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

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Abstract
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.