Unambiguously Enhanced Ultraviolet Luminescence of AlGaN Wavy Quantum Well Structures Grown on Large Misoriented Sapphire Substrate

High‐quality epitaxy consisting of Al_1?xGa_xN/Al_1?yGayN multiple quantum wells (MQWs) with sharp interfaces and emitting at ≈280 nm is successfully grown on sapphire with a misorientation angle as large as 4°. Wavy MQWs are observed due to step bunching formed at the step edges. A thicker QW width accompanied by a greater accumulation of gallium near the macrostep edge than that on the flat‐terrace is observed on 4° misoriented sapphire, leading to the generation of potential minima with respect to their neighboring QWs. Consequently, a significantly enhanced photoluminescence intensity (at least ten times higher), improved internal quantum efficiency (six times higher at low excitation laser power), and a much longer carrier lifetime are achieved. Importantly, the wafer‐level output‐power of the ultraviolet light emitting diodes on 4° misoriented substrate is nearly increased by 2–3 times. This gain is attributed to the introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation at the step‐bunched region thus forming a lateral potential well for carrier localization. The experimental results are further confirmed by a numerical modeling in which a 3D carrier confinement mechanism is proposed. Herein, the compositional modulation in active region arising from the substrate misorientation provides a promising approach in the pursuit of high‐efficient ultraviolet emitters.     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

Comparative research on the influence of varied Al component on the active layer of AlGaN photocathode

To theoretically research the influence of a varied Al component on the active layer of AlGaN photocathodes, the first principle based on density functional theory is used to calculate the formation energy and band structure of Al_xGa_1-xN with x at 0, 0.125, 0.25, 0.325, and 0.5. The calculation results show that the formation energy declines along with the Al component rise, while the band gap is increasing with Al component increasing. Al_xGa_1-x with x at 0, 0.125, 0.25, 0.325, and 0.5 are direct band gap semiconductors, and their absorption coefficient curves have the same variation tendency. For further study, we designed two kinds of reflection-mode AlGaN photocathode samples. Sample 1 has an Al_xGa_1-x active layer with varied Al component ranging from 0.5 to 0 and decreasing from the bulk to the surface, while sample 2 has an Al_xGa_1-x active layer with the fixed Al component of 0.25. Using the multi-information measurement system, we measured the spectral response of the activated samples at room temperature. Their photocathode parameters were obtained by fitting quantum efficiency curves. Results show that sample 1 has a better spectral response than sample 2 at the range of short-wavelength. This work provides a reference for the structure design of the AlGaN photocathode.     

Over 30% External Quantum Efficiency Light‐Emitting Diodes by Engineering Quantum Dot‐Assisted Energy Level Match for Hole Transport Layer

In the study of hybrid quantum dot light‐emitting diodes (QLEDs), even for state‐of‐the‐art achievement, there still exists a long‐standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn_1?x Cd_xSe/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn_1?xCd_xSe‐ based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m^?2, very low efficiency roll‐off at high current density (EQE ≈25% @ current density of 150 mA cm^?2), and operational lifetime extended to ≈1 800 000 h at 100 cd m^?2. These extraordinary performances make this work the best among all solution‐processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2‐ethylhexane‐1‐thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.     

The Role of Higher Lying Electronic States in Charge Photogeneration in Organic Solar Cells

The role of excess photon energy on charge generation efficiency in bulk heterojunction solar cells is still an open issue for the organic photovoltaic community. Here, the spectral dependence of the internal quantum efficiency (IQE) for a poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b]­dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)]:6,6‐phenyl‐C₆₁‐butyric acid methyl ester (PCPDTBT:PC60BM)‐based solar cell is derived combining accurate optoelectronic characterization and comprehensive optical modeling. This joint approach is shown to be essential to get reliable values of the IQE. Photons with energy higher than the bandgap of the donor material can effectively contribute to enhance the IQE of the solar cell. This holds true independently of the device architecture, reflecting an intrinsic property of the active material. Moreover, the nanomorphology of the bulk heterojunction plays a crucial role in determining the IQE spectral dependence: the coarser and more crystalline, the lesser the gain in IQE upon high energy excitation.     

