Detrapping Mechanism Research Articles

(Digital Presentation) Temporal Evolution of the Threshold Voltage in Organic Thin Film Memory Transistors

We present a procedure to monitor the temporal evolution of the trapped charge in floating gates of organic thin film transistors (OTFTs) based memories in dynamic regimes. The potential applications of organic semiconductors in flexible and wearable electronics and their known features such as low-temperature and solution fabrication make the OTFTs be considered as candidates also in the field of non-volatile memories. Past efforts have been devoted on the preparation and optimization of floating gates [1]. In the present work, we focus on the modelling of this kind of devices, in particular, on a model for the threshold voltage useful in dynamic experiments, such as hysteresis and transient regimes. The information extracted from the modelling and simulation of these devices can be related to technological parameters with the objective of finding a better memory design.Current-voltage curves impacted by hysteresis are usually reproduced assuming the threshold voltage to be constant or piecewise constant. Nevertheless, despite positive outcomes [2], this is not a precise approach since it does not consider the actual temporal evolution of the threshold voltage. Thus, a time dependent threshold voltage is mandatory in order to interpret hysteresis phenomena and transient experiments, which in turn can be associated to trapping and de-trapping mechanisms over time.In this work, a model that describes the evolution of the threshold voltage with the time, which is linked to the variation of the trapped charge in the channel, is proposed.The evolution of the trapped charge is described with a first-order linear differential equation with a term controlled by a time constant t, and another term, similar to the generation term in a typical continuity equation, which is proportional to the drain current flowing through the transistor channel. This model is introduced in a compact model previously developed for OTFTs that include contact effects [3–6]. The resulting model is combined with an evolutionary parameter extraction procedure, which is applied to experimental current–voltage curves taken from the literature that show both contact and hysteresis effects [7]. Figures (a) and (b) show the best fitting of our calculations (solid lines) and experimental output and transfer characteristics (symbols), respectively, of a pentacene-based organic thin film memory transistor using poly (methyl methacrylate) (PMMA) as the insulator [7]. Figures (c) and (d) show the time evolution of the threshold voltage during the experiments (a) and (b), respectively.The authors acknowledge support from the project PID2022-139586NB-44 funded by MCIN/AEI/10.13039/501100011033 and by European Union NextGenerationEU/PRTR.

Reinforcing Synaptic Plasticity of Defect-Tolerant States in Alloyed 2D Artificial Transistors.

While two-dimensional (2D) materials possess the desirable future of neuromorphic computing platforms, unstable charging and de-trapping processes, which are inherited from uncontrollable states, such as the interface trap between nanocrystals and dielectric layers, can deteriorate the synaptic plasticity in field-effect transistors. Here, we report a facile and effective strategy to promote artificial synaptic devices by providing physical doping in 2D transition-metal dichalcogenide nanomaterials. Our experiments demonstrate that the introduction of niobium (Nb) into 2D WSe2 nanomaterials produces charge trap levels in the band gap and retards the decay of the trapped charges, thereby accelerating the artificial synaptic plasticity by encouraging improved short-/long-term plasticity, increased multilevel states, lower power consumption, and better symmetry and asymmetry ratios. Density functional theory calculations also proved that the addition of Nb to 2D WSe2 generates defect tolerance levels, thereby governing the charging and de-trapping mechanisms of the synaptic devices. Physically doped electronic synapses are expected to be a promising strategy for the development of bioinspired artificial electronic devices.

Modeling and validation of mechanoluminescent strain sensing mechanism at quasi-static loading rates

Mechanoluminescence (ML) sensors offer full-field strain/stress measurements and have the advantages of easy implementation, non-invasiveness, and low cost. However, knowledge of the mechanism of carrier detrapping owing to mechanical forces remains elusive as it may include one or more complicated processes. Several attempts to develop calibration models for converting the light emitted by ML sensors into mechanical stress have been reported, but few studies have tested the ML response under static or quasi-static loading rates. In this study, we built a real-time measurement system to evaluate the constitutive strain-luminescence relationship of ML sensors. The ML response was investigated at loading rates as low as 5 µm/s (i.e., strain rates as low as 16.2 μst/s). The results showed that the light of ML sensors was still excited under quasi-static loading rates. A double-exponential function was applied to describe nonlinear changes in light intensity over time. We used this function to modify the previous model and predict the ML responses under variable loading rate conditions. The results predicted by the model proposed in this paper are in good agreement with the experimental results.

