Scanning electron microscope studies of AlGaN films grown by organometallic vapor phase epitaxy

Microcathodoluminescence (MCL) and electron beam induced current (EBIC) studies were carried out on AlGaN films grown by organometallic vapor phase epitaxy (OMVPE) on sapphire and SiC. It is shown that both non-uniformities of distribution of deep recombination centers and of electron concentration can exist in AlGaN layers and in some cases these non-uniformities can be associated with the presence of certain types of defects. For example, regions adjacent to pores in AlGaN films are often depleted of deep centers one of which is the center responsible for yellow luminescence in AlGaN. Electron concentration is enhanced in hexagonal hillocks commonly observed for unoptimized growth conditions of AlGaN. There were also found non-uniformities in concentration of deep centers or in electron concentration that could not be related to particular morphological features or structural defects. Diffusion lengths measurements for various AlGaN films are also reported. The diffusion lengths are shown to be in the 0.3-0.5 μm range for most of the samples. In some of the studied AlGaN films it was found necessary to introduce two diffusion length values to describe the experimental EBIC distribution profiles. The longer diffusion length is found to increase in value for lower donor densities. Minority carriers lifetimes are found to be longer in AlGaN compared to GaN films.