Carrier Confinement Effects Research Articles

Quantum‐Dots Matrix Encapsulated CsPbI3 Polycrystal Composite Films for Efficient and Bright Deep‐Red Light‐Emitting Diodes

AbstractAll‐inorganic CsPbI3 perovskite is a promising emitter for deep‐red light‐emitting diodes (LEDs). However, presently fabricated CsPbI3 polycrystalline films are composed of island‐like polycrystals, encountering the problems of serious interface current leakage and low‐efficiency carrier radiative recombination. Here, a CsPbI3‐xBrx quantum dots (QDs) matrix encapsulated CsPbI3 polycrystal film is reported to address the low‐efficiency issue of island‐like CsPbI3 polycrystalline film applied in deep‐red LEDs. The developed QDs matrix encapsulation strategy has two benefits. One is the filling of void space in the island‐like CsPbI3 polycrystal film to suppress the interface current leakage. The other more important benefit is utilizing the strong carrier confinement effect of QDs to strengthen the formation of excitons in the composite film and thus improve the electroluminescence efficiency. Moreover, interesting grain growth is found between CsPbI3‐xBrx QDs and CsPbI3 polycrystals, which further enhances the exciton transfer effect brought by the QDs matrix and optoelectronic properties of the fabricated composite films. Based on the obtained high‐quality QDs/polycrystal composite films, efficient and bright deep‐red LEDs are achieved with a peak external quantum efficiency of 15.24% and a maximum luminance of 3691 cd m−2.

Heterointerface Effects on Carrier Dynamics in Colloidal Quantum Dots and Their Application to Light-Emitting Diodes.

Colloidal quantum dots (QDs) are promising candidates for next-generation display technology because of their unique optical properties and have already appeared in the market as a high-end product. On the basis of their extraordinary properties, QD emissions with a given chemical composition can be tailored in a wide spectral window due to quantum size effects, which constitutes a key advantage of QDs in the display field. Specifically, investigations of structure-dependent and composition-dependent characterizations outside the quantum confinement effect have become an important part of practical applications. Therefore, from the perspective of designing nanostructures with well-defined heterointerfaces, strong quantum confinement effects with effective carrier confinement are desirable. Our results show that the photoluminescence (PL) intensity of CdSe/CdZnS core-shell QDs was enhanced 5.7 times compared with that of the CdSe core QDs. Supplementary analytical techniques involving transmission electron microscopy revealed the heterointerface configuration and composition distribution of the core and shell materials. The effects of the heterointerface on carrier dynamics in core-shell QDs were revealed by monitoring wavelength-dependent time-resolved PL. To further develop the QD light-emitting diodes (QD-LEDs), we produced an all-solution processed inverted QD-LEDs using CdSe/CdZnS core-shell QDs as the emitter. The electroluminescence spectrum of deep-red emissive QD-LEDs with CIE chromaticity coordinates of (0.68, 0.32) exhibited a peak at 638 nm.

Optical tuning of the terahertz response of black phosphorus quantum dots: effects of weak carrier confinement

Abstract In contrast to few-layer black phosphorus (BP) with a relatively larger area, BP quantum dots (BP-QDs) are expected to have distinctive electromagnetic response and carrier behaviors, especially in low-frequency range such as in the THz regime. Herein, we experimentally investigate the THz properties of BP-QDs as well as the optical control of these properties. It is demonstrated that the effects of weak carrier confinement, which is associated with diffusive restoring current in each BP-QD, contribute significantly to the effective THz conductivity of BP-QDs. Instead, spectral features of discretely spaced energy levels as shown for many kinds of semiconductor QDs in UV-visible range are not observed in the THz regime. This indicates an insignificant contribution of strong quantum confinement here. Based on the modified Drude–Smith formula, we show that the optical excitation/pump of a CW laser can induce photogenerated carriers and enhance the effects of weak carrier confinement in BP-QDs. Thus, a nonlinear enhancement of THz absorption can be observed by increasing the power of the excitation laser. These results not only deepen our understanding of the fundamental physics of BP nanomaterials but also provide an alternative approach to realize active control of BP-based THz devices.