采用AlN/SiNx钝化的高性能AlGaN/GaN HEMTs

两种不同的钝化层结构被应用到势垒层厚度为12 nm的AlGa/GaN 高电子迁移率场效应晶体管中。首先采用等离子增强原子层沉积(PEALD)技术生长5 nm的AlN薄膜,然后再覆盖50 nm的等离子增强化学气相淀积(PECVD)生长的SiNx。相比于传统的SiNx钝化,AlN钝化层的插入更有效地抑制了电流崩塌效应,同时获得了小的亚阈值斜率(SS)。AlN钝化层的插入增大了器件的射频跨导从而获得了较高的截止频率。另外,通过变温直流特性测试发现,AlN/SiNx钝化的器件在高温时饱和电流和最大跨导的衰退相对于仅采用SiNx钝化的器件都要小,表明AlN钝化层的插入改善了器件的高温稳定性。     

Growth of AlGaN alloys exhibiting enhanced luminescence efficiency

Interest in developing ultraviolet emitters using the III-Nitride family of semiconductors has sparked considerable effort in fabricating AlGaN alloys that exhibit enhanced luminescence based on strong carrier localization, similar to their InGaN brethren. In this paper, we report on the growth of such alloys by plasma-assisted molecular beam epitaxy (PA-MBE) without the use of indium. This enhancement is attributed to the presence of nanoscale compositional inhomogeneities (NCIs) in these materials. The emission wavelength in these materials has been tuned between 275 nm and 340 nm by varying growth conditions. The effects of dislocations on double heterostructures (DHs) that employ an NCI AlGaN active region has been investigated, with an internal quantum efficiency as high as 32% obtained for the lowest dislocation density samples (3×10¹⁰ cm⁻²). Prototype DH-ultraviolet light emitting diodes (DH-UVLEDs) emitting at 324 nm were fabricated employing an NCI AlGaN alloy as the active region.     

Room-Temperature Quantum Cascade Laser: ZnO/Zn1−x Mg x O Versus GaN/Al x Ga1−x N

A ZnO/Zn_1?x Mg _x O-based quantum cascade laser (QCL) is proposed as a candidate for generation of THz radiation at room temperature. The structural and material properties, field dependence of the THz lasing frequency, and generated power are reported for a resonant phonon ZnO/Zn_0.95Mg_0.05O QCL emitting at 5.27 THz. The theoretical results are compared with those from GaN/Al _x Ga_1?x N QCLs of similar geometry. Higher calculated optical output powers [ $ {P}_{\rm{ZnMgO}} $  = 2.89 mW (nonpolar) at 5.27 THz and 2.75 mW (polar) at 4.93 THz] are obtained with the ZnO/Zn_0.95Mg_0.05O structure as compared with GaN/Al_0.05Ga_0.95N QCLs [ $ {P}_{\rm{AlGaN}} $  = 2.37 mW (nonpolar) at 4.67 THz and 2.29 mW (polar) at 4.52 THz]. Furthermore, a higher wall-plug efficiency (WPE) is obtained for ZnO/ZnMgO QCLs [24.61% (nonpolar) and 23.12% (polar)] when compared with GaN/AlGaN structures [14.11% (nonpolar) and 13.87% (polar)]. These results show that ZnO/ZnMgO material is optimally suited for THz QCLs.     

Nanoscale Insight Into Lead‐Free BNT‐BT‐xKNN

Piezoresponse force microscopy (PFM) is used to afford insight into the nanoscale electromechanical behavior of lead‐free piezoceramics. Materials based on Bi_1/2Na_1/2TiO₃ exhibit high strains mediated by a field‐induced phase transition. Using the band excitation technique the initial domain morphology, the poling behavior, the switching behavior, and the time‐dependent phase stability in the pseudo‐ternary system (1–x)(0.94Bi_1/2Na_1/2TiO₃‐0.06BaTiO₃)‐xK_0.5Na_0.5NbO₃ (0 <= x <= 18 mol%) are revealed. In the base material (x = 0 mol%), macroscopic domains and ferroelectric switching can be induced from the initial relaxor state with sufficiently high electric field, yielding large macroscopic remanent strain and polarization. The addition of KNN increases the threshold field required to induce long range order and decreases the stability thereof. For x = 3 mol% the field‐induced domains relax completely, which is also reflected in zero macroscopic remanence. Eventually, no long range order can be induced for x >= 3 mol%. This PFM study provides a novel perspective on the interplay between macroscopic and nanoscopic material properties in bulk lead‐free piezoceramics.     

