Multiquantum-well Light-emitting Diodes Research Articles

Impact of Crystal Orientation on Modulation Bandwidth: Towards GaN LED-Based High-Speed Visible Light Communication

Light-emitting diodes (LEDs) with high modulation bandwidth are required for high-speed visible light communication applications. Crystal orientation in the GaN LED structure plays a key factor in its modulation bandwidth as the recombination lifetime is highly dependent on crystal orientation owing to the Quantum-Confined Stark Effect (QCSE). In this study, six different crystal orientation multi-quantum well (MQW) GaN LEDs are simulated to understand the impact of heterostructure orientation on modulation bandwidth, radiative recombination rates, and emission intensity. The results of this study demonstrate that semi-polar 101¯3¯ MQW LEDs provide the highest bandwidth in the current density range of 9–20 kA/cm2 compared to the other five orientations. For instance, the semi-polar 101¯3¯-based LED offers a modulation bandwidth of 912.7 MHz at 20 kA/cm2 current density. These results suggest that the semi-polar 101¯3¯ orientation-based LED has the potential to support a high-speed visible light communication system.

Modeling and study of MgZnO/CdZnO MQW LED with p–GaN/AlGaN cladding/EB layer

We propose a multi–quantum–well (MQW) light–emitting–diode (LED) design including p–GaN/p–AlGaN/MgxZn1–xO/CdxZn1–xO/n– MgxZn1–xO /n–ZnO structure. The effect of different Mg and Cd molar fraction, thickness of quantum barrier, number of quantum well and carrier concentration of p–GaN layer on the performance of the proposed structure are studied through 2D simulation by ATLAS. The optimal xMg= 0.22 and xCd= 0.003 is determined through optimization process. Also, the optimal thickness of 5 nm for quantum barrier and the optimal number of 2 quantum wells is achieved for the proposed structure. To increase/decrease as much as possible the internal quantum efficiency/efficiency droop of the device, different affecting parameters are varied. The carrier concentration of p–GaN layer was revealed as the most affecting factor on the increase of quantum efficiency and reduction of the efficiency droop.

Color‐Tunable 3D InGaN/GaN Multi‐Quantum‐Well Light‐Emitting‐Diode Based on Microfacet Emission and Programmable Driving Power Supply

AbstractColor‐tunable InGaN/GaN multi‐quantum‐well (MQW) light‐emitting diodes (LEDs) are reported based on GaN microfacet structure directly grown on c‐plane patterned sapphire substrate by metal organic vapor phase epitaxy (MOVPE) through promoting 3D growth. By adjusting GaN growth temperature and pattern arrangement, a GaN microfacet with almost pure {101} semipolar facets is obtained. The multifacetted InGaN/GaN MQW LED chip evolves three distinct emission peaks around 630, 530, and 450 nm in electroluminescence (EL) as injection current increases from 1 to 100 mA. The EL behavior originates from locally different facets of the complex 3D structure: MQWs grown on c‐planes and semipolar facets, respectively, which is confirmed by cathodoluminescence characterization in a scanning transmission electron microscope (STEM‐CL). Considering the dependence of emission wavelength and intensity on injection currents, a programmable power supply is designed to drive the LED. The specific color of the LED is tuned by time‐shared driving of the currents based on three channels with controllable magnitudes and duty cycle from the power supply, covering red, yellow, green, cyan, blue, and purple. Furthermore, white LEDs with high color rendering index (CRI) up to 96.1 and correlated color temperature (CCT) between 4000 and 10 000 K are achieved.

Analysis of InGaN Multiple-Quantum-Well Photoelectric Device of Visible Light Communication

The key to achieving high-speed and high-quality visible light communication is to increase the modulation speed of Light-Emitting Diode (LED). Therefore, in this study, the influence of the Composite Mechanism of Carrier (CMC) on the modulation speed of LED is studied by designing different structures of the InGaN Multi-quantum-well (MQW) LED active region. Because the carrier subspace waves function of narrow quantum well LED overlaps more frequently and the electron leakage effect is more significant, the compound rate is faster and the modulation bandwidth is higher. InGaN quantum barrier LED with a content of 1% can increase the weight of radiation recombination, which makes the modulation bandwidth higher than GaN quantum barrier LEDs; when the in content is 5%, electron leakage and Auger recombination have a dominant position. Moreover, because these two compounding mechanisms have a fast compounding rate, the modulation bandwidth is significantly increased. Then the 405 nm laser-excited photoluminescence (PL) is introduced to analyze the carrier dynamics in the LED and obtain the related processes of carrier distribution and transport. The proposed carrier microscopic model can well explain change characteristics of the PL luminescence peak, luminous intensity, and half-height width of InGaN/GaN MQW LED with different excitation wavelengths. At low temperature, the PL peak of the InGaN/GaN quantum well LED redshifts with the increase of temperature, because the weakly bound carrier transfers the obtained energy to the deeply bound energy level of high In content.

