Injection Current Level Research Articles

Electroluminescence and infrared imaging of fielded photovoltaic modules: A complementary analysis of series resistance-related defects

Fielded photovoltaic (PV) modules often exhibit series resistance-related defects, mainly due to solder bond degradation and metallization corrosion. To quantitatively analyze the series resistance increase in fielded modules, we conducted a detailed investigation on two sets of modules, employing two complementary techniques, electroluminescence (EL) and infrared (IR) imaging. The dependence of EL image characteristics on module temperature, as revealed in IR images, was a key focus of this study. The first and second sets of modules were exposed in Florida (hot and humid climate) and Arizona (hot and dry climate) over 10 years and 18 years, respectively. The resistive defect patterns obtained using EL and IR images showed a closer correlation for the Florida modules compared to the Arizona modules as the Florida modules primarily experience solder bond degradation. EL and IR images were acquired at five current injection levels (i.e., 0.1, 0.3, 0.5, 0.7, 1.0 x short circuit current) and two exposure times (i.e., 60 s and 300 s) and used to develop and report a new curve fitting method for estimating the external series resistance. The results indicate that inaccurate temperature determinations from IR images can lead to underestimations (up to 23 %) in EL-based external series resistance estimates. For the most accurate series resistance estimation, especially in modules with severe thermal defects and series resistance deterioration, the study recommends obtaining EL and IR images within 60 s of the current injection time. This study also reports a Monte Carlo simulation assessing the impact of EL and IR characteristics on the accuracy of external series resistance estimations.

Direct modulation of electrically pumped coupled microring lasers.

We demonstrate how the presence of gain-loss contrast between two coupled identical resonators can be used as a new degree of freedom to enhance the modulation frequency response of laser diodes. An electrically pumped microring laser system with a bending radius of 50 μm is fabricated on an InAlGaAs/InP MQW p-i-n structure. The room temperature continuous wave (CW) laser threshold current of the device is 27 mA. By adjusting the ratio between the injection current levels in the two coupled microrings, our experimental results clearly show a bandwidth improvement by up to 1.63 times the fundamental resonant frequency of the individual device. This matches well with our rate equation simulation model.

Shifting LED emission from blue to the green gap spectral range using In0.12Ga0.88N relaxed templates

In y Ga 1-y N templates are grown with y ≤ 13.5% and a few nm surface roughness . These templates are used successfully to address two of the main issues facing long wavelength emitting LEDs, mainly the low growth temperature and high values of strain in the quantum wells (QWs). In this work, three LED structures are investigated: the first is a blue LED grown on GaN, the second and third are green LEDs grown on relaxed In y Ga 1-y N templates with y of about 10% and 12%, respectively. The same multiple quantum wells (MQWs) were used in the three LED structures, with the same well width, barrier width, and growth temperature. The reduced strain in the QWs due to the use of InGaN templates enhances the indium incorporation rate in the QWs. Red shift in emission wavelength of about 100 nm, from 470 nm to 570 nm, was achieved, at low injection current. Optical output power and external quantum efficiency (EQE) measurements showed that at high level of current injection, performance of the blue LED is about twice of the green emitting LEDs on InGaN templates. The current results indicate the potential of the InGaN template approach, with high values of y , in addressing problems facing long wavelength InGaN LEDs. • Relaxed InGaN templates are used to shift blue LED emission from blue to the green gap spectral range. • Indium content>10% shows surface roughness of few nanometers and pit density in the 10 7 cm −2 to the mid 10 8 cm −2 range. • Performance of the blue the blue LED on GaN is about twice that of green LED on InGaN templates.

