High Brightness Light Emitting Diodes

Chapter 1

Chapter 2

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Materials issues in high-brightness light-emitting diodes

Techniques for production of III-V semiconductors for LEDs

Specific materials systems

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Techniques for production of III-V

semiconductors for LEDs Liquid-Phase Epitaxy (LPE) Vapor-Phase Epitaxial (VPE) Molecular Beam Epitaxy (MBE) Organometallic Vapor-Phase Epitaxy

(OMVPE)

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Liquid-Phase Epitaxy (LPE)

LPE is the simplest technique mechanically. It is an excellent technique for the

production of the very thick layers used in some high-brightness LED structures.

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Molecular Beam Epitaxy (MBE) MBE is the most powerful technique for the

production of superlattice and quantum-well structures.

MBE can be used for the growth of a wide range of materials, but a notable shortcoming is the difficulty experienced with the growth of the phosphides.

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Organometallic Vapor-Phase Epitaxy (OMVPE)

OMVPE is the most versatile technique for the production of III-V materials and structures for eletronic and photonic device.

It is also the most recent technique to be devoloped for the production of high-quality III-V semiconductors.

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Driving force of growth technique

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Specific materials systems AlGaAs AlGaInP AlGaInN

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AlGaAs AlGaAs was the first material for which very high

brightness LEDs were demonstrated. The AlGaAs system is nearly lattice-matched to

the GaAs substrates for all compositions. When the Al content increases, the bandgap

becomes large and indirect.With increasing Al composition, the wavelength will also decrease.

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AlGaInP The AlGaInP system was identified early as

one of the most promising for high-performance LEDs.

AlGaInP have high external quantum efficiency, like 20% at 630 nm, 10% at 590 nm, and 2% at 570 nm.

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AlGaInN AlGaInN are very differently from the

conventional III-V semiconductors.Due to they have large bond strengths, so they require high growth tempertures.

They can be grown on SiC and sapphire, but the lattice match between GaN and SiC is much better than for sapphire.

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Overview of device issues in high-brightness light-emitting diodes

Introduction Internal Quantum Efficiency Light Extraction

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Introduction 1962 : GaAsP LED (Red) 1970 : GaP:N LED (Green)

GaAsP:N (Red~Yellow) 1980s : AlGaAs LED (Red) 1990s : AlInGaP LED (Red~Green) 1993 : GaInN LED (entire visible spectral

region)

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Evolution of LED Performance

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Luminous performance of high performance LED

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Internal Quantum Efficiency High purity and low defect density

substrates and epitaxial structures A direct semiconductor energy gap

covering the desired color region A lattice-matched materials system enabling

the growth of heterostructure devices with low defect densities

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Light extraction The extraction efficiency is the fraction of

generated light that escapes from the semiconductor chip into the surrounding air.