Subsequent Electron Trapping Research Articles

Creating Deep Traps in Yttrium Aluminum Garnet for Long‐Term Optical Storage and Afterglow‐Intensity‐Ratio‐Based Temperature Sensing

AbstractDeep traps are needed for electron‐trapping‐based long‐term optical storage due to its resistance to thermal erasure. Current electron trapping materials have the erasing temperatures hardly beyond 600 K, limiting its storage time. Herein, an electron trapping material containing deep traps is achieved by co‐doping Tb and Eu in Y3Al5O12 via solid‐state reaction in reducing atmosphere. After 254 nm UV charging, strong thermoluminescence of Tb3+ with the glow curve peaking at 600, 693, and 765 K is observed. The comparative studies of the as‐made and the air‐annealed samples suggest that these new traps are related to the complex defects containing oxygen vacancy with adjacent Eu2+ and the charging process is the photoionization of Tb3+ with subsequent electron trapping. The 693 and 765 K thermoluminescence glow peaks in the charged sample show almost no decrease during 108 h storage in dark at room temperature. Images write‐in and read‐out via 808 nm laser stimulation are realized. Furthermore, the present phosphor also exhibits temperature‐sensitive afterglow spectra in the range of 553–803 K with the sensing span of 250 K wider than other afterglow thermometers. These findings indicate the great application potentials of Y3Al5O12:Tb,Eu phosphor in long‐term optical storage and temperature sensing.

Exciton-Scissoring Perfluoroarenes Trigger Photomultiplication in Full Color Organic Image Sensors.

We report an unprecedented but useful functionality of perfluoroarenes to enable exciton scissoring in photomultiplication-type organic photodiodes (PM-OPDs). Perfluoroarenes that are covalently connected to polymer donors via photochemical reaction enable us to demonstrate high external quantum efficiency and B-/G-/R-selective PM-OPDs without the use of conventional acceptor molecules. We investigated the operation mechanism of the suggested perfluoroarene-driven PM-OPDs, "How can covalently bonded polymer donor:perfluoroarene PM-OPDs perform as effectively as polymer donor:fullerene blend-based PM-OPDs?". By employing a series of arenes and conducting steady-state/time-resolved photoluminescence and transient absorption spectroscopy analyses, we find that interfacial band bending between the perfluoroaryl group and polymer donor is responsible for exciton scissoring and subsequent electron trapping, which induces photomultiplication. Owing to the acceptor-free and covalently interconnected photoactive layer in the suggested PM-OPDs, superior operational and thermal stabilities were observed. Finally, we demonstrated finely patterned B-/G-/R-selective PM-OPD arrays that enable the construction of highly sensitive passive matrix-type organic image sensors. This article is protected by copyright. All rights reserved.

Tuning Nb Solubility, Electrical Properties, and Imprint through PbO Stoichiometry in PZT Films.

Lead zirconate titanate (PZT) films with high Nb concentrations (6-13 mol%) were grown by chemical solution deposition. In concentrations up to 8 mol% Nb, the films self-compensate the stoichiometry; single phase films were grown from precursor solutions with 10 mol% PbO excess. Higher Nb concentrations induced multi-phase films unless the amount of excess PbO in the precursor solution was reduced. Phase pure perovskite films were grown with 13 mol% excess Nb with the addition of 6 mol% PbO. Charge compensation was achieved by creating lead vacancies when decreasing excess PbO level; using Kroger-Vink notation, NbTi• are ionically compensated by VPb″ to maintain charge neutrality in heavily Nb-doped PZT films. With Nb doping, films showed suppressed {100} orientation, the Curie temperature decreased, and the maximum in the relative permittivity at the phase transition broadened. The dielectric and piezoelectric properties were dramatically degraded due to increased quantity of the non-polar pyrochlore phase in multi-phase films; εr reduced from 1360 ± 8 to 940 ± 6, and the remanent d33,f value decreased from 112 to 42 pm/V when increasing the Nb concentration from 6 to 13 mol%. Property deterioration was corrected by decreasing the PbO level to 6 mol%; phase pure perovskite films were attained. εr and the remanent d33,f increased to 1330 ± 9 and 106 ± 4 pm/V, respectively. There was no discernable difference in the level of self-imprint in phase pure PZT films with Nb doping. However, the magnitude of the internal field after thermal poling at 150 °C increased significantly; the level of imprint was 30 kV/cm and 11.5 kV/cm in phase pure 6 mol% and 13 mol% Nb-doped films, respectively. The absence of mobile VO••, coupled with the immobile VPb″ in 13 mol% Nb-doped PZT films, leads to lower internal field formation upon thermal poling. For 6 mol% Nb-doped PZT films, the internal field formation was primarily governed by (1) the alignment of (VPb″-VO•• )x and (2) the injection and subsequent electron trapping by Ti4+. For 13 mol% Nb-doped PZT films, hole migration between VPb″ controlled internal field formation upon thermal poling.

