good properties are attributed to the smoother surface of the deposited disilane poly-I film and the more incorporation of nitrogen during the rapid thermal annealing (RTA) in N/sub 2/O ambient. It is suitable to be as the inter-polyoxide of the electrically-erasable programmable read only memory (EEPROM).