Electron Trapping Effect Research Articles

High-performance near-infrared polymeric phototransistors realized by combining cross-linked polymeric semiconductors and bulk heterojunction bilayer structures

Flexible polymeric phototransistors highly responsive to near-infrared (NIR) light are crucial for the room temperature biological imaging due to some unique properties of organic semiconductors, such as light weight, low cost, and good biocompatibility. However, the low mobility and/or the high dark off-current (DOC) of low-bandgap polymeric phototransistors limit their NIR light detectability. This work proposes the development of a solution-processable bilayer heterostructure (BHS) for polymeric phototransistors, which uses cross-linked polymeric semiconductors (CLPS) as the bottom conducting channel and a bulk heterojunction (BHJ) as the photoactive layer. The BHS architecture ensures fast carrier transport and efficient exciton separation. Besides, the electron-trapping effect generated between the CLPS and BHJ results in a significantly lower DOC compared with other controlled devices. Thus, the BHS phototransistor based on low-bandgap polymer can detect NIR light with wavelengths of 808 nm, 1064 nm, and even 1550 nm, going from NIR-I (750–1000 nm) to NIR-II (1000–1700 nm). Meanwhile, its responsivity (R) is two or three orders of magnitude higher than the controlled device. Furthermore, an ultra-flexible NIR phototransistor array (~5 μm thick) using this strategy could detect image barcodes with different gray levels, demonstrating its potential application in flexible NIR imaging.

Ion front acceleration in collisional nonthermal plasma

A three fluid-model consisting of electrons, ions, and neutral atoms including source terms of ionization and recombination is used to study the ion front evolution in an intense-laser created plasma. A parametric study for the ion front profiles as a function of electron nonthermality and trapping in the presence of different source terms is performed. The numerical results show that the ion front profiles are significantly affected by the nonthermal and trapping effects. In the case of ionization alone, these effects favor the stability of the ion acceleration process interpreted from the plateau appearing in the ion front profiles, whereas the case of recombination alone shows a more important ion beam energy. On the other hand, taking into account both ionization and recombination processes in the same nonequilibrium plasma model is more adequate to obtain highly monoenergetic ion beams. In addition, the same study is performed for three different target materials, H, C, and Al. It is found that proton and carbon ion energy profiles present a good trend and the same ion front position behaviors, unlike aluminum ion which show a slower acceleration. This work is motivated to improve the understanding and predictive capability of electron nonthermality, trapping and collision effects on the ion front profiles in high-intensity laser-plasma acceleration.

Structure and photocatalytic properties of TiO2/Cu3N composite films prepared by magnetron sputtering

In this work, TiO2/Cu3N composite films were prepared by using magnetron sputtering. Results show that the optical band gap of the composite films can be effectively controlled by changing the preparation parameters, and the optical band gap of the composite films is lower than that of the TiO2 films (approximately 3.0 eV). The TiO2 in the composite films mainly appears as anatase phase, and the nano-Cu3N films prepared under the condition of r=1/4 promoted the formation of TiO2 effective electron trap on the surface of the composite films. In terms of degradation of methyl orange solution, the degradation rate of TiO2/Cu3N composite film is higher than that of pure TiO2 film, which can reach 99.2 % (using mercury lamp for 30 min). The nano-Cu3N grain inserted to TiO2 is beneficial to the conversion of Ti4+ to Ti3+, which significantly improves the photocatalytic reduction activity of the films. The TiO2/Cu3N composite films has an excellent catalytic activity and potential application prospects in wastewater treatment.

Microwave Performance of ‘Buffer-Free’ GaN-on-SiC High Electron Mobility Transistors

High performance microwave GaN-on-SiC HEMTs are demonstrated on a heterostructure without a conventional thick doped buffer. The HEMT is fabricated on a high-quality $0.25~\boldsymbol {\mu }\text{m}$ unintentional doped GaN layer grown directly on a transmorphic epitaxially grown AlN nucleation layer. This approach allows the AlN-nucleation layer to act as a back-barrier, limiting short channel effects and removing buffer leakage. The devices with the ‘buffer-free’ heterostructure show competitive DC and RF characteristics, as benchmarked against the devices made on a commercial Fe-doped epi-wafer. Peak transconductances of 500 mS/mm and a maximum saturated drain current of ~1 A/mm are obtained. An extrinsic $\text{f}_{\sf T}$ of 70 GHz and $\text{f}_{\sf max}$ of 130 GHz are achieved for transistors with a gate length of 100 nm. Pulsed-IV measurements reveal a lower current slump and a smaller knee walkout. The dynamic IV performance translates to an output power of 4.1 W/mm, as measured with active load-pull at 3 GHz. These devices suggest that the ‘buffer-free’ concept may offer an alternative route for high frequency GaN HEMTs with less electron trapping effects.

