Multiple Trap Levels Research Articles

Critical role of the deep electron trap in explaining the inconsistency of sensitivity-corrected natural and regenerative growth curves of quartz OSL at high irradiation doses

Natural quartz minerals that contain impurities, complex defect structures, and multiple trap levels within their crystals have been widely used for optically stimulated luminescence (OSL) dosimetry. Accurate application of the single-aliquot regenerative-dose (SAR) protocol requires an equivalence between the sensitivity-corrected natural and regenerative growth curves. In this work, the sensitivity-corrected natural and regenerative growth curves of quartz OSL are modelled using a comprehensive kinetic model based on the band theory of solids. We demonstrate that the trapping behaviour of the deep electron trap exerts a critical influence on the inconsistency of the sensitivity-corrected natural and regenerative growth curves at high irradiation doses. A delay in saturation of the deep electron trap at high irradiation doses leads to a substantial increase in electron competition in the recombination centres during the irradiation of the test dose and a decrease in electron competition in the centres during the subsequent stimulation of the test dose OSL. As a consequence, the test dose OSL responses as functions of irradiation doses are inconsistent (incompatible) between natural and regenerative aliquots, and the sensitivity-corrected regenerative growth curve tends to be higher than the natural one in the high-dose region. There may be substantial deep electron traps within the crystal of natural quartz that cause their delayed saturation at high irradiation doses.

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories

Low‐Frequency‐Noise Spectroscopy of TaOx‐based Resistive Switching Memory

AbstractResearch and development into resistive switching memories, such as resistive random access memory (ReRAM) and memristors, are being actively promoted toward the realization of new computing techniques. To improve the reliability of these devices, it is important to clarify their resistance characteristics. Low‐frequency‐noise spectroscopy (LFNS) is a powerful method for investigating the nature of traps in conduction paths, even at the nanoscale. In this article, the results of LFNS measurements on a TaOx‐based ReRAM device are presented. The temperature dependence of the noise spectra at given frequencies reveals the existence of multiple trap levels, which are observed over a wide range of resistance values. These experimental results show that the ReRAM device is particularly advantageous when used in analog resistive switching applications.

Deconvolution of thermoluminescence curves and its implications: The case of CaAl2O4:Sm3+

Deconvolution of thermoluminescence(TL) curve has evolved as a subject of its own merit and has attained a mature state. Unfortunately, even with the availability of multiple methods id analysis and numerous deconvolution software, the pitfalls are many. This paper presents a comprehensive deconvolution of TL curves of a typical phosphor, namely CaAl2O4:Sm3+ and once for all settle the prevailing myths on evaluation of intrinsic trap parameters (Activation energy, E and frequency factor, s) for crystalline phosphors. The TL curves are that of four different kinds of excitation (UV, x-ray gamma-ray and electron beam) as well as ten doses of gamma-ray excitation(0.5–50 Gy). Our results show multiple discrete trap levels within the bandgap of the CaAl2O4 lattice that are a potential source for developing novel afterglow phosphors and thermoluminescence dosimeters. Finally, we provide some simple guidelines to overcome the misconceptions that have been perpetually being propagated in the literature of TL and its applications.

Physically Consistent Method for Calculating Trap-Assisted-Tunneling Current Applied to Line Tunneling Field-Effect Transistor

The default trap-assisted-tunneling (TAT) modeling framework, also known as the dynamic nonlocal TAT model, does not allow to specify multiple trap levels; the lone trap level allowed by the model is not considered in the Poisson-charge self-consistent loop. A method is presented here which allows specifying a trap distribution and the electrostatic degradation (ESD) caused by the trap distribution. The method relies on an individual simulation for each trap level in the distribution, with the total TAT current obtained by summing the TAT currents from the individual trap-level simulations. ${I}_{{\text {ds}}}$ – ${V}_{{\text {gs}}}$ data for an experimental L-shaped tunneling field-effect-transistor (LTFET) were fit using the proposed method, for validation of the proposed method. ESD was found to be an important factor in determining the TAT generation rate ( ${G}_{{\text {TAT}}}$ ). The calculated TAT current resulting from a trap distribution allowed to realistically estimate the lifetime parameter, which, if calculated based on a single trap, would be significantly underestimated. From the device point-of-view, this work demonstrates the severity of the TAT problem; TAT was found to dominate for the entire ${V}_{{\text {gs}}}$ range, including the ambipolar current, and for all ${V}_{{\text {ds}}}$ bias values.

