Microscopic-scale investigation of the degradation of InGaN-based laser diodes submitted to electrical stress

This paper presents an extensive analysis of the degradation of InGaN-based laser diodes submitted to electrical stress. The analyzed devices, with emission in the violet spectral region, were submitted to constant current stress; the degradation process was monitored by means of electro-optical measurements, which indicated that stress induced an increase in the threshold current of the devices, ascribed to the generation of non-radiative defects. After stress, the (thick) top metallization was removed, and the optical behavior of the samples was characterized by micro-cathodoluminescence and micro-photoluminescence investigation. Results indicate that (i) stress induced a significant degradation of the efficiency of the devices under the ridge, i. e. in the region which is crossed by high current densities during ageing. (ii) the darkening of the ridge was detected both by micro-cathodoluminescence measurements (in which carriers are generated both in the barriers and in the quantum wells) and by micro-photoluminescence analysis with sub-bandgap excitation (with respect to the barriers). The experimental evidence collected within this paper demonstrates that the degradation of the laser diodes can be ascribed to an increase in the rate of non-radiative recombination within the active region of the devices, possibly due to a defect diffusion process. Hypothesis on the nature of the defects involved in the degradation process are formulated based on capacitance Deep Level Transient Spectroscopy measurements.