Interactions between screw dislocation and twin boundary in high-entropy alloy: A molecular dynamic study

The interaction between dislocations and twin boundaries plays an important role in the plastic deformation of high-entropy alloys (HEAs), in which deformation twinning mechanisms are highly active. However, research concerning the twinning-induced strengthening in HEAs is lacking, especially atomistic studies. Therefore, molecular dynamics (MD) simulations are performed to elucidate the interaction mechanisms between the screw dislocation and the coherent twin boundary in the CoNiCrFeMn HEA and pure Ni. Dislocation transmission across the twin boundary is observed as the only mode in the CoNiCrFeMn HEA and pure Ni. However, the Shockley partial dislocations cannot simultaneously constrict on the twin boundary in the CoNiCrFeMn HEA because of the nanoscale segment detrapping mechanism related to the statistical fluctuation of chemical ordering, different from that in pure Ni. To quantify the interaction mechanism, generalized stacking fault energies and lattice friction are considered for predicting the interaction mode in the CoNiCrFeMn HEA. A new material parameter that considers the negative stacking fault energy and large lattice friction stress is proposed for the CoNiCrFeMn HEA.

The 3D magnetic topology and plasma dynamics in open stochastic magnetic field lines

The thermal quench triggered by locked modes is known to be mainly due to open stochastic magnetic field lines connected to the wall boundary. It is essential to understand the 3D structure of open stochastic field lines since it determines the overall plasma dynamics in the system. In this study, we analyze the 3D magnetic topology for two key concepts, the connection length Lc and the effective magnetic mirror ratio Meff, and present a comprehensive picture of electron and ion dynamics related to the magnetic topology. The connection length determines the 3D structure of the ambipolar potential, and a sharp potential drop across distinct Lc regions induces the E × B transport and mixing across the field line. The confinement of electrons and ions along the field line is determined by the ambipolar potential and Meff configuration. Electron and ion temperatures in magnetic hills (Meff<1) are lower than in magnetic wells (Meff>1) because particles in magnetic hills are more likely to escape toward the wall boundary along the field line. The mixing between the magnetic wells and hills by E × B and magnetic drift motions results in collisionless detrapping of electrons and ions, which reduces their temperature efficiently. Numerical simulations of two different magnetic configurations demonstrate the importance of the collisionless detrapping mechanism, which could be the main cause of plasma temperature drop during the thermal quench.

Electroforming free enhanced resistive switching in reduced graphene oxide films embedded with silver nanoparticles for nonvolatile memory applications

In this paper, the reduced graphene oxide (rGO)-based resistive random access memory (RRAM), embedded with Ag NPs, for nonvolatile memory applications has been investigated. The device exhibited enhanced I on/I off current ratio at lower switching voltage. The rGO-Ag thin film was deposited by spin-coating technique on an indium tin oxide coated glass substrate. Top contacts of aluminum (Al) were deposited by thermal evaporation technique for electrical measurements. The morphology of thin film has been characterized by scanning electron microscopy, and elemental quantification has been done using energy-dispersive x-ray spectroscopy. The structural and chemical analysis of thin film has been performed using x-ray diffraction (XRD) characterization and x-ray photoelectron spectroscopy, respectively. During applied potential, the device switches between two resistance states; low resistance state (LRS) and high resistance state (HRS). Initially when potential is applied across the device, the deposited thin film (rGO-Ag) was found in LRS which did not require initial forming voltage to activate the device. The requirement of this forming voltage may induce severe mechanical and electrical stress in the device and degrades the device performance. The rGO-Ag-based RRAM devices required (<1 V) to switch from LRS to HRS or vice versa with a large (∼103) on/off current ratio. The electrical stability of the devices has been tested for 4 × 103 s and repeatability for >60 cycles, which did not exhibit too much degradation. In rGO-Ag thin film-based RRAM devices, Ag NPs act as charge trapping sites, which results in enhanced switching characteristics. The trapping and detrapping mechanism of charges by Ag NPs is responsible for the enhanced switching characteristics in these films.

Coupled Strengthening Effects by Lattice Distortion, Local Chemical Ordering, and Nanoprecipitates in Medium-Entropy Alloys

Extraordinary mechanical properties can be achieved in high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) with nanoprecipitates. In the present study, the extra coupled strengthening effects by lattice distortion, local chemical ordering, and nanoprecipitates in the HEAs and MEAs with nanoprecipitates have been systematically investigated by large-scale molecular dynamics simulations. The moving of the dislocation can be slowed down, and the dislocation line shows a wavy configuration due to lattice distortion and local chemical ordering, resulting in strengthening. The degree of the wavy configuration increases and the sliding velocity of the dislocation decreases with increasing degrees of local chemical ordering. It is clearly indicated that the dislocation moves via nanoscale segment detrapping mechanism due to the effects of lattice distortion and local chemical ordering, resulting in roughened dislocation pathways for strengthening. The activated nanoscale segments are observed to be easier to detrap from the regions with stronger Co-Cr local chemical ordering and then propagate into the regions without such chemical ordering. These moving characteristics of the dislocation can delay the unpinning process from nanoprecipitates; thus, extra coupled strengthening effect has been revealed in the HEAs and MEAs with nanoprecipitates compared to pure Orowan’s strengthening.