Determination of thermoluminescence parameters in nanocrystalline CaAl2O4

This paper is a critical investigation of trap levels present in nanocrystalline CaAl2O4. The technique of thermoluminescence (TL) is used to determine the trap levels. Unlike most workers, we use Computerized Glow Curve Deconvolution (CGCD) technique to decode the TL curves that enables us to determine all the three key trapping parameters, namely, the activation energy (E), the frequency factor (s) and the order of kinetics (b). Using these as inputs we evaluate the electron lifetime (τ) of charge trapped in different traps. This provides the vital information of the possible use of CaAl2O4 based phosphors for practical application in dosimetry, scintillation and persistent luminescence.The phosphor contains multiple discrete trap levels in the band gap (Eg = 7.4 eV) with trap-depths 0.69 ± 0.02, 0.83 ± 0.01, 0.99 ± 0.02, 1.06 ± 0.02, 1.18 ± 0.03, 1.28 ± 0.02 and 1.41 ± 0.01 eV. The magnitudes of the frequency factors (s) for all the cases are ≈109 that translates the Urbach's coefficient as 23.47 ± 0.14. We believe this work is first of its kind in providing complete parameters of trap levels in CaAl2O4 phosphor. The life times at Room temperature (τ300) of the change in these varies are spread are a wide range of 18 min ≤ τ300 ≤ 1007 years which show that CaAl2O4 based phosphors can be designed for specific application with appropriate trap level modification. This explains the excellent persistence luminescence in CaAl2O4: Eu2+, Nd3+ that will be discussed in detail.

Modeling of H/D isotope-exchange in crystalline beryllium

Abstract A reaction-diffusion model with surface occupation dependent desorption [D. Matveev et al., Nucl. Instr. Meth. B 430 (2018) 23–30] has been updated to handle multiple hydrogen species to simulate hydrogen/deuterium isotope-exchange experiments performed on polycrystalline beryllium samples under ultra-high vacuum laboratory conditions. In the experiments subsequent exposures of a sample to hydrogen and deuterium ion beams in direct and reverse implantation order were followed by thermal desorption spectroscopy measurements under a constant heating rate of 0.7 K/s. The recorded signals of masses 2 to 4 (H2, HD and D2) indicate that the second implanted isotope dominates clearly the low temperature release stage ( ≈ 450 K), while both isotopes show a comparable contribution to the high temperature desorption stage ( ≈ 700 K) with only minor effect of the implantation order attributed to a slightly deeper penetration of deuterium compared to hydrogen. Simulations of the implantation and subsequent thermal desorption of hydrogen isotopes are performed to assess the atomic processes behind the isotope-exchange. Simulations were performed under the assumption that the low temperature release stage is attributed to hydrogen/deuterium atoms retained on effective open surfaces (e.g. interconnected porosity) represented in the simulations by a surface with an effective surface area exceeding the nominal exposed surface area by a factor up to 100. Kinetic de-trapping from vacancies with multiple trapping levels and enhanced desorption at surface coverages close to saturation are addressed in the model as possible mechanisms promoting the isotope-exchange. Simulation results suggest the applicability of the model to describe isotope-exchange processes in crystalline beryllium and give a qualitative explanation of the observed experimental facts.

