Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy

Scanning near-field photoluminescence spectroscopy has been applied to evaluate bandgap fluctuations in epitaxial AlGaN films with the AlN molar fraction varying from 0.30 to 0.50. A dual localization pattern has been observed. The potential of the small-scale (<100 nm) localization, evaluated from the width of the photoluminescence spectra, is between 0 and 51 meV and increases with increased Al content. These potential variations have been assigned to small-scale compositional fluctuations occurring due to stress variations, dislocations, and formation of Al-rich grains during growth. Larger area potential variations of 25-40 meV, most clearly observed in the lower Al-content samples, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. The density, size, and bandgap energy of these domains were found to be composition dependent. The lower bandgap domains were found to be strongly correlated with the regions with efficient nonradiative recombination. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3594239]