III-nitride LEDs Research Articles

760 Mbps ultraviolet wireless communication based on a transmitter with AlGaN/InGaN MQWs

III-nitride LEDs offer a solution for ultraviolet (UV) high-speed communication as a transmitter with high performance. This paper focuses on a transmitter with AlGaN/InGaN multi-quantum wells (MQWs) for UV communication. The transmitter is realized on a GaN-on-silicon platform by a double etching process. The emission region of the transmitter with a small area is beneficial for improving the data rate of UV communication. The emission peak keeps stable at 376.48 nm in the UVA band. The transmission with 300 Mbps is obtained in a UV communication system setup with on-off keying (OOK) modulation. We realize a digital signal transmission up to 760 Mbps by bit-loading discrete multi-audio (DMT) modulation.

InGaN-Based microLED Devices Approaching 1% EQE with Red 609 nm Electroluminescence on Semi-Relaxed Substrates

In this paper, we report the successful demonstration of bright InGaN-based microLED devices emitting in the red spectral regime grown by metal organic chemical vapor deposition (MOCVD) on c-plane semi-relaxed InGaN substrates on sapphire. Through application of an InGaN/GaN base layer scheme to ameliorate high defect density and maintain appropriate lattice constant throughout the growth, high-In quantum wells (QWs) can be grown with improved crystal quality. Improvement to the design of the growth scheme also yields higher power output resulting in an increase to the external quantum efficiency (EQE). Combined, these two improvements allow for an 80 × 80 μm2 microLED device emitting at 609 nm to achieve 0.83% EQE. Furthermore, the true In content of the QW is measured using atomic probe tomography (APT) to confirm the improved In incorporation during high temperature active region growth. These developments represent advancement toward the realization of bright, highly efficient red III-nitride LEDs to be used in RGB applications under one material system.

Role of V-defect density on the performance of III-nitride green LEDs on sapphire substrates

In this study, we experimentally investigated the role of V-defect density on the performance of green III-nitride LEDs grown on sapphire substrates by metalorganic chemical vapor deposition. We systematically varied the threading dislocation (TD) density from 4 × 108 to 1 × 109 cm−2 by changing the V/III ratio during initial high temperature GaN growth. A 30-period InGaN/GaN superlattice promoted V-defect formation and growth at TDs, where the density of V-defects was correlated to the TD density. By interrupting the LED growth and examining the surface of the active region, we quantified the average size and density of V-defects. In a series of LEDs, we measured a systematic decrease in forward voltage (VF) with V-defect density. At a V-defect density of 5.0 × 108 cm−2 and TD density of 1 × 109 cm−2, green LED devices were demonstrated with λ = 523 nm and VF = 2.94 V at 20 A cm−2. These results highlight the potential of using V-defect engineering to achieve low VF long wavelength LEDs on sapphire substrates, where opening of remaining threading dislocations into V-defects presents an opportunity for further VF reduction.

Fabrication and chemical lift-off of sub-micron scale III-nitride LED structures.

Nanoscale light emitting diodes (nanoLEDs, diameter < 1 µm), with active and sacrificial multi-quantum well (MQW) layers epitaxially grown via metal organic chemical vapor deposition, were fabricated and released into solution using a combination of colloidal lithography and photoelectrochemical (PEC) etching of the sacrificial MQW layer. PEC etch conditions were optimized to minimize undercut roughness, and thus limit damage to the active MQW layer. NanoLED emission was blue-shifted ∼10 nm from as-grown (unpatterned) LED material, hinting at strain relaxation in the active InGaN MQW layer. X-ray diffraction also suggests that strain relaxation occurs upon nanopatterning, which likely results in less quantum confined Stark effect. Internal quantum efficiency of the lifted nanoLEDs was estimated at 29% by comparing photoluminescence at 292K and 14K. This work suggests that colloidal lithography, combined with chemical release, could be a viable route to produce solution-processable, high efficiency nanoscale light emitters.

High Modulation Bandwidth of Semipolar (11-22) InGaN/GaN LEDs with Long Wavelength Emission.

Visible light communication requires III-nitride LEDs with a high modulation bandwidth but have c-plane limitations. General illumination requires green/yellow III-nitride LEDs with high optical efficiency that are difficult to achieve on c-plane substrates. Micro-LEDs with a low efficiency are used to obtain a high modulation bandwidth. This paper demonstrates a record modulation bandwidth of 540 MHz for our semipolar green LEDs with a broad area. Semipolar yellow and amber LEDs with modulation bandwidths of 350 and 140 MHz, respectively, have also been reported, and are the longest wavelength III-nitride LEDs. These results agree with differential carrier lifetime measurements.

