Electron Detrapping Research Articles

A charge trap–based MoSe2 device emulating bio-realistic synaptic functionalities

Abstract Synaptic devices based on two-dimensional (2D) materials are promising toward the development of high-performance and low-power neuromorphic systems. In this study, we report a single-channel, three-terminal-based nonvolatile and multistate device based on 2D MoSe2. The device operates on the principle of trapping and detrapping of electrons at the MoSe2/SiO2 interface, in response to an applied gate voltage, resulting in a nonvolatile modulation of the threshold voltage. The memory behavior is highly reproducible as verified for around 100 cycles of the gate voltage sweep. The multistate behavior of the MoSe2 device was exploited to demonstrate the characteristics of the biological synapse. The device exhibits various synaptic functions, such as potentiation, depression, spike rate–dependent plasticity, spike magnitude–dependent plasticity, and the ability to transition from short-term to long-term memory. The bio-realistic synaptic behavior of the MoSe2 device underscores its promising potential for neuromorphic hardware.

Abnormal Temperature Dependence of Huang-Rhys Factor and Exciton Recombination Kinetics in CsPbBr3 Perovskite Quantum Dots.

Anomalous thermal behaviors of excitonic luminescence in CsPbBr3 perovskite quantum dots (PQDs) were observed. It is found that the main luminescence peak originated from the excitonic radiative recombination assisted by the longitudinal-optical (LO) phonon, and its integrated intensity first declines as the temperature varies from 10 to 150 K and then turns to increase at ∼160 K, reaching a maximum value at 300 K. A model considering the thermal detrapping and transfer of electrons from a trap level is developed to interpret these abnormal thermal behaviors of the luminescence from the PQDs. On the other hand, the quantum-mechanical multimode Brownian oscillator model was employed to unravel that the Huang-Rhys factor exclusively characterizing the exciton-phonon coupling in the studied CsPbBr3 PQDs decreases from 1.65 at 10 K to 1.31 at 200 K.

Core-shell engineering of graphite nanosheets reinforced PVDF toward synchronously enhanced dielectric properties and thermal conductivity

Polymer composites integrating high dielectric permittivity (ε′) but low loss, large breakdown strength (Eb) and thermal conductivity (TC), have attracted widespread attention in electronic devices and power systems. To simultaneously achieve these properties in graphite nanosheet (GNS)/poly(vinylidene fluoride, PVDF), the GNS were first encapsulated by a layer of aluminum oxide (Al2O3), and then incorporated into PVDF, and the resulting PVDF nanocomposites’ dielectric properties and TC were explored in terms of the Al2O3 shell thickness and filler loading. The insulating Al2O3 shell serves as a barrier layer for the formation of leakage current and long-distance electron migration thus resulting in much lower dielectric loss and conductivity of the GNS@Al2O3/PVDF. It effectively mitigates the dielectric mismatch between the filler and host matrix, further introduces more traps that can capture charge carriers, and increases the barrier height for electron detrapping subsequently preventing the growth of electric trees and elevating the Eb. Moreover, the Al2O3 interlayer alleviates both the phonon density state and phonon impedance mismatches between the filler and matrix and facilitates the interfacial phonon transport thus leading to improved TC. The dielectric parameters and TC of the GNS@Al2O3/PVDF can be simultaneously modulated by optimizing the Al2O3′s thickness. This work offers an effective approach for designing and fabricating the polymeric nanodielectrics concurrently integrating high ε′ but low loss, enhanced Eb, and TC for prospective applications in power equipment and microelectronic devices.

Structural, Optical, Electrical and Photocatalytic Investigation of n-Type Zn2+-Doped α-Bi2O3 Nanoparticles for Optoelectronics Applications.

