EL Intensity Research Articles

Spectra of surface plasmon polariton enhanced electroluminescence from electroformed Al-Al2O3-Ag diodes

Narrow band-pass filters have been used to measure the spectral distribution of electroluminescent photons with energies between 1.8 eV and 3.0 eV from electroformed Al-Al2O3-Ag diodes with anodic Al2O3 thicknesses between 12 nm and 18 nm. Electroforming of metal-insulator-metal (MIM) diodes is a non-destructive dielectric breakdown that results in a conducting channel in the insulator and changes the initial high resistance of the MIM diode to a low resistance state. It is a critical step in the development of resistive-switching memories that utilize MIM diodes as the active element. Electroforming of Al-Al2O3-Ag diodes in vacuum results in voltage-controlled negative resistance (VCNR) in the current-voltage (I-V) characteristics. Electroluminescence (EL) and electron emission into vacuum (EM) develop simultaneously with the current increase that results in VCNR in the I-V characteristics. EL is due to recombination of electrons injected at the Al-Al2O3 interface with radiative defect centers in Al2O3. Measurements of EL photons between 1.8 eV and 3.0 eV using a wide band-pass filter showed that EL intensity is exponentially dependent on Al2O3 thickness for Al-Al2O3-Ag diodes between 12 nm and 20 nm thick. Enhanced El intensity in the thinnest diodes is attributed to an increase in the spontaneous emission rate of recombination centers due to high electromagnetic fields generated in Al2O3 when EL photons interact with electrons in Ag or Al to form surface plasmon polaritons at the Al2O3-Ag or Al2O3-Al interface. El intensity is a maximum at 2.0–2.2 eV for Al-Al2O3-Ag diodes with Al2O3 thicknesses between 12 nm and 18 nm. EL in diodes with 12 nm or 14 nm of Al2O3 is enhanced by factors of 8–10 over EL from a diode with 18 nm of Al2O3. The extent of EL enhancement in the thinnest diodes can vary significantly between samples. A narrow band of recombination centers was found in one Al-Al2O3-Ag diode with 12 nm of Al2O3; it had EL intensity 100 times greater at 2.15 eV than the diode with 18 nm of Al2O3. EL intensity for photons with energies greater than 2.6 eV is nearly the same for all diodes.

Analysis of recombination path for Cu(In,Ga)Se2 solar cells through luminescence

In this paper, the recombination through defects in absorber and CdS/Cu(In,Ga)Se2 interface states has been investigated by comparing photoluminescence to electroluminescence. The ranges of photoluminescence and electroluminescence collection region are 450 and 90nm, respectively. Thus, electroluminescence is more sensitive to CdS/CIGS interface recombination than photoluminescence. The devices experienced rapid thermal annealing at the temperature of 300°C. After annealing, the reduction of InCu defects in absorber layer increased PL and EL intensities at +0.7V bias by three times, while the decrease of interface states increased EL intensities from +1.0 to +1.5V bias by six times. The capacitance–voltage characterization confirmed that the density of interface states fell from 2.8×1013 to 1.5×1013cm−2.

Controlling potential barrier height by changing V-shaped pit size and the effect on optical and electrical properties for InGaN/GaN based light-emitting diodes

The internal quantum efficiency (IQE) of InGaN/GaN multiple quantum wells (MQWs) with blue light emission was improved by inserting an InGaN/GaN superlattice (SL) beneath the MQWs. While the SL technique is useful for improving the light-emitting diode (LED) performance, its effectiveness from a multilateral point of view requires investigation. V-shaped pits (V-pits), which generate a potential barrier and screen the effect of the threading dislocation, are one of the candidates for increasing the light emission efficiency of LEDs exceptionally. In this research, we investigated the relationship between the V-pit and SL and revealed that the V-pit diameter is strongly correlated with the IQE by changing the number of SL periods. Using scanning near-field optical microscopy and photoluminescence measurements, we demonstrated the distinct presence of the potential barrier formed by the V-pits around the dislocations. The relationship between the V-pit and the number of SL periods resulted in changing the potential barrier height, which is related to the V-pit diameter determined by the number of SL periods. In addition, we made an attempt to insert pit expansion layers (PELs) composed of combination of SL and middle temperature grown GaN layer instead of only SL structure. As a result of the evaluation of LEDs using SL or PEL, the EL intensity was strongly related to pit diameter regardless of the structures to form the V-pits. In addition, it was clear that larger V-pits reduce the efficiency droop, which is considered to be suppression of the carrier loss at high injection current.

