The localization of excitons on quantum-dot-like compositional fluctuations has been observed in temperature-dependent near-field magnetophotoluminescence spectra of InGaAsN. Localization is driven by the giant bowing parameter of these alloys and manifests itself by the appearance of ultranarrow lines (half-width <1 meV) at temperatures below 70 K. We show how near-field optical scanning microscopy can be used for the estimation of the size, density, and nitrogen excess of individual compositional fluctuations (clusters), thus revealing random versus phase-separation effects in the distribution of nitrogen.

Raman spectra of coherently strained layers of GaAs 1Ϫx N x grown on ͑001͒ GaAs with xϭ0 -0.05 by metalorganic molecular-beam epitaxy are reported. The optical phonons of the GaAs and GaN types, as well as disorder-activated acoustical phonons, are observed. A strongly confined GaAs optical mode at ϳ255 cm Ϫ1 , indicating the ordering of As and N atoms, is also detected. The GaAs-and GaN-type optical phonons exhibit strong diagonal components, forbidden for the zinc-blende structure. A bond polarizability analysis of the Raman selection rules shows that these components are activated by the trigonal distortion of the alloy lattice. The trigonal distortion arises from the formation of ordered ͕111͖-(GaN) m (GaAs) n clusters with nϭmϭ1.

We used near-field magneto-photoluminescence scanning microscopy to study structural and optical properties of quantum-dot-like compositional fluctuations in GaAsN and InGaAsN alloys. We show that these fluctuations manifest themselves by the appearance of narrow emission lines (halfwidth 0.5-2 meV) at temperatures below 70K. We estimated the size, density, and nitrogen excess of individual compositional fluctuations (clusters), revealing phaseseparation effects in the distribution of nitrogen in GaAsN and InGaAsN. We found a dramatic difference in the Zeeman splitting of cluster lines between GaAsN and InGaAsN, indicating a strong effect of In on the exciton g-factor.

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