Charge Carrier Trapping Effects Research Articles

Enhanced thermoelectric performance of vertically aligned silicon nanowires through the cold spot effect and charge carrier trapping effect of attached gold nanoparticles

The thermoelectric performance of vertically aligned silicon (Si) nanowires (NWs) with diameters of 100–150 nm fabricated through metal-assisted chemical etching is improved using electroless-deposited gold (Au) nanoparticles (NPs) (ф ≈ 10 nm). After the deposition of metal NPs, the thermal conductivity and Seebeck coefficient of the Si NWs decrease 5.78-fold and increased 2.71-fold, respectively. Despite the 3.23-fold increase in the electric resistivity of Si NWs through the deposition of Au NPs, the figure of merit of the Au NP-deposited Si NWs is enhanced by 1320% (0.444) compared to that of pristine Si NWs (0.0337). The power factor is enhanced 2.28-fold after Au NP deposition (0.175–0.400 mV·K−2m−1). Based on finite-element method simulations of Au NP-deposited Si NW, a cold spot is generated inside the Si NW by the attached Au NP. Moreover, charge carrier trapping at the interface between the Si NW and Au NP is anticipated due to interfacial Fermi-level pinning. Cold spot and charge carrier trapping effects are proposed as important factors for enhancing the thermoelectric performance of Au NP-deposited Si NWs.

Effects of Charge Carrier Trapping on Image Resolution of Multilayer Photoconductive Detectors: Application to Amorphous Selenium X-ray Detectors

The effects of charge carrier trapping on image resolution in multilayer photoconductive imaging detectors was analyzed by developing an analytical model for calculating the modulation transfer function (MTF) of the imaging detectors. The MTF model was developed by considering the charge carrier trapping/recombination in the blocking layer and the distributed trapping in the bulk photoconductor layer of multilayer photoconductive imaging detectors. The relative importance of the bulk and blocking layer trapping, as well as the K-fluorescence reabsorption on the resolution, was also analyzed. The model was applied to amorphous selenium- (a-Se) based multilayer detectors for both chest radiographic and mammographic applications. The theoretical model agreed well with experimental results. The charge carrier trapping/recombination in the blocking layer and K-fluorescence reabsorption mainly controlled the resolution of the a-Se mammographic detectors. In addition to these two effects, the bulk trapping also had a significant effect on the resolution of chest radiographic detectors. The blocking layer thickness had more of a profound effect on the resolution than the amount of trap centers in the blocking layer. The resolution deteriorated drastically with increased blocking layer thickness.

The Two-Dimensional Short-Circuit Detection Protection For SiC MOSFETs in Urban Rail Transit Application

This article is an extension of our research, which was presented at the PCIM Europe conference in Nuremberg 2019. After proving the potential of the two-dimensional (2-D) short circuit detection method for SiC mosfet s in the preceding work Hofstetter et al. , this article presents the use of the protection system in an urban rail transit application. This introduces many challenges. It is shown how to calibrate the detection system when facing the challenge of different Miller plateaus due to temperature and dc-link voltage ranges and charge carrier trapping effects. Additionally, the effects due to parallelization are examined. False detections due to drain current oscillation are prevented with a simple additional circuit, which is used to suppress the oscillation from the detection circuit. This method leads to a minor increase in current until detection in a short circuit type 2 event. All the techniques and circuitry are experimentally verified and validate the 2-D short-circuit protection as a perfect candidate for the use in urban rail transit application.

Impact of charge carrier trapping on amorphous selenium direct conversion avalanche X-ray detectors

A cascaded linear system model is developed to determine the detective quantum efficiency (DQE) considering trapping of charge carriers in the absorption layer of an amorphous selenium multilayer direct conversion avalanche detector. This model considers the effects of charge carrier trapping and reabsorption of K-fluorescent X-rays on the frequency-dependent DQE(f). A 2-D simulation is performed to calculate the actual weighting potential in the absorption layer, which is used to calculate the amount of collected charge. It is observed that the DQE(f = 0) reduces from 0.38 to 0.19 due to charge carrier trapping in the absorption layer having a thickness of 1000 μm when the electronic noise is 1500 electrons per pixel. The avalanche gain enhances the signal strength and improves the frequency dependent DQE(f) by overcoming the effect of carrier trapping and as well as the effect of the electronic noise. The simulations suggest that avalanche gain of 35 and 20 are required to overcome the effect of the electronic noise of 1500 and 700 electrons per pixel, respectively.