Photoluminescence and Electroluminescence of Eu Doped Y2O3

In this paper, Eu doped Y2O3 nanophosphors are prepared at temperature 600oC by the combustion method. X-ray diffraction patterns confirm the formation of pure cubic phase of Y2O3. Electroluminescence of Eu doped Y2O3 has been investigated at different voltages by sandwitching the thick films between the conducting glass plate and aluminium electrode. Conducting transparent electrode has been obtained by depositing SnO2 layers over heated glass substrate. AC voltage of different frequencies was applied and EL intensity was measured with the help of photomultiplier tube. The intensity of EL increases with increasing voltage. PL decay characteristics of the Eu doped Y2O3 nanoparticles in the form of powder were measured.

Periodic variation in the electroluminescence intensity on a single pattern from InGaN/GaN light-emitting diodes fabricated on lens-shaped patterns

We investigated the spatial distribution of light emission from InGaN/GaN light-emitting diodes (LEDs) fabricated on lens-shaped patterns (LSPs) by using confocal scanning electroluminescence microscopy (CSM). From three-dimensional CSM images measured by varying the position of the focal plane of a detector, the luminescence profiles from the InGaN/GaN LEDs were significantly changed with respect to the position of the LSP itself. In addition, it the EL intensity around the boundary region of the single LSP was found to fluctuate periodically with the distance of 671 nm between the adjacent bright patterns. From the simple theoretical analysis, the periodic fluctuation around the boundary of every single pattern in the CSM images is attributed to interference effects due to periodically-positioned LSPs.

Electroluminescence of germanium LEDs on silicon: Influence of antimony doping

AbstractAntimony‐doped Ge‐LEDs were subjected to electroluminescence studies at temperatures about 300 K and 80 K. The LEDs were grown on Si substrates by MBE. The thickness of the active layer was 300 nm. For the p+nn+‐LEDs the Sb concentrations were 1 × 1018, 1 × 1019, 3 × 1019, 4 × 1019, 7 × 1019 or 1 × 1020 cm–3, respectively. As reference a p+in+‐LED without intentional doping in the active layer was used. The investigated specimen exhibited dominance of the direct transition line at about 0.8 eV.Moreover, luminescence indirect transition was observed. In general, the spectra reveal higher EL intensities at room temperature as compared to 80 K. With decreasing temperature the direct peak was blue shifted. The highest EL intensity was found for the LED with Sb concentration of 3 × 1019 cm–3. With increasing Sb doping a red‐shift of the direct peak was observed, caused by band gap narrowing. In addition, a simulated curve of the direct transition was compared with our samples and fits well with the measured spectra. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The First Tandem, All-exciplex-based WOLED

Exploiting our recently developed bilayer interface methodology, together with a new wide energy-gap, low LUMO acceptor (A) and the designated donor (D) layers, we succeeded in fabricating an exciplex-based organic light-emitting diode (OLED) systematically tuned from blue to red. Further optimization rendered a record-high blue exciplex OLED with ηext of 8%. We then constructed a device structure configured by two parallel blend layers of mCP/PO-T2T and DTAF/PO-T2T, generating blue and yellow exciplex emission, respectively. The resulting device demonstrates for the first time a tandem, all-exciplex-based white-light OLED (WOLED) with excellent efficiencies ηext: 11.6%, ηc: 27.7 cd A−1, and ηp: 15.8 ml W−1 with CIE(0.29, 0.35) and CRI 70.6 that are nearly independent of EL intensity. The tandem architecture and blend-layer D/A (1:1) configuration are two key elements that fully utilize the exciplex delay fluorescence, providing a paragon for the use of low-cost, abundant organic compounds en route to commercial WOLEDs.

P‐88: All‐Inorganic Quantum‐Dot Light‐Emitting Devices Prepared by Solution‐Process Route

AbstractWe have fabricated all‐inorganic quantum‐dot light‐emitting devices(QDLED) with NiO as a hole transporting layer and TiO2 as a inorganic electron transporting layer. P‐type NiO and n‐type TiO2 charge‐transport layers were synthesized by sol‐gel method using spin‐coating technique. The thickness of TiO2 layer was optimized with different spin‐coating speed, and studied the luminescence characteristics of devices. Results indicated devices with 4000 rpm TiO2 layer has lower turn voltage 4.2V and higher EL intensity.