Photo- and electro-luminescence studies of semipolar (112¯2) InxAl1−xN

InxAl1−xN (0.15 < x < 0.4) single layers (thickness = 300 nm) and light emitting diodes (LEDs) with InxAl1−xN (x = 0.24 or 0.28) quantum wells (QWs) were grown on (112¯2) free-standing GaN substrates by metal organic chemical vapor deposition. The emission characteristics of InxAl1−xN films and QWs were then investigated by photoluminescence (PL), cathodoluminescence (CL), and electroluminescence (EL). PL emission down at 1.87 eV (∼660 nm) was observed from In0.37Al0.63N at room temperature. However, its emission wavelength and spectral width showed almost no temperature-dependence, strongly suggesting the radiative recombination process in In0.37Al0.63N is heavily affected by the potential fluctuations associated with the defect levels such as deep donor–acceptor pairs (DAPs). For In0.23Al0.77N, the emission peaks at 3.24 and 2.4 eV were observed at 300 K but CL showed no evidence of compositional fluctuations. Given the wide spectral width and weak In clustering, PL at 3.24 eV is likely from a shallow DAP transition involving an impurity-related shallow donor and valence band. PL emission at 2.4 eV, on the other hand, behaved more like In0.37Al0.63N, suggesting significant deep DAP transitions. Both LEDs with In0.24Al0.76N and In0.28Al0.72N QWs showed a discrete blueshift of the emission peak with increasing injection current, which is very different from the gradual shift observed in InGaN LEDs. The high-energy peak was much narrower than the low-energy peak, suggesting it could be the near-band edge emission. Si doping of In0.24Al0.76N QWs lowered the injection current level at which the high-energy peak appears.

Nearly Efficiency-Droop-Free AlGaN-Based Deep-Ultraviolet Light-Emitting Diode Without Electron-Blocking Layer

Abstract An aluminum-rich AlGaN layer is commonly implemented to act as an electron-blocking layer (EBL) to block electron overflow from the active region in the conventional deep-ultraviolet light-emitting diodes (DUV LEDs). Herein, we propose a DUV LED device architecture with specially designed band-engineered quantum barriers (QBs) to “serve” as an alternative approach to alleviate such overflow effect, suppressing the electron leakage, and facilitating the electron and hole injection into the active region for efficient radiative recombination. Intriguingly, a much smaller efficiency droop with a significant enhancement of light output power (LOP) by nearly 50% can be achieved at the injection current level of 120 mA in such EBL-free device, in comparison with the conventional EBL-incorporated DUV LED structure. Thus, the EBL-free device architecture provides us an alternative path toward the realization of efficient DUV light emitters.

Room-temperature operation of near-infrared light-emitting diode based on Tm-doped GaN with ultra-stable emission wavelength

Near-infrared (NIR) light with a wavelength of 650–950 nm is used for various biomedical applications. Although NIR emitters are typically based on GaAs-related materials, they contain toxic elements, and the emission wavelength can easily shift during the device operation due to temperature changes and current injection levels. On the other hand, Tm3+, which is one of the rare-earth ions, can generate ultra-stable NIR luminescence with a wavelength of ∼800nm, based on 3H4–3H6 transitions in a 4f shell, and we have recently focused on Tm-doped GaN (GaN:Tm) based light-emitting diodes (LEDs) as novel NIR emitters. In this paper, we present a demonstration of a NIR-LED based on GaN:Tm grown by the organometallic vapor phase epitaxy method with optimized growth conditions and structures, where the parasitic reaction is well suppressed. NIR luminescence from the GaN:Tm-based LED is derived from 3H4–3H6 transitions of Tm3+ ions and consists of three dominant peaks at 795, 806, and 814 nm. The turn-on voltage of the NIR-LED is ∼6.9 V, and it is significantly lower than the previously reported electroluminescent devices based on GaN:Tm with impact ionization processes. From a current dependence of the electroluminescence spectra and temperature-dependent photoluminescence for the NIR-LED, the peak shifts are determined to be <7.6 pm/mA and ∼1.3 pm/K, respectively.

Evaluation of polarization field in InGaN/GaN multiple quantum well structures by using electroluminescence spectra shift**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0400803 and 2016YFB0401801) and the National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, and 61574134).

