Relative External Quantum Efficiency Research Articles

MOCVD grown ultraviolet microlight-emitting diodes with an InGaN/GaN/AlGaN active region design

In this work, we present an InGaN/GaN/AlGaN active region design for high-efficiency UV LED grown by MOCVD. The use of the GaN layer allows the AlGaN layer to be grown in optimized conditions which will enhance the overall crystal quality and device performance. The InGaN/GaN/AlGaN structure showed an improvement in the internal quantum efficiency (IQE) simulation in comparison with the conventional InGaN/AlGaN structure, with 15% and 14% at operating current density of 20 and 30 A/cm2, respectively. The IQE advantage remains at more than 10% in the operating current density range from 0 to 100 A/cm2 due to an enhanced electron-hole wavefunction overlap. The InGaN/GaN/AlGaN quantum wells were utilized to grow UV-A structures and micro-LEDs with sizes ranging from 20 × 20 to 100 × 100 μm2 were fabricated. The fabricated micro-LEDs showed a reliable emission at around 372 nm with 9 nm FWHM. Micro-LED devices with smaller sizes showed an improvement in relative external quantum efficiency due to its advantage in current spreading. This manuscript presents the potential to achieve both improved crystal quality and high-efficiency device using InGaN/GaN/AlGaN active region design.

Design and optimization of high temperature optocouplers as galvanic isolation

The commercial InGaN-based (blue and green) and AlGaInP-based (red) multiple quantum well (MQW) lighting emitting diodes (LEDs) were studied in a wide range of temperatures up to 800 K for their light emission and detection (i.e., LEDs operated under reverse bias as photodiodes (PDs)) characteristics. The results indicate the feasibility of integrating a pair of selected LEDs to fabricate high temperature (HT) optocouplers, which can be utilized as galvanic isolation to replace the bulky isolation transforms in the high-density power modules. A detailed study on LEDs and PDs were performed. The external quantum efficiency (EQE) of the LED and PDs were calculated. Higher relative external quantum efficiency (EQE) and lower efficiency droops with temperatures are obtained from the blue and green LEDs for display compared with the blue one for lighting and red LED for display. The blue for lighting and red for display devices show superior responsivity, specific detectivity (D*), and EQE compared with blue and green for display when operated as PDs. The results suggest that red LED devices for display can be used to optimize HT optocouplers due to the highest wavelength overlapping compared with others.

Understanding the degradation mechanisms of InGaN/GaN multiple quantum well UV photodetectors submitted to different current stresses.

The degradation characteristics of InGaN/GaN multiple quantum well (MQW) photodetectors (PDs) stressed at 100 and 200 mA over 480 h are investigated. We have observed that the luminescence intensity, short circuit current density, and open circuit voltage decrease strongly, whereas the leakage current increases intensely due to the constant current stress. The strong activity of the Mg dopant and trap-assisted tunneling under the direct current stress are critical factors in the degradation of InGaN/GaN MQW PDs. Further, the photocurrent spectroscopy results reveal that for 100 mA stress current, the peak value of relative external quantum efficiency (EQE) slightly increases due to the widening of the space-charge region while, for the 200 mA of stress current, the peak value of EQE decreases (∼15.4%) due to some permanent damages in the active region and/or the metal/semiconductor interface, and the associated resistive effects.