Characterization of Electron-Induced Defects in Cu (In, Ga) Se2 Thin-Film Solar Cells using Electroluminescence

ABSTRACTCIGS solar cells were irradiated with 250 keV electrons, which can create only Cu-related defects in the cell, to reveal the radiation defect. The EL image of CIGS solar cells before electron irradiation at 120 K described small grains, thought to be those of the CIGS. After 250 keV electron irradiation of the CIGS cell, the cell was uniformly illuminated compared to before the electron irradiation and the observed grains were unclear. In addition, the EL intensity rose with increasing electron fluence, meaning the change in EL efficiency may be attributable to the decreased likelihood of non-irradiative recombination in intrinsic defects due to electron-induced defects. Since the light soaking effect for CIGS solar cells is reported the same phenomena, the 250 keV electron radiation effects for CIGS solar cells might be equivalent to the effect.

Study of shifting of recombination zone in multi-emissive layer organic light emitting devices and its effect on color stability

Color stability in multi-emissive layer organic light emitting devices (Me-OLEDs) has been investigated. Me-OLEDs were based on multiple emitters with a common host CBP doped with three dopants, BCzVBi, Ir(ppy)3 and Ir(btp)2acac for blue, green and red emission. A major variation in CIE coordinates were found from (0.312, 0.294) to (0.236, 0.267) with increase in voltage from 6 to 9V. This coordinate shift in Me-OLEDs has been attributed to the shifting of recombination zone with increase in voltage. In order to support our experimental observation, the EL spectrum of Me-OLEDs has been expressed as a linear combination of EL intensities of OLEDs with its individual constituting layers as emitters. In this way, the contribution of each layer in the EL spectrum of Me-OLEDs has been evaluated at each voltage. It has been observed from these calculations that the contribution of red emitter decreases from 47% to 33.33% and blue emitter increases from 38% to 51.67% with the increase in voltage from 6 to 9V. This supports our hypothesis of shifting of recombination zone with the change in voltage. This shifting has been attributed to the field dependency of electron mobility in CBP. Me-OLED with CBP layers between the emitters was fabricated to improve the color stability. Significant improvement in color stability was achieved without changes in current efficiency in Me-OLED with interlayers.

Study of Electroluminescence in Pr Doped ZnS

Electroluminescence study of Pr doped ZnS phosphor has been reported in this work. The phosphor is prepared by heat treatment technique. It is observed that the intensity of emission varies with the applied voltage and excitation frequency according to the universal laws. Pr is found to enhance the EL intensity of ZnS phosphor. The average grain size and micro strain (μ) of Pr-doped ZnS is found to be 37.25 μm and 0.46, respectively.

器件老化处理对有机发光磁效应的影响

We investigated the influence of electric stress condition on organic magneto-electroluminescence (MEL) in tri-(8-hydroxyquinoline)- aluminum (Alq₃)-based organic light-emitting diodes. The results show that the performance of the devices goes to bad as increasing the aging time; in particular, the threshold voltage becomes larger and the luminescence efficiency gets lower. Moreover, the magnitude of the MEL increases firstly and then gets smaller with the line shape unchanged as the device aging. Possible explanations for the above effects were proposed based on the mechanism of the device aging. We suggested that the quencher, Alq₃ cation-radicals, which formed in the emitting layer, reduce the EL intensity and enhance the magnitude of MEL. In addition, the increase in turn-on voltage and the decrease in MEL are assigned to the degradation at the interface of Al/Alq₃.