In order to investigate the inherent polarization intensity in InGaN/GaN multiple quantum well (MQW) structures, the electroluminescence (EL) spectra of three samples with different GaN barrier thicknesses of 21.3 nm, 11.4 nm, and 6.5 nm are experimentally studied. All of the EL spectra present a similar blue-shift under the low-level current injection, and then turns to a red-shift tendency when the current increases to a specific value, which is defined as the turning point. The value of this turning point differs from one another for the three InGaN/GaN MQW samples. Sample A, which has the GaN barrier thickness of 21.3 nm, shows the highest current injection level at the turning point as well as the largest value of blue-shift. It indicates that sample A has the maximum intensity of the polarization field. The red-shift of the EL spectra results from the vertical electron leakage in InGaN/GaN MQWs and the corresponding self-heating effect under the high-level current injection. As a result, it is an effective approach to evaluate the polarization field in the InGaN/GaN MQW structures by using the injection current level at the turning point and the blue-shift of the EL spectra profiles.

Doping-induced energy barriers to improve the current spreading effect for AlGaN-based ultraviolet-B light-emitting diodes

Current crowding effect for AlGaN based ultraviolet-B light-emitting diodes (UVB LEDs) is normally observed at high injection current levels, which can cause non-uniform optical emission and generate very high local heat. In this work, UVB LED with a PNP-AlGaN current spreading layer is fabricated and studied. The PNP-AlGaN current spreading layer is obtained by sandwiching the n-AlGaN layer into the p-AlGaN layer. Owing to the modulated energy band profiles by the different doing types, the formed energy barriers for holes in PNP-AlGaN layer can make the current more uniformly distributed laterally and therefore the hole injection efficiency at the mesa edges can be enhanced. As a result, the measured external quantum efficiency (EQE) and optical output power for the proposed UVB LED is increased. Moreover, thanks to the enhanced current spreading effect, the thermal droop for the optical output power is suppressed and the forward voltage decreases for the proposed architecture.

Experimental Investigation of Lasing Modes in Double-Lattice Photonic-Crystal Resonators and Introduction of In-Plane Heterostructures

Photonic-crystal surface-emitting lasers (PCSELs) are attractive for a wide range of applications such as material processing and remote sensing owing to their advantage of broad-area coherent lasing supported by a 2-D photonic crystal. Recently, we proposed double-lattice photonic-crystal resonators and achieved 10-W-class high-power and high-beam-quality (namely, high-brightness) operation. To further increase the brightness, it is important to understand the lasing mode behavior of PCSELs in detail. In this article, we experimentally investigate the change in the lasing mode depending on the current injection levels and compare the results with a theoretical analysis that accounts for the change in refractive index and gain distributions due to the current injection. We observed a transition from single-mode lasing to two-mode lasing at high injection currents. We develop a technique for resolving the near-field profiles of individual lasing modes and show that these profiles coincide with theoretical predictions. A comparison of experimental and theoretical results reveals that high-order modes appear due to a bandgap confinement effect induced by a change in the refractive index in the carrier injection area. To effectively remove this confinement effect and suppress the oscillation of high-order modes, we incorporate an in-plane heterostructure into the PCSEL.

Multiwavelength Single Nanowire InGaAs/InP Quantum Well Light-Emitting Diodes.

We report multiwavelength single InGaAs/InP quantum well nanowire light-emitting diodes grown by metal organic chemical vapor deposition using selective area epitaxy technique and reveal the complex origins of their electroluminescence properties. We observe that the single InGaAs/InP quantum well embedded in the nanowire consists of three components with different chemical compositions, axial quantum well, ring quantum well, and radial quantum well, leading to the electroluminescence emission with multiple wavelengths. The electroluminescence measurements show a strong dependence on current injection levels as well as temperatures and these are explained by interpreting the equivalent circuits in a minimized area of the device. It is also found that the electroluminescence properties are closely related to the distinctive triangular morphology with an inclined facet of the quantum well nanowire. Our study provides important new insights for further design, growth, and fabrication of high-performance quantum well-based nanowire light sources for a wide range of future optoelectronic and photonic applications.