Sky-blue-emitting cationic iridium complexes with oxadiazole/triazine-type counter-anions and their use for efficient solution-processed organic light-emitting diodes

Counter-anion control has emerged as a facile approach to tune the properties of cationic iridium complexes for optoelectronic applications. Here we report sky-blue-emitting cationic iridium complexes with electron-deficient oxadiazole/triazine-type counter-anions and their use for highly efficient solution-processed organic light-emitting diodes (OLEDs). A sky-blue-emitting complex [Ir(meoppy)2(dmapzpy)]+ (meoppy is 4-methoxy-2-phenylpyridine and dmapzpy is 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine) has been developed as the phosphorescent cation. Hexafluorophosphate (PF6−), 3,5-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzenesulfonate (OXD-7-SO3−), 4-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate (TRZ-p-SO3−) and 3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate (TRZ-m-SO3−) have been developed as the counter-anions in complexes R, 1, 2 and 3, respectively. In doped films, the complexes afford similar sky-blue emission peaked at around 472 nm with high phosphorescent efficiencies of around 0.80. Solution-processed, double-layer OLEDs using the complexes as dopants afford sky-blue electroluminescence peaked at around 476 nm. The devices using complexes 1–3 show largely enhanced performances compared to the device using the reference complex R, owing to the improved carrier-recombination balance imparted by the electron-trapping effects of the oxadiazole/triazine type counter-anions. The devices based on complexes 2 and 3 exhibit higher performances than that based on complex 1, because of the stronger electron-trapping effects of TRZ-p-SO3− and TRZ-m-SO3− compared to OXD-7-SO3−. The double-layer device using complex 3 shows a peak current efficiency of 29.4 cd A−1 and an external quantum efficiency of 12.4%, which are the highest for blue OLEDs using cationic iridium complexes reported so far.

Reliability Characterization of Gallium Nitride MIS-HEMT Based Cascode Devices for Power Electronic Applications

We present a detailed study of dynamic switching instability and static reliability of a Gallium Nitride (GaN) Metal-Insulator-Semiconductor High-Electron-Mobility-Transistor (MIS-HEMT) based cascode switch under off-state (negative bias) Gate bias stress (VGS, OFF). We have investigated drain channel current (IDS, Max) collapse/degradation and turn-on and rise-time (tR) delay, on-state resistance (RDS-ON) and maximum transconductance (Gm, max) degradation and threshold voltage (VTH) shift for pulsed and prolonged off-state gate bias stress VGS, OFF. We have found that as stress voltage magnitude and stress duration increases, similarly IDS, Max and RDS-ON degradation, VTH shift and turn-on/rise time (tR) delay, and Gm, max degradation increases. In a pulsed off-state VGS, OFF stress experiment, the device instabilities and degradation with electron trapping effects are studied through two regimes of stress voltages. Under low stress, VTH shift, IDS collapse, RDS-ON degradation has very minimal changes, which is a result of a recoverable surface state trapping effect. For high-stress voltages, there is an increased and permanent VTH shift and high IDS, Max and RDS-ON degradation in pulsed VGS, Stress and increased rise-time and turn-on delay. In addition to this, a positive VTH shift and Gm, max degradation were observed in prolonged stress experiments for selected high-stress voltages, which is consistent with interface state generation. These findings provide a path to understand the failure mechanisms under room temperature and also to accelerate the developments of emerging GaN cascode technologies.

Electric-Based Thermal Characterization of GaN Technologies Affected by Trapping Effects

This article presents an electric-based methodology for thermal characterization of semiconductor technologies. It is shown that for technologies such as gallium nitride (GaN) high electron mobility transistors, which exhibit several field induced electron trapping effects, the thermal characterization has to be performed under specific conditions. The electric field is limited to low levels to avoid activation of trap states. At the same time, the dissipated power needs to be high enough to change the operating temperature of the device. The method is demonstrated on a test structure implemented as a GaN resistor with large contact separation. It is used to evaluate the thermal properties of samples with different silicon carbide suppliers and buffer thickness.