GaN HEMT Reliability: From Time Dependent Gate Degradation to On-state Failure Mechanisms

ABSTRACTIn this paper, we compare degradation modes and failure mechanisms of different AlGaN/GaN HEMT technologies. We present data concerning reverse-bias degradation of GaN-based HEMTs, which results in a dramatic increase of gate leakage current, and present a timedependent model for gate degradation. Some of the tested technologies demonstrated to be immune from this failure mechanism up to drain-gate voltages in excess of 100 V. When this was the case, the main failure mode consisted of drain current degradation during on-state tests, resulting from charge trapping in the gate-drain access region attributed to hot-electron effects. Finally, the use of diagnostic techniques such as electroluminescence microscopy and Deep Level Transient Spectroscopy for the identification of failure modes and mechanisms of GaNbased HEMTs is reviewed. Concerning reverse-bias degradation of GaN-based HEMTs, we demonstrate that, (i) when submitted to reverse-gate stress, HEMTs can show both recoverable and permanent degradation. (ii) recoverable degradation consists of a decrease in gate current and threshold voltage, which are ascribed to the simultaneous trapping of negative charge in the AlGaN layer, and of positive charge close to the AlGaN/GaN interface. (iii) permanent degradation is manifested by the generation of parasitic leakage paths. Time-dependent analysis suggests that permanent degradation can be ascribed to a defect generation and percolation process. Results supports the existence of a time to breakdown for HEMT degradation, which significantly depends on the stress voltage level. On the contrary, AlGaN/GaN technologies which were found to be resistant to gate degradation (off-state critical voltage larger than 100 V for a 0.25 um gate device) were subjected to on-state tests at different gate and drain voltage levels. All tests showed a non-recoverable degradation of electrical parameters (drain saturation current, threshold voltage and on-state resistance) and electroluminescence signal EL, with a strong dependence on the EL value of the bias point, and a negligible dependence of temperature. Once verified that EL intensity represents a reliable estimate of channel hot electron effects, we attributed the degradation to hot electron trapping in the gate-drain access region. Using EL intensity as a measure of the stress acceleration factor, we derived an acceleration law for GaN HEMT hot electron degradation similar to the one already demonstrated for GaAs devices.

Development of free-standing InGaN LED devices on Al2O3/Si substrate by wet etching

Free-standing InGaN-based LEDs grown on Al2O3/Si (1 1 1) have been achieved using selective area wet etching. Conventional device design was used for LED fabrication, in which p-type and n-type contacts are located at the same side of the epilayers. These LED devices were bonded to a dual in-line package (DIP), and epoxy was used to protect the front side of the epilayers as well as the bonding wires. The silicon substrate was selectively removed by wet etching while the chip was mounted in a DIP which prevented the thin film from cracking or warping. No significant change in electrical characteristics, peak emission wavelength or EL intensity versus drive current was observed. The substrate-removal process and the challenges involved are discussed. Such packaging techniques could be beneficial for commercial-scale production of InGaN-based LEDs grown on silicon substrates.

Violet-blue LEDs based on p-GaN/n-ZnO nanorods and their stability

In this paper, we report a fabrication, characterization and stability study of p-GaN/n-ZnOnanorod heterojunction light-emitting devices (LEDs). The LEDs were assembled fromarrays of n-ZnO vertical nanorods epitaxially grown on p-GaN. LEDs showedbright electroluminescence in blue (440 nm), although weaker violet (372 nm) andgreen-yellow (550 nm) spectral components were also observed. The device characteristicsare generally stable and reproducible. The LEDs have a low turn-on voltage (∼5 V). Theelectroluminescence (EL) is intense enough to be noticed by the naked eye, at an injection current as lowas ∼ 40 µA (2.1 × 10 − 2 A cm − 2 at 7 V bias). Analysis of the materials, electrical and EL investigationspoint to the role of a high quality of p–n nano-heterojunction whichfacilitates a large rectification ratio (320) and a stable reverse current of 2.8 µA (1.4 × 10 − 3 A cm − 2 at 5 V). Stability of EL characteristics was investigated in detail. EL intensity showed systematicdegradation over a short duration when the LED was bias-stressed at 30 V. At smaller bias (<20 V) LEDs tend to show a stable and repeatable EL characteristic. Thus a simple lowtemperature solution growth method was successfully exploited to realize nanorod/filmheterojunction LED devices with predictable characteristics.