Electron Blocking Layer Free Full-Color InGaN/GaN White Light-Emitting Diodes

Indium gallium nitride (InGaN) compound semiconductor alloys have been intensively studied and utilized for white light-emitting diodes (LEDs) [1, 2]. Compared to conventional planar structures, III-nitride nanowires can exhibit significant advantages including greatly reduced dislocation densities and polarization fields, due to the effective lateral stress relaxation. Additionally, the nanowire LEDs with the incorporation of quantum dots/disks in the active region may lead to superior carrier confinement, promising for high efficiency LEDs with tunable emission [2]. However, nanowire LEDs still contain several challenges to further improve the quantum efficiency and light output power, which includes inefficient carrier confinement in the active region, nonuniform carrier distribution, and electron overflow. The integration of an electron blocking layer (EBL) in the LED mitigates electron leakage problem to an extent but it also lowers the hole injection into the active region if not designed properly causing poor hole transport problem. Hence, the LED without EBL, with efficient carrier confinement capability, negligible carrier leakage from active region is the optimal choice [3].In this work, we have demonstrated EBL free high-performance phosphor-free InGaN/GaN nanowire white LEDs. The proposed structure is grown on Si (111) substrate by RF plasma-assisted MBE (Veeco Gen II MBE) under nitrogen-rich condition, then fabricated devices for characterization. The proposed LED structure (LED3) consists of a 200 nm thick n-GaN nanowire template, 10 multiple quantum wells (MQWs) of 3 nm GaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) in the active region and a 100 nm thick p-GaN. In addition, we have incorporated 30 nm n-In0.2Ga0.8N QW between the n-GaN and the active region to control electron overflow. Moreover, to utilize the electrons escaped from the active region, a second 10 nm p-In0.2Ga0.9N QW is introduced between the active region and p-GaN. This second QW reduces electron loss to the p-GaN and contributes blue light emission to relatively control the white light emission from the LED device. The schematic diagram of the proposed structure is shown in Fig. 1(a). A comparison has been made with the conventional LED structure (LED1: 200 nm n-GaN, 10 MQWs (3 nm)/MQBs (3 nm), 100 nm p-GaN) and EBL based structure (LED2: 200 nm n-GaN, 10 MQWs (3 nm)/MQBs (3 nm),10 nm p-Al0.1Ga0.9N EBL, 100 nm p-GaN) to understand the enhanced performance of the proposed LED structure. As a result, we observed that proposed LED has better performance in terms of less electron leakage, high hole injection, improved internal quantum efficiency (IQE), external quantum efficiency (EQE) and output optical power as compared to other conventional and EBL based InGaN/GaN LEDs. The resulting device exhibits high IQE of 58.5% with highly stable emission characteristics. The relative EQE and output optical power of the LEDs are shown in Figs. 1(b) and 1(c). Such unique EBL free nanowire LED structure is a promising approach to achieve high-power phosphor-free LEDs for general illumination and display applications.

GaN/InGaN Blue Light‐Emitting Diodes on Polycrystalline Molybdenum Metal Foils by Ion Beam‐Assisted Deposition

Light‐emitting diode (LED) arrays fabricated on a polycrystalline metal substrate are demonstrated using a novel technique that enables the growth of epitaxial metal‐organic chemical vapor deposition (MOCVD) GaN layers on non‐single‐crystal substrates. Epitaxial GaN is deposited directly on metal foil using an intermediate ion beam‐assisted deposition (IBAD) aligned layer. For a single 170 μm‐diameter LED on the metal foil, electroluminescence (EL) spectrum shows a peak wavelength of ≈452 nm and a full width at half maximum (FWHM) of ≈24 nm. The current–voltage (I–V) characteristics show a turn‐on voltage of 3.7 V, a series resistance of 10 Ω. LEDs on metal show a relative external quantum efficiency (EQE) that is roughly 3× lower than that of similar LEDs fabricated on a sapphire substrate. InGaN LEDs on large‐area non‐single‐crystal substrates such as metal foils enable large‐area manufacturing, reducing production cost, and opening the door for new applications in lighting and displays.

Oblique-Angle Deposited SiO2/Al Omnidirectional Reflector for Enhancing the Performance of AlGaN-Based Ultraviolet Light-Emitting Diode

We report the improvement of the performance of AlGaN-based ultraviolet (UV)-light-emitting diodes (LEDs) using oblique-angle deposited SiO2-based omnidirectional reflectors (ODRs). The electron-beam (e-beam) deposition at angles of 60° and 80° resulted in the formation of porous SiO2 films. The refractive index (n) varied from 1.49 to 1.25 at 365 nm with changing angle from 0 to 80°. Simulation based on the transfer matrix method showed that the porous SiO2 (n = 1.25)/Al ODR gave the normal incidence reflectance of 95.4% at 365 nm, whereas the conventional SiO2 (n = 1.49)/Al and ITO/Al reflectors have the normal incidence reflectance of 93.8% and 79%, respectively. The UV-LED with different reflectors had a forward voltage in the range of 3.40–3.51 V at 20 mA. The UV-LEDs with porous SiO2/Al ODRs (40% mesh ohmic area) yielded 21% higher light output at 100 mA and 24% relative external quantum efficiency (EQE) at 6 mA compared with the one with a reference ITO/Al reflector. The plan-view emission images showed that the SiO2/Al ODR UV-LED displayed better current spreading than the reference one with the ITO/Al reflector experiencing current crowding near the p-pad.