Hybrid white organic light-emitting devices consisting of a non-doped thermally activated delayed fluorescent emitter and an ultrathin phosphorescent emitter

Hybrid white organic light-emitting devices (OLEDs) are fabricated by employing non-doped emitting layers (EMLs), which are consisted of a blue thermally activated delayed fluorescent (TADF) emitter 9,9-dimethyl-9,10-dihydroacridine-diphenylsulfone (DMAC-DPS) and an ultrathin yellow iridium complex bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium (acetylacetonate) [(tbt)2Ir(acac)]. With thickness optimization of DMAC-DPS, a white OLED achieves maximum current efficiency, power efficiency and external quantum efficiency of 34.9cd/A, 29.2lm/W and 11.4%, respectively, as well as warm white emission with relatively stable electroluminescence spectra. The results suggest that, bipolar charge carrier transport property and concentration independent property of DMAC-DPS, charge carrier trapping effect of the ultrathin (tbt)2Ir(acac), and balanced self-emission process and energy transfer process between DMAC-DPS and (tbt)2Ir(acac), contribute to high device performance.

Time response of Cd0.9Zn0.1Te crystals under transient and pulsed irradiation

A CdZnTe detector based on high-quality Cd0.9Zn0.1Te crystals was developed and tested as a monitor in high-intensity radiation fields. The current–voltage measurements were performed using thermally evaporated Au contacts deposited on the crystals, which revealed resistivity of 1010 Ω·cm. Typical leakage current for the planar devices was ∼3 nA for a field strength of 1000 V·cm–1. The test results show that the CdZnTe detector has a fast time response, with a rise time of approximately 2 ns, when exposed to transient and pulsed irradiation of X-rays or electron beams. The decay of current curves is observed and discussed according to charge carrier trapping effects and space-charge accumulation mechanisms. It is suggested that the current decreases quickly with strengthening of the electric field, possibly because of charge de-trapping.

Modelling of detective quantum efficiency of direct conversion x-ray imaging detectors incorporating charge carrier trapping and K -fluorescence

A cascaded linear system model is developed for calculating the frequency-dependent detective quantum efficiency, DQE(f), of a direct conversion x-ray imaging detector by incorporating the effects of charge carrier trapping and reabsorption of K-fluorescent x-rays. The present model considers a combination of series and parallel processes and interactions between them. The modulation transfer function for K-fluorescent x-ray reabsorption is modelled by determining the line spread function and subsequent one-dimensional Fourier transform. The DQE model is applied to amorphous selenium (a-Se) and polycrystalline mercuric iodide (poly-HgI2) detectors. The charge carrier trapping has a significant effect on DQE in both a-Se and poly-HgI2 detectors. The charge carrier transport properties have higher influences on DQE performance in a-Se detectors than that in HgI2 detectors, because of relatively low conversion gain in a-Se detectors. High conversion gain can minimise the adverse effect of incomplete charge collection. A simplified model for the calculation of zero spatial frequency detective quantum efficiency, DQE(0), under parallel cascaded system is also proposed in this study. There exists an optimum photoconductor thickness, which maximises the DQE(0). The proposed model is compared with the published measured data and shows good agreement.

Control of magnetoconductance through modifying the amount of dissociated excited states in tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes

Magnetoconductance (MC) is generally believed to be controlled by the ratio of singlet to triplet excited states. In this study, it is found that the MC magnitude of tris-(8-hydroxyquinoline) aluminum-based organic light-emitting diodes decreases substantially upon the introduction of narrow band gap fluorescent dopants. Since singlet to triplet ratio of excited states keeps unchanged in doped devices, this large reduction in MC means that other underlying mechanism affects the MC. The charge carrier trapping effect is proposed here to vary the magnitude of MC. By using this trapping effect, the controlling of the total amount of dissociated electron-hole pairs and consequently the magnitude of MC are realized by changing the dopant’s concentration or band gaps.