Collisionless plasma transport mechanisms in stochastic open magnetic field lines in tokamaks

The collisionless plasma transport in stochastic magnetic fields connecting to wall boundary has been studied to understand the underlying mechanisms of thermal quench physics in tokamak disruption. In this study, we present a comprehensive understanding of the plasma dynamics in three-dimensional stochastic open magnetic field lines, taking into account the consistent coupling of electron and ion dynamics through the ambipolar electric field. The open stochastic field lines act as a 3D magnetic mirror consisting of the magnetic well and hill regions. It was found that the magnetic hill regions play a critical role in enhancing electron thermal transport. When the plasma collapses to the wall, the 3D ambipolar potential arises in the stochastic layer to maintain the quasi-neutrality of the plasma. The E × B vortices induced by the 3D ambipolar potential mix the plasma across stochastic field lines and enhance the radial transport. Particularly, the E × B mixing between the magnetic well and hill regions provides a collisionless detrapping mechanism that plays a major role in the loss of high-energy trapped electrons. As a result, the electron temperature steadily decreases at a rate of about −0.5 keV ms−1, comparable to the rate observed in thermal quench experiments.

Novel TCAD Approach for the Investigation of Charge Transport in Thick Amorphous SiO2 Insulators

A TCAD approach for the investigation of charge transport in thick amorphous silicon dioxide is presented for the first time. Thick oxides are investigated representing the best candidates for integrated galvanic insulators in future power applications. The large electric fields, such devices experience and the preexisting defects in the amorphous material, give rise to a leakage current, which leads to degradation and failure. Hence, it is crucial to have a complete understanding of the main physical mechanisms responsible for the charge transport in amorphous silicon oxide. For this reason, metal–insulator–metal structures have been experimentally characterized at different high-field stress conditions and a TCAD approach has been implemented in order to gain insight into the microscopic physical mechanisms responsible for the leakage current. In particular, the role of charge injection at contacts and charge build-up due to trapping–detrapping mechanisms in the bulk of the oxide layer has been investigated and modeled to the purpose of understanding the oxide behavior under dc- and ac-stress conditions. Numerical simulations have been compared against experiments to quantitatively validate the proposed approach.

Analysis and Compact Modeling of Fast Detrapping From Bandgap-Engineered Tunneling Oxide in 3-D NAND Flash Memories

We present the comprehensive analysis and the compact modeling methodology of fast electron detrapping from the bandgap engineered tunneling oxide (BE-TOX) in 3-D NAND flash memories. To this end, we develop a physics-based detrapping model considering the detailed electron dynamics in the BE-TOX and various emission paths of the detrapping mechanism. By the aid of the carrier density rates from the accurately calibrated model, we find that the major path of the charge loss can be direct tunneling (DT) or trap-to-band tunneling (TBT) depending on the temperature and the nitride layer position in the BE-TOX. In addition, we compare the model result and the fitted stretched exponential function. From those comparisons, we correlate several indicators (time constant and curve shape) of the retention characteristics with the major paths of the charge loss. Finally, our model facilitates the quantitative separation of the detrapping mechanism into different paths.

Particle Trapping in Axisymmetric Electron Holes

AbstractElectron orbits are calculated in solitary two‐dimensional axisymmetric electrostatic potential structures, typical of plasma electron holes, in order to establish the conditions for the particles to remain trapped. Analytic calculations of the evolution of the parallel energy caused by the perturbing radial electric field (breaking magnetic moment invariance) are shown to agree well with full numerical orbit integration Poincaré plots. The predominant mechanism of detrapping is resonance between the gyrofrequency in the parallel magnetic field and harmonics of the parallel bounce frequency. A region of phase space adjacent to the trapped‐passing boundary in parallel energy is generally stochastic because of island overlap of different harmonics, but except for very strong radial electric field perturbation, more deeply trapped orbits have well‐defined islands and are permanently confined. A simple universal quantitative algorithm is given, and its results plotted as a function of magnetic field strength and hole radial scale length, determining the phase space volume available to sustain the electron hole by depression of the permanently trapped distribution function.