Optical properties of Mn2+ doped CsCdF3: A potential real-time and retrospective UV and X-ray dosimeter material

Luminescence measurements of CsCdF3:Mn reveal material properties suitable for UV and X-ray sensing. Mn2+ substitutes for Cd2+ with octahedral crystal field parameters 10Dq = 7020 ± 120 cm−1 and B = 773 ± 13 cm−1. UV stimulation at 250 nm results in intense Mn2+ emission and also in the production of F-centers and additional defects. Continuous 250 nm stimulation charges the material, enhancing the Mn2+ emission intensity over time due to the filling of shallow traps that contribute to persistent luminescence. When the stimulation is ceased, Mn2+ emission is observed as an afterglow. F-centers give rise to optically stimulated luminescence (OSL) that is temporally stable and can be stimulated at wavelengths near 300 nm. We propose a model that describes these effects, where UV stimulation results in electron transfer from Mn2+ to the conduction band and subsequent electron trapping in various defect states. X-ray stimulation produces Mn2+ radioluminescence that varies in intensity by 0.004%/Gy after a 300 Gy priming dose. The X-ray irradiation also produces OSL-active F-centers and the defects that cause the afterglow. Thermoluminescence measurements reveal low-temperature peaks that are correlated with the afterglow and high-temperature peaks that are correlated with the OSL, after both X-ray and UV irradiations. The luminescence of Mn2+ in CsCdF3:Mn may be useful for radiation sensing, where the real-time changes in Mn2+ emission under X-ray and UV stimulation allow for real-time dose measurements, and the OSL induced by each irradiation allows for retrospective dose monitoring.

Scanning photocurrent microscopy of electrons and holes in the pigment semiconductor epindolidione

Abstract Photocurrent microscopy is used to characterize the kinetics of electrons and holes in organic field-effect transistors (FETs) with the hydrogen-bonded pigment epindolidione as active layer. The method relies on electrons and holes, generated on local illumination, which are provided after exciton splitting, to probe charge trapping. In the dark, hole conduction is observed for negative gate voltage while no electron conduction is observed for positive gate voltage. However, under illumination, a fast displacement current with 60 μs onset time and 1 ms exponential decay occurs for positive gate voltage, which can be explained by exciton splitting underneath the semitransparent top contact followed by subsequent electron trapping and hole extraction. Afterward, trapped electrons hop via further trap states within the film to the insulator into interface traps (13 ms exponential decay) which induce a positive threshold voltage shift in the FET transfer curves for hole transport. Photocurrent microscopy confirms that the displacement current occurs only for illumination under and near the semitransparent source/drain contacts, which act here as metal-insulator-semiconductor (MIS) diodes. For negative gate voltage instead, the photocurrent comprises an enhanced hole current in the FET channel between the contacts. In the channel region, the detrapping of holes at the interface with the insulator (3 ms time constant) enhances the transistor current at low frequencies 10 kHz. Thus, we show here that photocurrent microscopy allows to identify the kinetics of electrons and holes in traps close to the contacts and in the FET channel of pigment transistors.

Achieving high sensitivity in hybrid photodetectors based on an organic single crystal and an inorganic nanocrystal array

We demonstrate an approach to enhance the photosensitivity of an organic single-crystal photodetector by combining it with a nanocrystal (NC) array. A systematic study of the dependence of the photodetector performance on illumination wavelength and light power together with the charge carrier mobility reveals that charge separation at the single-crystal/CdSe NC interface and subsequent electron trapping in the CdSe NCs generates effective photoconductive gain by hole circulation through the high-mobility single crystal. As a result, the responsivity and specific detectivity reached values up to 50 A W−1 and 2 × 109 cm Hz1/2 W−1, respectively.

