Electroluminescence Experiments Research Articles

Strain modulation effect of superlattice interlayer on InGaN/GaN multiple quantum well

The strong piezoelectric field in InGaN/GaN heterostructure quantum wells severely reduces the light emission efficiency of multiple quantum well (MQW) structures. To address this issue, a strain modulation interlayer is commonly used to mitigate the piezoelectric polarization field and improve the luminescence performance of the devices. To investigate the influence and mechanism of strain modulation in the InGaN/GaN superlattice (SL), epitaxial wafers with an n-type InGaN/GaN SL interlayer sample, and their corresponding control samples are prepared. The measured temperature-dependent photoluminescence (PL) spectra of the epitaxial wafers, show that the introduction of an SL interlayer leads to a shorter-wavelength emission and enhancement of internal quantum efficiency. As the temperature increases, a blue shift of the PL peak is observed. However, for the sample with an SL interlayer, the blue shift of the PL peak with temperature increasing is relatively small. Electroluminescence (EL) experiments indicate that the introduction of an SL interlayer significantly increases the integrated intensity of the EL peak and reduces its full width at half maximum. These phenomena collectively indicate that the incorporation of a superlattice interlayer can partly suppress the quantum-confined Stark effect (QCSE) that affects the light emission efficiency. Theoretical calculations show that the introduction of a superlattice strain layer before growing an active multiple quantum well can weaken the polarization-induced built-in electric field in the active quantum well, reduce the tilt of the energy band in the multiple quantum well active region, increase the overlap of electron and hole wave functions, enhance the emission probability, shorten the radiative recombination lifetime, and promote competition between radiative recombination and non-radiative recombination, thereby achieving higher recombination efficiency and improving light emission intensity. This study provides experimental and theoretical evidence that the strain modulation SL interlayer can effectively improve the device performance and offer guidance for optimizing the structural design of devices.

Issue Information

Journal of the American Ceramic SocietyVolume 106, Issue 7 p. 3979-3982 ISSUE INFORMATIONFree Access Issue Information First published: 05 May 2023 https://doi.org/10.1111/jace.18564AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract Cover Photograph: Ultrafast high-temperature sintering (UHS) and flash sintering are novel methods for rapid sintering of ceramics, often completed in just a few seconds. The authors of https://ceramics.onlinelibrary.wiley.com/doi/full/10.1111/jace.19070 this issue's feature article show that both also share two additional features: an abrupt rise in electrical conductivity, which is electronic, and electroluminescence, through simple yet elegant experiments on a commercial gas-fired lantern mantle. Read the article https://doi.org/10.1111/jace.19070 for more information. Volume106, Issue7July 2023Pages 3979-3982 RelatedInformation

Understanding the degradation mechanism of TTA-based blue fluorescent OLEDs by exciton dynamics and transient electroluminescence measurements.

The lifetime of blue organic light-emitting diodes (OLEDs) has always been a big challenge in practical applications. Blue OLEDs based on triplet-triplet annihilation (TTA) up-conversion materials have potential to achieve long lifetimes due to fusing two triplet excitons to one radiative singlet exciton, but there is a lack of an in-depth understanding of exciton dynamics on degradation mechanisms. In this work, we established a numerical model of exciton dynamics to study the impact factors in the stability of doped blue OLEDs based on TTA up-conversion hosts. By performing transient electroluminescence experiments, the intrinsic parameters related to the TTA up-conversion process of aging devices were determined. By combining the change of excess charge density in the emitting layer (EML) with aging time, it is concluded that the TTA materials are damaged by the excess electrons in the EML during ageing, which is the main degradation mechanism of OLEDs. This work provides a theoretical basis for preparing long-lifetime blue fluorescent OLEDs.