Fabrication and Characterization of GaN-Based Micro-LEDs on Silicon Substrate**Supported by the National Key Research and Development Program of China under Grant No 2016YFB0400100, the National Natural Science Foundation of China under Grant Nos 61674076, 61674081 and 61605071, the Natural Science Foundation of Jiangsu Province under Grant Nos BY2013077, BK20141320 and BE2015111, the Six Talent Peaks Project of Jiangsu Province under

GaN-based micro light emitting diodes (micro-LEDs) on silicon (Si) substrates with 40 μm in diameter are developed utilizing standard photolithography and inductively coupled plasma etching techniques. From current-voltage curves, the relatively low turn-on voltage of 2.8 V and low reverse leakage current in the order of 10−8 A/cm2 indicate good electrical characteristics. As the injection current increases, the electroluminescence emission wavelength hardly shifts at around 433 nm, and the relative external quantum efficiency slightly decays, because the impact of quantum-confined Stark effect is not serious in violet-blue micro-LEDs. Since GaN-LEDs are cost effective on large-area Si and suitable for substrate transfer or vertical device structures, the fabricated micro-LEDs on Si should have promising applications in the fields of high-resolution display and optical communication.

Surface Passivation toward Highly Stable Mn4+‐Activated Red‐Emitting Fluoride Phosphors and Enhanced Photostability for White LEDs

AbstractMn4+‐activated red‐emitting fluoride phosphors are indispensable candidates for white light emitting diodes (WLEDs) with enhanced color rendition, however, their intrinsic hydrolysis characteristics seriously restricted the durability applications. Here, a facile and general postsynthetic hydrogen peroxide (H2O2) surface passivation strategy is proposed to treat the K2XF6:Mn4+ (KXF, X = Ti, Si, Ge) phosphors, in which a Mn4+‐rare surface protective layer appears and further covers the corresponding particles. The environment moisture can be effectively isolated by the Mn4+‐rare surface passivation layer with low solubility, which will be sacrificed to protect the phosphor particles even if under extreme hydrolysis conditions. The relative external quantum efficiency of the optimized phosphors still maintains over 96% after the passivation treatment, and the relative luminous intensity still remain 97% even when soaked in water after 12 h. The correlated color temperature of high‐power WLEDs fabricated by the passivated phosphors has no remarkable change during aging process (100 days) in the high temperature (85 °C) and high humidity atmosphere (85%). It is expected that such a surface‐redox strategy can be expanded to other doped materials systems, and also opening a new perspective for the development of luminescence materials with enhanced surface stability and device duration.

Enhanced near-infrared response of a silicon solar cell by using an up-conversion phosphor film of Yb/Er – co-doped CeO2

Co-doped CeO2 thin films were grown from a bulk target with starting composition Ce0.95Yb0.04Er0.01O2 by pulsed laser deposition (PLD) on a p+-n-n+ single crystal silicon diode. The PLD laser fluence was varied between 1.7 J/cm2 and 3.7 J/cm2. The device with the film grown for a laser fluence of 2.3 J/cm2 delivers the highest performance taking advantage of the up conversion (UC) effect provided by this film. Namely, the increase in the relative power conversion efficiency of the device is 12.1% and 39.2% for illumination under 1 and 2.1 sun, respectively, and its relative external quantum efficiency is 8.2% when illuminated with 980 nm light. The film grown for the optimum 2.3 J/cm2 fluence shows good target-film composition transfer and a granular morphology with a low roughness. The UC mechanism consists of efficient energy transfer between spatially separated Yb3+ and Er3+ ions, i.e. the absorption of infrared light photons by the Yb3+ ions (2F7/2 → 2F5/2 transition) is followed by a two-step energy transfer process to neighboring Er3+ ions and by their characteristic luminescent emissions ((2H11/2, 4S3/2) → 4I15/2) and (4F9/2 → 4I15/2).

Measuring the Internal Quantum Efficiency of Light-Emitting Diodes at an Arbitrary Temperature

We present a method of measuring the internal quantum efficiency (IQE) of light-emitting diodes (LEDs) as a function of driving current at an arbitrary temperature only with experimentally measurable quantities of optical power and current. The measurement procedure starts to find an exact value of the IQE at the reference point where the injection efficiency is considered as 100% and the radiative efficiency is calculated from the infinitesimal change of the relative external quantum efficiency (EQE). Once the IQE at the reference point is exactly known, the IQE at any other point is calculated by the relative ratio of the EQE values to the reference one. We show the theoretical formalism of the proposed method and demonstrate its validity using a commercial InGaN blue LED as an example. The comparison with the IQE obtained by the conventional temperature-dependent electroluminescence is also given for various temperatures.

Addressing limitations of photoluminescence based external quantum efficiency measurements

The spectral response of photoluminescence is a contactless method that provides a measurement of the relative external quantum efficiency of silicon solar cells and wafers. This method is accurate only if the measured luminescence originates from the radiative recombination of voltage dependent carriers. This paper investigates the impact of luminescence from voltage independent carriers in heavily diffused regions and other spurious sources of luminescence, such as luminescence from dielectric layers. A method, based on partial shading, is then demonstrated to record luminescence from only the voltage dependent carriers. This method is shown to provide accurate relative external quantum efficiency on cells and partially processed wafers. The relevance of the dependence of the measured data on the angular distribution of the incident light is demonstrated in this context, which explains errors in previously published data.