Generated Carrier Dynamics in V-Pit-Enhanced InGaN/GaN Light-Emitting Diode

We investigate the effects of V-pits on the optical properties of a state-of-the-art, highly efficient, blue InGaN/GaN multi-quantum-well (MQW) light-emitting diode (LED) with a high internal quantum efficiency (IQE) of >80%. The LED is structurally enhanced by incorporating a pre-MQW InGaN strain-relief layer with low InN content and a patterned sapphire substrate. For comparison, a conventional (unenhanced) InGaN/GaN MQW LED (with an IQE of 46%) grown under similar conditions was subjected to the same measurements. Scanning transmission electron microscopy reveals the absence of V-pits in the unenhanced LED, whereas in the enhanced LED, V-pits with {10-11} facets, emerging from threading dislocations (TDs), were prominent. Cathodoluminescence mapping reveals the luminescence properties near the V-pits, showing that the formation of V-pit defects can encourage the growth of defect-neutralizing barriers around TD defect states. The diminished contribution of TDs in the MQWs allows indium-rich localization...

Characterization of the InGaN/GaN Multi-Quantum-Wells Light-Emitting Diode Grown on Patterned Sapphire Substrate with Wide Electroluminescence Spectrum

We report the characterization of the InGaN/GaN multi-quantum-well (MQW) light-emitting diode (LED) grown on a patterned sapphire substrate by metal organic chemical vapor deposition (MOCVD) using the selective area growth (SAG) method. The SAG patterns were designed to be circular and their diameters were 700 and 200 µm. After the growth, the InGaN/GaN MQW LED of 200 µm diameter had various crystal facets and a shape similar to volcanic craters, which were not observed in the 700-µm-diameter sample. We obtained an active layer with compositional nonuniformity and superior optical properties. We found wide electroluminescence (EL) spectral peaks near 470, 570, and 600 nm. The distribution of the EL spectrum of the sample was similar to that of a conventional phosphor-converted white LED.

Enhanced Performance of Nitride-Based Blue LED with Step-stage MQW Structure

A step-stage InGaN/GaN multiquantum-well (MQW) structure can enhance the efficiency of GaN-based light-emitting diodes (LEDs). Compared to dual-stage MQW LEDs, the step-stage MQW LEDs have lower forward voltage and higher light output. The measured light output power of step-stage LEDs operating at 350 mA shows an increase of approximately 23% with an external quantum efficiency (EQE) increase of 6.6%, when compared to dual-stage LEDs.

222 nm single‐peaked deep‐UV LED with thin AlGaN quantum well layers

AbstractWe fabricated 222 nm AlGaN multi‐quantum well (MQW) deep‐ultraviolet (DUV) light emitting diode (LED) on high quality AlN buffers grown by ammonia (NH3) pulse‐flow multilayer growth method on sapphire substrate. We obtained remarkable enhancement of emission intensity of 220 nm‐band DUV LED by introducing thin (<1.6 nm) quantum well. We realized a single peaked operation of a 222 nm DUV AlGaN MQW LED under pulsed current injection, which is the shortest wavelength of AlGaN LED on sapphire substrate. The maximum output power and external quantum efficiency of 222 nm AlGaN MQW LED were 14 μW and 0.003%, respectively. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Milliwatt power 270 nm‐band AlGaN deep‐UV LEDs fabricated on ELO‐AlN templates

AbstractWe demonstrated CW milliwatt power operations of 270 nm‐band AlGaN multi‐quantum well (MQW) deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) fabricated on epitaxial lateral overgrowth (ELO) AlN templates on sapphire. An initial AlN stripe layer was grown directly on sapphire by using ammonia (NH3) pulse‐flow multilayer (ML) growth method. An AlN stripe structure with a width of 5 μm and a spacing of 3 μm was completely coalesced after the growth of an approximately 15 μm‐thick ELO‐AlN layer grown by low‐pressure metal‐organic chemical vapor deposition (LP‐MOCVD). The edge‐type threading dislocation density (TDD) of the wing region of the ELO‐AlN layer was 3×108 cm‐2, as observed by cross‐sectional transmission electron microscope (TEM) image. The maximum output power of 2.7 mW was obtained from an AlGaN MQW LED with emission wavelength at 273 nm fabricated on ELO‐AlN template under room temperature (RT) CW operation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction in efficiency droop, forward voltage, ideality factor, and wavelength shift in polarization-matched GaInN/GaInN multi-quantum-well light-emitting diodes