InGaN micro-pixellated light-emitting diodes with nano-textured surfaces and modified emission profiles

We present the fabrication details and performance characteristics of InGaN light-emitting diodes (LEDs) consisting of arrays of interconnected micro-pixels where each micro-pixel is nano-textured via nano-imprinting. We have taken the further step of embodying the pixels in a rhomboidal geometry. It is found that the power output of these nano-textured micro-LEDs with rhomboidal geometries is 57% higher than that of conventional square-shaped broad-area reference LEDs. The series resistance of the textured LEDs is reduced, owing to the multi-finger electrodes introduced. Furthermore, these LEDs can sustain higher operation current of up to 500 mA without encapsulation, suggesting improved thermal dissipation capability. Finally, the combined effects of surface texturing, micro-LED configuration, and geometric shaping on the light extraction are analyzed. It is found that the power enhancement by surface texturing, micro-pixellating and the rhomboidal geometry are 32%, 16%, and 9%, respectively, implying that surface texturing is the most effective contribution to increasing the light extraction efficiency in our design. The angular dependent far-field beam profile is also remarkably changed, compared with the standard Lambertian emission pattern of the conventional square-shaped LEDs. Substantial increase in the EL intensity is evident from both the top surface and the sidewall.

Growth behavior and growth rate dependency in LEDs performance for Mg-doped a-plane GaN

We investigated the influence of growth rate of Mg-doped a-plane GaN on the surface morphological and electrical properties, and the characteristics of InGaN-based nonpolar LEDs. Mg-doped a-plane GaN layers were grown on r-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cathode luminescence (CL) analysis exhibited that the surface morphology changed from stripe features with large triangular pits to rough and rugged surface with small asymmetric V-shape pits, as the growth rate increased. The Mg incorporation into a-plane GaN layers increased with increasing growth rate of Mg-doped a-plane GaN, while the activation efficiency of Mg dopants decreased in a-plane GaN. Additionally, it was found that operation voltage at 20 mA decreased in characteristics of LEDs, as the growth rate of Mg-doped a-plane GaN decreased. Meanwhile, the EL intensity of LEDs with p-GaN layers grown at higher growth rate was improved compared to that of LEDs with p-GaN layers grown at lower growth rate. Such an increase of EL intensity is attributed to the rougher surface morphology with increasing growth rate of Mg-doped a-plane GaN.

Enhanced luminescence of GaN-based light-emitting diodes by selective wet etching of GaN/sapphire interface using direct heteroepitaxy laterally overgrowth technique

We introduce a method to enhance the luminescence of GaN-based LEDs by combining the direct heteroepitaxy laterally overgrowth (DHELO) technique with selective wet etching process. The epitaxial overgrowth of GaN layers on sapphire substrate with SiO 2 micro-rods array exhibited a reduced dislocation density and improved the crystal quality. The EL intensity of LEDs with SiO 2 micro-rods array was 6.5% higher than conventional LEDs at 20 mA. The selective wet etching process was then used to texture the LED sidewalls into inverted pyramid shape. Finally, the EL intensity could be further enhanced about 12.5% as compared with LEDs with SiO 2 micro-rods array when adopting the textured sidewalls.

Study of green light-emitting diodes grown on semipolar (11–22) GaN/m-sapphire with different crystal qualities

We investigated the anisotropic optical and structural properties of semipolar (11–22) InGaN-based green light emitting diodes (LEDs) grown on GaN templates with the different crystallographic properties. By introducing the N 2-GaN as a seed layer grown at a N 2 atmosphere, the full width at half maximum (FWHMs) of X-ray rocking curves (XRCs) for semipolar GaN templates were decreased from 1331 to 727 arcsec and from 1955 to 1076 arcsec with the incident beam directions of [11–2–3] and [1–100], respectively. It was found that the interfacial qualities of InGaN/GaN quantum wells (QWs) would be improved by reducing the FWHMs of XRCs with regardless of crystallographic directions. However, the thickness uniformity of InGaN QWs was significantly deteriorated for the direction of [11–2–3] rather than [1–100]. In addition, the EL intensity of semipolar green LEDs would be increased by enhancing the crystal quality of semipolar GaN template, which could also be resulted in the formation of abrupt interface and the enhancement of homogeneity at InGaN/GaN QWs.

The Joint Measurements of Electroluminescence and Photoluminescence of PET Films under Low Temperature

The EL intensity and the spectrum of the PET films were tested under the low temperature by home-made experimental set-up on the conditions of DC high electric fields and irradiated by xenon lamp. The result shows that there are emission peaks at about 350~450nm, 500~600nm and 750nm in the EL spectrum of the PET films. The intensity of light emission increases with the applied electric fields; the pre-breakdown field is about 3.3MV/cm, this threshold value is no significant change which compares with the normal temperature.