High Brightness Light Emitting Diodes Chapter 1 Chapter

High Brightness Light Emitting Diodes Chapter 1 Chapter 2 屠嫚琳

Materials issues in high-brightness lightemitting diodes n n Techniques for production of III-V semiconductors for LEDs Specific materials systems 屠嫚琳 2

Techniques for production of III-V semiconductors for LEDs n n Liquid-Phase Epitaxy (LPE) Vapor-Phase Epitaxial (VPE) Molecular Beam Epitaxy (MBE) Organometallic Vapor-Phase Epitaxy (OMVPE) 屠嫚琳 3

Liquid-Phase Epitaxy (LPE) n n 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 4

Molecular Beam Epitaxy (MBE) n n 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 5

Organometallic Vapor-Phase Epitaxy (OMVPE) n n 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 6

Driving force of growth technique 屠嫚琳 7

Specific materials systems n n n Al. Ga. As Al. Ga. In. P Al. Ga. In. N 屠嫚琳 8

Al. Ga. As n n n Al. Ga. As was the first material for which very high brightness LEDs were demonstrated. The Al. Ga. As system is nearly lattice-matched to the Ga. As 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 9

Al. Ga. In. P n n The Al. Ga. In. P system was identified early as one of the most promising for highperformance LEDs. Al. Ga. In. P have high external quantum efficiency, like 20% at 630 nm, 10% at 590 nm, and 2% at 570 nm. 屠嫚琳 10

Al. Ga. In. N n n Al. Ga. In. N 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 Si. C and sapphire, but the lattice match between Ga. N and Si. C is much better than for sapphire. 屠嫚琳 11

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

Introduction n n 1962 : Ga. As. P LED (Red) 1970 : Ga. P: N LED (Green) Ga. As. P: N (Red~Yellow) 1980 s : Al. Ga. As LED (Red) 1990 s : Al. In. Ga. P LED (Red~Green) 1993 : Ga. In. N LED (entire visible spectral region) 屠嫚琳 13

Evolution of LED Performance 屠嫚琳 14

Luminous performance of high performance LED 屠嫚琳 15

Internal Quantum Efficiency n n n 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 16

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