ABSTRACT
GaN and related materials, such as AIN and InN, have attracted attention because of their application to the short-wavelength luminescent devices for displays, lightings and data storage devices as well as high-temperature and high-power electronic devices. Ultra-bright GaN based LEDs grown on sapphire substrate were first demonstrated by Nakamura, who adopted a metalorganic chemical vapor deposition (MOCVD) as the growth method [1]. Recently, GaN based high-power ultraviolet light-emitting diodes (UV-LED) have been realized using the lateral epitaxy on patterned substrate (LEPS) technique by MOCVD [2]. Ammonia gas source molecular beam epitaxy (NH3 GS-MBE) was also promising growth method [3]. However, since appropriate GaN substrates are still not commercially available, Π -nitride-based optical devices were commonly fabricated on lattice-mismatched substrates such as sapphire. Those devices contain a large number of structural defects in their active layers due to the large lattice-mismatch and/or the large difference in thermal expansion coefficient between ΙΠ-nitrides epitaxial films and substrates. The structural characterization of the epitaxial films is one of the most important keys to obtain high-quality heteroepitaxial GaN for high-power LEDs. In addition, the correlation between crystalline structures and optical properties is still unclear.