CdS nanoparticles modified Ni@NiO spheres as photocatalyst for oxygen production in water oxidation system and hydrogen production in water reduction system

The exploration of bifunctional catalysts to produce both hydrogen and oxygen will accelerate the progress of overall water-splitting. This paper introduces a facile method to fabricate bifunctional photocatalysts toward the water splitting by doping nano CdS with S vacancies on Ni@NiO nano-spheres. A serious of photocatalytic catalysts were developed and subsequently characterized, the two half-reactions of photocatalytic water splitting were studied in detail by using sacrificial agents, the hydrogen output at 41699 μmol g−1 h−1 (AQE 28.5%, λ = 420 nm) while the evolution of O2 is as high as 55120 μmol g−1 h−1 (AQE 35.7%, λ = 450 nm). It is a strategy that the presence of Ni@NiO can trigger the S vacancy in CdS to act as an effective electron trap, the p-n junction of NiO-CdS, as well as the Schottky barrier between NiO and Ni, can change the dynamic characteristics of photogenerated electrons and prolong their life. These prolonged photogenerated electrons can initiate stable and effective photocatalytic redox reactions.

Phase Control Behavior of Lateral Polymer Photodetectors Using Strong Aggregation Bulk Heterojunction Film

Back Cover: In article number 1900474 by Tao Han, Chunzhi Jiang, Xiaoyi Huang, and co-workers, a phase control of lateral polymer photodetectors with strong aggregation film is realized, with which effective electron trapping under high PCBM ratio can be obtained. Doping highly concentrated acceptors can achieve photomultiplication and introducing 1,8-diiodooctane can promote response speed.

Effects of trapped electrons on the ion temperature gradient mode in tokamak plasmas with hollow density profiles

The ion temperature gradient (ITG) mode in the presence of impurity ions and trapped electrons (TEs) is numerically investigated in tokamak plasmas with hollow density profiles, using the gyrokinetic integral eigenmode equation. It is found that in the inverted density plasma, the increase of the ITG enhances the growth rate and frequency of the ITG, and the density gradient plays an important role in the ITG modes. For weak density gradient situations, the trapped electron effects increase the instability of the ITG, while the impurity has an obviously stabilizing effect. For the strong density gradient cases, both the impurities and trapped electrons enhance the ITG instabilities. In addition, it is shown that the growth rate of the ITG decreases with positive magnetic shear s while the real frequency increases with positive magnetic shear. The growth rate of the ITG increases with negative magnetic shear s while the real frequency decreases with negative magnetic shear. The length of the calculated mode structure in the positive and negative magnetic shear intervals is also presented.

Light-Induced Formation of MoOxSy Clusters on CdS Nanorods as Cocatalyst for Enhanced Hydrogen Evolution.

Metal and metal-oxide particles are commonly photodeposited on photocatalysts by reduction and oxidation reactions, respectively, consuming charges that are generated under illumination. This study reveals that amorphous MoOxSy clusters can be easily photodeposited at the tips of CdS nanorods (NRs) by in situ photodeposition for the first time. The as-prepared MoOxSy-decorated CdS samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP) to determine the composition and the possible formation pathways of the amorphous MoOxSy clusters. The MoOxSy-tipped CdS samples exhibited better hydrogen evolution performance than pure CdS under visible-light illumination. The enhanced activity is attributed to the formation of intimate interfacial contact between CdS and the amorphous MoOxSy clusters, which facilitates the charge separation and transfer. Through time-resolved photoluminescence (TRPL) measurements, it was clearly observed that all MoOxSy-decorated CdS samples with different loadings of MoOxSy showed a faster PL decay when compared to pure CdS, resulting from the effective trapping of photogenerated electrons by the MoOxSy clusters. Kelvin probe force microscopy (KPFM) was further used to study the surface potentials of pure CdS NRs and MoOxSy-decorated CdS NRs. A higher surface potential on MoOxSy-decorated CdS NRs was observed in the dark, indicating that the loading of MoOxSy resulted in a lower surface work function compared to pure CdS NRs. This contributed to the effective electron trapping and separation, which was also reflected by the increased photoelectrochemical response. Thus, this study demonstrates the design and facile synthesis of MoOxSy-tipped CdS NRs photocatalysts for efficient solar hydrogen production.