Carrier confinement for mobility enhancement in dual-gate organic thin-film transistors

Mobility of charge carriers in organic thin-film transistors (OTFTs) is one of the most important parameters, which is affected by the disorders in the semiconducting films. In general, realization of higher carrier concentration facilitates improved charge transport against these disorders. Here, we demonstrate in amorphous indacenodithiophen-co-benzothiadia (IDT-BT) based dual-gate organic thin-film transistors (DG-OTFTs) that the mobility can be robustly increased from 0.8 to 1.37 cm2 V−1 s−1, when voltage biases of opposite signs (negative one accumulate holes, positive one deplete holes) are applied on the dual independent gate electrodes. In details, the positive bias can confine the spatial distribution of the accumulated carriers near the negatively biased gate, leading to a higher effective carrier concentration with higher mobility. In addition, the interface confinement effect of carriers was further verified by two-dimensional numerical simulation, and calculations based on the variable range hopping method were performed to accurately reproduce the experimentally observed charge transport behavior in DG-OTFTs.

Near infrared photoluminescence of Si1–xGex quantum dots fabricated by double hot Ge+/Si+ implantation into SiO2 layer

In this study, we developed very simple and ULSI (ultra large scale integration) compatible fabrication processes for group-IV (Si1–xGex and Si) semiconductor quantum dots (QDs) to apply hybrid ULSIs with photonic and electron devices, using double Ge+/Si+ hot-ion implantation into a SiO2 layer with larger bandgap EG and the post-furnace annealing. We successfully demonstrated the near-infrared (IR) photoluminescence (PL) from Si1–xGex-QDs. Transmission electron microscopy observations of single-crystallized Si1–xGex-QDs revealed that the diameter and the QD density were 3.6 ± 0.9 nm and (2.6 ± 0.4) × 1012 cm−2, respectively. In addition, Ge atoms were detected in the Si1–xGex-QDs by the energy dispersive x-ray spectroscopy analysis, and the Ge fraction of Si1–xGex-QDs was varied from 0.06 to 0.26 by changing the Ge ion dose. The increase in the PL intensity by forming gas annealing was attributed to the dangling-bond reduction by the H-atom termination method. The PL spectrum of Si1–xGex-QDs was fitted by PL components of two QD structures containing Si1–xGex and Si materials. The PL intensity and PL-peak photon energy of Si1–xGex-QDs strongly depended on the Ge fraction. The Si1–xGex-QDs achieved the maximum PL intensity at x ≈ 0.13. High PL-peak photon energy (∼1.31 eV) of Si1–xGex-QDs is attributed to the quantum confinement effect of carriers in QDs. Consequently, group-IV semiconductor QDs including Si1–xGex, Si, SiC, and C, through the simple hot-ion implantation into the SiO2 layer, exhibited a wide range of PL emissions from the near-IR to ultraviolet regions.

Water-induced construction of Cs4PbBr6/CsPbBr3 heterojunction for efficient perovskite light-emitting diode

Perovskite Cs4PbBr6/CsPbBr3 heterojunction with a strongly carrier-confined effect is emerging as a promising luminescent material for perovskite light-emitting diodes (PeLEDs). However, it is a big challenge to construct the heterojunction on a molecular scale to fully excavate this confinement effect for efficient electroluminescence efficiencies of PeLEDs. Herein, we report a water-induced construction of Cs4PbBr6/CsPbBr3 heterojunction for efficient PeLEDs via adding trace water with strong polarity. Benefiting from its strong carrier confinement effect, this heterojunction has a higher exciton binding energy (94.7 meV) and a lower nonradiative decay rate constant (0.007 ns−1) compared to that of single CsPbBr3 nanocrystals (NCs) (14.1 meV and 0.031 ns−1). Based on it, the as-fabricated PeLEDs exhibit a greatly-improved maximum current efficiency of 43.62 cd A−1 and a peak EQE of 11.31%, approximately 1.5 times that of CsPbBr3 NCs (28.43 cd A−1 and 7.78%). Evidently, this work provides an important way toward the construction of Cs4PbBr6/CsPbBr3 heterojunction and then promotes the commercial development of heterojunction-like optoelectronics.