Simulation of the Carrier Trapping Effect in a Schottky CdTe Detector by Considering a Nonuniform Electric Field

In order to simulate the low-energy tail in the response function of a Schottky CdTe detector, which is a high-resolution room temperature X- or γ-ray spectrometer, we introduced the effect of a nonuniform electric field within the detector to the response function by a Monte Carlo calculation using the EGS5 code. The simulated response functions were compared with measurements for γ-rays from 109Cd, 241Am, 133Ba, and 57Co sources. The present method successfully reproduced the low-energy tail due to the charge carrier trapping effect, while the conventional model assuming a uniform electric field underestimated the low-energy tail significantly, particularly for higher energy γ-rays.

Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin films

Abstract The effect of post-deposition isothermal annealing (30 °C ⩽ T A ⩽ 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin films deposited at the substrate temperatures T S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) films ( T S , T A Z ″( ω )– ω curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z ″( ω )– Z ′( ω ) diagrams of illuminated polycrystalline Se samples (50 °C ⩽ T S , T A ⩽ 70 °C) exhibited almost full semicircles, whereas their Z ″( ω )– ω curves revealed prominent loss peaks at well-defined frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersections of these semicircles with the Z ′-axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental findings were explained in terms of a significant increase in electrical conductivity of selenium films due to thermally-induced crystallization at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination.

Effects of charge carrier trapping on polycrystalline PbO x-ray imaging detectors

The effects of charge carrier trapping on the x-ray sensitivity, resolution, and detective quantum efficiency (DQE) in polycrystalline lead oxide (PbO) x-ray imaging detectors are theoretically analyzed. The theoretical models for calculating carrier trapping-limited x-ray sensitivity and modulation transfer function (MTF), and DQE incorporating polyenergetic x-ray spectrum are described. A cascaded linear system model is developed for calculating the spatial frequency f dependent DQE of PbO detectors by incorporating the effects of bulk charge carrier trapping on the MTF and the x-ray interaction depth dependent charge collection efficiency and noise. The theoretical calculations are compared with the published experimental data and show a very good agreement. From the fittings of the sensitivity and MTF curves, the electron and hole ranges in polycrystalline PbO are found to be 3.5×10−7 and ∼10−8 cm2/V, respectively. The f dependent noise power spectrum NPS(f) and DQE(f) performances are analyzed as a function of applied electric field and carrier lifetimes. The calculation shows that the carrier trapping is responsible for a 21% reduction in the MTF at the applied field of 1.0 V/μm at the Nyquist frequency (fN) and the aliased NPS at fN is reduced to 63% of that at zero spatial frequency. The sensitivity, MTF, NPS(f), and DQE(f) strongly depend on the applied electric field and carrier ranges. It is found that the DQE(f) for fluoroscopic applications can be improved dramatically by simply enhancing the carrier lifetimes through improving material properties of the PbO sample.

