Blue-Green Light-Emitting Diodes and Violet Laser Diodes
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duced to the green region, the external quantum efficiency drops sharply because the band structure of AlInGaP becomes nearly indirect. Therefore highbrightness pure green LEDs, which have a high efficiency above 1% at the peak
p-electrode p-GaN — p-Alo.2Gao.sN — Ino.45Gao.ssN n-GaN —
GaN buffer layer.
Sapphire substrate —
Figure 1. The structure of a green single-quantum-well (SOW) lightemitting diode (LED).
100 -
400
450 500 550 600 Wavelength (nm)
650
Figure 2. Electroluminescence (EL) of (a) blue, (b) green, and (c) yellow SQW LEDs at a forward current of 20 mA.
700
wavelength of 510-530 nm with a narrow full width half maximum (FWHM), have not been commercialized yet. GaN and related materials such as AlGalnN are III-V nitride semiconductors with the wurtzite crystal structure and a direct energy band structure suitable for light-emitting devices. The bandgap energy of AlGalnN ranges between 6.2 and 1.95 eV, depending on its composition at room temperature (RT). Therefore these III-V nitride semiconductors are useful for light-emitting devices, especially in the short-wavelength regions. The spinel (MgAl2C>4) (9.5%)" and SiC substrates (3.5%),10 which have a small lattice mismatch in comparison with that (13%) between GaN and sapphire, also have been used for the growth of the III-V-nitride-based blue LED and LD structures. Recent research on III-V nitrides has paved the way for the realization of high-quality crystals of GaN, AlGaN, and GalnN, and of p-type conduction in GaN and AlGaN.11 13 The mechanism of the acceptor compensation, which prevents obtaining low-resistivity p-type GaN and AlGaN, has been elucidated.14 In Mg-doped p-type GaN, Mg acceptors are deactivated by atomic hydrogen, which is produced from NH 3 gas used as the N source during GaN growth. High-brightness blue LEDs have been fabricated on the basis of these results, and luminous intensities over 2 cd have been achieved.15 These LEDs are now commercially available. Also highbrightness, single-quantum-well (SQW)structure blue, green, and yellow InGaN LEDs with a luminous intensity above 10 cd have been achieved and commercialized.1617 Also recombination of localized excitons has been proposed as an emission mechanism for these InGaN quantum-well-structure LEDs.18 By combining these high-power and highbrightness blue InGaN SQW LEDs, green InGaN SQW LEDs, and red AlInGaP LEDs, many kinds of applications, such as LED full-color displays and LED white lamps for use in place of light bulbs or fluorescent lamps, are now possible with characteristics of high reliability, high durability, and low energy consumption. At present, the main focus of III-Vnitride research is the realization of a current-injected laser diode that can be operated under a continuous wave (CW) at RT. Recent developments have yielded optically pumped stimulated emission from GaN films,19-20 InGaN films,21'22 AlGaN/InGaN double heterostructures,23 and GaN/AlGaN double heterostructures.24'25 However stimulated emission had been observed only with optical 29
Blue-G
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