Benzimidazole-substituted bisanthracene: a highly efficient deep-blue triplet–triplet fusion OLED emitter at low dopant concentration

Two anthracene derivatives, MAnBz and DiAnBz, have been developed as emitters for deep-blue triplet–triplet fusion (TTF) fluorescence organic light emitting diodes (OLEDs). DiAnBz bearing two anthracene units at a closed distance, separated by one phenylene moiety, shows high photoluminescent quantum yields and deep-blue emission. The remarkable maximum external quantum efficiency (EQEmax) of 6.7% and 8.3% are obtained from DiAnBz-OLEDs with pristine prompt fluorescence and TTF fluorescence models, respectively. In particular, effectively diffusive TTF characteristics still can be detected at DiAnBz-OLED with an extreme low emitter concentration of 1 wt%, whose device performance exhibited a high EQEmax of 7.0% with deep blue emission at CIE (0.148, 0.078). The diffusive TTF characteristics with 20 times shorter delayed emission lifetime (τ2 of 3–4 μs), in comparison with that of MAnBz (τ2 of 60–70 μs) in the time-resolved electroluminescence experiments. This implies that pairing and TTF of the DiAnBz triplet excitons are more effective than that of MAnBz in the light emitting layer.

Influence of an atomistic protrusion at the tip apex on enhancing molecular emission in tunnel junctions: A theoretical study.

Light emission from the gap of a scanning tunneling microscope can be used to investigate many optoelectronic processes at the single-molecule level and to gain insight into the fundamental photophysical mechanisms involved. One important issue is how to improve the quantum efficiency of quantum emitters in the nanometer-sized metallic gap so that molecule-specific emission can be clearly observed. Here, using electromagnetic simulations, we systematically investigate the influence of an atomic-scale protrusion at the tip apex on the emission properties of a point dipole in the plasmonic nanocavity. We found that such an atomistic protrusion can induce strong and spatially highly confined electric fields, thus increasing the quantum efficiency of molecular fluorescence over two orders of magnitude even when its dipole is oriented parallel to the metal surface, a situation occurring in most realistic single-molecule electroluminescence experiments. In addition, our theoretical simulations indicate that due to the lightning rod effect induced by the protrusion in a plasmonic nanocavity, the quantum efficiency increases monotonically as the tip approaches the dipole to the point of contact, instead of being quenched, thus explaining previous experimental observations with ever-enhancing fluorescence. Furthermore, we also examine in detail how the protrusion radius, height, and material affect the protrusion-induced emission enhancement. These results are believed to be instructive for further studies on the optoelectronic properties of single molecules in tip-based plasmonic nanocavities.

Physical Insights Into the Impact of Surface Traps on Breakdown Characteristics of AlGaN/GaN HEMTs—Part I

Impact of surface traps on the breakdown characteristics of AlGaN/GaN HEMT devices is revealed using detailed TCAD computations and supporting experiments. Detailed mechanism explaining the role of surface traps in modulating channel electric field under various surface trap, device design, gate-stack, and applied voltage conditions is discussed. Experimental analysis with different surface conditions shows the measured electric field profile, using electro-luminescence experiments, to be in complete agreement with the computational findings. Device design guidelines with respect to design parameters affecting drift region's electric field and gate-stack in presence of surface traps or varying surface trap concentration is presented.

Self-organized micro-light-emitting diode structure for high-speed solar-blind optical wireless communications

The origin of the fast modulation characteristics of deep ultraviolet (DUV) AlGaN light-emitting diodes (LEDs) grown on AlN/sapphire templates with vicinal off-angles is reported by employing time-resolved electroluminescence (EL) and micro-imaging experiments. The LEDs have recently demonstrated Gbps-class optical wireless communication (OWC) under both room-lighting and direct-sun. The frequency response (f3dB) of the LED reached 184 MHz, which is far beyond expectations by considering the size of the LEDs. Since self-organized micro-LED structures with a low electric capacitance (C) are observed by the EL experiments, the compatibility of high efficiency and fast modulation nature of the AlGaN LEDs is explained. Our approach can overcome the dilemma, where micro-LEDs can be modulated fast but have low power, and therefore, the self-organized micro-LED structure is an ideal solution to realize practical DUV OWCs.

