Electron Trapping Process Research Articles

Trapping Phenomena in InAlN/GaN High Electron Mobility Transistors

Electron trapping-detrapping phenomena were studied for InAlN/GaN high electron mobility transistors (HEMTs) by drain current-drain voltage static and pulsed current voltage (I-V) characteristics, gate current I-Vs, transfer characteristic measurements, current deep level transient spectroscopy (CDLTS) with gate voltage pulsing, and by drain current transient measurements following both drain and gate voltage steps. The electron trapping processes are temperature activated with activation energies of 1.1 eV for drain voltage steps and 1 eV and 0.75 eV for gate voltage steps. Detrapping processes show activation energies 0.9 eV and (0.7–0.75) eV. Drain current transients are accurately described by the sum of two stretched exponents. The amplitudes of CDLTS peaks corresponding to electron trapping and electron detrapping processes show la ogarithmic dependence on the width of the injection pulse, indicating that the traps are related to dislocations.

Cathodoluminescence studies of defects in coated boron nitride

Optical emission properties of boron nitride (BN) substrates, BN with alumina (Al2O3) coating, and thermally-annealed alumina-coated boron nitride (an-BN/Al2O3) were investigated under electron irradiation using cathodoluminescence (CL) measurements. Tests were performed, temperatures ranging from ~100 K to ~300 K, with monoenergetic beams from 5 keV to 30 keV, and electron flux densities from 1 nA · cm−2 to 500 nA · cm−2. These experiments were conducted to identify the effects of coating and thermal annealing on the nature and occupation of defect states in different samples with BN substrates. Previous studies have shown that these treatments can limit the charging of BN substrates. Consequently, thorough investigations of electron trapping and recombination processes as a function of low temperature, dose and charging/discharge were performed in order to explain the differences of electrical behaviour and compare the CL spectra of the three different samples studied. Broad features associated with the BN and sharper features resulting from the annealed alumina coating were observed. Changes in the intensity, energy, and width of the features with sample treatments were observed. Different incident beam parameters were used to associate these features with specific types of defect states. The effects of charging, temperature- and dose-dependent conductivity, and thermal annealing and aging of the samples on the CL spectra were investigated. These were used to study defect creation and occupation and to understand the predominant physical mechanisms and main structural and chemical differences between these ceramic configurations.

Hydrothermal synthesis of reduced graphene oxide-CoFe2O4 heteroarchitecture for high visible light photocatalytic activity: Exploration of efficiency, stability and mechanistic pathways

RGO-CoFe2O4 heterostructure nanocomposite was prepared by hydrothermal method and was characterized by various analytical techniques such as Powder X-ray Diffraction method (PXRD), UV–vis absorbance, Photoluminescence (PL), Fourier Transform Infra Red (FTIR) spectroscopic techniques, BET surface area measurements, Field Emission Scanning Electron Microscopy (FESEM), Raman Spectroscopy and Vibrating Sample Magnetometer (VSM). The results confirmed the formation of hybrid structure with CoFe2O4 particles embedded in RGO sheets. Photocatalytic activity of the nanocomposites was probed for the degradation of 4-Chlorophenol (4-CP) as the model compound under the visible light illumination. The photocatalytic activity decreases in the following order RGO-CoFe2O4>CoFe2O4>RGO. Further the activity of RGO-CoFe2O4 composite was explored in the presence of peroxymonosulfate (PMS) as an oxidant. LUMO of PMS can accommodate photogenerated electrons, thereby suppresses the recombination process. The enhanced activity of RGO-CoFe2O4 hybrid is compared to its individual counterparts and the higher activity is accounted to its unique electronic structure. RGO serves as electron acceptor from CoFe2O4 and electron donor to the oxygen molecule. During the photocatalysis, transformation of the native structure from normal spinel to inverse spinel and vice versa may take place continuously from the process of electron trapping and detrapping by Fe3+ and Co2+ions. The observed continuous absorption for RGO-CoFe2O4 composite in the UV–vis spectra implies active d–d transitions involving transition metals present in the nanocomposite.

High-Performance Nonvolatile Organic Field-Effect Transistor Memory Based on Organic Semiconductor Heterostructures of Pentacene/P13/Pentacene as Both Charge Transport and Trapping Layers.

Nonvolatile organic field‐effect transistor (OFET) memory devices based on pentacene/N,N′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic diimide (P13)/pentacene trilayer organic heterostructures have been proposed. The discontinuous n‐type P13 embedded in p‐type pentacene layers can not only provide electrons in the semiconductor layer that facilitates electron trapping process; it also works as charge trapping sites, which is attributed to the quantum well‐like pentacene/P13/pentacene organic heterostructures. The synergistic effects of charge trapping in the discontinuous P13 and the charge‐trapping property of the poly(4‐vinylphenol) (PVP) layer remarkably improve the memory performance. In addition, the trilayer organic heterostructures have also been successfully applied to multilevel and flexible nonvolatile memory devices. The results provide a novel design strategy to achieve high‐performance nonvolatile OFET memory devices and allow potential applications for different combinations of various organic semiconductor materials in OFET memory.