Controlled synthesis of GaSe microbelts for high-gain photodetectors induced by the electron trapping effect

A GaSe microbelt-based photodetector has been realized with a high responsivity of ∼3866 A W−1 and a photoconductive gain of up to ∼1.06 × 104, which are attributed to the built-in electric field between the GaSe microbelt and the surface GaOx layer.

Prolonged electron lifetime in sulfur vacancy-rich ZnCdS nanocages by interstitial phosphorus doping for photocatalytic water reduction

Sulfur vacancy-rich ZnCdS nanocages with interstitial P dopant atoms were fabricated. The promoted Fermi level caused by interstitial P doping facilitates the S vacancy level to be an effective electron trapping center, thus enhancing the photocatalytic performance.

Ti3C2 MXene-modified Bi2WO6 nanoplates for efficient photodegradation of volatile organic compounds

Air pollution of volatile organic compounds (VOCs) threatens human health. Developing advanced photocatalysts for VOCs elimination thus attracts much interest. Bi2WO6 shows the merits of stability, nontoxicity and visible light absorbance but suffers from its rapid electron/hole recombination. Herein Ti3C2 MXene nanoparticles were electrostatically adsorbed on Bi2WO6 nanoplates for efficient electron/hole separation. Upon hybridization, electrons in Bi2WO6 flow to Ti3C2 Mxene as evidenced by XPS analysis, implying that Ti3C2 Mxene has impressive capability of trapping photoinduced electrons. As-prepared BT4 reveals increased photocurrent and weak photoluminescence under light irradiation owing to the electron-trapping effect of Ti3C2 Mxene. DFT simulations reveal that Ti3C2 Mxene has strong chemical adsorption for HCHO and CH3COCH3 for while Bi2WO6 shows weak physical adsorption. Such strong adsorption of Ti3C2 Mxene is attributed to the charge transfer between Ti3C2 Mxene and VOC molecules as disclosed by differential charge density and Bader charge analyses. The superoxide radical (O2−) and hydroxide radical (∙OH) participated in the photo-oxidation process. As-obtained BT4 revealed 2 times and 6.6 times higher photocatalytic activity towards the degradation of HCHO and CH3COCH3 in comparison with Bi2WO6. Such superior performance of BT4 can be ascribed to favourable adsorption of VOCs and efficient electron/hole separation of Ti3C2 MXene.

Obliquely propagating ion-acoustic solitary and shock waves in magnetized quantum degenerate multi-ions plasma in the presence of trapped electrons

The properties of obliquely propagating ion-acoustic waves have been investigated in multi-ions magnetized plasma comprising of inertial, positively and negatively charged ion fluids, trapped electrons, and negatively charged stationary heavy ions. The propagation of the waves is oblique to the ambient magnetic field which is along the z-direction. Only fast type of modes exists in the linear regime. The reductive perturbation method was adopted to derive the Korteweg– de Vries (KdV) and Burger equations, as well as the solitary and shock wave solutions of the evolved equations, have been used to analyze the properties of the small but finite amplitude waves. The effects of the constituent plasma parameters, namely, the trapping effect of electrons, the electron degenerate temperature and the viscosity coefficient on the dynamics of the small amplitude solitary and shock waves have been examined. The influence of the magnetic field and the obliquity parameter on the propagation characteristics of ion-acoustic waves are discussed.

Visible-light-responsive photocatalyst with a microsphere structure: preparation and photocatalytic performance of CQDs@BiOCl

In this study, the effects of carbon quantum dot (CQD) doping on the photocatalytic performance of semiconductor BiOCl microspheres were investigated. Highly dispersed CQDs with up-conversion luminescence properties were prepared using the hydrothermal method, and visible-light-responsive CQDs@BiOCl photocatalysts with regular morphology were prepared via CQD doping. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible (UV–Vis) spectroscopy were used to investigate the morphology and light absorption properties of the materials. The degradation rate of rhodamine B (RhB) was 76.1% after 180 min of visible-light irradiation when CQDs@BiOCl were used and only 19.4% when pure BiOCl was used. The photoluminescence (PL), UV–Vis diffuse reflectance spectra (UV–Vis DRS) and electron paramagnetic resonance (EPR) results were analyzed to determine the possible reasons for the increased photocatalytic activity of CQDs@BiOCl microspheres. The results showed CQD doping expanded the visible light absorption range, CQDs exhibited fast photoinduced electron transfer, and CQDs@BiOCl possessed high mesoporosity, which promoted the effective separation of photogenerated electron–hole pairs. In addition, the microsphere structure of CQDs@BiOCl exhibited a larger specific surface area and a more regular morphology than its sheet-like structure. These features increased the number of photocatalytic reaction sites and the surface adsorption of the catalyst. In addition, the electronic conjugated structure of CQDs was demonstrated to function as an effective electron trap. CQD doping effectively inhibited the photogenerated electron–hole pair recombination of the composite photocatalyst, which enhanced the photocatalytic performance of the system.