Informative Aspects of Molten KOH Etch Pits Formed at Basal Plane Dislocations on the Surface of 4H‐SiC

Despite being a destructive technique, molten KOH etch pits at basal plane dislocations are found to have informative aspects, as revealed by photoluminescence imaging and optical microscopy, on the 4° off‐cut (0001) surface of 4H‐SiC p–i–n diode chips, where single Shockley‐type stacking faults have expanded during electroluminescence experiments. Smaller etch pits are observed aligning on single lines along ±[100] that are the shallowest sides of stacking faults. Through closer observation of the pits, the facing directions are found to agree well with a model explaining the two types of partial dislocations constituting the line. Other basal plane dislocation pits are observed by scanning electron microscopy on the sublimation‐grown wafer surface before epitaxial growth. The distances between the partial dislocation cores are measured at each pit. No basal plane dislocation etch pit facing toward ±[100] is found, but a weak correlation is noted between the partial dislocation distance at the cores and the direction that the basal plane dislocations face. The corresponding results are discussed in terms of the image force and the force exerted on steps perpendicular to the (0001) plane, to understand the mechanisms of basal plane dislocation propagation and/or conversion to threading edge dislocations.

GaSb‐based heterostructure with buried vacuum pocket photonic crystal layer

The vacuum pocket retaining molecular beam epitaxial regrowth of the nano-patterned GaSb surface was demonstrated. The high contrast 2D photonic crystal layer was incorporated into the test 2 µm emitting laser heterostructure. The photonic dispersion determined from angle-resolved electroluminescence experiment showed four well-resolved bands corresponding to the model predictions for the square lattice. The single-mode lasing near 2 µm has been observed at the temperature corresponding to the alignment of the photonic crystal band-edge states and the quantum well gain peak. The reference devices without the photonic crystal layer emitted trivial spectra and did not lase at any temperature.

Electrically Injected GeSn Vertical-Cavity Surface Emitters on Silicon-on-Insulator Platforms

An efficient electrically injected group-IV light source compatible with the complementary metal-oxide-semiconductor (CMOS) process is the holy grail for realizing functional, intelligent electronic-photonic integrated circuits for a wide range of applications. The group-IV GeSn material is considered as a promising solution for efficient light sources because its bandgap can be fundamentally transformed from indirect to direct with appropriate Sn compositions. However, an important challenge in realizing efficient electrically injected light emitters is the incorporation of an optical cavity with electrical structures. Here we demonstrate, to the best of our knowledge, the first electrically injected GeSn vertical-cavity surface emitter on the silicon-on-insulator platform. A vertical cavity employing a buried oxide layer and a deposited SiO2 top layer as reflectors is developed for enhancing the electroluminescence in the GeSn active layer. Room-temperature electroluminescence experiments reveal clear c...

Fabrication of tunable n-Zn1-xCdxO/p-GaN heterojunction light-emitting diodes

Zn1-xCdxO films with different Cd contents obtained by controlling the growth oxygen pressure (Op) were deposited on both c-sapphire (Al2O3) substrates and p-GaN substrates, by means of the pulsed laser deposition (PLD) method. Photoluminescence (PL) measurement revealed that the variation of Op influenced the amount of Cd doping into Zn1-xCdxO films deposited on Al2O3, leading tunable luminescence from the ultraviolet (UV) to blue emission extended to the green band. Then all the fabricated n-Zn1-xCdxO/p-GaN heterojunction possessed good ohmic contacts and exhibited typical rectifying characteristic of the diode. Indeed, the high luminescence from the ZnCdO layer could be attained by inserting a MgO insulator to the heterojunction interface. The fabrication of tunable n-Zn1-xCdxO/p-GaN heterojunction light-emitting diodes is available which was supported by the results of electroluminescence (EL) experiment.

