Optical and structural characterization of self-organized stacked GaN/AlN quantumdots

Abstract

Self-organized GaN/AlN stacked quantum dots (QDs) have been studied by means ofcathodoluminescence (CL), near field scanning optical microscopy (NSOM), photoluminescence,μ-Raman, and transmission electron microscopy. Assignment of the optical emissions wasmade on the basis of the structural parameters, power-dependent optical studies anddepth-resolved CL.Power-dependent studies allowed us to distinguish between quantum confined and bufferemissions. On increasing the power injection conditions, a QD-size-dependent blueshift due to the screening of the internal electric fields was found together with atrend to saturation observed in the high injection limit. The possible evidenceof excited states has also been shown by power-dependent photoluminescenceand CL. Different blue shifts in specimens with different numbers of stackedlayers suggested possible different residual strain values as confirmed byμ-Raman studies.Depth-resolved CL investigations performed at constant power injection per unit volumeallowed us to distinguish between QD layers with different nominal GaN coveragesand a linear dependence of peak energy versus GaN monolayer number has alsobeen found. Adding 1 ML of GaN resulted in an average shift of about 150 meV.The existence of QDs with different size distributions along the growth axis was also found.The observations were confirmed by NSOM spectroscopy.