Band offset measurement at the MAPbBr3/Al2O3 heterointerface by X-ray photoelectron spectroscopy

Recently, Al2O3 has been demonstrated to be an effective material type for perovskite photoelectric device performance improvement. It is therefore essential to investigate the interfacial electronic characteristics and the energy-level diagrams of perovskite/Al2O3 heterojunctions in depth. In this research, we fabricated an MAPbBr3 microcrystal/Al2O3 film heterojunction and determined the band offset by performing X-ray photoelectron spectroscopy measurements. The resulting valence band offset of ΔEv = 0.87 ± 0.01 eV and the conduction band offset of ΔEc = 3.03 ± 0.01 eV indicated occurrence of a type-I band alignment at the MAPbBr3/Al2O3 heterointerface. The carrier confinement effect was reflected in the photoluminescence spectra obtained for the heterojunction sample. This work will be valuable for optimization of the energy level alignment of perovskite/metallic oxide heterojunctions during the design of photoelectric devices.

Determinants Affecting the Performance of CZTSSe: Antisite Defects and Multiple Quantum Confinement for Photon-Sensitive Devices

For its earth availability and non-toxicity, kesterite (CZTS, CZTSe and CZTSSe) material has gained much importance in photon-sensitive(PS) applications. Additionally, effect of multiple carrier confinement adds the performance of the devices.The high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\textit {oc}} $ </tex-math></inline-formula> deficit in chalcogenide materials is caused by structural antisite defects. The use of carrier confinement using Quantum Wells with CZTSSe and CZTS as well and barrier material has been investigated in this study. The defect distribution on the CZTS material and the effects of creating more potential wells with variations in the (S,Se) composition of CZTSSe is explored. The analsysis has a substantial influence on the electrical and optical properties of the absorber layer. The quantum efficiency is also calculated in order to observe sensitivity of device w.r.t solar spectrum. A comparative analysis is performed between four major aspects: first, the performance of with and without defect structures, second, the compositional change of S,Se in CZTSSe material, third, the growth in the number of QWs, and finally, the variation in defect density. According to the findings of the study, recombination in the barrier region has a significant impact in the performance of QW structures, particularly higher QW structures. According to the acquired results, at a defect density on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> , a range of nearly 16 to 18% efficiency has been achieved for the 70QW structure for various CZTSSe compositions.

Improved Performance of InGaN/AlGaN Multiple‐Quantum‐Well Near‐UV Light‐Emitting Diodes with Convex Barriers and Staggered Wells

The physical mechanism of improving the photoelectric performance of InGaN/AlGaN‐based near UV light‐emitting diode (LED) with convex quantum barrier and staggered quantum well (QW) is studied by numerical simulation. The simulation results indicate that the voltage–current characteristics of the LED structure with convex quantum barrier and staggered QW are effectively improved compared with the traditional multiple quantum well (MQW) structure, and its electroluminescence (EL) intensity and light output power are significantly improved. The main physical mechanisms are: on the one hand, the convex quantum barrier with a lower average Al component can reduce the polarization electric field at the interface between the quantum barrier and QW as well as the effective potential barrier of holes, improve the spatial separation of electron and hole wave functions, promote the injection efficiency of carriers, and improve the uniformity of carrier distribution in the MQWs active region; On the other hand, staggered QWs can provide stronger carrier confinement effect and further increase the overlap of electron and hole wave functions, so as to improve the carrier radiative recombination efficiency; In a word, this work provides a valuable reference for obtaining high‐performance near‐ultraviolet LED.

