Optical and structural characterization of GaN/AlN quantum dots grown on Si(111)

GaN/AlN-based heterostructures made from stacked GaN quantum dots (QDs) have been studied by means of the cathodoluminescence (CL), photoluminescence (PL), near-field scanning optical microscopy (NSOM) and micro-Raman techniques. The influence of the number of stacked layers (2-85) and of the different electron beam injection conditions on the main optical emissions was studied by means of CL, revealing transitions from 2.5 and 4.4 eV. Power-dependent cross-sectional CL studies revealed a large (87-180 meV) blue-shift only for the optical bands located in the 2.5 and 3.1 eV spectral range. This observation enabled us to assign a zero-dimensional character to those bands. The results were confirmed by PL and NSOM studies. Different values of the blue-shift were found for specimens with different numbers of stacked layers. This suggested the presence of different residual strains inside the structures, as confirmed by micro-Raman studies. An inhomogeneous distribution of the QD emissions was also observed both in the plane and along the growth direction.