Experimental Characterization of Charge Trapping Dynamics in 100-nm AlN/GaN/AlGaN-on-Si HEMTs by Wideband Transient Measurements

This article deals with the characterization of charge trapping dynamics in a novel 100-nm double-heterojunction AlN/GaN/AlGaN-on-Si radio frequency (RF) HEMT process. In order to study the detrapping mechanisms, we perform the wideband acquisitions of the transient behavior by sweeping the pulsed voltages to cover the entire device operating area. The fast acquisition also enables the characterization of the charge capture behavior, a key aspect for RF performance. From the analysis of the drain current transients, time constants are extracted, showing a fundamental release time constant in the order of 0.1–1 ms, and more than one capture constants, the fastest being in the order of 300 ns. To the best of authors’ knowledge, this is the first time that trapping dynamics under large-signal regime are characterized for this type of process.

Enhanced Resistive Switching Effect in Ag Nanoparticles Embedded in Graphene Oxide Thin Film

In this paper, the effect of different concentrations of Ag nanoparticles embedded in graphene oxide (GO) for resistive random-access memory (RRAM) has been investigated. The spin-coating method was used for the deposition of a GO-Ag layer on indium tin oxide-coated glass substrate. The structural studies of the samples were carried out using x-ray diffraction. The morphology of the as-deposited layer was determined using scanning electron microscopy and atomic force microscopy. It has been observed that Ag-doped memory devices require low voltage to switch from OFF to ON and vice versa. The switching voltage was reduced by half in Ag-doped devices as compared to undoped devices with a high OFF/ON current ratio of ∼ 103. The electrical stability of Ag-doped devices was tested for 4 × 103 s. Also, the SET/RESET behavior was tested for up to 60 cycles at a read voltage of 0.2 V. It was observed that both electrical stability and SET/RESET behavior did not exhibit excessive degradation. The switching speed of the RRAM device was calculated using an oscilloscope and found to be ∼ 200 ns for the low-resistance state (LRS) and ∼ 1 μs for the high-resistance state (HRS). The capacitance in Ag-doped GO devices was recorded and found to be ∼ 400 pF in the HRS as compared to 20 pF in case of undoped GO devices. It has been concluded that the charge trapping and de-trapping mechanism in case of Ag-doped GO is responsible for the enhanced properties of fabricated RRAM devices.

Quartz optically stimulated luminescence configurational coordinate model

In a recent study using pulsed stimulation we revealed that optically stimulated luminescence (OSL) and photo-transferred thermoluminescence (PTTL) in quartz are characterised by a common pulse response. We interpreted this result as indicating that both emissions are governed by the same detrapping mechanisms. In the present study we develop a common configurational coordinate model to describe the mechanisms. Within our model we identify the energy associated with the thermal assistance effect as the vibrational energy of the ground state of the dimer that represents the trap. Further, we model the OSL stimulation process as an optical transition between a vibrationally-excited ground state level and a dimer excited state. By introducing additional luminescence parameters, we reveal that the processes that underlie thermal quenching and the room temperature OSL response to pulsed stimulation can be represented by competing modes which empty the excited state. In particular, we find that the room temperature pulse response can be modelled in terms of an optical transition from the excited state to a lower antibonding level from which a hole is ejected. In contrast, thermal quenching is explained by excited state vibrational dissociation at large dimer component separations.

Photoluminescence and thermoluminescence properties of ZnGa2O4 prepared by a microwave assisted solid state reaction

A novel microwave assisted solid state chemical reaction was used to synthesize ZnGa2O4, from stoichiometric amounts of ZnO and Ga2O3 that were calcined at 1100 °C for 8 h under the assistance of 1200 W microwave power. The formation of a single phase was confirmed by X-ray diffractometry (XRD). The optical properties were probed using UV–Vis–NIR absorption, photoluminescence (PL) and Thermoluminescence (TL) spectroscopic techniques. The energy band gap of the compound was approximated to 4.68 eV from diffuse reflectance spectrum using the Kubelka-Munk model. The displayed greenish emission was centred at 528 nm, and the thermoluminescence measurements proved that the electron trapping and de-trapping mechanisms followed second-order kinetics. The variable heating rate (VHR) and initial rise (IR) methods were used to determine the depth of the electron trapping centres.