Subsiding strain-induced In-Ga intermixing in InAs/InxGa1−xAs sub-monolayer quantum dots for room temperature photodetectors

Strain – induced intermixing in sub – monolayer (SML) quantum dots (QDs) affects primarily the dot size distribution in an erratic way and also the inter – dot coupling efficiency for carrier transport. In this study, we have explored the role of strain and carrier confinement effects in order to control the dot size distribution by varying InAs QDs coverage (X: 0.3 and 0.5 ML) and stacks (Y: 4, 6, and 8) simultaneously. The size variation affects the position of localized levels inside QDs primarily as it is the inter – dot coupling that decides the luminescence efficiency. Next, through multiple stacking of QD layers, strain variation becomes inhomogenous that facilitate a higher degree of In – Ga intermixing which necessitates to estimate the amount of Indium (%) present inside such In–rich islands to progress for a superior optical performance. Hence, we combine and address these above concerns in great detail. The 20 K ground-state (GS) photoluminescence (PL) energy for 0.3 and 0.5 ML samples were centered at 1.24–1.33 eV, and 1.11–1.19 eV respectively. The PL linewidth and activation energy variation with temperature validated that there exists two possible thermal escape pathways for carrier recombination. Besides, PL – excitation (PLE) studies comprehended the influence of phonon and excited states (ES) in the samples. Raman analysis helped us calculate the In% inside QDs by figuring out the shift in InAs QD phonon modes (amount of compressive strain). Furthermore, the structural analysis by high-resolution X-ray diffraction (HRXRD) was done to calculate the strain moderated defect density and also the effect of in – plane compressive strain to the reduction in quantum confinement. The increase in compressive strain inside QDs caused more In–Ga intermixing in 0.3 ML samples and higher defect densities. Altogether we see that sample with 0.5 ML, 4 stacks showed vertically-correlated dot growth (from HR-XTEM) with minimum strain (−0.03383 a.u.), dislocation density (4.887 x 1011 cm−2) and higher In content (62.41%). This high degree of excitonic carrier confinement and strain tuning makes SML QDs an attractive candidate for room-temperature (RT) operable photodetector (PD) devices.

Fast and effective catalytic degradation of an organic dye by eco-friendly capped ZnS and Mn-doped ZnS nanocrystals.

Undoped and manganese doped ZnS nanocrystals encapsulated with thioglycolic acid(ZnS-TGA) were synthesized and characterized with different techniques, and finally tested in the photodegradation of a methyl orange in aqueous solution under UV and sunlight irradiations. FTIR and X-ray diffraction results confirmed the functionalization of these nanocrystal surface by thioglycolic acid and the formation of crystalline structures of ZnS and Mn-doped ZnS with cubic and hexagonal phases. Calculated average size of ZnS nanocrystals was in the range of 2-3nm. It was observed a blue shift of the absorbance threshold and the estimated bandgap energies were higher than that of Bulk ZnS thus confirming the quantum confinement effect of charge carriers. Photoluminescence spectra of ZnS nanocrystals exhibited emission in the range of 410-490nm and the appearance of an additional emission band around 580nm (2.13eV) connected to the 4T1 → 6A1 transition of the Mn2+ions. Photodegradation of methylene orange with undoped and Mn-doped ZnS-TGA nanocrystals was investigated. Dye adsorption prior to photocatalysis using nanocrystals was studied via kinetic and equilibrium experiments. The maximum dye adsorption capacity on doped ZnS-TGA was ~ 26.98mg/g. The adsorption kinetic was found to follow the pseudo-second-order kinetic model. A statistical physics model was used to analyze the equilibrium data where the calculated adsorption energy was 17-18kJ/mol. It was concluded that the dye adsorption was associated to the hydrogen interaction where the removal process was feasible and multi-molecular at 25°C. The photocatalytic activity of undoped ZnS nanoparticles under UV irradiation showed better efficiency than doped nanocrystals thus indicating that manganese doping generated a dropping of the photocatalytic degradation of the dye. Dye degradation efficiency of 81.37% using ZnS-TGA nanocrystals was achieved after 6min, which indicated that ZnMnS-TGA nanocrystals may be considered an alternative low cost and environmental friendly material for facing water pollution caused by organic compounds via photodegradation processes.

