Electron microscopic characterization of III-nitride films grown by MOCVD

The original promise of III-nitride materials was the bright emitting LEDs, which initiated the extensive research of GaN and related materials. Although the first GaN layers contained high number of defects, the LEDs worked and there was a hope to develop and prepare blue lasers in a simple way. By now it has been learned, that the defect density must be decreased in order to get device quality layers. A second major application of nitrides, the use of AlGaN layers for HEMT devices gave a new boom to the research in this field. III-nitrides are grown in the form of epitaxial layers on single crystalline substrates, in most cases on sapphire or on 6H-SiC. Metal organic chemical vapor deposition (MOCVD) is widely used to grow several micrometer thick epitaxial nitride layers and gives relatively high quality layers, while TEM is a very useful method to characterize the grown layers. Among the ternary layers AlGaN with different Al concentration and also AlN/GaN superlattices are investigated. Our results show that hexagonal inverted pyramids (also called V-shaped defects) are formed in AlGaN with similar shape, to those observed in InGaN. However, there are substantial differences in the formation of those defects in InGaN and in AlGaN. In the latter one the V-shaped defects are connected to inversion domains. Segregation of aluminium is observed as well. The defect density of nitride layers should be decreased further especially for laser devices. Less promising (like multiple buffer layers) and very promising methods like ELO (epitaxial lateral overgrowth) will be discussed as well. (Authors)