Relative External Quantum Efficiency of Crystalline Silicon Wafers From Photoluminescence

The external quantum efficiency is routinely measured to determine the carrier collection probability of solar cells as a function of illumination wavelength. The common method used to perform these measurements requires a fully metalized device such that the short circuit current can be measured under monochromatic illumination. This paper demonstrates the extraction of relative external quantum efficiency data from open circuit photoluminescence measurements in a contactless fashion. Good agreement is observed between the photoluminescence-based technique and traditional external quantum efficiency data for illumination wavelengths in the range 400 to 625 nm. Deviations are observed for longer illumination wavelengths and their origin is discussed.

Simultaneously Enhancing Light Emission and Suppressing Efficiency Droop in GaN Microwire-Based Ultraviolet Light-Emitting Diode by the Piezo-Phototronic Effect.

Achievement of p-n homojuncted GaN enables the birth of III-nitride light emitters. Owing to the wurtzite-structure of GaN, piezoelectric polarization charges present at the interface can effectively control/tune the optoelectric behaviors of local charge-carriers (i.e., the piezo-phototronic effect). Here, we demonstrate the significantly enhanced light-output efficiency and suppressed efficiency droop in GaN microwire (MW)-based p-n junction ultraviolet light-emitting diode (UV LED) by the piezo-phototronic effect. By applying a -0.12% static compressive strain perpendicular to the p-n junction interface, the relative external quantum efficiency of the LED is enhanced by over 600%. Furthermore, efficiency droop is markedly reduced from 46.6% to 7.5% and corresponding droop onset current density shifts from 10 to 26.7 A cm-2. Enhanced electrons confinement and improved holes injection efficiency by the piezo-phototronic effect are revealed and theoretically confirmed as the physical mechanisms. This study offers an unconventional path to develop high efficiency, strong brightness and high power III-nitride light sources.

Localized Surface Plasmon-Enhanced Electroluminescence in OLEDs by Self-Assembly Ag Nanoparticle Film.

We fabricated Ag nanoparticle (NP) film in organic light emission diodes (OLEDs), and a 23 times increase in electroluminescence (EL) at 518 nm was probed by time-resolved EL measurement. The luminance and relative external quantum efficiency (REQE) were increased by 5.4 and 3.7 times, respectively. There comes a new energy transport way that localized surface plasmons (LSPs) would absorb energy that corresponds to the electron-hole pair before recombination, promoting the formation of electron-hole pair and exciting local surface plasmon resonance (LSPR). The extended lifetime of Alq3 indicates the existence of strong interaction between LSPR and exciton, which decreases the nonradiative decay rate of OLEDs.

Understanding peculiarities in the optoelectronic characteristics of light emitting diodes based on (In,Ga)N/GaN nanowires

We investigate the effect of the p-type top contact on the optoelectronic characteristics of light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. We compare devices fabricated with either Ni/Au or indium tin oxide (ITO) top contact. The NW-LEDs with ITO exhibit a number density of NWs emitting electroluminescence about ten times higher, significantly lower turn-on voltage and series resistance, and a relative external quantum efficiency more than one order of magnitude higher than the sample with Ni/Au. These results show that limitations in the performance of such devices reported so far can be overcome by improving the p-type top-contact.

Carrier lifetimes in green emitting InGaN/GaN disks‐in‐nanowire and characteristics of green light emitting diodes

AbstractImprovement in the internal quantum efficiency (IQE) of InGaN/GaN disks‐in‐nanowires by surface passivation is demonstrated. The highest IQE achieved through surface passivation for green emitting (λ=540 nm) InGaN/GaN disks‐in‐nanowires is ∼53%. Radiative and nonradiative carrier lifetimes are calculated for as‐grown and surface passivated green emitting disks‐in‐nanowires. Passivated green sample exhibits a room temperature radiative lifetime of ∼748 ps, which is much smaller than that of equivalent quantum wells. Electroluminescence measurements on passivated green light emitting diodes containing InGaN disks demonstrate no roll over or efficiency droop up to 375 A/cm2, and exhibit a blue‐shift of 7 nm in peak wavelength. An enhancement in the light output due to surface passivation is observable in the relative external quantum efficiency of the surface passivated devices as compared with the as‐grown samples. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN계 청색 발광 다이오드에서 저전류 스트레스 후의 광 및 전기적 특성 변화