Efficient Photocatalytic Nitrogen Fixation: Enhanced Polarization,Activation, and Cleavage by Asymmetrical Electron Donation to NN Bond

Photocatalytic nitrogen (N₂) fixation suffers from low efficiency due to the difficult activation of the strongly nonpolar N?N bond. In this study, a Ru–Co bimetal center is constructed at the interface of Ru/CoS_x with S‐vacancy on graphitic carbon nitride nanosheets (Ru‐Vs‐CoS/CN). Upon adsorption, the two N atoms in N₂ are bridged to the Ru–Co center, and the asymmetrical electron donation from Ru and Co atoms to N₂ adsorbate highly polarized N?N bond to double bond order. The plasmonic electric‐field‐enhancement effect enables the Ru/CoS_x interface to boost the generation of energetic electrons. The Schottky barrier between Ru and CoS_x endows the interface with electron transfer from CoS_x to Ru. The Ru‐end bound N at the Ru–Co center is preferentially hydrogenated. As a result, the Ru‐Vs‐CoS/CN photocatalyst shows an NH₃ production rate of up to 0.438 mmol g^?1 h^?1, reaching a high apparent quantum efficiency of 1.28% at 400 nm and solar‐to‐ammonia efficiency of 0.042% in pure water under AM1.5G light irradiation.     

High Efficiency Deep‐Blue Phosphorescent Organic Light‐Emitting Diodes with CIE x,y (≤ 0.15) and Low Efficiency Roll‐Off by Employing a High Triplet Energy Bipolar Host Material

Recently, bipolar host materials are the most promising candidates for achieving high performance phosphorescent organic light‐emitting diodes (PHOLEDs) in order to maximize recombination efficiency. However, the development of host material with high triplet energy (E _T) is still a great challenge to date to overcome the limitations associated with the present PHOLEDs. Herein, a highly efficient donor‐π‐acceptor (D‐π‐A) type bipolar host (4′‐(9H‐carbazol‐9‐yl)‐2,2′‐dimethyl‐[1,1′‐biphenyl]‐4‐yl)diphenylphosphine oxide (m‐CBPPO) comprising of carbazole, 2,2′‐dimethylbiphenyl and diphenylphosphoryl as D‐π‐A unit, respectively, is developed. Interestingly, a high E _T of 3.02 eV is observed for m‐CBPPO due to highly twisted conformation. Furthermore, the new host material is incorporated in PHOLEDs as emissive layer with a new carbene type Ir(cb)₃ material as a deep‐blue emitter. The optimized devices show an excellent external quantum efficiency (EQE) of 24.8% with a notable Commission internationale de l'éclairage (x, y) ≤ 0.15, (0.136, 0.138) and high electroluminescence performance with extremely low efficiency roll‐off. Overall, the above EQE is the highest reported for deep‐blue PHOLEDs with very low efficiency roll‐off and also indicate the importance of appropriate host for the development of high performance deep‐blue PHOLEDs.     

Radiative recombination via direct optical transitions in In1 −x GaxAs (0≤x≤0.16) solid solutions

Photoluminescence from In_1?x Ga_xAs (0≤x≤0.16) solid solution epilayers LPE-grown on (111)InAs substrates and electroluminescence from p-n junctions on their bases have been studied in the temperature range 77–450 K. Despite the negative lattice mismatch between epilayer and substrate, radiative recombination in epilayers occurs via direct optical transitions ensuring a high internal quantum efficiency of luminescence (6% at 295 K).     

