Phase separation in metalorganic vapor-phase epitaxy AlxGa(1−x)N films deposited on 6H–SiC

The microstructure of AlxGa(1−x)N films deposited on 6H–SiC substrates by metalorganic vapor-phase epitaxy is studied by transmission electron microscopy and energy-dispersive x-ray analysis. It is shown that in the first step of the growth (three-dimensional), the deposit is constituted of a thin Al-rich AlGaN wetting layer covering the substrate surmounted by pure GaN islands. In a second step, a homogeneous AlxGa(1−x)N film, with nominal Al concentration, is deposited and smoothing is observed. The results of the high-resolution transmission electron microscopy about the strain state of the different parts of the deposit indicate that the driving force for the phase separation phenomena is the decrease of the interfacial energy due to the low mismatch between Al-rich AlGaN and 6H–SiC (1% for pure AlN).