Modeling of charge transport in insulating dielectric for gas-insulated transmission line based on multiple trap levels

Space charge behavior in polymeric materials is of interest to the electrical engineering community and has significant effects on the behavior of the polymers that are used in gas-insulated transmission lines (GILs). For example, the space charge in certain insulating materials can cause severe distortion of the electric field that may even lead to breakdown of the materials. It is therefore important to understand the trapped charge distributions in these materials because they can be used as novel indicators for effective evaluation of the aging status and the electric withstand strength of these insulating materials. This paper addresses recent developments in the modeling of charge transport in polymers for use in GIL applications. A numerical model of bipolar charge transport in polymeric insulators is presented in this paper. This model was studied based on use of multiple trap levels and consideration of symmetrical parameters for the generation and transport of electrons and holes.

Study of Exciton Hopping Transport in PbS Colloidal Quantum Dot Thin Films Using Frequency- and Temperature-Scanned Photocarrier Radiometry

Solution-processed colloidal quantum dots (CQDs) are promising materials for realizing low-cost, large-area, and flexible photovoltaic devices. The study of charge carrier transport in quantum dot solids is essential for understanding energy conversion mechanisms. Recently, solution-processed two-layer oleic-acid-capped PbS CQD solar cells with one layer treated with tetrabutylammonium iodide (TBAI) serving as the main light-absorbing layer and the other treated with 1,2-ethanedithiol (EDT) acting as an electron-blocking/hole-extraction layer were reported. These solar cells demonstrated a significant improvement in power conversion efficiency of 8.55% and long-term air stability. Coupled with photocarrier radiometry measurements, this work used a new trap-state mediated exciton hopping transport model, specifically for CQD thin films, to unveil and quantify exciton transport mechanisms through the extraction of hopping transport parameters including exciton lifetimes, hopping diffusivity, exciton detrapping time, and trap-state density. It is shown that PbS-TBAI has higher trap-state density than PbS-EDT that results in higher PbS-EDT exciton lifetimes. Hopping diffusivities of both CQD thin film types show similar temperature dependence, particularly higher temperatures yield higher hopping diffusivity. The higher diffusivity of PbS-TBAI compared with PbS-EDT indicates that PbS-TBAI is a much better photovoltaic material than PbS-EDT. Furthermore, PCR temperature spectra and deep-level photothermal spectroscopy provided additional insights to CQD surface trap states: PbS-TBAI thin films exhibit a single dominant trap level, while PbS-EDT films with lower trap-state densities show multiple trap levels.

Novel elastico-mechanoluminescence materials CaZnOS:Mn2+ and CaZr(PO4)2:Eu2+

This paper highlights our work on the recent progress in novel elastico-mechanoluminescence (EML) materials CaZnOS : Mn 2+ and CaZr ( PO 4)2: Eu 2+, which can simultaneously "feel" (sense) and "see" (image) the applied mechanical stress as an intense and proportional luminescence. CaZnOS : Mn 2+ can sense various types of mechanical stress, including ultrasonic vibration, impact, friction and compression because of the large piezoelectric coefficient. CaZr ( PO 4)2: Eu 2+ with multiple trap levels shows a broad measurement range for dynamic load. Both of CaZnOS : Mn 2+ and CaZr ( PO 4)2: Eu 2+ belong to the defect-controlled type piezoelectric phosphors, and their EML mechanisms could be explained using the piezoelectrically induced carrier de-trapping model.

Persistent luminescence, TL and OSL characterization of beta irradiated SrAl2O4:Eu2+, Dy3+ combustion synthesized phosphor

The persistent luminescence (PLUM), thermoluminescence (TL) and optically stimulated luminescence (OSL) properties of strontium aluminates co-doped with Eu+2 and Dy+3 exposed to beta radiation is reported. The phosphor was synthesized by the combustion synthesis method employing a highly exothermic redox reaction between the metal nitrates [Al(NO3)3, Sr(NO3)2, Eu(NO3)3 and Dy(NO3)3] and organic fuel carbohydrazide (CH6N4O). The long decay PLUM emission, TL and OSL were measured as a function of beta radiation dose. A wide emission band centered at 510nm (green) related to Eu2+ ions and lattice defects were observed for the synthesized samples. The presence of a variety of defects and aggregates were responsible for the observed broad 100°C peaked TL glow curve of the irradiated sample which is composed of several overlapped TL peaks. The existence of multiple trapping levels, with different trapping/detrapping probabilities, is behind the particular features for the PLUM, TL and OSL emissions. We conclude that in the SrAl2O4:Eu2+, Dy3+ phosphors, the low temperature TL peaked around 30–75°C is responsible for the PLUM emission and those around 100°C were related to very stable trapping states which provide suitable radiation storage properties to be used as a PLUM/TL/OSL radiation phosphor.

