R. S. Kern

Bio: R. S. Kern is an academic researcher. The author has contributed to research in topics: Light-emitting diode & Quantum efficiency. The author has an hindex of 5, co-authored 6 publications receiving 732 citations.

High-power AlGaInN flip-chip light-emitting diodes

Abstract: 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.

Performance of high-power AlInGaN light emitting diodes

Abstract: 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/cm2, these LEDs exhibit power conversion efficiencies ranging from ≈26% (428 nm) to ≈10% (527 nm).

III-Nitride light emitting devices with low driving voltage

Abstract: III-Nitride light emitting diodes having improved performance are provided. In one embodiment, a light emitting device includes a substrate, a nucleation layer disposed on the substrate, a defect reduction structure disposed above the nucleation layer, and an n-type III-Nitride semiconductor layer disposed above the defect reduction structure. The n-type layer has, for example, a thickness greater than about one micron and a silicon dopant concentration greater than or equal to about 1019 cm−3. In another embodiment, a light emitting device includes a III-Nitride semiconductor active region that includes at least one barrier layer either uniformly doped with an impurity or doped with an impurity having a concentration graded in a direction substantially perpendicular to the active region.

Group iii nitride light-emitting element with low-drive voltage

Abstract: PROBLEM TO BE SOLVED: To provide a semiconductor light-emitting element, more specifically, a group III nitride light-emitting element SOLUTION: This group III nitride light-emitting element has improved performance In one embodiment, this light-emitting element contains a substrate, a nucleating layer arranged on the substrate, a defect-reducing structure arranged above the nucleating layer, and an n-type group III nitride semiconductor layer arranged above the structure The semiconductor layer has a thickness of about >1 μm and contains a silicon dopant at a concentration of about 10 cm or higher In another embodiment, this light-emitting element contains a group III nitride semiconductor active region, having at least one barrier layer doped uniformly with an impurity or doped with the impurity, in a state where the concentration of the impurity changes in stepwise manner in a direction substantially perpendicular to the active region

Performance of high-power AlInGaN light emitting diodes

Abstract: 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).

Status and Future of High-Power Light-Emitting Diodes for Solid-State Lighting

Abstract: Status and future outlook of III-V compound semiconductor visible-spectrum light-emitting diodes (LEDs) are presented. Light extraction techniques are reviewed and extraction efficiencies are quantified in the 60%+ (AlGaInP) and ~80% (InGaN) regimes for state-of-the-art devices. The phosphor-based white LED concept is reviewed and recent performance discussed, showing that high-power white LEDs now approach the 100-lm/W regime. Devices employing multiple phosphors for "warm" white color temperatures (~3000-4000 K) and high color rendering (CRI>80), which provide properties critical for many illumination applications, are discussed. Recent developments in chip design, packaging, and high current performance lead to very high luminance devices (~50 Mcd/m2 white at 1 A forward current in 1times1 mm2 chip) that are suitable for application to automotive forward lighting. A prognosis for future LED performance levels is considered given further improvements in internal quantum efficiency, which to date lag achievements in light extraction efficiency for InGaN LEDs

Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening

Abstract: Roughened surfaces of light-emitting diodes (LEDs) provide substantial improvement in light extraction efficiency. By using the laser-lift-off technique followed by an anisotropic etching process to roughen the surface, an n-side-up GaN-based LED with a hexagonal “conelike” surface has been fabricated. The enhancement of the LED output power depends on the surface conditions. The output power of an optimally roughened surface LED shows a twofold to threefold increase compared to that of an LED before surface roughening.

Origin of efficiency droop in GaN-based light-emitting diodes

Abstract: The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

Illumination with solid state lighting technology

Abstract: High-power light-emitting diodes (LEDs) have begun to differentiate themselves from their more common cousins the indicator LED. Today these LEDs are designed to generate 10-100 lm per LED with efficiencies that surpass incandescent and halogen bulbs. After a summary of the motivation for the development of the high-power LED and a look at the future markets, we describe the current state of high-power LED technology and the challenges that lay ahead for development of a true "solid state lamp." We demonstrate record performance and reliability for high-power colored and white LEDs and show results from the worlds first 100-plus lumen white LED lamp, the solid state equivalent of Thomas Edison's 20-W incandescent lightbulb approximately one century later.

Auger recombination in InGaN measured by photoluminescence

Abstract: 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.