Blue light-emitting diodes (LEDs) with polarization-matched GaInN/GaInN multi-quantum-well (MQW) active regions are grown by metal-organic vapor-phase epitaxy. The GaInN/GaInN MQW structure reduces the magnitude of polarization sheet charges at heterointerfaces in the active region. The GaInN/GaInN MQW LEDs are shown to have enhanced light-output power, reduced efficiency droop, a lower forward voltage, a smaller diode ideality factor, and decreased wavelength shift, compared with conventional GaInN/GaN MQW LEDs.

The Effect of Temperature on the Efficiency of Nitride-based Multi-quantum well Light-emitting Diodes

AbstractWe have recently developed a self consistent light-emitting diode (LED) model that accounts for the current transport and internal heating in AlGaAs-GaAs LEDs. In this paper we extend the model to describe multi-quantum well (MQW) active regions and III-N materials, within the limits of the currently known values and temperature dependencies of the recombination parameters in these materials. The MQW description accounts for the effect of the reduced wave function overlap to the recombination. We present simulation results obtained for an InGaN MQW LED with 4 wells at selected temperatures and discuss the factors limiting the efficiency and luminescent output of LEDs.

Improving the Performance of Green LEDs by Low-Temperature Annealing of p-GaN with PdZn

This article reports the electrical properties of p-GaN annealed at low activation temperature by using a PdZn film in green InGaN/GaN multiquantum well (MQW) light-emitting diodes (LEDs). Electroluminescence (EL) intensity of green MQW LED annealed at 600°C using PdZn was improved by 33% at 20 mA compared to that annealed at 800°C without PdZn. These results are attributed to an increase of the hole concentration of p-GaN due to removal of hydrogen in p-GaN by PdZn and a decrease in thermal damage of MQW at low activation temperature.

Lattice Constant Effect of Photonic Crystals on the Light Output of Blue Light-Emitting Diodes

Photonic crystal (PC) slabs with triangular lattice constants of 230, 345, 460, and 690 nm have been fabricated onto InGaN-GaN multiquantum-well light-emitting diodes with a wavelength of 461 nm. For the shallow nanoholes in the depth range of 10-38.5 nm, the current-voltage characteristics were not changed but the electroluminescence intensities were enhanced by a factor of 3.5. The light output was most enhanced with the lattice constant corresponding to the wavelength due to the PC diffraction with constructive interference but was suppressed with the lattice constant of the half wavelength by forming a photonic bandgap.

Enhanced characteristics of blue InGaN∕GaN light-emitting diodes by using selective activation to modulate the lateral current spreading length

We have studied the characteristics of blue InGaN∕GaN multiquantum-well light-emitting diodes (LEDs) after reducing the length of the lateral current path through the transparent layer through formation of a peripheral high-resistance current-blocking region in the Mg-doped GaN layer. To study the mechanism of selective activation in the Mg-doped GaN layer, we deposited titanium (Ti), gold (Au), Ti∕Au, silver, and copper individually onto the Mg-doped GaN layer and investigated their effects on the hole concentration in the p-GaN layer. The Mg-doped GaN layer capped with Ti effectively depressed the hole concentration in the p-GaN layer by over one order of magnitude relative to that of the as-grown layer. This may suggest that high resistive regions are formed by diffusion of Ti and depth of high resistive region from the p-GaN surface depends on the capped Ti film thickness. Selective activation of the Mg-doped GaN layer could be used to modulate the length of the lateral current path. Furthermore, the external quantum efficiency of the LEDs was improved significantly after reducing the lateral current spreading length. In our best result, the external quantum efficiency was 52.3% higher (at 100mA) than that of the as-grown blue LEDs.

Nitride-Based Dual-Stage MQW LEDs

Nitride-based light emitting diode (LED) with dual-stage multiquantum well (MQW) structure is proposed and fabricated. It was found that we could improve crystal quality, reduce reverse leakage current, and reduce forward voltage of the LED by inserting the electron emitter MQW structure. With current injection, it was found that measured output powers were 3.2 and for the conventional single-stage MQW LED and the dual-stage MQW LED, respectively. Furthermore, it was found that electrostatic discharge characteristics of the dual-stage MQW LED are better.

