Gain suppression phenomena observed in InxGa1−xN quantum well laser diodes emitting at 470nm

Optical gain spectra were measured for InGaN-based laser diodes (LDs) emitting at 406nm (LD406) and 470nm (LD470) by employing the Hakki-Paoli method. The internal loss coefficient was as large as 35cm−1 for the LD470 compared to 25cm−1 for LD406. Moreover, gain saturation was observed at about 490nm just below the lasing threshold, and a gain band appears at higher photon energies for further carrier injection resulting in lasing at 470nm. Spontaneous emission peaks of electroluminescence were measured as a function of injection current density below threshold for both samples. The authors attribute the huge blueshift of the spontaneous emission of LD470 up to 450meV to a filling of the localized tail states in addition to that caused by the screening of the piezoelectric field. The blueshift for the LD406 was as small as about 30meV and can be interpreted as a result of the screening by both injected carriers and dopants.