High-efficiency White Phosphorescent Organic Light-Emitting Diodes

High-efficiency white organic light-emitting diodes (WOLEDs) based on phosphorescent dye bis[2-(4-tert- butylphenyl)benzothiazolato-N,C 2′]iridium (acetylacetonate) [(t-bt) 2Ir(acac)] phosphor doped 4,4′-bis(carbazol-9-yl) biphenyl (CBP) as a yellow emitting layer were fabricated. The structure of the devices is indium-tin oxide (ITO)/ N, N′-bis-(1-naphthyl)- N, N′-biphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/CBP:(t-bt) 2Ir(acac)/NPB/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/8- hydroxyquinoline aluminum (Alq 3)/Mg:Ag, and the second NPB layer from the anode was used as a blue fluorescent emitter and the first NPB layer was used as a hole transporting layer, the BCP and Alq 3 were used as hole blocking and electron transporting layers, respectively. The results showed that the turn-on voltage of the device was 3 V with a maximum luminance of 15460 cd·m −2 at 16.5 V and a maximum luminance efficiency of 7.5 lm·W −1 at 4 V, respectively. The Commission Internationale de l′Eclairage (CIE) coordinates located at (0.33, 0.32) remained stable at the driving voltage higher than 8 V, which is almost consistent with the optimum white CIE of (0.33, 0.33). The purity of white light could be adjusted by doping concentration. The effects of charge carrier trapping and energy transfer on the device performance were also discussed.

Low-voltage operation of a pentacene field-effect transistor with a polymer electrolyte gate dielectric

Large operating voltages are often required to switch organic field-effect transistors (OFETs) on and off because commonly used gate dielectric layers provide low capacitive coupling between the gate electrode and the semiconductor. We present here a pentacene OFET gated by a solution-deposited polymer electrolyte film in which the current was modulated over four orders of magnitude using gate voltages less than 2V. A subthreshold slope of 180mV per decade of current was observed during transistor turn on at a source-drain bias of −1V; the estimated dielectric layer specific capacitance was 5μF∕cm2. Sweep rate-dependent hysteresis may be attributed to a combination of ion migration and charge carrier trapping effects. Strategies to improve switching speeds for polymer electrolyte-gated OFETs are also discussed.

Modulation transfer function of photoconductive x-ray image detectors: effects of charge carrier trapping

An analytical expression for calculating the modulation transfer function (MTF) due to distributed carrier trapping in the bulk of the photoconductor of a direct conversion pixellated x-ray image detector is derived using the trapped charge distribution across the photoconductor. The analytical expressions of trapped charge distributions are also derived by solving the continuity equation for both types of carriers (electrons and holes). The MTF of photoconductive x-ray detectors is analysed in terms of normalized parameters, namely (a) the normalized x-ray absorption depth (absorption depth/photoconductor thickness) and (b) normalized carrier schubwegs (schubweg/thickness). Trapping of the carriers that move towards the pixel electrodes degrades the MTF performance, whereas trapping of the carriers that move away from the pixels improves the sharpness of the x-ray image. The MTF model is applied to polycrystalline CdZnTe detectors and is fitted to recently published experimental results. The theoretical model shows very good agreement with reported experimental data.

Dependence of the detective quantum efficiency of photoconductive x-ray image detectors on charge transport parameters and exposure: Application to a-Se

The effects of charge carrier trapping (i.e., incomplete charge collection) on the detective quantum efficiency (DQE) of a photoconductive detector are studied by using a cascaded linear system model. The model includes signal and noise propagations in the following stages: (1) x-ray attenuation, (2) conversion gain, (3) charge collection, and (4) the addition of electronic noise. We examine and discuss the DQE of an a-Se fluoroscopic detector as a function of charge transport parameters and x-ray exposure. The theoretical model shows a very good agreement with experimental DQE versus exposure characteristics.