Strengthening in multi-principal element alloys with local-chemical-order roughened dislocation pathways

High-entropy and medium-entropy alloys are presumed to have a configurational entropy as high as that of an ideally mixed solid solution (SS) of multiple elements in near-equal proportions. However, enthalpic interactions inevitably render such chemically disordered SSs rare and metastable, except at very high temperatures. Here we highlight the wide variety of local chemical ordering (LCO) that sets these concentrated SSs apart from traditional solvent-solute ones. Using atomistic simulations, we reveal that the LCO of the multi-principal-element NiCoCr SS changes with alloy processing conditions, producing a wide range of generalized planar fault energies. We show that the LCO heightens the ruggedness of the energy landscape and raises activation barriers governing dislocation activities. This influences the selection of dislocation pathways in slip, faulting, and twinning, and increases the lattice friction to dislocation motion via a nanoscale segment detrapping mechanism. In contrast, severe plastic deformation reduces the LCO towards random SS.

A voltage-transient method for characterizing traps in GaN HEMTs

Trapping effects in GaN HEMTs still limit their performance. The current-transient methodology has shown advantages in characterizing traps in the device. However, the voltage drift may cause errors in measurements with high accuracy requirements. In this paper, we present a methodology to characterize traps in GaN HEMTs using the voltage-transient measurements. We demonstrate the advantages of this method in terms of simplicity and effectiveness. In particular, it avoids the said problem due to the optimized measuring circuit. With this method, we have identified the time constants and energy levels of traps in the AlGaN barrier layer and the GaN buffer layer, respectively, in the devices. Their trapping and de-trapping mechanisms were also demonstrated at various temperature measurements. A classic exponential dependence of the degradation rate on the channel current was identified.

Flexible, transparent resistive switching device based on topological insulator Bi2Se3-organic composite

Two-dimensional topological insulator bismuth selenide (Bi2Se3) nanosheets (NSs) embedded in poly-methyl methacrylate (PMMA) are employed for the first time for the resistive switching (RS) application. Hexagonal 2D Bi2Se3 NSs are synthesized by a simple solvothermal method and combine with PMMA at different weight percentages of 2D Bi2Se3. Field emission scanning electron microscopy and transmission electron microscopy along with other characterizations such as X-ray photoelectron spectroscopy and Raman spectroscopy were performed for the characterization of Bi2Se3@PMMA hybrid system. The composite was deposited on a transparent, flexible polyethylene terephthalate substrate to form Ag/Bi2Se3@PMMA/indium doped tin oxide memory cell. I-V characteristics of the device revealed a stable and non-volatile memory effect. The device shows a significantly high resistance (RHRS/RLRS) ratio, more than 103, high retention time (more than 9000 s) with high reproducibility over a large number of (105) ac cycles. From the experimental data, RS performances are explained by using a charge trapping–detrapping mechanism. Owing to the increasing interest in flexible electronics, bending tests are carried out at various bending diameters (10–30 mm) to show the mechanical robustness of the proposed device.

Stable charge retention in graphene-MoS 2 assemblies for resistive switching effect in ultra-thin super-flexible organic memory devices

This study investigated the nonvolatile memory characteristics of devices fabricated with PMMA embedding composite of graphene and molybdenum disulphide, which were employed as charge-trapping centres. X-ray diffraction measurements, Scanning and transmission electron microscope image analysis along with other characterisation techniques were performed for the assessment of the nature of graphene MoS2 hybrid system. Bistable I-V characteristics of the device revealed stable and rewritable memory effect. A significantly high resistance ratio > 104 (Roff/Ron) was observed even after 10 days. The reliability and reproducibility of the devices were tested for 104 cycles. The conduction mechanisms of the fabricated nanocomposite based memory cell were discussed on the basis of experimental data using a charge trapping–detrapping mechanism in the graphene MoS2 hybrid. Owing to the increasing interest in the flexible electronics, bending tests were carried out on the devices fabricated on PET substrates. Bendability test at various bending diameters (40-5 mm) for 100 cycles was carried out to show the mechanical robustness of the device.

Electric field dependence on the conductivity of Kapton-HN: Integration into a 1D physical model used for the description of charge transport in dielectric materials under space environment

In this paper, the development of a 1D physical conductivity model that describes charge transport in dielectric materials under space radiations with application to Kapton-HN is discussed. This model allows a good description of the charging behavior in dielectric materials. Former experiment run at ONERA on Kapton-HN showed that conductivity is temperature and electric field dependent. These results confirmed the need of implementing these dependences to our 1D model. This paper focuses on the electric field dependence. After a thorough analysis in the literature, several physical theories have been found that can explain this electric field dependence on the conductivity. They are the Onsager theory and the electric field assisted de-trapping mechanisms: direct tunneling, hopping transport, and the Poole-Frenkel theory. This paper is divided as follows: first the experiments that brought into evidence the electric field dependence of the conductivity in Kapton-HN are pointed out. Then the theories that could explain this dependence are described and implemented into the model. Finally, numerical and experimental conductivity results are compared and presented in the last sections.