Pd-nanoparticle-decorated ZnO nanowires: ultraviolet photosensitivity and photoluminescenceproperties

ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm(Pd–ZnO NWs) via a chemical solution route. The microstructural characterizations havebeen done using field emission scanning electron and high-resolution transmissionelectron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnONWs have been investigated by studying the ultraviolet (UV) photosensitivityand photoluminescence (PL) properties. The surface-modified NWs show a UVphotosensitivity more than double and a response seven times faster compared to the bareNWs. The photocarrier relaxation under the steady UV illumination conditionis quite different in Pd–ZnO NWs. The higher and faster photosensitivity hasbeen explained on the basis of photocarrier transfer from the conduction band ofZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbedO2 molecules on the NWs’ surface, which have been presented through a proposed model. ThePL spectrum of Pd–ZnO NWs shows that the intensities of the band-edge anddefect-related emissions decrease and increase, respectively, due to Pd anchoring, the effectbeing pronounced as the density of Pd nanoparticles increases.

Quartz as a natural luminescence dosimeter

Abstract Luminescence from quartz is commonly used in retrospective dosimetry, in particular for the dating of archaeological materials and sediments from the Quaternary period. The phenomenon of luminescence is related to the interaction of natural radiation with mineral grains, by the activation of and subsequent trapping of electrons at defects within the quartz lattice. The latent luminescence signal (i.e. the trapped electrons) is released when the grains are exposed to stimulation energy in the form of light or heat. Despite the fact that quartz is most nominally pure SiO2, the mineral forms in several different geological settings, i.e. under different pressure and temperature conditions. The luminescence emitted from quartz is complex and shows a variety of different components with diverse physical properties. This complexity is explained by the variety of defects in quartz that are either intrinsic (e.g., Si and O vacancies) or related to impurity atoms (e.g., Al or Ti). The concentration of impurity-related defects is dependent on the conditions of mineral formation or subsequent alteration. Experimental data have shown that the luminescence properties of quartz are highly variable with geological source and vary even at a grain-to-grain level within a sediment. As a consequence, caution is needed when making any general statements about the luminescence properties of quartz. When using luminescence measurements as a dating technique, it is necessary to adjust the measurement procedures for each geological provenance. Furthermore, some quartz has luminescence properties that make it problematic, or even unsuitable, for certain applications. These problems can arise from low and changing luminescence sensitivity, thermal transfer of trapped electrons, thermal instability of the trapped electrons and low saturation dose. Reviewing the present knowledge reveals that insufficient information is available either to unambiguously link distinctive lattice defects with characteristic luminescence components, or even to explain problems observed in application studies by potential dynamics of the defects within the crystal. This paper gives some ideas on how future research could utilise innovative analytical tools to identify or map the distribution of lattice defects and how practitioners could relate lattice defects to measured luminescence properties of quartz.

Modeling of the I– V characteristics of high-field stressed MOS structures using a Fowler–Nordheim-type tunneling expression

The gate current–voltage characteristic of a high-field stressed metal-oxide-semiconductor structure with trapped charge within the insulator barrier is consistent with a Fowler–Nordheim-type tunneling expression. Instead of considering a correction for the cathode electric field as usual, we use an effective local electric field that takes into account the distortion of the oxide conduction band profile caused by the trapped charge. An energy level at the injecting interface, introduced as an optimization parameter of the model, controls the tunneling distance used for calculating the effective field. Trap generation in the oxide is induced by high-field constant current stress and subsequent electron trapping at different injection levels is monitored by measuring the associated flat band voltage shift. The model applies for positive gate injection regardless the stress polarity and the involved parameters are obtained by fitting the experimental data without invoking any particular theoretical model for the trapping dynamics. In addition, it is shown how the presented model accounts for consistently both the current–voltage and voltage–current characteristics as a function of the injected charge through the oxide.

Quadruplevoids in amorphous Si : H

The mechanism of the Staebler–Wronski effect is still in question. We assume that a defect precursor A is transformed into the actual, metastable defect B by excitation and subsequent electron trapping. Calculations on two large, distorted silicon clusters are presented which contain a bent and a pyramidal void, respectively, created by removal of four adjacent silicon atoms. A QM/QM embedding procedure is employed using a density functional (BP86) and a semiempirical method (AM1). Full geometry optimization results in novel and unprecedented structure information. Excitation energies and electron attachment energies are presented. These findings substantiate the new ideas regarding the nature of A and B.