Herein, n-type pure and Zn2+-doped monoclinic bismuth oxide nanoparticles were synthesized by the citrate sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) analysis, ultraviolet-visible (UV-vis) spectroscopy, and Hall effect measurements were used to study the effect of Zn2+ on the structural, optical, and electrical properties of nanoparticles. XRD revealed the monoclinic stable phase (α-Bi2O3) of all synthesized samples and the crystallite size of nanoparticles increased with increasing concentration of dopant. Optical analysis illustrated the red shift of absorption edge and blue shift of band gap with increasing concentration of dopant. Hall Effect measurements showed improved values (2.79 × 10-5 S cm-1 and 6.89 cm2/V·s) of conductivity and mobility, respectively, for Zn2+-doped α-Bi2O3 nanoparticles. The tuned optical band gap and improved electrical properties make Zn2+-doped α-Bi2O3 nanostructures promising candidates for optoelectronic devices. The degradation of methylene blue (MB, organic dye) in pure and zinc-doped α-Bi2O3 was investigated under solar irradiation. The optimum doping level of zinc (4.5% Zn2+-doped α-Bi2O3) reveals the attractive photocatalytic activity of α-Bi2O3 nanostructures due to electron trapping and detrapping for solar cells.

Charge carrier trapping management in Bi3+ and lanthanides doped Li(Sc,Lu)GeO4 for x-ray imaging, anti-counterfeiting, and force recording

Discovering energy storage materials with rationally controlled trapping and de-trapping of electrons and holes upon x-rays, UV-light, or mechanical force stimulation is challenging. Such materials enable promising applications in various fields, for instance in multimode anti-counterfeiting, x-ray imaging, and non-real-time force recording. In this work, photoluminescence spectroscopy, the refined chemical shift model, and thermoluminescence studies will be combined to establish the vacuum referred binding energy (VRBE) diagrams for the LiSc1−xLuxGeO4 family of compounds containing the energy level locations of Bi2+, Bi3+, and the lanthanides. The established VRBE diagrams are used to rationally develop Bi3+ and lanthanides doped LiSc1−xLuxGeO4 storage phosphors and to understand trapping and de-trapping processes of charge carriers with various physical excitation means. The thermoluminescence intensity of x-ray irradiated LiSc0.25Lu0.75GeO4:0.001Bi3+,0.001Eu3+ is about two times higher than that of the state-of-the-art x-ray storage phosphor BaFBr(I):Eu2+. Particularly, a force induced charge carrier storage phenomenon appears in Eu3+ co-doped LiSc1−xLuxGeO4. Proof-of-concept non-real-time force recording, anti-counterfeiting, and x-ray imaging applications will be demonstrated. This work not only deepens our understanding of the capturing and de-trapping processes of electrons and holes with various physical excitation sources, but can also trigger scientists to rationally discover new storage phosphors by exploiting the VRBEs of bismuth and lanthanide levels.

Characterization of program and erase speed of memory capacitors with nanolaminated HfO2/Al2O3 stacks for application in non-volatile memories

The programming and erase performance of atomic layer deposited HfO2/Al2O3 memory stacks is investigated depending on the thickness of the tunnel oxide and the annealing of the structures. The as-grown stacks require applying of voltage pulses with duration of about 10-3 s for a measurable electron trapping (programming). The time dependence of the electron trapping process is quite sharp initially - the increase of the charging time from 4×10-4 s to 4×10-3 s provides more than 80% of the saturation value of the trapped charge obtained at durations above 1 s. The annealing increases the time needed for the onset of the electron trapping. For the chosen voltage amplitudes the programming characteristics does not depend on the tunnel oxide thickness. The electron detrapping (erase) is observed even at durations of ~10-7 s, but with a low efficiency. The erase characteristics is more gradual then the program one, and shows some dependence on tunnel oxide and annealing. The complete erase is achieved for times comparable to those for programming the cell. Erase pulses with duration > 10-2 s results to accumulation of positive charge in the capacitors.

MoS2 Quantum Dot-Optimized Conductive Channels for a Conjugated Polymer-Based Synaptic Memristor.