Effects of interfaces and current spreading on the thermal transport of micro-LEDs for kA-per-square-cm current injection levels.

The three-dimensional thermal characteristics of micro-light-emitting diodes (μLEDs) on GaN and sapphire substrates were studied with forward-voltage methods, thermal transient measurements, and infrared thermal imaging. The μLEDs on the GaN substrate showed an approximately 10 °C lower junction temperature and smaller amplitude of the K factors than those on the sapphire substrate under the current injection level of 4 kA cm−2. The thermal transient measurement showed that the spreading thermal resistances of the mesa, the GaN epilayer, and the interface of the GaN/substrate were reduced significantly for μLEDs on the GaN substrate because of the high-quality GaN crystal and the homogeneous interfaces. The infrared thermal images showed lower total average junction temperatures and more uniform temperature distributions for the μLEDs on the GaN substrate, which were also simulated with APSYS software. The thermal transport mechanisms are discussed for the lateral and vertical directions in the μLEDs.

Spikeling: A low-cost hardware implementation of a spiking neuron for neuroscience teaching and outreach.

Understanding how neurons encode and compute information is fundamental to our study of the brain, but opportunities for hands-on experience with neurophysiological techniques on live neurons are scarce in science education. Here, we present Spikeling, an open source in silico implementation of a spiking neuron that costs £25 and mimics a wide range of neuronal behaviours for classroom education and public neuroscience outreach. Spikeling is based on an Arduino microcontroller running the computationally efficient Izhikevich model of a spiking neuron. The microcontroller is connected to input ports that simulate synaptic excitation or inhibition, to dials controlling current injection and noise levels, to a photodiode that makes Spikeling light sensitive, and to a light-emitting diode (LED) and speaker that allows spikes to be seen and heard. Output ports provide access to variables such as membrane potential for recording in experiments or digital signals that can be used to excite other connected Spikelings. These features allow for the intuitive exploration of the function of neurons and networks mimicking electrophysiological experiments. We also report our experience of using Spikeling as a teaching tool for undergraduate and graduate neuroscience education in Nigeria and the United Kingdom.

Total Ionizing Dose Effects of SiGe HBTs Induced by 60Co Gamma-Ray Irradiation

For evaluating the SiGe heterojunction bipolar transistors (HBTs) performance during and after gamma-ray irradiation, the characteristics of the direct current (dc) gain of the test samples changed with increasing irradiation dose, and collector current injection levels during 60Co gamma irradiation were measured and analyzed. The experimental results of the typical dc and alternating current (ac) electronic parameters before and after irradiation revealed that the base current Ib, the collector current IC, the dc current gain, and the maximum oscillation frequency fmax exhibited degradation. While other electronic parameters including the cut-off frequency fT, the ac current gain |H21|, and output capacitance CBC did not exhibit any significant change compared with those of pre-irradiation. The degradation mechanisms of the typical dc and ac parameters measured in this work were primarily analyzed. The tested results of SiGe HBTs offered some reference data for evaluating the behavior of the test type of device operated in an ionizing radiation environment.

Terahertz light-emitting graphene-channel transistor toward single-mode lasing

AbstractA distributed feedback dual-gate graphene-channel field-effect transistor (DFB-DG-GFET) was fabricated as a current-injection terahertz (THz) light-emitting laser transistor. We observed a broadband emission in a 1–7.6-THz range with a maximum radiation power of ~10 μW as well as a single-mode emission at 5.2 THz with a radiation power of ~0.1 μW both at 100 K when the carrier injection stays between the lower cutoff and upper cutoff threshold levels. The device also exhibited peculiar nonlinear threshold-like behavior with respect to the current-injection level. The LED-like broadband emission is interpreted as an amplified spontaneous THz emission being transcended to a single-mode lasing. Design constraints on waveguide structures for better THz photon field confinement with higher gain overlapping as well as DFB cavity structures with higher Q factors are also addressed towards intense, single-mode continuous wave THz lasing at room temperature.