Alternating Current III-Nitride Light-Emitting Diodes With On-Chip Schottky Barrier Diode Rectifiers

We report in this paper the design and fabrication of single-chip alternating current-LEDs (AC-LEDs) by monolithically integrating, for the first time, high-breakdown Schottky barrier diodes (SBDs) and micro-LED arrays using III-nitride LED epi-wafers of standard specs for volume production. A technique of cyclic mixed-etching has been introduced to restore the surface of inductively coupled-plasma (ICP) etching processed gallium nitride to device quality for fabricating high-breakdown Schottky junctions. Proof-of-concept single-chip AC-LED devices and a prototype driver-free white AC-LED lamp were demonstrated, showing high-efficiency LED emission, high chip area utilization efficiency, low power loss of the on-chip SBD bridge rectifier, and good luminous efficacy of the prototype AC-LED lamp. This paper paves the way toward mass-producing reliable and low-cost driver-free AC-LED lamps for solid-state lighting with existing LED manufacturing infrastructures.

Prospects for 100% wall-plug efficient III-nitride LEDs.

The possibility of a III-nitride LED with 100% or greater wall-plug efficiency is examined considering recent observations of the phenomenon for smaller bandgap mid-IR LEDs under extremely low-bias operation [Phys. Rev. Lett. 108, 1 (2012)]. Thermoelectric pumping of carriers by lattice heat enables ≥ 100% WPE, but this effect is relatively weaker for the wider band gap III-nitrides. This work assesses the electrical and optical performance of several state-of-the-art nitride devices and summarizes the requirements and prospects for approaching ≥ 100% WPE for III-nitride LEDs operating at technologically relevant current densities (> 1 A/cm2).

Polarization-induced hole doping in N-polar III-nitride LED grown by metalorganic chemical vapor deposition

Polarization-induced doping has been shown to be effective for wide-bandgap III-nitrides. In this work, we demonstrated a significantly enhanced hole concentration via linearly grading an N-polar AlxGa1-xN (x = 0–0.3) layer grown by metal-organic chemical vapor deposition. The hole concentration increased by ∼17 times compared to that of N-polar p-GaN at 300 K. The fitting results of temperature-dependent hole concentration indicated that the holes in the graded p-AlGaN layer comprised both polarization-induced and thermally activated ones. By optimizing the growth conditions, the hole concentration was further increased to 9.0 × 1017 cm−3 in the graded AlGaN layer. The N-polar blue-violet light-emitting device with the graded p-AlGaN shows stronger electroluminescence than the one with the conventional p-GaN. The study indicates the potential of the polarization doping technique in high-performance N-polar light-emitting devices.

Nanoscale Electronic Conditioning for Improvement of Nanowire Light-Emitting-Diode Efficiency.

Commercial III-Nitride LEDs and lasers spanning visible and ultraviolet wavelengths are based on epitaxial films. Alternatively, nanowire-based III-Nitride optoelectronics offer the advantage of strain compliance and high crystalline quality growth on a variety of inexpensive substrates. However, nanowire LEDs exhibit an inherent property distribution, resulting in uneven current spreading through macroscopic devices that consist of millions of individual nanowire diodes connected in parallel. Despite being electrically connected, only a small fraction of nanowires, sometimes <1%, contribute to the electroluminescence (EL). Here, we show that a population of electrical shorts exists in the devices, consisting of a subset of low-resistance nanowires that pass a large portion of the total current in the ensemble devices. Burn-in electronic conditioning is performed by applying a short-term overload voltage; the nanoshorts experience very high current density, sufficient to render them open circuits, thereby forcing a new current path through more nanowire LEDs in an ensemble device. Current-voltage measurements of individual nanowires are acquired using conductive atomic force microscopy to observe the removal of nanoshorts using burn-in. In macroscopic devices, this results in a 33× increase in peak EL and reduced leakage current. Burn-in conditioning of nanowire ensembles therefore provides a straightforward method to mitigate nonuniformities inherent to nanowire devices.

Field-assisted Shockley-Read-Hall recombinations in III-nitride quantum wells

The physical process driving low-current non-radiative recombinations in high-quality III-nitride quantum wells is investigated. Lifetime measurements reveal that these recombinations scale with the overlap of the electron and hole wavefunctions and show a weak temperature dependence, in contrast to common empirical expectations for Shockley-Read-Hall recombinations. A model of field-assisted multiphonon point defect recombination in quantum wells is introduced and shown to quantitatively explain the data. This study clarifies how III-nitride LEDs can achieve high efficiency despite the presence of strong polarization fields.

High wall-plug efficiency blue III-nitride LEDs designed for low current density operation.