Donor-acceptor-type conjugated polymers are widely used in memristors due to their unique push-pull electron structures and charge transfer mechanisms. However, the inherently inhomogeneous microstructure of polymer films and their low crystallinity produce randomness that destabilizes formed conductive channels, giving polymer-based memristors unstable switching behavior. In this contribution, we prepared a synaptic device based on PM6-MoS2 QD (molybdenum disulfide quantum dot) nanocomposites. In the composites, MoS2 QDs provided the active centers for forming conductive channels via electron trapping and detrapping. They also controlled the directional formation of conductive channels between PM6 and MoS2 QDs, reducing randomness and giving devices a narrow switching voltage range and cycling longevity. The device exhibited continuous multistage conductance states under a direct current voltage sweep and simulated a variety of synaptic functions, including long-term potentiation, long-term depression, short-term potentiation, short-term depression, paired-pulse facilitation, spiking-rate-dependent plasticity, and "learning experience" behavior. The memristor could also perform arithmetic, including "counting" and "subtraction" operations. This work provides a new approach to improving the performance of memristors for neuromorphic computing.

Symmetric bipolar resistive switching in copper oxide nanostructure/ITO lateral device under exposure to atmospheric oxygen and application in artificial synaptic devices

Capacitively coupled resistive switching behaviour was observed in a planar-geometry two-terminal device based on nanostructures of copper oxide as active material and indium tin oxide (ITO) as electrodes. When tested in ambient atmosphere, current-voltage I−V characteristic was found to be symmetric with pronounced hysteresis loop signifying two different states of resistance, namely the low resistive state (LRS) and high resistive state (HRS). Capacitive effect was apparent in the low voltage regime due to the presence of offset voltage at zero current and offset current at zero bias. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of native oxygen vacancies in copper oxide. Additionally, exposure to atmospheric oxygen played the dominant role in creation of excess vacancy sites by electrochemical reaction near positive electrode. Creation of excess density of oxygen vacancy near the electrodes in conjunction with trapping and detrapping of electrons from these defect sites is perceived as the prime mechanism for the observed switching behaviour. The device ON/OFF ratio, ION/IOFF∼1.5 was found to be stable over at least 100 continuous cycles. When tested under vacuum, the device I−V characteristics was linear in shape without any hysteresis signifying ohmic transport. Analysis of ultraviolet photoelectron spectroscopy (UPS) showed the Fermi energy level of copper oxide is at ∼ 4.69 eV, which is in a close match with the work function of ITO-electrode. The essential synaptic characteristics such as learning and forgetting curves and paired pulse facilitation (PPF) are established. The nonlinearity factors (NLF) are very low indicating the copper oxide-based devices are promising candidates for neuromorphic applications.

High temperature operation of Beta-Ga2O3 transistors

We studied the application of Beta-Ga2O3 for high temperature operation up to 500C. Field effect transistors were fabricated using epitaxial films grown on insulating Beta-Ga2O3 substrates. Variable temperature DC measurements were performed in vacuum and air ambient. Measurements revealed a reduction in on/off ratio for the devices due to increase in thermionic emission over the gate/dielectric barrier of MOSFET and over the gate/semiconductor barrier of MESFET. Devices also exhibited detrapping of electrons from interface traps with the increase in temperature. After the devices were tested intermittently at different high temperatures in vacuum or in air ambient, suggesting no change in semiconductor and contact properties.

Advancements in Complementary Metal-Oxide Semiconductor-Compatible Tunnel Barrier Engineered Charge-Trapping Synaptic Transistors for Bio-Inspired Neural Networks in Harsh Environments.

This study aimed to propose a silicon-on-insulator (SOI)-based charge-trapping synaptic transistor with engineered tunnel barriers using high-k dielectrics for artificial synapse electronics capable of operating at high temperatures. The transistor employed sequential electron trapping and de-trapping in the charge storage medium, facilitating gradual modulation of the silicon channel conductance. The engineered tunnel barrier structure (SiO2/Si3N4/SiO2), coupled with the high-k charge-trapping layer of HfO2 and high-k blocking layer of Al2O3, enabled reliable long-term potentiation/depression behaviors within a short gate stimulus time (100 μs), even under elevated temperatures (75 and 125 °C). Conductance variability was determined by the number of gate stimuli reflected in the maximum excitatory postsynaptic current (EPSC) and the residual EPSC ratio. Moreover, we analyzed the Arrhenius relationship between the EPSC as a function of the gate pulse number (N = 1-100) and the measured temperatures (25, 75, and 125 °C), allowing us to deduce the charge trap activation energy. A learning simulation was performed to assess the pattern recognition capabilities of the neuromorphic computing system using the modified National Institute of Standards and Technology datasheets. This study demonstrates high-reliability silicon channel conductance modulation and proposes in-memory computing capabilities for artificial neural networks using SOI-based charge-trapping synaptic transistors.

