We report on the degradation of high-power blue semiconductor lasers, with nominal wavelength of 450 nm and output power of 1.6 W, used for lighting, projection and metrology applications. Our analysis consists in constant current stress, aimed at investigating the changes in electrical and optical characteristics of the lasers. For the first time, we describe the degradation of the spectral characteristics during high current stress.
The study showed three main effects: (i) the decrease in the sub-threshold optical power, which shows two different slopes, that we ascribe to the regions where A, Shockley-Read-Hall (SRH) recombination coefficient, and B, radiative coefficient, dominate. (ii) a logarithmic decrease during the stress time of the characteristic temperature T<sub>0</sub>. (iii) the presence of a parasitic peak, with energy close to the main emission peak. This peak is ascribed to recombination in a second quantum well with slightly different energy, due to the different internal field. The intensity of this excited emission decreases during stress time, possibly due to a change in the injection efficiency.
We have also found an initial increase in the optical power at very low current levels, followed by a decrease with increasing stress time. This behavior is ascribed to an initial annealing, that favors the activation of magnesium, followed by an increment of the density of defects in the material caused by the stress.