A Study of Efficiency Droop Phenomenon in GaN-Based Laser Diodes before Lasing.

Carrier recombination behavior in c-plane GaN-based laser diodes (LDs) is numerically investigated by using the commercial software LASTIP. It is found that efficiency droop phenomenon does exist in GaN-based LDs before lasing, which is confirmed by experimental results. However, the current density corresponding to the peak efficiency of GaN-based LDs before lasing, Jmax, is nearly 40 A/cm2, which is much lower than that reported by other studies. The reported Jmax, measured from the cavity facet side is modulated by the absorption of quantum wells, which shifts the Jmax to a higher value. In addition, the currents due to various recombinations are calculated. It is found that Auger recombination affects the threshold current greatly, but it only plays a small role at high current injection levels.

Waveguide-coupled nanopillar metal-cavity light-emitting diodes on silicon

Nanoscale light sources using metal cavities have been proposed to enable high integration density, efficient operation at low energy per bit and ultra-fast modulation, which would make them attractive for future low-power optical interconnects. For this application, such devices are required to be efficient, waveguide-coupled and integrated on a silicon substrate. We demonstrate a metal-cavity light-emitting diode coupled to a waveguide on silicon. The cavity consists of a metal-coated III–V semiconductor nanopillar which funnels a large fraction of spontaneous emission into the fundamental mode of an InP waveguide bonded to a silicon wafer showing full compatibility with membrane-on-Si photonic integration platforms. The device was characterized through a grating coupler and shows on-chip external quantum efficiency in the 10−4–10−2 range at tens of microamp current injection levels, which greatly exceeds the performance of any waveguide-coupled nanoscale light source integrated on silicon in this current range. Furthermore, direct modulation experiments reveal sub-nanosecond electro-optical response with the potential for multi gigabit per second modulation speeds.

Reverse recovery transient characteristic of PEDOT:PSS/n-Si hybrid organic-inorganic heterojunction

We study the junction behavior of poly (3,4-ethylenedioxythiophene):polystyrenesulphonate/n-Si hybrid organic/inorganic heterojunction by reverse recovery transient (RRT) characterization. RRT response for PEDOT:PSS/n-Si hybrid junction is reported for various n-Si doping concentration and forward bias current injection level. The presence of settling time of 8.3–23.5 μs in the RRT response in contradiction to Schottky junction model commonly assumed for PEDOT:PSS/n-Si hybrid structure. The decrease in the minority carrier lifetime from 126.8 μs to 39.5 μs with increased n-Si doping concentration, suggests that minority carriers are stored at n-Si side of the junction, which is consistent with a p+-n junction model for the hybrid structure. The minority carrier lifetime is found to depend on forward bias current injection level, attributed to trap-saturation effect of the recombination-centers at the PEDOT:PSS/n-Si junction. The DC-IV characteristics of the PEDOT:PSS/n-Si hybrid junction are also consistent with the notion of diffusion and trap assisted recombination dominated dark current. The diffusion dominated transport of PEDOT:PSS/n-Si leads to an ideal p+-n junction behavior that leverages on the good transport properties of Si. Our findings are important in the modeling and optimization of the characteristics of electronic devices based on the organic/Si hybrid junction.

Simultaneous Bayesian Estimation of Excitatory and Inhibitory Synaptic Conductances by Exploiting Multiple Recorded Trials.