Resistive switching characteristics of MnO2-based thin film for transparent non-volatile ReRAM

The present study covers the fabrication of indium tin oxide (ITO)/manganese dioxide (MnO2)/ITO capacitor-based transparent resistive random access memory (ReRAM) device. The ITO/MnO2/ITO device exhibits 80 % transparency in the visible region along with steady bipolar resistive switching characteristics. The conduction mechanism investigation reveals that the current in low resistance state (LRS) is governed by Ohmic conduction. However, the dominance of trap-controlled space charge limited conduction (SCLC) mechanism in high resistance state (HRS) was observed. The resistive switching phenomenon in ITO/MnO2/ITO device was caused due to the formation and rupture of conduction filament via electron trapping and de-trapping in oxide-related traps (oxygen vacancies). In terms of reliability, the MnO2-based device exhibits good endurance up to 104 cycles and long retention for 104 s at room-temperature as well as at 85 °C. Cumulative probability distribution shows resistance ratio of 103 between LRS and HRS, which is sufficient to read memory states. These outcomes suggest that the ITO/MnO2/ITO-based transparent ReRAM could be a futuristic see-through electronic device for artificial intelligence (AI) application.

Nonvolatile and volatile resistive switching characteristics in MoS2 thin film for RRAM application

Resistive random access memory (RRAM) is one of strong candidates for future memory technology. The volatile and nonvolatile properties of devices are important foundations for the construction of neuromorphic hardware systems, which still represents a challenge. In this study, we fabricated the Cu/MoS2/ITO RRAM by RF magnetic sputtering with different thicknesses of MoS2 thin films, and explored the possibility of obtaining the nonvolatile and volatile memristive effects of RRAM devices through different operating voltages. By systematically investigating the resistive switching (RS) characteristics, we proposed and discussed the nonvolatile and volatile conceptual models to elaborate on the RS mechanism of the fabricated devices. The nonvolatile and volatile behaviors were explained respectively by the forming and breaking of Cu conductive filaments between Cu and ITO, and the trapping and de-trapping of electrons leading to the change of the Schottky barrier at the ITO/MoS2 interface.

Impact of high-temperature operating lifetime tests on the stability of 0.15 μm AlGaN/GaN HEMTs: A temperature-dependent analysis

We present a detailed analysis of the effect of low- and high-temperature operating lifetime (LTOL and HTOL) carried out on 0.15 μm GaN HEMTs for RF applications. Based on several stress experiments carried out at different temperature levels, a) we demonstrate the existence of two degradation modes consisting, respectively, in the negative and positive shift in the threshold voltage; b) temperature-dependent I-V measurements indicate that no modification in the Schottky barrier height takes place after stress; c) the negative VTH shift is ascribed to the temperature- and field-assisted de-trapping of electrons from the passivation/AlGaN stack under the gate, consistent with previous reports; d) the positive VTH shift, which only occurs at high temperature levels, is ascribed to the trapping of hot electrons, possibly at the AlGaN/SiN interface.

Sub-bandgap photon-assisted electron trapping and detrapping in AlGaN/GaN heterostructure field-effect transistors

We have investigated photon-assisted trapping and detrapping of electrons injected from the gate under negative bias in a heterostructure field-effect transistor (HFET). The electron injection rate from the gate was found to be dramatically affected by sub-bandgap laser illumination. The trapped electrons reduced the two-dimensional electron gas (2DEG) density at the AlGaN/GaN heterointerface but could also be emitted from their trap states by sub-bandgap photons, leading to a recovery of 2DEG density. The trapping and detrapping dynamics were found to be strongly dependent on the wavelength and focal position of the laser, as well as the gate bias stress time prior to illumination of the HFET. Applying this phenomenon of trapping and detrapping assisted by sub-bandgap photons, red, green, and purple lasers were used to demonstrate photo-assisted dynamic switching operations by manipulation of trapped carriers at the surface of an AlGaN/GaN HFET. A physical model based on band diagrams, explaining the trapping and detrapping behavior of electrons, has been presented.

