Wide-band-gap semiconducting GaN is a promising candidate for fabricating blue light emitting diodes (LEDs) and UV-emitting laser diodes (LDs). In order to realize the production of these emitting devices, a method for obtaining high-quality GaN films and highly p-type GaN films must be developed. It has been very difficult to obtain high-quality GaN films because of the lack of high-quality lattice-matched substrates. Also, it has been impossible to obtain a p-type GaN film possibly due to the presence of high concentrations (typically above 1018 cm-3) of shallow donors, generally attributed to N vacancies. The surface morphology of these GaN films was markedly improved when an A1N buffer layer was initially deposited on the sapphire, as shown first by Yoshida et al. [162, 163]. More recently, Amano et al. [42, 156] and Akasaki et al.  have obtained high-quality GaN films using this A1N buffer layer by means of the metalorganic chemical vapor deposition (MOCVD) method. They showed that the uniformity, crystal quality, luminescence and electrical properties of the GaN films were markedly improved, and they developed GaN p-n junction LEDs for the first time using this A1N buffer layer technique [41, 153, 158]. Nakamura et al. succeeded for the first time in obtaining high-quality GaN films using GaN buffer layers instead of A1N buffer layers, as discussed in Sects. 4.5 and 5.1 [154, 159]. The crystal quality and p-type conductivity control of GaN films grown with GaN buffer layers were far superior to those of films grown with A1N buffer layers as demonstrated in Sect. 4.6. With the use of this GaN buffer layer technique, GaN p-n junction blue LEDs suitable for practical use were fabricated for the first time . In such p-n junction blue LEDs, Si was used as an n-type dopant because undoped GaN films had a carrier concentration as low as 1016 cm−3 .
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