Role of intervalley scattering in the radiative recombination in Pb1 − x Eu x Te alloys (0 ≤ x ≤ 1)

Measurements of the photoluminescence from epitaxial layers of Pb_{1 ? x} Eu _x Te alloys with 0 ≤ x ≤ 0.32 are carried out. It is found that the luminescence intensity decreases with increasing Eu content and, already for x as low as about 0.1, drops by more than one order of magnitude. No luminescence is observed for 0.2 < x ≤ 0.32. This behavior is explained by the fact that, for x ≈ 0.1, the absolute minimum in the conduction band changes from the L to X point, which results in the scattering of nonequilibrium electrons to the X valley and, thus, causes a decrease in the quantum efficiency of the emission. According to published data, for x > 0.85, optical transitions also take place with the participation of the X valley; in this case, the emission is governed by the formation of magnetic polarons. The temperature dependences of the band gap are determined for 0 ≤ x ≤ 0.11. These dependences have a wide linear region characterized by a positive dE _g /dT coefficient, which decreases with the Eu content to become negative in pure EuTe.     

Morphology Engineering in Monolayer MoS2‐WS2 Lateral Heterostructures

In recent years, heterostructures formed in transition metal dichalcogenides (TMDs) have attracted significant attention due to their unique physical properties beyond the individual components. Atomically thin TMD heterostructures, such as MoS₂‐WS₂, MoS₂‐MoSe₂, MoS₂‐WSe₂, and WSe₂‐WS₂, are synthesized so far via chemical vapor deposition (CVD) method. Engineering the morphology of domains including size and shape, however, still remains challenging. Here, a one‐step CVD strategy on the morphology engineering of MoS₂ and WS₂ domains within the monolayer MoS₂‐WS₂ lateral heterostructures through controlling the weight ratio of precursors, MoO₃ and WO₃, as well as tuning the reaction temperature is reported. Not only can the size ratio in terms of area between WS₂ and MoS₂ domains be easily controlled from less than 1 to more than 20, but also the overall heterostructure size can be tuned from several to hundreds of micrometers. Intriguingly, the quantum well structure, a WS₂ stripe embedded in the MoS₂ matrix, is also observed in the as‐synthesized heterostructures, offering opportunities to study quantum confinement effects and quantum well applications. This approach paves the way for the large‐scale fabrication of MoS₂‐WS₂ lateral heterostructures with controllable domain morphology, and shall be readily extended to morphology engineering of other TMD heterostructures.     

Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open‐Circuit Voltage

In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (E_D*) and the energy of the charge transfer state (E_CT) will optimize the open‐circuit voltage (V_oc). In this work, this energy loss E_D*‐E_CT is measured directly via Fourier‐transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI‐1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI‐1 are similar (1.4 eV–1.5 eV), the optimized photovoltaic devices show large differences in V_oc and internal quantum efficiency (IQE). For P3TI:PC₇₁BM blends a E_D*‐E_CT of ～ 0.1 eV, a V_oc of 0.7 V and an IQE of 87% are found. For PTI‐1:PC₆₁BM blends an absence of sub‐gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher V_oc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI‐1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between E_CT and qV_oc due to radiative and non‐radiative recombination are quantified for both material systems, indicating that for the PTI‐1:PC₆₁BM material system, V_oc can only be increased by decreasing the non‐radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (<0.1 eV) and a high V_oc.     

CuIn1−xGaxSe2‐based thin‐film solar cells by the selenization of sequentially evaporated metallic layers

Polycrystalline CuIn_1−xGa_xSe₂ (CIGS) thin films were deposited by the non‐vacuum, near‐atmospheric‐pressure selenization of stacked metallic precursor layers. A study was carried out to investigate the influence of significant factors of the absorber on the solar cells performance. An efficiency enhancement was obtained for Cu/(In+Ga) atomic ratios between 0·93 and 0·95. The slope of the observed energy bandgap grading showed a strong influence on the V_OC and the short circuit current density J_SC. An increase of the Ga content in the active region of the absorber was achieved by the introduction of a thin Ga layer on the Mo back contact. This led to an improvement of efficiency and V_OC. Furthermore, an enhanced carrier collection was detected by quantum efficiency measurements when the absorber layer thickness was slightly decreased. Conversion efficiencies close to 10% have been obtained for these devices. Copyright © 2005 John Wiley & Sons, Ltd.     