Interplay between electron and ion plasma waves

We report the observation of the interplay between the first kind of high-frequency electron-dominated plasma waves (EPWs) and the second kind of low-frequency ion-dominated plasma waves (IPWs) in a DC glow discharge plasma experiment. On application of positive DC voltage on a probe immersed in plasma when the drift velocity of the electrons (vd) ≈1.3× the thermal velocity of electrons (vte), it gives rise to an instability resulting in the formation of double layers (DL). Under DL conditions, the anode sheath extends deep into plasma. The DL represents a nonequilibrium plasma condition that is known for the instabilities and excitation of plasma waves. In this experiment, the effect of the monotonic increase of plasma density fluctuations (δn≈0.01%) on the evolution of DL has been studied. The interplay between EPWs and IPWs is observed in the course of this investigation. The overall plasma response on density fluctuations is recorded as floating potential fluctuations (FPFs) which vary with an increase of δn. The FPFs exhibit order-chaos-order transition with the evolution of DLs. Further analysis of the fluctuations using the wavelet method gives a spectrum in both the time and frequency domains simultaneously. Wavelet spectra reveal that the nonlinear evolution of FPFs is a consequence of the interplay between EPWs and IPWs due to the trapping and detrapping of electrons in the extended anode sheath region. These results explain that the ordered oscillation corresponds to dominant EPWs and the chaotic fluctuations are due to the coexistence of EPWs and IPWs. This new understanding may assist to study the complex plasma behaviour observed in laboratories as well as space plasmas.

Research Process of Carbon Dots in Memristors

AbstractThe explosive growth of digital communication promotes advanced memory and computing devices in Big Data and artificial intelligence era. In particular, memristors hold great promise for in‐memory computing and artificial synapses, expected to break through restrictions on hardware computational power and storage capacity caused by the von Neumann bottleneck and declining Moore's Law. The memristance controllability is vital to memristors, in which functional layer materials are key. As novel functional layer materials, carbon dots (CDs) are expected to overtake silicon‐based materials due to their satisfactory modulation effects on memristance and distinguished memristive performances. The combination of CDs and matrix materials may carry out multimode sensation memristors for interactive intelligent systems. This review first gives a brief introduction to memristors and their functional layer materials, especially different CDs and their relations with memristance. Then the modulation effects of CDs on memristance are highlighted, mainly including the local electric field enhancement effect, the electron trapping and detrapping effect, and the photosensitization effect, accompanied by applications of CDs‐based memristors (CDMs). Lastly, challenges and perspectives of CDMs are pointed out. This work is rewarding to understand the role of CDs in memristors, to guide relevant research about CDMs, and to promote implementation for intelligent memory and computing.

Highly efficient yellow emission and abnormal thermal quenching in Mn2+-doped Rb4CdCl6.

In this paper, Mn2+-doped Rb4CdCl6 metal halide single crystals were prepared by a hydrothermal method. The Rb4CdCl6:Mn2+ metal halide exhibits yellow emission with photoluminescence quantum yields (PLQY) as high as 88%. Due to the thermally induced electron detrapping, Rb4CdCl6:Mn2+ also displays good anti-thermal quenching (ATQ) behavior with thermal quenching resistance (131% at 220 °C). The increase in the photoionization and the detrapping of the captured electrons from the shallow trap states were appropriately attributed to this exceptional phenomenon based on thermoluminescence (TL) analysis and density functional theory (DFT) calculations. The relationship between the fluorescence intensity ratio (FIR) of the material and temperature change was further explored using the temperature-dependent fluorescence spectrum. It was used as a temperature measuring probe based on absolute sensitivity (Sa) and relative sensitivity (Sb) with the change in temperature. The phosphor-converted white light emitting diodes (pc-WLEDs) were fabricated using a 460 nm blue chip with a yellow phosphor, which has a color rendering index (CRI = 83.5) and a low correlated color temperature (CCT = 3531 K). Because of this, finding new metal halides with ATQ behavior for high-power optoelectronic applications may be made possible by our findings.

