High Brightness Light Emitting Diodes Chapter 1 Chapter 2 屠嫚琳

2 Materials issues in high-brightness light- emitting diodes Techniques for production of III-V semiconductors for LEDs Specific materials systems

屠嫚琳 3 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)

屠嫚琳 4 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.

屠嫚琳 5 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.

屠嫚琳 6 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.

屠嫚琳 7 Driving force of growth technique

屠嫚琳 8 Specific materials systems AlGaAs AlGaInP AlGaInN

屠嫚琳 9 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.

屠嫚琳 10 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.

屠嫚琳 11 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.

屠嫚琳 12 Overview of device issues in high- brightness light-emitting diodes Introduction Internal Quantum Efficiency Light Extraction

屠嫚琳 13 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)

屠嫚琳 14 Evolution of LED Performance

屠嫚琳 15 Luminous performance of high performance LED

屠嫚琳 16 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

屠嫚琳 17 Light extraction The extraction efficiency is the fraction of generated light that escapes from the semiconductor chip into the surrounding air.