(Invited) Fabrication of Ge-on-Insulator By Epitaxial Growth and Ion-Implanted Exfoliation for Electronics and Optoelectronics Applications

1. IntroductionGe has been received much interest as a CMOS channel material and a near-infrared optical material due to its superior characteristics [1,2]. For application use, Ge-on-Insulator (GOI) structure is necessary to suppress large leakage current originated from the narrow bandgap. A method combined wafer bonding and layer splitting by hydrogen ion (H+) implantation, known as Smart-CutTM and realized for Si-on-Insulator fabrication, has been tried to apply for GOI with large diameter, uniform thickness, and single crystal [3-5]. In this study, we fabricated GOI by Smart-CutTM technique and demonstrated electronic and optoelectronic devices on the GOI. Besides, we combined the epitaxial growth method to improve the quality of GOI.2. Experimental, Results, and DiscussionsGOI fabrication procedure by conventional Smar-CutTM method is shown in Fig. 1 [5]. H+ implantation with the dose of 4×1016 cm-2 and energy of 120 keV into Ge was performed through 100 nm-thick SiO2. After SiO2 removal, 3 nm-Al2O3 and 50 nm-SiO2 were formed on the Ge and Si substrates, respectively. Then, the Al2O3/Ge and the SiO2/Si were manually bonded. After annealing in N2 ambient at 300 °C to enhance bonding strength [6], layer splitting was carried out by annealing at 400 °C. GOI thinning by wet etching and surface flatten by chemical mechanical polishing (CMP) were performed. We fabricated both p- and n-MOSFETs with metal source/drain (S/D) on the GOI. The p-MOSFET is an accumulation mode device with ohmic PtGe/Ge S/D. On the other hand, n-MOSFET is an inversion mode device with rectifying TiN/Ge S/D. Both devices work successfully as shown in Fig. 2. We also demonstrate asymmetric metal/Ge/metal photodiode on the GOI. PtGe and TiN are also used as electrodes for hole and electron injection, respectively. Figure 3 shows electroluminescence (EL) spectra for the photodiodes fabricated on the GOI and the bulk Ge. The GOI device shows higher EL intensity than that of bulk Ge. It is considered carrier confinement effect by thin Ge crystal.However, the MOSFETs show a large leakage current at the high drain voltage region. Also, the photodiode on GOI has a broader spectra peak. Figure 4 is the cross-sectional TEM image of the GOI before GOI thinning. There are many defects as black dots near both the split surface and the buried oxide (BOX) interface. The defects near the BOX are serious because this region will be kept after GOI thinning, and cause the degradation of device characteristics (leakage current and broad PL peak). They may be introduced by H+ implantation.To avoid the introduction of the defect during GOI fabrication, we propose an alternative method which is epitaxial growth of a new Ge layer on H+ implanted surface, as shown in Fig. 5. In this method, all defective layer was removed after layer splitting and only epi-Ge layer remains on the handle Si. Figure 5 also shows electron concentrations and mobilities of the GOIs evaluated by Hall effect. The conduction type of the GOI by conventional Smart-CutTM changes from n-type to p-type due to defect introduction. On the other hand, the proposed method successfully keeps the original conduction type (n-type). Although electrical characteristics should be improved more, it is clarified that defect elimination before bonding is important for GOI fabrication by Smart-CutTM.3. ConclusionWe fabricated GOI by Smart-CutTM and demonstrated MOSFETs and photodiode on the GOI. Although both devices successfully work, TEM analysis clarified there are lots of defects near the GOI/BOX interface. Alternative way combined epitaxial growth to eliminate the defects succeeded to improve the electrical characteristics of the GOI.AcknowledgmentThis work is supported by JSPS KAKENHI (No. 19K15028 and 19H05616) and the Research Institute of Electrical Communication, Tohoku University.