Commercial LEDs for solid-state lighting are often designed for operation at current densities in the droop regime (~35 A/cm2) to minimize costly chip area; however, many benefits can be realized by operating at low current density (J ≈1 - 5 A/cm2). Along with mitigation of droop losses and reduction of the operating voltage, low J operation of LEDs opens the design space for high light extraction efficiency (LEE). This work presents detailed ray tracing simulations of an LED design for low J operation with LEE ≈94%. The design is realized experimentally resulting in a peak wall-plug efficiency of 78.1% occurring at 3.45 A/cm2 and producing an output power of 7.2 mW for a 0.1 mm2 emitting area. At this operation point, the photon voltage Vp=hνq exceeds the forward voltage (V), corresponding to a Vp/V = 103%.

Semipolar (202̅1) III-Nitride P-Down LEDs with in situ anneal to reduce the Mg memory effect

P-down LEDs (PDLEDs) have the potential to open up new design schemes for III-nitride LEDs compared to conventional n-down LEDs (NDLEDs). For light emitters operating above 480nm, the PDLED design enables the epitaxial advantages of semipolar (202̅1) and gains the polarization benefits of semipolar (202̅1̅). Here, we investigated semipolar (202̅1) InGaN-based PDLEDs in terms of their photoluminescence (PL) spectra and compositional profile. Despite concerns of the Mg memory effect degrading PDLED performance due to Mg-related non-radiative recombination centers, the PL intensities were nearly identical between the NDLED and PDLEDs, which emitted at wavelengths centered near 500nm. Secondary ion mass spectrometry revealed that the Mg doping levels in the multiple quantum well (MQW) active region were comparable for each structure, with average values of 2.9×1018cm−3 for the NDLED and 1.8×1018cm−3 for the PDLED. Prior to growing the active region MQW, a 700°C in situ anneal was carried out to reduce the average Mg concentration in the PDLED MQW to 3.7×1017cm−3. Its hydrogen concentration remained at 5×1019cm−3 in the p-type GaN region, which suggests that hydrogen passivation occurs during the growth of subsequent epitaxial layers in ammonia.

High-speed 405-nm superluminescent diode (SLD) with 807-MHz modulation bandwidth.

III-nitride LEDs are fundamental components for visible-light communication (VLC). However, the modulation bandwidth is inherently limited by the relatively long carrier lifetime. In this letter, we present the 405 nm emitting superluminescent diode (SLD) with tilted facet design on semipolar GaN substrate, showing a broad emission of ~9 nm at 20 mW optical power. Owing to the fast recombination (τe<0.35 ns) through the amplified spontaneous emission, the SLD exhibits a significantly large 3-dB bandwidth of 807 MHz. A data rate of 1.3 Gbps with a bit-error rate of 2.9 × 10-3 was obtained using on-off keying modulation scheme, suggesting the SLD being a high-speed transmitter for VLC applications.

Growth of monolithic full-color GaN-based LED with intermediate carrier blocking layers

Specially designed intermediate carrier blocking layers (ICBLs) in multi-active regions of III-nitride LEDs were shown to be effective in controlling the carrier injection distribution across the active regions. In principle, the majority of carriers, both holes and electrons, can be guided into targeted quantum wells and recombine to generate light of specific wavelengths at controlled current-densities. Accordingly we proposed and demonstrated a novel monolithic InGaN-based LED to achieve three primary colors of light from one device at selected current densities. This LED structure, which has three different sets of quantum wells separated with ICBLs for three primary red-green-blue (RGB) colors, was grown by metal-organic chemical vapor deposition (MOCVD). Results show that this LED can emit light ranging from 460 to 650 nm to cover the entire visible spectrum. The emission wavelength starts at 650 nm and then decreases to 460 nm or lower as the injection current increases. In addition to three primary colors, many other colors can be obtained by color mixing techniques. To the best of our knowledge, this is the first demonstration of monolithic full-color LED grown by a simple growth technique without using re-growth process.

Silver Nanowire Transparent Conductive Electrodes for High-Efficiency III-Nitride Light-Emitting Diodes.

Silver nanowires (AgNWs) have been successfully demonstrated to function as next-generation transparent conductive electrodes (TCEs) in organic semiconductor devices owing to their figures of merit, including high optical transmittance, low sheet resistance, flexibility, and low-cost processing. In this article, high-quality, solution-processed AgNWs with an excellent optical transmittance of 96.5% at 450 nm and a low sheet resistance of 11.7 Ω/sq were demonstrated as TCEs in inorganic III-nitride LEDs. The transmission line model applied to the AgNW contact to p-GaN showed that near ohmic contact with a specific contact resistance of ~10−3 Ωcm2 was obtained. The contact resistance had a strong bias-voltage (or current-density) dependence: namely, field-enhanced ohmic contact. LEDs fabricated with AgNW electrodes exhibited a 56% reduction in series resistance, 56.5% brighter output power, a 67.5% reduction in efficiency droop, and a approximately 30% longer current spreading length compared to LEDs fabricated with reference TCEs. In addition to the cost reduction, the observed improvements in device performance suggest that the AgNWs are promising for application as next-generation TCEs, to realise brighter, larger-area, cost-competitive inorganic III-nitride light emitters.

