Cluster size and composition variations in yellow and red light-emitting InGaN thin films upon thermal annealing

We study thermal annealing effects on the size and composition variations of indium-aggregated clusters in two InGaN thin films with photoluminescence ͑PL͒ in the yellow and red ranges. The methods of investigation include optical measurement, nanoscale material analysis, and theoretical calculation. Such a study is important for determining the relation between the band gap and the average indium content of InGaN. In one of the samples, the major part of the PL spectrum is shifted from the yellow band into the blue range upon thermal annealing. In the other sample, after thermal annealing, a broad spectrum covering the whole visible range is observed. Cathodo-luminescence ͑CL͒ spectra show that the spectral changes occur essentially in the photons emitted from the shallow layers of the InGaN films. Photon emission spectra from the deeper layers are essentially unaffected by thermal annealing. The spectral changes upon thermal annealing are mainly attributed to the general trend of cluster size reduction. This interpretation is supported by the CL, x-ray diffraction, and high-resolution transmission electron microscopy results. To obtain a basic physics picture behind the spectral blue shift upon thermal annealing in the yellow emission sample, we theoretically study the quantum-confinement effects of InGaN clusters based on a quantum box model. The theoretical results can generally explain the large blue shift of PL spectral peak position.