500 °C operation of β-Ga2O3 field-effect transistors

We demonstrated 500 °C operation of field-effect transistors made using ultra-wide bandgap semiconductor β-Ga2O3. Metal–semiconductor field-effect transistors were fabricated using epitaxial conductive films grown on an insulating β-Ga2O3 substrate, TiW refractory metal gates, and Si-implanted source/drain contacts. Devices were characterized in DC mode at different temperatures up to 500 °C in vacuum. These variable-temperature measurements showed a reduction in gate modulation of the drain current due to an increase in gate leakage across the gate/semiconductor Schottky barrier. Devices exhibited a reduction in transconductance with increasing temperature; despite this, drain current increased with temperature due to a reduction in threshold voltage caused by the de-trapping of electrons from deep-level traps. Devices also showed negligible change in semiconductor epitaxy and source/drain contacts, hence demonstrated recovery to their room-temperature electrical properties after the devices were tested intermittently at different high temperatures in vacuum. The mechanism of gate leakage was also explored, which implicated the presence of different conduction mechanisms at different temperatures and gate electric fields.

Electrical instabilities in amorphous Si-Zn-sn-O thin film transistors under ultra-violet irradiation depending on oxygen content

The effect of different oxygen (O2) flow rates during the radio-frequency (RF) sputter deposition of amorphous silicon-zinc-tin-oxide (a-SZTO) thin film transistors (TFTs) has been studied. The threshold voltage (Vth) shifts towards the positive bias by increasing the O2 flow rate during the deposition, which decreases the electron concentration by reducing oxygen vacancies (VOs). The post-treatment of the devices under ultraviolet (UV) irradiation in the air atmosphere has been performed to analyze the reliability of light. The post-treatment of UV irradiation on the a-SZTO TFT for different time intervals significantly shifts the threshold voltage (Vth) from enhance- to depletion mode owing to the creation of VOs. Interestingly, we observed that UV irradiation is more effective on the parameters of Vth and the current ON/OFF (ION/OFF) ratio for the TFTs prepared at lower O2 flow rates. The reason may be that by introducing more oxygen flow, the de-trapping of electrons is reduced under UV irradiation due to the reduced VOs. UV irradiation does not significantly impact the field-effect mobility (µFE) for different O2 flow rates. The negative bias stress measurement of the devices was performed and analyzed for different O2 flow rates under UV irradiation treatment.

Protons: Critical Species for Resistive Switching in Interface‐Type Memristors

AbstractInterface‐type (IT) resistive switching (RS) memories are promising for next generation memory and computing technologies owing to the filament‐free switching, high on/off ratio, low power consumption, and low spatial variability. Although the switching mechanisms of memristors have been widely studied in filament‐type devices, they are largely unknown in IT memristors. In this work, using the simple Au/Nb:SrTiO3 (Nb:STO) as a model Schottky system, it is identified that protons from moisture are key element in determining the RS characteristics in IT memristors. The Au/Nb:STO devices show typical Schottky interface controlled current–voltage (I–V) curves with a large on/off ratio under ambient conditions. Surprisingly, in a controlled environment without protons/moisture, the large I–V hysteresis collapses with the disappearance of a high resistance state (HRS) and the Schottky barrier. Once the devices are re‐exposed to a humid environment, the typical large I–V hysteresis can be recovered within hours as the HRS and Schottky interface are restored. The RS mechanism in Au/Nb:STO is attributed to the Schottky barrier modulation by a proton assisted electron trapping and detrapping process. This work highlights the important role of protons/moisture in the RS properties of IT memristors and provides fundamental insight for switching mechanisms in metal oxides‐based memory devices.