Advanced statistical methods have enabled trial-by-trial inference of the underlying excitatory and inhibitory synaptic conductances (SCs) of membrane-potential recordings. Simultaneous inference of both excitatory and inhibitory SCs sheds light on the neural circuits underlying the neural activity and advances our understanding of neural information processing. Conventional Bayesian methods can infer excitatory and inhibitory SCs based on a single trial of observed membrane potential. However, if multiple recorded trials are available, this typically leads to suboptimal estimation because they neglect common statistics (of synaptic inputs (SIs)) across trials. Here, we establish a new expectation maximization (EM) algorithm that improves such single-trial Bayesian methods by exploiting multiple recorded trials to extract common SI statistics across the trials. In this paper, the proposed EM algorithm is embedded in parallel Kalman filters or particle filters for multiple recorded trials to integrate their outputs to iteratively update the common SI statistics. These statistics are then used to infer the excitatory and inhibitory SCs of individual trials. We demonstrate the superior performance of multiple-trial Kalman filtering (MtKF) and particle filtering (MtPF) relative to that of the corresponding single-trial methods. While relative estimation error of excitatory and inhibitory SCs is known to depend on the level of current injection into a cell, our numerical simulations using MtKF show that both excitatory and inhibitory SCs are reliably inferred using an optimal level of current injection. Finally, we validate the robustness and applicability of our technique through simulation studies, and we apply MtKF to in vivo data recorded from the rat barrel cortex.

Noninvasive Electric Current Induction for Low-Frequency Tissue Conductivity Reconstruction: Is It Feasible With a TMS-MRI Setup?

Noninvasive quantification of subject-specific low-frequency brain tissue conductivity () will be valuable in different fields, for example, neuroscience. Magnetic resonance (MR)-electrical impedance tomography allows measurements of . However, the required high level of direct current injection leads to an undesirable pain sensation. Following the same principles, but avoiding pain sensation, we evaluate the feasibility of inductively inducing currents using a transcranial magnetic stimulation (TMS) device and recording the magnetic field variations arising from the induced tissue eddy currents using a standard 3 T MR scanner. Using simulations, we characterize the strength of the incident TMS magnetic field arising from the current running in the TMS coil, the strength of the induced magnetic field arising from the induced currents in tissues by TMS pulses, and the MR phase accuracy required to measure this latter magnetic field containing information about . Then, using TMS-MRI measurements, we evaluate the achievable phase accuracy for a typical TMS-MRI setup. From measurements and simulations, it is crucial to discriminate the incident from the induced magnetic field. The incident TMS magnetic field range is ±10−4 T, measurable with standard MR scanners. In contrast, the induced TMS magnetic field is much weaker (±10−8 T), leading to an MR phase contribution of ∼10−4 rad. This phase range is too small to be measured, as the phase accuracy for TMS-MRI experiments is ∼10−2 rads. Thus, although highly attractive, noninvasive measurements of the induced TMS magnetic field, and therefore estimations of , are experimentally not feasible.

Systematic study of GeSn heterostructure-based light-emitting diodes towards mid-infrared applications

Temperature-dependent characteristics of GeSn light-emitting diodes with Sn composition up to 9.2% have been systematically studied. Such diodes were based on Ge/GeSn/Ge double heterostructures (DHS) that were grown directly on a Si substrate via a chemical vapor deposition system. Both photoluminescence and electroluminescence spectra have been characterized at temperatures from 300 to 77 K. Based on our theoretical calculation, all GeSn alloys in this study are indirect bandgap materials. However, due to the small energy separation between direct and indirect bandgap, and the fact that radiative recombination rate greater than non-radiative, the emissions are mainly from the direct Γ-valley to valence band transitions. The electroluminescence emissions under current injection levels from 102 to 357 A/cm2 were investigated at 300 K. The monotonic increase of the integrated electroluminescence intensity was observed for each sample. Moreover, the electronic band structures of the DHS were discussed. Despite the indirect GeSn bandgap owing to the compressive strain, type-I band alignment was achieved with the barrier heights ranging from 11 to 47 meV.