MBE grown germanium tunnel-junctions—burstein-moss effect and band-edge luminescence in the Ge Zener-Emitter

We investigated the Burstein-Moss (BM) shift by electroluminescence experiments in the Ge Zener-Emitter. Due to fast transit times through a tunneling barrier (few ps), electrons can be accumulated in the indirect as well as the direct conduction band by Zener band-to-band tunneling. The resulting shift can thus be assigned to the band-filling effect in the indirect conduction band (BM shift of 39 meV) as well as the direct conduction band (BM shift of 58 meV). Theoretical modelling has been carried out and shows good agreement with our experimental data from the Zener-Emitter.

Ultrafast gain recovery and large nonlinear optical response in submonolayer quantum dots

Submonolayer quantum dots combine the zero-dimensional charge-carrier confinement of self-assembled quantum dots with the large density of states of a quantum well. Electroluminescence and pump-probe experiments on a submonolayer-based optical amplifier show that the system exhibits a high gain of $90{\mathrm{cm}}^{\ensuremath{-}1}$ and an ultrafast gain recovery. We propose a rate equation system describing the microscopic carrier dynamics which quantitatively reproduces the observed behavior and provides deeper theoretical understanding of the material system. In contrast to Stranski-Krastanov quantum dots, the fast gain recovery is enhanced by a strong interdot coupling. Optically inactive submonolayer states form an efficient carrier reservoir and give rise to a large nonlinear optical response.

Single step deposition of an interacting layer of a perovskite matrix with embedded quantum dots.

Hybrid lead halide perovskite (PS) derivatives have emerged as very promising materials for the development of optoelectronic devices in the last few years. At the same time, inorganic nanocrystals with quantum confinement (QDs) possess unique properties that make them suitable materials for the development of photovoltaics, imaging and lighting applications, among others. In this work, we report on a new methodology for the deposition of high quality, large grain size and pinhole free PS films (CH3NH3PbI3) with embedded PbS and PbS/CdS core/shell Quantum Dots (QDs). The strong interaction between both semiconductors is revealed by the formation of an exciplex state, which is monitored by photoluminescence and electroluminescence experiments. The radiative exciplex relaxation is centered in the near infrared region (NIR), ≈1200 nm, which corresponds to lower energies than the corresponding band gap of both perovskite (PS) and QDs. Our approach allows the fabrication of multi-wavelength light emitting diodes (LEDs) based on a PS matrix with embedded QDs, which show considerably low turn-on potentials. The presence of the exciplex state of PS and QDs opens up a broad range of possibilities with important implications in both LEDs and solar cells.

Effects of pn Doping in Thiophene/Phenylene Co-oligomers Thin Films

Doping effects in thiophene/phenylene co-oligomers (TPCOs) have been studied by means of extreme-UV (EUV) excited photoelectron spectroscopy (EUPS) and electro-luminescence experiments. Doped poly-crystalline films were fabricated by p-type doping with MoO3 to inherent p-type TPCO, BP1T (2,5-bis(4-biphenylyl)thiophene), and by n-type doping with Cs2CO3 to n-type AC5-CF3 (1,4-bis{5-[4-(trifluoromethyl)phenyl]thiophen-2-yl}benzene), respectively. Doping concentrations were 2% in both cases. The work function of BP1T was shifted from 4.0 to 4.3 eV and that of AC5-CF3 was shifted from 4.8 to 3.9 eV. The energy shifts are reasonable directions for p- and n-type doping. We also confirmed higher current injection with the doping films.

On optical polarization and charge carrier statistics of nonpolar InGaN quantum wells

AbstractAbstractauthoren Optical polarization is a fundamental property of light emission from m‐plane InGaN quantum wells. It is a result of band structure, anisotropic strain, the degree of polarization depending on thermal occupation, and band‐filling of valence subbands. We analyze the optical‐polarized light of nonpolar samples at low and at room temperature over a wide range of excitation density. We observe that the measured energy separation and linewidth of the orthogonally polarized emission depend on carrier density, too. A consistent explanation of these effects is possible in the context of Fermi–Dirac statistics for a degenerate carrier density and inhomogeneously broadened density of states. In addition, pulsed electroluminescence experiments were performed to validate our conclusions for optoelectronic devices based on nonpolar InGaN quantum wells.