Microstructure of local defects in amorphous Si:H: A quantum chemical study

The mechanism of the Staebler-Wronski effect, which is responsible for the light-induced degradation of photovoltaic devices based on amorphous, hydrogenated silicon $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}),$ is still in question. It is, e.g., assumed that a defect precursor A is transformed into the actual, metastable defect B by excitation and subsequent electron trapping. We present calculations on large, distorted silicon clusters containing vacancies/voids of up to three missing atoms. A sophisticated embedding procedure is employed where the atoms surrounding the vacancy/void are treated by means of a density functional (BP86) and the outer atoms are taken into account semiempirically (AM1). Full geometry optimization results in novel and unprecedented structure information. Excitation energies and electron attachment energies are presented. These findings substantiate ideas regarding the nature of A and B.

Failure mechanisms of AlGaAs/InGaAs pseudomorphic hemt's: Effects due to hot electrons and modulation of trapped charge

The reliability of AlGaAs/InGaAs pseudomorphic HEMT's has been investigated by means of thermal and hot-electron accelerated tests. Two commercially available devices have been tested, together with prototypes fabricated by a European supplier. Different failure modes have been observed after hot-electron testing, depending on the device type, i.e. (a) increase of drain current, I D and threshold voltage, | V T|, which can be attributed either to thermally-activated electron detrapping or to charge compensation by holes generated by impact ionization; (b) decrease of I D at low drain to source voltages, V DS, with the development of a kink in the output characteristics due to the generation of deep levels under the gate and subsequent electron trapping. In the former case, (a), hot carriers and/or high temperature storage only modulate the charge present on deep levels, leading to recoverable alterations of device characteristics. In the latter case, (b), the presence of additional deep levels under the gate leads to a permanent degradation. The link between the observed failure modes and the underlying physical mechanism is investigated by means of different techniques, and the main functional effects of the degradation modes are addressed.

Photoinduced hysteresis changes and optical storage in (Pb,La)(Zr,Ti)O3 thin films and ceramics

Pb(Zr,Ti)O3 and (Pb,La)(Zr,Ti)O3 thin films and bulk ceramics are shown to exhibit two distinct, but related types of photoinduced changes in their hysteresis behavior: (1) a photoinduced suppression of the switchable polarization and (2) a photoinduced voltage shift. Both effects give rise to stable and reproducible hysteresis changes and, thus, either could be the basis of an optical memory. Both phenomena can be explained by trapping of photogenerated charge at domain boundaries to minimize internal depolarizing fields. The space-charge field that causes the voltage-shift effect is primarily due to the migration and subsequent trapping of electrons. However, the thickness dependence of the voltage shift implies that the trapped charge is not confined to the interface. The voltage-shift kinetics exhibit a stretched-exponential dependence, whereas the polarization-suppression effect follows an exponential time dependence. However, both effects exhibit similar relaxation times. In addition, the relaxation time for the voltage-shift effect decreases with increasing light intensity according to a power-law relationship, τ∝I−n, where 0.67<n<0.75.

A temperature effect in the luminescence emission from electron-irradiated MgO

The temperature dependence of luminescence emissions from electron-irradiated CaO and MgO single crystals has been studied by time resolved luminescence spectroscopy after pulsed nanosecond irradiation with 0.20 to 0.60 MeV electrons. Emissions from CaO at 375 nm at both 293 and 83 K, show similar threshold characteristics for atomic displacement. These have been attributed to the displacement of oxygen ions and subsequent electron trapping, resulting in the formation of F+ centers. The threshold energy of 0.32±0.01 MeV corresponds to an oxygen displacement energy of 58±2 eV. A 380 nm emission from MgO, also attributed to oxygen displacement and F+ center formation, shows no temperature effect, with a displacement threshold virtually identical to that for CaO. A 235 nm emission from MgO shows a significant temperature effect. The threshold energy at 293 K is 0.31±0.01 MeV, whilst at 83 K two thresholds are observed, 0.31±0.01 and 0.41±0.01 MeV.