We analyzed the changes in electrical and optical characteristics of 1 multiple-quantum-well (MQW) blue LEDs grown on a c-plane sapphire substrate after a stress test. Experiments were performed by injecting 50 mA current for 200 hours to TO-CAN packaged sample chips. We selected the value of injection current for stress through the junction-temperature measurement by using the forward-voltage characteristics of a diode to maintain a sufficiently low junction temperature during the test. The junction temperature at the selected injection current of 50 mA was 308 K. Experiments were performed under the assumption that the average junction temperature of 308 K did not affect the characteristics of the ohmic contact and the GaN-based materials. Before and after the stress test, we measured and analyzed current-voltage, light-current, light distribution on the LED surface, wavelength spectrum and relative external quantum efficiency (EQE). After the stress test, it was observed experimentally that the optical power and the relative EQE decreased. We theoretically investigated and experimentally proved that these phenomena are due to the increased nonradiative recombination rate caused by the increased defect density.

On the quantum efficiency of InGaN light emitting diodes: Effects of active layer design, electron cooler, and electron blocking layer

AbstractEfficiency and efficiency retention in InGaN LEDs has recently received considerable attention. In this realm, we investigated internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) of c‐plane InGaN LEDs designed for emission at ∼420 nm from the active region which contains multiple quantum wells (MQWs) of different barrier height (either In0.01Ga0.99N or In0.06Ga0.94N barriers) and thickness (3 and 12 nm) as well as a 9‐nm double heterostructure (DH). Pulsed electroluminescence (EL) and laser excitation power‐dependent measurements indicated that both the relative EQE and the IQE were enhanced due to the incorporated two‐layer InGaN stair‐case electron injector (SEI) with indium mole fraction steps of 4 and 8% as compared to the conventional AlGaN electron blocking layer (EBL). Furthermore, the lowered In0.06Ga0.94N interwell barriers (LB) instead of the traditional In0.01Ga0.99N barriers improved the EQE and the IQE of MQW LEDs. Specifically, the MQW LEDs with the 6‐period 2‐nm In0.2Ga0.8N quantum well and 3‐nm In0.06Ga0.94N barrier structure showed 6% higher IQE at an injected carrier density of 6 × 1018 cm−3 and 35% higher EQE as compared to that of the same structure with a higher In0.01Ga0.99N barrier. The DH LEDs showed 30% higher EQEs compared to MQW LEDs, albeit at a relatively higher injection current density of 150 A/cm2. The relatively low EQE in the DH LEDs at low injection levels is attributed to spatial separation of electrons and holes due to confinement in the interfacial triangular well and thus the associated decrease in radiative efficiency and possible increase in nonradiative recombination due to degradation of material quality with increasing InGaN layer thickness.

InGaN based light emitting diodes utilizing Ga doped ZnO as a highly transparent contact to p‐GaN

AbstractWe report on the electrical and optical properties of InGaN based light emitting diodes (LEDs) utilizing Ga doped ZnO (GZO) grown by MBE as a highly transparent contact p ‐electrode (GZO‐LEDs). For comparison, the LEDs with semi‐transparent thin Ni/Au (5nm/5nm) current spreading layers were also fabricated on the same wafer (Ni/Au‐LEDs). Pulsed electroluminescence measurements revealed that the relative external quantum efficiency (EQE) of the GZO‐LED is 1.7‐2 times higher than that of Ni/Au‐LED at high current densities. Moreover, GZO‐LEDs can withstand much higher current densities (4700 A/cm2) than the Ni/Au LEDs (3500 A/cm2) under pulsed mode. Unpackaged GZO‐LEDs with 200 μm diameter showed negligible light output degradation for up to 30 mins under CW current of 100 mA (corresponding to a 318 Acm‐2 current density), while the light output for Ni/Au‐LEDs was reduced by 85% after only 5 mins of operation due to the severe current crowding effect (or current filamentation). (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement of efficiency droop of GaN-based light-emitting devices by a rear nitride reflector

GaN-based light-emitting devices (LEDs) with and without a rear distributed Bragg reflector (DBR) were grown by metal–organic chemical vapor deposition. The integrated electroluminescence (EL) intensity of LED with a rear nitride DBR showed a super-linear increase up to a current density of 135 A/cm 2 and the relative external quantum efficiency (EQE) kept monotonously increasing with increase in injection current density. For the LED without a rear DBR, however, the emission efficiency reached the maximum at a very low current density of 10 A/cm 2. The improvement is mainly attributed to reflection of light from the DBR to the “top side” and decrease in light absorption in the active region with increase in injection current density. Moreover, the improvement in relative EQE was attributed to the resonant cavity enhancing spontaneous emission at its resonant wavelength too.