DQE of photoconductive x-ray image detectors: application to a-Se

A cascaded linear system model that includes incomplete charge collection and interaction-depth dependent conversion gain and charge collection stages is considered for the calculation of the zero spatial frequency detective quantum efficiency, DQE(0), of a direct conversion x-ray image detector. The model includes signal and noise propagations in the following stages: (1) x-ray attenuation, (2) conversion gain, (3) charge collection, and (4) addition of electronic noise. The primary x-ray photon interaction and also the secondary K-fluorescent photon interaction are included in determining the interaction-depth dependent conversion gain across the photoconductor. We examine DQE(0) of a-Se detectors for fluoroscopic applications as a function of photoconductor thickness with varying amounts of electronic noise and x-ray exposure under (a) constant field, and (b) constant voltage operating conditions. We show that there is an optimum photoconductor thickness, which maximizes DQE(0) under a constant voltage operation. The optimum thickness depends on the added electronic noise, x-ray exposure, charge collection efficiency and bias voltage. For the quantities mentioned above that are appropriate for a-Se detectors and fluoroscopic applications, the optimum a-Se thickness is ∼700 μm and the corresponding DQE is ∼0.4. It is shown that the DQE depends strongly on the charge transport properties of the photoconductors. With the radiation-receiving electrode negatively biased, the DQE is more dependent on electron lifetime (τe) than hole lifetime (τh). Full electron trapping, (τe = 0) reduces the DQE by about 73.3% at the detector thickness of 1000 μm whereas full hole trapping (τh = 0) reduces the DQE by about 43.7%. The DQE for the negative bias is lower than for the positive bias, and the difference in DQE, as expected, increases with the photoconductor thickness because of the asymmetric transport properties of holes and electrons in a-Se. The present results show that the DQE generally does not continue to improve with greater photoconductor thickness because of charge carrier trapping effects. The DQE of a polyenergetic x-ray beam is only slightly lower than a monoenergetic x-ray beam with the same average photon energy. The theoretical model shows a very good agreement with the experimental DQE versus exposure characteristics published in the literature.

Charge carrier trapping effect by luminescent dopant molecules in single-layer organic light emitting diodes

We investigated electroluminescent (EL) characteristics of single-layer organic light emitting diodes (SOLEDs). Our SOLED devices are composed of an inert polymer as a binder, in which hole transport molecules, emissive electron transport molecules (ETMs), and highly fluorescent dopants as luminescent centers are dispersed. We examined two typical dopants: rubrene and coumarin 6. These exhibited different charge carrier recombination and emission mechanisms. The dopant concentration dependence of the current density–voltage–luminance relationships clearly showed the importance of carrier trapping by dopant molecules for obtaining high luminance. When the dopant was rubrene, we observed that charge carriers were well trapped by the dopant molecule. This means that direct recombination of holes and electrons occurred on the dopant molecules and trapping significantly enhanced the external EL quantum efficiency ΦEL. For coumarin 6, on the other hand, we observed that charge carriers primarily recombined at the emissive ETMs and that the energy transfer from the host to the guest coumarin 6 molecule dominated the EL process. A comparison of these distinct processes revealed that carrier trapping by dopant molecules was necessary to enhance ΦEL in SOLED devices. In our best SOLED device with rubrene as a dopant, we measured luminance of 2800 cd/m2 at J=100 mA/cm2, which corresponds to ΦEL=0.85%.

Resolution enhancement of gamma-spectroscopy data from neutron-damaged coaxial n-type HPGe-detectors

A new method has been developed to correct gamma-spectroscopy data for effects of charge carrier trapping in neutron-damaged coaxial n-type HPGe-detectors. In addition to the pulse height, the risetime of each signal is measured with standard nuclear spectroscopy electronics and both data are recorded on tape. From this data the amount of the pulse height deficit, which is due to the neutron damage, can be calculated for each event. The correction of the pulse height is done by software during the normal analysis of the listmode data. For the determination of the correction coefficients a calibration of the detector with a standard gamma-calibration source is performed. With this new technique the energy resolution of neutron-damaged coaxial n-type HPGe-detectors can be improved significantly.

Effect of charge carrier trapping on germanium coaxial detector line shapes

A theoretical method to predict and quantify the effects of charge carrier trapping on germanium coaxial detector line shapes has been developed. This model was used to calculate line shapes which closely matched the measured line shapes of both conventional and reverse electrode high-purity germanium coaxial detectors that had suffered fast neutron damage. In accordance with experimentally established behavior, the theory predicts that reverse electrode detectors are vastly more resistent to the effects of neutron damage than conventional electrode detectors. The theory also predicts that the energy resolution and line shape of neutron-damaged conventional electrode detectors are far more dependent on detector diameter than are detectors of the reverse electrode configuration. The generally observed variation of the energy resolution and line shape as a function of bias voltage is shown to arise from an ¢E −1 dependence of the trap cross section on electric field.