Improvement of Radiative Recombination Rate and Efficiency Droop of InGaN Light Emitting Diodes with In‐Component‐Graded InGaN Barrier

The influences of In‐component‐graded InGaN barrier on the performances of InGaN light emitting diodes (LEDs) are studied. The results show that LEDs with In‐component linearly graded InGaN barrier exhibit better performances of radiative recombination rate and efficiency droop than reference LED with constant‐In‐component InGaN barrier. At 150 mA, the external quantum efficiencies for LED A with constant‐In‐component InGaN barrier, LED B with the In‐component linearly increasing in the InGaN barrier, and LED C with the In‐component linearly decreasing in the InGaN barrier are 30.30%, 36.66%, and 34.03%, respectively. Among the three sets of LED samples, LED B, with the In‐component linearly increasing in the InGaN barrier, has the highest radiative recombination rate due to suppression of quantum confinement stark effect and enhancement of carrier confinement effect in the active layer. The efficiency droop for LED A, LED B, and LED C is 34.83%, 25.67%, and 16.67%, respectively. Analysis indicates that the efficiency droop is mainly determined by the hole injection efficiency and electron leakage from the active layer. LED C with the In‐component linearly decreasing in the InGaN barrier has the lowest efficiency droop due to improved hole injection efficiency and reduced electron leakage.

Germanium Nanocrystal Properties from Photoluminescence

Morphological information has been obtained using the strong near-infrared photoluminescence emitted by germanium (Ge) nanocrystals (NCs) coherently imbedded in SiGe alloy layers, grown by molecular beam epitaxy on Si substrates. The emission spectra are analyzed for the effects of strain, carrier confinement, and disorder over a wide range of Ge concentrations in the surrounding SiGe medium. This analysis provided significant insight into the properties of the Ge nanocrystals, including their size and shape. We also discuss the mechanisms leading to the high quantum efficiency observed for emission from the Ge nanocrystals at low temperatures. We indicate how direct gap behavior might be achieved for Ge NCs lattice matched within dilute Ge1-ySny alloys, where tensile strain would be present in the NCs in all three directions.

Improved carrier confinement and distribution in InGaN light-emitting diodes with three-layer staggered QWs

Compared with conventional InGaN Quantum Wells (QWs), staggered InGaN QWs offer improved optical and electronic properties. This work studied the carrier concentration, band structure, overlap of hole and electron wave functions, and polarization field of three-layer staggered QWs in the blue spectral region and analyzed them in detail theoretically to explore the source and the dominant mechanism for improvement. Although theoretical studies indicate that the polarization field in QWs of staggered InGaN QWs is larger, the carrier confinement effect is stronger, and the carrier distribution is more uniform. Therefore, three-layer staggered QWs can improve overlapping of the hole and electron wave functions and then enhance the recombination rate so as to increase the optical output power and electroluminescence intensity. Moreover, the performance of the staggered structure C with the lowest indium content at the center of the well is better than that of the step-staggered structure B.