Elastico-mechanoluminescence in CaZr(PO_4)_2:Eu^2+ with multiple trap levels

We report on a novel elastico-mechanoluminescence (EML) phosphor of CaZr(PO4)2:Eu2+ for simultaneous luminescent sensing and imaging to mechanical load by the light-emitting of Eu2+ ions. The EML properties of CaZr(PO4)2:Eu2+ show an intense luminance (above 15 mcd m(-2)), a low load threshold (below 5 N), a broad measurement range for the dynamic load (up to 2000 N), and an accurate linear relationship of EML intensity against the applied load. The excellent EML characteristics are considered to originate from the piezoelectric crystal structure and the multiple trap levels with appropriate depths. An EML mechanism based on the electrons as the main charge carriers is proposed.

RETRACTED: The initial rise method extended to multiple trapping levels in thermoluminescent materials

The well known Initial Rise Method (IR) is commonly used to determine the activation energy when only one glow peak is presented and analysed in the phosphor materials. However, when the glow peak is more complex, a wide peak and some holders appear in the structure. The application of the Initial Rise Method is not valid because multiple trapping levels are considered and then the thermoluminescent analysis becomes difficult to perform. This paper shows the case of a complex glow curve structure as an example and shows that the calculation is also possible using the IR method. The aim of the paper is to extend the well known Initial Rise Method (IR) to the case of multiple trapping levels. The IR method is applied to minerals extracted from Nopal cactus and Oregano spices because the thermoluminescent glow curve's shape suggests a trap distribution instead of a single trapping level.

Electronic noise due to multiple trap levels in homogeneous solids and in space-charge layers

A near-exact expression is derived for carrier-density fluctuations associated with multiple trap levels, interacting with one band only (say, the conduction band), using the many-variate Master Equation. The canonical constraint is handled with the Darwin–Fowler method. For a homogeneous solid with a discrete number of traps the spectrum consists of a sum of Lorentzians, as expected. For the case of a continuously distributed energy range, the results do not support a Bernamont–Surdin–McWhorter 1/f-like envelope spectrum; on the contrary, no more than two decades of 1/f noise can be expected. The situation is different, however, when band-bending occurs. Employing a Green’s function approach, developed by us previously, we give a formal complete result, which can be evaluated when Ec(r) is known.

Effect of thermal annealing on hole trap levels in Mg-doped GaN grown by metalorganic vapor phase epitaxy

Deep trap levels in a Mg-doped GaN grown by metalorganic vapor phase epitaxy are studied with deep level transient spectroscopy (DLTS). The Mg concentration of the sample was 4.8×1019 cm−3, but the hole concentration was as low as 1.3×1017 cm−3 at room temperature. The DLTS spectrum has a dominant peak D1 with an activation energy of 0.41±0.05 eV, accompanied by two additional peaks with activation energies of 0.49±0.09 eV (D2) and 0.59±0.05 eV (D3). It was found that the dominant peak D1 consists of five peaks, each of which has different activation energy and capture cross section. In order to investigate these deep levels further, we performed heat treatment on the same samples to observe the variations of activation energy, capture cross section, and amplitude of DLTS signals. It was found that the longer the heat treatment duration is, the lower the amplitude of DLTS peaks become. This suggests that the decrease of the DLTS signal originates from hydrogen atom outgoing from the film during the annealing process. The possible originality of multiple trap levels was discussed in terms of the Mg–N–H complex.