Single occupancy spectroelectrochemistry of freely diffusing flavin mononucleotide in zero-dimensional nanophotonic structures.

Zero-mode waveguides (ZMW) have the potential to be powerful confinement tools for studying electron transfer dynamics at single molecule occupancy conditions. Flavin mononucleotide contains an isoalloxazine chromophore, which is fluorescent in the oxidized state (FMN) while the reduced state (FMNH2) exhibits dramatically lower light emission, i.e. a dark-state. This allows fluorescence emission to report the redox state of single FMN molecules, an observation that has been used previously to study single electron transfer events in surface-immobilized flavins and flavoenzymes, e.g. sarcosine oxidase, by direct wide-field imaging of ZMW arrays. Single molecule electron transfer dynamics have now been extended to the study of freely diffusing molecules using fluorescence measurements of Au ZMWs under single occupancy conditions. The Au in the ZMW serves both as an optical cladding layer and as the working electrode for potential control, thereby accessing single molecule electron transfer dynamics at μM concentrations. Consistent with expectations, the probability of observing single reduced molecules increases as the potential is scanned negative, E(appl) < E(eq), and the probability of observing emitting oxidized molecules increases at E(appl) > E(eq). Different single molecules exhibit different electron transfer properties as reflected in the position of E(eq) and the distribution of E(eq) among a population of FMN molecules. Two types of actively-controlled electroluminescence experiments were used: chronofluorometry experiments, in which the potential is alternately stepped between oxidizing and reducing potentials, and cyclic potential sweep fluorescence experiments, analogous to cyclic voltammetry, these latter experiments exhibiting a dramatic scan rate dependence with the slowest scan rates showing distinct intermediate states that are stable over a range of potentials. These states are assigned to flavosemiquinone species that are stabilized in the special environment of the ZMW nanopore.

Magnetoelectroluminescence of organic heterostructures: Analytical theory and spectrally resolved measurements

The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magnetoelectroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solution using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. To validate the theory, spectrally resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. These observations are consistent with the model.

Effect of uniaxial stress on electroluminescence, valence band modification, optical gain, and polarization modes in tensile strained p-AlGaAs/GaAsP/n-AlGaAs laser diode structures: Numerical calculations and experimental results

The effects of uniaxial compression in [110] direction on energy-band structures, heavy and light hole mixing, optical matrix elements, and gain in laser diodes with “light hole up” configuration of valence band levels in GaAsP quantum wells with different widths and phosphorus contents are numerically calculated. The development of light and heavy hole mixing caused by symmetry lowering and converging behavior of light and heavy hole levels in such quantum wells under uniaxial compression is displayed. The light or heavy hole nature of each level is established for all considered values of uniaxial stress. The results of optical gain calculations for TM and TE polarization modes show that uniaxial compression leads to a significant increase of the TE mode and a minor decrease of the TM mode. Electroluminescence experiments were performed under uniaxial compression up to 5 kbar at 77 K on a model laser diode structure (p-AlxGa1−xAs/GaAs1−yPy/n-AlxGa1−xAs) with y = 0.16 and a quantum well width of 14 nm. They reveal a maximum blue shift of 27 meV of the electroluminescence spectra that is well described by the calculated change of the optical gap and the increase of the intensity being referred to a TE mode enhancement. Numerical calculations and electroluminescence data indicate that uniaxial compression may be used for a moderate wavelength and TM/TE intensity ratio tuning.

Spatial Modeling of Thin-Film Solar Modules Using the Network Simulation Method and SPICE

The use of the network simulation method (NSM) for the modeling of thin-film solar modules is discussed. It will be shown that for the accurate modeling of small defects using the NSM, a variable mesh is required to keep the computation time within reasonable limits. A simulation tool for thin-film solar modules is developed that implements the NSM with a variable, adaptive mesh. The results of this model will be compared with electroluminescence experiments on a Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> solar module with a defect (shunt). Furthermore, it is demonstrated that the program can handle complex-shaped inhomogeneities such as local variations in solar cell properties, illumination, and contact layer resistance.