Achieving a Low-Voltage, High-Mobility IGZO Transistor through an ALD-Derived Bilayer Channel and a Hafnia-Based Gate Dielectric Stack

Ultrahigh-resolution displays for augmented reality (AR) and virtual reality (VR) applications require a novel architecture and process. Atomic-layer deposition (ALD) enables the facile fabrication of indium-gallium zinc oxide (IGZO) thin-film transistors (TFTs) on a substrate with a nonplanar surface due to its excellent step coverage and accurate thickness control. Here, we report all-ALD-derived TFTs using IGZO and HfO2 as the channel layer and gate insulator, respectively. A bilayer IGZO channel structure consisting of a 10 nm base layer (In0.52Ga0.29Zn0.19O) with good stability and a 3 nm boost layer (In0.82Ga0.08Zn0.10O) with extremely high mobility was designed based on a cation combinatorial study of the ALD-derived IGZO system. Reducing the thickness of the HfO2 dielectric film by the ALD process offers high areal capacitance in field-effect transistors, which allows low-voltage drivability and enhanced carrier transport. The intrinsic inferior stability of the HfO2 gate insulator was effectively mitigated by the insertion of an ALD-derived 4 nm Al2O3 interfacial layer between HfO2 and the IGZO film. The optimized bilayer IGZO TFTs with HfO2-based gate insulators exhibited excellent performances with a high field-effect mobility of 74.0 ± 0.91 cm2/(V s), a low subthreshold swing of 0.13 ± 0.01 V/dec, a threshold voltage of 0.20 ± 0.24 V, and an ION/OFF of ∼3.2 × 108 in a low-operation-voltage (≤2 V) range. This promising result was due to the synergic effects of a bilayer IGZO channel and HfO2-based gate insulator with a high permittivity, which were mainly attributed to the effective carrier confinement in the boost layer with high mobility, low free carrier density of the base layer with a low VO concentration, and HfO2-induced high effective capacitance.

(Invited) Heteroepitaxy and Strain Engineering of Germanium-Silicon-Tin Ternary Alloy Semiconductor Thin Films for Energy Band Design

Ternary alloy semiconductor of germanium-silicon-tin (GeSiSn) has attracted for electronic and optoelectronic applications, because this group-IV alloy material provides energy band engineering independent on the lattice constant like group III-V compound semiconductors [1]. In addition, GeSiSn/Ge(Sn) heterostructure realizes type-I energy band alignment with a sufficiently large band offset at both conduction and valence band edges with no or very small lattice mismatching [2,3], and that is preferable for effective carrier confinement for light emitting devices and high mobility transistors. On the other hand, there are still challenges for crystal growth with strain and band engineering of group-IV ternary alloy semiconductor thin films. GeSn is one of promising candidates for optoelectronic applications since it realizes direct transition group-IV semiconductor. GeSn with an in-plane lattice constant larger than Ge is better to enhance the emission efficiency, as the conduction band edge of the gamma-valley lowers than that of the L-valley with large lattice constant and/or in-plane tensile strain. Thus, a buffer layer such as GeSiSn with a lattice constant sufficiently larger than Ge is required to obtain direct transition GeSn thin films, and lattice-constant engineering technology of buffer and GeSn layers should be developed with well-controlled strain relaxation technique. Recently, we explored effective strain engineering technique of GeSiSn ternary alloy epitaxial layers on Ge and Si substrate to obtain much larger in-plane lattice constant. We investigated the effect of low-temperature growth of Ge buffer layer on the strain relaxation of GeSiSn layer [4,5]. Also, we examined the ion implantation technique into substrate to enhance the strain relaxation of GeSiSn epitaxial layers grown on Ge substrate [6]. We can achieve GeSiSn epitaxial layers with the enhancement of strain relaxation and the lattice constant larger than Ge, although the crystalline quality of these epitaxial layers should be still improved. We previously reported that the strain direction as compressive and tensile ones is significantly influence of GeSiSn epitaxial layers [7]. The control of strain structure and its relaxation process is keys to obtain high crystalline quality of GeSiSn and GeSn heteroepitaxial layers for improving the electronic and optoelectronic properties of group-IV heterostructures.