Study of dual-blue light-emitting diodes with asymmetric AlGaN graded barriers

A dual-blue light-emitting diode （LED） with asymmetric A1GaN composition-graded barriers but without an AlGaN electron blocking layer （EBL） is analyzed numerically. Its spectral stability and efficiency droop are improved compared with those of the conventional InGaN/GaN quantum well （QW） dual-blue LEDs based on stacking structure of two In0.18Ga0.szN/GaN QWs and two In0.12Ga0.88N/GaN QWs on the same sapphire substrate. The improvement can be attributed to the markedly enhanced injection of holes into the dual-blue active regions and effective reduction of leakage current.     

Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light‐Emitting Diodes

Efficient blue‐, green‐, and red‐light‐emitting organic diodes are fabricated using binuclear platinum complexes as phosphorescent dopants. The series of complexes used here have pyrazolate bridging ligands and the general formula C^∧NPt(μ‐pz)₂PtC^∧N (where C^∧N = 2‐(4′,6′‐difluorophenyl)pyridinato‐N,C^2′, pz = pyrazole ( 1 ), 3‐methyl‐5‐tert‐butylpyrazole ( 2 ), and 3,5‐bis(tert‐butyl)pyrazole ( 3 )). The Pt–Pt distance in the complexes, which decreases in the order 1 > 2 > 3 , solely determines the electroluminescence color of the organic light‐emitting diodes (OLEDs). Blue OLEDs fabricated using 8 % 1 doped into a 3,5‐bis(N‐carbazolyl)benzene (mCP) host have a quantum efficiency of 4.3 % at 120 Cd m^–2, a brightness of 3900 Cd m^–2 at 12 V, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.11, 0.24). Green and red OLEDs fabricated with 2 and 3 , respectively, also give high quantum efficiencies (～ 6.7 %), with CIE coordinates of (0.31, 0.63) and (0.59, 0.46), respectively. The current‐density–voltage characteristics of devices made using dopants 2 and 3 indicate that hole trapping is enhanced by short Pt–Pt distances (< 3.1 Å). Blue electrophosphorescence is achieved by taking advantage of the binuclear molecular geometry in order to suppress dopant intermolecular interactions. No evidence of low‐energy emission from aggregate states is observed in OLEDs made with 50 % 1 doped into mCP. OLEDs made using 100 % 1 as an emissive layer display red luminescence, which is believed to originate from distorted complexes with compressed Pt–Pt separations located in defect sites within the neat film. White OLEDs are fabricated using 1 and 3 in three different device architectures, either with one or two dopants in dual emissive layers or both dopants in a single emissive layer. All the white OLEDs have high quantum efficiency (～ 5 %) and brightness (～ 600 Cd m^–2 at 10 V).     

Spin-orbit splitting dependent resonant third-order nonlinear optical susceptibility in InGaN/GaN multiple quantum wells

For the transition between valence band and conduction band, the third-order nonlinear optical susceptibility χ⁽³⁾ for degenerated four-wave mixing in In_xGa_1−xN/GaN multiple quantum wells (MQWs) has been calculated. The contributions of spin-orbit split-off energy to the resonant third-order nonlinear optical susceptibility of the modes, whose polarization is vertical to the [0 0 1] direction of the MQWs, are discussed in detail. The correlations between the peaks of χ⁽³⁾, which are due to the transitions from the spin-orbit split-off energy level to first conduction subband, and the width of the quantum well and the constituents of the semiconductor material are obtained.     

Improved Performance of White Phosphorescent Organic Light‐Emitting Diodes through a Mixed‐Host Structure

Highly efficient white phosphorescent organic light‐emitting diodes with a mixed‐host structure are developed and the device characteristics are studied. The introduction of a hole‐transport‐type host (N, N’‐dicarbazolyl‐3‐3‐benzen (mCP)) into an electron‐transport‐type host (m‐bis‐(triphenylsilyl)benzene (UGH3)) as a mixed‐host emissive layer effectively achieves higher current density and lower driving voltage. The peak external quantum and power efficiency with the mixed‐host structure improve up to 18.9% and 40.9 lm/W, respectively. Moreover, this mixed‐host structure device shows over 30% enhanced performance compared with a single‐host structure device at a luminance of 10,000 cd/m² without any change in the electroluminescence spectra.