Nathan F. Gardner

Auger recombination in InGaN measured by photoluminescence

The Auger recombination coefficient in quasi-bulk InxGa1−xN (x∼9%–15%) layers grown on GaN (0001) is measured by a photoluminescence technique. The samples vary in InN composition, thickness, and threading dislocation density. Throughout this sample set, the measured Auger coefficient ranges from 1.4×10−30to2.0×10−30cm6s−1. The authors argue that an Auger coefficient of this magnitude, combined with the high carrier densities reached in blue and green InGaN∕GaN (0001) quantum well light-emitting diodes (LEDs), is the reason why the maximum external quantum efficiency in these devices is observed at very low current densities. Thus, Auger recombination is the primary nonradiative path for carriers at typical LED operating currents and is the reason behind the drop in efficiency with increasing current even under room-temperature (short-pulsed, low-duty-factor) injection conditions.

High-power AlGaInN flip-chip light-emitting diodes

Data are presented on high-power AlGaInN flip-chip light-emitting diodes (FCLEDs). The FCLED is “flipped-over” or inverted compared to conventional AlGaInN light-emitting diodes (LEDs), and light is extracted through the transparent sapphire substrate. This avoids light absorption from the semitransparent metal contact in conventional epitaxial-up designs. The power FCLED has a large emitting area (∼0.70 mm2) and an optimized contacting scheme allowing high current (200–1000 mA, J∼30–143 A/cm2) operation with low forward voltages (∼2.8 V at 200 mA), and therefore higher power conversion (“wall-plug”) efficiencies. The improved extraction efficiency of the FCLED provides 1.6 times more light compared to top-emitting power LEDs and ten times more light than conventional small-area (∼0.07 mm2) LEDs. FCLEDs in the blue wavelength regime (∼435 nm peak) exhibit ∼21% external quantum efficiency and ∼20% wall-plug efficiency at 200 mA and with record light output powers of 400 mW at 1.0 A.

Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200A∕cm2

Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200A∕cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432nm have an external quantum efficiency of 40% and output power of 2.3W at 2A.

InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures

Electrical operation of InGaN/GaN quantum-well heterostructure photonic crystal light-emitting diodes (PXLEDs) is demonstrated. A triangular lattice photonic crystal is formed by dry etching into the top GaN layer. Light absorption from the metal contact is minimized because the top GaN layers are engineered to provide lateral current spreading, allowing carrier recombination proximal to the photonic crystal yet displaced from the metal contact. The chosen lattice spacing for the photonic crystal causes Bragg scattering of guided modes out of the LED, increasing the extraction efficiency. The far-field radiation patterns of the PXLEDs are heavily modified and display increased radiance, up to ∼1.5 times brighter compared to similar LEDs without the photonic crystal.

Carrier distribution in (0001)InGaN∕GaN multiple quantum well light-emitting diodes

We study the carrier distribution in multi quantum well (multi-QW) InGaN light-emitting diodes. Conventional wisdom would assume that a large number of QWs lead to a smaller carrier density per QW, enabling efficient carrier recombination at high currents. We use angle-resolved far-field measurements to determine the location of spontaneous emission in a series of multi-QW samples. They reveal that, no matter how many QWs are grown, only the QW nearest the p layer emits light under electrical pumping, which can limit the performances of high-power devices.

Polarization anisotropy in the electroluminescence of m-plane InGaN–GaN multiple-quantum-well light-emitting diodes

InGaN–GaN multiple-quantum-well light-emitting diodes were fabricated on (101¯0) m plane GaN films grown on (101¯0) m plane 4H–SiC substrates. The [0001] axis of the epitaxial film is parallel to the [0001] axis of the substrate. The surface is striated, with features running perpendicular to the c axis and a maximum surface height difference of 45nm. Electroluminescence shows strong polarization anisotropy, with 7× more light emitted with polarization perpendicular to the c axis compared to parallel to the c axis. An Ahrrenius fit of the polarization ratio indicates that there is a 49meV difference in the energy gap between the two polarization states. This suggests that a high polarization ratio can be maintained at the high temperatures (>150°C) and drive current densities required for high-power light-emitting diode applications.

High Power LEDs – Technology Status and Market Applications

High power light emitting diodes (LEDs) continue to increase in output flux with the best III-nitride based devices today emitting over 150 lm of white, cyan, or green light. The key design features of such products will be covered with special emphasis on power packaging, flip-chip device design, and phosphor coating technology. The high-flux performance of these devices is enabling many new applications for LEDs. Two of the most interesting of these applications are LCD display backlighting and vehicle forward lighting. The advantages of LEDs over competing lighting technologies will be covered in detail.

Performance of High‐Power AlInGaN Light Emitting Diodes

The performance of high-power AlInGaN light emitting diodes (LEDs) is characterized by light output-current-voltage (L-I-V) measurements for devices with peak emission wavelengths ranging from 428 to 545 nm. The highest external quantum efficiency (EQE) is measured for short wavelength LEDs (428 nm) at 29%. EQE decreases with increasing wavelength, reaching 13% at 527 nm. With low forward voltages ranging from 3.3 to 2.9 V at a drive current density of 50 A/cm 2 , these LEDs exhibit power conversion efficiencies ranging from 26% (428 nm) to 10% (527 nm).

High‐Power III‐Nitride Emitters for Solid‐State Lighting

High-power, large-area InGaN/GaN quantum-well heterostructure light-emitting diodes based on an inverted, or flip-chip, configuration are described. These devices are mounted in specially designed high-power (1-5 W) packages and exhibit high extraction efficiency and low operating voltage. In the blue wavelength regime, output powers greater than 250 mW (1 x 1 mm 2 device) and 1 W (2 x 2 mm 2 device) are delivered at standard operating current densities (50 A/cm 2 ), corresponding to wall-plug efficiencies of 22%-23%. Employing phosphors for the generation of white light, these same devices achieve luminous efficiencies greater than 30 lm/W.

Droop in III-nitrides: Comparison of bulk and injection contributions

We study mechanisms which are thought to contribute to efficiency droop in III-nitrides. We first observe droop in a photoluminescence (PL) experiment on bulk GaN, which confirms the existence of a bulk contribution to droop, unrelated to piezoelectric fields or alloy fluctuations. We then perform biased-PL on a series of InGaN light-emitting diodes to estimate the potential impact of carrier leakage on PL experiments. We conclude that carrier leakage is only significant at very low pump densities and does not contribute to droop, thus validating the use of PL to characterize droop.