Transition of Infrared Absorption Peaks in Thermally Annealed GaInNAs

We measured the infrared absorption of GaInNAs to clarify the origin of the large bandgap shift induced by thermal annealing. In the as-grown sample, an absorption peak due to the transverse-optical (TO) mode of Ga–N bonds was clearly observed at 469 cm-1. At higher annealing temperatures, the peak intensity decreased, and a new peak with an increased intensity appeared at 489 cm-1. Because the total absorbance of both peaks after the annealing was almost the same as that before the annealing, this peak transition resulting from thermal annealing is probably due to the alternation of the Ga–N bonding state. Because absorption peaks of larger bandgap semiconductors are generally observed in the higher wavenumber region, the observed transition of the peaks to the higher wavenumber side is consistent with the increased bandgap of GaInNAs. Therefore, we found that the variation in bonding causes the blue shift in the bandgap of GaInNAs during thermal annealing.