Morphological and optical characterization of GaN/AlN heterostructures grown on Si(111) substrates by MBE

We report an investigation of wurtzite GaN/AlN heterostructures grown on Si(111) substrates by MBE, based on scanning tunneling microscopy measurements in air and photoluminescence. The scanning tunneling microscopy analysis of the sample surfaces demonstrates that the morphology of the GaN layers depends strongly on the thickness of the thin AlN buffer layer and has a weaker dependence on the substrate temperature during growth. GaN layers grown directly on the Si substrate or on a thin (<10 nm) AlN buffer layer present surface defects that can be associated with the formation of screw dislocations with their axis parallel to the growth direction. The photoluminescence spectra show strong extrinsic emission lines for all the investigated samples.