Light-emitting diode quality investigation via low-frequency noise characteristics

There are presented a comprehensive investigation of noise characteristics (the fluctuations spectra of both the emitted light power and the voltage of light-emitting diodes (LEDs) at constant direct current (d.c.), and their simultaneous cross-correlation factor) for nitride-based green (InGaN) and phosphide-based red (AlInGaP) LEDs. The noise characteristics of LEDs before aging and during their aging have been carried out. It is shown, that large level of electrical fluctuations of the red LEDs did not show on high defectiveness of the active layer. These electrical fluctuations are related with the charge carrier recombination and capture processes in defects outside the active region, and they only weakly influence the emitted light power fluctuations. On the contrary, the large cross-correlation factor for the green LEDs shows that low-frequency electrical and optical noises are mainly due to defects in the active layer and its interfaces. It is shown, that during 1800 h aging the red LEDs characteristics are more stabile: their light output power has negligible changes, while analogical characteristics for the green LEDs gradually decreases over 30%.