Piezoelectric photothermal spectroscopy (PPTS) measurements were carried out on In x Ga 1Àx N (x ¼ 0:01{ 0:32) thin films grown by radio-frequency molecular beam epitaxy. We found that the band energy shifts to the lower energy side of the spectrum (red shift) with an increase in the indium composition from 0.01 to 0.32. For samples with a lower indium composition, we were able to observe the exciton contribution, and the binding energy was estimated to be 27 meV (x ¼ 0:01). Since conventional photoreflectance (PR) spectroscopy was unable to observe signals for the samples with a higher indium content (x ¼ 0:13, 0.2, and 0.32), the usefulness of this PPTS method for samples with phase fluctuation is demonstrated.
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