A COMPREHENSIVE STUDY AND METHODS OF ELIMINATION OF OVAL DEFECTS IN MBE-GAAS

We have studied the mechanisms of formation and microstructure of oval defects in GaAs films grown by molecular-beam epitaxy, and discovered a novel technique for elimination of Ga-cell related oval defects. Plan-view and cross-sectional transmission electron microscopy (TEM) suggest that the nucleation of oval defects involves imperfections on the surface of the growing crystal such as excess Ga, Ga droplets, or contamination. TEM study also reveals massive dislocation tangles in the core region of oval defects. The majority of the large size oval defects with a core have dark centers in spatially resolved photoluminescence images. which suggests that the central part of such defects is optically inactive. Because of "Ga spitting", the initial density of oval defects in our system was very high, varying between 103 and 105 cm-2 depending on the GaAs thickness, growth rate, and growth conditions. By evaporating Ga from an Al-treated PBN crucible, Ga spitting and formation of Ga-cell related oval defects were eliminated. MBE-GaAs layers as thick as 20 μm were totally free from the Ga-cell related oval defects. Remaining oval defects were those related to particulates or substrate contamination and their densities were 100 and 500 cm-2 on 5 and 20 μm thick layers of GaAs on GaAs substrates. These particulates related oval defects were, however, hardly seen on up to 4 μm thick GaAs layers grown on Si substrates. Aluminum wets and reacts with the PBN crucible when heated up to 1300°C during evaporation of Al. Condensed gallium near the orifice wets the aluminum treated surface and is not expected to spontaneously flow to the Ga charge in the form of droplets. As a result, Ga-spitting does not occur and related oval defects do not form. With this technique, MBE-GaAs layers on contamination and particular free substrates can be free from all kind of oval defects. © 1990.