Improvement in performance of GaN-based light-emitting diodes with indium tin oxide based transparent ohmic contacts

InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) with indium tin oxide (ITO) and Ni/Au p-contacts were fabricated. ITO (500 nm) and Ni/Au (2 nm/9 nm) films were deposited onto p-GaN epitaxial layers by an e-beam evaporation system. For the LEDs using in situ annealed ITO and Ni/Au films as p-contacts, the forward voltage at 20 mA was 3.5 and 3.2 V, respectively. Under the same amount of injection current, the LED with in situ annealed ITO p-contact had higher output electroluminescence (EL) intensity and larger light output power. The EL intensities and the light output power of ITO LEDs were enhanced by 85% and 60%, respectively, at 20 mA. As a result, the light output and power conversion efficiency of ITO LEDs on GaN were greatly improved at high injection currents. The fabricated LEDs were subjected to a stress test at 30 mA and 55 °C and showed a very small degradation of optical power (<1% decrease) for 24 h. The light output of MQW LEDs keeps 80% of the original value after 1000 h stressing. Therefore, the fabricated LED devices have demonstrated a good reliability.

High dose Co-60 gamma irradiation of InGaN quantum well light-emitting diodes

InGaN multiquantum-well light-emitting diodes (LEDs) in the form of unpackaged die with emission wavelengths from 410to510nm were irradiated with Co60 γ-rays with doses in the range 150–2000Mrad (Si). The forward turn-on voltage for all the irradiated LEDs was increased slightly (e.g., by only ∼0.1–0.15V for 500MRad dose irradiation) while the reverse breakdown voltage was unchanged within experimental error. The light output intensity for the 410nm diodes was decreased by 20% after a dose of 150MRad and 75% after ∼2GRad. The current transport in the LEDs was dominated by generation-recombination (ideality factor ∼2) both before and after irradiation. The morphology and appearance of the p and n-Ohmic metallization did not show any detectable change as a result of even the highest γ-ray dose.

Effect of inductively coupled plasma damage on performance of GaN–InGaN multiquantum-well light-emitting diodes

InGaN multiquantum-well light-emitting diodes (LEDs) in the form of unpackaged die with emission wavelengths from 420to505nm were exposed to either Ar or H2 inductively coupled plasmas as a function of both rf chuck power (controlling incident ion energy) and source power (controlling ion flux). The forward turn-on voltage is increased by both types of plasma exposure and is a function of both the incident ion energy and flux. The reverse bias current in the LEDs is much larger in the case of H2 plasma exposure, indicating that preferential loss of nitrogen leads to increased surface leakage. The current transport in the LEDs is dominated by generation-recombination (ideality factor ∼2) both before and after the plasma exposures.

Optical study of spin injection dynamics in InGaN∕GaN quantum wells with GaMnN injection layers

The spin injection dynamics of GaMnN∕InGaN multiquantum well (MQW) light emitting diodes (LEDs) grown by molecular beam epitaxy were examined using picosecond-transient and circularly polarized photoluminescence (PL) measurements. Even with the presence of a room temperature ferromagnetic GaMnN spin injector, the LEDs are shown to exhibit very low efficiency of spin injection. Based on resonant optical orientation spectroscopy, the spin loss in the structures is shown to be largely due to fast spin relaxation within the InGaN MQW, which itself destroys any spin polarization generated by optical spin orientation or electrical spin injection. Typical photoluminescence decay times were 20–40ns in both commercial GaN MQW LEDs with emission wavelengths between 420–470nm and in the GaMnN∕InGaN multi-quantum well MQW LEDs. In the wurtzite InGaN∕GaN system, biaxial strain at the interfaces give rise to large piezoelectric fields directed along the growth axis. This built-in piezofield breaks the reflection symmetry of confining potential leading to the presence of a large Rashba term in the conduction band Hamiltonian which is responsible for the short spin relaxation times.

InGaN/GaN multi‐quantum dot light‐emitting diodes

It has been demonstrated that InGaN/GaN blue light-emitting diodes (LEDs) with multiple quantum dot (MQD) were successfully fabricated by metal-organic chemical vapor deposition. We have formed nanoscale InGaN self-assembled QDs in the well layers of the active region with a typical 3-nm height and 10-nm lateral dimension. With a 20-mA DC injection current, the forward voltage was 3.1 and 3.5 V for MQD LED and conventional nitride-based multi-quantum well (MQW) LED with the same structure, respectively. It was also found that EL peak position of the MQD LED is more sensitive to the amount of injection current, as compared to the conventional MQW LEDs. (C) 2003 Elsevier B.V. All rights reserved.