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

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

REVIEW High-efficiency light-emitting diodes emitting amber, green, blue, and ultraviolet light have been obtained through the use of an InGaN active layer instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based emitting devices. The blue and green InGaN quantum-well structure light-emitting diodes with luminous efficiencies of 5 and 30 lumens per watt, respectively, can be made despite the large number of threading dislocations (1 x 10(8) to 1 x 10(12) cm-2). Epitaxially laterally overgrown GaN on sapphire reduces the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. InGaN multi-quantum-well structure laser diodes formed on the GaN layer above the SiO2 mask area can have a lifetime of more than 10,000 hours. Dislocations increase the threshold current density of the laser diodes.

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The Roles of Structural Imperfections in InGaN-Based Blue Light-Emitting Diodes and Laser Diodes

Recent advances in fabrication technologies for the semiconducting nitrides of the group III elements have led to commercially available, high-efficiency solid-state devices that emit green and blue light Light-emitting diodes based on these materials should find applications in flat-panel displays, and blue and ultraviolet laser diodes promise high-density optical data storage and high-resolution printing

Nitride-based semiconductors for blue and green light-emitting devices

During the last few years the developments in the field of III–nitrides have been spectacular. High quality epitaxial layers can now be grown by MOVPE. Recently good quality epilayers have also been grown by MBE. Considerable work has been done on dislocations, strain, and critical thickness of GaN grown on different substrates. Splitting of valence band by crystal field and by spin-orbit interaction has been calculated and measured. The measured values agree with the calculated values. Effects of strain on the splitting of the valence band and on the optical properties have been studied in detail. Values of band offsets at the heterointerface between several pairs of different nitrides have been determined. Extensive work has been done on the optical and electrical properties. Near band-edge spectra have been measured over a wide range of temperatures. Free and bound exciton peaks have been resolved. Valence band structure has been determined using the PL spectra and compared with the theoretically calcu...

III–nitrides: Growth, characterization, and properties

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.

Auger recombination in InGaN measured by photoluminescence

The electrical, optical, and structural properties of light emitting diodes (LEDs) fabricated from the III–V nitride material system have been studied. LEDs with external quantum efficiencies as high as 4% were characterized by transmission electron microscopy and found to contain dislocation densities in excess of 2×1010 cm−2. A comparison to other III–V arsenide and phosphide LEDs shows that minority carries in GaN‐based LEDs are remarkably insensitive to the presence of structural defects. Dislocations do not act as efficient nonradiative recombination sites in nitride materials. It is hypothesized that the benign character of dislocations arises from the ionic nature of bonding in the III–V nitrides.

High dislocation densities in high efficiency GaN‐based light‐emitting diodes

Data are presented demonstrating the operation of transparent‐substrate (TS) (AlxGa1−x)0.5In0.5P/GaP light‐emitting diodes (LEDs) whose efficiency exceeds that afforded by all other current LED technologies in the green to red (560–630 nm) spectral regime. A maximum luminous efficiency of 41.5 lm/W (93.2 lm/A) is realized at λ∼604 nm (20 mA, direct current). The TS (AlxGa1−x)0.5In0.5P/GaP LEDs are fabricated by selectively removing the absorbing n‐type GaAs substrate of a p‐n (AlxGa1−x)0.5In0.5P double heterostructure LED and wafer bonding a ‘‘transparent’’ n‐GaP substrate in its place. The resulting TS (AlxGa1−x)0.5In0.5P/GaP LED lamps exhibit a twofold improvement in light output compared to absorbing‐substrate (AS) (AlxGa1−x)0.5In0.5P/GaAs lamps.

Very high‐efficiency semiconductor wafer‐bonded transparent‐substrate (AlxGa1−x)0.5In0.5P/GaP light‐emitting diodes

Currently, commercial LEDs based on AlGaInN emit light efficiently from the ultraviolet-blue to the green portion of the visible wavelength spectrum. Data are presented on AlGaInN LEDs grown by organometallic vapor phase epitaxy (OMVPE). Designs for high-power AlGaInN LEDs are presented along with their performance in terms of output power and efficiency. Finally, present and potential applications for high-power AlGaInN LEDs, including traffic signals and contour lighting, are discussed.

High-brightness AlGaInN light-emitting diodes

Packaging challenges of high-power LEDs for solid state lighting

Green phosphor-converted LEDs using a blue pump InGaN diodes have advantages over the direct green InGaN LED with regards to color stability with drive and/or temperature. Added manufacturing steps are outweighed by higher color yield, as a range of pump colors can be used without changing the final chromaticity. The conversion losses can be smaller than the decrease in wall-plug efficiency from blue towards green, which has been reported by many sources. A distinct disadvantage of the concept is due to only one color and phosphor proven - SrGa 2 S 4 :Eu 2+ and 535 nm peak wavelength.

Dan A. Steigerwald

Papers

High dislocation densities in high efficiency GaN‐based light‐emitting diodes

Very high‐efficiency semiconductor wafer‐bonded transparent‐substrate (AlxGa1−x)0.5In0.5P/GaP light‐emitting diodes

High-brightness AlGaInN light-emitting diodes

Packaging challenges of high-power LEDs for solid state lighting

Green phosphor-converted LED