Time-dependent dielectric breakdown characterizations of interlayer dielectric damage induced during plasma processing

We proposed an electrical evaluation method on the basis of leakage current–stress time (I–t) measurement for plasma-induced damage (PID) in interlayer dielectric films. The I–t measurement, which is widely used for the time-dependent dielectric breakdown (TDDB) test is applied to the evaluation of PID in thick SiO2 films. We focus on the electron trapping process during the TDDB test. The initial stage of time evolution of leakage current is monitored, and compared among the samples damaged under various plasma conditions. It was found that the initial electron trapping rate depends on the amount of damage, i.e., the rate increases with increasing in process time (tpr) and dc self-bias voltage (Vdc). The proposed technique can be used to assess the reliability degradation of interlayer dielectric films caused by PID.

Interaction of an ion bunch with a plasma slab

Charge neutralization of a short ion bunch passing through a plasma slab is studied by means of numerical simulation. It is shown that a fraction of plasma electrons are trapped by the bunch under the action of the collective charge separation field. The accelerated electrons generated in this process excite beam−plasma instability, thereby violating the trapping conditions. The process of electron trapping is also strongly affected by the high-frequency electric field caused by plasma oscillations at the slab boundaries. It is examined how the degree of charge neutralization depends on the parameters of the bunch and plasma slab.

Investigation of kink effect in normally-off AlGaN/GaN recessed-gate MOS-heterostructure FETs

Kink effects, anomalous increase of the output current, were observed at room temperature in normally-off AlGaN/GaN metal-oxide-semiconductor (MOS)-heterostructure field effect transistors with recessed gate. The kink phenomenon occurred only at certain bias-sweeping conditions and is suggested to result from electron trapping and subsequent detrapping process which gave rise to temporary shift of the threshold voltage. The magnitude of the kink is related to the positive gate bias and the temperature. Positive bias applied on the gate induced the negative charge build-up at the MOS interface and hot electrons released trapped electrons by impact ionization.

Long persistent luminescence in Mn^2+-activated sodium gallium germanate glass and glass ceramics induced by infrared femtosecond laser pulses

A distinct long persistent luminescence (LPL) was obtained from home-made Mn2+-activated sodium gallium germanate glass samples which were excited by an infrared femtosecond laser. The LPL, with a central wavelength longer than 600 nm, can be seen by the naked eye for more than one hour in the dark at room temperature. It has been proven that the LPL originated from the 4T1→ 6A1 transition of Mn2+ ions rather than from femtosecond laser induced extrinsic defects with photoluminescence (PL) bands peaking at 1.85 and 2.2 eV. The LPL properties of Mn2+ in the same host glass-ceramics were also investigated. An obvious green emission band appeared in PL spectra while absent in LPL spectra, which indicated a process distinguished from traditional heat-assisted tunneling effect. A new mechanism based on the process of strong-field excitation, collision excitation, electron trapping and direct capture of de-trapped electrons by excited states of Mn2+ ions is proposed to elucidate the generation of LPL.

Resolving carrier recombination in CdS quantum dots: A time-resolved fluorescence study

Trap states in colloidal quantum dots are known to play an important role in exciton recombination dynamics. CdS is an interesting quantum dot system because it exhibits both exciton and trap photoluminescence (PL) at room temperature. Previous ultrafast work has uncovered both electron and hole trapping processes, but the relationship between exciton and trap PL and the nature of the trap states remains poorly understood. In this article we take a detailed look at exciton and trap PL in several CdS samples. By varying temperature as well as excitation and emission energies, we develop a detailed picture of carrier recombination dynamics in these materials and, with the help of dynamical models, assign two distinct recombination pathways involving trapped electrons and holes. These results help us to understand the factors that govern charge carrier trapping in colloidal quantum dots and could help us to optimize their role in future applications.

