Degradation of InGaN/GaN laser diodes investigated by cross-sectional electron beam induced current imaging

In this study, the degradation mechanism of InGaN/GaN laser diodes has been investigated using cross-sectional electron beam induced current (EBIC) imaging, combined with scanning electron microscopy (SEM). By comparing the EBIC images of an untested and a degraded laser from the same laser bar, we show that the p-n junction position of the degraded laser has shifted towards the n-doped region and the carrier interface at the p-side of the p-n junction of the degraded laser became less sharp than in the original laser diode. This p-n junction position shift leads to carrier compensation in the active and the n-doped regions, which subsequently leads to an imbalance in the hole and electron concentrations in the active region and a reduction in the emission efficiency of InGaN quantum wells. This occurs because a displacement of the p-n junction, from its optimal position, results in a minority carrier loss, as the latter has a higher probability of recombining non-radiatively en-route to the active region. The minority carrier diffusion lengths were also calculated using two different theoretical models. The results show that the electron diffusion length in the p-doped region in the degraded laser is longer than that in the untested laser. The reason may also be related to the compensation effect or the reduced recombination rate in the degraded laser, which will result in an increase in the threshold current of the laser and the degradation of the laser.