Perovskite-WS2 Nanosheet Composite Optical Absorbers on Graphene as High-Performance Phototransistors.

High-responsivity phototransistors with a structure of perovskite-WS2 nanosheet composite optical absorber and a reduced graphene oxide (rGO) channel layer is demonstrated via a facile and low-cost solution-processing method. The WS2 nanosheets are dispersed within the perovskite matrix, forming the perovskite-WS2 bulk heterojunction (BHJ). The hybrid phototransistor exhibits excellent figures of merit including high photoresponsivity of 678.8 A/W, high specific detectivity of 4.99 × 1011 Jones, high EQE value of 2.04 × 105% and rapid response to photoswitching. The high photoresponsivity could be attributed to the WS2 nanosheets induced photo-generated electron-hole separation promotion effects due to the selective electron trapping effects in the WS2 nanosheets, together with the high carrier mobility of the rGO channel. This work provides a promising platform for constructing high-responsivity photodetectors.

Role of hole trapping in the unintentionally doped GaN layer in suppressing the two-dimensional electron gas degradation in AlGaN/GaN heterostructures on Si

We identify the spatially resolved trapping mechanism and clarify the role of the unintentionally doped (UID) GaN layer in suppressing the two-dimensional electron gas (2DEG) degradation in AlGaN/GaN heterostructures on Si. The trapping mechanism is characterized by measuring C–V dispersion after three different configurations of bias stress: high drain-substrate voltage stress, high drain-gate voltage stress and combined stress (with both high drain-gate voltage and drain-substrate voltage stress). Under the combined stress, the 2DEG degradation is the overall effect of electron trapping and hole trapping. By comparing samples with and without the UID GaN layer, we confirm the role of the UID layer in suppressing the 2DEG degradation by hole trapping in that layer. The electron and hole trap states are further identified by reversed vertical stress and current transient measurements. The electron trap with an activation energy of 0.53 eV and the hole trap with an activation energy of 0.81 eV are distinguished.

A Highly Sensitive Perovskite/Organic Semiconductor Heterojunction Phototransistor and Its Device Optimization Utilizing the Selective Electron Trapping Effect

AbstractHybrid organic–inorganic perovskite materials have recently attracted attention due to their impressive physical properties and promising application in future optoelectronic devices and systems. In this study, a high‐performance and broadband photodetector comprising vertically stacked Cs‐doped formanidinium lead iodide (FAPbI3) perovskite/dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene heterojunction, which shows a high responsivity of 778 A W−1 and a specific detectivity of 1.04 × 1013 Jones, is reported. In addition, the photodetector displays excellent stability and broadband responsivity to illumination ranging from deep‐ultraviolet to near‐infrared light. It is interesting to note that doping a small amount of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) into the perovskite film results in a substantial increase in responsivity, specific detectivity, and photoconductive gain. Specifically, the specific detectivity is as high as 7.96 × 1013 Jones, which is much higher than that of other devices with similar geometries. The device optimization can be ascribed to an enhanced electron–hole separation ability and increase in electron accepting sites that can selectively trap electrons in the perovskite–PCBM bulk heterojunction, according to results from the experimental analyses.

Effects of electron trapping on nonlinear electron-acoustic waves excited by an electron beam via particle-in-cell simulations

By performing one-dimensional particle-in-cell simulations, the nonlinear effects of electron-acoustic (EA) waves are investigated in a multispecies plasma, whose constituents are hot electrons, cold electrons, and beam electrons with immobile neutralized positive ions. Numerical analyses have identified that EA waves with a sufficiently large amplitude tend to trap cold electrons. Because EA waves are dispersive, where the wave modes with different wavenumbers have different phase velocities, the trapping may lead to the mixing of cold electrons. The cold electrons finally get thermalized or heated. The investigation also shows that the excited EA waves give rise to a broad range of wave frequencies, which may be helpful for understanding the broadband-electrostatic-noise spectrum in the Earth’s auroral region.