Study of binary and ternary organic hybrid CdSe quantum dot photodetector

The hybrid binary and ternary photodetectors (PDs) were fabricated from P3HT–PC71BM with CdSe quantum dot (QD) materials. The absorption spectra of P3HT:PC71BM (named as B1), P3HT:CdSe (B2) and P3HT:CdSe:PC71BM (T) active blended material were analyzed in the wavelength range from 350 to 800 nm. The current density–voltage characteristics of the device were measured in dark and under illumination for study of detector detectivities and the contact with electrode. The \({J_{{\text{under}}\,{\text{illumination}}}}/{J_{\text{dark}}}\) ratio at −0.5 V for PDs B1, B2 and T is 1.1 × 102, 1.9 × 102 and 1.8 × 103, respectively. The values of detectivity \(({D^*})\) for B1, B2 and T are 1 × 1010, 2 × 1010 and 7 × 1011 Jones, respectively. The \({D^*}\) for PD T is ten times in comparison with B1 and B2 PDs. The linear dynamic range (LDR) value for ternary device is more than double to both binary PDs. The absorption by CdSe QD increases the photon efficiency in the ternary detector, and at the same time the ternary detectors have high detectivity in broad spectral range. The responsivity of current to the light intensity exponent θ for detector B1, B2 and T is ~0.55, 0.55 and 0.62, respectively, which represents a complex process of electron hole generation, recombination and trapping within active material.

Frequency-Modulated Gigahertz Complex Conductivity of TiO2 Nanoparticles: Interplay of Free and Shallowly Trapped Electrons

In a quest to explore the prominent electronic properties of titanium dioxide (TiO2), precise knowledge on the electron mobility and trapping process is an immensely important part of organic photovoltaic applications. We have investigated the frequency dispersion of complex photoconductivity of TiO2 nanoparticles by laser-flash frequency-modulated time-resolved microwave conductivity (FM-TRMC) in the 9–34 GHz region. The absolute ratio of imaginary to real transient conductivities was analyzed by the combination of Drude–Smith and Drude–Zener models, which allows for determining the trap depth (ET) and population of trapped electrons (ftrap). We have identified that the shallowly trapped electrons (ET = 70–110 meV, ftrap > 0.8) have a large impact on the negative imaginary conductivity, while the free electron confined in TiO2 nanoparticle significantly contributes to the positive real part in spite of its small population (<0.2). The kinetic mismatch of real and imaginary conductivities suggests the inv...

Trapping of electrons and acceleration of the electron bunch in a wake wave

The process of electron trapping by a wake wave excited by a laser pulse in a plasma channel in the case where the electron bunches are injected into the vicinity of the maximum of the wakefield potential at a velocity lower than the wave phase velocity is considered. The mechanism for the formation of a compact electron bunch in the trapping region when only the electrons of the injected bunch that are trapped in the focusing phase mainly undergo the subsequent acceleration in the wakefield is analyzed. The influence of the spatial dimensions of the injected bunch and its energy spread on the length of the trapped electron bunch and the fraction of trapped electrons is studied analytically and numerically. For electron bunches with different ratios of their spatial dimensions to the characteristic dimensions of the wake wave, the influence of the injection energy on the parameters of the high-energy electron bunch trapped and accelerated in the wake-field is studied.

Effect of silane coupling agent chemistry on electrical breakdown across hybrid organic-inorganic insulating films.

Dielectric breakdown measurements were conducted on self-assembled monolayer (SAM)/native silicon oxide hybrid dielectrics using conductive atomic force microscopy (C-AFM). By depositing silane coupling agents (SCAs) through a diffusional barrier layer, SAM roughness was decoupled from chemistry to compare the chemical effects of exposed R-group functionality on dielectric breakdown. Using Weibull and current-voltage (I-V) analysis, the breakdown strength was observed to be independent of SCA R-group length, and the addition of a SAM was seen to improve the breakdown strength relative to native silicon oxide by up to 158%. Fluorinated SCAs were observed to suppress tunneling leakage and exhibited increased breakdown strength relative to their hydrocarbon analogs. Electron trapping, scattering, or attachment processes inherent to the fluorinated moieties are thought to be the origin of the improved breakdown properties.

Uniform bipolar resistive switching properties with self-compliance effect of Pt/TiO2/p-Si devices

We report uniform bipolar resistive switching characteristic with self-compliance effect of Pt/TiO2/p-Si devices in which TiO2 thin films were prepared directly on p-Si substrates by chemical solution deposition method. The resistive switching parameters of the Pt/TiO2/p-Si cell obtained, such as distribution of threshold voltages, retention time, as well as resistance variation of high resistance state (HRS) and low resistance state (LRS), were investigated, and the conduction mechanisms of HRS and LRS were analyzed. The conductive mechanism at LRS and low voltage region of HRS was dominated by Ohmic law. At the high voltage region of HRS, the conductive mechanism followed the space charge limited current theory. The resistive switching phenomenon can be explained by electron trapping and de-trapping process, in which the defects (most probably oxygen vacancies) act as electron traps. Our study suggests that using p-type silicon as bottom electrode can provide a simple method for fabricating a resistive random access memory with self-compliance function. In addition, the Pt/TiO2/p-Si configuration is compatible with complementary metal oxide semiconductor process.

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remain unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors (anthraquinone, methylviologen, and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ∼0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Franck-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes.