Two-dimensional modeling of Cd(Zn)Te strip detectors

Although there has been significant recent progress in the development of CdTe and CdZnTe radiation detectors, these materials are still plagued by charge trapping and recombination problems. Recent work has shown that fabrication of arrays with small anodes can overcome these problems to some extent. The purpose of this simulation study was to investigate this small anode effect in relatively large strip detectors of various geometries. Unlike previous simulations, the current study includes the effects of both trapping and detrapping, significant inter-pixel spacing, and non-linear electric field profiles. In addition, the methods have the potential to account efficiently for spatial variations in trapping behavior and for multiple trapping levels. The results of the simulation predict that 8 mm thick strip detectors with 1 mm anode thickness and 0.3 mm anode spacing will provide adequate performance for our medical imaging application.

Effect of Hydrogenation on the Electrical and Optical Properties of GaSb

AbstractThe effect of hydrogen plasma treatment on the optical and electrical properties of Gallium Antimonide bulk single crystals is presented. Plasma exposure gives rise to a layer of defects on the surface. These defects introduce multiple trap levels in the band gap from which a slow emission of carriers is observed during the capacitance - voltage measurements. On removal of the defect layer by controlled etching, the effects of hydrogen passivation are seen. The results of optical measurements indicate that passivation of shallow acceptors is more efficient than that of the donors and in general the passivation efficiency depends on the doping level. Passivation of deep levels and extended defects like grain boundaries and dislocations has also been observed. The thermal stability of the passivated deep level and extended defects is higher than that of the shallow level.

Models for optical bleaching of thermoluminescence in sediments

In this paper, we examine several models for the description of TL, focusing most attention on the properties of optical bleaching, in an effort to determine if realistic models can be devised which accurately describe the observed behavior. The models considered include multiple trapping and recombination levels, radiative and non-radiative processes, trap interaction effects, retrapping of released charge, and include transitions into and out of the recombination centers as well as the traps. All such phenomena are likely to occur in real materials and should be included in any model purports to describe such materials. This paper concludes that models which account for the recombination of feed electrons with trapped holes during the bleaching period appear to be the most successful at describing the observed phenomena.

Modeling of short-pulse threshold voltage shifts due to DX centers in Al/sub x/Ga/sub 1-x/As/GaAs and Al/sub x/Ga/sub 1-x/As/In/sub y/Ga/sub 1-y/As MODFET's

The DX-center-related short-pulse threshold voltage shifts (SPTVS) in Al/sub x/Ga/sub 1-x/As-based MODFETs is modeled using CBAND, a simulator that solves Poisson equations self-consistently with Schrodinger equations and donor statistics. Using values given in the literature for the DX energy level in Al/sub x/Ga/sub 1-x/As this technique gives good agreement between measured and simulated SPTVS for Al/sub 0.3/Ga/sub 0.7/As/GaAs and Al/sub 0.3/Ga/sub 0.7/As/In/sub 0.2/Ga/sub 0.8/As MODFETs. Both simulation and experiment show that the use of Al/sub 0.2/Ga/sub 0.8/As in the donor layer reduces the SPTVS relative to the structures using Al/sub 0.3/Ga/sub 0.7/As. However, the measured shifts at this composition are considerably lower than the simulated values, indicating a DX energy level that may be higher than the value extrapolated from the literature, possibly due to the existence of multiple trap levels. Despite this discrepancy, these results support the use of strained-channel layers and lower Al/sub x/Ga/sub 1-x/As compositions in MODFETs for digital and other large-signal applications requiring good threshold stability. >

Experimental observations of trapping levels in BSO

The photorefractive effect in BSO has been used to examine deep trapping levels. Observations of the decay dynamics of laser-induced phase gratings are consistent with the assumption of multiple shallow and deep trapping levels. A close correlation between diffraction intensities during write and read modes has been observed. The possibility of multiple phase-shifted gratings is suggested by these results. Electron and hole trapping effects appear to be simultaneously present with the hole traps exhibiting a long decay time constant.