Growth by molecular beam epitaxy and characterization of Al(x)Ga(1-x)N alloys and heterostructures
Abstract
Research in semiconductors has recently been focusing on III-V nitrides due to their potential optoelectronic applications in the visible and ultraviolet. This thesis studies AlsbxGasb1-xN thin films and heterostructures grown by molecular beam epitaxy over the entire alloy composition. The study addresses heteroepitaxial growth onto a variety of substrates (sapphire and 6H-SiC), n- and p-type doping, film structure and optoelectronic properties. Reflection high energy electron diffraction and atomic force microscopy were used to probe the early stages of epitaxial growth. Techniques were developed for the growth of AlsbxGasb1-xN films directly on 6H-SiC without a buffer layer. The as-grown films had no cubic domains and the dislocation density close to the interface was 2 x 10sp9 cmsp{-2}. The initial stages of growth on sapphire were found to have a pronounced effect on the films' optoelectronic properties. X-ray diffraction was used to study issues related to strain, long range atomic ordering and superlattice effects. For the first time, kinetic conditions were identified for the growth of ordered AlsbxGasb1-xN films over the entire alloy composition. Superlattice structures of AlsbxGasb1-xN/GaN were grown and characterized. X-ray diffraction studies on off-axis peaks showed that the superlattices were strained but coherent. Furthermore, the in-plane lattice parameters of the superlattices assumed the in-plane lattice constant of the underlying bulk film. The diffraction pattern of on-axis peaks indicated that the interfaces of the structures were abrupt. The optoelectronic properties of the AlsbxGasb1-xN films grown were investigated. Photoluminescence and transmission experiments showed that the bandgap of these materials varied linearly with the c-lattice parameter. The films were doped n- and p-type with Si and Mg respectively. The donor activation energy was found to increase with Al concentration while the electron mobility decreased as the AlN mole fraction increases. Light emitting diode structures based on AlsbxGasb1-xN/GaN multi quantum wells were formed and the electroluminescence spectra indicated recombination across the bandgap of GaN.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1998
- Bibcode:
- 1998PhDT.......178K