Influence of Formate Adsorption and Protons on Shallow Trap Infrared Absorption (STIRA) of Anatase TiO2 During Photocatalysis

ATR-IR spectroscopy has been used to probe the spectral changes resulting from UV irradiation of thin particulate films of anatase TiO2 under aqueous anoxic conditions. Irradiation under circumneutral solutions removed adsorbed impurities and produced a transient broad IR absorption peaking at ∼880 cm–1 that has been attributed to excitations associated with shallow electron traps. This shallow trap IR absorption (STIRA) was long-lived under acidic conditions with peak absorbance rising with decreasing pH. Formate was used as a hole scavenger to probe the impact of hole removal on the electron trapping processes. The adsorption of formate to anatase TiO2 was investigated in the absence of UV irradiation with the spectra showing a mixture of inner- and outer-sphere adsorbed species. UV irradiation of anatase films in the presence of formate at low concentrations enhanced STIRA intensity substantially. The STIRA scaled with TiO2 mass was more pronounced from larger crystallite anatase and was sensitive to [H+], suggesting that formation of the shallow trap state is accompanied by proton intercalation.

Distribution Function of Electrons in an Ion Channel

The problem on the evolution of the distribution function of mutually noninteracting electrons in an infinite ion channel (channeling in plasma) is considered for an infinite electron beam (electron current) and the process of electron trapping into the channel is simulated.

What Limits Photoconductance in Anatase TiO2 Nanostructures? A Real and Imaginary Microwave Conductance Study

The macroscopic electron transport in porous films of sintered 9 nm sized nanocrystals (NC-TiO2) is known to be 4–6 orders of magnitude worse than in polycrystalline (Pol-TiO2) dense films. To obtain fundamental knowledge regarding this large difference, we investigated the effects of spatial confinement and electron trapping processes on the charge transport in these samples. We determined the time-resolved real and imaginary microwave photoconductance (TRMC) on pulsed excitation. Large amounts of the photoexcited electrons are readily immobilized in deep traps in NC-TiO2 as concluded by comparing the TRMC decay kinetics with previously published transient absorption measurements. Our results show that trapped electrons do not give rise to a microwave photoconductance response, nor do they affect the motion of conduction band electrons. Additionally, by comparing a bare NC-TiO2 film with a dye-sensitized NC-TiO2 sample, the influence of the photogenerated holes on the photoconductance as a function of their locus is investigated. The positive charges either inside or outside the TiO2 nanocrystals contribute insignificantly to the photoconductance. On a nanosecond time scale only a minor fraction (maximum 2%) of the photoexcited electrons resides in the conduction band of NC-TiO2. Importantly, in both NC-TiO2 and Pol-TiO2 these electrons have the same intraparticle microwave mobility of 1.7 cm2/(V s) due to frequent backscattering events at a mean time interval of 85 fs. This mobility value represents the upper limit for the trap-free dc electron mobility in anatase TiO2 irrespective of the crystallite size. Hence, the photoconductance across a NC-TiO2 layer can be strongly enhanced by reducing the electron trap density and by eliminating the relatively inefficient electron hopping steps between adjacent nanocrystals.

Effect of Discharge Plasma Potential on Diffusion Plasma Parameters Controlled by a Mesh Grid in a Double Plasma Device

AbstractThe plasma region under investigation is separated from the discharge region by a mesh grid. Plasma potential and electron number densities and electron temperatures under bi‐Maxwellian approximation for electron distribution function of the multi‐dipole argon plasma are measured. The cold electrons in the diffusion region are produced by local ionization. The hot electrons are the ionizing electrons behaving as Maxwellian. The electron trapping process in the discharge region is produced by potential well due to positive plasma potential with respect to the anode and by a repulsive grid. The dependence of ratios of the density of the hot to the cold electrons NE (=Neh/Nec) and hot to cold electron temperature T(=Teh/Tec) in the diffusion region on the depth of the potential well has been investigated. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Insulating properties and charge characteristics of natural ester fluid modified by TiO2 semiconductive nanoparticles

In order to improve dielectric performance of natural esters, TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> semiconductive nanoparticles were added to obtain an ester-based nanofluid. AC, lightning impulse voltage and partial discharge tests were applied to the natural ester and the ester-based nanofluid to check their dielectric performances. It was found that TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles can not only enhance AC breakdown voltage of the ester based nanofluid by a factor of 1.3 but also improve the partial discharge properties of the ester based nanofluid compared to that of the natural ester. Pulse electroacoustic (PEA) and thermally stimulated current (TSC) techniques were applied to investigate charge transport and trap characteristic in natural ester and ester-based nanofluid. It reveals that TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles can accelerate the charge dissipation rate and uniform the electric field and increase the shallow trap density in the ester-based nanofluid. It is proposed that electron trapping and de-trapping processes in the shallow traps could be one of the main charge transport processes in the ester based nanofluid.