Visible electroluminescence from a ZnO nanowires/p-GaN heterojunction light emitting diode.

In the current paper we apply catalyst assisted vapour phase growth technique to grow ZnO nanowires (ZnO nws) on p-GaN thin film obtaining EL emission in reverse bias regime. ZnO based LED represents a promising alternative to III-nitride LEDs, as in free devices: the potential is in near-UV emission and visible emission. For ZnO, the use of nanowires ensures good crystallinity of the ZnO, and improved light extraction from the interface when the nanowires are vertically aligned. We prepared ZnO nanowires in a tubular furnace on GaN templates and characterized the p-n ZnO nws/GaN heterojunction for LED applications. SEM microscopy was used to study the growth of nanowires and device preparation. Photoluminescence (PL) and Electroluminescence (EL) spectroscopies were used to characterize the heterojunction, showing that good quality of PL emission is observed from nanowires and visible emission from the junction can be obtained from the region near ZnO contact, starting from onset bias of 6V.

ABC-model for interpretation of internal quantum efficiency and its droop in III-nitride LEDs: a review

The paper reviews applications of ABC-model to interpret internal quantum efficiency and its droop in III-nitride light-emitting diodes. Advantages of the model, its intrinsic limitations, and tentative mechanisms responsible for deviation of the model predictions from available observations are discussed. New experimental information on recombination processes in the LED active regions coming in terms of the ABC-model is considered along with still open questions and tasks for further experimental and theoretical studies.

Characterisation of Defects at Non-Polar GaN/InGaN Junctions in Novel Materials for Application in Light Emitting Diodes

Despite the significant progress in the area, III-nitride LEDs still suffer from reduced efficiency due to a high dislocation density associated with a lack of suitable growth substrates and strong polarisation fields along the c-axis, the predominant growth direction, resulting in the quantum confined stark effect (QCSE). The novel materials studied here have been developed with the aim of reducing both of these effects and consist of a series of GaN and InGaN layers deposited by chemical vapour deposition on the non-polar, m-plane facets of core structures prepared from c-axis GaN thin films. A dual-beam FIB has been used to prepare a and c-plane sections through these novel structures in order to characterise the defects in the core and non-polar junction using, both diffraction contrast and high resolution, TEM and STEM. The dislocations in the c-axis GaN were generally observed to be confined to this region but the non-polar layers had a high density of defects. The latter include unusual flag type defects which are shown to be comprised of pyramidal dislocations and basal-plane stacking faults, with the Burgers vectors determined via g·b analysis.

Guest Editorial Recent Advances in Solid State Lighting

This Special Issue on Solid State Lighting, comprising the April/May/June 2013 issues of the IEEE/OSA Journal of Display Technology, focuses on the material physics and innovations, device physics and simulations, new device concepts and engineering, and system implementation of high efficiency solid state lighting technologies. The 45 papers of this Special Issue on Solid State Lighting covers both the innovations in inorganic and organic LEDs technologies, as well as the new concepts required for system implementation of the technology. We are very fortunate to have several excellent manuscripts in the key topics areas related to the progress for achieving high internal quantum efficiency and novel approaches for improving light extraction in III-Nitride LEDs, new concept for achieving phosphor-converted white LEDs, and key advances in the important field of organic LEDs. The progress in both organic and inorganic LEDs is very important, and the advances are complementary of one another attributed to the different key applications enabled by these two technologies. This Special Issue on Solid State Lighting covers three separate categories: 1) innovations for addressing the internal quantum efficiency and efficiency-droop in III-Nitride LEDs; 2) novel approaches for addressing light extraction efficiency in III-Nitride LEDs, and enabling circuits and systems for LEDs; and 3) novel approaches for white LEDs, and key advances and review on the material and device concepts in organic LEDs.

Optimization of Light Extraction Efficiency of III-Nitride LEDs With Self-Assembled Colloidal-Based Microlenses

Improvement of light extraction efficiency of InGaN LEDs using colloidal-based SiO2/polystyrene (PS) microlens arrays was demonstrated. The size effect of the SiO2 microspheres and the thickness effect of the PS layer on the light extraction efficiency of III-nitride LEDs were studied. The monolayer rapid convective deposition conditions for SiO2 microspheres were also investigated. Ray tracing simulations show that the use of microlens arrays can lead to increase in light extraction efficiency of InGaN LEDs by 2.64 times. This is consistent with experiments that demonstrated 2.49 times improvement in light extraction utilizing SiO2/PS microlens arrays. The enhancement in light extraction efficiency is attributed to increase in effective photon escape cone due to SiO2/PS microlens arrays, and reduced Fresnel reflection within the photon escape cone due to the grading of refractive index change between GaN/SiO2/PS/air interface.