Gold nanoparticles supported on mesostructured oxides for the enhanced catalytic reduction of 4-nitrophenol in water

In this work, Au nanoparticles supported on aluminum oxide (Au/ANT) and titanate (Au/TNT) nanotubes were synthesized for their use as catalysts in the reduction of 4-nitrophenol to produce 4-aminophenol with NaBH4 as the reducing agent. The catalysts were prepared with a 0.5 % metal loading employing the nanotube supports modified with 3-aminopropyl-trimethoxysilane (APTMS) to provide plentiful anchoring sites to trap the Au nanoparticles and prevent their agglomeration. All materials were characterized using a range of analytical techniques, and it was found that Au zero-valent nanoparticles were homogenously supported on the inner/outer surfaces of the nanotubular-structured carriers. Both catalytic systems were highly active and selective in the reduction of 4-nitrophenol, giving TOF values of 20,561 and 19,560 h−1 for Au/TNT and Au/ANT, respectively. The excellent catalytic activity was attributed to the highly dispersed Au clusters on the support surfaces through enhanced functionalization with APTMS, and the confinement effect of the nanotubular carriers. Furthermore, Au/TNT exhibited a high operational stability for the reduction of 4-nitrophenol reaching 10 catalytic cycles with only a moderate decrease in the conversion level after the seventh cycle, which was attributed to a degree of metal leaching. Finally, the catalytic reduction performance of the Au/TNT catalyst was tested in different nitroarene-substituted pharmaceuticals, and revealed a high activity (>99 % after 60 min of reaction) and selectivity toward production of the desired substituted anilines, thereby highlighting the potential of this catalyst for application in these processes.

Effect of low-temperature GaN cap layer thickness on the optoelectronic performance of InGaN green LEDs with V-shape pits

The low-temperature GaN cap layer was designed after quantum wells for the V-pits contained InGaN-based green light-emitting diodes (LEDs). It is found that the LEDs’ performances are greatly improved with a thicker cap layer. The indium distribution in InGaN/GaN multiple quantum wells (MQWs) becomes more homogeneous. The quality of interface between the quantum well and quantum barrier is promoted, and the carrier confinement effect is enhanced. Thereby the external quantum efficiency (EQE) increases at high current density. However, the sample with a relative thicker cap layer exhibits the lower EQE at low current density, which is attributed to there being more defects at the upper interface of QWs, and the increased probability of carriers meeting at these non-radiative recombination centers. Meanwhile, only the major emission peak emitted from c-plane MQWs can be observed in the electroluminescence spectrum of 100 K, and other emission peaks are hardly visible, which indicates that the sample with the thicker cap has better carrier transport performance.

Analysis of Electrical Characteristics of InAlGaN/GaN-Based High Electron Mobility Transistors with AlGaN Back Barriers.

In this study, the effect of an AlGaN back-barrier on the electrical characteristics of InAlGaN/GaN high electron mobility transistors (HEMTs) was investigated. The dependence of the thickness and the Al composition of the AlGaN back-barrier on the off-state current (Ioff) of the devices was investigated. An InAlGaN/GaN HEMT with an Al0.1GaN back-barrier of thickness 20 nm exhibited lower Ioff because of the carrier confinement effect, which was caused by the back-barrier. The carrier confinement effect also improved the maximum output current density and the transconductance (gm). Thus, the obtained cut-off frequency (fT) and maximum oscillation frequency (fmax) values for the InAlGaN/GaN HEMT with the 20 nm thick AlGaN back-barrier were 2.6% and 13% higher than those without the AlGaN back-barrier. In addition, the impact of the buffer trap density and GaN channel thickness were evaluated. In the case of a thickness of 20 nm for the Al0.1GaN back-barrier, a low Ioff was maintained although the trap density in the buffer layer was changed. In addition, as the gate length (LGa) decreased to 50 nm, the InAlGaN/GaN HEMT with the 20 nm thick Al0.1GaN back-barrier achieved better Ioff characteristics, lower drain-induced barrier lowering (DIBL) of 85.8 mV/V, and subthreshold swing (S) of 269 mV/dec owing to a reduction in the short-channel effect.