Generation of Oxide Charge and Interface States by Ionizing Radiation and by Tunnel Injection Experiments

Results of irradiation and high field tunnel injection experiments on MOS capacitors are discussed. The midgap voltage shift as a function of dose is caused by hole trapping only. In the case of tunnel injection, the generation of electron-hole pairs by impact ionization requires a much larger electron density and high fields. Thus a model of charge build-up is established which takes into account the hole trapping in neutral oxide states, the subsequent electron trapping in now positively charged states and detrapping of captured electrons. By means of this model, the prediction of the radiation hardness of MOS devices is feasible, provided that the impact ionization coefficient a is known accurately. If this is not the case, the combined techniques of ionizing irradiation and tunnel injection can be utilized to determine ? = ?o exp(-H?/F) as a function of the electrical field F. Electron capture and detrapping crosssections ?n and sn, resp., can be deduced by fitting the model to the experimental results. An F-3 dependency for ?n and an exp(-Hs/F) dependency for sn are found. Only a weak dependence on different processing parameters is observed. The proposed model is verified by a sequence of irradiation and injection steps. The generation of oxide charge is accompanied by an increase in interface state density Dit with a distribution, which peaks at about 0.15 eV above midgap, in both experiments. The results indicate that the generation of interface states is proportional to the amount of trapped holes.

Effect of long-term stress on IGFET degradations due to hot electron trapping

The threshold voltage shift through the long-term stress is measured for IGFET's. The gate bias dependence shows that the hot electron trapping is affected strongly by the electric field in the gate insulator. The threshold voltage shift versus time is well explained with the theory modified by the effect of the trapped charge on the subsequent electron trapping. The effect of transistor dimensions and temperature are also discussed.

Effect of Long Term Stress on Hot Electron Trapping

The threshold voltage shifts through the long term stress are measured for IGFETs. The gate bias dependence shows that the hot electron trapping is affected strongly by the electric field in the gate insulator. The threshold voltage shift versus time is well explained with the simple theory modified by the effect of the trapped charge on the subsequent electron trapping. The access time shifts are measured for actual 16 Kbit dynamic MOS RAM's. The access time degradations correspond well relatively to the threshold voltage degradations for the IGFET's.

Point defect generation in lif by plastic deformation and X-irradiation at low temperature

LiF crystals have been plastically deformed in compression while at liquid nitrogen temperature and have been subjected to X-irradiation without intermediate warming. The rates of formation of F and self-trapped hole centers are enhanced in the deformed samples in the early stages of irradiation, but are not influenced at longer irradiation times. These results can be simply interpreted in terms of the generation of negative ion vacancies during plastic deformation and the subsequent trapping of electrons at these vacancies to form F centers during irradiation. The rate of formation of self-trapped holes is enhanced because the rate of electron trapping is enhanced. The saturated enhancement of the self-trapped hole band is proportional to the saturated enhancement of the F band. Since enhancement of the F band is not observed as a result of plastic deformation at room temperature, these results suggest that negative ion vacancies produced during deformation at room temperature are sufficiently mobile to cluster, annihilate or otherwise become unavailable for F center formation during irradiation. The results at low temperature at longer times of irradiation indicate that the rate of generation of new negative ion vacancies during irradiation is insensitive to dislocation density as reported previously for room temperature irradiation. The saturated enhancements of the F and self-trapped hole bands at low temperature as a function of plastic deformation indicate a threshold for vacancy generation at approximately 1 % compression for the conditions of this experiment.

Intrinsic absorption and photoconductivity in single crystal SiTe 2

LiF crystals have been plastically deformed in compression while at liquid nitrogen temperature and have been subjected to X-irradiation without intermediate warming. The rates of formation of F and self-trapped hole centers are enhanced in the deformed samples in the early stages of irradiation, but are not influenced at longer irradiation times. These results can be simply interpreted in terms of the generation of negative ion vacancies during plastic deformation and the subsequent trapping of electrons at these vacancies to form F centers during irradiation. The rate of formation of self-trapped holes is enhanced because the rate of electron trapping is enhanced. The saturated enhancement of the self-trapped hole band is proportional to the saturated enhancement of the F band. Since enhancement of the F band is not observed as a result of plastic deformation at room temperature, these results suggest that negative ion vacancies produced during deformation at room temperature are sufficiently mobile to cluster, annihilate or otherwise become unavailable for F center formation during irradiation. The results at low temperature at longer times of irradiation indicate that the rate of generation of new negative ion vacancies during irradiation is insensitive to dislocation density as reported previously for room temperature irradiation. The saturated enhancements of the F and self-trapped hole bands at low temperature as a function of plastic deformation indicate a threshold for vacancy generation at approximately 1 % compression for the conditions of this experiment.